BioCycle February 2004, Vol. 45, No. 2, p. 43
King County, Washington’s food residuals processing capacity is expected to take a step forward when permits are finalized at Cedar Grove Composting near Maple Valley. After years of handling preconsumer source separated vegetative feedstocks – primarily from grocery stores – Cedar Grove will be able to accept postconsumer feedstocks, including food scraps and soiled paper, from residential routes and commercial generators. The facility processes about 195,000 tons/year of primarily yard trimmings, making it one of the largest operations of its type in the U.S.
Cedar Grove Composting began operations in 1989. The composting site grew steadily over the years. In the very beginning, materials were processed in windrows, but climatic conditions, space constraints and odor issues led to a shift to static piles in 1992. While static piles enabled Cedar Grove to bring in more yard trimmings, the size of the piles limited oxygen transfer, both slowing down decomposition and increasing anaerobic conditions. In 1995, a shift was made to large aerated static piles and biofiltration to improve process control and have the ability to treat odors.
In 1997, due to a warm, wet spring and an avalanche of yard trimmings, the facility’s volume suddenly spiked 30 percent. Odors from the overloaded site prompted numerous complaints to the Puget Sound Clean Air Agency, and eventually a lawsuit and a settlement payment with the neighbors. Faced with the choice of radically changing its operations or shutting down, Cedar Grove chose to change (see “Compost Site Comes Back from the Brink,” May, 1999). The company’s survival strategy included major capital investments to upgrade its facilities and implementation of an Environmental Management System (EMS) that included strict controls on the height, weight, porosity, and grass content of aerated static piles. The new operational procedures were effective in managing odors. Yard trimmings continued to be the dominant feedstock, with about 10,000 tons/year of preconsumer vegetative materials.
UPGRADING TO POSTCONSUMER RESIDUALS
In King County, food scraps and compostable paper comprise more than 30 percent of single-family wastes, adding 139,524 tons to the disposal stream each year. The commercial sector adds another 100,355 tons of food and compostable paper waste. In Seattle in 2002, commercial businesses were estimated to dispose of around 70,000 tons of food scraps and compostable paper, according to waste stream composition studies. Residential customers in the city in 2003 were estimated to dispose of nearly 40,000 tons of food residuals and compostable paper in residential garbage. Modeling of food waste recycling weekly collection programs for the city of Seattle project that around 34,000 tons of food scraps/compostable paper could be collected from businesses and another 12,000 tons from Seattle residents. As a result, food residuals have been targeted for diversion by both the King County Solid Waste Division and Seattle Public Utilities.
Modeled on successful programs in San Francisco and Canada, in April, 2002, King County conducted a series of pilot curbside food residuals recycling projects. Close to 2,000 households in the cities of Kirkland, Redmond, Issaquah, and Lake Forest Park participated in the initial nine-month study. In addition to vegetable scraps, residents were allowed to add cheese, eggshells, fruit peels, meat, bones, fish, coffee filters, pizza boxes, tea bags, milk cartons, wilted flowers, and soiled paper along with yard trimmings in their existing green waste carts. The combined food, paper and yard trimmings were collected weekly at no extra charge and transported to Cedar Grove for composting, which received permission for the pilot from the Department of Health – Seattle and King County and the Puget Sound Clean Air Agency. The site’s permits required that these materials be composted during the initial phase in “Zone 7,” an enclosed structure that houses aerated static piles with treatment of air in a biofilter. (During the secondary phase, these materials could be added into larger piles, however the batches had to be tracked.) Typically, materials were unloaded in the enclosed tipping building, amended with yard trimmings, then ground in a Diamond Z tub grinder to 2-inch minus prior to being transported to Zone 7.
With positive customer response, in December, 2003, Kirkland expanded its combined food and yard trimmings program from 225 households to more than 45,000 households. Neighboring cities of Redmond and Bellevue will follow suit this spring, and Seattle Public Utilities is considering initiating a residential program. “We did projections for diversion of 20 percent and 50 percent of the food residuals generated in these four communities, which represents about 51,000 households in total,” says Beth Humphreys, who oversees food residuals recycling initiatives in King County’s Solid Waste Division. “At 20 percent, that is about 2,500 tons/year. At 50 percent, it’s around 6,100 tons/year. Added to that are about 27,000 tons of yard trimmings currently diverted from those communities.”
INSTALLATION OF FABRIC-COVERED SYSTEM
Cedar Grove Composting applied to the King County Health Department and the Puget Sound Clean Air Agency for full-scale permits to receive the postconsumer materials, both from residents and commercial businesses and institutions. “The regional air agency is the lead permitting authority, and we expect to get the permits this spring,” says Jerry Bartlett, General Manager of Cedar Grove. It was recognized early on that for full-scale operations with postconsumer feedstocks, some sort of enclosed composting technology would facilitate the permitting process, mostly to control odors and other air emissions, and address concerns about pathogens.
In 2002, Cedar Grove decided to procure the GORE™ Cover System, opting for a 16-windrow set-up with the capacity to process 40,000 tons/year. The Gore technology is being used in North America at two sites (see sidebar) and is marketed by SEC in Lynden, Washington. Components include W.L. Gore’s membrane laminate cover, a concrete or asphalt composting pad, a 10-foot high concrete back wall with a winding device for handling the covers, in-ground trenches for positive aeration and leachate collection, one 2-hp blower per windrow, stainless steel temperature and oxygen probes, and a computer control system. Construction management at Cedar Grove was provided by Earth Tech, an engineering and operating services company based in Canada.
With permission from the health department and air agency, Cedar Grove began cocomposting postconsumer food, soiled paper and yard trimmings in the Gore system in May 2003. Regulatory concerns about odors and pathogens appear to be addressed with this system. Odors were negligible and pile temperatures reached 170°F to 180°F, notes Bartlett, insuring pathogen reduction. “The Puget Sound Clean Air Agency approved the Gore cover system as a Best Available Control Technology,” he says. “We are able to meet the standardized emissions requirements.” The initial phase under the covers is four weeks. Then materials are mixed (and moisture adjusted if necessary) and moved to another pad, and covered and composted for an additional two weeks. “We then uncover the piles and keep them on the pad for two more weeks, after which we age the material for an additional two to four weeks,” says Bartlett. Cedar Grove sells finished compost primarily for residential uses, both in bags (about 500,000 sold annually) and in bulk.
Rate negotiations between the haulers, communities with curbside service and Cedar Grove resulted in keeping the tipping fee the same for processing the postconsumer feedstocks. An article in the December issue of BioCycle (“Seattle Studies Anaerobic Solution for Source-Separated Food Residuals”) reported that compost tipping fees ranged from $35 to $42/ton. Bartlett expects that by using the Gore technology, it will be able to process food residuals “at the same cost as yard wastes,” primarily because of the shorter composting time. He adds that the new system also provides significant energy savings. “The positive aeration system is about 95 percent more efficient than the negative aeration we use on the large, outdoor aerated static piles,” he says. SEC estimates that energy requirements are about 1 kWh of electricity per ton of compost processed.
Because Cedar Grove’s permit for the Maple Valley site limits total annual tonnage processed to 195,000 tons, the postconsumer food residuals will take the place of an equivalent tonnage of yard trimmings. That shift will be facilitated by the opening of a second composting plant in neighboring Snohomish County near the town of Everett. Cedar Grove Composting is constructing an 82,000-ton/year facility, which also will use the Gore technology. Land use permits were approved in November, 2003 and construction is anticipated to be completed this summer. Plans are to expand the facility in two phases, to an ultimate capacity of more than 120,000 tons/year, says Bartlett. As the system expands, Snohomish County may consider including food residuals with yard trimmings recycling.
In December 2003, the Seattle City Council unanimously approved an ordinance that bans the disposal in the garbage of certain materials, effective January 2005. Businesses will be required to separate out recyclable paper, cardboard and yard debris for recycling, while residents will be required to recycle paper, cardboard as well as all cans and bottles (a requirement to recycle yard trimmings has been in place for the residential sector since 1989). The Council also approved funding for the Mayor’s “60 percent recycling” package of programs, which also includes a citywide program to offer curbside collection of food scraps to interested Seattle businesses. A program which would add food scraps to the existing residential subscriber collection program for yard trimmings has been delayed to await the city’s experience with commercial sector food scrap collection. “We are ready to take the commercial stream,” says Bartlett. “Our tipping fee is a lot lower than what these generators are paying to have their wastes taken to the landfill.” The two haulers servicing the city of Seattle, Waste Management and Rabanco, are putting programs together, he adds.
MEMBRANE COVERED INSTALLATIONS IN NORTH AMERICA
Membrane-covered aerated static pile technology was first introduced in Germany in 1993 by W.L. Gore & Associates GmbH for soil bioremediation. The material was soon utilized to compost biosolids, yard trimmings and food residuals in covered windrows. W.L. Gore’s solid waste treatment group took a whole systems approach, designing an integrated composting system around the breathable qualities of the company’s semipermeable ePFTE membranes (see “Enhanced Fabrics Spur Composting Innovations,” November 2001). Today, notes the company, there are more than 100 membrane-covered systems installed. The largest plant, processing 130,000 tons of household waste annually, is located in Israel.
The three-layer membrane laminate repels rainwater while allowing CO2 to escape from the pile. Odorous gasses in the humid process air are condensed on the underside of the cover and drop back into the pile, which essentially acts as a biofilter.
Prior to Cedar Grove Composting’s start-up in the spring of 2003, two other facilities were operating the GORE™ Cover System in North America. Alvin Starkenburg, owner of Green Earth Technologies in Lynden, Washington, installed a 10,000 tons/year capacity system, built for processing yard trimmings. It began operating in December, 2002 to produce custom composts for residential customers and the landscape industry.
A much bigger facility using the technology is in Edmonton, Alberta, at the city-owned Edmonton Composting Facility operated by Earth Tech Canada. The primary processing plant is in a 2.5-hectare stainless steel building, where 195,000 tons/year of mixed municipal solid waste (MSW) is cocomposted with about 22,000 dry tons/year of biosolids. With Edmonton’s continued growth it was determined that within a few years, the facility would require additional capacity to effectively process all of the city’s biosolids.
Therefore, the decision was made to construct a smaller, separate facility to complement existing operations using the Gore technology. Construction began in July, 2002, and a 16-windrow, 40,000 tons/year system was completed and in operation by the end of September. Based on initial trials it was determined that, by volume, the system could operate at a ratio of 2.5 parts bulking agent to 1 part biosolids cake. Chipped pallets and green waste are used as bulking agents to provide porosity and available carbon.
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