Methane Recovery from Animal Manures:
The 1997 Opportunities Casebook

Philip D. Lusk
Resource Development Associates

Abstract

Growth and concentration of the livestock industry in the United States creates opportunities for the proper disposal of the large quantities of manures generated a dairy, swine and poultry farms. Economic evaluations and case studies of operating systems indicate that the anaerobic digestion (AD) of livestock manures is a commercially-available technology with considerable potential for providing profitable co-products, including a renewable fuel. The 1997 Casebook provides an updated examination of some of the current opportunities for the recovery of methane from the AD of animal manures. Revised case studies of operating digesters, including project and maintenance histories, and the operator's "lessons learned," are included as a reality check. Factors necessary for successful projects, as well as a list of reasons explaining why some AD projects fail, are provided. The role of farm management is key; not only must digesters be well engineered and built with high quality components, they must also be sited at farms willing to incorporate the uncertainties of a new technology.

The growth and concentration of the livestock industry in the United States creates new opportunities for proper disposal of manures generated at dairy, swine, and poultry firms. Principal pollutants from decomposing livestock manures are methane emissions, ammonia, excess nutrients and pathogens, along wit biochemical oxygen demand. The major pollution problems associated with these wastes are surface and groundwater contamination, surface air pollution due to odors, dust, volitile organic acids, and ammonia. There is also concern about the contribution of methane emissions to global climate change.

One manure management system provides not only pollution prevention but also can convert a problem into a new profit center. Economic evaluations and case studies of operating systems indicate that the AD of livestock manures is a commercially-available technology with considerable potential for providing profitable co-products including a renewable fuel.

USA livestock operations currently employ four types of AD technology:

• Slurry
• Plug flow
• Complete mix
• Covered lagoon

Figure 1 provides a numerical status report of farm-based anaerobic digesters in the USA. The data presented only includes the digesters that are installed on working dairy, poultry, and swine farms. It excludes 65-70 digesters that are installed on or were planned for beef feedlots, and those digesters which were primarily university research-oriented.

The numerical status is not the whole story. Surveyed farmers who have installed and continue to operate a digester are generally satisfied with their investment decision. Some chose to install a digester for non-economic reasons, primarily to control odor or contain excess nutrient run-off. Farmers have found that the returns provided from electricity and other co-product sales from the digester, however limited, are preferred to the sunk-cost of conventional disposal providing zero return on investment.

Moreover, without the environmental benefits provided by AD technology, some might have been forced out of livestock production. AD is sometimes the only technology that allows growth in the livestock production business. Turning a waste liability into a profit center that generates annual revenues can moderate the impacts of declining commodity prices and can diversify farm income.

None of the farmers surveyed to date with an operating anaerobic digester said that they regret their basic decision. Most would have preferred to spend less money on its design and installation, but they are unsure of exactly how costs could have been cut. Many seek new ways to increase profitability by the sales of co-products, primarily the digested solids. They would like additional assistance determining how best to optimize the added-value of co-products.

The reasons for digester failure go beyond bad design or equipment; as many as 16 digesters in the U.S.A.were shut down when farmers were sold or shut down. One encouraging note is the resurgence of interest in farm-based AD technology. For example, in the past two years, seven new systems were installed. Several others are under construction or in a design phase.

Still, at face-value, the data in Figure 1 is not encouraging to a farmer considering whether to install an anaerobic digester as a waste treatment option. Overall, the chance for failure, that is having a non-operating anaerobic digester, is about 63 percent in the USA. This is roughly 1.7 times greater than the possibility of successfully employing a working digester.

Figure 1: Status of Farm-Based Anaerobic Digesters in the US.

All Politics are Local

Most analyses cannot provide a quantifiable impact for some of the subjective advantages that can result from the adoption of farm-based AD technology. For example, no value is usually calculated for environmental externalities.

Unrecovered methane in biogas produced by the inevitable decomposition of animal manures is an agent of global climate change. Methane is potent greenhouse gas. According to the Intergovernmental Panel on Climate Change (IPCC), the Global Warming Potential (GWP) is an attempt to provide a simple measure of the relative radiative effects of the emissions of various greenhouse gases. For example, carbon dioxide has a GWP of 1. The IPCC's 1996 GWP estimate for methane was estimated to be 21. This means that a given mass of methane could increase the atmosphere's radiative forcing by an amount 21 times greater than the forcing associated with the same mass of carbon dioxide.

As a portion of the methane emission reduction component of the Climate Change Action Plan announced by President Clinton and Vice-President Gore in 1993, the U.S.EPA and the U.S.Departments of Energy and Agriculture expanded a voluntary pollution prevention program with the livestock industry. In the AgSTAR program, a livestock producer agrees to explore profitable methane reduction activities. AgSTAR producers will install systems for the recovery and use of methane only where it is profitable to do so.

A recent survey of dairy and swine farms in the U.S.A.re-affirmed the conclusion that AD is a technology with considerable potential. Assuming that the end use of the biogas manufactured at these facilities is used to generate electricity internal combustion engines, two key factors determining digester profitability are farm size and electricity rates.

The inventory of these economically recoverable emissions suggests that about 0.425 Tg of methane are potentially recoverable from 3000 dairy and swine farms in 19 states. The resultant supply curve of recoverable emissions in Figure 2 demonstrates deploying digesters in just three key states, North Carolina, California, and Illinois, could potentially recover 79 percent of these methane emissions.

Assuming that all of the recoverable methane emissions are converted into electricity suggests that slightly over 165-mW of new capacity could be brought on-line. However, given methane's GWP, it represents the potential to offset about 3500-mW of generating capacity. To give some sense of scale, this represents about one-half of the total current biomass generating capacity in the USA.

It is apparent that most voluntary greenhouse gas control programs are not working as effectively as planned. As a result, there has been much recent discussion about the need to impose more stringent greenhouse gas control efforts. Although these may take the next two to three years to be fully developed, broad-based carbon taxes are among the most promising and important of the emerging tools for promoting reductions in carbon emissions. Five European nations have adopted such taxes, but initiatives in the USA, in Australia, and in the European Union have failed or stalled.

A carbon tax cannot guarantee how great the reductions in emissions will be, but any tax will be excellent for biogas projects because of methane's GWP. Some planning units at the U.S.Department of Energy currently use a placeholder of $15 per ton of carbon dioxide ($55 per ton carbon) in their current planning models. A modest carbon tax like this could provide a value-adder of about 11.3 cents per kWh to an AD project.

Gray line represents AgSTAR Climate Change Action Plan Goal

Summary

The key to increasing the deployment rate of farm-based AD technology is cost-effectiveness. More cost-effective and easily managed manure management techniques are still needed, especially for smaller farms, to encourage farmers to use their animals' waste for conversion into energy and nutrients. Not only will farmers benefit monetarily, the use of AD will also help mitigate animal manure's contribution to air, surface, and groundwater pollution. There are additional indirect benefits for sustainable rural economic development from the implicit multiplier effect resulting from the direct jobs that can be created by providing, installing and maintaining the digester system equipment.

Current developments in applied R&D could provide some very exciting advances. One Florida project uses a fixed film system to treat liquid dairy manure. Current liquid waste management practices use a conventional anaerobic lagoon for treatment and storage. A lagoon can be a source of odors because of incomplete digestion either from overloading or a too short retention time. Current efforts are using an engineered lagoon and cover are an advancement, but because of the large volume required may not always be a solution due to shallow water tables or other reasons. The fixed-film reactor provides the potential to reduce the processing time of a conventional lagoon from 40 days to as little as two days. Another project in California uses an Advanced Integrated Ponding System (AIPS). AIPS use a submerged canopy covering a faculative pond, where the organic wastes are completely converted into methane, nitrogen, carbon dioxide, and stable residues. The submerged canopy is potentially longer-lived than conventional covered lagoons because it is not exposed to weather and other elements. One intriguing aspect of AIPS is that their effluent is discharged into secondary pools and expressly used as a growth culture for algae, thereby offering a more complete solution for carbon dioxide sequestration.

This paper was prepared for the National Renewable Energy Laboratory under Subcontract EKG-7-17098-01 and was sponsored by the Regional Biomass Energy Program of the U.S.Department of Energy.

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