Comparative biomethane production from three wastewater-derived algal biomasses during co-digestion with fat, oil and grease

Wastewater-derived algal biomass is increasingly generated from emerging algae-based treatment systems; however, its anaerobic digestion performance and response to co-digestion remain less understood across different wastewater treatment configurations. This study investigated the anaerobic co-digestion of fat, oil and grease (FOG) and three wastewater-derived algal biomasses harvested from pilot-scale systems treating anaerobic digestion centrate, municipal wastewater using algal-bacterial photogranules, and Upflow Anaerobic Sludge Blanket (UASB) effluent. Biochemical methane potential tests and kinetic modelling revealed substantial differences in mono-digestion performance among the three algal biomasses, with methane yields ranging from 142 to 321 L CH 4 kg −1 VS and apparent hydrolysis rates from 0.10 to 0.37 d −1 . During co-digestion, methane yields increased proportionally with FOG addition and closely matched values predicted from mono-digestion performance, indicating limited synergistic interactions. In contrast, digestion kinetics responded strongly to algal origin. FOG increased model-derived apparent hydrolysis rates by up to 2.8-fold for slowly degradable centrate-algae, whereas only minor kinetic improvements were observed for photogranules and UASB-algae. Comparison with published algae co-digestion studies further demonstrated that wastewater algae-FOG systems consistently exhibited predictable methane production while maintaining favourable kinetic enhancement. These findings demonstrate that wastewater algal origin is a key determinant of anaerobic digestion performance and that FOG functions primarily as a kinetics-enhancing rather than a methane-yield-synergistic co-substrate. This engineering framework provides a practical basis for co-substrate selection and process optimisation to facilitate energy recovery from algae-based wastewater treatment systems.