Effect of High Strength Food Wastes on Anaerobic Co-digestion of Sewage Sludge

Anaerobic co-digestion has been practiced at wastewater treatment plants to increase methane production, but the impact of many variables is still not well understood. In this study, the potential benefits and feasibility of co-digesting fats, oils and grease (FOG) and other high strength wastes (HSWs) with municipal sewage sludge were investigated.Four lab-scale, continuously mixed digesters were operated at a working volume of 9.75 L, 15 day SRT and temperature of 37 °C. Wastes from different food processing industries, including whey (cheese), juice, grease trap waste (GTW) and dissolved air flotation waste (DAF), along with municipal sewage sludge, were fed to the digesters in varying amounts. The volume and methane content of biogas were measured regularly. Additionally, the optimal amount of polymer needed for sludge dewatering was determined through the capillary suction time (CST) method, and odor measurements were made on the dewatered sludge.The results showed that the addition of HSWs led to higher methane production at lower organic loadings (6% whey, 18% GTW and 76% sludge mix on a volumetric basis). However, at higher organic loadings, the GTW appeared to be toxic to the methanogens, leading to digester failure. During this period of failure, there was a decrease in pH and biogas production, and an accumulation of volatile fatty acids within the digester.