Multistep Anaerobic Digestion (AD) as a Tool to Increase Energy Production”, in proceedings of IV International Symposium on Energy from Biomass and Waste

Multistep Anaerobic Digestion (AD) has been verified experimentally using two carbon sources: glucose, the most easily fermentable, and a lignocelluloses material (coffee seed skin) waste from a coffee production facility. In the latter case, a basic-thermal pre-treatment has been used. The energy production has been analyzed by evaluating the produced hydrogen and the methane using two parameters: the first, efficiency (x ), takes into account the quantity of energy that the process is able to extract, compared to the available amount of energy embedded in the refuse, which is evaluated by Low Heat Value (LHV); the second, efficacy (η), compares the energy conversion efficiency of the bioprocess, using the refuse and the same energy conversion parameter obtained using glucose. The tests have been carried out in a bench bioreactor (14 L stirred-batch reactor) after a pre-treatment step in the case of coffee seed skin. The pre-treatment step consisted of mechanical (kitchen blade mixer) and chemical/thermal (NaOH for 3 h at 90 °C) processes. The specific methane production reached 0.98 L CH4/ gVS in the case of coffee seed skin, which is three times the value reported in the literature. The paper confirms the great opportunity offered by two-step AD as a processes able to extract energy from organic refuse, even if a Life Cycle Analysis and Energy Sustainability analysis are necessary to valorise the approach.