The role of fungi in the formation and instability of halophilic aerobic granular sludge in a continuous flow reactor

Abstract

The role of fungi in aerobic granular sludge remains unknown. This study explored the formation and instability of halophilic aerobic granular sludge (HAGS) induced by fungi in a continuous flow reactor (CFR) figured with an aeration tank and a two-zone sedimentation tank. HAGS with a particle size of 420.10 ± 1.13 μm was successfully cultivated by increased hydraulic shear force (HSF). Increasing HSF stimulated microorganisms such as unclassified_f_Rhodobacteraceae, Muricauda, and Raineyella to secrete more extracellular polymeric substances (EPS) with a high protein (PN) content (81.03 ± 2.65 %). The peeled filamentous fungal fragments could act as colonization carriers to connect microorganisms. The mature HAGS had excellent removal efficiency of chemical oxygen demand (COD; 96.20 ± 1.19 %) and ammonia (92.13 ± 5.36 %) from 20 g/L saline wastewater. However, HAGS was disintegrated due to the fast growth of fungi, mainly unclassified_f_Nectriaceae, resulting in the deterioration of the sludge settling performance (SVI5 = 499.05 ± 7.80 mL/g). Filamentous fungi can use EPS secreted by bacteria under anaerobic and aerobic conditions. The excess EPS consumption finally destroyed the EPS and bacteria network. This study revealed the role of fungi in the formation and instability of HAGS in CFR, providing the basis for maintaining long-term HAGS stability.