Development of Wet-Weather Sewage Treatment Technologies and their
Application to the Combined Sewer System Improvement Plan of Osaka
City
Enao Takayanagi*, Makoto Fujita**Environment and Sewerage Bureau, Osaka City Government, 1-14-16 Nanko-kita, Suminoe-ku,
Osaka 559-0034 Japan
Abstract: In Osaka City, since 97% of the city area is served by combined sewer systems (CSS),
the Osaka City Government has been originally carried out researches and experiments on CSS
improvement methods, particularly wet weather combined sewage treatment technologies.
Among them, ³wet weather sewage activated sludge process´ and ³sedimentation using inclined
plate settlers with coagulant addition´ have been successfully developed to a practical level, which
can effectively increase continuous wet weather sewage treatment capacity of existing sewage
treatment facilities with relatively lower cost and shorter period. The city government has revised
its CSS improvement plan by applying these technologies, with the objective of reducing discharge
pollutant loads in effluent and overflow from the CSS during wet weather to the equivalent level ofseparate sewer system. This paper presents the performance of the above newly developed
technologies and the basic concept of the CSS improvement plan.
Key words: combined sewer system, plate settler sedimentation, wet-weather activated sludge
process

Introduction
Osaka is the largest city in western Japan witha resident population of about 2.63 million (daytime
population of about 3.66 million) and area of about
220 km2. Most of the city area lies below the riverflood level, and gravity drainage area is only about
10% of the city area. Therefore, most of thestormwater needs to be collected once at sewage
treatment plants or pumping stations then pumped
and discharged to public water bodies.
In the modern sewerage construction projects
in Osaka City started in 1894 in response to cholera
epidemics, CSS was adopted in order to improve
public hygiene and control flooding at lower cost andin a shorter period. Currently, the sewer systemcovers almost 100% of the city area, and 97% of thesewered area are served by CSSs (Figure 1).
- 361 1-

Fig. 1: Areas Served by Combined Sewer Systems

In CSSs, when sewage flow exceeds a certain level during wet weather, untreated sewage
may be discharged directly from combined sewer overflow (CSO) outfalls and pumping stations topublic water bodies. A survey conducted by the City Government revealed that, among thepollutant loads flowing into sewage treatment plants, the proportion discharged to public water
bodies was 14% in terms of BOD and 36% in terms of SS. Out of this pollutant load discharged to
public water bodies, 70% of BOD and 80% of SS are discharged during wet weather conditions.
This result shows that it is the key issue to reduce the pollutant load in wet-weather discharges forthe further improvement of the quality of public water bodies.
Since the 1970s, when the coverage of the sewer system increased and the safety against
floods was improved to a certain level in the city, the City Government has been carried out
researches and studies on technologies for CSS improvement. Especially, in Osaka City, almost
the entire city area has been already urbanized, therefore, it is very difficult to acquire lands for theconstruction of new treatment facilities. Furthermore, the cost for CSS improvement has to beminimized as possible because the scale of CSSs in Osaka is so huge. From these points of view,
the City Government has been actively developed and introduced original technologies to increase
continuous wet-weather sewage treatment capacities by fully utilizing existing facilities, instead ofother CSS improvement methods commonly adopted in Japan such as construction of wet-weather
sewage storage facilities and sewer separation.
This paper introduces the major technologies for CSS improvement newly developed by the
Osaka City Government and the city¶s CSS improvement basic plan applying these technologies.

Wet-Weather Sewage Treatment Technologies for CSS Improvement
Developed by the Osaka City Government
Wet-weather sewage activated sludge process
In Osaka City, sewage treatment plants are designed to receive up to three times of the design
peak dry weather flow (3Qsh) in wet weather conditions. In the conventional method, 1Qshamong 3Qsh was treated by the activated sludge process and the remaining 2Qsh was treated byonly primary sedimentation and discharged (Figure 2). In the wet-weather activated sludge
process, this 2Qsh of wet-weather sewage is fed into the latter section of aeration tanks and istreated by the activated sludge process (Figure 3).
Activatedsludgetreatment(1Qsh)

Wet
weatherwastewater(3Qsh)
1Qsh
Primary
sedimentationtank

Discharge after
sedimentation(2Qsh)

Aeration tankFinal
sedimentation
tank

Wet
weatherwastewater(3Qsh)
Primary
sedimentationtank

Discharge after
sedimentation(2Qsh)

Return sludge

Fig. 2: Conventional Wastewater Treatment during Wet Weather- 362 2-

2Qsh
1Qsh

Aeration tank

Activatedsludgetreatment(3Qsh)
Final
sedimentation
tank

Return sludge

Fig. 3: Wet Weather Activated Sludge Process

With the conventional activated sludge process, pollutants are absorbed by activated sludge inthe first 30 minutes or so, and then gradually decomposed. In the wet-weather activated sludge
process, the hydraulic retention time of wet-weather sewage fed into the latter section of aeration
tanks is about 30 minutes, and pollutants in the wet-weather sewage are quickly removed due to theabsorption by the activated sludge suspended in the aeration tanks.
Furthermore, the aeration tanks in sewage treatment plants in Osaka City have been equippedwith channels for step feeding to be applicable for both the conventional activated sludge process
and the step feed aeration process. Thus, this new process could be introduced with relatively
small modifications of existing facilities.
The Osaka City Government started design studies on this treatment process in 1988, andcarried out performance surveys at treatment facilities using this process from 1992 to 2002. The
results verified stable operations while processing sewage volume ranging from 1.48 to 4.62 Qsh(2.70 Qsh on average) with the average effluent quality at 9.3 mg/L in SS and 7.7 mg/L in BOD.
In addition, results from four surveys verified that this new method could reduce the amount ofpollutant discharge by 59 to 91% (73% on average) in SS, and by 27 to 78% (61% on average) in
BOD for a single rainfall event, when compared with the conventional method employing primary
sedimentation (Figure 4).
In September 2000, when there was a prolonged rain event with the total rainfall of 165 mm,
this process was continuously operated for 45 hours stably, and it was verified that the new process
was capable of prolonged continuous operation.
Since these results have demonstrated the effectiveness of the wet-weather activated sludge
process, the Osaka City Government has been introducing this process to all 12 sewage treatment
plants by March 2007.
Comparison of discharge pollutant load in SS

Comparison of discharge pollutant load in BOD
100

80

60

40

by 73%

84%

80

Reduction

60

Primary treatment
Activated sludge
treatment

40

20

27%

20

Wet weather
activated sludge
process

078%

by 61%

Primary treatment
Activated sludge
treatment

Reduction

100

39%
16%
0

Conventional
treatment

22%
Conventional
treatment

Wet weather
activated sludge
processFig. 4: Effect of the Wet Weather Activated Sludge Process- 363 3-

Sedimentation process using inclined plate settlers with coagulant addition
Sedimentation process using inclined plate settlers is adopted in the treatment of drinking
water supplies. The Osaka City Government has carried out studies on this process for treating
wet-weather sewage, which tends to increase rapidly, within limited land area. From the late1970s, the City Government carried out monitoring on the characteristics of wet-weather sewage
and experiments on the plate settler sedimentation process, and obtained the following results:
1) Wet-weather sewage contains high content of suspended solid (SS) with a high sedimentation
velocity, therefore, it is reasonable to use this process to separate this high settleable SS component.
2) At an average flow rate of 0.5 to 0.6 m/min. in the plate settler module with the detention time ofabout 10 minutes, effluent quality is equivalent to the one from a conventional sedementation tank.
Besides, experiments were carried out to investigate the effect of coagulant dose to
sedimentation efficiency and resulted in the following:3) Poly-aluminum chloride (PAC) is most suitable as a coagulant for the wet-weather sewage
sedimentation process.
4) By adding PAC at the concentration of 2.5 to 10 mg-Al/L, influent sewage with BOD of 100 to
200 mg/L can be treated to produce an effluent with BOD of 20 to 25 mg/L with the detention time
not exceeding 10 minutes.
However, as mentioned earlier, it is very difficult to acquire lands in Osaka City for theconstruction of new treatment facilities, therefore, the City Government started design studies on
modifying existing primary sedimentation tanks to plate settler sedimentation tanks as an alternativeengineering option, and installed inclined plate settler modules to an existing primary sedimentation
tank in the Osaka City¶s Chishima Sewage Treatment Plant as a pilot treatment facility for thisprocess. The performance surveys were carried out for 29 rainfall events for one-year period from
October 2004. The specifications of the experimental facility are as follows:- Inclined plate settler type: Horizontal flow, flat plate type
- Length of inclined plates: 3 to 6 m- Angle of inclination:
60º- Height of a settler module: 2.5 m- Intervals of inclined plates: 100 mm- Inclined plate material:
Vinyl chloride resin and recycled plywood- Coagulant:
PAC- Rapid Mixing:
Utilize pre-aeration- Flocculation:
No special equipment/device is provided- Inclined plate cleaning:
Air blowing and water spraying- Sludge collector:
N/A- Sludge removal:
Discharge sludge by water flushing after rainfall events
In order to effectively utilize the existing sedimentation tank, it is necessary to place multiple
inclined plate settler modules in the tank and distribute the influent sewage to each module.
Therefore, influent main pipes are placed in the tank and connected with perforated pipes at theirend as shown in Figure 5.
- 364 4-

Fig.5

Design of Inclined Plate Settler in Chisima Sewage

ᵹ౉

Inflow Conduit TubeǾ600

Inclined Plate Unit

Inflow Multi-hole DuctǾ2505Inflow Multi-hole DuctǾ3005Plan View of Inflow Conduit
OutflowConduit TubeǾ600

OutflowConduit TubeǾ500

OutflowConduit TubeǾ600

Outflow

Outflow Multi-hole DuctǾ2005Outflow Multi-hole DuctǾ3006OutflowMulti-hole DuctǾ2004

Plan View of Inflow Conduit
OutflowConduit Tube
Inflow Multi-hole Duct

Inflow Conduit Tube

Outflow Multi-hole Duct

Stectional View

The hole size of perforated pipes must be determined to distribute influent sewage to theindividual modules equally. Perforated pipes are vertically placed near the inlet cross-section ofplate settler modules to distribute the influent sewage evenly in horizontal and vertical directions of
the module cross-section and mitigate density currents.
For the inclined plates, vinyl chloride resin plates and recycled plywood plates (made fromwaste paper and plastic) are used. In the survey, treatment performance was measured under
different surface loading rates by changing the length of the inclined plates.
Sludge collectors were not installed, therefore, sludge is stored on the bottom of tank during
rainfall events. Then after rainfall events, water in the tank is once drained together with most of
the stored sludge, and the remaining sludge is flushed from the rear end of the tank using settled
water from the adjacent primary sedimentation tank.

Influent SS(mg/L)

Effluent SS (mg/L)

140
120

50
150
250
350
450
550

100
80
60
40
20
0
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Flow Rate (m/min)Fig.6 Influence of Flow Rate to Effluent SS

Influent
BOD(mg/L)

140

Effluent BOD (mg/L)

Influence of flow rate in the plate settler
modulesFigures 6 and 7 show the relationship between theflow rate in the plate settler modules and the effluent
quality when the surface loading rate is less than 35
m3/(m2·d). SS concentration in the treated effluent
increased when the flow rate exceeded 0.5 m/min. andthe BOD concentration increased when the flow rate
exceeded about 0.55 m/min. It is observed that the
impact of the flow rate is less evident in BOD than in SS.
This may be attributed to the soluble BOD. From theseresults, it is considered that the flow rate in the plate
settler modules should be set 0.5 m/min or less.

120

50
150
250
350
450

100
80
60
40
20
0
0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Flow Rate (m/min䋩

Fig.7 Influence of Flow Rate to Effluent BOD- 365 5-

Influence of surface loading rate
Figure 8 shows the relationship between thesurface loading rate and the treated effluent quality
when the flow rate is below 0.5 m/min. As shownin the graph, it is observed that the effluent with
sufficient quality is obtained stably when the surface
loading rate is 40 m3/(m2·d) or less.

Influent SS(mg/L)

Effluent SS (mg/L)

60
50

50
150
250
350
450
550

40
30
20
10
0
0

10

20

30

40

50

Surface Loading Rate (m3/(m2·day))Fig.8 Influence of Surface Loading Rate to Effluent SS

Treated effluent quality
Based on the above observations, the relationship between the influent sewage quality and thetreated effluent quality was analyzed under the conditions when the flow rate is less than 0.5 m/min.
and the surface loading rate is less than 35 m3/(m2·d). The results are shown in Figures 9 and 10.
The SS values were averaged within each 100 mg/L range; and the BOD values were averaged
within each 50 mg/L range. The vertical bars show the standard deviation. It is considered thatpollutants can be removed to the level of the average effluent quality approximately at 20 mg/L in
SS and 20 to 30 mg/L in BOD, except when the pollutant concentration of the influent sewage is

45
40
35
30
25
20
15
10
5
0

60

Effluent BOD (mg/L)

Effluent SS (mg/L)

extremely high. Since little data was available under the condition of the influent sewage withhigh pollutant concentrations, such data need to be collected furthermore.
50
40
30
20
10
0
0

100

200
300
400
Influent SS (mg/L)

500

600

Fig.9 Average Influent and Effluent SS

0

50

100

150
200
250
Influent BOD (mg/L)

300

350

Fig.10 Average Influent and Effluent BOD

Removal rate
Under the same conditions, the relationship between the influent sewage quality and the

100
90
80
70
60
50
40
30
20
10
0

BOD Removal Rate (%)

SS Removal Rate (%)

removal rate is shown in Figures 11 and 12. The removal rate is approximately around 90% in SS
and 80% in BOD, although it shows low value when the pollutant concentration of the influent is at
low level.
100
80
60
40
20
0

0

100

200
300
400
Influent SS (mg/L)

500

0

600

50

100
150
200
250
Influent BOD (mg/L)

300

Fig.12 Average BOD Removal Rate

Fig.11 Average SS Removal Rate
- 366 6-350

From these findings based on the survey results, it is estimated that existing primary
sedimentation tanks can be capable of treating wet-weather sewage flow equivalent to five times its
present design maximum daily dry-weather flow by installing inclined plate settler modules withinclined plate intervals of 100 mm, flow rate in the plate settler module of less than 0.5 m/min., andsurface loading rate of 25 m3/(m2·d).
Furthermore, the city government is conducting performance surveys when applying this plate
settler sedimentation process to the primary sedimentation process under dry weather conditions.
It is expected that the plate settler sedimentation process will be able to have the equivalenttreatment capacity of the existing primary sedimentation tank with 1/2 to 1/3 of its original surface
area. In this case, the remaining space can be used for wet-weather sewage treatment by the plate
settler sedimentation process with coagulant addition. Then, the wet-weather sewage treatment
capacity can be increased without constructing new tank facilities.

The Newly Revised CSS Improvement Plan of Osaka City
In the previous CSS improvement plan of Osaka City, the objective was set as ³to reduce theannual discharge pollutant load including dry weather period to the equivalent level of separated
sewer system´, and various programs have been implemented such as eliminating sediment traps at
the bottom of manholes, introducing the wet-weather sewage activated sludge process, andconstructing stormwater reservoirs for storage of wet-weather sewage. However, in 2004, theSewerage Law Enforcement Order was amended and the wet weather effluent quality standard for
CSSs was newly stipulated. On the other hand, the city government has been originally studied
the plate settler sedimentation process as a CSS improvement method and developed to a practicallevel. In response to these changes and new developments, the Osaka City Government
established a new CSS improvement plan in March 2006.
Objectives
Mid-term objectives (target year: 2018)- Within each treatment area, the annual discharge pollutant load including dry weather periods
should be reduced to the equivalent level of separated sewer system. Here, it is assumed that the
average quality of wet weather discharge from a separated sewer system is 18 mg/L in BOD.
- The wet weather effluent quality should meet the standard stipulated in the Sewerage Law
Enforcement Order amended in 2004, that is the average quality of wet weather effluent during asingle rainfall event with the total rainfall of 10 to 30 mm should be not more than 40 mg/L in BOD
in each treatment area
- The number of overflow events from CSO outfalls should be reduced by half.
Long-term objective- The wet weather effluent quality should be the same level of dry weather effluent quality.
Specifically, the annual average effluent quality from each outlet should be not more than 15 mg/L
in BOD.

- 367 7-

Major approaches
In order to reduce discharge pollutant load to public water bodies, increasing wet weather
sewage treatment capacity was selected as the basic strategy, and the above objectives should beachieved mainly by the combination of following four major approaches.
1) Introduction of the wet weather sewage activated sludge process
2) Introduction of the plate settler sedimentation process with coagulant addition
3) Storage of wet-weather sewage utilizing large stormwater sewers designed for flood control
4) Newly construction of facilities for storage of wet-weather sewage such as stormwater reservoirs
In the implementation of the CSS improvement program, priority should be given to projects
concerning those public water areas that are important in terms of water quality protection andlandscape conservation.
Modification of primary sedimentation tanks
There are two necessary steps when the plate settler sedimentation process for wet weather
sewage treatment is introduced in the existing sewage treatment plants. First, the required area forprimary sedimentation of dry weather flow should be reduced to 1/2 to 1/3 by modifying existing
sedimentation tanks to the plate settler sedimentation process (without coagulant addition) or thehigh rate filtration process. Then, the plate settler sedimentation process with coagulant addition
should be introduced using the remaining 1/2 to 2/3 of the area for wet weather sewage treatment.
Cost and Benefit
Simulation analysis was conducted for one treatment area in the case of wet weather sewage
treatment up to 6Qsh by introducing the wet weather activated sludge process and the plate settler
sedimentation process with coagulant addition modifying existing facilities. The result shows thatthe following effects can be obtained.
1) 84% of wet weather sewage flows and 94% of the pollutant loads in the sewage can be captured
for treatment, and 83% of captured pollutant loads can be removed.
2) The annual discharge pollutant loads can be reduced to the equivalent level of a separate sewer
system.

79.2 72.1 84.0

96.7 91.8 94.2

Direct Discharge
81.7 83.4 83.1

Approach"C"

Approach"B"

Approach"A"

Approach"C"

Approach"B"

Approach"A"

Approach"C"

Approach"B"

Captured
Pollutant Loads
Captured Sewage
flows

35

1083

30
25

Construction Costs(billion JPY )Reccurent Costs(million JPY/year)

1200
1000
800

20
15
10
5

600

30.7

400

263
210
7.3

0

Reccurent Costs(million JPY/year)

Pollutant Removal
Rate

Approach"A"

100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%

Construction Costs (billion JPY)

3) Comparing the conventional CSS improvement approach of constructing stormwater reservoirs,
the construction costs and the annual recurrent costs can be reduce to about 20% for removing theequivalent amount of pollutant load.

200
6.3

0
Approach"A" Approach"B" Approach"C"
Fig 14: Construction Costs and Annual Recurrent Cost

Fig 13: Comparison of effect in alternative approaches

Approach ³A´: Stormwater reservoir (13mm) + sedimentation (2Qsh)Approach ³B´: Wet-weather activated sludge process (3Qsh) + Stormwater reservoir (3mm)Approach ³C´: Wet-weather activated sludge process (3Qsh) + Plate settler sedimentation with coagulant addition (3Qsh)- 368 8-

Exceeding 3Qsh (Direct discharge)

2Qsh

Final
SedimentationTanks

Influent flow

Temporarystorage

Reaction
Tanks(for
1Qsh)㩷

⚳ೋᴉ෻Primary
SedimentationTanks(for3Qsh)㩷

1Qsh㩷

Temporary Direct
storage discharge(bystormwater
Sedimentaion
reservoirs)
only

2Qsh

Secondary treatment

1Qsh

3Qsh

Time

Stormwater
Reservoirs(newly constructed)㩷

Fig 15: Image of Conventional Treatment Process in Wet Weather Events

Exceeding 6Qsh (Direct discharge)

Reduction in direct discharge
Increase in WWF
treatment capacity

Install additional treatment
devices for remaining area

3Qsh
Temporary
Storage
Stormwater Sewers
Storage Facilities

Final
Sedimentation
Tanks

1Qsh
3Qsh

Influent flow

ೋ
ᴉ

Wet
Weather
Activated
SludgeProcess
(for
3Qsh)

⚳

Plate
Settler
SedimentationPrimary
sedimentation

෻

3Qsh

3Qsh
Temporary Direct
storage discharge
Plate settler
sedimentation
with coagulant
addition
Wet weather
activated◲ᤃ᡼
sludge process 㩷

Reduction in the area for primary sedimentation
by high rate filtration or plate settler
sedimentation without coagulant addition

Fig 16: Image of Wet Weather Sewage Treatment in the New CSS Improvement Plan

- 369 9-

3Qsh

2Qsh
1Qsh
Time

Table 1: Estimated Project Cost for CSS Improvement
Furthermore, the total project
Approaches
Cost (billion JPY)
costs for the previous improvement
Wet Weather Activated Sludge Process
18
Plate Settler Sedimentation Process57
plan was estimated to be about 300
Construction of Storage Facilities
87
billion yen. However, based on the
Other Approaches
Filling Sediment Traps in Manhole Bottoms
30
above results, the total project costs toReplacement of Screens, etc.
achieve the mid-term objectives for the
Total
192
entire city area was estimated to beabout 200 billion yen as shown in Table 1, a saving of about 100 billion yen.

Future issues- In order to significantly increase the amount of sewage processed at sewage treatment plants,
further studies are required regarding transfer of sewage to treatment plants, distribution of sewage
to various treatment processes in each plant, capacity of effluent channels, etc.
- For actual operations of storing wet weather sewage utilizing large stormwater sewers,
technologies such as real-time control need to be studied to ensure CSO control and flood control
without lowing safety level against floods.

Conclusions
The Osaka City Government has selected CSS to achieve the flood control and public hygiene
improvement at the same time. Thereby, the sewer systems were constructed rapidly over theentire city area. CSSs were able to cope with increase in wastewater flow due to urban density
growth. Currently in Japan, the problem of wet weather discharge pollutant load by CSOs is oneof the priority issues for water environment protection. The Osaka City Government is now
planning to take comprehensive countermeasures by combining various methods including someoriginally developed technologies, and expects to be able to eventually capture and treat almost allof wet-weather sewage except in the case of extremely heavy rains comparable to the probablemaximum precipitation. If such improvements are achieved, CSS has the potential to become an
excellent system that can also deal with non-point source pollutants released in wet weather
conditions.
The City Government will continuously monitor CSOs at main overflow outlets in order toadequately investigate CSO characteristics and clearly identify the benefits and costs of various
program components for implementing the improvement plan with public consensus.

References
Takayanagi E., Horita K., Fukui S., Taruya T. and Yamamoto S. (1997): Measures against
Combined Sewer Overflow in Osaka City, Proceedings the 7th Japanese-German Workshop on
Wastewater and Sludge Treatment, pp.8-3-1-11, 1997
Takayanagi E., Abe T, Taruya T and Fukui S. (1997): Introduction of Plate Settler for Combined
Sewer Overflow Treatment, Water Science and Technology, Vol. 36, No. 8-9, pp.207-212, 1997,
IAWQ- 370 10-

- 371 

1

3/40

Resident population:
2.63 million
 Daytime population:
3.66 million
2
 Area: 220 km
 Sewers: 4,830 km
 Pumping Stations: 57
 Sewage Treatment Plants(STPs):12
 Total Treatment Capacity:
2,844,000 m3/day

Profile of Osaka City

Manager for Project Assessment and Evaluation
Administrative Division
Environment and Sewerage Bureau
Osaka City Government

Makoto Fujita

Development of Wet Weather Sewage
Treatment Technologies and their
Application to the Combined Sewer
System Improvement Plan of Osaka City

Geographical Feature

4/40

2/40

Combined sewer systems (CSSs) improvement
plan of Osaka City, which was recently revised
applying the above technologies.

Wet weather sewage activated sludge process
Plate settler sedimentation process with coagulant
addition

Most of the city area lies
below the river flood level.
 Gravity drainage area isonly about 10% of the cityarea.
 Therefore, most of thestormwater need to becollected once at STPs or
pumping stations, thenpumped and discharged
to public water bodies.








Profile and features of Osaka City and its
sewerage system Wet weather sewage treatment technologies
developed by Osaka City Government to apractical levelPresentation Topics

- 372 -





5/40

7/40

From the these points of view, the City
Government has been actively developedand introduced original technologies toincrease continuous wet weather sewage
treatment capacities by fully utilizing
existing facilities, instead of other CSS
improvement methods commonly
adopted in Japan, such as construction ofstormwater reservoirs and sewer
separation.

Selected Alternatives for CSS
Improvement in Osaka City

In the modern sewerage
construction projects
stared in 1894, combined
sewer system (CSS) was
adopted for publichygiene improvement andflood control at the sametime with lower cost andin a shorter period.
 Currently, the sewer
system covers almost
100% of the city area,and 97% is served by
CSSs.

Sewage Collection System

Hirano STP

Ono STP

6/40

Imafuku STP

1980s Experimental studies, 1988 Design studies,1990 Pilot plant studies,1998 Introduction to full scale treatment facilities started
Late 1970s Experimental studies, 1987 Design studies, 2002
Experiments on coagulant addition, 2004 Pilot plant studies
1980s Experimental studies, 1987 Design studies,1998 Installation of a full scale unit
1983 Research studies, 2000 Operation started

1984 Effectiveness surveys in pilot areas, 1995 Modifications
started for the entire city area, 2003 Completion
2003 Pilot plant studies

Wet Weather Sewage
Activated Sludge
Process
Inclined Plate Setter
Sedimentation Process
Fine Mesh Screens for
Wet Weather Sewage
Storage of Wet Weather
Sewage utilizing
Stormwater Sewers
Filling Sediment Traps at
the Bottom of Manholes
Wet weather sewage
high rate filtration

8/40

1971 Design, 1974 Performance survey

Wet Weather Sewage
Sedimentation Tank

R&D on CSO Control Technologies
carried out by Osaka City Government

Tsumori STP

Almost the entire city area has been alreadyurbanized, therefore, it is very difficult toacquire lands for the construction of new
facilities.
 Because the scale of CSSs in is so huge,
the cost for CSS improvement has to beminimized as possible.
Constraints in CSS Improvementin Osaka CityIn the “wet weather
activated sludge
process”, 2Qsh is fed
Wet
2Qsh
into the latter section of weather
wastewater
aeration tanks and(3Qsh)
1Qsh
treated by the activatedPrimary
sedimensludge process.
tationAeration tanktank
 The results of 27
surveys verified stable
Discharge after
operations in processing
sedimentation
Return sludge
sewage volume of 1.48(2Qsh)
4.62 Qsh with theaverage effluent quality
at 9.3 mg/L in SS and7.7 mg/L in BOD.



9

11/40

Final
sedimentation
tank

Activatedsludgetreatment(3Qsh)

Wet Weather Sewage ActivatedSludge Process

Wet Weather Sewage
Activated Sludge Process

Return sludge

Aeration tank

1Qsh

Pollutant Removal in the
Activated Sludge Process
In the conventional
activated sludge process,pollutants are absorbedby activated sludge in thefirst 30 minutes andgradually decomposed.
 In the wet weather
activated sludge process,the hydraulic retention
time of 2Qsh wet weather
sewage fed into the latter
section is about 30 min,therefore, pollutants inthe sewage are quickly
removed.


In Osaka City, STPs aredesigned to receive up to
3 times of the designWetweatherpeak dry weather flow
wastewater(3Qsh).
(3Qsh)
 In the conventional
Primary
sedimenmethod, 1Qsh wastationtanktreated by the activatedsludge process, and theDischarge after
sedimentation
remaining 2Qsh was(2Qsh)
treated by only primary
sedimentation anddischarge.

Absorption Decomposition(approx. 30 min)

12/40

Time

10/40

Final
sedimentation
tank

Activatedsludgetreatment(1Qsh)

Conventional Sewage Treatment
during Wet Weather

Concentration- 373 -

SS, BOD (mg/L)

0

30

60



After (automatically
13/40

4:00

9:00

14:00

19:00

0:00

5:00
Time

10:00

15:00

20:00

1:00

Treated Wet Weather Sewage Flow
Influent SS (mg/L)
Influent BOD (mg/L)
Effluent SS (mg/L)
Effluent BOD (mg/L)

15/40

0

1

2

3

4

5

In September 2000, when there was a prolonged rain
event with the total rainfall of 165 mm, this process wascontinuously operated for 45 hours stably, and it wasverified that the new process was capable of prolonged
continuous operations.

Result of Prolonged Continuous
Operation (45 hours)

Before (hand

treatment

Conventional

16%

84%

Wet weather
activated sludge
process

27%

0

20

40

60

80

treatment

Conventional

22%

78%

Activated sludge
treatment

Primary treatment

Comparison of discharge pollutant load in BOD
100

Wet weather
activated sludge
process

39%

14/40

23:00

4:00

0
18:00

0

2

3

4

5

1
13:00

Treated Wet Weather Sewage Flow
Influent SS (mg/L)
Influent BOD (mg/L)
Effluent SS (mg/L)
Effluent BOD (mg/L)

50

100

150

200

16/40

In January 2001, when the influent pollutant load washigh for a long time (the highest BOD was 160 mg/L,
the average BOD was 120 mg/L), it was verified thisprocess was capable of operations under high pollutant
loading condition.

Operation under High Pollutant
Loading Condition

0

20

40

60

Activated sludge
treatment

Primary treatment

by 73%

80
Reduction

100

Comparison of discharge pollutant load in SS

Results from 4 surveys verified that this new process
could reduce the amount of pollutant discharge by 5991% (73% on average) in SS, and by 27-78% (61% on
average) in BOD for a single rainfall event, comparedwith the conventional wet weather sewage treatment.

by 61%

The aeration tanks in STPs in Osaka City have beenequipped with channels for step feeding to be
applicable for both the conventional activated sludge
process and the step feed aeration process.
 Thus, this new process could be introduced withrelatively small modifications of existing facilities.

SS, BOD (mg/L)

Reduction

Advantages

Ratio of treated sewage flow rate to
Design dry weather peak flow（Qsh）

Effect of Wet Weather ActivatedSludge Process

Ratio of treated sewage flow rate to
Design dry weather peak flow （Qsh）- 374 -

- 375 -Sedimentation Process using
Inclined Plate Settlers

Inclined
Plate Settler
ModulePlate settler sedimentation process is adopted in thetreatment of drinking water supplies.
 Osaka City Government has carried out studies andexperiments on the application of this process fortreating wet weather sewage since the late 1970s.





19/40

17/40

From the results of performance surveys, it was verified; The operation of this process can run stably, while
treating up to 3 times of the design peak dry weather
flow .
 This process is capable of prolonged continuous
operations and under high pollutant loading condition.
 The amount of pollutant discharge can be considerably
reduced, compared with the conventional method.
 Since these results have demonstrated the effectivenessof the “wet weather activated sludge process”, Osaka City
Government has been introducing this process to all 12
STPs by March 2007.

Summary - Wet Weather ActivatedSludge Process

Wet weather sewage contains high content of
SS with a high settling velocity, therefore, it isappropriate to use this process to separate thehigh settleable SS components.
 At an average flow rate of 0.5-0.6 m/min in theplate settler module with the detention time ofabout 10 min, effluent quality is equivalent to
the one from conventional setting tanks.
 PAC is most suitable as a coagulant for wet
weather sewage sedimentation process.
 By adding PAC at the concentration of 2.5-10
mg-Al/L, influent sewage with BOD of 100-200
20/40
mg/L can be treated to produce an effluent with


18

Findings from Basic Studies andExperiments

Plate Settler Sedimentation
Process with Coagulant
Addition- 376 -In order to utilize
the existing
sedimentation tankeffectively, it isnecessary to place
multiple plate
settler modules inthe tank anddistribute theinfluent sewage toeach module.

Inffluent Main Pipe

Effluent

23/40

Effluent Main Pipe

300mm x 6 200mm x 4

Cross Section A-A
Perforated Pipes Perforated Pipes

Perforated Pipes 200mm x 5

Plan of Effluent Pipes
Effluent Main Pipe 600mm Effluent Main Pipe 500mm

Perforated Pipes 300mm x 5

Inclined Plate Module

Perforated Pipes 250mm x 5

Influent Main Pipe 600mm

Plan of Influent Pipes

Installation of Inclined Plate
Settler Modules

21/40

Since it is very difficult to acquire lands for theconstruction of new facilities, the city government
started design studies on modifying existing primary
sedimentation tanks to plate settler sedimentation
tanks using a pilot treatment facility at Chishima STP.

Chishima Sewage Treatment Plant

Modifying Existing Primary
Sedimentation Tanks

Influent

Vinyl chloride resin
Recycled plywood

Per

24/40

Influent main pipes are placed in the tank and connectedwith perforated pipes at their ends.
 Perforated pipes are vertically placed near the inlet crosssection of plate settler modules in order to distribute theinfluent sewage evenly in horizontal/vertical directions of
the module cross-section and to mitigate density currents.


22/40

Equalization of Influent Sewage
Distribution to Each Module

for
ate
1
#
for d pipe
n
i
in
s
ma
#2 distri fluen
ent
n
u
t
i
b
l
f
utio
Ef
ma
n
nt aineulmf
Ef entlu
InfIntervals of inclined plates:
100 mm Height of a settler module: 2.5 m
 Coagulant: PAC (JIS K 1475
Al2O3:10.0-11.0 wt%) Dosage of PAC: 3-10 mg-Al/L

Length of inclined plates: 3-6 m
 Angle of inclination: 60º
 Plate Material:Specifications





0

10

20

30

0

20

30

40

Surface Loading Rate (m3/m2·day)

10

50

50
150
250
350
450
550

50

40

Influent SS(mg/L)

60

27/40

Under the condition that the flow rate is less than
0.5m/min, it was observed that the effluent with
sufficient quality was obtained stably, when the surface
loading rate was less than 40m3/(m2·day).

Results of Performance Surveys
2) Influence of Surface Loading Rate

25/40

Sludge collectors were not installed, therefore, sludge
is stored on the bottom of the tank during rainfall
events. After rainfall events, water in the tank is once
drained together with most of the stored sludge. Then,the remaining sludge is flushed from the rear end of thetank using settled water from the adjacent settling tank.

Sludge Removal

Effluent SS (mg/L)

0

40
20

60

80

100

120

140

0

40
20

80
60

Under the condition thatthe flow rate is less than
0.5m/min and the surface
loading rate is less than
35m3/(m2·day), It is considered thatpollutants can beremoved to the level ofthe average effluent
quality at 20mg/L in SS
and 20-30 mg/L in BOD,
expect when the influent
pollutant load isextremely high.

0

10

20

30

40

50

60

45
40
35
30
25
20
15
10
5
0

0

0

50

100

100

150
200
250
Influent BOD (mg/L)

200
300
400
Influent SS (mg/L)

350

600

28/40

300

500

26/40

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Flow Rate in the Plate Setter Module (m/min)

Influent
BOD(mg/L)
50
150
250
350
450

50
150
250
350
450
550

Influent SS(mg/L)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Flow Rate in the Plate Setter Module (m/min)

100

120

140

Results of Performance Surveys
3) Treated Effluent Quality

Under the condition thatthe surface loading rate isless than 35m3/(m2·day), Effluent SS concentration
increases when the flowrate exceeds 0.5m/min.
 Effluent BOD concentration
increases when the flowrate exceeds 0.55m/min.
 It is proposed that the flowrate in the plate settler
modules should be set
0.5m/min or less.

Results of Performance Surveys
1) Influence of Flow Rate
Effluent SS (mg/L)
Effluent BOD (mg/L)
Effluent SS (mg/L)
Effluent BOD (mg/L)- 377 -

- 378 -

0

0

50

100

100
150
200
250
Influent BOD (mg/L)

200
300
400
Inf luent SS (mg/L)

350

600

29/40

300

500

31/40

The city government is also conducting performance
surveys when applying this process to the primary
sedimentation process under dry-weather conditions.
 It is expected that the plate settler sedimentation process
will be able to have the equivalent treatment capacity ofthe existing primary sedimentation tank with 1/2-1/3 of its
original surface area.
 In this case, the remaining space can be used for wetweather sewage treatment by the plate settler
sedimentation process with coagulant addition.



0

20

40

60

80

100

100
90
80
70
60
50
40
30
20
10
0

Next Application of Plate Settler
Sedimentation Process

Under the same condition, The removal rate isapproximately 90% in SS
and 80% in BOD,
although it shows low
value when the influent
pollutant concentration is
at low level.

Results of Performance Surveys
4) Pollutant Removal Rate
SS Removal Rate (%)

BOD Removal Rate (%)



Newly Revised CSS
Improvement Plan of Osaka City

32

30/40

From the results of performance surveys, it isestimated that the existing primary sedimentation
tanks can be capable of treating wet-weather
sewage flow equivalent to 5 times its present
design maximum daily dry-weather flow, by
installing inclined plate settler modules with thefollowing specifications.
 Intervals of inclined plates: 100mm
 Flow rate in the settler modules: less than
0.5m/min Surface loading rate: 25m3/(m2·day)

Summary - Plate Settler
Sedimentation Process











Wet weather activated sludge process Plate settler sedimentation process Storage of wet-weather sewage using large
stormwater sewers designed for flood control
 Construction of wet-weather storage facilities

In the newly revised CSS improvement plan

Wet weather activated sludge process Filling sediment traps in manhole bottoms
 Replacement of screens Construction of stormwater reservoirs

In the previous CSS improvement plan

Major Approaches

The plate settler sedimentation process has
been developed to a practical level.

35/40

33/40

“The average quality of wet weather effluent during asingle rainfall event with the total rainfall of 10-30mm
should be not more than 40mg/L in BOD in eachtreatment area. (The tentative standard is 70mg/L
for the next 20 years.)”

In 2004, “Sewerage Law Enforcement Order”
was amended, and the wet weather effluent
quality standard for CSSs was newly stipulated.

Needs/Background for Revision(newly constructed)

Stormwater
Reservoirs

Temporary
Storage

3Qsh

反応槽

初
Primary
Sedimen
Reaction沈
tation
Tanks
等
Tanks
(for
(for
1Qsh)
3Qsh)

Exceeding 3Qsh (Direct discharge)

Final
Sedimentation
Tanks

1Qsh

2Qsh

36/40

Time

1Qsh

Sedimentation
2Qsh
only

Directdischarge

Secondary treatment

Temporarystorage(bystormwater
reserviors)

Image of Conventional Treatment
Process during Wet Weather

34/40

Mid-term Objectives (target year: 2018)
 Within each treatment area, the annual
discharge pollutant load including dry weather
periods should be reduced to the equivalentlevel of a separate sewer system (18mg/L in
BOD).
 The wet weather effluent quality should meetthe standard stipulated in the Sewerage Law
Enforcement Order amended in 2004.
 The number of overflow events from CSO
outfalls should be reduced by half.

Objectives of the Revised CSS
Improvement Plan

Influent flow

- 379 終沈Project Cost

終沈

- 380 -

1Qsh
3Qsh
Wet簡易放流
weather
activatedsludge process

Table 1: Estimated Project Cost for CSS Improvement
Approaches
Cost (billion JPY)
Wet Weather Activated Sludge Process
18
Plate Settler Sedimentation Process
57
Construction of Storage Facilities
87
Other Approaches
Filling Sediment Traps in Manhole Bottoms
30
Replacement of Screens, etc.
Total
192

1Qsh

2Qsh

Time

37/40

Temporary Direct
storage discharge

39/40

For the previous improvement plan, the total project
cost was estimated to be about 300 billion yen.
 Whereas in the new improvement plan, which fully
utilizes existing facilities to increase the wet-weather
sewage continuous treatment capacity, the total projectis estimated to be about 200 billion yen, a saving ofabout 100 billion yen.



Stormwater
Reservoirs

Final
Sedimentation
Tanks
3Qsh



Wet
Weather
Activated
SludgeProcess
(for
3Qsh)
Final
Sedimentation
Tanks
1Qsh
3Qsh

1Qsh

2Qsh

3Qsh

Time

40/40

“Wet weather activated sludge process” and “plate
settler sedimentation process” have been developed toa practical as widely applicable economic continuous
treatment technologies, which can;
 Utilize existing facilities effectively,
 Reduce CSS improvement costs significantly,
 Show rapid impacts on CSS improvement, Facilitate the progress of CSS improvement, and Suggest that CSS has the potential to become themost appropriate water pollution control system fordensely populated urban areas where flood control isalso a critical issue.

38/40

Temporary Direct
storage discharge
Plate settler
sedimentation
with coagulant
addition
Wet簡易放流
weather
activatedsludge process

Reduction in the area for primary sedimentation
by high rate filtration or plate settler
sedimentation without coagulant addition

Plate
初Settler沈
Sedime等
ntationPrimary
sedimentationSummary

Stormwater Sewers
Storage Facilities

Temporary
Storage

3Qsh

3Qsh

Increase in WWF
treatment capacity

Reduction in direct discharge

Install additional treatment
devices for remaining area

Exceeding 6Qsh (Direct discharge)

反応槽(newly constructed)

Temporary
Storage

3Qsh

反応槽

Wet
初
Primary
Weather
Sedimen
沈
Activatedtation等
SludgeTanks
Process
(for
(for
3Qsh)
3Qsh)

Exceeding 3Qsh (Direct discharge)

Influent flow

Image of Wet Weather Sewage
Treatment in the New Plan

Influent flow

Image of Wet Weather Sewage
Treatment in the Previous Plan

終沈