Improvement of biogas production by co-digestion of swine manure and residual glycerine

Abstract

Anaerobic digestion of swine manure supplemented with residual glycerine from a biodiesel production plant was investigated as a means for increasing biogas production on livestock farms treating residues by the anaerobic digestion process. Two different manures were tested proceeding from two local facilities, being evaluated under batch digestion. Semi-continuous co-digestion was studied using single-stage completely mixed anaerobic digesters. Mixtures were prepared at 2, 5 and 8% (v/v) of glycerine. Improvement of biogas production was observed with the increment in the content of residual glycerine fed to the digester, increasing the biogas production from 2.1 to 5.4 l/day. Inhibition by ammonium was presented in all systems evaluated (with free ammonia concentrations above 300 mg/l), resulting in volatile fatty acids build-up, being higher as the content of glycerine was increased in the feeding mixture. Although systems were submitted to a 30-day adaptation period and the time of study was performed until completing three-times the hydraulic retention time, volatile fatty acid measurements presented great variability being indicative that a time greater than three volume turnovers is needed if conditions of steady state are to be set.