Long-term assessment of anaerobic co-digestion of food waste and microalgae: Process stabilization, methane yield and agronomic properties of digestate

This study aimed at assessing the anaerobic co-digestion of food waste and microalgal biomass. Microalgae were harvested from an open treatment pond used for digestate treatment. For that, two continuous laboratory-scale reactors were operated over 200 days at ambient temperature. Results obtained showed that the reactor fed with a 75:25 mixture of food waste and microalgae (based on volatile solids) improved process stability, overcoming volatile fatty acid accumulation and pH drop, compared to mono-digestion of food waste. Consequently, co-digestion increased methane yield in 20–32 % (0.22 L CH 4 /g VS). Moreover, the agronomic assessment revealed that digestates exhibited potential for use as a biofertilizer, i.e. adequate organic matter and nutrient contents heavy metals concentrations below limits established by different regulations (i.e., Zn, Ni, Cr, and Cu). These findings indicate that the anaerobic co-digestion of food waste and microalgae may offer potential for biomass valorisation in a circular approach, generating bioenergy and biofertilizer. • Anaerobic digestion of food waste and its co-digestion with microalgae were examined. • Microalgae contributed to maintaining pH buffer capacity and process stability. • Co-digestion showed more stable and highest methane yield in all phases. • Digestates could be used as biofertilizer according to agronomic characterization.