Alkalinity role in recovery of disintegrated halophilic aerobic granular sludge treating high salt-alkali wastewater: Insights into community ecology of fungi and bacteria

The overgrowth of filamentous fungi is a major factor causing the disintegration of halophilic aerobic granular sludge (HAGS). However, controlling filamentous fungal overgrowth and sustaining the niche balance between fungi and bacteria remain largely unknown. In this study, the influence of alkalinity on the fungal and bacterial community ecology was revealed when treating high salt-alkali wastewater, proposing a strategy to restore disintegrated HAGS. Changes in alkalinity resulted in remodeling the community ecology of fungi and bacteria. With alkalinity increasing from 600 to 3000 mg CaCO₃/L, the population diversity of fungi and bacteria declined; fungi in the Nectriaceae family were enriched, and the relative abundance of bacterial genus TM7x gradually increased. The growth of filamentous fungi was significantly inhibited under high alkalinity (3000 mg CaCO₃/L), and bacteria obtaining a greater substrate competitive advantage. The ammonia removal contribution of fungi decreased from 51.22 % to 39.73 % with increasing alkalinity, while the contribution of autotrophic bacteria increased from 28.76 % to 42.31 %. When the alkalinity was restored to 600 mg CaCO₃/L, the decrease in the fungal population size and the increase in the secretion of extracellular polymeric substances promoted the reconstructed of dense granules (mean size 762.9 ± 1.95 μm). This study found that alkalinity could make influences on the substrate competitive advantage of fungi and bacteria, and thus filamentous fungal overgrowth can be controlled. This finding provided a solid theoretical basis for controlling fungal overgrowth and restoring the structural stability of disintegrated HAGS.