Can two-stage instead of one-stage anaerobic digestion really increase energy recovery from biomass?

The supremacy of two-stage on traditional one-stage anaerobic digestion (AD), in terms of overall energy recovery (ER) from biomass has often been proved. However, the process conditions ensuring this result, as well as the reasons for higher efficiency, have always been unclear. In this work, a new standardized approach is proposed: optimization at lab-scale of both hydrogen and methane generation processes allowed comparing the maximum potential ER of both two-stage (as H2 + CH4) and one-stage AD (as CH4). Relatively high bio-hydrogen yields were obtained testing four different organic substrates (ER of 1–1.6 MJ kg−1VS-added). Biomethane generation resulted in ER in the range of 9–19 MJ kg−1VS-added., similarly for two-stage and one-stage systems. The overall ER resulted in significantly higher (8%–43%) for the two-stage in the large majority of experimental conditions and never significantly lower. These preliminary results should drive further research to better understand the conditions that can drive the two-stage AD to higher performance.