5 Water treatment
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 tip leachate containing heavy metals and organic compounds, farm run-off containing nitrates and 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.
In this course we shall be looking at the treatment of water after it has been abstracted from a suitable source.
While the prime function of water treatment is to produce a safe product, several stages are involved:
the removal of suspended matter and rendering of the water clean, colourless and free from disagreeable taste and odour;
the disinfection of the water so that the numbers of bacteria are reduced to an appropriate level;
the removal of chemicals harmful to health and the reduction to low levels of chemicals that might otherwise interfere with normal domestic and industrial requirements;
the reduction of the corrosive properties of the water and protection of the pipe supply system;
the minimisation of the amount of material passing into the supply system which might encourage biological growth.
In Europe, the quality of water for potable supply has to comply with the EU Drinking Water Directive (98/83/EC) passed in 1998.
Raw water is usually abstracted from a river and pumped to a reservoir for storage and settlement. In the reservoir, the number of faecal bacteria is reduced through natural processes such as predation by protozoa and 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.
In some situations, particularly in hilly areas, rainwater is abstracted from a storage reservoir made by damming a valley in an upland catchment area, instead of from a river. In other instances, water may be drawn from aquifers. (These waters usually require little treatment due to their often unpolluted nature.)
The basic treatment for river water is shown in Figure 19. It should be noted that not all the processes shown will be required for water from every source. The treatment used will depend on the quality of the abstracted water. For water that has little pollution, it may only be necessary to use preliminary settlement, rapid sand filtration and chlorination, whereas poor quality water may require even more treatment than that shown.