&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

-------
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

-------
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

-------
      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

-------
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

-------
United States                      Center for Environmental Research                                Fees Paid
Environmental Protection              Information                                                 Environmental
Agency                           Cincinnati OH 45268                                          Protection
                                                                                         Agency
                                                                                         EPA 335

Official Business
Penalty for Private Use $300


RETURN POSTAGE GUARANTEED                                                                Third-Class
                                                                                         Bulk Rate
                      LUU  W  TILLEY
                      REGION  V  EPA
                      LIBRARIAN
                      230  S  DtAKBURN  ST
                      CHICAGO IL  60&Q4

-------