Two years of full-scale storm operation experience.

Current practice in Flanders is to limit the hydraulic capacity of the wastewater treatment plant to 6O14 (O14 = 1.7 dry weather flow O(DWF)). A maximum of 3O14 is treated in the biological system, the excess flow undergoing only physical treatment in the stormtank. This practice has been challenged by a new concept, consisting of the treatment of the full storm sewage flow in the biological train and of the use of the stormtank(s) as additional secondary clarifier(s). This paper reports on the long-term experience gained on a total of 12 full-scale plants. The analysis focused on the parameters subject to regulatory discharge (BOD, COD, suspended solids, total nitrogen and total phosphorus) plus ammonia. Special attention is paid to the performance of the clarification and of the nitrification processes. The significance of the proposed way of operation in attenuating the overall pollution impact on the receiving water body is shown.

A practical application of integrated urban pollution modelling in Flanders (Belgium): the catchment of Tielt.

In view of a European Innovation Project (DG XIII) on the application of the Urban Pollution Management (UPM) procedure, a European consortium was set up to carry out pilot studies in Belgium, France, Ireland, Italy and Portugal. On the Belgian pilot catchment of Tielt the impact from sewer system and sewage treatment plant (STP) on the receiving water courses was studied for both the existing situation and for a number of possible improvement schemes, using the Intermittent Standards, described in the UPM procedure. The integrated modelling revealed some interesting conclusions on the relative impact of the schemes considered.

Full-scale application of the IAWQ ASM No. 2d model.

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.

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

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.

Development of a risk assessment based technique for design/retrofitting of WWTPs.

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.

Membranebioreactor with external membranes.

The recent development of less expensive and more performant ultrafiltration membranes has generated a new concept in biological treatment known as membrane bioreactors (MBR). In the field of wastewater treatment, the MBR process is used as a modification of the conventional activated sludge (CAS) process, where the clarifier is replaced by a membrane module for the separation of the solid and the liquid phase. Membrane technology can, amongst others, be applied for the fast retrofit of relatively high loaded activated sludge systems to nutrient removal. In the last year Aquafin has built up extensive experience with submerged membrane systems. The research presented here was aimed at evaluating a particular external tubular membrane unit for MBR. More specifically it was evaluated whether higher stable flux levels could compensate the higher energy consumption, typical for external membrane systems. Both (1) treatment performance and (2) possible operational drawbacks were tested on a 5 m3/h activated sludge pilot-scale. The membrane bioreactor was tested in parallel with a conventional activated sludge system.