Anaerobic digestion of glycerol and co-digestion of glycerol and pig manure

The potential of glycerol obtained from transesterification of waste cooking oil as a main carbon source for biogas production was investigated. The glycerol was highly contaminated with oils and fats and was pretreated with sulfuric acid. Using a carbon source of glucose as a control, we compared biogas production from the acid-treated glycerol in a synthetic medium and the acid-treated glycerol mixed with pig manure. The anaerobic digestion of acid-treated glycerol with supplement in a synthetic medium was found to be satisfactory at organic loading rates (OLR) between 1.3, 1.6 and 2.6 g chemical oxygen demand (COD) L−1 d−1. The maximum methane yield of 0.32 L at Standard temperature and pressure (STP) g−1 COD removal was achieved at an OLR of 1.6 g COD L−1 d−1 and the methane content was 54% on an average. At a higher organic loading rate of 5.4 g COD L−1 d−1, the propionic acid to acetic acid ratio was higher than the critical threshold limit for metabolic imbalance. Anaerobic digestion of acid-treated glycerol with pig manure was also investigated at the COD ratio of 80:20 (glycerol:pig manure). The anaerobic digestion of acid-treated glycerol with pig manure was found to be satisfactory at organic loading rates between 1.3, 1.7, 2.9 and 5.0 g COD L−1 d−1 in terms of COD reduction (>80%) and methane content of (62% on an average). However, the biogas production rate was found to significantly decrease at the highest load. The maximum methane yield of 0.24 L STP g−1 COD removal was achieved at an OLR of 1.3 g COD L−1 d−1.

► Acid-treated glycerol was the main carbon source for biogas production.
► The maximum organic loading rate was found to be 5.0 g COD L−1 d−1.
► High H2S production was observed at the higher organic loading rates.
► Methanogenesis inhibition was observed at the higher organic loading rates.
► The modified Stover–Kincannon model can describe the kinetics of the co-digestion.