8/2/2019 1-s2.0-S0011916404006800-main

1/12

ELSEVIER Desalination 173 (2005) 1-12DESALINATION

www.elsevier.com/locate/desal

Op tim ization o f seawater RO system s des ignM ark Wi lt '* , C ra ig Bar t e l sHydranautics, Oceanside, CA, USATel. +1 (760) 901-2548; Fax: +1 (760) 901-2664; email: mwilf@hydranautics.com

Received 19 March 2004; accepted 21 June 2004

Abstrac tThe paper describes the configuration a nd operating parameters of current large seaw ater desalination systems.Major advances o f RO seaw ater desalination techn ology hat lead to a remarkable decrease o f desalted water costs areevaluated. Process improvements th at enable compliance with mo re stringent requirements o f permeate wate r qualityare discussed. Results of field tests con ducte d to demonstrate a new process approach are described. Som e examplesof process optimization resulting in lower pow er consumption an d more e fficient system operation are presented.

Keywords : Seawater reverse osmosis; Po we r recovery; Boron reduction

1. I n t r o d u c t i o nI n th e l a s t f e w y e a r s R O s e a w a t e r d e s a l i n a t i o n

t e c h n o l o g y h a s g o n e t h r o u g h a r e m a r k a b l e t r a n s -f o r m a t io n . T h e n u m b e r a n d c a p a c i t y o f l ar g e R Op l a n ts h a v e i n c r e a s e d s i g n i f ic a n t l y . S y s t e m s w i t hp e r m ea t e cap ac i t y u p t o 3 0 0 ,0 0 0 m 3 / d a r e cu r -r en t l y b e i n g b u i l t . I n a p a r a l l e l s h i f t t h e cap i t a la n d o p e r a t i n g c o s t h a s d e c r e a s e d . D e s a l t e d w a t e rco s t , s u p p l i ed t o cu s t o m er , d ec r ea s ed f r o m$ 2 .0 / m 3 i n 1 9 9 8 d o w n t o cu r r en t ( 2 0 0 4 ) p r i ce o fab o u t $ 0 .5 / m 3. T h i s d ec r ea s e o f w a t e r co s t i s ev e nm o r e r e m a r k a b l e i f o n e c o n s i d e r s t h a t , o n t h ea v e r a g e , th e p e r m e a t e w a t e r q u a l i t y r e q u i re m e n t sa r e m o r e s t r i n g e n t n o w t h a n t h e y w e r e 5 y e a r s*Corresponding author.

a g o . T h e d r i v e r s b e h i n d t h e s e e c o n o m i c a l i m -p r o v e m e n t s a r e c o m p e t i t io n a n d i m p r o v e m e n t o fp r oc e s s a n d m e m b r a n e t e c h n o l o g y . A m a j o r it y o fl a r g e R O s y s t e m s a r e b u i l t t o p r o v i d e w a t e r t om u n i c i p a l i t ie s , u s u a l l y i n t h e f r a m e w o r k o f b u i ld ,o w n a n d o p e r a t e a r r a n g e m e n t s . T h e d e s a l i n a t io np r o j e c ts a r e a w a r d e d a s r e s u l t o f a v e r y c o m -p e t i t i v e b i d d i n g p r o c e s s . T h i s c o m p e t i t i v e b i d -d i n g p r o c e s s a f f e c t e d p r ic e s o f th e e q u i p m e n tc o m p o n e n t s o f R O s y s t e m s ( in c l u d in g m e m b r a n ee l em en t s ) an d r e s u l t ed i n a b r o ad p r i ce d ec l i n e .B e t t e r p e r f o r m a n c e o f e q u i p m e n t a n d o p t i -m i z a t i o n o f p r o c e s s d e s i g n r e s u l te d i n l o w e ro p e r a t i n g c o s ts . T h e r e c e n t t r e n d s o f w a t e r c o s tf r o m l a r g e s e a w a t e r R O i n s t a l la t i o n s is s u m -m a r i z e d i n F i g . 1 a n d T a b l e 1 .

0011-9164/05/$- See front matter 2005 E lsevier B.V. All r ights reserveddoi: 10.1016/j.desal.2004.06.206

8/2/2019 1-s2.0-S0011916404006800-main

2/12

2 M. Wilf, C. Barrels/De salination 173 (2005) 1-1 2Table 1Water cost in recently built RO seawater plantsLocation Permeate capacity, m 3 / d S t a t u s W ater price, $/m3Eilat Israel 20,000Larnaea, Cyprus 56,000Tam pa, FI 106,000Ashkelon, Isr ae l 272,000

10,000 m3/d;commenced operat ion in June 199 7 0.72Com menced operation in M ay 2001 0.83Com menced operation in May 2003 0.56Under construction, to be completed in 2004 0.54

e q 2 .0,61" ~ e - t 1 , 2

o o ..ll l l U | u~ 0 , 0 1980 1990 1991 1995 1996 1997 2000C o n t r a c t y e a r

Fig. l . C ost of water for large seawater RO plants.

T h e c u r r e n t l a r g e R O s y s t e m s a r e c h a r a c -t e r i zed b y t h e fo l l o w i n g t ech n i ca l f ea t u r e s : u t il iz a t io n o f h i g h - e f f i c i e n c y p u m p i n g a n d

p o w e r r e c o v e r y e q u i p m e n t , i n c l u d i n g u s e o fv a r i a b le s p e e d d r i v e r s ( V F D ) ,

o p t i m i z a ti o n o f r e c o v e r y r at e in r e s p e c t o fp o w e r c o n s u m p t i on ,

i n c r ea s e d n u m b e r o f e l e m e n t s p e r p r e s s u rev es s e l co mb i n ed w i t h s h i f t t o a s i n g l e - s t ag ea r r ay ,

m o r e e f f i c ie n t t w o - p a s s c o n f i g u r a t io n u t il iz a t io n o f p o w e r p la n t c o o l i n g w a t e r a s a

f e e d to R O , w i d e s p r e a d f i e ld t e s ti n g o f U F / M F a s p r e -

t r ea t m e n t f o r s e a w a t e r R O , s t r i n g en t p e rmea t e q u a l i t y r eq u i r emen t s ,

s o m e t i m e s i n c l u d i n g l i m i t s o n m a x i m u mb o ro n co n cen t r a t i o n ,

b e tt e r p e r f o r m a n c e o f s e a w a t e r R O m e m -b ran es : h i g h e r p e rmeab i l i t y an d h i g h e r s a l tre j ec t ion .

2 . W a t e r c o s t d i s t r i b u t i o nT h e c o n t r i b u t i o n o f v a r i o u s c o m p o n e n t s t o

c o s t o f p r o d u c t w a t e r f r o m a s e a w a t e r R O s y s t e mi s p r e s en t ed i n F ig . 2 . T h e l a rg es t co m p o n e n t s a r ep o w e r a n d f i x e d c h a r g e s c o s ts . F i g. 3 s h o w s ab r e a k d o w n o f p o w e r c o n s u m p t i o n i n a p a rt ia lt w o -p as s s y s t em. A s ex p ec t ed , t h e f i r s t - s t ag ep u m p i n g s y s t e m i s t h e m a j o r p o w e r c o n s u m e r .T h e o v e r a l l p o w e r c o n s u m p t i o n c a n b e r e d u c e db y u t i l i z i n g m o r e e f f i c i e n t p u m p i n g s y s t e m a n db y o p t i m i z i n g t h e r e c o v e r y r a te .

2 .1 . C o n f i g u r a t io n o f h i g h - p r e s s u r e p u m p i n gs ys t em

I n s e a w a t e r R O s y s t e m s t h e f e e d p r e s s u r er e q u i re d t o p r o d u c e d e s i g n o u t p u t c a p a c i t y f lu c -t u a t e s w i t h f eed w a t e r s a l i n i t y , t emp e ra t u r e ,d e g r e e o f m e m b r a n e s u r f a c e f o u l i n g an d m e m -b r a n e c o m p a c t i o n . T h e l a s t t w o p a r a m e t e r s a r eu s u a l l y b u n d l e d t o g e t h e r i n t o a " f l u x d e c l i n e "f a c t o r ( F D F ) , w h i c h r e f l e c t s m e m b r a n e p e r m e a -b i l it y d e c l in e w i t h t im e . T o a c c o m m o d a t e v a r i a-b i l i t y o f r eq u i r ed f eed p r e s s u re w i t h t ime , w i t h o u tn eces s i t y t o t h ro t t l e h i g h -p re s s u re p u mp s o rp o w e r r e c o v e r y t u r b i n e s , a f l e x i b le h i g h - p r e ss u r ep u m p i n g s y s t e m i s r e q u i re d . T h e s u it a b le e q u i p -m e n t s h o u l d b e a b l e t o p r o c e s s c o n s t a n t f l o w i nt h e p r o j e c t e d r a n g e o f f e e d a n d c o n c e n t r a t ep re s s u re s w i t h o u t s i g n i f i c an t l o s s e s o f tr an s fo r -ma t i o n e f f i c i en cy . T h e f l ex i b i l i t y o n t h e f eedw a t e r s u p p l y s i d e i s u s u a l l y a c h i e v e d b y i n c o r -p o r a t i n g a v a r i a b l e f r e q u e n c y d r i v e ( V F D ) i n t o

8/2/2019 1-s2.0-S0011916404006800-main

3/12

Product transfer pumps, 6.7%

High pressure pump2ndt pass, 3,8%

M. Wilf, C. Barrels/Desalination 173 (2005) 1-12Miscellaneous, 1.8%/

~water supply,%Pretreatmentsystem, 2 .6%

, ,,~,, ~, . . . . . e pumps, 1stpass, 80.6%

Fig. 2. Power usage in a RO seawater plantwith partial second stage.

Maintenance, 8%

C he

JElectric power, 33%

Labor, 4% M em bra ne~lacement, 3%

a.EquipmentAmortization, 48%

Fig. 3. Water cost components in a RO sea-water plant.

e lectr ic motor unit that dr ives the h igh-pressurepump. The Pelton wheel or posi t ive d isplacementdevices can provide suff ic ient operat ional f lexi-bi l ity as a powe r recov ery device . The advantageof the Pelton wheel is the f la t eff ic iency curve ina wide range of concentra te f lows. Howe ver ,concentra te exi ts the Pelton wheel a t a tmosphericpressure; therefore, gravitation hea d or additionalpumping is required for concentra te d isposal .F ig. 4 is a d iagram o fa RO system equipped witha high-pressure pump and a Pel ton wheel powe rrecovery unit . In such a syst em the requiredpressure of the feed s tream (F) is generated by acentr ifugal pump (HP). Product (P) leaves the ROunit at a pressure required to flow to the storagetank (-1 bar) . The pressure o f the concentra te (C)is lower than the feed pressure due to the h ydrau-lic friction loses in the RO unit. The concentrate

f lows through the Pelton wheel power recoveryunit (T) where i ts energ y turns the dr ive of theelectr ical motor (M) and conne cted high-pressurepump (HP). A higher eff ic ienc ypositive displace-ment power recovery devices (pressure exchan-gers), that in the past were on ly used in small ROseawater units , are a lso s lowly gaining acceptancein large desalinat ion plants . H ydraulic eff ic iencyof th is type of equipme nt is in the range of 94-96%. Some of these devices u t i l ize p is tons; o thertransfer energy through a d irect contact betweenconcentra te and the feed s tream. A diagram o f aRO unit u t i l iz ing such equipment is shown inFig. 5. Ac cor din g to the diagram , feed (F) is splitin to two s treams. One s tream (F 1), which has af low rate equivalent to the permeate f low (P) , ispumped to the feed pressure by the main high-pressure pump (HP). The second s tream (F2) ,

8/2/2019 1-s2.0-S0011916404006800-main

4/12

M. Wi l f, C . Bar te l s / Desa l inat ion 173 (2005) 1- 12

~ 3.00

i 2 .602 .20

1.8030 35 40 45 50 55 60

Fig. 4. RO unit with a conventional energy recoverysystem.

P er m ea te r ecover y r a te, %Fig. 6. Specific power consumption in SW RO M editer-ranean fee d. Conventional and advanced pumpingsystem.

(C

FFig. 5. RO unit with a pressure exchanger type energyrecovery system.

wh ich f lo w r a t e i s eq u iv a len t t o t h e co n cen t r a t ef l o w , f l o w s t h r o u g h p r e s s u r e e x c h a n g e r a n de x c h a n g e s p r e s s u r e w i t h t h e c o n c e n t r a t e s t r e a m( C ) . Th e p r e ssu r e o f s t r eam F2 a t th e ex i t f r o mt h e p r es s u r e e x c h a n g e r i s a f u n c t i o n o f c o n c e n -t r at e p r e s s u r e a n d e f f i c i e n c y o f t h e p r e s s u r ee x c h a n g e r d e v i c e . T h e p r e s s u r e o f s tr e a m F 2 i sl o w e r b y 3 - 5 b a r s t h a n t h e p r e s s u r e o f s t r e a m F 1a t t h e d i s c h a rg e o f p u m p H P . T h e p r e s s u re o fs t r eam F2 i s i n c r ea se d to th e p r e ssu r e o f s t reamF t b y a b o o s t e r p u m p ( B ) . B o th s t r eam s ( F 1 + F2 )a r e c o m b i n e d a t t h e e n t r a n c e t o t h e m e m b r a n ef e e d m a n i f o ld . T h e p r e s s u r e e x c h a n g e r s a r e p o s i -t i ve d i s p l a c e m e n t d e v i c e s a n d t h e r e f o r e h a v e h i g ht r an s f e r e f f i c i en cy . Ho we v e r , so f a r t h e i r f l o w

capac i t ies a re l imi te d to less than 100 m3/h, and al a r g e s y s t e m r e q u i r e s a s i g n i f i c a n t n u m b e r o fpara l le l un i ts .

Th e u n i t s t h a t o p e r a t e w i th d i r ec t co n tac t o fc o n c e n t r a t e a n d f e e d e x p e r i e n c e s o m e m i x i n g ,wh ich r e su l t s i n i n c r ea sed f eed sa l in i ty , i n t h er a n g e o f 3 % . T h e o t h e r t y p e o f p r es s u r e e x c h a n g ed ev ice th a t u t i l i z e s p i s to n s d o es n o t ex p e r i en cean y s ig n i f i can t sa l in i ty i n c r ea se d u e to m ix in g ,b u t i t r eq u i r e s f l o w r eg u la t in g v a lv es . Th esed e v i c e s o p e r a t e a t h i g h f r e q u e n c y o f o p e n /c l o s ec y c l e s a n d m a y r e q u i r e a s i g n i f i c a n t d e g r e e o fm a i n t e n a n c e . R e g a r d l e s s o f s o m e o p e r a ti o n a lp r o b l e m s , t h e s e p o w e r r e c o v e r y d e v i c e s p r o v i d es i g n i fi c a n t r e d u c t io n o f p o w e r u s a g e . F i g . 6s h o w s s p e c i fi c p o w e r c o n s u m p t i o n c a l c u l a t ed f ora s e a w a t e r R O s y s t e m , a s s u m i n g u s e o f l o we f f i c i e n c y c o n v e n t i o n a l p u m p i n g e q u i p m e n t ( A ) ,h i g h e f f i c i e n c y c o n v e n t i o n a l p u m p i n g e q u i p m e n t( B ) a n d p u m p i n g e q u i p m e n t , w h i c h i n c l u d e sp r e s s u r e e x c h a n g e r s ( C ). T h e e f f i c ie n c i e s o fp u m p i n g e q u i p m e n t a n d p a r a m e t e r s o f c al c u la -t i o n s a r e l is t ed in Tab le 2 . Th e ca l cu la t io n s we r ec o n d u c t e d f o r a R O s y s t e m p r o c e s s i n g M e d i t e r -r a n e a n f e e d ( 40 , 6 0 0 p p m T D S ) a t a t e m p e r a t u reo f 2 2 C , av e r ag e p e r m ea te f lu x ra t e o f 1 3 .8 l /m L hu t i l i z i n g H y d r a n a u t i c s S W C 3 + e l e m e n t s a n da s s u m i n g a 3 - y m e m b r a n e a g e , w h i c h c o r r e -sp o n d s to 2 0 % f lu x d ec l in e . Fo r t h e ca l cu la t io n sco r r e sp o n d in g to a sy s t em u t i l i z in g p r e ssu r e

8/2/2019 1-s2.0-S0011916404006800-main

5/12

M. Will C. Bartels / Desalination 173 (2005) 1-12Table 2Efficiencies of pumping equipment

Case A Case B Case CConfigurationPump efficiency, %Pelton w heel/pressure exchangerefficiency, %Electric mo tor efficiency, % 94VFD efficiency, % 98Raw water and pretreatment 4pressure losses, barConc entrate discharge pressure, bar 0.5Permeate pumping, bar I0.0Auxiliary equip., kWh/m3 0.05

Low efficiency pump + H ig h fficiency pump + High pressure pump +Pelton wheel Peltonwheel pressure exchanger82 88 8882 88 94

96 9698 984 40.5 0.510.0 10.00.05 0.05

e x c h a n g e r a 3 % f e e d s a l i n it y i n c r e a s e w a sap p l ied . Th e r e su l t s i n F ig . 6 sh o w th e e f f ec t o fp u m p i n g e q u i p m e n t e f f i c i e n c y a n d r e c o v e r y r a teo n p o w e r c o n s u m p ti o n . T h e p o w e r c o n s u m p t i o nd e c r e a s e s w i t h a n i n c r e a s e o f p u m p a n d m o t o re f f i ci e n c y , a n d t h e m i n i m u m s h i ft s t o a l o w e rr eco v e r y r a t e . I t is ev id en t t h a t i n r e sp ec t o fp o w e r c o n s u m p t i o n a l o n e , p r e s s u r e e x c h a n g e re q u i p m e n t h a s a s i g n i fi c a n t a d v a n t a g e o v e r t h eco n v en t io n a l co n f ig u r a t io n , i . e . , a h ig h - p r e ssu r ep u m p c o m b i n e d w i th a P e l to n w h e e l p o w e rr eco v e r y u n i t. Ad d i t io n a l ly , o p e r a t io n a t l o werr eco v e r y wi l l re su l t in im p r o v ed p e r m ea te q u a l i t y .D e c r e a s i n g r e c o v e r y r a te f r o m 4 5 % t o 3 5 % w i l lr e su l t in a d ec r ea se o f p e r m ea te sa l in i ty b y 1 0%.Ho wev e r , i t sh o u ld b e n o ted th a t o p e r a t io n a t al o w e r r e c o v e r y r a t e w i l l i n c r e a s e t h e p r e t r e a t m e n tcost .

2.2. RO unit configurationC o n f i g u r a t io n o f t h e m e m b r a n e u n i t h a s a

s i g n if i c an t e f f e c t o n p e r f o r m a n c e a n d e c o n o m i c so f t h e R O p lan t. Th e u su a l d e s ig n co n s id e r a t io n sin c lu d e th e a r r ay : sh o u ld seawa te r R O t r a in s b eco n f ig u r ed a s a s in g le - o r two - s t ag e u n i t s? Ho w

m a n y e l e m e n t s p e r p r e s s u r e v e s s e l s h o u l d b eu sed ? I f a p a r t i a l two - p ass p r o ce ss in g i s r eq u i r ed ,h o w i t s h o u l d b e c o n f i g u r e d ?

in th e p a s t , t h e seawa te r u n i t s we r e u su a l lyc o n f i g u r e d a s t w o - s t a g e , s i x - e l e m e n t s - p e r - v e s s e lu n i t s . Th e r a t io n a le b eh in d th e two - s t ag e d es ig ni s th a t i t r e su l t s i n a h ig h f e ed - co n cen t r a t e f l o w,w h i c h r e d u c e s c o n c e n t r a t i o n p o l ar i z at i o n. I n s e a -w a t e r R O s y s t e m s s c a l i n g i s n o t a r e c o v e r yl im i t in g f ac to r , b u t l o we r co n cen t r a t io n p o la r i za -t i o n wi l l r e su l t i n l o we r io n co n ce n t r a t io n a t t h em e m b r a n e s u r f a c e a n d , t h e r e f o r e , s o m e w h a tl o w e r p e r m e a t e s a l i n i t y . H o w e v e r , w i t h h i g h e rf e e d f l o w , h i g h e r f e e d p r e s s u r e i s r e q u i r e d d u e t oan in c r ea se d p r e ssu r e d r o p ac r o ss t h e R O t r a in s .D e s i g n e f f o r t s t o r e d u c e p o w e r c o n s u m p t i o n a n ds y s t e m c o s t r e s u l t e d i n t h e t r a n s i ti o n o f s e a w a t e rp l an t d e s ig n to a s in g le - s t ag e co n f ig u r a t io n an di n c r e a s e d th e n u m b e r o f e le m e n t s p e r v e s se l .

T h e m a j o r it y o f c u r r e n t s e a w a t e r R O s y s t e md e s i g n s a r e s i n g l e s ta g e w i t h s e v e n e l e m e n t s p e rv e s s e l . S o m e e v e n v e r y l a r g e s y s t e m s a r ed e s i g n e d a n d o p e r a t e d w i t h e i g h t e l e m e n t s p e rv esse l . Th e r e i s an o b v io u s co s t ad v an tag e in th ei n c r e a s e d n u m b e r o f e l e m e n t s p e r v e s se l . A R Os y s t e m u s i n g s i x e l e m e n t s p e r v e s s e l w i l l r e q u ir e

8/2/2019 1-s2.0-S0011916404006800-main

6/12

6 M. W ilf C. Bartels/Desalination 173 (2005) 1-123 4 % m o r e p r e s s u r e v e s s e l s t h a n a s y s t e m u s i n gt h e s a m e m e m b r a n e a r e a b u t c o n f i g u r e d w i t he i g h t e le m e n t s p e r v e s se l . C o m p a r i n g t h e a b o v eco n f ig u r a t io n s , t h e c r o ss sy s t em p r e ssu r e d r o p ina s in g le - s tag e u n i t w i ll b e o n ly 1 .1 b a r co m p ar edto 3 .4 b a r f o r t h e two - s t ag e u n i t , r e su l t i n g in a2 . 5 % h i g h e r p o w e r r e q u i r e m e n t o f t h e l a tt e r c o n -f ig u r a tio n . I n th e p a s t t h e e ig h t - e l em en t , s in g le -p ass co n f ig u r a t io n was c r i t i c i zed b ecau se i tr e su l t ed in u n ev en f lu x d i s t rib u t io n : l e ad e l em en t so p e r a t in g a t v e r y h ig h f lu x , wh ich m ay r e su l t i ne x c e s s i v e fo u l in g . H o w e v e r , e x a m i n i n g t h e f e e dsa l in i ty an d p r e ssu r e d i s t r ib u t io n , i t is ev id e n t t h a tth e f lu x d i f f e r en ce b e tw een th e l e ad an d t a i lp o s i t i o n in a s in g le - s t ag e sy s t e m i s l o we r th an ina t w o - s t a g e s y s t e m o p e r a t i n g a t t h e s a m e r e c o v -e r y r a t e . An ad d i t i o n a l p r e ssu r e d r o p in a two -s t a ge s y s t e m r e s u l ts i n h i g h e r f e e d a n d l o w e r c o n -cen t r a t e p r e ssu r e a s co m p ar ed to th e s in g le - s t ag eco n f ig u r a t io n .

T h e o n l y p r a c t i c a l w a y t o i m p r o v e f l u xd i s t r ib u tio n in a seawa te r two - s t ag e s y s t em i s tou se an in t e r s t ag e b o o s t e r . Th e ab o v e so lu t io n i sso m e t im es ap p l i ed , b u t i t r e su l t s i n h ig h e r eq u ip -m e n t c o s t s w i t h o u t a n y s i g n i f i c a n t b e n e f i t s o fr e d u c e d p o w e r c o n s u m p t i o n .

2 . 3. O p t i m i z a t i o n o f t h e t wo -p a s s d es i g nR e c e n t c o m m e r c i a l s e a w a t e r m e m b r a n e e l e -

m e n t s h a v e b e e n c h a r a c t e r i z e d b y v e r y h i g h s a l tr e j ec t io n : 9 9 .7 - 9 9 .8 %. Fo r t h e sam e ap p l i ca tio n s ,d u e to h ig h f eed sa l in i ty o r t em p er a tu r e , a two -p ass p r o cess i s r eq u i r ed to p r o d u ce co n s i s t en t lyd es ig n p e r m ea te sa l in i ty . A two - p ass co n f ig u r a -t i o n i s a l so n ecessa r y f o r R O sy s t em s wi ths t r in g en t lim i t s o n p e r m e a te q u a l i t y , su ch a s v e r ylo w ch lo r id e co n cen t r a t io n o r l o w b o r o n l im i t s .F o r R O s y s t e m d e s i g n a s a f u l l t w o - p a s s c o n -f ig u r a t io n , a p a r t i a l two - p ass p r o cess m ay b es u f f ic i e n t d u r i n g t h e o p e r a t in g p e r i o d w h e n m e m -b r an es s t i ll m a in ta in su f f i c i en t ly h ig h r e j ec t io n o rd u r i n g t h e s e a s o n s o f l o w f e e d w a t e r t e m p e r a -tu r e s . Th e co n v en t io n a l p a r t i a l two - p ass sy s t em

o p e r a t e s i n t h e s a m e w a y a s a f u l l t w o - p a s sd es ig n . Pe r m ea te f r o m th e f i r s t p a ss i s co l l ec t edin a s to r ag e t an k , an d th e r eq u i r ed f r ac t io n i sp r o c e s s e d b y t h e s e c o n d p a s s . I t i s k n o w n t h a tp e r m e a t e s a l i n i t y a l o n g t h e s y s t e m i n c r e a s e sp a r a l l e l t o t h e in c r ea se o f f eed sa l in i ty . I t i sl o w e s t a t t h e f e e d e n d o f t h e p r e s s u r e v e s s e l a n dh ig h es t a t t h e co n c en t r a t e o u t l e t .

A ty p ica l p e r m ea te sa l in i ty d i s t ri b u t io n a lo n ga n e i g h t - e l e m e n t p r e s s u r e v e s s e l i s s h o w n i nF ig . 7 . I t is p o ss ib l e t o t ak e ad v an tag e o f t h i ssa l in i ty d i s t r i b u t io n b y co l l ec t in g p e r m ea te f r o ms e p a r a te s t a g e s o r f r o m b o t h e n d s o f p r e s su r ev e s s e ls i n a g i v e n m e m b r a n e s t ag e . T h i s p r o c e s sh a s b e e n p r o p o s e d i n t h e p a s t b y B r a y [ 1] .R e c e n t l y , i t h a s b e e n i m p l e m e n t e d i n a l a r g eseawa te r sy s t em [ 2 ] . I n t h i s co n f ig u r a t io n th eh ig h sa l in i ty fr ac t io n ( co l l ec t ed f r o m th e co n cen -

Table 3Conve ntional and s plit partial two-pass configuration o fRO systemsprocessing Mediterranean seawater or a totalpermeate capacity of 10,000 m3/dand CI concentration ncombined permeate of 100 ppmFirs t pas s Sec ond ass

Conventional design:Perm eate flow , m3/d 10,580 5,000Recovery ratio, % 50 90No. of pressure vessels 120 20No. of elements 960 160Feed pressure, bar 66.3 12.4Combined power 3.25requirement, kW h/m3Spl i t -part ia l design:Perm eate flow , m3/d 10,250 2,000Recovery ratio, % 50 90No. of pressure vessels 116 8No. of elements 928 64Feed pressure, bar 67.3 14.4Combined power 3.07requirement, kW h/m3

8/2/2019 1-s2.0-S0011916404006800-main

7/12

M. Wilf, C. Bartels / Desalination 173 (2005) 1-1 2 75000.

~ 3

0,

4500,4000,3500,3000,:ZOO0,15001000500,00 1 2 3

/ Indi,,idual6~O/n r~c . .n. . . . . . . . . jy

ComNr~4 5

8elementpoSit~nW

~ C o m b i n e d- ,I ,- , tndi',4dual

A Relative lux

Fract}on or 2 '~ Pass

Fig. 7. Salinity distribution from SWC elements in a pressure vessel.

Low salinitypermeate

High salinitypermeate

CombinedI. permeateLow salin ity~ .~ - Membrane ~ Membrane ~ " Nembrane ~ ~ i g h ~ialinitypermeate~"T[1 etement ~ element ~ elernent j~T]'r'---'P~Pe meate

ConcentrateFeed port portFig. 8. Flow diagram of a split partial two-pass RO system.

trate end) is processed with the second-pass ROunit and blended with the low sa l ini ty f rac t ion(col lec ted f rom the feed end) . A diagram of sucha sys tem is show n in Fig. 8. The advantage of thissplit partial, two-pass design is smaller permeatecapacity required from the first- and second-passuni t , a higher e ffec tive recovery ra te o f the com-

bined system and lower power consumption.Table 3 summarizes an example of such design.In this particular case the split partial system issmaller with 13% fewer elements and pressurevessels and 6.5% lower power consum ption thanthe equivalent conventional partial two-pass sys-tem configuration.

8/2/2019 1-s2.0-S0011916404006800-main

8/12

M. Wilf C. Bartels / Desalination 173 (2005) 1-122 .4 . F e e d w a t e r s u p p l y a n d t r e a tm e n t

A s e l e c ti o n o f s e a w a t e r s u p p l y s o u r c e s f o r R Od e s a l in a t i o n sy s t e m s d e p e n d s t o s o m e e x t e n d o ns i t e c o n d i t i o n s , b u t i s m a i n l y d e t e r m i n e d b ysys tem s ize . Fo r l a rge sys tem s the on ly v iab les o u r c e o f s e a w a t e r s u p p l y i s f r o m a n o p e n i n ta k e .The pos s ib le op t ions a re e i the r a s a s t and-a lones y s t e m o r o n e c o n t i g u o u s t o s o m e o t h e r l a r g eseawate r u se r such as a power p lan t . A supp lyf r o m a d e d i c a t e d i n t a k e u s u a l l y i m p l i e s a d e d i -ca ted ou t f a l l f ac i li ty as we l l . Cur ren t r egu la t ion sr e q u i r e c a r e f u l d e s i g n t h a t w i l l m i n i m i z e a n ypo ten t ia l env i ron m e n ta l e f f ec t [3] .

T h e l e n g t h y p r o c e s s f o r o b t a i n i n g p e r m i t s f o rc o n s t r u c t io n a n d o p e r a t i o n o f i n t a k e a n d o u t f a l lf ac i li t ie s m akes loc a t ion o f RO p lan t s ad jace n t too n - s h o r e p o w e r p l a n t s a v e r y c o n v e n i e n t s o lu t i o n .I n t h i s p r o c e s s c o n f i g u r a t i o n t h e R O s y s t e mu t i li z e s s e a w a t e r d i s c h a r g e d f r o m t h e h e a t r e j e c ts e c ti o n o f t h e p o w e r p l a n t a s a f e e d b e f o r e i tf lows to the ocean . In a s im i la r f ash ion , ROc o n c e n t r a t e is d i s c h a r g e d t o t h e s a m e l in e , d o w n -s t re a m o f th e f e e d u p t a k e . T h e t e m p e r a t u r e o fs e a w a t e r a t t h e o u t l e t f r o m t h e p o w e r p l a n t i su s u a l l y h i g h e r b y 3 - 5 C t h a n t h e w a t e r a t t h ei n t a k e . F o r t h e l o c a t i o n o f l o w s e a w a t e r t e m p -e r a t u r e , t h e t e m p e r a t u r e i n c r e a s e d u e t o p o w e rp lan t opera t ion i s benef ic ia l a s i t inc reasesm e m b r a n e p e r m e a b i l i t y , a n d R O s y s t e m c a no p e r a t e a t l o w e r f e e d p r e s s u r e . D u r i n g t h e p e r i o d so f h i g h s e a w a t e r t e m p e r a t u r e, a b o v e 3 0 C , f u r t h e ri n c r ea s e o f f e e d w a t e r t e m p e r a t u r e d o e s n o t r e s u l tin any s ign i f i can t dec rease o f f eed p res su re .D e p e n d i n g o n f e e d s a l i n it y a n d r e c o v e r y r a te i nt h e t em p e r a t ur e r a ng e o f 3 0 - 4 0 C , h i g h e r m e m -b r a n e p e r m e a b i l i t y a t h i g h e r t e m p e r a t u r e s i sa d v e r s e l y c o m p e n s a t e d f o r b y i n c r e a s e d o s m o t i cp res su re . H igher f eed wate r t em pera tu re r esu l t s inh ighe r s a lt pas sage ( sh ow n in F ig . 9 ) . I f th i sinc rease o f s a lt pas sage r equ i r es inc rea sed opera -t i o n o f t h e s e c o n d p a s s , h i g h e r f e e d w a t e r t e m p -e r a t u r e c a n a c t u a l l y r e s u l t i n h i g h e r p o w e rc o n s u m p t i o n o f t h e p l an t .

0 80 070 0G)

~; 500

~ | |looZoU_

db

I I; ;. , I I

I I " "

~( I ' ~ ,Ba r

I I T D S

00 5 10 15 20 25 30 35 40 45

F e e d te mp e ra tu re , CFig. 9. Temperature effec t on seawater RO sys tems.

L o c a t i n g th e R O s y s t e m c o n t i g u o u s t o ap o w e r p l a n t m a y r e s u l t i n s o m e f e e d w a t e r q u a li t yp r o b l e m s . I t is c o m m o n p r a c t i c e o f p o w e r p l a n tsto ch lo r ina te the in take s t ruc tu re in te rm i t t en t ly tor e d u c e b i o g r o w t h . A n a d d i t i o n a l p e r io d i c e v e n tt h a t m a y a f f e c t s e a w a t e r q u a l i t y i s c l e a n i n g o f t h eh e a t t r a n s f e r s u r f a c e s o f t h e c o n d e n s e r . A s aresu l t o f c lean ing , sm al l pa r t i c le f r agm en ts a r er e l e a se d t o t h e c o o l i n g s e a w a t e r t h a t c o u l d e n d u pi n th e R O f e e d. B o t h p e r i o d i c e v e n t s a t t h e p o w e rp l a n ts , i n t a k e c h l o r i n a t i o n a n d c l e a n i n g o f t h ec o n d e n s e r h e a t e x c h a n g e s u r f a c e s , s h o u l d b ea d d r e s s e d i n R O s y s t e m d e s i g n a n d o p e r a t i o n top r e v e n t p o te n t ia l m e m b r a n e d a m a g e .

T h u s f a r, l a r g e s e a w a t e r R O p l a n t s h a v e b e e nb u i l t e x c l u s i v e l y w i t h f e e d w a t e r p r e t r e a t m e n ttha t inc ludes m e d ia f i l t r a tion to r em ov e co l lo ida lp a r t i c l e s . T h e p e r f o r m a n c e r e c o r d o f t h i s a p -p r o a c h , w i t h f e w e x c e p t i o n s , i s m a i n l y p o s i ti v e .H o w e v e r , a t s o m e l o c a t i o n s w i t h d i f fi c u l t f e e dwate r , m ed ia f i l t r a t ion , even w i th a two-s tageconf igu ra t ion , is no t su f f i c ien t to p roduc e s a ti s -f ac to ry qua l i ty f eed wate r . Th i s l eads to exces s ivem e m b r a n e f o u l i n g . F o u l i n g r e s u l t s i n r a p i di n c r ea s e o f p r e s s u r e d r o p a n d / o r d e c r e a s e o fm e m b r a n e p e r m e a b i l i t y . A s a r e s u lt , h i g h e r t ha nd e s i g n e d f e e d p r e s s u r e h a s t o b e a p p l i e d t op r o d u c e t h e r a t e d o u t p u t . F r e q u e n t m e m b r a n e

8/2/2019 1-s2.0-S0011916404006800-main

9/12

M . W i lf , C . B ar t e l s /D e sa l i na t i on 173 (2005) 1 -1 2 9c l e a n i n g r es u lt s i n a h i g h e r c o n s u m p t i o n o fc h e m i c a l s a n d d e c r e a s e o f t h e o n - l i n e p l a n tf ac to r . Fo r those p lan t s u sua l ly , f r eq uenc y o f ca r t-r idge f i l t e r r ep lac em en t inc reases as we l l . Th esec o n d i t i o n s m a y r e s u l t i n a s h a r p i n c r e a s e o f th eo p e r a t i n g co s ts . I n t h e B u i l d , O w n a n d O p e r a t et y p e o f c o m m e r c i a l a r r a n g e m e n t , s u c h a s i t u a ti o nm ay r esu l t in heavy f inanc ia l lo s ses to the p lan topera to r .

M e m b r a n e p r e t r e a t m e n t t e c h n o l o g y t h a t h a sbeen succes s fu l ly app l ied in l a rge - s ca le was te -w a t e r r e c l a m a t i o n s y s t e m s i s b e i n g e x t e n s i v e l ytes ted fo r s eawate r app l ica t ions . A la rge num bero f s e a w a t e r p i l o t s t u d i e s o f m i c r o f i l tr a t i o n ( M F )and u l t r a f i lt r a t ion (UF) have been c ond uc te d [5, 6 ]w i t h v e r y p o s i t i v e r es u l ts . R e c e n t l y t w o l a r g e R Oseawate r p lan t s (30 , 000 m 3/d and 125 , 000 m 3/d ),o n e o n t h e R e d S e a a n d a n o t h e r o n t h e M e d i t e r -r a n e a n, h a v e b e e n d e s i g n e d a n d w i l l b e b u i l t w i t hU F / M F p r e tr e a tm e n t . T h e i m p e d i m e n t o f w id e -s p r e a d i m p l e m e n t a t i o n o fM F / - U F p r e t r e a t m e n t i nR O s e a w a t e r s y s t e m s i s e q u i p m e n t c os t , w h i c h i ss ti ll h i g h e r t h a n m e d i a f il t ra t io n . H o w e v e r , m e m -b r a n e p r e t r e a t m e n t s y s t e m p r i c e s a r e d e c r e a s in g ,and i f , in add i t ion to tw o-s tage f i l tr a t ion , ano therp r e t r e a tm e n t s te p i s r e q u i r e d , M F / U F t e c h n o l o g ym ay a l r eady be a cos t - e f f ec t ive a l t e rna t ive . Them a j o r a d v a n t a g e o f m e m b r a n e p r e t r e a t m e n t is t h e

e x i s t e n c e o f a b a r r i e r t h a t h a s c o m p l e t e re j e c t io nof wa te r -bo rne co l lo ida l pa r t i c les tha t cou ldo t h e r w i s e f o u l t h e m e m b r a n e s u r f a c e o r b l o c km e m b r a n e e l e m e n t fe e d c h a n n el s . M F / U F m e m -b r a n e t e c h n o l o g y c o u l d b e p r e s su r e o r v a c u u mdr iven , and i t can opera te r e l i ab ly a t a low d r iv ingpres su re , in a w id e r ange o f r aw wate r qua l i t i e s.

T h e s c h e m a t i c d i a g r a m o f a R O s y s t e m i n c o r -p o r a t i n g s u b m e r s i b l e , v a c u u m - d r i v e n , m e m b r a n epre t r ea tm en t i s shown in F ig . 10 . A r ecen t p i lo ts t u d y , w h i c h i n c l u d e d p a r a l le l o p e r a t i o n o f s u b -m e r s i b l e m e m b r a n e f i lt r a ti o n a n d c o n v e n t i o n a l ,t w o - s t a g e , d u a l - m e d i a f i l tr a t io n ( D M F ) , d e m o n -s t ra t e d t h a t t h e p e r f o r m a n c e o f m e m b r a n e f il tr a -t i o n is m o r e r e l ia b l e t h a n a D M F s y s t e m d u r i n gt h e p e r i o d s o f p o o r q u a l i t y s e a w a t e r . A l s o , t h em e m b r a n e f i l t ra t i o n s y s t e m i s c a p a b le o f m a i n -t a i n i n g a s t a b l e c a p a c i t y a n d p r o d u c e g o o d -q u a l i ty e f f l u e n t u t il i z i n g a m u c h l o w e r q u a n t i t y o fc h e m i c a l s c o m p a r e d t o a c o n v e n t i o n a l f i lt r at io ns y s t e m .

T a b l e 4 p r o v i d e s a c o m p a r i s o n o f f il tr a te c a p a -c i t y b e t w e e n c o n v e n t i o n a l g r a v i t y f i l t e r s a n d as u b m e r s i b l e m e m b r a n e u n i t ( H y d r a s u bTM,H y d r a -nau t ics ) , in s ta l l ed in the s a m e foo tp r in t a s a p re -t r ea tm en t fo r a l a rge s eawate r RO p lan t . Tab le 4i l lu s tr a tes tha t even in co m p ar i so n to s ing le - s tageD M F f i lt r a ti o n , m e m b r a n e p r e t r e a t m e n t h a s a

0.4 barP o w e r c o n s u m p t i o n 2 . 4 2 k W h r / m 3

( 9 .2 k W h r l k g a l l o n ) 2bar

o.3

62

b a r6 0bar

2 bar ~ 1 bar

I n t a k e M e m b r a n epretreatmant Clearwell Pressure exchanger

Fig. 10. Seawater RO system with work exchanger.

8/2/2019 1-s2.0-S0011916404006800-main

10/12

10 M. Wi lf , C . B ar re l s / D esa l i n a t i on 173 (2005) 1 - 12T a b l e 4C o m p a r i s o n o f a s u b m e r s i b l e M F m e m b r a n e f i l tr a t io n u n i t w i t h a g r a v i t y m e d i a f i l t r a ti o n s y s t e m . G r a v i t y f i l t e r fo o t p r i n t3 , 7 0 0 m 2 , f i l t r a t io n a re a 2 , 3 0 0 m 2G ra v i ty f i l t e ra v e ra g e f i l t r a t io nr a t e, m 3 / m L h

G ra v i ty f i l t e r n e tcapacity, single-stagef iltration, m3/d

G ra v i ty f i l t e r n e tc a p a c i ty , tw o -s t a g efi l t ra t ion , m3/d

H y d r a s u bTM n e tcapacity @ 17 lmhfiltration rate, m3/d

Capac ity ratio:HydrasubVM/twostage gravity filters4.9 270,000 135,0007.4 401,000 201,0009.9 534,000 267,000

12.3 666,000 333,00014.8 799,000 399,000

h ig h e r cap a c i ty p e r u n i t o f f i l t r a ti o n sy s t e m a r ea .F o r l o c a t io n s w i t h p o o r s e a w a t e r q u a l i t y w h e r e atwo - s t ag e d u a l m e d ia f i l t r a ti o n i s r eq u i r ed , m em -b r a n e p r e t r e a t m e n t w o u l d h a v e a d e f i n i t e f o o t -p r in t ad v an tag e .

2.5. Perm eate quali tyC u r r e n t r e q u i r e m e n t s f o r p e r m e a t e q u a l i t y

u su a l ly in c lu d e s t r in g en t sp ec i f i ca t io n s f o r an u m b e r o f c o n st i tu e n t s , i n c l u d i n g b o ro n . B e c a u s eb o r o n i s p o o r l y r e j e c t e d b y R O m e m b r a n e s a ta m b i e n t p H , s p e c i f ic a t i o n s f o r b o r o n c o n c e n -t r a t io n in p e r m ea te a f f ec t s sy s t em co n f ig u r a t io na n d t h e s c o p e o f t h e s e c o n d - p a s s t r e a t m e n t . An u m b e r o f p ro c e s s c o n f i g u r a t i o n s h a v e b e e np r o p o s e d t o a c h i e v e l o w b o r o n c o n c e n t r a t i o ne f f e c t i v e l y i n t h e p e r m e a t e [ 6] o f s e a w a t e r R Os y s t e m s . A m a j o r i t y o f th e s e i n v o l v e R O t r ea t -m e n t o f f ir s t- p a ss p e r m e a t e a t e l e v a t e d p H a n d / o ru t i l iz a t io n o f b o r o n - sp ec i f i c i o n ex ch an g e .

B o r o n r e j e c ti o n b y R O m e m b r a n e s i s a fu n c -t io n o f pH , c l o s e l y f o l l o w i n g t h e d i s s o c i a t i o nr a t io o f b o r i c ac id. F ig . 11 sh o w s th e d i s so c ia t io nr a t io o f b o r i c ac id to b o r a t e v s . p H. T h i s f i g u r ea l so sh o ws th a t t h e d i s so c ia t io n r a t io i n c r ea se swi th th e f eed sa l in i ty ( io n ic s t r en g th ) . W i thin c r ea sed sa l in i ty , t h e eq u iv a len t d i s so c ia t io nr a t io sh i f ts t o l o wer p H. A cco r d in g ly , a re l a t iv e lym i n o r i n c r e a s e o f p H o f s e a w a t e r f e e d w i ll r e s u lt

708,000 5.2708,000 3.5708,000 2.7708,000 2.1708,000 1.8

1 .0i 0 .9 ] ,o w s alinity / ~ _ ~

0.8 . . . . S e a w a te r . t ~ / - -, ' ~ 0 . 7~ ~'~ 0.40"50"6- - Concen trate/s/l/ts / 1!~ '~ 0 .3 " / 1e 0 . 2 / , , I I~ o . 1 " " I0.0 I6 7 8 9 10 11 12p H o f s o lu t i o nFig. 1 . B oron species distribution at 25C.

in s ig n i f ican t d ec r ea s e o f b o r o n p assag e . Tab le 5s u m m a r i z e s t h e r e s u l ts o f m o n i t o r i n g b o ro np a s s a g e i n a c o m m e r c i a l s e a w a t e r R O s y s t e m a sa f u n c t i o n o f f e e d p H . I n t h e f e e d p H r a n g e o f 7 -8 .8 , b o r o n p assag e d e c r ea s ed b y 5 0 %. N a tu r a lseaw a te r h a s a p H o f ab o u t 8 .1 an d lo w a lk a l in i tyo f a b o u t 1 4 0 - 1 6 0 p p m . T h e r e f o r e , a r e l a ti v e l ys m a l l q u a n t i ty o f N a O H is r e q u i r e d to i n c r e a s ep H th a t w o u ld r e su l t i n s ig n i f i can t b o r o n p assag er ed u c t io n .

2.6. M emb rane perform ance restorationI t i s a c o m m o n o c c u r r e n c e t h a t m e m b r a n e

p e r f o r m a n c e d e t e r i o r a t e s w i t h o p e r a t i n g t i m e .B o t h m e m b r a n e p e r m e a b i l i t y a n d s a l t r e j e c t i o nm a y d e c l i n e a t a r a te t h a t d e p e n d s m a i n l y o n f e e d

8/2/2019 1-s2.0-S0011916404006800-main

11/12

M. Wilf C. Bartels / Desalination 173 (2005) 1-12 11Table 5Boron rejection in a seawater RO system as a function offeed pHFeed pH System boron Relativerejection, % boron passage7.0 (acidified feed) 78.6 1.08.1 (no acid dosing) 81.6 0.868.3 (caustic addition) 83.5 0.778.6 (caustic addition) 86.6 0.638.8 (caustic addition) 89.6 0.49

w a t e r q u a l i t y . I n i t i a l l y m e m b r a n e p e r f o r m a n c ec a n b e r e s t o re d w i t h e f f e c t i v e c l e an i n g . H o w e v e r ,a f t e r p r o l o n g e d e x p o s u r e t o f o u l i n g c o n d i t i o n s ,p e r f o r m a n c e r e s t o r a t i o n t h r o u g h m e m b r a n ec l ean i n g i s l e s s e f f ec t i v e , an d t h e l i mi t s o f s y s t emp e r f o r m a n c e ( f e e d p r e s s u r e a n d / o r p e r m e a t e q u a l -i ty ) a re e x c e e d e d . T h e n , r e p l a c in g o l d m e m b r a n e sw i t h n e w e l e m e n t s i s t h e o n l y m e a n s t o r e s t o r es y s t e m p e r f o r m a n c e .

T a b l e 6 d e m o n s t r a t e s t h e e f f e c t o f m e m b r a n erep l acemen t o n p e rmea t e s a l i n i t y . I n t h i s p a r t i -c u la r c as e th e R O s y s t e m w i th n e w m e m b r a n e sp r o d u c e d a p e r m e a t e s a l i n it y o f 2 4 0 p p m . T h ecu r r en t p e rmea t e s a l i n i t y i s 5 0 0 p p m an d t h ed e s i g n p e r m e a t e s a l i n i t y i s 4 0 0 p p m . A s c a n b eex p ec t ed , t o r e s t o r e s a l t p a s s ag e t o t h e i n i t i a lv a l u e , al l m e m b r a n e e l e m e n t s i n t h e s y s t e m h a v et o b e r e p l a c e d . T o a c h i e v e d e s i g n p e r m e a t es a li ni ty v a l u e o f 4 0 0 p p m , 3 7 % o f m e m b r a n ee l e m e n t s w o u l d h a v e t o b e r e p l a c e d . T a b l e 7d e m o n s t r a t e s t h e e f fe c t o f m e m b r a n e r e p l a c e m e n to n s y s t e m c a p a c it y . T o i m p r o v e s y s t e m c a p a c i t yf r o m 7 0 % o f n o m i n a l f l o w t o 8 0 % ( d e s i g n v a l u e ) ,3 3 % o f m e m b r a n e e l e m e n t s h a v e t o b e r e p l a ce d .C o r r e c t io n f r o m 6 0 % t o 8 0 % w o u l d r e q u i r e 5 0 %e l e m e n t r e p l a c e m e n t . T h i s e x t e n s i v e e l e m e n tr e p l a c e m e n t c a n b e r e d u c e d i f t h e s y s t e m i sd e s i g n e d t o e n a b l e t h e a d d i ti o n o f e le m e n t s . T op e r f o r m t h e a b o v e c o r r e c ti o n s , 1 0 % a n d 2 0 % o fn e w m e m b r a n e e l e m e n t s w o u l d h a v e t o b e a d d ed ,r e s p e c ti v e ly . H o w e v e r , a s a r e s u lt o f m e m b r a n e

Table 6Membrane replacement schedule for salt rejectionrestoration. RO seaw ater system: Me diterranean seawater,40,000 ppm TDS, tem p. 29 C, recovery 45%, flux13.8 lmh, nom inal salt rejection of new elements 99.7%.Initial permeate salinity 240 ppm, design permeatesalinity 400 ppm , current permeate salinity 500 ppmTarget permeate System salt El em en ts o besalinity after passage, % replaced, %replacement, ppm240 0.43 100300 0.38 80400 0.71 37500 0.88 - -

Table 7Membrane replacement schedule for salt rejectionrestoration. RO seawater system, M editerranean seawater40,000 ppm TDS, tem p. 29 C, recovery 45%, flux13.8 Imh. Design flux decline 20%, actual flux decline30-40%Flu x Cu r r e n t T a rg et E l e m en t sorestor at ion permeate permeate be replacedmode capacity, % capacity, % or added, %Replacement 70 80 33Replacement 60 80 50Addition 70 80 10Addition 60 80 20

a d d i t i o n , m e m b r a n e a r e a i n c r e a s e s a n d s a l t p a s -s ag e w i l l in c r ea s e p ro p o r t i o n a l l y . T h i s e f f ec t o np e r f o r m a n c e h a s t o b e c o n s i d e r e d w h e n s e l e c ti n gt h e m o s t e f f e c t iv e m e t h o d o f s y s t e m p e r f o r m a n c eco r r ec t i o n .

2.7. Improved RO membranesR e c e n t a d v a n c e s i n m e m b r a n e t e c h n o l o g y

h a v e r e s u l t e d i n m o r e e f f i c i e n t p r o d u c t i o n o fp o t a b l e w a t e r f r o m s e a w a t e r . I n a ty p i c a l s e a w a t e rR O s y s te m , t h e o s m o t i c p r e ss u r e m a y v a r y f r o m

8/2/2019 1-s2.0-S0011916404006800-main

12/12

12 M. W ilf , C Bartels / Desalination 173 (2005) 1-1 23 4 t o 5 9 b a r f ro m t h e f ee d i n l e t t o t h e b r i n e o u t l e t .I n t h e p a s t 5 y ea r s , i t w o u l d b e r ea s o n ab l e t o u s ea b o u t 6 8 b a r f e e d p r e s s u r e t o a c h i e v e 1 3 . 6 l m hf l u x a t 2 5 C o n 4 0 , 0 0 0 m g / L s e a w a t e r . T h u s , o na v e r a g e , a b o u t 21 b a r w a s u s e d t o p e r m e a t e w a t e rt h r o u g h t h e m e m b r a n e a n d 4 7 b a r w a s u s e d t oo v e r c o m e o s m o t i c p r e s s u r e . R e c e n t i m p r o v e -m e n t s i n s e a w a t e r m e m b r a n e p r o d u c t s h a v ere s u l t ed i n f eed p r e s s u re s i n t h e r an g e o f 6 5 b a rf o r t h e s a m e d e s i g n , w h i c h i s a 4 . 5 % o v e r a l lr e d u c t io n i n p r e s su r e . H o w e v e r , t h e a c t u a l r e d u c -t i o n i n p r e s s u re r eq u i r ed fo r p e rmea t i o n h asd e c r e a s e d b y 1 4 % , a v e r y s u b s t a n t i a l i m p r o v e -m e n t . T h e a b o v e 3 b a r d e c r e a s e o f f e e d p r e s s u r ei s e q u i v a l e n t t o a p u m p i n g p o w e r r e d u c t i o n o fa b o u t 0 . 1 2 k W h / m 3, w h i c h i n l a r g e R O p l a n tsl ead s t o s i g n i f i c an t co s t s av i n g s . A t t h e s amet i m e , t h e i m p r o v e m e n t s i n s e a w a t e r R O m e m -b r a n e s h a v e l e d t o e v e n h i g h e r r e j e c t io n p r o d u c t s .M a n y v e n d o r s o f fe r s ea w a t e r m e m b r a n e s w h i c hh av e 9 9 . 7 % t o 9 9 . 8 % re j ec t i o n a t s t an d a rd t e s tco n d i t i o n s o f 3 2 , 0 0 0 mg /1 N aC I a t 8 0 0 p s i an d2 5 C . T h i s c o m b i n a t i o n o f i n c r e a se d w a t e r p e r m -eab i l i t y an d l o w er s a l t p a s s ag e h as c o n t r i b u t ed t ot h e r e d u c e d c o s t o f p o t a b l e w a t e r p r o d u c t i o n .

3 . C o n c l u s i o n sT h e c o s t o f se a w a t e r d e s a l i n a ti o n b y R O h a s

s i g n i f i c an t l y d ec rea s e d . Bo t h g r ea t e r co mp e t i t i o na n d i m p r o v e d t e c h n o l o g y h a v e c o n t r i b u t e d t o th i s

r ed u c t i o n i n d e s a l t ed w a t e r p r i ce s . T h e ma i nt e c h n o l o g i c a l i m p r o v e m e n t s h a v e c o m e f r o mo p t i m i z e d p r o c e s s d e s i g n a n d i m p r o v e d e q u i p -m e n t . P r o c e s s d e v e l o p m e n t s u c h a s t w o - p a s s ,s p l i t p a r t i a l p e rmea t e t r ea t men t , o n e - s t ag e a r r ayc o n f i g u ra t i o n , a n d h i g h p H s e a w a t e r f e e d f o rb o r o n r e m o v a l h a v e p r o v e n t o b e c o s t e f f e c ti v e .F i n a l l y , d e s a l i n a t i o n co s t s h av e d ec l i n ed d u e t oh i g h e r e f f i c i e n c y e n e r g y r e c o v e r y d e v i c e s a n dh i g h e r p e r m e a b i l i t y h i g h r e j e c t io n m e m b r a n e s .

R e f e r e n c e s[1] D .T . Bray , U.S . Patent 4,046,685, 1977.[2] L. Stevens, J. Kowal, K. He rd, M. Wilf and W.Bates, Tampa Bay seawater desalination facility:

start to finish, Proc. IDA Congress, Baham as, 2003.[3] H. Iwahori, M. Ando, R. Nakahara, M. Furuichi,

S. Tawata and T. Y amazato, Seven year operationand environmental aspects of 40 ,000 m3/d seawaterRO plant at Okinawa, Japan, Proc. IDA Congress,Bahamas, 2003.

[4] P. Glueckstern, M. Prie l and M. W ilf, Field evalu-ation of capillary UF technology as a pretreatmentfor large seawater RO systems, Proc. EDS Con-ference, Toulouse, 2002).

[5] G. Pearce, J. Allan and K. Chida, Ultrafiltration pre-treatment at Kindasa water services, Jeddah, SaudiArabia, Proc. IDA Con gress, Bahamas, 2003.

[6] P. Glueck stern and M. Priel, Optim ization o f boronremoval in old and new SWRO systems, Desali-nation, 156 (2003) 219-228 .