Sewage

Sewage, liquid and semisolid wastes from dwellings and offices, industrial wastes, and surface and storm waters. Sewage systems collect the sewage, transport and treat it, then discharge it into rivers, lakes, or the sea. Vaulted sewers had been developed by the Romans, but from the Middle Ages until the mid-19th century sewage flowed through the open gutters of cities, constituting a major health hazard. Later sewage was discharged into storm-water drains that were developed into sewers. Because the dumping of large amounts of untreated sewage into rivers led to a serious water pollution, modern treatment methods arose, at least for major cities. An early solution (still sometimes practiced) was sewage farming, in which raw sewage was used as fertilizer. Noting that natural watercourses can purify a moderate amount of sewage, sanitary engineers imitated natural conditions by allowing atmospheric oxidation of the organic matter by the activated-sludge process, in which compressed air is passed through a sewage tank, where the sludge is decomposed by the many microorganisms that it contains. A by-product is sludge gas, chiefly methane, burned as fuel to help power the treatment plant. Sedimentation is carried out before and after decomposition; the filtered solids are buried, incinerated, or dried for fertilizer. Dwellings not connected to the sewers have their own septic tanks.