Wastewater from households and industry, commonly together with water runoff from urban areas, is conveyed by the sewerage system to sewage treatment plants for safe and economic treatment of sewage, and treatment and disposal of the resulting sludge. This Information page describes the key processes involved.IntroductionMains water supplied to households is used for many purposes, other than drinking and food preparat ion, notably bathing and showering, toilet flushing and the washing of utensils, dis hes and clothes. Except where main drainage is not installed, the used water gravitates to the local sewer and becomes ‘sewage’. Domestic wastewater will contain both solid and dissolved pollutants including faecal matter, paper, urine, sanitary items, food residues and a variety of other contaminants. The sewer network will usually also receive wastewaters from office and commercial properties and from industrial premises. Rainwater from roofs and roads may also drain into the sewer network . The combined flow from these various sources travels through the sewer system and ultimately to a ‘sewage works’ where it receives treatment before discharge of the treated effluent to a stream, river, estuary or the sea. Collecting and treating wastewater has been even more beneficial to human health than the health service because it stopped water-borne diseases such as cholera and typhoid. Sewage TreatmentWhy do we need to treat Sewage? Treatment of sewage is essential to ensure that the receiving water into which the effluent is ultimately discharged is not significantly po lluted. However, the degree of treatment required will vary according to the type of receiving water. Thus, a very high degree of treatment will be required if the effluent discharges to a fishery or upstream of an abstraction point for water supply. A lower level of treatment may be acceptable for discharges to coastal waters where there is rapid dilution and dispersion. Effluent StandardsStandards for the quality of effluents from sewage works discharging to rivers and coastal waters have been applied in the UK since early in the last century but the EC Urban Waste Water Treatment (UWWT) Directive 1991 (Reference 1) now defines standards for sewage effluents discharging to rivers, estuaries and coastal waters. What does Sewage Treatment involve?Sewage treatment involves: The removal of solids by physical screening or sedimentation
