Seawater Desalination Facility on Okinawa

Toru Yamazato
Seawater Desalination Center, Chatan Water Administration Office,Okinawa Prefectural Enterprise Bureau

1. Introduction
Okinawa Seawater Desalination Center was completed with full capacity in1997. The facility was constructed to ease water shortage on Okinawa byconverting surrounding clean seawater into drinking water. With ten years ofoperation, the facility has proved to be successfully functioning to meet demand fordrinking water especially in drought situation.
This presentation covers an overview of Okinawa Seawater Desalination
Center, issues including product water quality and facility maintenance, andfinally, the facility’s overall performance of the past ten years.
2. Overview
2.1 Need for desalination facility on Okinawa
Due to population growth and economic development, the average water
consumption on Okinawa doubled from approximately 200,000 m3/day in 1972 to
420,000 m3/day in 2002. Water rationing occurred in 14 years out of 34 years since1972. The longest water rationing continued for 326 days from 1981 to 1982. The
most recent water rationing occurred in 1993.
Shortages were anticipated for water resources dependent on thedevelopment of dams and rivers. As a countermeasure, the Enterprise Bureau ofOkinawa Prefecture decided to promote the desalination project. The first survey
was conducted by the Japanese government in 1977, and construction began in1993.
2.2 Reverse osmosis seawater desalination facility
The construction cost of Okinawa Seawater Desalination Center was 34.7
billion yen, 85% of which was subsidized by the national government.
The facility exploits reverse osmosis technology to desalt seawater. The
production capacity is 40,000 m3/day, which is nearly equal to 10% of the averagewater consumption on Okinawa. Fig.1 shows the process flow.
The facility needs approximately 9,000 kW of electricity for full operation.
1-27 Miyagi, Chatan-cho, Okinawa 904-0113 Japane-mail: yamaztot@pref.okinawa.lg.jp- 281 -High pressure pumps, which are necessary to bring about reverse osmosis,
consume most of the energy. Energy recovery turbines retrieve approximately 30%of the energy from the brine, which still has high pressure.
The desalted fresh water is sent to adjoining Chatan Water Purification Plant
and mixed with potable water produced from inland water.
SeawaterPrimary andSecondary Sand Filters

SandSedimentation
Basin

Intake Pump

Intake Tower

Security
FilterAdjustment Tank

RO SupplyPump

FreshWater

High Pressure Reverse
OsmosisPump
Unit

Fresh Water
Tank

Brine

Energy Recovery
TurbineMulti-Nozzle Discharge Tower

Fig.1

Process flow of Okinawa Seawater Desalination Center

3. Water quality
3.1 Effect of desalination
In the reverse osmosis (RO) process, most of the substances in seawater areremoved. Table.1 compares quality of the RO feed seawater, the desalted productwater, and the mixed potable water.
Table.1

Comparison of water quality
Item

RO feed water

Desalted water

Mixed water

Total Dissolved Solids

34,800278

256

Chloride Ion

19,800119

46.5

Sodium and its compounds

11,40096.1

38.8

Sulfate Ion

2,490

5.9

36.8

Hardness

6,360

10113

Electrical conductivity

50,800456

436

Boron and its compounds(Units:

4.68

1.14

0.12

[μS/cm] for electrical conductivity and [mg/l] for others)- 282 -

The mixing strategy has several advantages, as explained in the following sections.
3.2 Hardness
Desalted fresh water has an extremely low hardness and is considered
corrosive. Usually, a mineralizing process follows the RO process as apost-treatment in order to prevent corrosion. However, since the water produced in
Chatan Water Purification Plant has relatively high hardness, the hardness of themixed water becomes moderate. Therefore, the post-treatment is not needed.
3.3 Conductivity
Salt rejection rate is a great concern in the desalination facility. Electricalconductivity is a good scale of salinity and easy to monitor continuously. Okinawa
Seawater Desalination Center has set 720μS/cm as the maximum operational
conductivity for the product water. This value corresponds to the allowableconcentration of chloride ion according to water quality regulations.
3.4 Boron concentration
Boron is hard to remove in a single stage reverse osmosis process. Japanesewater quality regulations demand boron concentration to be less than 1.0 mg/l.
Table.1 shows that boron concentration was not sufficiently reduced by thedesalination process alone but successfully suppressed by mixing.
4. Maintenance
4.1 Biofouling
The RO process utilizes polyamide membrane, which does not tolerateoxidizing agents. Since the feed water does not contain chlorine, biofilm tends togrow inside the seawater channel and obstruct the feed flow in the RO elements.
This phenomenon, biofouling, may cause irreversible damage to the membraneelements. Okinawa Seawater Desalination Center conducts sulfuric acid shock
treatment and membrane cleaning to control biofouling. An effective cleaning
method has been established. However, the shock treatment still needs to beinvestigated to achieve reliable efficiency.
4.2 Element replacement
As the RO elements are used, the salt rejection rate declines, and average
conductivity of the product water increases. The deteriorated elements must bereplaced in order to maintain quality of the product water. The life span of an RO
element depends on the operating conditions such as quality of pretreatment.
Okinawa Seawater Desalination Center began element replacement in 2000 andhad replaced about a half of the originally installed 3,024 elements by 2006.

- 283 -

5. Overall performance
5.1 Production
Ten years have passed since the facility started operating. For the first fiveyears, Okinawa had enough rainfall, and full operation of the desalination facility
was not necessary. Since 2001, there have been occasional drought, and Okinawa
Seawater Desalination Center has performed long term full operations. Fig.2
shows the annual production and the operating rate of each year. The total
production through 2005 had reached over 37 million cubic meters. Due to recent
droughty climate, the operating rate reached 44.2%, the facility’s highest record.

Annual Production and Operating Rate

（％）
100

6,455

6,000

5,523

80

×103m3
7,000

5,000
60

20

2,489

2,318

37.5
26.0

24.5

530

17.0

15.9

12.2

29.3

27.6

18.9

22.0

4,000
44.2

2,766

40
1,783

4,028

3,794

3,580

4,2862,000
1,000001995

1996

1997

1998

Annual Production (×1000 ｍ3)

Fig.2

3,0001999

2000

2001

2002

2003

2004
2005
Fiscal year

Operating Rate（％）

Annual production and operating rate

5.2 Cost
As of 2004, the production cost of the fresh water produced in Okinawa Seawater
Desalination Center is approximately 132 yen/m3. Fig.3 shows the components ofthe cost. As shown, electricity is the most dominant factor of the cost.

Chemicals
9%Personnel
Expenses
6%

Maintenance6%

Depreciation
15%

Interest
8%

Electricity
48%

Fig.3

Others8%Components of the production cost- 284 -5.3 Water quality and element replacement
Table.2 lists some of the water quality items maintained in Okinawa
Seawater Desalination Center. Fig.4 shows the ten year trends of the productwater quality. The standards have been maintained for all the items except boron.
Fig.4 also shows the numbers of replaced membrane elements within the sameperiod. The figure clearly shows the effect of the membrane replacement on theproduct water quality.
Table.2

Water quality items
StandardItem

Max Operational Value
Water Quality Regulations

720 μs/㎝

Total Dissolved Solids

500 mg/l

Chloride Ion

200 mg/l

Boron1.0 mg/l*(mg/l)＊Boron378

Conductivity

378378273
TDS98
Chloride Ion
1995

Fig.4

Conductivity

Water Qualty and Membrane Replacement(mg/l, μs/㎝)

800
700
600
500
400
300
200
1000Value

1996

1997

1998

Number of replaced membrane1999 2000 2001
Fiscal Year

2002

2003

1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00

2004

Water quality and membrane replacement

6. Conclusion
With ten years of operation, Okinawa Seawater Desalination Center has
accumulated valuable technical experiences. The membrane cleaning and theelement replacement have been successful in maintaining quantity and quality ofthe product water. Since biofouling is a drawback for long term full operation, aneffective shock treatment method is sought. The operating rate of the facility has
been increasing due to the droughty climate. Therefore, the facility has beenfunctioning as a crucial water resource on main island of Okinawa.
Reference

Japan Water Works Association, Design Criteria for Waterworks Facilities (2000) 352- 285 -Introduction

Seawater Desalination Facility on Okinawa

1. Overview
2. Water quality
3. Facility maintenance
4. Overall performance

Toru Yamazato
Seawater Desalination Center
Chatan Water Administration Office
Okinawa Prefectural Enterprise Bureau

1．Overview

Needs for desalination

 Needs for desalination
 Reverse osmosis seawater
desalination facility

 Water demand doubled in 30 years
 Supply dependent on dams and rivers
 Merit of desalination- Abundant water from the sea- Short construction period- Location near consumers

Reverse osmosis

Water rationing

Semi-permeable
membrane
P > ΔπOperating pressure : P

SeawaterFresh water
Osmotic Pressure ：Δπ

Flux ： F Water rationing occurred in 14 years out of 34 years since 1972.
 The longest continued 326 days.
 No water service restriction occurred since 1993.

Okinawa Seawater Desalination Center

Reverse osmosis (RO) membrane element

Seawater

Differential Pressure
Feed

Primary andSecondary Sand
Filters

Brine

ROSupplyPump

Permeate

RO membrane

Center pipe
Adjustment
Tank

RO module

Intake Sand
Pump Sedimentation
Basin

Intake Tower

HighPressure
Pump
Security
FilterFreshWater

ReverseOsmosis Unit

Energy Recovery Turbine

Fresh Water
Tank
Brine

Multi-Nozzle Discharge Tower- 286 -Okinawa Seawater Desalination Center

2. Water quality

 Production capacity : 40,000m3/day
 Recovery rate : 40%
 Maximum power consumption : 9,000 kW
 378 elements / unit × 8 units = 3,024 elements
 Product water is sent to adjoining water purification
plant to be mixed with water from inland.
 Construction cost : 34.7 billion yen 85% subsidized by the national government Partly operational in 1995
 Fully operational in 1997

Item

Mixed water

34,800278

256

Chloride Ion

19,800119

46.5

Sodium11,40096.1

38.8

Sulfate Ion

2,490

5.9

36.8

Hardness

6,360

10113

Electrical conductivity

50,800

456

4.68

1.14

Countermeasure for biofouling Sulfuric acid shock treatment- Acid injection to the feed
- pH = 2.5 ～ 3.0 30 minutes- Advantage : Long running time of RO units- Polyamide membrane and oxidizing agents- Biofilm in the feed channel

 Membrane cleaning- Off-line cleaning- pH = 10.5 3 days
- Advantage : Fundamental and efficient- Damage to the membrane elements

Membrane Cleaning

Element replacement

2.0
No.５
－０．４９

1.9

 Salt rejection declines

No.３
＋０．０１

1.8
1.7

436
0.12

(Units: [μS/cm] for electrical conductivity and [mg/l] for others)

 Biofouling

No.６－０．０８RO feed water

Boron

3. MaintenanceDifferential Pressure
Kg/cm2

Desalted water

Total Dissolved Solids

No.７
－０．２７

No.４
－０．０４

- Conductivity of the product water increases.

1.6

No.３
－０．２５

- Replacement started in 2000.

1.5
1.4
No.４
－０．３８

1.3

Alkaline cleaning only
1.2
2004/2/16

2004/2/23

2004/3/1
No.2

No.3

- About a half of the elements were replacedby 2006.

Alkaline cleaning + SBS soaking
2004/3/8
No.4

2004/3/15
No.5

No.6

2004/3/22
No.7

2004/3/29

No.8

Production and Operating Rate

４．Overall performance

×103m3（％）

100

7,000
6,455

 Production
 Cost Water quality and membrane elements

6,000

5,523

80

5,000
4,286

60

4,028

3,794

3,580

4,000
44.2

2,766

40

2,489

2,318
1,783

530

29.3

27.6

17.0

15.9

12.2

1,000

00
1995

1996

1997

1998

Annual Production (×1000 ｍ3)- 287 -

2,000

18.9

22.0

20

3,000

37.5
26.0

24.5

1999

2000

2001

Operating Rate（％）

2002

2003

2004 2005
Fiscal year

Long Term Full Operation

Production Cost

Prod uction［ｍ 3／ d a y ］
450

4 0 ,0 0 0300

2 5 ,0 0 0250

2 0 ,0 0 0200

1 5 ,0 0 0150

1 0 ,0 0 0100

5 ,0 0 050

35,000

391
363

350

3 0 ,0 0 030,000280

277

17,626219

15,05010,335

9,838
6,347

7,545

25,000270

280

11,000186

10,0006,794

5,00001997

Ja

03
n-r
Ap

3-0

Ju

03
l-c
O03
t-

Ja

04
n-r
Ap

4-0

Ju

04
l-

20,00015,00011,69600c
O

40,000408

400

3 5 ,0 0 002
t-（ m 3 / day ）（ Ye n / m 3 ）

4 5 ,0 0 0

1998

1999

2000
2001
2002
（ Fiscal year ）Average daily production

20032004

2005

Production cost per m3DateComponents of Production Cost

Example of standards maintained( 132 yen / m3 , as of 2004 - 40,000m3/day operation is assumed )Personnel
Expenses
6%

Chemicals
9%

Maintenance6%

Depreciation
15%

Standard

Item

Max Operational Value

Conductivity
TDS

Water Quality Regulations Chloride Ion
Boron

Interest
8%
Others8%

Electricity
48%

Water Quality and Membrane Replacement( m g/ l, μ S/ ㎝ )* ( m g/ l)

800

1.60

700
600

1.00

400
300

Conductivity

200

TDS

100
0378

378

378

273

0.80
0.60
0.40

98
Chloride Ion

Replaced Eelements

500 mg/l
200 mg/l
1.0 mg/lConclusion

1.20

500

720 μS/㎝

 Okinawa Seawater Desalination Center accumulated
valuable technical experiences.
 Membrane cleaning and element replacement have beensuccessful.
 An effective shock treatment method is needed.
 The operating rate has been increasing due to droughty
climate.
 Therefore, the facility has been functioning as a crucial
water resource on Okinawa.

1.40＊BoronValue

0.20
0.00

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
Fisc al Y e arＥＮＤ- 288 -