A couple of years ago I had an altercation with a shop owner as I was emptying my waste tank (TM) down a Foul Drain near the rear of his shop. He threatened to report me to the authorities. I knew I was in the clear but just in case contacted the local United Utilities Trade Effluent officer, to explain the problem.
He told me it was a very grey area and would get back to me. He did so a couple of days later. He told me they were going into a meeting and would make a ruling on the matter to clear up any misunderstandings.
He contacted me again to tell me that they had come to a decision and that this had now been laid down.
Basically it was this, Carpet Cleaners can dispose of their waste water (trade effluent) down ANY Foul Drain, but never any storm drain.
They prefer that we empty our waste water down the customers foul drain (this is usually the one that the kitchen and bathroom sink pipes drain into) if that's not possible (some modern houses don't have open drains) they say to take it home and empty down own Foul Drain or as a last resort down any KNOWN Foul Drain.
Road side and house runoff water (From gutters etc) drains often go straight into streams and rivers untreated and we must NOT use those.
This of course is the situation within my area and may differ in other areas, but it may help if you contacted your own Trade Effluent officer.
Dave.