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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.5 ALTERNATIVE TECHNOLOGIES FOR METALS REMOVAL
6.5.4 Evaporation
6.5.4.1 Overview
6.5.4.2 Development and Commercialization
6.5.4.3 Applications and Restrictions
6.5.4.4 Technology/Equipment Description
6.5.4.5 Costs
6.5.4.6 Performance Problems
6.5.4.7 Operational and Maintenance Problems
6.5.4.8 Residuals Generation
Evaporation is a commonly used technology for the recovery of plating chemicals from rinse waters, as discussed in Sections 3.2 and 3.3. In contrast, this section discusses the use of evaporation as an end-of-pipe technology. These applications are differentiated from those in Section 3 by the fact that no plating chemicals are recovered with the end-of-pipe applications. Rather, the evaporation technology is employed solely to reduce or eliminate the discharge of wastewater. It should be noted that several survey respondents used evaporation as a recovery technology and achieved zero-discharge (PS 071, PS 080, PS 193, PS 195). In three of these cases (all except PS 193), the evaporation needed to recover the rinse water/drag-out and attain zero-discharge was provided by heated process tanks (i.e., recovery rinsing). The fourth shop, PS 193, used atmospheric evaporators to provide the needed headroom in their process tanks (nickel and chrome plating) for attaining zero-discharge (their experience is discussed in Section 3.2).
A total of 8 (or 2.5%) of the respondents employ evaporation as an end-of-pipe technology. Of these eight respondents, seven have attained zero-discharge. The one plant with a discharge (PS 036) has an average daily discharge flow of 2,704 gpd. That shop indicated that they installed the evaporator as a treatment process for difficult to treat wastes (copper strip, mixed metal wastes and spent electroless nickel solution). Different types of equipment are used by the respondents, with vacuum evaporators being the most predominant. Five of the respondents use vacuum evaporators, two use atmospheric evaporators and one uses a boiler.
Most of the shops that attained zero-discharge did so recently. Four of the seven zero-discharge shops reached this goal in 1992. One shop reached zero-discharge in 1985, one in 1988 and the remaining shop did not indicate a date. All but one of these shops was discharging less than 1,000 gpd the year prior to reaching zero-discharge. The other shop appears to have jumped from 8,000 gpd to zero-discharge (PS 100). Several of the zero-discharge shops operated a limited number of plating processes. PS 282 and PS 006 only perform chromium plating and PS 021 performs chromium and electroless nickel plating. Only two of the shops gave any reasons for going to zero-discharge. One shop indicated that there was a $12,000 a year savings in self-monitoring costs. Another shop provided the following explanation (PS 202):
ìTo avoid ever increasing costs and goon squad regulation by the City of _____ Department of Sanitation-We severed our connection to the POTW. We recycle all rinse water that doesnít become contaminated. We use evaporation and a filter press to reduce waste to 55 gal DOT barrels and ship off-site for recycle. Crude-but effective for our operation.î
6.5.4.2 Development and Commercialization
The development and commercialization of atmospheric and vacuum evaporators are discussed in Sections 3.2.2 and 3.3.2, respectively.
6.5.4.3 Applications and Restrictions
Four basic configurations of end-of-pipe evaporator use were employed by the eight survey respondents. These configurations are described in Exhibit 6-38. Application EVEOP-1 employs an atmospheric evaporator and there is no reuse of evaporated water. The other three configurations reuse the evaporated water for rinsing or, in the case of the boiler system (application EVEOP-4), the steam condensate is reused.
Evaporators not directly connected to a wastewater system may require a RCRA permit to operate them. Installation and operation of an atmospheric evaporator may require an air permit. Plating shops contemplating this type of treatment process should investigate the regulatory requirements for their specific application.
6.5.4.4 Technology/Equipment Description
This subsection contains references to commercially available evaporation equipment that is manufactured and/or sold by vendor survey respondents or identified during the literature search. 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.
The evaporation equipment used for end-of-pipe treatment is usually the same equipment used for chemical recovery. Atmospheric and vacuum evaporation units are discussed in Sections 3.2.4 and 3.3.4. Some atmospheric and vacuum evaporation equipment is specifically designed for concentrating wastes. Applicable examples of atmospheric evaporation equipment include the Technotreat and SAMCO products discussed in Section 3.2.4. A type of vacuum evaporation equipment that is used for end-of-pipe treatment applications, but not usually applied to recovery, is the flash evaporator (e.g., LICON Flashvap). This equipment is discussed in Section 3.3.4.
Capital and operating costs for atmospheric and vacuum evaporators are presented in Sections 3.2.5 and 3.3.5. Capital costs can be expected to be approximately the same for recovery and end-of-pipe applications. In some circumstances, pretreatment of the feed stream may be warranted to prevent corrosion of the evaporator, necessitating additional tankage and chemical control equipment (e.g., pH control). Operating costs will be somewhat higher for end-of-pipe applications because solutions will typically be concentrated to a higher solids concentration in an effort to reduce the volume of concentrate produced and therefore lower off-site hauling costs. This will result in increased scaling within the evaporation unit and result in higher maintenance costs. These higher costs are specified in the operating costs for vacuum evaporators (Section 3.3.5.2).
6.5.4.6 Performance Experience
A summary of the Users Survey data is provided in Exhibit 6-39. The following information and data summarize the performance experience of the survey respondents.
- Shops using evaporation for end-of-pipe treatment gave mixed satisfaction levels for manufacturersí support and the technology itself. The low satisfaction level from PS 282 is mostly due to their inability to use their evaporator for chromium recovery. This problem may have been caused by an inappropriate selection of materials of construction. They stated: ìThe Water Technology system was purported to be a totally recyclable system, generating chrome acid concentrate back to the plating tank and distilled water. The chromic acid was contaminated and poisoned the plating baths. The fluoride catalyst attacks the titanium heat exchanger and the water sometimes gets contaminated with chrome. The system is a total nightmare.î
- The following is a breakdown of the reasons why shops purchased this technology (includes data from six respondents):
- To meet or help meet effluent regulations: 3
- To reduce plating chemical purchases: 1
- To reduce the quantity of waste shipped off-site: 3
- To reduce wastewater treatment costs: 4
- To improve product quantity: 1
- Other: 0
- The use of evaporation had a little impact on production quality and the rate of production. The following responses were provided (includes data from six respondents):
Production Rate Product Quality
Improved 1 0
No Change 4 4
Decreased 1 1
- None of the shops indicated that evaporation applied as an end-of-pipe treatment method was the cause of an effluent compliance excursion (includes data from six respondents).
- Respondents reported operating cost savings from reduced water use, reduced treatment chemical use, reduced sludge disposal and from the elimination of discharge monitoring.
- PS 006 reported: ìUnit will not process our effluent as well as Co [manufacturer/vendor] lead us to believe when purchasing unit. Technology was grossly oversold.î The supplier stated capacity of the unit was 800 gal/24 hr and the actual capacity was 250 to 300 gal/24 hr.
- PS 233 reported that the supplier stated capacity of their unit was 450 gpd and that the actual capacity was 125 gpd.
6.5.4.7 Operational and Maintenance Problems
The following summarizes the respondentís O&M experiences and provides operating labor information.
- Five shops provided operating labor data. For these shops, the average number of annual operating hours per evaporator were: 468 hrs/yr (645 hr/yr excluding PS 100). The following is a breakdown of the responses for skill requirements (includes data from six respondents):
- Environmental Engineer: 0
- Process/Chemical Engineer: 0
- Chemist: 0
- Consultant: 0
- Plumber/Pipe Fitter: 1
- Electrician: 2
- Vendor: 0
- Senior-Level Plater: 3
- Junior Level Plater: 1
- Trained Technician: 4
- Wastewater Treatment Plant Operator: 2
- Common Labor: 0
- Other: 0
- PS 133 reported that use of their boiler for end-of-pipe treatment has resulted in the need to replace it.
- PS 036 indicated that ammonia, contained in their spent copper strip and electroless nickel solutions, is found in their distillate and that a second pass through the evaporator is necessary for complete removal.
- PS 282 reported: "Maintenance nightmare. No support from vendor-screwed up plating baths for at least one year."
- PS 233 reported: "Material failure: i.e., valves, piping, seals, vacuum pump, product pumps, refrigeration system, overflow protection."
Evaporators generate a concentrated waste product that can be directly shipped off-site for recovery or disposal or can be further processed (e.g., filter press) to increase its solids content. The concentration of solids from the discharge of the evaporator will depend on the type of unit selected. Common atmospheric evaporators used for chemical recovery are unable to concentrate waste much beyond typical plating bath strength. Some vacuum evaporators, such as the climbing film type, can achieve concentrations of 500,000 mg/l or more. Residuals data provided by respondents using evaporation as an end-of-pipe technology are shown in Exhibit 6-40.