Effects of sludge retention time (SRT) on nitrogen and phosphorus removal and the microbial community in an ultrashort-SRT activated sludge system

The ultrashort-sludge retention time activate sludge (USAS) process is considered promising for the recovery of energy and resource from wastewater. In this study, we developed five USAS systems (with sludge retention time (SRT) of 0.5, 1, 2, 3, and 4 d), and investigated the nitrogen and phosphorus removal, as well as the microbial community, in the USAS systems. The results revealed that the activated sludge in the USAS system with a shorter SRT can capture more organic matter from wastewater, with organic matter mineralization, capture, and loss rates of 12.9-45.9, 42.4-62.3, and 18.2-24.8%, respectively. In addition, ammonia oxidization only occurred in the USAS systems with SRTs of 3 and 4 d, and all the USAS systems with different SRTs exhibited phosphorus-removal ability. The microbial community analysis results revealed that the relative abundance of nitrifying and denitrifying bacteria (such as Thauera) increased with an increase in the SRT.