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

Abstract

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 <1 to 85 g NaCl/L. While the activated sludge-inoculated reactor initially formed larger granules, the halophile-inoculated reactor better retained granule structure at hypersaline conditions (>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.