As I understand it, to comply with the law, you need:
To collect dirty water in a tank on yor vehicle IE wash the bin over the tank so no effluent spils in street and goes down surface water gullies.
Have a waste carriers licence to cover the effluent you are transporting
Have a licence and training to discharge effluent into a foul sewer
If you fill up from a hydrant in street, a local water authority "key" and paid for.