This tool is intended to guide you through technical feasibility of installation of on-farm anaerobic digestion. Refer to the

user manual for guidance on the use of this tool. This tool will take you through the following steps:

This tool will provide recommendations for the most appropriate technology at your site based on both your management practices and preferences for the system. Please rate the following criteria based on importance to your facility on a scale of 1-5, where 1 is lowest priority and 5 is highest priority. You may select any value between 1 and 5 for any of the criteria and you may select the same number for more than one criteria.

Co-digestion has shown to be economically viable in other states. When agricultural producers and related industries (eg. food manufacturers) are located nearby, there are typically efficiencies in feedstock availability and transport, as well as the energy end product. The economic efficiency improves when the biogas can be used onsite or nearby.

Feasibility studies have shown that co-digestion projects might be economically viable in Colorado (Stewart Environmental, 2008; Keske, 2009; City of Greeley and Symbios, 2009). However, at this writing, there is not a co-digestion project currently operating in Colorado. In summary, while there have been national efforts to compile information about AD and co-digestion, Colorado and the Intermountain West possess unique environmental characteristics that affect the economic feasibility of these systems. Low humidity, scarce water resources, western farm management practices, and regional policies are examples of such unique issues. If you or your community has an interest in a co-digestion project, it is suggested that you review one of the attached reports, and contact the corresponding author to learn more information.

References

City of Greeley and Symbios Technologies, LLC.Final Report to the State of Colorado Governor's Energy Office. Phase 1 Engineering and Business Feasibility Analysis of a Multi-Feedstock Waste-to-Energy Facility at the Greeley Clean Energy Park. November 13, 2009.

Keske, Catherine M.H. 2009. Economic Feasibility Study of Colorado Anaerobic Digester Projects Prepared for the Colorado Governor's Energy Office. Grant 09-205. August 28, 2009.

Stewart Environmental Consultants. Report of the Feasibility Study on Utilizing Anaerobic Digesters to Generate Biogas from Diary Cattle. Report submitted to Colorado Department of Agriculture. October 20, 2008.

Answering the next round of questions will provide more information on the feasibility of installation of anaerobic digestion at your site.

Covered lagoons are considered the cheapest and simplest anaerobic digestion technology available. Anaerobic digestion and subsequent production of methane takes place naturally in lagoons which contain animal wastewater. A synthetic cover, typically high density plastics or rubber is used to trap and store the biogas. Covered lagoons are difficult to heat and they are only recommended in warm climates where freezing temperatures are rarely observed. Often times too little methane is generated by covered lagoons during cold winter months to justify installation of biogas capture and use equipment. Covered lagoons have the lowest operations and maintenance of the technologies described here, but also have the lowest energy yields. Covered lagoons can have a low capital cost, but are greatly affected by economics of scale and are not practical for very small operations. You should consider installing a covered lagoon if complaints of odor or greenhouse gas reduction are the major factors driving interest in AD technology. Carbon credits can be obtained through reduction in methane release from the lagoon compared to an uncovered lagoon, but the carbon market is volatile. Installing a covered lagoon can also add complexity to the required cleaning and maintenance of existing lagoons due to issues with removing covers. Covered lagoons have low volatile solids destruction rate compared to other technologies, meaning that there will be a larger amount of solid end product to handle when this technology is applied compared to others.

Plug flow digesters are a low tech anaerobic digestion technology for treatment of high solids content waste. The thick, high solids content waste travels down the digester in a "plug," as a continuous mass. Plug flow digesters can be a good fit with the high solids content waste generated by animal feeding operations in the arid west. Some of the biggest advantages of plug flow digesters are that they have low probability of upset, low maintenance, and require lower energy input compared to other types of digesters. Plug flow digesters are the most trusted and tested digester on the market, with roughly 55% of all digester implementations containing either a plug flow or modified plug flow design. Plug flow systems require a larger foot print than other digesters and can have issues with consistent gas production. Given the simple nature of plug flow AD systems, most on-farm installations can be monitored and maintained by a committed staff.

Complete Mix Anaerobic Digester (figure developed by Lucas Loetscher, Colorado State University)

Complete mix reactors are large, often cylindrical, tanks which have a mechanism to keep the reactor completely stirred (Figure 3). This mechanism can be injected biogas, or a motorized paddle. Mixing produces an ideal environment for anaerobic microorganisms by spreading the nutrients evenly throughout the reactor, while simultaneously helping to prevent acute toxicity. Complete mix reactors operate best when solids content is between 5-10% (Table 2). Because solids content of waste produced at most Colorado feeding operations is higher than 5-10%, complete mix reactors are often not a good fit unless an external source of water or wastewater is readily available.

Complete Mix Anaerobic Digester (figure developed by Lucas Loetscher, Colorado State University)

Upflow sludge blanket reactors are similar in design to a complete mix reactor, except that there is no integrated mechanism for homogenizing the waste. Instead, settling of solids is encouraged so that a sludge blanket is formed, maintaining biomass within the system, thus reducing the required holding time. These reactors are highly efficient and have been successfully up-scaled to the commercial scale. Sludge blankets are considered an emerging technology and are still in development phases. In general, waste generated at intermountain west animal feeding operations is too high in solids for application of an upflow sludge blanket reactor. Upflow sludge blanket systems are capable of producing very large amounts of methane biogas per volume of reactor compared to other technologies. They also have a very low retention time requirement, resulting in decreased foot print. However, when treating manure they can have longer start up times, often taking six to eight months to form a proper sludge blanket. This is critical when considering the variability of manure streams and that a single upset event could shut down operations for the better part of a year. Given the sensitive nature of the bacterial cultural it is important that an upflow digester be properly monitored by technical experts.

Complete Mix Anaerobic Digester (figure developed by Lucas Loetscher, Colorado State University)

In a fixed film digester, bacteria colonize a provided support structure within the reactor. This support structure is a high surface area material suitable for colonization, such as PVC pipe or shredded plastic. Fixed film reactors have successfully been implemented with low solids content (<3%) dairy manure wastewaters in Florida, but are not likely to be a good fit with wastes produced in the arid west unless a large quantity of wastewater is available from a nearby source. Fixed films digesters have low retention times (comparable to the upflow sludge blanket system) while also having the capacity to generate large amounts of methane biogas. Fixed film digesters do not require solids removal as all excess bacteria is passed through the system and have lower startup times than upflow sludge blanket or complete mix reactors. Capital cost for fixed film systems can be offset with grants and other funding mechanisms, however costs remain higher than other technologies. The operational complexity of fixed film digesters can often lead to potential variations in gas production or improper substrate removal rates and it is recommended that these systems are monitored by technical experts.

Complete Mix Anaerobic Digester (figure developed by Lucas Loetscher, Colorado State University)