Enhancing robustness of halophilic aerobic granule sludge by granular activated carbon at decreasing temperature

High salinity seriously inhibits the growth and metabolism of microorganisms, resulting in poor settleability, excessive biomass loss and low treatment efficiency of biological wastewater treatment systems. The development of halophilic aerobic granular sludge (HAGS) is a feasible strategy for addressing this challenge. However, there are problems with the granulation of HAGS and the stability of granules at decreasing temperatures. In this study, granular activated carbon (GAC) with a large specific surface area and good biocompatibility was used to enhance the robustness of HAGS. The results showed that the addition of GAC shortened the granulation time from 60 d (control system) to 35 d (GAC-addition system). The proteins contents of extracellular polymeric substances (EPS) in the GAC-addition system was significantly higher (p < 0.05) than that in the control system during granulation. Satisfactory NH₄⁺-N and chemical oxygen demand (COD) removal efficiencies reached more than 96% in both systems at 18-26 °C. When the operating temperature was lower than 15 °C, the GAC-addition system exhibited better NH₄⁺-N removal performance (>80%) than the control system (<60%). Moreover, the abundance of almost all nitrogen metabolism-related genes in the GAC-addition system was higher than that in the control system. During the granulation process, the enrichment of functional microorganisms, including family Flavobacteriaceae, Rhodobacteraceae, and Cryomorphaceae, may promote the production of EPS by significantly upregulating (p < 0.05) the metabolic pathway "Signaling Molecules and Interaction" in the GAC-addition system. The overexpression of the nitrogen assimilation gene glnA in heterotrophic bacteria (Halomonas and Marinobacterium) may promote the conversion of inorganic nitrogen to extracellular proteins to adapt to the decreased operational temperature. Our findings confirm that GAC addition is a simple but effective strategy to accelerate granulation and enhance the robustness of HAGS in saline wastewater treatment.