Question: What Specifications Should I Look For In A Membrane?

Answer: With the exception of a few proprietary RO system manufacturers, all residential membranes are the same size and all fit in the same membrane housing. When the residential RO market first started to come alive, nearly all of the membranes where 8 to 10 gallons per day systems. Many of the proprietary membranes are still under 25 gallons per day production capacity. But the vast majority of the RO systems being sold today are 50, 75 and 100 gallons per day units.

To better understand how membrane manufacturers rate the capacity of their membranes, you need to be aware that membrane manufacturers establish the gallons per day production for a membrane based on the following criteria: 1) Incoming water at 77 degrees Fahrenheit 2) Incoming water at 65 PSI 3) TDS (Total Dissolved Solids) of 200 ppm.Not very often do we find these three conditions existing in real life situations. Correspondingly, most RO system users are unable
to achieve the level of production indicated by the manufacturers ratings. Bear in mind that it is possible to achieve greater production capacity than the rating from the membrane manufacturers if your water temperature is greater than 77 degrees (F) or you have more than 65 PSI flowing from the incoming water. Some marketers of membranes rate the membranes they are selling to have production capacity greater than the ratings listed by the manufacturer. Some will indicate 85 gpd from a truly 75 gpd membrane. They could well have achieved those numbers but they have altered the water temperature or increased the PSI upward and are in fact quoting unachievable ratings in the real world unless your real world includes the increased PSI and temperature.

RO membranes are self-flushing provided there is a match between the RO membrane and the flow restrictor. To operate properly (and to extend the life of the RO membrane) there needs to be approximately 3 times the amount of water going to drain as compared with the product water going to the storage tank unless you have include a Permeate Pump which will reduce the drain water flow. The ratio of water flowing through the membrane is the function of the flow restrictor.
If the flow restrictor is too small, you will be causing pre-mature fouling of the RO membrane. If the capacity of your flow restrictor is too high, you are sending too much water to the drain, this will cause a small increase in water usage.

The life of a membrane is directly related to the care and support given by the owner. If you do not change your pre-filters every 6 months, you will likely be shortening the life of your RO membrane. Proper care of the pre-filters could extend the life of a RO membrane out to 5 years or more. Chlorine will quickly foul up a TFC membrane. You must be certain that there is sufficient carbon capacity in your carbon pre-filter to avoid chlorine wiping out your RO production capacity