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Should we use DI vs distilled vs r/o water

We manufacture small stainless steel tubing and we currently use city water to clean with . We are going to switch to DI, distilled or r/o. What is the preferred water for stainless steel cleaning?

Ron Laird
- El Cajon, California, USA

The relative quality of these treatments is pretty much the same. At its best DI, or mixed bed DI, will produce a little better water than RO, but that's assuming better is better and not just interesting.

The capital costs will be much higher with RO than with DI, but the operating costs are lower, and the safety issues are better with RO. Everything in this argument is truly relative and depends on your company's quality, safety, and economic needs.

As for actual information, anything less than about 50 PPM TDS will produce a spot free finish on stainless, and will give excellent results in salt-spray if coated.

Jeff Watson
Jeff Watson
- Pearland, Texas

First of two simultaneous responses --

Either RO or DI water will make a difference. Distilled water is usually only used for pharmaceutical applications. DI water has slightly lower dissolved solids than RO water, typically less than 1-2 mg/L, while RO water will typically have 3-20 mg/L of TDS.

If you want to eliminate water spots on the surface of your products, then you need to remember that it is not just the quality of the water you send to the rinse that matters, It is the quality of the last water that stays on the surface when it dries.

Lyle Kirman
consultant - Cleveland Heights, Ohio

Second of two simultaneous responses --

Jeff is correct. Having experienced both systems (DI & RO) I really, really prefer RO. Not having to deal with regenerations was a tremendous load off my waste treatment and it allowed me to assign personnel elsewhere. The RO practically runs itself and the water quality is at least consistent. My advice though if you are considering ordering one is OVERSIZE the flow you'll anticipate using. Once you have good water you'll be surprised at all the applications around your plant.

Trent Kaufman
Trent Kaufman
electroplater - Galva, Illinois

What's missing in these responses is what water quality specification is required. While Reverse Osmosis typically achieved 98 to 99% reduction of dissolved solids (3-20 mg/L of TDS) mixed bed deionization can achieve 0.036 mg/L. Water in the parts per billion range is achieved via deionization. Also if they are using a relatively small volume of water the economics of portable exchange deionization may be the best choice. Reverse Osmosis may require more pretreatment based on the feedwater than onsite deionization or portable exchange deionization. If they also require water for their quality assurance lab then side polishing of a stream from any of these choices may be called for.

Doug Bibler
- Grandville, Michigan, US
April 10, 2008

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