Heat recovery from composting processes and its use as a source of bioenergy have come into prominence in recent years. In this study, the selection of an adequate bulking agent for the composting of the solid fraction of pig slurry has been based on the heat produced by the microbial activity during aerobic degradation of the organic matter. Residues of five plant species typical of the Mediterranean area were mixed with the solid fraction of pig slurry, as bulking agents. The residues were: the prunings of three common cultivated tree species (orange, palm, and olive) and the biomass of two herbaceous species (milk thistle and sunflower). Self-heating tests were run for 18 days in thermally insulated laboratory-scale reactors; the energy released as heat was calculated from the temperature increase inside the reactors. The mixtures were sampled at the beginning and end of the experiment, and the microbial stability was evaluated by aerobic respiration tests. The heat energy produced was highest in the mixture with milk thistle (18.3 ± 1.78 MJ kg-1 of total solids -TS), followed by that with orange tree prunings (16.0 ± 1.094 MJ kg-1 TS), and was lowest with olive tree prunings (6.3 ± 0.23 MJ kg-1 TS). The microbial biodegradability of the mixtures and the physico-chemical characteristics (mainly the C/N ratio, cellulose concentration, and density) of the different bulking agents controlled the heat energy produced. The orange tree prunings were considered the most suitable bulking agent for composting of pig slurry solids as they showed high degradability and energy recovery potential.
Keywords: Compost; Energy release; Higher heating value; Lignocellulosic biomass; Microbial degradation.
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