Nitrifier characteristics in submerged membrane bioreactors under different sludge retention times

Three submerged membrane bioreactors (MBRs) were operated continuously for 230 days by feeding with synthetic inorganic wastewater (NH4+-N, 100 mg L−1) under different solids retention times (SRTs. M30d, 30 days; M90d, 90 days; Minfinite, no sludge purge) to examine the influence of SRT on nitrification performance and microbial characteristics. All the reactors could oxidize NH4+-N to NO3−-N effectively without accumulation of NO2−-N. M30d with the shortest SRT showed significantly higher specific ammonium oxidizing rate (SAOR, 0.22 kg NH4+-N kg−1 MLSS day−1) and specific nitrate forming rate (SNFR, 0.13 kg NO3−-N kg−1 MLSS day−1) than the other two MBRs (0.12–0.14 kg NO3−-N kg−1 MLSS day−1 and 0.042–0.068 kg NO3−-N kg−1 MLSS day−1, respectively). Short SRT led to low extracellular polymeric substances (EPS) concentration and long operating cycle. The nitrite oxidizing bacteria (NOB) ratios by both the fluorescence in situ hybridization (FISH) (3.6% for M30d and 2.1–2.2% for M90d and Minfinite) and MPN (1.4 × 107 cells g−1 MLSS for M30d and 6.2 × 105 and 2.7 × 104 cells g−1 MLSS for M90d and Minfinite) analyses showed that M30d favored the accumulation of NOB, which was in accordance with the SNFR result. However, the ammonia oxidizing bacteria (AOB) ratios (3.5%, 3.2% and 4.9% for M30d, M90d and Minfinite) were not in accordance with the SAOR result. PCR-DGGE, clone library and FISH results showed that the fast-growing Nitrosomonas and Nitrobacter sp. were the dominant AOB and NOB, respectively for M30d, while considerable slow-growing Nitrosospira and Nitrospira sp. existed in Minfinite, which might be an important reason why Minfinite had a low SAOR and SNFR.