A Breath of Fresh Air
The control of fugitive dust at aggregate sites is something that members of the Ontario Stone, Sand and Gravel Association (OSSGA) take very seriously. From the licensing process through to final rehabilitation, operators must consider the potential impacts of fugitive dust and ensure that “dust is mitigated onsite,” a phrase embedded in the conditions on almost every site plan.
Dust is a broad term, which includes the potential for both nuisance and health impacts due to fine grained particulate matter suspended in the air. The finer the material, the greater the potential for health impacts, but even larger dust particles have the potential to create nuisance impacts on neighbouring properties. Thankfully, while some techniques are better at controlling larger dust particles, all of these techniques offer a degree of effectiveness on a wide range of particle sizes.
Controlling fugitive dust comes down to four key principles.
1. REDUCE THE GENERATION OF DUST
The obvious first step is to reduce the number of dust particles created in the first place – a challenging measure for the aggregate industry since the fundamental process of turning large rocks into small rocks creates dust. Each loader transfer, crusher, screen, conveyor, transfer point and stacker can generate dust. Where possible, streamlining these processes can reduce the amount of dust being generated. Something as simple as using a longer conveyor instead of several shorter conveyors makes a difference. Enclosing processing plants is also a way to reduce dust generation, though this may not be feasible at all sites.
Even more significant, using conveyors instead of haul trucks can result in the single largest reduction in dust emissions from an aggregate site. This is typically achieved by conveying material from a portable screen and primary crusher at the working face back to a centralized plant near the site entrance. Setting up a site this way also reduces the highway distance trucks travel for shipping, providing yet another reduction in emissions.
Good housekeeping practices help as much as possible to remove accumulations of dust particles and help prevent them from becoming airborne. All sites can practice this.
2. INCREASING DUST PARTICLE SIZE
Almost all site plans refer to using “water or another provincially approved dust suppressant” to mitigate dust. Water and dust suppressants cause dust particles to stick together, increasing the particle size. Larger particles are less likely to become airborne and settle faster if they do.
With water, the key concept is to offset evaporation once the road surface has been wetted. The best way to do this is to monitor the weather, since watering once dust is observed is often already too late. A predictive watering scheme may be useful for some operations. This also helps to reduce water use, which is something all OSSGA members strive to achieve.
Ministry-approved chemical dust suppressants work in a similar manner, but last longer and can therefore reduce water use. These pose additional costs and must be re-applied several times each season, but for sites with limited water supplies, this can be an effective solution.
Paving onsite roads is also an option. While costly, it is a long-term solution. However, roads must be kept clean through sweeping or washing. Track-out of silt onto paved road surfaces, especially public roads, can make paved surfaces worse than unpaved! Some sites have employed permanent water spray systems along paved haul routes to great effect.
3. DECREASING WIND SPEEDS
Decreasing the speed of winds moving through a site can be very effective when other measures fail. The tendency of particles to become airborne is closely related to wind speed – even short gusts can pick up significant amounts of fine dust.
Using solid barriers is not an ideal solution unless the process or operation is being completely enclosed. Wind speeds can in fact increase around solid barriers, making things worse in some cases. Engineered windscreens, or even dense mature vegetation, can provide an effective solution since they are able to reduce the wind speed rather than attempt to block the wind. This reduces the ability of the wind to pick up dust particles and causes some of the particles already carried in the wind to drop out. This is most dramatic with larger dust particles but does help with the smaller size particles as well. When using vegetation to help reduce wind speeds, the vegetation needs to be dense enough to affect wind speeds. A dense stand of conifers is best for this.
4. REDUCING POTENTIAL IMPACTS
When all methods of control have been exhausted, the final option is to operate in a manner that reduces the potential impact to neighbours. Increased separation distances between operations and neighbours can sometimes help, where possible.
The next best option is to keep enough flexibility at a site to curtail or restrict operations when the wind is blowing toward the residence and only operate when weather conditions are favourable. The exact distance varies based on the scale of operations and intervening terrain. When the winds are blowing from the residence toward the operations, or if conditions are wet (rain drizzle), ramp up operations as much as possible. This places additional restrictions on site operations but is an alternative to increased setbacks.
Nearly all these techniques depend on having accurate weather data for the site itself and understanding weather patterns and predictions. Installing meteorological equipment at a site is highly recommended, especially if there is no reliable, local source of weather data. Knowing wind speeds and wind directions alone is critical in planning for and responding to potential dust events. Should a complaint occur, that data is also often invaluable in investigating the complaint and helping to identify the root cause of the issue.
The most effective approach for controlling fugitive dust is to have a toolbox full of techniques available. The techniques noted above provide some of the most effective methods currently in use, but the combination is highly site-specific. While fugitive dust cannot be eliminated entirely, these measures will ensure impacts to your neighbours will be minimized.