Deionized water vs. reverse osmosis method

A. Nick,

The major problems with RO is the high pressure water that need to be generated and that only about 30% of the water becomes pure - the rest is flushed away. RO water is not as pure as ion exchanged (DI) water and is often used before the ion exchange process. Often RO water is pure enough. RO water can easily become contaminated by biological species if the membrane is not kept clean.

A. Nick, We have been using RO water for several years now for all processes in which water comes in contact with metal parts including finishing and machining processes. As I recall, its cost is supposed to be on the order of 10% of that of ion exchange columns. We are very pleased with the results. It should be remembered that RO leaves about 5% of the salts in the water. In the few cases where RO water is not pure enough (such as for our chemistry laboratory) we use RO water as the feed to the ion exchange column and extend its service life by a factor of 20.

A. Sir,

An ion-exchange plant with an anion exchange and a cation exchange column and a double-bed column gives water of very high purity of up to 0.1 microsiemen/cm conductance if the raw water is also passed through RO.This type of plant I have seen in the processing in semi-conductor manufacture. It is also important that the deionized water is pumped through PVC PIPING and a flash deionizer is used at the user"s end. hope my comments are of some relevance to the topic under discussion.thanks.

A. The standard RO systems available today typically run less than 300 psi and will recover 50-75% of the water removing 95% of the TDS. IX will treat all of the water and with a cation anion produce water from .5 to 5 TDS and a mix bed polisher would yield 15-18 megohm water. The disadvantage with IX is the need for regeneration with acid and caustic and water which all need to be discharged and will contain much higher salt content than the discharge of the RO. The advantage is the ease of operation as long as you have it plugged in and the chemical day tanks filled it will put out water. An RO requires a more robust pretreatment of the water to prevent fouling of the membranes compared to an IX system. However IX will not remove bacteria and water borne parasites where an RO will so really it depends on the quality of water you are looking to produce and the source of the feed water. In many cases it makes sense to use a combination of the systems.

Hope this helps.

Tan, are you doing research for your thesis or do you intend to set up a system?

A. Fundamentals of Deionization by Ion Exchange. Deionization is a method used most often by laboratories to produce purified water on demand. All naturally-occurring water contains dissolved mineral salts. In solution, salts separate into positively-charged cations and negatively-charged anions. Deionization can reduce the amounts of these ions to very low levels through the process of ion exchange. Cations are removed by cation exchange resin. It replaces sodium, calcium, magnesium, and other cations with hydrogen ions (H+). This exchange produces acids which must be removed or neutralized by anion exchange resin. Two general types of anion resin are used for deionization: weak base resin and strong base resin. Weak base resin adsorbs strong acids, while strong base resin exchanges chloride, sulphate and alkaline anions for hydroxide ions (OH-). The hydrogen ions from the cation exchange process combine with the hydroxide ions from the anion exchange process to form water (HOH or H2O). Because the deionization process is so effective, the water quality is usually measured by the water's resistance to electric current (in OHM-cm). Deionized water quality depends on a variety of factors, including raw water composition, ion exchange types and quantities, and the number of resin tanks in the system. Two-bed deionizers use separate tanks, one containing cation resin, the other containing anion resin. A two-bed weak vase deionizer typically produces water with electrical resistance of about 50 kOHM-cm. A two-bed strong base deionizer typically produces water with electrical resistance of about 200 kOHM-cm. In a mixed bed deionizer, cation and anion resins are thoroughly mixed in a single tank. The mixed resins act like a series of alternating cation and anion exchange tanks to produce very high quality water. A mixed-bed deionizer typically produces water with greater than 10 megOHM-cm resistance which is equivalent to less than 0.05 mg/L of sodium chloride. The resins need regeneration when they no longer produce the desired water quality. In the case of a two-bed deionizer, the cation tank is backwashed for 5 to 10 minutes, then washed with a solution of acid. Then the anion tank is backwashed and washed with a solution of caustic. After rinsing the residual chemicals from each tank, water flows through both tanks to drain until the water reaches the desired quality. In a mixed-bed deionizer, the resins have to be separated before regeneration. After regeneration and rinsing, they have to be remixed, using air, before returning to service. Although the process is fairly simple in concept, its application is complicated by different variables.

Ed. note: Hi Ardako, please try your best to post your question in terms of the previous answer ... because we're not sure whether you haven't read it, or you don't accept the previous advice and insist on an alternative, or if you've already separated the resins. Thanks!

A. Hi Sana. A mixed bed DI system can produce water much purer than an RO system . . . no comparison.

Regards,

A. Sorry, Leonardo, I'm not sure what you are asking or trying to suggest. But anodizing is a water-based process, so reverse osmosis can be used to offer cleaner water for make-up of the anodizing bath, and for rinsewater.

Regards,