sun and water: an overview of solar water treatment
the simplest solar water purification devices are the solar box and the solar still. solar boxes are a well-known method for cooking food and can be used for water pasteurization. a solar box consists of a cardboard or wooden box with insulated bottom and sides and a glass or clear-plastic lid. the inside surfaces should be painted black. a covered pot with water (ideally, also black) is placed inside. the pot needs to remain in the box until the water is at 150 f (65 [degrees] c) for a few minutes. generally, a solar box can pasteurize about 1 gallon of water in 3 hours on a very sunny day in, say, southern california. pasteurization kills bacteria, viruses, and cysts, but does not remove chemical contaminants (2,3).
a very low-tech method, using direct solar radiation to reduce pathogens, was field-tested by researchers from the royal college of surgeons in dublin, ireland. the researchers gave 206 masai children clear, 1.5-liter plastic bottles. the children in the test group were told to fill the bottles (from the contaminated water supply) and place them on the roof, from dawn to midday. the control group kept their bottles inside. diarrhea incidence in the two groups was tracked over 12 weeks. the researchers found that this solar radiation method may significantly reduce diarrheal disease for communities that have no other way to disinfect water (4).
boiling water is not necessary to kill pathogens, but reaching water pasteurization temperature is critical. so how do you measure temperature, especially in a remote area, after the glass thermometer breaks? an ingenious solution is the water pasteurization indicator (wapi). a prototype was developed by dr. fred barrett (u.s. department of agriculture, retired) in 1988. the current wapi was developed by dale andreatta and other graduate engineering students at the university of california, berkeley. the wapi is a polycarbonate tube, sealed at both ends, and partially filled with a blue soybean fat that melts at 156 [degrees] f (69 [degrees] c). the wapi is placed inside the water container, with the fat end up. the user can easily tell when the water reaches 156 [degrees] f (69 [degrees] c) because the fat melts and runs to the bottom of the tube. the wapi is reusable and durable. this device can be placed in a pot in a solar box or over a stove or fire. since pasteurization occurs at a lower temperature than boiling, less fuel is needed for heating water. this is important where fuel is scarce or expensive (3,5).
if materials for a box are not available, or if cost is a factor, then a solar puddle may be the answer. a shallow pit about 3 feet (1 meter) square and 4 inches deep is dug and is insulated with native materials (paper, straw, grass, leaves). over this are placed layers of clear and then black plastic, with the edges extending out and over the sides of the pit. the bottom should be flat, except for a trough along one side. a wapi could go in the trough, the coolest part. a drain siphon is installed in the lowest part of the trough, and weighted down with rocks. water is added to a depth of 1 to 3 inches. a layer of clear plastic is laid over the water, with the edges extending over the edges of the pit. spacers, such as wadded paper, are placed on this layer of plastic. then a final layer of plastic is placed over the spacers; the plastic layers should be at least 2 inches apart. rocks and dirt are used to weigh down the edges of the plastic. water needs to be added each day. during tests in berkeley, sunny days produced 17 gallons of water in the device, which cost about $4 (3).
a higher-tech version of the solar box was developed by sun utility network, inc. they produce a sunthermos bottle, essentially an oversized insulated tube. they claim that it will kill bacteria, protozoa, and viruses as it reaches pasteurization temperature. a single bottle lists for $150, and sun utility also offers accessories such as containers, filters, and stand. the company is offering cash awards and marketing assistance to developers of simple, practical, and cost-effective applications of their product for developing countries, campsites, or eco-tourist developments (6).
the concept of a solar still is familiar to anyone who has been inside a steamy greenhouse. a solar still usually consists of a large flat surface, about the size of a dining room table (about 3 by 5 feet). this can be mounted on legs or on top of a shed or house. a short wall is built around the top of the table and is lined with impermeable material (ideally black, high-temperature silicone rubber) to make a small pool on the top of the table. a pane of glass or plexiglas is mounted at a slight angle above the table. as the water is heated, it forms water vapor, which condenses on the pane. gravity pulls the condensate to the lower edge of the pane, which overhangs the pool, and drips into a trough, and then through a hose or tube into a collection jug. a still 3 feet by 6 feet large produces about 3 gallons a day in the summer, with winter production about half. the solar still is being used by the solar water purification project of the texas state energy conservation office. the el paso solar energy association (epsea) runs this project. epsea's demonstration project has installed solar stills at a colonia along the texas-mexico border. the colonia residents have truck-delivered water that is stored in 55-gallon drums. the solar still treats this water to provide good tasting drinking water that the children will drink. epsea estimates that a single still costs between $200 and $300 and requires basic tools for installation. epsea will provide free plans for nonprofit organizations and nongovernmental organizations (ngos) (7). a commercial product is available from aqua del sol, of pima, arizona (8). free plans are also available from the solardome's school of solar thermal energy web site (9).
a somewhat more complex method has been developed by the pax world service: a flow-through unit. using a standard solar box cooker, they added about 50 feet of black plastic tubing; a storage tank for untreated water; a prefilter (sand, gravel, and charcoal); and a thermostatic valve and container on the treated side. since the tubing only contains about one-half gallon (1.5 liters), it is rapidly heated to the valve opening temperature of 182[degrees] f (83.5[degrees] c) (a mass-produced automotive radiator thermostat valve is used). as the heated water drains into the container, untreated water is drawn into the tubing. when the untreated water reaches the valve, the valve closes. this device has two main advantages over the solar box or still. potable water is available throughout the day, depending upon the amount of solar heat. also, the process is automatic; no "pot-watching" is required, and there is no guessing about when the water has reached pasteurization. the device has been successfully field tested by pax world service and the pakistan council of appropriate technology, showing yields of 4 to 6 gallons (20 liters) a day. it costs about $50 (5).
although the pax system is a distinct improvement on the solar box or still, it does not produce enough water for a village. solar devices often use heat exchangers to increase efficiency. heat exchangers preheat the untreated water with the heat from the treated water. as a result, output can be increased by about 400 percent, or 20 to 25 gallons (80 to 96 liters) per day. a tubular or flat metal plate can be used as the heat exchanger. another benefit is that the outflow of treated water is much cooler than with the pax device, so that the chance of burns is reduced. devices that use heat exchangers are marketed by safe water systems (3,10). the kathmandu post reported that the liver foundation nepal is interested in working with the friendly appropriate solar technology, a california company, to manufacture these devices in kathmandu (11).
another variation on the solar theme is the device developed at the florida solar energy center. this device uses a parabolic trough to concentrate solar energy on a copper pipe placed across the center of the trough. the copper pipe uses the heat exchange principle, with three nested pipes that create an outer and inner channel. the untreated water is pumped into the outer channel, where it is heated directly by the concentrated solar energy after the water reaches the end of the pipe, it returns through the middle channel, and preheats the untreated water. this device also uses a standard automotive thermostatic valve. a photovoltaic panel supplies electricity for the pump. experiments at the florida solar energy center showed that this device would produce up to 660 gallons (2,500 liters) of drinking water per day, using a 92-square-foot (28-square-meter) concentrator. the developers have estimated that the cost of materials for this device is about $60 per square yard (1 square meter), or about $1,680. when the cost of the pump and reservoirs is added, this becomes one of the more expensive devices (12).
conventional technology also can treat water in remote areas, by combining photovoltaic (p) cells that produce electricity with compact, sturdy equipment. ultraviolet light disinfection may be preferred where chlorination is unavailable or unwanted. major research by a lawrence berkeley laboratory team has produced a uv system for field use. this system is designed to provide water for a large village, with the uv light running on electricity. it can, however, run on two 6.5-square-foot (2-square-meter) pv panels or on a 12-volt car battery. the system relies on gravity for water flow through the unit. the base unit for the uv system is $500, including valve, fittings, and labor. the price does not include storage tanks and sand filters, which are provided by the villagers. the unit can disinfect 4 gallons per minute. this system was field tested in india, and has been proposed for areas in south africa (13,14). the system, uv waterworks, will be available through waterhealth international, inc., of napa, california. waterhealth international intends to contract with local dealers in selected countries and to work with agencies such as the world health organization to bring the system to more remote areas (16). waterhealth international estimates operation of the system would cost about 15 cents per person per year, for a village of 1,500 to 2,000 people (1). similar small-scale devices are available from o-so pure and aqua sun international. each of these devices consists of a self-contained "briefcase" and can produce 1 to 2 gallons a minute. they use filtration and uv light for disinfection. aqua sun also produces larger stationary or truck-mounted units that can produce up to 15,000 gallons per day. (16,17).
pv cells also can provide the power for mixed-oxidant disinfection systems. these systems use sodium chloride (table salt) to produce a highly active oxidant mixture. linda venczel, ph.d., of the university of north carolina at chapel hill, has examined the effectiveness of this system in the laboratory and in a village in bolivia. during the village study, the test group received mixed-oxidant disinfectant solution, narrow-necked plastic water storage bottles, and hygiene education. the control group continued to draw water from the village wells, which were open, shallow wells with high levels of contamination. after six months, the test group had significantly lower levels of e. coli and c. perfringens and fewer diarrheal episodes than the control group. at the end of the study, the test group continued to use the disinfection system, and the control group also wanted to use it. the study concluded that on-site production of mixed-oxidant disinfectants can provide inexpensive and effective disinfection where conventional treatment and distribution are unavailable (18). global water, inc., wants to install these systems in 100 villages in bolivia and peru. they estimate that in a village of 750 people, the initial cost would be about $17 per person and would cost about 20 cents per person per year to operate. global water, inc., is an international, nonprofit, nonsectarian, and ngo dedicated to the development and implementation of safe water and health-related projects in developing countries (19).
successful installation of any of these methods requires that the users also receive instruction on sanitary waste disposal; proper containers for water storage, transport, and dispensing; and maintenance of the system. some past efforts at chlorine disinfection have failed because of a lack of training and the high maintenance required by those systems (1). most of the developers of the systems discussed here have emphasized the need for education and are looking for ngos and other relief organizations that can use the systems, especially where electricity and fuel are scarce and expensive. this brief overview of the various types of solar-powered water disinfection devices only touches upon the opportunities for their use in developing countries. interested readers may want to review a recent publication by the national renewable energy laboratory that provides a detailed description of these devices, analyzes the feasibility for their use in developing countries, and identifies research opportunities (20). the journal of environmental health encourages the submission of articles about the field application of these devices.
1. "brighter light, better water" (1996), environmental health perspectives, 104(1), as reported at http://ehpnet1.niehs.nih.gov/docs/1996/104(1)/innovations.html.
2. solar box cookers northwest, 7036 18th ave. ne, seattle, wa 98155. http://solarcooking.org/spasteur.htm.
3. dale andreatta, of sea, inc., columbus, oh 43085 (1094), "a summary of water pasteurization techniques," document based on d. andreatta, d.t. yegian. l. connelly, and r.h. metcalf, "recent advances in devices for the heat pasteurization of drinking water in the developing world," paper presented at the 29th intersociety energy conversion engineering conference, american institute of aeronautics and astronautics, inc.
4. conroy, r.m., m. elmore-meegan, t. joyce, kg. mcguigan, and j. barnes (1996), "solar disinfection of drinking water and diarrhoea in maasai children: a controlled field trial," lancet, 348(9043): 1695.
5. metcalf, r. (1994), "recent advances in solar water pasteurization," solar box journal, 16(february): http://solarcooking.org/metcalf.htm.
6. solar thermos bottle, sunthermos bottle, sun utility network, inc., 626 wilshire boulevard, suite 711, la, ca 90017; http://www.sunutility.com/thermos.htm.
7. solar water purification project, sponsored by the texas state energy conservation office. el paso solar energy association, p.o. box 26384, el paso, tx.
8. aqua del sol, p.o. box 1114, pima, az 85543; http://www.zekes.com/~actuadelsol/index.htm.
9. solardome school of solar thermal energy, http://wwv.solardome.com/solardome84.html.
10. safe water systems, hawaii, division of grand solar, http://solarcooking.org/solarwat.htm.
11. "pasteurized water can help prevent most illnesses" (may 3, 1997), as reported at http://www.south-asia.com/ktmpost/may/may3/may3-lc.htm.
12. anderson, r. (1996), "solar water disinfection," proceedings of solar '96. the 1996 american solar energy society annual conference, boulder, colo.: american solar energy society.
13. gadgil, a.j., and l.j. shown (february 1, 1995), "to drink without risk: the use of ultraviolet light to disinfect drinking water in developing countries," unpublished paper, center for building science, lawrence berkeley laboratory, berkeley, ca 94720.
14. yards, l. (june 7, 1996), "water purification system, materials library selected for discover's awards," http://www.weea.org/news/uvwaterworks/2.htm.
15. meites, e. (november 15, 1996), "do drink the water," the daily california, as reported at http://www.dailycal.org/issues/11.25.96/ultraviolet.txt.
16. o-so pure solar powered model, 2240 w. desert cove ave., phoenix, az 85029-4913: http://dci-press.com/business/osopure/solution.html
17. aqua sun solar powered water purification systems, aqua sun international, p.o. box 2919, minden, nevada 89423; http://www.aqua-sun-intl.com.
18. venczel, l., and m.d. sobsey (1997), "evaluation and application of a mixed oxidant disinfection system for prevention of waterborne disease," ese notes, chapel hill, n.c.: department of environmental sciences and engineering, university of north carolina; as reported at http://bes.isis.unc.edu.
19. global water, inc.; http://www.mountainhawk.com/globalwater.
20. burch, j., and k.e. thomas (1998), "an overview of water disinfection in developing countries and the potential for solar thermal water pasteurization," national renewable energy laboratory of the u.s. dept. of energy. golden, colorado, nrel/tp 550-23110. copies available from the national technical information service, u.s. dept. of commerce, 5285 port royal road, springfield, va 22101, (703) 487-4650.
corresponding author: trudy c. rolla, senior environmental health specialist,
seattle-king county department of public health, 999 3rd ave., suite 700, seattle, wa
98104. e-mail: email@example.com.