Challenges in Dust Control
There’s no shortage of sites that need dust control these days.
“A number of construction projects are caught up in the economy,” says Morgan Kost, chief executive officer of Superior Hydroseeding in Watsonville, CA. “Property owners want to develop them, but they’re still in a holding pattern. When you have large tracts of open land, that’s when dust control comes in.”
Dust control is also necessary on smaller areas, such as materials stockpiles and haul roads. Because dust is responsible for so many problems, from sedimentation and water pollution to respiratory health problems to inhospitable environments in which to work and live, federal, state, and local agencies all require dust control to some degree.
The 1970 Clean Air Act authorized the US Environmental Protection Agency to establish air-quality standards for pollutants that were shown to threaten human health and welfare. Three of these “criteria pollutants” are particulate matter less than 10 microns (PM10), particulate matter less than 2.5 microns (PM2.5), and lead. The EPA also requires state and local agencies to operate ambient air monitoring networks.
|Photo: MIDWEST INDUSTRIAL SUPPLY
The polymers in Road Pro NT asphalt emulsion change the properties of liquid asphalt so it can stand up to sun, wind, weather, and traffic.
|Photo: SUPERIOR HYDROSEEDING
Superior Hydroseeding, in Watsonville, CA, uses Soilfloc by Hydrosorb Inc. in its hydroseeding slurry to establish temporary vegetation, which costs less than reapplying
temporary dust suppressants.
The choice of measures to use to control these particulates, the application rate, and the frequency involve a complex interplay of factors. The first consideration is the client’s needs, Kost says.
On bare areas, construction sites, stockpiles, and landfills, the needs may be for short-term dust control or long-term soil stabilization, depending on the project or the stage of the project. Activities such as excavation, demolition, grading, and transporting soil demand greater dust control. The amount can be reduced by disturbing small areas of the site in stages, but it’s usually less cost-effective than clearing and grading an entire site at once.
While gravel roads and haul roads used for mining and other industrial sites usually need long-term stabilization, some haul roads are built for short-term projects. The amount of dust that’s generated on these roads depends on both the surface of the road and on the number and weight of the vehicles using it. According to the EPA, studies have shown that as much as 1 ton of aggregate per mile is lost each year for each vehicle that passes over a road daily.
The site conditions, including topography, soil type, climate, weather, and traffic, also come into play. “All of these sorts of things impact our work,” Kost says. “There can be more than one application rate on a single project. For example, on flat areas you can go lighter, provided they’re not being driven on.”
The terrain may be flat or sloped, or surrounded by slopes. There may be building pads or detention basins on the site. Dust control measures are especially important when the soil is extremely dry, because it’s vulnerable to winds. Interestingly, though, although most people think of the desert as an unending source of dust, undisturbed desert soil is generally very hard, almost like a pavement surface. It’s only when activities such as construction, traffic, or recreation disturb this soil crust in arid and semiarid areas that it becomes unstable and produces excessive dust.
In areas of the country that receive a high amount of moisture, the problem is greatly reduced. On the other hand, says Kost, dust control is also needed just before the rainy season, because contractors have to seal sites up for the winter.
The EPA has set minimum measures that must be taken for dust control. State and local agencies may have more specific—and more rigorous—requirements. These range from sprinkling the ground with water to establishing vegetation to spraying chemical treatments. Each has advantages and disadvantages, depending on the dust control need and the site and weather conditions. In general, according to the EPA, methods using water and chemical controls require more monitoring than structural or vegetative ones.
The simplest measure is sprinkling or irrigating the ground surface with water until it’s moist. This can be effective, but it can also be time intensive and can result in runoff from the site, or in vehicles tracking mud onto public roads.
Covering the soil slows the wind velocity at the ground surface, which reduces the potential for dust to become airborne. In areas where there will be no traffic, planting vegetation or hydroseeding works well, as does covering the soil with a mulch of materials as diverse as compost and stones. Mulch can reduce wind erosion by up to 80%. Deep tilling brings soil clods to the surface, which also prevents dust from becoming airborne. According to the EPA, gravels that contain plastic clays bind with the soil and are more effective than limestone gravels.
Using wind barriers, whether trees or shrubs that are already onsite, wind fences, snow fences, tarp curtains, hay bales, or sediment walls, also slows the wind velocity. Their effectiveness depends on their permeability.
There is a wide variety of spray-on adhesives, some of which use the latest technology, that can be very effective on especially challenging sites. Adhesives can help preserve road surfaces by reducing washboarding and keeping the gravel from being whipped off to the edge of gravel roads. They include polymer-based emulsions, latex emulsions, and asphalt emulsions, which don’t contain the amounts of fuel oil or kerosene that cutback liquid asphalts do.
Resin-water emulsions are made of lignin sulfonate, which binds cellulose fibers together in wood and soil particles when they’re incorporated into surface gravel. Such salts as calcium chloride, magnesium chloride, and sodium chloride are hygroscopic, which means they draw moisture from the air, thereby keeping road surfaces damp. They’re effective when they’re used correctly. According to the EPA, these chemical applications should be used sparingly on mineral soils (sand, loam, and clay). They shouldn’t be used at all on “muck” or peat soils, because their misuse could create additional surface water pollution from runoff or they could contaminate groundwater. They could also create a health risk if excessive amounts are used. Before selecting one, it’s important to consider whether the chemical is biodegradable or water soluble and what effect its application could have on the surrounding environment.
Construction Sites in Northern California
Superior Hydroseeding works on a wide variety of dust control projects, from landslide repairs to commercial developments and subdivisions to bridge widenings and levees. The company’s service area includes some very steep slopes, from Watsonville on the Pacific Coast as far east as Fresno, and from the Golden Gate Bridge to King City.
It’s the company’s clients, from agencies such as the California Department of Transportation (Caltrans) and flood control agencies to counties and cities to private owners, that are affected by air-quality control regulations.
The company add Soilfloc, from Hydrosorb Inc. in Orange, CA, to its hydroseeding slurry on projects virtually every day. It hasn’t been specified for dust control, but when you have a bare site, you have dust, she says, and when Soilfloc is added to the slurry, it works very well in controlling dust.
Crews usually hydroseed to help establish temporary vegetation, which costs less than reapplying temporary dust suppressants. The size of the machine used depends on the size of the site, but the slurry mix is consistent. It contains a fiber (paper, cellulose, or wood), a specified seed mix, fertilizer, and Soilfloc, a polyacrylamide. Soilfloc binds the slurry together but is also the key to effective dust control.
“The way it binds with soil is the important part,” Kost says. “It seals the soil, and every time it rains, it resticks itself.”
In areas where vegetation is not part of the long-term plan, the slurry also can be diluted and used without the seed mix and the fertilizer so the contractor doesn’t have to deal with grass later on.
The effectiveness depends on the dilution rate of Soilfloc to water. Typically, the dilution rate is for six months of effectiveness, but it can be adjusted to provide dust control for as little as three months or as long as three years.
“Soilfloc is really, really sticky,” Kost says. “It’s very effective. It’s important to avoid spilling it and getting it on your hands or shoes. Its only drawback is that it does its job.”
A Haul Road in Iowa
At the SSAB electric arc furnace in Muscatine, IA, huge forklifts called coil haulers transport tons of steel coils from the mill to the yard, and then from the yard to the shipping area.
“We have permits and permit limitations for paved and unpaved roadways,” says Tom Sanicola, the company’s environmental manager. “One of our concerns was that during transport a lot of dust was being generated because of the equipment. It needed to be controlled as much as possible.”
The mini-mill, which is owned by a Swedish company, can produce about 1.25 million tons of steel coil and plate per year—from commodity steels to very high strength—for equipment for companies including John Deere and Caterpillar. It sits on about 450 acres of the 2,200-acre property, which drops down a bluff to the Mississippi River.
It gets windy at times, and although the soil isn’t particularly fine, there’s a lot of peripheral dust. The mill is in a farming area, and its roads are prepared with slag, a byproduct of steel making that cools into a coarse aggregate. The dust the coil haulers stirred up used to waft into the workshop where the coils are made.
It isn’t easy complying with the Iowa Department of Natural Resources (DNR) air-quality control permits, Sanicola says. “For the most part, they’re permits for our facility,” he says. “They put limitations in there that are hard to test to.”
Some states require permitees to employ best management practices (BMPs), such as sweeping, vacuuming, or the use of a dust control product, to show they’re managing dust on their roadways. The SSAB mill had to perform testing and maintain the BMPs for the roadways.
This is difficult for a number of reasons, Sanicola says. The test method is undefined, and very few labs do the testing. The permit provides a single, consistent number to comply with, but the testing is very dependent on the conditions at the time, which change, for example, depending on when and how much it last rained, how much loading there was, how and when the road was paved, and even how the sample is collected.
The company chooses when to do the testing, but the permit states that it must be done under worst-case conditions, which also can be difficult to define. It’s usually midwinter, when the roads haven’t been watered or swept, although the permit allows the company not to treat the roads if the temperature is below 35°F.
“It’s basically just a snapshot in time,” he says. “Depending on the conditions, we can get very high or very low numbers.”
Traditionally the company had paved its roads with asphalt, Sanicola says. There are a number of problems with asphalt paving, though. In hot weather, it gets soft, resulting in ruts and tracks. In hot sun, volatile compounds in the asphalt evaporate; the surface oxidizes and becomes brittle. In cold weather, the surface cracks. In addition, at the mill, the weight of the equipment simply tore it up.
“We came across Midwest and they suggested Road Pro NT,” he says. “We’re trying to use it essentially as a BMP to reduce dust loading as much as possible. We’ve been using it for approximately three years.”
Road Pro NT, from Midwest Industrial Supply Inc. in Canton, OH, is a polymer-modified asphalt emulsion. The polymers change the properties of liquid asphalt so it can stand up to sun, wind, weather, and traffic loadings. Road Pro NT is not only considered nonhazardous, but it also can help lock in place the hazardous polynuclear aromatic hydrocarbons (PAHs) in asphalt.
For the past year, Midwest has been applying Road Pro NT to the unpaved roads in the coil area at the mill once every two weeks. It soaks into the unpaved road base and binds with it, trapping the dust so it can’t be released into the air. Some of the previous application remains on the roadways, so each treatment establishes a more solid base. Sanicola says this potential for reducing treatments and costs is another benefit of the product.
The emulsion takes about one day to dry. Vehicles can drive on it while it’s wet, but it does have a tendency to dilute or wash away if rain falls the day before or after it’s applied, so the application time should be adjusted if rain is expected. Once the emulsion dry, it essentially acts like asphalt.
“The product has worked very well for us. It costs more initially than lignin-based emulsions, but it’s much, much more effective,” Sanicola says. “Overall, from a cost-benefit standpoint, it’s been very effective and good for us. And the employees are very happy with the lack of dust entering the shop area.”
Battery Recycling Facility
And then there are sites like the Marjol Battery recycling facility adjacent to the Lackawanna River in a residential location in Throop, PA.
For some 30 years, from Marjol’s beginnings the 1950s to its closing in the late 1980s, lead was removed from lead acid batteries at this facility and recycled. At the time the facility closed, environmental regulations were much less stringent than they are today, says Jody Cordaro, president of SCE Environmental Group Inc. in Dickson City, PA. The owner cleaned up the residential properties in the vicinity, took the soil to the recycling facility, and covered it with plastic. And there it remained for more than 15 years, while the owner studied the site and determined a final remedy.
“There was contamination throughout the site,” Cordaro says. SCE, a full-service environmental contracting company that provides a wide range of services, including landfill construction, closure, and demolition as well as emergency response work, was selected to clean it up. The company excavated approximately 290,000 cubic yards of affected ground, constructed a 9.5-acre permanent landfill on the 40-acre site, and transported the contaminated soil to the new landfill.
There was a danger of contaminated dust being released during each step of the process: the excavation, the transportation, and the redepositing of the soil at the new landfill. Cordaro’s main concern was a 29,000-cubic-yard “high hazard” pile that contained high levels of lead and had been covered with a plastic liner for 15 years. There was a very real possibility that large amounts of toxic dust could be created when the plastic cover was pulled back.
Because of the danger of this dust migrating offsite, the owner, the consulting engineer, the USEPA, and the Pennsylvania Department of Environmental Protection (PA DEP) stipulated that any visible dust in the air was cause to shut down the area until it could be saturated with water.
“In my 20-something years in this business, I’ve never had such a stringent rule before,” Cordaro says. “Even if this only happened occasionally, and even if it only cost us a couple hours of total downtime each week, it’s a potential loss of more than $30,000 a month.”
According to the Web site set up to report on the Marjol Battery Site project, airborne particulate levels were measured in three ways. Real-time visual monitors provided immediate information about the amount of dust, including lead, that was being generated during activities such as excavation and backfilling and ensured that it wasn’t migrating away from the work site.
Six site-perimeter monitors on the boundaries monitored ambient air for lead. They drew air through a filter and the samples were analyzed at a lab. All the sample concentration averages were below the National Ambient Air Quality Lead Standard of 0.15 μg per cubic meter. In addition, workers wore personal air monitors to determine if lead was present. Representatives of the owner, the consulting engineer, the USEPA, the PA DEP, and the US Army Corps of Engineers were all on hand to oversee the monitoring.
Work began in May 2008, with 25 to 30 pieces of heavy equipment moving about the job site at any given time. Trucks brought in as many as 175 loads of clean fill each day. Five water trucks sprayed the site 15 hours a day, but by midsummer the ground dried out very quickly, and spraying more water just created mud.
Cordaro did some research on the Internet and found DustBoss, by Dust Control Technology in Peoria, IL. The company is a global leader in dust and odor control solutions for demolition, recycling, scrap processing, and the mining and rock industries. SCE began using the DustBoss DB-60 on the high-hazard pile in the fall of 2008.
“We saw right away that DustBoss had great coverage,” Cordaro says.
DustBoss has a portable ducted fan design with a 25-horsepower motor that generates 30,000 cubic feet per minute of air volume and can cover an area of 21,000 square feet from a fixed position. The methodology that creates the mist system and the distance of spray is the key, he says.
“What was really nice about it was that it doesn’t create mud, but it does suppress dust. It almost creates a ‘mist curtain’ when water vapors are sprayed onto an area of concern.”
In addition, DustBoss is automated and can run unattended for long periods of time. Because the unit is portable, SCE crews were able to move it to other areas of the site, including the haul roads and loading areas, as well as position it directly over the high-hazard pile while they transported material to the landfill.
The project was completed in September 2010. The cost was probably about a third the cost of conventional dust suppression, Cordaro says. For this project they would have needed a truck, a driver, fuel, and additional maintenance. With DustBoss, all they needed was one unit.
Most importantly, the company successfully complied with the zero-dust standard and avoided downtime and potential fines. “It’s very unusual to be able to control dust levels to this level on a 40-plus-acre site through three construction seasons,” he says. “The DustBoss was a key component of this success.”
Author's Bio: Janet Aird is a California writer specializing in agricultural and landscaping topics.