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» Different types of a RO membrane:
» What is a reverse osmosis membrane?
» Membrane out put subject to?
» Difference between the 50, 75, 100 & 150 GPD membranes?
» Effects of RO water on the DI or post filter:

Different types of a RO membrane:
The RO membrane, though somewhat fragile, is pretty tough. You can handle it without any problem! We at Air, Water & Ice prefer to use FilmTec membranes over any other.

FilmTec membranes can process seawater, brackish water or tap water (from either a city or well source). This is done by utilizing the thin film composite (TFC) material.

The TFC material is great for removing TDS, but can be damaged by chlorine or other oxidants. This is why pre-filters and pre-treatment of the water is important.

How do you tell them apart?
Simple. Each membrane has alphanumeric ID number. The first two letters indicate which type of membrane is used:
SW - Seawater - TDS removal of 32,000 to 40,000. These membranes are made with a thick fiberglass shell and operates under 700 - 1,000 psi (800 is the normal pressure used for seawater).

BW - Brackish water - TDS removal of 2,000 to just below SW membranes. These membranes also have a fiberglass shell, but it's thinner than the SW membranes. The BW membranes work with 200 - 600 psi.

TW - Taped wrapped - This is normally used referred to as a tap water membrane. TDS removal of below 100 to 2,000. Since these are taped wrapped, they operate under low pressures between 35 - 200 psi.

The next two digits (normally the number 30) is the chemistry element. The following numbers are the size of the membrane. For SW and BW membranes, there are no more numbers after the size. For the TW membranes, there are extra numbers to tell you the GPD (gallon per day) rating of the membrane.
SW30-4040 = Seawater(30) - 4.0" x 40"(4040)
TW30-1812-100 = Taped wrapped(30) - 1.8" x 12"(1812) - 100 (GPD)

How can a 1.8" diameter membrane fit and seal in a 2" diameter housing?
The Brine Seal. It's a rubber piece that's wrapped to the TW membranes to form a seal. Since the length of the membranes stays the same, you don't need to go with a different membrane, even if the diameter is slightly smaller.

Are all membranes the same standard size?
As with pre-filters, the answer is no. Different companies can use membranes that are specially made for their unit. For instance; the GE Merlin uses a TW membrane that is 17" long. Just make sure you know the dimensions of the membrane before ordering a new one.

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What is a reverse osmosis membrane?
A reverse osmosis membrane is a very tightly wrapped microfiber that has a .001 micron rating. Water is forced through the membrane by water pressure. Only the purest water molecule can penetrate the membrane.
There are two kinds of water coming out of the membranes: the pure and the waste. The pure water is also called the permeate, product or dilute solution. The waste water is also know as the brine or concentrate.
The pure water has low TDS. The waste water, however, is normally higher in TDS than your tap water. This is because of the pre-treatment of water before the RO membrane.
Membrane out put subject to?
All RO membranes are subject to three things; your water quality, water pressure and water temperature.
The membrane is rated at 77 degrees and 50 psi.
For every pound of pressure lower than 50(psi) subtract 2% of the output. For every degree(F) below 77, subtract 1.4%.
Example: Let's take 40 psi water at the 77 degree temperature with a 50 GPD membrane.
50 GPD - 20% (40 psi) = 40 GPD.
Now let's take the 50 psi, but reduce the temperature to about 40 degrees:
50 GPD - 51.8% (since it's 37 degrees colder x 1.4) = about 26 GPD
Difference between the 50, 75, 100 & 150 GPD membranes?
The major difference, besides how much water they can make, is the rejection rate. Normally, the lower the GPD volume, the higher the removal rate is.
The 50 and 75 GPD membranes have about the same rejection rate of 97%. The 100 GPD has about a 90% rejection rate. The 150 GPD membrane is a different story. It allows for high volume without a big drop in the rejection rate. It has about a 97% - 99% rejection rate.
Effects of RO water on the DI or post filter:
As mentioned before, the RO water has a direct impact on the life on the DI. For calculations, please view the DI resin page on this FAQ.
It also has an impact on a final filter's (aka - inline carbon) lifespan. Since carbon filters are there normally to treat things like sediment and chlorine, the final filter has a good lifespan to begin with. Since RO water is free of those things, the final filter can last for at least 1-2 years.
This holds true to any other post filter. Since the RO water is already very clean, post filters have a better lifespan than if you were to put them as stand alone filters or have no pre-filtration.
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