Anaerobic co-digestion of dairy manure, meat and bone meal, and crude glycerol under mesophilic conditions: Synergistic effect and kinetic studies

Anaerobic digestion is a potential renewable energy, climate independent and robust technology, which is able to treat different kinds of organic wastes and by-products. This study investigated the anaerobic co-digestion of meat and bone meal (MBM) with dairy manure (DM) and crude glycerol (CG). Three sets of batch experiments were conducted at mesophilic condition; one set of anaerobic mono-digestion and two sets of anaerobic co-digestion. In experiment I, each substrate was mono-digested at inoculum to substrate ratio of 1. In experiment II, MBM and DM were co-digested at ratios of 1.0:1.0, 1.0:2.0, 1.0:1.0, and 2.0:1.0, while in experiment III CG was co-digested with MBM at ratios of 1.0:3.0, 1.0:1.0 and 3.0:1.0, at a fixed amount of DM. The results of anaerobic mono-digestion showed that CG produced the highest methane yield (0.48 L/gVS) followed by MBM (0.41 L/gVS) and DM (0.17 L/gVS). In the anaerobic co-digestions, methane yield increased with the increase of MBM content, while it increased together with CG content. The kinetic studies showed that the physico-chemical characteristics of the co-digested substrates influenced hydrolysis rate constant and lag-phase, which increased with the increase of CG content. However, synergistic effect was decreased when MBM content was increased, whereas the opposite was observed to that with CG. Therefore, carbon to nitrogen ratio was an important parameter determining synergistic effect in anaerobic co-digestion, while the physico-chemical characteristics influenced the hydrolysis rate constant and lag-phase.