US Farm Digester Saves Cost by Using Waste Tyres in Fixed Film Reactor

Northwest project to build tire-filled digester
By Ryan C. Christiansen

Biomass Magazine - Dec. 19, 2008, at 10:51 a.m. CST

Portland, Ore.-based Northwest Natural Gas Co. is partnering with BEF Renewable Inc. and Threemile Canyon Farms LLC to build the first anaerobic digester under NW Natural’s Smart Energy program. Launched in September 2007, the NW Natural’s Smart Energy program is a customer-funded carbon offset project to reduce methane emissions from dairy manure.

BEF Renewable is a for-profit business that is a wholly owned subsidiary of the nonprofit Bonneville Environmental Foundation in Portland. Threemile Canyon Farms is a 93,000-acre crop and dairy farm near Boardman, Ore.

According to NW Natural Gas, the anaerobic digestion technology that will be used at Threemile Canyon has been developed by engineers at the J-U-B Engineers Inc. office in Kennewick, Wash., and will digest manure from 1,200 dairy cows, approximately 144,000 pounds per day.

Work to build and install the digester begins this month and it is expected to be fully operational by mid-March. The digester will produce enough biogas to supply 102 homes, but the biogas will be used on site by Threemile Canyon.

It’s estimated that the digester will reduce carbon emissions by 1,500 tons per year, equal to the emissions from using natural gas in more than 400 homes.

The anaerobic digester that will be used at Threemile Canyon is based on the fixed film anaerobic digester design. According to the U.S. EPA, fixed film anaerobic digesters are typically used on dairy farms that use water to flush the manure from their barns into holding ponds. The digesters consist of a tank filled with plastic media on which bacteria grows. As the manure wastewater passes through the media-filled reactor, the bacteria on the media convert the organic matter in the wastewater into biogas.

The J-U-B anaerobic digester design incorporates a lined and covered lagoon instead of a higher-priced concrete or steel tank. The covered lagoon is filled with discarded automobile tires which trap and retain bacteria from the cow manure. The trapped bacteria convert the manure into biogas. According to Troy Green, executive vice president for J-U-B, the patented stacked tire matrix in the lagoon improves upon problems that have been associated with fixed film anaerobic digesters.

“Every media that I'm aware of today had plugging issues,” Green said. “You put media in a digester in a manure stream and it plugs up.” Using tires takes care of that problem, he said. Green said using tires also helps to bring down the price of the digester operation. “There is a price barrier out there for manure digesters,” he said, noting that most dairy farmers aren’t ready to shell out $1,200 to $1,400 per cow for a more expensive digester design. “By using tires, that media is a pretty cheap source,” he said. “It's actually a free source, or even a net source, depending on where we get those tires, and they are beneficially used as a media instead of thrown in a landfill.” Green said the lagoon basin will hold up to 35,000 tires. J-U-B worked with Washington State University to develop the technology, he said.

Under NW Natural’s Smart Energy program, residential customers can pay $6 per month or $.10 cents per therm of gas to offset their emissions and fund the Smart Energy program. Commercial customers can pay $10 per month or choose to contribute more. As of Nov. 30, more than 6,300 NW Natural customers, mostly residential customers and small businesses, were participating in the Smart Energy program. See more of this Tyre Digester article here.