Improvement of Methane Yield from Maize Silage by a Two-stage Anaerobic Process

Growing interest in processes that involve the conversion of biomass to renewable energies, such as anaerobic digestion, has stimulated research in this field and a considerable number of research projects have been developed to asses ideal digestion conditions for different energy crops. Among these potential crops, great interest has been addressed to maize silage. Most of the research on maize silage has been carried out on 1 stage batch digestion tests which have shown a great variability of results depending on hybrid type, harvest time, silage time and particle size. In this research project, an experimental procedure for the assessment of a two stage anaerobic digestion process was developed: a bench-scale reactor system was assembled for the evaluation of methanogenic activity of maize silage submitted to an acidification step, which could reduce the variability issues previously mentioned. The system consisted of two pairs of 5 L batch-fed completely stirred reactors, of which one was employed for the hydrolytic step and one for the acetogenic-methanogenic step. The Reactors were kept at mesophilic conditions (38°C). For the acidogenic reactors an inoculum, taken from an operating full scale acidogenic reactor, was diluted and used as starter for the hydrolysis of the maize. The acidification rate was evaluated by means of the carbon dioxide production, total COD and pH. Samples of the hydrolyzed substrate were then used as feed for the methanogenic reactor, in order to evaluate the methane yield of the produced biogas, which was 0.3 m3 CH4/kg CODremoved, the COD removal efficiency, that was over 80% and methane productivity in terms of volatile solids (VS), which was 0.38 m3/kg VS. The final totalCOD mass balances had a mean error lower than 5%.