Rethinking growth and decay kinetics in activated sludge – towards a new adaptive kinetics approach

Growth kinetics in activated sludge modelling (ASM) are typically assumed to be the result of intrinsic growth and decay properties and thus process parameters are deemed to be constant. The activity change in a microbial population is expressed in terms of variance of the active biomass fraction and not actual shifts in bacterial cellular activities. This approach is limited, in that it does not recognise the reality that active biomass is highly physiologically adaptive. Here, a strong correlation between maximum specific growth rate (μ_max) and decay rate (b_e) of ordinary heterotrophic organisms was revealed in both low solids retention times (SRT) and high SRT activated sludge systems. This relationship is indicative of physiological adaptation either for growth (high μ_max and b_e) or survival optimization (low μ_max and b_e). Further, the nitrifier decay process was investigated using molecular techniques to measure decay rates of ammonia oxidizing bacteria and nitrite oxidizing bacteria over a range of temperatures. This approach revealed decay rates 10–12% lower than values previously accepted and used in ASM. These findings highlight potential benefits of incorporating physiological adaptation of heterotrophic and nitrifying populations in future ASM.