Dust suppression on demolition sites: rate and reapplication

The monitor at the site boundary logged 480 micrograms per cubic metre of PM10 at 11:20, against a limit of 150. The demolition contractor had sprayed that morning at seven, the wind had picked up to 6 m/s from the west, and the excavator was working the third floor of a brick building with a soft-strip that had barely started. The spray was not the problem. The interval between sprays was.

Water alone is a poor dust suppressant on demolition because it evaporates in twenty minutes under a summer sun and because a brick rubble pile has an enormous surface area per tonne. A polyacrylate added to the bowser holds the water in the surface layer for hours instead of minutes, and that changes the economics of the whole operation — fewer passes, less bowser traffic, and a boundary reading that stays inside the limit.

What the polymer is doing, and what it is not

The granule swells and holds water in the top few millimetres of the rubble, releasing it slowly as the surface dries. That keeps the fines damp, which is the whole game: dust comes from the fraction below 75 microns, and a damp fines layer does not lift. It does not cement the surface, it does not stop dust from a fresh cut, and it will not help on a windy day if the material has already dried.

That last point matters for planning. Polymer-treated water is not a one-off application that lasts a week. It buys you hours, often four to eight on a still day, and two to three in wind. The saving is in the number of passes, not in the elimination of them.

Rate: grams per square metre, not litres per truck

The useful rate is 0.4 to 0.8 litres of diluted solution per square metre of exposed surface, at a polymer concentration of 0.3 to 0.5 percent by weight. That works out at roughly 2 to 4 g of dry granule per square metre. On a 2,000 m² demolition footprint that is 4 to 8 kg per application, which is far less than most contractors guess.

Dilute it properly. Powder poured into a bowser will gel on the surface and block the pump strainer; the correct order is water first, then powder into the vortex, then twenty minutes of recirculation before spraying. A venturi eductor does this in one pass and is worth the cost if you are dosing daily.

Spray the surfaces that generate dust, not the whole site. The active face, the stockpiles, the haul route and the load-out area are where the reading comes from. Spraying the clean hardstanding is a waste of water and creates a slip hazard for the plant.

The reapplication schedule that survives an inspection

Build the schedule around wind and work activity rather than the clock. A simple rule that holds: reapply when the surface colour has gone from dark to matte, when a boot scuff raises a puff, or every four hours during active demolition, whichever comes first. Log the time, the area, the concentration and who sprayed.

That log is what a regulator or a client’s environmental clerk will ask for, and a spray record with real times is far more persuasive than a method statement written before the job started. Add the boundary monitor readings to the same sheet if you have one, so the two columns sit side by side.

The failure that costs a site its permit

I have seen one job where the polymer was dosed at 2 percent instead of 0.4, on the theory that more must be better. The result was a slick, soapy surface across the haul road, a loaded tipper that slid two metres at the junction, and a stop-work while the surface was scraped and re-graded. The dust reading was excellent. The safety report was not.

Over-dosing also produces a crust that peels in sheets after a rain, which then becomes a new dust source as it breaks down under the tracks. If you see flaking, halve the concentration before you change anything else.

Take a bucket and a jug to the first application and measure the area you actually covered. Most bowsers deliver 6 to 8 litres a minute through a hand lance, so a 2,000 m² face at 0.6 L/m² is 1,200 litres — three bowsers, not one. Knowing that before the first pass is the difference between a plan and a wish. If you are setting up the dilution for the first time, the mixing guidance for super absorbent powder covers the wetting-in order that keeps a lance from clogging.