Metal Finishing Guide Book


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SYSTEM SELECTION CRITERIA Four major factors contribute to the size, complexity, and cost of conventional wastewater treatment systems. Pollutant Type The complexity of the treatment system needed to effectively remove pollutants from a wastewater is determined by the type and nature of the pollutants encountered. A basic system will only require simple neutralization and chemical precipitation prior to solids separation for certain, although few, metal finishers. The process use of complexing or chelating agents in production baths would increase system complexity, often requiring two-stage treatment or neutralization and the need to apply chemical coagulants or specialty metal precipitants to reduce metal solubility. Other pretreatment processes, including hexavalent chromium reduction and cyanide oxidation, are only required when the plating operation utilizes these common chemicals. Oil separation on a segregative basis may be necessary in facilities where oil and grease concentrations in the combined raw wastewater exceed 200 mg/L. Increasingly, today's metal finishers are modifying processes and getting rid of certain finishes to eliminate problem pollutants and the resultant system complexity, or simply to reduce discharge violations. Over the years, there has been a major industry shift to noncyanide bath finishes. Curbing or modifying the use of complexing chemicals and conversion to trivalent chromium finishes has further reduced system complexity through changes in pollutant type. Pollutant Loading Treatment chemical costs and solids handling equipment sizes/costs increase proportionally to pollutant loading to the wastewater treatment system. Clarification, sludge storage, filter presses, and sludge dryers are sized in accordance to projected loads and solids generation. Increased size requirements result in higher capital equipment costs and higher disposal costs for waste residuals. Proper selection of plating baths with reduced metal maintenance levels and precise control of bath concentrations will reduce loadings. Other common loading minimization practices include implementing a rigorous housekeeping program to locate and repair leaks around process baths, replacing faulty insulation on plating racks to prevent excessive solution drag-out, installing drip trays where needed, etc.; using spray rinses or air knives to minimize solution drag-out from plating baths; recycling rinsewater to plating baths to compensate for surface evaporation losses; using spent process solutions as wastewater treatment reagents (acid and alkaline cleaning baths are obvious examples); using minimum process bath chemical concentrations; installing recovery processes to reclaim plating chemicals from rinsewaters for recycle to the plating bath; and using process bath purification to control the level of impurities and prolong the bath's service life. Hydraulic Flow Rates The size and capital costs for wastewater treatment are largely dependent on the instantaneous flow rate of wastewater requiring treatment. The major contributor to the volume of wastewater requiring treatment is rinsewater used 586

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