??? 20 gallons get reused, 100% of it goes back into the system. If somehow 5% was destroyed from showering we wouldn't have any water left.
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.