Current practice in Flanders (Belgium) is to limit the hydraulic capacity of the wastewater treatment plant to 6Q14. A maximum of 3Q14 is treated in the activated sludge system, the excess flow undergoes only physical treatment (stepscreen, sand trap and settling). This paper focuses on an alternative storm management operation strategy aiming at maintaining plant performance and reducing the total pollutant discharge towards the receiving waterbody. Given the observed dilution of incoming wastewater under storm conditions, the idea was put forward that higher hydraulic loadings could be treated within the biology if additional secondary clarifier volume was supplied. The new storm operation strategy would consist of treating 6Q14 biologically using the available storm tanks as additional clarifier volume. The outcome of this study clearly shows that 6Q14 can be treated biologically using the storm tank as an extra clarifier. It was shown that doing so the overall pollutant discharge was significantly reduced. The proposed strategy does not entail any extra operational costs. On the contrary it offers a potential cost saving of 244 million Euro in view of a possible future change of environmental legislation regarding storm tank spill frequencies.

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