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Pollution Prevention and Control Technologies for Plating Operations
Section 1 - Overview of Project Results
1.2 Overview of Users Survey Results
1.2.4 Pollution Prevention
1.2.4.1 Drag-Out and Rinse Water Reduction
1.2.4.2 Chemical Recovery Technologies
1.2.4.3 Solution Maintenance Methods and Technologies
1.2.4.4 Substitute TechnologiesVarious terms are used to group or categorize the different methods employed by industry in their pollution control strategies, e.g., pollution prevention, source control, waste minimization, and in-plant changes. One popular term used extensively by EPA since 1988 is pollution prevention. This term is defined by EPA as the maximum feasible reduction of all wastes (wastewater, solid waste and air emissions) generated at production sites. It involves the judicious use of resources through source reduction, energy efficiency, reuse of input materials during production and reduced water consumption. Pollution prevention covers a broad range of pollution control methods employed by plating shops, such as drag-out and rinse water reduction, chemical recovery from rinse waters, bath maintenance, and material substitution. Each of these aspects of pollution prevention have been addressed by the Users Survey. An overview of the survey results are presented here and details are contained in Sections 2, 3, 4 and 5.
For platers, pollution prevention has emerged as an important set of tools used to help attain compliance and reduce operating costs. Widespread success has been achieved using simple methods and techniques that reduce drag-out losses and rinse water use. More than 90 percent of the shops indicated that they utilize these tools and have benefited from them. Although some shops have had great success with chemical recovery technologies, these have generally been much less frequently applied than drag-out and rinse water reduction efforts. The most successful of the chemical recovery technologies is atmospheric evaporation, which is also generally regarded as the least technically sophisticated recovery technology. Bath maintenance technologies are less frequently purchased than are chemical recovery technologies, but have generally been more successful. Exhibit 1-12 shows ratings given by the respondents for some common pollution prevention methods. The following subsections provide overviews of key pollution prevention topics.
1.2.4.1 Drag-Out and Rinse Water Reduction
For the typical electroplating job shop, the drag-out of process solutions and the subsequent contamination of rinse waters are the major pollution control problems. Section 2 contains an explanation of the basic principles of drag-out theory and explores the function and applicability of the various drag-out minimization techniques in use today. Because of the importance of drag-out and drag-out loss prevention, numerous questions in the Users Survey were related to this topic. The responses to these questions are statistically evaluated in Section 2 and summarized in this subsection.
The Users Survey asked respondents to indicate and rate the usefulness of the methods that they employ to reduce the formation or loss of drag-out and the usage rate of rinse water. A summary of their responses is presented in Sections 2.4 and 2.5. The most frequently used drag-out reduction methods are: allowing parts/racks to drip over process tanks; the use of drag-out rinses; reducing the speed of rack/part withdrawal; use of drip shields; and positioning the workpiece to minimize solution holdup. On the average, all of the drag-out reduction methods that are used by the respondents have been successfully applied. Some shops had specific problems with one or more methods (e.g., buildup of bath contaminants). These problems are discussed in Section 2 along with potential solutions.
The most frequently used methods of reducing water use involve the application of: flow restrictors; counterflow rinses; manually turning off water; and air agitation. As with drag-out methods, the rinse water reduction methods have been generally successful, with the highest success ratings given to the use of flow restrictors and counterflow rinsing.
1.2.4.2 Chemical Recovery Technologies
Chemical recovery technologies are used by platers to separate plating chemicals from rinse waters or other solutions or to concentrate them, thereby making them available for reuse/recycle. According the respondents of the Users Survey, chemical recovery technologies are most frequently purchased to (in order of frequency): help meet effluent regulations; reduce wastewater treatment costs; reduce plating chemical purchases; and reduce the quantity of waste shipped off-site.
The Users Survey requested platers to provide detailed technical, performance and operating cost data for chemical recovery technologies. Also, during their survey, vendors were requested to provide technology descriptions, operating data and capital cost data. As a result of obtaining data from these two sources, plus the information from the extensive literature review, this text contains a substantial quantity of information for the following chemical recovery technologies: electrodialysis, electrowinning, atmospheric evaporators, vacuum evaporators, ion exchange, reverse osmosis and meshpad mist eliminators. A separate subsection of the report is devoted to each of these technologies. Within each subsection, the following are provided: technology overview; development and commercialization; applications and restrictions (with diagrams showing different potential configurations); technology/equipment description; capital costs; operating costs; performance experience; and residuals generation.
Exhibit 1-13 presents a summary of the chemical recovery applications addressed by the Users Survey data.
Many installations of chemical recovery technologies and advanced bath maintenance (see Section 1.2.4.3) have not been successful (approximately 30 to 40 percent). The survey respondents indicate that failure is most frequently caused by: maintenance problems, misapplication of the technology (often due to ignorance on the part of manufacturers' representatives and/or the plating shop personnel), poor design, inability to purchase replacement parts (usually manufacturer went out of business), poor technical support by manufacturers, improper operation of technology by shop personnel, technically too complex for employees, chemical recovery caused a build-up of contaminants in plating bath, recovery process destroyed plating chemicals, recycled water was of insufficient quality, chemical product was insufficiently concentrated for return to plating bath, inadequate capacity, and high residuals generation.
Maintenance problems were the most frequent cause of system failure. The maintenance problems most often reported with advanced technologies are: low quality system components, mechanical problems with pumps and valves, damage to or fouling of components by plating chemicals, and excessive labor requirements for system cleaning.
Exhibit 1-14 indicates the operational status of the chemical recovery and bath maintenance technologies purchased by survey respondents.
1.2.4.3 Solution Maintenance Methods and Technologies
Metal finishing solutions are subjected to a variety of forces that cause them to become unusable. The key contributing factors are: (1) depletion of bath chemicals; (2) chemical break-down of process chemicals or chemical side reactions; (3) contamination from impurities in make-up water, chemicals or anodes; (4) anodic/cathodic etching of parts and inert electrodes; (5) corrosion of parts, racks, bussing, tanks, heating coils, etc.; (6) drag-in of non-compatible chemicals; (7) buildup of by-products (e.g., carbonates); (8) breakdown of maskant, fume suppressant and wetting agents; (9) errors in bath additions; and (10) airborne particles entering the tank.
Solution maintenance replaces the practices of: (1) using a fresh chemical solution until it is degraded and replacing it with fresh solution or (2) decanting a portion of a degraded solution and replacing it with fresh solution. In both cases, the spent solution is usually either treated on-site or transported to a treatment/disposal site. On-site treatment is not always possible because concentrated wastes may upset treatment facilities designed primarily for treating dilute rinse waters.
Two major categories of solution maintenance were identified during the project: preventative and corrective. Within this text, preventative solution maintenance refers to the practices that avoid bath contamination or involve monitoring and adjusting of solution chemistry. Corrective solution maintenance refers to the practice of removing contaminants from the bath, whether they are dissolved or particulate, organic or inorganic. Both preventative and corrective solution maintenance involve the use of methods, techniques and technologies. Methods and techniques are typically procedural in nature or low capital items that can be implemented quickly and have an almost immediate payback. Technologies are generally equipment packages that have a moderate to high capital cost and payback periods of one year or greater. Most preventative measures are either methods or techniques. Corrective measures include both methods/techniques such as dummy plating and technologies such as microfiltration.
Within this text, the corrective technologies, which are generally less familiar to platers, are covered in detail. The methods of preventative and corrective solution maintenance that are commonly applied by plating shops (e.g., filtration) are more familiar to platers and therefore are covered less extensively. The bath maintenance technologies addressed by the project include: microfiltration, ion exchange, acid sorption, ion transfer, membrane electrolysis and diffusion dialysis. Exhibit 1-15 presents a summary of the corrective technology use by respondents to the Users Survey.
1.2.4.4 Substitute Technologies
The results of the Users Survey show that respondents have made significant strides in reducing or eliminating the use of chlorinated solvents, cadmium, cyanide and chromium. Sometimes referred to as the four Cs, these materials have been identified by EPA as key targets for control within the metal finishing industry. Approximately 60 percent of the respondents attempted material input changes that potentially reduce or eliminate the use of one or more of the four Cs or another pollutant problem. Based on the comments received from respondents, these changes were made in an effort to reduce the impacts of their processes on the environment and worker health, to help meet environmental regulations and to reduce operating costs.
Although most of the material input changes attempted by survey respondents have been successful, there have been some failures and in many cases, even with successful changes, there have been adverse production impacts. Section 5 summarizes the status of change in these areas and conveys the attitudes and concerns of the respondents.