Effect of nitrite on anoxic phosphate uptake in biological phosphorus removal activated sludge

Results from a series of batch experiments are presented, in which activated sludge obtained from an alternating type biological phosphorus removal process was exposed to nitrite or mixtures of nitrite and nitrate at various concentration levels. By comparing the course of phosphate, nitrite and nitrate (measured on-line) and the internal storage components PHB and PHV (measured manually for some experiments) with batches exposed to nitrate only, the effect of nitrite on anoxic phosphate uptake was investigated. How nitrite exposure affects subsequent aerobic phosphate uptake was also examined in one experiment. The experiments show that nitrite at low concentration levels (up to about 4 to 5 mg NO2-N/l) is not detrimental to anoxic phosphate uptake and can serve as electron acceptor for anoxic phosphate uptake. Exposure to higher concentration levels (roughly 8 mg NO2-N/l and greater) inhibits anoxic phosphate uptake completely, and aerobic phosphate uptake severely. The critical nitrite concentration above which nitrite inhibition of phosphate uptake occurs appears to be dependent on sludge conditions, lying in the range of from 5 to 8 mg NO2-N/l for the experiments performed in this study. The inhibiting effect of nitrite was found to last for at least several hours after the nitrite exposure.