As a result of this, the hardness is also reduced. CTA membranes break down over time due to bacterial attack whereas TFC membranes are more or less impervious to this. TFC membranes are very sensitive chemically to the chlorine found in most water supplies. RO filters work by forcing water under pressure against the membrane. The membrane allows the small water molecules to pass through while rejecting most of the larger contaminants.
RO units waste a lot of water.
The membrane usually has 4-6 times as much water passing by it as it allows though. Unfortunately, the more water wasted, the better the membrane usually is at rejecting pollutants. Also, higher waste water flows are usually associated with longer membrane life. What this means in practice is that 300 gallons of total water may be required to produce 50 gallons of purified water. Like any filter, RO membranes will eventually clog and need to be replaced.