Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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instantaneous flow rate of wastewater requiring treatment. The major contrib- utor to the volume of wastewater requiring treatment is rinsewater used in the production processes coming in direct contact with the workpiece. The conver- sion to air-cooled rectifiers from water-cooled rectifiers, and installation of chillers and cooling towers for reuse of bath and rectifier cooling water, have large- ly eliminated noncontact hydraulic loadings. Other common practices used to reduce wastewater volume include imple- menting rigorous housekeeping practices to locate and repair water leaks quick- ly; employing multiple counterflow rinse tanks to reduce rinsewater use sub- stantially; employing spray rinses to minimize rinsewater use; using conductivity cells to avoid excess dilution in the rinse tanks; installing flow regulators to min- imize water use; and reusing contaminated rinsewater and treated wastewater where feasible. Negative results impacting treatment system operation, however, have result- ed from zealous water-reduction programs. Rinsewater reductions invariably result in increased contaminant concentrations undergoing treatment, and occasion- ally to problem levels. Increases in alkaline cleaner and chelating chemical con- centrations, in particular, commonly impede conventional treatment, result- ing in poor coagulation and floccuation. Environmental Regulations The stringency of the concentration-based discharge limitations affecting a metal finisher is often the leading criterion in selecting treatment processes and systems. Generally, conventional chemical precipitation systems, perhaps with polishing filtration, are suitable to attain compliance with federal regulations or reasonable local standards. For those firms residing in communities that have adopted local standards with metals limitations ranging from 0.1 to 1.0 mg/L, cost and complexity of the sys- tem can be substantial. Multiple conventional treatment trains in series opera- tions are relatively simple, but effective. Advanced microfiltration, cation exchange polishing, reverse osmosis, and complete evaporation may be necessary to meet stringent standards or totally eliminate the discharge. CONVENTIONAL METHOD OF WASTEWATER TREATMENT To this day, the majority of metal finishers are meeting, or attempting to meet, effluent limitations by treating wastewater by conventional physical-chemical treatment. The process basically involves the use of chemicals to react with sol- uble pollutants to produce insoluble byproduct precipitants, which are removed by physical separation via clarification and/or filtration. Conventional treatment systems often include hexavalent chromium reduc- tion, cyanide oxidation, and chemical precipitation in a neutralization tank. Typically, these steps are followed by clarification. As clarification is not a 100% solids separation device, additional polishing is often required using one of many filtra- tion devices. Increasingly, it is becoming common to eliminate the clarification stage totally, and its polymer flocculation step, in favor of direct microfiltration. The sludge from either separation stage is stored/thickened in a sludge tank, then dewatered via a filter press. Chromium Reduction Chromium in metal finishing is normally used in the hexavalent ion form (Cr6+ 528 ) in

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