Many factors are required to successfully implement and operate an AD/biogas system. The following list briefly introduces the 10 key factors essential for a successful farm-based digester project. Click to learn more.
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- Plan for success.
During the planning stage, identify and define clear project goals. To establish these goals, site-specific farm information should be collected, including ownership and managerial goals and projections, animal information (e.g., number, types, maturity, bedding type), type(s) of manure recovery, volume of manure, manure analytical information, past and current disposal practices, and operational costs. Working towards project parameters is also crucial in addressing and meeting goals. This includes, often in iterative planning stages, identifying available feedstock, defining the type of digestion system, conversion efficacy of that feedstock, economic or financial factors and limitations, and project risks associated with developing an AD/biogas system.
- Recruit and secure an experienced team.
Seek out and work with an experienced and qualified team to help initiate and successfully implement the project. At project initiation, the &#;core&#; team should include:
- an engineer/permitting specialist who knows the farm history and local regulations and
- an engineer or specialist with seasoned experience implementing various AD/biogas systems.
Verify the project references, experience, and historical success of the engineers and specialists. This &#;core&#; team should help identify an applicable system and ensure the development is feasible and planned to meet the owner&#;s goals and expectations. As the project progresses, the team may also expand to include technology vendor(s), equipment provider(s), a project developer, investors/bankers/lender(s), and/or operators to supplement the initial &#;core&#; team. The farm owner or operating personnel should also be included in the &#;core&#; team early on if the project is not being developed by the farm itself.
- Develop a sustainable business model.
A successful AD/biogas system requires a sustainable business model. The project should not only be cost-effective, but it must also meet financial goals. The economic factors include well-defined project costs, expenses, revenue or income, liabilities, among many others. Personal goals for the project&#;s liquidity and profitability potential define the financial factors. The business model could consider involving partners, utilizing third party investments, or other traditional &#;cooperative&#; models.
- Secure suitable feedstock supply.
During the planning and engineering phase, identify all suitable feedstocks. The digester must be supplied with a consistent quality and type of feedstock (manure and co-substrates) to maintain a productive microbial community. This will result in consistent organic destruction and biogas production and minimize operational issues. It is of high value to ensure that feedstocks are free of toxic and inorganic contaminants that will &#;upset" the intended microbial and mechanical processes. Sand, gravel, and other inert material should be removed to the degree possible to minimize sediment accumulation in the digester. Feedstocks from outside sources should be routinely characterized to monitor consistency. Projects that focus on co-digestion feedstocks (i.e., feedstock supplementing manure) should include contractual agreements to specify material quantity and quality, testing frequency, revenue received, and duration to ensure the right type and amounts of materials and revenue are provided. Co-digestion feedstocks should be designed for flexibility as external supplies are likely to vary over time.
- Use the most appropriate technology.
The AD technology needs to be carefully evaluated to match the type and amount of feedstock that is expected to be processed. There is no single AD technology that can be used for all situations or feedstock.
Among the many key factors that need to be considered include:
- the type of manure and co-digestion feedstocks,
- how the manure is collected,
- conversion efficiency goals,
- the climate where the digester is located,
- bedding type and mass,
- amount of allocated maintenance, and
- other factors.
AD technology selection should also consider management goals and needs and future plans of the farm.
- Analyze options for biogas and digestate use.
During the planning stage, considerations should include market availability, capital and operating costs, and potential revenue to determine how the biogas is best monetized, which can include on-site use or off-site sales.
Potential uses include:
- on-site use of thermal and/or electrical energy,
- off-site sale of thermal and/or electrical energy,
- off-site sale of compressed natural gas or liquified natural gas (CNG/LNG) typically used for transportation fuel or other applications,
- on-site use of renewable natural gas (RNG),
- off-site sale of RNG, and/or
- bio-based material generation.
The need for on-site digestate use as fertilizer or bedding should also be determined, and the market for digestate final products should be assessed, including fertilizer, salable compost, or other value-added digestate products. Proper management of digestate, whether recovered for its nutrient value or disposed of in an environmentally correct manner, is critical to the success of the project.
- Develop off-take agreements.
It is critical to execute off-take agreements or legal contracts with users of the AD/biogas products and byproducts (e.g., biogas, electricity, heat, RNG, digestate, fertilizers) early in the development stage. These agreements&#;including power purchase agreements (PPA), biogas/RNG sale agreements, or digestate sales agreements&#;define the price and detailed specifications for all materials that any third party will purchase.
- Evaluate added benefits.
Consider the added benefits of AD, which may be difficult to quantify, but could be critical reasons for implementing an AD/biogas project. These benefits may include odor control or reduction in greenhouse gas (GHG) emissions. Digesters often are installed to reduce odor problems, particularly on farms where there is public development encroachment.
- Conduct community outreach.
Community outreach and education is critical to obtain buy-in and approval from the community, including, but not limited to, regulatory approval and the community and neighborhood approval where the project is located.
- Plan for operation and maintenance.
Good operation and maintenance practices are key for effective operation of AD/biogas systems. This includes continuous monitoring and management to ensure the biological processes and mechanical equipment are working properly. Often, AD/biogas system operating expenses (OPEX) are underestimated. Analyzing other similar operating systems that have several years of operational history can assist in predicting OPEX accurately. It is also important to consider whether current staff or a third party will be used to perform these functions.
Introduction
A great variety of organic material can be used in anaerobic digesters as a feedstock for generating biogas. However, there are scientific, engineering and legal limits to what can be added successfully to a digester. In addition, the feedstock needs to be a liquid mixture with an appropriate moisture content. For example, mesophilic complete mix tank digesters (the type most commonly used today) typically operate best with a mixture of 4 to 8% solids in water. Digesters require various moisture contents, depending on the design and operation of the system.
Feedstocks for Anaerobic Digestion
Manure in dairy barn.
Most easily biodegradable biomass materials are acceptable as feedstocks for anaerobic digestion. Common feedstocks include livestock manure, food-processing waste, and sewage sludge. The energy production potential of feedstocks varies depending on the type, level of processing/pretreatment and concentration of biodegradable material. Listed below are feedstocks that can be commonly used in anaerobic digesters:
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- Livestock manures
- Waste feed
- Food-processing wastes
- Slaughterhouse wastes
- Farm mortality
- Corn silage (energy crop)
- Ethanol stillage
- Glycerine as the product from biodiesel production
- Milkhouse wash water
- Fresh produce wastes
- Industrial wastes
- Food cafeteria wastes
- Sewage sludge
Livestock manures are generally lower-energy feedstocks because they are predigested in the gastrointestinal tracts of the animals. Manure, however, is an easy choice for anaerobic digestion because it generally has a neutral pH and a high buffering capacity (the ability to resist changes in pH); contains a naturally occurring mix of microbes responsible for anaerobic degradation; provides an array of nutrients, micronutrients, and trace metals; is available in large quantities; and can be transferred by pump.
Animal wastes containing bedding such as chicken litter with substantial quantities of wood chips or sawdust can be used successfully in anaerobic digestion. The woody material, which degrades very slowly because of its lignin structure, is essentially passed through without digestion, and retention times are based on digestion of the manure.
Blending of energy-dense feedstocks with livestock manure is a common practice to maximize biogas production by optimizing nutrient levels and providing buffering capacity. The use of manure as a base for anaerobic digestion is important because many of the energy-dense feedstocks, such as food-processing waste and ethanol stillage, are acidic, contain little if any naturally occurring microbes, and oftentimes lack the nutrients (nitrogen, trace elements, vitamins, etc.) necessary for anaerobic digestion. Potentially, farms operating anaerobic digestion systems could take on additional wastes and benefit from increased gas production as well as tipping fees.
Materials to Be Excluded from Anaerobic Digesters
Materials that should be excluded as feedstock from anaerobic digesters include those containing compounds known to be toxic to anaerobic bacteria, poorly degradable material, and biomass containing significant concentrations of inorganic material. Poorly biodegradable materials require higher retention times, meaning they must spend more time in the anaerobic digester to be broken down and converted into biogas.
Biogas equipment for electricity generation. Photo: Daniel Ciolkosz, Extension Associate, Penn State.
Inorganic materials, on the other hand, contain no carbon and cannot be converted into biogas. Materials such as sand bedding do not contribute to the biogas potentialo and may cause operational problems such as pipe clogging, premature equipment wear and volume reduction due to sludge accumulation. Also, the feedstock containing too much ammonium or sulfur should be avoided, because ammonium and sulfur inhibit anaerobic organisms.
Evaluating Feedstock Biogas Potential
The biogas potential of different feedstock materials or feedstock combinations is often difficult to predict due to differences in the source, processing, volatile solids concentration, chemical oxygen demand, moisture content, and/or inclusion of toxic compounds. The total biogas potential assay, also known as the biochemical methane potential (BMP) assay, provides an efficient and economic method for estimating biomass conversion and biogas yield of feedstocks or feedstock blends.
BMP assays are a multifaceted approach to evaluating the potential to produce biogas. BMPs are a practical, lab-based approach to identifying and evaluating potential feedstocks for anaerobic digestion. Potential anaerobic digestion feedstocks are commonly evaluated by three criteria.
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Feedstock characterization: Both before and after BMP assay, includes pH, chemical oxygen demand (COD), total solids (TS), and volatile solids (VS). Characterization results found prior to the experiment are used to determine the quantity of feedstock needed to maintain the BMP assay for as much as 30 days. Characterization results following the completion of the BMP assay are used to evaluate the anaerobic digestion process in terms of the destruction of the organic material.
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Total biogas production: Is measured throughout the BMP either through manual means or continuously by commercial software designed for tracking gas production. Biogas can be scrubbed of the carbon dioxide by running it through a potassium/sodium hydroxide solution to monitor only methane production or can be left unscrubbed to monitor the total biogas production.
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Biogas analysis: Biogas composition can be investigated by means of a gas chromatograph during the BMP assay. Though the capital investment is large, gas chromatographs provide accurate measurements of the constituents of the biogas produced during the BMP. Gas chromatographs can be set up to determine the concentrations of methane, carbon dioxide, nitrogen, and hydrogen sulfide gases.
The BMP assay is a combination of a single feedstock or feedstock blend, inoculum, and stock solutions in a batch system. Inoculum is used to seed the feedstock with an active anaerobic culture to initiate activity and reduce any lag time required for establishment of a culture. Stock solutions are added to assure that macronutrients, micronutrients, and vitamin deficiencies do not limit biogas production. BMP evaluations should always be completed in replication and results should be verified at pilot or full-scale, and it is strongly recommended that full-scale designs not be based on BMP results because full-scale digesters are often run at continuous mode while BMP tests are batch mode.
Considerations
The biogas potential of feedstocks is an important factor when considering anaerobic digestion on your farm. But other considerations, such as economics, regulatory issues, feedstock availability on and off the farm, and end use of the biogas, should also be evaluated.
References
- Chynoweth, D.P., C.E. Turick, J.M. Owens, D.E. Jerger and M.W. Peck. . Biochemical Methane Potential of Biomass and Waste Feedstocks. Biomass & Bioenergy 5:95-111.
- Liu, Y., S.I. Miller, and S.A. Safferman. . Screening co-digestion of food waste water with manure for biogas production. Biofuels, Bioproducts, Biorefining 3:11&#;19
- Owen, W.F., D.C. Stuckey, J.B. Healy, L.Y. Young, and P.L. McCarty. . Bioassay for monitoring biochemical methane potential and anaerobic toxicity. Water Research 13:485-492.
- Steffen, R., Szolar, O., and Braun, R. . Feedstocks for Anaerobic Digestion. Institute for Agrobiotechnology Tulln, University of Agricultural Sciences, Vienna.
- Speece, R.E. . Anaerobic Biotechnology for Industrial Wastewater. Archae Press, Nashville.
Contributors to This Article
Authors
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Dana M Kirk, Michigan State University
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Louis Faivor, Michigan State University
Peer Reviewers
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