Seeking clear skies: at construction and demolition job sites as well as recycling facilities, keeping the air free from dust is critical.

Hazards and environmental concerns at job sites take many forms. One of the quieter but more omnipresent nuisances (and potential hazards) is the generation of dust at demolition and construction job sites and at facilities where almost any material is recycled.

The creation of dust can be the byproduct of an industrial process, such as concrete crushing, or it can be caused by factors beyond the control of site supervisors--such as high winds blowing across an unpaved job site.

Regardless of where dust comes from, neighboring property owners are bound to agree that it is unwelcome--and they expect dust prevention steps to be taken.

For contractors and recyclers, knowing their dust control options can help them keep both nuisance complaints and compliance costs down.

AVOIDING THE CRUSH

Operating a crusher involves using strong machinery powered by plentiful energy, but even the strongest machines can be laid low by nuisance complaints.

What needs to be accomplished in terms of dust control is obvious. But as Fred N. Kissell, editor of the Centers for Disease Control (CDC) "Handbook for Dust Control in Mining" writes, "If controlling dust were a simple matter, dust problems in tunnels and mines would have been eradicated years ago."

The fact that the CDC handbook is 132 pages in length helps demonstrate that the number of problems and potential solutions can be quite extensive.

In terms of crushing and other operations that carry over to recycling, the CDC notes that "dust from crushers is controlled by water sprays and local exhaust ventilation from the crusher enclosure."

But as recyclers will be the first to acknowledge, the complications arise when trying to design a water suppression system that gets the job done without also wasting water or causing a mess. "The amount of water needed to do the job is hard to specify," the CDC handbook concedes. "It depends on the type of material crushed and the degree to which water will cause downstream handling problems. If the rock is dry, a starting point is to add a water quantity equivalent to one percent of the weight of the material being crushed."

Business owners and makers of dust control systems have found that applying water gently is critical. "The nozzle pressure of sprays at the grizzly and crusher jaw should be below 60 psi to avoid stirring the dust cloud and reducing the capture efficiency of the ventilation system," writes Kissell and CDC co-author Gregory J. Chekan.

Before and after material is crushed, it can also generate dust during its journey on conveyors. Belt scrapers and belt washers can help reduce blowing dust or prevent the conditions that create dry dust from the "carryback" stream.

"Carryback is that portion of the carried material that sticks to the belt instead of falling off at the head pulley," note Kissell and Chekan. "It becomes airborne dust as the belt dries and [meets] the return idlers."

While belt scrapers can be helpful, they may miss the smallest particles that have the greatest chance of becoming airborne dust. Thus, systems placed between scrapers that spray the belt with water can add another layer of prevention.

Operators of both fixed and portable crushing systems may wish to avoid the added cost and hassle of setting up mist or spray systems, but the costs of being perceived as a generator of dust can be even higher.

At press time, a concrete crushing plant in southeastern Michigan is idle because a township zoning board of appeals has denied a request for a permit from the operators.

The Pamar Enterprises plant in Macomb County, Mich., has been unable to secure a permit that would allow the company to resume operations at the crushing plant.

According to a report in the Macomb Daily (of Mt. Clemens, Mich.), the facility has generated complaints because of its proximity to an elementary school and the dust it generates.

GOOD NEIGHBORS

Recyclers as well as contractors have dust control aspects beyond material down-sizing to consider in their efforts to be good neighbors.

Trucks operating on unpaved surfaces as well as stockpiled material (before or after it is processed) can produce dust during periods of dry, windy weather.

Guidelines from the City of Albuquerque, N.M.--a part of the country that can be both dry and windy--provide several recommendations for what the city's Air Quality Division calls "Fugitive Dust Control Methods."

For storage piles, the city notes that watering with spray bars, hoses and water trucks can be a first line of defense, as can three-sided storage barriers that will greatly reduce the amount of wind exposure for stored materials.

Employees and facility designers aware of prevailing wind directions can also play a part by confining "loading and unloading procedures to the downwind side of storage piles," according to the Albuquerque guidelines.

Material that exceeds its bunker height may need to be covered by tarps that should be "anchored to prevent [the] wind from removing them."

Construction and demolition sites or unpaved roads can have dust generation problems that will require similar methods, plus some additional techniques.

Simply installing fencing and reducing vehicle speed are suggestions from Albuquerque, as is "locating haul roads as far from existing housing as possible."

Additionally, watering can be beneficial, as can paving or chip sealing of high-traffic surfaces.

Demolition contractors or the property owners they are working with can also take steps to make sure an abandoned site does not become a source of airborne dust. Vegetation should be established "as quickly as possible when active operations have ceased," the Albuquerque guidelines recommend.

Fencing, watering and chemical stabilization can also be considered for inactive sites.

Controlling dust, like the dust itself, can indeed be a nuisance. Contractors and recyclers looking for a set of practical suggestions can find the Albuquerque guidelines on the Web at www.cabq.gov/airquality/pdf/ dustcontrolmethods.pdf.

The author is editor of Construction & Demolition Recycling and can be contacted at [e-mail].