Enhancement of the anaerobic digestion of the organic fraction of municipal solid waste through nutrient supplementation

The effect of the addition of trace-elements on the anaerobic digestion of the simulated organic fraction of municipal solid waste was studied. Experiments were conducted at the one-litre, five-litre, seven-litre and twenty-five litre scale, and included both batch and semi-continuos low-solids digestion at mesophilic temperatures. Trace elements were added in the form of a defined chemical media, and the enhancement of digestion efficiency was measured in terms of biogas production and organic degradation.Adjustment of the carbon:nitrogen ratio of the substrate was necessary for stable batch digestion. Various organic and inorganic nitrogen sources were tested and the most effective nitrogen supplement was shown to be urea. The addition of a trace element mix provided enhanced digestion in both batch and semi-continuous operation. When the component elements were added to the digestion substrate individually the principal stimulatory element within the mix was shown to be cobalt, with an optimum added concentration of 0.15mg/1. The gas production and solids removal in digesters containing cobalt equalled that observed in digesters containing the entire trace element mix.Specific inhibitors (vancomycin and 2-bromoethanesulfonic acid) were used to study the effect of the addition of cobalt on the various trophic groups involved in the anaerobic digestion of the substrate. Analysis of intermediate products showed that the main degradation pathway was via the production of organic acids (and not alcohols). Cobalt was found to be stimulatory to acetoclastic methanogenesis. In the absence of methanogenesis, addition of cobalt also slightly increased the rate of production of volatile fatty acid intermediates. The principal rate-limiting step in the anaerobic digestion of the OFMSW substrate was propionic acid degradation.