Anaerobic co-digestion of municipal waste sludge with fat, oil and grease : effectiveness of process configurations, and point of failure Okanagan

Fat, oil and grease (FOG) collected from restaurant grease trap is a well-known co-substrate for anaerobic digestion (AD) process to improve the process efficiency, since FOG has theoretically high biogas production potential. FOG hydrolysis leads to fast long chain fatty acid (LCFA) formation; however, LCFA accumulation can occur at a high FOG loading rate, which can lead to AD process failure. This study investigated the effect of FOG addition to the AD process utilizing municipal sludge and identified the optimum operational condition to reach the highest stable biogas production. Initially, two semi-continuous flow single-stage anaerobic digesters were operated under mesophilic condition (35°C) as a control and a co-digester. FOG addition to the co-digester was increased in a stepwise manner, this corresponded to organic loading rates (OLR) of 1.84 and 3.15 g volatile solids (VS)/L/d for control and co-digester, respectively. Results indicated that increasing FOG/sludge ratio up to 40% VS led to a stable digester performance and at 50% VS FOG, digester failure was observed. As a result of the addition of FOG at 40%, up to 99, 28 and 13% improvements in methane yield, VS removal, and chemical oxygen demand (COD) removal was achieved, respectively. Moreover, Class B biosolids were produced from the digesters for potential land application as fertilizer. In a reattempt to achieve a stable AD process at 50% VS FOG addition, two sets of temperature--phased AD (TPAD) systems were run according to the following scenarios: TPAD-1 with acid and phase temperatures of 55 and 38°C, respectively, and TPAD-2 with acid and methane phase temperatures of 70 and 38°C, respectively. They were run at 50%VS FOG addition and methane phase digester failure was observed in TPAD-1 and TPAD-2 after 35 and 49 days of operation, respectively. Hence, no improvement over the single-stage co-digester was achieved. Finally, batch experiments were conducted with three different biochar dosages and it was concluded that biochar has the potential to remove inhibitory LCFAs from AD. However, further studies are needed for biochar addition to continuous-flow AD process to identify the optimal biochar dose and AD configuration for an effective LCFA inhibition mitigation.Applied Science, Faculty ofEngineering, School of (Okanagan)Graduat