Put my 4040 R/O together a couple of weeks ago. My water is at a tds of about 90 - 100. Flushed unit for a short time, maybe 5 minutes and then adjusted the waste gate so I could get a ratio of about 50/50, ie., 2 litres pure to 2 litres waste. Within a few minutes the pure was at a tds of 2. It very quickly dropped from 12 to 2. HF5 membrane and water at 40 psi according to pressure gauge.
I put in an inline tds meter on Monday and the tds fluctuated between 2 and 3 from the membrane on this meter.
My original hand held tds meter has recently be calibrated at a tds of 34.
So according to my maths/calculator the membrane is about 97 - 98% efficient, which in my books is about right and where it should be. Not quite as efficient as my old RoMan 450 R/O but does it produce water fast.