Wheel and tyre wash systems, and the dust rules that require them

Closer to it here than almost anywhere else. The dust mitigation measures for construction and demolition sites state that all vehicles, including those carrying construction material or debris of any kind, should be cleaned and wheels washed before leaving the construction site — and the official monitoring checklist asks whether wheels and under body were washed. Those measures are issued as directions under s.18(1)(b) of the Air (Prevention and Control of Pollution) Act 1981, so they bind the implementing agencies that inspect you. Above 20,000 m² of built-up area the same rules add anti-smog guns and blacktopped internal roads.

It is treated as a dust-control failure rather than a road offence: environmental compensation levied by the State Pollution Control Board, and in the National Capital Region a materially worse position when GRAP tightens. Because the inspection is a checklist, the practical exposure is not whether you own a wheel wash but whether you can show it was running — which is why the log matters as much as the bay.

Sometimes — they sit at the same point on the site and guidance treats them as alternatives. The difference is measurable: the US Environmental Protection Agency puts sediment removal at less than 30% up to 60% for gravel pads and shaker racks, against 75% or more for a properly installed wheel washing rack. On a dry site with light traffic over a stabilised stone access, a grid is often enough. On sticky clay, in the wet, or where the road outside is a residential street, it generally is not — and a dry control has nothing to say about the undercarriage.

A single pass, without stopping, in under a minute — the vehicle drives through at a crawl and the jets run while the wheel turns. A 4 m bay clears around 50 vehicles an hour, which is why the sizing question is peak hour, not daily average: the queue that matters is the one at shift change. Bay length is the lever, because it is also how many times the tread turns under spray — 4 m turns it once, 8 m twice, 12 m three times.

Far less than hosing by hand, because the water is recirculated rather than consumed. Up to 90% of the wash water is recovered every cycle: it drops into a tank below the bay, the solids settle out in stages, an oil skimmer and coagulant dosing clean it up, and it goes round again. The system draws make-up water to replace what leaves on the vehicle and in the sludge. Containment is also what keeps the discharge question out of scope — nothing goes to the drain.

The plan that names the control, evidence that it was working on the day, and the sludge disposal record. Several jurisdictions ask for a wheel-wash log specifically, and dust-control regimes that inspect against a checklist treat "was it running" as a separate question from "do you have one". A control you cannot evidence is, for audit purposes, a control you do not have.

The regulator does not care which, only that the outcome holds for as long as the site does. Portable wins where the exit moves: no excavation, commissioned inside a workday, relocated across phases and onto the next project — and it can be hired rather than bought. Concrete earns its civil works where the gate outlives the programme: a quarry, a batching plant, a depot, a landfill or a long-life mine.

It settles out of the water in the tank under the bay and leaves as sludge. With a plain settling tank that is a scheduled job, usually done by plant already on site moving spoil. With a PCET scraper-conveyor dirt trap — the trailing C in the model code — a chain-and-flight mechanism lifts the solids out continuously into a chute, so nothing is shut down to clear it. Where the sludge goes is worth settling before anyone quotes: it is a waste stream with its own paperwork.