Two-stage anaerobic digestion: State of technology and perspective roles in future energy systems

[Display omitted] •Industrial waste is with 35.2% most common substrate used in two-stage systems.•High methane contents of 85% can be reached with two-stage anaerobic digestion.•The pH in the first stage influences process stability and performance parameters.•With a higher process efficiency, the two-stage approach saves material costs.•Biomethane by two-stage high-pressure anaerobic digestion substitutes fossil fuels. Two-stage anaerobic digestion (TSAD) systems have been studied on a laboratory scale for about 50 years. However, they have not yet reached industrial scale despite their potential for future energy systems. This review provides an analysis of the TSAD technology, including the influence of process parameters on biomass conversion rates. The most common substrate (35.2% of the 38 selected studies) used in the analysed data was in the category of rapidly hydrolysable industrial waste with an average dry matter content of 7.24%. The highest methane content of 85% was reached when digesting food waste in a combination of two mesophilic continuously stirred tank reactors with an acidic (pH 5.5) first stage and alkaline (pH 7) second stage. Therefore, the review shows the limitations of the TSAD technology, future research directions, and the effect of integration of TSAD systems into the current strategy to reduce greenhouse gas emissions.