A SYSTEMATIC-APPROACH FOR REDUCING COMPLEX BIOLOGICAL WASTE-WATER TREATMENT MODELS

Biological wastewater treatment systems comprise a variety of processe s which occur at vastly different rates. Biological growth, mass trans fer, hydraulics and chemical reactions all occur simultaneously and ar e all inter-dependent. In this paper we address the question ''to what extent can we de-couple these processes, and what are the associated issues? We aim to introduce people who work with biological wastewater treatment models to analytical tools which may be used for model redu ction. We present a quantitative technique to compartmentalise states into fast, medium and slow. From this we have provided an algorithm fo r eliminating state variables from a model based on whether they affec t the process in the selected ''time scale of interest''. Through the technique presented we provide a means of quantifying the interaction between state variables, the ''speed'' of a state and whether it is it candidate for reduction. A simple case study of a biological wastewat er treatment process is presented. We were able to reduce four biologi cal and 19 settler differential equations into algebraic equations. Th is resulted in significant savings in integration time. Application of the technique also highlighted the strong coupling between the slower biomass states and the rest of the model. (C) 1997 Elsevier Science L td.