Frequently Asked Questions

Recent news articles relating to harmful chemicals leaching from plastic containers to the contained water or food have caused public concern. These concerns are valid in very specific circumstances, but the tone of many reports has often lead to exaggerated public fears for two reasons.

1. There are many different types of plastic, and the components of each plastic are very different.
2. Chemical leaching from plastics requires long periods of contact, high temperatures, or both. For the times and temperatures of contact common in day-to-day life, including the SODIS process, leaching is not a concern.

The chemicals that have been in the news lately because of leaching into water from plastics are Bisphenol A (BPA) and antimony. BPA is not present in PET bottles (the bottles used for SODIS), so it poses no threat at all to SODIS users. BPA is used in the manufacture of polycarbonate (PC), so it is in PC containers that BPA leaching can occur.

Antimony is present in minute quantities in PET bottles, and antimony does leach into water from PET. However, rigorous studies have proved that the level of antimony that leaches into SODIS bottles during treatment is not a health threat. The World Health Organization (WHO) publishes lists of the minimum levels that could possibly pose a health threat for different water contaminants. They are conservative in their guidelines, to be on the safe side. Antimony levels reached during SODIS treatment are well below the WHO’s guideline for drinking water.

The following is an excerpt from a letter to Water School, written by Dr William Shotyk, who has studied antimony leaching into water from PET bottles. Dr Shotyk is the world’s leading expert in this field.

“Without reservation, I fully endorse the work of Water School in bringing clean water to the people of Africa by placing contaminated water in PET plastic bottles for a few hours in the sun…. The extent to which antimony is expected to be released by a few hours of exposure to the sun is so small that it represents no potential harm to the consumer…. I very much hope that your important work will continue (Shotyk).”

The full letter from Professor Shotyk is available upon request.

There are a few other contaminants that can leach from PET bottles into water, but they too remain well below the WHO’S guidelines. There is no health risk from chemicals leaching into the water during the SODIS process.

Types of Plastic – It is important to note that there are many different types of plastic used in the production of food and beverage containers. Since recycling has become common in most countries, these different plastics are identified by a code inside a small triangle somewhere on the container, usually the bottom. This allows for proper sorting of the plastics to be recycled. The numbers in the triangle range from 1 to 7 which identify the plastic type. The plastic used for clear disposable plastic water bottles is almost universally PET (polyethylene terephthalate) which corresponds to #1 on the chart. PET is the plastic used in the bottles that Water School works with in the SODIS process in developing countries.

Plastic Additives – When molten plastic is processed and formed, small amounts of chemicals are added to give specific properties to the final product (clear or opaque, flexible or rigid, heat resistant, etc). Studies have shown that minutes quantities of these chemicals can be released into the contained liquid on a time/temperature based relationship. Longer contact times and higher temperatures result in the release of these chemicals in increasing amounts. The amounts released are in the parts per billion or parts per trillion ranges.

Scientists continue to disagree on the exact health impact of these minute quantities of chemicals but err on the side of caution, such as in the recent baby bottle concern. Plastic baby bottles were removed from the shelf in Canada when it was shown that placing hot liquid into the polycarbonate bottle extracted minute quantities of the chemical Bisphenol A from the container into the liquid. Even though there was no scientific evidence to support a negative health impact from the minute quantities measured, there was general agreement to remove polycarbonate baby bottles from the shelf.

The chemicals of concern are listed and explained below. In every case, scientific evidence is cited to show that the respective contaminant levels stay well below the WHO safe drinking water guidelines during SODIS treatment.

Bisphenol A and Chlorinated Hydrocarbons – Bisphenol A and chlorinated hydrocarbons are not used in PET bottle production and are therefore not leached in any amounts into liquids from PET containers.

Antimony – Antimony is used as a catalyst in the PET production process. It has been shown to leach into water in minute quantities (parts per trillion) at rates that vary with time, in the range of several months at room temperature (Shotyk et al) and with very high temperatures (Westerhoff et al). For the temperatures and durations relevant to SODIS treatment, antimony levels have been found to be far below the WHO drinking water guideline. In fact, the only test samples that even approached the WHO’s antimony guideline were held at 80°C for over 150 hours (Westerhoff et al) – this far exceeds the exposure temperatures and durations reached in SODIS.

Plasticizer Phthalate and Adipate – These plasticizers are components of some plastics which make them bendable and soft. They are not used in the production of PET, but cross contamination with other plastics can lead to low levels of plasticizers in PET. Several published experiments regarding the leaching of plasticizers into water from PET bottles during SODIS are described below.

In one test, water samples treated with SODIS at various temperatures were tested for phthalate (di(2-ethylhexyl) phthalate or DEHP) and adipate (Di(2-ethylhexyl)adipate or DEHA), and it was found that they had levels comparable to those found in water that had never been in contact with PET, which means that these samples were well below the WHO standards for plasticizers (Kohler et al).

Another experiment analyzed a number of samples that had and hadn’t been exposed to sunlight in new and used bottles. All of the samples remained well below the WHO standards (Montouri, P., et. al.).

Aldehyde and Formaldehyde – A study published in 2000 shows that formaldehyde leaches into the water from PET bottles, but for the durations of sunlight exposure used in SODIS, the formaldehyde levels remain far below the Swiss government’s formaldehyde standard. The study also demonstrates that aldehyde does not leach into the water (Wegelin, M., et. al.). (Swiss guidelines are cited here because the WHO does not give guidelines for formaldehyde or aldehyde, since they “[occur] in drinking-water at concentrations well below those at which toxic effects may occur.”)

Conclusion – None of the chemicals of concern reach the health-concern threshold levels specified in the WHO’s guidelines for drinking water quality during SODIS treatment. Therefore, chemical leaching is not a concern for SODIS users.

Contaminant WHO Drinking Water Guideline (parts per billion) Maximum concentration after SODIS treatment (parts per billion) Comment/Reference Bisphenol A not present in PET Chlorinated Hydrocarbons not present in PET Antimony 20 15 This concentration only reached under conditions much more extreme than SODIS (150 hours at 80°C) (Westerhoff et al) Phthalate 8 0.71 (Kohler et al) Adipate 80 0.46 (Kohler et al) Aldehyde Concentration does not change with SODIS treatment (Wegelin et al) Formaldehyde 15,000* 44 (Wegelin et al)

* No WHO Guideline given; Swiss guideline provided instead

References:

Kohler et. al., Migration of organic compounds from polyethylene terephthalate (PET) bottles to water. EAWAG EMPA. (2003).

Montouri et al., Assessing human exposure to phthalic acid and phthalate esters from mineral water stored in polyethylene terephthalate and glass bottles. Food Additives and Contaminants. (2007).

Shotyk at al., Contamination of bottled waters with antimony leaching from polyethylene terephthalate (PET) increases upon storage. Environ. Sci. Technol. (2007).

Shotyk. Personal correspondence with Robert Dell. (2007).

Wegelin et. al., Does sunlight change the material content of polyethylene terephthalate(PET) bottles? AQUA. (2000).

Westerhoff et. al., Antimony leaching from polyethylene terephthalate (PET) plastic used for bottled drinking water. Water Res. (2007).

Yes and no. Boiling water is fairly expensive, because it requires a lot of fuel, while SODIS is very inexpensive. Boiling may be more effective than SODIS, although both SODIS and boiling reduce the pathogen content to meet the WHO’s guidelines for water quality. The overall effect of both technologies is a significant reduction in water-related illness, but Water School teaches SODIS because it is so affordable.

One of the primary goals of development organizations like Water School is to find and implement “appropriate technologies.” This concept encompasses how well each technology works, how much it costs, whether the necessary supplies are available locally, whether it is culturally appropriate in a given region, etc. The Water School has concluded that SODIS is a more “appropriate technology” to fight waterborne disease than boiling water. These judgment calls are never entirely one-sided, and involve weighing positives and negatives. Some of the key factors in this decision are explained below.

Effectiveness – Boiling is an effective way to destroy pathogens in water. When done properly (by maintaining a rolling boil for 2 to 20 minutes, depending which expert you ask) boiling can kill most of the germs in the water, because these microorganisms cannot withstand the thermal and physical shocks. The proteins of a microorganism denature at high temperatures, and while germs can self-repair small “wounds,” much like people can, sustained boiling caused more damage than they can heal. A recent field study found that the drinking water of self-professed boilers in Vietnam met the WHO’s microbial guidelines for drinking water in 37% of cases, and was in the “low risk” category for another 38% (Clasen et al) – it is not perfect, but it is quite good.

SODIS is also effective at destroying pathogens, but it may be less effective than boiling. Details about the effectiveness of SODIS can be found in FAQ #18. However, water treated with SODIS meets the WHO’s microbial guidelines, so the difference between SODIS and boiling is fairly small – both processes kill over 99.9% of the germs. SODIS has the added benefit that the water can be consumed directly from the container where it was treated, whereas boiled water is generally transferred to at least one secondary container before consumption, increasing the likelihood of recontamination.

Cost – SODIS is very inexpensive because the only supplies needed are a SODIS table and some emptied plastic bottles. In fact, even the SODIS table is not strictly required, and in some areas where The Water School works SODIS users have come up with innovative, lower cost alternatives. Water School encourages this type of adaptation not only because it minimizes costs, but it also helps the users gain “ownership” of the technology as their own way to treat their water.

Boiling water is quite expensive because it requires a lot of fuel (1 kg of wood is required to boil 1 litre of water). In many developing regions firewood is sparse and costly, having been collected at unsustainable rates for many years. Where stores offer propane canisters or other gaseous fuel, it too is expensive. It is difficult enough for many families to obtain enough fuel for cooking, without the added burden of boiling water.

Effect on the environment – A secondary factor is the effect that each treatment method has on the environment. Boiling requires a fire that can bring several litres of water to a boil and hold it there for as long as 20 minutes. When many families in a region do this several times a day, it adds up to a lot of pollution, which is often released directly into peoples’ homes.

SODIS produces no pollution, and in fact reuses plastic bottles that would otherwise be discarded. Water School is considering adding another stage of recycling as well, because even when the bottles are no longer fit for SODIS they can be put to good use in other innovative ways that further help the families.

Other factors – There are a number of other problems with boiling water. Families that boil the water have to watch the fire closely, because it is common for children to get burned. Boiled water has a different (“flat”) taste that many people do not like, while SODIS does not change the taste of the water. However, boiling is a faster way to treat water than SODIS, although the time to collect fuel (or to earn the money to pay for it) and the time it takes for the water to cool both add to the net time requirement of boiling water.

Having considered these and other arguments, Water School has chosen to teach SODIS rather than boiling water. However, Water School does not target communities that already have effective and appropriate means of treating their water, which is sometimes the case where boiling is a viable option (for example, where firewood is readily available and affordable).

Water School is a teaching and water treatment organization. If a circumstance arises where SODIS is not the most appropriate approach to reduce a region’s water contamination problems, The Water School will investigate other technologies to use.

References:

Clasen et al., Microbiological effectiveness and cost of boiling to disinfect drinking water in rural Vietnam. Environ. Sci. Technol. (2008).

For SODIS to work, light has to be able to penetrate the water. The “finger test” is used to check the water’s clarity: if you can count your four fingers through the water-filled bottle, it is clear enough for SODIS treatment.

If you cannot count your fingers, the water needs to be clarified either by settling out the suspended dirt particles, or by passing it through a cloth filter. In Water School’s experience, these measures make the water clear enough for SODIS in almost every case.

Because SODIS relies on UV light reaching the water, and light cannot penetrate water that is cloudy, SODIS requires that the water be fairly clear. Furthermore, crevices in small dirt particles can act as hiding places for pathogens, so cloudy water often has germs that cannot be reached with sunlight.

The official position of most SODIS researchers is that the turbidity should be less than 30 NTU (nephelometric turbidity units; a measure of light scattering due to suspended particles), but this has little practical value in the field. “The “finger test” that Water School teaches is this: Hold the filled bottle with your fingers spread behind it: if you can count your fingers through the bottle the water is clear enough for SODIS.” What to do when the water is not clear enough? Many people stop at this point and declare the water unfit for treatment with SODIS. However, in almost every case if the water is left to stand, a layer of mud will settle to the bottom within a very few hours and the rest of the water will be clear. This should be done in a larger container, like a jerry can, so that the user can then draw some clear water off the top for SODIS treatment, leaving the mud undisturbed at the bottom of the settling container.

In very rare cases settling does not work, possibly because the suspended particles are very light or electrically charged, and thus repel each other, rather than accumulating on the bottom. If this is the case, a simple cloth filter is used. This can be done by simply folding a piece of cloth so that it is 4 or 8 layers thick, and then pouring the water into a can or jar through the cloth. Through physical straining, the particles get caught in the cloth, and the water is made clearer. In its years of experience Water School has found that it is extremely uncommon for these methods not to make the water clear enough for SODIS.

UV light is absorbed more easily than visible light – in fact, most of the UV from the sun is absorbed in the earth’s atmosphere, and only a small fraction of it reaches Earth. So even though visible light can penetrate water very easily, most of the UV light that reaches the bottle is absorbed within the first 10 cm (4 inches) of water. Of course this depends also on the clarity of the water, but we have found that this is a reliable generalization when the “finger test” is used (see FAQ #3).

For this reason, we recommend that bottle sizes up to 2 litres be used for SODIS. For practical reasons, a 1 litre container seems to be an optimum size as it is easy for a child to hold and drink from using one hand, and because 1 litre bottles are widely available. Half litre bottles are also plentiful and acceptable for SODIS.

We recommend that if at all possible, SODIS users drink the water directly from the bottle, rather than pouring it into a cup or another container before drinking it. Every time the water contacts another surface (a different container, dirty fingers, etc) there is a risk of recontamination. This risk is minimized when the water is consumed directly from the SODIS bottle.

Just as we still receive some visible light from the sun on cloudy days, we receive some UV light through the clouds as well. This is shown by the fact that we can still get a sun burn when it is cloudy, since sunburns are caused by UV light. About half of the sun’s UV light is absorbed by clouds, and the other half still reaches Earth. For this reason we recommend that SODIS bottles be exposed for one full sunny day or two full cloudy days. Our experiments and others have shown this to be a conservative timetable for SODIS to work. If it is a totally rainy day, we ask people not to count that day, even though evidence suggests significant microbial reductions on rainy days as well. On rainy days, rainwater can be collected and consumed.

The easy rule is that SODIS works in a “35 degree latitude window,” either side of the equator. This is the region of the earth that receives the most direct sunlight. These boundaries include almost all the other regions where waterborne disease is a serious problem.

Since the early days of SODIS it has been widely accepted that SODIS only works in the “35 degree window,” that is, the region between 35° North latitude and 35° South latitude. There is plenty of evidence that the “middle” of our planet gets the most direct sunlight (which is, of course, intuitively obvious), but there has been very little investigation into the relative effectiveness of SODIS at different latitudes. It is known that latitude is not the only factor effecting solar intensity (geographic and climactic factors, as well as altitude are also important), and that solar intensity is not the only factor effecting SODIS (temperature can be an important parameter as well), so the 35° window is probably a simplistic approach. In fact, there is evidence that even at latitudes north of 40°, SODIS can be effective, at least in the summer months. Sunlight intensity in Toronto on June 21 is higher than at the equator.

These details however, are not really important in Water School’s day to day work, because the 35° window includes the complete continent of Africa, and almost all of the places in the world where children suffer from waterborne disease. SODIS can be used with confidence in these regions. If a decision is made to use SODIS outside this latitude range, investigations should be conducted at that time.

SODIS has been found to work faster above 45°C, and even three times as fast when the water exceeds 50°C (Berney et al). However, it is rare for ambient temperatures to get this high, and because SODIS bottles do not absorb much infrared radiation (which is the form of light that causes an increase in temperature) the water does not usually reach this threshold. For these reasons it is uncommon for this temperature factor to play a role in actual SODIS treatment.

Water School has done a number of tests in Kisoro, Uganda, which is a mountainous area. In all of these tests the water temperature never approached 50°C, yet 100% kill of E. coli (the standard test organism) was observed. The highest temperature was 43°C and in most cases it was in the low 30′s. The important thing is that SODIS is effective at the temperatures we experience in this area. It can be even more effective at higher temperatures, but does not impact our current projects.

Some have recommended SODIS bottles should be painted black on one side to increase the heat absorption from sunlight. Our experience does not support this recommendation. It adds another step and detracts from sustainability. This issue is addressed more fully in FAQ#10.

References:

Berney et al., Efficacy of solar disinfection of Escherichia coli, Shigella Flexner, Salmonella Typhimurium and Vibrio cholera. J Appl Microbiol. (2006).

Currently worn out SODIS bottles are returned to whatever disposal stream is locally available. This usually means going out with the garbage, which is where the bottles were headed before they were recycled for SODIS use. A bottle that was previously disposed of after one use, can now last up to one year and 80 fillings. SODIS adds substantially to the bottles’ useful life, but it does not keep them from ultimately ending up in the waste stream.

Water School is investigating how to better dispose of the used plastic bottles after SODIS. In some areas, they are now filled with water and inserted upside down into the ground at the base of newly planted saplings to irrigate the roots. They may also be cut in half and used to plant seedlings or used as construction materials.

Where reuse is not realistic, the bottles could possibly be manually crushed and containerized, to be sent to central recycling facilities.

There are inorganic salts in most water (iron, calcium, magnesium, etc). SODIS destroys the water’s biological contaminants (which is by far the biggest problem), and has no effect on these inorganic chemicals. Some people are also concerned that treating their water will change its taste, but SODIS also has no effect on the taste of the water.

SODIS does have a few minor effects on water chemistry, but they are inconsequential to the health or satisfaction of the user. SODIS disinfects through photochemical reactions (light rays provide the activation energy for reactions in the water) to kill germs. These reactions pose no health threat to the user.

Another chemical effect of SODIS is that minute amounts of chemicals from the bottles can get into the water. These are not a health threat either. A full explanation of this phenomenon is provided in FAQ#12.

Some people believe it is important to paint the bottles black on one side to absorb heat energy, but our experiments found that this does not help the SODIS process, and it is an unnecessary complication that makes SODIS difficult to spread.

There is a widespread belief that SODIS bottles should be painted black on one side, to trap heat and increase the temperature. Water School’s experimentation suggests that this does not affect the efficiency of treatment. Reports from Nepal and Bolivia suggest that painting the bottles can actually decrease the effectiveness of SODIS.

We strongly recommend that the bottles not be painted, because any added complication makes the technology much more difficult to implement. The strength of the SODIS process is its simplicity. Water School has encountered a number of regions where SODIS is not used simply because there was no black paint available. Even where the paint is available, people are more likely to continue using a method that is simple and convenient, so having them paint the bottles is not helpful. It adds an unnecessary step to a simple process.

We have been asked why we don’t teach people to put SODIS bottles on their roofs instead of on SODIS tables. Children can only learn, use, and teach SODIS when they can reach the bottles, and children are Water School’s best students and teachers. SODIS tables are also more effective for a number of reasons. And furthermore, using SODIS tables helps the technology by word of mouth.

Water School has found that the most effective way to teach SODIS is to partner with schools. Children learn SODIS in class and practice it at school, and they teach their families at home as well, especially when their parents notice their improved health. Children cannot reach bottles on the roofs of their schools or homes, so The Water School uses SODIS tables instead.

The metal SODIS table was first used because it was shown that the steel would reflect the light back through the bottles, so we would get two “passes,” and hence somewhat faster disinfection. Water School did some tests which suggested that the use of a corrugated steel SODIS table may make the treatment process work as much as 20% faster, but at the end of the day, the exposure times recommended (1 full day in the sun) are conservative and the impact from the corrugated metal is small.

The scientific benefit of the steel tables may be questionable, and SODIS tables are an added cost for the end users, but we continue to use SODIS tables for a number of reasons:

1. They are a very good way to hold the bottles. The bottles lie in the curves of the corrugated metal, so they cannot roll around or fall.
2. It is easy to position a SODIS table in a location that receives direct sun all day, whereas one cannot reposition a roof if it lies in the shade of a tree or another building.
3. Corrugated steel is available and inexpensive all over the world, because it is a very common building material.
4. The SODIS table becomes a conversation piece that neighbors gather around, and the knowledge of SODIS spreads. This is particularly effective when the neighbors realize that the children in SODIS families are no longer getting sick – they want to know why! We have found that for every family that learns about SODIS from Water School’s programs, two or more families start to use it from word-of-mouth. That’s good leveraging! So the SODIS tables help increase our audience and spread the word more widely.
5. It is easy for SODIS users to develop their own versions of the SODIS table, and this helps them gain “ownership” over the technology as something they have adapted for their specific needs and resources. Recently we have seen SODIS tables made with scavenged bamboo and even rope.
6. We believe that when a family owns a SODIS table, SODIS becomes a more permanent fixture in their family’s lifestyle and culture – more so than if they just lay bottles on the roof or on the ground. This small investment adds some weight to the family’s commitment to treating their water with SODIS.

PET (polyethylene terephthalate) is the most common material for plastic water bottles because it is inexpensive, durable, and transparent. Because of the popularization of bottled water in recent years these bottles are available all over the world.

Unlike other plastic bottles, such as those made from PC (polycarbonate), PET bottles have very thin walls, so more UV light can penetrate to the inside. Also, some plastics leach hazardous amounts of chemicals into food or water that they contact, but this is not a concern for PET during SODIS treatment (see FAQ#1).

Glass bottles are thicker, allowing less UV light into the bottle, are heavier and are breakable. PET SODIS bottles can be used for about a year before they become scratched to point of needing replacement.

Some early procedures called for the water to be shaken in the bottle to trap oxygen. We have found this step to cause no measurable improvement. The strength of SODIS is in its simplicity and we do not recommend this step be used.

Many believe that SODIS bottles should be filled partway, shaken, and then filled to the top, to increase the oxygen content of the water before treatment. Our tests have showed that this is not necessary – in fact, it has no effect that we have been able to detect.

Oxygen is a key ingredient for the photochemical reactions that kill germs in the water during SODIS (Reed, R. H). However, the water that is used for SODIS has generally had plenty of air contact, so it has sufficient oxygen content already, without our help. Oxygen is constantly diffusing from the air into the lakes and rivers where drinking water is collected. Even the act of pouring water into a bottle stirs it up and adds air bubbles, which infuse the water with oxygen while they rise to the surface. Additional oxygenation is not necessary.

Water from certain sources (such as deep wells and springs) does not have much oxygen at all, but these sources also have little or no biological contamination– microorganisms cannot survive without oxygen! Possibly the strongest argument we can give is this: Water School has had excellent results without teaching anyone to shake water in the SODIS bottles.

One of the key principles of Water School’s work is that simplicity is crucial. Evidence is abundant to show that unnecessary complexities kill development projects. This is one of the primary reasons that we choose to teach SODIS– it is a simple and easy to teach technology. We need very little in the way of supply chains, training modules, complex strategies, etc, and SODIS users only need to be taught a few basic principles to perform SODIS effectively, and even to teach others. For this reason, we do not teach extra steps like shaking the bottles or painting them half black, which have little or no added benefit.

References:

Reed, R. H., Solar inactivation of faecal bacteria in water- the critical role of oxygen. Lett. Appl. Microbiol. (1997).

Water School has been very successful by partnering with school teachers, who are naturally motivated, capable, and enthusiastic to teach SODIS to their students. We have found that this is an effective entry point for our community projects, with SODIS use spreading quickly to the children’s families and communities. This is the advantage of spreading knowledge instead of a product.

Water School’s implementation model is this: Children learn SODIS in class and practice it at school, under the guidance of their teacher, who has been trained by Water School’s staff. Their parents are given the opportunity to come and learn SODIS at the school as well, or else the children can tell their parents at home about what they have learned. Even skeptical parents generally “buy in” when they see their children’s health improving, and families can get SODIS tables and bottles for their own use. Once the children are using SODIS at school and at home other families in the area find out about SODIS. The Water School has found that for every family we teach, two or more families start using SODIS through word of mouth. It is a simple technology that does not require everyone to be trained directly. This encourages grass-roots growth.

We introduce SODIS through the schools for a number of reasons:

1. Water School does not spread a product, we spread knowledge. This enables us to build on the strength of the education system, rather than creating our own structures and facilities.
2. Teachers are a tremendous asset. They tend to have more education than other people in a village, and are respected in their communities. Teachers are very interested in the children’s health, and they can see the benefit of SODIS very clearly because it is directly reflected in school attendance and performance.
3. Children come to school to learn, so this is a natural place to teach them about SODIS.

By teaching children directly we ensure that the next generation of parents, teachers, and leaders understands SODIS. 5. Children are the people most affected by waterborne disease, so it makes sense to teach them how to fight it.

Water School recommends that people leave their SODIS bottles out from sun-up to sun-down for one day if it is sunny and two days if it is cloudy. We recommend that entirely rainy days not be counted but that rain water be collected and consumed on these days.

It has been shown that on a sunny day SODIS can make the water safe to drink in 3 to 5 hours (EAWAG/SANDEC). However, many people do not have an easy way to measure hours, and because the bulk of the UV radiation for the day is delivered between 10 am and 2 pm, it is much safer to have people leave the bottles out for a whole day. This also gives us a “factor of safety” by exposing the bottles a little longer than the minimum allowable time, and it gives the water a chance to cool down a bit as evening approaches, which is important because nobody likes to drink warm water. If we were to teach a “6 hour rule” it is possible that some people would put the water out from, say, 6:00 am to noon, instead of 10:00 am to 4:00 pm or so. The water would get a lot less irradiation than it needs and it would be unpleasantly warm when the person would drink it at lunch time.

Furthermore, the simplicity of our “one day/sunny, two days/cloudy” rule allows SODIS to be passed on by word of mouth without much risk of being misunderstood.

References:

EAWAG/SANDEC. SODIS news no. 1. Internal Report. (1997).

SODIS is very effective at killing bacteria, viruses, and protozoa that are not in the cyst stage (Boyle et al; Dejung et al; Berney et al; Wegelin et al). These make up the great majority of waterborne disease-causing microorganisms, so by killing these germs SODIS greatly reduces sickness. For more details, see FAQ #18.

Cysts are more resistant to most threats to the organism, including SODIS. Studies suggest that when organisms are in the cysts stage of development SODIS does not harm them, although at temperatures above 50°C they do appear be susceptible (Feachem et al).

References:

Berney et al., Efficacy of solar disinfection of Escherichia coli, Shigella flexneri, Salmonella typhimurium and Vibrio cholerae. J. Appl. Microbiol. (2006).

Boyle et al., Bactericidal effect of solar water disinfection under real sunlight conditions. Appl. Environ. Microbiol. (2008).

Dejung et al., Effect of solar water disinfection (SODIS) on model microorganisms under improved and field SODIS conditions. J. Water SRT- Aqua. (2007).

Feachem et al., Sanitation and disease, health aspects of excreta and wastewater management. John Wily & Sons. (1983).

Wegelin et al., Solar water disinfection: scope on the process and analysis of radiation experiments. J. Water SRT-Aqua. (1994).

We can get skin cancer from too much exposure to sunlight, because solar UV radiation causes small but important changes in our genetic material. Every person has about one quadrillion cells, each with a complete copy of his or her DNA. Most microorganisms, however, have just one cell and one copy of their DNA, so they are more susceptible to UV radiation. For this reason UV light from the sun kills microorganisms, or “inactivates” them (stops them from reproducing, which renders them unable to infect people), very effectively.

Sunlight also causes reactions in the water that make the water uninhabitable for microorganisms. These reactions do not affect the person who drinks the water.

SODIS is not yet thoroughly understood. However, it is believed that the following mechanisms play a role in the process. First, UV light is absorbed by DNA, which causes thymine bases to bond covalently, forming dimers. These thymine dimers terminate the DNA replication process prematurely (Acra; McGuigen). Furthermore, incorrect repair of thymine dimers can cause genetic mutation (Raven). The DNA absorbance of UV light is strongest in the UV-C region, peaking at about 265nm (Setlow), but a sufficient dose of UV-A light, which ranges from 320-400nm, can still inactivate microorganisms.

Second, when the water’s naturally occurring dissolved organic matter absorbs UV light, photochemical reactions produce highly reactive species, such as hydroxyl radicals (OH), superoxides (O2-), and hydrogen peroxide (H2O2) (Reed; Stum).These oxidize cellular components of microorganisms, thereby damaging or killing them (McGuigen et al; Reed).

Third, red and infrared light is absorbed by the water, which raises the water temperature. Beyond the maximum growth temperature, additional heat causes denaturation, impeding protein function and often killing the organism (Brock et al). This heat energy has a synergistic effect with the UV mechanisms at and above 45°C (McGuigen et al). When the temperature exceeds 50°C only one third of the fluence is required for SODIS to work effectively, compared to solar disinfection at lower temperatures (Wegelin et al).The rule of thumb is that below these temperatures 3 to 5 hours of solar radiation above 500 W/m2 is adequate to render microorganisms inactivated (EAWAG/SANDEC).

Further research into the inactivation mechanisms is ongoing. It has been shown that the primary damage of E. coli cells due to SODIS treatment effects cytoplasmic membrane transport processes (EAWAG). Clearly this is a distinct mechanism from the DNA damage described above, although little is known about the specifics of this transport interruption mechanism. Future investigations will use a variety of methods to gain insight into the way the cells are injured and killed, including determining the ATP content of cells, traditional plating procedures, and methods that use flow-cytometry (EAWAG). The hope is that by detecting damage at the protein, lipid, and DNA levels the inactivation mechanisms will become clear.

References:

Acra et al., Water disinfection by solar radiation: assessment and applications. International Development Research Center. (1984).

Brock et al., Biology of microorganisms. Prentice Hall. (2000).

EAWAG/SANDEC. SODIS news no. 1. Internal Report. (1997).

EAWAG. Solar disinfection of drinking water. Accessed from the World Wide Web on January 15, 2009. http://www.eawag.ch/…

McGuigan et al., Solar disinfection of drinking water contained in transparent plastic bottles: characterizing the bacterial inactivation process. J. Appl. Microbiol. (1998).

Reed. Sol-air water treatment. 22nd WEDC Conference Discussion Paper. (1996).

Setlow. The wavelengths of sunlight effective in producing skin cancer: a theoretical analysis. Proc Natl. Acad. Sci. (1974).

Stumm et al., Chemical equilibria and rates in natural waters 3rd Edition. Aquatic Chemistry. (1995).

Wegelin et al., Solar water disinfection: scope of the process and analysis of radiation experiments. J Water SRT – Aqua. (1994).

SODIS reduces bacteria by 99.999% and viruses by 99.9 – 99.99%. It is also effective in treating protozoa, except in the cyst stage of their lifecycles. A Water School SODIS project in Uganda reduced the incidence of diarrhea from 42% – 13% during the testing period.

Extensive experimentation shows that SODIS kills more than 99.999% of bacteria (Boyle et al; Dejung et al; Berney et al) and between 99.9 and 99.99% of viruses (Wegelin et al). These are the most common disease-causing microorganisms. The challenge is with protozoa that become cysts as part of their lifecycle (although it does work during the other stages of development), in which case SODIS can still work after longer exposures (McGuigan et al) or at temperatures above 50°C (Wegelin et al).

Perhaps a more meaningful measure is the effect that Water School’s programs have had on the regions where there they were implemented. Twenty years ago in Kisoro, Uganda, water borne disease was so bad that a hospital was built and known as the dysentery hospital. The hospital expanded over the years and the dysentery wing remained. Water intervention programs including rain water harvesting and SODIS caused a dramatic decrease in waterborne disease over the 6 year period from 2001 to 2007 and the dysentery wing of the hospital was permanently closed.

In 2008 EAWAG (the Swiss Federal Institute of Aquatic Science and Technology) sent a representative to Uganda to do a third-party impact study of The Water School’s work. They found that, with Water School’s training, the incidence of diarrhea dropped from 42% to 13%, and school attendance increased from 43% to 78% (EAWAG).

References:

Berney et al., Efficacy of solar disinfection of Escherichia coli, Shigella flexneri, Salmonella typhimurium and Vibrio cholerae. J. Appl. Microbiol. (2006).

Boyle et al., Bactericidal effect of solar water disinfection under real sunlight conditions. Appl. Environ. Microbiol. (2008).

Dejung et al., Effect of solar water disinfection (SODIS) on model microorganisms under improved and field SODIS conditions. J. Water SRT-Aqua. (2007).

EAWAG. Draft report about Health Impact Study in Kisoro district in May 2008. Internal Report. (2008).

McGuigan et al., Batch solar disinfection inactivates oocysts of Cryptosproidium parvum and cysts of Gardia muris in drinking water. J. Appl. Microbiol. (2005).

Wegelin et al., Solar water disinfection: scope of the process and analysis of radiation experiments. J. Water SRT – Aqua. (1994).

Clean water is crucial to overall health, but hygiene is also important. Studies have shown that improved drinking water quality and hand-washing with soap decreases deaths due to diarrheal disease by 50%, and safe disposal of excreta decreases it another 30% (Esrey et al; Hutley et al).

For these reasons Water School includes sanitation education in our SODIS training. When we teach a family that germs in their water make them sick, it is easier to help them understand how to avoid the other ways that germs infect them, such as through dirty hands and contaminated food.

It is doubly important for us to teach proper sanitation because if people are taught SODIS but have poor sanitation, they will still get sick. They will lose faith in SODIS, and will have continued poor health.

References:

Esrey et al. Effects of improved water supply and sanitation on ascariasis, diarrhoea, dracunculiasis, hookworm infection, schistosomiasis, and trachoma. WHO. (1991).

Hutley et al. Prevention of diarrhoea in young children in developing countries. WHO. (1997).

A recent study of SODIS in Bolivia has attracted substantial media attention for its negative findings on implementation of the SODIS process. The investigators reported that the 19% reduction of diarrhoeal diseases was not significant. The intervention group included both SODIS users and non-users, and the reported rate of SODIS use was only about 30%. The actual consumption of SODIS-treated water in the household may have been as low as 14%. It is also not clear how much SODIS water versus untreated water actually was consumed by the small number of user households.

The report concludes:

“Despite an extensive SODIS promotion campaign we found only moderate compliance with the intervention and no strong evidence for a substantive reduction in diarrhoea among children. These results suggest that there is a need for better evidence of how the well-established laboratory efficacy of this home-based water treatment method translates into field effectiveness under various cultural settings and intervention intensities. Further global promotion of SODIS for general use should be undertaken with care until such evidence is available” (Mausezahl et al)

An estimated 3 million people in 30 countries are currently using SODIS to reduce their risk of water-borne disease. This includes over 100,000 users from Water School interventions in Uganda and Kenya. Numerous studies have reported positive health benefits of SODIS when it is correctly and consistently used. In one such study, the incidence of cholera during an epidemic in Kenya was 88% lower among SODIS users than non-users.

Implementation procedures for SODIS are very important. We continue to recommend that education of SODIS users should focus on establishing the regular practice of the method and also on eliminating the consumption of any untreated and contaminated water. Success is also dependent on strong “buy-in” by local leadership before the implementation begins as well as an integrated teaching program on health and sanitation issues (washing hands, protection of food, etc).

The most important question raised in the Bolivian study is this: why was there such a small user uptake of the process (30%) in spite of what is described as “an extensive SODIS promotion campaign.”This is in contrast to the experiences of SODIS implementations in many other countries with other organizations.

References:

Conroy, R. M., Meegan, M. E., Joyce, T., McGuigan, K., Barnes, J. Solar disinfection of drinking water protects against cholera in children under 6 years of age. Arch Dis Child .(2001) http://adc.bmj.com/cgi/content/abstract/85/4/293

EAWAG. SODIS Impact Studies http://www.sodis.ch/Text2002/T-HealthImpact.htm

Solar disinfection of water for diarrhoeal prevention in southern India http://adc.bmj.com/cgi/content/abstract/91/2/139

Mausezahl, D., Christen, A., Duran Pacheco, G. Alvarez Tellez, F., Iriarte, M., Zepata, M. E., Cevallos, M., Hattendorf, J., Daigle Cattaneo, M., Arnold, B., Smith, T. A., Colford, J. M. Jr. Solar Drinking Water Disinfection (SODIS) to Reduce Childhood Diarrhoea in Rural Bolivia: A Cluster-Randomized, Controlled Trial. PLoS Med. (2009)

http://www.plosmedicine.org/article/info%3Adoi%2F10.1371%2Fjournal.pmed.1000125