Unraveling the two-sided effect of nitrate on aerobic granulation

Aerobic granular sludge (AGS) is a promising technology in wastewater treatment which has gained considerable attention. This study aims to unravel the two-sided effect of nitrate on AGS granulation. By feeding different concentrations of nitrate (30 and 66 mg/L), better granulation was achieved in experimental groups compared to control group without nitrate feeding. However, after three-months cultivation, the mean granule size of control group drastically exceeded the nitrate-feeding groups where disintegration of granules occurred. The variation in extracellular polymeric substances (EPS) was consistent with the trend of granulation, suggesting its important role in AGS granulation. An in-deep analysis indicated that this two-sided effect of nitrate was rooted in denitrification, which had been promoted by nitrate feeding. The short-term promoting effect of granulation was due to the alkalinity generated by denitrification, which was beneficial for calcium retention, thereby providing micronuclei for granule growth and promoting EPS secretion. The inhibiting effect of granulation stemmed from the suppressive concentration of c-di-GMP caused by denitrification, and it became pronounced under long-term cultivation, inducing a decline in EPS. In microbial level, nitrate significantly affected the denitrifying and EPS-producing bacteria, such as Micropruina, Thauera and Flavobacterium. The changes in these two types of bacteria were related to denitrification and EPS variation. This given further evidence for the two-sided effect of nitrate on AGS granulation. This study has highlighted the potential effects of nitrate on aerobic granulation and provided references for utilizing AGS in nitrate-containing wastewater.