Energy 101: Anaerobic Digestion

Many companies specialize in installing anaerobic digesters, with some having decades of experience. Recent technological advancements have improved the success of on-farm digesters, but it's essential to educate yourself or hire a qualified consultant to guide you through the technology selection and installation process. Exploring different financing options available through banks or technology providers is also necessary.

Anaerobic digestion requires manure with a solids content of less than 17%. Manure collected from concrete surfaces typically meets this requirement, making it suitable for conventional anaerobic digestion technology.

Rocks, sand, and soil can cause significant operational issues for anaerobic digesters and must be removed before processing. This removal process can add complexity, capital costs, and maintenance requirements to your anaerobic digestion system.

Co-digestion, which involves combining manure with nearby wastewater sources, can enhance digester operations and increase biogas production.

Operating an anaerobic digester requires more maintenance than other manure management practices, such as composting or waste lagoon management. Depending on the size of your operation, you may need to hire additional employees for routine maintenance.

• Average energy costs of at least $5,000 per month?
• Frequent and/or credible complaints about odor?
• Poultry or swine operation?
• Potential for co-digestion?

Anaerobic digesters represent a significant financial investment. To justify this investment, you should be able to offset other operational costs, such as energy use or lawsuit mitigation. Conducting a detailed analysis of potential revenues and costs over the life of your digester is prudent.

Anaerobic digestion is a widely accepted technology for managing organic waste from livestock manure. This biological process involves microorganisms breaking down organic material to produce biogas, which typically contains 60-70% methane (CH₄) and carbon dioxide (CO₂), along with small amounts of hydrogen sulfide, ammonia, water, and trace hydrocarbons. Biogas can be used to generate electricity or be refined and supplied to natural gas lines. Utilizing methane from livestock manure can significantly reduce greenhouse gas emissions, replace fossil fuels, decrease carbon dioxide emissions, mitigate odors from agricultural facilities, and improve water quality. Additionally, anaerobic digestion can lower on-farm energy costs or increase revenue from energy sales.

The Anaerobic Digestion Process

Anaerobic digestion requires feed material with less than 17% solids by weight. Manure collected on dry lots often exceeds this threshold, making it unsuitable without additional processing. The microorganisms responsible for converting organic materials into methane are sensitive to pH and temperature, requiring a pH of around 7 and temperatures near 95°F. A 20-degree drop in temperature can halve gas production, and nearly half of the biogas generated may be needed to maintain the digester's temperature. More heat input is required during winter months, but methane production remains stable if the temperature is maintained at 95°F.

System Configuration

Anaerobic digesters are typically large reactors made of concrete or steel, with the volume depending on the waste processed and the required retention time. Conventional digesters, which are flow-through systems, need a holding time of 20-30 days to convert manure solids into methane. Manure is added continuously or daily, and biogas can be used on-site, as fuel for electricity generators, or purified for natural gas lines. Cogeneration, which captures heat generated as a byproduct to maintain the digester's temperature, is the most common use for methane produced by anaerobic digestion.

Biogas Purification and End Products

There is growing interest in purifying biogas for natural gas lines or converting it to liquid natural gas due to the high maintenance requirements of generators not designed for biogas from manure. This purification process involves removing components other than methane, such as carbon dioxide, hydrogen sulfide, ammonia, water, and trace organics. Hydrogen sulfide, which is corrosive, must be removed for cogeneration, often by passing biogas through iron particles.

The end product of anaerobic digestion is a nutrient-rich slurry. While the liquid volume of waste does not reduce during digestion, expect a 50-60% reduction in solids without sacrificing nutrients, making the end product valuable for agriculture. Nearly 95% of pathogens are killed during anaerobic digestion, converting organics to methane and conserving nutrients like nitrogen and phosphorus. The resulting low-odor, stabilized waste is suitable for land application.

Feasibility

Anaerobic digestion may not be suitable for all animal feeding operations. It's essential to ensure the feasibility of anaerobic digestion at your farm before committing to a system. Various technologies are available, including covered lagoons, plug flows, complete mixes, upflow sludge blankets, and fixed film reactors. Consult the selection guide for detailed information on these technologies and seek expert advice to determine the most appropriate technology for your operation. Once you establish that anaerobic digestion is technically and economically feasible, explore which types of anaerobic digester technologies best fit your site.

Biogas Handling

Methane concentrations between 6% and 15% mixed with air can be explosive. Since methane is lighter than air, it can accumulate under rooftops and in enclosed areas, making detection difficult due to its relative odorlessness. Extreme caution and special safety features are necessary when designing a digester system and storage tank, especially if the gas is compressed.

Corrosive Biogas

Biogas generated by anaerobic digesters contains highly corrosive hydrogen sulfide, which must be removed before feeding the biogas into a generator. Passing the biogas through iron particles is a simple, low-cost method for removing sulfides. The sulfides attach to the iron surfaces, removing them from the gas. The iron must be replaced every six to twelve months, depending on the size-to-gas-flow ratio.

Dry Wastes

Some collected animal wastes can have high solids content. Dairy operations are often considered suitable for anaerobic digestion technology, but waste management methods vary. Manure scraped from concrete floors or dry lots can have solids content as high as 90%. For wastes with more than 17% solids, substantial quantities of water may be required for anaerobic digestion, increasing operating costs. Clean water can absorb nutrients and pathogens, becoming a nuisance if added to an anaerobic digester. Dilution of waste with water is most practical when there is an available source of wastewater, such as from domestic or food processing uses.

High Inorganic Content

Manure collected from dry lots often contains high inorganic content, including rocks and soil particles, which cause major operational problems and must be removed before processing. This has been a prominent cause of failure in on-farm anaerobic digesters. Sand in bedding can also be problematic if it ends up in the waste supply. Removing rocks, soil, and sand typically involves adding water to the waste and allowing particles to settle, adding complexity, capital cost, and maintenance.

Co-Digestion

Combining animal feeding operation wastes with wastewater generated onsite or at nearby facilities can increase water content and methane production capacity, a process known as co-digestion. This practice is growing in popularity. However, the ability to combine manure with other wastes must be carefully evaluated before installation and operation of an anaerobic digester. Waste streams should remain consistent, as microorganisms in the digester are sensitive and can take up to three months to adjust and resume methane production when the waste source changes. Ensure that additional waste other than manure will be available daily throughout the year.

Handling End Products

Anaerobic digester effluent is a slurry containing 1% to 15% solids, depending on the waste fed into the system. This stabilized product is suitable for land application, being low in pathogens and high in nutrients. Processed material containing solids can be applied by a honey wagon, or solids can be separated for land application separate from liquids. When solids are separated, they can be composted and applied by a manure spreader. Solids separation combined with composting can result in a lower-weight product, reducing transportation costs compared to slurry for land application. The weight of processed slurry containing liquid and solids may be too expensive to transport over large distances. Using the nutrient-rich liquid component for irrigation, known as fertigation or chemigation, is regulated in most states. When fertigation systems are connected to a freshwater source, measures must be taken to avoid contamination, such as including a backflow preventer and shutoff valve. Fertigation systems must adhere to state and local regulations. If land application is not an option, find alternative methods for storage or on-site treatment.

The predicted energy production for different types of animal wastes is shown in the table below.

To calculate the energy production per day (EPD) in kWh:

EPD = No. of animals × kWh per 1,000-lb animal per day × typical animal weight (in lbs)

For example, if you have 100 dairy cattle, each weighing 1,000 lbs:

EPD = 100 × 4.7 × 1,000 = 470,000 kWh/day

To estimate savings associated with biogas for on-site heating, first determine the available energy after biogas (AEB) is used for heating the digester. Assuming 50% of the produced biogas is used for heating:

AEB = EPD × 0.5

Next, calculate cost savings:

Cost Savings = AEB × 0.65 × cost of energy per day

Note: The cost of energy per day should be in dollars per kWh. Gas bills often report energy in Btu (1 kWh = 3,412 Btu).

If you intend to install a generator for on-site use or sell electricity to a utility, determine your on-site electricity demand (OED) by reviewing your utility bills over the past year. Energy in excess of OED can be sold to your utility if they are amenable to such an arrangement. Research net metering possibilities if interested in selling generated energy to the utility.

Calculate electricity available (EA) from the generator, assuming an efficiency of 35% for biogas use:

EA = EPD × 0.35

Estimate savings from on-site use of energy:

Cost Savings = OED × cost of energy per day

If EA exceeds OED, determine the price (P) the utility is willing to pay per kWh and estimate revenue from electricity sales:

Revenue = (EA−OED) × P

Other energy savings can be achieved by recovering heat from the water used in the engine generating electricity, which is particularly beneficial for dairy operations.

According to the U.S. Environmental Protection Agency's AgSTAR program, the capital cost of an on-farm anaerobic digester ranges from approximately $400,000 to $5,000,000, depending on the size of the operation and the technology used. A typical on-farm anaerobic digestion unit costs around $1.2 million. Costs vary based on the size, design, and features of the unit. The type of anaerobic digester necessary for your operation, and therefore the cost, depends on technical considerations and the number of livestock. Additionally, most digesters are somewhat customized by the provider, so capital outlay and operating and maintenance costs can vary. Annual operation and maintenance costs, including repairs, parts, labor, and insurance, must also be factored in.

Funding Programs

Given the capital-intensive nature of anaerobic digesters, it's essential to thoroughly understand the parameters of any funding programs before investing in a system. Discussing loan risks associated with a methane digester with an agricultural loan officer can help ensure that additional debt will not compromise your ability to access capital for your operation.

Utility Contracts and Net Metering

Utility contracts can vary considerably throughout rural communities in Maryland. Some utilities have net metering policies that allow small energy generators, like those with an anaerobic digester, to offset energy consumption by producing electricity. However, the value of net-metered power varies by utility. Some utilities credit net-metered power at the retail rate, providing a direct offset for every kilowatt-hour of electricity produced, while others credit it at a discounted or wholesale rate. In agricultural operations, demand charges for electricity may not be offset in net metering but can comprise up to half of your electrical expense.

Increasing Profitability

To increase profitability, focus on reducing operational and maintenance costs and offsetting energy usage with an anaerobic digester system. Be cautious about relying on an anaerobic digestion system to generate revenue by selling electricity to a utility, as they may be unwilling to enter into such agreements. If they are willing, the rate offered is typically a wholesale price. Ensure there are no contractual terms that might be problematic, such as clauses requiring a guaranteed amount of power supply or obligations to notify the utility of changes in electrical production.

Expected Costs and Revenues

Outline your expected costs and revenues over the life of the digester as you determine what is best suited for your operation. Once you contact a technology provider, you can obtain more detailed information necessary to calculate actual costs.

There are five key indicators that can help determine if an anaerobic digester might be economically feasible for your operation. These indicators serve as a screening tool to decide whether you should pursue a comprehensive feasibility study. If your operation meets at least two of the criteria, consider conducting a more detailed analysis:

1. Confined Animal Feeding Operation (CAFO): If your operation meets the definition of a CAFO, it must comply with state and federal laws governing waste management practices. An anaerobic digester might complement a CAFO’s plan for air emissions, nutrient, or waste management. CAFOs must adhere to strict waste management regulations. An anaerobic digester can enhance compliance with air emissions, nutrient management, and waste management plans.
2. Co-Digestion Potential: When agricultural producers and related industries, such as food manufacturers or municipal waste treatment facilities, are located nearby, there may be efficiencies that can improve the economic viability of a project. Feasibility studies have shown that co-digestion projects might be economically viable. Combining waste streams from agricultural producers and nearby industries can create efficiencies and improve project viability. Review feasibility studies and reports for more information on co-digestion projects.
3. Odor Complaints: Anaerobic digestion units can provide a measurable reduction in odor, improving relations with neighbors and mitigating nuisance lawsuits. Avoiding potential lawsuits and accompanying financial liability may help justify the capital expenditure of an anaerobic digestion project. Reducing odor through anaerobic digestion can prevent nuisance lawsuits and improve community relations. The financial risk associated with odor-related lawsuits can be difficult to estimate, but avoiding these risks can justify the investment.
4. Dairy, Swine, or Poultry Operation: Many nuisance claims involve these types of operations, which have also seen high punitive damage awards. Swine and poultry producers might consider adopting anaerobic digestion units to reduce the risk of nuisance claims. These operations are often subject to nuisance claims due to odor. Anaerobic digesters can mitigate these risks and improve management practices.
5. High Energy Expenditures: If your operation incurs more than $5,000 in average electricity or heating expenditures per month, an anaerobic digester might be economically feasible. Offsetting these costs can significantly impact the economic viability of the system. High monthly energy costs can be offset by using biogas generated from anaerobic digestion. This can make the system economically feasible, especially if the operation incurs significant electricity or heating expenses.

• Water Costs: Include the cost of water in your financial analysis.
• Greenhouse Gas Offsets: Do not rely on revenues from greenhouse gas offsets to fund the system, as these markets are volatile.
• Tipping Fees: Review state guidelines for waste transport policies before calculating potential tipping fees.
• Maintenance and Labor Costs: Account for these costs in addition to the capital outlay of an electricity generator.
• Backup Energy Systems: Include costs for backup systems in case of maintenance downtime.
• Net Metering and Buyback Programs: Understand state and utility policies regarding net metering and energy buyback programs.
• Infrastructure Costs: Consider the location of the digester relative to utility infrastructure, as tying into the grid can be expensive.
• Methane Generation Potential: Maintain a realistic perspective of energy costs that can be offset.
• Risk Assessment: Factor in risk by considering the most likely and worst-case scenarios.

Several anaerobic digestion technologies are available, including covered lagoons, plug flows, complete mixes, upflow sludge blankets, and fixed film reactors. The selection of technology depends heavily on the solids content of the waste being processed (see table below). For example, swine waste typically comes in the form of a slurry with less than 15% solids, making it suitable for conventional anaerobic digester technology. Conversely, cattle waste collected from dry lots can have solids content as high as 50%. Dairy cattle manure collected on concrete generally has solids content between 10% and 16%, whereas flushed manure can have solids content less than 3%, varying significantly based on the amount of water used for flushing. Use the online decision support tool for additional guidance on technology selection based on your current waste management methods.

Covered Lagoons

Covered lagoons are among the cheapest and simplest anaerobic digestion technologies. Anaerobic digestion and methane production occur naturally in lagoons containing animal wastewater. A synthetic cover, typically made of plastic or rubber, traps and stores the biogas. Covered lagoons are challenging to heat, so they are recommended only in warm climates where freezing temperatures are rare. Insulated covers could make covered lagoons more feasible in colder climates.

Plug Flow Digesters

Plug flow digesters are a low-tech anaerobic digestion technology suitable for high-solids-content waste. The thick waste travels down the digester in a "plug," as a continuous mass. Plug flow digesters can be a good fit for the high-solids-content waste generated by animal feeding operations.

Complete Mix Reactors

Complete mix reactors are large tanks with stirring mechanisms, such as injected biogas or motorized paddles, to keep the reactor circulating. Mixing creates an ideal environment for anaerobic microorganisms by evenly distributing nutrients and dampening shock loads of toxins. Complete mix reactors operate best with solids content between 5% and 10%.

Upflow Sludge Blanket Reactors

Upflow sludge blanket reactors are similar to complete mix reactors but lack an integrated mechanism for homogenizing waste. Instead, solids settle to form a sludge blanket, maintaining biomass within the system and reducing required holding time. These reactors are highly efficient and scalable for commercial use.

Fixed-Film Digesters

In fixed-film digesters, bacteria colonize a support structure within the reactor, such as PVC pipe or shredded plastic. Fixed-film reactors are effective for low solids content (less than 3%) dairy cattle manure wastewaters. The waste must be diluted effectively and inexpensively for these digesters to perform correctly.

With a solid understanding of the appropriate anaerobic digestion technologies for your operation, you are ready to contact technology providers. You may choose to hire a consultant to guide you through the selection process, but ensure they are not tied to a specific provider. Some technology providers may assist with project financing, but it's crucial to consider all financing options.

Consider asking these questions to evaluate potential technology providers:

• Experience and Location: How many on-farm anaerobic digesters does your company currently have in operation, and where are they located? Choosing a company with many successful projects can lower risk. Newer companies may offer innovative systems but come with higher risk. Ensure technologies have been successfully demonstrated on a large scale and ask to speak with producers involved in demonstrations. Review published case studies if available.
• Specialization: Of the operating digesters, how many are applied to manure management in animal feeding operations? Companies specializing in manure digestion may be more suitable. Technologies developed for urban food and yard waste may not work well for manure.
• Regional Success: Where are successfully operating anaerobic digesters located? Are you willing to take on projects in Maryland? Some providers may have regional preferences and may not be willing to move outside their current service area.
• Technology Types: What types of anaerobic digestion technologies does your company provide? Ensure the provider offers technologies suited to the waste generated at your farm, such as complete mix, plug flow, upflow sludge blanket, or fixed film.
• Services Provided: What services does your company offer? Determine what services are included and whether additional support is needed.
• Case Studies: Are there case studies of your technology that you can share? Reviewing case studies can help compare performance, system lifetime, and insurance policy details.
• Pretreatment Requirements: Is pretreatment required? Some technologies require pretreatment of waste, adding substantial capital and maintenance costs.
• Project Timelines: How long are your project design, construction, and system timelines on average? Understand the installation duration and expected system lifetime.
• Performance Guarantees: Does your company provide a performance guarantee or warranty? Compare the details of guarantees or warranties between different providers.
• End Product Handling: Does your company provide support and guidance for handling end products? The end product of anaerobic digestion is a slurry that must be applied to land or otherwise disposed of. Consider the costs and maintenance of handling end products in the project feasibility study.
• Subcontractors: Will your company hire any subcontractors to complete portions of the project design and construction? Understand who will comprise the project team and ensure you are comfortable with the process.
• Client Training: What kind of training does the client receive from the technology provider? Installation of an anaerobic digester requires more maintenance than composting or lagoon management. Ensure the provider offers a clear plan and training for maintenance activities.
• Project Financing Coordination: Will the technology provider help coordinate project financing? Research financing options, including federal and state funding programs, grants, reduced interest loans, and tax credits. The U.S. Environmental Protection Agency’s AgSTAR website is a good starting point for funding information.

Several technology providers offer loans directly for anaerobic digestion projects and may help navigate federal and state grants or loan programs. They might also connect you with privately funded niche programs, including greenhouse gas mitigation programs. Your local agriculture lender may be your best financial resource. Ensure you fully understand the project financing package offered.