November 15, 2010 | General

Calculating Feasibility Of Codigestion At Wastewater Treatment Plants

BioCycle November 2010, Vol. 51, No. 11, p. 39
Interactive map identifying food waste sources and processors, and spreadsheet to help WWTPs determine economic feasibility of codigestion, are available from EPA’s Pacific Southwest Region.
William Smallen, Matthew Ward and Laura Moreno

PROPOSALS to build new biogas facilities or to expand existing ones must demonstrate adequate source material as well as the potential for a reasonable return on investment. Recognizing the environmental value of anaerobic digestion and biogas use, the U.S. Environmental Protection Agency’s (EPA) Pacific Southwest Region developed two project planning and development tools that aim to bring more clarity to project developers. The first is an interactive map of the region that identifies sources and potential users of organic waste such as food processing facilities and wastewater treatment plants. The other tool, a downloadable Excel spreadsheet, is designed to help wastewater treatment plants determine the economic feasibility of codigesting food scraps as well as fats, oils, and grease (FOG).
EPA’s Pacific Southwest Region, encompassing California, Arizona, Nevada, Hawaii, and the Pacific Islands, contains some of the most productive agricultural land in the world. Combined with its large population, this ensures that there is a steady supply of organic waste, including food scraps, manure, biosolids and FOG. Currently, most of these materials end up in landfills, lagoons, or the sanitary sewer system. Diverting organic material to anaerobic digesters allows it to be converted to biogas and a digestate, generating energy and a soil amendment.
If anaerobic digestion were to reach its full market potential, a vast amount of energy could be harvested from organic materials. A study performed for the Northern California Power Agency in 2008 determined that agricultural, wastewater treatment and food processing wastes in California alone could be digested to obtain 453 MW of energy. This is enough to run a utility scale power plant while preventing 3.7 million bone dry tons of organic materials from ending up in landfills. If this biogas were used to displace natural gas, it would have a climate change abatement potential equal to taking approximately 160,000 cars off the road. This clean energy also could help California meet its Renewable Portfolio Standard.


The Waste to Biogas Mapping Tool (, currently in Beta version, covers EPA’s Pacific Southwest Region and is designed to connect facilities that generate organic waste with those that can utilize the material in anaerobic digesters. Streamlining the process of acquiring and disposing of digestible organic materials, this tool enables users to determine where potential partner facilities are located and transportation distances. This data assists in gauging the concentration of facilities in a given area that produce organic waste, allowing for an initial assessment of the suitability of an area for expanded codigestion or construction of a new digester facility. Users can determine the types of facilities in their area, where clusters are located, and the distance between a waste producer and an anaerobic digester. The tool functions in reverse as well, allowing generators of organic waste to find partner facilities that will accept it.
The types of facilities displayed include organic waste generators such as supermarkets, dairies, and food processing facilities and those that utilize the materials such as wastewater treatment plants and dairy digesters. Currently, public data regarding facility locations can be sporadic and difficult to obtain and some data is available only for a select number of states. In this initial version of Biogas Mapping Tool, select groups of waste generating and utilizing facilities were included. To make future versions of the tool more comprehensive, EPA will work to fill these data gaps and assemble more consistent information across states. Users of the tool can influence future versions by providing comments on desired data and potential data sources.
In some areas, such as California’s San Joaquin Valley and South Coast, poor air quality has resulted in stringent regulations of many pollutants, including nitrous oxide (NOx) and particulate matter. NOx, a precursor to ozone (smog), is emitted from internal combustion engines that are currently the norm for generating energy at biogas facilities. These engines cannot meet the more stringent regulations without costly improvements. In these regions, more innovative methods for using biogas, including gas upgrading and pipeline injection, should be considered. As an appendix to the Biogas Mapping Tool, information on natural gas utility service territories is presented as a static map. In future versions, this information will be displayed as part of the interactive map.
Natural gas service territories are displayed as a translucent color layer rather than as pipes, both for practical reasons as well as for security. Natural gas delivery pipes vary in size and capacity. Some can handle injection while others cannot. The information is provided so that project developers can more easily contact the utility to determine the feasibility of pipeline injection.
EPA is seeking feedback to improve this tool. Potential features for the next version include the addition of direct biogas users, quantities of organic waste at different sites, and estimated excess digester capacities. Stakeholder suggestions were the catalyst for many of the current features of this tool, and EPA welcomes ideas that will increase its utility across many sectors.


Codigestion of food scraps and FOG at wastewater treatment plants (WWTP) makes sense environmentally and economically. This emerging practice has the potential to use existing digester capacity to divert waste, mitigate climate change and generate renewable energy. Despite these benefits, the economic feasibility of codigestion is still relatively unknown and can vary greatly from place to place. The Co-Digestion Economic Analysis Tool, or CoEAT, enables wastewater treatment operators and solid waste managers to perform an initial economic analysis of codigesting food scraps and FOG at a WWTP. CoEAT users enter site-specific data to calculate the economic and operational viability of codigestion, as well as provide limited environmental data, including mitigated methane emissions from landfills and solid waste diversion potential.
No single set of parameters makes codigestion economically feasible. It depends heavily on the amount of excess capacity available in the WWTP digester(s), the quantity and quality of feedstock that can be readily sourced, as well as other factors. The various examples of codigestion in the United States have different models for collecting, processing and digesting the organic material. No single method has become the norm for preprocessing, either. The lack of existing programs and the diversity among cities, especially in the residential and commercial collection systems and infrastructures, make modeling difficult. For these reasons, EPA is looking for feedback for future iterations of CoEAT.
CoEAT utilizes the current publicly available data on codigestion at WWTPs to calculate key outputs from site-specific user inputs (Figure 1). These outputs include: 1) Fixed and recurring costs; 2) Solid waste diversion savings; 3) Required capital investments; 4) Biogas production and associated energy value; and 5) Methane reductions from landfills. CoEAT does not require preexisting WWTP digesters, and will calculate results with no digester in place. The model was intended, however, to help WWTP operators assess the viability of codigestion with existing anaerobic digesters.
A comprehensive analysis calculates the costs and savings at every step, from sourcing the feedstock to management of the digestate. With CoEAT, users can source feedstock from residential, commercial and industrial sources as well as fats, oils and grease (FOG). They can review and analyze existing collection infrastructure, bins and trucks, access costs, and tipping fees. Users can also include existing digestion infrastructure and processing or maintenance needs. Finally, they can choose how the digestate and biogas will be managed. CoEAT does require a basic amount of technical information. Realizing that some communities may have more detailed data on feedstock availability or other parameters, the tool is designed to accommodate varying availability of data. To promote transparency and flexibility, the key parameters and assumptions are accessible and can be easily changed by the user.
CoEAT is not intended to generate a full economic feasibility study, but merely to provide an initial assessment to understand the potential for a wastewater treatment plant to begin codigestion. If CoEAT shows that codigestion may be feasible, a professional feasibility study may still be needed.
As we come full circle from the “rag and bone” recyclers of a century ago to landfills as the norm and back to more comprehensive recycling, we are now considering our “wastes” as valuable commodities. Waste to biogas projects are a prime example of this trend, generating renewable energy and soil amendments from discarded food. While these projects have multiple environmental benefits, there is still much uncertainty around their implementation. These tools can help advance waste to biogas projects by reducing the uncertainty in sourcing feedstocks and economic feasibility.

William Smallen and Matthew Ward are Masters Candidate at the Monterey Institute of International Studies and interns in U.S. EPA Region 9’s Sustainable Infrastructure Office and Office of Pollution Prevention and Solid Waste, respectively. Laura Moreno is an Environmental Scientist in the U.S. EPA Region 9 Office of Pollution Prevention and Solid Waste (

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