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Up to now, within the design/retrofit of wastewater treatment plants (WWTPs), deterministic models were used to evaluate different scenarios on their merits in terms of effluent compliance. This paper describes an approach in which a Monte Carlo engine is coupled to a deterministic treatment plant model, followed by risk interpretation in the form of concentration-duration-frequency (cdf) curves of norm exceedance. The combination of probabilistic modelling techniques with the currently available deterministic models allows to determine the probability of exceeding the effluent limits of a WWTP. This percentage of exceedance is accompanied by confidence intervals resulting from the inherent uncertainty of influent characteristics and model parameters. The approach is illustrated for a hypothetical case study, consisting of a denitrifying plant model inspired by the benchmark model described by Spanjers et al.

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In the framework of the EU-funded TTP-UPM project (Technology Transfer Project--Urban Pollution Management) the waste water treatment plant (WWTP) of Tielt was modelled with the recently issued IAWQ ASM No. 2d model. Up to 41 % of the total COD load is originating from a textile industry. A measurement campaign was conducted during a period with industrial discharge and a period with only domestic sewage. The stop of the industrial discharge resulted in a highly dynamic response of the system. Based on an expert-approach the calibration was obtained changing only four parameters (anaerobic hydrolysis reduction factor etafe, reduction factor for denitrification etaNO3, the decay rate of autotrophs bAUT and the decay rate of the bio-P organism building blocks bPAO, bPHA, bPP). Influent fractionation remains a critical step within the model calibration. A proven procedure to characterise the influent determinants by standard physical chemical analysis failed to assess the influent COD fractions when the textile waste water is discharged to the WWTP. Selected bench-scale experiments, instead, succeeded in providing the adequate influent characterisation accuracy. For characterising the readily biodegradable COD fraction respirometry is to be preferred.

Current practice in Flanders (Belgium) is to limit the hydraulic capacity of the waste water 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 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 6Q14 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 6Q14 can be treated biologically using the storm tank as anextra 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.