Reverse Osmosis (RO) is the highest end water treatment technique as far as membrane technology is concern. It is capable of removing particle down to minute size ion and capable of achieving >98% salt rejection. RO removes bacteria, viruses, colloidal matter and other substances found in water. It is also the method of choice in desalination plant for treating Sea Water for drinking. Dissolved solids content in Sea Water typically in the range of 20,000 to 30,000 mg/l.

Nanofiltration (NF) can replace the Reverse Osmosis process if the treatment objective is just to reduce the dissolved solid content. The different between NF and RO is the degree of salt rejection of the formal is slightly below that of RO. NF typically achieve salt rejection of <95% compared to >98% for RO. As a result, the operating pressure will be lower for NF systems in the range of 5 to 10 bar. Product water from NF system will not contain hardness ion due to the preferential rejection of higher molecular weight and charged ion possessed by hardness ion. NF shall be suitable to treat brackish water up to 5,000 ppm dissolved solids content and to achieve drinking water quality.

When NF and RO technologies are used to treat brackish water, typically 15 to 25% of raw water shall be wasted as reject. Typical operating pressure shall be within the range of 7.5 to 15 bar depends on system design.

When RO is used in desalination plant, the operating pressure can be as high as 50 bar and reject/waste can also be as high as 50%. As a result, sea water desalination RO system normally quite costly and it is often the last choice only when fresh water source is not available.