Reverse Osmosis Membranes

Reverse osmosis membranes will reject dissolved and suspended materials including monovalent salts. Since essentially all dissolved and suspended material is rejected by the membrane, the RO permeate is pure water.

Reverse osmosis membranes are made with various rejection rates for different applications.

For example, water can be softened with a nanofiltration (NF) membrane that rejects 85% of salt (sodium chloride) but 99% of the hardness ions (calcium and magnesium). The highest salt rejection rates (99.7% or higher), which can be provided by RO membranes, are required for seawater desalination.

Typically, Reverse osmosis membranes have a very high removal capacity of at least 3 log units (>99.9% rejections) for bacteria and viruses, in many cases even higher.

Pores in reverse osmosis membranes are so small they have not yet been resolved, even by the most advanced microscopic techniques. They are generally regarded to be in the 4 to 8 range, four orders of magnitude smaller than the finest of the normal-flow particle filters.

FILMTEC membranes tapwater elements are used in single-element commercial and residential systems to treat pre-processed/municipal water

Reverse osmosis is a moderate to high pressure (80-1200 psig) driven process for separating larger size solutes from aqueous solutions by means of semi-permeable reverse osmosis membranes. This process is carried out by flowing a process solution along a membrane surface under pressure. Retained solutes (such as particulate matter and dissolved salts) leave with the flowing process stream and do not accumulate on the membrane surface. The amount of salt and other impurities is often referred to as TDS, or total dissolved solids. The higher the TDS, the more feed pressure required.

Eight-inch brackish water RO membranes are used to treat water with salt concentrations between 50 and 10,000 mg/l. In commercial applications, these elements are used in multiple-element systems to treat pre-processed or municipal water.

Nanofiltration is a term coined about six years ago to define membranes, which were already in use, referred to then as "loose RO." They have pores close to one nanometer diameter (1-) and affect partial salt rejection. Typical NF membranes pass a higher percentage of monovalent salt ions than diva-lent and trivalent ions. Most NF membrane polymers carry formal charges which exclude higher valence ions more than monovalents from passing through the membrane with the solvent water. Nanofiltration membranes span the gap between RO and UF classes.

Nanofiltration membranes are used for selective removal of impurities where the high level of salt rejection provided by reverse osmosis is not required. Nanofiltration elements reduce hardness and remove pesticides, organics, color, bacteria, and other impurities from raw water.

Seawater membrane elements are used for treatment of water with salt concentrations greater than 10,000 mg/l.

Since RO membranes require both extremely small pores and significant water sorption tendency, only two materials are in common use: cellulose acetate and polyamide polymers. The CA membranes tolerate chlorine at levels used for microbial control, while PA membranes will be destroyed by even low levels of chlorine. However, the PA membranes produce both higher rejection and flux, and tolerate a wider pH range on a continuous basis and a higher continuous temperature than CA membranes (pH 2-8 for CA, 2-11 for PA, 40C [104F] for CA, 65C [149F] for PA).