Approach to modeling protozoa grazing on the basis of the current state of knowledge

Protozoa are significant components of activated sludge which purify the effluent of free swimming bacteria as well as trigger floc formation. In addition, their presence is often used as an indicator of process quality. In classical models, the impact of protozoa on biomass is implicitly included in the bacteria decay rates, which in most cases gives a sufficient level of detail. However, modeling of certain processes, such as bioaugmentation, would greatly benefit from a functional model including protozoa grazing explicitly. To further establish the approach for protozoa grazing modeling, the authors have summarized the current state of knowledge in this area, as well as pointed out crucial elements that have to be considered. Aspects of the endogenous oxygen uptake rate (OUR), the preference of protozoa towards particular bacteria groups, and alternative sources of nutrient are presented and discussed. Based on the drawn conclusions, the authors have proposed a modeling concept towards protozoa grazing that will maintain both stability and accordance with generally accepted activated sludge models (ASM). The presented approach includes a division of each bacteria group into dispersed and flocculated bacteria that emerge from newly formed flocculation and deflocculation processes, with a different level of grazing on both components.