Oil and solids seperation is what you want to do. Need to start with what local codes are. Might be as simple as the same system as a power car wash uses. Solids trap in first stage. Oil floats to top later in the process and gets recovered. The less you have to dispose of, the lower the cost to be compliant.

Here in Spain do the same, in our workshop waste water acummulates in an underground tank and these residues are removed by an authorized recycling company.

Regards

we have a system that captures the waste water in an underground tank, and it gets hauled off by the same company that buys our waste oil, but they charge for hauling off the steam cleaner water.
I think the Government would 'string us up' if we ever just let it run into the sewer, and I know our would be competitors would have a field day if it ever got out that we let even a little run into the ground.

I worked at a place a few years ago as a 3rd party contractor which made a recovery system for pressure washing equipment. It was a rather large platform (probably 20x20 ?) which was strong enough to hold about 10,000lbs (working on memory here), was about a foot thick, & recycled the same water thru itself as you pressure washed. Around here (outside Phila)- you must contain, collect & dispose of wastewater from pressure washing.

If you need further information- let me know- I'll see what I can dig up