Water can be contaminated by the following agents:
■ Pathogens – disease-causing organisms that include bacteria, amoebas and viruses, as well as the eggs and larvae of parasitic worms.
■ Harmful chemicals from human activities (industrial wastes, pesticides, fertilizers).
■ Chemicals and minerals from the natural environment, such as arsenic, common salt and fluorides.
Some non-harmful contaminants may influence the taste, smell, colour or temperature of water, and make it unacceptable to the community. Water from surface sources is often contaminated by microbes, whereas groundwater is normally safer, but even groundwater can be contaminated by harmful chemicals from human activities or from the natural environment. Rainwater captured by a rooftop harvesting system or with small catchment dams is relatively safe, provided that the first water is allowed to flow to waste when the rainy season starts. The amount of water to be treated should also be assessed.
This can be estimated by assuming that each person will need a minimum of 20–50 litres of water a day for drinking, cooking, laundry and personal hygiene. A community should be consulted when choosing a water-treatment system and should be made aware of the costs associated with the technology. In particular, community members should be made aware of the behavioural and/or cultural changes needed to make the system effective over the long-term and thus be acceptable to them. Communities may also need to be educated about protecting water sources from animal or human contamination, and mobilized. It should be emphasized that all the positive effects of a water-treatment system could be jeopardized if the water is not drawn, stored and transported carefully and hygienically. The Fact Sheets in this section deal with both community and household methods for treating water.