In the UK each person produces on average 180 litres of sewage daily. Sewage may also include surface or storm water from pavements and roads, which may be disposed through a separate system. Waste water from industrial processes is now commonly treated separately from sewage.
Sewage treatment is divided into a series of stages.
Preliminary treatment involves the removal of gross solids - oil, grease, and grit. In primary treatment the sewage passes slowly through sedimentation tanks, where larger particles sink to the bottom as a sludge. Chemicals may be added to encourage smaller particles to coagulate. Sewage discharged into the sea may receive only preliminary and perhaps primary treatment, although this procedure is now discouraged. In this, dissolved and colloidal organic material are oxidized by microorganisms, such as bacteria .