Effect of environmental parameters on autotrophic decay rate (bA).

Activated sludge treatment systems for industrial wastewater effluents, or mixtures of industrial and urban wastewater, can be subjected to nitrifying nitrogen starvation periods. Because of the autotrophic decay, the maximum nitrification rate could be significantly reduced after a few days of nitrogen starvation. As published data on the autotrophic decay rate (b(A)) are scarce, little is known about the parameters influencing its value. The autotrophic decay rate of nitrifiers (b(A)) was estimated in an activated sludge pilot plant treating industrial wastewater under stable conditions of load, temperature and hydraulic parameters. The effect of environmental parameters on b(A) was investigated. Influent quality had a significant impact on b(A): at 30 degrees C, under aerobic conditions, changes in the quality of an industrial effluent from the same source induced b(A) variations from 0.08 to 0.16 d(-1). The b(A) value doubled when small amounts of urban wastewater were added to the pilot plant influent. When 30% of the load was composed of urban wastewater, b(A) reached a value of 0.36 d(-1). Microfauna grazing of autotrophic bacteria was shown to be partly responsible for b(A) variations, explaining why the b(A) value was found to decrease by at least 50% under anoxic conditions.