Laboratory scale examination of the effects of overloading on the anaerobic digestion by glycerol

The anaerobic digestion of pure glycerol, which produces a baseline acetic acid to propionic acid ratio of 0.2, was studied in laboratory scale reactors (3 l working volume) at mesophilic temperature (37 °C) with 3000 mg chemical oxygen demand (COD) l−1d−1. During the experiment tVFA and C2-C6 VFA analysis and daily biogas yield measurement were carried out. Following 10 days of a 15% d−1 increase in the organic loading rate (OLR) of 3.0–10.5 g COD l−1d−1, the concentration of propionic acid increased to 6200–8000 mg l−1. Then the inoculum was divided into three parts feeding with 100% glycerol, 50% glycerol + 50% acetic acid, and 50% glycerol + 50% thick stillage, (presented in % of 2.60 g COD l−1d−1 OLR), respectively. The application of co-substrates reduced the recovery period by 5 days compared to feeding with pure glycerol. When the reactors were loaded with glycerol again (10% OLR raise per day) the previously applied co-substrates had a positive effect on the VFA composition and the biogas yield as well.