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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.5 ALTERNATIVE TECHNOLOGIES FOR METALS REMOVAL
6.5.1 Overview
For many plating shops, the conventional hydroxide precipitation/clarification process will be the most economical and reliable end-of-pipe method of meeting metals discharge limitations. However, for various reasons, shops may want to employ an alternative metals removal process. Some of the more common reasons include:
- Some local jurisdictions have adopted discharge standards more stringent than the Federal regulations. Because the Federal regulations are based on the technical capabilities of conventional treatment, these processes are often unable to achieve the lower concentration limits.
- Metal finishing wastewaters often contain compounds that interact with dissolved metals and interfere with their precipitation as metal hydroxides. Such compounds as ammonia, phosphates, tartrates, and ethylene-diaminetetraacetic acid (EDTA) are commonly used in plating and printed circuit board operations and consequently find their way into the wastewater. These compounds, called chelates, combine with the dissolved metal ion to form a complexed ion that is relatively soluble in neutral or slightly alkaline solutions. In many cases, waste streams containing high levels of chelates (e.g., printed circuit board shops) cannot be treated with conventional precipitation to the level required by Federal regulations.
- In some cases, metal discharge requirements are not being met, even though the level of dissolved metals in the effluent is low. In cases of this kind, the solids separation component of the process may be allowing too much suspended matter, including precipitated metals, to pass into the discharge. This condition can result from overloaded or poorly designed clarifiers, ineffective conditioning (coagulation or flocculation) of the clarifier feed, or poor pH control.
- Some shops may find that the capital and/or operating costs of conventional processes are too high. Alternative technologies may reduce capital costs under certain conditions (e.g., if a centralized waste treatment plant is located nearby) and/or specific components of operating costs (e.g., shops located long distances from sludge disposal sites may focus on reducing sludge volumes).
The alternative technologies discussed in this section are the ones that have been installed by the respondents to the Users Survey; they include: ion exchange, microfiltration and evaporation. Each of these alternatives offers solutions to one or more of the technical problems encountered with conventional treatment or, in some cases, offers cost savings through a reduction in equipment requirements or operating expenses such as chemical reagent purchases. In all cases, however, alternative technologies offer a trade-off. Their advantages are gained at the expense of other benefits. Such trade-offs are often site-specific and must be evaluated case by case.
In addition to the three alternative technologies for metals removal discussed in this report, there exists many other alternatives that have been reported in the literature and/or utilized in plating shops. Examples of these include sulfide precipitation, sodium borohydride precipitation, dissolved air flotation, freeze crystallization, and insoluble starch xanthate treatment. Of these technologies, sulfide precipitation is the most widely applied in the metal finishing industry. Although it is not used by any respondents to the Users Survey, it is used in the plating industry, as documented in the literature (e.g., ref. 39, 393, 348). Also, the sulfide precipitation chemistry is sometimes used with end-of-pipe membrane filtration systems. This application is discussed in Section 6.5.3.