The mesophilic and thermophilic anaerobic digestion of coffee waste containing coffee grounds

The anaerobic digestion of waste water containing significant levels of coffee grounds was assessed in mesophilic and thermophilic batch studies and CSTRs fed daily. A 58% reduction in VS was seen in both batch studies. Proximate compositional analysis showed that the waste had a high lipid component (26–33%). Levels of lipid, hemicellulose, α-cellulose and lignin were determined before and after digestion. These components were reduced as follows: lipid by 87% in the mesophilic study and 65% in the thermophilic study, α-cellulose by 51% in both mesophilic and thermophilic batch studies, hemicellulose by 22% in the mesophilic studies and 64% in the thermophilic studies. The lignin component was not reduced in either study. Mesophilic continuous digestion was achieved at a loading rate of 1.3 kg COD m−3 day−1 (25 day HRT) for 99 days. Addition of sodium bicarbonate alone was not sufficient for long term anaerobic digestion. Addition of Ca(OH)2, nitrogen, phosphorus and trace elements, however, gave successful digestion with COD and VS removal of 60% and a gas production rate of 0.34 11−1 day−1 (65–70% methane). Low levels of TVFA and high levels of bicarbonate alkalinity were present. Thermophilic digestion could be established at 1.6 kg COD m−3 day−1 (20 day HRT) with the addition of sodium bicarbonate alone, or Ca(OH)2 with nitrogen, phosphorus and trace elements. However long term digestion could not be established beyond 50 days without a increase in TVFA occurring.