BioCycle October 2005, Vol. 46, No. 10, p. 25
Improved understanding of how feedstock decomposition impacts building materials leads to better choices for handling corrosion challenges.
NOT TOO many years ago, when compost operators began sheltering their active piles to protect them from the elements, they were entering uncharted territory. Over time, composters have learned much more about the best ways to enclose and/or cover compost piles without constricting the desired flow of oxygen and moisture.
One of the major issues for those opting to enclose their composting operation is choosing building materials, protective coatings and structural elements that can stand up to corrosive gases and moisture. In spite of its innate strength, experts say that uncoated carbon steel is very susceptible to corrosion from the combination of heat, moisture, oxygen and nitrogen compounds. Certain types of protective coatings can provide a solution, along with other building material options such as aluminum, stainless steel, fabric and concrete.
In the corrosive environment of indoor composting facilities, “the key issue is, how do you seal it so that gas can’t get in behind the coatings,” says Craig Coker, chief of engineering for North Carolina-based McGill Leprechaun, a composting company with several enclosed facilities. “A secondary question is dealing with the inevitable damage that occurs from front-end loaders being on the site.”
It’s a problem the wastewater treatment industry has been dealing with for years, he notes. “Wastewater treatment plants have more significant issues with corrosion caused by hydrogen sulfide,” explains Coker. “But there are a couple of high-solids, epoxy-based coatings that have been well-proven in that industry.” In the composting business, “virtually everybody I talk to is either going with stainless-steel or epoxy-coated steel buildings,” adds Coker.
Corrosion from typical composting off-gases has become an issue for the 10-year old epoxy-coated steel building that houses Davenport, Iowa’s 28 dry-ton/day biosolids composting operation. The most serious corrosion has been occurring in the facility’s totally enclosed composting hall, according to manager Scott Plett, “especially in the joints along the building’s bar joists.”
The corrosion has not posed a threat to the building’s structural integrity. Steel support-members in the 120,000 square foot structure have been protected by two coatings of a high-performance, epoxy-based Tenemec paint, each 4 mils thick. But the original epoxy coating “probably didn’t get into the joints very well,” Plett explains.
City officials want to remedy the problem by coating the inside of the building, including joists, structural steel and roof panels, with a layer of foam covering, possibly using the spray-applied system developed and marketed by Cleveland-based Preferred Solutions (PSI). With an estimated cost of $1.2 million to $1.5 million, the project “will probably take place in stages over the next three to five years, as money becomes available in the budget,” Plett says. “We’ll probably do a section of the building each year.”
PSI, which developed its Stayflex technology over 20 years ago, has found a direct correlation between longevity of structure interiors and the thickness with which protective coatings are applied. “With even the most current (structural) coatings, the problem is essentially a function of thickness,” says Jack Stahl, vice-president of operations for PSI. “The thinner the coating, the more permeable it tends to be – allowing moisture and vapor to pass through the paint-film to the substrate. Once the substrate begins corroding, because the paint is so thin, it doesn’t have the strength to withstand the force of corrosion. The growth of rust and corrosion breaks the paint bond.” PSI’s materials are a half-inch to 4-inches thick.
One key step composting facilities can take to prevent corrosion is to properly manage air movement, Stahl adds. “It’s important that plants be designed to keep the amount of vapor and gas in the air as low as possible. Doing that will also mean the (HVAC) equipment will be more efficient, which should lead to better composting and worker safety. The more moisture you can get out of it, the longer a building is going to last.”
STAINLESS STEEL ALTERNATIVE
Stainless steel is another option to prevent corrosion of building components. Behlen’s preengineered Corr-Span 300-series stainless steel building system has been in use at the 300,000-square foot Edmonton (Alberta) composting facility since it began operations in 2000. (See “Five Years of Composting In Edmonton, Alberta, Part I” in September 2005 and Part II in this issue, starting on page 30.) With no internal supporting columns, the main composting structure has smooth sidewalls and ceiling to promote air flow.
The Behlen system has “held up quite well,” according to Allan Yee, general supervisor of composting facilities. Small, metal components attached to the building’s interior also have resisted corrosion, even though they’re not stainless steel, along with nongalvanized metal panes in the main aeration hall. The only corrosion has taken place in the buildings’ stainless steel “man-doors,” Yee says. “We’ve had to replace doors because the hinges and door locks had started corroding. It’s cheaper to do that than to install stainless steel hardware.” A total of six doors were replaced, three in the finishing building and three in the download/trommel area, according to Jim Lapp, a facility technician. Both of those are high moisture areas.
The Edmonton facility has a couple of major facility improvements in the works, Yee reports – one to replace the entire compost “finishing circuit,” and the other a HVAC retrofit to direct gases from the drums in the discharge area to the odor control system. “When we download from the drums, the gases that come along with that material have a tendency to build up, including ammonia and carbon monoxide. We want to make sure we capture those and not have them in the atmosphere.”
In Delaware County, New York, a long-awaited cocomposting facility opened its doors in mid-September. The $21 million project is designed to process 45,000 tons/year of MSW organics, biosolids, manure and commercial food grade solids. The plant includes a 128,000 sq ft processing building that houses a waste receiving area, a rotating bioreactor, a primary refining and sorting area for separation of recyclables and inorganics, an IPS composting system, a product refinement area and storage and curing. Building and process air is treated through a biofilter. Behlen supplied the preengineered Corr-Span stainless steel structure. More details on this new facility will be included in next month’s special report on municipal solid waste composting.
FABRIC STRUCTURE OPTIONS
Technology has also added to the options available for composting structures in the form of “macrofiber” fabric covers that allow moisture to escape compost piles while keeping out rain and snow. Since 2003, the village of Bartlett, Illinois has been using Cover-All Legend buildings to cover aerobic digesters at the wastewater treatment plant. The four buildings are 62 by 62 feet in size, covering two sludge tanks each. “One of the problems we had in the past was controlling the temperatures during the winter, as well as containing odors,” says Ron Johnson, Wastewater Supervisor. “Since we installed the buildings, we’ve been able to hold the heat in during the winter so the bacteria can do its job and we no longer have odor complaints from nearby residential areas.”
The standard galvanized steel-frame system is clad with a triple-coated anticorrosion barrier and a Sunseal protective coating applied to fasteners and cables. The polyolefin of the Cover-All DuraWeave membrane is inert to ammonia gasses and is not biodegradable.
Johnson says the agency considered using a PVC system designed to completely cover the digesters, but it would have hampered access to the sludge for sampling, temperature readings and other purposes, Johnson says. Each of the buildings has a side access, enabling workers to lower a hoist down within the tank, clean the digesters or install equipment. Another benefit is that the translucence of the fabric membrane allows natural light to enter, eliminating the need for light fixtures.
Steve Wisbaum of Champlain Valley Compost Co. in Charlotte, Vermont has been a composting entrepreneur for over 10 years. Along with providing custom composting services for about 15 to 20 horse, dairy and vegetable farms in northern and central Vermont, Wisbaum makes about 2,500 cubic yards/year of manure-based compost, which he delivers to home gardeners, nurseries and landscapers in northwestern Vermont.
Faced with the challenge of composting “extremely wet” dairy manure, Wisbaum was seeking ways to change the moisture balance and prevent additional moisture from entering his compost piles. “I was introduced to the concept of compost covers, which were developed in Europe but still relatively unknown in the U.S.,” says Wisbaum. He bought a few Compostex covers made by Texel in Quebec, and was impressed with the results and the relatively low cost. Champlain Valley Compost has since become a distributor for the covers, which are permeable to oxygen, carbon dioxide and water vapor.
“They absorb rainfall and then wick it down the gravitational gradient,” he explains. “If piles are too wet, covers can be used during rainfall events and removed during dry conditions. In dry climates and conditions, they can reduce moisture loss by eliminating exposure to sun and wind.” The covers are made from 100 percent UV-protected material.
Some composting operators use standard impermeable plastic tarps for finished compost, “since breathability isn’t that critical,” Wisbaum adds. “But what people often find is those tarps are difficult to keep on piles in windy conditions. With the macropores in the permeable covers, wind can escape, making them easier to hold down with minimum anchoring.” In addition, plastic tarps degrade fairly quickly.
The city of Rockingham, North Carolina has been using Compostex covers at its composting site since about 1995, according to plant operations director Larry Cobler. The municipal facility composts about 1,800 tons/year of biosolids with wood chips purchased from a local lumber company. Sludge is dewatered with a belt press, mixed with the wood chips in a 2-to-1 (chips to sludge) ratio, then placed in windrows 100 feet long and 30 feet wide on the facility’s 2.5-acre concrete pad. Piles are covered and after a minimum of 21 days, each windrow is turned using a front-end loader. The covers stay off (weather permitting) to allow faster drying. After another 35 to 40 days, the compost is delivered to the North Carolina Wildlife Resources Commission, which uses it on gamelands scattered throughout a three-country area.
The city spends about $100,000 a year to buy the hardwood chips. When it started composting, the plant originally tried using straw as a bulking agent, but found that chips work more effectively; the air spaces between the chips allow for faster drying, Cobler notes. He adds that in the winter months, the covers help get the pile temperatures up a lot faster. When it rains, the covers “wick” the water off and don’t allow it to penetrate the pile. “We had tried covering the piles with a blanket of wood chips, but it took so many chips that the rain wouldn’t run off as it does with the covers. We also experimented with plastic tarps, but we didn’t get any evaporation; the compost would just sort of rot.”
Concludes Wisbaum: “These covers aren’t for every operation. A lot of places don’t have problems with excess moisture, but that depends on the size of the pile and type of feedstock. Woody materials are less susceptible to excess moisture conditions, as opposed to biosolids and food waste mixed with yard waste – feedstocks that tend to hold water more. In terms of finished compost, just about any fine particle-size product has greater susceptibility to excess moisture. For sites that encounter excess moisture problems at any stage of the compost process, this is an option to consider.”
October 25, 2005 | General
COMPOST BUILDING PRESERVATION STRATEGIES
BioCycle October 2005, Vol. 46, No. 10, p. 25