Contrary to belief, it is not necessary to boil water to make it safe to drink. Heating water to 65°C (149°F) for 6 minutes, or to a higher temperature for a shorter time, will kill all germs, viruses, and parasites. This process is called pasteurisation.
A simple method of pasteurising water is to put blackened containers with water in a solar cooker. The solar cooker reflects sunlight onto the container, which heats up the content. The cooker may be an insulated box made of wood, cardboard, plastic, or woven straw, with reflective panels to concentrate sunlight onto the water container. The box cooker should be frequently repositioned to ensure it is catching all available sunlight and never in shade. A thermometer or indicator is needed to tell when the required temperature is reached for pasteurisation to monitor the required exposure time of six minutes. Common devices for monitoring the water temperature use either beeswax, which melts at 62°C (144°F), or soybean fat, which melts at 69°C (156°F).
Effectiveness and Health Impact
As the water heats due to solar radiation, the increased temperature between 65° and 75°C (149-167°F) will kill or inactivate an important part of commonly waterborne pathogenic bacteria, viruses, helminths, and protozoa. But spores are more resistant to thermal inactivation than vegetative cells; treatment to reduce spores requires a thermal treatment up to boiling point and must ensure sufficient temperature and time. Furthermore, solar pasteurisation does not improve turbidity, odour, taste, colour, or chemical contamination.
In rural Kenya, a simple thermo indicator which changes colour at 70°C was applied to show household members when pasteurisation temperature had been reached. This increased the number of households whose drinking water was free of coliforms from 10.7 to 43.1% and significantly reduced the incidence of severe diarrhoea compared to a control group.
Solar pasteurisation has proven to be a very low-cost disinfection method to produce drinking water out of non-turbid fresh water. However, solar pasteurisation is not that easy to implement and monitor, thus it is not a widespread method for point-of-use water treatment. Heating the water to the pasteurisation temperature rather than the boiling point reduces the energy required by at least 50%. But solar pasteurisation is only effective if treated water is protected from post-treatment contamination during storage.
The solar pasteurisation method seems more suitable on household level rather than for producing high quantities of drinking water. Because it relies on solar energy its effectiveness depends on the daily hours of sunshine in the area of application. Solar pasteurisation might be an option on both, the village and household level, but household usage is more competitive because village-scale alternatives have much lower treatment costs. Existing solar devices have water disinfection costs that are an order of magnitude lower than boiling.
- The system requires no additional inputs (electricity, chemicals or fossil fuels) after installation
- Simple designs are available at very low cost, and this device may be built with parts available in most countries
- Anyone can be trained to construct a solar cooker and there are no specific manufacturing hazards
- Solar pasteurisation boxes can also be used as solar cookers for cooking meals
- Compared to boiling, the pasteurisation process does not consume wood, charcoal, or other biomass as energy supply (environmentally more sustainable) and does not take time and energy for its procurement
- Requires sunny weather and does not work during continuous rainfall, on very cloudy days, or under freezing conditions
- Recontamination is possible after the water has cooled because it contains no residual disinfectant; subsequent safe storage is essential
- Does not reduce turbidity, odour, taste, or colour and does not remove chemical pollutants from water
- Users require a thermometer or pasteurisation indicator device
- Water needs to cool down before use
- Cookers are made from lightweight and easily breakable materials