Dust to Earth
The use of suppressants is a key component of three dust-control strategies.
Environmental issues such as water and air pollution resulting from manufacturing processes may garner the majority of media coverage, but regulators are concerned about the release of fine dust from naturally occurring materials into the air, too. This article looks at three cases in which dust suppressants have been successfully applied to naturally occurring materials to prevent the release of fines into the air—a situation that has proved harmful to human health and, in some cases, the environment.
Treating Contaminated Soils
The US Department of Energy’s River Corridor Closure Project at the Hanford Site in southeastern Washington state is a major environmental cleanup effort, scheduled to be completed in 2013. The site was used for plutonium production for national defense programs for nearly 50 years. Main contractor Washington Closure Hanford manages the $2.2 billion effort to safely clean up and close the Columbia River corridor, an area of about 210 square miles along the outer edge of the Hanford Site; the entire cleanup site covers 586 square miles.
The effort involves decontaminating and removing 486 facilities, closing or remediating 370 waste sites, placing four deactivated plutonium production reactors in interim safe storage, and disposing of about 4 million tons of contaminated material. In such an environmentally sensitive area, one that is so close to one of the largest rivers in the western United States, preventing the generation of dust in contaminated soils is an important priority.
Knoxville, TN–based Safety and Ecology Holdings Corp. is one of the numerous contractors involved in the effort and has been remediating soil covering a total of about 50 acres in a project that began in August 2006 and is scheduled to be completed in 2011. Preston Anderson, project specialist for Safety and Ecology, reports that numerous designated waste sites within the overall cleanup area have been slated for cleanup based on historical evidence and sampling. Several since-demolished production facilities were located on Safety and Ecology’s portion of the site, and much of the soil is contaminated.
|Photo: Safety and Ecology Holdings Corp.
Stabilizing soil is especially important at the arid Hanford site.
Radionuclides are prevalent in the area soils, and Anderson says that acidic, basic, and heavy-metal substances have been found. The contractor is excavating the contaminated soil from the sites and trucking it to a disposal facility. After the excavated areas have been tested to ensure that the contaminated soils have been removed and regulators provide approvals, new soil will be trucked in and the excavations filled.
Anderson describes the project area as an arid semi-desert, a potentially major generator of dust. For the past several years, Safety and Ecology has applied Earth Chem’s EarthBound, a soil stabilizer and mulch tackifier, to its excavations at the site to control dust. The product was designed to protect soil colloids, the chemically active component of soil, to reduce soil erosion by up to 85% at low application rates of 5 to 20 pounds per acre. The product is designed with a long molecular structure to bind mulch to the soil and keep dust down for four to 12 months.
Anderson points out that Safety and Ecology is also using a great deal of water for dust control. “We use the soil fixative on a daily basis to control dust, but we also use a tremendous amount of water without the dust additive just to maintain a wet soil so we don’t have a lot of dust blowing out of the excavations,” he says, adding that the contract specifies zero dust emissions from the excavations. Regulators monitor dust by means of visual checks and air testing, and the contractor’s industrial hygiene department monitors the site and workers for airborne dust.
Due to regulatory limitations, excavation dust-control measures are confined to the additive and/or water, according to Anderson. In some locations directly adjacent to the river, he adds, silt fences or detention basins are used, but these measures are used more to control stormwater run-on and runoff than to control dust. Dust on roadways is also suppressed with a magnesium-chloride solution that is applied once a year using a drip bar. The solution is worked into the roadway with a blade and compacted with a vibratory roller.
Pure water is used on a more continual basis, whereas EarthBound is used when overnight or over-the-weekend dust control is needed, Anderson reports. Noting that the wind is fairly constant and the soils are sandy in the area, he says, “We’ll actually stop excavation when the wind gets up and blows the native soils around. That’s when we add the fixative to the final coating of water, if you will, and lock down the excavation. When we’re out at the excavation, we can control the dust locally, with water. At the end of the day and end of the week, we coat the area and bind the soil with the fixative.”
Using a scoop, Safety and Ecology adds the EarthBound wettable powder to water tanks mounted on two trucks. The company has a 4,000- and a 5,000-gallon-capacity truck. The solution can be applied with either a water cannon or with a spray bar.
Truck Driver Training Site Suppressed
A 4-acre gravel lot used as a truck-driver training course at the Apollo Truck Driving Academy in Lima, OH, was a major potential generator of dust when the academy moved onto a new site in 2007. Every year about 150 students train for their commercial driver’s licenses at the academy, part of the Apollo Career Center, on six tractor-trailers and one straight truck, according to training manager Jim Rosen. Heavy traffic on the site quickly ground down the gravel, Rosen reports.
“We’re constantly grinding gravel,” he says. “We drop and hook trailers like they do at a regular truck terminal, but we also back up, pivot, and go forward. We just go back and forth and just grind up the surface.” Rosen says that, even though the academy has not been warned or fined by environmental regulators, some site attributes gave him cause for concern in regard to dust.
“I wanted to be proactive,” he says. One concern was neighbors—primarily residential—although they are not located within a few feet of the academy. “And I had concerns about just having people out there. I have instructors out there who are far enough away from the trucks to be upwind, but when I’d see their cabs getting dusty by the end of the day, I knew they were [inhaling] some dust. There’s also the equipment issue: We’d try to reuse and blow out filters, but everywhere we’d grease, dust was going to collect, and so we’d have more friction. I had one oil sample from a tractor showing excessive dirt, too.”
In 2008, the academy applied a water-soluble polymer product to the surface, but the crust quickly broke and he was back where he started, Rosen says. He needed more resilient dust control.
The next year, the academy had SynTech Products Corp. apply Extreme Dust Control (EDC), an agriculturally engineered dust suppressant that uses a formulation combining carbohydrates and surface modifiers that bond dust particles together for road base stability and dust control. The product is designed to work in subzero temperatures and extreme heat alike, and for application using a standard water truck. The manufacturer says that the product is nontoxic, nonhazardous, and environmentally sensitive. The user can apply it either full-strength or diluted infinitely with water for use as a water additive. EDC is said to easily wash off of vehicles and heavy equipment. Application rates are 0.10 to 0.50 gallon per square yard, and roadways can be opened to traffic immediately after application, according to the company.
Given the location of the site, the product had to have a minimal impact on the environment. “That was also a concern,” he says. “We had our maintenance guy go over the Material Safety Data Sheet and see if we had runoff in our applications—we do have a creek not far away. That was one issue with the previous product as far as cleaning out our equipment: We had to make sure our product didn’t flow into it. This product is pretty easy for us to wash off of our equipment, and we don’t have to worry about hurting the environment.”
Rosen reports that, while training is in session in the warmer weather, the product requires application about every three or four weeks. “The drier it is, the more we have to apply,” he points out. “Occasionally we do have to drag a harrow across the top to rework it a bit; on days where you have a heavy dew, it reabsorbs that pretty well and takes you through most of the day.” He adds that busier training periods can necessitate more frequent product application. “We might have 10 in a class running five trucks as opposed to six in a class running three trucks, so we could put more wear and tear on the surface,” Rosen says. “It seems to depend more on weather, though. If we have a long dry spell, we have problems unless we reactivate the EDC by adding water.”
One plus is the product’s regenerative quality, he says. “Sometimes we use water to regenerate it. The fire department required the school to have another fire hydrant installed for some of the buildings, and I asked the fire department if they could put it by our lot. This coming year, we’re going to have access to a little more water for prewetting, so we think that should stretch our product.”
Rosen says that having the manufacturer apply the product the first time in the spring of 2009 gave him and the maintenance staff an idea of how much to prewet the surface and how much product to apply. But first, the maintenance staff spread out a top layer of gravel, and training was conducted for about three weeks to start breaking down the surface. “Any applications after that depended on weather,” says Rosen. “This product seems to regenerate itself really well when we get moisture. It’s almost a visual thing—you can look at the surface and see where it lights up.”
For subsequent applications, the academy used its own water tank, which mounts on the straight truck. “Sometimes our biggest problem is to get enough moisture that we can put the product down without trying to prewet a lot, because we don’t have a big tanker—we only have a 500-gallon water tank and we can spray that out really quickly, so we always try to apply the product after a rain. We went three, maybe four weeks before we would put down 500 gallons of product mixed with equal amounts of water. Sometimes we go 50-50, sometimes we knock down the EDC content to 30%, depending on how wet the surface is. Having that kind of flexibility makes it very user-friendly.”
Application time is reasonably short, Rosen says. “How wet the surface is will make a big difference in how strong a solution we’ll use. As long as the ground is not too wet, we can use closer to pure product so that we don’t have to go to the fire hydrant to dilute or top off our tanks. We can cover our lot and use probably two totes [500 gallons of EDC] and 2,000 gallons of water. We can apply it in an hour to an hour and 20 minutes, and it’s not a big deal for us on labor.”
An attribute that provides more flexibility is the product’s shelf life, Rosen says. “The first year, we ended up having 200 gallons left over, but I was told that the shelf life is close to five years,” he says. “There’s no issue there; we keep it stored.”
After a full year of using the product, Rosen reports plenty of anecdotal evidence to support the improvement in dust control. The instructors have told him that the amount of dust is much lower than in the year prior.
Additionally, he says, gravel roads leading to the training course were constructed using recycled aggregate from the facility’s front parking lots, which were repaved. The product was applied to the new roads and has stabilized them and prevented dust, Rosen reports.
Treating County Roads—Sustainably
Larimer County, CO, located north of Denver, and its road and bridge director, Dale Miller, have come to grips with a major realization in regard to sustainability issues in recent years. All told, the county stabilizes 332 miles of nonpaved roads. As do other Colorado cities and counties, Larimer County must apply dust-control treatment to urban roads with more than 150 daily trips and to rural roads with more than 200 daily trips. A few years ago, the county teamed up with the Colorado State University Department of Bioagricultural Sciences and Pest Management to study the effects of salt compound magnesium chloride—which the county had used to stabilize and control dust on nonpaved, i.e., gravel, roads for many years—on roadside vegetation and stormwater runoff. Additionally, there were concerns that the use of the compound on asphalt roads would cause the release of hydrocarbons.
Three years into a five-year study, indications were that the compound can flow from road surfaces onto roadside areas and possibly affect trees adversely, although no negative impacts on streams or aquatic life were discovered. Having committed to phasing out the use of the compound by 2012, the county began searching for a non-chloride alternative and offered to test any manufacturer’s product on a 1-mile stretch of a county road, with a promise to pay for any treatment that works.
The county has tried several non-chloride products, according to Miller. A few years ago, the use of lignin sulfonate, which had been used with and without magnesium chloride, was discontinued. Several petroleum- and enzyme-based products were also tried, with unsatisfactory results.
About five years ago, the county began using EnviroTech Services’ X-hesion for stabilization and dust control. In 2009, the county treated 192 miles (58% of its roads) with the product and plans to treat 261 miles (79%) in 2010 with the product. “It’s the new reality,” Miller says of using more expensive non-chloride products. “We’re dealing with products today that we believe to be friendlier to the environment. The payment you make to use those products is that it typically takes more gallons of concentrate per square yard, so that means more gallons per mile, and the material is more expensive per gallon than mag chloride.”
Dust control is actually a byproduct of the county’s efforts to stabilize its nonpaved surfaces to reduce the frequency of necessary maintenance, Miller adds. “The impetus for us treating the roads is not necessarily dust control—that’s a component—but I would say the reason we began treating and at least half of the reason we continue treating is that there are stabilizing effects from using the dust-control products and, obviously, a stable grade is going to be less likely to send off fugitive dust under traffic.”
X-hesion uses a proprietary formulation of agriculturally derived complex organic polymers and natural humectants. The formulation is engineered to bind base materials and small dust particles together, reducing the release of fugitive dust particles smaller than 10 microns (PM-10)—such as that generated by vehicles traveling on gravel roads—into the air.
Larimer County prepares the road surface, grading it to eliminate potholes and washboarding and establishing a relatively steep crown of 4% to aid drainage, and then prewetting the surface. The manufacturer handles the actual application by spray bar so that the county can avoid any uncertainty about adding the correct X-hesion dosage. “How many times we apply it in a year depends on traffic, weather, and the longitudinal grade of any stretch of road,” Miller says. “It can also depend on the composition of the road base material.” Because closing the roads after application usually is not an option, the county erects temporary signs alerting drivers about the stabilization and dust control treatment so that they can take an alternative route and prevent their vehicles from getting soiled.
Miller says the county uses additional measures to control dust off of the road surface. These include straw bales, silt fences, erosion logs, and revegetation of roadside ditches via hydroseeding or drill seeding in the fall. For the road surface itself, the material composition may impact the amount of dust released into the air, Miller argues. On its nonpaved roads, the county uses aggregate crushed to meet the requirements of the Colorado Department of Transportation’s Standard Specifications for Road and Bridge Construction, Section 703.03 and Table 703-3 for Class 5 Aggregate Base Course (modified).
Miller explains that the specification for gradation of this material is 100% passing the one-and-a-half-inch sieve, 95% to 100% passing the 1-inch sieve, 30% to 70% passing the No. 4 sieve, and 3% to 15% passing the No. 200 sieve. The county modifies the specification to run at the top of the band of the No. 200 sieve (12% to 15%, rather than the specified 3% to 15%), a tweak that Miller believes improves the durability and stabilization of the road surface. A byproduct of greater durability and stabilization may be enhanced dust control, he adds.
Sealing nonpaved surfaces with an eye on eventually paving them is another way of controlling dust, says Miller. “We have a history, a fair amount of experience and success, in double-chip sealing a gravel road, which takes it to a low-volume paved status,” he says. “If we’ve got a good, sound, minimum 6- to 8-inch-thick crushed aggregate road base that’s been stabilized and dust treated for a number of years, the next step in the evolutionary process is potentially a chip seal. When you do that, you’re done grading that road, and so you’re done stirring up those fines and you’ve effectively contained the dust below the surface of the road, and there’s your erosion control.”
Miller concludes that a big part of the reason why Larimer County stabilizes its county roads is that gravel is a precious commodity. Without treatment, the fine aggregate particles get removed from the road surface; the next step is that the larger coarse aggregate particles also separate from the road surface, and the road loses its cohesiveness. Chip sealing or paving the road solves that problem.
Don Talend specializes in covering sustainability, technology, and innovation.