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

The City of Davenport Compost Facility, located in a 120,000-square-foot building on a 13-acre site in Eastern Iowa, serves as a regional collection center for green waste in Scott County. But the operation is so successful that mulch and compost produced there are sold throughout many additional counties in Iowa and Illinois.

“We’re in the busy part of the season now,” says Facility Manager Scott Plett. “Winter inventory is gone and at the moment we’re selling it as fast as we can generate it.”

Plett may specify his sales reach by county, but the interest in what they’re doing in Davenport can be gauged by country. Over the years tours have come from as far away as Kosovo, Russia, China and Korea.

The composting 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, the facility produces 25,000 cubic yards of compost and about 6,000 cubic yards of wood mulch annually.

Materials processed include: grass clippings, leaves, brush and trees that are brought in by area residents, contractors and a local waste reduction and curb-side recycling programs.” A tipping fee, ranging from $6 to $10 depending on the size of material, helps fund the facility. Brush and logs smaller than 6 inches in diameter cost $6 per cubic yard (to drop off), while logs more than 6 inches in diameter cost $10 per cubic yard. Compacted yard waste costs $8 per cubic yard.

The Davenport Compost Facility started 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; the goal was to reduce landfill capacity 25 percent by 1995. It has grown into a world-renowned composting facility and has helped the county reduce its landfill volume by 28 percent.

About 80 percent of the end-product is sold in bulk to contractors and developers. The other 20 percent is sold in bulk and bags to residents. The facility sells end-product compost on a sliding scale starting at $9 per cubic yard, to $3 per cubic yard in volume (more than 1,000 cubic yards purchased at once). The cost of mulch is $15 per cubic yard.

The Facility
Composting of the biosolids using the “extended aerated static pile” method occurs in a totally enclosed and insulated 66,000-square-foot building that features an in-floor aeration system. A 40-foot-wide central access aisle separates the east and west aeration zones. Pre-cast polymerized concrete trenches are 72 feet in length and spaced 6 feet on center to provide aeration to the compost piles.

The pre-cast concrete trenches eliminate the expense (and landfill space) required to periodically replace thin-walled, perforated PVC pipe that can be damaged from being run over by equipment.

An “engineered hole pattern” was used in the heavy-duty cast-iron trench covers and provides uniform aeration down the length of each trench. Four trenches are serviced by one of 24 aeration stations, each capable of providing 1400 cfm at 8 inches of water column. Each blower station was designed to operate in negative or positive aeration mode, depending on operator preference. Plett has chosen to operate in negative aeration mode during the active composting phase to more effectively control odors.

The aeration rate is controlled with a temperature feedback control system that is operated through the facility computer. Up to four aeration rates are provided for each individual compost pile, based on the variations in temperature. Higher temperatures result in increased blower run time. Three thermocouples placed in the pile provide temperature readouts, with the low temperature automatically selected as the temperature feedback controller. This way, the oxygen cooling and drying requirements for each individual pile are controlled independently.

Aerated curing is provided utilizing portable blower stations and disposable perforated, high-density polyethylene pipe. This area is located under cover adjacent to the screening area and is sized to handle 30 days of screened compost production. Cycling timers, operated through the facility computer, control aeration cycles as necessary in this stage of the process.

Because of the high priority to effectively manage odors at the facility, all exhaust from the mixing and composting buildings is collected and scrubbed through two large biofilters. Each filter is sized to process 105,000 cfm of exhaust gas at a residence time of 45 seconds. A 4-foot-deep mixture of yard waste compost and wood chips is used as the biofilter media. Each filter is divided into four independent zones with an individual booster fan and controls. This allows for redundancy during scheduled and unscheduled maintenance activities.

The Process
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.

The brush and trees are then processed twice using a 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. Once the processing phase is complete, the material is composted using the static pile method for a minimum of 30 days to provide pathogen control. The resulting landscape mulch is ready to sell to local residents and contractors.

Clippings and leaves are also processed twice using the tub grinder, and a 4-inch screen creates a uniform bulking-agent product for the composting stage.

Waste requiring size-reduction is ground with an 800 hp Vermeer TG800 tub grinder prior to using it as a bulking agent.

Plett says he purchased the tub grinder after a local Vermeer dealer demonstrated it. “The machine was brought to our site and operated under our conditions with facility operators, equipment and materials. It handled the large logs quickly and easily, and increased overall productivity,” he says. High productivity using a grinder of that size and power is critical, he says, because operators are able to spend less time performing this function and move on to other important duties.

Once processed, the yard clippings will be used as a bulking agent and mixed with biosolids, delivered in dump trucks, from the Davenport Waste Water Treatment Plant located adjacent to the composting facility. Biosolids are a dewatered sludge that has been anaerobically digested, but still contains 8082 percent water, so it has a thick, mud-like consistency.

Mixing of the yard waste and biosolids occurs in a totally enclosed room utilizing an automated feed system. Biosolids and bulking agents are loaded into live-bottom hoppers for metering into the automated system. Using a computer program designed for the facility, the operator controls the variable-speed drives discharging the biosolids and bulking agents onto a conveyor leading to the mixers.

The biosolids and bulking agents are thoroughly blended in continuous-feed pugmill mixers with the resulting product conveyed to a bunker located in the composting hall. Operators use 10-cubic-yard front-end loaders to place the mix in new extended piles to begin the composting process.

The composting facility design allows for a 1-foot base of new wood chips to be placed over the aeration trenches, followed by 8 feet of mix and a 1-foot insulative cover of recycled compost. Constructed compost piles are 90 feet long and 26 feet wide. In times requiring additional capacity, the mix height can be increased to 10 feet without adversely affecting the process.

The material is composted for 21 days using the in-floor aeration system, and pile temperatures are constantly monitored using stainless-steel thermocouples. Temperatures are recorded to the facility computer each 12-hour period. These recordings are transferred to a graph to prove “PFRP” (process to further reduce pathogens) has been achieved for each pile.

Negative aeration is used during the high-rate decomposition composting process with exhaust gases being collected and vented directly to biofilters for treatment. Offgas from the compost piles and the building are collected via centralized ducting for treatment with the compost pile exhaust through biofilters.

After composting, the pile is screened again using a CEC deck screen with 5/8-inch “z” wire. The screened compost is placed in cure piles for an additional 30 days using positive aeration and temperature thermocouples to monitor temperatures. After stabilizing, the cured compost is placed in storage windrows ready to be sold.  Oversized materials from the screening plant are blended with yard waste and recycled through the composting process again.

The Reputation
The Davenport Compost Facility has received numerous awards over the years, including “Outstanding Civil Engineering Project Achievement” in 1997, “Best Local Government Recycling Program for 1999” and “Recycled Product Manufacturing Award” in 2000 — both from the Iowa Recycling Association. The U.S. Composting Council recognized Plett with the “Composter of the Year” award in 2000 for excellence in compost operations and market development.

Plett says the Davenport Compost Facility has accommodated visitors from across the country, as well as those from other parts of the world. “We have tours that come through on a regular basis,” he says. “Local colleges and high school science classes schedule regular tours each semester to educate students about local recycling and environmental issues.”

To what does he attribute the facility’s success?

“Quality employees and design are both requirements for any business to succeed,” he says. “You absolutely must have dedicated employees who want to see themselves and the business succeed, while the facility must provide a design that meets current needs and is flexible enough to accommodate future operational requirements.”