ALN logo; link to Arid Lands Newsletter home page No. 57, May/June 2005
Water efficiency II: Rural areas and agriculture

Making the most of a scarce resource: Solar disinfection of drinking water in the arid zone of western Rajasthan

by Rob Reed, Isaac Bright Singh, Shibu Mani, Ranjit Kanjur, Neil Simon and Surekha Mol

"Field trials in rural Rajasthan showed a measurable reduction in the incidence of gastroenteritis and diarrhea in a case-controlled trial over a 1-year period, demonstrating that solar disinfection can enable such households to widen their sources of treated drinking water."


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Rajasthan is the largest state in India, occupying just over 10% of the total area of the country. The Aravali range of hills, an important physiographic feature of the region, divides the state into two; the western region, which includes the Thar desert, forms an arid zone with mostly sandy soils, covering around 175 000 km2 (~67,570 mi2). The groundwater at handpumps and borewells at many locations in western Rajasthan is saline, making it unsuitable for drinking, and at some locations it also contains high levels of fluorides, leading to skeletal fluorosis when such sources are used for drinking. Given the low annual rainfall of western Rajasthan, human development in rural locations has been driven by the need to use water with maximum efficiency.

Exposure to waterborne infectious diseases is a significant issue in rural India, where almost three-quarters of the population lack access to a reliable source of safe water and where diarrhea and similar diseases linked to the consumption of water contaminated with pathogenic microbes represents a continuous threat to the local people. This is especially so in arid areas such as western Rajasthan, where treated water is a scarce resource.

Traditional approaches to water use efficiency - rainwater harvesting systems

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Since many villages have no direct access to a satisfactory supply of groundwater, various traditional systems have been developed to harvest and store the small amount of rainwater that falls during the single annual monsoon in July-September, which typically represents around 85% of the annual rainfall of 150-300 mm (~6-12 in). These traditional systems include:

  • nadis:
    • most rural villages have their own pond, or nadi, constructed by the further hollowing out of a natural surface depression; the position and size of a nadi has often dictated the location and scale of a village. Most nadis retain water for several months of the year but many run dry before the next rainy season, especially in recent years when the region has experienced repeated severe droughts.
  • talabs:
    • these are surface reservoirs, usually created either (i) by building an embankment around a nadi or (ii) by building a wall or bund on the lower part of a sloping site, to retain rainwater. In some communities, the words nadi and talab are interchangeable. The water in nadis/talabs is often quite turbid (typically up to 100 turbidity units or more) and/or colored (due to algal growth), and therefore is not usually the first choice as a source of drinking water for the local people, when an alternative source is available. For example, villages near to major roads often have a piped supply from an external source, but these are highly unreliable. As a result, such communities would benefit from having an effective means of treating the locally available water when the external water supply fails.
  • tankas:
    • typically, these are underground cisterns, lined with lime mortar or cement. Village community tankas are used for collection of surface runoff and also for storage of water collected from ponds or elsewhere (stored pond water is thereby reduced in turbidity); household tankas are used for water storage at the family level, and in a few buildings they may be linked to rooftop rainwater harvesting systems. In times of drought, such as in 1998-2003, water may be brought in by road-going tanker, funded by government or relief agencies, and then stored in the village tankas. Water is drawn from the tanka by bucket ("balti") and rope and then transferred to metal pots or earthenware pitchers for transport and further storage.

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  • baoris:
    • also known as stepwells, these are unique to Rajasthan and Gujarat, combining the practical functions of water harvesting and storage - they are typically shallow rectangular wells, built at the center of a depression and designed to catch the rainfall from the surrounding area.. One side is a broad set of stone steps, giving direct access to the water. The stepwells also performed religious and social functions in traditional societies, and the most complex examples have been described as "temples of water," as a result of their ornate arched galleries and landings (Livingston 2003).

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Link to Figures 2a and 2b, ~52K

Microbiological quality of harvested/stored water

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While traditional systems can provide a supply of water for rural villagers, its microbiological condition is often poor, since it will have been harvested and collected as surface runoff and then stored under conditions that will often cause further contamination. For example, it is common for people to step into the water within a baori or talab in order to fill their pitchers. When such surface water is collected, individual villagers may cover the mouth of their pitcher with a cloth, or may pour the water into a second cloth-covered pitcher to sieve out the zooplankton that are carriers of guinea worm (Dracunculus medinensis).

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For the same reason, if a woman should need to drink water directly from a nadi or talab, she will often use her headscarf veil (known locally as a "dupatta") or the edge of her sari to filter the water during drinking. However, apart from such simple filtration procedures, water is usually drunk straight from the source, without any treatment. This is partly due to a lack of awareness of the risks of transmission of disease via the water supply, but is also due to the absence of a suitable method of treatment. For example, fuelwood is scarce in the region, so very few people are able to boil their water before drinking, and the inconsistent availability of chlorine tablets (usually through the community health worker) for the treatment of water stored in tankas means that people cannot always rely on this approach. Similarly, while alum ("fitkari") can be used as a coagulant to treat highly turbid water in village community tankas, it is expensive and erratically available (often, only in cities and larger towns). As a result, such untreated supplies represent a potential means of transmission of waterborne infectious diseases, including dysentery, cholera and gastroenteritis, which are not removed by the traditional filtration procedures described above. In our routine testing of such waters using field kits for H2S-producing bacteria, we have found that almost all of the surface water and stored water supplies are grossly contaminated.

Water is usually stored on a daily basis in traditional earthenware pitchers ("matkas"), where evaporation from the surface reduces the water temperature, making it more pleasant to drink in the hot climate. Thus villagers will often select water based on such behavioral patterns, for example from the coolest available source in summer, rather than considering the potential for waterborne disease. However, there is also a traditional belief that storage in pitchers made from either copper or brass helps improve the quality of the water, thereby reducing stomach upsets and aiding well-being. The microbiological basis for this has been confirmed by recent studies demonstrating that fecal coliform bacteria are inactivated by storage for 48 hours in a traditional copper and brass vessels (Tandon, Chhibber and Reed 2005). However, water is rarely stored for that long, being usually collected and consumed daily. In recent years, villagers have moved away from copper vessels, such as when they use plastic containers to transport water to school or work. This behavioral change may be driven by the ready availability of such plastic items; for example, discarded plastic mineral water bottles used by tourists are obtainable in villages near to highways. Additionally, plastic is lighter, less expensive than traditional metal pitchers, and may be seen as more "modern" than brass and copper vessels. In our studies in rural Rajasthan, we have found that while most of the villagers are aware of the traditional belief in the effectiveness of copper and brass pots, they do not make use of such metal pitchers for water storage, though a few make use of brass pitchers for collecting and carrying water to the household.

Given the limited amount of safe water for drinking and the recent problems with drought conditions in western Rajasthan, we felt that it would be worthwhile to see whether solar disinfection might be a useful means of providing rural villagers with an alternative source of safe drinking water. The Wellcome Trust UK sponsored a project to evaluate the development and application of small-scale solar water treatment systems in India, including two field locations in Rajasthan, from 2001 to 2005.

Solar disinfection as a point-of-use water treatment system

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The use of sunlight is one of the oldest recorded methods of water purification in India, dating back to at least 2000 BCE. The ceremonial ritual would often involve placing water in a shallow bowl in direct sunlight and reciting a prayer at the start and the end of the day (Patwardhan 1990). However, it is only in more recent times that water purification by sunlight has been systematically evaluated. In the 1980s Aftim Acra and co-workers at the American University in Beirut proposed the practical application of sunlight for the disinfection of drinking water and oral rehydration solution (ORS) contaminated with waterborne pathogens, describing the process as "solar disinfection" (Acra, Karahagopian and Raffoul 1980). Their vision was that sunlight could be used to provide a low-cost, sustainable and simple method of treating contaminated drinking water in those developing countries with consistently sunny climates, under circumstances where people had no access to alternative water treatment systems.

At its simplest ("batch-process" solar disinfection), the method involves filling a transparent plastic or glass vessel with contaminated water and then keeping this vessel in full-strength sunlight for several hours, to inactivate pathogenic microbes. Using discarded plastic (PET) drinks bottles in laboratory and field experiments, research has demonstrated that solar radiation can inactivate a wide range of microbes, including fecal indicator bacteria such as E. coli, waterborne pathogenic bacteria such as those responsible for cholera and dysentery, and certain viruses and protozoal parasites. Continuous-flow solar water treatment systems have also been developed and evaluated, though these are more complex and sophisticated in design and operation (for a more detailed review of solar water treatment, see Reed 2004).

Solar disinfection is the result of two processes:

  1. thermal inactivation: absorption of solar infrared radiation can raise the temperature of illuminated water to the point where microbes are inactivated, as a result of thermal damage to heat-sensitive biological structures such as membranes. When this is carried out in a black-painted container or in a solar cooker, it is often termed "solar pasteurization," by analogy with commercial pasteurization. However, such temperatures are not always readily achieved during solar water treatment in a transparent bottle, especially outside the summer season.
  2. optical inactivation: ultraviolet and visible wavelengths of sunlight may be absorbed by photosensitizer molecules, found either within microbial cells (e.g. porphyrins and other pigments), or in the surrounding water (e.g. humic substances). The excited photosensitizer will then usually react with molecular oxygen, leading to the production of various reactive oxygen species including superoxide and hydrogen peroxide. These reactive oxygen species then interact with the macromolecular constituents of microbial cells, causing irreversible inactivation ("solar photo-oxidative disinfection").

It is generally accepted that optical inactivation due to solar UV radiation is the main component of solar disinfection. However, research has shown a synergistic interaction between optical and thermal inactivation at temperatures above 45°C (~113°F), where the combined optical and thermal effects act to inactivate bacteria at a faster rate than would be predicted from the effect of each factor in isolation (Reed 2004).

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Since the optical inactivation process involves the sunlight-driven production of reactive oxygen species, solar disinfection is strongly influenced by the level of dissolved oxygen in the water, being optimum only under oxygen-saturated conditions; however, this is relatively easily achieved by leaving a small air gap during filling, and then shaking the bottle (Reed 1997).

The optical inactivation process is also sensitive to water turbidity, being most effective with water of low-to-moderate turbidity (typically up to100 turbidity units). For water of higher turbidity, a three-stage process can be used:

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  1. Clarification - by sedimentation, filtration or coagulation/flocculation (in Rajasthan, the villagers sometimes add dried sand, known locally as "balu mitti," to reduce the turbidity of water with a high level of suspended solids before use).
  2. Oxygenation - by vigorous mixing of the container before exposure to sunlight.
  3. Illumination - in full-strength sunlight, for as long as possible (ideally, a whole day).

Optimizing the solar disinfection procedure prior to field trials

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An important aspect of our initial research was to design and evaluate a custom-made solar disinfection bottle, taking into account the following factors:

  • Size: one liter of water was selected as appropriate for individual use, especially for children, being readily transported to school or the workplace.
  • Shape: a flattened, rectangular cross-section was chosen, to provide the largest surface area (23 cm x 9.5 cm [~9 in x 3.7 in]) and minimum depth (5.2 cm [~2 in]) for sunlight penetration. The number of surface grooves and features designed to provide strength and rigidity to the reactor was kept to a minimum, to maximize transmission of sunlight.
  • Backing surface: trials were conducted with (i) standard, fully transparent bottles; (ii) bottles where the rear surface was painted black (to absorb solar infrared radiation and thereby enhance thermal inactivation); and (iii) bottles given a reflective backing (stainless steel holder) to return the sunlight through the water under treatment, thus boosting the optical inactivation.

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Laboratory trials, carried out at Cochin University of Science and Technology, Kochi, India, have compared the effectiveness of bottles with transmissive, reflective and absorptive rear surfaces by adding a suspension of Escherichia coli to pre-sterilized distilled water. Our results have shown that while the black-backed bottles were the most effective in inactivating fecal indicator bacteria under full-strength sunlight, a reflective backing was best under sub-optimal sunlight conditions (cloudy skies) where the rate of solar disinfection was slowest, showing around 25% enhancement in the rate of inactivation under low sunlight conditions, compared to transmissive and absorptive rear surfaces. As a result of these experiments, we decided to evaluate the custom-made 1-liter bottles with stainless steel reflective backings under field conditions in western Rajasthan, since it is important to have the best rate of inactivation under sub-optimal light conditions.

Field study

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The overall aims of the field-based research were to:

  1. investigate the practical application of solar disinfection as a means of providing a sustainable water treatment system where none was currently available;
  2. establish the effect of solar water treatment on the incidence of waterborne disease; and
  3. obtain feedback on the opinions of the end-users, including their level of satisfaction and their perceptions as to the advantages or drawbacks of the process.

Field locations

The villages of Tulesar Purohitan and Tulesar Charnan, lying within the rural interior of Jodhpur district approximately 30 km (~19 mi) east of Jodhpur city, were selected as suitable field locations to evaluate the household-level use of solar disinfection. Each village has a population of around 1500 persons. Tulesar Puohitan is mainly composed of members of the Rajpurohit caste, a vegetarian community that maintains links with other ruling and religious groups such as the Brahmins, while Tulesar Charnan mostly comprises the Meghwal caste, a non-vegetarian occupational group engaged in tanning/weaving and similar manual activities.

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Link to Figures 7a and 7b, ~57K

While each village has its own talab, both villages are connected by a pipeline that can provide safe, non-saline water from a borewell around 45 km (~28 mi) away. However, this pipeline also supplies 40 other villages along the route, and water is only available intermittently, due to frequent failures in the electricity supply and to lack of an adequate supply in the borewell. Typically, water is pumped to each village once every 10 days or so. In Tulesar Purohitan, the piped water is stored in a large village community tanka from where it is collected by bucket and rope, whereas in Tulesar Charnan the external supply is via a standpost and community tap on the edge of the village, which is supplemented by government water tanker every few days during the dry season. Regardless of the source, water is transported to the home in metal vessels, and then stored in earthenware matkas before drinking, especially in the summer months, when the surface water temperatures can exceed 50°C (~122°F).

The lack of a consistent supply of safe groundwater means that both communities would benefit from an effective treatment system for their local surface water. The villagers of Tulesar Purohitan maintain and utilize their talab to a greater extent than the villagers of Tulesar Charnan, with appropriate religious rituals/observances that give due regard to its traditional role as a source of water; thus, cattle dung is regularly removed from the immediate vicinity of the talab, and anyone found urinating or defecating in the catchment area is charged 50 rupees (just over US $1) as a penalty. In Tulesar Charnan, the people prefer to use the external water supplies (public standpost or government tanker), and only use the talab water when these external supplies are unavailable. In both villages, animals are allowed free access to the talab, to drink and bathe, creating further contamination and increasing the turbidity of the water.

We began our initial work in 2002, selecting families in each village for a year-long baseline study prior to the introduction of solar disinfection as part of a further year's follow-up study. However, the ongoing drought meant that all local talabs and nadis were dry, and the people were entirely reliant on additional supplies of clean water brought in by government tanker. The drought eased only in 2004, and a change in plan was required, due to the shortened timescale of the project: 40 families in each village were randomly assigned either to the treatment group, who were to use solar disinfection, or to the control group, who were to continue with their previous practices (20 families per group, with around 4 adults and 3 children per family). Each family in the treatment group received 12-15 custom-made 1-liter solar disinfection bottles and reflectors.

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Most of the families live in "pakka" (good quality) houses, soundly constructed with bricks and cement, with a tiled roof and a terrace, whereas the remainder are in "kachcha" (makeshift) houses, with clay walls and grass or brushwood roofs. Irrespective of the type of dwelling, all families had either a cemented terrace or a cleared area close to the dwelling where solar disinfection bottles could be kept in full sunlight for most of the day.

In all families, the women and children are responsible for collection and storage of water. In contrast with some other areas of India, all of the women in the villages are poorly educated and illiterate, with no women's groups and with tribal protocols that limit their interaction with outsiders. Consequently, their awareness of the various health issues associated with unsafe water is extremely poor.

Evaluation of field trials

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Monthly surveys of the incidence of gastroenteritis/diarrhea were carried out, alongside testing of the local water supplies (all talab and tanka water was found to be highly contaminated on every test occasion, whereas the piped water supply tested negative for bacteria contamination). Despite the turbidity and color of the surface waters, solar experimental studies carried out in the laboratory at Kochi using E. coli added to pre-sterilized water samples showed that the rate of solar inactivation of fecal indicator bacteria in such waters was broadly equal to that of non-turbid distilled water to which the same strain of E. coli had been added.

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Link to Table 3, ~5K

The families in the treatment group who were using solar disinfection to treat their drinking water reported substantially fewer incidences of gastroenteritis/diarrhea than the control group in both villages, with an overall reduction of around 70% across all seasons. There was also a widely held view among those in the treatment group that drinking solar treated water led to a greater sense of overall well-being and an enhanced ability to work, attend school, etc. In the later stages of the field trials, this view also spread to members of the control group, who felt a sense of exclusion since they had not been given the solar disinfection bottles.

Implementation of solar disinfection began in January 2004, which is the cool season in Rajasthan. At such times, the ambient daytime temperature is typically 20-25°C (~68-77°F), and people prefer to drink warmer water; thus the solar treated water could be consumed at the end of a day's sunshine, and was consistent with the traditional preferences of the local people. As the temperature increased towards the annual maximum of around 42-45°C (~107.5-113°F) in May-June, the water had to be kept indoors overnight in order to reduce its temperature; this reduced its appeal to some of the families, who were intermittent in their use of the bottles. However, many of the villagers in the treatment groups were still positive about the use of the system, since they could appreciate that a closed bottle also blocked the access of flies and other insects to the treated water. Some families developed their own procedures to reduce the temperature of the water prior to consumption, for example by keeping the bottles overnight in a half-filled earthen matka. Individuals who took the bottles to work or school often covered them with cloth to prevent the water from overheating on the following day, while others cooled them in shaded ditches.

A more significant drawback to the use of the solar water treatment bottles in summer was reported for those using unfiltered water directly from the talab; in such cases the treatment process gave the water a foul smell, presumably due to the decomposition of phytoplankton and other microflora. For those families who filtered their water before solar disinfection, it proved necessary to carry out cloth filtration three times before treatment, in order to reduce this problem.

Around half of the bottles were lost or damaged over the year-long implementation period; however, laboratory tests also showed that while the remaining containers were often badly scratched by sand and grit, they were still able to provide effective disinfection, even after a year of regular use. Several users commented that the 1-liter bottles supplied by the project were too small, and should be replaced by larger containers (e.g. 2-liter or 5-liter bottles). In addition, several villagers commented that because the containers were given entirely free of charge, they were not held in such high regard as they would have been if a small charge had been levied. Any follow-up work should investigate the idea of charging a nominal fee for the container and reflector, in order to increase its worth in the eyes of the end-users. Similar conclusions have been reached for other small-scale water treatment systems (UNDP-World Bank Water and Sanitation Program 1999).

Children were often found to be the most enthusiastic adopters of solar water treatment, taking responsibility for laying out the bottles each day, and then making use of the bottles at school. More recently, youth groups have been formed in both of the villages, and these have considerable promise for the continued implementation of solar water treatment. In hindsight, the use of a male researcher based outside the village created a barrier to communication with the women of each household, as face-to-face discussions could only be held in the presence of a village elder. A better strategy would be to work through a female worker sited within the village as part of a larger health awareness program, though this was outside the scope and funding of our project.


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Our work has shown that solar disinfection can be optimized under low sunlight conditions by using a custom-made container with a reflective rear surface.

Field trials in rural Rajasthan showed a measurable reduction in the incidence of gastroenteritis and diarrhea in a case-controlled trial over a 1-year period, demonstrating that solar disinfection can enable such households to widen their sources of treated drinking water.

The historical record of the use of sunlight for the traditional ceremonial purification of drinking water should assist its wider acceptance as a practical approach to water treatment in rural India, where sustainable implementation will be the next challenge.


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This research study was supported by the Wellcome Trust, UK and by Northumbria University.


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Acra, A., Y. Karahagopian and Z. Raffoul. 1980. Water disinfection by solar radiation. Lancet II:1257-1258.

UNDP-World Bank Water and Sanitation Program. 1999. Willing to pay but unwilling to charge. South Asia Field Note. Online:

Livingston, M. 2003. Temples for water: The stepwells of western India were a magnificent architectural solution to the seasonality of the water supply. Natural History May: 52-56.

Patwardhan., A. R.1990. Our water our life. Delhi: Council for Advancement of People's Action and Rural Technology.

Reed, R. H. 1997. Sunshine and fresh air: A practical approach to combat waterborne disease. Waterlines 15:27-29.

_____. 2004. The inactivation of microbes by sunlight: Solar disinfection as a water treatment process. Advances in Applied Microbiology 54:333-365.

Tandon, P., S. Chhibber, and R. H. Reed. 2005. Inactivation of Escherichia coli and coliform bacteria in traditional brass and earthenware water storage vessels. Antonie van Leeuwenhoek 88:35-48.


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Author information

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Professor Rob Reed holds a personal chair in Microbiology at Northumbria University, Newcastle upon Tyne, UK. Contact via email:

Professor Isaac Bright Singh is Reader in Environmental Microbiology, Cochin University of Science and Technology, Kochi, India, and Visiting Professor of Microbiology at Northumbria University, UK. Contact via email:

Mr. Shibu Mani is a Senior Research Associate, School of Environmental Studies, Cochin University of Science and Technology, Kochi, India. Contact via email:

Mr. Ranjit Kanjur is a Research Associate, School of Environmental Studies, Cochin University of Science and Technology, Kochi, India. Contact via email:

Mr. Neil Simon is a field-based Research Associate, A259 Shastri Nagar, Jodhpur 342003, India. Contact via email:

Miss Surekha Mol is a Research Associate, School of Environmental Studies, Cochin University of Science and Technology, Kochi, India. Contact via email:


Additional web resources

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Maps of Rajasthan
A broad range of maps, including physical features, agriculture, district maps (including Jodhpur region).

Rajasthan drought
Information and statistics from the UN disaster management team.

Rainwater harvesting
Details of traditional and contemporary approaches in India.

Solar disinfection
A comprehensive guide to solar water treatment, with technical notes, educational materials and case studies.

Guidelines for solar disinfection of water and oral rehydration solutions
Link to the UNICEF publication by Aftim Acra and colleagues

Household water treatment
A report/review of small-scale interventions and health gains of point-of-use water treatment systems, including solar disinfection

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