Ag & Energy - Lesson 7 - Agriculture & Biogas Energy

Ag & Energy - Lesson 7 - Agriculture & Biogas Energy

Target Grade Level / Age Range:

Grades 9-12


50 minutes


By the end of this lesson, students will be able to:

  1. Understand what an anaerobic digester is and how it can be used to create methane to generate electricity or burned for heat.
  2. Identify the different types of infrastructures that allow for the capture of methane in farm operations.
  3. Identify how methane can be a source of fuel for farms.


  • Projector
  • Screen
  • Large writing surface
  • 2 laptops or computers
  • Flip charts
  • Markers

Suggested Companion Resources


  • Biogas: mixture of different gases produced by the breakdown of organic matter in the absence of oxygen. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste
  • Anaerobic digestion: a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen. One of the end products is biogas
  • Methane: a chemical compound with the chemical formula CH4. It is the simplest alkane and the main component of natural gas

Background – Agricultural Connections

  • 8 million small-scale anaerobic digesters used in China for heating & cooking
  • 6,800 large-scale anaerobic digesters operating in Germany
  • 162 anaerobic digesters operating in the USA (producing 453 million kWh)
  • HUGE potential remains for the USA

Interest Approach or Motivator

Where on a farm or in a home do we use gas (i.e. propane, natural gas) as an energy source? Students may respond with barbeque grill, gas cooking stove, hot water heater, furnace, etc. It can also be burned in a gas turbine or steam generator to produce electrical energy that can be added to the grid.

Where does that gas come from? Most often it is piped into homes and businesses from utility companies. Utility companies typically extract gas from reserves underground. If not connected to a utility company, a rural home or farm might have a propane tank on site that needs to be refilled periodically.

What is natural gas? How is it produced? Natural gas is a naturally occurring hydrocarbon gas that is primarily methane. It is formed when layers of decomposing plant and animal organic matter is exposed to heat and pressure. It can be found deep underground typically near coal beds or petroleum deposits.

More recently methane is being harvested from garbage in landfills. Pass out printed copies of the two news articles (Garbage to Gas to Energy and Landfill Methane) and have students read them. How is methane being captured from landfills?

It takes a large amount of decaying biomass to produce methane. Where on a farm might we find a source of decaying biomass? Animal manure! Could this manure be used to produce methane to heat our homes and generate electricity?


Objective 1: Understand what an anaerobic digester is and how it can be used to create methane to generate electricity or burned for heat.    

Set up two stations on opposite sides of the classroom. Each station should have a laptop with sound and be queue up to one of two videos. Station 1 should be queue up with the video – Anaerobic Digestion. Print out the notes below on card stock and have it available at station 1. Station 2 should have the video The Anaerobic Digester. Print out copies of the article Methane Energy 4 Iowa and have it available at station 2.

Divide the class into two groups and assign each group to one of the stations. Students will be challenged to watch the video and read the notes/articles to glean as much information as they can about anaerobic digesters. They will then as a group be responsible for teaching the content that they learned from the video and the reading to the other group. They can teach the content in whatever way they want. Provide whiteboard, flipchart, markers, or other material as necessary to the students. They can teach the material in an audio/visual presentation, a skit, an interview, a debate, or any other creative presentation.

Give the students 5-7 minutes to watch the videos and read the notes. Give students another 5-10 minutes to discuss with their group how to present the information. Then allow 5 minutes for each group to present their material to the other group.


Anaerobic Digestion: Anaerobic digestion is the main process to get energy from animal waste (manure). Basically, manure and other organic waste is loaded into an airtight tank where special anaerobic bacteria (that live without oxygen) consume the waste and give off methane. Methane is a very flammable gas (like propane) that can power generators efficiently. The process also helps to dispose of the waste that can have harmful effects to water quality. On top of that, the process helps control the smell from livestock operations. In fact, smell complaints from nearby towns are a main reason why many farmers install a digester in the first place. And on top of that, the process prevents the methane from entering the atmosphere. Methane is a greenhouse gas that is about 23 times more powerful than carbon dioxide in trapping heat in the atmosphere. By trapping and burning it up in generators, GHG impacts are avoided, and farmers get free electricity and heat. A big digester can cost up to $1 million to install but with financial incentives and rising fuel prices, the payback is often within ten years.

  • Manure added to tank
  • Anaerobic bacteria consume waste
  • Resulting methane is piped out to storage tanks or to fuel generators and heaters
  • Sterilized solids are extracted for use as fertilizer and compost

Uses for captured methane

  • Fuel for generators to power farm & sell to electric utility
  • Boiler fuel for farm heating
  • Added to natural gas pipelines (after refinement)
  • Flared for odor & greenhouse gas control
Objective 2: Identify the different types of infrastructures that allow for the capture of methane in farm operations.

Now that students know what anaerobic digesters are, direct the discussion to different infrastructures to capture methane on farms. Use slides 2-6 in the PowerPoint to facilitate this discussion and new information. Have students capture notes into their notebooks.

Plug Flow: Manure must be scraped for collection; No mixing; High solids content only

Covered Lagoon: Low maintenance; Unheated; Lower productivity in cold weather. A covered lagoon digester operation is simple and straightforward. A pool of liquid manure is topped by a pontoon or other floating cover. As the manure is digested by bacteria in the lagoon, the methane is collected and sent directly to a modified natural gas generator and the micro turbine; both of which produce electricity. Covered lagoons are designed to use manure with two percent or less solid content. This type of digester requires high throughput in order for the bacteria to work on enough solids to produce gas. These systems are dependent on temperature and, as a result, methane production is greater in summer than in winter.

Complete Mix: Heated tank with mixing system; High moisture content acceptable

The capital costs of an anaerobic digester can be very high and may range from a few hundred thousand to a few million dollars, depending on the size. Payback periods can range from 5 to 16 years, depending on the conditions of operation. It is also important to keep in mind that the value of the biogas from the digester will vary depending on the type of fuel that it is replacing. A general rule of thumb is that an anaerobic digester will not be cost effective for a farm with fewer than 300 head of livestock. For smaller farms, a cooperative operation used by multiple farms may be an option.

Biological and chemical stages of biogas fermentation processes: The exact chemical species produced at each stage depend considerably upon the kinds of microorganisms used as well as processing conditions. Polymeric form of organics such as carbohydrate, protein, lipid, cellulose are degraded into their constitutive monomers by many kinds of microorganisms. The monomers are then converted mainly into organic acids (such as butyric acid or propionic acid) by the action of acid producing microorganisms. Next step is the conversion of these organic acids into acetic acids as well as hydrogen. Finally, methane can be recovered by the methane-producing bacteria. Since some of the hydrogen produced cannot be converted, the final product of biogas usually contains methane (CH4) and hydrogen (H2). These products (which are the main constituents of biogas), after desulphurization and deodorization, can later be used in the combustion system or fed to the fuel cell system to generate electricity. This is how we obtain energy from the biogas fermentation method. 

Objective 3: Identify how methane can be a source of fuel for farms.

Continue discussion with students using slides 7-8.

Application on farms:

  • Each year, 81 million tons of manure is created by Iowa’s confined livestock
  • With methane capture, this could meet the energy needs of 325,000 homes
  • Anaerobic digesters are site-specific, so site studies are needed to assess potential
  • Methane is one of the most potent greenhouse gases, trapping 25 times as much heat as carbon dioxide¹
  • ~65% of atmospheric methane is generated from agriculture, with over 1/4 of that coming from livestock²
  • If half of Iowa’s dairy operations had anaerobic digesters, 100 million pounds of methane could be captured each year (3.15 million tons of carbon equivalent)³
  • Methane produces more heat per mass unit (55.7 kJ/g) than other complex hydrocarbons, harnessing 1,000 BTU/scf

Economic Benefits of Digesters:

  • Minimizes time/labor in handling waste
  • Income from anaerobic digester byproducts & disposal fees
  • Income from power generation, tax credits & carbon markets from each kWh produced
  • Reduced on-farm costs for electricity, heat, livestock bedding and water
  • Increases overall self-sufficiency of farms
  • At $1.13/therm for natural gas, using biogas on-farm can provide a cost-effective replacement.
  • At $1.82/therm for propane, biogas can be a cost-effective replacement with profit potential.

Option 1: We have discussed the purpose, process, and types of anaerobic digesters to produce biogas energy. It is time to summarize the information into a 30 second report. The challenge is to summarize it into a report similar to a weather report. You will briefly mention how biogas is created and capture, describe how it is used, and describe how it can benefit farmers and/or the environment. Students will have eight minutes to script your weather report and get it ready to be delivered to the class. Students can work with a partner to develop the report.

If time allows, have the students present their summations or working with a partner record them on a mobile device. Recordings could be collected and graded or later showed to the class as a whole.

Option 2: Use Plickers cards to review and test student knowledge. Prior to class login to the Plickers website and register your account and enter your students on the website  Print out the cards and laminate them with non-glossy or matte lamination. Pass out cards to students. Student number should match their name that you entered on the website.

Each card has four sides (A, B, C, D) and will hold up the card facing the teacher with the letter they think is correct at the top of the card. Make sure students hold the card so that they are not covering up any part of the code on the front of the card. Teacher will then use their smartphone or tablet to scan the cards. You can connect your smart device to the project if you want students to see the feedback and responses in real time.

Have the questions and the correct answers preloaded on the website. Questions and choices are below and in slides 9-13. The correct answer is in bold below.

  1. Anaerobic bacteria help biomass decompose in the absence of oxygen.
    1. True
    2. False
  2. Covered lagoon anaerobic digesters produce more methane in the
    1. Winter
    2. Spring
    3. Summer
    4. Fall
  3. Anaerobic digesters are cost effective for small scale farms with fewer than 300 head of livestock.
    1. True
    2. False
  4. Iowa’s livestock produce _______________ tons of manure and could create enough biogas energy to supply 325,000 homes.
    1. 51 million
    2. 81 million
    3. 84 million
    4. 104 million
  5. Which of the following are economic benefits of anaerobic digesters?
    1. Reduced on-farm costs for electricity, heat, livestock bedding and water
    2. Minimizes time/labor in handling waste
    3. Increases overall self-sufficiency of farms
    4. All of the above

Essential Files (maps, charts, pictures, or documents)

Extension Activities

Conduct an experiment making a homemade anaerobic digester with this procedure .



Will Fett

Organization Affiliation

Iowa Agriculture Literacy Foundation

Agriculture Literacy Outcomes

  • Theme 1: Ag & the Environment
    • Describe resource and conservation management practices used in agricultural systems (e.g., riparian management, rotational grazing, no till farming, crop and variety selection, wildlife management, timber harvesting techniques)
    • Evaluate the potential impacts of climate change on agriculture
    • Evaluate the various definitions of “sustainable agriculture,” considering population growth, carbon footprint, environmental systems, land and water resources, and economics
  • Theme 4: STEM
    • Identify current and emerging scientific discoveries and technologies and their possible use in agriculture (e.g., biotechnology, bio-chemical, mechanical, etc.)
    • Evaluate the benefits and concerns related to the application of technology to agricultural systems (e.g., biotechnology)

Education Content Standards

  • SS.9–12.E.2: Essential Concept and/or Skill: Understand the role of scarcity and economic trade–offs and how economic conditions impact people’s lives.
  • HS-PS3-3. Design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy.*
  • HS-LS2-3. Construct and revise an explanation based on evidence for the cycling of matter and flow of energy in aerobic and anaerobic conditions.
  • HS-ESS3-2. Evaluate competing design solutions for developing, managing, and utilizing energy and mineral resources based on cost-benefit ratios.*

Common Core Connections

  • RI.9–10.4: Determine the meaning of words and phrases as they are used in a text, including figurative, connotative, and technical meanings; analyze the cumulative impact of specific word choices on meaning and tone (e.g., how the language of a court opinion differs from that of a newspaper).
  • RI.9-10.IA.1: Employ the full range of research-based comprehension strategies, including making connections, determining importance, questioning, visualizing, making inferences, summarizing, and monitoring for comprehension.
  • RI.9–10.2: Determine a central idea of a text and analyze its development over the course of the text, including how it emerges and is shaped and refined by specific details; provide an objective summary of the text.