Anaerobic Digester Lagoon with
Methane Gas Recovery: First year
Management and Economics

Leland M. Saele
Environmental Engineer
USDA-NRCS

Anaerobic lagoons are perhaps the most trouble free, low maintenance systems available for treatment of animal waste. This is particularly true in the southern U.S.where winter temperatures are mild, permitting anaerobic digestion the year around. The effluent from the digester is a valuable source of nitrogen for plants that can be field applied for improved crop production. Placing a cover over the lagoon for collecting biogas virtually eliminates odor from the lagoon. The collected biogas, a byproduct of the digestion process, is typically 60 to 70 percent methane that can be utilized as a valuable energy resource. Limited experience indicates that odor from field application of effluent from two cell covered lagoons is much reduced from what might be expected when applying untreated or uncovered lagoon effluent. A properly designed, constructed and operated anaerobic digester is a low maintenance system that is very forgiving and not likely to create emergency situations that can be expected with many alternative waste management systems. Adding methane recovery to the anaerobic digester increases maintenance but even in the event of failure of the gas collection system, it will not interrupt the waste stream and digestion process. It is well suited to the livestock industry.

AgSTAR is a voluntary program developed by the Environmental Protection Agency (EPA) to encourage livestock producers to consider methane gas recovery as part of their animal waste management system. Working in partnership with the U.S.Department of Energy (DOE) and Department of Agriculture (USDA), products, technical information and services are available to producers through the AgSTAR program. For general information on the AgSTAR program contact the AgSTAR hot line by dialing 1-800-95AgSTAR (952-4787). Natural Resources Conservation Service (NRCS) is the agency under USDA working with the AgSTAR program to assist producers with technical information.

In 1996, Julian Barham, a producer in Johnston County, NC, entered into an agreement with EPA for a pilot project on his farm show casing the technology and economic benefits of methane recovery from animal waste. Mr. Barham's operation consisted of a modern 4000 sow, farrow to wean, swine farm with an existing, 6 surface acre, anaerobic lagoon. A feasibility study using AgSTAR technical information and software indicated a five year pay back for a capital investment of approximately $250,000. This included a new, 20 foot deep, 1.6 surface acre anaerobic lagoon, a lagoon cover with gas collection system, and engine generator with heat exchanger for heat recovery and cogeneration. The anaerobic lagoon was designed and constructed in accordance with NRCS interim standards and criteria. The lagoon cover was designed by RCM Digesters¹ and manufactured by Reef Industries² using permalon, (a 20 mil reinforced HDPE material). The engine generator consisted of a CAT 3406 engine with a 120 KW induction generator. The lagoon was completed in the fall of 1996 and filled with effluent from the existing lagoon. The installation of the lagoon floating cover was completed in December 1996 and all gas system components including the engine generator installed by 3/97.

The start up experiences with the Barham project have shown that even with knowledgeable consultants and technical expertise, problems do occur. Two were significant: 1) An expensive engine generator (40% of capitol investment) sits idle while waiting for the lagoon to mature and reach predicted gas yields. 2) A manufacturing defect in the lagoon cover material resulted in having to replace the cover. On the positive side, we were surprised to find essentially no odor from the digester effluent, even during field application. Based on this first year of experience, this paper addresses measures in planning, design, operation and economics that I believe could help avoid similar problems for livestock producers considering methane gas recovery systems.

Use the AgSTAR Handbook³! "This handbook is for livestock producers, developers, and others considering biogas recovery systems as a livestock manure management and odor control option. The handbook provides a step-by-step method to determine whether a particular biogas recovery system is appropriate for your livestock facility. This handbook complements the guidance and other materials provided by the AgSTAR program towards promoting biogas recovery at commercial farms in the United States." ³

Feasibility study - The feasibility assessment is an evaluation of the producers livestock facility and the key to determining the economic benefits of methane recovery. Computer software developed under the AgSTAR program facilitates this process. Although relatively simple and straight forward to use, first time users are advised to review results with those experienced with the program. How the biogas will be utilized and the economic analysis to determine benefits is an important part of the process. A completed feasibility study should include a preliminary cost estimate, general layout of proposed operation, predicted biogas yields and identified economic returns.

Verify Feasibility - Compare the results of your study with experience of others. If feasibility is based on economic returns of biogas utilization, compare the predicted biogas yield with other similar operations. This can best be accomplished by visiting farms where existing methane recovery systems are functional and discussing with experienced operators. A list of known farms is available by contacting the AgSTAR hot line noted above. If there are no systems of the type proposed, either in operation or that you can visit, be very cautious before proceeding.

Secure contracts - When economic returns are based on assumed sales such as the sale of power to a utility company, contracts should be obtained prior to expenditure of funds. Don't assume this will happen after construction.

Experienced engineer - Hire an engineer with a proven track record. Ask for a list of jobs completed. Check them out by telephone or site visit or both. Be sure the design for your operation is similar to referenced work. Experience with one type of digester does not mean the person is knowledgeable in other types. Each system must be a site specific design. Lagoon cover design is still experimental. The manufacturer should provide a material/fabrication warranty in writing. One year is not be enough. Often times consultants are trying to make improvements or to improve the economics. Be sure you understand the purpose and function of each component and understand what it does in your system. Improvements may or may not work. It may cost you extra to correct if it does not work.

Complete drawings - The consultant or designer should provide a complete set of drawings and specifications for the work. The drawings should show each component of the system. It is important for the owner/operator go over the drawings and specifications prior to the beginning of construction, identify each component and its function. This is also a good time to ask the consultant if the specific component has been used on one of his jobs before. This might be something as simple as the type of joints in the gas pipe. The drawings and specifications should be accompanied by a design report that explains how the system works and the design assumptions and parameters. If these assumptions to not match the owner/operators intentions or farm operation, one or the other will require modification.

Operation and maintenance manual - Each job should come with a complete operation and maintenance manual. The manual should address startup operation, normal operation and emergency operations. It should address all elements of the system and any special precautions.

Regulations and certifications - Since this will be a change to your livestock waste management system, it may need to be certified or approved by state and/or local jurisdictions. If cogeneration is part of the project, a licensed electrician will need to certify design for interconnection to the utility. Verify any cogeneration agreements with utility company prior to start of construction.

Construction is often accomplished by a combination of available farm labor and hired contractors. Consultants will usually provide some assistance. It is recommended that consultants or manufacturer's representative provide onsite supervision for installation of the lagoon cover. Electrical wiring and connection to utility must be done under the supervision of licensed electricians and with approval of utility company.

Initial start up - Operation should be in accordance with the guidelines provided by the consultant. Expect the consultants to oversee the initial startup and stabilization of the system. If it is a new livestock operation, initial startup will be delayed while the lagoon matures. A temporary flare may be installed near the lagoon to burn off biogas while waiting for the lagoon to mature or completing construction on other elements of the system. Each system is unique and will require adjustments as the operator becomes familiar with peculiarities of the system.

Patience - Methane is one of the byproducts of anaerobic digestion (a biological process) in a lagoon. There are many variables that can affect the rate of production. The makeup of the waste stream and the temperature are the most critical. Both affect the rate of bacteria growth. More important, a new lagoon requires a number of cycles before the bacterial colony is sufficiently developed to produce the predicted volume of biogas. It is not unreasonable to wait 1 to 2 years for the lagoon to mature and methane production to reach predicted levels.

Be prepared for the unexpected - Methane recovery systems are still experimental and do not always perform as predicted. The objective is to collect the biogas from the lagoon surface and deliver it to the end use point without the presence of atmospheric air. The introduction of air can disrupt the performance of burners and more importantly engines in cogeneration operations. An air leak anywhere in the system can be time consuming to locate. This is particularly true if the problem is the lagoon cover and it can be even more difficult to fix.

Year one - don't expect a return the first year. It will take at least one year to get the bugs out and obtain consistent results. Also, there likely will be changes, this will cost money and could offset any revenue.

Phase capital investment - if cogeneration is part of the proposed system, begin the first year with only the gas collection components and flare the gas or burn for heat. Cogeneration systems are expensive (as much as 50 percent of the cost of construction) and adequate gas yield is critical to successful operation. Monitor the lagoon and gas production the first year to determine biogas yield (figure 1). After the first year, a cogeneration unit can be purchased that matches the gas production or less expensive alternatives can be pursued if the gas yield is limited.

Consider odor control an economic benefit. Public opinion on odor is becoming more vocal and without proper control, producers could be forced out of business.

Systems are experimental, look for and expect financial assistance. Methane is a renewable energy source and a greenhouse gas that contributes to global warming. Federal and state agencies often will provide financial assistance to promote alternative waste systems that reduce greenhouse gases and or utilize renewable energy. The AgSTAR Handbook provides guidance in looking for financial resources.

Barham Farm, 4000 sow, farrow-to-wean, anaerobic lagoon. Picture taken in January 1997, one month after the cover installation.

 
Figure 1

Biogas produced by the lagoon during the first year of operation measured 35 to 45% of the predicted gas yield.

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