I wonder what a WS355 is. Oh, look. Its a water softener. Nice of you to give us that clue.
Manual here:
http://www.bwt-uk.co.uk/en/business/products/Pages/DownloadManual.aspx
Overflow will go through the wall to the outside world
Drain would probably go to the same drain as your washing machine.
Why not contact the maker for their advice, instead of asking here?
XXX, August 2016
