How Are Sand and Oil Interceptors Sized for Car Washes?

I build the interceptor sizing from the equipment's simultaneous discharge rates — every wash function that can run at once during the peak hour, plus hose-down and reclaim bypass flows. That peak number selects the unit: the chamber volume must provide enough residence time for grit settling and oil flotation at that flow, and the oil-storage compartment must hold the accumulation between pump-outs. I check the manufacturer's rating basis carefully, since rated flows assume specific influent conditions, and I carry a margin for the days when the wash runs harder than the design hour. The sizing also sets the physical footprint and depth, which I coordinate with the site grading and the structural design of the buried unit.

The internal configuration is what makes the rated volume perform. I specify baffle arrangements that force the full flow path through the chamber — no short-circuiting from inlet to outlet — with deep grit compartments and hopper or sloped bottoms that collect sediment where the vacuum hose can reach it. Inlet piping enters to distribute flow and dissipate energy so incoming water does not resuspend settled grit; the outlet draws from the clean middle layer with oil-retention baffling. Invert elevations are set so every upstream trench drain flows by gravity with adequate slope, and I verify the hydraulic grade line through the whole drainage system. Where the site is too flat for gravity, the lift station goes downstream of the interceptor, never upstream where pumping would re-emulsify the oil.

The interceptor protects the sewer only if it is sized, configured, and serviceable. I check these items on each facility I design. • Sized on peak simultaneous wash discharge with residence time for grit settling and oil flotation • Deep grit compartments with pump-out access sized for vacuum-truck hoses • Baffle and outlet configuration that prevents short-circuiting and retains floating oil • Clean stormwater routed around the interceptor, never through it • Invert elevations set for gravity flow from every upstream drain • Sampling point at the discharge and accumulation monitoring for the permit program