SODIS

Solar Water Disinfection (SODIS) is a simple water treatment technology that can be used at household level. As a point-of-use water purification method, SODIS improves the microbiological quality of drinking water with solar radiation at almost zero investment and maintenance costs. Today, SODIS is one of the recommended household level water treatment technologies by the World Health Organisation (WHO).

PET (Polyethylene terephthalat) bottles or plastic bags with a volume of 1 to 2 litres are filled with water of low turbidity and exposed to the sun for a certain time, depending on the local weather conditions. The SODIS reference centre (www.sodis.ch) recommends, that 6 hours are sufficient under bright to 50% cloudy sky, whereas 2 days of exposure are required for 100% cloudy sky (OATES 2003, EAWAG/SANDEC 2006). The treatment efficiency can be improved if the plastic bottles are exposed on sunlight-reflecting surfaces such as aluminium- or corrugated iron sheets (MEIERHOFER 2002). During periods of continuous rainfall, SODIS does not perform satisfactorily and rainwater harvesting is recommended during these days.

UV-B light (280 to 320 nm) can directly be absorbed by DNA, leading to damage, but only 1% of natural solar light is of this type. Hence, sunlight used during the SODIS process consists mainly of UV-A light (320 to 400 nm), which makes 6% of the total solar light. This light can be absorbed by molecules, which are naturally present in the water. When these molecules (called photosensitizers) have absorbed the energy, they can pass it to the oxygen in the surrounding water, leading to the formation of reactive oxygen species (ROS). ROS include free radicals such as the superoxide anion as well as non-radicals such as hydrogen peroxide. ROS damage the membranes proteins, enhancing the oxidative stress of the cell and accelerating their aging process (BOSSHARD, 2009). But ROS can also lead to some DNA damage, such as single strand breaks and nucleic base modifications, which may be lethal and mutagenic. This type of pathogen inactivation Is called photooxidative stress. Photo- because it is the light which generates ROS, and oxidative stress, because these reactive oxygen species (ROS) lead to stress.

When the water temperature reaches more than 45°C, a synergistic effect of UV-A radiation (optical inactivation) and infrared light (thermal inactivation) is observed. At increased water temperatures, the exposure time needed to disinfect the water is reduced. But during cloudy weather conditions, the solar infrared light is reduced and bottles need to be exposed for 2 days to guarantee that the radiation dose received is sufficient to disinfect the water.

In contrast to infrared light (heat), UV-A radiation is only slightly attenuated by clouds. But turbidity in the water reduces the amount of UV radiation that penetrates the water. Therefore, the containers used for SODIS should be relatively flat (i.e. not exceed a depths of 10 cm). Water turbidity can easily be determined: Place the full bottle on a newspaper headline (the letters should have a size of about 1.5 cm) and look through the bottle from top to bottom. Water turbidity is less than 30 NTU if you can read the letters through the water. If not, the water must be filtered first so that SODIS will work.

Suitable containers for SODIS are PET or glass bottles. Ordinary window glass is almost opaque to UV light and cannot be used to construct SODIS containers. The use of PVC bottles is not recommended. It is normally marked on the bottle whether it is PET or PVC, however, the labels vary from country to country. PET and PVC bottles are normally marked accordingly. The labels can vary from country to country, though. If the bottles are not marked, you can only tell the difference between the bottles by setting fire to them. PET burns quickly and easily when it is held in a flame. When it is taken out of the flame, the fire goes out slowly, or it may keep burning. The smoke smells sweet. PVC does not burn easily. The material does not burn at all when not in the flame. PVC smoke smells acrid. Heavily scratched or old PET bottle should be replaced regularly as the scratching makes them opaque.

Today, over 4 million people of the world do drink SODIS treated water. To establish a successful application of SODIS, it is important to implement a comprehensive education and training program aiming at creating awareness and establishing a sustainable behaviour change accordingly. Important factors that support project success are: the local availability of PET-bottles and interventions that support the take-up of a certain behaviour, such as education and awareness creation (for example through using participatory methods such as PHAST), motivation and the use of convincing & emotional arguments. Tools applied in this phase are training through motivated health workers of groups and individual households, dissemination of information materials, demonstrations, demonstration of water quality tests, fun during educational events (songs, theatres, etc.), and education in schools. Generally, one year after project implementation, 20-80% of the trained people use SODIS on a regular basis (MEIERHOFER 2009).

After the practice has been established in a community, it is necessary, to implement interventions that support the continued practice of the behaviour and establish a habit such as regular household visits, tools that remind people of treating the water, mass media campaigns, regular application in schools etc.

The health impact of SODIS has been evaluated during several epidemiological health impact studies in various countries, which demonstrated that using SODIS for water treatment could reduce the diarrhoea incidence of children below five years of age between 16 and 88% (CONROY 2001; ROSE 2006, HOBBINS 2004, GRAF 2008; MAEUSEZAHL 2009). The health impact that can be achieved by a SODIS intervention depends on the following factors:

• The initial (microbial) quality of the water source: the impact is higher if the intervention community consumed highly contaminated water prior to the intervention.
• The general sanitary situation and hygiene practices in the community (e.g. presence of latrines and hand washing with soap): Diarrhoeal diseases are not only transmitted through drinking water. Therefore, the health impact of a drinking water intervention is limited if a community does not have latrines and proper hygiene practices.
• The percentage of safe drinks consumed by a user: A significant health impact can only be achieved if the consumption of unsafe drinks is avoided completely!