A new method was developed, carrying out the batch tests of oxygen utilization rate with and without the addition of Allythiourea solution (inhibitor) at the same time, to separately and simultaneously estimate both the biomass of heterotrophic and autotrophic bacteria in a mixed culture activated sludge of a sequential batch reactor system. The results of the batch tests showed that the YH kinetic parameter was equal to 0.77 g cell COD per g oxidized COD and the maximum specific growth rate and biomass of heterotrophic bacteria were 2.13 d(-1) and 212 mg l(-1) as COD (about 66% of the activated sludge), respectively. On the other hand, the maximum specific growth rate and biomass of autotrophic bacteria were 1.62 d(-1) and 2.51 mg l(-1) as COD (about 0.8% of activated sludge), respectively. The proposed method is easier and cheaper than other developed methods.

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