As far as I know, the only water that is discharged is potable water that comes from the river and is used on-site for drinking water and so on, and it is then treated like any sanitary sewage, which is then sent back to the river. That's what's called not process-affected; it hasn't been touched by the tar sands. Any process-affected water is not discharged.
There is other river water, for example, that might have gone through a heat exchanger, did not touch any tar sands, and could be discharged. As long as it hasn't been in contact with the oil sands, it is discharged. It's just the river water.
In the case of potable water or sanitary sewage, it would be treated the same way water in any other sewage plant would be before going back to the river.