The effect of recycled utilization of liquid digestate on methane production and system stability of 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. The present study contributed to prove that digestates recirculation operation could improve the bioenergy production in the anaerobic digestion of pig manure under the 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. 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. Moreover, the stable carbon isotope analysis of delta C-13(CO2) and delta C-13(CH4) in produced the biogas indicate different methanogenic pathways between the anaerobic reactors with and without digestate recirculation. (C) 2017 Elsevier Ltd. All rights reserved.