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From BioCycle Journal of Composting &Organics Recycling November 2000, Page 26 |
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INNOVATIVE MANAGEMENT
EDMONTON GIVES A NEW DIMENSION TO MSW COMPOSTING
Canadian city teams up with power utility to launch a 30-year agreement to cocompost over 200,000 metric tons per year of residential garbage and biosolids.
Jerome Goldstein
AFTER six months of operating experience at the Edmonton Compost Facility, manager Dwayne Simmons and his 36-person staff are justifiably proud of what’s been accomplished. “Our biggest challenge was to prove that a municipal solid waste composting project of this size would prove to be an economical and environmental success,” says Simmons. “I believe our experiences are moving in that direction — especially after receiving a record one-day delivery of 1,300 metric tons earlier this week.”
The Edmonton, Alberta facility — called North America’s largest cocomposting project — began receiving deliveries of about 500 metric tons/day of MSW last March. Biosolids piped in from the city’s wastewater treatment plant started arriving in mid-September. The facility is designed to compost 180,000 metric tons of residential garbage along with 22,500 tons of biosolids, generating 125,000 tons of finished product annually.
“Edmonton has become a world leader in large-scale composting and an example to other cities,” observes Susan Antler, executive director of the Composting Council of Canada, which held its 10th annual conference in Edmonton in late September. Conference attendees toured the facility, and the city of 650,000 was given special recognition for exceeding the 50 percent landfill diversion target set by the Canadian government in 1989. In the view of one veteran composting professional who toured the site, “Edmonton has much going that should lead to successful results — good location, good process design, attention to design details, and experienced operational and maintenance staff who are learning what it takes to manage mixed organic substrates.”
POWER UTILITY/CITY PARTNERSHIP
The $100 million composting facility is a public-private partnership, built and operated by TransAlta Corporation under a 30-year agreement with the city of Edmonton. Headquartered in Calgary, Alberta, TransAlta is an international electric utility with about $6 billion in assets. Pressures of deregulation, a search for sustainable/profitable development, and the “need to be agile” in the marketplace led TransAlta into the waste management arena.
To select a composting system, TransAlta used a local multidisciplinary engineering firm (GKO Engineering of Edmonton) to assess technology in various phases of the process. Based upon their experience with general facility design (not compost facility), GKO staff went process by process (i.e., aeration system, post-processing, etc.), analyzing alternatives and making specific recommendations. In the past, many compost project developers have relied on a single vendor with a turnkey system to serve as the “engineer of record.” The team at TransAlta took a far different proactive approach, which included visits to MSW composting projects overseas. The utility also had a specific interest in being a user of the finished compost, recognizing its value for reclaiming land near Edmonton used in its surface coal-mining operations.
For its part, Edmonton public works officials sought a long-term solution for its MSW that would not require construction of a new landfill or long distance hauling, and would help meet recycling goals. With the composting program in place, the city is paying TransAlta $62/ton and fulfilling sustainable objectives. According to Roy Neehall, chief engineer for the city’s Waste Management Branch, the composting facility is consistent with what Edmontonians want — “a reasonable cost system with a commitment to reduce and recycle waste, and a commitment to environmentally sensitive ways of disposal.” At the end of 30 years, Edmonton has the option of taking ownership of the plant.
TRANSLATING A CONCEPT INTO A WORKING REALITY
Operations take place on about 60 acres of the Clover Bar Waste Management Centre which includes a fully automated materials recovery facility (MRF), landfill gas recovery and leachate treatment plants as well as a developing Centre of Excellence for applied research. Three all-steel buildings (a total of 300,000 square feet) enclose the tipping, compost processing and finishing areas.
Since Edmonton introduced its Blue Bag program and the MRF opened, volume of recyclables put out by single-family residents increased almost ten percent. Last year, the city collected 32,700 tons through the Blue Bag and dropoff programs — almost 440 lbs/household. Recyclables include newsprint (65 percent), mixed paper (17 percent), cardboard (10 percent), metal cans (3 percent), plastic film (2 percent), plastic milk jugs (1 percent) and glass (1 percent). It’s estimated that 15 to 18 percent of the city’s residential MSW is recycled through operations at the MRF.
The residential MSW that now goes to the compost facility instead of the landfill continues to be placed at curbside; as before, recyclables and household hazardous waste are separated. Collection vehicles unload materials through one of eight doors on the tipping floor (43,200 sq ft area). Loads average 750/tpd, reaching as high as 1,300 tpd during the first six months. Oversized items that are nonorganic or hazardous materials are manually removed and taken to the landfill. (Previous waste stream audits have indicated that 60 percent of household trash is organic.) Front-end loaders push remainder to one of the five mixing drums through chutes on the tip floor. Fabricated by A.C. Equipment Services of Milwaukee, Wisconsin, each drum is 243 feet long by 16 feet in diameter. One full rotation is made each minute, and materials remain in the drums for 24 hours.
Dewatered biosolids are added to the MSW in the mixing drums to create a feedstock that has 48 percent moisture, 30:1 carbon to nitrogen ratio, and a 5.5 to 6.0 pH. After mixing, material is conveyed to one of two (Triple/S Dynamic) trommel screens that remove particles larger than 8 cm (which are landfilled); Material that passes through the screens is conveyed to the “aeration hall” (more than 247,500 sq ft — almost 5.7 acres) where composting takes place. This structure is described as the largest stainless steel building in North America.
COMPOSTING PROCESS AND BIOFILTER
Each of the three aeration bays — supplied by the Italian firm of Sorain Cecchini Tecno — has turning equipment, water nozzles and a ventilation system which pulls air through the feedstock. Composting material slowly moves through the bays for 28 days before being transferred by conveyors for final screening and finishing.
As explained by Stan Ziemniak, who is the facility’s lead laboratory technician, variables such as porosity, C:N ratios, temperature, oxygen content and moisture content are closely monitored. Each bay has a negative aeration system that draws air down as well as the turning mechanism. Air pulled through the feedstock is run through biofilters outside the aeration hall.
According to Simmons and Ziemniak, the biofilters were designed to manage 100 percent of all process air. In order to accomplish this management, such factors as area, mass, contact time and differential pressure (both new and seasoned media) were considered. Using reference information, the source of media was chosen for long life at a reasonable cost. Three beds were constructed, each on an area of 79 by 280 feet with a pile depth of four feet. The media for two of the beds were a ratio of fir tree bark and softwood peelings. The third bed was deliberately filled with a hardwood filler to evaluate the rate of degradation between the two different sources. “We expect a four year life on the fir bark and softwood biofilters, and a one to two year life on the hardwood biofilter,” note Simmons and Ziemniak.
The biofilter consists of an air handling system of fans and ducts, which direct the wet air stream to a cooling chamber for volume reduction followed by an acid scrubber to remove ammonia, and finally a manifold to distribute the air to the biofilter media. Cooling water is pumped from a pond up to a large heat exchanger to cool process gas before entering the biofilter. The Alberta Research Council is currently testing biofilter materials onsite using the raw odor from the composting bays as a source.
In the refining stage, compost is removed from the bays and passed through trommel screens to remove particles larger than 10 mm. Man-made inerts (glass, plastics, sharps, etc.) are removed with ballistic air separators, eddy current, magnetic separators as well as screens. The final product is transported by truck a short distance to a site at the east end of the facility. Compost is stored here for curing for three to four months.
To date, all compost has achieved a CCME (Canadian Council of Ministers on the Environment) Class B designation based on published guidelines. This classification allows significant land applications over several years. Two trace elements in the finished compost have prevented the Edmonton Compost Facility from achieving a Class A designation. These elements are copper and zinc. “Several thousand metric tons of compost have been sold to an environmental service company to decontaminate hydrocarbon contaminated soils. The remaining compost produced is aging to maturity in the curing area,” concludes Simmons.
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