Current practice in Flanders is to limit the hydraulic capacity of the wastewater treatment plant to 6Q14 (Q14=1.7 dry weather flow QDWF). A maximum of 3Q14 is treated in the biological system, the excess flow undergoing only physical treatment in the stormtank. This practice has been challenged by a new concept, consisting of the treatment of the full storm sewage flow in the biological train and of the use of the stormtank(s) as additional secondary clarifier(s).

This paper reports on the long-term experience gained on a total of 12 full-scale plants. The analysis focused on the parameters subject to regulatory discharge (BOD, COD, suspended solids, total nitrogen and total phosphorus) plus ammonia. Special attention is paid to the performance of the clarification and of the nitrification processes. The significance of the proposed way of operation in attenuating the overall pollution impact on the receiving water body is shown.

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