Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 527 of 707

ods of microfiltration and ion exchange polishing, and closed-loop, zero-discharge methods of reverse osmosis and evaporation. 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 pollu- tants from a wastewater is determined by the type and nature of the pollutants encountered. A basic system will only require simple neutralization and chemi- cal precipitation prior to solids separation for certain, although few, metal fin- ishers. The process use of complexing or chelating agents in production baths would increase system complexity, often requiring two-stage treatment or neu- tralization and the need to apply chemical coagulants or specialty metal pre- cipitants to reduce metal solubility. Other pretreatment processes, including hexavalent chromium reduction and cyanide oxidation, are only required when the plating operation utilizes these com- mon 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 get- ting 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 pro- portionally 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 housekeep- ing 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 com- pensate for surface evaporation losses; using spent process solutions as waste- water treatment reagents (acid and alkaline cleaning baths are obvious exam- ples); 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 impu- rities and prolong the bath's service life. Hydraulic Flow Rates The size and capital costs for wastewater treatment are largely dependent on the 526

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