Storm operation strategy: high-flow activated sludge process operation

Current practice in Flanders (Belgium) is to limit the hydraulic capacity of the waste water treatment plant to 6Q(14). A maximum of 3Q(14) 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 waste water 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 6Q(14) biologically using the available storm tanks as additional clarifier volume. Dynamic simulation was used to asses the feasibility of this strategy. In a next step a full scale test was run over several months. The outcome of this case study clearly shows that 6Q(14) can be treated biologically using the storm tank as an extra clarifier. This operation mode eliminates the direct overflow of only physically pre-treated waste water coming from the stormtank towards the receiving waterbody, it was shown that doing so the overall pollutant discharge was significantly reduced.