Resistance to salt stresses by aerobic granular sludge: sludge property and microbial community

Saline wastewater is regarded as a challenge for wastewater treatment plants because high-salinity conditions negatively affect on traditional biological technologies. Aerobic granular sludge (AGS) has gained attention as a promising technology for saline wastewater treatment because of its compact structure and the ability to withstand toxic loadings. Therefore, this study investigated the salt-resistance performance, sludge properties and microbial community of AGS under low-salinity and high-salinity conditions, with the saline concentrations ranging from 0 to 50 g/L. The results showed that AGS could withstand long-term saline stresses, and the maximum salinity reached 50 g/L within 113 d. Under salinities of 10, 30, and 50 g/L, the chemical oxygen demand (COD) removal efficiencies were 90.3%, 88.0% and 78.0%, respectively. AGS also its maintained strength and aggregation at salinities of 10 and 30 g/L. Overproduction of extracellular polymeric substances (EPS) by non-halophilic bacteria that enhanced sludge aggregation. The compact structure that ensured the microorganisms bioactivity helped to remove organic matters under salinities of 10 and 30 g/L. At a salinity of 50 g/L, moderately halophilic bacteria, including Salinicola, Thioclava, Idiomarina and Albirhodobacter, prevailed in the reactor. The dominant microbial communities shifted to moderately halophilic bacteria, which could maintain aerobic granular stabilization and remove organic matters under 50 g/L salinity. These results in this study provide a further explanation for the long-term operation of AGS for treating saline wastewater at different salinities. It is hoped that this work could bring some clues for the mystery of salt-resistance mechanisms.