Anaerobic digestion of crude glycerol from biodiesel production for biogas generation: Process optimization and pilot...

Anaerobic digestion of wastes and wastewater is a complex process that can be affected by many operational parameters. In this context, the purpose of the present study was to optimize biogas production using crude glycerol (GLY) generated in biodiesel production from waste cooking oil without pretreatment or nutrient supplementation. The study was divided into two parts: the first phase consisted of an experimental design based on central composite design (CCD) with two variables (food to microorganism (F/M) ratio and cycle time) at five levels (F/M of 0.20; 0.51; 1.02; 1.53 and 2.04 gCOD/gVS; t_c of 3, 4, 5, 6, 7 days) focusing on optimizing the biogas production from crude GLY in lab-scale batch reactors (500 mL). The second phase was conducted on a pilot-scale biodigester (1.2 m³) based on the optimized variables obtained from the CCD. The optimized results showed that the F/M ratio of 2.04 gCOD/gVS and a cycle time (t_c) of 6 days reached the highest specific methane production (SMP) of 46 LCH₄/kgVS. However, the highest SMP of 14.7 LCH₄/kgVSd was obtained during the operation of the pilot-scale biodigester for the optimized conditions of F/M ratio of 0.23 gCOD/gSV and a t_c of 7 days. Therefore, pilot-scale biogas production from crude GLY was demonstrated to be feasible without the use of nutrients or GLY pretreatment at 0.15 LGLY/m³ d.