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.

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 allythiourea (ATU) shock. The AOB (genera Nitrosomonas and Nitrosospira) and NOB (genera Nitrobecter and Nitrspira) communities were analyzed using fluorescence in situ hybridization (FISH). AOB 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 nitrifying bacteria under conditions favorable for nitrification, while Nitrosospira outcompeted Nitrosomonas under adverse conditions (low [NH 4 + ], low DO, short HRT, and ATU shock), and Nitrobecter outcompeted Nitrospira at high substrate concentrations (COD and [NH 4 + ]). Under ATU shock that inhibited the oxidation of NH 4 + to NO 2 -, the 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 2 oxidation, 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 in order to optimize process design and operation at different influent characteristics, aeration intensity, retention time, and potential influent toxic shock. KEYWORDSFluorescent in situ hybridization (FISH), nitrification, nitrifying bacteria, sequencing batch reactor (SBR), dissolved oxygen (DO), hydraulic retention time (HRT), allythiourea (ATU) shock -) by autotrophic nitrite-oxidizing bacteria (NOB) (Reaction 1 and 2). In anoxic denitrification, nitrite/nitrate is reduced to nitrogen gas (N 2 ) by heterotrophic denitrifiers with the presence of carbon source (e.g. methanol, acetic acid) as electron donor (Reaction 3). 5008 WEFTEC®.06 Ammonia oxidizing bacteria (AOB) Nitrite oxidizing bacteria (NOB) Anoxic denitrifiers 5009 WEFTEC®.06 The changes of nitrifying bacterial population in single parameter modification testsThe changes of nitrifying bacterial populations under specific operational condition (COD, DO, or retention time) were studied in eight single parameter tests (Table 1). Nitrifying bacterial populations were then compared with FISH results at mid-level operation status. 2.6%±0.4 8.9%±0.6 2.5%±0.7 5017 WEFTEC®.06