According to scientific resources, we need to discharge less sludge to increase the sludge age (SRT). As less sludge is discharged, the total biomass in the reactor will be increased for certain time. When the total biomass is increased, the organic load given to the unit sludge is reduced. As a result, the fraction of heterotrophs gradually tends to decrease, while the ototrophics start to increase using existing appropriate electron donors. However their dividing rates are lower than heterotrophic. Thus, the autotrophic fraction in the sludge is still lower than the fraction of heterotrophs. Nevertheless, remember that both of them use oxygen, and first consider the oxygen requirement for the oxidation of organic substances, then the oxygen requirement for the oxidation of nitrogenous substances.
SRT ~ 1/U, where U is the specific growth rate. At high SRT, the microbial diversity increases, however the specific microbial activity may be reduced as F/M ratio decreases.
Excuse me, but how could the respirometer give the biomass activity?. If you refer to the (Standard methods for the examination of water and wastewater) you will find the different methods to measure (MLVSS) which represents the biomass. This is done by a Muffle Furnace (Temp.>700 degrees centigrade), then calculating the ash residue.Please refer to the procedure. Thank you, friend.