Microbial activity: Mathematical expression and direct determination

A new approach is proposed for the definition of active fraction in activat ed sludge of both the heterotrophic and autotrophic biomass. In contrast to most existing approaches, it is based on the concept that biodegradable or ganic volatile suspended solids in an activated sludge system is composed o f both active and nonactive microbial cells. Applying this hypothesis, a ma thematical 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 express ion 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, wher eas 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 a utotrophic biomass. The effect of MCRT on nitrifiers' maximum specific grow th rate, commonly used parameter in Monod kinetics, was confirmed by experi ments. Hence, the maximum specific growth rate of nitrifiers, expressed in terms of generated autotrophs per unit of total autotrophic volatile suspen ded solids per unit time should be adjusted, according to the particular MC RT. An empirical equation for such adjustment is proposed. Coefficients pre sent in the mathematical expressions for both heterotrophic and autotrophic biomasses were experimentally determined.