research paper on water purification

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Research Article

Research on drinking water purification technologies for household use by reducing total dissolved solids (TDS)

Roles Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing

* E-mail: [email protected]

Affiliation Redlands East Valley High School, Redlands, California, United States of America

• Bill B. Wang

• Published: September 28, 2021
• https://doi.org/10.1371/journal.pone.0257865
• Reader Comments

This study, based in San Bernardino County, Southern California, collected and examined tap water samples within the area to explore the feasibility of adopting non-industrial equipment and methods to reduce water hardness and total dissolved solids(TDS). We investigated how water quality could be improved by utilizing water boiling, activated carbon and sodium bicarbonate additives, as well as electrolysis methods. The results show that heating is effective at lower temperatures rather than long boils, as none of the boiling tests were lower than the original value. Activated carbon is unable to lower TDS, because it is unable to bind to any impurities present in the water. This resulted in an overall TDS increase of 3.5%. However, adding small amounts of sodium bicarbonate(NaHCO 3 ) will further eliminate water hardness by reacting with magnesium ions and improve taste, while increasing the pH. When added to room temperature tap water, there is a continuous increase in TDS of 24.8% at the 30 mg/L mark. The new findings presented in this study showed that electrolysis was the most successful method in eliminating TDS, showing an inverse proportion where an increasing electrical current and duration of electrical lowers more amounts of solids. This method created a maximum decrease in TDS by a maximum of 22.7%, with 3 tests resulting in 15.3–16.6% decreases. Furthermore, when water is heated to a temperature around 50°C (122°F), a decrease in TDS of around 16% was also shown. The reduction of these solids will help lower water hardness and improve the taste of tap water. These results will help direct residents to drink more tap water rather than bottled water with similar taste and health benefits for a cheaper price as well as a reduction on plastic usage.

Citation: Wang BB (2021) Research on drinking water purification technologies for household use by reducing total dissolved solids (TDS). PLoS ONE 16(9): e0257865. https://doi.org/10.1371/journal.pone.0257865

Editor: Mahendra Singh Dhaka, Mohanlal Sukhadia University, INDIA

Received: June 22, 2021; Accepted: September 14, 2021; Published: September 28, 2021

Copyright: © 2021 Bill B. Wang. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: The author received no specific funding for this work.

Competing interests: The authors have declared that no competing interests exist.

Introduction

The concentration of total dissolved solids(TDS) present in water is one of the most significant factors in giving water taste and also provides important ions such as calcium, magnesium, potassium, and sodium [ 1 – 3 ]. However, water with high TDS measurements usually indicates contamination by human activities, such as soil and agricultural runoff caused by irrigation, unregulated animal grazing and wildlife impacts, environmentally damaging farming methods such as slash and burn agriculture, and the overuse of nitrate-based fertilizer [ 4 , 5 ], etc. Around tourist areas as well as state parks, these factors will slowly add up over time and influence the water sources nearby [ 5 ]. Water that flows through natural springs and waterways with high concentrations of organic salts within minerals and rocks, or groundwater that originates from wells with high salt concentration will also result in higher particle measurements [ 6 ].

Water sources can be contaminated by substances and ions such as nitrate, lead, arsenic, and copper [ 7 , 8 ] and may cause many health problems related to heavy metal consumption and poisoning. Water reservoirs and treatments plants that do not consider water contamination by motor vehicles, as well as locations that struggle to provide the necessary components required for water treatment will be more prone to indirect contamination [ 9 – 11 ]. Many plants are effective in ensuring the quality and reduction of these contaminants, but often leave out the secondary considerations, The United States Environmental Protection Agency(US EPA)’s secondary regulations recommend that TDS should be below 500 mg/L [ 2 ], which is also supported by the World Health Organization(WHO) recommendation of below 600 mg/L and an absolute maximum of less than 1,000 mg/L [ 3 ]. These substances also form calcium or magnesium scales within water boilers, heaters, and pipes, causing excess buildup and drain problems, and nitrate ions may pose a risk to human health by risking the formation of N -nitroso compounds(NOC) and less public knowledge about such substances [ 12 – 15 ]. Nitrates can pose a non-carcinogenic threat to different communities, but continue to slip past water treatment standards [ 15 ]. Furthermore, most people do not tolerate or prefer water with high hardness or chlorine additives [ 16 ], as the taste changes tremendously and becomes unpreferable. Even so, TDS levels are not accounted for in mandatory water regulations, because the essential removal of harmful toxins and heavy metals is what matters the most in water safety. Some companies indicate risks in certain ions and alkali metals, showing how water hardness is mostly disregarded and is not as well treated as commercial water bottling companies [ 17 , 18 ].

In Southern California, water quality is not as well maintained than the northern counties as most treatment plants in violation of a regulation or standard are located in Central-Southern California [ 19 ], with southern counties having the largest number of people affected [ 20 ]. This study is focused on the Redlands area, which has had no state code violations within the last decade [ 21 ]. A previous study has analyzed TDS concentrations throughout the Santa Ana Basin, and found concentrations ranging from 190–600 ppm as treated wastewater and samples obtained from mountain sites, taking into account the urban runoff and untreated groundwater as reasons for elevated levels of TDS but providing no solution in helping reduce TDS [ 22 ]. Also, samples have not been taken directly through home water supplies, where the consumer is most affected. Other water quality studies in this region have been focused on the elimination of perchlorates in soil and groundwater and distribution of nitrates, but such research on chemicals have ceased for the last decade, demonstrated by safe levels of perchlorates and nitrates in water reports [ 23 , 24 ]. In addition to these studies, despite the improving quality of the local water treatment process, people prefer bottled water instead of tap water because of the taste and hardness of tap water [ 25 ]. Although water quality tests are taken and documented regularly, the taste of the water is not a factor to be accounted for in city water supplies, and neither is the residue left behind after boiling water. The residue can build up over time and cause appliance damage or clogs in drainage pipes.

This study will build upon previous analyses of TDS studies and attempt to raise new solutions to help develop a more efficient method in reducing local TDS levels, as well as compare current measurements to previous analyses to determine the magnitude to which local treatment plants have improved and regulated its treatment processes.

Several methods that lower TDS are reviewed: boiling and heating tap water with and without NaHCO₃, absorption by food-grade activated carbon [ 26 , 27 ], and battery-powered electrolysis [ 28 – 30 ]. By obtaining water samples and determining the difference in TDS before and after the listed experiments, we can determine the effectiveness of lowering TDS. The results of this study will provide options for residents and water treatment plants to find ways to maintain the general taste of the tap water, but also preserve the lifespan of accessories and pipelines. By determining a better way to lower TDS and treat water hardness, water standards can be updated to include TDS levels as a mandatory measurement.

Materials and methods

All experiments utilized tap water sourced from Redlands homes. This water is partially supplied from the Mill Creek (Henry Tate) and Santa Ana (Hinckley) Water Sheds/Treatment Plants, as well as local groundwater pumps. Water sampling and sourcing were done at relatively stable temperatures of 26.9°C (80.42°F) through tap water supplies. The average TDS was measured at 159 ppm, which is slightly lower than the reported 175 ppm by the City of Redlands. Permission is obtained by the author from the San Bernardino Municipal Water Department website to permit the testing procedures and the usage of private water treatment devices for the purpose of lowering water hardness and improving taste and odor. The turbidity was reported as 0.03 Nephelometric Turbidity Units (NTU) post-treatment. Residual nitrate measured at 2.3mg/L in groundwater before treatment and 0.2 mg/L after treatment and perchlorate measured at 0.9 μg/L before treatment, barely staying below the standard of 1 μg/L; it was not detected within post-treatment water. Lead content was not detected at all, while copper was detected at 0.15 mg/L.

For each test, all procedures were done indoors under controlled temperatures, and 20 L of fresh water was retrieved before each test. Water samples were taken before each experimental set and measured for TDS and temperature, and all equipment were cleaned thoroughly with purified water before and after each measurement. TDS consists of inorganic salts and organic material present in solution, and consists mostly of calcium, magnesium, sodium, potassium, carbonate, chloride, nitrate, and sulfate ions. These ions can be drawn out by leaving the water to settle, or binding to added ions and purified by directly separating the water and ions. Equipment include a 50 L container, 1 L beakers for water, a graduated cylinder, a stir rod, a measuring spoon, tweezers, a scale, purified water, and a TDS meter. A standard TDS meter is used, operated by measuring the conductivity of the total amount of ionized solids in the water, and is also cleaned in the same manner as aforementioned equipment. The instrument is also calibrated by 3 pH solutions prior to testing.All results were recorded for and then compiled for graphing and analysis.

Heating/Boiling water for various lengths of time

The heating method was selected because heat is able to break down calcium bicarbonate into calcium carbonate ions that are able to settle to the bottom of the sample. Four flasks of 1 L of tap water were each heated to 40°C, 50°C, 60°C, and 80°C (104–176°F) and observed using a laser thermometer. The heated water was then left to cool and measurements were made using a TDS meter at the 5, 10, 20, 30, and 60-minute marks.

For the boiling experiments, five flasks of 1 L of tap water were heated to boil at 100°C (212°F). Each flask, which was labeled corresponding to its boiling duration, was marked with 2, 4, 6, 10, and 20 minutes. Each flask was boiled for its designated time, left to cool under open air, and measurements were made using a TDS meter at the 5, 10, 20, 30, 60, and 120-minute marks. The reason that the boiling experiment was extended to 120 minutes was to allow the water to cool down to room temperature.

Activated carbon as a water purification additive

This test was performed to see if food-grade, powdered activated carbon had any possibility of binding with and settling out residual particles. Activated carbon was measured using a milligram scale and separated into batches of 1, 2, 4, 5, 10, 30, and 50 mg. Each batch of the activated carbon were added to a separate flask of water and stirred for five minutes, and finally left to settle for another five minutes. TDS measurements were recorded after the water settled.

Baking soda as a water purification additive

To lower scale error and increase experimental accuracy, a concentration of 200 mg/L NaHCO₃ solution was made with purified water and pure NaHCO₃. For each part, an initial TDS measurement was taken before each experiment.

In separate flasks of 1 L tap water, each labeled 1, 2, 4, 5, 10, and 30 mg of NaHCO 3 , a batch was added to each flask appropriately and stirred for 5 minutes to ensure that everything dissolved. Measurements were taken after the water was left to settle for another 5 minutes for any TDS to settle.

Next, 6 flasks of 1 L tap water were labeled, with 5 mg (25 mL solution) of NaHCO₃ added to three flasks and 10 mg (50 mL solution) of NaHCO₃ added to the remaining three. One flask from each concentration of NaHCO₃ was boiled for 2 mins., 4 mins., or 6 mins., and then left to cool. A TDS measurement was taken at the 5, 10, 20, 30, 60, and 120-minute marks after removal from heat.

Electrolysis under low voltages

This test was performed because the ionization of the TDS could be manipulated with electricity to isolate an area of water with lower TDS. For this test, two 10cm long graphite pieces were connected via copper wiring to a group of batteries, with each end of the graphite pieces submerged in a beaker of tap water, ~3 cm apart.

Using groups of 1.5 V double-A batteries, 4 beakers with 40mL of tap water were each treated with either 7.5, 9.0, 10.5, and 12.0 V of current. Electrolysis was observed to be present by the bubbling of the water each test, and measurements were taken at the 3, 5, 7, and 10 minute marks.

Results/Discussion

Heating water to various temperatures until the boiling point.

The goal for this test was to use heat to reduce the amount of dissolved oxygen and carbon dioxide within the water, as shown by this chemical equation: Heat: Ca(HCO 3 ) 2 → CaCO 3 ↓ + H 2 O + CO 2 ↑.

This would decompose ions of calcium bicarbonate down into calcium carbonate and water and carbon dioxide byproducts.

Patterns and trends in decreasing temperatures.

The following trend lines are based on a dataset of changes in temperature obtained from the test results and graphed as Fig 1 .

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• TIFF original image

https://doi.org/10.1371/journal.pone.0257865.g001

To predict the precise temperature measurements of the tap water at 26.9°C, calculations were made based on Fig 1 . The fitting equations are in the format, y = a.e bx . The values for the fitting coefficients a and b, and correlation coefficient R 2 are listed in Table 1 as column a, b and R 2 . The calculated values and the target temperature are listed in Table 1 .

https://doi.org/10.1371/journal.pone.0257865.t001

Fig 2 was obtained by compiling TDS results with different temperatures and times.

https://doi.org/10.1371/journal.pone.0257865.g002

The fitting equations for Fig 2 are also in the format, y = a.e bx . The fitting coefficients a and b, and correlation coefficient R 2 values are listed in Table 2 . Based on the fitting curves in Fig 2 and the duration to the target temperature in Table 1 , We calculated the TDS at 26.9°C as listed in column calculated TDS in Table 2 based on the values we reported on Fig 2 .

https://doi.org/10.1371/journal.pone.0257865.t002

Based on the heating temperature and the calculated TDS with the same target water temperature, we obtained the following heating temperature vs TDS removal trend line and its corresponding fitting curve in Table 2 .

In Fig 1 , a trend in the rate of cooling is seen, where a higher heating temperature creates a steeper curve. During the first five minutes of cooling, the water cools quicker as the absorbed heat is quickly released into the surrounding environment. By the 10-minute mark, the water begins to cool in a linear rate of change. One detail to note is that the 100°C water cools quicker than the 80°C and eventually cools even faster than the 60°C graph. Table 1 supports this observation as the duration to target temperature begins to decrease from a maximum point of 94.8 mins to 80.95 mins after the 80°C mark.

As shown in Fig 2 , all TDS values decrease as the temperature starts to cool to room temperature, demonstrating a proportional relationship where a lower temperature shows lower TDS. This can partially be explained by the ions settling in the flasks. Visible particles can also be observed during experimentation as small white masses on the bottom, as well as a thin ring that forms where the edge of the water contacts the flask. When the water is heated to 40°C and cooled, a 3.8% decrease in TDS is observed. When 50°C is reached, the TDS drops at its fastest rate from an initial value of 202 ppm to 160 ppm after 60 minutes of settling and cooling. The TDS measurements in these experiements reach a maximum of 204 ppm at the 60°C mark. However, an interesting phenomenon to point out is that the water does not hit a new maximum at 100°C. meaning that TDS reaches a plateau at 60°C. Also, the rate of decrease begins to slow down after 20 minutes, showing that an unknown factor is affecting the rate of decrease. It is also hypothesized that the slight increase in TDS between the 5–20 minute range is caused by a disturbance in the settling of the water, where the temperature starts to decrease at a more gradual and constant rate. The unstable and easy formation of CaCO 3 scaling has also been the subject of a study of antiscaling methods, which also supports the result that temperature is a significant influence for scale formation [ 12 ].

In Table 2 , calculations for TDS and the time it takes for each test to cool were made. Using the data, it is determined that the test with 50°C water decreased the most by 16% from the initial measurement of 159ppm. This means that it is most effective when water is heated between temperatures of 40–60°C when it comes to lowering TDS, with a difference of ~7–16%. When water is heated to temperatures greater than 80°C, the water begins to evaporate, increasing the concentration of the ions, causing the TDS to increase substantially when cooled to room temperature.

Finally, in Fig 3 , a line of best fit of function f(x) = -0.0007x 3 + 0.1641x 2 –10.962x + 369.36 is used with R 2 = 0.9341. Using this function, the local minimum of the graph would be reached at 48.4°C.

https://doi.org/10.1371/journal.pone.0257865.g003

This data shows that heating water at low temperatures (i.e. 40–50°C) may be more beneficial than heating water to higher temperatures. This study segment has not been presented in any section within the United States EPA Report on water management for different residual particles/substances. However, warmer water temperatures are more prone to microorganism growth and algal blooms, requiring more intensive treatment in other areas such as chlorine, ozone, and ultraviolet disinfection.

Using the specific heat capacity equation, we can also determine the amount of energy and voltage needed to heat 1 L of water up to 50°C: Q = mcΔT, where c, the specific heat capacity of water, is 4.186 J/g°C, ΔT, the change in temperature from the experimental maximum to room temperature, is 30°C, and m, the mass of the water, is 1000 g. This means that the amount of energy required will be 125580 J, which is 0.035 kWh or 2.1 kW.

After taking all of the different measurements obtained during TDS testing, and compiling the data onto this plot, Fig 4 is created with a corresponding line of best fit:

https://doi.org/10.1371/journal.pone.0257865.g004

In Fig 4 , it can be observed that the relationship between the temperature of the water and its relative TDS value is a downwards facing parabolic graph. As the temperature increases, the TDS begins to decrease after the steep incline at 50–60°C. The line of best fit is represented by the function f(x) = -0.0142x 2 + 2.258x + 105.84. R 2 = 0.6781. Because the R 2 value is less than expected, factors such as the time spent settling and the reaction rate of the ions should be considered. To determine the specifics within this experiment, deeper research and prolonged studies with more highly accurate analyses must be utilized to solve this problem.

Boiling water for various amounts of time

Trend of boiling duration and rate of cooling..

Using the same methods to create the figures and tables for the previous section, Fig 5 depicts how the duration of time spent boiling water affects how fast the water cools.

https://doi.org/10.1371/journal.pone.0257865.g005

As seen in Fig 5 , within the first 10 minutes of the cooling time, the five different graphs are entwined with each other, with all lines following a similar pattern. However, the graph showing 20 minutes of boiling is much steeper than the other graphs, showing a faster rate of cooling. This data continues to support a previous claim in Fig 2 , as this is most likely represented by a relationship a longer the boil creates a faster cooling curve. This also shows that the first 5 minutes of cooling have the largest deviance compared to any other time frame.

The cooling pattern is hypothesized by possible changes in the orderly structure of the hydrogen bonds in the water molecules, or the decreased heat capacity of water due to the increasing concentration of TDS.

Effect on TDS as boiling duration increases.

In Fig 6 , all lines except for the 20-minute line are clustered in the bottom area of the graph. By excluding the last measurement temporarily due to it being an outlier, we have observed that the difference between the initial and final TDS value of each test decreases.

https://doi.org/10.1371/journal.pone.0257865.g006

Despite following a similar trend of an increase in TDS at the start of the tests and a slow decrease overtime, this experiment had an interesting result, with the final test measuring nearly twice the amount of particles compared to any previous tests at 310 ppm, as shown in Fig 6 . It is confirmed that the long boiling time caused a significant amount of water to evaporate, causing the minerals to be more concentrated, thus resulting in a 300 ppm reading. Fig 6 follows the same trend as Fig 2 , except the TDS reading veers away when the boiling duration reaches 20 minutes. Also, with the long duration of heating, the water has developed an unfavorable taste from intense concentrations of CaCO₃. This also causes a buildup of a thin crust of CaCO₃ and other impurities around the container that is difficult to remove entirely. This finding is in accordance with the introductory statement of hot boiling water causing mineral buildups within pipes and appliances [ 9 ]. A TDS reading of 300ppm is still well below federal secondary standards of TDS, and can still even be compared to bottled water, in which companies may fluctuate and contain 335ppm within their water [ 1 , 2 ].

This experiment continues to stupport that the cooling rate of the water increases as the time spent boiling increases. Based on this test, a prediction can be made in which an increased concentration of dissolved solids lowers the total specific heat capacity of the sample, as the total volume of water decreases. This means that a method can be derived to measure TDS using the heat capacity of a tap water mixture and volume, in addition to current methods of using the electrical conductivity of aqueous ions.

Adding food-grade activated carbon to untreated tap water

Fig 7 presents a line graph with little to no change in TDS, with an initial spike from 157 to 163 ppm. The insoluble carbon remains in the water and shows no benefit.

https://doi.org/10.1371/journal.pone.0257865.g007

The food-grade activated carbon proved no benefit to removing TDS from tap water, and instead added around 5–7 ppm extra, which settled down to around +4 ppm at 120 minutes. The carbon, which is not 100% pure from inorganic compounds and materials present in the carbon, can dissolve into the water, adding to the existing concentration of TDS. Furthermore, household tap water has already been treated in processing facilities using a variety of filters, including carbon, so household charcoal filters are not effective in further reducing dissolved solids [ 18 ].

Adding sodium bicarbonate solution to boiled tap water

As seen in Fig 8 , after adding 1 mg of NaHCO 3 in, the TDS rises to 161 ppm, showing a minuscule increase. When 4 mg was added, the TDS drops down to 158 ppm. Then, when 5 mg was added, a sudden spike to 172 ppm was observed. This means that NaHCO 3 is able to ionize some Ca 2+ and Mg 2+ ions, but also adds Na + back into the water. This also means that adding NaHCO 3 has little to no effect on TDS, with 4mg being the upper limit of effectiveness.

https://doi.org/10.1371/journal.pone.0257865.g008

To examine whether or not the temperature plays a role in the effectiveness in adding NaHCO 3 , a boiling experiment was performed, and the data is graphed in Fig 9 .

https://doi.org/10.1371/journal.pone.0257865.g009

Fig 9 presents the relationship between the amount of common baking soda(NaHCO₃) added, the boiling time involved, and the resulting TDS measurements. After boiling each flask for designated amounts of time, the results showed a downward trend line from a spike but does not reach a TDS value significantly lower than the initial sample. It is apparent that the NaHCO₃ has not lowered the TDS of the boiling water, but instead adds smaller quantities of ions, raising the final value. This additive does not contribute to the lowering of the hardness of the tap water. However, tests boiled with 5 mg/L of baking soda maintained a downward pattern as the water was boiled for an increasing amount of time, compared to the seemingly random graphs of boiling with 10 mg/L.

In some households, however, people often add NaHCO₃ to increase the pH for taste and health benefits. However, as shown in the test results, it is not an effective way of reducing TDS levels in the water [ 10 , 16 ], but instead raises the pH, determined by the concentration added. Even under boiling conditions, the water continues to follow the trend of high growth in TDS, of +25–43 ppm right after boiling and the slow drop in TDS (but maintaining a high concentration) as the particles settle to the bottom.

Utilizing the experimental results, we can summarize that after adding small batches of NaHCO3 and waiting up to 5 minutes will reduce water hardness making it less prone to crystallizing within household appliances such as water brewers. Also, this process raises the pH, which is used more within commercial water companies. However, the cost comes at increasing TDS.

Using electrolysis to treat TDS in tap water

Different voltages were passed through the water to observe the change in TDS overtime, with the data being compiled as Fig 10 .

https://doi.org/10.1371/journal.pone.0257865.g010

The process of electrolysis in this experiment was not to and directly remove the existing TDS, but to separate the water sample into three different areas: the anode, cathode, and an area of clean water between the two nodes [ 19 ]. The anions in the water such as OH - , SO 4 2- , HCO 3 - move to the anode, while the cations such as H + , Ca 2+ 、Mg 2+ 、Na + move to the cathode. The middle area would then be left as an area that is more deprived of such ions, with Fig 10 proving this.

As shown in Fig 10 , electrolysis is effective in lower the TDS within tap water. Despite the lines being extremely tangled and unpredictable, the general trend was a larger decrease with a longer duration of time. At 10 minutes, all lines except 10.5 V are approaching the same value, meaning that the deviation was most likely caused by disturbances to the water during measurement from the low volume of water. With each different voltage test, a decrease of 12.7% for 6.0 V, 14.9% for 9.0 V, 22.7% for 10.5 V. and 19.5% for 12.0 V respectfully were observed. In the treatment of wastewate leachate, a study has shown that with 90 minutes of electrical treatment, 34.58% of TDS content were removed, supporting the effectiveness of electricity and its usage in wastewater treatment [ 29 ].

This experiment concludes that electrolysis is effective in lowering TDS, with the possibility to improve this process by further experimentation, development of a water cleaning system utilizing this cathode-anode setup to process water. This system would be a more specific and limited version of a reverse osmosis system by taking away ions through attraction, rather than a filter.

The Southern Californian tap water supply maintains TDS values below the federal regulations. However, crystalline scale buildup in household appliances is a major issue as it is hard to clean and eliminate. To easily improve the taste and quality of tap water at home as well as eliminating the formation of scales, the following methods were demonstrated as viable:

• By heating water to around 50°C (122°F), TDS and water hardness will decrease the most. Also, the boiling process is effective in killing microorganisms and removing contaminants. This process cannot surpass 10 minutes, as the concentration of the ions in the water is too high, which poses human health risks if consumed. These, along with activated carbon and NaHCO₃ additives, are inefficient methods that have minimal effects for lowering TDS.
• Electrolysis is one of the most effective methods of eliminating TDS. Experiments have proven that increased current and duration of time helps lower TDS. However, this method has yet to be implemented into conventional commercial water filtration systems.

Also, some observations made in these experiments could not be explained, and require further research and experimentation to resolve these problems. The first observation is that TDS and increasing water temperature maintain a parabolic relationship, with a maximum being reached at 80°C, followed by a gradual decrease. The second observation is that when water is boiled for an increased duration of time, the rate of cooling also increases.

This experiment utilized non-professional scientific equipment which are prone to mistakes and less precise. These results may deviate from professionally derived data, and will require further study using more advanced equipment to support these findings.

Acknowledgments

The author thanks Tsinghua University Professor and PLOS ONE editor Dr. Huan Li for assisting in experimental setups as well as data processing and treatment. The author also thanks Redlands East Valley High School’s Dr. Melissa Cartagena for her experimental guidance, and Tsinghua University Professor Dr. Cheng Yang for proofreading the manuscript.

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Recent advancements in water treatment

For immediate release, acs news service weekly presspac: january 19, 2022.

Generating clean, safe water is becoming increasingly difficult. Water sources themselves can be contaminated, but in addition, some purification methods can cause unintended harmful byproducts to form. And not all treatment processes are created equal with regard to their ability to remove impurities or pollutants. Below are some recent papers published in ACS journals that report insights into how well water treatment methods work and the quality of the resulting water. Reporters can request free access to these papers by emailing  newsroom@acs.org .

“Drivers of Disinfection Byproduct Cytotoxicity in U.S. Drinking Water: Should Other DBPs Be Considered for Regulation?” Environmental Science & Technology Dec.15, 2021

In this paper, researchers surveyed both conventional and advanced disinfection processes in the U.S., testing the quality of their drinking waters. Treatment plants with advanced removal technologies, such as activated carbon, formed fewer types and lower levels of harmful disinfection byproducts (known as DBPs) in their water. Based on the prevalence and cytotoxicity of haloacetonitriles and iodoacetic acids within some of the treated waters, the researchers recommend that these two groups be considered when forming future water quality regulations.

“Complete System to Generate Clean Water from a Contaminated Water Body by a Handmade Flower-like Light Absorber” ACS Omega Dec. 9, 2021 As a step toward a low-cost water purification technology, researchers crocheted a coated black yarn into a flower-like pattern. When the flower was placed in dirty or salty water, the water wicked up the yarn. Sunlight caused the water to evaporate, leaving the contaminants in the yarn, and a clean vapor condensed and was collected. People in rural locations could easily make this material for desalination or cleaning polluted water, the researchers say.

“Data Analytics Determines Co-occurrence of Odorants in Raw Water and Evaluates Drinking Water Treatment Removal Strategies” Environmental Science & Technology Dec. 2, 2021

Sometimes drinking water smells foul or “off,” even after treatment. In this first-of-its-kind study, researchers identified the major odorants in raw water. They also report that treatment plants using a combination of ozonation and activated carbon remove more of the odor compounds responsible for the stink compared to a conventional process. However, both methods generated some odorants not originally present in the water.

“Self-Powered Water Flow-Triggered Piezocatalytic Generation of Reactive Oxygen Species for Water Purification in Simulated Water Drainage” ACS ES&T Engineering Nov. 23, 2021

Here, researchers harvested energy from the movement of water to break down chemical contaminants. As microscopic sheets of molybdenum disulfide (MoS2) swirled inside a spiral tube filled with dirty water, the MoS2 particles generated electric charges. The charges reacted with water and created reactive oxygen species, which decomposed pollutant compounds, including benzotriazole and antibiotics. The researchers say these self-powered catalysts are a “green” energy resource for water purification.

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Appropriate household point-of-use water purifier selection template considering a rural case study in western India

• Review Article
• Open Access
• Published: 30 April 2020
• volume  10 , Article number:  124 ( 2020 )

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• Ramprasad Venkatesha 1 ,
• Anand B. Rao 2 &
• Shireesh B. Kedare 3  

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9 Citations

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Cite this article

There is a wide range of household water treatment options available for a variety of contexts. Each water purifier has its own optimal range of operation. Simultaneously, the diverse environments and circumstances set different boundary conditions for such purifiers to operate successfully. In low-income countries, especially with unregulated and decentralised water supply mechanisms such as open wells, the use of point-of-use water purifiers is quite widespread. However, it is observed that the water purifier may not be appropriate to the prevailing context. Hence, this short review aims to introduce a wide range of water purification alternatives available for a family (of about 3–5 members) and the way they could be classified and reviewed. The perspective selected is that of a low-income rural household in coastal region of western India and the scenario of water quality which is primarily affected by physical and biological impurities and not necessarily severe chemical contamination. Based on this context, attributes are defined and prioritised; further, a scale to rate the purifiers is worked out. A selected number of point-of-use water purifiers for which data from the literature or field observations are available are reviewed against these attributes for the sample context chosen. This independent review methodology consists of setting the attributes and comparing the water purifiers based on the sum of prioritised scores and thus acts like a selection template and can be adopted to select the appropriate purifier for any other scenario accordingly.

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Introduction

The global population which did not have access to safe drinking water in the year 2012 was around 700 million. With household treatment of water, it has been reported that diarrhoeal illnesses could be reduced by 30–40% (Sobsey et al. 2008 ). In India, it has been observed that 70% of the surface water is microbiologically and chemically contaminated. Further, more than 33% of ground water sources in rural India is claimed to be polluted (Water Pollution 2013 ).

In rural areas due to the dispersed settlements, drinking water sources are usually decentralised in the form of dug wells, hand pumps or tube wells. Even in urban areas, despite the existence of centralised water supply, marginal communities may lack the access to such utilities or supply through such centralised systems may be prone to get contaminated. In such scenarios, household point-of-use water purifiers become indispensable. A review of such purifiers might be quite useful to zero in on the most suitable purifier for the prevailing context and hence this study.

Point-of-use purifiers which incorporate water treatment at or near the place of use are covered in the study. This study, primarily undertaken through the review of the literature, classifies and describes the different types of PoU purification alternatives for a family (of about 3–5 members). Based on the perspective of a rural household consuming water from a decentralised source, such as an open well in the coastal region of Maharashtra State in western India, the attributes used to compare the water purifiers are prioritised. Further, the study compares a selected number of point-of-use purifiers across different attributes, for which relevant data from the literature and field observations were available. This independent reviewing approach, considering various references, helps in identifying the water purifier which gets the highest total score which is to be considered the appropriate water purifier for the prevailing scenario. Finally, this review comes up with some emerging inferences.

Classification and description of point-of-use water purifiers

Classification of water purifiers.

The classification of purifiers has gradually evolved, and the description and methodology adopted in this review takes references from Peter-Varbanets et al. ( 2009 ) and Loo et al. ( 2012 ). The purifiers have been categorised into thermal- or light-based treatment techniques, physical removal methods, chemical treatment techniques and integrated water purification. Both sections on classification (“ Classification of water purifiers ” section) and description (“ Description of purifier alternatives “ section) cover a wide variety of water purification options; however, a special focus has been accorded to feasible options in the context of decentralised water sources prevalent in rural coastal areas in developing countries such as India. Most of the water purifiers in use or available in the market usually combine different treatment techniques and hence are integrated water purification methods. However, to make it clear to the reader the underlying water purification methodology, the classification in terms of the technique used for purification is adopted. The detailed classification chart is as shown in Fig.  1 .

Hierarchical chart depicting the classification scheme of the study

Description of purifier alternatives

Thermal or light based treatment techniques.

Boiling is perhaps the oldest method of water purification (Sobsey 2002 ) but is a highly energy intensive one. One minute of boiling at a temperature 100 °C (at mean sea level) ensures neutralisation of faecal and thermo-tolerant coliforms, protozoan cysts and viruses (Sobsey 2002 ; Loo et al. 2012 ). Since boiling does not provide residual protection, boiled water needs to be kept in closed and clean containers and preferably consumed within 24 h. The taste of water after boiling gets altered and is generally not easily adopted across all regions except in Asia due to sociocultural reasons (Lantagne and Clasen 2009 ).

Thermal pasteurisation

In thermal pasteurisation, temperature usually does not go beyond 75 °C which is suitable to eliminate E. coli by more than 5 LRV (Islam and Johnston 2006 ; Gupta et al. 2008 ). Coiled metal tubes can be retrofitted with cookstoves (Fig.  2 ) as in traditional earthen cookstove or Chulha in Chulli purifier or Lonera cookstove in water disinfection stove (WADIS). Due to its difficulty in use and mechanical issues, its adoption was limited in Bangladesh (Gupta et al. 2008 ; Loo et al. 2012 ).

Chulli purifier in the form of a coiled tube (Islam and Johnston 2006 )

It has been reported that up to 4 LRV of faecal coliform and viruses could be removed using solar water heaters with solar irradiation of just 2 h on sunny days and 4 h on cloudy days (Kang et al. 2006 ; Loo et al. 2012 ).

Solar distillation

Solar distillation combining the process of evaporation and distillation can be a convenient method for the removal of salts and non-volatile impurities using solar stills (Flendrig et al. 2009 ). Solar still consists of a vessel which holds the contaminated water and a transparent lid which aids condensation (Flendrig et al. 2009 ) (Fig.  3 ). This method involves large area, high upfront cost and low discharge rates of 0.5 L/d to 3 L/d (Flendrig et al. 2009 ; Loo et al. 2012 ).

Conceptual sketch of a solar distillation unit (Loo et al. 2012 )

Solar disinfection (SODIS)

For low volumes of filtered water with turbidity of less than 30 NTU, water can be filled in transparent polyethylene terephthalate (PET) bottles (Fig.  4 ) and kept under sunlight for at least six hours after forceful shaking for aeration (Peter-Varbanets et al. 2009 ). SODIS can be an effective way to use heat and UV radiations from the sun to targets microbes. In case of low intensity of solar radiation, solar collectors or additives like lemon juice or vinegar can be used to improve the efficacy (Loo et al. 2010). SODIS has low operating costs involved because easy availability of PET bottles, however, has a long treatment time on low volumes of water.

SODIS in operation (SODIS, 20.11. 2019 )

Ultraviolet (UV) treatment

For low turbid water UV treatment could be effective even on Crypto Giardia lamblia cysts and Cryptosporidium parvum oocysts by more than 3 LRV (Gadgil 1998 ; Berg 2010 ). UV systems generally rely on electrical power and do not offer residual protection (Berg 2010 ). Aquaguard Compact (Fig.  5 ) is an example of UV-based purifier (Aquaguard, 28.10. 2014 ).

Aquaguard Compact UV purifier (Aquaguard, 28.10. 2014 )

Chemical treatment techniques

Chlorination

Chlorination is a simple, affordable and scalable method of water disinfection through the use of sodium hypochlorite NaOCl (liquid) (Fig.  6 ), NaDCC (solid) and calcium hypochlorite (Ca(OCl) 2 ) (solid). It gives residual protection due to the availability of free chlorine; however, there may not be any improvement in terms of turbidity. With a dosage of 2 mg/L for about 0.5 h, chlorination can offer about 3 LRV of enteric bacteria (Gadgil 1998 ); however, there is the issue of generation of disinfection by-products (DBPs).

Plastic bottle containing sodium hypochlorite solution. Photo: Darpan Das, based on special arrangement with the authors

Combined flocculation/coagulation and disinfection (CFD/CCD)

For a reduction in turbidity as well as microbial disinfection, combined methods such as coagulant/flocculant as well as chemical disinfectant powders/tablets are used (Peter-Varbanets et al. 2009 ). These products (like PuR sachet in Fig.  7 ) combine calcium hypochlorite (or bleach) with coagulating agents like sodium carbonate and oxidisers like potassium permanganate. For 10 L of water, a PuR sachet of 4 g is added, stirred for 5 min and after sedimentation, and the water is filtered across a clean fabric and left undisturbed for 20 min for disinfection (CDC, 28.10. 2014 ). The method could offer 7–9 LRV for bacteria, 2–6 LRV for viruses and 3–5 LRV for protozoa (Sobsey et al. 2008 ). Flocculation–disinfection also has the problem of taste and odour like chlorination but also involves higher cost, multiple steps and resources (Lantagne and Clasen 2009 ).

Combined flocculant–disinfection (PUR) sachet (PUR, 31.10. 2014 )

To remove particulates, organic matter and chlorine/disinfectant leftovers, adsorbents like activated carbon are used. They are used in granulated form after disinfection methods like chlorination, UV, etc. like in commercial purifiers like Aquaguard Compact and HUL PureIt Classic (Aquaguard, 28.10. 2014 ; Pureit, 28.10. 2014 ) (Fig.  8 ). Biofilm growth compels frequent replacement of such cartridges (Peter-Varbanets et al. 2009 ). In Tata Swach, adsorption is through rice husk ash (activated silica and activated carbon) which is impregnated with silver nanoparticles to target microbes (Swach, 28.10. 2014 ).

Tata Swach Cristella Plus and HUL Pureit Classic (Swach, 28.10. 2014 ; Pureit, 28.10. 2014 )

Physical removal methods

Sedimentation or clarification.

Clarifiers like alum, lime, iron, seeds of Moringa oleifera (drumstick) (Fig.  9 ) and seeds Strychnos potatorum (clearing nut or Nirmali tree) (Fig.  10 ), Guar gum and Jatropha curcas have been used to reduce turbidity through sedimentation (Ndabigengesere and Narasiah 1998 ; Sobsey 2002 ). There are also claims that Strychnos potatorum and aluminium salts (alum) and iron salts could help in reduction in microbial contamination by up to 95% and 99%, respectively (Sobsey 2002 ; Khan et al. 1984 ).

Moringa oleifera tree and dried seed (unpeeled and peeled) (Moringa Tree, 30.10. 2014 )

Strychnos potatorum (clearing nut) seeds (Clearing nut, 31.10. 2014 )

Membrane based treatment methods

In these methods, filtration occurs across a semi-permeable membrane due to gravity or a difference in pressure, osmotic potential, temperature or electric potential (Mulder 2000 ). Depending on the pore size, microfiltration (0.1–1 µm) can retain only bacteria, ultrafiltration (0.005–0.1 µm) can remove both bacteria and viruses, nanofiltration (0.5–5 nm) cannot retain salts, while reverse osmosis (0.15–0.5 nm) can even filter out salts (Fig.  12 ) (Peter-Varbanets et al. 2009 ) (Fig.  11 ).

Different membrane purification regimes, the respective pore sizes and the particles that can be removed (Peter-Varbanets et al. 2009 )

Paper, fabric and fibre filters

Considering the pore size of paper and fabric filters, only pathogens like Vibrio cholera can be filtered to a extent of 95–99% (Sobsey 2002 ). Those made with multiple layers of polyester or nylon could remove cyclops and zooplankton (Agrawal and Bhalwar 2009 ). Up to 6 LRV of Escherichia coli and 3 LRV of Enterococcus faecalis , the removal is possible through bactericidal papers impregnated with silver nanoparticles due to inactivation offered by silver (Loo et al. 2012 ).

Microfiltration (MF)

There are ceramic- and polymer-based microfiltration systems. In ceramic filters (Fig.  12 ), clay is mixed with burnout material to make porous ceramic filters of varied shapes with pore size of about 0.2–3.0 mm depending on the sophistication of manufacture (Sobsey et al. 2008 ). These ceramic filters could be locally made and coupled with silver impregnation to provide disinfection. The efficiency of removing bacterial and protozoan contaminants is significant (2–6 LRV and 4–6 LRV respectively); however, it is not so significant on viruses (0.5–4 LRV) (Sobsey et al. 2008 ). These filters can help in visible reduction in turbidity, however, needs to be cleaned and handled safely (Loo et al. 2012 ). Katadyn Mini, Potters for Peace pot-based clay filter (PFP, 28.10. 2014 ) and Terafil filter (Terafil, 28.10. 2014 ) are all a silver impregnated ceramic filters.

Ceramic filter system and element in different forms (Simonis and Basson 2011 )

There are polymer-based microfiltration devices like FilterPen whose polymer size is about 0.15 mm with a surface area of 0.02 m 2 (Peter-Varbanets et al. 2009 ).

Coated textile candle filter is another example for microfiltration. After regular prefiltration and activated carbon treatment, water is passed through a coated textile candle which claims to remove pathogens larger than 1 µm and further ruptures microbes; however, the disinfection level is not specified. The whole setup is housed in PET containers (Fig.  13 ) (Livinguard, 29.10. 2014 ).

Livinguard Rural Filter (Livinguard, 29.10. 2014 )

Ultrafiltration (UF) and nanofiltration (NF)

With much lesser pressure potential, ultrafiltration and nanofiltration techniques can ensure a complete microbial removal (Peter-Varbanets et al. 2009 ). Although inlet water quality does not significantly affect the performance, periodical backwashing is required to prevent fouling. Some of the popular devices are Lifestraw (Lifestraw, 28.10. 2014 ) (Fig.  14 ), wherein purified water is sucked from a vessel containing impure water (Loo et al. 2012 ). Another product named Lifesaver bottle (Fig.  15 ) claims to achieve 7.5 LRV against bacteria and 5 LRV against viruses and treats about 4000 L of water (Lifesaver, 28.10. 2014 ).

Lifestraw in operation (Lifestraw in use, 31.10. 2014 )

Lifesaver bottle (Lifesaver, 28.10. 2014 )

Pedal-operated UF purifiers have also been attempted like He ( 2009 ), Saini et al. ( 2013 ) (Fig.  16 ) and BARC (28.10. 2014 ) where pressure of 4 bar generated through pedalling motion can accelerate discharge rate to about 36 L/hour (Saini et al. 2013 ). These systems greatly help in improving the visibility of water by reducing turbidity (44.7 NTU to 0.267 NTU) and TDS along with microbial (total coliform count from 300 cfu/100 mL to < 1 cfu/100 mL) (He 2009 ).

Pedal powered UF system. Photo: Ramprasad V

There are modular variants of UF purifiers which are suitable for community scale like SkyHydrant, Lifestraw Family and also mobile variants like Jaldoot and Perferctor E (Loo et al. 2012 ; Peter-Varbanets et al. 2009 ).

Several stationary household UF purifiers are available like Moselle (Fig.  17 ), Jaltara (Fig.  18 ) and Waterife Little Star Gold (Moselle, 29.10. 2014 ; Jaltara, 29.10. 2014 ; Waterlife, 29.10. 2014 ).

Moselle purifier (Moselle, 29.10. 2014 )

Jaltara filter (Jaltara, 29.10. 2014 )

There is a unique experiment with plant xylem-based ultrafiltration. Bacteria up to 3 LRV can get filtered out with sapwood (predominantly xylem) of trees like pine which is easily available, inexpensive, biodegradable and suitable for resource-constrained environments (Boutilier et al. 2014 ). A small branch of a pine tree is peeled and then inserted into a tube and clamped to make the filter (Fig.  19 ). Achieving high flow rates is difficult; however, a volume of 3 cm 3 of sapwood can meet the needs of an individual (Boutilier et al. 2014 ).

Preparation of the plant xylem purifier (Boutilier et al. 2014 )

Biopolymer-reinforced synthetic granular nanocomposites

Reverse osmosis (ro).

RO with pore size of < 1 nm and high water pressure filters out all types of pathogens and waterborne impurities (Fig.  20 ) (Loo et al. 2012 ). To avoid RO membranes getting fouled, prefiltration such as sedimentation, microfiltration and activated carbon filters (also in post-filtration) are adopted. RO-based water purifiers are generally expensive. RO systems can be coupled with photo-voltaic systems to power them (Loo et al. 2012 ) and can be mounted on vehicles to make them mobile (Peter-Varbanets et al. 2009 ).

Reverse Osmosis Purifier (Kent RO, 31.10. 2014 )

Most RO purifiers are integrated water purifiers with methods such as microfiltration, ultrafiltration and ultraviolet treatment in conjunction with reverse osmosis based water filtration.

Forward osmosis (FO)

In forward osmosis, a bag (e.g. Hydro Pack) made of semi-permeable membrane is filled with concentrated sugar solution and then dipped in impure water (HTI, 29.10.2014). Due to the osmotic potential, water enters the pouch and contaminants get trapped outside the bag (Fig.  21 ). The diluted sweet water packed with nutrients and minerals can be consumed directly (Peter-Varbanets et al. 2009 ). However, this method is suitable for individuals during emergencies, considering its high cost and low yield.

Forward Osmosis X-Pack (HTI, 20.11. 2019 )

Biosand filter

A biosand filter (BSF) consists of a container packed with sand where a biologically active layer (schmutzdecke) is allowed to develop on the top surface (Fig.  22 ) which restricts the passage of bacteria by around 2 LRV and protozoa by more than 3 LRV and viruses by about 1 LRV (Sobsey 2002 ; Peter-Varbanets et al. 2009 ). BSF can remove 95% turbidity and gives a discharge of about 20 L/hour (Peter-Varbanets et al. 2009 ). A diffuser plate is placed on bio-layer to avoid disturbance of schmutzdecke and the user just pours in water on top of the diffuser plate and collects filtered water from the outlet. NEERI - Zar developed by CSIR-National Environmental Engineering Research Institute (NEERI) is also a type of modified sand-based water filter (NEERI, 22.04. 2017 ).

Cross section of a biosand filter (Biosand, 31.10. 2014 )

Integrated water purification

Considering the advantages and limitations of different water treatment methods, some of the household water purifiers combine multiple types of water treatment techniques. For example, the RO systems generally are supported by microfiltration, ultrafiltration and ultraviolet treatment techniques. There are also some mobile purification units comprising of multiple methods of water treatment like micro-hydraulic mobile water treatment plant (MHMWTP) which have been developed. MHMWTP incorporates chlorination, sedimentation, filtration and optional granular-activated carbon (GAC) treatment (Garsadi et al. 2009 ). Similar mobile system is Jaldoot (Fig.  23 ) which involves multiple treatment mechanisms ranging from pressurised sand filtration, GAC module, microfiltration and ultrafiltration all integrated into one unit on a three-wheeler. This unit is capable of delivering 1500 L every hour (Jaldoot, 29.10. 2014 ).

Jaldoot mobile purifier (Jaldoot, 29.10. 2014 )

Another example of integrated water purification is microfiltration coupled with biopolymer-reinforced synthetic granular nanocomposites which release silver ions in water offer arsenic and microbiological disinfection at a low cost (Sankar et al. 2013 ). The system has a discharge rate of 10 L/hour and purifies 3600 L of water (Sankar et al. 2013 ) (Fig.  24 ).

Schematic diagram of the purifier prototype (A) and its actual photograph (B) (Sankar et al. 2013 )

Review of purifier alternatives

Attributes selected for review.

The reference for different attributes which evaluate the purifiers was primarily followed as in Peter-Varbanets et al. 2009 and Loo et al. 2012 . However, the list of attributes and their priority (Table  1 ) was selected based on their relevance to a low-income rural context. Specifically, the context is that of a coastal rural area in a developing country like India, wherein there were no major chemical contaminants identified in the decentralised water sources, mostly open wells.

The finalised list of attributes (and the reference for scoring) is selected for review as follows.

Sustainability (Peter-Varbanets et al. 2009 )

Purification performance (Peter-Varbanets et al. 2009 )

Rate of production (Peter-Varbanets et al. 2009 )

Maintenance (Peter-Varbanets et al. 2009 )

Energy requirement or dependence on utilities (Peter-Varbanets et al. 2009 )

Ease of use (Peter-Varbanets et al. 2009 )

Portability/ease of deployment (Aggregated from multiple sources including local references, primarily Loo et al. 2012 )

Supply chain requirement (Loo et al. 2012 )

Cost (in Rs/L) (Lifetime and investment adjusted) (Aggregated from multiple sources including local references, primarily Peter-Varbanets et al. 2009 )

Social acceptability (Peter-Varbanets et al. 2009 )

The above attributes can further be broadly classified based on the categories proposed by Pagsuyoin et al. ( 2015 ) into technological performance, environmental sustainability, economic viability and social acceptability. This is a simpler way of integrating the attributes for a given context. Based on the categorisation, the following attributes: purification performance, rate of production, maintenance, energy requirement or dependence on utilities, ease of use, portability/ease of deployment and supply chain requirement will mostly get categorised under technological performance. Further, sustainability, cost and social acceptability would get categorised into environmental sustainability, financial viability and social acceptability, respectively.

However, it is to be noted that this broad categorisation of attributes is simplistic and not exactly as proposed in Pagsuyoin et al. ( 2015 ). Since the scores for different attributes selected were mostly from Peter-Varbanets et al. ( 2009 ), Loo et al. ( 2012 ) and other sources, an independent methodology for review, hve been adopted.

Alternatives selected for review

The classification of water purification techniques in “ Classification of water purifiers ” section represents an overall academic approach to illustrate wide spectrum of options possible with a special focus towards possibilities in the context of decentralised water sources as in rural coastal areas in developing countries like India. However, to adopt a more practical approach for identifying the appropriate water purifier for a given scenario, the water purifiers for which sufficient information from primary sources (field observations) and secondary sources (literature review) was available were chosen for review. A thorough literature review was undertaken from multiple references, but primarily from Peter-Varbanets et al. 2009 and Loo et al. 2012 . The purifiers for which sufficient information for comparison against all attributes was chosen were as follows:

Household boiling

Solar stills

Solar disinfection

UV-based purifiers

Combined coagulation–disinfection (PuR sachet)

Biosand Filter

Household Ceramic Filters

Portable UF (Lifesaver bottle)

Bicycle powered UF

Small-scale Reverse Osmosis

FO reusable filter pouch

Analysis of review of alternatives

This section presents a detailed comparison based on preferential scores assigned for each purification method based on its performance against a particular attribute. The attributes have been accorded priority considering a low-income rural household as the case in focus. Several visits to Ransai, Vavoshi and Shiroshi villages on Pen-Khopoli road in Raigad District of Maharashtra State in India were undertaken with support from a social organisation named Rural Communes. Similarly, several visits to villages near Ganeshpuri in Palghar district of Maharashtra State in India were undertaken with support from a social organisation named Shree Nityananda Education Trust (SNET). Based on extensive visits to these villages, observation of water sources, feedback from villagers and discussions with teams of social organisations, the attributes used to rate the purifiers were accorded priority. Physical filtration of turbidity and removal of pathogens turned out to be some of the key needs of the villages. This formed the specifics of the context of the review methodology: a rural setup in the coastal area in a developing country like India wherein open wells are the primary water sources and the primary concerns of water quality are mostly physical (turbidity) and biological (microbial contamination). No specific chemical contamination has been focused in the review as no such major issue was identified in the field area under consideration.

The purification methods have been assigned scores based on their performance again each attribute out of a total score of 3. Finally, the total score of each purification method is calculated by summing up the product of the score against a particular attribute and the attribute’s priority (Table  2 ).

Description of score rating of purifiers against each attribute

Purification performance against pathogens

Purification performance is microbial removal efficiency against pathogens. This attribute primarily considers performance in terms of effectiveness in the removal of bacteria. This was observed as one of the key needs in the villages surveyed apart from physical treatment in terms of turbidity. While turbidity does not have immediate adverse impact over the health of the people, the removal of pathogens is one of the critical needs; hence, this attribute has been assigned the highest priority of 6 out of 6 (Table  1 ). Apart from boiling, coagulation–disinfection, RO, FO, UV and UF, there seem to be no “foolproof” method of microbial disinfection; hence, these have been assigned a score of 3 out of 3. All of these are expensive except boiling when firewood is easily available and hence probably indicates the huge dependency of rural areas on boiling. The remaining purification techniques are assigned a score of 2 out of 3.

Cost Rs/L (lifetime and investment adjusted)

In the villages which were considered, cost is a crucial consideration in the adoption of any water purifier because population is mostly composed of low-income households. Hence, once again the highest priority of 6 out of 6 has been assigned to the cost attribute (Table  1 ). Each purifier has a time limit within which the purifier’s operational efficiency is acceptable. To take into account the investment and operational costs of the purifier over its lifetime, cost is calculated for every liter of water purifier based on the information of lifetime of the purifier available through the literature or through field-based observation. The ratio of purifier’s investment cost to the volume purified over its lifetime is summed up with the operational cost over the purifier’s lifetime in consistent units and presented as a single attribute as cost in Rs/L. (This review assumes a conversion rate of 1 US$ = INR Rs.60.) In this regard, sand/ceramic-based and chlorination-based purification is the cheapest (score 3 out of 3), while UF, solar stills, RO and FO are quite expensive (score 1 out of 3). The remaining purification methods have been assigned a score of 2 out of 3.

Rate of production

This attribute implies the ability to cater to increased demands of water within a short period of time. Boiling, SODIS, chlorination and combined coagulation–disinfection can be scaled up and down to meet the flexible demands, methods like UV, household UF, bicycle UF and RO have production rates of > 10 L/hour; hence, these have been assigned a score of 3 out of 3. Solar distillation and forward osmosis have < 0.1 L/hour (score 1 out of 3), while purifiers like Tata Swach, HUL Pureit, ceramic, biosand filter, Terafil and Lifestraw have discharge rates which are in between 0.1 and 10 L/hour (score 2 out of 3). If the rate of production is too low, it often renders the purifier unusable; hence, this attribute is also assigned a high priority of 6 out of 6 (Table  1 ).

Ease of use

If a purifier is convenient to handle and use, there are higher chances of its continued usage. Based on the field observations, it was inferred that ease of use for the community matters quite heavily in terms of purifier’s continued usage. Hence, this is assigned a priority of 4 out of 6 (Table  1 ). Referring to Peter-Varbanets et al. ( 2009 ), some qualitative scores have been assigned as follows. A score of 3 (out of 3)/++ is given for most purifiers which can be handled easily and which require only filling of feed water and collection of purified water. A score of 2 (out of 3)/+ is allocated if any extra effort is required like chlorination and combined coagulation–disinfection demand stirring, boiling requires heating, SODIS and solar distillation require effort to place the purifiers under the sun and bicycle UF requires pedalling.

Maintenance

It is often seen in the field areas that once a water purifier is handed over to a rural community, its proper maintenance is often neglected. Improper maintenance may cause the purifier to malfunction, and hence, maintenance is accorded a priority of 4 out of 6 (Table  1 ). Maintenance in some form is required for all purifiers while the most common being cleaning water holding containers. Other maintenance operations include back flushing of membrane filters, removing depositions on candle filters and scraping off the top layer of sand in biosand filters. There is replacement of chemicals in chlorination, combined coagulation–disinfection and FO, while there is replacement of cartridges/membranes in ceramic microfiltration, household UF, bicycle UF and RO methods. Boiling, solar stills, SODIS, BSF, chlorination and combined coagulation and disinfection require the basic amount of maintenance and have been given score 3 out of 3 as the purifier would not severely malfunction in the absence of maintenance. Ceramic filters, UV and FO pouches require inexpensive replacements and hence have been given a score of 2 out of 3. RO and UF hugely depend on expensive module replacements and hence have been given a score of 1 out of 3.

Energy requirement or dependence on utilities

Different purification methods are dependent on different energy utilities, and this attribute needs to be considered for rural areas. Since it is observed that energy constraints are often prevalent in rural settings, this attribute is accorded a priority of 4 out of 6 (Table  1 ). Boiling requires fuel; UV and RO depended on electricity and hence have been assigned a score of 1 out of 3. UF systems require mechanical effort and SODIS & household stills depend on sufficient solar radiation and have been given a score of 2 (out of 3). The other methods do not need any external energy or depend on gravity and osmotic potential for their energy requirements and hence have a score of 3.

Ease of deployment

The ease of deployment is quite relevant in remote circumstances. Since ease of deployment plays a major role in handling of the purifier during deployment as well as shifting which could be common in a rural setting, this is assigned a priority of 2 out of 6 (Table  1 ). Purification methods which can be easily deployed and used (boiling, SODIS, chlorination, CCD, household UF, FO) have been given a score of 3 out of 3. However, if the purifier’s sophisticated make-up hampers its rugged use, then a value of 2 out of 3 is assigned. For example, biosand filter being bulky and taking considerably long start-up time and purifiers like solar stills, UV, RO and ceramic filters being delicate and prone to damage during handling/transporting.

Sociocultural acceptability

In the selected villages for the study, it appeared that sociocultural acceptability appeared to be an important consideration which cannot be neglected in terms of adoption of purifiers. Social acceptability has been assigned a priority of 2 out of 6 (Table  1 ). Boiling being the only traditionally practised method is given a score of 3 (out of 2). Most purifiers which were accepted when deployed (ceramic, UV, biosand, household UF and FO) have been assigned a 2 marks out of 3. Although chlorination and combined coagulation–disinfection have been in use for quite some time, they produce bad taste and odour. Some purifiers like SODIS have not been readily accepted even after deployment; further, bicycle-based UF requires some investment and pedalling effort and RO requires higher investments and electricity and hence have not been fully accepted. These methods have been assigned a score of 1 (out of 3).

Environmental sustainability

Environmental sustainability is difficult to quantify and can be sometimes subjective; however, usage of any purifier has environmental implications which is quite important to consider. Referring to Peter-Varbanets et al. ( 2009 ), some qualitative indications have been assigned as follows. Purification methods like boiling, household UF, RO and FO which are either energy intensive (electricity or firewood) or incorporate advanced systems and resources for short-term needs or have high rejection rates are assigned 1 out of 3. SODIS, solar distillation, BSF and household ceramic filter have been assigned 3 marks (out of 3) because they are either made from locally available materials with limited application of chemicals and are less dependent on non-renewable energy. Chlorination, combined flocculation–disinfection, UV and bicycle-operated UF have been classified with 2 marks (out of 3) due to their dependence either on chemical usage or due to the incorporation of exhaustible purifier components which cannot be locally sourced because of the use of sophisticated technology. Environmental sustainability has been accorded a priority of 2 out of 6 (Table  1 ).

Supply chain requirement

The selected areas of study are quite remote, wherein supply chain needs to be worked out from the nearby cities to nearby prominent villages/towns. Even if the water purifier is deployed once, unless the replacements and accessories are made available, the usage of purifiers in the long run may come to an end. Since supply chain is gradually improving, it has been assigned a priority of 2 out of 6 (Table  1 ). Biosand filters are assigned a score of 3 (out of 3) due to non-requirement of replaceables. Boiling, solar-based techniques, UV, ceramic filters, UF and RO need occasional supply in the form of fuel or replacement of a few accessories; hence, these have been given a score of 2 (out of 3). However, chlorination and FO-based techniques need strong supply chain network due to requirement of frequent replenishment and are given a score of 1 (out of 3).

Concluding remarks

This study adopts both academic and practical approaches towards water purifiers. The classification and description of water treatment alternatives are based on the operating technologies adopted for purification, so as to present a broad spectrum of possibilities. However, the review is based on the practical approach of evaluating water purifiers in the manner they can be used in the selected context. As presented in the review, there are a lot of alternatives available for a family-level water treatment even for a low-income household with decentralised water source in a developing country. The selection process of the optimal type of purifier for a given setting requires an assessment of different purification methods against several relevant attributes. However, the conditions in different places within a diverse country could be different. Based on the observations in the coastal region of Maharashtra in India, the attributes have been prioritised. The purifiers are then compared based on data available from primary and secondary sources. The review as an example indicates some of the purifiers suitable to the chosen scenario. Going further, this methodology can be used as a template to identify the best possible water purification technique relevant to the given scenario by tweaking the priority assigned to each attribute in the review based on the circumstance.

The viewpoint of this review was to classify, describe and review various household point-of-use water purifier based on prioritised attributes according to a local context. Some of the observations made through this review is as follows.

This is a wide spectrum of alternatives available in terms of technologies and water purifying devices.

Nowadays, membrane-based technologies in the realm of physical removal methods have a widespread adoption.

Based on the review, it can be inferred that no single purifier meets the mark in terms of all the attributes.

However, considering the summation of the product of the scores of the purifiers (> 90), it can be observed that boiling, biosand, ceramic and chlorination-based techniques seem to score the highest considering the priorities.

The above inference tends to match with the field observations in the selected villages, wherein these purifiers were used or deployed or seem agreeable to the people of the villages.

Based on field observation, it has been found that purifiers with low energy demands, those which are easy-to-use and handle and which can cater to flexible quantum of water requirements tend to have a higher adoption.

No matter what purification techniques are considered, certain attributes like purification performance will always be an important parameter for consideration.

It could probably be generalised that it would be hard to find a single water purifier which meets all the requirements as per all attributes in all contexts. Therefore, considering the specific needs and circumstances of the prevailing context, an optimum purifier could to be selected considering its advantages and limitations.

There is a huge scope for working on further fine-tuning low-cost, environmentally sustainable, easy-to-use water purifiers offering effective water treatment.

Point-of-use purifiers are especially suitable for water sourced from decentralised sources like tube wells, open wells, ponds and rain water harvesting tanks. It is observed that point-of-use purifiers are being increasingly used in developing countries irrespective of the water quality supplied by public utilities. This is so, because there is a high chance of contamination of water which is supplied even through centralised systems in areas like congested areas. PoU purifiers are also suitable for deployment during emergencies. Another observation noted is that most purifiers incorporate integrated treatment combing multiple purification methods, as generally observed in the case of household RO purifiers which generally incorporate micro-/nanofiltration and UV treatment.

It is, however, observed that household RO water purifiers are used as a common remedy for any type of water treatment need. RO purifiers are not only expensive and environmentally unsustainable (due to huge energy consumption and large release of discharge water) but may not be considered advisable for regular consumption (in case of routine water issues which are not very serious). It is in such a scenario that this review comes in hand, wherein the appropriate water purifier suitable to the relevant issue of water quality can be selected.

The future scope of this work could involve deeper market survey to consider more recent actual prices (apart from literature references) as these keep changing rapidly. Considering user perspective through surveys could also help in understanding the perceptions in terms of ease of use and sociocultural acceptability of the purifiers. This study does not include the ability of water purifiers to treat special chemical contaminants like arsenic, fluoride, etc.

This study considers an independent methodology for review of water purifiers based on references primarily from Peter-Varbanets et al. ( 2009 ) and Loo et al. ( 2012 ). However, there is further scope for undertaking a review based on a simpler way of integration of attributes could be as proposed in Pagsuyoin et al. ( 2015 ), which takes into account most of the attributes considered in this review.

Selecting a particular purifier depends on the several factors, among which some are changing needs, context of usage, development of technology, ease of use and market reach, etc. Amidst so many variables, identifying the appropriate purifier, is a challenge. However, as an example from this review, one can adopt a general methodology which could be used to identify the suitable water purifier for a given context. As described in earlier sections, the selection process first involves assigning the relevant priority to each attribute considering the prevailing situations in the given context. Secondly, rating all the water purifiers could possibly be adopted in the given context against each parameter. Finally, calculating the product of the score and the priority of the corresponding attribute identify the total score for each purifier. The appropriate water purifier is the one which has the highest score when calculated according this template.

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Acknowledgements

This work was carried out as part of a seminar coursework in IIT Bombay. The authors are grateful to the academic resources received from IIT Bombay, which contributed to most of this work. The authors also acknowledge the financial support of Tata Centre for Technology and Design (TCTD) for the project on Development of Clay based Water Purifier considering Local Needs, Skills and Materials , whose component was this study.

This study was conducted as part of a project supported by the Tata Centre for Technology and Design, Indian Institute of Technology Bombay.

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Venkatesha, R., Rao, A.B. & Kedare, S.B. Appropriate household point-of-use water purifier selection template considering a rural case study in western India. Appl Water Sci 10 , 124 (2020). https://doi.org/10.1007/s13201-020-01207-1

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• J Dent (Shiraz)
• v.16(3 Suppl); 2015 Sep

The Effectiveness of Home Water Purification Systems on the Amount of Fluoride in Drinking Water

Behrooz eftekhar.

a Dept. of Endodontic, School of Dentistry, Ahwaz Jondishapoor University of Medical Sciences, Ahwaz, Iran.

Masoume Skini

b Postgraduate Student, Dept. of Endodontic, School of Dentistry, Ahwaz Jondishapoor University of Medical Sciences, Ahwaz, Iran.

Milad Shamohammadi

Jaber ghaffaripour.

c DDS, School of Dentistry, Ahwaz Jondishapoor University of Medical Sciences, Ahwaz, Iran.

Firoozeh Nilchian

d Dental Students Research Center, Dept. of Dental Public Health, School of Dentistry, Isfahan University of Medical Sciences, Isfahan, Iran.

Statement of the Problem

Water purification systems for domestic use have drawn significant attention over the past few years. This can be related to the improvement of public health and concern for water contamination. 

The aim of this study was to evaluate whether home water purification systems eliminate the essential materials such as fluoride besides filtrating the heavy ions and other unwanted particles out of water.

Materials and Method

In this experimental study, six most frequently used commercial brands of water purifiers were evaluated and compared. Specimens were collected right before and after setting up the device, and 6 months later. Then, spectrophotometry (the Harrison device) was performed to compare fluoride clearance by each home water cleaner device.

Based on the data collected from all water purification devices in different locations, the amount of fluoride was significantly different before and right after using home water purifier and six months later ( p = 0.001 and p = 0.00, respectively).

The filtration of water significantly decreased its fluoride concentration. The fluoride content of purified water was approximately as much as zero in some cases.

Introduction

Fluoride is a natural element branched from Fluorine. This element can be found in all sorts of water and soil. Out of every kilogram of outer layer of earth, 0.3 gram is fluoride. Mineral waters have more amount of this element compared to other sources.( 1 )

About 60 years ago, Grand Rapids in Michigan State was the first city in which fluoride supplement was synthetically added to tap water. In US, adding fluoride to community water supplies of many cities has improved the oral health of millions of American citizens.( 2 )

Fluoridation of community water supplies is adding a specific amount of fluoride (0.7-1.2 ppm) to water in order to reduce the risk of dental caries. By 2002, almost 170 million Americans were provided with this privilege.( 3 )

Since most of the systemic fluoride is provided through tap water to population, many policies have been established to add fluoride to community water regarding its benefits for teeth and bones.( 4 )

In regions and countries that do not have water-fluoridation technology, there are natural supplements as previously mentioned. For example, Iran has many mineral water supplies that contain considerable amounts of fluoride. Amount of fluoride in natural mineral waters depends on weather conditions; the warmer the weather is, the higher the amount of fluoride can be detected. Mineral waters in southern regions that have warmer weather contain more fluoride. In Iran, the highest amount of fluoride has been found in southeast and northeast areas.

Water purification systems for domestic use have drawn much of attention over the past few years. This can be related to improvement of public health and concerns for water contamination. There are several types of home water purification systems that can be categorized into 3 different groups( 5 ) as filtered systems, systems using UV irradiation, and ion-exchange systems.

The aim of this study was to find out whether domestic water purification systems could eliminate the essential materials such as fluoride besides filtrating the heavy ions and other unwanted particles out of water.

In this study, 6 frequently used commercial brands of water purifiers in Ahwaz were compared. The commercial brands evaluated in the current study were CCK (Ceramic and Ceramic/Carbon Cartridges ; RTX-TS DLM filters, Korea), Soft Water (Ceramic Candles; Alpine TJ Series filters, W9332420, USA), Alkusar (Special media cartridges filters; PRB50-IN, USA), Puricom (Special media cartridges filters; Watts 4.5" x 10" Dual Housing, Korea), Water Safe (Granular Carbon Cartridges filters; LCV (Lead, Cysts, VOC's) (Carbon Block Filter Cartridges, Australia), and Aquafresh (Sediment String-Wound; Poly Spun and Pleated Washable Cartridges filters, K5520, USA). The main drinking water supply for Ahwaz is provided by governmental companies. After making arrangement with certain companies that supported these brands, the devices were setup in 6 different regions of Ahwaz. Samples were collected before and right after setting up the device. To reduce the errors and elevate the accuracy of the module, 5 samples were taken from each device. Another sample was collected from each single device 6 months later. A total of 64 samples were collected including 32 unfiltered (control) and 32 filtered samples of tap water (experimental) from 6 regions in Ahwaz. Fluoride sampling kits (Spands; EW-99574-08Hach ® Test Kits, USA) were used to test the amount of fluoride in sample waters. Samples were all collected in polyethylene sampling containers and were then coded. Spectrophotometry (AvaSpec-ULS2048L- USB2 UARS spectrometer, USA) was performed. In order to measure the characteristics of individual molecules, a mass spectrometer converted them to ions so that they could be moved about and manipulated by external electric and magnetic fields.

Atmospheric pressure was around 760 torr (mm of mercury). The pressure under which ions may be handled is roughly 10 -5 to 10 -8 torr (less than a billionth of an atmosphere). By varying the strength of the magnetic field, ions of different mass can be focused progressively on a detector fixed at the end of a curved tube and also under a high vacuum.

Latin alphabetic words were used to code each commercial device. Numbers were used for samples obtained before and after setting the device.( 6 )

The results were analyzed by using paired sample t-test, with alpha (ɑ) set at 0.05.

The amount of fluoride in water before and after using six brands of water purifier device is summarized in Table 1 .

The amount of fluoride before and after installing water purifier devices

Based on the data gathered from all water purification devices set in different regions, the level of fluoride was significantly different before and after using home water purifier ( p = 0.001). It was found that home water purifiers nearly eliminated fluoride from tap water. Table 2 represents the results of t-test.

Comparison of different study groups with t-test

* p< 0.05 is statistically significant.

Another round of sampling was done 6 months later from the same filters of home water purifier. Details are illustrated in Table 3 and 4.

The amount of fluoride in tap water after 6 months of using a water purification filter

Comparison of the study groups after six mounts with t-test

Fluoride absorption is mostly systemic or local; systemic absorption occurs through eating the element with food, water or fluoride pills, and local absorption by toothpastes and other fluoride-containing hygienic products. In many countries, the highest supply for fluoride absorption is systemic absorption through water consumption.( 6 ) In early 20 th century, the first attempts were made to fluoridate public water supplies, which eventually led to 40% decrease of dental caries in the target population.( 7 )Introduction of water fluoridation in the 1950-1960 and fluoride-containing dental products in the 1970 changed the situation. The main sources of fluoride in established market economies (EME) are drinking water, fluoridated salt, foods and beverages, baby cereals and formulas, fluoride supplements, toothpastes, mouth-rinses, and topical fluorides. Additionally, fluoride in water has a diffusion or halo effect; which means that the drinks and foods manufactured in fluoridated areas are also available to whole population including the residents of non-fluoridated areas.

Although adding fluoride to almost all oral hygienic products has restricted the effect of fluoride water (Halo effect), it is still common to fluoridate the city water supply.( 6 ) In many areas of the world, there is no systematic plan for fluoridation of community water and only the natural sources supply it. Therefore, sometimes the hardness of water and aggregation of different and sometimes poisonous elements drive the population to use bottled water or use home purification devices.

The findings of the present study revealed that all the 6 devices reduced the fluoride in tap water and most of them nearly eliminated it. Different home purification devices have been marketed each of which is claimed to eliminate certain kinds of elements from water.( 9 ) JK Mwabi et al. (2011) used 4 different filters to reduce the hardness and chemical contamination of water in poor villages in Africa, and reported that all of the four filters reduced the fluoride significantly. Bucket filter had the most significant effect and reduced fluoride element 99.9%. These results also indicated that fluoride was the most reduced element of all. Likewise, silver-impregnated porous pot (SIPP) filter reduced 90%-100% of elements.

Clasen et al. ( 5 ) in their study reported that 3 different home purification systems ,the ceramic candle gravity filter, iodine resin gravity filter, and iodine resin faucet filter, reduced bacterial contamination by four logs and decreased ions such as fluoride and arsenic, as well.

Moreover, there are certain methods to reduce the excessive amount of fluoride in the water. One of the best-known methods is absorption technique.( 7 ) Evaluation of 6 different commercial water purifiers has not been done in any other study; therefore, there is no similar study to compare the results exactly. More evaluations are suggested to be performed on home water purification systems, and more strategies should be devised to preserve the essential elements of tap water.

The current study found considerable differences between the amount of fluoride before and after filtration with home purification device; that is filtration significantly decreased the fluoride concentration even as much as 100% in some cases.

Conflict of Interest: None declared

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Nanotechnology: an approach for water purification-review

Rama Sharma 1

Published under licence by IOP Publishing Ltd IOP Conference Series: Materials Science and Engineering , Volume 1116 , International Conference on Futuristic and Sustainable Aspects in Engineering and Technology (FSAET 2020) 18th-19th December 2020, Mathura, India Citation Rama Sharma 2021 IOP Conf. Ser.: Mater. Sci. Eng. 1116 012007 DOI 10.1088/1757-899X/1116/1/012007

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Clean water is the global need and need of life for all the human kinds. But the clean water resources are being contaminated in present time. Nanotechnology is an easy and practical approach to clean waste water by using different methods. Different types of bacteria, toxic chemicals like arsenic, mercury etc., and sediments can be removed by using nanotechnology. Nanomaterial based devices are being used for water purification. Nano filtration method has advantages over other conventional method as low pressure is required to pass the water through filters and these filters can be cleaned easily by back flushing. Smooth interior of carbon nanotubes make them convenient for the removal of almost all types of water contaminants. Because of larger surface area nanostructured materials have advantages over conventional micro structured materials.

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• Published: 06 July 2020

Public health benefits of water purification using recycled hemodialyzers in developing countries

• Jochen G. Raimann   ORCID: orcid.org/0000-0002-8954-2783 1 , 2 , 3 , 4 ,
• Joseph Marfo Boaheng 4 , 5 ,
• Philipp Narh 4 , 6 ,
• Harrison Matti 4 ,
• Seth Johnson 1 , 4 ,
• Linda Donald 1 , 4 ,
• Hongbin Zhang 7 , 8 ,
• Friedrich Port 1 , 9 &
• Nathan W. Levin 1 , 4  

Scientific Reports volume  10 , Article number:  11101 ( 2020 ) Cite this article

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In rural regions with limited resources, the provision of clean water remains challenging. The resulting high incidence of diarrhea can lead to acute kidney injury and death, particularly in the young and the old. Membrane filtration using recycled hemodialyzers allows water purification. This study quantifies the public health effects. Between 02/2018 and 12/2018, 4 villages in rural Ghana were provided with a high-volume membrane filtration device (NuFiltration). Household surveys were collected monthly with approval from Ghana Health Services. Incidence rates of diarrhea for 5-month periods before and after implementation of the device were collected and compared to corresponding rates in 4 neighboring villages not yet equipped. Data of 1,130 villagers over 10 months from the studied communities were studied. Incidence rates showed a decline following the implementation of the device from 0.18 to 0.05 cases per person-month (ppm) compared to the control villages (0.11 to 0.08 ppm). The rate ratio of 0.27 for the study villages is revised to 0.38 when considering the non-significant rate reduction in the control villages. Provision of a repurposed hemodialyzer membrane filtration device markedly improves health outcomes as measured by diarrhea incidence within rural communities.

Introduction

Estimates from the World Health Organization and the World Bank place around 1.1 billion people in the world in a position of having to drink unsafe water. Water and sanitation, specifically access to clean water for the world population, were adopted as the Sustainable Development Goal-6 (SDG-6) by all member states of the United Nations. The deserved, widespread attention emphasizes the importance of the issue and the need for more improvement. Industrialized countries have to a large extent solved the problem and a majority of their populations has access to safe drinking water. This is mainly due to the effort of governments, strict laws, regular monitoring, efficient handling and cleaning of sewage, centralized and monitored provision of clean drinking water and lastly to a generally higher level of hygiene (including the use and provision of sanitary facilities). Due to high population growth rates, lack of economic development, and inadequate political efforts this remains a major problem in many countries with limited resources.

Rural areas in developing countries present problems of greatest magnitude. Water is still mainly carried from continually contaminated surface water such as ponds and rivers. Water is often polluted by coliform bacteria and viral pathogens. Factors such as a lack of sanitary facilities, inadequate hygiene practices and substantial flooding during rainy seasons aggravate the problem. Not only surface but also centralized, processed water are at high probability of being contaminated 1 . Wells may also be susceptible to pollution particularly when they are shallow or intermittently overcome by raising water tables. Further, in some low-income countries a flourishing business of sachet water exists, which is assumed to be safe for consumption. However, as shown in work from Nigeria these sachets are also in many cases contaminated due to improper packaging and storage, or inadequate hygiene in the processing. The incidence of diarrhea and its life threatening complications such as dehydration and acute kidney injury correlate with these factors 2 . Non-infectious contaminants in drinking water such as lead and other heavy metals, arsenic, and also organophosphates from pesticides and insecticides contribute to health hazards, problems that are not addressed with our work at present.

Since the first epidemiological studies by the physician John Snow in the nineteenth century, the deleterious effect of microbial pathogens in water has been well established. Estimates of the World Health Organization suggest that 88% of all diarrheal diseases are caused by the consumption of unsafe drinking water and the lack of adequate sanitation facilities 3 . A recent publication of the initiative has identified that a majority of cases of acute kidney injury in the developing world are (in contrast to the most frequently reported pathogenesis in first world countries) are associated with community-acquired disease and to a major part with diarrhea 4 . This is particularly evident in children 2 to 5 years of age in whom mortality is very high 5 . Overall, these data strongly corroborate why it must be a prime goal for the world community to jointly aim to achieve the SDG-6. These data provide a powerful stimulus for widespread joint action by the world community to achieve this goal.

Common approaches to counteract microbial pollution include various filtration devices: Microfiltration, ultrafiltration, nanofiltration and reverse osmosis. Membrane filtration has long been recognized as an effective and likely efficient approach to partly solve the problem in rural regions, however membranes and filtration devices are expensive, and filters are prone to clogging without proper functioning flushing methodologies. The great need that is also building the basis of the SDG-6 of the United Nations, will require an affordable solution to be made available that is not overly prone to malfunction, can sustain functionality over a long period of time and does not require too extensive maintenance in terms of parts and labor. Surface water is often polluted with parasites, bacteria and viruses that can cause serious health issues 6 . Of note, all these pathogens are larger than the pore size of the hemodialyzer that is approximately 0.003 µm. This pore size notably is smaller than most commercially available purification devices, the operation of which has been claimed to be a feasible technique for water purification 2 .

Hemodialysis is a renal replacement therapy modality that uses hemodialyzers in those suffering from renal failure to counteract the consequences of not having kidney function and to ultimately save them from dying. These hemodialyzers are mainly comprised hollow fibers in a plastic casing. This allows, after cannulation of the patient, to pass the patient’s blood inside the fibers, and along the semipermeable membrane of the fiber, until it leaves the hemodialyzer and is returned to the patient. At the same time, dialysis water, containing anions and cations in specifically defined concentrations, passes, in a countercurrent fashion, on the other side of the membrane resulting in gradient-driven diffusion allowing for toxin removal from the blood and by producing a hydrostatic pressure also removes excess water from the patient through volumetric ultrafiltration. These hemodialyzers were commonly being reused after sterilization, a practice that has changed since earlier days of dialysis and current clinical practice commonly uses hemodialyzers only once and discards them after use. Of note, this alone results in approximately 30 kg of annual waste for every (out of approximately 2 million worldwide) dialysis patient 7 . It was shown recently that used and re-sterilized hemodialyzers (a process possible at less than $2 per hemodialyzer) are effective in producing clean water from microbiologically contaminated water when pushed through these hemodialyzers under high hydrostatic pressure.

We, Easy Water for Everyone (EWfE), report here the experience and some preliminary data from the use of this relatively simple technique for preparation of drinking water from polluted river water in rural villages in Ghana that have no electricity. We provided villages with devices containing re-sterilized hemodialyzers uniquely repurposed from their hemodialysis past, which are capable of producing large volumes of water (up to 500 L/h) free of bacteria and viruses for domestic use. Here we report public health outcomes based on prospectively collected self-reported public health information on diarrhea incidence collected before and after implementation of this device in several villages.

Material and methods

Easy Water for Everyone (EWfE) is a 501(c)(3) non-profit, non-governmental organization (NGO) in the United States, Ghana (and with other countries in progress). With the help of local politicians and stakeholders a need for water purification in the estuary of the Volta River in Ghana was identified. For those living in this region the river is the main source for drinking water even though it is known to carry pathogens. Under the supervision of local committees and administrators, EWfE started to install and maintain a device in each of the villages. The chronological order was arbitrary and data collection was commenced on the islands around Ada Foah since 02/2018.

Water purification method

The membrane filtration device (NUF500; NUFiltration, Israel), consists of a set of 8 hollow-fiber hemodialyzers, appropriate tubings and a faucet. These hollow fiber hemodialyzers in this project have been used as hemodialyzers once, then reprocessed and sterilized according to FDA/AAMI standards before installation into the water-purification device. Each hemodialyzer contains around 12,000 capillaries providing a membrane surface area of nearly 2 square meters per hemodialyzer. The membrane pore size is 0.003 µm, notably preventing passage of bacteria, parasites and notably also of pathogenic viruses. The output of pure water can be as high as 500 L/h when actively pumped into the device or up to 250 L/h passed into the device by gravity after being pumped into an overhead tank as used in this study. The pressure by gravity is caused by a height of about 12 feet from which the polluted water enters the eight dialyzers placed in parallel (see Fig.  1 a, b).

Hemodialyzer membrane filtration device used for our project. Setting with ( a ) a manual pump (up to 500 L/h) and ( b ) gravitational force (up to 250 L/h) for driving the contaminated into the re-sterilized and repurposed hemodialyzer filters.

Contaminated river water enters the inside of the capillaries (“blood” compartment) while clean water collects outside of the capillaries (“dialysate” compartment in clinical hemodialysis). Only water (and dissolved salts) passes through the pores. Organic matter that accumulates on the inside of the capillary fibers needs to be rinsed away by intermittently reversing the pressures and filtering clean water back across the membranes (backwashing) through manual pumping. It takes less than 5 min for the backflow to change from dirty to clean appearance and then regain full efficiency for providing clean water.

Data collection

Following the approval of our research project, embedded in the non-profit endeavor, by Ghana Health Services, we initiated data collection with trained local community members to support our endeavor. Next to demographic data and water results before and after passing through the filter, we collected data monthly from the heads of households on self-reported diarrhea events in 8 villages during the months February through November 2018. This was a subset of villages served by EWfE.

In late June 2018, the hemodialyzer filtration devices became operational in 4 of these villages so that this ongoing monthly data collection started 5 months before the installation. It was concluded 5 months after the installation of the hemodialyzer filtration device. Simultaneously the same data was collected in the 4 villages without the device. For each village and each month, the count of diarrhea events and the number of persons exposed to the data collection were analyzed to estimate the monthly diarrhea incidence rates. Monthly data were summarized for each of the two groups of villages, the control group of 4 villages never exposed to the hemodialyzer water treatment and the group of 4 villages exposed to the water treatment during their second 5 months of the 10-months study period. This approach allowed comparison of the incidence rates during the first and second 5-months periods and incidence rate ratios (second/first 5 months) for the study group and the control group. Having this concomitant data allows us, in a univariate fashion, to use village populations as their own controls and consider the potential confounding effect of seasonality.

The results of water testing showed coliform bacteria at 558 CFU/100 mL in the source water (Volta River) and zero CFU in the filtrate water at the beginning of our installations in the villages of Big Ada. We studied 8 villages (4 were designated control villages and 4 were study villages) in rural Ghana. Table 1 shows the population characteristics of the study arms. Of the village populations studied in this cohort study, 11% and 8% were younger than 5 years of age and notably showed a remarkably high proportion of villagers (96% and 99%) had to resort to open defecation.

Monthly diarrhea incidence rates averaged 0.18 counts per exposure month during the baseline period of the study villages and 0.11 for the same 5 months of the control group. During the first 5 months after the installation of the hemodialyzer filtration device, the rate reduced to 0.05, yielding a rate ratio for the study group of 0.28. For the control group the second 5 months gave an average rate of 0.08, showing modest non-significant reduction from the prior 5 months period with a rate ratio of 0.73 (Table 2 ). Figure  2 a and b show the monthly data for the two periods in both village groups. The control villages of the same region and during the same calendar months allow consideration of a seasonal effect on the diarrhea incidence in the study group. Thus, using the incidence rate ratio for the second 5 months over the first 5 months gives a seasonally adjusted rate ratio of 0.38 (0.28/0.73), which translates to a diarrhea incidence rate that is reduced by 62% following initiation of the hemodialyzer filtration device in the study villages.

Monthly diarrhea incidence rates between February (Month − 5) and November (Month + 5) 2018 in ( a ) study villages, where the device was installed in late June 2018 and ( b ) control villages with no device installation during the same months.

In many countries microbiologically contaminated water is the underlying cause of gastrointestinal disease, mainly diarrhea, associated with deleterious consequences such as acute kidney injury resulting in a high mortality rate, particularly in weaned children younger than five and the elderly. Our data, collected in 4 rural communities in the Ada-East distric of Greater Accra Region in Ghana, before and after the implementation of a hemodialyzer membrane filtration device to produce clean drinking water, shows a substantially reduced risk (rate) of self-reported diarrhea by 72%. This is a major public health outcome particularly since diarrhea is well known to be associated with deleterious consequences such as acute kidney injury and death, particularly in younger children and the elderly. This finding is striking and the rigorous analytic design where each community serves as their own control allows for drawing solid conclusions. Studying and comparing our data to that of a control group which presented only with modest reduction in the incidence of diarrhea over the same time period, corroborates an effect that can be attributed to implementation of our approach. The only modest reduction of diarrhea incidence in the control villages also reduces concerns of seasonality in the incidence rates confounding our interpretation.

Discussion of our approach in comparison with other approaches

The methods used in the present study have been effective in removing pathogens from consistently polluted river or lake water sources. During the past 3 years the on-site implementation of the hemodialyzer filtration device have allowed us to demonstrate the success of providing clean and pathogen-free drinking water to villages where the source of drinking water had been consistently contaminated. This system works well even in remote areas without requiring electricity or other external power sources. No restrictions on water use need to be imposed and use of clean water can be encouraged also for handwashing with soap. When more water is needed, the filling of the main water tank can be increased from weekly to two to three times a week (or even daily). There are several key elements that contrast our approach to other methods to produce drinking water: (1) Rejection of pathogens is highly effective and includes particles as small as pathogenic viruses, given the pore size of 0.003 µm, (2) no need to add bactericidal agents such as chlorine to kill remaining pathogens in drinking water, (3) the simplicity of this design allows its use in isolated rural villages even in areas that have no electricity, (4) this system becomes almost self-sufficient after a few villagers have been trained to do the thrice daily backwashing, (5) excellent filtration rates have been observed with this setup for over one year, (6) visits by a trained technician once or twice weekly or more frequently when necessary for refilling the large water tanks using a gas-driven pump provide some monitoring of the continued function and service and (7) relatively low cost since the reprocessed hemodialyzers are inexpensive and have shown in our 3-year experience to maintain high output rates of nearly 250 L/h (by gravity feed) for over one year. Furthermore, in circumstances where larger volumes of purified water are needed, an expanded device, employing far more dialyzers could be utilized. It would also be feasible to equip the device with solar panels which would increase water production substantially but would add to the cost.

Comparison of efficacy with other approaches

Attempts to purify water from microbiological contamination have been undertaken in a multitude of studies discussing purification of water from springs, boreholes, and wells, all sources with many opportunities for contamination to occur between sources and point of use. The source water is detoxified and infectious agents are reduced or removed by methods such as chlorination, membrane filtration, flocculation and others. Direct systems include conventional filtration, for example using sand through granular media which removes parasites, bacteria and possibly some viruses. Conventional filtration also includes chemical coagulants such as potassium alum added to source water which produce clots (flocs) which are in turn filtered. These processes are not easy and require expert handling by trained individuals.

Quite commonly reported is household chlorination which is a simple technique with widespread use. It improves water quality and effectively prevent diarrheal diseases. Quantity and acceptance (because of the resultant taste of the water) are downsides of this approach 5 .

With direct filtration, water passes through a medium such as sand or diatomaceous earth, a process which removes giardia lamblia, cryptosporidia, and bacteria from the water. These methods also remove color and turbidity. Filtration bags are warm bags or cartridges containing a filament to strain the water. These bags are however not useful for anything smaller than the giardia. Ceramics may be impregnated with tiny colloidal particles and allows for eradication of most bacteria and protozoan parasites. However, also this method is not adequate for virus removal. Most of these methods however are laborious, require specialized knowledge and infrastructure, and also time.

Membranes are widely used to produce safe drinking water and are the only means available to produce water free of parasites, bacteria and all pathogenic viruses.

Membranes can be divided into groups largely defined by their characteristics in regard to pore sizes. Depending on the degree of pore size, they can also produce water free of many chemical components. In the case of biologically contaminated water some membranes can produce water free of bacteria, parasites and viruses.

Hemodialyzers that are contained in the device we have chosen to implement in village structures have a semi-permeable membrane made of polysulphone and polyethersulphone. The pore size is around 0.003 µm and will not let parasites, bacteria and viruses pass, while still providing an output as large as 500 L/h.

Decreased microbial quantity in drinking water is effective in decreasing diarrhea. Effectiveness does not solely depend on the presence of improved water supplies but will also be affected by the use of sanitization facilities and handwashing with diligent soap procedures. In concert with appropriate education, these interventions will play a powerful role in improving public health outcomes. Also important in the context of effectiveness is the amount of water that is being produced over a defined period of time. In this context it is of note that our approach, even with the use of the gravitational device where water is pumped into an overhead tank and gravitation is being used to transfer contaminated water into the filter, allows for up to 250 L/h.

Household efforts

Household efforts include: improved water storage, chlorination, solar exposure, filtration by filter media in relationship to pore size, combined flocculation and disinfection methods. A combination of efforts including improved water supply and storage, and improved sanitation results in better water supplies thus reducing the risk of developing diarrhea. Various authors provide a range of figures for reduction of diarrhea but overall it is expected that household interventions will provide a risk reduction for diarrhea incidence 8 . The WHO promotes water treatment and safe storage of household water. Affordability, acceptability, sustainability and scale ability are all important factors and these small-scale solutions do provide improvement.

A current technology comparable to our approach are the “Aqua Towers”, an approach that also uses gravitational forces to pass water through the filter. More than 1,000 of these are active in Asia Pacific and Latin America. It utilizes ultrafiltration but the manufacturer does not reveal the membrane type. Activated carbon is used to enhance the quality of the drinking water. In addition, part of the water supply is used for hand washing. The authors claim that viruses larger than 0.01 microns are removed. However, a membrane with pore sizes as large will not exclude the rotavirus (a causative pathogen of diarrhea in up to 40% in some reported populations), and hepatitis B and C viruses, unlike the hollow fiber hemodialyzer membrane as discussed above. Of note, no outcome data have been published for the communities using the “Aqua Towers”, to the best of our knowledge.

Strengths and limitations of our study

Surveys of diarrhea in households may be considered soft data, however the magnitude of a relative 72% reduction in the incidence of diarrhea per monitored population is strikingly large. It is also corroborated by many mothers reporting a sudden virtual absence of diarrhea in their children after availability of the hemodialyzer-filtered water. The marked reduction in the diarrhea incidence may be due to using sterile water instead of river water polluted with known pathogens, such as E. coli , as the main source of drinking water. Additionally, handwashing with clean water may be an important contributor to our observations. While our study cannot prove causation with certainty, the nearly stable rates in the control group suggests a causative role of the change in the water source from river water to filter-sterilized water.

Of note, we decided to not adjust for population characteristics for two reasons: the same population served as their own controls for each household and the groups of villages and secondly the incidence rates during the initial 5 months were similar for the two groups of villages.

Further considerations beyond water purification

The effectiveness of pure drinking water, sanitation and hygiene by the Campbell/Cochrane collaboration showed 66 rigorous evaluations and 71 interventions (accounting for 30,000 children in 35 countries). Point of use water quality was associated with positive outcomes and so did hand-washing with soap. The Cochrane data base of systemic reviews discussed the effect of hand washing promotion for preventing diarrhea induced nutritional deficiency 9 , retarded child development 10 and deaths in low- and middle-income countries. The list of interventions to improve water quality by eliminating or reducing pathogens with the objective of preventing diarrhea is substantial.

Our results on markedly reduced incidence of diarrhea after implementation of the hemodialyzer filtration device agree with prior studies. In Clasen’s data synthesis paper 11 on 42 studies in 21 countries showed that all interventions to improve the microbial quality of drinking water were effective in reducing diarrheal incidents even though variations in design and application of water cleansing systems limit comparability of their cited studies. Results are less consistent for the role of other common environmental interventions (such as sanitation, or instruction in hygiene) 12 .

Our study using monthly surveys of diarrhea in households may be considered soft data, however the magnitude of a relative 72% reduction in the incidence of diarrhea per monitored population is strikingly large. It is also corroborated by many mothers reporting spontaneously a sudden virtual absence of diarrhea in their children after availability of the dialyzer-filtered water. The marked reduction in the diarrhea incidence is likely due to using sterile water instead of using river water polluted with known pathogens, such as E. coli , as the main source of drinking water. It may be expected that combination of installing a membrane filtration device and combining it with WASH initiatives will have a strong amplified effect as compared to clean water provision alone. This however remains to be shown in further prospective research.

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Acknowledgments

First and foremost, we would like to thank those who have made this study possible by their generous donations. We further would like to thank all those that supported our work and helped us to get to the point we currently are. Last but certainly not least we would like to thank the village committees and everybody in the studied villages (Adzakeh, Agamakope, Alewusedekope, Amekutsekope, Anazome, Azizakope, Baitlenya and Tornyikope).

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Easy Water for Everyone, 10 Bank Street, Suite 560, White Plains, NY, 10606, USA

Jochen G. Raimann, Seth Johnson, Linda Donald, Friedrich Port & Nathan W. Levin

Research Division, Renal Research Institute, New York, USA

Jochen G. Raimann

Katz School at Yeshiva University, New York, USA

Easy Water for Everyone, Accra, Ghana

Jochen G. Raimann, Joseph Marfo Boaheng, Philipp Narh, Harrison Matti, Seth Johnson, Linda Donald & Nathan W. Levin

Department of Field Epidemiology and Applied Biostatistics, Kwame Nkrumah University of Science and Technology, Kumasi, Ghana

Joseph Marfo Boaheng

Ghana Health Services, Big Ada, Ghana

Philipp Narh

Department of Epidemiology and Biostatistics, CUNY Graduate School of Public Health and Health Policy, City University of New York, New York, USA

Hongbin Zhang

CUNY Institute for Implementation Science in Population Health, New York, USA

Departments of Medicine (Nephrology) and Epidemiology, University of Michigan, Ann Arbor, MI, USA

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Contributions

Conceptualization: J. R., S. J., L. D. and N. L.; Data curation: J. R., J. M. B., P. N. and F. P.; Formal analysis: J. R., J. M. B., H. Z. and F. P.; Funding acquisition: L. D. and N. L.; Investigation: J. R., J. M. B., H. Z., F. P. and N. L.; Methodology: J. R., J. M. B., H. Z., F. P. and N. L.; Project administration: P. N., L. D. and N. L.; Resources: J. R., P. N., S. J., H. Z. and N. L.; Software: J. R. and J. M. B.; Supervision: N. L.; Validation: J. R., H. Z., F. P. and N. L.; Visualization: J. R., J. M. B. and F. P.; Writing—original draft: J. R., F. P. and N. L.; Writing—review & editing, J. R., J. M. B., P. N., S. J., L. D., H. Z., F. P. and N. L.

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Correspondence to Jochen G. Raimann .

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Jochen Raimann and Seth Johnson are employees of Renal Research Institute/Fresenius Medical Care. All other authors have no financial disclosure.

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Raimann, J.G., Boaheng, J.M., Narh, P. et al. Public health benefits of water purification using recycled hemodialyzers in developing countries. Sci Rep 10 , 11101 (2020). https://doi.org/10.1038/s41598-020-68408-1

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DOI : https://doi.org/10.1038/s41598-020-68408-1

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Picking up interesting research paper topics on water purification.

There are a number of factors that go into writing a great research paper on water purification. But one of the most important is often glossed over as students simply want to get right down to the dirty work of researching and writing the paper – picking an interesting topic. Generally, academic experts suggest students pick topics that are original, manageable and ones that would be interesting to both the writer and reader. We know it can be hard to come up with ideas, so we’ve developed this list to help get the creative juices flowing:

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Preparing To Write

• Essentials for beginners
• Selecting a writing agency

Organizing time

Begin as early as possible and see to it to form a weekly schedule wherein you can dedicate a certain amount of time to work on your term paper research. Wisely decide the amount of time you will devote in the library, working on the computer, composing an outline, meeting your adviser, composing your rough draft, composing your final draft and the like. It should be realistic so this does not need to be a timetable that is set on stone.

Useful Links

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