Liquid digestate recycled utilization in anaerobic digestion of pig manure: effect on methane production, system stability and heavy metal mobilization

摘要

To improve bioenergy, methane, production in the anaerobic digestion plant application, the effects of recycled liquid digestate on anaerobic digestion of pig manure were investigated. Two continuous stirred tank reactors were operated for 230 days with varying organic loading rates (OLRs, from 1.5 to 6 g VS L-1 d-1); one reactor was implemented with liquid digestate recirculation and the other was set as the control without recirculation. It was demonstrated that the recirculation operation improved methane production and system fermentation stability, particularly for OLRs below 5 g VS L-1 d-1. The inhibition of methane production was found under an OLR of 6 VS L-1 d-1, which was caused by significantly increased viscosity from 30 to 1000 mPa·s and decreased mass transfer characteristics. The previously reported negative effects of accumulated ammonia and VFA on anaerobic digestion under digestate recirculation were not found in the present investigation of pig manure treatment. However, the heavy metals Pb, Mn, Cu and Zn accumulated in both liquid and solid fractions of the generated digestate in the digestate recycled reactor. The stable carbon isotope analysis of δ13CCO2 and δ13CCH4 produced the biogas may indicate different methanogenic pathways between the anaerobic reactors with and without digestate recirculation.