Energy production from agricultural residues: high methane yields in pilot scale two-stage anaerobic digestion

There is a large, unutilised energy potential in agricultural waste fractions. In this pilotscale
study, the efficiency of a simple two-stage anaerobic digestion process was
investigated for stabilisation and biomethanation of solid potato waste and sugar beet
leaves, both separately and in co-digestion. A good phase separation between hydrolysis/
acidification and methanogenesis was achieved, as indicated by the high carbon dioxide
production, high volatile fatty acid concentration and low pH in the acidogenic reactors.
Digestion of the individual substrates gave gross energy yields of 2.1–3.4 kWh/kg VS in the
form of methane. Co-digestion, however, gave up to 60% higher methane yield, indicating
that co-digestion resulted in improved methane production due to the positive synergism
established in the digestion liquor. The integrity of the methane filters (MFs) was
maintained throughout the period of operation, producing biogas with 60–78% methane
content. A stable effluent pH showed that the methanogenic reactors had good ability to
withstand the variations in load and volatile fatty acid concentrations that occurred in the
two-stage process. The results of this pilot-scale study show that the two-stage anaerobic
digestion system is suitable for effective conversion of semi-solid agricultural residues as
potato waste and sugar beet leaves.