Microbial Activity: Mathematical Expression and Direct Determination

A new approach is proposed for the definition of active fraction in activated sludge of both the heterotrophic and autotrophic biomass. In contrast to most existing approaches, it is based on the concept that biodegradable organic volatile suspended solids in an activated sludge system is composed of both active and nonactive microbial cells. Applying this hypothesis, a mathematical expression for calculating the active biomass fraction (ABF) of the biological mixed liquor volatile suspended solids at a given mean cell residence time (MCRT) was developed. ABFs calculated following this expression are less than those calculated according to the conventional approach. ABF varies between 0.8 and 0.55 for a MCRT of 3-10 days, respectively, whereas ABF calculated following the conventional approach varies between 0.87 and 0.67 for the same range of MCRT. A similar approach was developed for autotrophic biomass. The effect of MCRT on nitrifiersʹ maximum specific growth rate, commonly used parameter in Monod kinetics, was confirmed by experiments. Hence, the maximum specific growth rate of nitrifiers, expressed in terms of generated autotrophs per unit of total autotrophic volatile suspended solids per unit time should be adjusted, according to the particular MCRT. An empirical equation for such adjustment is proposed. Coefficients present in the mathematical expressions for both heterotrophic and autotrophic biomasses were experimentally determined.