VALUE OF BYPRODUCTS
BIODIGESTION
What is Anaerobic Biodigestion?
Anaerobic digestion is a process through which bacteria break down organic matter—such as animal manure, wastewater biosolids, and food wastes—in the absence of oxygen. Anaerobic digestion for biogas production takes place in a sealed vessel called a reactor, which is designed and constructed in various shapes and sizes specific to the site and feedstock conditions (learn more about AD system design and technology). These reactors contain complex microbial communities that break down (or digest) the waste and produce resultant biogas and digestate (the solid and liquid material end-products of the AD process), which is discharged from the digester.
Multiple organic materials can be combined in one digester, a practice called co-digestion. Co-digested materials include manure, food waste (i.e., processing, distribution, and consumer generated materials), energy crops, crop residues, fats, oils, and greases (FOG) from restaurant grease traps, and many other sources. Co-digestion can increase biogas production from low-yielding or difficult-to-digest organic waste. (EPA Anaerobic Digestion)
Biodigesters are becoming more widely used in converting dairy and feedlot manure into biogas. The technology for handling food waste is also growing in popularity. New developments are now making this technology accessible to meat and poultry processing operations.
Operating Temperature
Digesters are designed to run at different target temperature ranges. The temperature ranges are typically 86 - 100○ F for mesophillic and 122 - 140○ F for thermophilic. There are other populations of anaerobic microbes that thrive in these temperature zones.
Generally, thermophilic anaerobic digestion (AD) would be used when greater pathogen kill is necessary. This temperature range can produce "Class A Biosolids". Class A Biosolids is a designation for dewatered and heated sewage sludge that meets U.S. EPA guidelines for land application with no restrictions. Thus, Class A Biosolids can be legally used as fertilizer on farms and vegetable gardens and sold to home gardeners as compost or fertilizer. Thermophilic digesters require less time to process feedstocks but may have higher costs and be more challenging to operate.
In general, mesophillic digesters are easier to operate and maintain but will not result in sufficient pathogen kill to produce Class A Biosolids. (EPA Anaerobic Digesters)
