Kinetic study of the anaerobic digestion of the solid fraction of piggery slurries

This work deals with the determination of a kinetic model in an anaerobic digestion process. It is applied to the organic stabilization of the solid fraction of piggery slurry. The substratum results after filtering the farm wastewaters through a 1 mm mesh hydraulic sieve. Two pilot scale reactors were used, one digester was working in layered way — without agitation — and other was mixed with a stirrer. The experiments were carried out in a discontinuous process, 60 days hydraulic retention time, in the mesophilic interval (35°C). Average influent concentration was 68 and 97 g l−1 COD respectively for the non-stirred and stirred reactor, whereas the achieved reduction was 61 and 65%. Organic loading rates were 0.80 and 1.45 K VS m−3 d−1 . Specific biogas output was more efficient in the stirred reactor than in the stratified one. Several kinetic models were tried out in order to represent the methane production. A first order kinetic model applied in two stages was finally adapted for both reactors. The first stage presented the microbial growing as the limiting step, whereas the second stage was limited by the substratum availability. The effect of mixing on the kinetic parameters was analyzed. Significant differences were attained in the coefficients, thus K was 0.048 and 0.75 d−1 respectively for the non-stirred and stirred reactor.