Where waste water goes back to basically to a sewage plant. This is where household waste water goes (toilets, sinks etc). Drains on the road side tend to eventually just end up in the river system, hence why putting detergents in this system etc doesn't go down well with the authorities.
As for draining machines. Suck any left over clean water through to the dirty water side by allowing air to enter the tube at the same time. At the same time squeeze off any fluid left in the delivery pipe until that runs dry.
One word of warning. When sucking up clean detergent from one side to the other a lot of foam is created in the waste tank. Foam will not activate any trip device to switch off the power. Foam sucked into the motors is bad news. Either keep a close eye and switch off before this happens or make sure you have defoamer in the waste tank.