The total efficiency of combined sewerage systems and wastewater treatment plants (WTP) sbould be considered. By an overall planning procedure the benefits of the whole system can be optimized. For the estimation of the system behaviour with different rain events, computer simulations of the sewerage system including storage tanks and the treatment plant have to be carried out. The two systems are not coupled mathematically allowing separate simulation. Hydrographs of different scenarios computed for the sewerage system can be used for simulations of the WTP.

For comparing the effects of different throttle flows on the treatment plant an example bas been simulated. In this example the BOD5 and phosphorus loads could be reduced with a higher throttle flow. The large difference in total discharges from combined sewerage without storage tanks and with well-dimensioned tanks could also be demonstrated.

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