Decay processes of nitrifying bacteria in biological wastewater treatment systems

A knowledge of the decay rates of autotrophic bacteria is important for reliably modeling nitrification in activated sludge plants. The introduction of nitrite to activated sludge models also requires the separate determination of the kinetics of ammonia- and nitrite-oxidizing bacteria. Batch experiments were carried out in order to study the effects of different oxidiation–reduction potential conditions and membrane separation on the separate decay of these bacteria. It was found that decay is negligible in both cases under anoxic conditions. No significant differences were detected between the membrane and conventional activated sludge. The aerobic decay of these two types of bacteria did not diverge significantly either. However, the measured loss of autotrophic activity was only partly explained by the endogenous respiration concept as incorporated in activated sludge model no. 3 (ASM3). In contrast to nitrite-oxidizing bacteria, ammonia-oxidizing bacteria needed 1–2 h after substrate addition to reach their maximum growth rate measured as a maximum OUR. This pattern could be successfully modeled using the ASM3 extended by enzyme kinetics. The significance of these findings on wastewater treatment is discussed on the basis of the extended ASM3.