Originally Posted by J_Anton
I am curious as well if these are good...
I guess the primary factor in determining how good an RO
unit is, is the total dissolved solids (TDS
) rejection rate of the RO
membrane since the RO
membrane is the 'work horse' of the unit.
A greater rejection rate will increase the longevity of the DI
stage, which ideally should be be producing 0ppm TDS RO
An ideal RO
membrane should have a TDS
rejection rate of 98-99+%. A less than ideal RO
membrane may have rejection rate of less than 95%.
Every 2% increase in the rejection TDS rate doubles the longevity of the DI resin
. The DI
stage following a membrane with a 98% rejection rate lasts twice as long as the DI
resin following an RO
membrane with a 96% rejection rate and 4x's longer than one with a rejection rate of 94%.
The ultimate cost:
As an example, if the rejection rate of a "cheap" RO
membrane is 90% (unit A), then the system is expending the DI
resin 16x faster
than a high quality membrane with a 98% rejection rate (unit B). The owner of unit A will have gone through 16 DI
resin change outs by the time the owner of unit B replaces his DI
resin for the first time. A cheap unit may be saving money on the front end but will be wasting money in DI
resin replacement. (of course this is under the assumption that cheaper unit means less efficient RO
To determine the RO
membrane rejection rate, substract the TDS
of the RO
water (not the RO
water) from the TDS
of the tap water. Divide that figure by the TDS
of the tap water and multiply by 100.
tap water TDS
.9466 X 100=rejection rate of 94.6%