- from wastewater, electrotechnologies. Published by the EPRl Center for Materials Fabrication Vol. 7, No. 2, 1991.

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1 Published by the EPRl Center for Materials Fabrication Vol. 7, No. 2, 1991.\ Recycle and Recover Manufacturers of products from cutlery to automobiles to appliances are facing increasingly strict environmental regulations on wastewaters from their metal finishing operations: Electroplating Surface cleaning Machining Surface preparation (painting). M eta1 fin ish i ng wastewater comprises 9.6% of the industrial wastewater produced in the U.S. Typical operations generate anywhere from 200 to 100,000 gallons per day (gpd), primarily from metal plating baths, metal finishing baths, and rinsing steps. Common contaminants in metal finishing wastewater include acid, caustics, cyanide, and metals such as nickel, copper, and chromium. Table 1 shows the contaminants generated by various metal finishing processes. Metal finishers traditionally have treated wastewater by chemical or mechanical means and then have it discharged to the sewer, disposed of in a landfill, or incinerated (see box page5). But because of increasing pressure to reduce environmental hazards, they have begun to modify wastewater treatment techniques. Today many companies have completely changed their approach to waste management: first by eliminating waste at its source and implementing methods to recycle materials within the operation, then exam in i ng wastewater recovery techniques, and finally using wastewater treatment and discharge. Electrotechnologies provide many cost-effective methods to meet regulations for reducing hazardous wastes and contaminants in discharged water. By recovering and recycling pure materials - from wastewater, electrotechnologies can reduce-often by 90% or morethe costs of hazardous waste disposal, processing chemicals (including metals), and water. This issue of Techcommentary highlights cost-effective, electric-based technologies for wastewater recovery and discusses their applications. The best wastewater recovery technology for a specific application will depend on several factors: Type of contaminants present m Concentration of contaminant in the wastewater Throughput Degree of effluent purity required. Although each application requires individual evaluation, general guidelines for screening appropriate technologies are provided in Table 2. Reverse Osmosis Reverse osmosis (RO) is a membrane process which is effective for treating, before discharge, wastewaters with low concentrations of contaminants. A major application is in electroplating operations. As illustrated in Figure 1, RO can form a closed-loop system providing several benefits: Eliminates wastewater generation Recycles chemicals to the plating bath Recovers purified water for the rinse tank. RO saves $3,000 monthly by recovering the brightener chemicals on this tin/lead fluorborate line used for solder connectors on circuit boards. TechCommentaryiVol 7, No 2 1

2 Storaae - Recovered plating chemicals - Production Parts Rinse Figure 7. Closed-loop RO in electroplating. Overflow Rinse tank 1 Rinse tank 2 Rinse tank 3 Filter Reverse osmosis system Purified u rinse water Sources Standard Handbook of Hazardous Waste Treatment and Disposal and Water Technologies nc brochures Contaminated water from the first rinse tank is fed to the RO unit to recover purified water and a concentrate stream The purified water is recycled to the final rinse tank. The concentrate, sometimesafter further processing, is returned to the plating bath at a rate constant with the rate of evaporation. Process-A complete KO system includes membranes in modules, housings, electric pumps, prefilter equipment (when needed), tanks, membrane cleaning solutions, and controls. Common membrane materials include cellulose acetate, effective at ph 2-8, and thin film composites. RO semipermeable membranes pass only water and hydrogen-bonding solvents (the permeate) while retaining ions, salts, and other organic compounds (the concentrate). The RO membranes are built into a spiral-wound module, illustrated in Figure 2. Alternate layers of membrane, porous support, membrane, and spacer wound around a perforated center tube comprise the module. The feed is pumped through the spacer at one end of the unit, and permeate lnfeed Figure 2. Spiral-wound RO membrane module. Concentrate fh Permeate ane passes through the membrane into the center tube. Concentrate flows through the unit and is removed at the opposite end. Permeate recovery rates are Table 7. Wastewater Contaminants Generated by Metal Finishing Processes typically 75-90% but are 9598% in many nickel electroplating processes. To overcome water's natural tendency to migrate back into the concentrate, pumps generate pressures between 400 and 1000 pounds per square inch (psi) to transport water across the membrane. The operating pressure increases in direct proportion to the concentration in the feed. For contaminant concentrations above 1000 parts per million (ppm), economic considerations often dictate whether the allowable operating pressures required for RO are feasible. KO membranes are subject to fouling-particles collecting on the surface requiring cleaning, which increases equipment downtime and can reduce membrane life. New Developments-Cellulose acetate membranes degrade when exposed to extreme ph, which limits their use in the metal finishing industry to nickel electroplating processes (typically operated at ph 4-5). But newly developed membranes function in a ph range of KO systems using special membranes, along with sensors and controls that precisely monitor and maintain operating parameters, may be applied to plating baths of acid copper, Watts nickel, dilute chrome, electroless copper and nickel, and cyanide solutions of cadmium, copper, gold, and zinc. - Economics-Capital cost depends on the capacity needed and the composition of the wastewater. For a nickel electroplating operation, the general guideline is $20,000 + $2,000-$3,000 for each gallon per minute (gpm) of purified water. Operating costs may be $2-$5 per 1,000 gallons of purified water, including membrane replacement costs. Capital costs for RO systems with special membranes range from $40,000 for a -gpm system to $70,000 for a 5-gpm system. A 5-gpm system processing over 1 million gallons per year incurs operating costs of at least $8,000 per year. Special membranes are more expensive to replace, about $3,000 for each occurrence. The frequency of replacement varies with the process: 3-4 months for zinc cyanide plating and 2 years for nickel electroplating. - *Common heavy metals include Cr, Cd, Cu, Pb, Ni, Zn, Ag, Au, Sn, Fe. _1 Ultrafiltration (UF) in the metal finishing industry can be applied in Concentrating oily wastes and detergents from rinse waters Separating solvent from paint solids in electrodeposition paint baths Removing precipitated metal hydroxides from wastewater 2 TechCommentaryiVoi 7, No 2

3 Process-Metal finishers commonly use electrowinning, one type of electrolysis, to produce solid metal. They may also use other forms of electrolysis to destroy organics and change oxidation states of metal ions. n electrowinning, two metal electrodes are immersed in an electrolysis tank containing >% metal solution, and an electric potential is applied. The ions are plated onto the cathode as solid metal, which is used as an anode in a plating bath or stripped from the cathode and reclaimed. Electrowinning recovers copper, nickel, zinc, silver, and other metals. A separate electrolytic unit is required for each metal recovered because different bath conditions (temperature, ph, electric potential) are required. Economics-The cost of equipment to recover 32,000 pounds of nickel per year is approximately $70,000. Annual operating costs are approximately $12,000. Savings in sludge disposal and chemicals and the value of the reclaimed nickel can result in a simple payback of less than one year. A heat pump evaporator becomes an effective recovery method when the wastewater is too contaminated for RO or ion exchange or when these methods produce contaminated byproduct streams that require additional processing. Heat pump evaporators have two typical Table 2. Comparison of Wastewater Recovery Methods Electrotechnology Standard Reverse Osmosisb Special Membrane Reverse Osmosis Heat Pump Evaporators Freeze Crvstallization Ozone Ultrafiltration Electrowinnina n a closed-loop chromate conversion system, this heat pump recycles the rinse water (in holding tank at left) and reduces wastewater volume by 95%. applications to wastewater: closed-loop recovery and volume minimization before disposal. Electroplating, electropolishing, phosphating, or fluorboric acid processes can often be converted to closed-loop systems. n these cases, incorporating a heat pump evaporator into the process recovers water and recycles the concentrated metal solution for use in the operation. For spent electroless nickel plating baths, spent caustic cleaners, and spent acid solutions-any case where the contaminants cannot be recovered easily-a low-temperature heat pump evaporator reduces the wastewater volume by 85-99% The water can then be recycled or discharged and the concentrated sludge reconcentrated for reuse, sold for scrap value, or disposed. Concentration of Concentration of Applicable Contaminants in Contaminants in Contaminants Wastewater" Throughput Recovered H20 heavy metal 1,000 ppm unlimitedc 300 ppm salts heavy metal 1,000 ppm unlimitedc < PPm salts heavy metals up to 10% <10,000 gpd >005 ppm nonvolatile up to organics 100,000 gpd wlspecial equipment organics/heavy > 1% > 10,000 gpd <01 ppm metals 1 heavv metals in <90 1 5,000-M 1 Hz0 gpd <0.1 ppm 10-30% acid cmd cyanides and < 1% upto1 M 0 organics gpd" emulsified oil <1,000 ppm , ppm, and grease + gpd -1 ppm w/filter solids heavv metals > 1 o/o unlimitedc <lo rmm alo,ooo ppm = lolo, Ni electroplatinq example, Capacity is met by linking modules together Process-The equipment required for heat pump evaporators includes vacuum and standard pumps, condensers, compressors, heat exchangers, tanks, and controls. The wastewater is evaporated under vacuum, and pure water vapor is condensed back to a liquid. n a low-temperature heat pump, the water vaporizes at 45-65OF, while the contaminant remains behind as a concentrated liquid. Purified water may be recycled into the manufacturing process and the remaining concentrate can be returned to the process, sent for metal salvage, or disposed. Heat pump evaporators are less effective for wastewaters containing volatile organics or azeotropes (mixtures that vaporize and separate with water). Economics- Heat pump evaporators cost between $50,000 for a 500-gpd unit and $200,000 for a 10,000-gpd unit. Electricity and labor are the primary operating costs. Electricity costs are $0.03 to $0.04 per gallon; labor requirements are 1-2 hours per day. For metal finishing shops that generate more than 10,000 gpd of wastewater with mixed contaminants, freeze crystallization may be an economical alternative to using several treatment methods. Freeze crystallization recovers purified water and concentrates wastewater containing heavy metals, salts, heavy organics, and even volatile organics for recycle back to the process. For example, freeze crystallization recovers the chromelnickellcadmiumlphenol wastewater in a metal finishing shop, replacing cyanide oxid at ion, chrome reduction, metal hydroxide precipitation, organic adsorption on activated carbon, and sludge dewatering. Process-Freeze crystallization requires pumps, compressors, heat exchangers, a crystallizer tank, a crystal separation tank with ice scraper, a melter, and controls. n a mixture, each component freezes at a different temperature, forming crystals that can be removed. The freeze crystallization process separates wastewater by removing heat until the water crystallizes and can be separated. The feed is pumped into the crystallizer or crystal growth tank, heat is removed via a refrigeration system, and the water solidifies. Typically, one component of the wastewater may act as a refrigerant to facilitate crystal growth. Or, if the components have similar freezing points, a liquid refrigerant may be added. The refrigerant vaporizes, removing heat from the wastewater and crystallizing 4 TechCommentaryiVol 7, No 2

4 Economics-The capital cost of an ozone treatment unit ranges from $50,000 for a 5-gpm system to $150,000 for a 60-gpm system. The operating cost is 0.16 cents per thousand gallons, which is less expensive than purchasing water in some parts of the U.S. This pilot scale freeze crystallization unit concentrates nickel and chrome plating chemicals and purifies the rinse water for reuse. the water The liquid-crystal combination, called the slurry, is then pumped to a wash column and separator The water crystals are separated from the concen trated liquid and from other crystalline components mpurities are washed from the crystal surface producing pure crystals The pure water crystals are then recovered for reuse, as is the concentrated liquid Any added refrigerant is also removed and discarded or recycled Economics-The capital cost for freeze crystallization varies with the application and depends on the unit's size and type of construction materials Typical costs may range between $350,000 for a 300-gpd unit and $4,000,000 for a 144,000-gpd unit (100 gpm for 3 shifts) A freeze crystallization unit treating 25,000 gpd of hazardous waste incurs operating costs of approximately $005 per gallon X L Y.-. Ozone treatment is an environmentally sound, cost-effective alternative to chlorine for treating wastewaters containing organics and cyanides, including ferrous cyanide. Ozone destroys organics and cyanide without increasing the total solids in the water and without the toxicity of chlorine. After ozone treatment, water is suitable for reuse or discharge to the sewer. Process-Ozone equipment includes pumps, an ozone generator, an air or oxygen source, a reaction vessel containing UV lamps, an ozone destruction unit, and a filtration system. Dry air or oxygen is subjected to an electrical discharge to convert oxygen to ozone. The ozone gas is pumped to the reaction chamber and mixed with wastewater in the presence of ultraviolet light This process neutralizes the cyanide and organic contaminants Ozone treatment performs best for concentrations below 1 g/l, the heat generated during oxidation of higher concentrations becomes more difficult to control Electrodialysis-An established technologyjust beginning to find applications in U.S. metal finishing operations, electrodialysis is commonly used in Japan to recover metal from rinse waters and regenerate acid solutions. t effectively treats wastewater too contaminated for RO or X recovery and produces a more highly concentrated waste. Electrodialysis uses stacks of cationselective membranes placed alternately with anion-selective ones. An electric potential applied across the cell causes the cations and anions to migrate in opposite directions until halted by a membrane selective for the oppositely charged ion. ons are depleted from one reservoir and concentrated in the adjacent reservoir. n reversible electrodialysis, the polarity of the cells periodi- aced with high waste disposal costs and increasingly strict environmental regulations that limit wastewater effluents and restrict land disposal of hazardous wastes, metal finishers are implementing programs that focus on minimizing the waste generated Waste management programs stress source reduction-preventing or reducing the waste generated during the metal finishing process and typically follow this series of steps Facility Assessment-dentify the process steps where waste is generated and quantify the amounts A plant survey helps collect information on production processes, facility layout, waste stream generation, waste management costs, and raw material costs Analyzing this information helps Determine accurate costs associated with waste dentify and evaluate cost-effective waste reduction techniques Procedure modifications-examine the operational practices that may contribute to additional waste For example Extend the withdrawal and drain times over rinse tanks Orient parts so they do not trap solution when removed from the tank Materials changes-change raw materials to lower the amount or toxicity of the waste produced Some examples follow Substitute steam for acidlcaustic cleaners Replace hexavalent chromium plating solutions with less toxic trivalent chromium Replace cyanide plating solutions with cyanide free solutions Process changes-nstitute process changes or combinations of changes that increase efficiency and reduce waste For instance nstall mixing systems in cleaning and rinse tanks Spray or brush items instead of immersing in process solutions Waste minimization techniques are described in more detail in the sources listed under For More nformation TechCommenfaryiVol 7, No 2 5

5 - Chemical and Mecha-, Conventional methods for treating these waste products include: Heavy Metals-Precipitation techniques are used for treating solutions containing heavy metals. n hydroxide precipitation, lime is added to drastically shift the solution ph to around 12. The metal ions precipitate out as metal hydroxides, which are separated by sedimentation and treated as sludge. When metal complexes are present, chemicals may be added to lower the ph and break up the complexes. Adding a reducing agent and hydroxide to raise the ph promotes precipitation as metal hydroxides, which are separated by sedimentation. Cyanide-The most common method of treating cyanide wastewater is by oxidation with chlorine. Chlorine and sodium hydroxide convert toxic cyanide solutions into nitrogen, sodium chloride (table salt), and harmless sodium carbonate. This method has several disadvantages: total solids content of the water increases, making it unsuitable for reuse yet causing concern with discharge. For complete oxidation, excess chlorine must be present, which is harmful to aquatic life and forms toxic chlorinated compounds with organics that may be present. Precautions also must be taken when storing chlorine to prevent release of deadly chlorine gas. Oils and organics-common mechanical means to remove oils and toxic organics that settle in oils include skimming, coalescing, emulsion breaking, flotation, and centrifugation. These methods work best when the oils are segregated from other wastes. Wastes containing combinations of metals, cyanide, and oils are often treated by adsorption, settling, and volatilization. Sludge-Vacuum filters, centrifuges, or belt presses concentrate sludge and remove water. The sludge bed is dried to reduce the water content before mechanical collection and transport to landfill or incinerator. Process-Like RO, UF uses pressure and semipermeable membranes to separate and concentrate waste from water, but it passes particles up to 0.01 pm and operates at pressures of psi. The basic equipment is similar, although UF membrane modules can be built into a hollow fiber design as well as spiral wound. Common membrane materials are polysulfone and cellulose acetate. n one common hollow fiber design, shown in Figure 3, fibers made of porous material with membrane on the interior are bundled together and sealed in a chamber. The feed is pumped through the tubing, and the permeate flows through the tubing into the chamber. Many fibers are assembled into a a lnfeed t Permeate f- Hollow fibers Concentrate figure 3. Hollow fiber UF membrane module. bundle, creating a large surface area in a minimum of space. n other designs, the hollow fibers are wound in a helical configuration, and the feed flows axially across the bundles. Economics-Capital cost of a UF system also depends on the capacity needed and the wastewater's composition. A 5-gpm system would cost $34,000 and would incur operating costs of approximately $0.04 per gallon. on exchange (X) has two types of applications in metal finishing: separating metals from dilute rinse waters and regenerating acid baths containing metals. X effectively reclaims water from high volume, dilute wastewaters. t separates metal salts from the rinse waters of anodizing, etching, stripping and electroplating operations-including chrome, acid copper, and nickel plating baths. X-processed water can be recycled to the operation or discharged. The resulting concentrated metal-acid solution (regenerant) is often treated by electrodialysis or electrowinning to recover solid metal or metal salts. X also removes metal contaminants from concentrated acid solutions, such as those in pickling operations, for return to the process bath. Process-lX operates by a sorption process using resin beds, pumps, electronic controls, automatic valves, and prefilters. Wastewater is passed through a series of resin beds that selectively remove dissolved metal salt ions. As the ions contact the resin, they displace ions of like charge attached to the resin bed. When the resin bed reaches adsorption capacity, the system switches to a second set of beds, or stops, while the saturated resin is rinsed first with an acid and then with a caustic regenerant. The regenerant flushes the contaminant ions from the resin, restoring the resin bed to its original effectiveness. The metal-rich regenerant is then treated (often by electrolysis) to recover the metal. P Filter 4 Switch on exchange resin beds ' l l h figure 4. on exchange in a plating operation. or discharge Regenerant (either electrowinned or disposed) Economics- on exchange has relatively low capital and operating costs. For a 14,000-gpd system, capital costs are $35,000 for a fully automatic system with two sets of beds. Operating costs are approximately $3,500 per year based on a 150 ppm metal feed stream. Costs for processing or disposing of the regenerant are additional. Electrowinning recovers solid metals either from rinse waters generated by metal plating and etching operations, or from ion exchange regenerant. The solid metal may be used as a plating electrode or sold for reclaim value, while the purified water may be recycled to the operation or discharged. JechCommentaryiVol 7, No 2 3

6 cally reverses, reversing flow through the reservoir to redissolve and purge particles and surface films. Ceramic Membranes-Still in the development phase is the use of ceramic membranes for UF and microfiltration (a process that filters pm particles). Although they are more expensive than polymer membranes, they are expected to last indefinitely. An application expected to become commercial within two years S cross-flow filtration for solid/ liquid separation. Using microfiltration with ceramic membranes could replace the clarifying, filtering, and thickening steps currently required. Within five years, ceramic membranes are likely to be commercially applied to alkaline cleaners. Electrophoresis-Electrophoresis has not yet been used for recovering metal finishing wastewater, although energy and materials costs are considerably less than those for the electrodialysis and ion exchange methods. Potential applications include electroplating and other aqueous solutions. n electrophoresis, the feed is placed between membranes and subjected to an electric potential. Electrodes at the ends of the cell attract charged particles, but membranes allow only ions and water to pass. The information in this issue of TechCommentary gives an overview of the current and developing electrotechnologies applicable for recovering wastewater in the metal finishing industry. By using these technologies whenever practical, managers in the metal finishing industry can reduce Environmental hazards Costs for waste treatment and disposal Costs for water and valuable raw materials. For More nformation The following references may be useful in providing information on wastewater recovery USEPA April 1988 Waste Minimization Opportunity Assessment Manual EPA/625/ Environmental Protection Agency Hazardous Waste Engineering Research Laboratory Cincinnati OH A recommended pro cedure for identifying and implementing source reduction and recycling appli cations including a series of worksheets that guide plant personnel through the process of identifying and evaluating promising waste minimization strategies ncludes methodologies for analyzing the economic and technical feasibility of potential options Freeman H M Standard Handbook of Hazardous Waste Treatment and Disposal New York McGraw Hill Book Company 1989 Contact your state environmental agency about its assistance programs The EPRl Center for Materials Fabrication (CMF) is an R&D Applications Center sponsored by the Electric Power Research lnstitute (EPR) and operated by Battelle EPR, a nonprofit organization, encourages new and improved technologies io help the utility industry meet present and future electric energy needs in environmentally and economically acceptable ways CMF s mission is to assist EPR members and their customers (metal plastics ceramics composites and wood fabricators and processors) in implementing cost effective electrotechnologies that improve efficiency productivity and address environmental issues TechCommentary is one communication vehicle that the Center uses to transfer technology to industry through an electric utility network The Center also conducts applications development projects that demonstrate innovative uses of electrotechnologies This issue of TechCommentary was made possible through the cooperation of Fred Steward and Stan Karrs Alcoa Separations Technology nc Val Partyka CALFRAN nternational nc Jim Heist Freeze Technologies Steve Michels Osmonics Bill Martin Ozone Processes nc and Tom Von Kuster and Joel Malmberg Water Technologies nc The sources used in this issue of TechCommentary were California Department 01 Health Services Metai Waste Management Aiternatives 1989 Symposium Proceedings Center for Hazardous Materials Research University of Pittsburgh Applied Research Center Pittsburgh PA Hotline Cherry, K F, Rich, G "Electroplating Waste Reduction," Hazardous Waste Minimization in Ohio A Statewide Conference Series June 27-30, 1989 Davis. M W, Sandy, T "Zero SludgeiZero Discharge Pretreatment Systems tor the Metal Finishing and Plating ndustry," 44th Purdue industria/ Waste Conference Proceedings, West Lafayette, N, May 9-11, 1989 Estey, P, Hampton, H, Sedfidpour, S "Electrotechnologies for Waste and Water Treatment," EPRl Report EM~5418, October 1987 Hunt, G E "Waste Reduction in the Metal Finishing ndustry." JAPCA Vol 38, pp Jacobs Engineering Group, "Reducing California's Metal- Bearing Waste Streams," California Department of Health Services, August 1989 Salas, A C, Cunha, H J "Reduction of Costs and Liability Risks in Electroplating wastewater Treatment," industria/ Water Engineering. Vol 21, No 2, 1984, p 9-13 Sedfidpour, S "Separation and Concentration Technologies for ndustrial Wastewater Treatment," EPRl Report RP-22662~4, October, 1989 Steward, FA, McLay, WJ "Waste Minimization, Metai Finishing. August-December 1985 TechCommentary, Vol 1, No 2, 1988 Membrane Processes EPRl Process ndustry Coordination Office TechCommentary, Vol 1. No 1, 1988 Freeze Concentration EPRl Process ndustry Coordination Office Photos courtesy of Water Technologies. lnc, Freeze Technologies. and CALFRAN nternational, nc Applicable SC Codes All SCS are applicable Members are weicome to quote verbatim from /his publication provided CMF is crediied CMF approvai is required for editorial incorporation as pertaining to US copyright iaws For ordering information, call EPRl's Affiliate Member Program AM P For technical information, contact CENTER FOR MATERALS FABRCATON An EPRl R&D Applications Center 505 King Avenue Columbus Ohio (614) Copyright 1991 Electric Power Research institute Palo Alto CA 6 TechCommentaryiVol 7, No 2