Solids in combined sewage come from different sources. During dry weather flow, organic material will settle in sewer sections having low flow velocities. Microorganisms living from solubles out of dry weather flow are growing at the sewer wall. During low rains, mineral and organic solids from the surface will be washed into the sewer systems and part will settle at the sewer bottom. During storms, all these types of solids will be resuspended and mixed into the complex suspension called “combined sewage”. After five years' research it is apparent that settling velocity is a good instrument for describing sedimentation, transportation and erosion of settleable solids. The knowledge about the transportation of solids in dry weather flow will lead to better design of sewers. During wet weather, another major process for keeping solids back from receiving waters is the settling process. Up to 70% of settleable solids will be settled in a stormwater tank designed after the German guideline A 128. With knowledge of the settling behaviour of combined sewage the design of stormwater tanks could be improved and the receiving waters better protected from sewer pollution.

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