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Development of Wet-Weather Sewage Treatment Technologies and theirApplication to the Combined Sewer System Improvement Plan of OsakaCity
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 CSSimprovement methods, particularly wet weather combined sewage treatment technologies.Among them, wet weather sewage activated sludge process and sedimentation using inclinedplate settlers with coagulant addition have been successfully developed to a practical level, whichcan effectively increase continuous wet weather sewage treatment capacity of existing sewagetreatment facilities with relatively lower cost and shorter period. The city government has revisedits CSS improvement plan by applying these technologies, with the objective of reducing dischargepollutant 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 developedtechnologies and the basic concept of the CSS improvement plan.
Key words: combined sewer system, plate settler sedimentation, wet-weather activated sludgeprocess
IntroductionOsaka is the largest city in western Japan with
a resident population of about 2.63 million (daytimepopulation of about 3.66 million) and area of about220 km2. Most of the city area lies below the riverflood level, and gravity drainage area is only about10% of the city area. Therefore, most of thestormwater needs to be collected once at sewagetreatment plants or pumping stations then pumpedand discharged to public water bodies.
In the modern sewerage construction projectsin Osaka City started in 1894 in response to choleraepidemics, CSS was adopted in order to improvepublic 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).
Fig. 1: Areas Served by Combined Sewer Systems
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2
In CSSs, when sewage flow exceeds a certain level during wet weather, untreated sewagemay 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 waterbodies was 14% in terms of BOD and 36% in terms of SS. Out of this pollutant load discharged topublic 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 againstfloods was improved to a certain level in the city, the City Government has been carried outresearches and studies on technologies for CSS improvement. Especially, in Osaka City, almostthe 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 increasecontinuous wet-weather sewage treatment capacities by fully utilizing existing facilities, instead ofother CSS improvement methods commonly adopted in Japan such as construction of wet-weathersewage storage facilities and sewer separation.
This paper introduces the major technologies for CSS improvement newly developed by theOsaka City Government and the city s CSS improvement basic plan applying these technologies.
Wet-Weather Sewage Treatment Technologies for CSS ImprovementDeveloped by the Osaka City Government
Wet-weather sewage activated sludge processIn 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 sludgeprocess, this 2Qsh of wet-weather sewage is fed into the latter section of aeration tanks and istreated by the activated sludge process (Figure 3).
Fig. 2: Conventional Wastewater Treatment during Wet Weather
Discharge aftersedimentation
(2Qsh)
1QshPrimary
sedimen-tationtank
Return sludge
Wetweather
wastewater(3Qsh)
Activatedsludge
treatment(1Qsh)
Aeration tank
Finalsedimen-
tationtank
Fig. 3: Wet Weather Activated Sludge Process
Discharge aftersedimentation
(2Qsh)
1QshPrimary
sedimen-tationtank
Return sludge
Wetweather
wastewater(3Qsh)
Activatedsludge
treatment(3Qsh)
2Qsh
Aeration tank
Finalsedimen-
tationtank
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3
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 sludgeprocess, the hydraulic retention time of wet-weather sewage fed into the latter section of aerationtanks 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 processand the step feed aeration process. Thus, this new process could be introduced with relativelysmall 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. Theresults 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) inBOD for a single rainfall event, when compared with the conventional method employing primarysedimentation (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 processwas capable of prolonged continuous operation.
Since these results have demonstrated the effectiveness of the wet-weather activated sludgeprocess, the Osaka City Government has been introducing this process to all 12 sewage treatmentplants by March 2007.
Comparison of discharge pollutant load in SS
0
20
40
60
80
100
Conventional Wet weatheractivated sludge
process
Primary treatment
84%
Red
uctio
n
16%
by73
%
Activated sludgetreatment
treatment
27%
0
20
40
60
80
100
78%
22%39%
Red
uctio
n
by61
%
Primary treatmentActivated sludgetreatment
Conventional Wet weatheractivated sludge
processtreatment
Comparison of discharge pollutant load in BOD
Fig. 4: Effect of the Wet Weather Activated Sludge Process
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4
Sedimentation process using inclined plate settlers with coagulant additionSedimentation 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 treatingwet-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 sewageand 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 sedimentationvelocity, 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 tosedimentation efficiency and resulted in the following:3) Poly-aluminum chloride (PAC) is most suitable as a coagulant for the wet-weather sewagesedimentation process.4) By adding PAC at the concentration of 2.5 to 10 mg-Al/L, influent sewage with BOD of 100 to200 mg/L can be treated to produce an effluent with BOD of 20 to 25 mg/L with the detention timenot 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 onmodifying existing primary sedimentation tanks to plate settler sedimentation tanks as an alternativeengineering option, and installed inclined plate settler modules to an existing primary sedimentationtank 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 fromOctober 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 eventsIn 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.
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5
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 ofthe 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 underdifferent 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 duringrainfall events. Then after rainfall events, water in the tank is once drained together with most ofthe stored sludge, and the remaining sludge is flushed from the rear end of the tank using settledwater from the adjacent primary sedimentation tank.
Influence of flow rate in the plate settlermodules
Figures 6 and 7 show the relationship between theflow rate in the plate settler modules and the effluentquality when the surface loading rate is less than 35m3/(m2·d). SS concentration in the treated effluentincreased when the flow rate exceeded 0.5 m/min. andthe BOD concentration increased when the flow rateexceeded about 0.55 m/min. It is observed that theimpact 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 platesettler modules should be set 0.5 m/min or less.
Inflow Conduit Tube 600
OutflowConduit Tube 600 OutflowConduit Tube 600 OutflowConduit Tube 500
Inflow Multi-hole Duct 250 5 Inflow Multi-hole Duct 300 5
Plan View of Inflow Conduit
Inclined Plate Unit
Outflow
OutflowConduit Tube
Inflow Conduit Tube Stectional View
Fig.5 Design of Inclined Plate Settler in Chisima Sewage
Outflow Multi-hole Duct 200 5 Outflow Multi-hole Duct 300 6 OutflowMulti-hole Duct 200 4
Plan View of Inflow Conduit
Inflow Multi-hole Duct Outflow Multi-hole Duct
Fig.7 Influence of Flow Rate to Effluent BOD
0
20
40
60
80
100
120
140
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Flow Rate (m/min
Effl
uent
BOD
(mg/
L)
50150250350450
InfluentBOD(mg/L)
Fig.6 Influence of Flow Rate to Effluent SS
0
20
40
60
80
100
120
140
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8Flow Rate (m/min)
Efflu
entS
S(m
g/L) 50
150250350450550
Influent SS(mg/L)
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6
Influence of surface loading rateFigure 8 shows the relationship between the
surface loading rate and the treated effluent qualitywhen the flow rate is below 0.5 m/min. As shownin the graph, it is observed that the effluent withsufficient quality is obtained stably when the surfaceloading rate is 40 m3/(m2·d) or less.
Treated effluent qualityBased on the above observations, the relationship between the influent sewage quality and the
treated 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 averagedwithin 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 inSS and 20 to 30 mg/L in BOD, except when the pollutant concentration of the influent sewage isextremely high. Since little data was available under the condition of the influent sewage withhigh pollutant concentrations, such data need to be collected furthermore.
Removal rateUnder the same conditions, the relationship between the influent sewage quality and the
removal rate is shown in Figures 11 and 12. The removal rate is approximately around 90% in SSand 80% in BOD, although it shows low value when the pollutant concentration of the influent is atlow level.
Fig.11 Average SS Removal Rate Fig.12 Average BOD Removal Rate
0102030405060708090
100
0 100 200 300 400 500 600Influent SS (mg/L)
SSR
emov
alR
ate
(%)
0
20
40
60
80
100
0 50 100 150 200 250 300 350Influent BOD (mg/L)
BO
DR
emov
alR
ate
(%)
Fig.10 Average Influent and Effluent BOD
0
10
20
30
40
50
60
0 50 100 150 200 250 300 350Influent BOD (mg/L)
Effl
uent
BO
D(m
g/L)
Fig.9 Average Influent and Effluent SS
05
1015202530354045
0 100 200 300 400 500 600Influent SS (mg/L)
Efflu
entS
S(m
g/L)
Fig.8 Influence of Surface Loading Rate to Effluent SS
0
10
20
30
40
50
60
0 10 20 30 40 50Surface Loading Rate (m3/(m2·day))
Effl
uent
SS
(mg/
L)
50150250350450550
Influent SS(mg/L)
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7
From these findings based on the survey results, it is estimated that existing primarysedimentation tanks can be capable of treating wet-weather sewage flow equivalent to five times itspresent 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 platesettler 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 surfacearea. In this case, the remaining space can be used for wet-weather sewage treatment by the platesettler sedimentation process with coagulant addition. Then, the wet-weather sewage treatmentcapacity can be increased without constructing new tank facilities.
The Newly Revised CSS Improvement Plan of Osaka CityIn the previous CSS improvement plan of Osaka City, the objective was set as to reduce the
annual discharge pollutant load including dry weather period to the equivalent level of separatedsewer system , and various programs have been implemented such as eliminating sediment traps atthe 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 forCSSs was newly stipulated. On the other hand, the city government has been originally studiedthe 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 Governmentestablished a new CSS improvement plan in March 2006.
ObjectivesMid-term objectives (target year: 2018)- Within each treatment area, the annual discharge pollutant load including dry weather periodsshould be reduced to the equivalent level of separated sewer system. Here, it is assumed that theaverage 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 LawEnforcement 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 BODin 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/Lin BOD.
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8
Major approachesIn 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 process2) Introduction of the plate settler sedimentation process with coagulant addition3) Storage of wet-weather sewage utilizing large stormwater sewers designed for flood control4) 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 projectsconcerning 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 weathersewage 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 existingsedimentation tanks to the plate settler sedimentation process (without coagulant addition) or thehigh rate filtration process. Then, the plate settler sedimentation process with coagulant additionshould be introduced using the remaining 1/2 to 2/3 of the area for wet weather sewage treatment.
Cost and BenefitSimulation 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 settlersedimentation 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 capturedfor 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 sewersystem.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.
72.1 84.079.296.7 91.8 94.2
83.481.7 83.1
0%10%20%30%40%50%60%70%80%90%
100%
Appr
oach
"A"
Appr
oach
"B"
Appr
oach
"C"
Appr
oach
"A"
Appr
oach
"B"
Appr
oach
"C"
Appr
oach
"A"
Appr
oach
"B"
Appr
oach
"C"
Pollutant RemovalRate
Direct Discharge
CapturedPollutant Loads
Captured Sewageflows
Fig 13: Comparison of effect in alternative approaches Fig 14: Construction Costs and Annual Recurrent Cost
Approach A : Stormwater reservoir (13mm) + sedimentation (2Qsh)Approach B : Wet-weather activated sludge process (3Qsh) + Stormwater reservoir (3mm)Approach C : Wet-weather activated sludge process (3Qsh) + Plate settler sedimentation with coagulant addition (3Qsh)
30.7
7.3 6.3
263210
1083
0
5
10
15
20
25
30
35
Approach"A" Approach"B" Approach"C"
Con
stru
ctio
nC
osts
(billi
onJP
Y)
0
200
400
600
800
1000
1200
Rec
cure
ntC
osts
(milli
onJP
Y/ye
ar)
Construction Costs(billion JPY )Reccurent Costs(million JPY/year)
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9
Influentflow
1Qsh
2Qsh
Time
Temporarystorage
(bystormwaterreservoirs)
Directdischarge
Sedimentaiononly
2Qsh
3Qsh
Exceeding 3Qsh (Direct discharge)
1Qsh
ReactionTanks(for
1Qsh)
FinalSedimen
tationTanks
PrimarySedimen
tationTanks(for
3Qsh)
StormwaterReservoirs
(newly constructed)
Secondary treatmentTemporarystorage
Fig 15: Image of Conventional Treatment Process in Wet Weather Events
Influentflow 3Qsh
1Qsh
2Qsh
Time
Wet weatheractivated
sludge process
Temporarystorage
Directdischarge
Plate settlersedimentationwith coagulant
addition
3Qsh
PlateSettler
Sedimentation
Primarysedimen
tation
Exceeding 6Qsh (Direct discharge)
3Qsh
Reduction in the area for primary sedimentationby high rate filtration or plate settler
sedimentation without coagulant addition
Install additional treatmentdevices for remaining area
Increase in WWFtreatment capacity
Reduction in direct discharge
1Qsh3Qsh
WetWeatherActivatedSludgeProcess
(for3Qsh)
FinalSedimen
tationTanks
3Qsh
Stormwater SewersStorage Facilities
TemporaryStorage
Fig 16: Image of Wet Weather Sewage Treatment in the New CSS Improvement Plan
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10
Furthermore, the total projectcosts for the previous improvementplan was estimated to be about 300billion yen. However, based on theabove results, the total project costs toachieve the mid-term objectives for theentire 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 sewageto 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 controlwithout lowing safety level against floods.
ConclusionsThe 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 densitygrowth. 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 nowplanning 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 anexcellent system that can also deal with non-point source pollutants released in wet weatherconditions.
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 variousprogram components for implementing the improvement plan with public consensus.
ReferencesTakayanagi E., Horita K., Fukui S., Taruya T. and Yamamoto S. (1997): Measures againstCombined Sewer Overflow in Osaka City, Proceedings the 7th Japanese-German Workshop onWastewater and Sludge Treatment, pp.8-3-1-11, 1997Takayanagi E., Abe T, Taruya T and Fukui S. (1997): Introduction of Plate Settler for CombinedSewer Overflow Treatment, Water Science and Technology, Vol. 36, No. 8-9, pp.207-212, 1997,IAWQ
Table 1: Estimated Project Cost for CSS ImprovementApproaches Cost (billion JPY)
Wet Weather Activated Sludge Process 18Plate Settler Sedimentation Process 57Construction of Storage Facilities 87Other Approaches
Filling Sediment Traps in Manhole BottomsReplacement of Screens, etc.
30
Total 192
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1
Dev
elop
men
t of W
et W
eath
er S
ewag
e Tr
eatm
ent T
echn
olog
ies
and
thei
r A
pplic
atio
n to
the
Com
bine
d Se
wer
Sy
stem
Impr
ovem
ent P
lan
of O
saka
City
Mak
oto
Fujit
aM
anag
er fo
r Pro
ject
Ass
essm
ent a
nd E
valu
atio
nA
dmin
istr
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e D
ivis
ion
Envi
ronm
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ewer
age
Bur
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Osa
ka C
ity G
over
nmen
t
2/40
Pre
sent
atio
n To
pics
Pro
file
and
feat
ures
of O
saka
City
and
its
sew
erag
e sy
stem
Wet
wea
ther
sew
age
treat
men
t tec
hnol
ogie
s de
velo
ped
by O
saka
City
Gov
ernm
ent t
o a
prac
tical
leve
lW
et w
eath
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ewag
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e pr
oces
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late
set
tler s
edim
enta
tion
proc
ess
with
coa
gula
nt
addi
tion
Com
bine
d se
wer
sys
tem
s (C
SS
s) im
prov
emen
t pl
an o
f Osa
ka C
ity, w
hich
was
rece
ntly
revi
sed
appl
ying
the
abov
e te
chno
logi
es.
3/40
Pro
file
of O
saka
City
Res
iden
t pop
ulat
ion:
2.
63 m
illio
nD
aytim
e po
pula
tion:
3.66
mill
ion
Are
a:22
0 km
2
Sew
ers:
4,8
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mP
umpi
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tatio
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e Tr
eatm
ent P
lant
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:12
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4,00
0 m
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4/40
Geo
grap
hica
l Fea
ture
Mos
t of t
he c
ity a
rea
lies
belo
w th
e riv
er fl
ood
leve
l.G
ravi
ty d
rain
age
area
is
only
abo
ut 1
0% o
f the
city
ar
ea.
Ther
efor
e, m
ost o
f the
st
orm
wat
erne
ed to
be
colle
cted
onc
e at
STP
sor
pu
mpi
ng s
tatio
ns, t
hen
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ped
and
disc
harg
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to p
ublic
wat
er b
odie
s.
- 371 -
5/40
Sew
age
Col
lect
ion
Sys
tem
In th
e m
oder
n se
wer
age
cons
truct
ion
proj
ects
st
ared
in 1
894,
com
bine
d se
wer
sys
tem
(CS
S) w
as
adop
ted
for p
ublic
hy
gien
e im
prov
emen
t and
flo
od c
ontro
l at t
he s
ame
time
with
low
er c
ost a
nd
in a
sho
rter p
erio
d.C
urre
ntly
, the
sew
er
syst
em c
over
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mos
t 10
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f the
city
are
a,
and
97%
is s
erve
d by
C
SS
s.
6/40
Con
stra
ints
in C
SS
Impr
ovem
ent
in O
saka
City
Alm
ost t
he e
ntire
city
are
a ha
s be
en a
lread
y ur
bani
zed,
ther
efor
e, it
is v
ery
diffi
cult
to
acqu
ire la
nds
for t
he c
onst
ruct
ion
of n
ew
faci
litie
s.B
ecau
se th
e sc
ale
of C
SSs
in is
so
huge
, th
e co
st fo
r CSS
impr
ovem
ent h
as to
be
min
imiz
ed a
s po
ssib
le.
Tsu
mor
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Hir
ano
STP
On
o ST
PIm
afu
kuST
P
7/40
Sel
ecte
d A
ltern
ativ
es fo
r CS
S
Impr
ovem
ent i
n O
saka
City
From
the
thes
e po
ints
of v
iew
, the
City
G
over
nmen
t has
bee
n ac
tivel
y de
velo
ped
and
intro
duce
d or
igin
al te
chno
logi
es to
in
crea
se c
ontin
uous
wet
wea
ther
sew
age
treat
men
t cap
aciti
esby
fully
util
izin
g ex
istin
g fa
cilit
ies,
inst
ead
of o
ther
CS
S
impr
ovem
ent m
etho
ds c
omm
only
ad
opte
d in
Jap
an, s
uch
as c
onst
ruct
ion
of
stor
mw
ater
rese
rvoi
rs a
nd s
ewer
se
para
tion.
8/40
R&
D o
n C
SO C
ontr
ol T
echn
olog
ies
carr
ied
out b
y O
saka
City
Gov
ernm
ent
1983
Res
earc
h st
udie
s, 2
000
Ope
ratio
n st
arte
dS
tora
ge o
f Wet
Wea
ther
S
ewag
e ut
ilizin
g S
torm
wat
erS
ewer
s
2003
Pilo
t pla
nt s
tudi
esW
et w
eath
er s
ewag
e hi
gh ra
te fi
ltrat
ion
1984
Effe
ctiv
enes
s su
rvey
s in
pilo
t are
as, 1
995
Mod
ifica
tions
st
arte
d fo
r the
ent
ire c
ity a
rea,
200
3 C
ompl
etio
n Fi
lling
Sed
imen
t Tra
ps a
t th
e B
otto
m o
f Man
hole
s
1980
s E
xper
imen
tal s
tudi
es, 1
987
Des
ign
stud
ies,
1998
Inst
alla
tion
of a
full
scal
e un
itFi
ne M
esh
Scr
eens
for
Wet
Wea
ther
Sew
age
Late
197
0s E
xper
imen
tal s
tudi
es, 1
987
Des
ign
stud
ies,
200
2 E
xper
imen
ts o
n co
agul
ant a
dditi
on, 2
004
Pilo
t pla
nt s
tudi
esIn
clin
ed P
late
Set
ter
Sedi
men
tatio
n Pr
oces
s
1980
s E
xper
imen
tal s
tudi
es, 1
988
Des
ign
stud
ies,
1990
Pilo
t pla
nt s
tudi
es,
19
98 In
trodu
ctio
n to
full
scal
e tre
atm
ent f
acilit
ies
star
ted
Wet
Wea
ther
Sew
age
Act
ivat
ed S
ludg
e Pr
oces
s
1971
Des
ign,
197
4 P
erfo
rman
ce s
urve
yW
et W
eath
er S
ewag
e S
edim
enta
tion
Tank
- 372 -
9
Wet
Wea
ther
Sew
age
Act
ivat
ed S
ludg
e P
roce
ss
10/4
0
Con
vent
iona
l Sew
age
Trea
tmen
t du
ring
Wet
Wea
ther
In O
saka
City
, STP
sar
e de
sign
ed to
rece
ive
up to
3
times
of t
he d
esig
n pe
ak d
ry w
eath
er fl
ow
(3Q
sh).
In th
e co
nven
tiona
l m
etho
d, 1
Qsh
was
tre
ated
by
the
activ
ated
sl
udge
pro
cess
, and
the
rem
aini
ng 2
Qsh
was
tre
ated
by
only
prim
ary
sedi
men
tatio
n an
d di
scha
rge.
Dis
char
ge a
fter
sedi
men
tatio
n (2
Qsh
)
1Qsh
Prim
ary
sedi
men
-ta
tion
tank
Ret
urn
slud
ge
Wet
w
eath
er
was
tew
ater
(3
Qsh
)
Activ
ated
sl
udge
tre
atm
ent
(1Q
sh)
Aera
tion
tank
Fina
lse
dim
en-
tatio
nta
nk
11/4
0
Wet
Wea
ther
Sew
age
Act
ivat
ed
Slu
dge
Pro
cess
Dis
char
ge a
fter
sedi
men
tatio
n (2
Qsh
)
1Qsh
Prim
ary
sedi
men
-ta
tion
tank
Ret
urn
slud
ge
Wet
w
eath
er
was
tew
ater
(3
Qsh
)
Activ
ated
sl
udge
tre
atm
ent
(3Q
sh)
2Qsh
Aera
tion
tank
Fina
lse
dim
en-
tatio
nta
nk
In th
e “w
et w
eath
er
activ
ated
slu
dge
proc
ess”
, 2Q
sh is
fed
into
the
latte
r sec
tion
of
aera
tion
tank
s an
d tre
ated
by
the
activ
ated
sl
udge
pro
cess
.Th
e re
sults
of 2
7 su
rvey
s ve
rifie
d st
able
op
erat
ions
in p
roce
ssin
g se
wag
e vo
lum
e of
1.4
8-4.
62 Q
shw
ith th
e av
erag
e ef
fluen
t qua
lity
at 9
.3 m
g/L
in S
S a
nd
7.7
mg/
L in
BO
D.
12/4
0
Pol
luta
nt R
emov
al in
the
Act
ivat
ed S
ludg
e P
roce
ssIn
the
conv
entio
nal
activ
ated
slu
dge
proc
ess,
po
lluta
nts
are
abso
rbed
by
act
ivat
ed s
ludg
e in
the
first
30
min
utes
and
gr
adua
lly d
ecom
pose
d.In
the
wet
wea
ther
ac
tivat
ed s
ludg
e pr
oces
s,
the
hydr
aulic
rete
ntio
n tim
e of
2Q
sh w
et w
eath
er
sew
age
fed
into
the
latte
r se
ctio
n is
abo
ut 3
0 m
in,
ther
efor
e, p
ollu
tant
s in
th
e se
wag
e ar
e qu
ickl
y re
mov
ed.
Tim
e
Concentration
Dec
ompo
sitio
nA
bsor
ptio
n(a
ppro
x. 3
0 m
in)
- 373 -
13/4
0
Adv
anta
ges
Bef
ore
(han
d A
fter (
auto
mat
ical
ly
The
aera
tion
tank
s in
STP
sin
Osa
ka C
ity h
ave
been
eq
uipp
ed w
ith c
hann
els
for s
tep
feed
ing
to b
e ap
plic
able
for b
oth
the
conv
entio
nal a
ctiv
ated
slu
dge
proc
ess
and
the
step
feed
aer
atio
n pr
oces
s.Th
us, t
his
new
pro
cess
cou
ld b
e in
trodu
ced
with
re
lativ
ely
smal
l mod
ifica
tions
of e
xist
ing
faci
litie
s.
14/4
0
Effe
ct o
f Wet
Wea
ther
Act
ivat
ed
Slu
dge
Pro
cess
Res
ults
from
4 s
urve
ys v
erifi
ed th
at th
is n
ew p
roce
ss
coul
d re
duce
the
amou
nt o
f pol
luta
nt d
isch
arge
by
59-
91%
(73%
on a
vera
ge) i
n S
S, a
nd b
y 27
-78%
(61%
on
aver
age)
in B
OD
for a
sin
gle
rain
fall
even
t, co
mpa
red
with
the
conv
entio
nal w
et w
eath
er s
ewag
e tre
atm
ent.
Co
mpa
rison
of d
ischa
rge
pollu
tant
load
in S
S
02 0406080100
Conv
entio
nal
Wet
wea
ther
ac
tivat
ed s
ludg
e pr
oces
s
Prim
ary
treat
men
t
84%
Reduction
16%
by 73%
Activ
ated
slu
dge
treat
men
t
treat
men
t
27%
020406 080100
78%
22%
39
%
Reduction
by 61%
Prim
ary
treat
men
t Ac
tivat
ed s
ludg
e tre
atm
ent
Conv
entio
nal
Wet
wea
ther
ac
tivat
ed s
ludg
e pr
oces
s tre
atm
ent
Com
paris
on o
f disc
harg
e po
lluta
nt lo
ad in
BO
D
15/4
0
03060
4:00
9:00
14:0
019
:00
0:00
5:00
10:0
015
:00
20:0
01:
00Ti
me
SS, BOD (mg/L)
012345
Ratio of treated sewage flow rate toDesign dry weather peak flow（Qsh）
Trea
ted
Wet
Wea
ther
Sew
age
Flow
Influ
ent S
S (m
g/L)
Influ
ent B
OD
(mg/
L)Ef
fluen
t SS
(mg/
L)Ef
fluen
t BO
D (m
g/L)
Res
ult o
f Pro
long
ed C
ontin
uous
O
pera
tion
(45
hour
s)In
Sep
tem
ber 2
000,
whe
n th
ere
was
a p
rolo
nged
rain
ev
ent w
ith th
e to
tal r
ainf
all o
f 165
mm
, thi
s pr
oces
s w
as
cont
inuo
usly
ope
rate
d fo
r 45
hour
s st
ably
, and
it w
as
verif
ied
that
the
new
pro
cess
was
cap
able
of p
rolo
nged
co
ntin
uous
ope
ratio
ns.
16/4
0
Ope
ratio
n un
der H
igh
Pol
luta
nt
Load
ing
Con
ditio
n
050100
150
200
13:0
018
:00
23:0
04:
00012345
Ratio of treated sewage flow rate toDesign dry weather peak flow （Qsh）
Trea
ted
Wet
Wea
ther
Sew
age
Flow
Influ
ent S
S (m
g/L)
Influ
ent B
OD
(mg/
L)Ef
fluen
t SS
(mg/
L)Ef
fluen
t BO
D (m
g/L)
SS, BOD (mg/L)In J
anua
ry 2
001,
whe
n th
e in
fluen
t pol
luta
nt lo
ad w
as
high
for a
long
tim
e (th
e hi
ghes
t BO
D w
as 1
60 m
g/L,
th
e av
erag
e B
OD
was
120
mg/
L), i
t was
ver
ified
this
pr
oces
s w
as c
apab
le o
f ope
ratio
ns u
nder
hig
h po
lluta
nt
load
ing
cond
ition
.
- 374 -
17/4
0
Sum
mar
y -W
et W
eath
er A
ctiv
ated
S
ludg
e P
roce
ssFr
om th
e re
sults
of p
erfo
rman
ce s
urve
ys, i
t was
ver
ified
;Th
e op
erat
ion
of th
is p
roce
ss c
an ru
n st
ably
, whi
le
treat
ing
up to
3 ti
mes
of t
he d
esig
n pe
ak d
ry w
eath
er
flow
.Th
is p
roce
ss is
cap
able
of p
rolo
nged
con
tinuo
us
oper
atio
ns a
nd u
nder
hig
h po
lluta
nt lo
adin
g co
nditi
on.
The
amou
nt o
f pol
luta
nt d
isch
arge
can
be
cons
ider
ably
re
duce
d, c
ompa
red
with
the
conv
entio
nal m
etho
d.S
ince
thes
e re
sults
hav
e de
mon
stra
ted
the
effe
ctiv
enes
s of
the
“wet
wea
ther
act
ivat
ed s
ludg
e pr
oces
s”, O
saka
City
G
over
nmen
t has
bee
n in
trodu
cing
this
pro
cess
to a
ll 12
S
TPs
by M
arch
200
7.
18
Pla
te S
ettle
r Sed
imen
tatio
n P
roce
ss w
ith C
oagu
lant
A
dditi
on
19/4
0
Sed
imen
tatio
n P
roce
ss u
sing
In
clin
ed P
late
Set
tlers
Pla
te s
ettle
r sed
imen
tatio
n pr
oces
s is
ado
pted
in th
e tre
atm
ent o
f drin
king
wat
er s
uppl
ies.
Osa
ka C
ity G
over
nmen
t has
car
ried
out s
tudi
es a
nd
expe
rimen
ts o
n th
e ap
plic
atio
n of
this
pro
cess
for
treat
ing
wet
wea
ther
sew
age
sinc
e th
e la
te 1
970s
.
Incl
ined
Pl
ate
Sett
ler
Mod
ule
20/4
0
Find
ings
from
Bas
ic S
tudi
es a
nd
Exp
erim
ents
Wet
wea
ther
sew
age
cont
ains
hig
h co
nten
t of
SS
with
a h
igh
settl
ing
velo
city
, the
refo
re, i
t is
appr
opria
te to
use
this
pro
cess
to s
epar
ate
the
high
set
tleab
leS
S c
ompo
nent
s.A
t an
aver
age
flow
rate
of 0
.5-0
.6 m
/min
in th
e pl
ate
settl
er m
odul
e w
ith th
e de
tent
ion
time
of
abou
t 10
min
, effl
uent
qua
lity
is e
quiv
alen
t to
the
one
from
con
vent
iona
l set
ting
tank
s.P
AC
is m
ost s
uita
ble
as a
coa
gula
nt fo
r wet
w
eath
er s
ewag
e se
dim
enta
tion
proc
ess.
By
addi
ng P
AC
at t
he c
once
ntra
tion
of 2
.5-1
0 m
g-A
l/L, i
nflu
ent s
ewag
e w
ith B
OD
of 1
00-2
00
mg/
L ca
n be
trea
ted
to p
rodu
ce a
n ef
fluen
t with
- 375 -
21/4
0
Mod
ifyin
g E
xist
ing
Prim
ary
Sed
imen
tatio
n Ta
nks
Sin
ce it
is v
ery
diffi
cult
to a
cqui
re la
nds
for t
he
cons
truct
ion
of n
ew fa
cilit
ies,
the
city
gov
ernm
ent
star
ted
desi
gn s
tudi
es o
n m
odify
ing
exis
ting
prim
ary
sedi
men
tatio
n ta
nks
to p
late
set
tler s
edim
enta
tion
tank
s us
ing
a pi
lot t
reat
men
t fac
ility
at C
hish
ima
STP
.
Chi
shim
aSe
wag
e Tr
eatm
ent P
lant
22/4
0
Spe
cific
atio
nsLe
ngth
of i
nclin
ed p
late
s: 3
-6 m
Ang
le o
f inc
linat
ion:
60º
Pla
te M
ater
ial:
Vin
yl c
hlor
ide
resi
nR
ecyc
led
plyw
ood
Inte
rval
s of
incl
ined
pla
tes:
100
mm
Hei
ght o
f a s
ettle
r mod
ule:
2.5
mC
oagu
lant
: PA
C (J
IS K
147
5 A
l 2O3:1
0.0-
11.0
wt%
)D
osag
e of
PA
C: 3
-10
mg-
Al/L
23/4
0
Inst
alla
tion
of In
clin
ed P
late
S
ettle
r Mod
ules
In o
rder
to u
tiliz
e th
e ex
istin
g se
dim
enta
tion
tank
ef
fect
ivel
y, it
is
nece
ssar
y to
pla
ce
mul
tiple
pla
te
settl
er m
odul
es in
th
e ta
nk a
nd
dist
ribut
e th
e in
fluen
t sew
age
to
each
mod
ule.
Influ
ent M
ain
Pip
e 60
0mm
Effl
uent
Mai
n P
ipe
600m
m
Per
fora
ted
Pip
es 2
50m
m x
5
Pla
n of
Influ
ent P
ipes In
clin
ed P
late
Mod
ule
Influent
Effl
uent
Per
fora
ted
Pip
es 3
00m
m x
5
Pla
n of
Effl
uent
Pip
esE
fflue
nt M
ain
Pip
e 50
0mm
Per
fora
ted
Pip
es 2
00m
m x
530
0mm
x 6
200m
m x
4
Effl
uent
Mai
n P
ipe
Per
fora
ted
Pip
esP
erfo
rate
d P
ipes
Inffl
uent
Mai
n P
ipe
Cro
ss S
ectio
n A
-A
24/4
0
Equ
aliz
atio
n of
Influ
ent S
ewag
e D
istri
butio
n to
Eac
h M
odul
eIn
fluen
t mai
n pi
pes
are
plac
ed in
the
tank
and
con
nect
ed
with
per
fora
ted
pipe
s at
thei
r end
s.P
erfo
rate
d pi
pes
are
verti
cally
pla
ced
near
the
inle
t cro
ss-
sect
ion
of p
late
set
tler m
odul
es in
ord
er to
dis
tribu
te th
e in
fluen
t sew
age
even
ly in
hor
izon
tal/v
ertic
al d
irect
ions
of
the
mod
ule
cros
s-se
ctio
n an
d to
miti
gate
den
sity
cur
rent
s.
Efflu
ent m
ain
#2Ef
fluen
t mai
n #1
Influ
ent m
ain
Perfo
rate
d pi
pes
for i
nflu
ent
dist
ribut
ion
- 376 -
25/4
0
Slu
dge
Rem
oval
Slu
dge
colle
ctor
s w
ere
not i
nsta
lled,
ther
efor
e, s
ludg
e is
sto
red
on th
e bo
ttom
of t
he ta
nk d
urin
g ra
infa
ll ev
ents
. A
fter r
ainf
all e
vent
s, w
ater
in th
e ta
nk is
onc
e dr
aine
d to
geth
er w
ith m
ost o
f the
sto
red
slud
ge.
Then
, th
e re
mai
ning
slu
dge
is fl
ushe
d fro
m th
e re
ar e
nd o
f the
ta
nk u
sing
set
tled
wat
er fr
om th
e ad
jace
nt s
ettli
ng ta
nk.
26/4
0
Res
ults
of P
erfo
rman
ce S
urve
ys1)
Influ
ence
of F
low
Rat
eU
nder
the
cond
ition
that
th
e su
rface
load
ing
rate
is
less
than
35m
3 /(m
2·da
y),
Effl
uent
SS
con
cent
ratio
n in
crea
ses
whe
n th
e flo
w
rate
exc
eeds
0.5
m/m
in.
Effl
uent
BO
D c
once
ntra
tion
incr
ease
s w
hen
the
flow
ra
te e
xcee
ds 0
.55m
/min
.It
is p
ropo
sed
that
the
flow
ra
te in
the
plat
e se
ttler
m
odul
es s
houl
d be
set
0.
5m/m
in o
r les
s.
020406080100
120
140 0.
00.
10.
20.
30.
40.
50.
60.
70.
8Fl
ow R
ate
in th
e P
late
Set
ter M
odul
e(m
/min
)
Effluent SS (mg/L)
50 150
250
350
450
550
Influ
ent S
S
(mg/
L)
020
40
60
80
100
120
140
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Effluent BOD (mg/L)
50
150
250
350
450
Influ
ent
BO
D
(mg/
L)
Flow
Rat
e in
the
Pla
te S
ette
r Mod
ule
(m/m
in)
27/4
0
Res
ults
of P
erfo
rman
ce S
urve
ys2)
Influ
ence
of S
urfa
ce L
oadi
ng R
ate
Und
er th
e co
nditi
on th
at th
e flo
w ra
te is
less
than
0.
5m/m
in, i
t was
obs
erve
d th
at th
e ef
fluen
t with
su
ffici
ent q
ualit
y w
as o
btai
ned
stab
ly, w
hen
the
surfa
ce
load
ing
rate
was
less
than
40m
3 /(m
2·da
y).
0102030405060
010
2030
4050
Sur
face
Loa
ding
Rat
e (m
3 /m2 ·d
ay)
Effluent SS (mg/L)
50 150
250
350
450
550
Influ
ent S
S
(mg/
L)
28/4
0
Res
ults
of P
erfo
rman
ce S
urve
ys3)
Tre
ated
Effl
uent
Qua
lity
Und
er th
e co
nditi
on th
at
the
flow
rate
is le
ss th
an
0.5m
/min
and
the
surfa
ce
load
ing
rate
is le
ss th
an
35m
3 /(m
2·da
y),
It is
con
side
red
that
po
lluta
nts
can
be
rem
oved
to th
e le
vel o
f th
e av
erag
e ef
fluen
t qu
ality
at 2
0mg/
L in
SS
an
d 20
-30
mg/
L in
BO
D,
expe
ct w
hen
the
influ
ent
pollu
tant
load
is
extre
mel
y hi
gh.
051015202530354045
010
020
030
040
050
060
0In
fluen
t SS
(mg/
L)
Effluent SS (mg/L)
0 10
20
30
40
50
60 0
50
100
150
200
250
300
350
Influ
ent B
OD
(mg/
L)
Effluent BOD (mg/L)
- 377 -
29/4
0
Res
ults
of P
erfo
rman
ce S
urve
ys4)
Pol
luta
nt R
emov
al R
ate
Und
er th
e sa
me
cond
ition
, Th
e re
mov
al ra
te is
ap
prox
imat
ely
90%
in S
S
and
80%
in B
OD
, al
thou
gh it
sho
ws
low
va
lue
whe
n th
e in
fluen
t po
lluta
nt c
once
ntra
tion
is
at lo
w le
vel.
0 20
40
60
80
100 0
5010
0 15
020
025
0 30
0 35
0 In
fluen
t BO
D (m
g/L)
BOD Removal Rate (%)
0102030405060708090100
010
020
030
040
050
060
0In
flue
nt S
S (m
g/L)
SS Removal Rate (%)
30/4
0
Sum
mar
y -P
late
Set
tler
Sed
imen
tatio
n P
roce
ssFr
om th
e re
sults
of p
erfo
rman
ce s
urve
ys, i
t is
estim
ated
that
the
exis
ting
prim
ary
sedi
men
tatio
n ta
nks
can
be c
apab
le o
f tre
atin
g w
et-w
eath
er
sew
age
flow
equ
ival
ent t
o 5
times
its
pres
ent
desi
gn m
axim
um d
aily
dry
-wea
ther
flow
, by
inst
allin
g in
clin
ed p
late
set
tler m
odul
es w
ith th
e fo
llow
ing
spec
ifica
tions
.In
terv
als
of in
clin
ed p
late
s: 1
00m
mFl
ow ra
te in
the
settl
er m
odul
es: l
ess
than
0.
5m/m
inS
urfa
ce lo
adin
g ra
te: 2
5m3/
(m2·
day)
31/4
0
Nex
t App
licat
ion
of P
late
Set
tler
Sed
imen
tatio
n P
roce
ssTh
e ci
ty g
over
nmen
t is
also
con
duct
ing
perfo
rman
ce
surv
eys
whe
n ap
plyi
ng th
is p
roce
ss to
the
prim
ary
sedi
men
tatio
n pr
oces
s un
der d
ry-w
eath
er c
ondi
tions
.It
is e
xpec
ted
that
the
plat
e se
ttler
sed
imen
tatio
n pr
oces
s w
ill b
e ab
le to
hav
e th
e eq
uiva
lent
trea
tmen
t cap
acity
of
the
exis
ting
prim
ary
sedi
men
tatio
n ta
nk w
ith 1
/2-1
/3 o
f its
or
igin
al s
urfa
ce a
rea.
In th
is c
ase,
the
rem
aini
ng s
pace
can
be
used
for w
et-
wea
ther
sew
age
treat
men
t by
the
plat
e se
ttler
se
dim
enta
tion
proc
ess
with
coa
gula
nt a
dditi
on.
32
New
ly R
evis
ed C
SS
Im
prov
emen
t Pla
n of
Osa
ka C
ity
- 378 -
33/4
0
Nee
ds/B
ackg
roun
d fo
r Rev
isio
nIn
200
4, “S
ewer
age
Law
Enf
orce
men
t Ord
er”
was
am
ende
d, a
nd th
e w
et w
eath
er e
fflue
nt
qual
ity s
tand
ard
for C
SS
sw
as n
ewly
stip
ulat
ed.
“The
ave
rage
qua
lity
of w
et w
eath
er e
fflue
nt d
urin
g a
sing
le ra
infa
ll ev
ent w
ith th
e to
tal r
ainf
all o
f 10-
30m
m
shou
ld b
e no
t mor
e th
an 4
0mg/
L in
BO
D in
eac
h tre
atm
ent a
rea.
(Th
e te
ntat
ive
stan
dard
is 7
0mg/
L fo
r the
nex
t 20
year
s.)”
The
plat
e se
ttler
sed
imen
tatio
n pr
oces
s ha
s be
en d
evel
oped
to a
pra
ctic
al le
vel.
34/4
0
Obj
ectiv
es o
f the
Rev
ised
CS
S
Impr
ovem
ent P
lan
Mid
-term
Obj
ectiv
es (t
arge
t yea
r: 2
018)
With
in e
ach
treat
men
t are
a, th
e an
nual
di
scha
rge
pollu
tant
load
incl
udin
g dr
y w
eath
er
perio
ds s
houl
d be
redu
ced
to th
e eq
uiva
lent
le
vel o
f a s
epar
ate
sew
er s
yste
m (1
8mg/
L in
B
OD
).Th
e w
et w
eath
er e
fflue
nt q
ualit
y sh
ould
mee
t th
e st
anda
rd s
tipul
ated
in th
e S
ewer
age
Law
E
nfor
cem
ent O
rder
am
ende
d in
200
4.Th
e nu
mbe
r of o
verfl
ow e
vent
s fro
m C
SO
ou
tfalls
sho
uld
be re
duce
d by
hal
f.
35/4
0
Maj
or A
ppro
ache
sIn
the
prev
ious
CS
S im
prov
emen
t pla
nW
et w
eath
er a
ctiv
ated
slu
dge
proc
ess
Filli
ng s
edim
ent t
raps
in m
anho
le b
otto
ms
Rep
lace
men
t of s
cree
nsC
onst
ruct
ion
of s
torm
wat
erre
serv
oirs
In th
e ne
wly
revi
sed
CS
S im
prov
emen
t pla
nW
et w
eath
er a
ctiv
ated
slu
dge
proc
ess
Pla
te s
ettle
r sed
imen
tatio
n pr
oces
sS
tora
ge o
f wet
-wea
ther
sew
age
usin
g la
rge
stor
mw
ater
sew
ers
desi
gned
for f
lood
con
trol
Con
stru
ctio
n of
wet
-wea
ther
sto
rage
faci
litie
s
36/4
0
Imag
e of
Con
vent
iona
l Tre
atm
ent
Pro
cess
dur
ing
Wet
Wea
ther
Influent flow
1Qsh
2Qsh
Tim
e
Dire
ct
disc
harg
e反 応 槽
終 沈
初 沈 等
2Qsh
Exce
edin
g 3Q
sh (D
irect
dis
char
ge)
1Qsh
3Qsh
Sto
rmw
ater
Res
ervo
irs(n
ewly
con
stru
cted
)
Tem
pora
ry
Stor
age
Rea
ctio
n Ta
nks
(for
1Qsh
)
Prim
ary
Sed
imen
tatio
n Ta
nks
(for
3Qsh
)
Tem
pora
ry
stor
age
(by
stor
mw
ater
rese
rvio
rs)
Sed
imen
tatio
n on
lyS
econ
dary
trea
tmen
t
Fina
l S
edim
enta
tion
Tank
s
- 379 -
37/4
0
Imag
e of
Wet
Wea
ther
Sew
age
Trea
tmen
t in
the
Pre
viou
s P
lan
Influent flow
1Qsh
2Qsh
簡易
放流
Tim
e
Wet
wea
ther
ac
tivat
ed
slud
ge p
roce
ss
Tem
pora
ry
stor
age
Dire
ct
disc
harg
e反 応 槽
終 沈
初 沈 等
Exce
edin
g 3Q
sh (D
irect
dis
char
ge)
1Qsh 3Q
sh
3Qsh
Tem
pora
ry
Stor
age
Prim
ary
Sed
imen
tatio
n Ta
nks
(for
3Qsh
)
Stor
mw
ater
Res
ervo
irs(n
ewly
con
stru
cted
)
Wet
W
eath
er
Act
ivat
ed
Slu
dge
Pro
cess
(fo
r 3Q
sh)
Fina
l S
edim
enta
tion
Tank
s
38/4
0
Imag
e of
Wet
Wea
ther
Sew
age
Trea
tmen
t in
the
New
Pla
n
Influent flow
3Qsh
1Qsh
2Qsh
簡易
放流
Tim
e
Wet
wea
ther
ac
tivat
ed
slud
ge p
roce
ss
Tem
pora
ry
stor
age
Dire
ct
disc
harg
ePl
ate
settl
er
sedi
men
tatio
n w
ith c
oagu
lant
ad
ditio
n
反 応 槽
終 沈
初 沈 等
3Qsh
Pla
te
Set
tler
Sed
ime
ntat
ion
Prim
ary
sedi
men
tatio
n
Exce
edin
g 6Q
sh (D
irect
dis
char
ge)
3Qsh
Red
uctio
n in
the
area
for p
rimar
y se
dim
enta
tion
by h
igh
rate
filtr
atio
n or
pla
te s
ettle
r se
dim
enta
tion
with
out c
oagu
lant
add
ition
Inst
all a
dditi
onal
trea
tmen
t de
vice
s fo
r rem
aini
ng a
rea
Incr
ease
in W
WF
treat
men
t cap
acity
Red
uctio
n in
dire
ct d
isch
arge
1Qsh 3Q
sh
Wet
W
eath
er
Act
ivat
ed
Slu
dge
Pro
cess
(fo
r 3Q
sh)
Fina
l S
edim
enta
tion
Tank
s
3Qsh
Sto
rmw
ater
Sew
ers
Sto
rage
Fac
ilitie
s
Tem
pora
ry
Stor
age
39/4
0
Pro
ject
Cos
tFo
r the
pre
viou
s im
prov
emen
t pla
n, th
e to
tal p
roje
ct
cost
was
est
imat
ed to
be
abou
t 300
bill
ion
yen.
Whe
reas
in th
e ne
w im
prov
emen
t pla
n, w
hich
fully
ut
ilize
s ex
istin
g fa
cilit
ies
to in
crea
se th
e w
et-w
eath
er
sew
age
cont
inuo
us tr
eatm
ent c
apac
ity, t
he to
tal p
roje
ct
is e
stim
ated
to b
e ab
out 2
00 b
illio
n ye
n, a
sav
ing
of
abou
t 100
bill
ion
yen.
Tabl
e 1:
Est
imat
ed P
roje
ct C
ost f
or C
SS Im
prov
emen
t Ap
proa
ches
C
ost (
billio
n JP
Y)W
et W
eath
er A
ctiv
ated
Slu
dge
Proc
ess
18
Plat
e Se
ttler
Sed
imen
tatio
n Pr
oces
s 57
C
onst
ruct
ion
of S
tora
ge F
acilit
ies
87
Oth
er A
ppro
ache
s Fi
lling
Sed
imen
t Tra
ps in
Man
hole
Bot
tom
s R
epla
cem
ent o
f Scr
eens
, etc
. 30
Tota
l 19
2 40
/40
Sum
mar
y“W
et w
eath
er a
ctiv
ated
slu
dge
proc
ess”
and
“pla
te
settl
er s
edim
enta
tion
proc
ess”
have
bee
n de
velo
ped
to
a pr
actic
al a
s w
idel
y ap
plic
able
eco
nom
ic c
ontin
uous
tre
atm
ent t
echn
olog
ies,
whi
ch c
an;
Util
ize
exis
ting
faci
litie
s ef
fect
ivel
y,R
educ
e C
SS
impr
ovem
ent c
osts
sig
nific
antly
,S
how
rapi
d im
pact
s on
CS
S im
prov
emen
t,Fa
cilit
ate
the
prog
ress
of C
SS
impr
ovem
ent,
and
Sug
gest
that
CS
S h
as th
e po
tent
ial t
o be
com
e th
e m
ost a
ppro
pria
te w
ater
pol
lutio
n co
ntro
l sys
tem
for
dens
ely
popu
late
d ur
ban
area
s w
here
floo
d co
ntro
l is
also
a c
ritic
al is
sue.
- 380 -
