From fruit pulp wastes to biomethane: assessment of substrate shifts on the performance of a two-stage anaerobic system and biogas upgrading studies

Wastes with high organic content, such as food waste, are produced worldwide and can cause serious pollution problems when poorly managed. Thus, there is the need for the implementation of environmental friendly treatment systems for organic wastes. Anaerobic digestion has the potential to contribute for the sustainable treatment of these wastes while producing biogas which provides a renewable energy source, methane (CH4). In this study, a two-stage anaerobic system was operated treating three different fruit pulp wastes (peach, raspberry and white guava) in a sequential operation. The effect of substrate shifts and different operational conditions, such as hydraulic retention time (HRT), organic loading rate (OLR) and pH on the system’s performance was assessed. The shift of substrates caused no long-term instability issues. The differences observed in the acidogenic performance in terms of gas production between substrates were considerable. Conversely, only slight differences were observed in fermentation products (FP) concentration and profiles. No evident association was found between pH and HRT/OLR changes on FP concentration and profiles in the range studied. Overall, the sugar removal efficiencies obtained were between 93.8 – 97.8% and the acidification degree varied between 53.7% – 76.4%. In regard to the methanogenic reactor, biogas production (3.6 – 12.8 L d-1) increased as OLR increased up to 7.4 g COD L-1, while CH4 yield (0.30 – 0.37 L CH4 g-1 COD) and content (75.9– 80.6%) remained approximately constant. Maximal chemical oxygen demand (COD) removal efficiency (around 93%) was achieved at HRTs of 8.6 and 5 days (OLR of 1.9 – 3.7 g COD L-1 d-1). Currently, there is the need to develop effective and economical viable solutions for biogas upgrading. Thus, gas permeation studies using mixed-matrix membranes (MMMs) with two different metal organic frameworks (MOFs) - MIL-53 and MOF-5 - were carried out in other to assess the potential for CH4 and carbon dioxide (CO2) separation. Matrimid®5218 with 10% (w/w) MIL-53 membrane showed the best performance among the membranes tested