Energy Efficiency Measures

AGRICULTURAL: ANAEROBIC DIGESTION

Anaerobic Digestion is a manure management system that allows farmers to generate electricity from animal waste, thus offsetting their electric bills or even turning the manure into a new source of revenue. Although electricity generation provides the main financial benefit, it is often installed for reasons of odor mitigation, better water quality, reduced methane emissions (a potent greenhouse gas), and better soil nutrient additives.

Anaerobic digestion converts the organic matter in manure to methane, using bacteria in the absence of oxygen. The methane gas is then collected, scrubbed, and used to generate electricity in either an engine or microturbine. The heat from the electricity generation is then either fed back into the digester or used for any other farm process requiring heating or cooling, such as dairy refrigeration. In addition, the anaerobic digestion process creates co-products including fiber (which can be used for animal bedding or soil amendments), nutrient water, or struvite (a high-quality fertilizer).

Owners of dairies, feedlots, swine production, and other concentrated animal feeding operations can install an anaerobic digester if they meet the following conditions:

Minimum of at least 300 cows or 2000 swine (or equivalent). - this will yield about 50 kW of electricity
Manure is collected at a central point
The manure is relatively free of excess bedding, rocks, gravel, etc. (although a little is ok)
The manure is liquid, slurry, or semi-solid (in other words, not dry)

There are four general types of digesters: covered lagoon, complete mixed, plug-flow, and fixed-film, in addition to some other research and development efforts underway. The digester types differ in how long they take to process the manure, what level of solids they prefer, and what climates they are best in.

Electricity Production

Before the methane can be fed to an engine or microturbine to generate electricity, it must be cleaned of impurities and moisture, which can either clog up the small spaces of engines and turbines or corrode them. Most competent project developers, if they are familiar with on-farm biogas systems, will be familiar with this and will install a clean-up system on the front end of the engine or turbine; however, it’s best to confirm, to avoid BIG problems later. (Of course, even a well-designed system will require regular maintenance, just like cars.)

Whether the electricity is used exclusively on the farm, or sold to the utility, depends on how much electricity can be produced, how much the farm uses, and how much the local utility would be willing to pay for the renewable energy.

Grants and Loans

Look for outside sources of funds to help finance the construction (and possibly operation) of the digester. Three places to look include:

USDA Rural Energy for America Program (REAP) offers grants of up to 25% of project costs, or loans of up to $25 million, not to exceed a total of 75% of project costs. See details.
The DSIRE Database has the most comprehensive list of opportunities. See www.dsireusa.org. Once there, you can search by state or by technology (look for either biomass or combined heat and power)
The U.S. EPA AgStar Program keeps track of funding sources for anaerobic digesters in particular. See details.
Colorado has the "Advancing Colorado's Renewable Energy (ACRE)" program which provides funding to promote energy-related projects beneficial to Colorado's agriculture industry. Details here.

Other Tips

Go with an anaerobic digester designer/installer that has designed and installed plenty of other systems. It is a complicated technology and early designs have a likelihood of failing.
Talk to some of the designer and installer’s other customers.
Read the AgStar Handbook start to finish: it's a very clear yet comprehensive resource.

For More Information

U.S. EPA AgStar Program

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