Suitability of Agro-Industrial Residues as Co-Substrates for Anaerobic Co-Digestion of Microalgal Biomass

Microalgal biomass is a valuable substrate for anaerobic digestion and produces biogas with a high methane content. Its performance can be further improved through co-digestion with organic waste, which also enables the valorization of this waste. Three agricultural wastes were tested as co-substrates for Scenedesmus biomass: grape pomace (rich in carbohydrates), rapeseed oil cake from oil pressing (rich in protein), and expired rapeseed oil (rich in lipids). Each was added at 25% on a volatile solids (VS) basis. Co-digestion was compared with microalgal mono-digestion in a semi-technical digester with a working volume of 100 L under mesophilic conditions over a seven-day cycle. Biogas volume and composition, hydrogen sulphide concentration, energy efficiency, and process stability (pH, temperature, oxidation–reduction potential (ORP)) were measured in a single reactor over one feeding cycle, without independent replications. The three wastes differed in their suitability, and none performed best across all parameters. The highest biogas yield was obtained with grape pomace (432 L·kg−1 VS, compared with 379 L·kg−1 VS for mono-digestion), together with the highest energy efficiency (1.96 kWh·kg−1 VS), and the lowest hydrogen sulphide concentration (268 ppm), but the lowest methane content (56.8%). The highest methane content (62%) was obtained with expired rapeseed oil, with a yield close to that of the control (381 L·kg−1 VS). The lowest yield (252 L·kg−1 VS) and the highest hydrogen sulphide concentration (409 ppm) were obtained for rapeseed oil cake. Process conditions remained stable across all variants (pH, 6.9–7.1; temperature, 36.9–37.0 °C; ORP, −334 to −420 mV). Grape pomace was the most suitable co-substrate in terms of energy recovery and biogas quality, whereas rapeseed oil cake was the least suitable.