Household Water Treatments in Developing Countries - Journal of

Mar 18, 2011 - The intent of this article is to inform both high school and college chemical educators and chemistry students of this important applic...
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Household Water Treatments in Developing Countries Joanne A. Smieja* Department of Chemistry and Biochemistry, Gonzaga University, Spokane, Washington 99258, United States

bS Supporting Information ABSTRACT: Household water treatments (HWT) can help provide clean water to millions of people worldwide who do not have access to safe water. This article describes four common HWT used in developing countries and the pertinent chemistry involved. The intent of this article is to inform both high school and college chemical educators and chemistry students of this important application of chemistry and to inspire current and future chemists to participate in the global effort to develop effective, sustainable methods to purify water at the household level. KEYWORDS: General Public, High School/Introductory Chemistry, Environmental Chemistry, Internet/Web-Based Learning, Nonmajor Courses, Water/Water Chemistry

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umans need to consume approximately 2 L of water each day. Unfortunately, 884 million people worldwide do not have access to an improved water source such as a piped household connection, protected well, or public standpipe and must use contaminated water to meet their daily needs.1 Conventional treatment plants capable of distributing clean water reliably to individual households are the ideal solution but centralized systems are cost-prohibitive, especially in rural areas with low population densities. Household water treatments (HWT) have been proposed as viable, interim means to provide clean water until improvements in the water-supply infrastructure can be achieved.2 This article describes the four most common HWT used in developing countries and the pertinent chemistry involved. This is not an exhaustive review. Interested readers are directed to several excellent reviews for further details.3-8 The intent of this article is to inform both high school and college chemical educators and chemistry students of this important application of chemistry and to inspire current and future chemists to participate in the global effort to develop effective, sustainable methods to purify water at the household level.

are present in sufficient concentrations in some waters to cause adverse health effects if ingested over a given period of time. The two most prevalent harmful inorganic contaminants are arsenic9 and fluoride.10,11 Although the removal of these inorganic contaminants is critical for millions of people worldwide,12 this article will focus on the removal of biological pathogens. To be adopted and sustained by people in developing countries, a HWT must possess several features. • The implementation and maintenance costs must be low, thus, the materials used to produce and maintain a HWT must be available locally or an inexpensive distribution system must be in place. • The HWT must be easy to use and effective with little or no user training. • The HWT must treat a sufficient quantity of water in a reasonable amount of time. The amount of water depends on how the water will be used. If the water is used solely for drinking, 2 L per person per day is sufficient. However, if the water is also used for food preparation, approximately 8 L per person per day are needed.12 • The HWT must be culturally and socially acceptable.

’ NECESSARY CHARACTERISTICS OF HOUSEHOLD WATER TREATMENTS The most important attribute of a HWT is the ability to remove or eliminate contaminants that lead to waterborne diseases, that is, diseases associated with the ingestion of contaminated water. Many of the 884 million people who do not have access to an improved water source use surface water or shallow groundwater. In developing countries, the most common contaminants in these waters are pathogens from fecal material. Common fecal pathogens include protozoa, bacteria, and viruses. An effective HWT must be able to remove or eliminate these biological pathogens from a variety of water sources (streams, ponds, unprotected wells, etc.). In addition, inorganic contaminants Copyright r 2011 American Chemical Society and Division of Chemical Education, Inc.

’ DESCRIPTION OF METHODS An HWT can be classified as a thermal, photolytic, chemical, or filtration method. Current worldwide use is difficult to ascertain and complicated by a variety of factors.8,13 Survey data collected by the World Health Organization (WHO) indicates 1.1 billion people in 67 low- and medium-income countries practice some type of HWT. Extrapolation of these data to all 165 low- and medium-income countries suggests that the total number of people worldwide practicing some type of HWT exceeds 1.5 billion.13 Published: March 18, 2011 549

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• Absorption of solar radiation by water: Water strongly absorbs infrared light. If infrared absorption increases the water temperature above 41 °C, protein denaturation begins to occur, impeding cellular function. • Absorption of solar radiation by contaminants: If certain organic or inorganic contaminants are present, absorption of solar radiation in the presence of dissolved oxygen may result in the formation of reactive species such as superoxides or hydroxyl radicals. These reactive oxygen species can oxidize cellular components and impede cellular function. SODIS is used by more than 2 million people in 33 countries.24 SODIS users are instructed to place microbiologically contaminated water in a transparent container, shake to aerate, and expose to direct sunlight. Solar irradiance (measured in W/m2) and water turbidity determine disinfection time. Water with low to medium turbidity needs 3-5 h of solar radiation above 500 W/m2. Water with high turbidity must be filtered before photolysis. If skies are more than 50% cloudy, users are encouraged to expose the filled containers to sunlight for 2 consecutive days to ensure disinfection. Ideally, SODIS containers should be transparent to radiation above 280 nm, inexpensive, locally available, and sturdy. In many parts of the world, polyethylene terephthalate (PET) bottles are used. While they are inexpensive and locally available, PET bottles are not ideal SODIS containers. PET strongly absorbs ultraviolet radiation up to 320 nm, which prohibits UV-B radiation from reaching the contaminated water. Further, when exposed to 290-350 nm radiation, PET photodegrades and loses its mechanical integrity. Several recent studies have explored alternative SODIS containers and SODIS photocatalysts. Interested readers are directed to the online Supporting Information for additional details. Although SODIS has great promise as a sustainable HWT, to date this promise has not been realized. In a one-year, cluster-randomized, controlled trial designed to assess the effectiveness of SODIS, no significant differences were observed between the number of diarrhea episodes in 11 rural communities where the SODIS method was promoted and 11 rural communities that acted as a control group.25 A lack of compliance and the possibility of exposure via multiple transmission pathways may have contributed to the outcome.

Thermal Methods

Thermal methods are the most common HWT and are considered the benchmark by which other methods should be compared.8 When water, even turbid water, is heated to temperatures greater than 65 °C, all three classes of biological pathogens are killed or inactivated. If a combustion reaction is used as an energy source, no special equipment is needed. Thermal methods are culturally and socially acceptable to people who are accustomed to heating water. For these reasons, hundreds of millions of people currently use thermal HWT. Unfortunately, the use of combustion reactions may be problematic. Consider a family of four who chooses to follow the WHO recommendation to heat their water to the boiling point before drinking. Because water has a specific heat capacity of 4.18 J/g °C, the amount of energy needed to raise the temperature of 8 L of water from a room temperature of 25 to 100 °C will be approximately 2500 kJ. Wood containing 15% water has a caloric value of 16,000 kJ/kg. If a wood-burning stove with a heat transfer efficiency of 20% is used, approximately 780 g (1.7 pounds) of wood will be needed to boil drinking water for a family of four for one day. If this family also uses a combustion reaction for cooking, more than 3 kg of wood must be purchased or gathered daily. This quantity of wood or comparable fuel is too expensive for people living in extreme poverty. Consequently, many hours must be spent gathering fuel, a practice that limits a family’s earning power and can lead to deforestation or other environmental damage. To increase the efficiency of the process, large amounts of water are often boiled at one time and stored until needed. However, recent studies indicate that recontamination is common once the boiled water cools.14-16 In addition to prohibitive fuel costs, possible environmental damage, and potential for recontamination, the use of combustion reactions contributes to poor air quality in the home, which can lead to respiratory problems. Several options have been explored to overcome the problems associated with combustion reactions. To reduce the amount of fuel needed, reusable indicators have been developed to show when pasteurization temperatures have been reached,17,18 more efficient stoves have been designed,19,20 and the use of solar radiation has been explored as an alternative energy source.21,22 Solar radiation can be used to heat water to pasteurization temperatures if a heat-absorbing, opaque container surrounded with reflective material such as aluminum foil is used. The use of solar radiation does not have the same problems as the use of combustion reactions, yet the special equipment needed to achieve pasteurization temperatures with solar radiation is unavailable in many parts of the world.

Chemical Methods

A third type of HWT involves the addition of a chemical disinfectant. Done properly, chemical disinfection is one of the most effective, inexpensive treatments available.26,27 More than 13 million people are known to use a chemical HWT.8 The most commonly used disinfectant is hypochlorous acid, HOCl, which is capable of inactivating most waterborne pathogens.28 The mechanism of HOCl disinfection is uncertain but may involve inactivation of membrane proteins required for energy transduction29 or DNA replication.30 HOCl is a weak acid (pKa of 7.53). In natural waters where the pH is typically less than 8, the molecular species predominates (not the conjugate base, OCl-). The microbicidal activity of the molecular species is 150-300 times greater than the anion presumably owing to the molecule’s ability to penetrate cell walls. The pH dependence of HOCl disinfection was recently described in this Journal.31 In many parts of the world, HOCl solutions are available at low costs. Solutions of HOCl can be prepared locally by electrolysis of a sodium chloride solution to form chlorine,

Photolytic Methods

As discussed above, by using an opaque, heat-absorbing container, solar radiation can be used to induce thermal disinfection. But if a transparent container is used, photolytic disinfection is possible. The solar radiation that reaches Earth consists of UV-B (280-315 nm), UV-A (315-400 nm), visible (400-750 nm), and infrared (>750 nm) radiation. Solar disinfection (SODIS) occurs by multiple mechanisms.23 • Absorption of solar radiation by pathogens: DNA absorbs UV-B and high energy UV-A radiation. Absorption of UV radiation by adjacent DNA thymine molecules leads to the formation of thymine dimers. Unless repaired, the injured DNA cannot replicate and the cells cannot reproduce. 550

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which disproportionates to sodium hypochlorite and sodium chloride. 2NaClðaqÞ þ 2H2 O h Cl2 ðaqÞ þ H2 ðgÞ þ 2NaOHðaqÞ ð1Þ Cl2 ðaqÞ þ 2NaOHðaqÞ h NaOClðaqÞ þ NaClðaqÞ þ H2 OðlÞ ð2Þ Unfortunately, the use of a HOCl solution as a chemical disinfectant has several shortcomings. • To be effective, the correct dose must be added. Since proper dosing depends on a variety of factors, some user training is required. Interested readers are directed to the online Supporting Information for additional details. • Solutions of HOCl have a limited shelf life since HOCl decomposes upon heating or exposure to sunlight.31 2HOClðaqÞ þ hν h 2HClðaqÞ þ O2

Figure 1. Cross-section of a ceramic pot filtration system.

ð3Þ

Sodium dichloroisocyanurate (NaDCC) tablets have been explored as alternative sources of HOCl as NaDCC tablets have a longer shelf life than HOCl solutions.32

The chemistry of NaDCC and the use of NaDCC as a disinfection agent have been described in this Journal.33 • Suspended or dissolved organic matter may react with HOCl to produce disinfection by-products such as halomethanes, which may be mutagenic. In developed countries where the health risks due to microbiological contamination are low, the formation of disinfectant by-products is closely regulated. In developing countries, where risks due to microbiological contamination may be high, WHO recommends that microbiological quality take precedence over disinfectant by-product guidelines. Fortunately, the amount of halomethanes detected in HOCl treated waters is low.34 • Disinfection by a HOCl solution is ineffective in waters with high turbidity. A multistep process involving flocculation, filtration, and chemical disinfection has been developed by Procter and Gamble.35 Interested readers are directed to the online Supporting Information for additional details.

Figure 2. Cross-section of a BSF system.

complete ceramic filtration system is shown in Figure 1. When properly maintained, a ceramic pot typically has a lifetime of 2 to 3 years. The microbiological effectiveness of commercially available ceramic candle filters36 and locally produced ceramic pots has been examined.37 If constructed and used as directed, ceramic filters are effective at removing or inactivating many biological pathogens from both nonturbid and turbid water. The mechanism of pathogen inactivation is controversial. A limited number of peer-reviewed studies38-40 have been published and two primary mechanisms have been proposed: physical removal by size exclusion, and disinfection by silver ions impregnated in the ceramic. Interested readers are directed to the online Supporting Information for additional details. In general, there is agreement that simple porous ceramic filters are capable of decreasing bacteria and protozoa count, but not viral count. However, filters made from clays amended with iron or aluminum oxides prior to firing have been shown to be effective at removing viruses.41 The mechanism of viral inactivation is unknown and further studies are necessary. Biosand filters (BSF), the second common filtration device, consist of a concrete or plastic container filled with layers of sieved and washed sand and gravel, as shown in Figure 2. A detailed description of a BSF is provided in the online Supporting Information. Four different processes have been proposed: mechanical trapping in the spaces between sand grains, predation by microorganisms in the biolayer, adsorption to sand grains, and natural death.42-48 The most important operational parameter for pathogen removal is the pause period, that is, the time between runs. Tracer tests indicate that the flow through a BSF closely resembles plug flow44 and that the water that elutes first contains

Filtration Methods

The fourth type of HWT involves filtration. Multiple filtration devices have been developed, although this discussion will be limited to the two most common devices, porous ceramic filters and biosand filters. Porous ceramic filters have been used in developing countries to treat water since the late 1980s. At the onset, the filters were commercially produced with high-quality materials resulting in prices that were typically out of reach for many people in developing countries. In 1998, a U.S.-based nongovernmental organization called Potters for Peace developed a less expensive manufacturing process using local labor and starting materials. Clay, water, and a combustible organic material such as sawdust, flour, or rice husks are pressed into the shape of a pot and allowed to dry. Upon firing, combustion of the organic matter results in a porous material that is permeable to water. After cooling, the filters are painted with, or dipped in, a silver solution. The flow rate of the final product is measured and if it is between 1 and 2 L/h, the pot is deemed functional. A 551

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the lowest biological count.48 Similar to porous ceramic filters, BSF are more effective at removing bacteria and parasites than removing viruses. The two filtration methods have been compared to each other and to other HWT. In general, ceramic filters are better at removing pathogens but the BSF flow rate is more convenient.49,50 The sustained use of locally produced ceramic filters appears promising with a rate of disuse approximately 2% per month after implementation.51 The disuse is attributed primarily to breakage and a lack of readily available replacement parts. Using a meta-regression analysis of 28 separate studies of randomized controlled trials of HWT, the use of ceramic filters was found to be a more effective intervention than the SODIS method or chemical disinfection methods.52

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’ SUMMARY Millions of people worldwide must use contaminated water to meet their daily needs. Four different types of HWT have been developed to improve the microbiological quality of water. Thermal methods are the most common, but as the world population grows and natural resources are consumed, thermal methods may not be sustainable. Photolytic and chemical methods are the most affordable but filtration methods appear to be the most sustainable. Water treated by a filtration method looks better, tastes better, and smells better than untreated water. This is not necessarily the case when water is treated by thermal, photolytic, or chemical methods. Current and future chemists are invited to participate in the global effort to develop effective, sustainable methods to purify water at the household level. ’ ASSOCIATED CONTENT

bS

Supporting Information Additional details about HWT methods. This material is available via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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