Water for public supply can be obtained from underground sources by wells sunk into aquifers, or from surface sources, such as purpose-built reservoirs or lakes (collecting rainwater run-off or water from streams) and rivers. The safety of the water is of utmost concern - several million people die each year after consuming contaminated water. The primary aim in water treatment is the elimination of any pathogenic micro-organisms present. All the above-mentioned sources can be subject to pollution. In the case of underground water, polluted surface water can enter the saturation zone of an aquifer and so lead to its contamination. Pollution can come from waste containing heavy metals and organic compounds, farm run-off containing pesticides, and industrial wastes which may have been deliberately dumped down old coal mine shafts. River water can be affected by farm drainage, sewage works and industrial effluents, and also the run-off water from roads. Thus there is a need to maintain the quality of the aquatic environment to ensure that the water is suitable for treatment for public supply, and that the cost of treatment is kept as low as possible.
Raw water is usually abstracted from a river and pumped to a reservoir for storage and settlement. In the reservoir, the number of bacteria is reduced through natural processes, such as ultraviolet radiation from sunlight. Also, a large portion of the suspended solids settles out. The water is then conveyed from the reservoir to a treatment works.