Use of Halophilic Bacteria to Improve Aerobic Granular Sludge Integrity in Hypersaline Wastewaters

Aerobic granular sludge (AGS) reactors are often limited in their ability to treat high-salinity wastewaters. In this study, the performance of an AGS reactor inoculated with an enriched halophilic culture was compared with one seeded with activated sludge at salt concentrations ranging from 40 g NaCl/L), with average granule diameters between 0.8 and 1 mm, approximately double the size of those in the activated sludge reactor. The halophilic reactor also produced significantly higher amounts of both total extracellular polymeric substances (EPS) and alginate-like exopolysaccharides under hypersaline conditions. The halophilic inoculum also retained lower concentrations of intracellular Na+ (100 mg/g mixed liquor volatile suspended solids [MLVSS] vs. 125 mg/g MLVSS in the activated sludge reactor) at 85 g/L NaCl. In contrast, both cultures exhibited the same behavior with respect to Na+ substitution for divalent cations in extracted EPS. Illumina 16S targeted gene sequencing data analysis showed that the population of both reactors converged toward halophile-dominated systems, primarily by bacteria belonging to the phyla Proteobacteria and Bacteroidetes, at hypersaline conditions. This represented a substantial decrease in diversity for the activated sludge reactor compared with the initial inoculum, but a slight increase for the halophile-inoculated reactor. Despite the similarity in community structure, the differences in granule formation and stability indicate that adding halophilic organisms in the initial inoculum produces better granules under hypersaline conditions.