What is the main difference between nanofiltration (NF) membrane and reverse osmosis (RO) membrane in water filtration?

The main differences between nanofiltration (NF) membranes and reverse osmosis (RO) membranes in water filters are their filtration accuracy, the types of substances removed, and application scenarios. RO membranes have the filtration accuracy, with a pore size of about 0.0001 microns, and can remove all dissolved solids, including bacteria, viruses, heavy metals, salts, and all organic matter. In contrast, the filtration accuracy of NF membranes is between microfiltration and reverse osmosis, with a pore size of about 0.001-0.01 microns, and they mainly remove divalent and multivalent ions (such as calcium, magnesium, sulfate), some organic matter, and larger molecular pollutants, while allowing some monovalent ions (such as sodium and chloride) to pass through.

In terms of the types of substances removed, RO membranes can remove ions, dissolved solids, heavy metals (mercury, arsenic), nitrates, fluorides, bacteria, viruses, and  organic compounds in water, and all dissolved impurities will be removed. NF membranes mainly remove divalent and multivalent ions (such as hardness ions such as calcium and magnesium), some organic compounds (such as some pesticides and fertilizers), colors and odors, while retaining some monovalent ions (such as sodium and chlorine), so some natural minerals in the water can be retained.

RO membranes are suitable for applications that require  high purity water quality, such as drinking water purification, seawater desalination, industrial ultrapure water preparation and laboratory water. NF membranes are suitable for applications that require partial removal of hardness and some organic matter, such as household drinking water purification (retaining some minerals), mild industrial wastewater treatment, food and beverage processing, etc.