July 1, 2004 | General

Evolution Of A Composting Plant

BioCycle July 2004, Vol. 45, No. 7, p. 46
Davenport, Iowa facility turns out the right blend of biosolids and yard trimmings compost to be successful operationally and financially.
April Goodwin

Housed in a $1.1 million, 120,000-square foot building, the city of Davenport Compost Facility serves as a regional collection center for green waste in Scott County, Iowa and as a cocomposter of 150 cy (28 dry tons/day) of biosolids. Receiving an average of 90,000 cubic yards (cy) of yard trimmings plus 30,000 cy of brush annually, Davenport converts all feedstocks into 25,000 cy of compost and 6,000 cy of landscaping mulch/year. Started in 1989 as a pilot at the Scott County landfill to meet the state ban on landfilling yard trimmings, the original goal was to reduce landfilling by 25 percent. Efficient operations have actually reduced landfill volume by 28 percent.
“We’re in the busy part of the season now,” says facility manager Scott Plett, when interviewed in May. “Winter inventory is gone and at the moment, we’re selling it as fast as we can generate it.” Finished compost is sold on a sliding scale starting at $9/cy to $3/cy when more than 1,000 cy are purchased at one time. Price of mulch is $15/cy. About 80 percent of the compost is sold in bulk to contractors and developers; 20 percent is sold to local residents in bulk or bags. Tipping fees range from $6 to $10/cy, depending on the type of yard trimmings delivered. The charge for brush and logs smaller than 6-inch diameter is $6/cy, while logs over six inches is $8/cy.
Size Reduction and Mixing
Wood chips and yard trimmings are delivered and stored in the bulking agent storage area. Yard clippings and leaves are separated from the brush and trees. The latter is processed twice using an 800 hp Vermeer 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, chipped wood is composted outdoors in static piles 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 also are processed twice using the tub grinder, and a 4-inch screen creates a uniform bulking agent product for the composting stage. Anaerobically digested, dewatered biosolids (about 18 to 20 percent solids) are delivered in dump trucks from the Davenport Wastewater Treatment Plant located adjacent to the composting facility. Mixing of the yard trimmings and biosolids occurs in a totally enclosed room in the 121,000 sq. ft. building utilizing an automated feed system. Biosolids and yard trimmings are loaded into live-bottom hoppers with screw conveyors for metering into the automated system. The yard trimmings bins have 4- to 12-inch screws; the biosolids bins (72- cy capacity) have 9- to 12-inch screws. Using a computer program designed for the facility, the operator controls the variable-speed drives discharging the biosolids and yard trimmings onto a conveyor leading to the mixers.
The biosolids and bulking agents are thoroughly blended in two McClanahan continuous-feed pugmill mixers with the resulting product conveyed to a bunker located in the composting hall. In addition, chips made from recycled tires are added into the mix. Operators use 10 cy front-end loaders to place the mix in new extended piles to begin the composting process.
Cocomposting Phase
The biosolids/yard trimmings composting operation takes place in 66,000 sq. ft. of the epoxy-coated steel building. (See “Arresting Corrosion In Compost Structures,” June 2003 for a description of the building structure and steps taken to arrest corrosion. For a description of the overall design of the negatively aerated static pile biosolids composting system, see “The Big And Small Of Biosolids Composting,” April 1996.)
There is an in-floor aeration system. A 40-foot-wide central access aisle separates the east and west aeration zones. Precast polymerized concrete trenches are 72 feet in length and spaced 6-feet on center to provide aeration to the compost piles. The precast 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 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 facility uses 12 fans, 12,500 cfm each, to bring fresh air into the hall. Recurring problems with high moisture air getting into the motors and shutting down the fans led Davenport to switch to stainless steel waterproof motors.
When constructing a composting pile, a one foot base of new wood chips is placed over the aeration trenches, followed by 8-feet of mix and a one-foot insulated cover of recycled compost. Constructed composting 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 (including the mixing area) are collected via centralized ducting for treatment with the compost pile exhaust through biofilters.
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 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.
There are two large biofilters, each sized to process 105,000 cfm of exhaust gas at a residence time of 45 seconds. A 4-foot-deep mixture of yard trimmings 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, allowing for redundancy during scheduled and unscheduled maintenance activities.
Screening and Markets
After the active composting phase, material is screened using a two deck CEC screen. The top half of the top deck has three-inch wire mesh (one-quarter inch thick); the bottom half has a 1-1/2 inch punchplate. The second deck has 5/8-inch “z” wire “flex mat” panels. The screened compost is placed in cure piles for an additional 30 days using positive aeration and thermocouples to monitor temperatures. Aeration 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.
After stabilizing, the cured compost is placed in storage windrows ready to be sold. Oversized materials from the screening plant are blended with yard trimmings using front-end loaders. The next morning, the premixed bulking agent is processed through the mixing system adding biosolids and recycled through the composting process again.
Davenport has diversified compost markets, including sales in bulk and bags. When the original stationary screening plant was replaced with the CEC vibrating deck screen, that operation was moved to a different section of the building. The former screening area was enclosed and heated, with the feed hopper and conveyor used in a small bagging system. The system consists of a Verville bagger, hammer heat sealer and bag flattener. About 25,000 bags per year are produced for the local markets.
To encourage more use of compost, Davenport initiated a rental program for spreading equipment. The 24-inch push type is $5/day; the 42-inch is $10/day. Larger spreaders, e.g. for half a cubic yard, are $20/day; a 2 cy spreader is $50/day – and free if the customer purchases a semiload and has it delivered. Spreaders are rented to provide easier compost use on established turf while the larger spreaders can be used to lay down erosion control blankets (2-inch depth). “The program has been successful,” notes Plett, “with regular customers using the equipment. The equipment helps move the products when application is made easier.”
Davenport has tapped into the growing demand for compost as an erosion control Best Management Practice (BMP). “We are blending various ingredients for the erosion control markets,” he adds. “We make a filter berm material on demand using hardwood chips and compost and have a supply of our ESC erosion compost on hand for erosion control blankets and socks. Both sell for $10/cy.”
Design and Dedication
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 science classes from secondary schools and grade schools schedule regular tours each semester to educate students about local recycling and environmental issues.”
When thinking about the facility’s success after almost ten years of operation, Plett boils it down to a good design and dedicated staff. “Quality employees and design are both requirements for any business to succeed. 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.”
April Goodwin is a technical writer based in Des Moines, Iowa.

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