ABSTRACTThe Federal German Water Law of 1976 postulated for the first time that a licence for the discharge of wastewater into public watercourses might only be granted when the wastewater, prior to discharge, is treated according to laid‐down standards. In 1976 it was also decided that for the residual pollution load in sewage‐treatment plant effluents a pollution fee has to be paid.At present three pollution units have to be paidper capitaand annually for untreated domestic wastewater. One pollution unit is assessed at 17/annum, but the price will be increased to 30/annum in future.Since the pollution units encompass COD, nitrogen, phosphorus, heavy metals, and chlorinated hydrocarbon compounds, the pollution unit is a good tool to compare the effectiveness of treatment. For different drainage systems the residual pollution units (per litre and second) can also be calculated for low‐flow condi