Metal Finishing Guide Book

2013

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in the production processes coming in direct contact with the workpiece. The conversion to air-cooled rectifiers from water-cooled rectifiers, and installation of chillers and cooling towers for reuse of bath and rectifier cooling water, have largely eliminated noncontact hydraulic loadings. Other common practices used to reduce wastewater volume include implementing rigorous housekeeping practices to locate and repair water leaks quickly; employing multiple counterflow rinse tanks to reduce rinsewater use substantially; employing spray rinses to minimize rinsewater use; using conductivity cells to avoid excess dilution in the rinse tanks; installing flow regulators to minimize water use; and reusing contaminated rinsewater and treated wastewater where feasible. Negative results impacting treatment system operation, however, have resulted from zealous water-reduction programs. Rinsewater reductions invariably result in increased contaminant concentrations undergoing treatment, and occasionally to problem levels. Increases in alkaline cleaner and chelating chemical concentrations, in particular, commonly impede conventional treatment, resulting 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 system can be substantial. Multiple conventional treatment trains in series operations 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 soluble pollutants to produce insoluble byproduct precipitants, which are removed by physical separation via clarification and/or filtration. Conventional treatment systems often include hexavalent chromium reduction, 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 filtration 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+) in plating or chromating. As it soluble at all pH values, the chemical reduction step to its trivalent (Cr3+) form is necessary to ensure removal by precipitation. Commonly, 587

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