Effect of Addition of High Strength Food Wastes on Anaerobic Digestion of Sewage Sludge

Anaerobic co-digestion of municipal sludge and food wastes high in chemical oxygen demand (COD) has been an area of interest for waste water treatment facilities looking to increase methane production, and at the same time, dispose of the wastes and increase the revenue. However, addition of food wastes containing fats, oils and grease (FOG) to the conventional anaerobic digestion process can be difficult and pose challenges to utilities. Incorporating these wastes into the treatment plants can potentially inhibit the digestion process.

In this study four lab-scale, anaerobic digesters were operated under mesophilic conditions and fed municipal sludge. One of them served as the control, while the other three digesters were fed with different volumetric loadings of juice processing waste, cheese processing waste (whey), dissolved air flotation waste (DAF) from a food processor, and grease trap waste (GTW), in addition to the municipal sludge. The impact of these high strength wastes (HSWs) on digester performance was analyzed for a total period of 150 days.

Among the parameters analyzed were pH, total and soluble COD (tCOD and sCOD), Total and Total Volatile Solids (TS and TVS), Total Ammonia Nitrogen (TAN), Total Kjeldahl Nitrogen (TKN), Volatile Fatty Acids (VFA), Long Chain Fatty Acids (LCFA), and alkalinity. Biogas was collected and analyzed for methane content. The dewatering characteristics of digested sludge were also studied. Volatile organic sulfur compounds were analyzed on the dewatered sludge in order to monitor odors.

This study showed that different high strength wastes have different impacts on digester performance. HSWs have the ability to degrade along with municipal sludge and to increase biogas production. However, anaerobic digestion can be inhibited by the presence of FOG, and addition of these wastes might not always be cost effective. Careful selection of these wastes is necessary to ensure stable digester operation, while bringing about increases in gas production. Utilities need to be cautious before adding any high strength wastes to their digesters.