Lemna minor as Support Biomass for Enhancing the Biomethane Yield of Brewery’s Spent Grain Pulp When Used in Co-Digestion

Pursuing the so-defined biorefinery approach, residual biomass, such as agro-industrial wastes, should first be exploited for the extraction and production of high-value-added products and then processed for energy valorisation through anaerobic digestion (AD). However, the treatments applied to achieve the first goal could impact biogas yield. This problem can be solved by co-digesting the treated biomass with others. In this study, Brewery’ Spent Grain (by itself, a good biogas producer) was treated with an ionic liquid (IL) composed of triethylamine and sulfuric acid [TEA][HSO4] for lignin removal. The residual biomass (pulp, BSGp) was then used for biogas production. The tests revealed a marked reduction in the total quantity of biomethane (per unit of volatile solid—VS). In detail, 6.82 × 10−4 Nm3CH4/gVS of biomethane was produced with BSGp, against 1.31 × 10−3 Nm3CH4/gVS with BSG. The lack of organic nitrogen after the IL-based treatment prevented biogas production, resulting in a shorter production period. To compensate for the nitrogen deficiency and restore the optimal C/N ratio, BSGp was mixed with Lemna minor (LM), an aquatic weed with a high nitrogen content. By itself, LM cannot be considered a good biogas producer as proven in this study. However, the co-digestion of LM with BSGp extended the production period and kept the daily production close to that registered in test made with the sole BSGp, thus achieving a total biomethane production equal to 1.83 × 10−3 Nm3CH4/gVS, even higher than the one registered with untreated BSG.