A simulation supported scenario analysis for the upgrading of a municipal WWTP Hoogstraten (Belgium) with nitrogen removal is presented. The most cost-effective solution is to create optimal aerobic and anoxic conditions within the existing reactor volumes. The IAWQ-model No. 1 was used for the simulation of the biological reactor. A measuring campaign and some specific respirometric tests provided the necessary information for calibration and verification of the biological model.

The study consisted of two parts: 1) increase of the denitrification capacity, and 2) optimisation of operation. First, the impact of three measures, i.e. creation of anoxic zones, implementation of step feeding and introduction of internal recirculation, was evaluated by means of simulation. For this purpose a standard time variant influent was defined based on the measuring campaign. This approach made it easy to compare the different scenarios.

In the second part of the study some specific problems were handled. To avoid high nitrate concentrations during weekends three possible control actions were simulated. The simulation study also evaluated the usefulness of the primary clarifier in the upgraded design and the effect on the biological process of first flush during intensive rainfall.

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