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Pollution Prevention and Control Technologies for Plating Operations
Section 6 - Wastewater Treatment
6.2 CONVENTIONAL TREATMENT TECHNOLOGIES
6.2.1 Overview
Conventional treatment is a series of unit processes used extensively by industry that have provided reliable treatment for many electroplating operations. Exhibit 6-1 is a schematic of a conventional treatment system for electroplating wastes containing chromium and cyanides in addition to other heavy metals, acids, and alkalis.
The configuration of conventional treatment is relatively standard. It consists generally of the following unit processes:
- Chromium reduction of segregated chromium waste streams to reduce the chromium from its hexavalent form to the trivalent state, which can be subsequently precipitated as chromium hydroxide by additions of alkali.
- Cyanide oxidation of segregated cyanide-bearing waste streams to oxidize the toxic cyanides to harmless carbon and nitrogen compounds.
- Metals removal of the combined metal-bearing wastewaters using hydroxide precipitation techniques.
- Sludge dewatering using gravity thickening followed by a mechanical dewatering device to increase the solids content of the sludge and therefore reduce its volume. Within the past five years, thermal dehydration using sludge dryers has also become a conventional unit operation. This equipment further increases the solids content of the sludge.
The Federal electroplating and metal finishing pretreatment wastewater standards (Exhibits 6-2 and 6-3) were developed by EPA by identifying commonly used treatment practices and determining their effectiveness by collecting effluent data from well operated systems. Conventional treatment was selected by EPA as the standard system. Therefore, for most plating shops, use of conventional treatment will provide sufficient pollutant removal to meet discharge standards. There are two major exceptions to this rule. First, many plating shops are regulated by local discharge standards that are more stringent than the Federal standards and conventional treatment may be insufficient to meet these limitations. Second, the treatment systems selected by EPA for establishing the Federal standards were those systems that EPA determined to be "properly operating facilities." For example, EPA omitted facilities that: (1) did not have well operated treatment processes; (2) had complexing agents (e.g., non-segregated wastes from electroless plating); and (3) had dilution from non-plating wastewaters. As a result, some plating facilities may not meet the properly operated facility criteria used by EPA and may have difficulty meeting Federal standards using conventional treatment.
In cases where conventional treatment is insufficient to meet discharge limitations for a given facility, there are three basic choices for attaining compliance: (1) correct or upgrade the existing processes; (2) make internal changes (e.g., improve rinsing, add recovery, segregation of waste streams) to "normalize" the wastewater, (3) use conventional treatment plus additional treatment (i.e., polishing), and (4) use alternative treatment processes. Information on treatment system operation is reviewed in this section and details can be found in the literature (e.g., ref. 38). Methods for internal changes are discussed in Sections 2, 3, 4 and 5. Additional and alternative treatment processes are discussed in this section.