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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.5 ALTERNATIVE TECHNOLOGIES FOR METALS REMOVAL
6.5.2 Ion Exchange
6.5.2.1 Overview
6.5.2.2 Development and Commercialization
6.5.2.3 Applications and Restrictions
6.5.2.4 Technology/Equipment Description
6.5.2.5 Costs
6.5.2.5.1 Capital Costs
6.5.2.5.2 Operating Costs
6.5.2.6 Performance Experience
6.5.2.7 Operational and Maintenance Problems
6.5.2.8 Residuals
This section deals with ion exchange as an end-of-pipe technology. Ion exchange is a versatile pollution prevention and control tool for the metal finisher. It is utilized for raw water treatment, metals recovery, water recycle, bath maintenance and end-of-pipe treatment. Use of ion exchange for metals recovery/water recycle and process solution maintenance are discussed in Sections 3 and 4, respectively.
Approximately 6% of the respondents to the Users Survey use ion exchange as an end-of-pipe technology. Several different configurations are being employed by survey respondents and these are discussed in Section 6.5.2.3.
A general description of the ion exchange process is presented in Section 3.
6.5.2.2 Development and Commercialization
The development and commercialization of the ion exchange technology is discussed in Section 3. No direct references were found in the literature to indicate when ion exchange was first applied as an end-of-pipe technology. However, it can be assumed from the fully developed nature of the technology and the need to meet local discharge standards in certain locations of the U.S. that the technology was applied for this purpose in the 1950ís. The earliest application from the Users Survey was 1972 (PS 161). The EPAís regulatory document for the metal finishing industry indicated that 63 of the plants in their database used ion exchange (ref. 386). Their data collection efforts were performed in the late 1970ís and early 1980ís. The EPA document is unclear as to the total number of plants included in the database and therefore the percentage of shops that used ion exchange at that time cannot be calculated. However, as a comparison, EPA indicated that 154 of the plants in their database used hydroxide precipitation, a very common end-of-pipe technology. This would indicate that ion exchange had a very prominent role in pollution control at the time of the EPA study. (Note that a portion of the ion exchange users in the EPA report may have used the technology for raw water treatment, metals recovery or bath maintenance. EPA does not specify which applications were used by the shops in their database.)
Ion exchange is a very important end-of-pipe technology for shops utilizing the services of a centralized waste treatment (CWT) facility. Ion exchange resin beds can be shipped to a CWT facility more economically and safely than liquid wastes. CWT systems that utilized ion exchange were present in Europe and Asia in the 1970ís (ref. 387). Small U.S. operations were present in the 1970ís; however, the only major U.S. CWT plant (Minnesota Recovery Systems) was not operating until 1988 (ref. 419 ).
The Users Survey indicates that the role of ion exchange as an end-of-pipe treatment technology has continued through the 1980ís and into the 1990ís. Several shops have installed end-of-pipe equipment after 1990. Also, several of the respondents are using CWT services for regeneration of the columns, including four shops using the new CWT plant in Minnesota.
6.5.2.3 Applications and Restrictions
The shops that are discussed in this section use ion exchange for treatment of a mixed metal waste stream and/or they use it as the sole end-of-pipe method of meeting effluent limitations for metals.
In some cases, it is difficult to differentiate between recovery and end-of-pipe treatment applications of ion exchange and therefore several shops are discussed in both Section 3 and this section. These are shops that treat a single metal wastestream using ion exchange, but do not recover the metals on-site. Rather than recovering the metals on-site, these shops use a centralized waste treatment system for regeneration or they regenerate on-site and send the regenerant to an off-site metals recovery facility. In either case, their ion exchange system is the sole means of metals removal from their wastewater (i.e., no conventional treatment is present). Shops that fall into this category include: PS 061, PS 161, PS 136, PS 192, and PS 196.
Some shops in the Users Survey that employ ion exchange without any chemical recovery were placed solely into the recovery category (i.e., Section 3.4). This is a group of shops that does not perform chemical recovery; however, they treat a single metal rinse water using ion exchange, and therefore there exists a high potential for recovery. Also, each shop in this group has another technology present for end-of-pipe treatment (e.g., hydroxide precipitation). Because ion exchange is not used as their primary treatment system, this group of shops is only included in Section 3. As a result of this grouping, readers interested in ion exchange treatment should review both sections to benefit fully from the results of the Users Survey.
Ion exchange is employed as an end-of-pipe technology using three common schemes, as depicted in Exhibit 6-27. Variations of these three configurations were also observed. The first configuration employs ion exchange to remove the metals from large volumes of miscellaneous wastewaters and concentrate them into a smaller volume for subsequent treatment by conventional means (see application IXEOP-1). This was the most frequently used method of end-of-pipe ion exchange treatment by respondents to the Users Survey (PS 021, PS 036, PS 089, PS 121, PS 170, PS 187, PS 200, PS 221, PS 223, PS 209). This strategy reduces the capacity requirement of the conventional system and therefore may result in capital savings. With this configuration, all dilute rinses are sent to integrated ion exchange units (see definition in Section 3). The number of units needed will depend on the flow rate and the mixture of pollutant parameters present. As with conventional treatment, segregation of cyanide, chromium bearing and miscellaneous metal bearing rinse waters is needed. Pretreatment of the rinses may include one or more of the following processes: pH adjustment, cyanide destruction, filtration and carbon treatment. The pH adjustment step, which was performed by most shops using this configuration, is needed to ensure that the pH is within the operating range of the resin. Cyanide destruction, which was used by only one of the survey respondents (PS 223), will reduce the number of steps of the ion exchange process when cyanide complexes are present (see Section 3, Section 3.4.3 for a discussion of ion exchange treatment of cyanide-metal wastewaters). Filtration and carbon treatment, which are the most common pretreatment technologies, are needed to remove suspended solids and organics that may foul the resin beds. If water recycle is desired, the ion exchange system must consist of both anion and cation columns. Cation only systems can be used if water recycle is not desired and there are no chromates or cyanide present (these two pollutants are removed by anion resins). Concentrated wastes generated in the plating shop (e.g., batch dumps), should be discharged directly to the batch treatment system, rather than into the ion exchange feed stream. This practice prevents overloading of the ion exchange beds. Also, it should be noted that ion exchange is simply not a good technology for the treatment of concentrated wastes. If used for this purpose, the regenerant may be less concentrated than the original waste.
From the Users Survey, 11 respondents (or 3.5% of all respondents) utilized application IXEOP-1 for treatment of mixed metals waste streams. Of these, 7 shops recycle the water treated by ion exchange to rinse tanks (PS 021, PS 036, PS 170, PS 185, PS 187, PS 200, PS 209) and 4 shops do not recycle water, but rather discharge it directly to a sewer (sometimes after pH adjustment) or further process it with additional metals removal treatment (e.g., hydroxide precipitation) and then discharge it (PS 089, PS 121, PS 221, PS 223).
Shops that employed application IXEOP-1 with water recycle generally had a lower than average discharge rate. The average for all shops in the survey (318 shops) was 34,600 gpd and the average for the 11 shops using application IXEOP-1 was 10,064 gpd. One the shops using ion exchange for water recycle attained zero-discharge of effluent (PS 021). This shop batch treats regenerant in a 1,500 gallon tank (chromium reduction and metals precipitation), transfers the liquid waste to an atmospheric evaporator for concentration and dewaters the concentrate and sludge with a filter press. Approximately 1,100 lbs of sludge (20% solids) is sent off-site per year.
In the second configuration (application IXEOP-2), ion exchange is used as a polishing technology to remove residual pollutants following conventional treatment. Such applications are most common to facilities that directly discharge wastewaters to rivers or streams and are required to meet stringent metals limitations (ref. 39). Respondents to the Vendors Survey indicated that they have installed 21 polishing systems. However, only one Users Survey respondent used ion exchange in this manner (PS 068). The regulations for this particular plating and anodizing shop are below the 40 CFR 433 limitations for most parameters. PS 068 does not regenerate their spent resins on-site. Rather, they send them off-site for incineration and metals recovery, which is an unusual practice for this ion exchange application, especially considering the moderately high cost of resin. Their spent resins contain nickel, copper, silver and zinc. Presumably, the quantity of silver present in their resins justifies this practice.
IXEOP-2 is not a good configuration to use for recycling water. Water recycling requires the use of both anion and cation columns (i.e., deionization). Because the wastewater treatment process adds such an abundance of sulfates and chlorides to the wastewater, it is economically impractical (except under unusual conditions) to recycle it using ion exchange. The loading on the anion column and the resultant regeneration frequency would simply be too great. If water recycle is the goal, it is better to segregate wastewaters and recover the water as shown in application IXEOP-1 (ref. NAPCO file).
Ion exchange polishing applications generally use metal selective cation resins that have a high preference for heavy metal cations over the more common alkali or alkaline earth cations such as sodium, calcium, magnesium and potassium that are added by the conventional treatment process.
In the third configuration (application IXEOP-3), ion exchange is used as the primary end-of-pipe treatment for segregated metal and cyanide bearing rinse waters and the resin columns are sent to a CWT facility for regeneration. Because the wastestreams were segregated before ion exchange, the regenerants contain metals that can be recovered by relatively easy means (e.g., electrowinning). Application IXEOP-3 is based on sketches submitted by PS 192 and PS 237. Other shops using CWT had similar configurations (PS 136, PS 249), with only one significant variation, where resins were regenerated on-site and the regenerant was sent to an off-site metals recovery firm.
6.5.2.4 Technology/Equipment Description
This subsection contains a description of commercially available ion exchange equipment that is manufactured and/or sold by vendor survey respondents. This is intended to provide the reader with information and data on a cross section of available equipment. Mention of trade names or commercial products is not intended to constitute endorsement for use.
Descriptions of ion exchange system designs and commercially available equipment are presented in Section 3.4.4. Systems for applications IXEOP-1 and -2 will typically be integrated units and modular units are used for IXEOP-3. One additional commercial system is described in this section that is advertised specifically as end-of-pipe treatment. It would be employed as shown in Exhibit 6-27, application IXEOP-1.
Memtek Corporation manufactures the Rinse Water Maintenance System (RMS™), which is advertised as an end-of-pipe treatment/water recycle system. The unit has duplex cation and anion beds, employed in a parallel/standby mode of operation (see Section 3, Section 3.4.1.3 for definitions). It is advertised to be applicable to chromium, cyanide and acid/alkaline rinse waters. Optional equipment includes a batch treatment system for regenerants as well as an atmospheric evaporator or electrowinning unit for further processing the regenerants.
Capital costs for end-of-pipe ion exchange equipment will be similar to those presented in Section 3, Section 3.4.5.1 for integrated and modular systems.
Operating costs for end-of-pipe ion exchange equipment will be similar to those presented in Section 3.4.5.2 for integrated and modular systems. End-of-pipe applications may experience higher resin costs (approximately 25%) due to a shorter life span and higher fines loss (ref. Kinetic Recovery Corporation file).
Several respondents to the Users Survey provided operating cost data for using a CWT service (PS 161, PS 192, PS 237). The following data were provided by these respondents.
PS 161 leased their ion exchange columns that contain 3.2 ft3 of mixed (anion and cation) resin. During 1992 PS 161 sent 18 nickel columns and 8 chromium columns to Dayton Water Systems. The cost of the service was $336 per column ($105/ft3) for regeneration plus a $70 per month rental fee for the equipment (therefore, the total annual cost for 1992 is $6,888 for nickel and $3,528 for chromium). According to the respondent, Dayton Water Systems sent the regenerant to Inmetco for recovery of the metal (see Section 7 for information on off-site recycling).
PS 192 and PS 237 use the Metropolitan Recovery Systems CWT facility in Minnesota. Their operating costs are shown in Exhibit 6-28.
6.5.2.6 Performance Experience
Different sections and questions were contained in the Users Survey form for recovery (Section 5 of the survey form) and end-of-pipe systems (Section 6 of the survey form). Although respondents that use ion exchange for end-of-pipe were not asked to complete Section 5 for these applications, all of them did. Therefore, there are more data available for ion exchange end-of-pipe applications than for than for most other end-of-pipe applications.
A summary of the data for ion exchange applied to end-of-pipe treatment is presented in Exhibit 6-29.
The following information and data summarize the performance experience of the survey respondents.
- The performance of ion exchange, as an end-of-pipe treatment technology, was rated approximately the same as for recovery applications. The average satisfaction level for ion exchange applied to end-of-pipe treatment is 3.5 for IXEOP-1, 5.0 for IXEOP-2 (only one respondent) and 3.4 for IXEOP-3, whereas the average level rating for ion exchange used for chemical recovery is 3.5. Fourteen of the shops (or 74 %) indicated that this technology satisfied the need for which it was purchased (includes all applications), 4 (or 21%) indicated that it partially satisfied the need for which it was purchased and 1 (or 5%) indicated that it did not satisfy the need for which it was purchased (these data are not shown on Exhibit 6-29, but can be found in the database). The following is a breakdown of the reasons why shops purchased this technology (most shops had multiple reasons):
To meet or help meet effluent regulations: 15 To reduce the quantity of waste shipped off-site: 9 To reduce wastewater treatment costs: 11 To improve product quantity: 6 To reduce or eliminate effluent discharge: 3 To improve rinse water quality: 1
- The use of ion exchange for water recycle generally did not impact production quality or the rate of production. The following responses were provided (only for shops that recycle water):
Production Rate Product Quality Improved 1 1 No Change 10 8 Decreased 1 2
- Product quality was decreased for PS 121 and the production rate was decreased for PS 121 and PS 185. No explanations were given for these impacts.
- Based on their experience with this technology, 37% indicated that they would purchase the same technology from the same vendor. The following is a breakdown of all responses:
- Purchase the same technology from the same vendor: 7 (37%)
- Purchase the same technology from a different vendor: 4.5 (24%)
- Purchase a different technology: 6.5 (34%)
- Do nothing: 1 (5%)
- Two shops indicated that an ion exchange unit applied to end-of-pipe treatment was the cause of an effluent compliance excursion (PS 061, PS 136). Three shops did not respond to the question (PS 185, PS 187, PS 223).
- The major savings for this technology was water use reduction. PS 209 reported the greatest water reduction. In their survey form, PS 209 indicated that they achieved a flow reduction of 20,000 gpd since 1988 (current average flow is 11,500 gpd).
- The average plating shop effluent discharge for all shops that use ion exchange for end-of-pipe treatment and water recycling (i.e., application IXEOP-1) is 10,064 gpd. The average for all shops responding to the survey (318 shops) is 34,600.
6.5.2.7 Operational and Maintenance Problems
Reported O&M problems for end-of-pipe ion exchange applications relate mostly to resin fouling and complaints about the frequency of regenerating the resin beds. Of the 18 shops providing data, 16 (or 89%) were still operating their ion exchange equipment for end-of-pipe treatment. The average age of the operating systems was 5 years for IXEOP-1, 3 years for IXEOP-2 (only one respondent), and 4 years for IXEOP-3. The average percentage of downtime experienced by the respondents was 5.4% for IXEOP-1, 5.0% for IXEOP-2 and 14.1% for IXEOP-3 (however, just 4.0% excluding PS 061).
The following summarizes the respondentsí O&M experiences and provides operating labor information.
- The average number of annual operating hours per ion exchange system were: 835 hrs/yr for IXEOP-1, 600 hrs/yr for IXEOP-2, and 464 hrs/yr for IXEOP-3. The skill requirement commonly needed for operating this technology is a wastewater treatment plant operator and/or a trained technician. The following is a breakdown of the responses for skill requirements (multiple responses given by most respondents):
Environmental Engineer: 0 Process/Chemical Engineer: 3 Chemist: 5 Consultant: 3 Plumber/Pipe Fitter: 4 Electrician: 3 Vendor: 1 Senior-Level Plater: 2 Junior Level Plater: 4 Trained Technician: 8 Wastewater Treatment Operator: 8 Common Labor: 3 Other: 0
- Six shops (or 33%) indicated that resin fouling was an O&M problem (includes all applications). The causes of fouling were not always given, however, two respondents gave the following reasons: bacterial growth during warm weather (PS 170) and oil in the feed stream (PS 209). One shop indicated that they must frequently wash the resin (PS 223) and another estimated that fouling must be dealt with approximately four times per year. Two shops indicated that prefiltration was very important.
- Several shops complained that the resin beds load too quickly and must be regenerated (PS 068, PS 187, PS 170). One shop attributed a past problem with resin bed loading to an inefficient water softener that caused their bed to load with hardness causing ions.
- Only one shop indicated that resin life was an O&M problem (PS 223). One shop indicated that resin life was 4 to 5 years (PS 036).
The primary residuals from ion exchange end-of-pipe treatment applications are the regenerants (eluates) and backwash waters. Other residuals include cartridge filters and spent carbon. The regenerants are concentrated solutions and the backwash is dilute. Both solutions are either caustic or acidic, depending on the resin type and application. Regenerant or backwash from cation regenerations will typically contain acid, nickel, trivalent chromium, zinc, copper and/or other positively charged metals depending on the feed stream. The anion regenerant or backwash will typically contain caustic, cyanide, and hexavalent chromium. Regenerants are typically treated on-site or hauled to a metals recovery/disposal site. Backwash is typically treated on-site.
The volume of regenerant produced will depend on the regeneration requirement (e.g., lbs of acid per ft3 of resin) and the concentration of acid used (typically 1 to 5%). The regeneration requirement will depend on the resin type, application (metal or complex being recovered) and the configuration (co-current vs counterflow). Typical volumes of regenerant are 20 to 50 gal/ft3 of resin per regeneration. Resin loading capacities are discussed in Section 3. The volume of regenerant waste is sometimes reduced by reusing the last portion of the regenerant, which will be less contaminated with metal and contains free acid or caustic. Backwash volumes depend mostly on the equipment design and the application. Typically, backwashing generates 25 to 75 gal/ft3. The backwash is partly reused by some equipment vendors as make-up water for regenerant, in an effort to reduce the total waste volume generated. Because backwash contains only dilute concentrations of pollutants, it is typically not a major concern and is treated on-site and discharged. However, for shops working toward zero-discharge, the backwash volume could present a significant problem. Both backwash and regenerant can be processed by evaporation to reduce the volume requiring disposal. However, this increases the capital and operating costs of the system (see Sections 3.2 and 3.3). Also, if evaporation units are not directly connected to wastewater treatment systems, they may require a RCRA permit to operate them (see Section 2.7).
Waste treatment processes for these wastes generate sludge that is an EPA listed hazardous waste (F006). Unless chemical recovery is practiced, ion exchange (application IXEOP-1) with on-site batch treatment of regenerants will produce approximately the same quantity of F006 sludge as that produced from conventional treatment without ion exchange. Since application IXEOP-2 is an extra polishing step following conventional treatment, combined with the conventional system, it will result in slightly greater sludge generation than with conventional treatment alone.
Some plating shops send their ion exchange columns to an off-site CWT facility for the regeneration process. This practice can eliminate the on-site generation of F006 sludge. However, shops considering CWT should investigate the regulations and liabilities associated with the transport of the ion exchange columns and off-site treatment/recovery/disposal of the resultant materials.