Credit: Photos: Vermeer

Left: The successful Davenport Compost Facility serves as a regional collection center for green waste; mulch and compost produced there are sold throughout Iowa and Illinois.
Right: Regional collection trucks at the compost facility unload green waste into a staging pile waiting to be processed

An Iowa composting facility has become an example for industry experts and those aspiring to build their own world-class waste treatment plants.

The Davenport Compost Facility began in 1989 as a pilot program at the Scott Area Solid Waste Landfill, experimenting with open windrow technologies designed to meet 1991 state guidelines banning yard waste in landfills. Located in a 120,000-square-foot building on a 13-acre site in Eastern Iowa, this facility serves as a regional collection center.

The operation, housed in a $1.1 million, epoxy-coated steel building, is designed to process 28 dry tons of biosolids per day. From an average of 90,000 cubic yards of yard waste and 30,000 cubic yards of brush and trees received each year, it produces 25,000 cubic yards of compost and 6000 cubic yards of wood mulch annually.

Materials processed include grass clippings, leaves, brush, and trees brought in by area residents, contractors, and local waste reduction and curbside recycling programs. About 80% of the end product is sold in bulk to contractors and developers. The other 20% is sold in bulk and bags to residents.

Composting of the biosolids using the “extended aerated static pile” method occurs in an enclosed and insulated 66,000-square-foot building with an in-floor aeration system. Aeration rate is managed with a temperature feedback control system through the facility computer. Up to four aeration rates are provided for each individual compost pile, based on temperature variations.

Wood chips and yard waste are delivered and stored in the bulking-agent storage area. Yard clippings and leaves are separated from the brush and trees, which are processed twice with an 800-hp Vermeer TG800 tub grinder. On the first pass, a 4-inch screen is used to produce the landscape mulch. A 2-inch screen is used on the second pass. The processed material is then composted using the static pile method for a minimum of 30 days to provide pathogen control. The resulting mulch is ready to sell to local residents and contractors.

Clippings and leaves also are processed twice with the tub grinder, and a 4-inch screen creates a uniform bulking-agent product for the composting stage. Once processed, yard clippings are used as a bulking agent and mixed with biosolids in an enclosed room employing an automated feed system. Operators then use 10-cubic-yard front-end loaders to place the mix in new extended piles to begin the composting process.

Constructed compost piles are 8 feet high, 90 feet long, and 26 feet wide. Material is composted for 21 days using the in-floor aeration system, and pile temperatures are constantly monitored using stainless-steel thermocouples. After composting, the pile is screened again and placed in cure piles for an additional 30 days before being sold.

The Davenport Compost Facility has received numerous awards, including “Outstanding Civil Engineering Project Achievement” in 1997 and “Best Local Government Recycling Program for 1999” and “Recycled Product Manufacturing Award” in 2000-both from the Iowa Recycling Association.

“People from all corners of the world tour our facility on a regular basis,” said Scott Plett, facility manager of the Davenport Compost Facility, who was recognized in 2000 as “Composter of the Year” by the U.S. Composting Council. “Local colleges and high school science classes schedule regular tours each semester to educate students about local recycling and environmental issues.”

Tara Deering is a Des Moines, Iowa, technical writer.