So in theory it shouldnt run dry then? I mean if he has a 100GPD unit it should only be drawing less than 22ml of water a second and the waste water should be returned up to 18ml/s to the well. These arent mind blowing flow rates.
Not sure where you got 9000gal of waste water from. 98% rejection rate for the RO membrane means the membrane can filter out 98% of the TDS in the water, it does not mean it rejects 98% of the water going in.. Meaning water with 100TDS going in will have 2TDS going out. The amount of waste water an RODI produces depends on the restrictor valve on the waste water line, and its usually a 1:4 ratio. So if you make 100gal a day you produces 400 gal of waste water.