90% rejection rate is a bit low, if the filter is new I would expect better.
Are you letting it run a bit, and are you flushing the membrane?
I have about 370 ppm
input water, a common Dow filter, and am getting about 20 ppm
out of the RO
side, but for the first pint or so it is more like 35ppm (decreasing), even if I have flushed.
What I do is:
- Open the RO
tap (blocking DI
- Flush the filter about 3 minutes (i.e. no flow restriction)
- Add flow restriction so it is making RO
water, let that run for another 5 minutes
- At this point I'm getting about 20 ppm
- Switch it to go through the Di
- Let that flush for another 5 minutes or so (even though it is always 0 ppm
When done, close off the DI
, flush the membrane again for 3-5 minutes, and put it all away.
When the membrane is not under pressure there can be flow of dissolved solids across the membrane which means initial water out is higher.
On the other hand, if you are steadily getting 40ppm, it may just be the type of minerals -- any given membrane has widely varying rejection rates for different contaminants. I wouldn't worry unless this rises.