Long-term operation of reactors and membrane hybridizations.Table 3 shows average effluent concentrations during aeration periods from the five reactors for 1 year of operation.The reactors showed stable performance for most of the days sampled. However, short periods of efficiency instability occurred in all reactors, resulting in relatively high standard deviations for the effluent data shown in Table 3. Reactor C had the highest mean effluent ammonia concentrations (32 mg NH3-N liter1), and reactor B had the lowest mean effluent ammonia concentrations (22 mg NH3-N liter1). However,mean effluent ammonia concentrations were not significantly different among reactors (analysis of variance [ANOVA] P value of 0.7212;0.5), indicating that the AOB communities in all reactors could oxidize ammonia to similar levels
during aerated periods. Effluent nitrite and nitrate concentrations were significantly different among reactors (ANOVA P values of 0.003 and 0.004 for nitrite and nitrate, respectively 0.5). Reactor A, with the shortest nonaerated period, had the lowest mean effluent nitrite concentration. Reactor E, with the longest nonaerated period, had the highest mean effluent
nitrite concentration. These findings suggest that NOB are more sensitive than AOB to longer nonaerated periods. Reactor B had the lowest mean effluent nitrate concentration, and reactor C had the highest mean effluent nitrate concentration.
The fractions of AOB (total -AOB) and NOB (including Nitrobacter and Nitrospira) in the biomass were also monitored,and the average values are shown in Table 4. The mean fraction of AOB was not significantly different among reactors(ANOVA P value of 0.376;0.5). Nitrosomonas and Nitrosoccocus were the dominant AOB in all reactors, accounting for more than 70% of the total AOB fraction. Reactors C and D,both with the shortest aeration period (0.5 h), showed the lowest Nitrosomonas-Nitrosoccocus mean percentage of total-AOB among the reactors. Nitrospira was the dominant group of NOB in all reactors, accounting for more than 73% of total NOB. Reactors B and E, both with the longest nonaeration periods (3 and 4 h), showed the lowest mean Nitrobacter, Nitrospira,and total NOB (Nitrobacter plus Nitrospira) fractions among the reactors, suggesting that long nonaeration periods have a strong effect on the biomass levels of NOB. Reactors C
and D, both operated with 0.5 h of aeration, did not show significantly different total NOB biomass fractions than reactors with longer aeration periods, suggesting that short aeration
periods did not have considerable impacts on the NOB populations in the reactors.