Pollution Control for Anodizing

Q. I plan to install a small aluminum anodizing operation with a 500-amp rectifier. What pollution control equipment do you recommend? N.S.

A. We recommend three types of pollution control equipment: air pollution, secondary containment and wastewater pretreatment.

Whether or not your small anodizing operation needs an air pollution permit, we recommend a de-mister with periodic water wash down or a scrubber, because the anodizing tank emits a mist containing sulfuric acid due to electrolysis. Without these control devices, you will be spewing sulfuric acid mist outside your building.

We also recommend secondary containment with a protective coating surrounding the anodizing operation. At least 110 percent of the largest tank should be within the containment, which will protect you and your operations against leaks and other equipment failures.

Although we do not know your exact proposed anodizing process, especially the proposed sealing (hot water, chromate, nickel acetate, etc.), nor the discharge limits, we can give you some general wastewater guidance.

At a minimum, pH adjustment will be needed. Due to the overall acidic nature of the process, only caustic addition is likely needed to raise pH. If you plan to use chromate or nickel acetate sealing, you usually can eliminate treatment for these rinses by using three to four counterflow rinse tanks after the process tank, where the rinse water is used for evaporation makeup of this hot process tank.

Depending on the aluminum alloys that you plan to anodize, you may need to treat wastewater for metal removal in order to meet the Metal Finishing New Source Pretreatment Standards (40CFR433) and local limits, if more stringent. If this is the case, the typical pretreatment protocol, whether batch or continuous treatment, is holding/equalization, pH adjustment, coagulant addition, polymer flocculant addition, clarification or filtration, and sludge handling/dewatering.