Episode Details

If you’ve ever been near a farm with livestock, you might agree that farm odors can be carried far from the farm itself. Farms often face criticism from nearby residents over the offensive odor of manure. While manure is an unavoidable part of raising livestock, there is one technological innovation that can remove the smell from manure. Anaerobic digesters are a simple concept—instead of leaving manure rotting outside, the digester encloses the manure as bacteria decompose it, keeping the odor in. But could the same technology also deal with food waste and cut carbon emissions? 

How do Anaerobic Digesters work?

Anaerobic Digesters leverage the biology of decomposition to turn organic waste including manure and food scraps into useful products. The digestion process starts with pumping the waste into the digester, an enclosed tank containing microorganisms in the absence of air, hence the term anaerobic. In this digestor, a diverse community of different bacterial types ferment and feed off the waste in tandem with each other. 

First, bacterial hydrolysis breaks down large complex molecules like cellulose and carbohydrates into simpler forms that other bacteria can use. Acidogenic bacteria produce carbon dioxide, hydrogen, ammonia, and volatile fatty acids that are converted into acetic acid by acetogenic bacteria. Finally methanogenic bacteria take in these products to release methane and carbon dioxide. At the end of the process, the disgestor is left with biogas including methane and carbon dioxide, leftover solids called solid digestate, and leftover liquids called liquid digestate. Owners of digesters can aid these bacterial processes by adding water, heat, and supplemental nutrients, minerals, and pH buffers to keep the right conditions for fermentation. 

A farmer wanting to install anaerobic digesters will need to choose between many different forms of digesters that can suit different farm types. A major consideration is the solid content of the input waste, also called feedstock, which affects how difficult it is to mix and to heat, both of which keep fermentation going.  If the feedstock is more solid and difficult to mix, then it must be either diluted with water which requires more heat, or used with a digester that can handle more solid material, usually with less mixing. While the decision to install a digestor is a complex one, farmers can enjoy numerous benefits from having one on the farm.

Why Choose a Digester?

An anaerobic digester can be a source of money from the sale of its valuable products. In addition to trapping odor, digesters also hold biogas, and the deodorized end products, called digestate, all of which can be sold. Solid digestate can be repurposed as bedding for livestock or nutrient-rich soil material. Liquid digestate can be used around the farm as a fertilizer for crops. Biogas is a versatile alternative fuel that can be purified and used for cooking and heating, condensed and used for vehicle fuel, or burned and used as electricity. These products can bring in extra profit for farmers, and even before the digestion takes place, grocery stores and other companies that produce food waste will pay farmers to digest their waste. 

In addition to the economic incentives to digesters, they are also a climate solution. When food and manure decompose outside or in landfills, they release large quantities of methane into the air. Methane is an extremely potent greenhouse gas, a