Nitrogen removal pathways and microbial community structure in the aerobic granular sludge reactor with co-existence of granules and flocs

Granules and floc in the aerobic granular sludge (AGS) reactor undertook different roles in the nitrogen removal process and displayed significant disparities in microbial communities. This study quantified the contribution of different nitrogen removal pathways and analyzed the nitrogen removal mechanisms inside granules and flocs by metagenomics sequencing. The results showed that the AGS reactor performed excellent nitrogen removal efficiency and the removal efficiencies of NH4+-N and total nitrogen were more than 90.0 % and 60.0 %, respectively, with the main nitrogen form of NO2--N in the effluent. Batch experiments indicated that the nitrogen removal pathway was principally shortcut nitrification and denitrification (SCND), accounting for 51.6 %, followed by anaerobic ammonium oxidation (Anammox) accounting for 26.1 % and the remaining 22.3 % was simultaneous nitrification and denitrification (SND). The metagenomic sequencing results pointed out that the abundance of SCND-related genes in the granules was 0.18 %, and the abundance of Anammox-related genes was 0.02 %, which indicated that the granules mainly reduced nitrogen via SCND, and minor nitrogen was removed through Anammox. Inside the flocs, nitrogen was mainly reduced via the unconventional SND pathway (HAND). The in-depth analysis of the nitrogen removal process in the reactor with granules and flocs provides an important reference for further exploring the nitrogen removal mechanism, and supplies foundation for expanding the application of AGS.