RO or DI?

Ask an Expert From: Products Finishing, from Finishing Help, LLC

Posted on: 3/1/2003

Question: What are the differences between RO and DI water for electrocoat?

Question:

What are the differences between RO and DI water for electrocoat? T.T.

Answer:

Both reverse osmosis (RO) and deionized (DI) water are tap waters that are “purified” to remove ions that could affect electrocoat baths and post rinses. The “purity” is measured in micromhos of conductivity, as is the electrocoat bath. Since some pretreatment final rinse water is also carried into the electrocoat bath, RO or DI water is either used as a final fresh rinse going to drain or a “polished” recirculated rinse.

While almost all early (5-10 years old or older) electrocoat systems were set up with DI water, most new systems are using RO water. The benefits of RO water include 1) lower cost of production; 2) no acid or caustic disposal; 3) more consistent quality (less variation in conductivity); 4) lower maintenance cost (fewer components). Because of the normally lower production rates (gpm) of RO, a storage tank is required.

As with any support equipment for an electrocoat system the refined water system varies in cost based on production rate, amount of storage, regeneration/back flush time and other “bells and whistles.” While RO may have a higher initial cost than DI, operation, disposal, and maintenance costs are lower.

The electrocoat material suppliers also have switched their preference to RO from DI during the last few years. The more consistent quality is probably their largest reason. As far as I know, RO works well with all resin systems and both anodic and cathodic systems; however, it is always best to discuss your equipment sources or replacements with your paint and pretreatment suppliers as well as the equipment suppliers.

Older systems that have failing DI systems should probably consider replacing the DI system with a RO system. Fewer moving parts and lower consumable costs (acid and caustic versus salt) will normally offset the addition of a storage tank. Bigger is not always better in looking at production rates or storage tanks. While higher production rates are beneficial during initial fills and when dumping tanks, the higher initial costs may not be offset. Larger storage tanks take up valuable space and create a longer dwell time before use with the possibility of contamination before use. Again, look at both considerations and/or ask for references of other users. Other operations are quite often your best source of independent advice, and they are usually more than willing to share information with other users.



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