Anaerobic digestion of swine manure in conventional and hybrid pilot scale plants: Performance and gaseous emissions reduction

Abstract. This study aims to present the results of a series of experimental tests performed with pilot scale digesters operating with swine manure: two digesters were conventional high load, completely mixed type, while the third digester was a hybrid type, with a fixed bed up-flow section. The tests were performed at mesophilic regime, with separated (filtered) manure from fattening pigs. The hydraulic retention time varied from 10 to 20 days. During the entire period (270 days) the mean VS removal efficiency resulted slightly higher than 40%, with lower values corresponding to the lowest retention time. The volatile solids (VS) removal performance reached peaks of 70% for some periods of the tests, with the best results for the hybrid type digester. These results are aligned with those obtained by other authors, even if for several studies higher mean efficiency was obtained (in Moller et al., 2007, 61% removal was reached, but at thermophilic conditions). The analyses of biogas showed an average methane percentage higher than 60%, with a maximum of 75%: these values can be considered more than satisfactory, considering that they are very close to those obtained by Moller et al. in thermophilic conditions. The mean organic matter conversion rate reached a maximum of 0,33 m3/kgVS for the hybrid flow digester operating with the highest organic load (RT=10 days). The emissions of NH3, CH4, CO2, N2O and methyl-mercaptanes were also determined, by means of static chamber method, both from raw and digested manure. The results showed significant reductions of the emissions (about 90% for CH4, 32% for NH3, 77% for CO2, 78% for N2O and 43% for mercaptanes), confirming that significant advantage of the anaerobic treatment is the reduction of gases, and hence of odors, emitted by digested manure (compared to the untreated), which means an enhanced sustainability of agricultural utilization of manure.