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3.3 Urban waste water treatment

Urban waste water treatment protects surface waters from the adverse effects of waste water discharges, such as organic pollution, the associated development of bacteria and fungi, and oxygen depletion, which degrade aquatic life. This is achieved through the collection and treatment of waste water in settlements and areas of economic activity. The installation of waste water treatment facilities first requires a sewage collection system be established and then the provision of facilities to treat the collected waste water. “Primary treatment” refers to simple sieving of the effluent, to remove large objects, before discharge into the environment. “Secondary treatment” subjects the effluent to microbiological breakdown to reduce the oxygen demand on the receiving water. More stringent or tertiary treatment may be applied where the receiving water is particularly sensitive, and there is a need to substantially reduce nitrogen and phosphorus concentrations, for example. Disinfection techniques, such as ozonation, chlorination or UV treatment, may be applied to reduce the bacterial load, for instance to protect bathing waters. Typically, higher levels of treatment will involve higher costs, owing to higher energy and infrastructure costs.