What Is Reverse Osmosis Part - 7

 

SINGLE PASS RO VS DOUBLE PASS RO

Think of a pass as a stand alone RO system. With this in mind, the difference between a single pass RO system and a double pass RO system is that with a double pass RO, the permeate from the first pass becomes the feed water to the second pass (or second RO) which ends up producing a much higher quality permeate because it has essentially gone through two RO systems.

 

Besides producing a much higher quality permeate, a double pass system also allows the opportunity to remove carbon dioxide gas from the permeate by injecting caustic between the first and second pass. C02 is undesirable when you have mixed bed ion exchange resin beds after the RO. By adding caustic after the first pass, you increase the pH of the first pass permeate water and convert C02 to bicarbonate (HCO3-) and carbonate (CO3-2) for better rejection by the RO membranes in the second pass. This can't be done with a single pass RO because injecting caustic and forming carbonate (CO3-2) in the presence of cations such as calcium will cause scaling of the RO membranes.

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