The membranes have different efficiency rates.
Filmtec 50 gpd = 98% rejection rate
Filmtec 75 gpd = 98%
Filmtec 100 gpd = 90%
BFS 150 gpd = 98%
If your tap water is 225 ppm, and you're getting 18 ppm out of the RO, then your efficiency is about 92%. If I were you, I would change out your flow restrictor for a 75 gpd, and get a 75 gph membrane that is 98% efficient. This way you should get about 4-5 ppm out of the RO, and your DI resin will last much, much longer. This is probably your cheapest solution.
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~ Mindy
SPS fanatic.
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