ASM 1 does not describe biological or chemical phosphorus removal. It was necessary to expand it to 19 balances for 19 components and 12 processes. The reaction terms consist of up to 7 Monod parts. The expanded model, called ASM 2 (Henze et al. 2000), will not be described here. ASM 2 models biological phosphorous accumulation by organisms of two fractions, one of which is able to nitrify.
ASM 3 was developed to correct some ASM 1 problems:
• Limitations of nitrogen and alkalinity on growth rate of heterotrophic organisms.
• Inclusion of soluble and particulate organic nitrogen.
• The elimination of a differentiation between inert organic material, inert particles, influent or biomass decay compared to ASM 1.
All together, ten points were mentioned (Henze et al. 2000), showing that the ASM 1 model is too simple to accurately describe the activated sludge process. Therefore, a completely new model was generated, ASM 3, which will not be described or discussed here.
There is no single model which describes all the qualities and properties of a plant-scale activated sludge process. The power of modern computer systems tempts us to construct more and more complex models. But we should not forget that an activated sludge plant in non-steady operation is not only influenced by the complicated reaction terms of the ASM models. The fluid dynamics and the mass transfer in activated sludge plants influence the substrate removal and nitrification just as much if not more than do the micro-kinetics. We model completely mixed tanks and nothing else. Frequently, it is not easy to describe the measurements of retention time distributions mathematically. But there are only a few plants that have been studied with such measurements. For most basins, we do not have these results and we are not able to conclude whether we need a cascade model with six stages or only two. We should never forget such considerations when applying simple models or one of the ASM models!
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