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What to Do Before Getting a Digester

Several steps should be followed before deciding to get a digester. This article discusses the important things that you can do to help determine if a digester would be good for your farm.
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Updated:
October 1, 2024
Planning is an essential step in installing a digester (photo:adobestock)

Planning is an essential step in installing a digester (photo:adobestock)

Short Overview of Anaerobic Digesters (AD)

Anaerobic digestion occurs in an air-tight container where microorganisms break down organic material to produce biogas and digestate. The organic materials that can be broken down in the digester include animal waste, herbaceous plants, food waste, and sewage sludge.

Biogas is the primary product of an Anaerobic digester, and is composed of methane (CH4), carbon dioxide (CO2), hydrogen sulfide (H2S), and other impurities. Biogas can be used to generate heat and power on the farm, upgraded to provide fuel for cars, or injected into the pipeline or sold as compressed natural gas.

Digestate is a secondary product of an Anaerobic Digester, and in many respects, it functions much like raw manure, but with several enhanced properties (including dramatically reduced odor). It can be used as a fertilizer and soil amendment, the fibrous solids can be used for animal bedding, and the liquid fraction can serve as a nutrient-rich irrigation water.

Should every farm have a digester? Not necessarily. The up-front cost of a digester tends to be high, and economic returns vary. Many other factors also affect the practicality of adding a digester system to a farm, and farmers should look carefully at their situation before committing to installing a digester.

This fact sheet includes three comprehensive sections detailing the essential steps a farmer should take before installing a digester on a farm. These steps are:

  1. Learn about anaerobic digesters and their process.
  2. Analyze economic performance.
  3. Assess your site.

Learn About Anaerobic Digesters and Their Processes

Before installing an anaerobic digester, proper thought, consideration, and due diligence must be followed to learn and understand the key factors influencing anaerobic digesters. This includes visiting farms with digesters, asking various questions to get insight into the dynamics and practical experience of having anaerobic digesters, reading articles, and getting familiarized with the products and market available. Many people agree that nothing is as helpful as seeing another farm digester in operation. John Mclean of Mac Farms Inc. (Kentucky) says, "If you want to start a digester, go and learn from the ones operating." The US EPA "Agstar" website keeps a list of farm digesters and their locations. Another good source of information is the Penn State Extension website, where a variety of anaerobic digestion articles, recorded webinars, and other resources can be found; this can help in informing and preparing the farm for the task ahead.

"Proper preparation prevents poor performance." the same goes for assessing and constructing an anaerobic digester on a farm. EPA further buttresses this point in an article, where they highlighted ten critical points to digester success, which are stated below:

  1. Planning for the project's success: This decides the project's outcome. Therefore, the planning must be solid and based on facts required for the success of the project.
  2. Recruiting and securing an experienced team: Recruitment of teams that have worked on different digester projects is mainly required because they know the proper steps to take in securing and getting a digester working. Project verification is, however, required for the team by the farmer or operating personnel (it is also not harmful to try out new teams, but proceed with caution).
  3. Development of a sustainable business model: The project should be able to cover the cost of operation and provide revenue; therefore, a sustainable business model should be implemented.
  4. Securing suitable feedstock supply: The feedstock to be used should be determined before the beginning of the operation; if co-digestion is involved, the provision should be included in the construction of the digester and the source from which to obtain it.
  5. Using the most appropriate technology: The type of digester has to be evaluated based on the feedstock that will be used to generate the biogas and some other factors, including the manner of digestate collection and climate in the location, among others.
  6. Analyzing the use of biogas: The digester's end-use product, in this case, biogas, should be assessed, as well as its various markets. The question, "Will the biogas be used as a source of heat and electricity, renewable natural gas (RNG), or compressed natural gas (CNG)?" should be considered. 
  7. Agreement: Legal contracts and agreements should be made between parties involved in the construction and operation of the digester, most especially between a farmer and a developer; this is an essential component.
  8. Additional benefits of the digester: The digester produces biogas and reduces odor and greenhouse emissions.
  9. Community outreach: The acceptance of the digester by the community and people around has to be sought for the project to be successful.
  10. Planning for operation and maintenance: The health of the digester is based on the production of the biogas; the digester should have an operational maintenance routine, and it should be part of the plan from the beginning of the project.

Upon reviewing these key points, it is evident that feasibility study is often not emphasized adequately. According to the article "Economic Feasibility" by the University of Missouri Extension, five indicators should be considered to determine whether a comprehensive feasibility study is necessary. At least two of these conditions must be met. These indicators include:

  1. The farm is a Confined Animal Feeding Operation (CAFO).
  2. There is potential for co-digestion (adding another waste stream like food or grasses).
  3. The farm gets complaints about odor.
  4. The farm is a livestock farm.
  5. The annual cost of electricity incurred on the farm is more than $5,000.

In addition, the EPA has created an Initial Project Checklist that can help determine project viability.

Analyze Economic Performance

The overall cost of production and maintenance should be less than the income. Several sources of income can be obtained, such as energy credit sales and proceeds from the sales of biogas and digestate. In addition, the digestate can be used as animal bedding and fertilizer which reduces other costs on the farm.

The most significant cost associated with the digester is the initial capital cost, which is usually highly dependent on the scope of the operation (Redman, G. et al., 2010), and can be subsidized by securing loans and grants (Anonymous, 2020).

According to AgSTAR, analyzing the financial feasibility of an AD, which is part of the project planning and financing, consists of the following steps:

  1. Preliminary screening
  2. Technical feasibility
  3. Revenue and expenses
  4. Business model ownership structure
  5. Project finance
  6. Refine the project plan and select an approach

Tools for Analyzing Digester Economics

To analyze the digester's economic performance, one must know the capital and operating costs required. Some tools can be used to calculate the profitability or economic indicators, and they include:

  1. Agstar AD screening tool  
    Developed by the US Environmental Protection Agency, this tool generates estimates of biogas production from a given set of feedstocks.  It is a valuable aid for conducting feasibility analyses based on the feedstock used. Comprising 24 sheets, it estimates the annual biogas and digestate production, projects methane emission reductions, and identifies potential end uses by selecting the appropriate feedstocks and their characteristics.
  2. Penn State Extension digester economics tool
    This tool is designed to help stakeholders evaluate and estimate the economics of a farm-based anaerobic digester using animal manure as the major feedstock. It can also analyze the addition of food waste and warm season grasses to generate electricity from biogas (this tool does not include upgrading the biogas for Renewable Natural gas).
    The computer spreadsheet workbook consists of 15 sheets, clearly described on the welcome page; the tabs are color-coded: the yellow tab indicates data entry, the green tab for switchgrass data entry, and the grey tab for calculations. 

Economic Performance Indicators

A project has to have good economic performance for it to be viable. Several “economic performance indicators” estimate how profitable a project will be. These include:

  • Payback: The number of years it will take to recover the money invested in a project. A payback period of 5 years or less is usually considered favorable for a project.
  • Internal Rate of Return (IRR): The equivalent interest rate that you would have to receive on a bank account to get the same economic performance that the project is providing. An IRR of 15% or more is usually considered pretty favorable for a project.
  • Net Present Value (NPV): the amount of money that the project will be worth over the course of its lifetime, in terms of current dollars. The NPV for a project must be positive for it to be considered economically viable.

Economists look at these and other economic indicators to help determine if a project is economically attractive or not. When reviewing these performance indicators, it’s also important to consider risk. A project can have a high rate of return but also be very risky and may be less attractive than a project with a low return but a high likelihood of not failing.

Assess the Site

Assessing the site might be the last step before a farmer decides about a digester, but it is one of the essential steps in the process because it cannot change after construction.

Choosing the right location can affect the overall success of a digester project. The following steps should be followed when assessing the site for the digester:

  1. Verifying that local regulations, zoning, and deed restrictions allow for the construction and operation of an AD.
  2. It should be located close to the farm, most especially to the manure or biomass source.
  3. The land should be sloped to prevent water retention and allow the free flow of slurry.
  4. The area should be well-ventilated.
  5. The soil in the area should be strong enough to hold the structures required for the digester (about 2kg/cm2).
  6. The site should be sufficiently large to allow for all digester components to be installed without crowding other farm structures or impeding farm operations.
  7. The site should be accessible for equipment delivery and construction, as well as ongoing access for maintenance and operation.
  8. The AD should be properly isolated from ground or surface water.

These steps provide a list of options to consider before deciding to erect the digester on the farm to prevent unexpected problems or even failure due to site location.

Conclusions

Deciding whether or not to install a digester is a big decision for a farmer. So, it's important to consider the matter carefully before committing one way or the other. The guidelines presented here, namely to learn more about AD, analyze the economics, and assess the site, are a good starting point towards making a well-informed decision that is best suited for your farm. 

Further Reading

If you'd like to dig deeper on this topic, consider reading some of the following: 

References

Mary Akingbasote
Graduate Student
Penn State
mta5387@psu.edu

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