United States
Environmental Protection
Agency
/I
6
Municipal Environmental Research**
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-169 Oct 1981
Project Summary
Wastewater Dechlorination
State-of-the-Art Field
Survey and Pilot Studies
Ching-lm Chen and Henry B. Can
A study of dechlorination was con-
ducted in the County Sanitation
Districts of Los Angeles County to
determine the utility and efficiency of
the sulfur dioxide method and to
provide a cost-effective comparison
of sulfur dioxide and two other
methods of dechlorination, namely,
activated carbon and holding tank
processes. Study objectives were
accomplished through three main
phases of work: literature review,
pilot-scale testing, and full-scale
evaluation in the field.
The literature review involved an
extensive search on the practice of
dechlorination in the United States
and abroad, an assessment of the need
for reaeration and pH adjustment in
the dechlorinated effluent, and an
examination of the extent of bacterio-
logical aftergrowth in outfall pipelines
and receiving waters.
The pilot-scale testing indicated
that no degradation of physical and
chemical water quality occurred in the
dechlorinated effluents from any of
the three dechlorination processes
investigated. However, a one to two
order of magnitude increase in total
coliform density in the 10-minute
samples following dechlorination was
commonly observed among the three
dechlorination processes. The increase
seemed to originate from contamina-
tion by the existing microorganism
communities in the dechlorinated
effluent rather than from the reactiva-
tion of injured bacterial cells.
The field survey involved the can-
vassing of 55 operating plants in
California by mail, telephone, and site
visits to selected facilities. The feed
forward method of sulfur dioxide
dosage control with signals received
from both a flow and residual chlorine
controller appeared to be the most
commonly employed method. Although
overdosing of sulfur dioxide was
frequently necessary to meet the
residual chlorine discharge standards,
most installations found pH adjust-
ment and reaeration of the dechlori-
nated effluent unnecessary.
Process cost estimates based on the
field survey and pilot-plant study have
been prepared for all three dechlorina-
tion processes. The sulfur dioxide
process seems to be the most cost-
effective method for dechlorination.
The Project Summary was developed
by EPA's Municipal Environmental
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
Because of growing concern over the
effects of chlorine and chloramines on
the aquatic environment, dechlorination
has become an important unit process
to be considered as part of a wastewater
treatment system employing chlorina-
tion as its disinfection process. The
chlorine residuals, either free chlorine
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or chloramines, have been well demon-
strated to be toxic to fish and other
aquatic organisms. Therefore, the
regulatory agencies are in the process
of establishing, or have already estab-
lished, chlorine effluent standards for
wastewater discharges. These estab-
lished standards or proposed criteria for
chlorine residuals are normally dictated
by the residual chlorine detection limits.
The County Sanitation Districts of Los
Angeles County are required by the
California Regional Water Quality
Control Board to provide dechlorination
facilities at their water reclamation
plants for chlorine residual control. The
total chlorine residuals allowed in these
plant effluents, which are discharged
into nearby creeks or rivers, are equal to
or less than 0.1 mg/l. This study was
initiated as a result of the need for
dechlorination in the County Sanitation
Districts' facilities. The study had three
main objectives:
1. To establish on a pilot scale the
utility and efficiency of sulfur
doixide for dechlorinating chlori-
nated municipal wastewater treat-
ment plant effluent.
2. To demonstrate on a full scale the
cost-effectiveness of sulfur dioxide
dechlorination under actual oper-
ating conditions.
3. To examine the cost-effectiveness
of other methods of dechlorination
(i.e., activated carbon and holding
tank processes)
The pilot-plant study was conducted
at the County Sanitation Districts'
Advanced Wastewater Treatment Re-
search Facility, Pomona, California The
three methods of dechlorination evalu-
ated were sulfur dioxide, granular
activated carbon, and holding tank
impoundment. The schematic flow
diagram for these processes is shown in
Figure 1 Emphasis was placed on the
sulfur dioxide process because of its
potential for being the most cost-
effective method for dechlorination. The
granular activated carbon method had
been investigated previously at the
same research facility, and the results
are included in the final project report
The holding tank impoundment method
was evaluated concurrently with the
sulfur dioxide method.
The full-scale evaluation was con-
ducted by means of a field survey of all
California treatment plants that practiced
dechlorination by any means The
primary objectives of the field survey
were to assess the effectiveness and
reliability of actual full-scale dechlorin-
Sulfur Dioxide
Chorinated
Effluent
' *
Analyzer
\
1
XI
Drain
1
Sulfonator
I
Contact Chamber
Mixing
Chamber
Dechlorinated
Effluent
Activated Carbon
Chlorinated
Effluent
Dechlorinated
Effluent
Holding Tank
Chlorinated
Effluent
Holding Tank
Dechlorinated
Effluent
Note.
Electrical Signal
Liquid Flow
Figure 1. Flow diagram of dechlorination pilot-plant systems at Pomona, California
ation installations. Information on the
methods of control for the sulfonation
system, cost-effectiveness, and the
bacteriological aftergrowth in the
dechlonnated effluent were also re-
quested through the field survey, which
was conducted with both questionnaire
correspondence and site visit followup
Results
The most common layout of a sulfur
dioxide dechlorination system employed
in the plants visited is shown in Figure
2. As indicated in the figure, a feed
forward residual signal and a feed
forward flow signal were fed to the
sulfonator. These two signals were
sometimes combined into a product
signal through an electronic multiplier
before feeding to the sulfonator. This
was done to avoid having to excessively
overdose the chlorinated effluent with
sulfur dioxide.
The feed forward control system
requires an overdosing of sulfur dioxide
to accomplish the stringent dechlorina-
tion goals. Such an overdosing cost may
become a significant factor in large
dechlorination installations. Alternate
sulfur dioxide control systems as showntf
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Chlorinated
Effluent
Note-
pi
I ,
^ Chlorine
" Analyzer
\
Dram
Sample
Solution
ectncal Signal
heed horward
Residual Signal S02 Solution
1
1
i y~J Station \
₯,' \ Feed Forward
v ' Multiplier i ~i y
61
Dechlorinated
Effluent
Figure 2. Feed control system most commonly employed in sulfur dioxide dechlo-
rination facilities in California.
in Figure Swill reduce the sulfur dioxide
overdose requirement and hence the
operating chemical cost.
According to the field operators
contacted, the weakest link m a sulfur
dioxide feed control system was the
chlorine residual analyzer. The measur-
ing electrode of the chlorine analyzer
lost its sensitivity rapidly in a de-
( chlorinated effluent. The presence of
some amount of chlorine residual
seemed to help prevent polarization of
the electrode. The abrasive grits in the
measuring cell block were found unable
to prevent oxides from forming on the
electrode in the absence of chlorine
residual.
During the site visits, it was also
learned that poor performance was
experienced with the chlorine analyzer
when the plant received a high propor-
tion of industrial wastes. Plugging of the
measuring cell block frequently occurred.
However, the operators of the dechlorin-
ation facilities were generally satisfied
with the reliability of their sulfonators
No significant depletion of dissolved
oxygen or reduction in pH was observed
in the pilot-plant studies at a sulfur
dioxide to residual chlorine dosage ratio
of as high as 2 to 1. Therefore, no
reaeration or pH adjustment for the
dechlorinated effluent was found nec-
essary in the pilot-plant studies The
field survey results also indicated that
more than 97 percent of the dechlorina-
tion facilities in California did not have
the need for pH adjustment and reaera-
tion for their dechlorinated effluents.
The pilot-plant studies indicated that
both sulfur dioxide and activated carbon
I adsorption are very efficient and rapid
dechlorination processes, in contrast to
the rather slow holding tank process. In
the latter, the average dissipation rate
for residual chlorine was found to be
about 1 mg/l for every 20 hours. Thus,
the volume and land area requirements
for dechlorination by holding ponds are
considerable.
Bacteriological aftergrowth in some
microorganism populations was found
in all the dechlorination processes
investigated. This was observed pre-
Note.
dommantly in the total coliform group.
Figure 4 indicates typical results in a
sulfur dioxide dechlorination system.
Some increases in fecal coliforms and
other bacteria (as detected in the total
plate count) were also found in the
dechlorinated effluents Salmonella
A/vas not detected in most of the
samples. Fecal streptococci in the
effluent remained relatively unchanged
after dechlorination. The bacterial
increases in the dechlorinated effluents
seemed to be attributed to contamina-
tion by the microorganism communities
existing in the dechlorinated effluent
rather than reactivation of injured
bacterial cells. The rate of contamina-
tion after initial startup of the dechlorin-
ation systems is shown in Figure 5,
which indicates a saturation level of
contamination established after 5 days
of operation.
No significant chemical-physical
degradation of the dechlorinated ef-
fluent quality was found for the sulfur
dioxide, carbon adsorption and holding
tank dechlorination processes.
A comparison of total process costs,
including capital, operation and main-
tenance costs, for sulfur dioxide, carbon
adsorption and holding pond dechlorin-
ation processes is presented in Figure 6.
Clearly, the su If ur dioxide process is the
most cost-effective method for dechlorin-
Alternate No. 1
Chlormatet
t /ectncal signal
Liquid Flow
Sulfonator
j Set Point j
i Analyzer to |
| 70/7 Ratio of \
| Limit |
L J
f Alternate No. 2
Feed Back
Residual "Signal Cn/l
Ana
t
Feed Forward
Flow Sjqnal
1 /**\ Flowmeter >
\ t
Dram '
SO2 Solution
| Biased j
jrine | Residua/
lyzer | Chlorine |
| Signal \
L_ J
Sample Solution
Dechlorinated
Figure 3. Feed control systems used in dechlorination facilities to avoid excessive
SO 2 overdose
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Chlorine Dosage =13 mg/l
C/2 Res. Before DE C/2 = 3.5 mg/l
C/2 Res. After DE C/2 = 0 mg/l
Approx. SO2.C/2 Ratio =1-1
70%
20
Period
After
Chlor-
/nation
(hours)
1 2
Period
After
SO2
Dechlor-
mation
(hours)
Figure 4.
Pilot-plant observation of
total coliform before and
after dechlormation with
S02.
ation, particularly in the State of
California. The carbon adsorption
process is substantially more expensive
than the other two dechlormation
processes investigated. It is not eco-
nomically feasible to use the carbon
adsorption process solely for dechlorin-
ation purposes. The holding pond
process may become more cost-effective
than the sulfur dioxide process for
dechlorination, only if an inexpensive
land is available and a simpler pond
construction is acceptable.
Recommendations
Both field survey and pilot-plant study
results indicated that a more reliable
chlorine analyzer should be developed
to perfect the automation of the sulfur
dioxide feed control system The effects
of organic loading on the carbon
capacity for dechlorination should be
thoroughly evaluated
.C
5
o
o
<0
1 log 1 order of magnitude
Total Coliform MPN Prior
to DE C/2/ < 2 2/7 00 ml
Holding Pond
Sulfur Dioxide, ;* / /
10 Minute Sample
Carbon, 10 Minute Sample
I
Figure 5.
12341
Period After Startup, days
Rate of contamination after initial startup in clean dechlorination pilot-
plant systems.
The full report was submitted by the
Sanitation Districts of Los Angeles
County, Whittier, California, in fulfill-
ment of Contract Nos. 14-12-150 and
68-03-2745 under the partial sponsor-
ship of the U.S. Environmental Protec-
tion Agency.
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a
8*
tj 9
O ^
Plant Size = 438 I/sec (10 mgd)
Effluent Chlorine Residual ^ 0 05 mg/l
-Carbon Adsorption
Holding Pond*
'Sulfur Dioxide
18
16
-H
O
"§
468
Influent Chlorine Residual, mg/l
10
72
Figure 6. Process cost comparison among different dechlorination processes.
Ching-lin Chen and Henry B. Can are with the County Sanitation Districts of Los
Angeles, Whittier, CA 90607.
Albert D. Venosa and Irwin J. Kugelman are the EPA Project Officers (see
below).
The complete report, entitled "Wastewater Dechlorination State-of-the-Art
Field Survey and Pilot Studies." (Order No. PB 82-102 336; Cost: $11 00,
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 Officers can be contacted at'
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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