Increasing biogas production from sewage sludge anaerobic co-digestion process by adding crude glycerol from biodiesel industry

•Glycerol added to a primary sewage sludge in anaerobic digester.•Glycerol loaded from 0wt% organic loading rate to 70wt% organic loading rate.•Biogas production increased 2.8 times as compared to primary sewage sludge alone.•Inhibition dominated at organic loading rates greater than 65%. In an effort to convert waste streams to energy in a green process, glycerol from biodiesel manufacturing has been used to increase the gas production and methane content of biogas within a mesophilic anaerobic co-digestion process using primary sewage sludge. Glycerol was systematically added to the primary digester from 0% to 60% of the organic loading rate (OLR). The optimum glycerol loading range was from 25% to 60% OLR. This resulted in an 82–280% improvement in specific gas production. Following the feeding schedule described, the digesters remained balanced and healthy until inhibition was achieved at 70% glycerol OLR. This suggests that high glycerol loadings are possible if slow additions are upheld in order to allow the bacterial community to adjust properly. Waste water treatment plant operators with anaerobic digesters can use the data to increase loadings and boost biogas production to enhance energy conversion. This process provides a safe, environmentally friendly method to convert a typical waste stream to an energy stream of biogas.