Stable performance and microbial community dynamics under low HRT conditions in a propionate-fed UASB reactor

The propionate removal was achieved above 94.1% at hydraulic retention time (HRT) of 10-4 h in a propionate-fed upflow anaerobic sludge blanket. The microbial community characteristics under different HRT conditions were investigated by 454 pyrosequencing. The result showed that the microbial diversity showed a decreasing trend as the HRT decreases. When the HRT was lowered to 4 h from 10 h by stepwise, the dominant propionate-oxidizing bacteria were rather stable in composition and comprised relatives of Syntrophobacter. Their relative abundance was increased to 18.5% (HRT 6 h) and 23.0% (HRT 4 h) from 9.2% (HRT 10 h). Simultaneously, low abundance (0.1%-1.2%) of Smithella was observed in this system and its portion also was increased with HRT decreases. In addition, some syntrophic fatty acid-oxidizing bacteria (Syntrophomonas and Syntrophus) and fermentative acidogenic bacteria (Petrimonas, Kosmotoga and Aminiphilus) could be detected in three samples. Methanogens from four genera (Methanosaeta, Methanoculleus, Methanospirillum and Methanobacterium) were observed in three detected samples. Methanosaeta was the major acetotrophic methanogens with the relative abundance of 3.5%-7.1%. The hydrogenotrophic methanogens were Methanobacterium (0.4%) and Methanoculleus (0.1%) at HRT 10 h. When the HRT decreased to 6 and 4 h, the predominant hydrogenotrophic methanogens were shifted to Methanobacterium/Methanospirillum (HRT 6 h) and Methanospirillum (HRT 4 h).