Batch and Continuous Mesophilic Anaerobic Digestion of Food Waste: Effect of Trace Elements Supplementation

Trace metals are essential for the enzyme cofactors involved in the biochemistry of methane formation and are needed in a balanced anaerobic digestion process. Food and kitchen waste generally contains low concentrations of trace elements, especially metals. As a consequence the anaerobic digestion process may result instable. The aim of this study is to evaluate the effect of metals addition on mesophilic anaerobic digestion of food waste, both in batch tests and in laboratory scale CSTR reactors. Batch anaerobic trials using source-separated food waste as substrate with inoculums of different origins were carried out under mesophilic conditions. Reactions were operated both with and without trace elements (Co, Mo, Ni, Se, W) supplementation. Supplementation with trace metals had either neutral or slightly negative effects with inoculums originating from reactors with a high background level of metals, such as those for the co-digestion of biowaste and waste activated sludge. For inoculums from reactors treating food waste only, which inherently contain low levels of trace metals, supplementation with these metals increased methane production. In particular, Mo concentrations in the range of 3-12 mg/kgTSfed and Se concentrations of 10 mg/kgTSfed increased methane production to as high as 30-40 %. Supplementation with a metal mixture (Co, Mo, Ni, Se, W) increased the methane production to the range 45-65 % for inoculums with low background concentrations of trace metals. These findings demonstrate the importance of Co, Mo, Ni, Se, W for high-performance anaerobic digestion process. Trace metals additions that showed the best batch results (100mgNi/kgTSfed, 100mgCo/kgTSfed, 6mgMo/kgTSfed, 10mgSe/kgTSfed, 10mgW/kgTSfed) were selected for CSTR experimentation. Performances of a continuous anaerobic digester fed with trace elements were compared to a "control reactor" fed with the same substrate but without metals addition. Preliminary CSTR results showed that metals addition allowed for a stable anaerobic digestion process at Organic Loading Rate greater than 3 kgVS(fed) m(3).d but were not essential.