Anaerobic Digestion (AD) is a process by which microorganisms break down or "digest" biodegradable material in the absence of oxygen.  This process occurs naturally in locations such as swamps, bogs, and the inside of compost piles where organic matter is submerged in water or covered by many layers of material. Anaerobic bacteria in these environments gradually break down organic matter, releasing gases.  This naturally occurring process is replicated in constructed airtight, insulated vessels, that can be fed an extensive range of organic material including leftover food, cow manure, dairy whey, wastewater sludge, grass clippings, and fats, oils, and greases (FOG).  The reactions that occur inside the digester have numerous beneficial impacts - including a decrease of the volatile organic compounds, reduced odors, and destroyed pathogens. 
 
In the process of breaking down organic waste, anaerobic digesters produce two useful byproducts.  One being a biogas composed primarily of methane, which is released during digestion and can be captured for use in a generator; or upgraded to natural gas quality standards (becoming biomethane) and injected directly into the pipeline system or used to fuel compressed natural gas (CNG) vehicles.    ​​
 
The second useful byproduct is the solid and liquid effluent of the digestion process called digestate.  Digestate is a nutrient-rich substance that can be land applied as a soil amendment and fertilizer by farmers, residents, and gardeners; for landscaping municipally owned properties; in brownfield reclamation; or repurposed to be used as as cow bedding, bioplastics, low-grade building materials, and fireboard.  Due to the nature of the anaerobic digestion process, very little of the available nutrient value of the organic matter is changed - AD only stabilizes and converts the nutrients to more soluble and readily available forms for plant uptake.  Replacing the use and transportation of industrially produced chemical fertilizers (which are energy-intensive to manufacture and transport) with locally created digestate will assist in lowering the amount of nutrient runoff and accumulation in local waterways, groundwater contamination, and other negative environmental and human health effects.

 
Overall, anaerobic digestion reduces the energy and environmental footprint of waste management on many fronts, by reducing the total amount of waste, generating renewable energy in the process, and producing useful agricultural end products.
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