Reactor performance and microbial ecology of a nitritation membrane bioreactor

Partial nitritation is an indispensable pretreatment for anaerobic ammonium oxidation process. However, the nitritation is limited by the low growth rates of ammonia-oxidizing bacteria (AOB). In this study, a membrane bioreactor (MBR) was operated for 300 days to assess its nitritation performance and the shift of microbial community. Results showed that the reactor obtained satisfying nitritation after a startup period of 50 days, which finally achieved ammonium conversation rates of about 0.8 kg N/m3/d. The apparent half-saturation constant (Km) and maximum ammonium oxidation rate (rmax) of the AOB-enriched culture were determined to be 6.1 mg N/L and 1.1 kg N/g-VSS/d, respectively. In addition, lower fouling rates were found in the initial operating days that the reactor was fed with lower ammonium loads (day 0–day 150). However, the increased ammonium loads (>0.6 kg N/m3/d) in the following 150 days resulted in increases in extracellular polysaccharides, leading to much higher fouling rates. 16S rRNA high-throughput sequencing analysis showed clear changes in the microbial community populations during the MBR operation. Results also showed that ordinary heterotrophic organisms and nitrite-oxidizing bacteria were successively inhibited; finally Nitrosomonas dominated in the nitritation MBR, with relative abundance of 40–46%. Moreover, the AOB-enriched culture was of higher microbial diversity than the seeding sludge. This study could not only improve our understanding of the bacterial community dynamics in nitritation processes, but also provide more alternatives for MBR applications.