Dust control is not an option. Today’s increasing regulatory, safety, and social awareness demands compliance. So whether it’s a one-day project or an ongoing operation, dust control is definitely part of the plan.
Dust emissions-even for a short time-are never “OK,” but the duration of your project may affect how you approach the issue of dust control.
Sometimes, short-term, low-impact activities simply do not justify the application of a dust-control product. I recently attended a Civil War reenactment, which took place at a large ranch. The parking area was a grassy field and, even though the weather was sunny, it had rained the week before. As a result, the existing grass and slow vehicle speeds limited dust emissions. Under these types of short-duration, low-impact conditions, a water truck-available on an as-needed basis-is an adequate and low-cost solution. I think the cannon smoke created more particulate matter than dust from the parking area.
At the other extreme, such long-term operations as landfilling, mining, construction, and agricultural activities require more robust and durable solutions-solutions that are, more often than not, job-specific. In other words, what worked last year may not be the best choice for this year…or next. Job-site variation in terms of the gradation and dominant particle size of the dust often dictates what type of control mechanism will be most effective-and most economical.
In addition to project variation, increasing oil prices over the past year have resulted in significantly higher costs across the board-especially for petroleum-based dust suppression products. So again, last year’s choices may not fit this year’s issues.
The more you know about the problem, the better your chances of making the right decision.
Gravel Roads
Dave Prunty, public works director for Flathead County, MT, plans to hire a contractor to treat 50 miles of gravel road again this year using magnesium chloride (MgCl2). The county has tried various things, but according to Prunty, “Mag just seems to work best. And because of increasing oil prices, the cost is significantly less than emulsified oil.”
Magnesium chloride and its counterpart calcium chloride (CaCl2) are both deliquescent salts-chemicals that attract moisture from the atmosphere. When either of these chemicals is blended into a road’s surface (usually as a dissolved solution in water), it attracts moisture, thereby helping to prevent aggregate separation-and dust.
There are many providers of MgCl2 and other salts. North American Salt Co., a Compass Minerals Co., is one such producer. It provides various types of salts for many different applications, including dust control, erosion control, and deicing. Depending on your need, you can purchase a single drum or multiple railroad tankers.
According to Prunty, the MgCl2 application process-which is performed by a contractor-is the key to success. The four-step process includes the following:
- Pre-watering. Using a standard water truck, the contractor wets the road. This helps reduce dust during the blading process and, just as important, helps to retain and redistribute fines in the surface of the road.
- Blading. Once the road surface has adequate moisture (characterized by no dust), a motor grader loosens the top 2 inches or so. This of course helps smooth the road, but the primary purpose is to create a loose, well-graded layer of material to allow uniform penetration of the MgCl2.
- First Application. The first application of MgCl2 is performed with a boot truck at an application rate of 0.25 gallon per square yard. On a 22-foot-wide road, that works out to approximately 3,227 gallons of solution per mile.
- Second Application. The second application, done 24 hours later, is at the same application rate: 0.25 gallon per square yard.
The most important factor for successfully applying MgCl2 and other dust-suppressing salts is penetration. To achieve it, the road surface must first be loosened by blading or scarifying.
Over the years, Prunty has also identified some important gradation characteristics-criteria that must be met for MgCl2 treatment to be effective. “You need some fines, with clay and plasticity,” he says. “You can’t just apply mag to any old gravel road and expect it to work.” He goes on to explain, “We look for material with a plasticity index somewhere around 10. In terms of gradation, we also look for 8% to 14% material passing the 200 sieve.”
In essence, what he’s describing is not a clean gravel, but a well-graded gravel, leaning toward lots of fines. Table 1 shows some lab results for one of the county’s gravel mixes. The quantity of fines is clearly evident in this mix, with 13% passing the 200 sieve.
Ironic, isn’t it, that MgCl2 and other hydrating dust control chemicals are most effective on gravel mixes that have a higher component of fine particles-just the ones that would tend to create the most dust if not treated?
Based on this criterion, spraying MgCl2 on a marbly gravel road is a waste of time and money-and won’t be very effective at controlling dust.
Do local Flathead County residents appreciate this effort to control dust? You bet. The county’s dust-control program is based on a 50/50 cost-share program. Residents who want to participate are asked to pay their share in advance. The amount is based on the length of property frontage they own along county roads.
Flathead County has used another innovative approach to dust control, but this wasn’t something sprayed on the roads, it was someone who patrolled them. At one time, the Flathead County Sheriff’s office had a full-time position for what it called the “dust deputy.” Patrolling the county’s gravel roads, the dust deputy would issue citations for speeders. With regularly posted speed limits of 35 miles per hour, on down to 20 miles per hour during dusty conditions, the dust deputy-and his ticket book-were a great deterrent to speeding.
Speeding on gravel roads is hard on the road and unsafe-and, of course, creates a tremendous amount of dust. “We’d have guys driving 60 miles per hour on our gravel roads,” Prunty says, “but our dust deputy really helped to slow them down.”
When properly applied, MgCl2 can last all season. It keeps drawing moisture from the atmosphere to replenish what might be lost through evaporation. But for that to occur, there must be some moderate level of humidity-generally no problem for many areas. But in the Rockies and Southwest deserts, extended periods of hot, dry weather can deplete any chloride’s ability to retain moisture.
As a general rule of thumb, relative humidity levels above 30% are required to replenish the moisture of MgCl2 and other chloride treatments.
Typically, moisture that is depleted during the day is replenished at night when temperatures drop and humidity increases.
Where that is not happening, it’s possible to artificially replace the moisture by periodically spraying with a water truck. But remember: Those chlorides are water soluble and can be washed away if too much water is applied.
Getting Information
Decision makers for rural municipalities can get lots of good information about dust-control options by attending trade shows and conferences-although in this time of restrictive travel budgets, such events may be off limits for many people.
Another alternative source of training and information is the Local Technical Assistance Program (LTAP), which was created by the Federal Highway Administration in 1982. There are LTAPs in every state and Puerto Rico. A similar program, the Tribal Technical Assistance Program (TTAP), was established in 1991 to help meet the transportation needs of Native American tribes. Information on these two programs can be found at http://www.ltap.org.
Unsurfaced roads represent a significant source of dust. In fact, according to The New U.S.A. Gravel Road Manual, by Ali Selim and Ken Skorseth, there are over 1.6 million miles of unpaved roads in the United States. That’s a lot of unpaved roads-and consequently a lot of dust. But it’s certainly not the only source.
Construction Projects
Construction and demolition projects represent another type of dust source and often require another approach to control it. A typical construction project, such as site preparation for a subdivision, brings a more complex set of issues to the table. Portions of the project may generate lots of dust for a short period of time, while other portions of the project can be buttoned up quickly.
The solution will vary depending on job conditions and on the contractor’s approach. Some projects can efficiently be performed in small phases and may in fact be forced into a phased approach because of cash flow or permit restrictions. In these situations, the use of temporary treatments and permanent stabilization may overlap several times throughout the project.
Contractors must also consider soil conditions and the type of equipment used on the project. For example, when working in fine-grained soil, paddlewheel scrapers have the ability to generate a tremendous quantity of dust.
On many excavation projects, a common method of dust control is simply to spray the area with water. This is not necessarily done only for dust control, but also to add moisture to the soil-or as it’s referred to in the industry, moisture conditioning.
When compacted soil is used to construct roads, dams, building pads, and other structures, the right amount of moisture-referred to as the “optimum moisture content”-helps to lubricate and align the soil particles, which results in better compaction and increased soil density.
So, in the process of moisture conditioning the soil, contractors are also controlling dust. To increase the effectiveness of the water and speed up the construction process, contractors sometimes pre-wet the soil before it is excavated. This is usually done with a water truck, but we’ve also seen elaborate sprinkler systems-using agricultural-type irrigation pipes-used to pre-wet a job. Of course, between the excavation and fill areas, dust must also be controlled on the construction-site haul roads. Here again, the duration of the job is an important consideration in terms of selecting a method of dust control.
On a small project-especially one with a short haul road-a water truck may be the lowest-cost option. Chances are the contractor already has one onsite to moisture condition the soil, and it can also be used to spray the haul road a few times per day.
But on an extended project, or where the haul roads are long, the water truck option can quickly become very expensive.
Other Surface Treatments
When water alone isn’t the answer, you may need to apply a surface treatment. In addition to hydrating salts (which we’ve already discussed) your options include various types of polymers, oil-based products, or even natural materials such as mulch.
Most of these materials work by cohesion, binding dust-sized particles together. Some products are designed to withstand heavy traffic. These not only suppress dust, but also help increase the longevity of the road by keeping the larger gravels intact instead of raveling and drifting onto the shoulder. One important difference, though, is that some products form a rigid surface and others don’t. “What’s the difference?” you might ask. A rigid, tightly held surface is generally more durable under traffic, so for a high-use area, that’s what you’ll want. For non-traffic areas, where you simply want to suppress dust, either type would work.
However, if you want to control dust on those non-trafficked areas while also establishing some vegetation, you’ll want to avoid surface treatments that include chlorides, petroleum-based materials, or any treatment that forms a rigid crust.
Kevin Loucks, president of Earth Chem, explains, “We have different products to match different site conditions.” One of the products, EarthBound, is a polymer that helps bind fine soil particles, but in a non-rigid manner. EarthBound is most effective on completed, non-trafficked areas. “Because it does not form a rigid surface,” says Loucks, “EarthBound is very effective at retaining soil while vegetation is established.” It can be applied dry or in liquid form (known as EarthBound L) using a water truck or hydroseeding machine. But instead of forming a rigid surface, EarthBound forms a microscopic net-one that holds the soil particles in place. For this reason, it is most effective if applied after seeding. Because it is nonrigid, it is not intended for use in traffic areas.
The highest level of success occurs when the right product is combined with well-defined project sequencing and traffic management. We frequently work on landfill projects where broad areas of exposed soil can create lots of dust. Too often, we see surface treatments ruined because onsite traffic was not properly restricted. To avoid this problem when suppressing dust across large work areas, first figure out where the traffic will be. Then create a means of traffic control. Finally, select the appropriate dust suppression products.
In these situations, using an inexpensive surface treatment, along with limiting off-road traffic to specific driving areas, can go a long way toward decreasing dust-and dust suppression costs.
For projects where special consideration must be given to environmental issues (i.e., surface water), other types of products are available. Earth Chem also produces, among many other things, a product called WindBreaker, a vegetable-based surface treatment used for roads and other high-traffic areas. According to Loucks, it is “an environmentally friendly option for dust suppression in areas where users-or regulators-want a non-chloride, non-petroleum alternative.”
There are many different types of surface treatments designed especially for dust control, but on some projects surface treatments are ineffective because the dust is coming from the process itself, not from traffic or the ground surface. Examples include landfill operations, wood grinding operations, demolition projects, recycling operations, and quarries or mines. Controlling dust in these types of projects requires a different, more focused, approach. Let’s look at one very effective method: misters.
Demolition Projects
Demolition projects tend to ebb and flow in terms of the quantity of dust generated, but because they are typically located in developed areas, their dust-control solutions must shift accordingly. In the staging areas, such traditional dust control measures as water trucks or surface treatments can be effective, but it’s pretty difficult to get a water truck onto the roof of a building. So, when it comes to controlling dust on the demolition target itself, another approach is needed. Of course, some small demo projects, if performed during the wet season, may not produce much dust. But there are other considerations.
In the aftermath of Hurricane Katrina, officials in the New Orleans region were faced with a quandary. With 50,000 houses to demo-a project initially estimated to take three years at a rate of 50 houses per day-traffic dust was only one of the concerns; many of those older buildings also contained asbestos and mold.
But even on a smaller scale, most demolition projects require contractors to focus their dust-control efforts (usually water) on the actual work zone. One innovative method for doing this is with a misting device, like the one manufactured by DustBoss.
According to Rick Felde of DustBoss, the various units can be customized to match just about any dust-control need. With the ability to cover over 5,000 square feet of work area, one of the smaller units can be mounted on a skid-steer to provide mobile dust suppression. Others can be mounted on a pole or crane. The units do require connection to a water source-usually a hose or tank. However, for projects requiring more mobility, the units have also been mounted on a trailer (with a tank) or water truck to provide very mobile, untethered suppression. The largest unit, which can be equipped with 359-degree oscillation, can provide dust control over nearly 125,000 square feet.
The output of the DustBoss misters can be adjusted for angle, projecting distance (throw), and water flow as required to match the specific job conditions. They can even adjust water droplet size of the output mist, in order to match-and most effectively mitigate-the average dust particle size. Apparently, these units perform best when these two dimensions (water droplet and dust particle) are similar.
The units can be set up to start and stop, even to monitor and adjust to changing job conditions-all automatically. “We can integrate our system with a small weather station that’s set up to monitor job-site temperature and -humidity along with wind speed and direction,” explains Felde.
When we talk about demolition projects, most of us think about the actual process of taking down a building, bridge, or other structure, but that’s only step one. The material then must go someplace-typically to a recycling facility and/or a landfill.
And as that material arrives for processing, some of those same demolition issues-including dust-come with it. Landfills and recycling facilities, along with mines, agricultural operations, composting facilities, and other ongoing operations, must find effective and economical ways to suppress dust.
Steve Joseph, vice president and general manager at the PVT Landfill in Hawaii, oversees an operation that recycles several hundred tons of construction-and-demolition (C&D) waste each day. At the PVT Landfill, water trucks control dust along the access roads, but the recycling area also employs a DustBoss mister. According to Joseph, it uses approximately 800 gallons of water per hour to control dust at the recycling operation. Inbound C&D waste goes through a sorting process that includes on-the-ground sorting and a trommel (a rotating drum screen). The recycling crew extracts metal, concrete, and other recyclable materials-and in the process, it can get dusty. “The DustBoss not only controls dust,” explains Joseph, “but through evaporative cooling, it also helps lower the temperature by nearly 20 degrees. This is something the workers really like.”
Lots of Options
And that brings us back to the bottom line: When there are multiple solutions for dust control, the first impulse for anyone making such decisions is usually to go with the lowest-priced solution they can find. But lowest price and lowest (overall) cost are not always the same thing. When facing this decision, you should first ask whether everything is on the table: problems, solutions, side benefits, costs-and could there be other dust-control methods that you just haven’t heard about? Well, there’s always another option out there, with more being developed all the time, but to broaden your field of choices, you have to keep listening. If you turn over enough rocks, no telling what you might find.
One way to do this is to issue a performance-based request for proposal (RFP) instead of a prescriptive-type request for bid (RFB). Here’s the difference.
A performance-based RFP states the problem (i.e., too much dust) and asks for solutions (i.e., dust control). It simply asks for a solution, but leaves the method undefined. On the other hand, a prescriptive RFB states the solution (i.e., 0.25 gallon of CaCl2 per square yard) and directly asks for a price.
By asking contractors to come up with a solution, instead of simply asking for their cheapest price, you’re able to tap into their pool of experience and knowledge. Of course, this open-door type of solicitation has some risk. You’ll sometimes have to filter through some “overly creative” solutions that might not make sense for your project, and there is also a risk that you’ll pay first only to learn later that it didn’t work. But there are many innovative solutions out there, and if you aren’t asking, you may not hear about them. Contractors can be a great source of innovative and/or low-cost solutions. Remember, their livelihood depends on providing effective and competitive solutions.
Another great source of information is in the pool of vendors who actually manufacture and sell the various dust control products. While dust control may be an important item on your checklist, it’s still only one of many items. But for most of these folks, it’s all they do. Sure, they’re motivated to sell their products, but if it’s the right solution for your problem, then you’re also motivated to buy. It can definitely be a win-win arrangement.
The first thing to look for in a contractor or vendor is communication. It’s like playing catch. When you’re explaining your dust-control problem, they need to listen-you have the ball. But when it comes time for them to present a possible solution, the ball’s in their court-and it’s your turn to listen. This is an interactive process.
“Who else uses this product?” “Here’s a list.”
“How long does it last?” “Three to 12 months; here’s the data.”
“How well does it work?” “It depends on conditions. Here’s a case study.”
“Can you provide references?” “Here they are.”
“What’s the cost?” “It depends on volume; here’s a range.”
And so it goes, back and forth, and with each toss you learn more. But at some point, if either side won’t toss the ball back, it’s game over. To learn what you need to know about dust control, find vendors and contractors who want to play ball.
Dust control is a very broad topic with many complex issues. The good news is, there are many creative manufacturers and myriad of effective solutions. Spend some time talking-and listening-and you’ll find the ones you need.To control dust, many road crews and construction sites use a dust suppressant that works by attracting moisture from the air, especially at night, when humidity tends to be higher. Dust suppressants keep dust particles damp and less able to move into the air, while keeping expensive aggregate on the road instead of in the ditch or floating away in the breeze.One product that road maintenance crews have found especially helpful is a naturally occurring mineral, magnesium chloride, which is harvested using only the sun’s rays and wind. Magnesium chloride has been used successfully in road stabilization and dust control for over 40 years. Naturally occurring from the Great Salt Lake in Utah, magnesium chloride provides a nontoxic, environmentally responsible solution to help maintain gravel roads. Its easy-to-spread characteristics make it ideal for deep soil penetration and maximum control of dust.
According to Verlin Van Zee, assistant road supervisor of Lakes Highway District in Idaho, his use of DustGard magnesium chloride creates a significant impact on his ability to control dust while managing his budget.
“DustGard has reduced road grading from two or three times per month to one or two times per year. We’ve reduced use of our road equipment, allowing our manpower to do other maintenance needs,” he says. “In the nine years since we started the road treatment program, we have not re-graveled any of our gravel roads, and after three years of using the product, we have been able to cut our treatments in half.”
Because of the widespread problems caused by dust, the EPA enforces strict standards for the amount of dust that can be present in the air, and cities and counties are required to have air-quality plans in place in order to meet these standards.
Controlling dust is important because dust particles can easily enter the respiratory system, creating health problems in infants, children and the elderly. People with such respiratory conditions as asthma, allergies, bronchitis, and chronic obstructive pulmonary disease are especially at risk.
Protecting citizens’ health and safety, as well as complying with air-quality standards, are direct benefits of using magnesium chloride in gravel road maintenance programs. According to Van Zee, “Our biggest benefit is public safety and customer satisfaction. Since we’ve been treating our roads, we have reduced our complaints from the public by 90%.”