Reduction of biomass decay rate under anoxic and anaerobic conditions

The nitrification capacity of a nitrifying activated sludge which is not exposed to ammonium for several days is reduced by a significantly smaller amount if kept under anoxic and anaerobic rather than aerobic conditions, respectively. Since the enzyme system is not degraded as long as the micro-organisms are not in a starving state, this effect is probably due mainly to the anoxic and anaerobic biomass decay rate being smaller than the aerobic one. At 20°C the decay rate of nitrification activity reduces from about 0.2 to 0.03 d⁻¹ from aerobic to anaerobic conditions, respectively. From these results it becomes clear that aeration has to be properly controlled to reduce biomass decay. Nitrate respiration is reduced by 35–40% as compared to aerobic heterotrophic respiration. This is probably due to the reduced anoxic decay rate of heterotrophic micro-organisms and only partly because of the reduced anoxic hydrolysis rate.