A Pinch or a Dash

Utilities traditionally have used anaerobic digestion to treat the biosolids generated from wastewater treatment. However, in recent years this practice has expanded to include codigestion of fats, oils, and grease (FOG) and other food processing wastes (High Strength Wastes-HSW). Utilities interested in high strength waste (food processing, ethanol production wastestreams, biodiesel production wastestreams and many others) must also adopt to a commodity market where the availability of HSW is subject to competition from other utilities, production schedules, the impacts of the global oil market and agricultural commodity prices (corn, oils, etc). All of these factors must be incorporated into a business case evaluation to avoid investing capital into facilities that may be underutilized due to forces outside the control of a utility. The mindset for evaluating this approach must be adjusted from the norm, where all parties are willing (or required) to send wastewater to the POTW to one where wastes must be recruited and the services of the POTW marketed to potential waste producers.This paper will utilize the results of multiple business case evaluations, laboratory testing and full scale digestion results to explore the challenges of planning and implementing a co-digestion program. The paper will focus on the multiple paths to achieving increased gas production while accounting for the costs associated with HSW collection, pre-processing and post digestion treatment. Multiple project examples will demonstrate the value in a diverse HSW recruiting program with multiple feedstocks for co-digestion.Several case studies have conducted analysis of various HSWs to determine key characteristics including solids content, COD, Nitrogen and Phosphorus Content and Volatile Solids. These values in conjunction with biological methane potential testing and published values have been utilized to determine the potential for gas production from HSW. With this evaluation it has been possible to determine the volume and mass of feedstock necessary to increase gas production in mesophilic anaerobic digesters. Table 1 summarizes the multiple paths a POTW may take in order to provide sufficient feedstock to achieve a 9 MM BTU/Hr digester gas production rate from HSW.As the table demonstrates there are numerous approaches to achieving the necessary digester loading and the characteristics of the HSW are critical to the volume of waste necessary. As the protein and fat content of a HSW increases the potential digester gas production from those feedstocks increases. Those feedstocks that are primarily carbohydrate based have both a lower volume of digester gas produced per unit of COD and a lower methane % in the digester gas. In addition, the solids content of the organic waste directly impacts the volume of waste necessary and therefore the impact on both the digester volume that is utilized but also the volume of liquid that must be maintained at mesophilic temperatures within the digesters.The characterization of the waste may then be incorporated into a digester modeling tool to determine the mass of solids remaining after digestion and the mass of nitrogen and phosphorus that are released into the bulk liquid of the digester. As part of multiple case studies the costs associated with dewatering, residuals hauling and ultimate disposal are applied to the remaining solids produced as part of co-digestion. The costs associated with treating dewatering recycle streams including nitrification, denitrification and P removal have also been determined and incorporated into the model. With these costs and the value of the digester gas, either for heat, electricity production or as a natural gas equivalent, the modeling determines the minimum tipping fee (if any) that is necessary to offset the costs of treatment based on the value of the biogas. This tool has demonstrated that certain waste streams have greater value even though they have a lower energy density due to lower overall treatment costs.The final piece of the evaluation relates to outside competition for wastestreams. As digesters are best operated in a consistent manner, the reliability of a wastestream is part of the value of the product. In our case studies the variability and reliability of thin stillage from ethanol manufacturing has reduced the value of the product for co-digestion as the availability of the product is a function of the ethanol manufacturing market. Therefore, it is very difficult to maintain a consistent loading to digesters as the supply of thin stillage varies from feast to famine.This paper will present the tools utilized to determine the value of various HSWs and present successful mixes of products to increase digester gas production.