Pretreatment of lignocellulosic material to maximize its degradability and anaerobic digestion efficiency

Lignocellulosic material can be used as feedstock for anaerobic digesters to produce renewable energy as biogas or alternatively as isolated biomethane. Furthermore, if the biomass comes from waste sources, as in the case of FORSU (Frazione Organica del Rifiuto Solido Urbano), it allows us to get rid of them while valorizing their intrinsic qualities. In fact, the lignocellulosic material is potentially an excellent source of nutrition for microorganisms, but its complex structure makes it difficult to be metabolized; therefore, a pretreatment is necessary to increase its availability. Research on the state of the art of pretreatments is fundamental to select the most sustainable and efficient industrial process, but the high number of different methods available and the low maturity of pretreatment technologies render it particularly difficult; moreover, the wide typologies of substrate analyzed doesn’t allow a significant comparability between studies and often crop residues are used to measure pretreatment effectiveness. Therefore, the preliminary analysis has been carried out to have an overview on the state of the art, then pretreatment technologies have been chosen according to the company needs and availability. Experimental analysis has been done using thermal (80°C), acidic (Phosphoric acid: H₃PO₄), alkali (Potassium hydroxide: KOH) pretreatment and a combination of them (chemothermal pretreatments) on the solid fraction of digestate; in this way, it is treated the most recalcitrant portion of lignocellulosic material that has previously undergone anaerobic digestion without being digested. The results show a positive effect with every pretreatment analyzed and each efficiency is dependent on the pretreatment intensity.