I've never washed out into "drains or sewers" so maybe
you're too concerned. As long as you have grass,
any smart (that rules out many) tileman will wash out into
the grass and water it in real good. Maybe you better
stress this point to the tileman BEFORE you let him work
for you. The worst customer is one who brings up these
issues when I'm well into a job. I've always been real
particular about cleaning up 100%.
A couple of times I got trapped into jobs and found out
they wouldn't let me wash out ANYWHERE on the property,
telling me I had to take the grout water, dirty buckets etc
home to clean (an hour away).
Imagine that........their grass was too good for my washing
out, but _my_ yard met their standard.......... no problem,
I won't say what I did with it, but I left with clean buckets
and tools :-)