Microbial Stratification Engineering in Algal–Bacterial Biofilms for Energy-Efficient Wastewater Treatment and Resource Recovery

Algal–bacterial biofilms are being recognized as a promising bioresource technology for wastewater treatment and resource recovery, facilitating the simultaneous removal of pollutants, biomass production, and energy-efficient operation. Due to the vertically layered microbial structure of these biofilms, established by light and oxygen gradients, they can support the coexistence of aerobic, anoxic, and anaerobic microzones within a single biofilm matrix. The extracellular polymeric substances matrix in these biofilms has been recognized as a crucial component in maintaining biofilm structure, mass transfer, and functional microbial interactions under dynamic operating conditions. This review critically examines recent developments in the design and operation of algal–bacterial biofilms, focusing on nitrogen and phosphorus transformation mechanisms, microbial interactions, and the effects of key operational variables, including light intensity, biofilm thickness, and hydraulic retention time. The bioresource potential of novel reactor designs, such as inclined biofilm reactors, moving bed biofilm reactors, and membrane photobioreactors, is assessed based on energy efficiency, CO ₂ capture, and biomass productivity. Finally, the role of algal–bacterial biofilms in advancing carbon-conscious wastewater treatment through waste valorization, biogenic oxygenation, and integration with CO ₂ capture strategies is discussed, along with key challenges and future research needs.