86    TOXICITY STUDIES OF COMBINED METAL
FINISHING/MUNICIPAL WASTEWATERS
Frederic C. Blanc, Professor
Department of Civil Engineering
Northeastern University
Boston, Massachusetts 02155
James C. O'Shaughnessy, Head
Department of Civil Engineering
Worcester Polytechnic Institute
Worcester, Massachusetts 01609
Donald Walker, Engineer
Metcalf and Eddy Inc.
Wakefield, Massachusetts 01880
Scott Miller, Engineer
Haley and Ward, Inc.
Waltham, Massachusetts 02154
INTRODUCTION
This paper presents the results of treatability studies and engineering analysis for a case involving
the proposed discharge of a pretreated wastewater effluent from a metal finishing industry into a
small municipal POTW. The objective of the studies was to determine if the addition of the industry's
pretreated wastewater would in any way be detrimental to the performance of the POTW.
The municipal POTW is a small extended aeration oxidation ditch facility with brush aerators.
Degrited wastewater is treated directly in the aeration tank with no primary clarification. This plant
has a design flow of 0.32 MGD but is presently treating 170,000 gallons per day, at a hydraulic
retention time in excess of one day, in the oxidation ditch. Presently the POTW's sludge is being
stabilized in a large aerobic sludge holding tank. At one time it was planned that the stabilized
municipal sludge would be disposed of by land application. However, the present wastewater and
sludge is too high in cadmium, copper and nickel to meet the Massachusetts requirements for Type I
or Type II sludge disposal. Copper is a major problem for the municipality since the municipal
wastewater alone contains a significant amount with no identified industrial waste source. Corrosion
of household plumbing systems due to low pH , poorly buffered water supply has been mentioned as a
possible source of the copper but the evidence is not conclusive. Because of the inability of the
municipality to land apply its stabilized POTW sludge, the municipality is sensitive to possible
industrial discharges which contain metals.
The metal finishing company involved in this study is engaged primarily in aluminum anodizing
using sulfuric acid. Chrome plating is the major secondary production process along with copper
plating. A small part of the production involves the plating of precious metals such as silver. The work
pieces plated are parts for the electronics industry. Presently, the metal finishing company is discharging treated effluent to a stream somewhat upstream of the POTW. The average industrial wastewater
flow is 19,000 gallons per day. The company's treatment system operates on a continuous basis with
provisions for treating batch dumps as required. Wastewaters are segregated into sumps and treated.
A cyanide treatment system, equipped with pH/ORP control, destroys cyanide which is amenable to
chlorination. Chromium is first reduced to the trivalent state using bisulfite and then the chrome
wastewater along with the other wastewaters are treated with caustic and the resulting metal hydroxide
precipitates are removed by sedimentation. Table I lists some of the characteristics and metals concentrations for the treated metal finishing company's effluent.
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