A new concept for storm water treatment: full-scale experience in Flanders

Current practice in Flanders (Belgium) is to limit the hydraulic capacity o f the wastewater treatment plant to 6O(14). A maximum of 3O(14) is treated in the activated sludge system, the excess flow undergoes only physical tre atment (stepscreen, sand trap and settling). This paper focuses on an alter native storm management operation strategy aiming at maintaining plant perf ormance and reducing the total pollutant discharge towards the receiving wa terbody. Given the observed dilution of incoming wastewater under storm con ditions, the idea was put forward that higher hydraulic loadings could be t reated within the biology if additional secondary clarifier volume was supp lied. The new storm operation strategy would consist of treating 6O(14) bio logically using the available storm tanks as additional clarifier volume. T he outcome of this study clearly shows that 6O(14) can be treated biologica lly using the storm tank as an extra clarifier. It was shown that doing so the overall pollutant discharge was significantly reduced. The proposed str ategy does not entail any extra operational costs. On the contrary it offer s a potential cost saving of 244 million Euro in view of a possible future change of environmental legislation regarding storm tank spill frequencies.