Multistep anaerobic digestion (MAD) as a tool to increase energy production via H2 + CH4

In an effort to produce more energy using organic wastes as an energy source, multistep anaerobic digestion (MAD) has gained attention in recent years. The energy potentiality of the MAD approach has been verified experimentally using two carbon sources: glucose, as the more easily biodegradable carbon source, and a rich lignocelluloses material (coffee seed skin) waste. Energy production is analysed by evaluating the energy produced as hydrogen plus methane using the efficiency (η) parameter to determine the energy produced versus the energy embedded in the sources. Four fermentation tests, each repeated three times, were performed: one-step and two-step AD with glucose, and one-step and three-step AD with coffee seed skins (including a pretreatment step). The two-step AD efficiency (η = 74.5%) using glucose was 48% higher than the one-step AD (50.4%), while the efficiency using lignocelluloses material was 28.5% (2.4 times higher than the one-step AD). The specific methane production reached 0.15 NLCH4/gTVS using coffee seed skin, which is approximately 3-fold higher than the one-step AD (0.05 NL CH4/gTVS). This study confirms that the MAD process offers an opportunity to extract more energy from organic refuse that the one-step AD.