Heavy Metal Removal From Wastewater Treatment
Plant by Chemical Treatment
CHARLES M. WEST, Water Quality Specialist
Division of Water Quality Control
Tennessee Department of Public Health
Knoxville, Tennessee 37919
WILLIAM P. BONNER, Assistant  Professor
RAFAEL B. BUSTAMANTE, Associate Professor
Department of Civil Engineering
Tennessee Technological University
Cookeville, Tennessee 38501
INTRODUCTION
A great deal of interest in water quality has been shown by the public in recent years.
Particular concern has been demonstrated locally for the waters and streams receiving the
effluent of the Cookeville, Tennessee, sewage treatment plant. Investigation of the situation
revealed several facts:l) The current treatment plant is seriously overloaded, having been
designed to treat approximately two million gallons per day; 2) Sludge (solid waste) derived
from the sedimentation and skimming processes are normally treated by bacterial digestion
and subsequent dewatering, drying, and disposal as soil building material; 3) The effluent
has a low residual chlorine content and is fairly high in solids; 4) Sudden increases in inlet
flow rates after heavy rainfalls indicate contributions from storm drains for heavy leakage
of surface waters; 5) Several local industries, many of which have metal treating and plating
operations, are permitted to discharge their wastes into the city sewer system; and 6) An
investigation to determine the cause of digester inactivity had revealed the presence of
copper and chromium metals in the digester.
These facts gave rise to serious concern for the quality of the effluent with respect to
heavy metals. Further concern arose when it was discovered that plans had been made to
increase the capacity of the current plant and to add a tertiary treatment system (chemical
treatments followed by microscreening).
At this point it was decided that it would be of public and scientific interest to: 1)
determine the general quality of the effluent of the sewage treatment plant, with respect to
heavy metals, and 2) determine the effect of the proposed tertiary treatment system on the
quality of the effluent with respect to heavy metals.
Because of the suspected nature of the industrial wastes contributed to the system, six
heavy metals were chosen for investigation. Based on their toxicity and probability of
occurrence in these effluents, the metals were chromium, nickel, cadmium, copper, lead,
and mercury. The latter metal was added because of current interest in mercury pollution in
streams and rivers in the Tennessee area. There was little suspicion of the presence of this
metal, because its only potential source in the area would be accidental loss from pressure
measuring devices, pump and similar seals, or analytical laboratory effluents.
In order to acquire samples with as a high a concentration of heavy metals as possible
for the general survey, grab samples were taken over a thirty-day period in the late summer
(August, 1971). The average flow during this period was about two million gallons per day.
The flow rate during the wetter months may be as high as four to six million gallons per day.
If the metals are added at a fairly constant rate, this increase in volume could result in a two
or three fold dilution and reduce the metal concentration beyond the detection limits of the
analytical methods used. Samples taken for the study concerning the efficiency of the
proposed treatment system were acquired in the spring (April and May, 1972), when the
flow averaged four to six million gallons per day.
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