Higher capacity membrane and longevity?

[az2008]

I'm considering a Watts RO (some may recognize me from the undersink subforum yesterday). Their standard WP5 model is 24 gpd. It's also available on Amazon with a 50 gpd membrane.

I'm curious whether a higher-capacity membrane lasts longer. I.e., I only need 24 gpd. My water consumption would remain the same with the 50 gpd membrane. Therefore, does this mean the high-cap membrane will be stressed less due to operating for shorter durations?

In other words: I know all descriptions are qualified with "based upon water quality and usage." So, I'm wondering when they say "2-5 years" for both of those membranes, is it based upon something like 20% usage (5 and 10 gallons per day) and therefore the higher capacity membrane would have half the expected usage and last considerably longer?

Thanks!

[Andy CWS]

Advertised membrane capacity and actual capacity (gpd) can be vastly different. When you see those numbers on their marketing or from a manufacturer, they represent membrane testing (typically under NSF protocols) with specific parameters such 77˚F, certain constant pressure and a specified amount of TDS to give all membranes a level field. Your water conditions are going to be greatly different, especially temperature. So a 75gpd membrane may only produce 15 gpd in normal conditions. Mind you, that's over a 24 hour period.

Not trying to scare you away, just trying to let you in on the reality of RO production. Typically, the higher the incoming pressure, the greater the production. Membrane/permeate pumps can benefit production a great deal.

Another factor in membrane life is whether the incoming water is softened or not. Softened water will make the membrane last 3 to 10 times (or more) longer. Sodium is water soluble whereas calcium is not. Calcification of membrane can be destructive depending on usage, temeprature, pressure, and concentration--along with other contaminants.