Renovation of secondary treatment facility into the Step-feed Biological Nitrogen
Removal Process by the effective use of the existing structure

Kiyohiko Hayashi
Director, Toba Wastewater Treatment Plant,
Sewerage Department, Water and Sewerage Works Bureau, Kyoto City

Summery:
In order to improve the water environment in Kyoto city and in the down stream ofdischarging rivers, suitable advanced treatment processes have been adopted into
existing facilities by way of renovation. In the case of the existing secondary treatment
facility at Toba treatment plant, the step-feed biological nitrogen removal process waschosen since it can be applied without any large-scale remodeling of the structure of the
facility, and can be maintained easily.

Key Wards:
Advanced treatment, Nitrogen removal, Renovation, Step-feed biological nitrogen removal

1. Sewerage works of Kyoto city
(1) Outline
Kyoto city, which has more than 1200 years of history, is located in the midstream of
the Yodo River from Lake Biwa to Osaka Bay. The population of the city is around 1.47
million. Almost all of the water required is obtained from Lake Biwa through water
channels and all treated effluent from wastewater treatment plants is discharged to
the Yodo river water system. As the Yodo River water is used for drinking water bymore than 11 million people in Osaka and other cities, the sewerage system has asignificant role and its construction has been pursued vigorously. The sewered
population ratio as of 2006 is 99.1%. And the sewerage system has contributed to
improve the water quality of the Yodo River water system including rivers in the city.
However, as further strict design standards concerning BOD, COD, nitrogen andphosphorus will be established for each wastewater treatment plant, additional
improvement of effluent quality will be required.
(2) Promotion of Advanced Treatment
Considering of the location of the city, not only nitrogen and phosphorus but color are
fixed for the objective constituents of the advanced treatment in order to improve theappearance of river water. Biological removal processes have been applied for nitrogenand phosphorus while ozonation has been applied for color removal.
- 169 -

Because immense construction costs are required for introducing advanced
treatment, simultaneous removal processes for nitrogen and phosphorus are applied incase of renovation of the whole structure, and removal processes for one of theconstituents is applied in case of renovation of equipment such as diffusers in reaction
tanks. As compared with conventional activated sludge systems, biological advanced
treatment processes require longer reaction time, so facility volumeter of the advanced
treatment generally has to increase. But in Kyoto city, influent volume has been
decreasing after a volumetric peak in the early 90s on account of elevation of watersaving consciousness, and moving colleges and factories out of the city as well as alingering recession. Therefore, advanced treatment has been adopted by according as
the decrease of influent volume without any expansion of the facility volumeter.
(3) Outline of Toba Wastewater Treatment Plant
The first treatment line of the Toba plant, which is the 9th oldest one in Japan, began
operating in 1939. The amount of influent wastewater into the plant is about 63% of
the wastewater generated in the city. The present treatment capacity is 975,000m3/day.
As shown in Table-1, there are eleven treatment lines from A to K in the plant. Those
treatment lines had been constructed year by year, and conversion works of those lines
from conventional activated sludge process into advanced treatment has beenpromoted stepwise. A-line was converted into the anaerobic-anoxic-oxic (A2O) process
for the nitrogen and phosphorus removal on the occasion of the renovation of whole
structure. E and F lines were converted into the anaerobic-oxic (AO) process for thephosphorus removal by way of renovation of diffusers in the reaction tank.
Table-1 Treatment capacity of each line in Toba Plant㪫㪩㪜㪘㪫㪤㪜㪥㪫㩷㪣㪠㪥㪜㩷

㪘㩷

㪙㩷

㪚㩷

㪛㩷

㪜㩷

㪝㩷

㪞㩷

㪟㩷

㪠㩷

㪡㩷

㪢㩷

㪫㪦㪫㪘㪣㩷

㪚㪦㪤㪤㪜㪥㪚㪜㪛㩷㪦㪧㪜㪩㪘㪫㪠㪦㪥㩷

㩾㪐㪎㩷

㩾㪍㪋㩷

㩾㪍㪍㩷

㩾㪍㪎㩷

㩾㪍㪐㩷

㩾㪎㪈㩷

㩾㪎㪊㩷

㩾㪎㪍㩷

㩾㪎㪐㩷

㩾㪏㪋㩷

㩾㪏㪐㩷

㪄㩷

㪦㪩㪠㪞㪠㪥㪘㪣㩷㪚㪘㪧㪘㪚㪠㪫㪰㩷

㪄㩷

㪎㪏㩷

㪈㪌㪐㩷 㪈㪌㪏㩷㪐㪏㪐㩷㪘㪛㪭㪘㪥㪚㪜㪛㩷㪫㪩㪜㪘㪫㪤㪜㪥㪫㩷㪧㪩㪜㪪㪜㪥㪫㩷㪚㪘㪧㪘㪚㪠㪫㪰㩷

㪘㪉㪦
㪂㪘㪦㩷

㪈㪈㪐㩷

㪄㩷
㪍㪈㩷

㪎㪉㩷
㪄㩷

㪐㪇㩷
㪄㩷

㪌㪎㩷

㪘㪦㩷

㪪㪫㪜㪧㩷

㪄㩷

㪄㩷

㪄㩷

㪄㩷

㪏㪊㩷

㪌㪋㩷

㪐㪇㩷

㪈㪌㪐㩷 㪈㪌㪏㩷㪐㪎㪌㩷㧔Capacity:㧖103m3/Day㧕

2. Adoption of Advanced Treatment(1) Renovation ProjectԘ Investigation of introducing process
Low construction and maintenance costs, as well as maintenance efficiency havebeen the first priority in choosing the advanced treatment process. In case of E and F
lines, which have the same structure as G and H lines, the AO process was adopted forthe phosphorus removal by installing mixers instead of diffuser plates in reaction
tanks.
In case of the renovation of G and H lines, introducing process was investigated by- 170 -

adding Step-Feed Biological Nitrogen Removal Process (STEP), which had beendeveloped by Japan Sewerage Works Agency (JSWA) and was just authorized, as one of
the alternative processes. After careful investigation, the STEP process was chosen for
the following reasons; 1) Step feed channels are already installed in those treatment
lines and large scale construction is not necessary. 2) Equipping mixers in reaction
tanks instead of diffuser plates is adequate for a conversion into an advanced
treatment process. 3) Nitrogen concentration might not meet the future effluent water
quality standard.
ԙ STEP Process
Fig-1 is the illustration of the two-stage STEP process. Half the amount ofwastewater flows into the first stage. In the first denitrification tank, nitrified nitrogenin return sludge is mainly denitrified. In the first nitrification tank, ammonium
nitrogen in the influent wastewater into the first stage is nitrified. The nitrified liquor
flows into the second denitrification tank together with the remaining influent
wastewater is denitrified by using organic carbon in the wastewater. In the second
nitrification tank, ammonium nitrogen in the remaining half of influent wastewater isnitrified. Theoretical nitrogen removal ratio in the case of a 50% return sludge rate is67%.

Fig-1 Illustration of the two-stage STEP processԚ Renovation design
Table-2 shows the design factors of the STEP process introduced into G and H lines.
Table-2 Design factors of the STEP process
Number of Stages

2

Volume Ratio of 1st and 2nd Stage1䋺1.5

Volume Ratio of Denitrification and Nitrification Tank

1䋺1

Influent Volume Ratio into 1st and 2nd Stage

1䋺1

Treatment Capacity

54,000 m3/Day

Return Sludge Ratio

50%- 171 -

As will be mentioned afterward, the influent loading is relatively low, so two-stage
system was adopted for maintaining anaerobic condition in the denitrification tanks.
Influent volume ratio into the first and second stages is fixed at 1:1. Tank volume ratioof the first and second stages is set at 1:1.5 in order to equalize solids quantity in bothstages. Tank volume ratio of both the denitrification and nitrification tanks is fixed fora general value of 1:1. The designed treatment capacity becomes 54,000 m3/D which is
60% of the original conventional treatment capacity of 90,000m3/D.
(2) Renovation works
Ԙ Renovation of facilities
Renovation works of G and H line for advanced treatment was carried out by way of
renewal of diffusers in reaction tanks. The works of G line were completed in March
2003 after approximately a year of construction, and those of H line were completed in
October 2004. Table-3 indicates the outline of the renovation works. Reaction tanks
had been divided into four equal parts by baffle plates before the renovation, and it was
re-divided into four parts so as to make the volume ratio of the first denitrification tank,first nitrification tank, second denitrification tank and second nitrification tank into
1:1:1.5:1.5. In the first denitrification tank, 4 propeller type mixers per sub line were
installed and in the second denitrification tank, 6 mixers per sub line were installed. As
each treatment line consists of 4 sub lines, a total of 40 mixers were installed in eachline. In both the first and second nitrification tanks, super fine bubble membranediffusers made by urethane plastic were installed. Half the amount of wastewater
which flows into the second denitrification tank is led through unused step channels byconstructing the mouth of the channel near the inlet of the second denitrification tank.
Table-3 Outline of the renovation works
㩷Partition

Diffusion and
MixtureStep Feed Channel

before renovation

after renovation䋱䋺䋱䋺䋱䋺䋱

1䋺1䋺1.5䋺1.5
1st Denitrification Tank

Propeller type Mixer, 4units/tank

Ceramic Diffuser

1st Nitrification Tank

Urethane Diffuser Plate, pore 150Ǎm

pore 400Ǎm

2nd Denitrification Tank

Propeller type Mixer, 6units/tank

2nd Nitrification Tank

Urethane Diffuser Plate, pore 150Ǎm

Unuseduse by constructing inflow gatesԙ Construction cost
Table-4 shows the equipment and construction costs of the renovation into the STEP
process, and those of the suppositive renovation into the conventional process of which
treatment capacity is 90,000m3/D. The construction costs per treatment capacity of the
STEP process is 65% higher than that of the conventional process. But the construction
cost itself is actually almost the same for both processes due to the high cost of diffusers
compared with mixers, and due to the unnecessary of construction of the new stepchannel.

- 172 -

(3) Efficiency of introducing STEP processԘ Influent water quality
Table-5 indicates the influent quality into the reaction tank of G and H line. Water
quality of all constituents, except suspended solids, is lower than those researched by
Japan Sewerage Works Agency (Technical evaluation report of STEP Process, 2002)
and Public Works Research Institute (Report of investigation project for facility designof advanced treatment, 2003). This is mainly caused by dilution of the influent
wastewater due to a high underground water level. Maintenance of the biologicalnitrogen removal processes under such low pollutant loading is supposed to be difficult
because the low loading makes anaerobic conditions hard to maintain in the
denitrification tank
Table-4 Equipment and construction costs of the renovation into STEP process
STEP Process㩷equipment
cost

Quantity

unit cost

diffuserhalf of tankmixer

Conventional Process
cost

quantity

unit cost

52,000208,000

whole tank

77,00040

2,000

80,000---

inlet weir

8

4,000

32,0004

4,000

16,000others--

12,000--12,000

total

332,000

336,000

construction and other cost

308,000

308,000

total cost

640,000

644,000

cost
308,000(:103 yen)treatment capacity

54,000 m3/D

㩷

90,000 m3/D

total cost/capacity

11,900 yen/m3

㩷

7,200 yen/m3

Table-5 Influent water quality into reaction tank
Toba Plant

Report by JSWA

Report by PWRI

Average in 2005

Range

Average

BOD

47

60䌾147

121

SS

37

37䌾185

61

T-N

14

19䌾29

25

NH4-N

8.8

14䌾19

16

T-P

1.4

2.9䌾4.3

3.45

PO4-P

0.6

1.6䌾1.9

1.67

㩷

(: mg/L)ԙ Operational condition
Table-6 shows the operational condition of the STEP process in G line as well as the
AO process and conventional process for comparison. Average return sludge ratio is
49% which is almost the same as the designed value of 50%. Average MLSS in reaction
tank is 1,210mg/L which is quite low due to the low influent pollutant loading and isalmost same as both the AO and conventional processes. Average ratio of supplied air- 173 -

volume per treated wastewater is 1.9. In winter season, as influent loading becomes
rather high, so the ratio rises to 2.2 in order to keep MLDO in required degree.
Table-6 Operational condition of each process
STEP㩷

Average

Influent Volume(m3/D)

AO

Conventional

Max

Min

49,23053,14040,01066,98062,330Return Sludge Ratio

(%)

49

51

43

24

35

HRT in Reaction Tank

(hr)

12.6

14.0

11.8

9.1

9.7

MLSS

(mg/L)

1,210

1,400

1,050

1,120

1,180

BOD-SS Loading(kg/kg D)

0.08

0.10

0.05

0.11

0.10

A-SRT

(Day)

7.2

11.0

5.9

9.7

11.6

Air Volume Ratio(m3/m3)

1.9

2.2

1.6

2.9

2.3

(: monthly average in fiscal year 2005)

Ԛ Treatment result
In Table-7, the mean value of influent and effluent water quality of each treatment
process, as well as the removal rate, is shown from the period of April 2005 to March
2006.
Table-7 Water quality and removal rate
㩷

Influent

STEP

AO

Conventional

C-BOD(mg/L䋩

47

1.0

1.6

1.9

SS

(mg/L䋩

37

1.1

1.3

2.3

T-N

(mg/L䋩

14

4.1

8.4

8.4

NH4-N

(mg/L䋩

8.8

0.0

0.3

0.2

N Removal Rate (%)

䋭

70.0

39.0

39.0

T-P

(mg/L䋩

1.4

0.58

0.22

0.67

PO4-P

(mg/L䋩

0.60

0.52

0.13

0.57

P Removal Rate (%)

䋭

57.6

83.6

50.6

a. Nitrogen removal
Effluent total nitrogen concentration of the
STEP process is 4.1mg/L which is less thanhalf value of the AO and conventional
processes. As for the removal rate, 70% on
average is achieved in the STEP process
which is almost identical to the theoreticalvalue, while only 40% is achieved in the AO
and conventional processes. Fig-2 shows abreakdown of the nitrogen removal rate.

㪩㪼㫄㫆㫍㪼㪻㩷㪥㪹㫐㩷㪜㫏㪺㪼㫊㫊㪪㫃㫌㪻㪾㪼
㪉㪉㩼㪛㪼㫅㫀㫋㫉㫀㪽㫀㪼㪻㪥㪋㪏㩼㪜㪽㪽㫃㫌㪼㫅㫋
㪦㫉㪾㪸㫅㫀㪺㩷㪥㪍㩼㪜㪽㪽㫃㫌㪼㫅㫋
㪥㪦㫏㪉㪋㩼

Fig-2 Breakdown of nitrogen removal rate

- 174 -

㪈㪇㪇
㪙㪦㪛㪆㪥

㪥㩷㪩㪼㫄㫆㫍㪸㫃㩷㪩㪸㫋㪼㩷㩿㩼㪀Of this 70% total, 48% of nitrogen is removed by denitrification and 22% of it isremoved by excess sludge.
As shown in Fig-3, the nitrogen removal rate decreases to 63% in summer season. In
order to progress denitrification smoothly, organic matter concentration is regarded as
an important factor and the BOD/N ratio required for denitrification is reported to beat least 2 to 3gBOD/gN. As shown in Fig-4, the minimum ratio is 3.7gBOD/gN in Toba
plant, and as for ratio the sufficient value is maintained.

㪏㪇
㪍㪇

㪏
㪍
㪋
㪉
㪇
㪘㫇㫉㪅

㪋㪇
㪘㫇㫉㪅

㪘㫌㪾㪅

㪡㫌㫃㪅

㪛㪼㪺㪅

㪦㪺㫋㪅

㪡㪸㫅㪅

㪤㫆㫅㫋㪿

㪤㫆㫅㫋㪿

Fig-4 Variation of BOD/N ratio㪥㩷㪩㪼㫄㫆㫍㪸㫃㩷㪩㪸㫋㪼㩿㩼㪀Fig-3 Variation of nitrogen removal rate
But the relationship between influent BOD
concentration and the nitrogen removal rate
shown in Fig-5 indicates that both have astrong relation and the nitrogen removal rate
decreases as influent BOD decreases.

㪏㪇
㪎㪇
㪍㪇
㪌㪇
㪈㪌

㪊㪌

㪌㪌

㪎㪌

㪠㫅㪽㫃㫌㪼㫅㫋㩷㪙㪦㪛㩿㫄㪾㪆㪣㪀Fig-5 Influent BOD and N removal rate
This means that even if the BOD/N ratio is sufficient, it is difficult to keep anaerobic
conditions in the denitrification tank when MLDO concentration in the nitrification
tank come to be excessive, especially in summer season on account of low BOD loading.
b. Operational and maintenance cost
In Table-8, power consumption for mixers in the denitrification tank, air supply inthe nitrification tank of the STEP process, and that for air supply in the conventional
process is shown.
Table-8 Comparison of power consumption in reaction tank
STEP㩷

㩷

Conventional

Power for Mixer(kWh/D)1,633 㩷

䋭Power for Blower(kWh/D)

1,755 㩷2,689 㩷Total Power(kWh/D)3,388 㩷2,689 㩷Influent Volume(m3/D)

49,230 㩷

62,330 㩷

Power for Blower/Influent Volume(kWh/m3)

0.036

0.043

Total Power/Influent Volume(kWh/m3)

0.069㩷

0.043- 175 -

㩷

Even though the power consumption for blower per influent volume of the STEP
process (0.036kWh/m3) is lower than that of the conventional process (0.043kWh/m3),
total power consumption by adding the consumption for mixers (0.069kWh/m3)
becomes higher. Therefore, intermittent operation of mixers has been investigated anda blower with inlet-vanes has been adopted in order to reduce power consumption.

3. Conclusion
As a result of introducing the STEP process into the existing conventional facility at
Toba plant by way of renovation of diffusers, it was found that even though theconstruction cost of the STEP process per treatment capacity is 65% higher than that of
the conventional process and the power consumption of the process per influent volume
is higher than that of the conventional process by 60%, a nitrogen removal rate of
approximately 70% which is almost identical to theoretical value, was achieved.
Kyoto city will be promoting the adoption of the advanced treatment steadily andwill also be investigating its efficient operation and maintenance.

- 176 -

- 177 -Location of Kyoto City

Kyoto City

Water and Sewerage Works Bureau

Kiyohiko Hayashi

Renovation of secondary treatment
facility into the Step-Feed BiologicalNitrogen Removal Process byeffective use of the existing structure

Sewered Area

• More than 1200 years of history,• Around 1.4 million of population,• Located in midstream of the Yodo River,• The Yodo River water is used for drinking
by more than 11 million people in thelower part of the river,

Kyoto City- 178 -Ozonation• ColorAnaerobic-anoxic-oxic process
Anaerobic-oxic activated sludge process• Phosphorus

Anaerobic-anoxic-oxic process
Step-feed biological nitrogen removal process• Nitrogen

Adoption of Advanced Treatment

Percentage of Sewered Population

Ozonation• ColorAnaerobic-anoxic-oxic process
Anaerobic-oxic activated sludge process• Phosphorus

Anaerobic-anoxic-oxic process
Step-feed biological nitrogen removal process• Nitrogen

Adoption of Advanced Treatment

Ozonation• ColorAnaerobic-anoxic-oxic process
Anaerobic-oxic activated sludge process• Phosphorus

Anaerobic-anoxic-oxic process
Step-feed biological nitrogen removal process• Nitrogen

Adoption of Advanced Treatment

- 179 -• Started operation→ 1939• Treatment area
→ 8,191 ha• Population in the area → 771,200• Treatment capacity→ 975,000 m3/D

Toba Wastewater Treatment Plant

Ozonation• ColorAnaerobic-anoxic-oxic process
Anaerobic-oxic activated sludge process• Phosphorus

Anaerobic-anoxic-oxic process
Step-feed biological nitrogen removal process• Nitrogen

Adoption of Advanced Treatment

1993

1996

1999
year

2002

Photo of Toba Wastewater
Treatment Plant

6000006500007000007500008000002005

Decrease of Influent Wastewater
Influent to Reaction Tank
(m3/D)

- 180 -

-

AO
A2O

119

ADVANCED
TREATMENTPRESENT CAPACITY(*103m3/D)

61

78

-

ORIGINAL CAPACITY(*103m3/D)

B

A

TREATMENT LINE

57

-

72

C

57

-

72

D

83

AO

90

E

83

AO

90

F

54

S

90

G

54

S

90

H

90

-

90

I

J

K

-

159 158-

159 158

Treatment Capacity of Toba
Treatment Plant

Area of the Plant

Treatment Processes

Treatment Lines

- 181 -Reasons of Choosing STEP Process
1) Step feed channels are already installedin those treatment lines.
2) Equipping mixers instead of diffuser
plates is adequate for a conversion into
an advanced treatment process.
3) Nitrogen concentration might not meetthe future effluent water quality standard.

1) Step feed channels are already installedin those treatment lines.
2) Equipping mixers instead of diffuser
plates is adequate for a conversion into
an advanced treatment process.
3) Nitrogen concentration might not meetthe future effluent water quality standard.

1) Step feed channels are already installedin those treatment lines.
2) Equipping mixers instead of diffuser
plates is adequate for a conversion into
an advanced treatment process.
3) Nitrogen concentration might not meetthe future effluent water quality standard.

Reasons of Choosing STEP Process

Reasons of Choosing STEP Process

Layout of the Plant

- 182 -

PO4-P

T-P

NH4-N

T-N

SS

BOD

Report by
JSWA
Range

60～14737～18519～29
14～19
2.9～4.3
1.6～1.9

Toba Plant

Average in
2005

47
37
14
8.8
1.4
0.6

121
61
25
16
3.45
1.67

Average

Report by
PWRI

Influent Wastewater Quality

Illustration of two-stage STEP process

1：1

Volume Ratio of Denitrification
and Nitrification Tank
Influent Volume Ratio into 1st and 2nd
Stage

50%

Return Sludge Rate

Step Feed
ChannelDiffusion
andMixturePartition

unusedCeramic
Diffuser
pore
400μm

１：１：１：１

beforerenovation

use by constructing inflow gates

Urethane Diffuser
Plate, pore 150μm

Propeller type Mixer,
6units/tank2nd Denitrification
Tank
2nd Nitrification Tank

Urethane Diffuser
Plate, pore 150μm

Propeller type Mixer,
4units/tank
1st Nitrification Tank

1st DenitrificationTank

1：1：1.5：1.5

after renovation

Outline of Renovation Works

54,000 m3/D

Treatment Capacity

1：1

1：1.5

2

Volume Ratio of 1st and 2nd Stage

Number of Stages

Design Factors of STEP Process

- 183 90,000 m3/D
7,200 yen/m3

11,900 yen/m3
total cost/capacity(103 yen)

644,000

308,000

336,000

28,000-308,000

whole tankcost

54,000 m /D

44,00080,000quantity

Conventional
Process

treatment capacity3

640,000

332,000

total cost-

cost
208,000

308,000

total

others40

half of tankconstruction costequipment costmixer

diffuserquantity

STEP Process

Cost for Renovation

(hr)HRT in Reaction
Tank

7.2

1.9

(m3/m3)

0.08

(Day)

(kg/kg D)

1,210

12.6

49

49,230

2.2

11.0

0.10

1,400

14.0

51

53,140

Max

1.6

5.9

0.05

1,050

11.8

43

40,010

Min

2.9

9.7

0.11

1,120

9.1

24

66,980

AO

2.3

11.6

0.10

1,180

9.7

35

62,330Conventional

Denitrified N
48%

Effluent
Organic N
6%

Effluent NOx
24%

Removed N by
Excess Sludge
22%

Breakdown of Nitrogen Removal Rate

Air Volume Ratio

A-SRT

BOD-SS Loading(mg/L)(%)

Return Sludge
Ratio

MLSS

(m3/D)

Influent Volume

Average

47

1.0

0.58
0.52

1.4
0.60
－

(mg/L）(mg/L）PO4－Ｐ
P Removal Rate (%)

83.6

50.6

0.57

0.67

39.0

0.2

8.4

2.3

1.9

Conventional

40

60

80

100

Apr.

Aug.

Month

Dec.

Variation of Nitrogen Removal Rate

57.6

0.13

0.22

39.0

70.0

Ｔ－Ｐ

0.3

8.4

1.3

1.6

AO

0.0

－

4.1

1.1

N Removal Rate (%)

14

37

8.8

(mg/L）(mg/L）(mg/L）STEP(mg/L）

ＮＨ４－Ｎ

Ｔ－Ｎ

ＳＳ

Ｃ－ＢＯＤ

Influent(from Apr. 2005 to Mar. 2006)

STEP

Water Quality and Removal Rate

Operational Condition(monthly average in fiscal year 2005)

N Removal Rate (%)- 184 -

- 185 -55
Influent BOD(mg/L)

75

0.036
0.069

(kWh/m3)(kWh/m3)

Power for Blower/Influent
VolumeTotal Power/Influent Volume

0.043

0.043

62,330

2,689

2,689

• Approximately 70% of nitrogen removal
rate is achieved.
• Construction cost and operational cost of
the STEP process per influent volume arehigher than those of conventional processby 65% and 60% respectively.
• In order to reduce the operational cost,some measures have been investigated.

35

49,230(m3/D)

Influent Volume-

Conventional• Approximately 70% of nitrogen removal
rate is achieved.
• Construction cost and operational cost of
the STEP process per influent volume arehigher than those of conventional processby 65% and 60% respectively.
• In order to reduce the operational cost,some measures have been investigated.

15

3,388

(kWh/D)

Total Power

1,755

1,633

(kWh/D)(kWh/D)Power for Blower

Power for Mixer

STEP

Power Consumption in Reaction Tank

Conclusion

50

60

70

80

Influent BOD and Nitrogen
Removal Rate

Conclusion

Nitrogen Removal Rate(%)

- 186 -Discharging Rivers• Approximately 70% of nitrogen removal
rate is achieved.
• Construction cost and operational cost of
the STEP process per influent volume arehigher than those of conventional processby 65% and 60% respectively.
• In order to reduce the operational cost,some measures have been investigated.

ConclusionEND

- 187 -BOD/N

ORP(mV)

Apr.

Jul.
Month

Oct.

Jan.

-250

-150

-50

50

0

1

3

MLDO(mg/L)

2

4

MLDO in the 1st nitrification tank and
ORP in the 2nd denitrification tank

0

2

4

6

8

Variation of BOD/N Ratio

5

P removal rate（％）

(mg/L）（％）(mg/L）

N removal rate

T-N
T-P

1

2
MLDO(mg/L)

3

4

1.8

16

61

51.7

0.87

9.2

2.0

outflow
to river42.5

inflow toreactor

E：conventional
F：conventional
G：conventional
H：conventional
I：conventional

1995～1997

1.6

15

51

61.9

0.61

9.0

1.8

outflow
to river40.0

inflow toreactor

E：AO
F：AO
G：conventional
H：conventional
I：conventional

2000～2002

5

1.4

14

46

72.9

0.38

7.4

1.6

Outflow
to river47.1

inflow toreactor

E：AO
F：AO
G：STEP
H：STEP
I：conventional

2005

Outflow Quality to the River

0(mg/L）

0

0.2

0.4

0.6

0.8

in the 1st Nitrification Tank

MLDO and Residual NH4-N

BOD

Residual NH4-N(mg/L)

- 188 -(mg/L)(mg/L)

T-N

T-P

1.1

7.8

5

0.6

5.7

3

Master Plan (*2)

2nd Recycle Flow

1st Recycle Flow

Recycle Flow from
Sludge Treatment

Incinerator

Dewatering

Digestion Tank

Sludge Thickener

Main Source

60,200m3/D

35,300m3/D

Volume39
11

T-N (mg/L)
T-P (mg/L)

602

450

BOD (mg/L)
S S (mg/L)

17(mg/L)

T-P

70

967

610

(mg/L)(mg/L)(mg/L)

T-N

S S

BOD

Water Quality

Recycle Flow Quality(*1) Osaka bay, the Yodo river comprehensive basin-wide planning ofsewerage systems(*2) Master Plan of Kyoto City up to 2025, treatment by nitrogen andphosphorus removal, sand filtration and ozonation(mg/L)

BOD

Basin-wideplanning(*1)

Target Effluent Water Quality

SludgeTreatment
SectionSubsection 2

Wastewater
Treatment
Section 2

Subsection 1

Wastewater
Treatment
Section 1• Target year : 2025
1 sewered population of 100%
2 storm water management
3 improvement of CSO
4 advanced treatment
5 sludge treatment
6 renovation and earthquake proof
7 multi-use of facilities and resources

The master plan of sewerage
works of Kyoto City

Administration
SectionDirector

Organization of Toba Plant

- 189 -

14

4.1

70%

Effluent-N

Removal Rate

(mg/L)(mg/L)

Influent-N

3.3

Nitrifiable-N
0.8

3.1

Excess sludge-N

Unnitrifiable-N

0.8

10.1

Unnitrifiable-N

Nitrifiable-N

67%

3.3

10.1

Breakdown of Nitrogen Removal

Installation of Mixers

Benefit(after installation)

2006(before installation)

2005

28%

12,300 kWh/Day

17,100 kWh/Day

Power Consumption by Blower(May to August)

Effect of Installing
Blower with Inlet-Vanes