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