16.
Effective disinfection system with chlorine dioxide
Ｒ. Takahashi1 , T.Kirihara2 , M.Koeda3
Director１ , Chief Researcher２ ,Senior Researcher3
Second Research Department
Japan Institute of Wastewater Engineering Technology
KEYWORDS
Disinfection , Combined sewer system , Chlorine dioxide
ABSTRACT OF TECHNOLOGY
The principle of the technology
This disinfection technology injects chlorine dioxide into the upper position of the outlet of
untreated sewage and primary treatment water ( it is described with the following untreated
sewage.) in the combined sewerage, and number of total coliforms of disinfection treated
water are made to be 3,000/cm3 or less by the oxidizability.
Chlorine dioxide(ClO2) is powerful oxidizing agent with radical reaction, it selectively reacts
with high electron density position of the organic substance molecule, and it is reduced in
chlorite ion (ClO2-) (eq. 1).At that time, by inhibiting the protein synthesis function of virus
and bacterial cell, Chlorine dioxide seems to demonstrate the disinfection effect.
ClO2 + e−
→ ClO2−
（eq. 1）
ClO2 isn’t stored and formed from reaction of 2 chemicals(NaClO2 and HCl) by original
generator ( eq. 2).
5NaClO2 + 4HCl→4ClO2 + 5NaCl + 2H2O（eq. 2）
Equipment composition
Chlorine dioxide disinfection facility consists of chlorine dioxide generator, chemicals storage
facility, dilution water supply system.
When it was installed in the pumping station, untreated sewage which was made to circulate
from the wet well is supplied to the chlorine dioxide generator, and the chlorine dioxide
solution is injected into the settling basin upstream site(Figure 1). It is possible to make
reclaimed water in sewage-treatment plant to be dilution water, when it was installed in the
primary treatment channel.
Because using the methods of 2 chemicals, the space of the storage facility can be decreased,
and chlorine dioxide dilution water is little to other methods, so dilution water supply system
can be compact. In addition, the reduction of the use of the chemicals is possible by controlling
chlorine dioxide injection rate by continuous monitoring of the turbidity.
1
Chem ical storage facility
Special NaClO 2
tank
HCl tank
Scrubber
ClO2 generator
P
Dilution water supply system
P
Strainer
Grit cham ber
P
Figure 1. Installation example of the chlorine dioxide disinfection facility (
which it was installed in rain water pumping station )
The case in
DEVELOPMENT RESEARCH
Necessary performance and the criterion of technical evaluation
Necessary performance and the criterion of technical evaluation at SPIRIT21 are shown at
Table 1.
Table 1. Necessary performance and criterion
Technical
evaluation
Treatment
performance
Necessary performance
Number of total coliforms
3,000CFU/cm3 or less in
the effluent is achieved.
Efficiency of The time for obtaining the
disinfection disinfection effect is the
short time.
Safety
of Effect of the downstream
downstream water area on the aquatic
water area
organism is small.
Other
2
That the
chemicals
electric
attempted.
reduction of
quantity and
energy
is
Criterion
By the demonstration experiment, number of
total coliform 3,000CFU/cm3 or less of
treated water is achieved.
On shortening in reaction time, the necessary
reaction time is quantitatively confirmed, and
it is verified that the efficiency of the
disinfection is being attempted until now
from the comparison with the technology.
It is confirmed that the effect of the treated
water on the aquatic organism of the
downstream is slight by genotoxicity tests,
ecotoxicity tests, references, etc..
By the demonstration experiment, confirm
reduction of the utility of chemicals quantity
and electric energy and economical efficiency
of running cost.
Outline of the demonstration experiment
The outline of the demonstration experiment field is shown at Table 2. And photograph of
experimental equipment is shown in Figure 2, and experimental equipment flow chart is
shown in Figure 3.
Table 2. Outline of the demonstration experiment field
Sunamachi WWTP
Place
(Tokyo metropolitan)
Experiment
Proof experiment
ClO2
2004.5∼2004.8
Influent of primary
sedimentation basin
(untreated sewage)
Term
Raw water
Retention
time
Quantity of
raw water
injection
flow
treated water
5 min
1m3/min
Figure 2. Photograph of experimental equipment
Dilution water
supply pump
ClO2
generator
P
Untreated
sewage
Special NaClO2
tank
HCl tank
Dilution water tank
P
P
Sampling point
(raw water)
Sampling point
(treated water)
Water tank for experiment
inflow
P
P
Transportation pump
(treated water)
Intake pump
Water way from primary sedimentation basin
Figure 3. Experimental equipment flow chart
Development research result
Treatment performance
The demonstration experiment result is shown at Table 3. For untreated sewage, it was
confirmed to be possible to always make number of total coliforms 3,000CFU/cm3 or less in
the condition of retention time 5 minutes and chlorine dioxide injection rate 2∼15mg/L.
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Chlorine dioxide hardly remain, and chlorite ion(1.6∼12.0mg/L) and chlorate ion(0.1∼
2.3mg/L )which are disinfection sub-product are detected in the treated water.
Table 3.
Result of demonstration experiment
Experiment No.
Ⅰ
Ⅱ
Ⅲ
Ⅳ
precedence non-rain period (days)
2
10
4
15
Time largest amount of rainfall(mm/h)
13.0
2.5
5.0
5.0
Total amount of rainfall (mm)
96.5
15
15
19.5
ClO2-injection rate (mg/L)
2∼15
6∼15
7∼14
5.5∼15
Reaction time（min）
5
5
5
5
1.3×103∼
1.6×105∼
9.3×104∼
1.1×105∼
3.1×105
3.7×105
2.6×105
2.4×105
25∼2,600
460∼3,000
75∼2,150
185∼2,420
0.3∼3.4
1.7∼2.8
1.7∼3.1
2.0∼2.8
Inactivation rate (Log)
(mg/L)
ClO2
0.0
∼ 0.2
ClO2
1.6
∼ 12.0
ClO3
0.1
∼ 2.3
―
―
As
example
of
disinfection
experimental result, trend of amount of
rainfall, turbidity, chlorine dioxide
injection rate, number of total coliforms
in experimentⅠis shown in Figure 4.
Chlorine dioxide injection rate was
made to change by turbidity of raw
water of continuous monitoring. The
largest injection rate in the first flash
was 15mg/L , and it was possible to
make number of total coliforms to be
3,000CFU/cm3 or less after 6.5 hours
from rainfall start in low injection
rate(2 ∼ 5mg/L).
Average injection
rate in this experiment was about
5mg/L.
Verification of the safety
downstream water area
of
the
On the effect of the downstream water
area on the aquatic organism, the
comparison
examination
between
chlorine dioxide disinfection and
chlorine disinfection was carried out on
the point of "Safety assessment of
disinfectant itself" and "Effect by
adding disinfectant to the untreated
sewag".
Time amount of rain(mm/hｒ)
Residual level
12
10
8
6
4
2
0
600
Turbidity
Injection rate
500
Safety assessment of disinfectant
The result of「Algae, Growth Inhibition
4
15
400
12
300
9
200
6
100
3
0
0
15
6
10
20
25
経過時間(hr)
3,000CFU/cm3
5
10
4
10
3
10
2
10
1
Raw water
Treated water
10
0
10
0
(1)
18
5
10
15
30
35
20
Time (hr)
Figure 4. Result of experiment Ⅰ
25
40
ClO2 injection rate
(mg/L)
Treated water
Turbidity (NTU)
（CFU/cm3）
Raw water
Total coliforms(number/cm3）
Total coliforms
Test」on chlorine dioxide and chlorite ion and chlorate ion which are main sub-product, and
sodium hypochlorite which is the contrast disinfectant is shown in Table 4. It was proven
that the safety of chlorite ion and chlorate ion which were disinfection sub-product was over
100 times higher ( as a EC50 value ) than sodium hypochlorite.
Table 4. Result of algae-growth inhibition test
Chemical EC50 value(mg/L)
NaClO
0.08
ClO2
0.45
ClO2−
9
ClO3
25
−
(2)Effect by adding chlorine dioxide to the untreated sewage
For safety assessment, ①umu test, ② Microtox test ③measurement of THMs formation
were done using untreated sewage which respectively added chlorine dioxide and sodium
hypochlorite to 15mg/L as sample. The safety test result is shown at Table 5.
Table 5.
Result of safety test
Genotoxicity test.
(umu test)
Toxicity value
(+S9mix)
Toxicity value
(−S9mix)
ClO2
(15mg/L）
0.8
(Negative)
1.5
(Positive)
NaClO
(15mg/L)
0.9
(Negative)
1.7
(Positive)
Ecotoxicity test
(Microtox test)
EC50
0.451
0.153
0.027
0.037
Test
Total THMs formation（mg/L）
Note
100 times concentration
（raw water : 0.4）
100 times concentration
（raw water : 1.5）
raw water : 0.031
①Result of umu test
The genetic toxicity value of chlorine dioxide added sample was lower in comparison with that
of sodium hypochlorite added sample.
②Result of Microtox test
The EC50 value of chlorine dioxide added sample was higher in comparison with that of
sodium hypochlorite added sample.
③ Result of measurement of THMs formation
The concentration of total THMs formations of chlorine dioxide treatment was the lowest, and
the one of sodium hypochlorite treatment was the highest.
It was proven that chlorine dioxide was safer disinfectant for the aquatic organism in
discharge area than sodium hypochlorite from above result.
Economical efficiency comparison
For the facility of 10,000m3/h wastewater flow, the comparison of running cost/year between
in applying this disinfection technology and in applying sodium hypochlorite in usual
secondary treatment is shown in Table 6.
The running cost/year was estimated to about 4.2% of sodium hypochlorite consumed in the
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disinfection in secondary treatment annual, when this disinfection technology was applied to
the untreated sewage.
Table 6. Comparison of running cost/year
ClO2
NaClO
Advanced treatment
water
Raw water
Untreated sewage
Water quality
(raw water)
Injection rate of
disinfectant
(average)
Chemicals price
(yen / kg)
Running cost per quantity
of water to be treated
(yen / m3)
Total coliforms: (Max)3.4×106/cm3
Turbidity : under 200NTU
―
5.5mg/L
3mg/L
(NaClO:12%available
chlorine concentration)
Annual quantity of water
to be treated
Special NaClO2
HCl
140
20
30
6.47
0.75
40Q m3/year
8,760Q m3/year
(Q×24hrs×
365days/year)
（2×Qm3/h×3hrs×20days/year×
1/3）
Annual running cost
6,570Q
259Q
without electric power
3×8,760Q
（0.75yen/m
（6.47yen/ m3×40Q m3/year）
basis rate
3
m /year）
(yen / year)
4.2（Q＝10,000m3/h）
Annual running cost ratio
100
Notice )
1.Q shows design flow of wastewater(m3/h).
2.Quantity of untreated sewage were assumed 1/3 of primary treatment quantity of water.
『Quotation：The research report on improvement countermeasure of combined sewerage
(The Ministry of Land, Infrastructure and Transport City and Regional Development Bureau
Sewerage and Sewage Purification Department, Sewerage new technology driving
mechanism , 2002 March)』
3.Injection rate of NaClO was assumed 2∼4mg/L from ｢Sewerage facilities plan and design
guidelines｣( 2001 editions, Japan sewerage association ).
4.Maximum value of influent number of total coliforms was set referring to demonstration
experiment result (contain other company’s result ).
5.For set influent number of total coliforms, the average injection rate and the range of
influent turbidity were calculated from the demonstration experiment result.
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Evaluation of the technology
Result of evaluation of this technology is shown at Table 7.
Table 7. Experiment results in this disinfection system
Item of technical
evaluation object
Treatment
performance
Efficiency of
disinfection
the
development objective
(display requirement)
Number
of
total
coliforms
3,000CFU/cm3 or less in the
effluent is achieved.
The time for obtaining the
disinfection effect is the short
time.
Safety of
the downstream
water area
As a result of the disinfection, the
effect of the downstream water
area on the aquatic organism is
small.
Other
That the reduction of chemicals
quantity and electric energy is
attempted.
Experiment results
For the untreated sewage etc., it is possible to
make number of total coliform of treated water
to be 3,000CFU/cm3 or less within retention
time 5 minutes, and it is recognized that it has
the display requirement.
For the untreated sewage, when the
disinfectant was added under addition
condition
for
sufficiently
coming
the
disinfection, it is judged that the safety is
higher than the technology until now, and it is
recognized that it has the display requirement.
Chemicals quantity and electric energy are the
practical range, and it is recognized that the
integration to the existing facilities is possible.
FEATURES OF TECHNOLOGY
Features of the chlorine dioxide disinfectant
It is possible to disinfect in the short time
Chlorine dioxide disinfection of untreated sewage is possible within 5 minutes. And it takes
about 1/3 in contact time ( 15 minutes ) of the conventional chlorination. So chlorine dioxide
can adapt to the facilities which can not take enough contact time in chlorine contact chamber,
etc.
Disinfection effect keeps under ammonia existence
Chlorine dioxide can maintain it’s disinfection effect for the sewage including ammonia and
amine because it dosen’t react with ammonium ion and dosen’t form chloramine.
Chlorinated organic compounds such as the THMs are not formed
Since the chlorination of organic substance is not generated in the chlorine dioxide
disinfection, chlorinated organic compounds such as the THMs are not formed.
Safety for aquatic organism of the downstream water area is high
The toxicity of chlorite ion and chlorate ion which are disinfection sub-product are lower over
100 times than sodium hypochlorite, and both genetic toxicity and ecotoxicity of chlorine
dioxide treated water are lower than that of chlorinated water.
Features of the disinfection system
The installation space is compact
Because using the methods of 2 chemicals, the space of the storage facility can be decreased,
and chlorine dioxide dilution water is little to other methods, so dilution water supply system
can be compact.
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Quantity of chemicals can be reduced by injection rate control
Based on monitoring water quality and quantity of raw water, chlorine dioxide demand can be
controlled by changing chemicals’ flow which supplies to the generator. Therefore, it is
possible to easily carry out chlorine dioxide dosage control, and usage of chemicals can be
reduced.
CONDITION OF APPLICATION
This disinfection technology is installable without drastic remodeling for existing structure
such as grit chamber or rain water outlet channel in the pumping station and primary
treatment channel in WWTP, if having the installation spaces of chlorine dioxide generator,
chemicals storage facility, dilution water supply system.
However, chlorine dioxide, chlorous acid and chloric acid are regulated 0.6mg/L respectively
as water quality management setting objective item of the tap water, it is necessary to
examine the correspondence of confirming these concentration in the water intake site, when
the water purification plant is located in downstream water area of this system,.
TECHNOLOGY PROPONENT
KUBOTA Corporation
Address：1-1,Hama 1-chome,Amagasaki City, Hyogo Japan 661-8567
Tel
：81-6-6470-5806
Person in charge：Yuji Otsuka (Water & Sewage Engineering Dept.)
E. mail：[email protected]
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