The only real usage of water is evaporation and that's stuff like growing plants and cooling towers.
Many systems also output reclaimed water; it's clean, but not up to environmental standards for discharge or drinking; typically excess clorination. This is often used for municipal irrigation sometimes toliet flushing, etc; uses where water below drinking standards is fine.
A handful of systems discharge treated water into their reservoirs or into acquifer recharge ponds. But there's an ick factor, even when discharge water is often held to higher standards than drinking water, so it's only done when the situation outweighs the ick.
You have seperate drainage for shower water and effluent?
That’s certainly not the case here in Australia.
Here, typically storm water and household waste water are carried over a common system. Usually if it rains more than 3mm in 24hrs the treatment systems are overwhelmed and untreated waste is sent out to sea. Coastal areas anyways.