Frequently Asked Questions about Reverse Osmosis Membranes and Ion Exchange Resins (121-125)

121. What standards do Dow FILMTEC membrane products meet?

All Dow FILMTEC™ (TW30, BW30, BW30LE, XLE, SW30, SW30HR, NF90, NF200B, and NF270) membrane elements comply with FDA 21 CFR 177.2550.

Most Dow FILMTEC residential elements have received ANSI/NSF Standard 58 Component Certification and NSF Standard 58 Performance Certification, which can be found on the NSF website. NSF/ANSI Standard 58: "Applies to point-of-use (POU) and point-of-effect (POE) devices for reducing certain contaminants in water, including compounds, particulates, and microorganisms, which pose potential health hazards and affect the sensory properties of water quality. This standard also includes requirements for material and structural integrity." NSF/ANSI Standard 61: "Drinking water system components – Health impact standards for all devices, components, and materials in contact with drinking water."

122. How resistant are Dow FILMTEC membrane elements to chloramines?

In the laboratory, Dow FILMTEC™ membrane elements withstood 300,000 ppm-hrs, indicating that no dechlorination pretreatment is required. However, chloramines are formed when ammonia is added to water and reacts with chlorine. Even after ammonia is added, residual chlorine may still be present in the feed water of the membrane system. Dechlorination measures should still be considered, using activated carbon or reducing chemicals to convert the residual chlorine into harmless chloride ions.

123. What is the difference between reverse osmosis and nanofiltration?

Nanofiltration is a membrane liquid separation technology situated between reverse osmosis and ultrafiltration. Reverse osmosis can remove the smallest solutes, with molecular weights less than 0.0001 micrometers, while nanofiltration can remove solutes with molecular weights around 0.001 micrometers. Nanofiltration is essentially a low-pressure reverse osmosis method, used in situations where the purity of the treated water is not particularly stringent. Nanofiltration is suitable for treating well water and surface water. Nanofiltration is suitable for water treatment systems that do not require the high desalination rates of reverse osmosis, but it has a very high removal capacity for hardness components and is sometimes called a "softening membrane." Nanofiltration systems operate at low pressures and consume less energy than corresponding reverse osmosis systems.

124. What separation capabilities does membrane technology possess?

Reverse osmosis is currently the most sophisticated liquid filtration technology. Reverse osmosis membranes retain dissolved inorganic molecules such as salts and organic matter with a molecular weight greater than 100, while water molecules can freely pass through the membrane. Typical soluble salt removal rates are >95-99%. Operating pressures range from 7 bar (100 psi) for brackish water to 69 bar (1,000 psi) for seawater.

Nanofiltration removes impurities with particles as small as 1 nm (10 angstroms) and organic matter with molecular weights greater than 200–400. The removal rate of dissolved solids is 20–98%, with salts containing monovalent anions (such as NaCl or CaCl2) achieving a removal rate of 20–80%, while salts containing divalent anions (such as MgSO4) show a higher removal rate of 90–98%.

Ultrafiltration is effective for separating macromolecules larger than 100–1,000 angstroms (0.01–0.1 micrometers). All soluble salts and small molecules can permeate ultrafiltration membranes. Removable substances include colloids, proteins, microorganisms, and large organic molecules. Most ultrafiltration membranes have a molecular weight cutoff of 1,000–100,000.

Microfiltration removes particles ranging from approximately 0.1 to 1 micrometer. Typically, suspended solids and large colloids are retained, while macromolecules and dissolved salts can freely permeate the microfiltration membrane. Microfiltration membranes are used to remove bacteria, microflocculations, or total suspended solids (TSS). Typical membrane pressures are 1–3 bar.

125. Can Dow FILMTEC membrane elements be used to concentrate or purify organic solvents?

Hydrophobic solvents are incompatible with FILMTEC™ membrane elements. However, hydrophilic solvents such as ethanol are compatible. Hydrophilic solvents will not damage the membrane element but will cause membrane swelling, altering the membrane's desalination and permeate characteristics. Furthermore, high concentrations of organic solvents will extract from the membrane element's seal lines; prolonged contact will damage the seal. It is currently impossible to estimate the duration the membrane element can withstand. We recommend conducting compatibility tests on membrane sheets first, and if the results are good, then proceed with small-scale element testing.