February 23, 2010 | General

Cocomposting A Win For Municipalities

BioCycle February 2010, Vol. 51, No. 2, p. 22
Biosolids/yard trimmings cocomposting facilities expand due to cost-effectiveness of program and demand for finished product.
Nora Goldstein

THE CITY OF ST. PETERS, Missouri has relied on composting to manage its yard trimmings since 1990. In 1999, it began cocomposting biosolids cake from the Metropolitan St. Louis Sewer District with ground yard trimmings, and more recently, has been processing all of St. Peters’ biosolids as well. Now, 20 years later, St. Peters is completing construction of its most recent facility upgrade to an open bin aerated static pile (ASP) system.
“We compost about 6,000 wet tons of biosolids and 30,000 cubic yards of yard trimmings annually,” says Russ Batzel, Manager of Public Works Services. “We converted from static piles to an actively turned windrow process a number of years ago. Recently, the city decided to move to the aerated static pile system to more efficiently produce compost and manage odors generated during the active composting phases by cycling all the process air through biofilters. We are using a system supplied by Engineered Compost Systems where material is composted in aerated bunkers.”
When switching from turned windrows to an aerated static pile process, optimizing the initial feedstock mix becomes more critical, especially in terms of pile porosity and more uniform aeration. In 2008, the city received a $40,000 grant from the St. Louis-Jefferson County Solid Waste District that was used toward the purchase of a Roto-Mix 3410 (34 cubic yard) stationary electric mixer. Total capital cost of the mixer and electrical supply was about $150,000. The new aerated static pile system upgrade is about $2 million.
“The solid waste district uses fees they collect from landfills for grants to support diversion projects that reuse or recycle,” explains Batzel. “The mixer is operated by remote control by the front-end loader operator. We have a set recipe for ground yard trimmings and biosolids. The weight of each is recorded on a display. Material is discharged onto a conveyor, and then put in the windrows. In the future, it will be loaded into the ASP bunkers.” St. Peters also owns a Vermeer tub grinder to process yard trimmings and wood waste, both for cocomposting and mulch production.
St. Peters has an interest in composting produce and floral wastes and has had inquiries from a local grocery store and a national discount chain but no action has been taken yet. “We are set up to receive and process more food waste streams, but we are not in the business of going out on routes and collecting it,” says Bill Malach, Director of Utilities. “No one has pulled the trigger yet on doing collections, but the interest is definitely out there on the part of the generators.”

The city produces about 20,000 cubic yards/year of compost. It uses a Wildcat trommel to screen finished product. The compost is certified under the Seal of Testing Assurance program of the U.S. Composting Council. The City of St. Peters’ is registered as a certified supplier of compost and filter media for Filtrexx, a company that sells sediment and erosion control products such as compost filter media in a filter sock, and compost blankets. The city also has been involved in wetlands construction and other soil restoration projects. One included construction of a 4-mile long levee.
“The original specifications for the project called for the contractor to strip and save topsoil for use on top of the completed levee,” recalls Malach. “In practice, this method is seldom effective and is costly because of the problems with double handling and storing large amounts of material and the care needed to accurately place the topsoil on the finished slopes to create a true topsoil horizon. Therefore, we required the contractor to amend the finished grade in place using compost to add organics and nutrients to the soil.”
Because of the large surface to be treated – approximately 125 acres – 40,000 cubic yards of compost were transported to the site in walking floor trailers and placed in piles along the levee. It was then loaded into a Kuhn 2054 (20 cubic yard) ProPush spreader; a 2-inch blanket was distributed over the surface. The compost was tilled into the soil and the finished areas seeded. “This method was found to be effective at controlling erosion on the steep slopes because of the water retention capabilities of the compost and the quick sprouting of the grass to help hold the slope,” adds Malach. “The levee construction was done during a drought year and we still had good germination.”
St. Peters also uses the spreader to apply compost on its 200-acre farm growing crops of wheat, soy beans and corn. In addition, it land applies grass on the farm using the spreader. The grass is then incorporated into the soil as an amendment, providing a boost of nitrogren. “We run the grass through our grinder before we land apply to make sure there are no large sticks in it and to shred the paper bags,” says Batzel. Another new market is a local winery. “We just sold our first batch of compost to them,” he adds. “They like the nutrient value, and we’ve talked with them about developing special blends to meet their fertilization needs.”

The City of Cheney, Washington has been composting biosolids since 1994, using aerated static piles inside a building. It uses as much ground yard trimmings as it can produce, supplementing with purchased hog fuel (wood chips) as necessary. “It is getting hard to find and purchase hog fuel these days,” says Don Mangis, Wastewater Collection Supervisor for the City of Cheney. He adds that the operation recovers and reuses the wood chips a number of times before they pass through the final screen.
Cheney completed an upgrade of its wastewater treatment plant in November 2009, which increased the volume of biosolids generated. “We were okay on the hydraulic loading side of the treatment plant, but were short on the solids side,” explains Mangis. “Conditioning tanks were added for phosphorus removal and more aerators were installed in the aeration basins. The city added a second belt filter press and sludge storage tank. We also purchased two Roto-Mix stationary mixers, two holding hoppers located ahead of the mixers, a Bandit horizontal grinder for the yard waste and a Wildcat portable drum screen.” The size of the composting building was doubled in length, from 140-feet by 200-feet long to 400-feet long. It can hold 16 windrows, each about 350 cubic yards. About 3,200 cy of compost are produced annually.
Yard trimmings are processed and placed into windrows until they are needing for mixing with the biosolids. Solids content of the biosolids averages about 15 percent; operators use a ratio of roughly one-third biosolids to two-thirds hog fuel/ground yard trimmings. About 260 dry tons/year of biosolids are generated. Materials are composted for eight weeks indoors, then screened and moved outside for curing. The facility expansion included a larger biofilter. “Originally we used a biofilter contained in a 40 cubic yard roll-off box,” says Mangis. “The new biofilter is much larger. It is about 16-feet wide by 50-feet long and 8-feet high with alternating 18-inch layers of hog fuel and finished compost.”
In the early years of its cocomposting operation, the city had difficulty moving the finished product out the door. “We had mountains of the stuff and were mostly selling it to landscapers by word of mouth,” he adds. “Then, about six years ago we turned a corner, and now we can’t make enough!”

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