What is Anaerobic Digestion?

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. Learn more about how anaerobic digestion works top of page.

Anaerobic digestion uses bacteria to transform organic waste into energy in the complete absence of oxygen.

This transformation occurs in nature, in marshes, for example. In order to be useable on a larger scale, the process has been tamed and optimized in closed tanks called digesters. The micro-organisms digest the organic fraction of the waste and convert it into biogas, a source of renewable energy.

The digestion process begins with bacterial hydrolysis of the input materials in order to break down insoluble organic polymers such as carbohydrates and make them available for other bacteria.


Acidogenic bacteria then convert the sugars and amino acids into carbon dioxide, hydrogen, ammonia, and organic acids. Acetogenic bacteria then convert these resulting organic acids into acetic acid, along with additional ammonia, hydrogen, and carbon dioxide. Finally, methanogens convert these products to methane and carbon dioxide.

Basic Information about Anaerobic Digestion (AD)

An Anaerobic Digester is a unit designed for processing organics through the process of anaerobic decomposition. These containers house a variety of organics along with the organism needed to convert these items into biogases, which can be used as an energy alternative.These digesters have provided an opportunity for waste reduction. As some organic waste cannot be conventionally composted, decomposing the organics anaerobically allows them to be decomposed and using them in creating a new fuel source. At the same time, it prevents methane from entering the atmosphere.

The digestate produced is stored until required, and can be separated into liquid and solid fractions. Solid fractions can be processed further on site by being put into a composting operation for further processing or used directly on land. The liquid may also be used on the land as a biofertiliser.

The anaerobic digestion quality protocol (ADQP) has also now been published, providing information about the end of waste criteria for digestate. The ADQP is available from the environment agency website for download.

What Are the Main Benefits of AD?

Utilizing anaerobic digesters provides two principal benefits:

  • production of energy (in the form of biogas) and
  • environmental benefits.

In this case the manure, rather than being left in a lagoon, is kept in a closed anaerobic digester where the biogas is captured. Biogas is composed of about 60% methane, with most of the rest being carbon dioxide.

Biogas can be used as is or cleaned up to be used as a renewable substitute for natural gas, propane or other fossil fuels. In addition to the clean energy produced, there are also numerous environmental benefits.

The capture of greenhouse gas emissions that would otherwise have been emitted into the atmosphere can help combat global warming (methane is 18 times more powerful a greenhouse gas than CO2).

Other air quality improvements include significant reduction in odor, ammonia and particulates, and water quality improvements can include reduction of nutrient run-off issues (phosphates, nitrates, BOD materials).

So if AD is so great how come they’re not all over the place, you ask?

Until recently, use of AD in the U.S. has been limited to old-generation, small-scale technology. However, recent advances in technology have made AD more cost competitive with other types of energy.

Types of Anaerobic Digesters

Different anaerobic digesters process different categories of feedstock.

  • Wet digesters: the most common type. They process feedstock that contains less than 15 percent solids.
  • Dry digesters: process feedstock that contains more than 15 percent solids.

When multiple types of feedstock—for example, manure and food waste—are digested at the same time, the process is known as “co-digestion ”.

Our industry experience has taught us that it is never a “one-size-fits-all” solution each operator needs the system that best solves their organic waste and biogas needs.

Anaerobic digestion is a renewable energy technology that produces biogas. Biogas can be used to conduct work that has social benefits, such as the production of electricity or heat. Anaerobic digestion must be managed properly to generate biogas.

Feedstock, (also known as biomass), is fed to a digester tank to create biogas. Feedstocks vary in their biogas production potential.

Anaerobic digestion is a complex biochemical reaction carried out in a number of steps by several types of microorganisms that require no oxygen to live. This reaction produces biogas, which is primarily composed of methane and CO2.

A great variety of organic material can be used as feedstocks for generating biogas. All anaerobic digesters perform the same basic function. They hold organic matter in the absence of oxygen and maintain proper conditions for methane forming microorganisms to grow.

There is a wide variety of anaerobic digester types, each performing this basic function in a subtly different way.

Different types of biogas systems are better suited for different types of organic waste.

Waste and Wastewater

Anaerobic digestion has been widely applied in the treatment of municipal sludge and industrial wastewaters.

The taxonomic and functional composition of ad digesters treating high-strength industrial wastewater have not been extensively studied, especially those taxa and functions involved with the major AD steps.

Given the vast and increasing volume of high-strength industrial wastewater produced worldwide, the application of AD to wastewater is likely to grow. Therefore, it is important to thoroughly understand the microbiology involved in the anaerobic treatment of wastewater and how it differs from the more common sludge digesters.

Anaerobic digestion (AD) is the natural breakdown of biodegradable material by microorganisms in the absence of oxygen. Biodegradable materials such as animal manure, food scraps, wastewater treatment solids, restaurant grease, and municipal and industrial wastewater and residuals can be put to use to help with waste management and energy needs.

Biodegradable materials are put into a digester, where there is an absence of oxygen. The naturally occurring microorganisms found within the materials break down the organic matter, producing biogas and digestate. The digester is usually heated to a certain temperature, which depends upon what material is being digested and what type of system is used.

Often times a combined heat and power (CHP) unit that runs off of biogas is used, producing both electricity and heat. Most anaerobic digestion facilities share a similar process, but some facilities have separated the process into multiple stages. There are different types of digesters for different types of material, but all generally convert organics into biogas and digestate. via this link.

Waste and Wastewater Treatment

Many wastewater treatment plants (WWTP) already have on-site anaerobic digesters to treat sewage sludge, the solids separated during the treatment process. However, many US WWTPs do not have the equipment to use the biogas they produce, and flare it instead.

In the US, of the 1,269 wastewater treatment plants using an anaerobic digester, only around 860 use their biogas. If all the facilities that currently use anaerobic digestion—treating over 5 million gallons each day—were to install an energy recovery facility, the united states could reduce annual carbon dioxide emissions by 2. 3 million metric tons—equal to the annual emissions from 430,000 passenger vehicles.

Anaerobic digestion is frequently used in the treatment of municipal wastewaters, often in a process that also includes aerobic digestion (digestion in the presence of oxygen) and sedimentation. The amount of solids produced from wastewater treatment can be reduced though anaerobic digestion, which in turn reduces the costs associated with their disposal. Similar to human waste, animal waste may also provide the feedstock for anaerobic digestion.

We expect the sewage systems of our local communities to carry waste materials from our toilets, bathtubs, kitchen sinks, and ever plumbing fixture, to a treatment facility.

Anaerobic digestion (AD) is a process by which microorganisms break down organic matter such as that contained in sewage, producing various gases and a reduced volume of semi-solid residue.

The gas produced, called "biogas" or "digester gas," includes a high percentage of methane, which can be burned to produce heat and/or electricity.

Biogas from anaerobic digestion of biosolids and other organic residuals is a renewable, green fuel. The semi-solid residue from the digestion process, called "digestate" (and/or " biosolids ," if it is derived from wastewater solids and meets regulatory standards for use on land) is a stabilized material that can be used as a soil amendment, for animal bedding, or other uses, depending on the levels of further treatment.

The threats of climate change, population growth and resource constraints are forcing governments to develop increasingly stronger policy measures to stimulate the development of renewable energy technologies.

Bioenergy offers particular promise since it has the potential to deliver multiple benefits such as: improved energy security, reduced co2 emissions, increased economic growth and rural development opportunities.

Anaerobic digestion is one of the most promising renewable energy technologies since it can be applied in multiple settings such as wastewater and municipal waste treatment as well as in agriculture and other industrial facilities.

Mature Technology

It is a mature technology, which is applied around the world on farms and at municipal and industrial wastewater treatment plants. In the controlled environment of an anaerobic digester, a complex microbial food web breaks down organic wastes via hydrolysis, acidogenesis, acetogenesis, and methanogenesis to the main end product gaseous methane.

The process stability relies on syntrophic interactions between microbes carrying out different components of the anaerobic digestion pathway . In anaerobic digestion, multiple populations are often able to carry out the same function in the digester, which leads to a flexible community structure and increased process stability via redundancy.

In other words, the replacement of negatively impacted populations by others to fill their ecological niche [ 2 ]) in the microbiome ( 3 ). An increased understanding of how anaerobic digester microbiomes respond to disturbances is important from both microbial ecology and bioprocess operating perspectives.

Attractive features of the process include the production of a single end-product from a heterogeneous feedstock, and in-situ product separation of the gaseous end-product. Despite these intrinsic attractive properties of the process, the economic added value of the biogas produced is limited, enabling the development of alternative processes that yield higher-value end-products.

Typically the production of higher value end-products from low value feedstock and industrial wastewater proceeds via intermediate production of organic acids (and carbon dioxide and molecular hydrogen).

Optimization of organic acid production from particulate feedstocks and wastewater for development of the organic acid based resource recovery route receives significant research attention. The organic acid stream generated as such, has no economic value. But if organic acids can either be concentrated via membrane separation or (bio) converted to an end-product that can easily be separated from the liquid, an attractive biomass processing scheme can be developed.

Attractive end-products of organic acid processing include:

  • polyhydroxyalkanoates,
  • medium chain length fatty acids, or
  • other organic molecules using bio-electrochemical systems.

Overall we suggest that these novel bioprocessing routes for conversion of low value feedstock to higher added value products will contribute to a sustainable future and will change the economic status of organic waste.

A Mature Technology Available Right Now

The anaerobic process technology is available now! It is what is often called spade-ready!

It is most often used to treat organic wastes other than unsegregated MSW, such as sewage sludge (biosolids produced from treated sanitary sewage at a wastewater treatment plant), yard vegetation, agricultural wastes (both animal and plant) and some industrial waste sludge.  Depending on the waste feedstock and the system design, biogas is typically composed of 55 to 75 percent methane.

The anaerobic digestion process occurs in three steps (as in the diagram above:

  1. Decomposition of plant or animal matter by bacteria into molecules such as sugar.
  2. Conversion of decomposed matter to organic acids.
  3. Organic acid conversion to methane gas.

Anaerobic digestion (AD) can occur naturally in a landfill or in a controlled environment such as manufactured chamber or vessel. In an AD facility the incoming waste stream is initially processed for removal of potential contaminants, reduced in size and delivered to an air-tight reactor vessel, typically called a “digester”.

Single Reactor or Multireactor Systems

AD facilities are usually classified based on whether the system uses a single reactor or multireactor system, and if water is added for processing (wet systems) or absent during processing (dry systems).There are more than 200 AD facilities around the world with operating capacities greater than 2,500 tons per year (TPY).

These plants process not only the organic fraction of the MSW waste stream but also organic waste from food industries and animal manure.

Europe leads in the number of AD plants and total installed capacity principally due to the European Union Directive that requires member states to reduce the amount of landfilled organics by 65 percent by 2020.

There are more than 120 plants processing about 4.6 million tons per year of organic fraction of MSW in Europe.The “dry” system technology has been adapted to manage parts of the municipal solid waste (MSW) stream and is the current preferred technology in the U.S. for processing of organics in typical MSW.

Currently in North America there are four commercially operating “dry” AD facilities, where the feedstock is primarily organics from MSW.

Bioenergy and Anaerobic Digestion

Anaerobic digestion (AD) is a biochemical process that uses microorganisms to degrade organic materials. AD is a mature technology used for decades as a waste stabilization and/or bioenergy production process. AD transforms organic matter into biogas and a nutrient-rich effluent or “digestate. ”biogas is a mixture of gases, mainly methane (CH4) and carbon dioxide (CO2), which can be either burned directly for heat and power generation, or upgraded to be used as a transportation fuel. The effluent or digestate is rich in nutrients and is usually land-applied as a fertilizer and soil amendment.

"Zero waste energy" offers a high-solids, batch processing anaerobic digestion design for facilities starting as low as 5,000 tpy and up to 90,000 tpy of almost any organic material utilizing proprietary technology. The batch processing  is well suited for the production of biogas from stacked solid organic waste in a non-continuous  process.

Reactor technology development new wastewater process technologies the issues above are important for the prevention of water pollution by reducing organic solid waste and wastewater disposal, for saving water by enabling water reuse, for fossil energy saving and hence water saving.

And for progressing from water conservation to resource conservation.

Anaerobic technologies are key to enable resource recovery from wastewater and for new water technologies. The one key innovation/technology/development or hot topic on anaerobic digestion that will be transformational in this field is the development of high-rate anaerobic reactor technology that is independent of the presence of granular biomass.