Osmosis is a natural phenomenon whereby the lighter of two substances migrates through a semi permeable membrane toward a heavier solution.
This is the process by which our absorb nutrients, our lungs transport oxygen to our blood and plants extract moisture and nutrients from the soil. In Reverse Osmosis (RO) the heavier Solution is forced under pressure through the membrane leaving behind those molecules too large to pass through.
RO membranes work on the principle of rejection.
The higher the rejection rate the purer the water produced