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Reverse Osmosis/Nanofiltration (RO/NF)


The concept of reverse osmosis (RO) was first studied by Fick in 1855 (Glater, 1998) but pressure-driven RO membranes were not developed and tested until the mid-1950s by researchers at UCLA, University of Florida, and the US Office of Saline Water (Glater, 1998). RO and nanofiltration (NF) membrane manufacturers now produce many types of membranes, and different designs are optimized for productivity (high flux), lower energy, high salt rejection, fouling resistance, or other useful properties. Both RO and NF systems efficiency is limited by the solubility of species present in the feed.

RO and NF produce two streams of water, namely permeate and concentrate. The concentrate contains all of the salts removed by the membrane. Typical brackish RO/NF systems can operate at 75-85% recovery and are limited by the sparingly soluble salts such as CaSO4, CaCO3, and silica. The amount of salts in the concentrate is related to the recovery of the RO/NF; higher recovery systems will have higher concentrations. Disposal or treatment of the concentrate can be a limitation to the installation of RO or NF.

Both reverse osmosis and nanofiltration utilize pressure to produce permeate from a brackish water source using semi-permeable membranes installed in pressure vessels. RO is able to produce high quality product water; however, it requires higher pressure than NF and can be susceptible to fouling and scaling. Rejection of multi-valent and large molecules in brackish feedwater is similar for RO and NF membranes, but NF membranes have lower rejection of monovalent and uncharged molecules. NF can produce good quality permeate that meets primary drinking water standards and can require substantially lower feed pressure than RO.

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