The combination of heat, moisture, oxygen and nitrogen compounds like ammonia is pretty aggressive on carbon steel, says Jan Allen, a senior technologist with CH2M Hill, a Seattle-based engineering firm. But it seems that aluminum holds up pretty well, along with stainless steel, concrete and even wood.
Also, sheet metal with a factory-applied, baked-on epoxy paint seems to do better than structural members like beams and frames, Allen says. Any kind of field-applied paint tends to suffer corrosion; I haven't seen a good, field-applied coating. There is a debate going on whether that has to do with poor surface preparation or poor paint performance. And galvanizing doesn't do well; zinc tends to suffer in that environment.
Two other options, Allen points out, are a sprayed-on polyurethane liner (similar to a truck bedliner), or sprayed on fiberglass with a gel coat. The latter method has been used in containerized systems, but then you're getting into some pretty expensive stuff.
In some situations, a treated wood frame might be a better alternative than steel. It's worth talking about, Allen adds. Another thing people should consider is putting the frame on the outside with the sheet metal inside. It's a lot easier to replace a sheet-metal skin than replace a frame. In the future, concrete and wood, and putting the skin inside might make more sense.
Ventilation systems are another important issue. Fundamentally, people have to make a decision whether to aerate their piles with positive or negative aeration, he explains. The systems that have suffered the most are the ones with pressurized piles so that the entire building space becomes the capture device. Those that have a vacuum on the pile to capture all of the steam, odor and moisture in the piping system keep the indoor air-quality better. Some buildings have very low ventilation rates and are designed for those conditions.
Another design decision to make early on is the number of hourly air-changes a building will require. Allen says ventilation systems should be designed to turn over the air at least six times per hour. Some composting facilities have fewer than three air-changes an hour, which is not adequate. I would recommend six or more. And, some wastewater plants where they have high odor-loads and a high percentage of occupied space will regularly go to 12 per hour. But that may not be practical for big compost buildings, where there is more interior space.
WITHIN A BUILDING
Allen helped design a composting facility being developed in Chino, California by the the Inland Empire Utility Agency (IEUA). Rather than opting for a facility specifically built for composting, IEUA is in the process of converting a former furniture company warehouse for that purpose. The agency purchased the 410,000 square foot building, which is located next to its solid waste treatment plant, and is expanding it by 65,000 feet, according to John Gundlach, supervisor of organics management.
To reduce air volume in the concrete building, a dropped ceiling made of thin steel plating is being installed. The warehouse had a 42-foot high ceiling, but we only needed 22 feet of clearance, Gundlach explains. To protect the interior metal components against heat, moisture and gases, the composting facility is being developed as a building within a building, with seamless walls and ceiling protected by a coating system developed by Cleveland-based PSI. A half-inch layer of polyurethane will be sprayed on at the site.
The coating will serve the added purpose of providing insulation to reduce temperature gradients and condensation in the facility. PSI originally developed the coating for use in electroplating factories, where acid fumes could corrode ceilings and support components. It's overkill from our standpoint, but we couldn't find anything else to beat it in terms of protection and cost-benefit analysis, Gundlach points out. The Inland Empire Regional Composting Facility is scheduled to come online in November 2005.
Coatings that will provide impermeable barriers are essential. Because of the penetrating effect of the moisture and gases in composting facilities, even tiny openings in coatings designed to prevent corrosion of metal surfaces can lead to big problems. Within a year after Pierce County Recycling, Composting and Disposal LLC opened its new steel building in Puyallup, Washington, corrosion problems began to show up, according to Jeff Gage, owner of Olympia-based Compost Design Services. A former county employee, Gage was involved in designing the facility for what is now called The Compost Factory. The corrosion problems were allegedly caused because the building contractor failed to follow the exact building specs, according to a lawsuit filed by the county that is still working its way through the courts.
The building's factory-applied Kynar polymer finish was reasonably effective in preventing corrosion of the building's structural components, Gage says. However, some of the material substitutions made by the contractor led to corrosion problems. While the specs called for applying silicon beads as moisture seals, the contractor used a different, moisture-barrier adhesive, according to Gage. Screw-holes used to fasten the barrier to the building allowed moisture to reach the metal underneath. The contractor also used sheet metal screws that did not have stainless-steel shanks, as the specs called for. Because of that, we had to recoat all of the screws that penetrated into the building, and put paint over them, he recalls.
The interior paint coating also failed to properly protect the structure, possibly because the painting contractor added an accelerant for faster drying, which caused the coating to become porous, according to Gage. Although the paint carried a five-year warranty (two years for the application itself), within a year to 18 months, we were seeing blistering in the painted surfaces. The county is seeking compensation from the painter in its court action.
The building designers took care to prevent uncoated metal from exposure to the corrosive environment. We tried to design the building using full-length sheets of siding where we could, Gage explains. Places where the steel was cut were protected by overlapping steel. Any cut sheet has to be lapped so the cut portion is exposed to the outside, rather than the interior.
Pierce County's other composting facility, an open-sided structure which opened in 1992, has held up quite well, according to Gage.
One composting facility that has held up relatively well is the epoxy-coated steel building which houses Davenport, Iowa's 28 dry ton per day composting operation, which opened in 1995. Steel support-members in the 120,000 square foot structure have been safeguarded by two four mil thick coats of a high-performance, epoxy-based Tenemec paint. The coating has done its job, according to facility manager Scott Plett. But corrosion has damaged the uncoated steel bar joists which support the roof, and in some areas where the steel wasn't correctly prepared for painting, or the proper thickness wasn't applied.
Because the building is always full of materials, it's been difficult to get enough space to go up there and inspect it, Plett says. But, with the facility turning 10 years old in 2005, the city plans to hire an engineering firm to conduct a thorough inspection of the building components. City officials will also begin considering whether the facility should be expanded. We need to look long term and evaluate what we''ll need to be doing in five or 10 years, so we can get the capital budgeting process started, Plett explains.
Epoxy paint has been the most commonly used measure to prevent corrosion in steel composting buildings. Generally, epoxy coatings have worked okay, but if there is any defect in the coating, or if it is not applied correctly, you will wind up with problems, says Charley Alix, project engineer at Tetra Tech, a California based consulting engineering firm. The surface may not have been cleaned properly before painting, so there is dust under the paint. Epoxy paint must be applied to the full, specified thickness under moderate temperature and humidity conditions, he notes. Some steel facilities have had problems due to the use of enamel paint containing rust-preventative. A more effective, and more expensive, type of coating used by some facilities is essentially sprayed-on urethane foam with a fiberglass resin coating over it. It encapsulates everything, he notes.
Paints are always improving; coating systems are much better now than they were years ago, Alix points out. In designing facilities, the question has been whether to go with a traditional steel with some type of epoxy coating or encapsulation. He's also aware of several composting facilities equipped with false ceilings to protect the structural steel components above from exposure to the moisture and gases.
CORROSION IN MASSACHUSETTS
In March, 2003, Jordan, New York-based WeCare Environmental took over operation of the six-acre, five building biosolids/MSW cocomposting operation in Marlborough, Massachusetts. The firm made about $1 million in equipment upgrades and other improvements, along with replacing some aluminum ductwork damaged by corrosion. A coating of Dupont Carbolene epoxy paint protected the building's walls and ceilings from corrosion. But acidic condensation from the facility's aeration structures damaged the air-handling ductwork, the fire alarm system and hangers for utility and irrigation lines suspended from the ceilings.
The ductwork developed holes ranging in size from pinholes to about one-eighth in diameter. WeCare installed sealed PVC conduit to protect fire alarm systems and other components to corrosive gases. There were corrosion problems with the alarm system wiring, either because it wasn't enclosed, or there were openings that allowed moisture to drip down and short it out, says Bob Spencer, compliance manager for WeCare. Some sections of aluminum ductwork were replaced, and PitGuard, a liquid coating, was brushed on metal surfaces to protect other components.
Other work included rebuilding the facility's conveyor system, rehabbing the rotary kiln digesters, adding new chutes and air-piping, and rebuilding aeration trenches. Not enough time has passed since the plant's reopening in July to tell whether the new measures will prevent future corrosion problems, according to Spencer.
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 Edmonton (Alberta) Composting Facility, since it began operations in March, 2000. General Supervisor Allan Yee says the facility's stainless steel aeration hall is holding up very well. I'm sure the negative aeration system helps, but even if it wasn't negatively aerated, I think stainless steel would still be the way to go. With no internal supporting columns, the hall has smooth sidewalls and ceiling to promote air flow. The lack of internal columns is a nice feature in the building, but I'm also sure that the building could have been designed with columns that would have held up if they had been constructed of the proper materials or properly protected, Yee adds.
However, other areas of the facility which weren't done in stainless steel, but are exposed to high humidity and temperatures, are not holding up as well. In those locations, the designers 'cheapened out' by using inappropriate insulation, as well as not using proper coatings or giving sufficient thought to the materials and not doing a good job on the HVAC design. Corrosion in those areas of the plant is evident, but is not absolutely critical yet, so corrective measures are not high yet on our list of priorities, Yee explains.
ROLE OF HIGH-TECH
One of the newest solutions to prevent corrosion has been composting structures constructed with high-tech fabrics usually made of a fiberglass or PVC scrim mesh, which has been coated with kevlar or other UV-protective material. A fabric facility built by Universal Structures has been in use since 1999 at the cocomposting facility in Nantucket, Massachusetts. The 110-foot-by-286-foot aeration building is made of PVC-coated nylon fabric, supported by aluminum structural components. It's working very well, says Whitney Hall, president of Providence, R.I.-based Waste Options, which operates the Nantucket facility. The aluminum is showing no evidence of any corrosion, even though the facility's seacoast location makes it especially prone to fogging and indoor precipitation. It's always foggy in there; we have rain and, occasionally, snow.
Hy-Line International, a multinational purveyor of leghorn genetics, chose three fabric buildings by Cover-All to house its new chicken manure composting operation at its hatchery in Dallas Center, Iowa. Hy-Line plans to process about 570 tons of manure per year at the composting facility, which began operating in September, according to compost supervisor Jeff Withem. Hyline combines field by-products such as corn stocks and chicken manure with their hatchery by-product. The manure is moved from production at the end of the lay cycle and stored in three 40-foot wide Cover-All buildings. Feedstocks to be composted are mixed and deposited in the Cover-All buildings for a month-long heating process before being moved to an outside concrete storage facility. The final product is then sold to landscapers, golf courses, and the agricultural industry for fertilizer.
Production manager Travis Slusher says Hy-Line's interest in fabric buildings was generated from my experience with steel sided structures in the livestock industry and how much they sweat throughout the changes in our seasons. Through our research, we found that Cover-All buildings could be ventilated more easily to eliminate the condensation factor. The other reason for using the Cover-All buildings was this is a temporary site and the structures are easily relocatable. We are not locked into a long-term capital commitment at this site.
The natural light allowed in by the fabric structures is another plus factor, Slusher notes, reducing electric bills. The structures used for composting were designed with two-foot, side-curtain walls on each side for natural ventilation. The buildings are very airy. With the composting process, we have a lot of natural gases coming off the process itself which can easily corrode galvanized steel, notes Slusher. These buildings are much more conducive as the fabric itself doesn't degrade like a steel building would, says Slusher. He adds that after two years of use, any concerns about wind-load and snow-load have been put to rest.
Craig Coker, chief engineer with North Carolina-based McGill Environmental, emphasizes the importance of calculating the life-cycle cost of each building option before making a decision. McGill operates two solid waste composting plants in North Carolina, and is planning to build a third in Virginia. Each handles about a dozen different types of solid waste feedstocks, processing 80,000 tons per year of material ranging from sheet rock to agricultual wastes to wood. If it breaks down, we'll take it, he says. The third plant will be slightly larger than the first two, built to process 130,000 tons per year, according to Coker.
In choosing composting building components, a lot of composters look at the initial cost of everything, but don't think about the life-cycle costs, Coker notes. Is it wiser to spend a big chunk of money up-front to install a stainless steel building, or buy a regular steel building and coat it with an epoxy coating? Coker asks. Seven or eight years later, you will need to touch up the coating, and again seven or eight years after that. By the time you have reached year-20, is that cheaper than buying stainless? That's the kind of analysis you need to do at the front-end of these projects to make sure you are moving in the right direction.
In the wake of recent fire code additions, fire protection is another consideration. It's a foreign concept to composters; everybody thinks that because the material is wet it's not likely to catch fire, Coker says. But under the new International Building Code that has been adopted everywhere, composting facilities are lumped in under the F-1 category (factory, low to moderate fire-hazard). It could be argued they are ag buildings, but most building code officials I talk to don't buy that. So you have to have one-hour fire resistance in the structural steel, the roof-deck and exterior walls.
Sprinkler systems aren't a viable option in many locations, since composting operations tend to be located on the fringes of developed areas, where sufficient water pressure generally isn't available, according to Coker. Spray-on coatings may be the best solution, but the jury is still out on cost-effectiveness and other considerations, he notes.
CONSTRUCTION EXPERIENCES AT A PENNSYLVANIA SITE
DAMAGE due to metal
corrosion necessitated demolition and replacement of a building at a biosolids
composting facility operated by the University Area Joint Authority (UAJA) of
State College, Pennsylvania. The building, which went online in March 1992 was
'totally shot due to metal corrosion, says former plant manager Steve
Welch, who recently retired.
UAJA officials discovered corrosion problems in the facility about two and one-half years ago, according to Executive Director Cory Miller. Apparently, water leaked behind the protective coating that had been sprayed on and caused some of the structural members to deteriorate. A sampling indicated there were a lot of places where the same thing was happening. In some locations, the corrosion was significant enough that in another year or two there would be failure of some of the structural members. It was pretty bad. Since they needed to expand the facility, UAJA officials decided the best course would be to demolish the structure and start over.
Even though the (first) building had a special lining inside, it didn't hold up, Welch explains. In designing the new one, we went to great lengths to assure that wouldn't happen again, with an interior insulation and sealing system.
The project designers also changed the facility's air-handling system. Fresh air is now fed to the tipping floor at the front of the building as well as to the back end. Internal transfer fans pull air from those two areas and mix it with the air emanating from the bays, to be exhausted through the biofilter. The new building, which opened in July, has a 10-year warranty.
In preventing corrosion, we learned ventilation has lots to do with it, and also the application of the interior sealing system, Welch explains. Quality control is a big issue. We have been very careful about the specifications. We went to great lengths in inspection, making sure so many square feet of protective material was put in place, then inspected for adherence and proper thickness.
Welch says UAJA officials have been pleased with the functioning of the facility's U.S. Filter/IPS in-vessel composting system, so they also added six additional bays to increase processing capacity from 50 to 80 wet tons/day of biosolids. Hardwood sawdust is used as the source of carbon in the composting process. Central Pennsylvania has a good number of operating sawmills, Welch notes. So we don't have any problem acquiring enough sawdust.