I'm with southern water,I have an ro van mounted system,my waste water goes on my garden not down the drain.
I fitted a sub meter to my garden tap ,prior to using the tap I took the sub meter reading and also the main meter reading coming into my house.
When my metered bill arrived , I took a a reading on my sub meter, and gave it to southern water,I have just had a credit put on my account for 77 cubic meters of water,for non return to sewer,meaning I only pay for the waster used,and not charged for what would have gone down the drain.
Forgot to say that from the outset I had to fill a form in for non return to sewer.
Hope that helps.