Stability of a nitrifying activated sludge reactor

A nitrifying activated sludge unit operated in stable conditions was subjected to different instability actuations as overloads due to a sudden increase of the inlet ammonia concentration or a quick change of the inflow rate, pH shocks and limitations of dissolved oxygen (DO) concentration. The response of the unit to these different actuations was evaluated. During concentration overloads there were transitory nitrite accumulations but not during hydraulic overloads. However, efficiencies obtained for both kinds of overloads with respect to ammonia oxidation were very similar. The normal efficiency of the system was always restored and biomass was not washed out in spite of the low values of hydraulic retention time (HRT) used. The pH shocks of 9 and 10 had no appreciable effects, but the pH shock of 11 caused an irreversible loss of efficiency of 60%. An increase of the total organic carbon (TOC) in the effluent after this shock showed that biomass underwent a partial lysis. The specific activity of ammonia-oxidizers decreased from 0.6 to 0.3 g NH₄⁺-N/(g VSS day) but the nitrite-oxidizing specific activity remained practically constant. Finally, the unit was operated at different DO levels, ammonia being fully oxidized to nitrate at DO levels higher than 1 mg O₂/L. Different proportions of ammonia and nitrite appeared when the reactor was operated at DO values of 0.4 and 0.6 mg O₂/L.