Oil-Grit Separators

Runoff enters a chamber where velocity and internal flow paths allow dense grit to settle, floatables to rise, and free oil to remain at the surface. Treated flow exits toward downstream storm controls while larger flows may bypass. Stored sediment and oil must be removed periodically to preserve hydraulic and treatment capacity.

Generally no. A separator has limited internal storage and is designed primarily for sediment, floatable, and free-oil capture. It normally provides little meaningful peak-flow attenuation. Where a site requires quantity control, the separator is paired with a pond, tank, chamber system, oversized pipe, or another approved detention measure.

Ontario guidance identifies separators as typically used on small drainage areas below 2 hectares, but final selection is site-specific. Treatment flow, runoff capture, sediment loading, spill risk, bypass hydraulics, municipal criteria, and verified device performance determine sizing. The civil engineer and approval authority establish the accepted model and configuration.

The design compares groundwater uplift with the weight of the empty structure, soil cover, and any anchoring system. Where needed, a reinforced base slab, ballast, or structural connection resists uplift. Harrowgate completes the anti-flotation detail and balanced backfill before reducing dewatering around the installed unit.

A bypass carries flows above the water-quality treatment rate without creating damaging head loss, upstream flooding, sediment resuspension, or reverse flow. It can be internal or external. Harrowgate verifies flow direction, structure elevations, weirs, orifices, and bypass connections against approved hydraulic drawings before the system is buried.

It should be protected during upstream earthwork and cleaned before commissioning because construction sediment can rapidly consume the sump. Ongoing frequency depends on measured accumulation, oil storage, site use, and the approved maintenance plan. Inspection after major spills and high-sediment periods is essential rather than relying only on a calendar interval.

Yes. We plan storm bypass, excavation support, groundwater control, traffic staging, crane access, temporary protection, and tie-in windows. The old structure is isolated and removed, the new unit is set to surveyed inverts, and pavement is restored in phases. Commissioning confirms a clean, accessible, correctly oriented system.