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.