Extraneous water

Extraneous water is a major but undesirable flow component, influencing the size of a waste water system and the way it is operated. Extraneous water is largely made up of non-contaminated ground water or rain water entering the sewer system in different ways. The most common causes of infiltration include damaged sewers or pipes allowing ground water to enter through cracks or leaking connections; misconnected property drainage; river courses; and storm water discharge into the waste water sewer of a separate sewer system.

Extraneous water makes running a sewage treatment plant more complicated due to hydraulic stress, higher building and operating costs, and in particular an increased nitrogen input into the body of water. In the Ruhr catchment area, extraneous water infiltration is more than three times the German average with peak values in the Sauerland region which is high in precipitation but low in percolation.