EM SC 470
Applied Sustainability in Contemporary Culture

Anaerobic Digestion


You may recall from EM SC 240N that bioenergy is energy that comes from living or recently living things. Common examples include wood from trees used for heating and ethanol from corn used as a gasoline additive. Another form - and one that we will see while traveling - is called anaerobic digestion. "Anaerobic" refers to "without air" and the "digestion" part refers to the microorganisms that digest organic material. Putting it together, anaerobic digestion refers to microorganisms breaking down organic material when no oxygen is present. The following descriptions of anaerobic digestion are from the EPA's Anaerobic Digestion website. All points of emphasis (bold letters) are mine:

Anaerobic Digestion

Anaerobic digestion is the natural process in which microorganisms break down organic materials.  In this instance, “organic” means coming from or made of plants or animals.  Anaerobic digestion happens in closed spaces where there is no air (or oxygen).  The initials “AD” may refer to the process of anaerobic digestion or the built system where anaerobic digestion takes place, also known as a digester.

The following materials are generally considered “organic.”  These materials can be processed in a digester:

  • Animal manures;
  • Food scraps;
  • Fats, oils, and greases;
  • Industrial organic residuals; and
  • Sewage sludge (biosolids).

All anaerobic digestion systems adhere to the same basic principles whether the feedstock is food waste, animal manures or wastewater sludge.  The systems may have some differences in design but the process is basically the same

Byproducts of Anaerobic Digestion

Biogas is generated during anaerobic digestion when microorganisms break down (eat) organic materials in the absence of air (or oxygen).  Biogas is mostly methane (CH4) and carbon dioxide (CO2), with very small amounts of water vapor and other gases. The carbon dioxide and other gases can be removed, leaving only the methane. Methane is the primary component of natural gas.

The material that is left after anaerobic digestion happens is called “digestate.” Digestate is a wet mixture that is usually separated into a solid and a liquid. Digestate is rich in nutrients and can be used as fertilizer for crops

Uses of Anaerobic Digestion Byproducts

Biogas is produced throughout the anaerobic digestion process.  Biogas is a renewable energy source that can be used in a variety of ways. Communities and businesses across the country use biogas to:

  • Power engines, produce mechanical power, heat and/or electricity (including combined heat and power systems);
  • Fuel boilers and furnaces, heating digesters and other spaces;
  • Run alternative-fuel vehicles; and
  • Supply homes and business through the natural gas pipeline

How biogas is used and how efficiently it’s used depends on its quality.  Biogas is often cleaned to remove carbon dioxide, water vapor and other trace contaminants.  Removing these compounds from biogas increases the energy value of the biogas...Biogas treated to meet pipeline quality standards can be distributed through the natural gas pipeline and used in homes and businesses. Biogas can also be cleaned and upgraded to produce compressed natural gas (CNG) or liquefied natural gas (LNG).  CNG and LNG can be used to fuel cars and trucks.

Digestate is the material that is left over following the anaerobic digestion process.  Digestate can be made into products like:

  • Bedding for livestock;
  • Flower pots;
  • Soil amendments; and
  • Fertilizers.

When properly processed, dewatered digestate can be used as livestock bedding or to produce products like flower pots.

Digestate can be directly land applied and incorporated into soils to improve soil characteristics and facilitate plant growth. Digestate can also be further processed into products that are bagged and sold in stores. Some emerging technologies can be employed post-digestion to recover the nitrogen and phosphorus in digestate and create concentrated nutrient products, such as struvite (magnesium-ammonium-phosphate) and ammonium sulfate fertilizers. 

The video below from Michigan State University does a great job of explaining how they use anaerobic digestion to convert organic cafeteria and farm waste into useful energy and fertilizer. To view the transcript (and the video on YouTube, click this link.)

Click here for a transcript of The Anaerobic Digester at MSU video.

The South Campus anaerobic digester at Michigan State University aims to reduce odor and emissions from manure, food waste and biosolids, create a closed cycle for recycling organic material, and generate renewable energy. The digester will use a mix of dairy manure, dining hall food waste, biosolids (which are nutrient-rich organic materials), and other organic material from campus in the greater Lansing area. Once delivered, the food waste and biosolids will be treated (heated and pasteurized) at 160 degrees Fahrenheit for one hour to eliminate any potential pathogens. The material will then be mixed with manure from the MSU Dairy Teaching and Research Center and pumped into the digester.

The anaerobic digester is a sealed, airtight tank, which can hold 300,000 gallons. Organic material will be heated to 100 degrees Fahrenheit and will remain in the digester for 20 to 30 days. Inside the tank, mixers in the digester evenly distribute microorganisms. These microorganisms break down the mixture, producing biogas and nutrient-rich digestate. Biogas, which is roughly 60% methane, is a form of renewable energy which will be used to generate electricity. The system will generate enough electricity to offset the MSU DTRC, around 300 kilowatts per hour, with enough left over to help power MSU. The capturing and use of biogas will generate energy, shrink the carbon footprint of campus, and improve sustainability. What's left in the process is digestate, the nutrient-rich mixture that will be held in the final, storage tank until it can be utilized. Mixers in the storage tank will minimize settling for the potential 2.7 million gallons of digestate. Any unused biogas can also be stored in the sealed headspace. The nutrient-rich digestate will be used to fertilize croplands or other biological treatment processes to fuel research opportunities.

By utilizing anaerobic digestion technology, the organic materials from MSU will become resources. Renewable energy is created, while odors are reduced and emissions are eliminated. This project is one example of how Michigan State University is actively working to improve sustainability.

Thermodynamically speaking, the energy conversion process is:

  • Sunlight is converted to biomass by plants.
  • Plants are either used in the digester and converted to methane and CO2 or...
  • Biomass is eaten by animals and converted to another form of biomass.
  • Organic waste from animals is used in the digester and converted to methane and CO2.
  • The methane can then be used for electricity and/or heat.

It is extremely important to keep in mind that this is a natural process, and thus will occur any time organic material is subjected to low- or no-oxygen conditions. One important implication of this is that organic material that ends up buried in a landfill will convert partially to methane because there is very little oxygen underneath all of that "junk." As I'm sure you recall, methane is about 30 times more potent than CO2 in terms of its global warming impact. If you took the same organic material and let it biodegrade in the open air (i.e., with access to oxygen) it would release mostly CO2. The sad irony of this is that well-meaning people and companies can actually make the (climate change) problem worse if their biodegradable containers end up in the landfill. This methane can be captured, and in many places in the U.S. and throughout the world is. This is also why impoundment hydroelectric facilities (big dams) can cause methane emissions - organic material collects upstream of the dam, and low-oxygen conditions often occur near the bottom of the reservoirs, causing methane to be released. Systems thinking, everyone!