VAAFM - Anaerobic Digestion Technology - Manure Digester Technology

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Anaerobic Digestion Technology - Manure Digester Technology

There are three basic types of manure digesters in use in the United States: covered lagoons, plug flow, and complete mix.

Covered lagoons: These are built by covering the manure pond with a tailored tarp that catches rising methane and funnels it to pipes leading to a generator. These are the cheapest systems to build, but in Vermont the current designs are not practical because ambient temperatures are too low for much of the year, suppressing bacterial activity.

Plug flow: The most common systems for our climate are plug flow systems like the ones at Blue Spruce Farm or Foster Brothers, which hold manure at approximately 100°F for 20 days, or somewhat higher temperatures for shorter time periods. Most plug flow digesters are horizontal concrete tanks. The manure flows or is pumped into one end of the tank and travels to the other end where it flows out. Some tanks are “U” shaped so the manure flows down one side and then turns and returns on the other. Some systems incorporate agitation into the flow to minimize settling and crusting that can occur in a digester. Heating is generally done in the digester, which means that problems with heating system piping require the system to be shut down and drained for repairs. Systems to heat the manure before entering the digester are being developed.


	Soft Top - The Foster Brothers system, at left, is called a “soft top” because it features an expanding balloon that stores enough gas to produce more power during peak usage times. Their system is a self-generation installation, producing power for the farm without a grid interconnect to net meter.
Hard Top - The Blue Spruce system, right, is a “hard top” with a concrete roof. The system is grid-connected, so that gas is used as it is produced and the electricity generated at any time is sold to the utility.

Complete mix: These systems work on the same principle as plug flow, except that the effluent may have a lower percentage of solids and may be stirred to maintain a constant suspension. They can be more expensive to build than plug flow digesters, but they have the advantage of higher gas production and shorter retention times.

There are also hybrid systems, combining some attributes of both plug flow and mixed tank, such as the non-agitated vertical storage slurry system at Hinsdale’s Nordic Farm.

AgSTAR is an EPA-USDA-DOE joint program located at the EPA. They have a number of tools available to help in the decision of what,if any, digester to select. They have a list of vendors, a handbook of digester design principles, feasibility software and a list of most of the digester projects in the country on their data base the web address is: http://www.epa.gov/agstar/