DYNAMIC MODELING OF THE ACTIVATED-SLUDGE PROCESS - A CASE-STUDY

Description of the unsteady-state behaviour of the activated sludge process of wastewater treatment has attracted much attention, yet there have been few exhaustive assessments of the resulting models against field data from full-scale plants. This paper presents results for the evaluation of a model for carbonaceous and nitrogenous substrate removal with reference to a comprehensive set of data obtained from a 10-day monitoring exercise at the Norwich Sewage Works in eastern England. The model of the aerator is a simplified version of the IAWPRC model of the activated sludge process. The model of the secondary clarifier employs a conventional empirical expression for clarification and a conventional flux theory for thickening. Solute transport through both the aerator and clarifier is characterized by appropriate arrangements of fixed and variable-volume CSTR elements. Substrate removal and the production, thickening and recycle of biomass (suspended solids) are well replicated by the model. However, fundamental weaknesses are apparent in the currently available representations of the clarification function. Impacts of recycled flows from a sludge consolidation tank are also examined; sensitivity tests with the model suggest that these flows are a significant factor in the longer-term maintenance of aerator biomass concentration.