Iowa State Digester
DECEMBER 5, 2001, Revised September 2010 ISSUE #M21
The Dairy Foundation in cooperation with the Iowa State University has started up a new dairy manure digester system. The installation is at the model farm located at Northeast Iowa Community College in Calmer, Iowa.
The school has opened a new campus with a herd of 170 cows. The manure and bedding from one hundred of these cows are disposed of in a digester. The biogas generated is used to produce hot water used on the farm.
The manure digester is the plug flow type. This uses a concrete tank 10' deep by 13' wide by 68' long, holding a nominal 50,000 gallons. Foam insulation helps keep the liquid at 95o F, which is well suited for producing methane.
Biogas from the digester is burned in a boiler system designed and manufactured by Perennial Energy of West Plains, Missouri (417-256-2002). Rated at 150,000 BTU/hour, this heats water that is used to maintain the digester at temperature and to supply floor heat and hot wash water for the milk pipeline.
The manure sludge from the digester is pumped to a Vincent Model KP-6 screw press. The cake from this press is composted and used for bedding, while the press liquor flows to a treatment pond. Normal cake production is in the range of 150 to 250 pounds per hour, and press liquor flow of 34 gpm was measured recently.
Daniel Meyer of Fayette IA (319-425-3331) has been actively involved the $200,000 project since its inception. He favors the plug-flow type digester for dairy manure. He has also noted that scrape barn manure works better than flush barn for digesters because of the greater concentration of solids.
Mr. Meyer describes biogas production as a two stage process. In the first stage the solids are broken down with acid forming bacteria. In the second stage methane forming bacteria cause gasification. The pH must be kept about neutral during the process. The average electric production is one kilowatt of generator capacity per 10 cows.
Ray Crammond of Ankeny IA (515-965-8301) was a consultant on the design of the system.
TEN YEARS LATER
Like most manure digester projects, this one has been shut down.
When the digested plugged with solids (sand?) which settled out, a group of students were drafted into digging it out. The system was altered so that the manure was pressed ahead of the digester. Only the press liquor was put into the digester. This should have worked.