Which GPD should I get for the RO replacement filter?

[hefner413]

I've had a RODI for a while and now need a new RO filter. I can get just about any GPD online - 25 - 50 - 75 - 100. Do the 25 GPD do a better job of water quality than the 100? Or is the only difference the speed?

 

[florida joe]

The GPD in most RO/Di units are predicated on water temperature of 70-77 degrees F, 60-90 psi pressure and less than 200 ppm of TDS. If any of these parameters is not optimal, GPD decreases, sometimes up to 50%. Therefore, if you need a certain amount of water per day, and one or more of your parameters are outside the optimal range, you need to purchase a higher GPD rated unit.

 

[hefner413]

Thanks joe - yeah I knew about the change in GPD if not at the ideal parameters. I was asking more about the actual water produced by the RO filter. Do the lower GPD produce a "cleaner" water? And thus do you sacrifice quality of water produced by purchasing the higher GPD?

 

[ryancw01]

The 75 gallon per day membrane is rated to reject about 98% of the TDS. The 100 GPD Membranes is rated at about 90%. That difference changes the amount of work the DI has to do. If you do not need the large volume of water you are better off with the 75 gallon per day or smaller membrane. The non rejected TDS is mopped up by the DI so the less mopping up to do, the longer the DI lasts.
Example: Tap water 200PPM. The expected output of the 75 gpd would be 6-12 ppm. The expected output of the 100 gpm would be 20-40 ppm. Actual values are dependent on you tap water quality, pressure and temperature. 6800 > 12 = 566 gallons 6800 > 40 = 170 gallons.