Two Types of Reverse Osmosis Membranes

Always wondered what the difference between a CTA & TFC reverse osmosis membrane is? Well, keep reading to find out, and learn more "RO speak".

CTA membranes - cellulose triacetate: Use a CTA membrane when the source water is chlorinated. Usually a municipal or city water source. The CTA was originally used by reverse osmosis manufacturers. It's only advantage over a TFC is being chlorine resistant.
TFC membranes - thin film composite: Use a TFC membrane when the source water has a high TDS (total dissolved solids) and/or the source water is non-chlorninated. A TFC membrane is bacteria resistant. A TFC membrane has superior rejection and flow rates. The TFC was developed by NASA and will need a carbon filter(s) before the membrane when used with chlorinated water.

To use a CTA or TFC membrane, the feed water must be microbiologically safe with the following limits:

Iron - 0.1 mg/l, Manganese - 0.05 mg/l, Hydrogen Sulfide - 0 mg/l, Tannins - 0 mg/l

The difference between nominal and absolute membrane ratings

Nominal Ratings: If a membrane (or filter for that matter) has a nominal rating, it means that the tests that have been preformed on that membrane (or filter) have been within the stated range 85% - 90% of the time.
Absolute Ratings: If a membrane (or filter for that matter) has an absolute rating, it means that the tests that have been preformed on that membrane (or filter) have been within the stated range 95% - 99% of the time.

Below are some of the differences between a CTA and TFC membranes. The statistics are based on nominal membrane ratings.

Characteristic	CTA Membrane	TFC Membrane
Product water rate (to atmosphere @ 60 psi, 77 degrees Feranheit)	22 gpd	24 - 35 gpd
Maximum feed water TDS (with sufficient line pressure)	1200 mg/l	2000 mg/l
Maximum feed water hardness	10gpg	10 gpg
Rejection of TDS	88% - 93%	95% - 98%
Feed water temperature	40 °F - 86 °F 4 °C - 30 °C	40 °F - 113 °F 4 °C - 45 °C
Feed water pH	3.0 - 9.0	2.0 - 11.0
Maximum feed pressure (in suitable pressure vessel)	50 - 100 psi	50 - 100 psi
Booster pump pressure	80 psi	80 psi
Chlorine tolerance	excess of 10000 mgl	sensitive to chlorine & other oxidizers.

Nominal Membrane Rejections Performances*

Inorganic Contaminant	CTA membrane	TFC membrane	Inorganic Contaminant	CTA membrane	TFC membrane
Aluminum	90-95%	93-98%	Magnesium	90-95%	93-98%
Arsenic	90-95%	98-98%	Manganese 2	90-95%	93-98%
Barium	90-95%	98-95%	Mercury	90-95%	93-98%
Bicarbonate	85-90%	90-95%	Nickel	90-98%	93-98%
Boron	30-40%	55-60%	Nitrate 3	40-50%	85-90%
Cadmium	90-95%	93-98%	Phosphate	90-95%	93-98%
Calcium	90-95%	93-98%	Potassium	85-90%	90-95%
Chloride	85-90%	90-95%	Radioactivity	90-95%	93-98%
Chromate	85-90%	90-95%	Selenium	90-95%	93-98%
Chromium	90-95%	93-98%	Silver	90-95%	93-98%
Copper	90-95%	93-98%	Sodium	85-90%	90-95%
Cyanide	85-90%	90-95%	Strontium	90-95%	93-98%
Fluoride	85-90%	90-95%	Sulfate	90-95%	93-98%
Iron 2	90-95%	93-98%	Zinc	90-95%	93-98%
Lead	90-95%	93-98%
*Based on reverse osmosis membranes producing to atmosphere with 60 psi net pressure and 77 degrees Fahrenheit.

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