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Sunday, November 9, 2008

Water Purification Methods and Applications

De-ionization:

De-ionized water is used in clinical and medical laboratories for diluting important specimens that need to be untainted by dissolved solids, in the semi-conductor industry for cleaning silicone wafers, in the optics industry for cleaning lenses, and it is used in chemical laboratories for cleaning glassware. It is used in cosmetics, pharmaceuticals, PC water-cooling systems, car washes, window cleaning solution, freshwater/saltwater aquariums, and in fog-fire extinguishing systems.

De-ionization by ion exchange is the process of removing dissolved cations and anions from the water and replacing them with hydrogen and hydroxide ions which combine to form water molecules. First the positively charged cations are removed using the Strongly Acidic Cation Resin (SAC), followed by the removal of the negatively charged anions using a Strongly Basic Anion Resin (SBA). This is done by generating the each ion-exchange resin their respective highly acidic or highly basic solution for 30-60 minutes and then rinsing the excess solution out with feed water. This method works well on brackish waters with Total Dissolved Solids concentrations as high 3000 ppm, and can be followed by reverse osmosis and electrodialysis for even better results. They need to be regenerated whenever the exchange resin has reached exhaustion and no longer has any effect on hard water. They need to be backwashed too, about every 10 or so cycles to remove mudballs.

Note: Water softening by ion exchange uses the same process as deionization, but the resin is generated with sodium or potassium ions from salt instead of with hydrogen ions from strong acids.

Distillation:

Distilled water is used in chemistry and biology laboratories, in lead acid batteries, in automotive cooling radiators, steam irons, household aquariums, and in cigar humidors. In addition, distilled water can be drunk as a bottled beverage or in areas where the water is so salty that it is uneconomical to use other methods of desalination. Finally, distilled water is used in nuclear radiation cooling towers and on nuclear powered reactors on NAVY ships and nuclear submarines.

Distillation is a process where water is boiled and the dissolved solids which boil at a higher temperature are left behind to form a scale on the bottom of the evaporation dish. This method is the most energy inefficient method because water requires a lot of heat in order to boil, but the finished product can be considered completely sterilized from all bacterial, viral, and microorganism life forms. This is the best method for the desalination of extremely salty sea waters or groundwater.

Electro-dialysis:

Electro-dialysis is used for de-ionizing various aqueous solutions, in large scale seawater desalination, small scale drinking water production, pre-demineralization in semi-conductor and chemical industries, cooling towers, food processing plants, agricultural water applications, HVAC applications, and in the purification of glycerin.

Electrodialysis is a method used to remove ions by using a constant voltage potential between two electrodes. The water flows through the center plane while the ions are attracted to the cathode and the anodes. Cations, the positively charged ions, are attracted to the cathode, which is the negative terminal. Anions, being negatively charged, are attracted to the anode which is the positive terminal. There are alternating layers of ion-exchange resins which the ions get trapped by as they are drawn toward their terminals. An added feature that causes the process to reverse (reversal of the flow and the voltage potential on the electrodes) approximately every 10 minutes and then flush with 2 minutes has shown to considerably improve the effective lifetime of the ED-machine by preventing the formation of scale on the semi-permeable membrane. This method of deionization has been shown to work well on waters with a Total Dissolved Solids content of 3000 ppm and can go much higher up to, (maybe 20,000 ppm !?) for special applications.

Reverse Osmosis:

Water purified by reverse osmosis is used in drinking water, desalination, wastewater recycling, salt water aquariums, kidney dialysis machines, hydrogen fuel production, and rainwater recycling.

Reverse osmosis is the process of forcing water through a semi-permeable membrane by applying a high pressure using centrifugal pumps. The water can move through the membrane, but the salinity remains on the concentrated side of the membrane. These RO-filters can be used in series as much as 8 in a row, and can remove about 90% of the TDS. This method is good for waters up to 40,000 ppm Total Dissolved Solids, but the water must be treated first to remove colloidal matter, insoluble precipitates, and chlorine (in some cases) or else the membrane will clog up with debris. This process is made more efficient by the use of a concentrate turbine that is rigged up to the pump, and is considered much lower maintenance than any of the other systems.

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Sources:

(1) Course notes from a wastewater management class

(2) Answers.com

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