TREATMENT OF HEAVY METALS AT
SMALL ELECTROPLATING PLANTS
C. W. Sheffield, President
Bio Engineering Services, Inc.
Orlando, Florida 32806
It is estimated there are between 11,000 and 14,000 metal electroplating related firms
in the United States. Florida, and particularly central Florida, has its share of these firms
due presumably to the activities at the Kennedy Space Center.
Many of these smaller electroplating firms, which have been in operation for numerous
years, have recently decided to enlarge at the existing site or move to a new location. These
new and existing firms, like many other ones, come under the classification of toxic waste
in their process waters and, therefore, must comply with the P.L. 95-217 known as the
Clean Water Act of 1977. This act says basically if you are discharging into a city, county or publicly owned sewerage system that you must provide pretreatment to acceptable
limits.
The scope of this paper will be limited to three small electroplating plants with discharges into municipal sewers ranging from 3,000 gpd to 50,000 gpd (5-50 gpm over a
10-hour period). The heavy metals of concern will be copper or iron at these plants.
The actual object of this paper will be to indicate economical and practical means of
removing these heavy metals and provide engineering design criteria along with cost factors.
DISCUSSION OF EXISTING PROCESS
The literature indicated numerous methods of meeting the regulations by oxidation of
cyanide, reduction of hexavalent chromium, precipitation of heavy metals and adjustments
of the pH to acceptable levels.
For this paper the heavy metals copper, iron, nickel, chromium and lead will be precipitated by using lime (Ca(OH)2) and then gravity settling. Normally metal hydroxides are
formed when the pH is raised to a range of 6-10. One problem is the presence of complexing irons such as cleaners, EDTA, ammonia and others. These complexing irons may
have an effect on the ability of the metal to form a hydroxide precipitate. It has been
found that by precipitating the metals with a high pH lime treatment and then adding, in
the neutralization tank, a sulfate (FeS04) or sulfide (Na2S) will increase the efficiency of
the process.
Another problem lies in the fact not all heavy metals are insoluble or precipitate at the
same pH as indicated in Figure 1. The effluent limitation really dictates the amount of lime
being added so that the metal will be precipitated and be removed as a sludge. In most
cases this problem of solubility is increased when the waste stream has one or more heavy
metals and therefore when two or more are mixed, the choice must be the optimum pH of
one or a compromise between the two. These mixtures usually form a complex interaction
and therefore it is very important to ascertain the exact concentrations of the heavy metal
in the waste stream.
One of the important design aspects of any metal reduction process in an electroplating
waste is the effluent limitations. The raw waste characteristics that might be effluenting
into a municipal sewerage system from an electroplating plant are shown in Table I. The
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