&ER&
United States
Environmental Protection
Agency
Municipal Environmental Researc
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-167 Oct. 1981
Project Summary
Design Optimization of the
Chlorination Process:
Volume 1. Comparison of
Optimized Pilot System with
Existing Full-Scale Systems
Endel Sepp and Paul Bao
Parallel wastewater effluent chlo-
rination studies were done on a mobile
optimized chlorination pilot system
and the full-scale system at eight
different treatment plants. Disinfec-
tion efficiency was measured by total
coliform enumeration and chlorine
residual tests. Parallel flow-through
fish bioassays were also conducted at
each location. The objectives of the
study were as follows: achievement of
adequate disinfection with minimum
use of chlorine; reduction of chlorine-
induced toxicity; and writing of a
design manual. At 7 of the 8 plants
studied the optimized pilot plant
achieved an equivalent level of disin-
fection with significantly lower chlo-
rine dosage, in some cases more than
50% lower, than the full-scale plants.
The pilot plant chlorine residuals were
also lower by the same proportions.
The reasons for the better pilot plant
results were rapid initial mixing,
improved chlorine control, and plug
flow contact.
In most cases the bacterial survival
ratio could be expressed as a function
of the product of chlorine residual and
contact time. There appeared to be,
however, a limiting contact time to
which this relationship applied. The
degree of coliform reduction obtained
during initial mixing appeared to be a
function of chlorine residual.
This Project Summary was devel-
opedby EPA''s MunicipalEnvironmen-
tal Research Laboratory, Cincinnati,
OH, to announce key findings of the
research project that is fully docu-
mented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Increase in population and its mobility,
along with much greater use of waters
for recreation and water supply within
the past two decades have greatly
increased the opportunity for human
exposure to wastewaters discharged to
the environment. From a public health
standpoint, former natural safeguards
such as time, distance, and dilution
have been reduced due to the large
volumes of wastewater that are now
discharged and the numerous points of
use. Consequently, it is essential to
provide effective disinfection of waste-
waters prior to their release to the
environment. In almost all cases this is
accomplished by chlorination of the
treated effluent. Residual toxicity in the
wastewater has, in many instances,
adversely affected aquatic life and
since, in part, this toxicity has been
associated with the chlorination of
wastewater, there is a need to develop
effective chlorination techniques which
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will maximize disinfection and minimize
chlorine-induced toxicity.
The typical chlorination system which
has been used at wastewater treatment
plants does not provide efficient use of
chlorine because of design and opera-
tional deficiencies. Treatment plant
operators have attempted to meet
stringent bacteriological requirements
by changing the only process variable in
the disinfection process over which they
have direct control—the chlorine feed
rate. Increased chlorine dosages may
enable a waste discharger to meet
disinfection requirements but the
practice may result in the costly waste
of chlorine.
The investigation aimed to assess the
feasibility of achieving consistent and
reliable disinfection with the lowest
possible chlorine residual (and the
concurrent reduction of toxicity) through
improved chlorination process design.
Specific objectives were as follows:
1) Assess the efficacy of selected
treatment plants in meeting existing
disinfection standards as a function
of effluent quality, initial mixing,
residence time distribution, chlorine
residual and process control;
2) Assess the toxicity concentrations of
effluents as a function of chlorine
residual and other parameters;
3) Assess the efficacy of a pilot chlorin-
ation system of optimum design in
providing adequate disinfection of
effluents with minimum chlorine
residuals at different contact times;
4) Compare the results from the opti-
mized pilot plant to those of the full-
scale plant in regard to disinfection
efficiency, chlorine residuals and
toxicity; and
5) Identify cost savings achievable by
optimum design.
Results
Two weeks at each site were devoted
to the comparative performance of the
pilot system against the full-scale
system. Table 1 shows the chlorine dose
in the full-scale systems and the pilot
system used to achieve approximately
the same total coliform level after the
same contact time. In all cases shown,
the optimized pilot system used sig-
nificantly lower chlorine dose (18.6
percent reduction at San Pablo to 56.1
percent at Pinole).
The residence time distribution
curves of each chlorine contact tank
studied, along with those of the pilot
plant, were constructed from dye tracer
Table 1. Mean Chlorine Dosage Used by the Optimized Pilot Plant and Full-Scale
Plants in Comparative Studies"
Location
Chlorine Dosage, mg/lb
Full-Scale Plant Pilot Plant
Percent Reduction
in Pilot Plant
San Pablo
Pinole
Sacramento
Roseville
Dublin-San Ramon
Ross Valley
10.2 ± 0.9
37.4 ± 9.9
10.5 ± 2.0
10.6 ± 3.2
16.7 ± 3.7
14.8 ± 8.5
8.3 ± 0.4
16.4 ±8.1
5.8 ± 1.3
4.7 ±0.9
11.6± 1.1
6.7 ±0.4
18.6
56.1
44.8
55.7
30.5
54.8
Overall Mean
16.7 ± 10.5
8.9 ± 4.4
46.7
"During daytime sampling period.
^Arithmetic mean ± standard deviation.
studies performed during daily peak
flow periods. The calculated dispersion
data are shown in Table 2. It is evident
from the table that the pilot tank
provided better plug flow hydraulics
than most of the full-scale tanks. Also,
the calculated t, (time to the first
appearance of tracer) was longer than
that of 6 of the full-scale tanks shown.
However, 3 of the baffled tanks had a
smaller dispersion number than the
pilot tank. This indicates that use of the
dispersion number alone is not sufficient
for the proper evaluation of a tank, and
that the t, values and the extent of dead
space must also be considered.
From Table 2 it can also be seen that
the length-to-width ratio (L/W) does not
adequately describe plug flow charac-
teristics. The pilot tank had a much
higher L/W ratio than the full-scale
tanks but did not have a correspondingly
smaller dispersion number. Other
studies have indicated that the dispersion
number usually decreases with increas-
ing L/W ratio. Apparently factors other
than L/W ratio play a role here, such as
the depth-to-width ratio (H/W) and the
extent of dead space. It appears from the
data that the H/W ratio should be 1.0 or
less.
Another factor which may influence
contact tank design is the flow velocity.
All the full-scale baffled tanks had
average flow velocities greater than 105
cm/min. (3.5 ft/min.) at peak flow,
whereas the pilot tank had a flow
velocity of only 46 cm/min. (1.5 ft/min).
An adequate flow velocity may help to
keep the suspended solids in suspension
and reduce dead spaces. |
Mixing studies were carried out on
the pilot plant and at those full-scale
plants where it was possible to sample
immediately after mixing. The pilot plant
contained two static mixers: one a 76
mm (3-inch) tee in 76 mm (3-inch) pipe,
the other a 38 mm (1.5 inch) tee. The
chlorine solution was injected through
the tee into the flow stream through a
13 mm (1 /2-inch) tube. The 76 mm (3-
inch) mixer had a Reynolds Number
Table 2. Chlorine Contact Tank Dispersion Data from Dye Tracer Studies"
Treatment
Plant
San Leandro
San Pablo
Pinole
So. San Francisco
Sacramento
Roseville
Dublin -San Ramon
Ross Valley
Pilot Plant
Pilot Plant
L/W
—
15
40
—
—
60
22
42
135
270
H/W
—
0.6
1.0
—
—
1.0
1.0
0.2
2.0
2.0
t,/T
0.23
0.40
0.35
0.24
0.54
0.65
0.66
0.77
0.63
0.71
tm/T
0.57
0.68
0.66
0.75
0.74
0.87
0.89
0.85
0.82
0.92
fa/7"
1.11
0.80
0.76
1.80
1.00
0.92
0.89
0.90
0.94
1.03
d
0.170
0.038
0.029
0.260
0.050
0.008
0.008
0.010
0.023
0.021
T, min"
44
74
48
25
81
104
60
23
60
120
"L = length; W = width, H = height; T = theoretical detention time; t, - minimum
detention time; fm = modal (peak) detention time; ra = average detention time;
d = dispersion number.
tiAt time of test. 4
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38,500 and a G value of 115 sec \
whereas the 38 mm (1.5 inch) mixer had
a Reynolds Number of 77,000 and an
estimated G value of 875 sec"1. The G
value is the mean velocity gradient in a
shearing fluid. The G value is used to
estimate the energy spent on mixing the
fluid; it is defined as G = (P/u)05, where
P is power input per unit volume and u is
the absolute viscosity of the fluid. In
addition to the tee there was a 0.76 m
(30-inch) length of pipe before the
sampling point of the residual analyzer
was reached, and an additional length
of 50 mm (2-inch) pipe to a sampling tap.
A mixing length of 10 pipe diameters is
believed to be adequate in tubular
mixers provided that the chlorine
solution is injected into the center of the
flow stream. The total detention time in
piping to this sampling tap was approx-
imately 20 seconds.
The studies at San Leandro, San
Pablo and Pinole were done with the 76
mm (3-inch) tee; the studies at
Sacramento and Dublin-San Ramon
were done with the 38 mm (1.5 inch)
tee. At other sites the 38 mm tee was
used, but no adequate data were
obtained.
Results of the pilot plant mixing
studies are shown in Figures 1 and 2.
The data points depict the total coliform
reduction in the tubular mixer after 20
seconds detention time. The reduction
appears to be related to the chlorine
residual, i.e., the higher the residual,
the higher the percent reduction. Figure
1 shows the results with the 76 mm (3-
inch) mixer which has a low G value.
Although there is a wide scatter of
points, there is a clear trend toward
higher degree of coliform kill with
increasing chlorine residual. Figure 2
shows the results for the 38 mm (1.5
inch) mixer. There is much less scatter
in the data points. This indicates that the
38 mm (1.5 inch) tee does a better job of
mixing, apparently because it creates a
higher degree of turbulence. However,
the percent coliform reduction is lower
than in this mixer which may be due to
differences in water quality.
Regarding the full-scale plants, 4 of
them used turbine mixers and 2 used
hydraulic jumps. The G-values ranged
from 250 to 550 sec"1 and total coliform
reductions were all at least 99%. The
data obtained in this study are not
adequate to determine which type of
mixer is the best one. However, it is
clear that rapid mixing is very important
i achieving adequate disinfection
efficiency. It appears that a mixer that
achieves a total coliform reduction of at
least 99% is adequate. The turbine
mixers used for chlorination are usually
designed for a G value of 500-1,000
sec, and this appears to be an ap-
propriate range.
Conclusions
1. At all the seven wastewater
treatment plants where compara-
tive studies were made, the
optimized pilot system used sig-
nificantly less chlorine than the
existing full-scale systems. In
some cases the chlorine dosage
saved by the optimized system
was in excess of 50%. At these
treatment plants the chlorine
residuals in the pilot plant effluent
were significantly lower than
those in the full-scale effluents.
2. Tracer tests were necessary to
assess adequately the perform-
ance of chlorine contact tanks.
The use of the length/width ratio
alone was found to be not suf-
ficient.
90
z
= 33
.o
a
99.9
99.99
o
o
Log (/V,//V0 x 100)= -0.275R +7.722
r = 0.68
F = 37.3
Q
9
Figure 1.
24 6 8 JO
Chlorine Residual, mg/l
Coliform kill in 76 mm mixer as a function chlorine residual.
12
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90
99
99.9
99.99
Log (/V,//V0 x 700) = -0.223R + 2.087
r = 0.87
F = 69.3
6 a
Chlorine Residual, mg/l
10
12
better plug flow and longer mini-
mum contact time.
6. With effluents containing high
levels of suspended solids the
chlorine residual analyzer usec
clogged up and did not perform
well.
7. Significant cost savings can be
effected by good design anc
operation of chlorination systems
The cost savings realized are
mainly due to savings in chemical
costs.
8. The design of wastewaterchlorin
ation facilities, following second
ary or higher treatment, shouk
include the following optimum
features: a) rapid initial mixing, b
reliable and well adjusted auto
matic chlorine residual control
and c) adequate contact time (a
least 30 minutes at maximurr
flow) in a well designed contac
tank approaching plug flowcondi
tions.
9. Operator attendance is mandaton
to keep the chlorine controls ii
order, including daily cleanini
and calibration of the chlorine
residual analyzers. Therefore
better operator training is neces
sary in chlorination system operaj
tion and maintenance.
10. Small treatment plants whicl
cannot afford a closed loop chlo
rine control system should, as i
minimum, install flow proportiona
control paced on effluent flow.
The full report was submitted it
fulfillment of Grant No. S-803459 b'
the State of California Water Resource:
Control Board under the sponsorship o
the U.S. Environmental Protectioi
Agency.
Figure 2. Coliform kill in 38 mm mixer as a function of chlorine residual.
3. Total coliform destruction during
initial mixing was a function of
chlorine residual; i.e.,'the higher
the residual the higher the degree
of destruction.
4. The coliform destruction observed
in the full-scale chlorine mixers
was greater than 99%.
5. The pilot plant chlorine control
system performed significantly
better than the full-scale systems,
because of the following factors;
a) very short loop time, b) adequate
initial mixing, c) constant flow
rate, d) instrument compatibility,
and e) good operation and main-
tenance. In most cases the pilot
system maintained the control
residual within the desired ± 0.5
mg/l range. It appears that poorly
designed and/or operated chlorine
control systems were responsible
for a major portion of the excessive
chlorine dosage used at most of
the full-scale plants studied.
The pilot chlorine contact tank
also performed better than most
of the full-scale tanks due to
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Endel Sepp and Paul Bao are with the California Department of Health Services,
Sanitary Engineering Section, Berkeley, CA 94704.
Albert D. Venosa is the EPA Project Officer (see below).
The complete report, entitled "Design Optimization of the Chlorination Process:
Volume 1. Comparison of Optimized Pilot System with Existing Full-Scale
Systems," (Order No. PB 82-100 835; Cost: $ 12.50, subject to change} will be
available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
*U.S. GOVERNMENT PRINTING OFFICE :1 981--559-092/3313
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United States Center for Environmental Research Fees Paid
Environmental Protection Information Environmental
Agency Cincinnati OH 45268 Protection
Agency
EPA 335
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