Digestate recirculation through co-digestion with rice straw: Towards high biogas production and efficient waste recycling

The present study investigated the effect of solid digestates from first (FD) and second (SD) runs of anaerobic digestion on rice straw (S) degradation and biogas production. The results indicated that codigestion of digestate (D) increases the relative lignin content in favor of cellulose and hemicellulose. In addition, digestate enhanced the biomethane content of biogas under both mesophilic and thermophilic conditions. Therefore, different ratios of D:S were further studied. The lowest fiber contents after mesophilic co-digestion (14.0% cellulose, 11.1% hemicellulose and 7.4% lignin) were recorded with FD:S 2:1, which showed insignificant differences with those of FD:S 1:1. Thermophilic anaerobic digestion resulted in slight changes in fiber content among all studied digestate ratios, which confirms that addition of digestate during thermophilic anaerobic digestion speeds up the rate of the process rather than fiber degradation. All applied digestate ratios resulted in shift in the time of maximum biogas production. Therefore, the maximum biogas productivities of 8.5 and 12.4 L kg(-1) VS d(-1) under mesophilic and thermophilic anaerobic co-digestion were recorded using 1:1 FD:S and 1:1 SD:S, which represented 46.6% and 25.3%, respectively, over the corresponding control. The present study suggests a novel approach of solid digestate recycling that enhances pH stabilization and biogas production. (C) 2020 Elsevier Ltd. All rights reserved.