Modelling a nutrient deficient wastewater treatment process

In this paper a new model, tailor-made for nutrient deficient aerobic COD removal, is presented. The Activated Sludge Model No. 1 is chosen as a basis for model development. The original model is extended to include (1) phosphorus state variables; (2) phosphorus and nitrogen limited heterotrophic growth and (3) an additional active biomass component to model predation by higher-order organisms, e.g. protozoa. The phosphorus variables are included to model phosphorus consumption in biological growth. Nutrient limitations are modelled using Monod functions. The conventional heterotrophic biomass is assumed to be the only source of carbon and nutrients for the higher-order organisms. The model is reduced in the sense that it only regards aerobic conditions and it omits autotrophic organisms. A measurement campaign carried out at the wastewater treatment plant at Hylte mill in Sweden is used for model validation and influent fractionation. The basis for this plant design is a combined biofilm/activated sludge process. In the biofilm stage dispersed bacteria are produced, which are consumed by protozoa in the subsequent activated sludge stage. The significant presence of predating protozoa implies a negative observed yield coefficient for the activated sludge stage. An example of how the model can be used to determine operational parameters, such as sludge retention time and nutrient dosage, is shown.