Effects of a Mixture of Heavy Metals on
Sewage Treatment Processes
E. F. BARTH, Chemist
B.V. SALOTTO, Chemist
G. N. McDERMOTT, Sanitary Engineer
J. N. ENGLISH, Sanitary Engineer
M. B. ETTINGER, Chief, Chemistry & Physics
U. S. Department of Health, Education and Welfare
Public Health Service
Bureau of State Services
Robert A. Taft Sanitary Engineering Center
Cincinnati, Ohio
In previous reports from this laboratory the effects of copper, chromium,
nickel, and zinc introduced individually to the sewage feed of complete activated
sludge pilot plants have been discussed (1,2,3,4).
Using the results obtained from the individual studies as background information, we investigated the effects of a mixture of these four metals on the activated
sludge and anaerobic digestion processes. Detailed descriptions of the plant design
and operation can be found in the above references.
COMBINATION OF METALS EMPLOYED
The response of the activated sludge process was measured with each of three
metal combinations. The combinations, denoted as MC #1, MC #2, and MC #3
are given in Table I. Two of the metals, zinc and copper, were fed as soluble
complex cyanides. Chromium (VI) was introduced as potassium dichromate, and
nickel in the form of nickel (II) sulfate.
Previous studies have shown that once the activated sludge process acclimates
to cyanide no difference in effects on treatment exists between the metal as a
complex cyanide or free cation (2, 3).
Metal combination #1 was based on data from the previous studies on the individual metals and represents the approximate threshold limit for each metal. .
Chromium was deleted from MC #2 because chromium at four mg/1 should have
no effect on the activated sludge process (1); therefore, MC#2 should show the
same reaction as MC #1 if there was no interaction of the metals. MC #3 was
chosen in order to observe minimum effects and because the ratios of metals reflected average analyses obtained from field samples.
EXPERIMENTAL CONDITIONS
The run in which MC #1 was used was divided into two parts. In the first part
a supplemented weak domestic sewage was used as sewage feed (1,2,3,4). In the
second part a strong domestic sewage was employed with no supplement. The
studies with MC #2 and MC #3 were carried out entirely with the strong domestic
sewage.
The sewage was fed from a common storage tank to duplicate pilot plant units
operating at 350 1 per day capacity. One unit received no metal and served as a
control. The experimental unit received the combination of metals by constant-
head,  calibrated,   capillary tubes from which the metal solutions dripped into the
- 616 -