Wastewater treatment for electrocoating

Abstract

Because it is an immersion process, electrocoating employs the use of a large amount of water. Typically, pigment, resin, and additives make up only 10 to 15% of the contents of an electrocoating paint tank. Electrocoating is also an extremely efficient coating process due to recycling of paint through ultrafiltration, with usage typically ranging from 95 to 99%; however, the small amount of paint, which may at some point elude deposition, must be extracted from the waste stream prior to discharge. Wastewater treatment strategies for electrocoating-like those for pretreatment stages-are based on three main considerations: (1) removing impurities from the process tanks; (2) retaining useful materials in the process tanks; (3) minimizing the impurities for disposal. Waste streams from other sources, i.e., pretreatment stages, may not be initially compatible for treatment with electrocoating paint wastes. Problems arise often enough to warrant conservative strategies-the most basic of which is to separate the different wastes for differing methods of treatment before combining the resultant waste streams for common disposal (Fig. 1). Cleaning wastes are one category. Zinc and iron phosphate conversion coatings and their rinses are another. The chrome seal and the rinse(s) for that stage are a third category. Finally, there are the E-coat paint wastes themselves. Treatment for pretreatment wastes has been examined elsewhere in this publication. Here, we will consider treatment strategies specific to the electrocoating process.