Although there were already highly developed drainage systems especially in urban centres several millennia ago, it was the repeated occurrence of epidemics in the 19th century which led to the necessity of building sewers. Here, rainwater in particular but also washing water was diverted into open or closed channels, whereas faecal matter from privies was collected separately and reused owing to its value as fertiliser. Water-borne sewage systems of the present type only became established with increasing industrialisation and urbanisation.
The task of sewerage systems is to collect the waste water which is produced (contaminated and/or rainwater) and then to conduct it as quickly as possible to a sewage plant for cleaning and after treatment into a receiving water course in the least harmful manner possible.
This is intended to prevent the spread of diseases and epidemics by domestic waste water, avoid flooding of inhabited areas in the event of rain and minimise pollution of bodies of water by waste water. Inorganic pollutants (salts) are diluted, and organic components are degraded.
Sewer installations must be laid in such a manner that they are water-tight and frost-proof in the long term, have sufficient gradient for transporting dirt, be resistant to chemical and physical attack and able to cope with static loads.