Re-granulation and performance of anaerobically digested bacterial and algal-bacterial aerobic granular sludge

Effective treatment and sustainable waste sludge management are critical challenges for future biorefinery wastewater treatment plants (WWTPs). This study investigated the feasibility of re-granulating anaerobically digested bacterial and algal-bacterial aerobic granular sludge (AGS) for sustaining the continuous operation of the AGS-based WWTPs due to the requirement of long-term operation for granulation of flocculent activated sludge. Rapid re-granulation was achieved within 12 and 6 days respectively from digested bacterial AGS and algal-bacterial AGS, demonstrating their high stability and settleability even after anaerobic digestion (AD). The re-granulated bacterial AGS system exhibited >90% dissolved organic carbon (DOC) removal, probably attributed to its greater microbial diversity and richness and elevated extracellular polymeric substances (EPS) secretion. The re-granulated algal-bacterial AGS system featured enhanced functional adaptability. It showed a lower average effluent dissolved total carbon concentration (∼100 mg/L) and high total inorganic nitrogen removal (>89%) in addition to > 56% maximum total phosphorus removal. Morphological observations revealed that some granules retained their compact structure and cores after AD, providing a niche for their re-granulation. Aromatic proteins and fulvic acid-like organics were the critical promoters for AGS regranulation. Notable shifts in microbial community structure, particularly the increase in abundance of photosynthetic bacteria such as Erythromicrobium, Leptolyngbya, and Rhodobacter, played an essential role in enhancing the overall performance of the re-granulated algal-bacterial AGS. By validating the system's effectiveness and exploring the factors governing re-granulation, this study proposes a viable strategy for advancing the sustainability of AGS-based WWTPs and promoting circular bioeconomy practices. © 2025 Elsevier Ltd.