reverse osmosis (R.O.)

A water treatment process that removes undesirable materials from water by using pressure to force the water molecules through a semi-permeable membrane. This process is called “reverse” osmosis because the pressure forces the water to flow in the reverse direction (from the concentrated solution to the dilute solution) to the flow direction (from the dilute to the concentrated) in the process of natural osmosis. RO removes ionized salts, colloids, and organic molecules down to a molecular weight of 100. May be called hyperfiltration.

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