BioCycle September 2006, Vol. 47, No. 9, p. 29
There are a variety of corrosion resistant options for enclosed composting facilities, where the harsh environment shows no mercy to exposed metal surfaces.
MOISTURE is no stranger in Canada’s Maritime provinces, so when the Fundy Region Solid Waste Commission (FRSWC) in Saint John, New Bruns-wick embarked on a composting program, putting the operation indoors was a practical choice. The facility is located at the Crane Mountain Landfill, which opened in 1997. “The government of New Brunswick ordered all the open burning pit dumps closed and opened six municipal solid waste landfills around the province,” says Chris Harned, Waste Diversion Supervisor for the Commission. “One of our 13 operating conditions was to have a composting facility in place to conserve landfill capacity.”
Drop-off recycling depots were opened around the region in 2000. Municipal governments within the region initiated biweekly collection of source separated household organics at about the same time. The FRSWC purchased 40,000 60-gallon Rehrig Pacific aerated carts, which were supplied free-of-charge to households. A 2.5 gallon countertop container was provided as well. Residents are permitted to use compostable plastic bags, but they need to be approved by the Commission. “Right now, we’ve approved the Bio-Solo bag, Al-Pack and Ralston’s Bio-Sac,” says Harned. “They are used to line the kitchen buckets. Households have to purchase these.”
Participation in the source separated organics program is voluntary, but haulers have an incentive to encourage the households and commercial accounts they service to participate. “Our tipping fee at the landfill is $108/metric ton for MSW; the price for organics is $45/metric ton, which is more than half the cost,” he adds. “That’s a good incentive and in the case of their commercial accounts, haulers are passing on some of the savings they get from the lower tipping fee.”
About 35,000 metric tons of residential solid waste are generated annually. Of that amount, roughly 7,500 metric tons/year of organics are diverted to the composting facility. As more commercial generators come on line, Harned expects that volume to increase significantly.
The composting operation is housed in three Cover-All Titan buildings fabricated from epoxy-coated galvanized steel frames and DuraWeave fabric. The buildings are positioned on 4-feet thick by 5-feet high push wall foundations. The receiving hall is 120-feet wide by 60-feet long. Trucks unload on the floor and large contaminants are removed. Next the organic materials are loaded into an infeed hopper and conveyed through a sorting line to pick out contaminants. An electromagnetic separator removes metals, then the feedstocks are shredded, along with yard trimmings, to a 2-inch minus fraction in a Shred-Tech unit.
After size reduction, materials are conveyed to two composting buildings (80-feet by 180-feet). “Because we are on well water here, we opted for two composting halls so that we weren’t required to install sprinkler systems to meet the fire codes,” says Harned. “We decided to use the Cover-All buildings because when you bring composting inside, the environment is very harsh from the acidic process. Determining what structures to go with took a fair amount of research. We visited a number of other sites where the structures were deteriorating rapidly. The fabric on the Cover-All buildings is not affected by the corrosive environment and the steel tube framing requires minimal maintenance.” He adds that for housecleaning purposes, the inside of the buildings are hosed down twice a year with a pressure washer. “Very little maintenance is required for these structures, which helps with our budget.”
When the composting halls were constructed, aeration trenches were built into the floors to provide positive aeration to the piles. To date, the aeration system hasn’t been needed, notes Harned, because the “windrows are very manageable by just turning with a loader.” Residence time in the composting halls is 60 to 90 days – with materials transferred from one hall to the other about half way through – after which material is taken to an asphalt pad and turned with a KW windrow turner. “We do additional composting on the pad for about 40 days, then move the material to a dirt pad for about nine months of curing,” he says. “The whole process, from raw feedstocks through finished compost, takes 12 to 15 months.”
Compost is screened in a McCloskey trommel to three-quarter-inch minus. The overs are used as daily landfill cover. The compost is tested and then sold to residents at a fairly low price. “The compost is the fruits of their labor because we have a voluntary system and they are choosing to take the time to source separate,” explains Harned. “One customer said, ‘never in 100 years did I ever think I’d buy my garbage back.’ We also offer the municipalities a ‘buy one load, get one free’ offer. We want them to use the compost and promote it. Hopefully that helps build participation in the voluntary system as well.”
While the fabric structures have held up well, the original overhead doors were victims of the corrosive environment. The doors, rollers and chains corroded in all the buildings, causing safety issues with the doors seizing up and odor issues because they would not shut properly. Additionally, the metal plates over the aeration trenches deteriorated quickly. They were replaced with three-quarter-inch clear stone, which is holding up well. As for the doors, the situation was resolved by going with “old-fashioned wooden-style barn doors” that, Harned notes, are working very well. “The price was one-third the cost of a standard overhead door. We looked at rubber doors that were $20,000 to $30,000. Ultimately, we hired a contractor to build the barn doors.”
EVALUATING STRUCTURAL OPTIONS
After years of indoor composting – and many reports of significant corrosion problems – design engineers and composting practitioners are very aware of the choices they need to make when enclosing an operation. There are a variety of building and corrosion prevention options. Which direction to go in is dictated by whether the building is new or undergoing a retrofit, cost and climate. Stainless steel is most resistant to corrosion. “Nothing equals it in terms of corrosion prevention,” says Charles Alix, Senior Engineer with MWH Americas, Inc. “It can be dinged and banged and still not corrode. The next best option is probably encapsulation of the interior of the building with a product like Stayflex from Preferred Solutions, Inc. (PSI), assuming it is applied properly. Last is probably an epoxy paint system on steel.”
He adds that the fabric-covered buildings resist corrosion well, but aren’t insulated, which can be an issue in colder climates given the amount of moisture released during composting. “Those structures are reasonably inexpensive unless a lot of attachments are added. They hold up great and are easy to install.”
The challenge with epoxy paint systems on steel is that the application of the paint has to be done well. The surface has to be cleaned properly, and the paint applied at the right temperature and relative humidity. “If those conditions aren’t met, it shortens the life of the paint system,” notes Alix. “But even with proper cleaning and application, those structures won’t last forever in a corrosive environment.” Corrosion can begin with dust settling on the flat surfaces of the steel. As the dust gets wet from the high moisture environment, it leaves a small pile of mud-like material that wears away at the epoxy paint. Proper building ventilation can slow down the rate of corrosion, but it still is likely to occur, he adds. Open-sided steel buildings coated with epoxy paint systems are less vulnerable to corrosion and could be the most economical choice for covered, outdoor operations.
Recently, Alix was part of an engineering team working on a large-scale enclosed composting facility in California. Two public agencies, the Inland Empire Utilities Agency and the Los Angeles County Sanitation District, formed a joint powers authority called the Inland Empire Regional Composting Authority. The facility processes biosolids, manure and green waste in enclosed aerated static piles (see “Supersized Indoor Composting Facility,” March 2006). Located in an industrial area of Rancho Cucamonga, the composting operation is housed in a 410,000 sq. ft. building that previously was home to a furniture warehouse. “The building has a wooden truss roof that was 32-feet high,” he explains. “Not only was that way too much airspace to contend with, we didn’t want to expose the wood and galvanized connectors to corrosion and moisture.”
The solution was to install a drop ceiling (21-feet high) made from galvanized steel panels. New interior walls were fabricated from poured concrete and concrete blocks (up to 14-feet high) topped with gypsum wallboard held in by metal studs. Next, the Stayflex corrosion control and thermal insulation system was applied to the ceiling and gypsum board walls (not the concrete), starting with an insulating foam and covered with a polyester coat that is chemical resistant and acts as an air and moisture barrier. “Basically, we completely encapsulated the inside of the building, essentially creating a building within a building,” says Alix.
For building retrofits, especially structures made from epoxy paint systems on steel, the Stayflex product is commonly used. Alix cited biosolids composting facilities in Rockland County, New York and Davenport, Iowa as examples. John Stahl, President of Preferred Solutions, Inc., notes that the installed cost of the Stayflex System used to thermally insulate and prevent corrosion on the underside of metal roofs and supporting steel in preengineered steel buildings is under $25/sq.ft. The company has worked with about 10 composting operations around the country, including new buildings and retrofits.
Universal Fabric Structures, Inc. markets buildings to the composting market under the All-Site brand. The structure is made from extruded aluminum and a heavyweight PVC-coated polyester fabric that is corrosion resistant. The structures are manufactured in widths from 70-feet to 161-feet and whatever length the customer orders. The mixed waste composting site on Nantucket Island in Massachusetts uses a Universal Fabric building for its active composting phase.
INNOVATIONS IN CORROSION CONTROL
Much of the focus on corrosion control is protecting the metal surfaces in composting structures. A line of products from Cortec Corporation, Inc., however, addresses both the metal and the concrete surrounding it. Almost 30 years ago, Cortec developed the VpCI® (vapor phase corrosion inhibitor) technology based on carboxylated amine salts, explains Bob Boyle, a Technical Sales Manager with the company. Applied in a vapor form, it can protect multiple types of metals. Perhaps more directly applicable to composting structures, however, is the company’s Migratory Corrosion Inhibitors (MCI®), designed to protect the metal inside concrete. “On existing concrete surfaces, it is drawn in initially by capillary action, moves through the pore structure of the concrete as a vapor, and then ultimately, forms a film due to its ionic attraction to the metal,” explains Rae Jean Nicholl of S.M.A.R.T., N.A. (Structural Materials and Restoration Technologies) in Ohio, a Cortec distributor. “Basically, it forms a monomolecular film on the steel, protecting it from corrosion due to carbonation and chloride contamination.”
A common application of migrating corrosion inhibitors is restoration of building facades that have steel reinforced concrete with corrosion evident on the exterior. But Nicholl has also used the product in a small, outdoor composting project at Xavier University in Cincinnati. “They built a composting bin with steel reinforced concrete,” she says. “We applied the corrosion inhibitor on the concrete before the bin was loaded.”
With new construction, the MCI 2005 NS (normal set) is actually mixed in with the concrete at the ready-mix plant. “It is part of the concrete from the get-go,” adds Nicholl. “It is designed to protect the structure immediately, not just extending its overall life, but extending the time before the first repair.”
The corrosion inhibitor products can even be applied along with epoxy coated reinforcement, or when nicks or pinholes appear. Migrating corrosion inhibitor molecules are attraced to any place where there is exposed metal. “It forms a film where the nick or pit is in the paint,” she notes. The admixture product was used with epoxy coated rebar in a salt storage dome owned by Hennepin County, Minnesota. Five years after the corrosion inhibitor was applied to the interior of the dome, Cortec hired an engineering firm to find a rebar for field testing. Core holes were drilled and readings, as well as a visual inspection, were done on the metal embedded in the concrete. “Visual inspection of the bar over the course of the testing confirmed that there was no corrosion occurring on the steel at this time, even though chloride levels at the bar far exceeded the accepted values for corrosion initiation,” stated the engineer’s report. It concluded that the facility had no active corrosion and minimal potential for corrosion to occur. Nicholl says that the cost for MCI application is about .50 cents/sq.ft. when surface applied, or about $12 to $15/cubic yard if used for new construction in an admixture. – N.G.