Anaerobic digestion of spent coffee grounds

Abstract

Anaerobic digestion of spent coffee grounds (3 kg m −3 day −1) with efficient solids recycling achieved 99% conversion of solids and gas yields of 0·54 m 3 kg −1 (56–63% methane). Gas production declined steadily over the test period and after 80 days had fallen from an initial value of 1·70 litre litre −1 day −1 to 0·45 or 0·33 litre litre −1 day −1, with or without nitrogen supplementation respectively. Declining gas flows were not accompanied by other usual symptoms of digester failure and were not due to deficiency of nitrogen, phosphorus or trace metals. Inhibited digester liquors contained an unidentified component which inhibited gas production by fresh digestion liquors. This component was not a heavy metal, caffeine or brown ‘melanoid’ pigments. Digestion liquors could not be adapted to overcome this inhibition.