Case

Powdered activated carbon and intermediate chlorination to control
musty odor in the Tama River

water utility

Bureau of Waterworks Tokyo Metropolitan Government

General information of the utility (as of 2022)
Public (retail & industrial
Operation typewater supply※)

Service area(※ Industrial water supply
discontinued at the end ofMarch 2023.)(km2)

Population

13,695,575

servedServicecoverage (%)
NRW (%)
Number ofDistribution(m3/d)

1239.23

4,155,200

100

Pipe length (km)27,466

4.3

Number of staff

3,630

Surface water (4), Groundwater (278), Others (0)water sources(in case of 10m3 of water per month for

Water rates

1,067(JPY)

residential customers with 13mmdiameter)

The Ozaku Water Treatment Plant, which uses the Tama River as
its water source, has long been blessed with excellent raw water
quality and has conducted water treatment using a rapid filtration
system based on pre-chlorine injection. Around 2011, theconcentration of 2-Methylisoborneol (2-MIB), a substance thatcauses musty odor in raw water, began to rise sharply, and theplant responded by increasing the powdered activated carbon(PAC) injection facilities. However, the pre-chlorination method had
Summary

low 2-MIB removal efficiency of PAC, and faced problems such asincreased chemical costs and reduction in the volume of treatedwater.
Therefore, in order to implement more efficient PAC treatment,
the Bureau of Waterworks Tokyo Metropolitan Government verified
the introduction of intermediate chlorination through facility
upgrades, and completed the facility upgrades in 2016. The facility
has been used in combination with pre-chlorination facility andoperating without any problems up to the present, maintaining
good water quality.

1) Overview of the Ozaku Water Treatment Plant
The Ozaku Water Treatment Plant is located in the upper reachesof the Tama River and has a treatment capacity of 280,000 m3/day.
Raw water is all surface water from the Tama River, which is taken
at the Hamura intake weir located approximately 3km from thetreatment plant and is led to the plant by the water pump. The
water is treated by a rapid filtration system and is delivered to thewestern part of the Tama area.
2) 2-MIB in raw water
2-MIB in raw water of the Ozaku Water Treatment Plant bigan torise sharply around 2011 , reaching to a maximum of 220 ng/L in
August 2013. Surveys have confirmed that this increase in 2-MIB
concentration was caused by
the mass proliferation ofMicrocoleus autumnalis(Phormidium autumnale)(Figure 1), a type ofCurrent Status &Challenges

cyanobacteria attached toriverbeds, and that theconcentration of 2-MIB
increases as water temperaturerises.

Figure 1. Microcoleus autumnalis(Phormidium autumnale)3) PAC injection facilities
At the time of 2011, the plant had activated carbon injection
facilities at the Hamura Water Supply Pump Station and in theplant. To cope with the rise in raw water 2-MIB after 2011, atemporary injection facility was installed (2012), and then aninjection facility was added at the pump station (2014).
4) issues
The Ozaku Water Treatment Plant did not have an intermediate
chlorine facility, and sodium hypochlorite ("hypochlorite") wasinjected in the flush mixing tank which was placed before thesedimentation water culvert. In the pre-chlorination process, PAC
reacts with hypochlorite, releasingsome of the 2-MIB adsorbedon the PAC released (Figure 2) and the removal rate decreases,forcing an increase in the amount of PAC injected and a reduction
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in the water volume.

Figure 2. Effect of hypochlorite injection on 2-MIB treatment

In order to improve the 2-MIB removal rate by PAC, intermediate
chlorination in which hypochlorite is injected after coagulation
sedimentation process was considered to be effective. Therefore,
the Bureau examined the possibility of introducing this method byimproving the facility and verified the effectiveness of theintroduction.
1) Investigate feasibility of introducing facility improvements
Since this water treatment plant was designed on a narrow site
and it was difficult to incorporate new facilities and processes, it
was decided to inject and mix hypochlorite in the existing
Measures &Solutions

sedimentation water culvert and conduct the following experiments
and verifications.
1.1) experiment in practical facilities
A temporary hypochlorite injection facility was installed at thesite to investigate the state of hypochlorite mixing. Hypochlorite
was injected at one injection point in each group (four injection
points per system) at the inlet of the sedimentation basin, aiming
for mixing by water flow.
The results of the investigation showed that the mixing of
hypochlorite in the sedimentation water culvert was insufficient,resulting in fluctuations in the residual hypochlorite concentrationin the treated water. In addition, multiple hypochlorite injection
pumps and injection piping were required, and facility management
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issues were also identified.
1.2) hydraulic model experiments
To solve the problems identified in the previous experiment (1.1),
it was considered effective to install separation walls in the existing
sedimentation water culvert and use a one-point injection system
per system. However, the installation of these facilities could cause
a rise in the water level and an imbalance in the water flow to eachfiltration pond, so the shape of separation walls, etc. wasconsidered through a hydraulic model experiment (1/5 scale of theactual facility).
As a result of the investigation, it was found that a system withseparation walls, circulating water pump, and hypochlorite injection
point, as shown in Figure 3, and diversion wall at the opening ofseparation walls is optimal in terms of water level, water volume
balance, and hypochlorite mixing.

B-4
（１）

Rapid filter

（２）
（３）

（４）

（５）
Sedimentation tankgroup A

group B

group C

group Dsystem 1

Figure 3. Overview of the adopted sedimentation water culvert
improvement plan(1) sedimentation water culvert(2) circulating water pomp(3) hypochlorite injection point(4) separation walls(5) diversion wall installation position

1.3) Improvements and Verification in actual facility
Based on the results obtained from the hydraulic model tests, theconstruction work related to intermediate chlorination facility ,4 / 7

includingsedimentation water
culvert modification,
hypochlorite injection
facility installation, andmonitoring and control
modification, werecarried out and the
hypochlorite injection
facility started
operation in July 2016
(Figure 4,5). There

Figure 4. sedimentation water culvert
after improvement work

were no problems withthe water level in thesedimentation canal,
and the residual
chlorine concentration
was uniform after thediversion wall,indicating proper
mixing.
2) Verification of 2MIB removal
performance
To evaluate theremoval performance

Figure 5. Treatment process
at Ozaku WTP (after improvement work)of 2-MIB by PAC, the"amount of 2-MIB

removed per 1 mg of PAC (ng)" were compared between prechlorination and intermediate chlorination. As a result, when theraw water 2-MIB concentration was about 50 ng/L (the maximumin FY 2016), the amount of 2-MIB removed was 0.73 ng in prechlorination, while it was 1.4 ng in the intermediate chlorination,
confirming that the amount of PAC used could be reduced byintermediate chlorination (Figure 6).

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2-MIB removal per mg of PAC (ng)intermediate chlorination2

pre-chlorination

10020406080

1002-MIB in raw water (ng/L)
Figure 6. Comparison of 2-MIB removal of activated carbon

This case study shows that intermediate chlorination is possible
by improving existing pre-chlorination facilities, and that theamount of PAC injection can be reduced by intermediatechlorination. However, there are cases where pre-chlorination isFuture Plans

effective, such as when dealing with biological turbidity andmanganese, so it is necessary to devise flexible operation andmanagement according to the raw water quality.
The Bureau will continue to strive for a stable supply of safe,
good-tasting, high-quality water by conducting appropriate water
treatment according to the quality of raw water.
〇Tsunoda. T et al., “Results of genetic analysis of musty odorproducing cyanobacteria in the upper reaches of the Tama River”,
Journal of the Japan Society of Water Environment, vol. 37, No. 1, pp.
9-13, 2014 (in Japanise)〇Koda. T et al., “Investigation on musty odor-producing
cyanobacteria in the upper reaches of the Tama River: distribution
status and growth characteristics of Phormidium autumnale”,

References

Proceedings of the 2018 JWWA National Waterworks Conference, pp.
186-187, 2018 (in Japanise)〇Watanabe. A et al., “Measures to remove musty odor byintroducing intermediate chlorination at the Ozaku Water TreatmentPlant”, Proceedings of the 2018 JWWA National Waterworks
Conference, pp. 388-389, 2018 (in Japanise)〇Suetsugu. R et al., “Development of a simple detection method forcyanobacteria-derived fungal odor synthetase Genes and Utilization of
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Detection Results in Fact-Finding Surveys”, Proceedings of the 2018
JWWA National Waterworks Conference, pp. 618-619, 2022 (inJapanise)

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