Slug of Chromic Acid Passes Through a
Municipal Treatment Plant
J. N. ENGLISH, Senior Assistant Sanitary Engineer
E. F. BARTH, Chemist
B. V. SALOTTO, Chemist
M. B. ETTINGER, Scientist Director
Taft Sanitary Engineering Center
USPHS
Cincinnati, Ohio
Previous studies conducted at the Robert A. Taft Sanitary Engineering Center
using pilot scale, activated-sludge sewage treatment plants indicated that a 10
mg/1 slug of chromium lasting four hrs had no effect whatsoever on plant performance, while a 100 mg/1 and 500 mg/1 slug caused the plant efficiency, as
measured by BOD removal, to drop by three and 10 per cent, respectively, during
the first 24 hrs (1).
A field study was undertaken to complement the pilot scale study. The objective was to determine the effects of passage of a prearranged slug of chromic
acid on the efficiency of the Bryan, Ohio, municipal sewage treatment plant.
This paper presents the results of this study. In addition to the levels of chromium
in the plant processes attributable to the chromic acid slug, background data on
the concentrations of chromium, copper, zinc, and nickel are presented also,
CONDUCT OF STUDY
Plant Description
The Bryan, Ohio, sewage treatment plant serves a community of 7, 400 persons
with an equivalent population of 12, 000. The plant has conventional activated-
sludge secondary treatment, an average sewage flow of 0. 8 MGD, and a design
flow of 1.9 MGD.   A flow chart for the plant is shown in Figure 1.
The sewage, after passing through the grit chamber and comminutors, enters
a wet well and is then pumped into the primary clarifiers. The effluent from the
four primary clarifiers passes through the first of the three aeration tanks. The
mixed liquor from this tank is divided equally between the remaining two tanks
from where it flows into the final clarifiers. The effluent from the clarifiers is
discharged to drainage ditch number 40 and comprises 90 per cent of the flow.
The ditch discharges into Lick Creek, a tributary of the Tiffin River, 11.5 miles
below the plant outfall sewer.
Ninety-two per cent of the sludge pumped from the final clarifiers is returned
to the first aeration unit and the remaining eight per cent is wasted to the raw
sewage in the wet well. The return sludge flow averages 87 per cent of the raw
sewage flow at Bryan.
The sludge in the primary clarifiers is pumped to the primary digester twice
each day from 8 to 9 a.m. and 3 to 4 p.m. Supernatant is withdrawn from the
secondary digester and returned to the sewage wet well. Digested sludge is pumped
to sand drying beds and ultimately disposed of in fill areas.   Design data and load-
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