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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