Effect of Advanced Water Treatment on Behavior of Residual Chlorine in Drinking Water
and Development of Method for Chlorine Concentration Control in Water Distribution
Systems
T.Fuchigami*, M.Takeda** and K.Terashima***
*Water Examination Laboratory, Osaka City Waterworks Bureau, 1-3-14 Kunijima, Higashiyodogawa-ku, Osaka-shi,
Osaka 533-0024, JAPAN(E-mail: [email protected])
**Water Examination Laboratory, Osaka City Waterworks Bureau, 1-3-14 Kunijima, Higashiyodogawa-ku, Osaka-shi,
Osaka 533-0024, JAPAN(E-mail: [email protected])
***Water Examination Laboratory, Osaka City Waterworks Bureau, 1-3-14 Kunijima, Higashiyodogawa-ku, Osaka-shi,
Osaka 533-0024, JAPAN(E-mail: [email protected])
Annotation
At Osaka City Waterworks Bureau, we continuously monitor drinking water quality by telemeters located at key points
in the city's supply area. From 1998 to 2000, our water treatment process was switched from the mid-chlorination water
treatment(MCWT) to the advanced water treatment(AWT). As a result, we have successfully reduced the average
residual chlorine concentration in drinking water within the service area by 0.21 mg L-1 while satisfying the legally
required lowest concentration limit. The reason for this was because the AWT process reduced the amount of organic
matter dissolved in the finished water. By analyzing the data collected by telemeters, we confirmed that the overall
first-order chlorine reduction rate coefficients of the AWT were reduced to approximately half those of the MCWT. We
also researched into the the chlorine volatilization from the surface of reservoir water. Laboratory-scale experiments
confirmed that the reduction in chlorine concentration caused by volatilization can be analyzed by applying the
zero-order rate equation. Based upon these results, we created the simulation models for chlorine concentration control
in Osaka city. The models can be utilized to properly change the chlorine control values at the outflow in the water
purification plants and the injection rate of additional chlorine.
Keywords
advanced water treatment, reduction rate coefficient, residual free chlorine, simulation model, water distribution system
INTRODUCTION
The article 22 of the water works law in Japan describes sanitary measures of drinking water.
Based on the article, the ordinance for enforcement of the water works law defines as “All water
works authorities shall disinfect water by chlorine so that tap water shall contain 0.1 mg L-1 free
residual chlorine or more”. To satisfy the order, all waterworks authorities in Japan control the
chlorine concentration in their water supply network to maintain more than 0.1 mg L-1.
At Osaka City Waterworks Bureau (OCWB), we continuously monitor the quality of drinking
water using water quality remote monitoring devices (telemeters: TMs) installed at key points
throughout the city’s water supply network. These monitoring items include free residual chlorine
and other items. We use the monitoring data for the detection of accidental water quality
deterioration and to control the free residual chlorine concentration in the drinking water distributed
to the whole city area as low as possible at a predetermined target value.
From 1998 to 2000, we switched our water treatment process from the mid-chlorination water
treatment (MCWT) process to the latest advanced water treatment (AWT) process. We analyzed the
behavior of free chlorine concentration in the network before and after the introduction of AWT
using the stored data measured by telemeters. The analysis result shows that the behavior of
chlorine in the network is obviously different when the water treatment method has changed.
This paper reports the behavior of residual free chlorine in the supply network, especially on the
effects of the AWT process on the control of free chlorine concentration and the change in free
chlorine concentration reduction rate in the water distribution network. We also mention about the
residual chlorine simulation system developed for Osaka city based on the chlorine behavior
research mentioned above and the results of laboratory experiments.
WATER PURIFICATION PROCESS AND AUTOMATED WATER QUALITY REMOTE
MONITORING SYSTEM OF OSAKA CITY
Osaka City, which has an area of about 222 km2 and a population of about 2.6 million people, is
located close to the center of Japan. OCWB purifies drinking water at Kunijima Water Purification
Plant (purification capacity: 1.18 million m3day-1), Niwakubo Water Purification Plant (purification
capacity: 0.80 million m3day-1), and Toyono Water Purification Plant (purification capacity: 0.45
million m3day-1) and distributes purified water to every location within the city area. The quality of
raw water from the Yodo River was good until about the 1950s, but it worsened since then. So with
a view to improving the comprehensive quality of tap water, especially for eliminating musty odor
and reducing trihalomethane, OCWB has changed the water purification method to the AWT with a
two-stage process of ozonation and granular activated carbon (GAC) treatment. The conventional
MCWT and AWT processes are shown in Figure 1. Sodium hypochlorite solution is used as the
chlorine agent.
(a) MCWT (conventional)
Raw Water
chlorine
Coagulation/Sedimentation
chlorine
Rapid Sand Filter
Finished Water
chlorine
(b) AWT
Raw Water
Coagulation/Sedimentation
Figure 1
Mid-O3
Rapid Sand Filter
Post-O3
Finished Water
GAC
Flow chart of water purification treatment in Osaka City
φ1500
φ1500
OCWB has the automated water quality remote monitoring system in order to continuously check
the tap water quality and control residual free chlorine at the proper concentration in the water
distribution network. This monitoring system consists of 40 telemeters located at 31 taps and
outflows of 9 water distribution plants in the supply area. Each telemeter measures six water quality
items (free residual chlorine, water temperature, turbidity, color, pH value, and electric
conductivity) and transmits the data to Water Examination Laboratory and Water Distribution
Information Center using the dedicated line. The display device
Niwakubo W.P.P
installed in the laboratory can show the trend lines, list tables and
put data on the maps, which are based on updated measuring
values in any 1 minute, on the monitor screen. Each water quality
0
2km
instrument in the telemeter keeps an accurate measurement by
monthly routine maintenance and a yearly precise checkup.
φ150
0
φ1500
φ1500
W.D.P:
Water Distribution Plant
φ1500
W.P.P:
Water Purification Plant
Tatsumi
W.D.P
φ1350
TM③
φ1500
φ900
φ150
0
φ1000
(tap)
φ300
ESTIMATION OF OVERALL RATE COEFFICIENTS FOR
THE REDUCTION IN RESIDUAL FREE CHLORINE
CONCENTRATIONS IN THE WATER DISTRIBUTION
PROCESS
The reduction in chlorine concentrations in the water
distribution system in Osaka City was analyzed using a first-order
rate equation based on the results of telemeter measurement [1].
Figure 2 shows the water distribution system analyzed. As
represented in the figure, sodium hypochlorite is added to water
in Niwakubo Water Purification Plant. Telemeter ① was installed
at the outflow site of Tatsumi Water Distribution Plant and
Telemeters ② and ③ were at the taps. The residence time from
the outflow site of the water purification plant to Telemeters ①,
② and ③ was 6.5, 8.8 and 17.0 hours (in the case of the water
supply from Niwakubo Water Purification Plant: 364,400 m3
day-1).
TM: telemeter
φ： pipe diameter (mm)
TM①
(outflow)
φ1000
φ1200
φ1500
φ600
00
φ6
TM②
(tap)
Figure 2 Water distribution
system analyzed
The reduction rate of free chlorine concentration in the water distribution system, rall, is expressed
by the following equation:
rall = −
dC
= k all C
dt
(1)
where C is a free chlorine concentration measured by each telemeter installed in the water
distribution system, t is the residence time from the outflow site of the water purification plant to
each telemeter and kall is the observed reduction rate coefficient of free chlorine concentration in the
water distribution system, which is estimated as the slope of the regression line obtained from
plotting the logarithmic ratios of C to free chlorine concentration at the point where the water is
released from the water purification plant, C0 (ln C/C0) against t.
LABORATORY EXPERIMENTS
The two factors that reduce free chlorine concentration in the water distribution system was
determined separately by laboratory experiments, i.e., (a) bulk phase free chlorine consumption
caused by the reaction between free chlorine and dissolved organic matters [2], (b) reduction in free
chlorine concentration caused by volatilization from the water surface. Free chlorine concentrations
were measured by the N,N’-diphenyl-p-phenylenediamine colorimetric procedure using a
spectrophotometer (Shimadzu Corp.).
Test waters
All experiments were carried out with the MCWT and AWT waters which were treated at
Niwakubo Water Purification Plant. Both were the outflows of water stored in the clear water
reservoir for several hours after chlorine injection at the purification plant. Chlorine-solution-added
purified water adjusted at pH 7.5 with dilute sodium hydroxide solution and dilute sulfuric acid
solution was also used.
Apparatus and reagents
Glass-stoppered 25 mL test tubes and 2000 mL beakers (inner diameter: 125 mm, height: 200
mm) were used for the experiment. This glass apparatus was soaked in sodium hypochlorite
solution prior to the experiment and free chlorine consumption by the glass surface was eliminated.
To prepare the chlorine solution, chlorine gas was generated from sodium hypochlorite and
absorbed in sodium hydroxide solution, subsequently the solution was adjusted at pH 8 with
sulfuric acid and diluted with purified water. The purified water used was Milli-Q water (Millipore
Corp.).
Methods
(a)Experiment on the free chlorine consumption in bulk phase
Several glass-stoppered test tubes were filled with a water sample and sealed with a stopper.
Subsequently, the test tubes were kept dark in a constant-temperature oven at a specified
temperature. At the same time, free chlorine concentration in one test tube was measured and
regarded as the initial concentration (C0). Free chlorine concentration in each tube was measured at
predetermined intervals.
(b)Experiment on the reduction in free chlorine concentration caused by volatilization from a water
surface
First, 1500 mL of purified water was put into a 2000 mL beaker and the beaker was kept dark in a
constant-temperature oven to attain a predetermined water temperature. Subsequently, a chlorine
solution was added to purified water up to the predetermined free chlorine concentration and stirred.
Several glass-stoppered test tubes were filled with the mixture in the beaker and sealed with each
stopper. After accurately measuring the volume of solution in the beaker to be 1000 mL, the beaker
in which the water sample was continuously stirred and test tubes were kept dark in a
constant-temperature oven. Free chlorine concentration was measured simultaneously and regarded
as the initial concentration (C0). After a predetermined period, a 20 mL water sample was collected
from the beaker and the free chlorine concentration was measured. The free chlorine concentration
in one of test tubes was also determined. The constant-temperature oven was attached with a
vent-hole, so it was therefore not a closed environment.
RESULTS AND DISCUSSION
Change of the residual chlorine concentration in Osaka City after the introduction of AWT
We compared free chlorine concentrations measured Table 1 Comparison the residual free
at taps before and after the AWT process was chlorine concentrations measured by
introduced. Table 1 shows the maximum, minimum, telemeters in Osaka City
mean and standard deviation values of monthly average
MCWT
AWT
free chlorine concentrations that were measured in
（FY1997） （FY2000）
FY1997 and FY2000 at 20 telemeters excluding those
-1
mg
L
0.98
0.65
Max
at water distribution plants. The data for FY1997 were
-1
mg
L
Min
0.31
0.30
obtained before the AWT was introduced, whereas the
-1
mg
L
Mean
0.64
0.43
data for FY2000 were obtained after completion of the
-1
mg
L
SD
0.14
0.06
supply of the AWT water throughout the whole city. As
SD: standard deviation
can be seen from Table 1, the minimum free chlorine
concentrations in FY1997 and FY2000 were almost equal. This is because, in both fiscal years, we
controlled the free chlorine concentrations so that its minimum value would be maintained at 0.3
mg L-1 at each telemeter. The maximum and yearly average concentrations in FY1997 were higher
than in FY2000 with differences of 0.33 mg L-1 and 0.21 mg L-1, respectively. The standard
deviation, a statistical measure of data dispersion, was also higher in FY1997 than in FY2000.
To clarify the effects of water quality improvement due to the introduction of the AWT process
on reduction in the deviation of free chlorine concentration in Osaka City water service area, we
compared the free chlorine concentrations in the summer season (water temperature; 27－30°C)
that had been measured at nine telemeters in the area supplied by the Niwakubo Purification Plant.
The results are shown in Figure 3. This Figure is the histograms of daily average concentrations
with graduations of 0.05 mg L-1, which were measured in August of 1998 and 2000. In August 1998,
the Niwakubo Purification Plant was still treating water using the MCWT process, while the Plant
was operating the AWT process in August 2000. At the Niwakubo Purification Plant, the chlorine
concentrations of water released from the plant during those months were controlled to constant
values of 1.0 mg L-1 and 0.8 mg L-1, respectively. As is evident from Figures 3, the histogram for
the conventional MCWT water exhibited a flat, wide distribution pattern with a low peak value,
whereas the histogram for the AWT water showed a narrow distribution pattern with a mode of
approximately 0.4 mg L-1. Since both histo120
grams exhibited normal distribution patterns
*SD: standard deviation
MCWT
on the whole, we estimated the free chlorine
-1
100
SD
=
0.045mg
L
distributions in the summer season from the
AWT
80
standard deviations of the measured values
shown in the Figure. As a result, we
60
SD = 0.108mg L-1
determined that, for the MCWT process
40
during the summer season, 95% of the free
chlorine concentration in the investigated
20
service area had stayed within the range of
0
0.56±0.22 mg L-1, whereas for the AWT
process, the concentration had remained
within the range of 0.40±0.09 mg L-1. These
results indicate that water quality improveChlorine concentration (mg L-1)
ment by the introduction of the AWT process
is helpful in lowering and leveling residual Figure 3 Comparison of the histograms of free
free chlorine concentrations in Osaka City chlorine concentrations between the conventional
MCWT water and the AWT water
water service area.
*
frequency
AWT
0.80～0.85
0.75～0.80
0.70～0.75
0.65～0.70
0.60～0.65
0.55～0.60
0.50～0.55
0.45～0.50
0.40～0.45
0.35～0.40
0.30～0.35
0.25～0.30
MCWT
Changes of the overall free chlorine reduction rate in the water distribution system after the
introduction of AWT
To investigate changes in the reduction rate in free chlorine concentrations after the introduction
of AWT, the data obtained from the telemeters in the water distribution system shown in Figure 2
was analyzed using a first-order rate equation. The overall reduction rate coefficient of free chlorine
concentration, kall (hr-1) was estimated everyday from the free chlorine concentration measured at
the outflow of Niwakubo Water Purification Plant, the outflow of Tatsumi Water Distribution Plant
(Telemeter ①) and the taps (Telemeter ② and ③), C (mg L-1) and the residence time from the
water purification plant to each point, t (hr). The relationship between kall and water temperature T
(°C) in the MCWT and AWT is shown in Figures 4a and 4b, respectively. Free chlorine
concentrations at the water releasing point of the purification plant in the MCWT and AWT, C0
were 0.8－1.2 and 0.7－0.8 mg L-1, respectively, and t was recalculated separately each day using
the ratio of water supply quantity.
0.06
0.06
[a]MCWT
(conventional)
0.04
0.03
0.02
0.01
○
kall: distribution process(FY1998)
0.00
Bulk rate coefficient kb(hr )
-1
0.05
-1
Overall rate coefficient ｋall(hr )
-1
Overall rate coefficient kall(hr )
●
kall: distribution process(FY1999)
bulk water(test-tube study)
□ kb :
0.05
0.04
y = 0.0097exp(0.0388x)
[b]AWT
0.03
0.02
0.01
y = 0.0017exp(0.0742x)
0.00
0
5
10
15
20
25
Water temprature T（℃）
30
35
0
5
10
15
20
25
30
35
Water temperature T（℃）
Figure 4 Relationship between the overall reduction rate coefficients of chlorine concentration
and water temperature in Osaka City (a: MCWT, b: AWT)
Consequently, kall was estimated to be 0.015－0.059(hr-1) in the MCWT and 0.008－0.032(hr-1) after
introducing the AWT, and it was confirmed that kall in the AWT was reduced to almost half of that
in the MCWT. Based on the results of exponential regression obtained from plotting representative
kall values in Figure 4b, kall after the introduction of AWT is represented by the following equation:
kall = 0.0097 exp (0.0388 T )
(2)
Figure 4b also shows an observed reduction rate coefficient kb (hr-1), which was estimated from
analysis of the reduction in free chlorine concentrations in test tubes filled with the AWT water
using a first-order rate equation. The reduction rate due to free chlorine consumption in bulk phase
rb is represented by the following equation:
rb
= −
dC
dt
= k b C　(3)
kall was always greater than kb and the difference between kall and kb was probably derived from the
reduction of free chlorine due to contact with the inner surfaces of water supply pipes and structures
[3] and volatilization from the surface of reservoir water.
Factors that influence chlorine reduction and reaction kinetics analysis
(a) Chlorine consumption in bulk phase
kb (hr-1) of chlorine-solution-added purified water, water in the MCWT, water immediately after
the introduction of AWT and water in the AWT after a lapse of two years and five months from the
kb = 0.0017 exp (0.0742 T )
(4)
(b)Reduction in chlorine concentration caused by
volatilization from a water surface
The reduction in free chlorine concentration was
determined in chlorine-solution-added purified water in a
beaker. Consequently, it was found that the results plotting
the concentration C to time t fitted a linear regression quite
well (Figure 6). On the other hand, no reduction was observed
in the water in test tubes. When the reduction in free chlorine
concentration is approximated by linear regression, the rate of
free chlorine concentration caused by volatilization rv is
represented by the following zero-order rate equation:
rv
dC
= −
dt
= kv C0＝0.90～0.98mg L-1
at 20℃
0.014
-1
Bulk rate coeffcient ｋb(hr )
0.016
0.012
0.010
0.008
0.006
×　Purified water + Chlorine
△　MCWT water
●　AWT water（immediately）
◆　AWT water
0.004
0.002
（after 2 years and 5 months）
0.000
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
DOC （mg L-1)
Figure 5 Relationship between
bulk reduction rate coefficient
and DOC concentration
3.5
Chlorine concentration (mg L-1)
AWT introduction was estimated and the results plotting these
values against the concentration of dissolved organic carbon
(DOC) in each water sample are shown in Figure 5. kb of
water after the AWT introduction was always smaller than
that of water in the MCWT, suggesting that the variance in kb
between samples depended on the difference in DOC. The
dissolved organic matter immediately after the start of AWT
operation was low because of the high adsorption
performance of new GAC. The DOC concentration reached
nearly constant level after the AWT worked for approximately
one year. The relationship between kb and water temperature
after reaching the constant level is shown in Figure 4b and
based on the results of exponential regression, kb of the AWT
water is represented by the following equation:
3.0
2.5
test tube(dashed line)
beaker
(solid line)
2.0
1.5
1.0
0.5
*at 30℃
(5)
0.0
0
5
10
15
20
25
30
where, kv is an observed reduction rate coefficient of free
Time (hr)
chlorine reduction caused by volatilization. In addition, ΔCv
-1
-1
(mg L hr ) is defined as the slope of the regression line
Figure 6 Reduction in chlorine
obtained when plotting the difference between the initial
concentration by volatilization
concentration C0 and a concentration t hours after, Ct (= C0 –
from water surface
Ct) against t, and the surface area of a one liter water sample
in a beaker is estimated to be 0.012266 (m2). Under these conditions, the mass of chlorine
volatilizing from 1 m2 water surface per hour, w (mg m-2 hr-1), is estimated by the following
equation:
ΔCv ( mg L-1 hr -1 ) × 1 (L)
w　=
0.012266 ( m 2 )
(6)
When a water reservoir or tank is box- or cylindrical-shaped, the storage volume V (m3) can be
calculated as a product of the surface area of the water and the water depth, i.e., S (m2) × D (m).
Therefore, kv (mg L-1 hr-1) is represented by the following equation:
-2
kv
-1
2
-3
w ( mg m hr )　× S ( m )
w　× S
w × 10
=
=
=
3
S　× D　× 1000
D (m)
V ( m )　× 1000
(7)
As shown above, kv is proportional to w but inversely proportional to the water depth D. A similar
experiment at a different water temperature T (°C) was conducted and ΔCv obtained was plotted
against T, and w was estimated by Equation (6). Consequently, it was confirmed that w
exponentially increased as water temperature T rose. Based on the results of the regression, kv (mg
L-1 hr-1) is represented by the following equation:
kv =
0.0004 exp (0.0850 T )
D
(8)
To be specific, residual free chlorine is likely to disappear more when the water depth is shallower
and water temperature is higher under the same storage time.
Development of simulation models
Based on the analysis results of telemeter measurements and laboratory experiment results,
models for simulating residual free chlorine concentrations in Osaka City water supply area were
developed and the prediction accuracy was assessed.
(1) Overall Model
This model is applied to the water distribution processes in Osaka City excluding reservoir water
in the secondary distribution plant.
Ct = C0 exp (- k all t )
(9)
where, C0 (mg L-1) is the initial free chlorine concentration and Ct (mg L-1) is the free chlorine
concentration after t (hr). kall (hr-1) is calculated from the water temperature using Equation (2).
(2) Reservoir Model
Under the assumption that free chlorine concentration in water stored in a reservoir is reduced by
bulk free chlorine consumption and volatilization from the water surface, the reduction rate of free
chlorine concentration in stored water, rr is represented by the following equation based on
Equations (3) and (5):
rr = −
⎛
k ⎞
dC
= rb + rv = kbC + kv = kb ⎜ C + v ⎟
⎜
dt
k b ⎟⎠
⎝
(10)
Next, the free chlorine concentration when water flows into the reservoir (t=0) is defined as Cin (mg
L-1) and that when water flows out the reservoir after t hours storage is Cout (mg L-1). Subsequently,
Equation (10) is integrated and ordered as follows:
⎛
Cout = ⎜⎜ Cin +
⎝
kv ⎞⎟
k
exp ( - kbt ) − v
⎟
kb ⎠
kb
(11)
Therefore, Equation (11) is established as a model that can be applied to the reduction in free
chlorine concentration in stored water. kb (hr-1) and (mg L-1 hr-1) are calculated from the water
temperature and the depth of stored water using Equations (4) and (8).
(3) Validation of model prediction accuracy
The application method of the Overall Model (Equation 9) and the Reservoir Model (Equation
11) in this study is shown in Figure 7. The differences between the predicted and telemetermeasured concentrations at 19 points, where telemeters were installed at the taps, δ (mg L-1), was
calculated everyday throughout the year The distribution of the difference is shown in Figure 8. In
addition, the accuracy of the predicted
TM(tap)
TM(tap)
concentrations at the outflows of two
TM(outflow)
secondary water distribution plants was
W.P.P
1st W.D.P
2nd W.D.P
similarly assessed using δ values and the
Reservoir
Overall
Overall Model
results are shown simultaneously. Based on
Model
Model
W.P.P: Water Purification Plant
these results, it was confirmed that the
W.D.P: Water Distribution Plant
TM: telemeter
difference δ ranged within ±0.1 mg L-1.
Consequently, the models were considered Figure 7 Application method of simulation models
to sufficiently represent the changes in the
*δ ＝ measured-predicted
Overall Model
Overall Model +
Reservoir
Model(Case1)
Overall Model +
Reservoir
Model(Case2)
Verification period:
Jan.1 ～ Dec.31,2005
-0.24～-0.22
-0.22～-0.20
-0.20～-0.18
-0.18～-0.16
-0.16～-0.14
-0.14～-0.12
-0.12～-0.10
-0.10～-0.08
-0.08～-0.06
-0.06～-0.04
-0.04～-0.02
-0.02～-0.00
0.00～+0.02
＋0.02～+0.04
＋0.04～+0.06
＋0.06～+0.08
＋0.08～+0.10
＋0.10～+0.12
＋0.12～+0.14
＋0.14～+0.16
＋0.16～+0.18
＋0.18～+0.20
＋0.20～+0.22
＋0.22～+0.24
relative frequency
0.26
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
δ (mg L-1)
Figure 8 Prediction accuracy of simulation
models
control value of chlorine concentration at the outflow in W.P.P.
additional chlorine injection rate at the secondary W.D.P. [A]
additional chlorine injection rate at the secondary W.D.P. [B]
W.P.P.: water purification plant
W.D.P.: water distribution plant
40
35
30
Water
Temperature
25
20
0.4
Water temperature (℃)
Predicted value (mg L-1)
reduction rate associated with changes in water
temperature. The annual changes in water
temperature ranged from 6 to 30°C, the maximum
residence time from the water purification plant to
the telemeter was 33 hours, and the storage time
and water level in each secondary distribution plant
was 6 hours and 2.3 m in Case 1 and 15 hours and
2.4 m in Case 2, respectively.
Creation of the annual program for setting
chlorine control values in Osaka City using the
simulation models
To optimize the residual free chlorine concentrations throughout Osaka City, we created the
annual program for setting control values of the
chlorine concentration at the outflows in water
purification plants and the additional chlorine
injection rates at the secondary water distribution
plants using the above simulation models. One
example created is shown in Figure 9.
0.9
The water temperature used for the
0.8
creation of the program was the daily
0.7
average value measured by all telemeters
in city's service area for the past five
0.6
years.
0.5
Jul
y
Au
gus
t
Se
pte
mb
er
Oc
tob
er
No
vem
ber
De
cem
ber
y
Jun
e
Ma
Ap
ril
Jan
uar
y
Fe
bru
ary
Ma
rch
CONCLUSIONS
15
0.3
In Osaka City, the introduction of AWT
10
0.2
has allowed the city to lower and level
5
0.1
residual free chlorine concentrations in
0.0
0
water supply areas compared with
conventional treatment. The reason is that
elimination of dissolved organic matter is
improved through the AWT and bulk free
Figure 9 Annual chlorine control program created
chlorine consumption in the water using the simulation models for Osaka City
distribution system is reduced. The
behavior of residual free chlorine concentration is analyzed. The change of residual chlorine in the
water supply network is well defined and can be experienced as a first-order reaction. The observed
reduction rate coefficient is affected by temperature. Thus, water authorities must change the
chlorine control values properly depending on the progress of the season. OCWB has now set the
annual program for setting control values at each chlorine injection point using the developed
simulation models and controls residual free chlorine in an effective manner.
REFERENCES
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