Mathematical modelling of prefermenters—II. Model applications

This paper investigates several potential applications of a recently developed dynamic mathematical prefermenter model. Three case studies are presented. In the first case study, the model is used to analyse and optimise the performance of a prefermenter. The full-scale complete-mix prefermenter in Penrith, Australia, is used as an example. The model can predict the optimal residence time with respect to maximal mass of volatile fatty acids and soluble COD produced per day. The second study focuses on the effect of different feed types on prefermenter performance under different operating conditions. Four different feed types are defined for this case study, the main difference being their total COD concentrations. According to the simulation results, in-line prefermenters should generally be operated at a longer solids residence time than side-stream prefermenters for maximal rates of volatile fatty acids production. Optimal hydraulic retention times predicted by the model are considerably lower than what is currently used in practice for side-stream prefermenters. In the third case study the model is used to predict the dynamic behaviour of a prefermenter after a sudden disturbance. Experimental results from a pilot-scale activated primary tank undergoing dynamic transients and the corresponding simulation results are shown. The disturbances were initiated by adding glucose to the feed and by a step change in hydraulic residence time. The three case studies illustrate that the prefermenter model is a versatile tool for optimising the design and operation of prefermenters.