Pretreatment Rinse Tanks

Question:

I have a five-stage dip process. The first tank is a cleaner (liquid acid); second tank is a rinse (city water); third tank is a chrome tank (chrome phosphate and fluoride); fourth tank is a rinse (city water); and fifth tank is a rinse (city water). After water has been treated through my waste treatment system, instead of sending the water to the city I would like to use it in my final rinse tank. Would this be a problem? Each of my rinse tanks is continuously overflowed with city water at 6 gpm. J.C.

Answer:

This is a good question that I have been asked in the past. In general, I would not recommend this, but before dismissing it outright, there are a few factors that should be taken under consideration. You should first consider your rinsing needs and how you currently control them. Also consider what all the flows to the treatment system are and if any of these ions could contaminate your system. Since you mention a chrome phosphate pretreatment with fluoride, I assume you are performing a paint pretreatment to aluminum. Your paint type and performance requirements of the coating will help dictate your rinsing needs. Residual contaminant from the rinsing that is left between the paint and the pretreatment can shorten corrosion performance, hurt paint adhesion and decrease paint performance.

You should also consider how you are currently controlling your rinsing systems and strive to maintain the same quality. If not currently controlled, I would recommend that you document the current status of the system. This should involve at least conductivity measurements and, if possible, an analysis for specific contaminants (chloride, sulfate, calcium, magnesium, etc). I would recommend that this be done to the current incoming water, all rinse tanks and the wastewater outflow from your treatment system. There are some wastewater treatment systems that actually produce water of higher quality than the incoming plant water.

The bottom line is that you may be able to use this water in the rinsing system, but you should approach this cautiously. Document the current condition of the line and the performance of the product prior to making any changes, and then proceed to determine if you have adversely affected any aspect of the process and if the system still conforms to required specifications.