Summary

In this Study Session, you have learned that:

• Large-scale water treatment is required when the population needing water is large and surface water sources have to be used. Large-scale water treatment often involves seven stages.
• Screening involves trapping large floating and suspended solids using bar screens or devices such as micro strainers.
• Aeration helps expel any acidic gases and gaseous organic compounds from the water. Aeration also removes iron and manganese. Pre-chlorination is carried out instead of, or in addition to, aeration if there are excess algae in the raw water. The chlorine also oxidises taste- and odour-causing compounds.
• Coagulation is used to remove fine particles smaller than 1 µm in diameter. Aluminium sulphate and ferric chloride are two coagulants commonly used in water treatment.
• The next process is flocculation, where the water is stirred gently to enable large flocs to form.
• Once the large flocs have formed, the water goes to a sedimentation tank where the flocs settle out. Filtration follows sedimentation.
• After filtration the water is disinfected by chlorine. The chlorine stays in the water and protects it till it reaches the consumers.
• Supplementary treatment includes fluoridation of the water, to protect teeth. Defluoridation may be necessary in some areas to reduce excessive fluoride to safe levels.
• The wastes from a water treatment plant include screenings, sludge, backwash waters and packaging from the supply of chemicals and equipment.
• Sustainability in water treatment is enhanced by using simple processes, locally available materials, regular training of staff, designing for future water demand, using robust equipment and the use of renewable energy.
• Resilience of water treatment plant can be helped by taking protective measures against natural hazards and ensuring that all equipment and construction is of a high standard.
• Basic calculations for water supply can be carried out if the size of the population to be served and other water demands are known.