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Pollution Prevention and Control Technologies for Plating Operations
Section 2 - General Waste Reduction Practices
2.7 CLOSED-LOOP PROCESSING AND ZERO DISCHARGE
Closed-loop processing and zero discharge are two terms that are often used by platers, vendors, consultants and regulators (ref. 206, 223, 229, 258, 294, 397, 469, and 470). Various definitions are used for these terms, most of which recognize that 100 percent recovery/reuse of all materials (process chemicals, other chemicals, water, sludge, etc.) is not practical, economically feasible nor efficient from an energy standpoint.4 More realistically, all metal finishing shops as well as individual metal finishing processes generate some form of residuals. The residuals are typically in one or more of four common forms: wastewater, spent process solutions, sludge, or air emissions. Often, some recovery/reuse can be implemented, but with every technology scheme or configuration there is some residual generated. Often there is a trade-off between the quantity and characteristics of two or more of the four common residuals. As a simple example, closed-loop rinsing after chromium plating will result in a build-up of contaminants in the bath. The rinse water discharge can be eliminated, but the bath will have to be either discarded or ìpurified.î If the bath is purified, the purification process (e.g., porous pot, membrane electrolysis, or ion exchange) will result in a residual that must be properly discarded.
Of the 318 respondents to the Users Survey, 24 (or 7.5%) indicated that they have achieved zero discharge (these shops can be identified from Exhibit 1-8). Many other shops indicated that they are working towards this goal. Of the shops that have achieved zero discharge, 58% are primarily hard chrome platers. The hard chrome process is one the easiest to operate without a wastewater discharge because of the high ratio of evaporation to drag-out (i.e., permits use of recovery rinsing). Also, as indicated in Section 1.2.5, the respondents that have achieved zero discharge are generally smaller shops, which tend to be less complex and less automated. As a result, these shops generally have simple, manual processes that if given sufficient control can be operated without a wastewater discharge. The means implemented by these shops to achieve zero discharge is most frequently recovery rinsing and atmospheric evaporation. However, eight respondents have implemented advanced technology schemes in order to achieve zero discharge. The technologies implemented by these shops include vacuum evaporation, ion exchange, and membrane filtration. These systems are discussed in Section 6.
There are various economic benefits of operating closed-loop processes or a zero discharge shop, including: lower water and sewer charges, less effluent monitoring and paperwork, and reduced chance of compliance excursions/fines. Some firms are driven to zero discharge by specific regulatory conditions rather than economics, such as discharge permits with low numerical or mass loadings, TDS limits or aquatic-toxicity-based standards and low in-stream dilution factors. Furthermore, some facilities are practically isolated from sewers or streams (ref. 516). A negative aspect of zero discharge is the loss of flexibility that is provided by a wastewater treatment system/discharge. This loss can translate into higher costs for hauling spent process solutions to disposal sites, for example, if unexpected process solution contamination occurs. Also, one source indicates that closed-loop systems may pose regulatory conundrums, in that without a wastewater discharge permit, firms lose the RCRA treatment exemption and become subject to Part B permits (ref. 223) (see note 1). That source suggests that firms intent on eliminating wastewater discharge should negotiate a ìclosed-loopî Clean Water Act permit (i.e., NPDES or POTW) with the regulatory authority. Legal requirements and options for companies that wish to operate zero discharge facilities are discussed in the literature (ref. 516).
Zero discharge is generally achieved in stages: (1) prepare a plan (e.g., as part of a pollution prevention program, see Section 2.2); (2) implement good operating practices (Section 2.3); (3) minimize drag-out which causes wastewater generation (Section 2.4); (4) modify rinsing practices to reduce/eliminate flows to treatment (Section 2.5); (5) implement bath maintenance (see note 2) and chemical recovery (Sections 3 and 4); (6) continuously reassess flow rates, process chemical/contaminant concentrations in recovered streams, costs, and benefits. Additional suggestions for zero discharge programs can be found in ref. 223.
Note 1: The Resource Conservation and Recovery Act requires firms to obtain a permit for the operation of hazardous waste treatment processes. Such systems are exempt from permitting if they treat wastewaters that are discharged under the Clean Water Act.
Note 2: Some form of bath maintenance must be implemented concurrently with drag-out recovery, especially when high contaminate loads are present.