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Deionized water vs. reverse osmosis method

Q. Please provide information concerning deionization of water by reverse osmosis method. What are the advantages / disadvantages of this method vs. ion exchange resins method.

Nick Golas
- Athens, Greece

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.

Donald M. Mattox
Society of Vacuum Coaters
Albuquerque, New Mexico 1999

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.

Aryeh Asher
- Ashkelon, Israel

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.

- Bangalore, KARNATAKA, India

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.

John Ring
- Wheaton, Illinois

Q. Hi. Basically, I am doing research on application of deionized water. If anyone out there have any information, I would very grateful if you could respond . Please breakdown into the following category below:

1) Type of Industry:
2) Country:
3) Consumption per month:
4) Availability in your state:
5) Cost involved: Best regards.

Kheng Hock, Tan
electronics - Penang, Malaysia

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

Ng John
- Kuala Lumpur, Selangor, Malaysia
March 31, 2011

Q. I am looking for some information or an article on the basics of DI water. How does it work? What is Ion exchange? What is an Ion, anyway? How does this technology compare to RO and water are the criteria for selecting either technology?

Thank you,
Tom Morgan

Thomas J Morgan
- Princeton, Indiana

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.

Hamid Salari
- Rafsanjan, IRAN

Q. How can we regenerate mixed bed resins? Can you tell me. Thanks for all.

Ardako [last name deleted for privacy by Editor]
- Balýkesýr /TURKEY

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!

Q. I've read about the process to regenerate two bed systems as well as mixed bed and the requirement for separating the cation and anion resins. Can someone explain in more detail how mixed bed regeneration is accomplished in a single vessel? Is the acid wash and caustic wash washed through both resins?

Ron Curry
product designer - Pleasanton, California
April 12, 2008

Q. Please provide information about demineralization by ion exchange.
What is different process between single stage ion exchange (anion exchanger and cation exchanger bed) with mixed bed (multistage ion exchange)?
When we must use mixed bed?
Thanks for you attention.


Lelywaty Simamora
process engineer - Bontang, East Kalimantan, Indonesia
September 23, 2008

Q. I am looking for some information or an article on the basics of DI water. How does it work? What is Ion exchange? What is an Ion, anyway? How does this technology compare to RO and water are the criteria for selecting either technology?

Sohail Khan
- Karachi, Pakistan
April 9, 2009

Q. Please tell me either resins or reverse osmosis can give more pure water?

Sana Ullah
- Faisalabad, Pakistan
March 15, 2013

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


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
March 15, 2013

Q. I wonder if the precision of reverse osmosis is advised in the use of precision anodizing and coloring of anodized aluminum. Does anyone have any experience even if this will work?

I hope to have help from someone,

Thank you,

Leonardo Villalobos
technical - Sao Paulo - Brazil
May 8, 2013

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.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
May , 2013

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