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 in
1997. The facility was constructed to ease water shortage on Okinawa by
converting surrounding clean seawater into drinking water. With ten years of
operation, the facility has proved to be successfully functioning to meet demand for
drinking water especially in drought situation.
This presentation covers an overview of Okinawa Seawater Desalination
Center, issues including product water quality and facility maintenance, and
finally, 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 since
1972. 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 the
development of dams and rivers. As a countermeasure, the Enterprise Bureau of
Okinawa Prefecture decided to promote the desalination project. The first survey
was conducted by the Japanese government in 1977, and construction began in
1993.
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 average
water 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 Japan
e-mail: [email protected]
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.
Seawater
Primary and
Secondary Sand Filters
Sand
Sedimentation
Basin
Intake Pump
Intake Tower
Security
Filter
Adjustment Tank
RO Supply
Pump
Fresh
Water
High Pressure Reverse
Osmosis
Pump
Unit
Fresh Water
Tank
Brine
Energy Recovery
Turbine
Multi-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 are
removed. Table.1 compares quality of the RO feed seawater, the desalted product
water, and the mixed potable water.
Table.1
Comparison of water quality
Item
RO feed water
Desalted water
Mixed water
Total Dissolved Solids
34,800
278
Chloride Ion
19,800
119
Sodium and its compounds
11,400
96.1
38.8
Sulfate Ion
2,490
5.9
36.8
Hardness
6,360
10
113
50,800
456
436
Electrical conductivity
Boron and its compounds
(Units:
4.68
1.14
256
46.5
0.12
[μS/cm] for electrical conductivity and [mg/l] for others)
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 a
post-treatment in order to prevent corrosion. However, since the water produced in
Chatan Water Purification Plant has relatively high hardness, the hardness of the
mixed water becomes moderate. Therefore, the post-treatment is not needed.
3.3 Conductivity
Salt rejection rate is a great concern in the desalination facility. Electrical
conductivity 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 allowable
concentration of chloride ion according to water quality regulations.
3.4 Boron concentration
Boron is hard to remove in a single stage reverse osmosis process. Japanese
water quality regulations demand boron concentration to be less than 1.0 mg/l.
Table.1 shows that boron concentration was not sufficiently reduced by the
desalination process alone but successfully suppressed by mixing.
4. Maintenance
4.1 Biofouling
The RO process utilizes polyamide membrane, which does not tolerate
oxidizing agents. Since the feed water does not contain chlorine, biofilm tends to
grow inside the seawater channel and obstruct the feed flow in the RO elements.
This phenomenon, biofouling, may cause irreversible damage to the membrane
elements. 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 be
investigated 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 be
replaced 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 and
had replaced about a half of the originally installed 3,024 elements by 2006.
5. Overall performance
5.1 Production
Ten years have passed since the facility started operating. For the first five
years, 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
4,286
2,000
1,000
0
0
1995
1996
1997
1998
Annual Production (×1000 ｍ3)
Fig.2
3,000
1999
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 of
the cost. As shown, electricity is the most dominant factor of the cost.
Chemicals
9%
Personnel
Expenses
6%
Maintenance
6%
Depreciation
15%
Interest
8%
Electricity
48%
Fig.3
Others
8%
Components of the production cost
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 product
water quality. The standards have been maintained for all the items except boron.
Fig.4 also shows the numbers of replaced membrane elements within the same
period. The figure clearly shows the effect of the membrane replacement on the
product water quality.
Table.2
Water quality items
Standard
Item
Max Operational Value
Water Quality Regulations
Value
Conductivity
720 μs/㎝
Total Dissolved Solids
500 mg/l
Chloride Ion
200 mg/l
Boron
Water Qualty and Membrane Replacement
(mg/l, μs/㎝)
800
700
600
500
400
300
200
100
0
*(mg/l)
＊Boron
378
Conductivity
378
378
273
TDS
98
Chloride Ion
1995
Fig.4
1.0 mg/l
1996
1997
1998
Number of replaced membrane
1999 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 the
element replacement have been successful in maintaining quantity and quality of
the product water. Since biofouling is a drawback for long term full operation, an
effective shock treatment method is sought. The operating rate of the facility has
been increasing due to the droughty climate. Therefore, the facility has been
functioning as a crucial water resource on main island of Okinawa.
Reference
Japan Water Works Association, Design Criteria for Waterworks Facilities (2000) 352
Seawater Desalination Facility on Okinawa
Toru Yamazato
Seawater Desalination Center
Chatan Water Administration Office
Okinawa Prefectural Enterprise Bureau
Introduction
1.
2.
3.
4.
Overview
Water quality
Facility maintenance
Overall performance
1
1．Overview
ƒ Needs for desalination
ƒ Reverse osmosis seawater
desalination facility
Needs for desalination
ƒ Water demand doubled in 30 years
ƒ Supply dependent on dams and rivers
ƒ Merit of desalination
- Abundant water from the sea
- Short construction period
- Location near consumers
2
Water rationing
ƒ Water rationing occurred in 14 years out of 34 years since 1972.
ƒ The longest continued 326 days.
ƒ No water service restriction occurred since 1993.
Reverse osmosis
Operating pressure : P
Seawater
Semi-permeable
membrane
P > Δπ
Fresh water
Osmotic Pressure ：Δπ
Flux ： F
3
Reverse osmosis (RO) membrane element
Differential Pressure
Feed
Brine
Permeate
RO membrane
Center pipe
RO module
Okinawa Seawater Desalination Center
Seawater
Primary and
Secondary Sand
Filters
RO
Supply
Pump
Adjustment
Tank
Intake Sand
Pump Sedimentation
Basin
Intake Tower
High
Pressure
Pump
Security
Filter
Fresh
Water
Reverse
Osmosis Unit
Energy Recovery Turbine
Fresh Water
Tank
Brine
Multi-Nozzle Discharge Tower
4
Okinawa Seawater Desalination Center
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
ƒ
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
2. Water quality
Item
RO feed water
Desalted water
Mixed water
Total Dissolved Solids
34,800
278
256
Chloride Ion
19,800
119
46.5
Sodium
11,400
96.1
38.8
Sulfate Ion
2,490
5.9
36.8
Hardness
6,360
10
113
Electrical conductivity
50,800
456
436
Boron
4.68
1.14
0.12
(Units: [μS/cm] for electrical conductivity and [mg/l] for others)
5
3. Maintenance
ƒ Biofouling
- Polyamide membrane and oxidizing agents
- Biofilm in the feed channel
- Damage to the membrane elements
Countermeasure for biofouling
ƒ Sulfuric acid shock treatment
- Acid injection to the feed
- pH = 2.5 ～ 3.0 30 minutes
- Advantage : Long running time of RO units
ƒ Membrane cleaning
- Off-line cleaning
- pH = 10.5 3 days
- Advantage : Fundamental and efficient
6
Differential Pressure
Kg/cm2
2.0
Membrane Cleaning
No.５
－０．４９
1.9
No.３
＋０．０１
1.8
1.7
No.６
－０．０８
No.７
－０．２７
No.４
－０．０４
1.6
No.３
－０．２５
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
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
Element replacement
ƒ Salt rejection declines
- Conductivity of the product water increases.
- Replacement started in 2000.
- About a half of the elements were replaced
by 2006.
7
４．Overall performance
ƒ Production
ƒ Cost
ƒ Water quality and membrane elements
Production and Operating Rate
×103m3
（％）
7 ,00 0
10 0
6,455
6 ,00 0
5,523
80
5 ,00 0
4,286
4,028
60
4 ,00 0
3,794
3,580
44.2
2,766
40
2,489
2,318
1,783
26.0
24.5
18.9
22.0
20
530
29.3
27.6
17.0
15.9
12.2
3 ,00 0
37.5
2 ,00 0
1 ,00 0
0
0
19 95
19 96
19 9 7
1 99 8
Annual Production (×1000 ｍ3)
1 99 9
2 0 00
20 01
20 02
2 00 3
2 00 4 2 00 5
F isc al ye ar
Operating Rate（％）
8
Long Term Full Operation
Prod uction
［ｍ 3／ da y ］
4 5 ,0 0 0
4 0 ,0 0 0
3 5 ,0 0 0
3 0 ,0 0 0
2 5 ,0 0 0
2 0 ,0 0 0
1 5 ,0 0 0
1 0 ,0 0 0
5 ,0 0 0
Ju
l
Ap
r-
-0
4
04
-0
4
-0
3
Ja
n
Ju
l
O
ct
-0
3
03
-0
3
Ap
r-
Oc
t
Ja
n
-0
2
0
Date
Production Cost
（ m 3 / day ）
（ Ye n / m 3 ）
40,000
450
408
400
300
35,000
391
363
350
30,000
280
277
150
17,626
219
200
15,050
10,335
9,838
100
6,347
7,545
25,000
270
280
250
11,000
186
20,000
15,000
11,696
10,000
6,794
5,000
50
0
0
1997
1998
1999
2000
2001
2002
（ Fiscal year ）
Average daily production
2003
2004
2005
Production cost per m3
9
Components of Production Cost
( 132 yen / m3 , as of 2004 - 40,000m3/day operation is assumed )
Chemicals
9%
Personnel
Expenses
6%
Maintenance
6%
Depreciation
15%
Interest
8%
Others
8%
Electricity
48%
Example of standards maintained
Standard
Max Operational Value
Item
Value
Conductivity
720 μS/㎝
TDS
500 mg/l
Water Quality Regulations Chloride Ion
Boron
200 mg/l
1.0 mg/l
10
Water Quality and Membrane Replacement
( m g/ l, μS/ ㎝ )
* ( m g/ l)
800
700
600
1.60
1.40
＊Boron
1.20
500
400
300
200
100
0
1.00
378
Conductivity
378
378
273
0.80
0.60
0.40
TDS
98
Chloride Ion
Replaced Eelements
0.20
0.00
1995 1996 1997 1998 1999 2000 2001 2002 2003 2004
Fisc al Y e ar
Conclusion
ƒ Okinawa Seawater Desalination Center accumulated
valuable technical experiences.
ƒ Membrane cleaning and element replacement have been
successful.
ƒ An effective shock treatment method is needed.
ƒ The operating rate has been increasing due to droughty
climate.
ƒ Therefore, the facility has been functioning as a crucial
water resource on Okinawa.
11
ＥＮＤ
12