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.