Salinity decrease leads to recovery of filamentous bacterial bulking in moderately halophilic aerobic granules:...

Moderately halophilic aerobic granular sludge (M-HAGS) is an effective solution for treating ultra-high-salinity wastewater (≥ 50 g/L). In this study, filamentous bacterial bulking was observed for the first time. When influent salinity was reduced from 100 to 30 g/L, filamentous bacterial bulking in M-HAGS was recovered. The stable M-HAGS structure was maintained even after salinity was restored to 150 g/L. This study revealed the mechanism of granule bulking and recovery. The results indicated that at high salinity (≥ 100 g/L) a symbiotic relationship existed between filamentous bacteria and Halolactibacillus sp. within the bulking granules. Filamentous bacteria could feed on lactate and consume dissolved oxygen of granule surface, enabling in an anaerobic environment in the core of M-HAGS. When salinity was decreased from 100 to 30 g/L, the fungal population increased by 350-fold due to its low-salinity adaptation. Fungal hyphae penetrated the granules and formed oxygen-transport channels into the granule core, which disrupted the anaerobic habitat of Halolactibacillus sp. In addition, low salinity promoted the growth of Paracoccus sp., which secreted polysaccharide to promote granule adhesion. When salinity increased to 150 g/L, fungal overgrowth was limited by salinity inhibition, avoiding occurrence of fungal bulking. This study advances fundamental understanding of filamentous bacterial bulking and provides a feasible strategy for recovering disintegrated M-HAGS.