Well, I certainly wouldn't walk into a customers back garden and throw 2 or 3 buckets of water over the grass, river nearby or not!!!
Nor would I empty it down a storm drain, I agree with Ollie, A hefty fine will catch up with you at some time.
As I understand it from the roadshow, T/M's are allowed to empty into a sewer by lifting a manhole cover IF it is on private land! and portables, I use a downstairs loo where possible, if not then a drain that the kitchen sink leads to.
Did you know, T/M's have to have a waste disposal thingy license!!
Hope this helps
Chris