RO membrane in comparison to other kinds of membranes has the smallest pore size, a little larger than water molecule size. This is why membrane allows water molecules to pass by, while obstruct passing of sodium chloride, salts and impurities. The MSM 300 GPD RO membrane to be used in RO filter has the power of converting sea water into fresh drinking water. The minimum salt reduction rate of this membrane is 97% â 98%. 300 GPD in MSM 300 GPD RO membrane is permeate flow rate. Polyamide RO membrane sheets are used in the making of this kind of membrane. It performs exceptionally, while maintaining stability.