What is Anaerobic Digestion?

Anaerobic digestion is a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen. One of the end products is biogas, which is combusted to generate electricity and heat, or can be processed into renewable natural gas and transportation fuels.

A range of anaerobic digestion technologies are converting livestock manure, municipal wastewater solids, food waste, high strength industrial wastewater and residuals, fats, oils and grease (FOG), and various other organic waste streams into biogas, 24 hours a day, 7 days a week. Separated digested solids can be composted, utilized for dairy bedding, directly applied to cropland or converted into other products. Nutrients in the liquid stream are used in agriculture as fertilizer.

The Biological Process

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.

Digester Technologies

Many different anaerobic digester systems are commercially available. The following is an overview of the different types of technologies. In  general, the higher the solids content of the material, the less you’re able to mix it, which pushes you into choices: do you choose a technology that can handle higher solids, or do you dilute the material with water which makes mixing easier, but takes up space in the digester and more heat. Also, while not considered digester, landfills use the same anaerobic digestion process within the pile of material and produce biogas.

Complete mix digester

This is the most common style of digester in the US. Complete mix, or continuously stirred digesters are enclosed, heated tanks with a mechanical, hydraulic, or gas mixing system operated the way Goldilocks would prefer–just the right amount of mixing, heat, and pH balanced. The tanks are made of steel or concrete and are most often above grade. Complete mix digesters often dilute the mix of feedstock to aid mixing, like the consistency of a thick “soup.” 

Plug flow digester

Plug flow digesters put new feedstock in one end, pushing it to  the other end with minimal or no mixing and heating. They are often designed as long, narrow concrete tanks with a rigid or flexible cover. Most tanks are built partially or fully below grade to limit the demand for heat. Plug flow systems are common for dairy operations that collect manure by scraping.

Mixed plug flow digester

Mixed plug flow systems combine attributes of a complete mix/continuously stirred digester with a plug flow digester to include more mixing than a plug flow, but a higher solids content (less water) than a complete mix. They have mixing, heating, and are most often at or below grade concrete tanks. In the US, they are most used by dairy operations.

Dry digester

Instead of stirring the feedstock, dry digesters resemble a pile of material in an enclosed tank that often looks like a garage. Interior roof sprayers circulate a liquid, including microbes and some feedstock, onto the pile and collected through floor slats to be recirculated. Like other digesters, biogas rises to he top, collected in flexible bladders above the digester. Dry digesters have the highest solids content, are best for digesting yard waste, and can sometimes compost the material  after digestion by forcing air through the floor slats.

Covered lagoon system

Most covered lagoons used to be an open manure lagoon on a farm that have added an engineered flexible cover added to capture biogas. Most lagoons do not have heating or mixing, although this is changing. These are cheaper to build and operate compared to other digesters, but less productive in biogas production, which rises and falls with temperature, sometimes halting completely in the winter.


Landfills are biogas systems that use the same anaerobic digestion process digesters do to yield biogas. Landfills consist of layers of waste and alernate daily cover (often soil) with vacuum piping to extract the biogas. The biogas from landfills, also called landfill gas, often has a lower methane content, but much larger volumes of gas compared to digesters.