literature review on ro water purifier

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• Published: 22 July 2021

A critical review of point-of-use drinking water treatment in the United States

• Jishan Wu 1   na1 ,
• Miao Cao   ORCID: orcid.org/0000-0003-1564-7336 1   na1 ,
• Draco Tong 2 ,
• Zach Finkelstein 3 &
• Eric M. V. Hoek   ORCID: orcid.org/0000-0002-9674-1916 1 , 4 , 5  

npj Clean Water volume  4 , Article number:  40 ( 2021 ) Cite this article

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• Chemical engineering
• Water resources

An Author Correction to this article was published on 06 April 2022

This article has been updated

Ensuring safe water supply for communities across the United States is a growing challenge due to aging infrastructure, impaired source water, strained community finances, etc. In 2019, about 6% of public water utilities in the U.S. had a health-based violation. Due to the high risk of exposure to various contaminants in drinking water, point-of-use (POU) drinking water treatment is rapidly growing in popularity in the U.S. and beyond. POU treatment technologies include various combinations of string-wound sediment filters, activated carbon, modified carbon, ion exchange and redox media filters, reverse osmosis membranes, and ultraviolet lamps depending on the contaminants of concern. While the technologies are well-proven, highly commoditized, and cost-effective, most systems offer little in the way of real-time performance monitoring or interactive technology like other smart home appliances (e.g., thermostats, smoke detectors, doorbells, etc.). Herein, we review water quality regulations and violations in the U.S. as well as state-of-the-art POU technologies and systems with an emphasis on their effectiveness at removing the contaminants most frequently reported in notices of violations. We conclude by briefly reviewing emerging smart water technologies and the needs for advances in the state-of-the-art technologies. The smartness of commercially available POU water filters is critiqued and a definition of smart water filter is proposed.

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Introduction.

Access to clean water is of utmost importance for human health and society at large. References to water purification and filtration methods can be traced back to ancient Sanskrit and Egyptian writings—including descriptions of boiling, solar heating, and sand filtration 1 . Hippocrates, often referred to as the “father of medicine,” found that water could be made purer by filtering it and, in 500 BC, he designed a simple sediment filter by running water through cloth 2 . In modern times, sand filters were first documented as a water treatment device in 1804. By 1852, the Metropolis Water Act in London required the use of sand filters in part of the city 3 . The filters removed suspended solids, but did not address pathogenic microorganisms or chemical contaminants since microbiology and analytical chemistry were not yet adequately established 4 . In the United States, drinking water standards were gradually developed over the 20th century, culminating in the passage of the Clean Water Act (1972) and the Safe Drinking Water Act (1974), which were part of a landmark decade of promulgating new environmental regulations.

Water quality can be broken into numerous physical, biological, and chemical components 5 . Physical water quality descriptors include turbidity, total, settleable, filterable and dissolved solids, color, taste, odor, and temperature. Biological quality refers to protozoan, bacterial, and viral pathogens. Biological contamination is often an immediate health risk: crippling outbreaks of typhoid, cholera, salmonella, and other diseases have been spread through contaminated water supplies. Chemical components include trace organic and inorganic compounds, which may be toxic to humans and can also cause discoloration, poor taste, or odor 6 . Toxic chemicals may lead to both acute and chronic health effects. Water quality regulations in the U.S. were developed to address all three classes of contaminants. Primary drinking water standards are defined by maximum contaminant levels (MCLs) established by the U.S. Environmental Protection Agency (EPA) 7 . The standards focus on biological and chemical contaminants. The trace chemical contaminants are sometimes less than one part per billion and may be set at or near the limits of analytical detection methods 7 . Physical water quality components are mainly covered by the U.S. EPA’s secondary drinking water standards, which are unenforced unlike their primary counterparts 8 .

Although water quality is well-regulated in the U.S., there is considerable variation in contaminant levels by location. Consequently, consumers who are concerned about the quality of their water supply often purchase bottled water or various water-filtration devices to remove any remaining impurities. For instance, a recent set of studies conducted in Los Angeles, CA, USA has determined the following 9 , 10 :

Levels of distrust in tap water are high, especially among households of color (e.g., LA County had 2nd highest level of distrust among urban areas in the country before Flint).

Equating distrust with misperception in all cases (as many water systems and public health agencies do) is incorrect and generic “education” approaches to improve trust are neither effective nor respectful.

Much of distrust appears “rational” and stems from past/present experience of unclean, if not unsafe (we draw a distinction here) drinking water, much of it from premise plumbing.

Solutions to issues of premise plumbing are tough especially due to tenancy split-incentive issues, but legal and especially financial incentive approaches from other sectors can be brought to bear.

The consequences of distrust are severe for household health, finances, trust in the government, and the environment.

Since water quality degradation may occur in the distribution system, one solution could be widespread implementation of point-of-entry (POE) water treatment where a POE system is installed at a household’s or building’s water main intake ahead of the structure’s taps, faucets, or other dedicated outlets used to dispense water for drinking, cooking, and bathing. However, degradation can occur in premise plumbing (e.g., copper pipes) in older buildings, and hence, it may make the most sense to deploy point-of-use (POU) water treatment just ahead of the tap, faucet or dispensing outlet. In this review, we focus on POU water treatment.

Typical POU systems contain water treatment technologies such as media filtration, reverse osmosis (RO) membranes, UV disinfection, and remineralization (particularly after RO) 11 . Large particles, rust, and debris are first removed by filtration through string-wound sediment filters. Next, some form of selective separation may be employed such as redox media, activated carbon (AC), and/or ion exchange (IEX). Membrane technology, most commonly RO, removes nearly all suspended and dissolved contaminants such as dissolved organic chemicals, dissolved metals, minerals, and salts 11 . UV disinfection inactivates pathogenic microorganisms, rendering them non-infectious 11 . Remineralization after RO filtration is often used to add back the minerals removed by earlier stages to provide pH-buffered, better-tasting water 11 . In each step, there are various technologies available with different contaminant removal efficacies to satisfy a variety of situations and needs. In addition, emerging POU treatment technologies such as capacitive deionization (CDI) are attracting attention because of their selective contaminant removal 12 .

The rapid development of Internet technologies has encouraged many home-appliance manufacturers to provide “smart” products, including “smart” POU filters. There are various definitions of smart home appliances 13 , 14 . The consensus is that if a product is smart, it is one that can be remotely controlled by the user via a smart phone, tablet, or other device. Connectivity and interaction with the user via an “app” is achieved using WiFi or Bluetooth ® technology. Smart filter systems take many forms and have differing levels of sensor integration, but information on which filter media and sensors are included in home water treatment systems has been lacking. Moreover, different manufacturers seem to have different views on the smartness of water filters. Some products claim themselves to be “smart” because they can provide water with better quality, which does not satisfy the connectivity requirement of other smart home appliances.

This study reviews U.S. federal and (several) state regulations, the frequency and nature of water quality violations in the U.S., state-of-the-art POU water treatment technologies and their contaminant removal capabilities, especially emerging contaminants. Further, representative commercially available POU systems are compared, making note of filter types, any sensors employed, expected service life, and other details. Finally, the smartness of commercially available POU water filters is critiqued and a definition of smart water filter is proposed.

Water quality regulations, violations, and hazards in the US

Access to clean drinking water is imperative because of the potential for both acute and chronic health risks associated with drinking contaminated water. Federal regulations serve the purpose of reducing the likelihood of becoming ill from drinking the tap water. The EPA regulates contaminants by establishing MCLs for microbiological, organic, and inorganic contaminants based on health guidelines, research, and feasibility 15 . These standards delineate the maximum amount of a contaminant that can be allowed in drinking water to minimize exposure. States may build on the EPA’s standards by adding additional contaminants not regulated at the federal level and by further reducing MCLs for federally regulated contaminants.

Federal drinking water regulations

To regulate drinking water, the EPA establishes primary and secondary drinking water standards. Primary standards are enforceable by law and apply to all the U.S. public water systems; their goal is to limit levels of harmful contaminants in drinking water. The EPA 15 has a list of 88 contaminants regulated in the primary standards with the following contaminant categories and numbers: 3 disinfectants, 4 disinfection byproducts (DBPs), 16 inorganic chemicals, 8 microorganism categories, 53 organic chemicals, and 4 radionuclides. The EPA regulates most of these contaminants by establishing MCLs that can be present in the effluent of drinking water treatment plants. These MCLs are intended to keep people safe, but they are not necessarily safe. The maximum contaminant level goal (MCLG) is the amount of a contaminant in drinking water at which there is no known or expected risk. MCLs are determined by feasibility of measurement, removal, and enforcement in combination with MCLGs, so there may be some health risks even with MCLs in place.

To supplement the enforced primary standards, the EPA sets unenforced secondary drinking water standards. They are intended to improve aesthetic qualities of water such as taste, color, and odor. According to the EPA, these standards are important because if water looks, tastes, or smells bad, people may not drink it even if it is perfectly safe. Some other secondary standards help control scaling, which restricts water flow and corrosion, which can cause pipes to wear out or dissolve harmful contaminants previously fixed within the mineral scale 8 .

The EPA also maintains a contaminant candidate list (CCL) for compounds that are not currently regulated but are expected to be found in public water systems and may require regulation in the future 16 . The CCL serves an essential purpose in the process of enacting water quality regulations. Every 5 years, the EPA decides if it will regulate or not regulate at least five contaminants on the CCL. In February 2020, the EPA made preliminary decisions to regulate perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), but not to regulate six other chemicals including dichloroethane and acetochlor 17 . They make these decisions using data collected about these contaminants and compare it to the criteria for regulation under the Safe Drinking Water Act (SDWA). The CCL must be updated every 5 years, and the contaminants with the greatest potential health risks in drinking water shall be placed on the list 16 . Once the EPA decides to regulate a contaminant, it can take years before a regulation is enacted. For example, the EPA decided to regulate perchlorate in 2011, but as of 2020, the EPA still has not set a MCL for perchlorate 18 . Because it takes many years to regulate a chemical that it deems to be unsafe for human consumption 19 , there may be chemicals present in drinking water for which negative healthy effects are known, but no action has yet been taken.

State drinking water regulations

States are required to have standards at least as strict as EPA standards for primary drinking water treatment 20 . Yet, state standards may vary from the EPA standards, providing room for states to regulate certain contaminants more strictly or address contaminants that are not yet federally regulated 21 , 22 . For example, in California, contaminants are regulated because of determinations made by the California Office of Environmental Health Hazard Assessment, which sets public health goals based on the health impacts of individual contaminants 23 , 24 . For carcinogenic contaminants, they create regulations based on the risk of cancer from exposure to different amounts of the contaminant. Typically, the acceptable risk is for—at most—one person in a million to get cancer upon exposure over 70 years. After proposing a standard based on current research, they consult a group of scientific experts, make further revisions, and finally allow public comment. After setting a goal, they can establish an enforceable standard that is as close as possible to the goal while considering economic and technical feasibility. This process is similar to how the EPA sets its MCLs, but because it is separate from the EPA, they can regulate chemicals of local concern such as agricultural contaminants 25 .

Table S 1 7 , 26 , 27 , 28 compares the EPA’s primary drinking water standards to the drinking water regulations of several states; it also displays the health effects of exposure and the origins of these contaminants. Alaska, Texas, and California exhibit an exemplary range of different state’s approaches to regulations, with California being the most stringent 29 . Exposure to regulated contaminants can cause a variety of health issues including cancer, kidney problems, nervous system problems and more, which is why these chemicals are regulated by the EPA and states. In addition, one clear commonality amongst the origins of these contaminants is that they frequently come from industrial operations that discharge waste into the environment.

Violations of standards

Even though regulations exist to limit exposure to toxic contaminants, sometimes public water utilities violate existing standards. Public water utilities are categorized by the EPA as community water systems (CWSs), transient non-community water systems (TNCWSs), or non-transient non-community water systems (NTNCWSs) (Fig. S 1 ). The EPA then classifies the size of these public water systems in categories of very small, small, medium, large, and very large (Table 1 ) 30 . Fig. S 2 30 displays the amount of each type of public water system by size. It can be seen that CWSs represent a larger percentage of public water systems as the size of the population served increases, which means they end up serving residential communities, whereas smaller public water systems tend to be TNCWSs.

The EPA publishes a database with information about the types and sizes of public water systems and the violations that occur within these public water systems. Violations required to be reported under SDWA of EPA are grouped into the following categories 31 :

Health-based, including 3 categories: (1) exceedances of the maximum contaminant levels (MCLs) which specify the highest allowable contaminant concentrations in drinking water, (2) exceedances of the maximum residual disinfectant levels (MRDLs), which specify the highest concentrations of disinfectants allowed in drinking water, and (3) treatment technique requirements, which specify certain processes intended to reduce the level of a contaminant 31 .

Monitoring and reporting: failure to conduct regular monitoring of drinking water quality, or to submit monitoring in time, as required by SDWA 31 .

Public notice: systems are required to alert consumers if there is a serious problem with their drinking water or if there have been other violations of system requirements, as required by SDWA 31 .

Others: violations of other requirements of SDWA, such as failing to issue annual consumer confidence reports 31 .

Table 2 shows the number of serious violations by treatment plant size. A serious violation is when a public water system has unresolved serious, multiple, and/or continuing violations, which need to be returned to compliance or the system will be faced with formal enforcement action 30 . Many serious violators have violated monitoring and reporting guidelines; they fail to regularly monitor drinking water quality or promptly submit monitoring results to the EPA or a public health agency 32 . These violations indicate mismanagement or neglectful monitoring rather than an immediate health hazard.

However, some violations are health-based violations where public water systems exceed MCLs, maximum residual disinfectant levels, or have an incorrect treatment technique that is put in place to remove certain contaminants 30 . Especially, those violations that can pose immediate health effects are called acute health-based violations. There were over 6.5 million people affected by health-based violations in the United States in 2019. Violations including exceeding monthly allowed turbidity levels, treatment technique violations, Escherichia coli present in treated water, and nitrate violations have been reported 30 .

Allaire et al. 33 . evaluated spatial and temporal patterns in health-related violations of the SDWA using a panel dataset of 17,900 CWSs from 1982 to 2015. About 21 million people are affected by health-based water quality standard violations in the year 2015, according to the study 33 .

During each year between 1982 and 2015, 9–45 million people, up to 28% of US population, were affected 33 . Health-based violation was observed in about 8.0% of the 608,600 utility-year observations, while total coliform violation is observed in about 4.6% of all observations 33 . In total, 95,754 health-based violations were observed, and 37% of all violations are the total coliform type (Fig. 1a ). About 36% of violations are categorized as “other” contaminants, primarily DBPs. While violations of treatment rules and nitrate are less commonly observed (21% of total) 33 .

a Number of health-based violations, and b total violations per water system. In total, 95,754 health-based violations were observed from 1982 to 2015, affecting up to 28% of US population. Rural areas have a larger compliance gap than suburban and urban areas; however, fewer violations with DBP violations were observed in rural areas with higher incomes (reprinted with permission from 33 ; Copyright © PNAS, 2018).

The number of violations per CWS (Fig. 1b ) differs between rural and urban areas. Rural areas have a larger compliance gap than suburban and urban areas, however, fewer violations with DBP violations were observed in rural areas with higher incomes 33 . Differences between rural and suburban areas were exaggerated after new DBP rules in the early 2000s 33 , corresponding to the spike in Fig. 1b . Due to limited financial resources and technical expertise, regulatory compliance is a challenge for rural systems 33 . In contrast to large systems, small systems face restricted access to loans and outside financing 34 . Moreover, smaller customer base has less revenue for infrastructure improvements, repayment of debt, and salaries to attract technically skilled operators 34 . All these factors make the rural system operations and development challenging, and eventually may trigger the violations.

Violations also vary geographically. The distribution of the total number of violations, from 1982 to 2015, per CWS in a given county is shown in Fig. 2A . The majority of violations are observed in rural areas, located in Texas, Oklahoma, and Idaho. Total coliform violations, as shown in Fig. 2B , are primarily observed in the West and Midwest. Differences of violations across counties can be attributed to the difference of quality of source water as well as the state-level enforcement 33 . Other factors such as different temperatures at different seasons can also contribute to the regional difference of violations across the U.S. For instance, high summer temperatures might cause the Southwest region to be particularly susceptible to DBP violations. SDWA violations are mostly identified in Oklahoma and parts of Texas, based on local spatial autocorrelation, shown in Fig. 2C . 11% of the CWSs have repeat violations, including two or more subsequent years of a violation 33 . The states with the greatest proportion of CWSs with repeat violations are Oklahoma (43% of CWSs in the state), Nebraska (35%), and Idaho (33%) 33 .

A Total violations. B Total coliform violations. C Spatial clusters (hot spots) of health-based violations, 1982–2015. Violations also vary considerably across geographic locations. Some of the counties with the highest prevalence of violations are rural, located in Texas, Oklahoma, and Idaho (reprinted with permission from 33 , Copyright © PNAS, 2018).

Table S4 shows the breakdown of the size of treatment plants and the source of water. Larger treatment plants tend to use surface water, whereas smaller treatment plants predominantly use groundwater. From the above table and information about the different types and sources of violations of drinking water treatment plants, the percentage of violations by water source can be determined. The values in Table S5 were computed using the number of surface water and groundwater violations by size and comparing that to the total number of treatment plants using either surface water or groundwater as a source by size (data from Table S4 ). The percentages of CWSs, NTNCWSs, and TNCWSs were computed as well, using the number of violations of those types by size and comparing that to the total numbers of treatment plants by type and size (data from Table S4 ). Table S5 shows that with every type of violation, treatment plants that use surface water as a source tend to have a higher percentage of violations than treatment plants that use groundwater as a source. As the size of the treatment plant increases, the percent of violations amongst public water systems that use surface water tends to decrease. The only exception seen here is for treatment plants of very large size. In addition, CWSs typically have slightly higher percentages of violations (Table S5 ). This analysis, presented in Table 3 , shows that CWSs tend to have a higher percentage of surface water sources compared to NTNCWSs and TNCWSs.

Non-grid-tied water resources

Domestic wells (private or homeowner wells) are the dominant source of drinking water for people living in rural parts of the United States 35 . Population distribution using domestic supply wells per square kilometer is shown in Fig. 3a . Over 43 million people, 15% of the U.S. population, rely on domestic (private) wells as their source of drinking water 36 . These private wells are not regularly tested for known contaminants, and thus, may pose unknown health risks. The water safety of domestic wells is not regulated by the Federal Safe Drinking Water Act or, in most cases, by state laws. Instead, individual homeowners are responsible for maintaining and monitoring their own wells 36 .

a Population using domestic wells, and b domestic wells affected by arsenic. Over 43 million people, 15% of the U.S. population, rely on domestic (private) wells as their source of drinking water. About 2.1 million people in the conterminous U.S. were using water from private wells with predicted arsenic concentration >10 μg/L (reprinted with permissions from 35 , 36 ; Copyright © Elsevier, 2017; Copyright © ACS, 2017).

In a study of 2100 domestic wells, water in about 20% of the wells is contaminated with one or more contaminants at a concentration greater than MCLs 35 , 36 . Table 4 summarized some common contaminants in domestic wells which frequently exceeding health standards (MCLs regulated by USEPA or U.S. Geological Survey (USGS) Health-Based Screening Levels) in tests. The most common contaminants that were found to exceed health standards were metals including lead and arsenic, radionuclides, and nitrates 37 . Nitrates in drinking water supplies can cause harm such as methemoglobinemia in young children, but nitrates rarely cause direct harm to adults 36 . Microbial contaminants (for example, bacteria) were found in about 30% of wells tested, about 400 wells in total 36 . Ayotte et al. 35 . developed a logistic regression model of the probability of having arsenic >10 μg/L (“high arsenic”) from 20,450 domestic wells in the U.S. As shown in Fig. 3b , approximately 2.1 million people in the conterminous U.S. were using water from private wells with predicted arsenic concentration >10 μg/L 35 . Some states have both relatively large population, over 1 million people, and high percentages, over 1%, of total state populations with arsenic >10 μg/L. It is noteworthy that 60% of all counties with the largest population with high-arsenic wells are located in New England; other top-10 counties are located in Ohio, North Carolina, California, and Idaho, respectivly 35 . Considering the high risk of exposure to the various contaminations, it is therefore imperative to apply additional treatments, such as POU, before using the well water in households.

Contaminants of emerging concern with no regulations

Contaminants of emerging concern (CECs) are chemicals or microorganisms that are not commonly monitored in drinking water because they do not have established MCLs 38 . A USGS study found that over 80% of streams in the U.S. contained some form of emerging contaminant including pharmaceuticals, hormones, detergents, plasticizers, fire retardants, pesticides, and more. Although these were generally found at low concentrations, a growing number of research report their close relationships with some human diseases 39 , 40 . In addition, a more recent study found that about 8% of groundwater sources used for drinking water contain hormones and pharmaceuticals 41 . The unregulated status of these contaminants makes them unmonitored by treatment plants in many cases. It is also unknown how much of them end up in drinking water after drinking water treatment. Thus, there is potential health risk for people consuming these contaminants in drinking water.

Table 5 shows the features of several typical CEC types in drinking water. N -Nitrosodimethylamine (NDMA) is a semi-volatile organic compound used to help produce liquid rocket fuel, antioxidants, and additives for lubricants. Animal studies have found that NDMA causes cancer in the liver, respiratory tract, kidneys, and blood vessels 39 . NDMA is also expected to be carcinogenic to humans 42 , while EPA has not set a MCL for NDMA yet. However, it has been placed on the fourth contaminant candidate list (CCL4). Also, several states have guidelines (not regulations) for levels of NDMA that could exist in water. In California, several nitrosamines have guidelines set that were above a specified level (in the instance of NDMA, 300 ng/L), and a response is recommended. Potential treatments for NDMA include photolysis with UV radiation 43 , biological treatment, microfiltration, and RO treatment. Despite these treatments, it may still be present in water because it is a byproduct of chlorination, which occurs after treatment 39 .

Pharmaceutical and personal care products (PPCPs) are commonly found in sources of drinking water and enter these sources through domestic wastewater, hospital discharges, improper manufacturer disposal, and wastewater treatment plants 44 . PPCPs typically enter wastewater through human excrement or bathing and washing activities 45 . The amounts of PCCPs found in these treatment plants is low with concentrations between ng/L and μg/L. However, their long-term health effects are unknown and they can cause health issues through accumulation in the food chain 46 . In addition, some PPCPs containing amine groups demonstrate the potential to react with chloramines in the disinfection process to form toxic nitrosamines such as NDMA, which is not federally regulated and can cause adverse health effects as stated before 47 .

1,4-dioxane is another concerning contaminant given its classification as a probable human carcinogen. Approximately 30 million people in the U.S. have levels of 1,4-dioxane exceeding the health reference level for cancer, which indicates that it poses a serious risk to human health 48 . It is currently on the EPA’s CCL4 and has been on prior CCLs, which indicates 1,4-dioxane’s recognition as an emerging contaminant 39 . The problems with 1,4-dioxane include that it is highly soluble in water and does not react easily with other chemicals. In addition, AC filters do not absorb it. The best-known removal method appears to be RO 48 .

Methyl tert-butyl ether (MTBE) is an additive used in gasoline, designed for more efficient fuel combustion thus to improve overall air quality. It can cause liver, kidney, immune system, testicular, central nervous system, uterine, headache, and lung problems 40 . Like other CECs, no regulations have been established for MTBE by the EPA. In California, an established MCL for drinking water is 13 μg/L and a secondary maximum contaminant level (SMCL) is 5 μg/L 49 . The SMCL was established for water quality aesthetic properties such as taste and odor 42 .

Perfluorinated compounds such as PFOS are extremely hazardous emerging contaminants that enter the environment through their applications in the metal industry, firefighting foam, coatings on paper and textiles, and semiconductor production 50 . They can also occur due to biotransformation of dipolyfluoroalkyl, phosphates, fluorotelomer alcohols, and other chemicals 51 . They are persistent in the environment and tend to accumulate in red blood cells 48 . PFCs can cause pancreatic, liver, and Leydig cell cancers 40 . They are frequently found in treated drinking water with levels of up to 1000 ng/L, and over 6 million people receive water from systems that exceed health advisory levels for PFAS 48 . Studies have concluded that people who drink water with PFAS in it have higher levels of PFAS in their blood, indicating the contaminant’s health risk 48 . PFOS are easily removed by using granular activated carbon (GAC) filters which can remove over 90% of them and ROMs which can remove more than 99% of them 44 . The EPA decided in 2020 to regulate PFOA and PFOS in drinking water, but it may take many years before a MCL can be established as was the case with other contaminants taking over 10 years between the decision to be regulated and actual regulation 44 .

Antibiotics are another concerning contaminants that can be found in water. Antibiotics in water can cause the rise of antibiotic-resistant genes and antibiotic-resistant bacteria. This can make the use of antibiotics less effective against human and animal pathogens. As of now, there are approximately 2 million people who die in the U.S. from antibiotic-resistant bacteria per year, which is why it is important for them not to end up in aquatic environments 48 . Antibiotics can be detected at very low levels across the United States in the sources of drinking water (levels of between 20 and 60 ng/L) 52 . They are rarely detected in treated drinking water, and if they are detected, the levels are even lower (5–20 ng/L), and thus present little risk to human consumption themselves 53 .

Another concerning area of emerging contaminants is DBPs which are produced when chemical oxidants (e.g., chlorine, ozone, chloramine, etc.) are used for disinfecting microbes in drinking water. Over 700 DBPs have been identified by EPA, while only 11 types are regulated 48 . DBPs have been known to cause cancer and birth defects 48 . Thus, they too pose a risk to human health despite regulations that exist.

In summary, although well-intended and well-developed, the U.S.’s drinking water regulations do not fully assure the quality of tap water to prevent either short-term or long-term illness from drinking it. Improving upon water treatment technologies and moving them closer to the POU is a way to help remove contaminants that are not regulated yet or are introduced during distribution from the treatment plant to the tap. GAC, RO, UV radiation, and combinations of the above are all advanced water treatment technologies that remove emerging contaminants effectively. Although there has been much research on the mechanics and removal efficacies of these water treatment technologies, little information is available on their application in POU water treatment.

POU water treatment technologies

POU drinking water treatment systems are installed on the water supply lines ahead of water taps, showers, and dispensers to provide on-site purification of water for drinking, bathing, or cooking. A wide range of POU technologies have emerged in the past two decades including AC, redox media, ROMs, UV disinfection, CDI, and others. They are usually combined in a specific sequence to form a POU system (Fig. 4 ). The systems are thus expected to remove hazardous contaminants exceeding regulation limits while keeping those substances that are healthy and essential for human health.

A wide range of point-of-use technologies have emerged in the past two decades including activated carbon, redox media, RO membranes, UV disinfection, CDI, and others.

Media filtration

Sediment filters.

The most basic type of POU filter is the sediment filter, a form of physical filtration. It removes suspended solids from water, such as insoluble iron and manganese, and reduces water turbidity. In Fig. 5 , we present a schematic of the flow configuration and a photo of typical string-wound sediment filter 54 . Suspended solids from untreated water will accumulate throughout the depth of the filter material, while dissolved contaminants are not retained. Therefore, the classification and removal efficacy of sediment filters is highly dependent on the pore size of the filter media. For example, a “5 μm” filter is able to capture sediments as small as about 5 μm 55 . In addition, the filter rating is commonly described as either “nominal” or “absolute.” Nominal filters are expected to trap >90% of particles larger than the pore size ranting, while absolute filters should trap about >99%.

Suspended solids from untreated water will accumulate throughout the depth of the filter material, while dissolved contaminants are not retained. Therefore, the classification and removal efficacy of sediment filters is highly dependent on the pore size of the filter media (reprinted with permissions from 54 , 57 ; Copyright © Filters Fast LLC, 2020; Copyright © UNISUN, 2020).

The string-wound sediment cartridge filter is a common type of POU sediment filter, which is made from a central cartridge wrapped in string (Fig. 5 ). This type of filter typically has a micron-rating from 0.5 to 200 depending on the diameter of the string. The filter functions by mechanically trapping particles that are larger than the characteristic space between strings; sometimes thinner string is used near the core center and thicker string on the outside. In this way, the string-wound sediment filter can capture particles not only on the cartridge surface, but also through its depth and at the core surface.

The first string-wound filter cartridge entered the U.S. market around the mid-1930s. It was made of a woven wire mesh core surrounded by cotton yarn 56 . Today, this type of cartridge filter has evolved considerably by improving the filter material and the media arrangement. For example, by adding silver ions to the polypropylene yarn, the string-wound filter inhibits the growth of microbes. Also, to be compatible with corrosive solutions or high-temperature fluids, stainless-steel can be used as the core material to enhance the polypropylene stability and prevent it from swelling or softening 57 .

Ion exchange resins

Ion exchange (IEX) is a reversible chemical reaction between compounds in the aqueous phase and fixed charged functional groups on and within a solid phase. Polymeric resins are the most common IEX materials, widely used not only in POU filtration, but also in large-scale water and wastewater treatment, hydrometallurgy, chromatography, and sensors 58 . Although there seem to be a variety of IEX resins for water treatment in the market, they can be roughly categorized into five groups based on their framework (Table 6 ): strong-acid cation (SAC) resins, weak-acid cation (WAC) resins, strong-base anion (SBA) resins, weak-base anion (WBA) resins, and metal-selective chelating (MSC) resins 59 . Cation resins are extensively used in the water softening process, removing hardness ions (e.g., Ca 2+ , Mg 2+ , and other divalent cations). WAC resins are more effective for treating feed waters containing a high concentration of hydrogen peroxide or chlorine than SAC resin because they have a higher resistance to corrosive oxidants and are more stable. It is worth noting that different anion resins have a varied affinity to different acids, and WBA resins cannot remove weak acids such as carbon dioxide and silica 60 .

For POU applications, IEX resins meet the needs of water softening and demineralization. An acidic environment with high lead concentration might occur when solders and/or pipes corrode 61 . Cation softening resin is used to adsorb positively charged lead and many other metal ions. With a high molecular weight and a 2 + valence, lead has a high affinity towards the cation resin, MSC resin 62 . In addition, perchlorate can be removed to a very low level by SBA resin 63 . The major concerns for POU versions of the IEX resin involve its regeneration process. Even under strong acids, lead-laden cation resin cannot fully strip lead from the resin 61 . Similarly, perchlorate attaches strongly to the anion resin. The safe and economical disposal of high-concentration regenerant brines is also an environmental concern.

Activated carbon

Activated carbon (AC) is the most commonly employed commercial POU filter in the United States 64 . AC is created from charcoal by treating it with extremely hot gases, leaving pure carbon with many microscopic pores. Granular activated carbon (GAC) is a ubiquitous form of AC water filters in residential water filter systems. The carbon particles repel water and strongly attract nonpolar organic compounds via intermolecular Van der Waal’s and hydrophobic interactions. Van der Waal’s interactions are almost universally attractive and based on permanent, temporary, and induced dipole interactions between the atoms of the GAC and chemical compounds; hydrophobic attraction occurs between nonpolar compounds or nonpolar moieties within complex molecules. Highly polar and charged compounds can experience electrostatic and hydrophilic repulsion, which makes them less likely to be well removed by GAC. The surface area available for adsorption is extremely large due to the large quantity of micropores in the carbon. GAC is thus used to remove organic contaminants, some heavy metals 65 , and DBPs such as trihalomethanes (THMs).

In addition to bare GAC, several surface modification methods have been researched to enhance the affinity to different impurities, including chemical treatment, impregnation, and plasma treatment 66 (Fig. S 3 ). AC can be chemically modified to have an acidic or basic surface. In most cases, an acidic surface has typical functional groups of carboxylic acid, lactone, phenol, or lactol groups, while a basic surface is represented by the existence of chromene, ketone, pyrone, and nitrogen groups 67 . AC can also be impregnated with metals and metal oxides. These additive crystallites will disperse in carbon pores and become active sites for contaminant adsorption 66 . Silver-impregnated activated carbon (SIAC) is one promising POU filter medium based on this removal mechanism. Another class of surface modification is plasma treatment. Under vacuum or atmospheric conditions, AC is treated with air or oxygen plasma to create oxygen functional groups, which makes AC more active 66 .

The novel SIAC has extensive use in POU water treatment. This filter type has high removal efficiency towards the natural organic matter (NOM), disinfection byproducts (DBPs), trihalomethanes (THMs), and many other key drinking water contaminants. Rajaeian et al. discussed the silver leaching mechanism of SIAC (Fig. S 4 ). Their research reported that if the solution pH is properly controlled, additional bromide removal can be achieved, while minimizing silver leaching 68 . With preconditioning at pH 10.4, the release of silver is only 3%, which makes SIAC more competitive with longer service life 68 . Watson et al. found that combined with an enhanced coagulation pre-treatment, SIAC (0.1% Ag) can reduce tTHMs by over 98%, bromide by 95 ± 4%, and total dihaloacetonitriles (tDHANs) by 97 ± 3% (tDHANs = sum of dichloroacetonitrile (DCAN), bromochloroacetonitrile (BCAN), and dibromoacetonitrile (DBAN) concentrations) 69 . The enhanced removal rate for Cr 6+ has also been studied, over 94% Cr 6+ removal by SIAC can be achieved by cost-effective H 2 SO 4 pre-treatment 70 .

KDF redox media

Kinetic degradation fluxion (KDF), a type of copper-zinc filtration, relies on the redox potential between these two metals to remove certain contaminants. Figure 6 diagrams the removal mechanism of KDF for various contaminants. Heavy metals dissolved in the water are reduced into an insoluble form and thus precipitate out so they can be retained in the filter medium 71 . KDF filters also reduce free chlorine and inhibit bacterial growth. Experiments showed that the oxidation-reduction potential (ORP) rapidly drops from 200 mV to −500 mV when feedwater passes through KDF 72 . This sharp decrease makes the environment unsuitable for bacteria to survive. However, KDF filters do not remove organic contaminants. For this reason, KDF is often used as a prefilter or combined stage with GAC (Fig. S 5 73 ).

Heavy metals dissolved in the water are reduced into an insoluble form and thus precipitate out so they can be retained in the filter medium (reprinted with permission from 180 ; Copyright © Elsevier, 2019).

There are two primary types of KDF filters in the market for POU applications: KDF55 and KDF85 (see Table 7 for a comparison). Based on their differing composition, KDF55 is more suitable for chlorine, heavy metal, and bacteria removal, while KDF85 is a better choice for eliminating iron and hydrogen sulfide 74 . A higher portion of zinc in the material enhances the reduction ability of KDF55. Thus, it is more effective for free chlorine removal. However, in large-scale applications, maintaining high performance of the KDF filter requires a backwashing procedure with a high flow rate, about 30 gallons per minute per square foot of bed surface area 75 . The backwash rate is also supposed to be tunable to the environment temperature. Fouling problems and poor efficacy might result if backwashing procedures are not properly followed. Innovative modification of KDF process media is thus expected to improve the filter. Nano-KDF is a pioneering nano-sized filter medium which originated from KDF 76 , 77 . Its specific surface area is over 100 times larger than the conventional type. Even under high initial chlorine concentration of 3 mg/L, the removal efficiency can be over 99.9% 77 .

Novel green filter media

In residential water filtration systems, cheaper and greener alternatives are often preferred by consumers. Many companies have developed such eco-friendly water filters in recent years. By using recyclable raw materials, the carbon footprint and manufacturing cost is greatly reduced. For example, Glanris ( https://www.glanris.com ) is a 100% green product made from rice hulls, reducing 98% of carbon emissions during the manufacturing process; hence, the “green” aspect of their filter media 78 . The biodegradable raw material makes it non-toxic and easy to dispose. At the same time, the widespread availability of its raw material lowers the price to $3–6 per lb for specialty metal removal and $3–10 per lb for nutritional/vitamin grade. By combining features of both GAC and IEX resins, this hybrid technology achieves in a single step removal of a wide range of organics and heavy metals 79 . High chlorine removal capacity is developed with fast kinetics and color in drinking water can be eliminated as well. Another example is Swift Green Filters ( https://swiftgreenfilters.com ) who makes AC from renewable coconut shells, which has the advantage of 50% more micropores than other plant-based shells 80 . Since the global consumption of coconuts has rapidly increased in recent years, the massive amount of coconut husk waste can be sustainably reused, providing a steady and environmentally friendly raw material source for the company. Key water impurities like turbidity, lead, mercury, chlorine (taste and odor), and asbestos can all be effectively reduced 81 . Swift’s green water filters are categorized into refrigerator filters, under the sink systems, tap filters, alkaline water filters, etc.

Membrane filtration

Compared with media filtration, membrane filters require no chemical additives to achieve a target separation, and act as absolute barriers 82 . Pressure-driven membranes are divided into 4 categories 83 (Fig. S 7 ): microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) 84 . Most of MF/UF membrane products are made of commodity polymers such as polytetrafluoroethylene (PTFE), polyvinylidinefluoride (PVDF), polyethylene (PE), polypropylene (PP), polysulfone (PSU), and polyethersulfone (PES), although ceramic analogs are also widely available 85 , 86 . Both NF and ROMs are most often composite (multi-layered) structures where a denser film coats over a MF/UF-type membrane to provide enhanced selectivity towards dissolved substances. Depending on the pore size, membranes have distinct target pollutants and removal efficiencies (Table 8 ).

Among the four types, RO is the most popular membrane type in POU systems. Unlike MF, UF, and NF membranes, RO membranes are the “tightest,” allowing water to pass, but virtually everything else in water is retained down to simple salts (e.g., NaCl) and small organic molecules >100 Da 87 . It is thus highly efficient in rejecting dissolved organic and inorganic contaminants (Table 9 ). Even for pollutants with low molecular weight, only trace concentrations exist after RO filtration, and the values are typically below health limits 88 . ROMs also completely reject pathogens, with no E. coli or viruses detectable in RO permeate 89 . Moreover, with the feedwater concentration ranging from 0.5 to 1500 mg/L, over 99% of PFOS can be rejected by ROMs 90 . However, ROMs may not be as effective for carcinogenic nitrogen DBPs, as only 50–65% or less NDMA may be removed by RO 91 . Currently, UV treatment is an effective method for removal of NDMA. NDMA in aqueous solutions undergoes direct photolysis upon UV exposure, which further leads to dimethylamine, and nitrite and nitrate ions as the major degradation products 92 . A combination of RO and UV disinfection is thus preferred to improve efficiency to 59–75% 91 . The previously mentioned emerging carcinogen 1,4-dioxane can also be reduced by nearly 96% by a removal system combining RO and GAC 93 .

While ROMs are common in POU water filters, they are more expensive than GAC and sediment filters. Many consumers find less expensive filters sufficient for their needs. However, RO units produce purer water than other commercially available technologies. Also, pre-treatment is necessary during this process; otherwise, membrane fouling or damage can quickly occur. Given sufficient pressure, nearly all dissolved solutes can theoretically be removed, but realistic removal rates on the order of 90–99% are possible for contaminants that cannot be removed by other filtration methods. RO systems are thus in demand among those with a high standard for water quality. Some water filter companies address differing customer standards by selling versions of systems that differ only in the inclusion of a ROM.

UV disinfection

UV irradiation has been increasingly used in water disinfection to inactivate microorganisms because it adds no chemicals, does not produce harmful DBPs, and does not cause disinfectant resistance in bacteria 94 . The radiation penetrates the microorganisms and results in photochemical damage by impairing nucleic acids (DNA or RNA). Such damage further disables microorganisms from replication and infection. In this way, microorganisms are rendered unable to function or reproduce and might even be killed 95 , 96 .

Mercury-based lamps are often used as the UV emission source for the disinfection system. After the excitation of mercury vapors in the lamp, UV rays are generated. UV mercury lamps are mainly categorized into two types: low pressure and medium pressure. With pressure under 10 torr, the emission of conventional low-pressure mercury lamps is monochromatic at 254 nm 97 ; often used at low flow rates where the exposure time to UV can be longer. Medium pressure (approximately 1000 torr) UV lamps have higher emission intensity and cover a broader range of wavelengths (200–400 nm) 97 . Because of the high energy demand for emission, medium-pressure UV lamps are exclusively used in more commercial or regulatory contexts such as drinking water or wastewater treatment plants.

UV-LED has emerged as a viable alternative over the past decade to achieve a more sustainable, low-energy UV disinfection (Fig. S 6 ) 94 , 98 . Its small size (5–9 mm diameter) enables easy transport and disposal in POU application 99 . As LED does not need warm-up time, it saves energy and allows intermittent use, thus leading to lower system cost. The germicidal efficiency of UV-LEDs is reported to be at least as good as low-pressure UV disinfection lamps 100 . In most cases, the germicidal effect of UV-LEDs is enhanced compared to conventional UV mercury lamps as they can incorporate an LED array of differing UV wavelengths. UV radiation with different wavelengths have varied microorganism inactivation efficiencies 101 ; thus, UV-LED can maximize its combined germicidal effect. Pulsed irradiation by UV-LEDs can also be used to improve germicidal effects 102 . At 272 nm under pulsed UVC, the log inactivation rate for E. coli is 3.8 higher than continuous illumination with the same UV dose 103 .

Remineralizing media filters

Remineralization aims to adjust the alkalinity of RO-filtered water by re-introducing healthy minerals; tap water RO permeate is deficient in healthy minerals, has a slightly dry, burning feeling to the tongue, and is slightly corrosive with pH < 6.0 104 . Product water after remineralization not only makes RO-treated water more palatable, but also non-corrosive 105 , fulfilling the following water quality criteria: pH between 6.5 and 8.5; alkalinity >80 mg/L as CaCO 3 , and calcium carbonate precipitation potential (CCPP) range of 80 < CCPP < 120 mg/L as CaCO 3 106 , 107 .

Figure 7 gives an example of a 5-in-1 alkaline, remineralization, and far-infrared filter 108 . There is a bit of marketing mythology surrounding some remineralization filters with statements such as “Like the flow of a mountain spring, water passes through mineral rocks in sequence.” For other media, such as the Maifan Mineral stone (a.k.a., “Japanese & Chinese Medicine Stone”), product claims include “releasing beneficial microelements, stabilizing water pH, and absorbing chlorine and heavy metals, Maifan stone is widely used in traditional Chinese treatments of many conditions like digestive problems and high blood pressure” 84 . The “alkaline ceramic ball” claims a “capacity to break down a big water molecule groups into smaller ones” and to “activate water” 100 . Another claim is that ceramics can generate far-infrared rays (FIR), and hence, the alkaline ceramic ball offers enhanced filter performance in antibacterial, activation, absorption, and water purification 100 . We find no scientific evidence supporting any of the above claims and assume they are largely marketing stories; however, such 5-in-1 remineralizing filters appear popular following ROMs in high-end POU filtration systems.

Remineralization aims to adjust the alkalinity of RO-filtered water by re-introducing healthy minerals; tap water RO permeate is deficient in healthy minerals, has a slightly dry, burning feeling to the tongue, and is slightly corrosive with pH < 6.0 (reprinted with permission from 108 ; Copyright © EZFILTER, 2020).

Emerging technologies: an example of CDI

Apart from the above technologies, a growing number of novel water treatment technologies have emerged to meet the increasing removal needs for emerging contaminants and from higher regulatory requirements. Water treatment based on electrochemical principles is one promising technology, which is presently emerging. Electrochemical water treatment technologies include electro-oxidation, electro-reduction, electro-coagulation, electro-flotation, electro-decantation, capacitive deionization (CDI), and others 109 . In this section, CDI is chosen as an exemplar electrochemical water treatment technology. Its working principles, developing history, and comparison between different types are discussed.

CDI, and its various derivatives, are promising POU water treatment technologies that use applied electric fields to separate dissolved ions by various mechanisms. Ions in feed water can be immobilized to two paired porous electrodes by applying a low-voltage electric field between the electrodes 110 . This process generally follows electric double layer theory where the charge on the electrode surfaces (from the applied potential) is neutralized by the accumulation of counterions from solution 111 . Positively charged ions such as Ca 2+ , Mg 2+ , and Na + will be adsorbed to the negative cathode, while negatively charged ions such as Cl − and SO 4 2− will be adsorbed to the positive anode. The electrodes are regenerated (e.g., ions released) by reversing the applied potential releasing electrosorbed ions.

After years of research and development, different architectures of the CDI module have been developed with various advantages and disadvantages (Table 10 ). Flow between electrodes is the most conventional format of CDI designed by Blair and Murphy in 1960 112 . The desalination efficiency of CDI was improved through an innovation by Johnson et al. 113 . by pumping feed water through the porous electrodes rather than between the solid electrodes. Researchers are devoted to combining CDI with existing filtration technologies such as membranes 114 , 115 or modifying the surface and material for the electrodes 116 , 117 , 118 . However, high manufacturing cost limits widespread deployment of CDI-based POU water treatment products.

More recently, a form of CDI called “capacitive coagulation” has emerged as an electrically driven alternative to chemical coagulation with the advantages of no chemical use, no sludge production, and higher energy-efficiency than conventional chemical adsorption ( https://electramet.com ). A wide range of heavy metals like lead, copper, manganese, iron, zinc, nickel, and cobalt have been removed with over 99% selectivity 119 .

Summary of POU technology efficacy

A summary of available peer-reviewed studies on POU water treatment technologies is provided in Table 11 , where we compare contaminant types, specific contaminants, treatment technologies, and removal rates. The reported removals for nearly all of the technologies are over 90% and, in many cases, >99%. It is fair to assume that the reported removals can be expected from the technologies as tested and reported. What is not known from peer-reviewed studies and from for-profit companies’ product performance claims, is how long a given technology maintains the reported level of performance in terms of time, volumetric throughput, or contaminant mass loading. This is difficult to evaluate due to the lack of data from published scientific studies or company claims.

Commercially available POU systems

Here we consider the filtration and purification components most commonly employed, in what formats (e.g., under the sink, countertop, etc.), and how they are combined to create various POU water filtration systems. Mass-produced water filters need to be effective at removing contaminants, and also, must be compact, low-cost, and easy to maintain. These constraints place some limits on the purification technologies that consumers can access, which drives most consumer products to use highly commoditized filter media. We compiled data from 11 POU water filter companies’ websites, and the configurations of their systems are summarized below. Individual filters and system components are listed in order when they could be determined. Price ranges are listed where available; the range of products were selected to reflect the lowest and highest priced products within the category. The prices shown were listed on the companies’ websites, including discounts, as of December 2020. Many available products were not included in this analysis as the one’s shown are simply indicative of industry norms.

Under-the-sink water filters

One of the most popular POU locations is under the kitchen sink, for filtering water just before it comes out of the tap. Most households with these systems use untreated tap water for most of their water needs (such as showers and washing machines) and a small amount of filtered water for drinking and cooking. Under-the-sink filters are designed to purify only a few hundred gallons of water before needing replacement of filter media. They may be further divided based on the number of filtering stages or the presence of a ROM. There is considerable flexibility in the number of stages depending on the needs of customers, but may comprise up to seven stages including a sediment prefilter (SED), ion exchange (IEX), KDF or GAC media filters, activated carbon block (ACB) filters, RO membrane (ROM), remineralization media filters (ALK), UV sterilization (UVS) and/or postfiltration activated carbon (PAC) (Fig. 8 ).

There is considerable flexibility in the number of stages depending on the needs of customers, which may contain up to seven stages including a sediment prefilter (SED), ion exchange (IEX), KDF or GAC media filters, activated carbon block (ACB) filters, RO membrane (ROM), remineralization media filters (ALK), UV sterilization (UVS) and/or postfiltration activated carbon (PAC) (reprinted with permission from 181 ; Copyright © Express Water Inc., 2020).

Table 12 summarizes the abundance of under-the-sink water filter media across several brands. Among the units including an ROM, APEC Water offers the most models. It has three five-stage models with sediment, two carbon blocks, RO, and GAC postfilter ($190–280), two models with added remineralization ($230–320), two models with added UV before postfilter ($280–290), one model with added UV and remineralization ($310), two models with added pumps for low-pressure households ($370–400), and one compact four-stage model with sediment, GAC, RO, and GAC ($250) 120 . Similarly, Aquasana has one model with four stages: sediment, AC, RO, and “Claryum”, which is a special design consisting of AC, sediment, and IEX ($200) 121 . Culligan has one tankless model with RO only 122 and two models with storage tank and four stages: sediment, AC, RO, and specialized carbon block 123 , 124 . Whirlpool has three models with three stages and a tank: sediment/AC combined prefilter, RO, and AC postfilter 125 , 126 . Pelican has one six-stage model with 20 μm SED, GAC prefilter, RO, two GAC postfilters, and calcite remineralization ($220) 127 , 128 . GE and Kinetico provide comparatively limited choices, with one model for each brand. The former brand provides a model with GAC pre- and postfilters and tank ($180) 129 , 130 , while the latter has four stages: prefilter, RO, storage tank, and AC postfilter.

There are also many under-the-sink water filters without an ROM. For example, Aquasana has two models with SED and two “Claryum” stages ($142–175) and one model with two Claryum stages ($99) 131 , 132 . GE has one parallel-flow dual GAC model ($130) 133 and two single-stage GAC filters ($70–80) 134 , 135 . Products from iSpring are more complicated. This brand provides one four-stage system with sediment, UF membrane, hybrid KDF/GAC, and carbon postfilter ($170) 136 ; one similar compact model, but with UF as the final filter ($130) 136 ; one three-stage model with sediment and two AC blocks ($120) 136 ; and one two-stage system with GAC and AC block ($194) 137 . Similarly, Pelican has one single-stage GAC ($74) 138 and one three-stage system with SED and two catalytic GAC filters ($154) 139 . Finally, Whirlpool has one model with two AC stages (the first might have a combined sediment stage) 140 , one three-stage system designed for microbiological purification containing AC, and one single-stage AC system designed for kitchen and bath use ($90) 141 .

Countertop and pitcher water filters

Table 13 summarizes configuration details of available countertop and pitcher water filters. Countertop systems, among the cheapest of home water filters, often include only one or two stages which may combine multiple filter media (e.g., sediment and GAC). Some are pressurized and are essentially compact versions of under-the-sink systems. However, many are not pressurized: tap water is poured in and gravity alone moves the water through a small filter. Gravity-only filters are very popular and sold in retail hardware, grocery, and mega-stores; water filtration can be slow and most are designed to improve water taste (with some health protection benefits) removing residual chlorine, dissolved organics, and some metals like lead and copper. For some pitcher filters, the filtration process takes only around 30 s. Customers can simply pour water into the pitcher, then get clean drinking water in the pitcher reservoir as feedwater passing through the filter cartridge. In addition, the price of a common water filter pitcher is relatively lower than other filter types, typically less than $40. However, since most pitcher filters only contain GAC, IEX, and/or KDF media, not all contaminants are removed, especially some heavy metals, volatile organic compounds, and hormones.

Refrigerator water filters

Many refrigerators are designed to deliver filtered cold water and ice. All water that passes through a refrigerator is filtered first using a replaceable filter cartridge. This cartridge may be any combination of GAC/ACB, WAC, SBA, KDF, and/or media, but most often is solely some form of GAC or ACB. For example, GE 142 and Whirlpool 143 have numerous GAC/ACB models for refrigerators, while iSpring has one single-stage GAC ($39) model (Fig. S 8 ) 144 , 145 , 146 , 147 , 148 and one two-stage model with GAC and remineralization ($40) 149 .

Faucet-mounted water filters

By design, faucet head filters are among the smallest filters available. They usually consist of a single stage with a granular filter medium, which may consist of a couple components mixed in one housing (e.g., KDF and GAC). For example, Brita has two POU models with sediment filter and carbon block filter ($19–30) and PUR has four models with slightly more expensive prices ($20–35) 150 . iSpring has two models with what appears to be KDF, GAC, and calcium sulfite remineralization according to a picture on their website ($29–35) (Fig. S 8 ) 147 .

Showerhead water filters

These filters are designed to filter shower water, so drinking water purity is not strictly required; however, many volatile organics could be inhaled while showering and other contaminants could be taken up by dermal absorption from showering or bathing. Many shower filters are designed to remove chlorine (which dries out some people’s skin) and may include one or more remineralization stages for adding minerals deemed to be beneficial to the skin. Both GE and Pelican only have one model for showerhead water filters. The model from GE is similar to a KDF filter ($23) 151 , while Pelican’s model is more complex with copper-zinc, GAC, and remineralization media ($42) 152 . Aquasana has three two-stage models which are KDF followed by AC ($55–65) 49 , 51 . Models from iSpring are even more complicated (Fig. S 8 ), with three models of 15 stages each. In order, the stages are sand, stainless-steel mesh, particulate, many remineralization and ion exchange stages, KDF, GAC, particulate, stainless-steel mesh, and finally sand ($19–26).

Smart POU water filters

Definition of “smart water filter”.

One definition of “smart” originates from computer science, where SMART is a fault detection and monitoring system short for Self-Monitoring, Analysis and Reporting Technology 153 . The expectation about “smart” is higher now with the expansion of Internet coverage and WiFi technology. Nonliving things are becoming animate and “smart” through artificial intelligence (AI) and machine learning (ML) by interacting with human beings. Therefore, the present study proposes a new, expanded definition for “smart” with the following equation: Self-Monitoring, Analysis, and Reporting + Interaction with Human through Internet = SMART. In recent years, “smart” is an increasingly attractive product description for home equipment and appliances, including water filters. According to Investopedia.com 14 , “smart home” refers to a convenient home setup where appliances and devices can be automatically controlled remotely from anywhere with an Internet connection using a mobile or other networked device. Devices in a smart home are interconnected through the Internet, allowing the user to control functions such as home security systems, access to the home, temperature, lighting, music, and home theater equipment. Here are a few key takeaways about smart home technology.

A smart home allows homeowners to control appliances, thermostats, lights, and other devices remotely using a smartphone or tablet through an Internet connection.

Smart homes can be set up through wireless or hardwired systems.

Smart home technology provides homeowners with convenience and cost savings.

Security risks and bugs continue to plague makers and users of smart home technology.

Further, “smart products” include features such as context awareness through data collection, autonomous operation via AI/ML algorithms, and WiFi or Bluetooth connectivity and connection to other devices and/or the Internet 154 . For example, one smart air filter can not only actively track the filter life, but also provide environmental air quality information and tips for the user through a convenient mobile app 155 . Similar to a smart air filter, a smart water filter could allow the user to control the appliance remotely and keep track of important details such as filter lifespan and filtered-water quality. However, there is little consensus on what constitutes a smart water filter. Moreover, current smart water filters are not actually smart based on the fact that they can neither be remotely controlled nor provide necessary water quality information to the consumers.

Most “smart” water filter systems in the U.S. market only have a timer, flow counter, or “# of times used” counter to remind the user when to replace a filter. These rely on preset assumptions of filter usage and water quality, and do not directly measure the quality or quantity of water consumed. Water filter companies which use a battery-powered timed replacement feature include GE, Pelican, and Whirlpool. Kinetico claims to use a smart reminder, but its products simply use a flow counter to shut off the system when a prespecified number of gallons have been used 156 . Flow sensors record the rotation of an impeller wheel as water moves through the device 157 . Brita pitchers use a different method: a sensor in the lid counts how many times the lid has been opened to fill the pitcher, approximating the output of a flow sensor. Some sensors, such as those used in PUR faucets 158 , track both time and water flow. While more sophisticated than a time-based reminder, a flow-based reminder still does not use actual water quality information to assess filter performance or treated-water quality. Moreover, water quality can vary considerably between different households in different regions with different water quality and based on the age of premise plumbing. Pre-programmed filter change reminders have limited ability to adapt to local water quality or assess whether a system is functioning as designed.

It seems obvious to incorporate smart technology and elements into POU water filters, in particular, to reliably notify the user when to replace the filter. Conductivity, pH, and ORP sensors are widely available and some for less than $10 (presumably even less when purchased in bulk). However, we find limited evidence of such technologies in any commercially available POU products. A filter’s ability to remove lead, arsenic, and other harmful contaminants may be well-advertised, but it is not clear for how long that performance persists in a given household installation. A time- or flow-based change reminder is consistent in that a manufacturer can expect attentive customers to buy a replacement at predictable time intervals. This increases the ability of the manufacturer to plan their production and finances. Many customers may never replace their filter cartridges due to inconvenience or switching to a different filter brand. Customers in this group would also be unlikely to benefit from a more intelligent reminder system.

It seems likely that most consumers who will eventually replace their filters would prefer to do so when their system wears out or breaks rather than at a predefined time interval. If a product is developed which includes this feature, perhaps including sensors more sophisticated than TDS (total dissolved solids) (conductivity), then these capabilities could prove attractive. Users could be warned that contaminants have reached unacceptably high levels at the filtration system’s output, providing a more compelling reason to replace one or more components. The availability of in-line TDS sensors which can be plugged into existing water supplies indicates that measurement capabilities are in demand 159 , 160 . Given the trend towards an Internet of Things, a smart water filter with electronics which accurately measures water quality would be a fitting contribution to the idea of a smart home.

Sensors that could make water filters smarter

In a smart water filter, evaluation of various parameters using sensors is necessary to monitor water quality. Many sensors have been developed to measure physical qualities of water or the presence of chemical contaminants. These include sensors for electrical conductivity (EC)—a surrogate for TDS, ORP, pH, turbidity, ion-selective electrodes as well as emerging optical, fluorescent, and spectrophotometric devices. Adding these sensors to water filter systems could allow users to check the quality of their water without worrying about silent product expiry. Moreover, the widespread use of sensors could push manufacturers to improve removal efficacy and address more diverse contaminants. Moreover, even if sensors like those described below were to be deployed in POU water treatment systems, they would not be truly SMART until they communicate to the system owner directly through some smart device like an Internet connected phone, tablet, or PC.

TDS sensors

One of the most common sensors is an EC sensor, which requires only a pair of electrical contacts to measure the resistance of water by applying a small current. EC can be closely correlated with TDS because dissolved ions in water allow electricity to flow more freely between the contacts 161 , 162 . For instance, there is a fairly robust conversion factor from EC to TDS for fresh water, namely 1 mS/cm EC = 640 ppm TDS 163 . However, this is a crude measurement because conductivity fails to account for the specific ionic composition of the water. The TDS sensor is small and cheap, with simple versions available for close to $10 164 . The least expensive sensors can measure TDS within 10% accuracy 164 , while more expensive ones can achieve 1% accuracy or better 165 . Multiple forms of the sensor are available; Fig. S 9 160 , 166 depicts one sensor with exposed metal contacts and one in-line sensor which covers the contacts in a way that allows them to be placed in series with a water supply 159 . Due to their simplicity and adaptability, TDS sensors are one of the most common devices for measuring water quality. HM Digital TM TDS Meter can achieve EC-to-TDS conversion easily, and some meters can even have selectable conversion factors. The Dual Inline TDS Meter is different from the first one as it can measure two different water lines together. As a result, customers can get TDS information about both tap water and filtered water at the same time.

Uncharged contaminants such as soluble hydrocarbons, DBPs, and some pharmaceuticals and pesticides cannot be detected by conductivity sensors because they do not change the ability of water to conduct electrical current. Some charged contaminants such as lead, chromium (VI), and arsenic—while they can be ionic—are toxic at levels in the parts-per-billion range, too small to be detected by all but the most precise conductivity sensors 167 . Despite limitations, an EC/TDS sensor’s measurements could be used to evaluate whether a filter system is working. If an ROM is breached, downstream EC/TDS sensors would register an increase in the measured EC, which would indicate system performance has declined or failed 168 . Other filter stages such as WAC/SAC or SBA/WBA IEX resins may become less effective at removing ions from water when nearing saturation, and this difference may be indicated in conductivity measurements. Conductivity thus provides useful information on the status of a filtration system in the absence of more comprehensive water quality data.

pH and ORP sensors

Other types of water quality sensors, which do not appear to be implemented in any commercial POU systems, include pH 169 and ORP 170 sensors. Several types of pH sensors are available, of which the most common variation is the combination pH sensor. Two electrodes measure either side of a specially designed glass membrane, which contains a reference solution. The measured electrical potential is proportional to the pH of the test solution 171 . Like conductivity measurements, pH measurements can also indicate the successful operation of a RO Or IEX system 172 . Like pH probes, ORP probes consist of a test and reference electrode. The test electrode either gains or loses electrons from the solution, resulting in a measurable potential across the two electrodes. ORP is measured in millivolts and depends on the substances present in the solution as well as their concentrations 173 . These sensors are more complex and correspondingly more expensive, starting at around $30 for the cheapest pH sensors 169 and $90 for the cheapest ORP sensors 170 . Although not present in water filters due to their cost, the combination of pH and ORP measurements can provide significant insight into the chemical makeup of water such as speciation of metals (e.g., Fe 2+ /Fe 3+ ), oxyanions (e.g., H 2 AsO 4 − /HAsO 4 2− ), and multi-protic anions (e.g., HCO 3 − /CO 3 2− ) as well as corrosivity (e.g., Langlier saturation index).

Future smart POU system using nanotechnology-enabled sensors

Nanomaterial-enabled sensors, also called nanosensors, are invented for high-efficacy, multiplex-functionality, and high-flexibility sensing applications 174 . Interest in developing these sensors in POU applications origins from their potentials on facile, in-field contaminant detection. Many existing nanosensors are capable of sensing and monitoring the water safety. However, these sensors require further development into consumer- and operator-friendly products with the high compatibility of POU systems 174 . While monitoring the water safety, nanosensors have ultralow multiplex detection and rapid analysis times, due to their novel properties 175 , 176 . However, the great achievements in the laboratory and in the literature about nanosensors have seldom been translated to successfully commercialized products 174 .

In principle, nanosensor is comprised of (1) a nanomaterial, (2) a recognition element, and (3) a mechanism for signal transduction 177 . The interaction between the analytes and the recognition element will induce a detectable signal 174 . The specificity of the nanosensor is endowed by detecting an intrinsic signal from the analyte or by employing highly specific recognition elements that ideally bind only to a given target 177 . Moreover, the properties of the nanomaterial and the transduction method determine the sensitivity of the nanosensor 174 .

Electrically based nanosensor typically employs nanomaterials such as silicon, noble metal nanoparticles (Pt, Ag, Au), carbonaceous nanomaterials (graphene, carbon nanotubes), and inorganic two-dimensional nanosheets due to their high conductivity and electrochemical stability 174 . The electrically based nanosensors have enabled sensitive detection of waterborne contaminants such as E. coli 174 . Figure 9 illustrates representative electrically based nanosensor architecture for environmental analyte detection. A glassy carbon electrode was functionalized via multiple steps, including the treatment with reduced graphene oxide, electro polymerization of pyrrole, electrodeposition of gold, and the co-deposition of silica and acetylcholinesterase 174 . This nanosensor is capable of sensing organophosphorus pesticides.

A glassy carbon electrode was functionalized via multiple steps, including the treatment with reduced graphene oxide, electro polymerization of pyrrole, electrodeposition of gold, and the co-deposition of silica and acetylcholinesterase (reprinted with permission from 182 ; Copyright © RSC, 2014).

In contrast to the electrochemical method, magnetic transduction shows less background signal and therefore can detect contaminants with low concentrations 177 . The analytes that can then be detected are magnetically isolated via the functionalization of nanomaterials with analyte-specific biomolecules 174 , 178 . Figure 10 illustrates representative magnetically based nanosensor architecture for environmental analyte detection. Combined magneto-fluorescence approach is applied to sense and detect bacteria via fluorophore labeled magnetic nanoparticles. Fluorescence and magnetic bacterial sensing are achieved by functionalizing specific antibodies of E. coli with magneto-fluorescent nanosensors.

Combined magneto-fluorescence approach is applied to sense and detect bacteria via fluorophore labeled magnetic nanoparticles. Fluorescence and magnetic bacterial sensing are achieved by functionalizing specific antibodies of E. coli with magneto-fluorescent nanosensors (reprinted with permission from 183 ; Copyright © ACS, 2016).

Compared to traditional conductivity sensors or ORP sensors, nanosensors can be capable of detecting and monitoring a wider spectrum of contaminants by tailoring the sensor compositions 174 , 179 . However, the challenges associated with transitioning novel nanosensors into POU system are particularly vexing due to the lack of capital sources powering product research, development, and marketing. In general, if a novel technology is to gain a foothold then the potential profits associated with it have to be considerable, while the risks of adoption must be acceptable 174 .

Concluding remarks

Current household tap water quality in the United States is as good as anywhere else where drinking water is treated to regulated quality. That said, violations for a wide array of regulated contaminants by public water systems, unregulated non-grid-tied groundwater wells, and unregulated emerging contaminants still pose serious acute and chronic health risks. For example, some very-high-profile cases of impaired municipal drinking water have occurred in recent years (e.g., lead in Flint, Michigan and Newark, New Jersey). Moreover, long timelines for implementing new regulations in the U.S. cause some concern over the purity and healthfulness of municipal drinking water, especially with so many toxic, carcinogenic, endocrine disrupting, and pharmaceutically active chemicals known to be in drinking water’s source waters. Treatment plants that use surface water as a source tend to have a higher frequency of violations compared to those using groundwater. For smaller systems, many of the violations are for lack of reporting versus reports of known violations, so the potential risk to the populations served is difficult to assess. As the size of treatment plants increase, the percent of violations and, in particular, those that use surface water both tend to decrease; however, the number of people potentially at risk is quite high due to the large populations served.

These well-documented drinking water violations, unregulated off-grid groundwater wells, and emerging contaminants all give rise to consumers’ lack of confidence in drinking water quality and justify the use of POU drinking water treatment systems. Although there has been much research on the mechanisms and removal efficacies of the types of water treatment technologies employed in POU applications, most peer-reviewed studies are framed in the context of large-scale municipal or industrial treatment applications and few independent studies have evaluated their efficacy in POU water treatment applications. Components in commercially available POU water filter systems are highly commoditized and standardized across the industry. Sediment, KDF, AC (either GAC or ACB), RO, remineralization, and UVS are the most commonly employed technologies. This level of homogeneity in the production of filter systems is good in that it drives down costs to the consumer making POU water treatment widely accessible; however, the lack of regulations, monitoring and control of POU systems make it difficult to know if and when a POU system stops working as it was designed.

The smartness of POU water filters may be defined as their ability to perform tasks such as monitoring and reporting water quality, monitoring filter performance and expected lifetime, controlling filtration remotely, and connecting with consumers through personal smart devices. By this definition, currently there are no commercially available SMART POU water filtration products. Sensors providing water quality information is an essential feature for a water filter to become SMART. However, a SMART water filter is also expected to be connected to WiFi or Bluetooth and deliver the information to a mobile app. Interaction and control through the use of Internet is a key characteristic of any smart home technology. Future design and production of SMART POU water treatment systems should consider moving beyond timers and counters to flow meters and (at least) basic water quality sensors (e.g., EC/pH/ORP) along with Internet connectivity and interactive consumer apps. Finally, these technological innovations must be accomplished at very low cost to assure widespread accessibility for the most vulnerable and underprivileged populations.

Data availability

The authors declare that no datasets were generated or analyzed during the current study.

Change history

06 april 2022.

A Correction to this paper has been published: https://doi.org/10.1038/s41545-022-00159-0

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Acknowledgements

The authors are grateful for financial support for this study provided by Pacifica Water Solutions, the UCLA Samueli Engineering School, the UCLA Department of Civil & Environmental Engineering, and the UCLA Sustainable LA Grand Challenge.

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These authors contributed equally: Jishan Wu, Miao Cao.

Authors and Affiliations

Department of Civil & Environmental Engineering, University of California, Los Angeles (UCLA), Los Angeles, CA, USA

Jishan Wu, Miao Cao & Eric M. V. Hoek

Department of Electrical Engineering, University of California, Los Angeles (UCLA), Los Angeles, CA, USA

Department of Computer Science & Engineering, University of California, Los Angeles (UCLA), Los Angeles, CA, USA

Zach Finkelstein

California NanoSystems Institute, University of California, Los Angeles (UCLA), Los Angeles, CA, USA

Eric M. V. Hoek

Institute of the Environment & Sustainability, University of California, Los Angeles (UCLA), Los Angeles, CA, USA

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E.M.V.H. devised the project, the main conceptual ideas and proof outline of this review work. M.C., Z.F., and D.T. reviewed and drafted up the early manuscript. J.W. revised and improved the manuscript according to reviewers’ comments. All authors provided critical feedback and helped shape the research, analysis, and manuscript.

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Correspondence to Eric M. V. Hoek .

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Wu, J., Cao, M., Tong, D. et al. A critical review of point-of-use drinking water treatment in the United States. npj Clean Water 4 , 40 (2021). https://doi.org/10.1038/s41545-021-00128-z

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DOI : https://doi.org/10.1038/s41545-021-00128-z

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A Review on Reverse Osmosis and Nanofiltration Membranes for Water Purification

Affiliations.

• 1 Department of Materials Science and Engineering, The Ohio State University, 2041 N. College Road, Columbus, OH 43210, USA. [email protected].
• 2 Department of Materials Science and Engineering, The Ohio State University, 2041 N. College Road, Columbus, OH 43210, USA.
• 3 State Key Laboratory of Precision Measurement Technology and Instrument, Tianjin University, Tianjin 300072, China.
• 4 Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China.
• 5 William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA. [email protected].
• PMID: 31362430
• PMCID: PMC6723865
• DOI: 10.3390/polym11081252

Sustainable and affordable supply of clean, safe, and adequate water is one of the most challenging issues facing the world. Membrane separation technology is one of the most cost-effective and widely applied technologies for water purification. Polymeric membranes such as cellulose-based (CA) membranes and thin-film composite (TFC) membranes have dominated the industry since 1980. Although further development of polymeric membranes for better performance is laborious, the research findings and sustained progress in inorganic membrane development have grown fast and solve some remaining problems. In addition to conventional ceramic metal oxide membranes, membranes prepared by graphene oxide (GO), carbon nanotubes (CNTs), and mixed matrix materials (MMMs) have attracted enormous attention due to their desirable properties such as tunable pore structure, excellent chemical, mechanical, and thermal tolerance, good salt rejection and/or high water permeability. This review provides insight into synthesis approaches and structural properties of recent reverse osmosis (RO) and nanofiltration (NF) membranes which are used to retain dissolved species such as heavy metals, electrolytes, and inorganic salts in various aqueous solutions. A specific focus has been placed on introducing and comparing water purification performance of different classes of polymeric and ceramic membranes in related water treatment industries. Furthermore, the development challenges and research opportunities of organic and inorganic membranes are discussed and the further perspectives are analyzed.

Keywords: ceramic membranes; desalination; nanofiltration; polymeric membranes; reverse osmosis; water purification.

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Conflict of interest statement

The author declares no conflict of interest.

Classification of membranes for water…

Classification of membranes for water purification in terms of pore size and retained…

Representative reverse osmosis (RO) and…

Representative reverse osmosis (RO) and nanofiltration (NF) membranes for water treatment.

Thin-film composite membrane structure.

Mechanism of interfacial polymerization.

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Reverse Osmosis Literature Review

• 1.1 Google Scholar
• 2 Reverse Osmosis
• 3 Importance of Reverse Osmosis
• 4.1 A Review on Reverse Osmosis and Nanofiltration Membranes for Water Purification
• 4.2 Nanoparticles in reverse osmosis membranes for desalination: A state of the art review
• 4.3 Desalination Technologies for Developing Countries: A Review
• 4.4 Sustainable seawater reverse osmosis (SWRO) system design for rural areas of developing countries
• 4.5 Engineering antifouling reverse osmosis membranes: a review
• 4.6 Reverse osmosis technology for water treatment: State of the art review
• 4.7 Reverse osmosis membrane fabrication and modification technologies and future trends: a review
• 4.8 The challenges of reverse osmosis desalination: solutions in Jordan
• 4.9 Reverse Osmosis Water Purification by Cycling Action
• 4.10 Field evaluation of a community scale solar powered water purification technology: A case study of a remote Mexican community application
• 4.11 Purification of Contaminated Water with Reverse Osmosis: Effective Solution of Providing Clean Water for Human Needs in Developing Countries
• 5.1 DIY Maple Sap Reverse Osmosis (RO) Unit
• 5.2 DIY Reverse Osmosis For Home Drinking Water by Isopure Water
• 5.3 Build Your Own Reverse Osmosis System for Maple Syrup
• 5.4 How to Make an RO Water Filter at Home
• 5.5 Development and Filtration Performance of Polylactic Acid Meltblowns
• 6.1 Fundamentals of Membranes for Water Treatment
• 6.2 Tubular Membranes
• 6.3 A review of polymeric membranes and processes for potable water reuse
• 7.1 A critical overview of household slow sand filters for water treatment
• 8 Components
• 9 What Contaminants do Reverse Osmosis Systems Remove?
• 10 There are generally four stages in the Reverse Osmosis Process
• 11 Some factors that may affect the performance of a Reverse Osmosis System
• 12 References

Search Terms [ edit | edit source ]

Google scholar [ edit | edit source ].

• "reverse osmosis" membrane
• reverse osmosis "drinking water" international
• reverse osmosis "international development" OR "developing countries"
• drinking water treatment "reverse osmosis"

Reverse Osmosis [ edit | edit source ]

From Wikipedia: " Reverse osmosis ( RO ) is a water purification process that uses a partially permeable membrane to separate ions, unwanted molecules and larger particles from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by chemical potential differences of the solvent, a thermodynamic parameter."

Importance of Reverse Osmosis [ edit | edit source ]

RO is used to purify water by extracting particles of up to 0.0001 microns, the most powerful system for membrane purification. Completely removes dissolved salts in addition to everything said above. Using membrane technology can have many benefits such as:

• Allows to remove most of the solids (inorganic or organic) dissolved in the water (up to 99%).
• Removes suspended materials and microorganisms.
• Performs the purification process in a single stage and continuously.
• It is an extremely simple technology that does not require much maintenance and can be operated by non-specialized personnel.
• The process is carried out without phase change, with the consequent energy saving.
• It is modular and requires little space, which gives it exceptional versatility in terms of plant size: from 1 m3/day to 1,000,000 m3/day.
• Treatment of municipal and industrial effluents for pollution control and/or recovery of valuable reusable compounds.

Literature [ edit | edit source ]

A review on reverse osmosis and nanofiltration membranes for water purification [ edit | edit source ].

Yang, Zi, Yi Zhou, Zhiyuan Feng, Xiaobo Rui, Tong Zhang, and Zhien Zhang. 2019. "A Review on Reverse Osmosis and Nanofiltration Membranes for Water Purification" Polymers 11, no. 8: 1252. https://doi.org/10.3390/polym11081252

Abstract: "Sustainable and affordable supply of clean, safe, and adequate water is one of the most challenging issues facing the world. Membrane separation technology is one of the most cost-effective and widely applied technologies for water purification. Polymeric membranes such as cellulose-based (CA) membranes and thin-film composite (TFC) membranes have dominated the industry since 1980. Although further development of polymeric membranes for better performance is laborious, the research findings and sustained progress in inorganic membrane development have grown fast and solve some remaining problems. In addition to conventional ceramic metal oxide membranes, membranes prepared by graphene oxide (GO), carbon nanotubes (CNTs), and mixed matrix materials (MMMs) have attracted enormous attention due to their desirable properties such as tunable pore structure, excellent chemical, mechanical, and thermal tolerance, good salt rejection and/or high water permeability. This review provides insight into synthesis approaches and structural properties of recent reverse osmosis (RO) and nanofiltration (NF) membranes which are used to retain dissolved species such as heavy metals, electrolytes, and inorganic salts in various aqueous solutions. A specific focus has been placed on introducing and comparing water purification performance of different classes of polymeric and ceramic membranes in related water treatment industries. Furthermore, the development challenges and research opportunities of organic and inorganic membranes are discussed and the further perspectives are analyzed."

• Usage of inorganic and ceramic membranes
• Nanotechnology development for nanofiltration process
• Tunable pore structure
• Excellent mechanical and thermal tolerance
• Challenges in the future for the application of RO technology

Nanoparticles in reverse osmosis membranes for desalination: A state of the art review [ edit | edit source ]

Haleema Saleem, Syed Javaid Zaidi, Nanoparticles in reverse osmosis membranes for desalination: A state of the art review ,Desalination,Volume 475,2020,114171,ISSN 0011-9164, https://doi.org/10.1016/j.desal.2019.114171.

Abstract: The development of thin-film nanocomposite (TFNC) membranes utilizing nanoparticles present remarkable opportunity in the desalination industry. This review offers a comprehensive and in-depth analysis of TFNC membranes for reverse osmosis (RO) desalination by focusing on different issues existing in the RO process. Recent researches on nanoparticle incorporated TFNC membranes for application in water purification have been critically analyzed. The widely tested nanoparticles in these researches include carbon-based (carbon nanotube, graphene-oxide), metal and metal oxides-based (silver, copper, titanium dioxide, zinc oxide, alumina and metal-organic frameworks), and other nano-sized fillers like silica, halloysite, zeolite and cellulose-nanocrystals based. These nanoparticles demonstrated pronounced effect in terms of water flux, salt rejection, chlorine resistance, and anti-fouling properties of TFNC membranes relative to the typical thin-film composite (TFC) membranes. Here, we also focus on the environmental impact, commercialization, and future scope of TFNC membranes. From the current review, it is evident that the nanomaterials possess exclusive properties, which can contribute to the advancement of high-tech nanocomposite membranes with improved capabilities for desalination. Despite all the developments, there still exist significant difficulties in the large-scale production of these membranes. Hence, additional studies in this field are required to produce TFNC membrane with increased performance for commercial application.

• Review of recently developed TFNC RO membranes for desalination
• Improvement in the properties of TFNC membrane due to beneficial effect of nanoparticle
• Challenges associated with TFNC membranes and methods to overcome these
• Environmental impact of nanomaterials and their TFNC membranes
• Future prospects for advancement of TFNC membranes and their commercialization

Desalination Technologies for Developing Countries: A Review [ edit | edit source ]

Islam, M. S., Sultana, A., Saadat, A. H. M., Islam, M. S., Shammi, M., & Uddin, M. K. (2018). Desalination Technologies for Developing Countries: A Review. Journal of Scientific Research , 10 (1), 77–97. https://doi.org/10.3329/jsr.v10i1.33179

Abstract: Fresh water is rapidly being exhausted due to natural and anthropogenic activities. The more and more interest is being paid to desalination of seawater and brackish water in order to provide fresh water. The suitability of these desalination technologies is based on several criteria including the level of feed water quality, source of energy, removal efficiency, energy requirement etc. In this paper, we presented a review of different desalination methods, a comparative study between different desalination methods, with emphasis on technologies and economics. The real problem in these technologies is the optimum economic design and evaluation of the combined plants in order to be economically viable for the developing countries. Distillation plants normally have higher energy requirements and unit capital cost than membrane plants and produces huge waste heat. Corrosion, scaling and fouling problems are more serious in thermal processes compare to the membrane processes. On the other hand, membrane processes required pretreatment of the feed water in order to remove particulates so that the membranes last longer. With the continuing advancement to reduce the total energy consumption and lower the cost of water production, membrane processes are becoming the technology of choice for desalination in developing countries.

• Comparison of different desalination technologies.
• Low energy requirements and brackish water treatment are most common in developing countries.
• Unit capital cost and damage caused by corrosion or fouling are unusual in RO process.
• Pre-treatment of intake water is required in RO.

Sustainable seawater reverse osmosis (SWRO) system design for rural areas of developing countries [ edit | edit source ]

van Asselt, J., & de Vos, I. W. (2021). Sustainable seawater reverse osmosis (SWRO) system design for rural areas of developing countrie s .

• Solar-powered system, Kuwait
• Physical membrane types pros and cons: plate/frame, tubular, spiral, hollow
• Pretreatment types pros and cons: sand, cartridge, micro, ultra, nano (lists pore sizes)
• Debated: Open seawater vs. subsurface seawater intake

Engineering antifouling reverse osmosis membranes: a review [ edit | edit source ]

Zhao, S., Liao, Z., Fane, A., Li, J., Tang, C., Zheng, C., Lin, J., & Kong, L. (2021). Engineering antifouling reverse osmosis membranes: A review . Desalination , 499 , 114857. https://doi.org/10.1016/j.desal.2020.114857

Abstract: "Over the past decades, water scarcity and security have significantly stimulated the advances of reverse osmosis (RO) technology, which dominates the global desalination market. However, deterioration of membrane separation performance caused by inevitable fouling, including organic fouling, inorganic fouling, colloidal fouling and biofouling, calls for improved RO membranes with more durable antifouling properties. In this review, we analyze the correlations between membrane properties (e.g. surface chemistry, morphology, hydrophilicity, and charge) to antifouling performance. We evaluate the three key strategies for engineering fouling resistant thin film composite RO membranes, namely: (1) substrate modification before interfacial polymerization, (2) incorporating (hydrophilic/biocidal/antifouling) additives into the selective layer during interfacial polymerization, and (3) post (surface) modification after interfacial polymerization. Finally, we offer some insights and future outlooks on the strategies for engineering next generation of high performance RO membranes with durable fouling resistance. This review provides a comprehensive, state-of-the-art assessment of the previous efforts and strategies as well as future research directions for engineering antifouling RO membranes."

• Different membranes pros and cons
• Membranes get fouled: organic, inorganic, bio (most problematic), and colloidal
• Improve by being hydrophilic, neg. charge, smooth

Reverse osmosis technology for water treatment: State of the art review [ edit | edit source ]

Lilian Malaeb, George M. Ayoub, Reverse osmosis technology for water treatment: State of the art review , Desalination,Volume 267, Issue 1,2011,Pages 1-8,ISSN0011-9164,https://doi.org/10.1016/j.desal.2010.09.001

Abstract: This paper presents a review of recent advances in reverse osmosis technology as related to the major issues of concern in this rapidly growing desalination method. These issues include membrane fouling studies and control techniques, membrane characterization methods as well as applications to different water types and constituents present in the feed water. A summary of the major advances in RO performance and mechanism modeling is also presented and available transport models are introduced. Moreover, the two important issues of RO brine discharge and energy costs and recovery methods are discussed. Finally, future research trends and needs relevant to RO are highlighted.

• Research areas include brine discharge, fouling and removal of specific compounds.
• Modeling is important for better membrane characterization and for plant reliability.
• Existing cost assessment methodologies are not sufficiently accurate.
• Developing less energy-intensive systems is a main concern.
• Using new membrane materials is also a subject of future research.

Reverse osmosis membrane fabrication and modification technologies and future trends: a review [ edit | edit source ]

Hailemariam, R. H., Woo, Y. C., Damtie, M. M., Kim, B. C., Park, K.-D., & Choi, J.-S. (2020). Reverse osmosis membrane fabrication and modification technologies and future trends: A review. Advances in Colloid and Interface Science , 276 , 102100. https://doi.org/10.1016/j.cis.2019.102100

Abstract: "Reverse osmosis (RO) is the most widely used technology in water treatment and desalination technologies for potable water production. Since its invention, RO has undergone significant developments in terms of material science, process, system optimization, methods of membrane synthesis, and modifications. Among various materials used for the synthesis of an RO membrane, the polyamide thin-film composite (PA-TFC) is by far the most common, owing to its excellent water permeability high salt rejection, and stability. However, a tradeoff between membrane permeability and salt rejection and membrane fouling has been a major hindrance for the effective application of this membrane. Thus, a broad investigation has been carried out to address these problems, and among which co -solvent interfacial polymerization (CAIP) and the surface modification of substrates and active layers of RO membrane have been the most effective approaches for controlling and improving the surface properties of the PA-TFC membrane. In this review paper, the problems associated with the RO membrane processes and strategies has been discussed and addressed in detail. Furthermore, as the focus of this review, the major advancements in the strategies used for enhancement of RO membrane performance through CAIP, and surface modifications were scrutinized and summarized."

• Reverse osmosis steps
• Four steps in reverse osmosis plant: pre-treat for compatibility, pumping/pressure (overcome osmotic pressure), membrane separation, and post-treat
• Issues of reverse osmosis (+their solutions): membrane deterioration via fouling (smooth membrane, small neg. charge, high hydrophilicity), permeability/salt rejection, chlorination, boron extraction (run multiple times w/ pH balance), brine waste

The challenges of reverse osmosis desalination: solutions in Jordan [ edit | edit source ]

Maureen Walschot, Patricia Luis & Michel Liégeois (2020) The challenges of reverse osmosis desalination: solutions in Jordan , Water International, 45:2, 112-124, DOI: 10.1080/02508060.2020.1721191

Abstract: Desalinating water through reverse osmosis is becoming more economically affordable. Identifying the challenges in adopting desalination technology may help countries address water security concerns. In this article, we examine these challenges and present some of the solutions implemented in the Kingdom of Jordan, such as the creation of a cooperative water project to reduce financial investment and transportation costs and the coupling of renewable energy to desalination technology. Reverse osmosis desalination can play a role in promoting regional cooperation.

• Type of feed water (seawater or brackish)
• Energy source depending of the local availability and the cost of an energy source
• Plant size (most high and medium-income countries can afford large-scale desalination technology)
• Brine disposal to water bodies as the sea or open spaces
• CO2 emissions of, at least, 20%
• Energy consumption for its operation

Reverse Osmosis Water Purification by Cycling Action [ edit | edit source ]

Ravi V.K., Sushmitha V., Kumar M.V.P.., and Thomas A. (2017). Reverse Osmosis Water Purification by Cycling Action . International Journal of Latest Engineering Research and Applications , 2 (5), 54-59.

Abstract: "Pure water is very much essential to survive, but now a days the water is getting contaminated due to Industrialisation which leads to many water-releated diseases. Reverse Osmosis(RO) Water Purification by Cycling Action meets the needs of people without requiring any electrical energy. RO is a physical process that uses the osmosis phenomenon, that is, the osmotic pressure difference between the salt water and the pure water to remove the salts from water. Water will pass through the membrane, when the applied pressure is higher than the osmotic pressure, while salt is retained. As a result, a low salt concentration permeate stream is obtained and a concentrated brine remains at the feed side. A typical RO system consists of four major subsystem: pre-treatment system, high-pressure pump, membrane module and post treatment system. In operation by pedaling the cycle, man power is converted into mechanical energy which is further converted into hydraulic energy in RO pump."

• Human powered (electricity free) reverse osmosis
• "4 Stage = Sediment + Pre-Carbon + RO Membrane + Post-Carbon"
• Collect water, cycle, and clean when home

Field evaluation of a community scale solar powered water purification technology: A case study of a remote Mexican community application [ edit | edit source ]

Elasaad, H., Bilton, A., Kelley, L., Duayhe, O., & Dubowsky, S. (2015). Field evaluation of a community scale solar powered water purification technology: A case study of a remote Mexican community application . Desalination , 375 , 71–80. https://doi.org/10.1016/j.desal.2015.08.001

Abstract: "Lack of clean water in small remote communities in the developing world is a major health problem. Water purification and desalination systems powered by solar energy, such as photovoltaic powered reverse osmosis systems (PVRO), are potential solutions to the clean water problems in these small communities. PVRO systems have been proposed for various locations. However, small PVRO systems with production on the order of 1 m3/day for remote communities present some unique technical, cost and operational problems. This paper reports on a project in which a PVRO system is designed, fabricated and deployed in remote village in the Yucatan Peninsula of Mexico. The community residents are indigenous people who are subsistence farmers and beekeepers. Technical and economic models used to configure the system for the community are presented. A plan is developed in cooperation with the community aimed at making the system self-sustaining in the long term. Methods and materials are developed to permit the community members to operate and maintain the system themselves. The results provide insights for the design and deployment of small community-scale PVRO systems in remote communities."

• Photovoltaic reverse osmosis system for drinking water
• Issues with system: cost of shipping parts, language differences for training, hands-on training needed, water source quality
• Physical parts in reverse osmosis system (solar panel, membrane, filters, pump, testing, electronics, batteries, UV lamps) and diagram of process shown
• Cost: $10,000 USD to start and $1,342 USD annually

Purification of Contaminated Water with Reverse Osmosis: Effective Solution of Providing Clean Water for Human Needs in Developing Countries [ edit | edit source ]

Wimalawansa, S. J. (2013). Purification of Contaminated Water with Reverse Osmosis: Effective Solution of Providing Clean Water for Human Needs in Developing Countries. International Journal of Emerging Technology and Advanced Engineering, 3 (12).

Abstract: "Approximately 25% of the world's population has no access to clean and safe drinking water. Even though freshwater is available in most parts of the world, many of these water sources contaminated by natural means or through human activity. In addition to human consumption, industries need clean water for product development and machinery operation. With the population boom and industry expansion, the demand for potable water is ever increasing, and freshwater supplies are being contaminated and scarce. In addition to human migrations, water contamination in modern farming societies is predominantly attributable to anthropogenic causes, such as the overutilization of subsidized agrochemicals―artificial chemical fertilizers, pesticides, fungicides, and herbicides. The use of such artificial chemicals continue to contaminate many of the precious water resources worldwide. In addition, other areas where the groundwater contaminated with fluorides, arsenic, and radioactive material occur naturally in the soil. Although the human body is able to detoxify and excrete toxic chemicals, once the inherent natural capacity exceeded, the liver or kidneys, or both organs may fail. Following continual consumption of polluted water, when the conditions are unfavourable and the body's thresholds are exceeded, depending on the type of pollutants and toxin, liver, cardiac, brain, or renal failure may occur. Thus, clean and safe water provided at an affordable price is not only increasingly recognized, but also a human right and exceedingly important. Most of the household filters and methods used for water purification remove only the particulate matter. The traditional methods, including domestic water filters and even some of the newer methods such as ultra-filtration, do not remove most of the heavy metals or toxic chemicals from water than can harm humans. The latter is achieved with the use of reverse osmosis technology and ion exchange methods. Properly designed reverse osmosis methods remove more than 95% of all potential toxic contaminants in a one-step process. This review explains the reverse osmosis method in simple terms and summarizes the usefulness of this technology in specific situations in developing countries."

• Spiral-wound membrane shape + nanometer pore size for reverse osmosis
• Why RO > other filtering methods
• Physical parts in reverse osmosis system and process (including different options for each step)
• Lowered contaminant removal via fouling (backwashing helps).

DIY [ edit | edit source ]

Diy maple sap reverse osmosis (ro) unit [ edit | edit source ].

rsook74. DIY Maple Sap Reverse Osmosis (RO) Unit . Instructables. https://www.instructables.com/DIY-Maple-Sap-Reverse-Osmosis-RO-Unit/

• Physical parts needed

DIY Reverse Osmosis For Home Drinking Water by Isopure Water [ edit | edit source ]

Isopure Water . DIY Reverse Osmosis System for Home Drinking Water by Isopure Water . (2020, December 12). Isopure Water. https://www.isopurewater.com/blogs/news/diy-reverse-osmosis-system

• Cost: max $150 for parts + annual filters

Build Your Own Reverse Osmosis System for Maple Syrup [ edit | edit source ]

Michelle. (2019, January 8). Build your own Reverse Osmosis system for maple syrup . Souly Rested.https://soulyrested.com/2019/01/08/build-your-own-reverse-osmosis-system-for-maple-syrup/

• Cost: roughly $300-$350

How to Make an RO Water Filter at Home [ edit | edit source ]

Derek. (2017, June 20). How to Make a Reverse Osmosis Water Filter at Home . best-ro-system.com. https://www.best-ro-system.com/make-your-own-water-filter/

Development and Filtration Performance of Polylactic Acid Meltblowns [ edit | edit source ]

Liu, Y., Cheng, B. and Cheng, G., 2010. Development and filtration performance of polylactic acid meltblowns. Textile research journal, 80(9), pp.771-779. https://doi.org/10.1177/0040517509348332

Polylactic acid (PLA) is a biodegradable material that can be used to make meltblowns (MBs, which are fabrics made by the meltblowing method) using direct melt spun. PLA MBs were successfully produced in a 20 cm laboratory meltblown line. The relationships between the processing parameters and the filtration performance of PLA MBs were explored in this study. The key parameters regarding the filtration performance of PLA MBs, including the PLA chip drying process, the melt temperature, the hot air temperature, and the width of the air gap, were thoroughly investigated using scanning electronic microscopy, filtration efficiency, and breathability tests. It was found that the processing parameters were significant to the structure, thus the filtration performance of PLA MBs. PLA turned out to be a favorable material for meltblowing. The preferred spinning temperature was 220°C for optimal web quality. The diameter of PLA MB fibers became larger with the increase of hot air temperature. With the increase of air gap width, the diameter of PLA MB fibers went up, whereas the crimp level went down. This information may be useful for the future development of a commercialized production line of PLA MBs.

• basic schematics of MB system and spinning die; could base on recyclebot and winding system

Fabricating RO Membranes [ edit | edit source ]

The production is divided into the following process stages:

• Mechanical conditioning of the pulp: The pulp is fibrillated by different types of crushers, such as hammer mills and disc refiners, where the successive arrangement of both types of crushers ensures optimal dissolution.
• Chemical pretreatment: The fibrillated cellulose is treated with acetic acid with moderate agitation at 25°C to 50°C for approximately 1 h, resulting in continuous evaporation and condensation of the acetic acid in the spaces between the fiber particles. In addition to this acetic acid steam pretreatment, there is also a fine pulp state pretreatment. In this process, the cellulose is introduced in large quantities of water or diluted acetic acid and is vigorously stirred. Subsequent process steps, such as pressing or centrifugation, constantly increase the concentration of cellulose in the pulp.
• Cellulose Acetylation: In the commercial production of cellulose acetates, the acetic acid process or the methylene chloride process is often used for acetylation. In acetic acid processes, the pretreated cellulose mass is reacted in an acetylation mixture of acetic acid solvent with excess acetic anhydride, which serves as esterification agent, and with sulfuric acid as catalyst under vigorous mechanical mixing. In the methylene chloride process, methylene chloride is used in the acetylation mixture as a solvent instead of acetic acid. Since low boiling methylene chloride can be easily removed by distillation, process control is achieved even with highly viscous solutions. Even at low temperatures, it can dissolve cellulose triacetate very well. A small amount of sulfuric acid can be used as a catalyst, but often perchloric acid as well.
• Partial Hydrolysis: To obtain the desired secondary cellulose acetate types, cellulose triacetate is obtained by hydrolysis. For this purpose, the triacetate solution is typically heated to 60-80°C in the presence of an acid catalyst (usually sulfuric acid) by adding water while stirring and heating. Hydrolysis is controlled by the concentration of sulfuric acid, the amount of water and the temperature in such a way that the desired molecular degradation is achieved. The hydrolysis process is then stopped by adding basic salts that neutralize the acid catalyst.
• Cellulose acetate precipitation: When precipitating cellulose acetate from the reaction solution using dilute acetic acid, it is important to obtain uniform and easily washable cellulose acetate flakes. Before precipitation, any methylene chloride present must be completely removed by distillation. Acetic acid is then recovered.
• Washing and drying: By means of intensive washing, which is usually carried out against the current, the acetic acid must be removed from the flakes down to the smallest traces, otherwise damage ("charring") will occur during the drying process. After pressing the washing liquid, the flakes are dried in a conveyor dryer through which hot air flows to a residual moisture content of approx. 2-5%. For the further production of very high-quality, thermally stable, brightly colored and color-stable thermoplastic molding compounds, the cellulose acetate flakes are also bleached and stabilized in additional process steps before final drying.
• Flake Mixing: Before transporting the cellulose acetate flakes to a collection container from where they are transported to the appropriate processing plants, the flakes are mixed in a precisely controlled manner. This is to compensate for deviations of the cellulose acetates from different production batches. [1]

Fundamentals of Membranes for Water Treatment [ edit | edit source ]

Sagle, A. and Freeman, B., 2004. Fundamentals of membranes for water treatment. The future of desalination in Texas, 2(363), p.137. https://texaswater.tamu.edu/readings/desal/membranetechnology.pdf

• Good intro to the tech
• Commercial cellulose acetate (CA) membranes used for reverse osmosis have a degree of acetylation of about 2.7

Tubular Membranes [ edit | edit source ]

Daicen Membrane-Systems Ltd. (n.d.). Tubular Type Module . Tubular type module. Retrieved September 22, 2021, from https://daicen.com/en/products/membrane/chube.html .

• Treats human waste
• Specs for membrane (# of tubes, inner diameter, area)

PCI Membranes Filtration Group. (2021, August 25). C10 Series Tubular Membrane Modules . PCI Membranes. https://www.pcimembranes.com/products/c10-series-tubular-membrane-modules/

• Data Sheet: Components of a tubular membrane (ex. O Ring)

A review of polymeric membranes and processes for potable water reuse [ edit | edit source ]

David M. Warsinger, Sudip Chakraborty, Emily W. Tow, Megan H. Plumlee, Christopher Bellona, Savvina Loutatidou, Leila Karimi, Anne M. Mikelonis, Andrea Achilli, Abbas Ghassemi, Lokesh P. Padhye, Shane A. Snyder, Stefano Curcio, Chad D. Vecitis, Hassan A. Arafat, John H. Lienhard. (2018). A review of polymeric membranes and processes for potable water reuse , Progress in Polymer Science , Volume 81, Pages 209-237, SSN 0079-6700. https://doi.org/10.1016/j.progpolymsci.2018.01.004.

Abstract: Conventional water resources in many regions are insufficient to meet the water needs of growing populations, thus reuse is gaining acceptance as a method of water supply augmentation. Recent advancements in membrane technology have allowed for the reclamation of municipal wastewater for the production of drinking water, i.e., potable reuse. Although public perception can be a challenge, potable reuse is often the least energy-intensive method of providing additional drinking water to water stressed regions. A variety of membranes have been developed that can remove water contaminants ranging from particles and pathogens to dissolved organic compounds and salts. Typically, potable reuse treatment plants use polymeric membranes for microfiltration or ultrafiltration in conjunction with reverse osmosis and, in some cases, nanofiltration. Membrane properties, including pore size, wettability, surface charge, roughness, thermal resistance, chemical stability, permeability, thickness and mechanical strength, vary between membranes and applications. Advancements in membrane technology including new membrane materials, coatings, and manufacturing methods, as well as emerging membrane processes such as membrane bioreactors, electrodialysis, and forward osmosis have been developed to improve selectivity, energy consumption, fouling resistance, and/or capital cost. The purpose of this review is to provide a comprehensive summary of the role of polymeric membranes and process components in the treatment of wastewater to potable water quality and to highlight recent advancements and needs in separation processes. Beyond membranes themselves, this review covers the background and history of potable reuse, and commonly used potable reuse process chains, pretreatment steps, and advanced oxidation processes. Key trends in membrane technology include novel configurations, materials, and fouling prevention techniques. Challenges still facing membrane-based potable reuse applications, including chemical and biological contaminant removal, membrane fouling, and public perception, are highlighted as areas in need of further research and development.

Pre-filters [ edit | edit source ]

A critical overview of household slow sand filters for water treatment [ edit | edit source ].

B.L.S. Freitas, U.C. Terin, N.M.N. Fava, P.M.F. Maciel, L.A.T. Garcia, R.C. Medeiros, M. Oliveira, P. Fernandez-Ibañez, J.A. Byrne, L.P. Sabogal-Paz, A critical overview of household slow sand filters for water treatment ,Water Research,Volume 208,2022,117870,ISSN 0043-1354,https://doi.org/10.1016/j.watres.2021.117870.

Abstract: Household, or point-of-use (POU), water treatments are effective alternatives to provide safe drinking water in locations isolated from a water treatment and distribution network. The household slow sand filter (HSSF) is amongst the most effective and promising POU alternatives available today. Since the development of the patented biosand filter in the early 1990s, the HSSF has undergone a number of modifications and adaptations to improve its performance, making it easier to operate and increase users' acceptability. Consequently, several HSSF models are currently available, including those with alternative designs and constant operation, in addition to the patented ones. In this scenario, the present paper aims to provide a comprehensive overview from the earliest to the most recent publications on the HSSF design, operational parameters, removal mechanisms, efficiency, and field experiences. Based on a critical discussion, this paper will contribute to expanding the knowledge of HSSF in the peer-reviewed literature.

• Household slow sand filter is one of the most promising home scale treatments.
• HSSF is efficient in improving drinking water quality in isolated communities.
• Modification in the HSSF design and operation may encourage research.
• There is a lack of literature on protozoa, cyanobacteria, and emerging pollutants.

Components [ edit | edit source ]

• Valve descriptions
• "These tubular membranes were 250 mm in length with internal diameter of 7 mm."
• Pore size for ultra, micro, and nano filtration

What Contaminants do Reverse Osmosis Systems Remove? [ edit | edit source ]

Public water suppliers work hard to provide clean water for their customers. The problem is that there are many contaminants, especially those that cause taste and odor issues, which are simply not EPA regulated. These contaminants can easily penetrate aquifers, streams and rivers, bringing impurities straight to your water lines.

That's where Reverse Osmosis comes in. With a Reverse Osmosis filtration system, you can filter out impurities and produce outstanding drinking water for your home or business.

How Much Of A Contaminant Can A Reverse Osmosis System Remove?

• Fluoride (85-92%)
• Lead (95-98%)
• Chlorine (98%)
• Pesticides (up to 99%)
• Nitrates (60-75%)
• Sulfate (96-98%)
• Calcium (94-98%)
• Phosphate (96-98%)
• Arsenic (92-96%)
• Nickel (96-98%)
• Mercury (95-98%)
• Sodium (85-94%)
• Barium (95-98%

There are generally four stages in the Reverse Osmosis Process [ edit | edit source ]

SEDIMENT FILTER: This pre-filter stage is designed to strain out sediment, silt, and dirt and is especially important as the sediment filter protects dirt from getting to the delicate RO membranes that can be damaged by sediment. Learn more about sediment filter.

CARBON FILTER: The carbon filter is designed to remove chlorine and other contaminants that affect the performance and life of the RO membrane as well as improve the taste and odor of your water.

REVERSE OSMOSIS MEMBRANE: The semipermeable RO membrane in your RO system is designed to allow water through, but filter out almost all additional contaminants.

POLISHING FILTER: In a four-stage RO System, a final post filter (carbon filter) will "polish" off the water to remove any remaining taste and odor in the water. This final filter ensures you'll have outstanding drinking water.

Some factors that may affect the performance of a Reverse Osmosis System [ edit | edit source ]

• Incoming water pressure (most on municipal city tap water have 40-85 psi, but if water pressure is too low, RO system will not operate properly)
• Water Temperature (i.e. cold water takes longer to filter to filter)
• Type and number of total dissolved solids (TDS) in the tap water
• The quality of the filters and membranes used in the RO System (see operating specifications for your system)

References [ edit | edit source ]

Centers for Disease Control and Prevention. (2020, August 4). Technical information on Home Water Treatment Technologies . Centers for Disease Control and Prevention. Retrieved October 1, 2021, from https://www.cdc.gov/healthywater/drinking/home-water-treatment/household_water_treatment.html.

Michelle. (2019, January 8). Build your own Reverse Osmosis system for maple syrup . Souly Rested. https://soulyrested.com/2019/01/08/build-your-own-reverse-osmosis-system-for-maple-syrup/

Ravi V.K., Sushmitha V., Kumar M.V. P., and Thomas A. (2017). Reverse Osmosis Water Purification by Cycling Action . International Journal of Latest Engineering Research and Applications , 2 (5), 54-59.

Zhao, S., Liao, Z., Fane, A., Li, J., Tang, C., Zheng, C., Lin, J., & Kong, L. (2021). Engineering antifouling reverse osmosis membranes: A review. Desalination , 499 , 114857. https://doi.org/10.1016/j.desal.2020.114857

• ↑ https://en.wikipedia.org/wiki/Cellulose_acetate#Method
• Literature reviews
• FAST literature reviews
• Pages with no lead text
• Pages with no main image
• Pages with references but no references tag

Nanofiltration for drinking water treatment: a review

• Review Article
• Published: 26 November 2021
• Volume 16 , pages 681–698, ( 2022 )

Cite this article

• Hao Guo 1   na1 ,
• Xianhui Li 2   na1 ,
• Wulin Yang 3 ,
• Zhikan Yao 4 ,
• Ying Mei 5 ,
• Lu Elfa Peng 1 ,
• Zhe Yang 1 ,
• Senlin Shao 6 &
• Chuyang Y. Tang 1  

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In recent decades, nanofiltration (NF) is considered as a promising separation technique to produce drinking water from different types of water source. In this paper, we comprehensively reviewed the progress of NF-based drinking water treatment, through summarizing the development of materials/fabrication and applications of NF membranes in various scenarios including surface water treatment, groundwater treatment, water reuse, brackish water treatment, and point of use applications. We not only summarized the removal of target major pollutants (e.g., hardness, pathogen, and natural organic matter), but also paid attention to the removal of micropollutants of major concern (e.g., disinfection byproducts, per- and polyfluoroalkyl substances, and arsenic). We highlighted that, for different applications, fit-for-purpose design is needed to improve the separation capability for target compounds of NF membranes in addition to their removal of salts. Outlook and perspectives on membrane fouling control, chlorine resistance, integrity, and selectivity are also discussed to provide potential insights for future development of high-efficiency NF membranes for stable and reliable drinking water treatment.

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Acknowledgements

This work is supported by Senior Research Fellow Scheme of Research Grant Council (Grant No. SRFS2021-7S04) in Hong Kong and Seed Fund for Translational and Applied Research at The University of Hong Kong, China (Grant No. 104006007).

Author information

These authors contributed equally to this work.

Authors and Affiliations

Membrane-based Environmental & Sustainable Technology (MembEST) Group, Department of Civil Engineering, The University of Hong Kong, Hong Kong, China

Hao Guo, Lu Elfa Peng, Zhe Yang & Chuyang Y. Tang

Key Laboratory for City Cluster Environmental Safety and Green Development of the Ministry of Education, Institute of Environmental and Ecological Engineering, Guangdong University of Technology, Guangzhou, 510006, China

College of Environmental Science and Engineering, Peking University, Beijing, 100871, China

College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027, China

Research and Development Center for Watershed Environmental Eco-Engineering, Advanced Institute of Natural Sciences, Beijing Normal University, Zhuhai, 519087, China

School of Civil Engineering, Wuhan University, Wuhan, 430072, China

Senlin Shao

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Correspondence to Senlin Shao or Chuyang Y. Tang .

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Guo, H., Li, X., Yang, W. et al. Nanofiltration for drinking water treatment: a review. Front. Chem. Sci. Eng. 16 , 681–698 (2022). https://doi.org/10.1007/s11705-021-2103-5

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Received : 13 May 2021

Accepted : 28 July 2021

Published : 26 November 2021

Issue Date : May 2022

DOI : https://doi.org/10.1007/s11705-021-2103-5

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Our 7 Best Reverse Osmosis System Picks (2024)

We may be compensated if you purchase through links on our website. Our Reviews Team is committed to delivering honest, objective, and independent reviews on home products and services.

Investing in and installing a reverse osmosis water filter gives you cleaner, more pure water by removing dirt, chemicals, microorganisms, heavy metals, and other contaminants and impurities. These systems can vary greatly in price, with budget-friendly models starting around $150 – $300 and higher-end systems running anywhere from $500 – $1,000.

Our team has reviewed some of the best reverse osmosis systems for your home to help you choose the right one for your home. Overall, our top pick is the Waterdrop G3P800 8-Stage Tankless RO System , which uses a nine-stage filtration system to eliminate over 100 harmful contaminants from your water. Read our guide below for more of our top picks.

Reverse Osmosis System Overall

Waterdrop G3P800 8-Stage Tankless RO System

Best Countertop Reverse Osmosis System

Aquatru 4-stage countertop ro system.

Best Drinking Water Filtration System

Apec water systems roes-50 5-stage ro system, top 7 reverse osmosis systems.

• Best Reverse Osmosis System Overall : Waterdrop G3P800 8-Stage Tankless RO System
• Best Countertop Reverse Osmosis System: AquaTru 4-Stage Countertop RO System
• Best Drinking Water Filtration System: APEC Water Systems ROES-50 5-Stage RO System
• Best Value: iSpring 6-Stage Reverse Osmosis System
• Best Undersink Reverse Osmosis System: Express Water RO5DX 5-Stage Under Sink RO System
• Best Budget Model: Whirlpool WHAROS5 RO Water Filtration System
• Most Compact: Ecoviva Countertop Reverse Osmosis System

Compare Top Reverse Osmosis Systems

Best Reverse Osmosis System Overall

Prices taken at time of publishing.

• $999 at Amazon

What Are People Saying About the Waterdrop G3P800 8-Stage Tankless RO System?

Customers praised this system’s effectiveness. Several customers purchased tap water testing kits to determine the efficiency of this system and found that the water it provided was comparable to filtered bottled water. However, we also saw that some customers found its faucet to be flimsy, and some said it was overly loud while in operation.

“The reverse osmosis system is a high-quality product. The installation process is extremely easy and quick, as any person can install the system due to the clear instructions manual. The filtered water tastes crisp, refreshing, and pure. It tastes no different from bottled water. I would recommend it to anyone who wants refreshing clean water. The customer service was very quick to respond and was extremely patient with my questions and responses.” — Guo L. Huang via Amazon

“Tankless system requires an internal pump, which has to run every time you open the faucet. And also in the middle of the night when it does its flushing process. Spec is 65 dB, but to make it simple, we can hear the downstairs system flushing in the middle of the night from our upstairs bedroom with the door closed. Our kitchen is open to our family room, where we watch TV – every time someone gets water, we have to pause the TV. Some are claiming the noise isn’t that bad – I don’t understand how unless the unit is in a pretty thick cabinet or somehow farther away from where the humans are.” — Jim via Amazon

• $469 at Amazon

What Are People Saying About the AquaTru 4-Stage Countertop RO System?

Customers we found said that this reverse osmosis system was strong enough to remove even solid particles from the gallons of water it filtered. However, customers who weren’t satisfied with their purchase complained that it stopped working after a short period of time, while others were disappointed in how often this filter needed to be refilled.

“I have purchased other big-name brand water filters. I was very disappointed in the taste of their filtered water-it tasted of minerals and just plain yucky, for lack of a better word. I’ve heard good things about the AquaTru filter and thought I’d give it a try. I just received it and put it together yesterday. The water is so pure and delicious! It was very easy to assemble and prime to get it going! It is well made and it looks beautiful on my kitchen counter. I love the convenience of having it right where I need water! I highly recommend this AquaTru water filter! You’ll love it.” — Janet Tope via Amazon

“I drink a lot of water during the day and the amount of filtered water in the pitcher only lasted one day before having to be refilled again. The design seemed inefficient in that a lot of ‘waste’ water had to be emptied each time, and everything cleaned daily with soap and water. Too much of a hassle.” — Susan P. via Amazon

• $199.95 at Amazon

What Are People Saying About the APEC Water Systems ROES-50 5-Stage RO System?

Reviews we found complimented the efficiency of this system, which multiple customers said saved them more energy and money than previous reverse osmosis systems they had used. However, some customers weren’t satisfied with the system’s installation process, which required a  cordless drill  to drill holes. Others found that it didn’t reduce their water level’s TDS by as much as they were hoping.

“I am submitting this review 3 months after installation. I couldn’t be happier with this system! I have a lot of iron in my well water, use a softener to remove most of the iron and harness. However, the water still has a slightly funny taste and is yellow from suspended iron. This system finished the job. Not only does my water taste good, it’s perfectly clear now. I am so pleased, that I took the time to run a line from this system to my ice maker in my refrigerator .” —Michigan Shopper via Amazon

“For whatever reason, this system did not work very well for me. I was mostly trying to reduce total dissolved solids in our well water for use with an ice maker. Our TDS value is about 67 from our well water. With the system installed, TDS was reduced, but only down to about ~42, so maybe a ~35% reduction. Not great, not terrible. We never had any taste/odor issues with our water, so I can’t comment on the efficacy of the carbon filters— the RO seems marginal at best though.” — Clay N. Cowgill via Amazon

iSpring 6-Stage Reverse Osmosis System

• $218.99 at Amazon

What Are People Saying About the iSpring 6-Stage Reverse Osmosis System?

Our team found that customers frequently complimented how easy to install this system was, with the process being easy enough for even those with no DIY experience to manage. Reviewers who rated this product poorly complained that the system’s parts leaked.

“Love this system! Super easy to install with video and easy instructions. Had extremely high nitrates (+20). After installation, nitrates were down to 7! Water tastes amazing. The only issue I had was with my new RO system faucet leaking. I contacted customer service via email and received a response…. Wow! I cant say enough about the service I received!”  — Stacy via Amazon

“I really want to like this product. Actually, I have bought it three times since the price is reasonable. However, 3 out of the 3 installations had something leaking. The first two times were an o-ring on one of the large filters. I used Vaseline as recommended, but for some reason, the o-ring area leaked. They fortunately provide spare o-rings (probably knowing this is a common issue) so I replaced the o-ring, re-installed and everything was just fine. So either their o-rings have some kind of quality issue, or their design does, but it looks like the design is a very commonly used design.” — Turbo97se via Amazon

Best Undersink Reverse Osmosis System

Express Water RO5DX 5-Stage Under Sink RO System

• $152.99 at Amazon

What Are People Saying About the Express Water RO5DX 5-Stage Under Sink RO System?

Our team noticed that customers praised this RO’s filter design because the swivel connections allowed for quick-change filters without removing any tubing. However, dissatisfied customers complained about the system’s maintenance process, which required regular intensive cleaning in addition to filter changes. Others felt the flavor of the water after it passed through this RO system still did not taste right.

“We live in Hawaii and usually had to fill up water jugs to get filtered water. No more! It’s an investment, but man, it is worth it! It does take a minute to install and figure out, so definitely dedicate some time to putting it together. We’ve had it for several months and no complaints. The water tastes great, and the tap works amazing. It looks sleek and clean as well. 10/10 recommend.” — Amazon Customer via Amazon

“The countertop osmosis filter didn’t meet my flavor expectations. Despite using it consistently for two weeks, it retains an unpleasant plastic aftertaste that doesn’t resemble refreshing, clean water. Additionally, the need to frequently refill the container due to the waste water occupying most of it is inconvenient. This results in having to fill both sides regularly, consuming a significant amount of water. Furthermore, the instructions for setting up the product are unclear.” — Julian Kwan via Amazon

Best Budget Model

Whirlpool WHAROS5 RO Water Filtration System

• $220.21 at Amazon

What Are People Saying About the Whirlpool WHAROS5 RO Water Filtration System?

We found that customers who were impressed with this product particularly liked that it was easy to install, made for great-tasting clean water, and that it produced quick results. On the other hand, we also saw some customers who felt that some of the parts were not high quality, and others felt as though it wasted too much water.

“I have well water at home, it’s very hard (>30gpg) and has fairly high TDS (500ppm), I run through a sediment filter (5micron) then a water softener, then for drinking water it goes though this RO filter. Immediately after installation the TDS went from 500 down to 60 (ppm). After running it for a few days the TDS dropped even further to about 26ppm — this was my expectation and I’m pleased with it’s performance. I will keep this system for as long as it keeps producing water of this quality.” — C. Farmer via Amazon

“RO system like this one is known to produce clean and waste water which is discharged to the drain. Filter-based filtration systems, on the other hand, don’t waste water. So I expected waste water, but I didn’t know it wastes so much water until I measured it. Since none of the documents specify the amount of wasted water, I decided to measure it and was surprised. To be fair, the quality and taste of the produced, clean water is great.” — hermits via Amazon

Most Compact

Ecoviva Countertop Reverse Osmosis System

• $230.25 at Amazon

Good for: Homeowners seeking a countertop model with a sleek design that will blend in with other kitchen appliances.

What Are People Saying About the Ecoviva Countertop Reverse Osmosis System?

Our team found that customers who gave this product a positive review liked that it had a compact design and that it worked quickly to filter water. Additionally, others were impressed that it was easy to use and significantly improved water taste. However, others found it laborious to have to empty the tank after each use, and others said it didn’t always fully fill the carafe. 

“This counter top water purifier is excellent. The water tastes so much better than my unfiltered tap water. I was surprised to see that my water quality is actually pretty good, but this device still made a significant difference in the taste, and there is no more chlorine smell or presence. It actually removes impurities in the water rather than just improving taste, like some gravity-fed pitcher filters.” — fire_lion via Amazon

“The product did not fill the carafe even if the tank was full. Tried it several times and it won’t work. I waited for two days and worked again, but on and off. The other frustrating issue is that it doesn’t have an off/on button, so you have to unplug it all the time to start. It doesn’t start if you leave it plugged. For me, it is not safe to be plugging and unplugging it.” — ML via Amazon

Buying Guide To Reverse Osmosis Systems

A reverse osmosis water filtration system is an easy form of water treatment. It works by removing contaminants from your tap water during the filtration process.

However, before deciding on which RO system works best for your home, there are several factors you should take time to consider to ensure you’re making the right purchase. First, you should examine the various types of reverse osmosis systems and determine if you want a countertop or sink reverse osmosis system.

From there, you can determine what water filters the system has and if it includes a storage tank or other area for keeping water. You can also consider factors such as the capacity of the system, the water pressure levels, and the amount of safety features.

How do you determine the right size for an RO system?

Reverse osmosis systems can vary greatly in size. Most models we examined are designed to be stored underneath a sink, although countertop versions also exist. Measure how much space you have under or around your sink before deciding whether you need a tankless reverse osmosis system or if you have room for something larger.

Do reverse osmosis systems have water tanks?

Some RO filtration systems have a tank that stores water before it’s transferred to your sink. Others forgo the tank to save space. The type you purchase should depend on how much space you have in your kitchen.

What are the different types of RO system filters?

Reverse osmosis systems have multiple stages of filtration, moving water through three to eight filters to remove contaminants. Common types of filters we found include:

• Sediment: This type of filter removes dirt and other large particles.
• Carbon: This type removes chlorine.
• Reverse osmosis: An RO membrane removes particles that are larger than water molecules.
• UV: This type of filter kills microorganisms.
• Targeted: These filters remove specific types of contaminants, such as nitrate or fluoride. They’re only needed if these contaminants are present in large quantities.
• Remineralization: This filter introduces calcium and magnesium into the purified water to balance its pH level.

How is the capacity of an RO system measured?

The amount of water that a reverse osmosis system can purify at one time, also called the flow rate, is typically measured in gallons per day, or GPD. Choose a capacity based on how much water you and your family use in a day. For example, if you live alone, a system that purifies four gallons of water will be enough. However, if you have a family of five, you will need a reverse osmosis system that can purify at least 20 gallons of water.

What safety features should you look for in a reverse osmosis system?

We’ve found that some reverse osmosis systems have built-in features to ensure that they run smoothly. Common features we noticed include leak detectors and replacement filter indicators.

How do you measure the water pressure of an RO system?

In order for a reverse osmosis water filter to work properly, our team found it should have at least 50 PSI. RO water filters with above an 80 PSI are ideal for homes where the water supply lacks pressure.

Learn more about the cost of reverse osmosis systems .

How To Install a Reverse Osmosis Filter

While you can always hire a professional to install a reverse osmosis filter in your home, passionate DIYers can also try it on their own if they have the right materials, including an adjustable wrench , pencil or marker, screwdriver , and drill .  

In the video below, This Old House ’s Richard Trethewey takes you through the step-by-step process of installing a reverse osmosis filter underneath a kitchen sink to remove impurities from your home’s water. 

What is an NSF Certification?

National Sanitation Foundation (NSF) is an independent, third-party certification organization that tests and certifies products and services to ensure they meet certain public health and safety standards.

When a water filter is NSF certified, it means that it has been tested and found to meet or exceed specific performance standards for the reduction of a variety of contaminants, such as lead, chlorine, bacteria, and other impurities. This can provide peace of mind to consumers, knowing that the filter they are using has been independently verified to be effective at removing potentially harmful substances from their drinking water.

Furthermore, many government agencies and organizations require NSF certification for water filters to ensure that they meet specific standards for public health and safety. For example, many states require NSF certification for water filters used in schools, hospitals, and other public facilities.

Frequently Asked Questions About Reverse Osmosis Systems

How does a reverse osmosis system work.

A reverse osmosis system removes dirt, bacteria, and chemicals from your drinking water. It does this by passing water through a series of filters that each target a specific contaminant. Since the filters are semipermeable membranes, they allow water to pass through them while trapping undesirable molecules. The resulting water is purer than standard tap water.

Why do I need a reverse osmosis system?

A reverse osmosis system can provide your whole house with healthier and better-tasting water. However, our team notes that it’s especially important to use a reverse osmosis system if you live in an area with poor water quality.

How often do I need to replace the filters in my reverse osmosis system?

In our experience, you should replace the filters or filter cartridges in your reverse osmosis system at least once per year. If a filter gets clogged or damaged, replace it sooner. Some reverse osmosis systems have warning systems that tell you when a filter is failing or nearing the end of its life.

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To provide our readers with the best recommendations possible, we rely on several key sources of information to help guide our selection process.

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Assessing ro and nf desalination technologies for irrigation-grade water.

1. Introduction

2. methodology, 2.1. irrigation water quality, 2.2. salinity and sodium hazards (ec and sar).

Click here to enlarge figure

2.3. Specific Ion Toxicity

2.3.1. sodium toxicity, 2.3.2. chloride toxicity, 2.3.3. bicarbonate toxicity, 2.4. the model of irrigation water quality index (iwqi), 3. experimental data, 4. desalination system and simulation, 5. experiments and validation of the model, 6. results and discussion, 6.1. assessment of source water quality, 6.2. permeated water assessment, 6.4. rejection, 6.5. specific energy consumption, 7. conclusions, supplementary materials, author contributions, data availability statement, conflicts of interest.

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Share and Cite

Elmenshawy, M.R.; Shalaby, S.M.; M. Armanuos, A.; Elshinnawy, A.I.; Mujtaba, I.M.; Gado, T.A. Assessing RO and NF Desalination Technologies for Irrigation-Grade Water. Processes 2024 , 12 , 1866. https://doi.org/10.3390/pr12091866

Elmenshawy MR, Shalaby SM, M. Armanuos A, Elshinnawy AI, Mujtaba IM, Gado TA. Assessing RO and NF Desalination Technologies for Irrigation-Grade Water. Processes . 2024; 12(9):1866. https://doi.org/10.3390/pr12091866

Elmenshawy, Mohamed R., Saleh M. Shalaby, Asaad M. Armanuos, Ahmed I. Elshinnawy, Iqbal M. Mujtaba, and Tamer A. Gado. 2024. "Assessing RO and NF Desalination Technologies for Irrigation-Grade Water" Processes 12, no. 9: 1866. https://doi.org/10.3390/pr12091866

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Exploring Perfluoroalkyl and Polyfluoroalkyl Substance Presence and Potential Leaching from Reverse Osmosis Membranes: Implications for Drinking Water Treatment

Mohammad sadia.

† Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, P.O. Box 94240, Amsterdam, GE 1090, The Netherlands

Thomas L. ter Laak

‡ KWR Water Research Institute, P.O. Box 1072, Nieuwegein, BB 3430, The Netherlands

Emile R. Cornelissen

§ Centre for Advanced Process Technology for Urban Resource Recovery (CAPTURE), Ghent University, Frieda Saeysstraat 1, Gent 9052, Belgium

Annemarie P. van Wezel

Associated data.

Reverse osmosis (RO) is increasingly used in drinking water production to effectively remove micropollutants, such as perfluoroalkyl and polyfluoroalkyl substances (PFAS). However, RO membranes themselves may contain PFAS, which can potentially leach into treated drinking water. Leaching experiments and direct total oxidizable precursor assays revealed the presence and leaching potential of PFOS (branched and linear), PFBA, PFHxA, PFNA, and PFOA in five selected commercial RO membranes. This resulted in the release of tens of milligrams of ΣPFAS per membrane element used in drinking water production. Depending on assumptions made regarding leaching kinetics and volume of produced water per membrane element, predicted concentrations of ΣPFAS in the produced water ranged from less than one up to hundreds of pg/L. These concentrations are two to four orders of magnitude lower than those currently observed in Dutch drinking waters. The origin of PFAS in the membranes remains unclear. Further research is needed to bridge the gap between the laboratory conditions as used in this study and the real-world conditions and for a full understanding of potential leaching scenarios. Such an understanding is critical for water producers using RO technologies to proactively manage and mitigate potential PFAS contamination.

Short abstract

The presence of PFAS in commercial reverse osmosis membranes has been confirmed, but further research is needed to assess their potential leaching into drinking water during full-scale production.

1. Introduction

Developments in water technology as applied during drinking water production have contributed importantly to the protection of human health. The presence of micropollutants (MPs) and their transformation products in drinking water sources received attention as an emerging global challenge toward the latter part of the previous century. 1 − 3 To address this challenge, drinking water utilities are increasingly adopting advanced treatment technologies, 4 such as sorption to activated carbon, oxidation by ozonation or ultraviolet light, and size separation by membrane treatment, 5 to effectively remove undesired chemicals and produce safe and clean drinking water.

Reverse osmosis (RO) has emerged as an effective technology for removing many MPs in drinking water treatment. Nowadays, polyamide (PA) thin-film composite (TFC) membranes are the most commonly applied commercial RO membranes for the production of high-quality water. 6 The RO membrane itself consists of three layers: a top layer (PA); a porous middle layer (poly(ether sulfone) (PES) or polysulfone (PS)); and a bottom layer (a nonwoven fabric support sheet of polyester (PE)).

The top layer is responsible for perm-selectivity and is usually formed by interfacial polymerization (IP) at the interface between two immiscible solutions: an aqueous solution containing a diamine, such as m -phenylenediamine, and a nonpolar organic solution containing a triacyl, such as trimesoyl chloride. This process results in the formation of an ultrathin selective PA layer on the PS support layer. The support layer provides mechanical strength during operational processes and is usually fabricated using the phase inversion method. This involves dissolving the polymer in a solvent to form a casting solution, followed by achieving phase separation using a certain physical technology. 7 After synthesis, the PA-TFC membranes are stored in a solution (deionized water or sodium bisulfite solution) until use. The same applies to the support layer, which is stored to prevent pore collapsing with a storage solution until it is used for the IP process. 8

The performance of PA-TFC membrane can be improved through optimization of the PA layer, achieved by tuning monomer ratio and concentration, reaction temperature, and reaction time, and incorporating additives such as organic and inorganic chemicals, surfactants, cosolvents, and ionic liquids. 9 − 15 Additionally, optimizing the pore structure and the hydrophilicity of the support layer of TFC membranes can further improve the membrane performance. This optimization involves tuning factors such as polymer concentration, solvent composition, processing temperature, and the use of additives such as polyethylene glycol and polyvinylpyrrolidone. 16 − 19 These optimization techniques have been successfully applied to both the PA and the support layers in commercial membranes, resulting in improved performance of PA-TFC membranes. 20

Recently, the leaching of dissolved organic carbon from commercial RO membranes during the RO process has been confirmed, indicating the release of monomers used in the IP process. 21 However, to the authors’ knowledge, studies concerning the leaching of additives used in membrane production are currently still lacking.

Per- and polyfluoroalkyl substances (PFAS) are a group of synthetic chemicals known for their advantageous physicochemical properties, including their surfactant behavior, heat resistance, and fat and water repellency, making them valuable for many industrial applications.

While the explicit use of PFAS in the fabrication of TFC membranes has not been reported by industry, fluoropolymer-based polymerization reactions including additives are commercially applied for TFC membrane production. 22 They are approved in Europe as a processing additive on plastic materials and articles intended to come into contact with food by Commission Regulation (EU) 10/2011 of 14 January 14, 2011, and reported to be used in industries such as coating of equipment in chemical processing industry such as ducts, reactors, impellers, tanks, pipes, and fasteners. 23 Additionally, the use of surfactants as additives can improve the IP process by aiding monomers in moving from the amine phase into the organic phase. 24 , 25 , 12 Their synergistic use enhances membrane permeance and selectivity in solvent environments during TFC membrane synthesis. 26 , 27 At the same time, RO membranes have been proven to relatively effectively remove long and short-chain PFAS. 28 − 30

PFAS are recognized for their high persistence, accumulation potential, and associated hazards, leading to regulations of their occurrence in human relevant exposure media across different regions, 31 − 35 although still, many data gaps in our knowledge on the occurrence and (eco)toxicity exist for the broader set of PFAS. 36 Despite these efforts, elevated levels of PFAS continue to be found in environmental media. 37 − 39 , 30 , 40 Current efforts are being made to reduce this emission by proposing a ban on the production, use, sale, and import of all PFAS in the EU while exempting PFAS used as a pesticide and pharmaceutical and proposed derogations. 41

The potential emission of PFAS during water treatment due to leaching from the RO membrane materials, whether unintentionally introduced or intentionally added during membrane fabrication, has not been studied previously. This study aims to investigate PFAS residues in five commercially available RO membrane filters using leaching experiments, direct total oxidation precursors assay (TOPA), and analysis using a high-resolution mass spectrometer. The primary objective is to evaluate the presence of PFAS in RO membranes used for water purification and to quantify potential PFAS leaching during RO application based on assumptions regarding leaching kinetics.

2. Method and Material

2.1. standards and materials.

Native and isotopic mass labeled standards ( Table S1 ) were purchased from Wellington Laboratories (Guelph, Canada), excluding n -deuteriomethylperfluoro-1- n -octanesulfonamidoacetic acid- d 3 ( N -MeFOSAA- d 3 , > 99%) and n -ethylperfluoro-1- n -octanesulfonamidoacetic acid- d 5 ( N -EtFOSAA- d 5 , > 99%) which were purchased from Chiron (Trondheim, Norway), perfluoropropanoic acid (PFPrA, > 97%) from Sigma-Aldrich (Darmstadt, Germany), perfluoroethanesulfonic acid (PFEtS, > 98%) from Kanto Chemical (Japan), and n -methylperfluorobutanesulfonamide (MeFBSA, > 97%) from Apollo Scientific (Manchester, United Kingdom). A standard solution containing 45 PFAS analytes was made at 0.15 ng/μL in methanol, combining the mixture carboxylates (C3–C14), sulfonates (C3–C10; linear and branched), and a variety of precursors (C4–C12).

Additionally, LC-MS grade methanol and acetonitrile were purchased from Biosolve Chimie (Dieuze, France), while ammonium acetate (≥99%), hydrochloric acid (33%), and glacial acetic acid (≥99%) were obtained from Sigma-Aldrich. The ammonia solution (25%, analytical reagent grade) was sourced from Fisher Scientific (Massachusetts, United States).

2.2. Membrane Selection

Five different thin film composite commercial RO membranes, frequently investigated and widely employed in the literature and drinking water production, were randomly selected. 21 , 42 A comprehensive summary of the selected membranes and their corresponding material properties is provided in Table 1 .

The investigated spiral-wound membranes were manually opened. Following this, a few drops of Milli-Q water were applied to the membrane sheet and wiped with a paper tissue to remove residual salt of the storage solution before being cut into fragments measuring 1 cm 2 . All tools used for opening and cutting the membrane sheets were prewashed with methanol to prevent contamination.

2.3. Experimental Setup

PFAS presence in membranes and their leaching were assessed using two approaches. First, the membrane fragments were extracted with Milli-Q water to examine possible leaching from the membranes ( Section 2.3.1 ). Second, the total oxidizable precursor assay (TOPA) was directly applied to the membrane fragments ( Section 2.3.2 ).

The TOPA assay enables the indirect measurement of both known and unknown PFAS precursors by converting them into known measurable perfluoroalkyl carboxylic acids (PFCA) and perfluoroalkanesulfonic acids. 43 The harsh oxidizing conditions by TOPA might lead to oxidation of the polymers in the membrane materials, potentially enhancing the extraction of PFAS from the polymers. While TOPA is not specifically designed for extracting substances from polymer materials, it has been used on various polymers, including artificial turfs and textiles to detect PFAS. 44 − 46 The decision to employ direct TOPA was motivated by the objective of gaining a comprehensive understanding of the PFAS presence in the RO membrane, particularly on those PFAS that did not leach during the first leaching experiment. Those PFAS might encompass both extractable/leachable and nonextractable/nonleachable PFAS in the RO membrane sheet. Leaching to Milli-Q water was used instead of drinking water, as drinking water or any other water source potentially contains PFAS that would bias the results.

2.3.1. Membrane Leaching Experiment

A total of 40 membrane fragments of 1 cm 2 were placed into a 50 mL polypropylene falcon tube containing 35 mL of Milli-Q water (pH = 7). Each of the tested commercial membranes was assessed in separate tubes. Mass-labeled extraction standard solution (10 μL, 100 pg μL –1 ) in methanol was spiked into the tubes, which were then placed in the sonication bath for 48h. After sonication, the water samples were adjusted to pH 4 using acetic acid, followed by solid-phase extraction (SPE) using an Oasis weak anion exchange WAX SPE cartridge (3 mL, 60 mg, 30 μm; Waters Corporation Milford, USA). The cartridges were preconditioned by passing subsequently 3 mL each of 0.1% ammonium hydroxide in methanol, pure methanol, and Milli-Q water. Subsequent to sample loading, the cartridges underwent a wash with 3 mL of ammonium acetate buffer (pH 4), followed by vacuum drying for 1 h. PFAS was subsequently eluted using 3 mL of 0.1% ammonium hydroxide in methanol. The resulting extracts were evaporated under nitrogen to achieve a volume of 65 μL, followed by the addition of 175 μL of 0.05% acetic acid in water and 10 μL of mass-labeled injection standard solution (100 pg μL –1 ). The 250 μL extract underwent vortex-mixing and centrifugation (5 min at 4000 rpm) and was then transferred to an LC vial for further chemical analysis.

2.3.2. Total Oxidizable Precursor Assay

The TOPA was carried out directly on the membrane samples, following the method outlined by Lauria et al. 47 In short, for each investigated membrane, 20 membrane fragments of 1 cm 2 , were placed in a 50 mL falcon tube to which were added 30 mL of Milli-Q water, 0.48 g of potassium persulfate, and 0.456 mL of NaOH (10 M). The tubes were then placed in an oven at 85 °C for 16 h. After cooling, the samples were spiked with 10 μL of a mass-labeled extraction standard solution (100 pg μL –1 ), and their pH was adjusted to 4 using HCl (33%). Then, the water samples were extracted using SPE following the procedure described in Section 2.3.1 .

2.4. Quantification and Quality Control

The chemical analysis was performed on a Nexera UHPLC system (Shimadzu, Kyoto, Japan) coupled to a Bruker maXis 4 G q-TOF-high-resolution mass spectrometer (HRMS) and equipped with an IB-ESI source. Mass spectra were recorded in negative mode with a range of 50–1000 m / z and a 2 Hz sampling rate. Aliquots of 5 μL were injected into an Acquity UPLC CSH C18 column (130 Å, 2.1 × 150 mm, and 1.7 μm). The mobile phase consisted of 0.05% acetic acid in water (A) and 0.05% acetic acid in acetonitrile (B); details on eluent gradient and chromatographic separation can be found elsewhere. 30 Identification and confirmation of target compounds were achieved by accurate mass within a mass window of 2 ppm, retention time match (≤0.20 min) of analytes detected in samples with corresponding standards in calibration solution, and confirming the presence of at least one fragment ion. The list of all target analytes and their exact mass used for confirmation are shown in Table S1 . For branched isomers of PFOS, branched isomer standards were used for quantification. No branched isomers were detected for other PFAS.

The sample extraction procedure was conducted in triplicate for both the leaching experiment and the TOPA experiment. The relative standard deviation of the triplicate analyses was calculated to assess data reproducibility (RSD% < 20% for all samples). In the leaching experiment, procedural blank (Milli-Q water) and quality control (Milli-Q water spiked with native standards) were simultaneously extracted in triplicate with the samples. For the TOPA experiment, the procedural blank (tube without membrane) and quality control (20 μL perfluorooctanesulfonamide (FOSA) 500 pg μL –1 ) were also oxidized and extracted in triplicate alongside the samples, and full oxidation of FOAS was confirmed. A quality control sample (Milli-Q water spiked with native standards) was used for the SPE after TOPA was extracted alongside with TOPA samples, Table S2 .

For HRMS instrument quality control, methanol injections were conducted before and after standard injections to assess any contamination in the LC system. Internal mass calibration for each analysis was performed by infusing a 50 mM sodium acetate solution in a water:methanol mixture (1:1, v:v) at the beginning of the analysis (0.1–0.5 min). The limit of quantification LOQ was determined using average analyte concentration in the procedural blanks plus ten times the standard deviation. In case one of the targeted 45 PFAS was not detected in the procedural blank, the LOQ was defined as the lowest point in the calibration curve, 40 Table S2 .

The procedure blank showed high and inconsistent contamination, particularly with PFBS, leading to its exclusion from further evaluation. A limited number of PFAS were detected in the blank samples; details are provided in Table S2 . This table also includes information on the limit of quantification and recoveries for the quality control samples for each of the 45 target PFAS.

2.5. Data Analysis

To extrapolate the findings from the leaching experiments to the industrial context of RO operations, typical parameters of commercial RO systems were considered. These parameters include a membrane surface area of 40 m 2 , a membrane lifespan ranging from 8 to 12 years, and average water permeate flux rates of 20 L m –2 h –1 . In reality, these parameters may vary depending on the type of feedwater and membrane dimensions. 48 , 49

Furthermore, three different scenarios for the kinetics of PFAS leaching from the RO membrane during operation were proposed: (1) complete leaching occurring during the initial week of operation, (2) complete leaching taking place during the initial month of operation, and (3) continuous leaching throughout the entire lifespan of approximately 12 years of the membrane. Equilibrium was not assumed as the membranes are continuously exposed to new clean permeate water. The PFAS concentration in the permeate water was calculated by dividing the amount of PFAS released per element by the volume of permeate water produced at specific time intervals in each scenario, an example of calculation provided in the SI .

By examining these scenarios, we aim to better understand the potential long-term implications of PFAS leaching in industrial RO processes and the risk of contaminating permeate water with PFAS.

3. Results and Discussion

3.1. pfas presence in ro membranes: leaching and topa experiments.

Among the 45 investigated PFAS, 6 PFAS (specifically Br-/L-PFOS, PFBA, PFHxA, PFNA, and PFOA) were detected in the water from the leaching experiment with concentrations ranging from 17 to 38 pg/cm 2 ( Figure ​ Figure1 1 , Table S3 ). In the direct TOPA experiment, elevated concentrations of Br-/L-PFOS and PFOA were observed for all tested membranes ( Figure ​ Figure1 1 , Table S3 ). Only the TM720D-400 membrane exhibited the detection of PFBA, PFDA, and PFHxA in the TOPA, while these three PFAS were not found in the TOPA extracts of the other membranes. PFNA was not detected in the direct TOPA experiment for any of the membranes, whereas it was detected in the leaching experiment. This might be attributed to the higher LOQ for PFNA in the TOPA experiment compared to the leaching experiment ( Table S2 ).

Concentrations (pg/cm 2 ) of the detected PFAS in both leaching experiment and direct TOPA experiment for one cm 2 of investigated RO membrane and in procedure blank.

The results for the different membranes are more variable for the TOPA compared to the data from leaching experiments. In general, TOPA resulted in higher PFAS loads extracted per cm 2 than the water from the leaching experiment, Figure S1 . This either suggests the presence of PFAS precursors that are oxidized into PFCA during TOPA, or PFAS that did not leach during the water leaching experiment but were mobilized by the TOPA treatment. 45 Especially, the PFOS showed up to 16 times higher concentration per cm 2 of membrane after TOPA treatment. The direct TOPA reported that high concentrations of oxidant can catalyze the hydrolysis of sulfonamides, leading to the formation of perfluoroalkyl sulfonates instead of carboxylates. 46 Unlike standard TOPA, where radical oxidation by hydroxyl radicals primarily produces perfluoroalkyl carboxylates, the direct TOPA conditions favor the formation of perfluoroalkyl sulfonates, such as PFOS, due to the specific reactivity of sulfonamides under high oxidant concentrations.

Interestingly, in all tested membranes, only even-chain lengths (C4, C6, C8, and C10) were found after the TOPA, with no odd-numbered PFCA detected. This observation suggests that the PFAS precursors present are more likely to be sulfonamides rather than fluorotelomer compounds, as the fluorotelomer would result in chain-length shortening during the TOPA, producing odd-numbered PFCA. 50 This finding aligns with studies on PFAS fingerprinting, which indicate specific precursor compounds leading to such patterns. 46

The presence of PFAS observed in the membranes may be attributed to intentional or unintentional introduction during the manufacturing process of the membrane sheet by the incorporation of PFAS in the raw materials, such as monomers and solvents, or contamination during packaging or transportation. PFAS are also known to be used as polymer processing aids and might be used during the IP process. 51 , 52 , 23 Additionally, contamination may be introduced unintentionally during the production where PFAS or fluoropolymer are known to be used in industry, such as using PFAS in lubricants and greases, cleaning solutions, and coating of industrial equipment. 23 , 52 − 54 The dominant presence of the already globally banned PFOS and PFOA 55 , 56 in all tested membranes suggests potential contamination, from the production environment or raw materials, which may originate from currently not banned PFAS precursors. The lack of public information on the use of PFAS during the production processes of the membranes in literature or industrial reports disables further confirmation of the source(s) of the PFAS presence in the membranes.

3.2. Implementations of PFAS Presence in the RO Membrane for Drinking Water Production

The identification of the PFAS presence in the RO membrane material, as confirmed in this study, prompts a critical consideration of the potential leaching behavior in real-world scenarios for applications of these membranes during drinking water production. While the experimental setup presented in this study provides valuable insights into the presence of PFAS within the composite membrane, both the leaching experiments and direct TOPA assays cannot fully simulate the complex dynamics of processes occurring during RO operation in practice.

The current leaching experiment, employing sonication as a harsh condition to induce PFAS leaching under controlled conditions, offers applicable insights for potential leaching. The sonication has a physical effect on membrane materials, by accelerating membrane aging and degrading the membrane surfaces, increasing pore density and porosity over time. 57 This degradation process enhances PFAS extraction and release into the water phase. 58 Thereby, it might reflect membrane aging over its lifetime. In the real-world application of RO, the membrane surface experiences different conditions influencing PFAS leaching. For example, the membrane is prewashed before operation to remove stabilizing agents for a period of one to a few hours, based on the technical manual of the producer for each membrane. 48 This prewashing might lead to the removal of part of the PFAS before operation. Furthermore, the membrane during the RO operation process is exposed to an elevated pressure, varying temperatures, chemical cleaning agents, and biofilm formation on the membrane surface (fouling), all of which will contribute to membrane wear.

Furthermore, the 48 h sonication of membrane fragments extracted by Milli-Q water from both sides of the membrane does not fully replicate real-world conditions. In reality, the bottom side of the PA layer and the support layers (PS and PE) are exposed to the produced water, while the top side of the PA layer is exposed to the reject water. Therefore, leaching during RO operation in practice will differ from the 48 h sonication, as a disproportional part of the leached PFAS might actually end up in the reject water. Nevertheless, the sonication might provide an indication of the leaching potential.

Therefore, in this study, we can only preliminary indicate the scaling-up of the results from the PFAS leaching experiment conducted on a 40 cm 2 membrane area to an industrial scale using commercial membranes. Thereby, as the worst realistic case, it was assumed that all PFAS in the RO membrane that leach do end up in the produced water, which potentially results in a release of ΣPFAS mass in the order of tens of milligrams per membrane element (first row in Table 2 ). The proposed scenarios did not consider PFAS removal during the prewashing steps before operation.

The experimental setup used in this study does not elucidate the kinetics of PFAS leaching from the RO membranes during the production process. Extrapolating the results from the laboratory leaching and TOPA experiments to the context of common industrial RO operations, according to the suggested kinetic scenarios ( Section 2.5 ), predicts a concentration range in the permeate water for the ΣPFAS originating from the membrane, varying between less than one to hundreds of pg/L ( Table 2 ).

The concentrations of individual PFAS in both the month and year scenarios are currently undetectable using available analytical methods and fall well below the concentrations as observed in the drinking water produced from vulnerable groundwater or surface water in The Netherlands ( Figure ​ Figure2 2 , Table S4 ), which are treated using different treatment processes such as sorption, oxidation, or RO filtration. 30 Similarly, these concentrations are also well below those observed in drinking water produced outside The Netherlands. 59 − 61 In the week scenario, the PFAS might be detectable, but resulting concentrations are still 2 orders of magnitude lower compared to concentrations in drinking water ( Figure ​ Figure2 2 ). 30

Box whisker plots present ΣPFAS concentrations (logarithm scale) in drinking water sourced from surface water and groundwater (in red, sum of only the seven PFAS detected in the tested membrane, Table S4 ), 30 as compared to the predicted leaching kinetics scenarios (1 week, 1 month, 12 year) for the five tested membranes (blue for the leaching to water, green for direct TOPA). The whiskers represent the minimum and maximum concentrations, and the lower border of the box represents the first quartile (25%), the line inside the box the median, and the upper border is the third quartile (75%).

The PFAS concentrations, both as individual and sum of group PFAS ( Table S5 ), in the permeate water in all suggested kinetic scenarios comply with the existing guidelines for safe drinking water. 31 − 33 , 62 However, in the first and second scenarios, they exceeded the recently restricted lifetime health advisory level (LHAL) proposed by US EPA, 35 for both PFOS (EPA-LHDL 0.02 ng/L) and PFOA (EPA-LHDL 0.004 ng/L), with a few exceptions for PFOS in the second scenario ( Table S5 ). It must be noted that membranes in practice are used much longer than a week to months. Nevertheless, these different scenarios for leaching kinetics highlight the need for a comprehensive understanding of the kinetics in practice.

While this study primarily focuses on the implications of PFAS presence and potential leaching in RO membranes for drinking water treatment, it is important to acknowledge that membranes are also widely used in various other industries, including the food industry. 63 In these applications, membranes are utilized to prepare or treat aqueous matrices, presenting additional routes of exposure to PFAS. Therefore, the findings and considerations presented here are also relevant to other industries that utilize membrane technologies. Future research should take into account these broader applications and potential exposure routes to ensure comprehensive risk assessment and mitigation strategies.

The present study provides an initial step in understanding the presence of PFAS in the RO membranes and their potential for leaching. Further research is needed to understand the sources of PFAS within the membrane and to determine from which layer of the membrane the PFAS are leaching. Moreover, the kinetics of the leaching process needs further investigation under real-life conditions to assess the (temporal) variations in PFAS levels in permeate water and associated risks, particularly concerning critical windows during the development of unborn or newborns. Future research should aim to bridge these gaps between the current laboratory-scale experiments and full-scale industrial RO applications during drinking water treatment. Such research could assist the membrane-producing companies in providing protocols for proactive measures, such as adapting membrane conditioning and washing protocols before operation, to ensure safe application. Furthermore, this underscores the importance of ongoing research to prevent PFAS residues in drinking-water-contact materials, aligning with recent EU hygiene standards for materials and products that come into contact with drinking water, aimed at reducing such risks.

Acknowledgments

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 860665 (ITN PERFORCE3).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.4c04743 .

• Calculation of ΣPFAS concentration in permeate water; leaching and TOPA experiment results (Figure S1); analytical standards and quality control data (Table S1,S2); PFAS concentrations in RO membrane and drinking water (Tables S3 and S4); predicted permeate concentrations (Table S5) ( PDF )

The authors declare no competing financial interest.

Supplementary Material

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