Effect of electrochemical treatment of oleic acid on anaerobic digestion

Abstract

Electrochemical pre-treatment of oleate on anaerobic digestion was investigated using two anaerobic filters (AF and AFm) working, respectively, with increasing concentrations of electrochemically pre-treated oleate and oleate. Influents containing 25% and 50% of chemical oxygen demand (COD) of these substrates did not cause relevant alterations in either digester's performance. Comparatively, the pre-treated oleate unit (AF) showed a higher gas production but a lower COD removal than AFm (2.0 vs 1.5 m3 CH4 m−3 d−1; 76-85 vs 91-93% COD removal). The raised AFm COD removal together with the larger proportion of VSS in its effluent than in AF (5.69 vs 0.26 kg m−3) indicate that the biomass washout can be a consequence of the encapsulated solids by lipidic compounds that were not fairly degraded. Further increases of oleate in AFm feed (75 and 100% COD) led to decreases of biogas production to half and COD removal from values higher than 90% to 83-75%. The simultaneous increase of effluent VSS concentrations to 10-12 kg m−3 emphasizes the oleate negative effect. The use of pre-treated oleate as the only carbon source did not influence negatively the AF behaviour in terms of methane production. COD removal was maintained at values higher than 80% and the effluent VSS at low concentrations (0.15 kg m−3). The significantly higher methane yield achieved by AF than AFm indicates a higher accumulation of oleate than pre-treated oleate. Therefore, electrochemical process stands for a suitable pre-treatment for lipid rich effluents, allowing the application of higher organic loads keeping a higher methane yield.