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Operation and reliability of very high-recovery seawater desalination technologies by brine conversion two-stage RO desalination system
DESALINATION ELSEVIER
Desalination 138 (2001) 191-199 www.elsevier.com/locate/desal
Operation and reliability of very high-recovery seawater desalination technologies by brine conversion two-stage RO desalination system •
a*
Masaru Kurihara , Hiroyuki Yamamura b, Takayuki Nakanishi b, Synichirou Jinno e °R&D Division, bWater Treatment Technology Center, Toray Industries, lnc., Otsu, Shiga, 520-0842 Japan Tel. +81 (77) 533-8380; Fax +81 (77) 533-8695; email: [email protected] cWater Treatment System Department, Toray Industries, Inc., Urayasu, Chiba, 279-6555 Japan Received 9 February 2001; accepted 23 February 2001
Abstract A reverse osmosis (RO) seawater desalination system has many advantages such as saving energy and using less installation space, and has become regular technology to obtain fresh water from seawater. A significant way to lower energy and installation space is to raise system recovery, and we have developed a new RO seawater desalination system which provides 60% recovery of fresh water for 3.5% seawater. The new technology is called a brine conversion two-stage SWRO system (BCS). This system includes several new technologies such as system configuration, energy recovery, operating condition, etc.; high-performance membrane technology; anti-biofouling technology and a new analysis method. A pilot plant has been operated successfully at Toray's Ehime plant site since 1997. The first commercial plant of 4500m3/d (1.2 mgd) has been operating successfully since March 1999 in Mas Palomas (Gran Canada, Spain). The Tortola and Curacao plants in the Caribbean have been installed with the full BCS (first- + secondstage RO system), and also operated under good conditions. A new application of the BCS, installed at the Muroto plant in Japan, has been in operation to obtain bottled drinking water and high concentrated mineralized water from deep seawater. Furthermore, other plants are under construction in Spain and the Caribbean. The BCS is presumed to be the standard SWRO system for the 21 st century. Keywords: Seawater desalination; Higher recovery; BCS; Saving energy; High-performance membrane; Anti-biofouling
1. Introduction A reverse osmosis (RO) seawater desalination system has many advantages for saving energy *Corresponding author.
and requiring less installation space, and has become the usual technology to obtain fresh water. This RO technology produces enough fresh water without building new dams. However, in order to be recognized as a popular method for supplying fresh water world wide, it
Presented at the European Conference on Desalination and the Environment: Water Shortage. Lemesos, Cyprus, 28-31 May 2001. 0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 1 ) 0 0 2 6 4 - 8
192
M. Kurihara et al. / Desalination 138 (2001) 191-199
is necessary to develop techniques for higher water recovery using less energy with lower installation costs. Toray, as a leading company in membrane technologies, have developed a new type of RO membrane with a process for seawater desalination, which provides 60% recovery of fresh water [1-3]. Ohya et al. [4,5] also suggested that higher recovery of RO seawater desalination was the most effective technique for saving energy and lower operating costs. This paper describes the development of a high-recovery RO seawater desalination system (brine conversion system, BCS) and its operation conditions at a pilot plant and commercial plants.
2. Brine conversion system (BCS) technology
Recent progress with the development of high-pressure/high-rejection spiral-wound RO elements, combined with proven and innovative energy recovery and pumping devices, have
opened up new possibilities to reduce investment and operating costs. Most seawater RO desalination systems in use today are confined to approximately 40% conversion of the feed water (salt concentration 3.5%), since most of commercially available RO membranes did not allow for high-pressure operation more than around 7.0 MPa. In order to obtain low-cost desalination water, a 60% water recovery RO seawater desalination system is required. Process flow of the BCS is shown in Fig. 1. The concentrated brine water (salt concentration 5.8%, produced 6.0-6.5 MPa) from the first-stage RO modules is pressurized to 8.0-10.0MPa by a pressure booster; then this brine water is supplied to the second-stage RO modules. In the secondstage RO modules, additional fresh water is obtained and the brine water is finally concentrated to approximately 8.7% (produced under 8.0-10.0 MPa) at the end of second-stage RO modules. This can obtain a total of 60% recovery of fresh water: 40% from the first stage and 20% from the second stage.
.....................................1 Conventional System ] ............................................. Seawater [
I
High Pressure
RO Element
Product Water
Total Product Water
Pretreatment Brine Water (60) c=5.8% sw
RO Element
BaosterPumo oos er ump ' ( ): Water Flow Ratio
6.0 -. max. 10.0MPa
~
Water Recovery 60%
,I --.,,,,d pLqduc ~x1.,~.t Brine Water (20) Water (40) Cffi8.7%SW
Fig. 1. Typical flow diagram of brine conversiontwo-stage RO seawaterdesalinationsystem.
M. Kurihara et al. / Desalination 138 (2001) 191-199
The total recovery is determined considering the precipitation phenomena in seawater, and the operating pressure is determined to enable to desalt second-stage brine water (the concentration is approximately 8.7%). The increase in the stage number of RO modules can reduce energy to operate a high-pressure pump; however, a two-stage RO unit can minimize the total cost, including both capital and running costs. For saving energy, the BCS can provide high recovery, and thus this new system can reduce the total amount of feed water needed by a conventional RO system to produce the same amount of fresh water. The BCS can provide a fresh water recovery of 1.5 times in comparison with a conventional system. The space of the plant is reduced to approximately 2/3 due to reducing the space of the pretreatment process by using this system. The advantage of this BCS compared with the conventional (40% recovery) system, is as follows: • Cost saving for producing fresh water can be 20%.
• Plant installation space can be reduced to 2•3. • Plant capacity is easily expanded to 1.5 times by only adding a second-stage BCS to the conventional plant. • Disposed concentrated brine water is reduced to 2/3,so that cost for disposal can be reduced. For these reasons, this new brine conversion two-stage RO seawater desalination system has proven to be very useful in saving plant space, energy and cost. Energy consumption of seawater desalination RO plants has improved from 12kWh/m 3 (1980s) to around 5.5kWh/m 3 (1990s). However using the two-stage brine conversion system, energy consumption has been improved much more to the level of 4.6kWh/m 3 as a result of higher conversions and the use of energy recovery devices [6].
3. High-performance membrane technology It is very important to increase the water recovery ratio on seawater desalination systems
Structure of RO element •
Petmeme
Brine S
193
arm Weler
Max. allowable Operating Pressure 1st stage RO " 7MPa BCS 2nd stage RO" 10MPa
Feld Pemt~lto: Pom~emo :~tmcer
i ~U.~K:.v ~:~--MZx.-~?'.~ ..........................................
Structure of Brine Conversion Seawater RO Membrane
Product Water
Fig. 2. Schematic figure of spiral-woundRO membrane and membrane cross section.
194
M. Kurihara et al. / Desalination 138 (2001) 191-199
Table 1 Typical performanceof Toray's seawater RO membranes Ultra-high-pressureseawater RO membrane,SU-820BCM Membrane material Membrane morphology Membrane substrate Feed water spacer Permeate spacer Performance: Rejection, % Water permeability, m3/d Test conditions: Feed cone., mg/L Operating pressure, MPa Max. pressure, MPa
SeawaterRO membrane, SU-820
Crosslinkedfully aroamtic polyamide Composite membrane Flat, unwoven fabric Unwoven fabric or taffeta Special Normal Ultra-high-pressureresistant High-pressureresistant 99.70 16.0
99.75 16.0
58,000 9.0 10.0
35,000 5.5 7.0
for further cost reduction. For achieving the 60% RO seawater desalination system, it is absolutely necessary to make RO membrane elements which can be operated under very severe operating conditions with high pressure (8.0-10.0 MPa) and high feed water concentration (5.8%). Such a high-performance RO membrane element for seawater desalination was first developed by Toray Industries in 1995 [7-9]. A cross-linked, fully aromatic polyamide ultra-thin composite membrane, designated UTC-80, has excellent features, and the SU-820 element using the UTC-80 membrane has been used in many seawater desalination plants around the world. However, since the maximum operating pressure of the UTC-80 was around 7.0MPa, it was not possible to apply this membrane to a BCS, which requires high pressure (8.0-10.0 MPa) and high-concentration (5.8-8.7%) operation. In these situations, Toray Industries has recently developed a highperformance membrane, designated UTC80BCM, which can be operated under such highpressure and high-concentration conditions as shown in Table 1.
The SU-820BCM elements using UTC80BCM are applied to the second stage of the BCS; the chemical structure of the BCM membrane is basically the same as that of the UTC-80, which has proven its good performance world wide. A schematic figure of the spiral-wound RO membrane element and membrane cross-section is shown in Fig. 2.
4. Anti-biofouling technology Biofouling has been regarded as the most serious problem with the operation of SWRO plants. The usual method to prevent biofouling is continuous chlorine dosing to intake seawater with SBS dosing during RO phase. However, membrane performance deterioration occurred by oxidation for both polyamide and cellulose acetate membranes, and the problem of biofouling was not yet solved. We developed a new method that is effective in preventing biofouling on SWRO membranes and verified its effectiveness in actual plants.
M. Kurihara et al. / Desalination 138 (2001) 191-199
Continuous Chlorine
Sample Viable Cells x 1000/ml
Sea Water
195
Continuous SBS Dosing
Filtered Sea Water
0.3
Feed Water of RO Module
<0.1
1.5
Fig. 3. Viable cell count assessment in the RO plant for the case of biofouling. First o f all, by measuring the viable counts of bacteria at a plant adopting the continuous chlorine/SBS dosing method, we discovered that the number o f bacteria drastically increased immediately after SBS dosing (as shown in Fig. 3), and most o f these bacteria were very different from those in raw seawater. Therefore, we investigated a new membrane sterilization method in detail. Currently, the addition of SBS to feed water at a relatively high concentration has been used for sterilization o f RO membranes. However, when SBS was added to seawater, the pH dropped to 6, and most of the bacteria in the water were still alive. This result indicates that the sterilization ability o f SBS is due to lower pH and oxygen consumption, and SBS only plays a role to repress cell growth. Ultimately, we have developed a new agent, MT-901, which is effective in preventing biofouling on RO membranes. Adding MT-901 to seawater instead of SBS lowered the pH and was more effective in killing bacteria in a few samples of seawater within a short time.
0.04 Conventional ster~Hzatiqnmethod " i 0.03 : : : -:....... r r . . . . . .,-,-,., 0.02 . . . . . -: !
;
~
0.01
~ - ~%: -~.
i
_
i
: ....
-0.02
o
: 10
i
t
i
i
i
_ ~:. , , -.. , ~ :oedb . . . . . ~ . . .~. ~ , . . . . . . ,k. : .....
-0.01 . " ~ . i ' . . . ~ . . _ . _ ~ = . . = ' ,
: 5
i
QI
.... - ..... ~_.=_~....
Ne~ ~ t i o . n mel!md' 15 20 25 30 35 40
Operating time (days) Fig. 4. Comparison o f conventional and new sterilization methods. Conventional sterilization method: continuous chlorine dosing/intermittent SBS dosing; new sterilization method: intermittent chlorine dosing/intermittent MT901 dosing.
Finally, the effect of this method was verified at our SWRO plant. Here, when feed water was chlorinated and dechlorinated continuously with SBS and RO membrane modules were treated with SBS intermittently, the differentiation pressure between the entrance and exit of the module increased gradually, as shown in Fig. 4.
196
M. Kurihara et al. / Desalination 138 (2001) 191-199
From this point, the MT-901 was used for membrane module treatment in place of SBS, and the differential pressure decreased within 10 d. Moreover, using an intermittent chlorination method together was very effective in maintaining the initial differential pressure with less concentration of the MT-901 [10,11].
5. New analysis method for RO desalination technology It is effective to establish a new analysis method for the design of a RO seawater desalination process. The analysis method was developed based on the concentration polarization theory and mass transport theory, and it enabled obtaining the membrane transport parameters from the operating data and predict the plant performance from the conventional single-stage process to the new two-stage process. This method was verified by comparing the calculated product flow rate and permeate concentration with experimental values under various operating conditions in the RO test plant, shown in Fig. 5. This figure shows that this
analysis method is very reasonable and can be used for the analysis of membrane performance and design ofa RO plant [12,13].
6. Pilot and commercial plant operation The BCS first proved its excellent performance in three RO pilot plants around the world. The first one is located in the Ehime Prefecture (Japan) as shown in Fig. 6. Total capacity of producing fresh water is 210 m3/d, 140 m3/d from first-stage RO modules and 70 m3/d from secondstage ones. The pilot plant began operations in May 1997, and has proved to produce both good quality and a good quantity of fresh water for more than 3 years, as shown in Fig. 7. Its quality satisfies the WHO drinking water standard. Two other pilot plant operations were also carried out in Mas Palomas (Spain) and Ibiza Island (Spain); both performed excellently [14-16]. The first commercial brine conversion SWRO plant was constructed and started in March 1999 in Mas Palomas in Spain, with a total capacity of 4500 m3/d. There is a great demand for water in the Mas Palomas area and many SWRO desalination plants are in operation there. On the
2OO
~ 150
2000 ................................................. ~ ..................................................................
"a ~
~~
1500
lO0
....../.....- ................... i.................................
t-
,,ae"o
New Analysis i
.................................................................................................
New Method Analysis /
Method . . . ~ .
lOOO
........i..................
s
,~ 500 ..~lm_ ~~%'~°
~o-o ~ y s i s
0 0
C_.onventional
Method
50 100 150 200 Product flow rate [m3/dl (calculation result)
0 !
0
500 1000 1501) 2000 Permeate TDS [mg/l]
(calculation result)
Fig. 5. Calculationresult of new analysismethodbased on the concentrationpolarizationtheory.
M. Kurihara et aL / Desalination 138 (2001) 191-199
197
I r ......L-~'g~ '='"~ Seeweier Tank
SscKI Filer
Filtrate Tank
Pdishing Filter
Safety Filer
~'~o%~"
Htgh-Presm~e Pump
Fig. 6. Overview of test plant in Japan. 300 200
2_...~_-- _ ~- ~ . _ - - T ~
150 ,oo 0
: . . . . . .
•
F
1st Stage
:
:_'__~--,_~,
__,
Total
, .-
~
2ndStage
,,
.~ 6OO ~ 500 cr E 400
,
,
WlIO s ~
Calculated TDS from Standard of C l i l ~ ion C¢~ntmdon
"~ co 300 A- "-" ~oo 0
O
90
180
270
360
Fig. 7. Operating results o f the test plant in Japan.
450
540
630 720 Days
810
900
990 1080 1170 1260
198
M. Kurihara et al. / Desalination 138 (2001) 191-199
Mas ~
(Cmary ~ Spatu) t 4,S00~/day (L2MGD)
Curacao (the Netherluds Antilles)
9,000m~/day(2AMGD)
Tortoh (British ~
11,406m3/dayO.0MGD)
lshuds)
690m31day(0.1$MGD)
Fig. 8. BCS commercial plants around the world. Table 2 Seawater RO plants using the brine conversion two-stage system Plant
Type
Capacity
(m3/d)
Recovery, %
Start-update
Ehime, Japan Mas Palomas, Spain Ibiza, Spain Mas Palomas I, Spain KAE Curacao 1, Dutch Antilles KAE Curacao 2, Dutch Antilles Tortola, Caribbean Mas Palomas 2, Spain Mas Palomas 3, Spain Kochi, Japan Mas Palomas 4, Spain Mas Palomas 5, Spain Total
Full plant Retrofit Retrofit Retrofit Full plant
55 kgpd 71 kgpd 71 kgpd 1.2 mgpd 1.5 mgpd
(210) (270) (270) (4500) (5700)
60 60 60 60 57
10/1996 10/1997 1/1999 3/1999 9/1999
Full plant
1.5 mgpd
(5700)
57
10/1999
Retrofit Retrofit Retrofit Full plant Retrofit Retrofit
180 kgpd 1.2 mgpd 1.2 mgpd 130 kgpd 2.5 mgpd 1.2 mgpd 10.9 mgpd
(690) (4500) (4500) (480) (9360) (4500) (40,680)
60 60 60 60 60 60
11/1999 11/1999 11/1999 3/2000 8/2000 11/2000
M. Kurihara et al. / Desalination 138 (2001) 191-199
basis o f the successful operation o f the brine conversion pilot test plant, installation o f largescale brine conversion plants was determined, and the brine conversion two-stage RO seawater desalination system with the SU-820BCM element has been operated in Mas Palomas, a town in Gran Canaria Island, Spain [14]. Furthermore, with a new application of the BCS, the Muroto Plant in Kochi Prefecture (Japan) is in operation for desalting deep seawater (200 m or more in depth), and making bottled drinking water and high concentrated mineralized water. Currently 12 brine conversion SWRO plants are in operation, as shown in Table 5; Fig. 8 shows these operating facilities as well as the numbers o f other plants expected to be installed globally in the future.
7. Conclusions Toray Industries have developed the brine conversion two-stage RO seawater desalination system, which provides advantages o f high water recovery, low energy cost and lower plant installation costs. A new type o f RO membrane element, namely the SU-820BCM, was developed, which can operate under BCS conditions of high pressure (8.0-10.0 MPa) and high feed water concentration (5.8-8.7%). The continuous operating performance with SU-820BCM has been proven under good conditions in a BCS pilot plant for more than 3 years, and other commercial seawater desalination plants with the BCS have begun to operate. The high-recovery seawater desalination BSC will become the most popular process for RO seawater desalination in the near future.
199
References [1] H. Yamamura,M. Kuriharaand KatsunosukeMaeda, Japanese Patent Laid Open, No.08-108048, 1996. [2] M. Kurihara, H. Takeuchi and H. Yamamura, IDA World Congress on Desalination and Water Reuse, Madrid, 1 (1997). [3] H. Takeuchi, Proc., New Membrane Technology Symposium $2-1-1, 1998. [4] H. Ohya, T. Suzuki, S. Nakao et al., Bull. Soc. Sea Water Sci., Japan, 50(6) (1996) 389. [5] S. Nakao, Bull. Soc. Sea Water Sci., Japan, 50(6) (1996) 406. [6] I. Moth, The case for and feasibility of very high recovery sea water reverse osmosis plants, Proc., ADA Conference,Lake Tahoe, 2000. [7] M. Kurihara, Y. Himeshima and T. Uemura, Proc., InternationalCongresson Membranesand Membrane Processes, Chicago, 2 (1990) 1034. [8] M. Kurihara, Y. Fusaoka and T. Uemura, Proc., InternationalCongresson MembranesandMembrane Processes, Heidelberg, 1993, Poster Session, 1.12. [9] M. Kurihara, Y. Fusaoka, T. Sasaki, R. Bairinji and T. Uemura, Desalination, 96 (1994) 133. [10] Y. Nakaoki,Y. Ito and T. Kimura, 50th Proc., Suidou Kenkyu Happyoukai, Miyazaki, Japan, 1999, pp. 230-23 I. [11] M. Kurihara, Y. Fusaoka and H. Yamamura,Recent progresson membranetechnologyand challengesfor 21st century,Proc., IDA IntemationalConferenceon Desalination, Singapore, 2001, in preparation. [12] M. Taniguchi, M. Kurihara, H. Yamamura and S. Kimura,New analysismethod for reverse osmosis seawater desalination membranes and its process, IDA World Congress on Desalination and Water Reuse, Session 4A, San Diego, 1999. [13] M. Taniguchi,M. Kurihara and S. Kimura, J. Chem. EngineeringJpn., in press. [14] K. Kallenberg, J.L. Perez-Vera, H. Yamamura and M. Kurihara, Brine conversion (BCS) enhances SWRO desalination case histories, operating data, novel design features, IDA World Congress on Desalination and Water Reuse, San Diego, 1999. [15] M. Kurihara, H. Yamamura and T. Nakanishi, Desalination, 125 (1999) 9. [16] H. Yamamura,Y. Fusaoka and M. Kihara, Preprints of the Society of Chemical Eng. Conference Japan, 1999.
Desalination 138 (2001) 191-199 www.elsevier.com/locate/desal
Operation and reliability of very high-recovery seawater desalination technologies by brine conversion two-stage RO desalination system •
a*
Masaru Kurihara , Hiroyuki Yamamura b, Takayuki Nakanishi b, Synichirou Jinno e °R&D Division, bWater Treatment Technology Center, Toray Industries, lnc., Otsu, Shiga, 520-0842 Japan Tel. +81 (77) 533-8380; Fax +81 (77) 533-8695; email: [email protected] cWater Treatment System Department, Toray Industries, Inc., Urayasu, Chiba, 279-6555 Japan Received 9 February 2001; accepted 23 February 2001
Abstract A reverse osmosis (RO) seawater desalination system has many advantages such as saving energy and using less installation space, and has become regular technology to obtain fresh water from seawater. A significant way to lower energy and installation space is to raise system recovery, and we have developed a new RO seawater desalination system which provides 60% recovery of fresh water for 3.5% seawater. The new technology is called a brine conversion two-stage SWRO system (BCS). This system includes several new technologies such as system configuration, energy recovery, operating condition, etc.; high-performance membrane technology; anti-biofouling technology and a new analysis method. A pilot plant has been operated successfully at Toray's Ehime plant site since 1997. The first commercial plant of 4500m3/d (1.2 mgd) has been operating successfully since March 1999 in Mas Palomas (Gran Canada, Spain). The Tortola and Curacao plants in the Caribbean have been installed with the full BCS (first- + secondstage RO system), and also operated under good conditions. A new application of the BCS, installed at the Muroto plant in Japan, has been in operation to obtain bottled drinking water and high concentrated mineralized water from deep seawater. Furthermore, other plants are under construction in Spain and the Caribbean. The BCS is presumed to be the standard SWRO system for the 21 st century. Keywords: Seawater desalination; Higher recovery; BCS; Saving energy; High-performance membrane; Anti-biofouling
1. Introduction A reverse osmosis (RO) seawater desalination system has many advantages for saving energy *Corresponding author.
and requiring less installation space, and has become the usual technology to obtain fresh water. This RO technology produces enough fresh water without building new dams. However, in order to be recognized as a popular method for supplying fresh water world wide, it
Presented at the European Conference on Desalination and the Environment: Water Shortage. Lemesos, Cyprus, 28-31 May 2001. 0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All rights reserved PII: S 0 0 1 1 - 9 1 6 4 ( 0 1 ) 0 0 2 6 4 - 8
192
M. Kurihara et al. / Desalination 138 (2001) 191-199
is necessary to develop techniques for higher water recovery using less energy with lower installation costs. Toray, as a leading company in membrane technologies, have developed a new type of RO membrane with a process for seawater desalination, which provides 60% recovery of fresh water [1-3]. Ohya et al. [4,5] also suggested that higher recovery of RO seawater desalination was the most effective technique for saving energy and lower operating costs. This paper describes the development of a high-recovery RO seawater desalination system (brine conversion system, BCS) and its operation conditions at a pilot plant and commercial plants.
2. Brine conversion system (BCS) technology
Recent progress with the development of high-pressure/high-rejection spiral-wound RO elements, combined with proven and innovative energy recovery and pumping devices, have
opened up new possibilities to reduce investment and operating costs. Most seawater RO desalination systems in use today are confined to approximately 40% conversion of the feed water (salt concentration 3.5%), since most of commercially available RO membranes did not allow for high-pressure operation more than around 7.0 MPa. In order to obtain low-cost desalination water, a 60% water recovery RO seawater desalination system is required. Process flow of the BCS is shown in Fig. 1. The concentrated brine water (salt concentration 5.8%, produced 6.0-6.5 MPa) from the first-stage RO modules is pressurized to 8.0-10.0MPa by a pressure booster; then this brine water is supplied to the second-stage RO modules. In the secondstage RO modules, additional fresh water is obtained and the brine water is finally concentrated to approximately 8.7% (produced under 8.0-10.0 MPa) at the end of second-stage RO modules. This can obtain a total of 60% recovery of fresh water: 40% from the first stage and 20% from the second stage.
.....................................1 Conventional System ] ............................................. Seawater [
I
High Pressure
RO Element
Product Water
Total Product Water
Pretreatment Brine Water (60) c=5.8% sw
RO Element
BaosterPumo oos er ump ' ( ): Water Flow Ratio
6.0 -. max. 10.0MPa
~
Water Recovery 60%
,I --.,,,,d pLqduc ~x1.,~.t Brine Water (20) Water (40) Cffi8.7%SW
Fig. 1. Typical flow diagram of brine conversiontwo-stage RO seawaterdesalinationsystem.
M. Kurihara et al. / Desalination 138 (2001) 191-199
The total recovery is determined considering the precipitation phenomena in seawater, and the operating pressure is determined to enable to desalt second-stage brine water (the concentration is approximately 8.7%). The increase in the stage number of RO modules can reduce energy to operate a high-pressure pump; however, a two-stage RO unit can minimize the total cost, including both capital and running costs. For saving energy, the BCS can provide high recovery, and thus this new system can reduce the total amount of feed water needed by a conventional RO system to produce the same amount of fresh water. The BCS can provide a fresh water recovery of 1.5 times in comparison with a conventional system. The space of the plant is reduced to approximately 2/3 due to reducing the space of the pretreatment process by using this system. The advantage of this BCS compared with the conventional (40% recovery) system, is as follows: • Cost saving for producing fresh water can be 20%.
• Plant installation space can be reduced to 2•3. • Plant capacity is easily expanded to 1.5 times by only adding a second-stage BCS to the conventional plant. • Disposed concentrated brine water is reduced to 2/3,so that cost for disposal can be reduced. For these reasons, this new brine conversion two-stage RO seawater desalination system has proven to be very useful in saving plant space, energy and cost. Energy consumption of seawater desalination RO plants has improved from 12kWh/m 3 (1980s) to around 5.5kWh/m 3 (1990s). However using the two-stage brine conversion system, energy consumption has been improved much more to the level of 4.6kWh/m 3 as a result of higher conversions and the use of energy recovery devices [6].
3. High-performance membrane technology It is very important to increase the water recovery ratio on seawater desalination systems
Structure of RO element •
Petmeme
Brine S
193
arm Weler
Max. allowable Operating Pressure 1st stage RO " 7MPa BCS 2nd stage RO" 10MPa
Feld Pemt~lto: Pom~emo :~tmcer
i ~U.~K:.v ~:~--MZx.-~?'.~ ..........................................
Structure of Brine Conversion Seawater RO Membrane
Product Water
Fig. 2. Schematic figure of spiral-woundRO membrane and membrane cross section.
194
M. Kurihara et al. / Desalination 138 (2001) 191-199
Table 1 Typical performanceof Toray's seawater RO membranes Ultra-high-pressureseawater RO membrane,SU-820BCM Membrane material Membrane morphology Membrane substrate Feed water spacer Permeate spacer Performance: Rejection, % Water permeability, m3/d Test conditions: Feed cone., mg/L Operating pressure, MPa Max. pressure, MPa
SeawaterRO membrane, SU-820
Crosslinkedfully aroamtic polyamide Composite membrane Flat, unwoven fabric Unwoven fabric or taffeta Special Normal Ultra-high-pressureresistant High-pressureresistant 99.70 16.0
99.75 16.0
58,000 9.0 10.0
35,000 5.5 7.0
for further cost reduction. For achieving the 60% RO seawater desalination system, it is absolutely necessary to make RO membrane elements which can be operated under very severe operating conditions with high pressure (8.0-10.0 MPa) and high feed water concentration (5.8%). Such a high-performance RO membrane element for seawater desalination was first developed by Toray Industries in 1995 [7-9]. A cross-linked, fully aromatic polyamide ultra-thin composite membrane, designated UTC-80, has excellent features, and the SU-820 element using the UTC-80 membrane has been used in many seawater desalination plants around the world. However, since the maximum operating pressure of the UTC-80 was around 7.0MPa, it was not possible to apply this membrane to a BCS, which requires high pressure (8.0-10.0 MPa) and high-concentration (5.8-8.7%) operation. In these situations, Toray Industries has recently developed a highperformance membrane, designated UTC80BCM, which can be operated under such highpressure and high-concentration conditions as shown in Table 1.
The SU-820BCM elements using UTC80BCM are applied to the second stage of the BCS; the chemical structure of the BCM membrane is basically the same as that of the UTC-80, which has proven its good performance world wide. A schematic figure of the spiral-wound RO membrane element and membrane cross-section is shown in Fig. 2.
4. Anti-biofouling technology Biofouling has been regarded as the most serious problem with the operation of SWRO plants. The usual method to prevent biofouling is continuous chlorine dosing to intake seawater with SBS dosing during RO phase. However, membrane performance deterioration occurred by oxidation for both polyamide and cellulose acetate membranes, and the problem of biofouling was not yet solved. We developed a new method that is effective in preventing biofouling on SWRO membranes and verified its effectiveness in actual plants.
M. Kurihara et al. / Desalination 138 (2001) 191-199
Continuous Chlorine
Sample Viable Cells x 1000/ml
Sea Water
195
Continuous SBS Dosing
Filtered Sea Water
0.3
Feed Water of RO Module
<0.1
1.5
Fig. 3. Viable cell count assessment in the RO plant for the case of biofouling. First o f all, by measuring the viable counts of bacteria at a plant adopting the continuous chlorine/SBS dosing method, we discovered that the number o f bacteria drastically increased immediately after SBS dosing (as shown in Fig. 3), and most o f these bacteria were very different from those in raw seawater. Therefore, we investigated a new membrane sterilization method in detail. Currently, the addition of SBS to feed water at a relatively high concentration has been used for sterilization o f RO membranes. However, when SBS was added to seawater, the pH dropped to 6, and most of the bacteria in the water were still alive. This result indicates that the sterilization ability o f SBS is due to lower pH and oxygen consumption, and SBS only plays a role to repress cell growth. Ultimately, we have developed a new agent, MT-901, which is effective in preventing biofouling on RO membranes. Adding MT-901 to seawater instead of SBS lowered the pH and was more effective in killing bacteria in a few samples of seawater within a short time.
0.04 Conventional ster~Hzatiqnmethod " i 0.03 : : : -:....... r r . . . . . .,-,-,., 0.02 . . . . . -: !
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Operating time (days) Fig. 4. Comparison o f conventional and new sterilization methods. Conventional sterilization method: continuous chlorine dosing/intermittent SBS dosing; new sterilization method: intermittent chlorine dosing/intermittent MT901 dosing.
Finally, the effect of this method was verified at our SWRO plant. Here, when feed water was chlorinated and dechlorinated continuously with SBS and RO membrane modules were treated with SBS intermittently, the differentiation pressure between the entrance and exit of the module increased gradually, as shown in Fig. 4.
196
M. Kurihara et al. / Desalination 138 (2001) 191-199
From this point, the MT-901 was used for membrane module treatment in place of SBS, and the differential pressure decreased within 10 d. Moreover, using an intermittent chlorination method together was very effective in maintaining the initial differential pressure with less concentration of the MT-901 [10,11].
5. New analysis method for RO desalination technology It is effective to establish a new analysis method for the design of a RO seawater desalination process. The analysis method was developed based on the concentration polarization theory and mass transport theory, and it enabled obtaining the membrane transport parameters from the operating data and predict the plant performance from the conventional single-stage process to the new two-stage process. This method was verified by comparing the calculated product flow rate and permeate concentration with experimental values under various operating conditions in the RO test plant, shown in Fig. 5. This figure shows that this
analysis method is very reasonable and can be used for the analysis of membrane performance and design ofa RO plant [12,13].
6. Pilot and commercial plant operation The BCS first proved its excellent performance in three RO pilot plants around the world. The first one is located in the Ehime Prefecture (Japan) as shown in Fig. 6. Total capacity of producing fresh water is 210 m3/d, 140 m3/d from first-stage RO modules and 70 m3/d from secondstage ones. The pilot plant began operations in May 1997, and has proved to produce both good quality and a good quantity of fresh water for more than 3 years, as shown in Fig. 7. Its quality satisfies the WHO drinking water standard. Two other pilot plant operations were also carried out in Mas Palomas (Spain) and Ibiza Island (Spain); both performed excellently [14-16]. The first commercial brine conversion SWRO plant was constructed and started in March 1999 in Mas Palomas in Spain, with a total capacity of 4500 m3/d. There is a great demand for water in the Mas Palomas area and many SWRO desalination plants are in operation there. On the
2OO
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s
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50 100 150 200 Product flow rate [m3/dl (calculation result)
0 !
0
500 1000 1501) 2000 Permeate TDS [mg/l]
(calculation result)
Fig. 5. Calculationresult of new analysismethodbased on the concentrationpolarizationtheory.
M. Kurihara et aL / Desalination 138 (2001) 191-199
197
I r ......L-~'g~ '='"~ Seeweier Tank
SscKI Filer
Filtrate Tank
Pdishing Filter
Safety Filer
~'~o%~"
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Fig. 6. Overview of test plant in Japan. 300 200
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:
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Fig. 7. Operating results o f the test plant in Japan.
450
540
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810
900
990 1080 1170 1260
198
M. Kurihara et al. / Desalination 138 (2001) 191-199
Mas ~
(Cmary ~ Spatu) t 4,S00~/day (L2MGD)
Curacao (the Netherluds Antilles)
9,000m~/day(2AMGD)
Tortoh (British ~
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lshuds)
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Fig. 8. BCS commercial plants around the world. Table 2 Seawater RO plants using the brine conversion two-stage system Plant
Type
Capacity
(m3/d)
Recovery, %
Start-update
Ehime, Japan Mas Palomas, Spain Ibiza, Spain Mas Palomas I, Spain KAE Curacao 1, Dutch Antilles KAE Curacao 2, Dutch Antilles Tortola, Caribbean Mas Palomas 2, Spain Mas Palomas 3, Spain Kochi, Japan Mas Palomas 4, Spain Mas Palomas 5, Spain Total
Full plant Retrofit Retrofit Retrofit Full plant
55 kgpd 71 kgpd 71 kgpd 1.2 mgpd 1.5 mgpd
(210) (270) (270) (4500) (5700)
60 60 60 60 57
10/1996 10/1997 1/1999 3/1999 9/1999
Full plant
1.5 mgpd
(5700)
57
10/1999
Retrofit Retrofit Retrofit Full plant Retrofit Retrofit
180 kgpd 1.2 mgpd 1.2 mgpd 130 kgpd 2.5 mgpd 1.2 mgpd 10.9 mgpd
(690) (4500) (4500) (480) (9360) (4500) (40,680)
60 60 60 60 60 60
11/1999 11/1999 11/1999 3/2000 8/2000 11/2000
M. Kurihara et al. / Desalination 138 (2001) 191-199
basis o f the successful operation o f the brine conversion pilot test plant, installation o f largescale brine conversion plants was determined, and the brine conversion two-stage RO seawater desalination system with the SU-820BCM element has been operated in Mas Palomas, a town in Gran Canaria Island, Spain [14]. Furthermore, with a new application of the BCS, the Muroto Plant in Kochi Prefecture (Japan) is in operation for desalting deep seawater (200 m or more in depth), and making bottled drinking water and high concentrated mineralized water. Currently 12 brine conversion SWRO plants are in operation, as shown in Table 5; Fig. 8 shows these operating facilities as well as the numbers o f other plants expected to be installed globally in the future.
7. Conclusions Toray Industries have developed the brine conversion two-stage RO seawater desalination system, which provides advantages o f high water recovery, low energy cost and lower plant installation costs. A new type o f RO membrane element, namely the SU-820BCM, was developed, which can operate under BCS conditions of high pressure (8.0-10.0 MPa) and high feed water concentration (5.8-8.7%). The continuous operating performance with SU-820BCM has been proven under good conditions in a BCS pilot plant for more than 3 years, and other commercial seawater desalination plants with the BCS have begun to operate. The high-recovery seawater desalination BSC will become the most popular process for RO seawater desalination in the near future.
199
References [1] H. Yamamura,M. Kuriharaand KatsunosukeMaeda, Japanese Patent Laid Open, No.08-108048, 1996. [2] M. Kurihara, H. Takeuchi and H. Yamamura, IDA World Congress on Desalination and Water Reuse, Madrid, 1 (1997). [3] H. Takeuchi, Proc., New Membrane Technology Symposium $2-1-1, 1998. [4] H. Ohya, T. Suzuki, S. Nakao et al., Bull. Soc. Sea Water Sci., Japan, 50(6) (1996) 389. [5] S. Nakao, Bull. Soc. Sea Water Sci., Japan, 50(6) (1996) 406. [6] I. Moth, The case for and feasibility of very high recovery sea water reverse osmosis plants, Proc., ADA Conference,Lake Tahoe, 2000. [7] M. Kurihara, Y. Himeshima and T. Uemura, Proc., InternationalCongresson Membranesand Membrane Processes, Chicago, 2 (1990) 1034. [8] M. Kurihara, Y. Fusaoka and T. Uemura, Proc., InternationalCongresson MembranesandMembrane Processes, Heidelberg, 1993, Poster Session, 1.12. [9] M. Kurihara, Y. Fusaoka, T. Sasaki, R. Bairinji and T. Uemura, Desalination, 96 (1994) 133. [10] Y. Nakaoki,Y. Ito and T. Kimura, 50th Proc., Suidou Kenkyu Happyoukai, Miyazaki, Japan, 1999, pp. 230-23 I. [11] M. Kurihara, Y. Fusaoka and H. Yamamura,Recent progresson membranetechnologyand challengesfor 21st century,Proc., IDA IntemationalConferenceon Desalination, Singapore, 2001, in preparation. [12] M. Taniguchi, M. Kurihara, H. Yamamura and S. Kimura,New analysismethod for reverse osmosis seawater desalination membranes and its process, IDA World Congress on Desalination and Water Reuse, Session 4A, San Diego, 1999. [13] M. Taniguchi,M. Kurihara and S. Kimura, J. Chem. EngineeringJpn., in press. [14] K. Kallenberg, J.L. Perez-Vera, H. Yamamura and M. Kurihara, Brine conversion (BCS) enhances SWRO desalination case histories, operating data, novel design features, IDA World Congress on Desalination and Water Reuse, San Diego, 1999. [15] M. Kurihara, H. Yamamura and T. Nakanishi, Desalination, 125 (1999) 9. [16] H. Yamamura,Y. Fusaoka and M. Kihara, Preprints of the Society of Chemical Eng. Conference Japan, 1999.