| The combined effects of solids retention time (SRT) and dissolved oxygen (DO) concentration on nitrification, nitrifying bacterial communities, and aeration needs in the activated sludge process were studied. Nitrification was almost completed in the 10, 20, and 40-day SRT reactors with a minimum DO of 0.37, 0.25, and 0.16 mg/L, respectively. Under long-term low DO conditions, the endogenous decay of nitrifiers was slowed down and then nitirfier biomass concentration increased, thereby reducing the adverse effect of low DO on nitrification. Under long-term low DO conditions, the oxygen affinity of nitrite-oxidizing bacteria (NOB) increased significantly and as a result, NOB became a better competitor for oxygen than ammonia-oxidizing bacteria (AOB). Nitrosomonas europaea/eutropha-like AOB were dominant with all tested SRTs and DO levels. Nitrobacter-like NOB and Nitrospira-like NOB played the main role in the nitrite oxidation in the 5 and 40-day SRT reactors, respectively. In all reactors, Nitrospira increased considerably when the DO was reduced to ≤ 0.5 mg/L. Compared to a baseline condition (SRT = 10 days and DO = 2 mg/L), aeration need was reduced by about 20% under these two conditions (SRT = 10 days and DO = 0.37 mg/L; SRT = 40 days and DO = 0.16 mg/L). It was found that the boom of filamentous bacteria inhibited the oxygen transfer efficiency considerably in the 20-day SRT reactor.;In the activated sludge process, a combination of effluent ammonia and nitrite could effectively report insufficient or excessive DO, which was proposed as the indicator for aeration control. A simple control approach can be developed to reduce the aeration when both effluent ammonia and nitrite concentrations are ≤ 0.5 mg-N/L. When either is ≥ 2 mg-N/L, increase the aeration. |
