Pollution Prevention and Control Technologies for Plating
Section 6 - Wastewater Treatment
6.2 CONVENTIONAL TREATMENT TECHNOLOGIES
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
- 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
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.
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