Optimisation of a Large Wastewater Treatment Plant: Plant-Wide Modelling to Anticipate Major Modifications

A model of a 950 000 p.e. wastewater treatment plant has been developed to anticipate the impact of major changes on plant operation. Specific challenges were encountered in building the model, notably the need for a sub-model to predict intermittent aeration cycles. By analysing various scenarios, the impact on the plant's overall performance of a partial bypass of the primary settlers was assessed for different bypass strategies, as well as the implementation of anaerobic digesters (AD) and (internal and external) sludge handling strategies. As the plant is underloaded, the additional load due to bypassing the primary clarifiers can be effectively treated, but the fraction of influent that can bypass this step is limited by the additional aeration costs. While the implementation of AD is very beneficial in terms of the energy balance, it degrades effluent quality, which could be a problem under the forthcoming new European directive. This model is a very effective tool for helping to choose between different treatment methods in a holistic way.