Enhanced AGS by granular activated carbon loaded with nano iron when treating low strength and low COD/TN ratio municipal wastewater: microbial metabolism, electron transfer and enhancement mechanism

In order to accelerate aerobic granular sludge (AGS) formation and stabilization, four SBR reactors were used and labeled R1 (no added material), R2 (nano iron added, NI), R3 (granular activated carbon added, GAC) and R4 (GAC loaded with NI added, NI@GAC) to improve the granulation process when treating low strength and low COD/TN ratio municipal wastewater. R4 had excellent performance of granulation time (shortened nearly 40%), nitrogen removal performance (promoted the start-up and recovery time) and PO43--P removal efficiency (improved about 25%) compared with R1. The increased electron transport system activity (ETSA) and adenosine triphosphate (ATP) of R2 and R4 indicated that NI of NI@GAC is beneficial for promoting microbial metabolic activity and maintaining granules stability during granulation stage and famine recovery stage. Meanwhile, NI@GAC, acting as the core of AGS, stimulated the secretion of extracellular polymeric substances (EPS), which promoted AGS formation. Electrochemical analysis of R1-R4 showed that GAC of NI@GAC improved the electron transfer efficiency and maintained high electron transfer ability of EPS. In addition, NI@GAC promoted the enrichment of Bacteroidetes and Planctomycetes belonging to denitrifiers and nitrifiers genera, which is crucial for the nitrogen removal process. Combined with the above results, the enhancement mechanism of NI@GAC on AGS formation and granules stability was established through microbial metabolic activity, EPS secretion, electron transfer characteristic and bacterial community. This study provided a theoretical basis for rapid granulation and stability of AGS technology.