The purpose of this study was to correlate the population size of ammonia-oxidizing bacteria (AOB) and nitrite oxidizing bacteria (NOB) with nitrification performance under various operational conditions (chemical oxygen demand (COD) concentration, dissolved oxygen (DO), and hydraulic retention time (HRT)) and influent allylthiourea (ATU) shock. The AOB (genera Nitrosomonas and Nitrosospira) and NOB (genera Nitrobacter and Nitrospira) communities were analyzed using fluorescent in situ hybridization (FISH). Ammonia-oxidizing bacteria and NOB accounted for 6.2 ± 0.9% and 2.5 ± 0.3% in total biomass, respectively. The population sizes of AOB and NOB varied with different levels of COD, DO, and HRT. Nitrosomonas and Nitrospira were dominant under conditions favorable for nitrification, while Nitrosospira outcompeted Nitrosomonas under adverse conditions (low [NH4 + ], low DO, short HRT, and ATU shock), and Nitrobacter outcompeted Nitrospira at high substrate concentrations (COD and [NH 4 + ]). Under ATU shock that inhibited the oxidation of NH 4 + to NO 2 -, AOB population was substantially reduced with the stepwise increase of ATU dosage, and led to a corresponding decrease of NOB population. There was a discrepancy between nitrifying bacterial populations and their functions. Although AOB outnumbered NOB in all tests and became more dominant at low DO and short HRT, NH 4 + oxidation, instead of NO 2oxidation, was the rate-limiting reaction for nitrification and susceptible to the adverse conditions. The study demonstrated the importance of elucidating the shifts of nitrifying bacterial population to optimize process design and operation at different influent characteristics, aeration intensity, retention time, and potential influent toxic shock.

Nitrogen dissimilation (nitrification and denitrification) and assimilation (uptake by cell growth) under different operational conditions (chemical oxygen demand (COD) and dissolved oxygen (DO)) were evaluated in a sequencing batch reactor (SBR) system. Nitrogen dissimilation played an important role for nitrogen removal at low to mid CODs, while nitrogen assimilation became more significant with biomass concentration steadily increasing at high COD. Specific denitrification rate increased at low to mid COD (C:N < 15), but decreased at high COD (C:N > 20). Both COD and C:N ratio should be kept in proper ranges to obtain sufficient biomass concentration for nitrogen assimilation in treatment systems. With alkalinity being consumed in nitrification and produced in denitrification, effluent alkalinity indicated the corresponding nitrogen concentrations under different COD loadings. ΔAlk between influent and effluent was also well correlated with assimilation and dissimilation. The discrepancy between ΔAlkTheory and ΔAlkExperi was less than 15 mg/L when dissimilation played a predominant role in nitrogen removal, while the discrepancy increased to 30 mg/L when assimilation became dominant.