Leominster Wastewater Treatment Plant: Leominster, Mass


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LEOMI NSTER WASTEWATER TREATMENT PLANT
LEOMINSTER, MASS.
MARCH 24, 1972
General :
On March 24, 1972, at the request of the United States Environmental
Protection Agency’s (EPA) Region I, Design & Engineering Section, personnel
from Technical Studies Section conducted a study on the chlorine detention
tank at the Leominster Wastewater Treatment Plant (WWTP) to evaluate the
detention time through the tank at peak flows. This request was predicated
on the fact that constLa.iction grant monies was being request d by the To zn
for partial payment of the circular chlorine detention tank. Studies per-
formed on similar tanks in other parts of the country have shown the actual
detention time to be significantly less than the design time
Treatment Plant :
The WWTP is divided into two sections (See figure 1). The old sec-
tion is a conventional activated sludge plant while the new section is a
modification of the conventional activated sludge process known as step
aeration. After secondary clarification, both systems discharge into
a common chlorine contact chamber, to the chlorine detention tank and
finally through an outfall to the North Nashua River.
The circular chlorine detention tank has an inside diameter of
55 feet and a side water depth of 10 feet. This design theoretically
allows a detention time of 15 minutes at a peak flow rate of approxi-
mately 16.5 million gallons per day (mgd). In theory, flow enters the

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the tank at its periphery two feet above the tank floor and spirals to-
ward the overflow weir at the center of the tank.
Study ;
The detention time study occurred between 0900 and 1145 hours on
March 24, 1972, as past operating records show this to genex ally be the
time of peak flow at the WWTP. During the study, the flow nto the plant
as measured manually at the influent Parshall flumes remain d nearly con-
stant at 11.3 mgd. (See table 1). Temperature profiles usfii1 a YSI dis-
solved oxygen/temperature probe indicated no temperature strhtification
in the tank. (See table 2)
Using Rhodamine B dye diluted with acetic acid to a s ecific gravity
of one and a flow-through recording fluorometer, EPA personnel on three
separate runs, recorded the detention time of the dye between, the chlorine
contact chamber and the WWTP outfall. During the first run, ’the Eluoro-
meter pump clogged shortly after the peak dye concentration had passed,
thus allowing only the recording of the time of first dye trace and peak
concentration. The remaining two runs(using an increasing quantity of
dye on each run) recorded the time of first dye trace, peak concentration,
and final significant dye trace. (See table 3 & figure 2)
Results :
Table 3 shows the respective times for the occurrence of first dye
trace, peak dye concentration, center of mass, and final significant dye
trace.. After the dye was dumped, the first dye traces appeared at the
outfall in 2 to 3 minutes, peak concentrations in 4 to 4.5 minutes, center
of mass in 9.5 minutes, and final significant traces in 23 to 26 minutes.
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Surface dye dispersion patterns in the chlorine detention tank were
approximately the same for each run. Figure 3 shows the tyjical surface
,.i it e un 1 :iil ci 3 w nti I c s : lieu th ’ dye ha ci I CCfl dtiiiipc d . A I Ic r 3 in 11111 t: ;
the dye pacteril rcn tined essentially the S iI1lC throughout the rest of each
dye run.
Conclusions ;
At the time the dye study was being conducted, the actual average
detention time in the chlorine detention tank was approximately 9.5 min-
utes as opposed to the theoretical design detention time of 15 minutes
even though the actual flow through the plant was 11.3 mgd or 68 percent
of the 16.5 rngd design flow.
-3-

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