Why not invest in your own meters? I have a meter on my pure and another on my waste, both post RO. That way I know exactly how much pure I'm making and how much waste it costs me to make it. I keep records of the water usage and I can prove exactly how much water is for business use and so put it through my books. When it comes to paying for the sewerage, I can deduct the pure water because I can prove that doesn't go back into the sewer.
A couple of meters and associated pipework will set you back less than £50