Algal-bacterial granular sludge process for sustainable wastewater treatment: Technological advances and challenges

The algal-bacterial granular sludge (ABGS) process is a promising solution for sustainable wastewater treatment, owing to its synergistic contaminant removal, carbon sequestration potential, and reduced energy demand. This review critically synthesizes recent advances in the ABGS process, emphasizing key determinants of granule formation and stability under dynamic and static conditions. Existing knowledge and uncertainties throughout the entire treatment continuum are systematically discussed. Laboratory-scale explorations that rely on synthetic wastewater and batch operations fail to fully capture the complexity of real influent characteristics, geographic variability, and seasonal fluctuations. Sustaining a balanced algal-bacterial consortium is essential yet challenging, as uncontrolled proliferation disrupts community structure and diminishes efficiency in long-term practical operation. These limitations call for novel reactor designs and process optimizations that regulate mass transfer, dissolved oxygen (DO), and light availability while ensuring performance consistency at scalable capacities. Imbalanced algal-bacterial symbiosis may result in overestimated biodegradation performance, greenhouse gas (GHG) mitigation claims, and underestimated N₂O emissions, while the real environmental footprint and gene transfer risks still require further validation. Integrating artificial intelligence (AI) strategies is crucial for predicting system performance, optimizing microbial activities, and enhancing system efficiency. Critical knowledge gaps and emerging opportunities for future research are also outlined. This review reconceptualizes ABGS as a biotechnological innovation and an integrated, scalable solution within the broader context of the circular bioeconomy.