The sewerage system of a settlement has the task to transport rain- and drainage water and municipal and industrial sewage to the treat-ment plant or to the discharge points. Regarding the structure of the sewerage system combined and separate systems will be used:
• Combined system: Municipal and industrial sewage and rainwater is transported in one channel.
• Separate system: Municipal and industrial sewage is transported in one channel, the rain- and drainage water in another channel.
Combined systems needs stormwater overflows, because the rainwater can be much bigger than the sewage. The rainwater from rainfalls with less intensity or duration can be treated in the wastewater treatment plant. In separate systems the rainwater is fed directly to a river, usually with-out any treatment. In German cities the old parts of a town have often a combined system. New settlements in the surroundings are dewatered with separate sys-tems, where the sewage is fed to the combined system. This leads to a higher concentration in the combined system and following also to a higher discharge of loads into the river. The influent point of higher polluted industrial wastewater has to be considered for the technical calculations of the discharge loads. In some cases a separate pipe from the industry to the wastewater treat-ment plant is a suitable measure to reduce the direct discharge of loads to a river.
To reduce the negative impact to the receiving rivers by the stormwater overflows from combined systems, storage tanks or further measures can be installed. The hydraulic loads of the sewerage system and also the concentration of the relevant components, which have an impact to the river quality, are changing in a wide range. Because of the structure of a sewerage system, with many different pipes and channels, storage tanks and treatment facilities and the big differences in loads and quantities, by a management procedure for the sewerage system, significant reduction of the discharged loads can be achieved. Some examples will be explained following.