The Barnstead NanoPure Water apparatus are in room 230 of building 65 as well as in room W357 building 95. Both supply deionized water. Reverse osmosis treatment reduces the concentration of dissolved solids, including a variety of ions and metals and very fine suspended particles such as asbestos, that may be found in water (see below). An RO device may be installed following a water softener to reduce the concentration of sodium ions exchanged for hardness ions. RO also removes certain organic contaminants, some detergents, and specific pesticides. Check the ASTM specifications for additional data.
Ions and Metals: Arsenic, Barium, Bicarbonate, Cadmium, Calcium, Carbonate, Chloride, Chromium, Copper, Fluoride, Iron, Lead, Magnesium, Manganese, Mercury, Nitrate, Potassium, Radium, Selenium, Sodium, Sulfate, etc.
Organic Compounds: Benzene, Carbon tetrachloride, Dichlorobenzene, Trichloroethylene, Toluene, 1,2,4-trichlorobenzene
Particles: Asbestos and Protozoan cysts
Pesticides : 2,4-D, Atrazine, Endrin, Heptachlor, Lindane, Pentachlorophenol
Although RO membranes can remove virtually all microorganisms, it is currently recommended that only micro biologically safe (i.e., coliform negative) water be fed into RO systems. Some RO systems, however, may be used for removing waterbome protozoan cysts (such as Giardia and Cryptosporidium) found in surface drinking water supplies. Only treatment systems certified for cyst reduction by NSF, International should be used for this purpose.
Reverse osmosis will not remove all contaminants from water. Dissolved gases such as oxygen and carbon dioxide pass through RO membranes into the treated water. Unfortunately, hydrogen sulfide gas, with its notorious odor of rotten eggs, also passes through the RO membrane. RO is not a very effective treatment for trihalomethanes (THMs), some pesticides, solvents, and other volatile organic chemicals (VOCs). If the water source is severely polluted or untreatable, a public water supply or a reliable private water source must be used.
The Reverse Osmosis Process
In the reverse osmosis process a cellophane-like membrane separates purified water from contaminated water. An understanding of osmosis is needed before further describing RO. Osmosis occurs when two solutions containing different quantities of dissolved chemicals are separated by a semipermeable membrane (allowing only some compounds to pass through). Osmotic pressure of the dissolved chemical causes pure water to pass through the membrane from the dilute to the more concentrated solution (see the drawing below at the left) - There is a natural tendency for chemicals to reach equal concentrations on both sides of the membrane. In reverse osmosis, water pressure applied to the concentrated side forces the process of osmosis into reverse. Under enough pressure, pure water is "squeezed" through the membrane from the concentrated to the dilute side (See the drawing below on the right). Salts dissolved in water as charged ions are repelled by the RO membrane. Treated water is collected in a storage container. The rejected impurities on the concentrated side of the membrane are washed away in a stream of waste water, not accumulated as on a traditional filter. The reverse osmosis, pressure is applied to the concentrated solution reversing the natural direction of flow, forcing water across the membrane from the concentrated solution into the more dilute solution. The RO membrane also functions as an ultrafiltration device, screening out particles, including microorganisms, that are physically too large to pass through the membranes pores. RO membranes can remove compounds in the 0.0001 to 0.1 micron size range (thousands of times smaller than a human hair).
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