Stability of partial nitrification and microbial population dynamics in a bioaugmented membrane bioreactor.

Bioaugmentation of bioreactors focuses on the removal of numerous organics, with little attention typically paid to the maintenance of high and stable nitrite accumulation in partial nitrification. In this study, a bioaugmented membrane bioreactor (MBR) inoculated with enriched ammonia-oxidizing bacteria (AOB) was developed, and the effects of dissolved oxygen (DO) and temperature on stability of partial nitrification and microbial community structure, in particular on nitrifying community were evaluated. The results showed that DO and temperature played the most important roles in the stability of partial nitrification in the bioaugmented MBR. The optimal operation conditions were found at 2-3 mgDO/L and 30 degrees C, achieving 95% ammonia oxidization efficiency and 0.95 of nitrite ratio (NO2-/NOx-). High-DO (5-6 mg/L) and low-temperature (20 degrees C) had negative impacts on nitrite accumulation, leading to its drop to 0.6. However, the nitrite ratio achieved in the bioaugmented MBR was higher than that in most previous literatures. Denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH) were used to provide an insight into the microbial community. It showed that Nitrosomonas-like species as the only detected AOB remained predominant in the bioaugmented MBR all the time, which coexisted with numerous heterotrophic bacteria. The heterotrophic bacteria responsible for mineralizing soluble microbial products (SMP) produced by nitrifiers belonged to Cytophaga-Flavobacterium-Bacteroides (CFB) group, alpha-, beta-, and gamma- Proteobacteria. The fraction of AOB ranging from 77% to 54% was much higher than that of NOB (0.4-0.9%), which might be the primary cause for the high and stable nitrite accumulation in the bioaugmented MBR.