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On a practical note what is the real difference between washing down a drilling rig that has some mud and a bit of fuel on it or washing down a transit van that has a bit of mud and some oil on it? A busy garage is going to get several filthy commercial vans in every day to be cleaned and some of those will even have the engine compartment steam cleaned, releasing grease and oil into the drainage. Presumably the garage has a permit to discharge this water into the sewer?