Respiration rates for nitrate and oxygen are compared. Raw wastewaters vary much in these rates and the same holds for activated sludge systems. The denitrifying activity may range from 0-80 per cent of the oxygen activity. It is important to take these variations into consideration, when modelling complex activated sludge processes. Of similar importance are the amounts of active biomass in raw wastewater, as these amounts can exceed those produced in the activated sludge plant itself, and thus significantly can influence the overall composition of the active biomass.