Nitrifier characteristics in submerged membrane bioreactors under different sludge retention times

Abstract

Three submerged membrane bioreactors (MBRs) were operated continuously for 230 days by feeding with synthetic inorganic wastewater (NH(4)(+)-N, 100 mg L(-1)) under different solids retention times (SRTs. M(30d), 30 days; M(90d), 90 days; M(infinite), no sludge purge) to examine the influence of SRT on nitrification performance and microbial characteristics. All the reactors could oxidize NH(4)(+)-N to NO(3)(-)-N effectively without accumulation of NO(2)(-)-N. M(30d) with the shortest SRT showed significantly higher specific ammonium oxidizing rate (SAOR, 0.22 kg NH(4)(+)-N kg(-1) MLSS day(-1)) and specific nitrate forming rate (SNFR, 0.13 kg NO(3)(-)-N kg(-1) MLSS day(-1)) than the other two MBRs (0.12-0.14 kg NO(3)(-)-N kg(-1) MLSS day(-1) and 0.042-0.068kg NO(3)(-)-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 M(30d) and 2.1-2.2% for M(90d) and M(infinite)) and MPN (1.4 x 10(7)cells g(-1) MLSS for M(30d) and 6.2 x 10(5) and 2.7 x 10(4)cells g(-1) MLSS for M(90d) and M(infinite)) analyses showed that M(30d) 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 M(30d), M(90d) and M(infinite)) 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 M(30d), while considerable slow-growing Nitrosospira and Nitrospira sp. existed in M(infinite), which might be an important reason why M(infinite) had a low SAOR and SNFR.