Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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they will carry more TDS over a given period of time than bath drag-out. In a similar fashion the use of segregated closed-loop treatment rinses allows the first station of the rinse system (drag-out tank) to be as high as 10 to 15% of the TDS of the process bath, greatly extending the opportunity to recycle subsequent higher quality rinses. There is increasing interest in this country to further close the loop by desali- nating a treated effluent for maximum recycle and reuse. A number of large plants have been constructed with all of the TDS being concentrated into a small vol- ume of brine, which is hauled from the plant. While this may be necessary and economical in some cases it is not logical for most cases. Unless the plant is located near a seacoast, disposal of the brine is like- ly to be problematic. It is highly corrosive to concrete and steel structures and more difficult to assimilate in the environment than a high volume effluent at 1,000 mg/L TDS. The real answer lies in reducing the consumption of chemicals in the metal-finishing operation and thus the quantity of TDS requiring discharge. For situations where desalination and recycling of a treated effluent is desir- able or necessary the following treatment technologies can be considered. Ion Exchange Recycling of metal-finishing wastewater through ion exchange equipment has been practiced for decades in Germany and for many years in Japan. Practical expe- rience shows the need for segregated collection and treatment of not only batch dumps but also the first rinse after each process that flows at a rate to take away approximately 90% of the chemical load. Secondary and/or tertiary rinses can then be recirculated through ion exchange equipment after very thorough particulate filtration and carbon filtration. Cyanide and hexavalent chromium are problematic because they are poorly released from the anion exchange resins and tend to exist as perpetual low-lev- el contaminants throughout the plant's rinsewater system. Aside from high cost, the major drawback of this approach is that it actually increases the TDS discharge from the plant. In theory, if regeneration of ion exchange resins could be perfectly efficient, the process would multiply the TDS removed from the recirculated water by a factor of two. In practice, however, a 100 to 300% excess of regenerant chemical is typically required. This can be reduced to the range of 50 to 100% excess by holding and reusing certain fractions of the regenerant waste stream at the cost of additional capital investment and operating complexity. As a result of this need for excess regenerant, the TDS removed from the recirculating rinsewater is multiplied by a factor of three to six. Since it is the TDS that presents the problem for the environment and not the water, this approach does not hold long-term promise for the metal-finishing industry. In Germany, the population density has exacerbated the problem with TDS accumulating in the rivers. Practicing water chemists now recognize the counterproductive nature of this treatment process. Where either waste heat or reliable solar energy are available, vacuum evaporation or multistage vacuum distillation can be an attractive alternative for producing clean water. Capital costs are high but the ability to concentrate the brine is vir- tually unlimited and the equipment is rugged and reliable. Evaporation/Distillation Reverse Osmosis RO technology has been refined and extensively applied to the desalination of sea 560

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