CO₂ Emission Avoidance in Organic Waste Management: Co-Digestion of Brewery’s Spent Grain Pulp with Lemna minor

This study focuses on biogas production within lab-scale semi-batch bioreactors using agro-industrial wastes and dry biomass of an invasive aquatic species. In particular, the primary objective is to increase the yield of anaerobic digestion processes, with a specific focus on reducing CO₂ emissions associated with the degradation of biomass, by co-digesting different raw biomasses and agro-industrial wastes. In detail, the experiments concerned the pulp of Brewery’s Spent Grain (BSGp), consisting of the residual of Brewery’s Spent Grain after fiber deconstruction with ionic liquids–based treatment, and Lemna minor L. (LM). The two biomasses were studied separately and then co-digested. Co-digestion was carried out using a 1:1 (VS basis) mixture of Lemna minor and Brewery’s Spent Grain pulp. Due to the lack of organic nitrogen, BSGp showed low biogas production if compared with untreated BSG (1.14 × 10⁻³ vs. 1.71 × 10⁻³ Nm³/gVS). Differently, LM has a high nitrogen content and, when digested alone, produced 9.79 × 10⁻⁴ Nm³/gVS. The co-digestion tests allowed us to reach the highest performance: 2.94 × 10⁻³ Nm³/gVS. In terms of bioenergy production, the two biomasses showed high synergy when used in co-digestion. The amount of energy produced was calculated using a lower heating value (LHV) of CH₄ equal to 52 MJ. The results showed that co-digestion yielded 64.9 ± 0.6 MJ/kgVS, followed by BSG (43.3 ± 5.3 MJ/kgVS), BSGp (25.6 ± 0.3 MJ/kgVS), and LM (19.3 ± 1.0 MJ/kgVS). In addition, in terms of CO₂ avoided, the following results were achieved: 0.38–0.40 gCO₂/gVS with BSGp, 0.73–0.8 gCO₂/gVS with LM. Conversely, co-digestion tests allowed for the avoidance of 1.68–1.91 gCO₂/gVS. In conclusion, co-digesting BSGp with Lemna minor yields more methane and less CO₂ per unit processed, providing an effective way to convert readily available waste and biomass into bioenergy.