Microbial community evolution in partial nitritation/anammox process: From sidestream to mainstream

[Display omitted] •Microbial evolution from sidestream PN/A to mainstream PN/A was investigated.•For both side and mainstream PN/A, anammox bacteria mainly grew in granular sludge.•Ca. Kuenenia outcompeted Ca. Brocadia in the mainstream PN/A reactor.•Heterotrophic bacteria constitute the majority of microorganism in PN/A reactor.•Phylum Chloroflexi were enriched under mainstream conditions. This study investigated the microbial evolution in a mainstream partial nitritation/anammox (PN/A) reactor started by inoculation from sidestream PN/A. The reactor was fed with pre-treated sewage and operated for 120 days at room temperature (24–26 °C). It was found that for both sidestream and mainstream PN/A, anammox bacteria preferentially grew in granular sludge while ammonium-oxidizing bacteria (AOB) were mainly resided in flocculent sludge. After 120 days operation, the abundance of anammox bacteria in the reactor decreased from 6.6 × 1011 to 3.2 × 1011 copies/L. Besides, a shift of dominant anammox genera from Ca. Brocadia to Ca. Kuenenia was observed. In contrast, the dominant genera of AOB was Nitrosomonas throughout the operation. Furthermore, high-throughput sequencing revealed that heterotrophs constitute the majority of microorganisms in PN/A reactor. Especially, Chloroflexi, which can utilize cell decay materials from autotrophs, were enriched under mainstream conditions. This study provided a better understanding of the microorganisms in mainstream PN/A process.