Hilan N., Al-Zoubi H., Darwish N.a.-a Comprehensive Review of Nanofiltration Membranes_ Treatment,...

8/20/2019 Hilan N., Al-Zoubi H., Darwish N.a.-a Comprehensive Review of Nanofiltration Membranes_ Treatment, Pretreatm…

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ELSEVIER

Desalination 170 (2004) 281-308

DES LIN TION

www.elsevier.com/locate/desal

A com prehen sive review o f nanofil tration mem branes:

Treatment pretreatm ent m odel l ing and atomic force m icroscopy

N . H i l a l a , H . A 1 - Z o u b P , N . A . D a r w i s h b, A . W . M o h a m m a d c, M . A b u A r a b i d

aAdvanced Water Treatment Group, School of Chemical, Environmental an d Min ing Engineering,

The U niversity of Nottingham, NG 7 2RD , United Kingdom

Tel. +44 (115) 951-4168; Fax: +44 (115) 951-4115; em ail: [email protected]

t Department of Ch emical Engineering, College of Engineering, Jordan University of Science and Technology,

PO Box 3030, Irbid, Jordan -

CDepartment o f Chem ical a nd P rocess Engineering, Universiti Kebangsaan Malaysia,

43600 UK M Bangi, Selangor Darul Ehsan, Malaysia

d he

Middle Eas t Desalination Research Centre, PO B ox 21, Al Khuwair, Postal Code 1333, Oman

Rece ived 12 No vem ber 2003; accepted 5 January 2004

b s t r a c t

N an o f i lt r a tio n mem b r an es ( N F) h av e ap p l ica tio n s in sev er a l a r eas. O n e o f th e main ap p l ica tio n s h as b een in w a te r

t r ea tmen t f o r d r in k in g w a te r p r o d u c t io n as w e l l a s w as tew ate r t r ea tmen t . N F can e i th e r b e u sed to t r ea t a ll k in d s o f

wate r including ground, sur face, and w astew ater or used as a pretreatment for desal inat ion . The in troduction of NF as

a p r e t r ea tmen t i s co n s id e r ed a b r eak th r o u g h f o r th e d esa l in a tio n p ro cess . N F mem b r an es h av e b een sh o w n to b e ab le

to remove turb id i ty , microorganisms and hardness , as well as a f ract ion of the d isso lved sal ts . This resu l ts in a

s ignif ican tly lower operat ing pressure and thus provides a much more energy-ef f ic ien t process . S imilar to o ther

mem b r an e p r o cesses , a m a jo r p r o b lem in N F mem b r an e ap p l ica tio n s i s f o u lin g . Sev er a l s tu d ies h av e in v es t ig a ted th e

meeh an is rn s o f f o u lin g in N F mem b r an es an d su g g es ted meth o d s to min imize an d co n t r o l th e f o u l in g o f N F m em -

b r an es. Fo r N F mem b r an e ch ar ac te r iza t io n s an d p r o cess p red ic tio n , mo d e l in g o f N F p r o cesses an d th e u se o f a to mic

f o r ce mic r o sco p y ( A FM) a r e v e r y im p o r tant . Th e ab i l ity to p red ic t th e p e r f o r man ce o f N F p r o cesses w i l l l ead to a lo w er

n u mb e r o f ex p er imen ts , s av in g o f t ime an d mo n ey , an d h e lp to u n d er s tan d th e sep ar a t io n mech a n isms d u r in g N F . A

co mp r eh en s iv e r ev iew o f N F in w a te r t r ea tmen ts i s p resen ted in c lud in g a r ev iew o f th e ap p l ica t io n s o f N F in t r ea tin g

w ate r a s w e l l a s in th e p r e t r ea tmen t p r o cess f o r desal in a tion ; th e mech a n ism as w e l l a s m in imiza t io n o f N F mem b r an e

fouling problem s; and theor ies for modell ing and transpor t o f salt , charged an d noncharge d organic co mpo unds in NF

mem b r an es . Th e r ev iew w i l l a l so ad d r ess th e ap p l ica t io n o f A FM in s tu d y in g th e mo r p h o lo g y o f memb r an e su r f aces

as p a r t o f th e N F mem b r an e ch ar ac te r iza t ion .

Keywords:

Nanof i l t ra t ion ; Pretreatment; Fouling; Modeling; Atomic force microscopy; Desal inat ion

*Con'esponding author.

0011-.9164/04/ - Se e front matter © 2004 Elsevie r B.V. All f ights reserved

doi: 10.1016/j.desal.2004.01.007


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282 N. Hilal e t al . / Desalinat ion 170 2004) 281 -308

1 I n t r o d u c t i o n

The nanof i l tr a t ion (NF) me mb rane i s a type o f

p ressu re -d r iven membrane wi th p roper t ie s in

be tween reve rse osmo s is (RO ) and u l t ra f il t ra t ion

(UF) mem branes . NF o f fe r s seve ral advan tages

such as low operat ion pressure , h igh f lux, h igh

re ten tion o f mu l t iva len t an ion sa lt s and an o r -

gan ic molecu la r above 300 , re la t ive ly low inves t-

me n t and low opera t ion and m ain tenance cost s.

Because o f these advan tages , the app l ica t ions o f

NF wor ldw ide have inc reased [ 1 . T he h i s to ry o f

NF da tes back to the 1970s w hen RO mem branes

with a reasonable water f lux operat ing a t re la-

t ive ly low p ressu res were deve loped . Hence , the

high pressures t radi t ional ly used in RO resul ted

in a cons ide rab le ene rgy cos t . Thus , mem branes

wi th low er re jec t ions o f d i s so lved compon en ts ,

bu t wi th h igher wa te r pe rmeab i l i ty , wo u ld be a

g rea t improve men t fo r sepa ra tion techno logy .

Such low-pressu re RO membranes became

k n o w n a s N F m e m b r a n e s [2 ]. B y t h e s e c o n d h a l f

o f the 1980s, NF had be com e es tab li shed , and the

first application s w ere rep orte d [3,4].

Th is paper rev iews seve ra l a spec ts o f NF

mem branes such as the app l icat ions in t rea tmen t

of ground, surface , and w astewater ; as a pre treat-

m ent for desal inat ion, foul ing, and the separat ion

m e c h a n is m ; a n d m o d e l l i n g o f N F m e m b r a n e s . I n

add i tion , the app l ica t ion o f a tomic fo rce mic ros -

c o p y ( A F M ) i n s t u d y i n g th e m o r p h o l o g y o f t h e

NF mem brane sur faces wi l l a l so be rev iewed .

2 N F m e m b r a n e s i n w a t e r t r ea t m e n t s

In add i t ion to hav ing the capab i l i ty to reduce

the ion ic s treng th o f the so lu t ion , NF mem branes

can remove hardness , organics and par t icula te

con taminants . Man y resea rche rs have u sed NF to

ach ieve those ob jec t ives and the fo l lowing

sec t ions h igh l igh t the i r m a jo r f ind ings.

2 .1 . G r o u n d w a t e r

The so f ten ing o f g round wa te r us ing NF has

been s tud ied by m any inves t iga to rs [4 -8 ]. Schaep

e t a l. [4] used seve ra l d i f fe ren t types o f NF me m-

branes fo r ha rdness remova l . R e ten t ions h igher

than 90 were found fo r mul t iva len t ions,

whereas mo nova len t ions were re ta ined fo r abou t

60-70 . Som bekke e t a l. [5 ] comp ared NF

membranes wi th pe l le t so f ten ing and g ranu la r

ac t iva ted ca rbon fo r so f ten ing wa te r. B o th me th -

ods gave good resu l ts ; howeve r , NF mem branes

h a v e s e v e ra l a d v a n ta g e s f r o m t h e p o i n t o f v i e w o f

heal th aspects and low er investm ent costs .

Rem ova l o f na tu ra l o rgan ic ma t te r (NOM ) and

d is in fec t ion by-p roduc t (D BP) p recurso rs were

also s tudied [9-16] . Ja cang elo e t a l . [11] reported

tha t bo th RO and NF a re used in the US fo r

r e m o v a l o f N O M . To d a t e , N F h a s b e e n em -

p loyed p r imar i ly fo r g roundwate r con ta in ing

re la t ive ly low to tal d i s so lved so l ids bu t w i th h igh

tota l hardness , color , and DB P precursors . Esco-

bar e t a l . [12] found that the re je ct ion of ass imi-

lab le o rgan ic ca rbon was g rea te r than 90 a t

pH 7.5 and greater than 75 a t pH 5.5 by using

TFC-S NF membranes .

The remova l o f pes t ic ides and some mic ro -

po l lu tan ts f rom groun d wa te r has bee n s tud ied by

many resea rche rs [17 -25] . Van de r Brugg en e t a l.

[17 ] show ed tha t the NF70 mem brane can re jec t

pest ic ides such as a trazine , s imazine , d iuron and

isopro tu ron over 90 . S im i la rly , r emova l o f

t r ich lo roe thy lene and te t rach lo roe thy lene can be

ach ieved us ing seve ra l d i f fe ren t types o f NF

mem branes [25 ].

Ke t tunen and Kesk i ta lo [26 ] s tud ied the

remova l o f f luo r ide and a lum inum f rom ground-

water sources in Finland. In addit ion to the

rem oval of hardness , n i t ra tes , i ron and s tront ium,

f luo r ide remov a l was a l so m en t ione d by Pe rvov

e t a l. [27 ]. T he rem ova l o f d i s so lved u ran ium

from na tu ra l wa te rs has been condu c ted by Ra f t

and Wi lken [28] . The mul t iva len t an ion com-

p lexes o f u ran ium were re jec ted to abou t 95 .

F ina lly , a r sen ic remova l and the e f fec t o f pH in

the re jec t ion p rocess w ere s tud ied by U rase e t a l .

[29 ]. I t w as show n tha t the re jec t ion o f a r sen ite


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N. Hil al et al. / Desalination 170 2004) 281-30 8

283

A s ( I l l ) i n c r e as e d w i t h t h e i n c r e a s e o f p H f r o m

5 0 a t p H 3 t o 8 9 a t p H 1 0 , w h i le t h e d e g r e e o f

inc rease 'w i th As(V ) was sma l l e r ; f rom 87 a t

pH 3 to 93 a t pH 10.

2 . 2 . S u r f i t c e w a te r

Surface wa te rs o f t en have a chang in g chemis -

t ry o r compos i t i on due to seasona l changes o r

after d i lu t ion wi th ra in . NF is a re l iable opt ion for

su r face wa te r t rea tmen t , a l t hough the focus i s

he re ra the r on remov a l o f o rgan ics t han on so f t-

en ing [21] . Hardness rem ova l f rom a l ake i n

Ta iwan has bee n s tud i ed by Y eh e t a l . [30 ] u s ing

d i f fe ren t me thods such as a co nven t iona l p rocess

fo l lowed by ozo ne , GAC and pe l l e t so f ten ing ,

a n d a n i n t e g r at e d m e m b r a n e p r o c e s s ( U F / N F ) a n d

the conven t iona l p rocess . So f t en ing was ach i eved

b y a l l p ro c e s se s , b u t w a t e r p r o d u c e d b y t h e m e m -

brane p rocess had the bes t qua l i ty a s measu red by

tu rb id i ty (0 .03 NTU) , t o t a l ha rdness re j ec t i on

(90 ) , and d i sso lved o rgan ics re j ec t i on (75 ) .

R e m o v a l o f D B P p r e c u r so r s a n d N O M f r o m

s u rf a c e w a t e r s o u r c e s h a s b e e n s t u d ie d b y m a n y

invest igators [31-35]. Visvanathan e t a l . [32]

s tud i ed the e f fec t o f t he p re sence o f i ons , ope ra t -

i ng p ressu re , feed , pH, and suspen ded so l id s on

re j ec t ion p roper t i e s , and conc luded tha t h ighe r

p ressu re and suspended so l id s con ten t i nc rease

re j ec ti on , wh i l e re j ec t i ons a re som ewh a t l ower a t

h ighe r i on i c s t reng th . A l so , t hey conc lude tha t

b o t h h ig h a n d l o w p H o f t h e f e e d w a t e r p r o d u c e

low re j ec t i on , wh i l e be t t e r re j ec t i on has been

found a t a pH rang e o f 7 -9 , wh ich ind i ca t es t ha t

pH ad jus tmen t o f the feed m ay no t be requ i red .

R e m o v a l o f o r g a n i c m i c r o p o l l u ta n t s w a s a l so

invest igated by o ther researchers [36,37].

K o y u n c u a n d Y a z g a n [ 37 ] h a d g o o d r e s u l ts i n

t rea t i ng sa l t y and po l lu t ed wa te r f rom Kucuk-

c e k m e c e L a k e i n I s ta n b u l u s i n g t h e T F C - S N F

m e m b r a n e . N F m e m b r a n e s w e r e a l so f o u n d t o b e

ab le to rem ove v i ruses and ba c t e r i a f rom su r faces

qui te successful ly [11,38-41]. Reiss e t a l . [42]

u s e d a n i n t eg r a te d m e m b r a n e s y s t e m o f m i c r o -

f i l t ra t i on wi th NF to remove

B a c i l l u s s u b i l l u s

spores f rom 5 .4 t o 10 .7 l og . Us ing a nove l NF

m e m b r a n e ( N F 2 0 0 ), r e m o v a l o f o r g a n ic s ( u p t o

96 ) in a large NF plant in Paris [43 ,44] w as

a c h ie v e d . N F h a s b e e n a l so u s e d t o r e m o v e b o t h

a rsen i c (V) and a rsen i c ( I I I ) f rom su r face wa te r

[45 -48] . These s tud i es showed tha t (u s ing NF

membranes ) t he re j ec t i on o f As(V) i s re l a t i ve ly

h igh , up to 90 , wh i l e As( I I I ) is j u s t a round

30 .

An inves t i ga t ion o f t he re j ec t i on o f a lky l

p h t ha l a te s u s i n g f o u r t y p e s o f N F m e m b r a n e s w a s

cond uc ted by Ki so e t a l . [49 ]. M onov a len t i ons,

l i ke n i t ra t e s , how ever , we re found to have a l ower

e x t e n t o f r e j e ct i o n b y N F m e m b r a n e s [ 1 9 ,5 0 - 5 2] .

M ol ina r i e t al . [53] com pared re j ec t i on o f seve ra l

po l lu tan t s such as s i l ica , n i tra t e , manga nese and

h u m i c a c i d s b y m e a n s o f R O a n d N F m e m b r a n e s .

At a pH of 8 , RO rejec ted s i l ica , NO 3-, M n 2÷, and

hum ic ac id i n a ra t e equa l t o 98 , 94 , 99 and

9 5 .5 , r e sp e c t iv e l y , w h i l e N F h a d a m u c h l o w e r

reject ion , i .e ., 35 , 6 , 80 and 35 , respect -

i ve ly . Su lfa t e cou ld be rem ove d f rom ch lo ra lka l i

p l an t b r ine s t reams by NF [54 ] . The re su l t s

s h o w e d t h a t t h e N F m e m b r a n e s e f f e c ti v e ly re -

j ec t ed SO~- up to 95 in concen t ra t ed b r ines .

2 . 3 . W a s t e w a t e r

NF i s an e f f i c i en t and e co log ica l ly su it ed

t e c h n o l o g y f o r d e c o n t a m i n a t i o n a n d r e c y c l i n g o f

was t ew a te r genera t ed in man y indus t ri e s . Afonso

and Y af i ez [55] u sed N F to t rea t f i sh mea l was t e -

wa te rs . NF re duce d the o rgan ic l oad in t he was te -

wa te rs and p romoted the i r pa r t i a l desa l ina t ion ,

m a k i n g w a t e r r e u s e p o s s ib l e . A m a j o r p r o b l e m o f

the was t ewa te r t rea tmen t i s t he wa te r recovery

ra t e, wh ich shou ld be c lose t o 100 . To ach ieve

th i s t a rge t , many resea rche rs i nves t i ga t ed an

in t eg ra t ed membrane sys t em [56 -59] . Rau ten -

bach and Linn [56 ] and Rau tenbach e t a l . [57 ]

u s e d a n e w c o n c e p t o f i n t e g r a t e d m e m b r a n e s

c o n s i s t i n g o f R O / N F / h i g h - p r e s s u r e R O . T h e

in t eg ra t i on can ach i eve wa te r recovery ra t e s o f


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284 N . H i l a l e t a l . / D e s a l i n a t i o n 1 7 0 2 0 0 4 ) 2 8 1 - 3 0 8

mo re than 95 in t he case o f dum ps i t e l eacha t e ,

wh ich p romises an a lmos t ze ro d i sch a rge p rocess .

Hafiar le e t a l . [60] invest igated the removal of

c h r o m a te , f r o m w a t e r u s i n g a T F C - S N F m e m -

brane as a poss ib l e a l t e rna t ive to t he conven t iona l

m e t h o d s o f C r ( V I ) r e m o v a l f r o m a n a q u e o u s

so lu t ion . The re su l t s showed tha t t he re j ec t i on

depended on the i on i c s t reng th and pH. Be t t e r

re t en t ion was ob ta ined a t bas i c pH (up to 80 a t

a pH o f 8) . Resu lt s a l so show ed tha t NF i s a ve ry

p r o m i s in g m e t h o d o f t r e a t m e n t f o r w a s t e w a t e r

c h a r g e d w i t h h e x a v a l e n t c h r o m i u m . T h e s a m e

m e m b r a n e w a s u s e d b y K o y u n c u [ 6 1 ] t o t r e a t

op ium a lka lo id p rocess ing indus t ry e f f l uen t s .

App l i ca t i on o f N F in t he t rea tmen t o f t ex ti l e

w a s t e w a t er w a s c o n d u c t e d b y m a n y r e s e a rc h e r s

[62 -65] . Tang and Chen [62 ] u sed NF to t rea t

t e xt il e w a s t e w a t e r w h i c h w a s h i g h l y c o l o r e d w i t h

a h igh load ing o f i no rgan ic sa lt s. A t an ope ra t i ng

pressu re o f 500 kPa , t he f l ux ob ta ined was ve ry

h i g h , w h i l e t h e r e j e c ti o n o f d y e w a s 9 8 a n d t h e

NaC1 re j ec t i on was l e ss t han 14 . Thus , a h igh

q u a l it y o f r e u s e w a t e r c o u l d b e r e c o v e r e d . O n t h e

o the r hand , Vo ig t e t a l . [64 ] sugges t ed a new

TiO2-NF ce ramic membrane to deco lo r t ex t i l e

was t ewa te r u s ing an in t eg ra t ed p i lo t p l an . The

resu lt s show tha t i t i s poss ib l e t o c l ean co lo red

w a s t e w a t er w i t h d y e r e t e n ti o n v a r y i n g f r o m 7 0

to 100 , CO D reduc t ion o f 45 -80 , and sa lt

r e te n t io n o f 1 0 - 8 0 . A n e w c e r a m i c K - N F m e m -

brane was s tud i ed by Webar a t a l . [65 ] w i th

a lmos t t he same resu l t s a s t ha t o f t he T iO2-NF

m e m b r a n e .

Jakobs and Baum gar t en [66 ] show ed tha t NF

cou ld rem ove l ead up to 90 f rom a n i t r ic ac id

so lu tion . A l so , pu r i fi ca t i on o f phosph or i c ac id by

NF (D S5D L NF) was s tud i ed [67 ] u s ing d i ffe ren t

c o m m e r c i a l m e m b r a n e s o f c o m m o n i n d u s t r i a l

u se . The re su l t s show tha t t he re j ec t i on o f impur i -

t ies i s aroun d 99.2 .

3 N F a s p r e t r e a t m e n t f o r d e s a l i n a t i o n

Feed p re t rea tmen t i s one o f t he m a jo r fac to rs

de t e rmin ing the success o r fa i l u re o f a desa l i -

na t ion p rocess . D i f fe ren t me thods o fp re t rea tmen t

f o r d e s a li n a ti o n p r o c e s s w e r e s u g g e s t e d b y m a n y

resea rche rs . Trad i ti ona l p re t rea tmen t i s based on

mec han ica l t rea tmen t (med ia f i l te r s , ca r t r idge f i l -

t e rs ) suppor t ed by ex t ens ive chem ica l t rea tmen t

[68]. In the past , con ven t iona l pre t rea tmen t ( i.e . ,

coagu la t i on , f l occu la t i on , ac id t rea tmen t , pH

ad jus tmen t , add i t i on o f an t i - sea l an t and med ia -

f i lt ra ti on ) was usu a l ly u sed [69 ]. T he m a in p rob -

l em in us ing co nven t iona l p re t rea tmen t i s co r ro -

s ion and co r ros ion p roduc t s . Fo r ex ample , i n t he

ac id dos ing sys t em, co r ro s ion o f me ta l l ic su r faces

a n d c o r r o s i o n p r o d u c t s w i l l r o u g h e n t h e s u r f a c e

o f t he equ ipmen t , wh ich p rov ides ac t i ve s it e s fo r

precip i ta t ion o f m ore sca le depos i t s [69 ,70]. In

add i t ion , t h is p re t rea tmen t i s kno wn to be com-

p lex , l abo r i n t ens ive and space consu ming [71 ].

P ressu re -d r iven me m bran e p rocesses [mic ro -

f i l t ra t ion (MF), u l t raf i l t ra t ion (UF), and NF] are

the new t rends i n des ign ing a p re t rea tmen t sys -

t em. The in t roduc t ion o f NF as a p re t rea tmen t

wi l l l ead to a b reak th ro ugh in t he app l i ca t i on o f

RO and MS F becau se i t has imp l i ca t i ons fo r t he

desa l ina t ion p rocess i t se l f , and no t on ly on

quan t i t y o f t he feed wa te r [68 ]. M F can rem ove

suspended so l id s and low er t he s i lt dens i ty i ndex

(SDI) wh i l e i n UF , no t on ly suspen ded so l id s and

la rge bac t e r ia a re re t a ined , bu t a l so (d i s so lved )

macromolecu les , co l lo ids and smal l bac t e r i a .

Red ondo [72 ] s t rong ly p re fe r red M F o r UF as a

p re t rea tmen t ove r conven t iona l p re t rea tmen t t o

t reat d i ff icul t eff luent water .

Bou -Ha ma d e t a l. [73 ] t e s t ed t h ree p re trea t -

men t me thods (conven t iona l , a beachwel l sea -

wa te r i n t ake sys t em, and MF) p r io r t o RO.

A c c o r d i n g to t h e r e su l ts o f S D I , C O D a n d B O D

remova l and the t o t a l un i t cos t , t he beachwel l

seawate r in t ake and MF sys t ems we re i den t if i ed

as p romis ing t echn iques t o rep l ace conven t iona l

p re t rea tmen t . Ho we ver , a l l o f t he p re t rea tmen t s

m e n t i o n e d d i d n o t l o w e r t h e T D S v a l u e.

Severa l s t ud i es have a l so been c a r r i ed ou t on

the use o f UF m em bran e as p re t rea tmen t [70,71 ,


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N. Hi la l e t a l . / Desa l ina t ion 170 2004) 28 1- 30 8 285

74 ,75 ]. \ ran Hoo p e t a l . [74 ] con c luded tha t UF

can p rov ide a feed wa te r qua l i t y w i th a SDI

be low 2 , and opera t i ona l cos t s were v e ry a t t rac -

f ive . The same resu l t s were ob t a ined by A1-

A h m a d a n d A d b u l A l e e m [ 7 0 ] , w h o c o n c l u d e d

tha t the ho l low-f ibe r UF p roce ss p romises t o be a

s imple , easy - to -use , and robus t a l t e rna tive t o t he

c u m b e r s o m e c o n v e n t i o n a l f o rm s o f p r e t r e at m e n t .

T h e s u r f ac e s e a w a te r S D I o f 1 3 - 2 5 w a s r e d u c e d

be low 1 by UF , whereas conven t iona l p re t rea t -

men t fai l ed t o reduce i t be lo w 2 .5 [75 ] . The U F

pre t rea tmen t a l so p rov ide f i l te red wa te r w i th h igh

and cons t an t qua l i t y t ha t enhan ced the re l i ab i l i t y

o f t he RO desa l ina t ion p l an t . T eng e t a l. [76 ]

c o m p a r e d t h e u s e o f U F a n d M F a s a p r e tr e a t-

men t . The re su l ts show tha t t he h ighe r ope ra t i ng

f lu x o f t h e M F p i lo t s y s t e m w a s a c h i e v e d a t t h e

expense o f i nc reas ing t ransmembrane p ressu re ,

w h i c h h a d t o b e k e p t c o n s t an t . H o w e v e r , t he S D I

of t he f il tra t e p rod uced b y the U F p i lo t sys t em

was super io r t o t ha t p roduced by the MF p i lo t

sys t em. In genera l , the f i l tra t e qua l i t y o f t he M F

sys t em was in fe r io r t o tha t o f U F p i lo t sys t em.

Recen t ly , Va i l and Do ussau [77] co nduc ted a

p i lo t t e s t u s ing a new 0 .1 m M ic roza ho l low-

f i be r M F m e m b r a n e a s a p r e tr e a t m e n t to R O o n

Me di t e r ranean seawate r . A p i lo t r i g was op e ra t ed

at d i fferent f luxes ran ging fr om 80 to 140 1/h .m2

wi th and wi thou t fe r ri c ch lo r ide add i t i on . Dur ing

the test, t]hem i c r o f i lt e r w a t e r S D I r e m a i n e d b e l o w

2 under t he op t imized opera t i ng cond i t i ons .

L o n g e r r u n s b e t w e e n c h e m i c a l c l e a n i n g a n d

be t t e r f i l t e red wa te r qua l i t y were ob t a ined wi th

fe r ri c ch lo r ide add i ti on to t he raw seaw ate r and

b y p e r f o r m i n g b a c k w a s h e s w i t h s o d i u m h y p o -

c h lo r it e. A g a i n , t h e s a m e m e m b r a n e u n d e r w e n t

p i lo t te s ts a t t h ree d i f fe ren t l oca t ions on Med i t e r -

ranean seawate r [78 ] . The same resu l t s were

o b t a i n e d b y t h e m e m b r a n e , w h i c h p r o v i d e d

c l a r if i ed wa te r t o RO hav ing a S DI o f 1 .8 o r l e ss,

w h i c h a l l o w s R O o p e r a t io n a t h i g h e r r e c o v e r y ,

enhanc ing the t o ta l sys t em runn ing cos t .

The Casp ian Sea i n Russ i a i s cha rac t e r i sed by

h a v i n g l o w T D S b u t a r e l a t i v e l y h i g h b o r o n

c o n t e n t ( 4 - 5 p p m ) , w h i c h e x c e e d s t h e h y g i e n ic

s t andards in Russ i a (0 .4 -0 .5 ppm) . T o ach ieve the

need ed remov a l o f bo ron , P e rvov e t a l. [79 ]

sugges t ed a t ech no log ica l schem e inc lud ing c l a r i-

f i ca ti on , ac id and an t i sca l an t dosage as a p re t rea t-

men t , and doub le -s t age RO wi th caus t i c dos ing

into the f i rs t s tage produ ct .

NF was used fo r t he f i r st time by Hassan e t a l.

[80] a s p re t rea tm en t fo r seaw a te r reve rse osmos i s

(SWRO), mu l t i s t age f l a sh (MSF) , and seawate r

reve rse osmos i s re j ec t ed in mu l t i s t age f l a sh

( S W R O ~ m ~ - M S F ) p r o c es s e s . In t h e se in t e gr a te d

p r o c e ss e s , N F m i n i m i z e d h a r d n e s s , m i c r o o rg a n -

i sms and tu rb id i ty . A t a p ressu re o f 22 ba r , NF

r e m o v e d h a r d n e s s i o n s o f C a 2+, M g 2+, SO4 -,

HCO3, and to t a l ha rdness by 89 .6%, 94 .0%,

97 .8%, 76 .6% and 93 .3% , re spec t ive ly . In add i -

t io n , N F r e d u c e d t h e m o n o v a l e n t i on s o f C l+, N a÷,

and K÷; each ion was sepa ra t ed by 40 .3% and

overa l l seawa te r TDS by 57 .7%, w h i l e the t o ta l

h a r d n e s s w a s r e m o v e d b y 9 3 . 3 % . T h e p e r m e a t e

thus ob t a ined was fa r supe r io r t o seawa te r a s a

f e e d t o S W R O o r M S F . T h i s m a d e i t po s s ib l e to

opera t e a SWRO and MSF p i lo t p l an t a t a h igh

recovery : 70% a nd 80% , re spec t ive ly .

T h e i n te g r a ti o n o f N F w i t h M S F p r o c e s s e s

mak es i t poss ib l e t o ope ra t e MS F p l an t s on NF-

p r o d u c t

o r S W R r e j ~t e d

f r o m a N F - S W R O u n i t a t

a h igh d is t i l la t ion tem pera ture of 120°C to 160°C

wi th h igh d i s t i l la t e recovery , and a ga in wi thou t

c h e m i c a l a d d i t i o n . T h u s , M S F a n d N F -

S W R O ~ m ~ - M S F c o u l d b e o p e r a t e d a t a t o p

br ine t empera tu re o f 120°C wi thou t any sca l ing

prob lems [81 ] . These i n t eg ra t ed desa l ina t ion

s y s te m s , c o m b i n e d w i t h a r e d u c t i o n o f c h e m i c a l s

a n d e n e r g y , a l lo w s p r o d u c i n g f r e sh w a t e r f r o m

s e a w a t e r a t a 3 0 % l o w e r c o s t c o m p a r e d t o c o n -

ven t iona l SWR O [82] .

NF p re t rea tmen t o f seawa te r i n desa l ina t ion

p l an t s: ( 1 ) p r e v e n t e d S W R O m e m b r a n e f o u l in g

by the remo va l o f t u rb id i ty and bac t e r i a , (2 ) p re -

v e n t e d s c a l i n g ( b o t h i n S W R O a n d M S F ) b y

r e m o v a l o f s c a l e f o r m i n g h a r d n e s s i o n s a n d

( 3 ) l o w e r e d r e q u i re d p r e s s u re t o o p e r a te S W R O


6/28

286 N . H i l a l e t a l. / D esa l i na t i on 17 0 2004) 281 - 30 8

p lan ts by reduc ing seawate r feed TDS by 30-

6 0% , d e p e n d i n g o n t h e t y p e o f N F m e m b r a n e a n d

operat ing condit ions.

La te r, a dem ons t ra t ion p lan t was bu i l t a t Um m

Lujj , Saudi Arabia , consis t ing of s ix spira l-woun d

N F m o d u l e s ( 8 x 4 0 ) f o l l o w e d b y t h r e e S W RO

elements [83] . The resul ts obta ined from the

demons t ra t ion un i t con f i rmed the above resu lt s

obta ined from the pi lo t p lant s tudy. Criscuoali

and Dr io l i [84] co nc luded tha t the in t roduc t ion o f

NF fo r wa te r p re t rea tmen t leads to an improve-

me n t in the pe r fo rmance by m ain ta in ing a lmos t

invarian t the e ne rgy requ i remen t . M ohsen e t a l .

[85] invest igated NF to t reat brackish w ater col-

lec ted from Zarqa bas in , Jo rdan . The resu lt s show

than NF i s e f f ic ien t fo r reduc ing the o rgan ic and

inorgan ic con ten ts and g ives a h igh wa te r re -

covery up to 95%. P ontie e t a l. [86] s tudied the

poss ib il i ty o f ob ta in ing a pa r t ia l deminera l iza t ion

of seawate r us ing two success ive NF s tages. The

t reated wa te r ( sa lin i ty 9 g /L ) cou ld be used in the

f ie ld of hum an heal th ( i .e ., preparat ion o f nasal

sprays, m edical d ie te t ics and hot min eral springs) .

Recen t ly , seve ra l d i f fe ren t types o f in teg ra ted

mem branes were used as a p re t rea tmen t to desa l i -

nat ion. Driol i e t a l. [87] in trodu ced m em bran e

dis t i l la t ion (MD) to t reat the RO brine . This

enhanced the recovery fac to r o f RO . U s ing the

n e w l y i n t eg r a te d M F / U F + RO + M D , t h e p r e li m i -

na ry exper imen ta l re su lt s conf i rm ed the poss i -

b i l i ty o f reach ing a seawate r recovery fac to r o f

87%. Ohaa e t a l . [88] proposed an in tegrated

sys tem fo r the comple te usage o f compon en ts in

seawate r wi th a h igh recov ery o f wa te r up to 9 0%

as show n in F ig. 1 . An o the r in teg ra ted sys tem

(NF+RO +MC ) was sugges ted by Dr io l i e t al . [89]

where MC is a mem brane c rys ta ll i ze r . MC w as

used to ach ieve the to ta l recovery o f desa l ted

wate r. The p resence o f the NF in th i s in teg ra t ion

a l lowed an inc rease in the wa te r recove ry o f the

RO un i t up to 50%. M oreover , in t roduc t ion o f a

MC led to a 100% recovery and e l imina t ion o f

the b r ine d i sposa l p rob lem, rep resen t ing the pure

c rysta ls p rodu ced as a va luab le p roduc t. Th e m os t

impor tan t ma in tenance p rob lem assoc iated wi th

RO opera t ion is the m em brane fou l ing , e spec ia l ly

biological foul ing (biofouling) .

Visvanathafl e t a l . [90] used a membrane

b io reac to r (MB R) as a p re t rea tmen t p rocess to

e l imina te th i s b io fou l ing . M BR is the p rocess in

which a conven t iona l b io log ica l sys tem i s

coup led wi th the me mb rane p rocess (MF o r UF) .

The resu lt s show tha t the MB R sys tem produced

be t te r e f fluen t wi th 78% DO C rem ova l , and

the f i l t r a te y ie lded the h ighes t pe rmea te f lux

improvement , wh ich was approx imate ly 300%

compared wi th no n-p re t rea ted seawate r. A hybr id

NF/RO /MSF c rys ta ll i za tion sys tem of fe red ve ry

promis ing pe r fo rmance : h igh wa te r recovery

(77 .2%) and w a te r cos t s as low as $0 .37 /m3 were

sugges ted by Turek and Dyd o [91 ] .

4 M e m b r a n e fo u l in g

Membrane fou l ing i s caused by d isso lved

inorganics BaSO4, CaCO3) o r o rgan ic compo-

nen ts (hum ie ac ids ) , co l lo ids ( suspen ded pa rt i-

c les) , bacter ia or s uspe nde d sol ids [92,93] . Foul-

ing may o ccur in pores o f mem branes by pa r tia l

pore s ize reduc t ion caused by fou lan ts adsorb ing

on the inner pore wa l l s , po re b lockage and

surface fouling such as cake and gel layer for-

mat ion [94]. F ou l ing wi l l l ead to h igher opera -

t ion cos t s : h igher ene rgy demand , inc rease o f

c l e an i n g a n d r e d u c e d l i fe - t im e o f t h e m e m b r a n e

e lemen ts [95] . Ex is t ing fou l ing ind ices to mea-

su re the co l lo ida l fou l ing po ten tia l o f feedwate r

a re the SDI , app l ied wor ldw ide , and the Modi f ied

Fou l ing Index (M FI) . Bo th te s t s a re des igned to

s imula te membrane fou l ing by feed ing wa te r

th rough a 0 .45 /~m mic ro f i l t e r in dead-en d f low

[96].

The fo rmat ion o f a membrane fou l ing laye r

was m oni to red dur ing the t rea tmen t o f po llu ted

sur face wa te r us ing sp i ra l -woun d NF mem branes

[97]. The p lan t fee d was no t d i s in fec ted and


7/28

N. Hilal e t al . / Desalina t ion 170 2004) 281 -30 8 287

0 . 4

03BO

- ' - [ 5 O 5 l

( 4 . 0 8 ~ W a t e r o e ¢ ' ~

o . ~ / e

N a - ~ O 3

[ o . ~ l

o . 2 1 )

~ ,

< o . o o i

c c o 3 c c o 3

/ W a t c r / ~ ~ ] . H, " I - l , . , , ] ~

\ IHPRO I

0 0 6 6 7

I I

[ l e , O l , , I

. . v o M s o

M u l t i - i o n

( o s ~ )

o . w C r y s l ~ l i z o r i l , 9 5 × lO '-

A d s o r p t i o n 1

i

W t t e ¢ 0 . 1 0 8

s s × i o - S M F o r M E o o s s ~ . I

t ~ o / r , . . . . . . . : . ^ r - ~ 4 , ~ ' , q r - -

l , ~ s . o l ) ¢ 2 3 2 ~

( 1 o . 4 ) / r ( 0 7 7 )

s o ~

btjCI 0 .0248

M o n o - i o n

A d o s o r p t o n

Fig. 1 . Weight and com posit ion of each process s tream in the ease of 90% reco very of desal ted w ater, using an HPRO

system for NF scale remova l (based on I kg o f raw seawater). Total weight , kg; wa ter weight, kg; m ono and mult ivalent

ion, wt% (mo novale nt ion, wt%; m ult ivalent ion, wt%) [88].

d i s p la y e d a h i g h f o u l i n g te n d e n c y . T h e f o u l i n g

l a y e r w a s s h o w n t o b e a m i x t u r e o f b i o l o g ic a l ,

o r g a n i c a n d in o r g a n i c s p e c i e s . C a l c i u m a n d p h o s -

p h a t e w e, e t h e m a i n i n o r g a n i c c o m p o u n d s p r e -

s e n t w i t h m i n o r a m o u n t s o f i r o n a n d a l u m i n u m .

T h e h i g h l e v el o f p h o s p h o r u s f o u n d w a s p r o b a b l y

d u e t o th e u s e o f s o d i u m h e x a m e t a p h o s p h a t e a s a

p r e t r e a t m e n t a g e n t ( T a b l e 1 ).

B o e r l a g e e t a l. [ 9 6] d e v e l o p e d a M F I u s i n g U F

m o d u l e s a s a w a t e r q u a l i t y i n d i c a t o r a n d t o p r e -

d i c t f l u x d e c l i n e o c c u r r i n g i n m e m b r a n e f i lt r a ti o n

i n st a ll a ti o ns . T h e M F I - U F w a s f o u n d t o be a

p r o m i s i n g t o o l f o r m e a s u r i n g t h e c o l lo i d a l f o u l i n g

p o t e nt ia l f o r R O n a n o - a n d U F s y s te m s .

Table I

Concentration o f various elem ents in the fouling layer

[97]

E lemen t Concen tra tion #g /cm 2

mem brane area

Calcium 41.9

Phosphorus 32.2

Sulphur 2.6

Magnes ium 1 .6

Iron 17.5

Organ ic carbon 13.7


8/28

288 N. Hilal et al. / Desalination 170 2004) 281-308

T h e e x t e n t t o w h i c h U F m e m b r a n e s a r e

capab le o f re t a in ing co l lo ida l pa r t i c le s d epends

on the UF pore s i ze expressed as a m o lecu la r cu t-

o f f M W CO ), su r face mo rpho lo gy e .g ., po ros i t y )

and the na tu re o f t he me mb rane ma te r i a l e .g .,

hydroph i l i c it y ) . These U F m em brane p roper t i e s

a l so i n f luence the f i lt ra ti on mec han i sm s occu r r ing

during the tes t e .g . , cake an d bloc king f i l t ra tion) .

T h e r e f or e , th e i n f l u e n c e o f U F m e m b r a n e p r o p e r -

t ie s o n th e M F I - U F w a s e x a m i n e d t o e x p l ai n th e

occur rence o f f i lt ra ti on mec han i sm s in t he t e st .

T h e M F I - U F t e st m a y b e e q u a l l y a p p li e d f o r fl u x

dec l ine p red i c t i on and a d i f fe ren t i a l p ressu re

i n c re a s e i n N F a n d U F t o d e t e r m i n e th e b a c k w a s h

f requency o f a feed w a te r .

B i o f o u li n g - - d e f i n e d a s a c c u m u l a t io n o f

mic roo rgan i sms , i . e . , b io f i lm on a su r face by

growth and /o r depos i t i on a t such a l eve l t ha t

causes ope ra ti ona l p rob lem s - - i s d if f i cu l t t o

iden ti fy . I t may cause p rob lems such as dec l ined

normal i zed f l ux and /o r i nc reased no rmal i zed

pressu re d rops du r ing NF o r RO [93 ]. Co n t ro l l i ng

b io fou l ing can be ach i eve d by 1 ) fa r -reach ing

r e m o v a l o f d e g r a d a b l e c o m p o n e n t s f r o m t h e

feedwate r , 2 ) secu r ing the pu r i t y o f t he chemi -

ca l s dosed , and 3 ) pe r fo rming e f fec t ive c l ean ing

procedures . A l so a c l ean ing p rocedure app l i ed

w h e n f o u l i n g i s n o t a p r o b l e m m i g h t d e l a y

b io f i lm fo rmat ion .

V r o u w e n v e l d e r a n d K o o i j [ 9 8] s h o w e d t h a t

the d i agnos i s o f t he t ype / cause o f fou l ing i s an

essent ia l f irs t s tep to co nt ro l l ing furth er foul ing .

A qu ick scan au topsy ) g ives conc lus ive i n fo r -

ma t ion abou t t he t ypes and ex t en t o f fou l ing o f

the mem brane f i l tra t ion p l an t. Th i s o pens w ays

fo r p reven t ion and con t ro l o f fou l ing . The too l s

AOC -test , B io f i lm mon i to r ) can be used to t e s t

chem ica l s dosed on the r i sk o f b io ) fou l ing . In

add i ti on , Vro uw enve lde r e t a l . [95 ] m ade an

o v e r v i e w f o r c o h e r e n t to o l s f o r f o u l i n g o f N F a n d

R O m e m b r a n e s. T h e s e t o o l s - - d e v e l o p e d f o r t h e

d i agnos is , p red i c t ion , p reven t ion , and con t ro l o f

f o u l i n g - - h a v e b e e n a p p l i e d in p r ac t i ce a n d h a v e

proven the i r va lue i n con t ro l l i ng fou l ing . An

overv i ew i s sho wn in Ta b le 2 [95 ].

The spec i f i c oxygen consumpt ion ra t e i s a

S O C R ) p a r a m e t e r f o r d e t e r m i n i n g th e p r e s e n c e

o f a n a c t iv e b io m a s s i n a m e m b r a n e s y s t e m b y

m e a s u r i n g t h e o x y g e n d e m a n d , w h i l e S c a l e G u a rd

i s a con t inuous on - l i ne mon i to r w i th a s ing l e

sp i ra l -wound m em brane e l em en t tha t de t ec t s sca l-

i n g i n a n e a r l y s t a g e w h e n f e d w i t h t h e c o n -

cent ra te from ei ther a p i lo t or fu l l -scale p lant .

Sca l ing was obse rved on the Sca l eGuard be fo re

i t occurre d on the fu l l -scale insta l la t ion .

T h e p r e s e n c e o f c a l c i u m a n d h u m i c s u b st a nc e s

o r N O M i n s u r f a c e w a t e r s c a n c a u s e s e v e r e

fou l ing o f NF me mb ranes [99 ]. I t was foun d tha t

a t h igh ca l c iu m conc en t ra t i ons fou l ing i s de t r i-

m e n t a l, w i t h t h e m e c h a n i s m d e p e n d i n g o n s o lu -

t i o n c h e m i s t r y . C a l c i u m - h u m a t e c o m p l e x e s

caused the h ighes t f l ux de c l ine due to t he i r h igh ly

compac tab l e f l oc - l i ke s t ruc tu re compared to

ca l c i t e p rec ip i t a t e s . Depos i t i on o f ca l c ium and

organ ics i nc reased wi th pH due to t he p rec ip i -

t a t ion o f ca l c i te and ad so rp t ion o f o rgan ics on the

ca l c i t e su r face . Humic ac id caused the h ighes t

f l ux dec l ine o f t he t h ree o rgan ics u sed due to i ts

h ighes t concen t ra t i on in t he boundary l aye r ,

wh ich was re l a t ed t o i t s l a rges t mo lecu la r we igh t

a n d t h u s a l o w e r d i f f u s i o n a w a y f r o m t h e

membrane . As a re su l t , p rec ip i t a t i on occu r red .

Opera t ion a t l ow f lux , low t ransm emb rane p res -

su re and h igh shea r reduced the depos i t i on o f

in so lub le ma t t e r a t t he membrane su r face and

thus foul ing.

I n a m o r e d e t a i le d s t u d y , H o n g a n d E l i m e l e c h

[100] i nves t i ga t ed the ro l e o f chemica l and

p h y s i c a l in t e ra c t io n s i n N O M f o u l in g o f N F

mem branes . Th e ro l e o f so lu t ion ion i c s t reng th ,

p H , a n d d i v a l e n t c a t i o n s i n N O M m e m b r a n e

fou l ing , a s we l l a s t he fou l ing mechan i sms in -

vo lved , a re rep ro duce d in F ig . 2 [ 100] . O f t hese

chem ica l fac to rs, t he p re sence o f d iva l en t ca t i ons

s u c h a s c a l c i u m a n d m a g n e s i u m h a d a m a r k e d

ef fec t on NO M fou l ing . In add i ti on , i t was show n


9/28

N. Hi la l e t aL / Desa l ina t ion 170 2004) 28 1- 308 289

Table 2

Overview of tools available for determining the fouling potential of feed wa ter and fouling diagnosis of NF and RO

mem branes used in water treatment [95]

Tools Fouling diagnosis Com ment

Integrated diagnosis (autopsy)

Biofilm m onitor and AOC

SOCR

MFI-UF

ScaleGuard

Biofouling, inorganic,

compounds and particles

Biofouling

Biofouling

Particulate

Scaling

Diagnosis of foulant in membrane elements

Predictive and prevention of biofouling by

determining the (growth) potential o f water

Non-destructive method for determining

active biomass in mem brane systems

Particulate fouling potential o f water

Optimizing recovery, acid dose and anti-

sealant dose

C h . em i ca l C o n d i t i o n s

N ~ in ~ u t i o n N O M o n e m ~ r ~ S u r f a c e

presencea divalent~ n s

C on ~J ~t ~ I N ~ ~ a 9 t a ~ r

~ r e p e r m n t s

~ aecl~r e

h l gh p H 6n d

a ~ , . ~ o f d h , a t ~ t

c a t i o n s

~ ~ r m th~ f ou~ l~ ~ y ~

Fig. 2. Schematic description o f the e ffect of solution chemistry on the conformation o f NOM macrom olecules in the

solution and on the mem brane surface, and the resulting effect on mem brane permeate flux. The NO M fouling described

in the diagram is applicable for permeation rates above the critical flux. The difference between the two chemical

conditions shown becom es less clear a t ver y high permeate flux . At low perm eate flux (belo w the critical flux), no

significant fouling is observed f or both co nditions [ 100].

t h a t r a p i d m e m b r a n e f o u l i n g o c c u r s a t h i g h p e r -

m e a t i o n r a te s , e v e n u n d e r c h e m i c a l c o n d i t i o n s n o t

f a v o r a b l e f o r f o u l i n g , s u c h a s l o w i o n i c s t r e n g t h ,

l o w l e v e l s o f d i v a l e n t c a t io n s , a n d h i g h p H . T h e

r a te o f f o u l i n g i s c o n t r o l l e d b y a n i n t e r p la y

b e t w e e n p e r m e a t i o n d r a g a n d e l e c t r o s ta t i c d o u b l e


10/28


l aye r repu l s ion ; t ha t is , NOM fou l ing o f NF

mem branes i nvo lves i n t e r re la t i onsh ip coup l ing )

be tween phys i ca l and chem ica l i n te rac t ions . A l so ,

t he add it i on o f a s t rong che l a t ing agen t ED TA )

to feed wa te r reduces NO M fou l ing s ign i f ican t ly

b y r e m o v i n g f re e an d N O M - c o m p l e x e d c a l c i u m

ions.

A m o r e n e g a t i v e ly c h a r g e d m e m b r a n e s u r f a c e

co r re l a ted wi th i nc reased fou l ing , i nd i ca t ing e l ec -

t ro st a ti c repu l s ion , was no t e f fec t i ve fo r fou l ing

con t ro l . Inc reased su r face a rea due to g rea t e r

su r face roughness fo r t he more nega t ive ly

c h a r g e d m e m b r a n e s m a y h a v e c o n t r i b u t e d t o

o rgan ic adso rp tion fou l ing . Fou l ing was redu ced

w i t h a d d i t io n o f m o n o c h l o r a m i n e , l o w e r f l ux , a n d

lower recovery . Exper im en t s ope ra t ed unde r such

cond i t i ons d id no t p roduce fou l ing and a f f i rmed

the v i ab i l i t y o f i n t eg ra t i on membrane sys t ems

IMSs) fo r t rea tmen t o f t h i s so u rce [42] .

T h e e f f ec t o f m e m b r a n e f o u l i n g o n r e j e c t io n

was p resen ted by Sch afe r e t al . [94] . The s tudy

w a s b a s e d o n e x p e r im e n t s w i t h M F , U F , a n d N F

m e m b r a n e s . I t w a s c o n c l u d e d t h a t f o u l in g w a s

d e p e n d e n t o n p o r e s i z e a n d w a s c a u s e d b y l a r g e

co l lo ids 250 nm) o r coagu lan t f l oes i n M F, sma l l

co l lo ids , o rgan ic -ca l c ium f loes and aggrega t es

wi th a dense s truc tu re fo rm ed s lowly ) in UF , and

by a ca l c ium-organ ic p rec ip it a t e i n NF . T he fou l -

ing in f luenced the re j ec t i on o f co l lo ids in M F and

t h a t o f N O M i n U F a n d N F . I f a h i g h l y c h a rg e d

l a y e r w e r e d e p o s i t e d o n N F m e m b r a n e s , c a t io n

re j ec t ion was a l so i n f luenced . The cha rac t e r i -

za t i on o f pe rme a te o rgan ics revea l ed tha t l ow

m o l e c u l a r w e i g h t a c i d s p a s s e d t h r o u g h t h e N F

membranes and tha t t he re j ec t i on o f t hese

ac ids was a l so dependen t on the depos i t on t he

m e m b r a n e .

T h e m e c h a n i s m s w h i c h c a n e x p l a i n s u c h a n

inc rease i n re j ec t i on a re d i f fe ren t fo r t he t h ree

m e m b r a n e p r o c e ss e s . I n M F , p o r e p l u g g i n g a n d

cake fo rmat ion was found respons ib l e fo r fou l ing .

Th i s reduces t he po re s i ze and inc reases re j ec ti on .

In UF , i n t e rna l po re adso rp t ion o f ca l c ium-

organ ic f l oes reduc es t he i n t e rna l po re d i amete r

and sub sequen t ly i nc reases re j ec t ion . In NF , t he

key fac to r appea rs t o be t he ch a rge o f t he depos i t.

Th i s was i nves t i ga t ed wi th t he depos i t i on o f a

fe r r ic ch lo r ide p rec ip it a t e . I f t he p rec ip i ta t e w ere

a h igh pos i t i ve cha rge , t he re j ec t i on o f ca t i ons

i n c r e a s e d a n d t h a t o f n e g a t i v e l y c h a r g e d l o w

m o l e c u l a r w e i g h t a c i d s d e c r e a s e d c o m p a r e d to

mo re neu t ra l o r nega t ive p rec ip i t a te s . In e ssence ,

the re j ec t i on cha rac t e r i s t i c s o f membranes de -

p e n d e d m o r e o n t h e f o u l in g s t a te o f th e m e m -

branes and the na tu re o f fou l an t s t han on the

in i t i a l membrane cha rac t e r i s t i c s . On the o the r

hand , K osu t i c and Ku ns t [ 101 ] conc lu ded tha t the

pore s i ze d i s tr i bu t ion cu rv es and e f fec t ive num ber

o f p o r e s o n t h e m e m b r a n e s u r f a c e i n d i c at e d pl u g -

g i n g o f t h e t ig h t n e t w o r k p o r e s i n t h e m e m b r a n e

su r face and even the i r d i sappea rance du r ing

pro longed fou l ing . The neg l ig ib l e changes o f t he

t h e N F m e m b r a n e p e r f o r m a n c e p ar a m e t er s d u r i n g

mem brane fou l ing wi th hum ie ac id cou ld be a t tr i -

bu t ed to i n s ign i f i can t changes i n t he po re s i ze

d i s tr i bu t ion and the e f fec t i ve num ber o f po res i n

t h e s u r fa c e o f t h e m e m b r a n e . A d e l a y o f c h e m i c a l

c l e a n in g o f t h e f o u l e d m e m b r a n e l e d t o i rr e v er -

s ib le changes i n t he po ro us s t ruc tu re o f bo th t he

R O a n d N F m e m b r a n e s , w h i c h w e r e c a u s e d b y

microbia l ac t iv i ty .

Dep end ing on the feed com pos i t i on , fl ux de -

c l i ne rang ing f rom a few pe rcen t t o a comple t e

loss o f wa te r f l ux can be found . Fo r aqu eous

so lu t ions con ta in ing o rga n ic com ponen t s , i n t he

absence o f suspended so l id s o r h igh concen-

t ra ti ons o f i ons t ha t m ay cause sca l ing , adso rp t ion

of o rgan ic ma te r i a l on the membrane su r face i s

the ma jo r fou l ing me chan i sm [92 ]. Iden t i f i ca t ion

of t he pa ramete rs t ha t p l ay a ro l e i n t he p rocess o f

adso rp t ion on the m em bran e su r face shou ld lead

t o a b e t te r u n d e r s t a n d i n g o f t h e m e c h a n i s m t h at

re su l t s i n a h ind rance o f t he wa te r f l ux , and

even tua l ly t o po re b lock ing . In t ha t s tudy , m any

paramete rs w ere se l ec t ed fo r a de t a i l ed i nvest i -

ga t ion o f t he adso rp t ion p rocess such as d ipo le

mom en t , po l a r i zab i l it y , d i e l ec t r i c cons t an t , so lu -

b i l i t y i n wa te r , oc t ano l -wa te r pa r t i t i on coe f f i -


11/28

N. Hila l et al. / Desalination 170 2004) 281 -30 8 291

c ien t , con tac t ang le membrane /wa te r , mo lecu la r

s i ze, and pKa . A l l o f t hese pa ramete rs we re

e v a l u a te d i n t he f r a m e w o r k o f a d s o r p t io n o n N F

m e m b r a n e s f r o m a n a q u e o u s s o l u t io n . T h e u s e o f

each pa ramete r fo r desc r ib ing adso rp t ion was

d i scussed . The pKa was re j ec t ed on theo re t i ca l

g rounds ; o the r pa ramete rs such as t he so lub i l i t y

in wa te r p roved to be imprac t i ca l. Ev en tua l ly , the

d ipo le momen t , oc t ano l -wa te r pa r t i t i on coe f f i -

c i en t, and mo lecu la r s i ze were se l ec t ed .

T h e a d s o r p t io n o f o r g a n i c m o l e c u l e s f r o m a n

a q u e o u s s o l u t i o n o n t h e m e m b r a n e s u r f a c e o r

in s ide t he mem brane po res has a l a rge i n f luence

o n ~ a t e r f l u x . T h e m o r e a g i v e n c o m p o n e n t i s

adso rbec l on the membrane , t he l a rge r t he f l ux

dec l ine . Th i s m ay be up to a f l ux dec l ine o f 60

fo r a concen t ra t i on o f 10 mmole /1 o f a s ing l e

o rgan ic componen t i n pu re wa te r . Schaep e t a l .

[4 ] sugges t ed UF as p re t rea tmen t , add ing hydro -

ch lo r i c ac id t o min imize t he fou l ing , e spec i a l l y to

p reven t ca l c ium ca rbona te . F ina l ly , Shaa l an [ 102]

sugges t ed an empi r i ca l equa t ion to p red i c t t he

i m p a c t o f s o m e w a t e r c o n t a m i n a n t s o n f l u x

d e c l in e o f th e N F m e m b r a n e s . T h e s e f o r m u l a e

enab le dec i s ion -mak ing conce rn ing the su i t ab l e

p re t rea tmen t fo r NF sc hem es and a l so se l ec t i on o f

the mos t app ropr i a t e c l ean ing cyc l e . The c o r re -

la t ion [Eq. (1)] i s appl icable for the fo l lowing

condi t ions: t ( t ime) = 10-80 h , I ( ion s t rength)=

1.6-75 rnM, TO C con cent r a t ion = 0--20 mg/1 , P

(opera t ing p ressu re ) = 80 -14 7 ps i and d (mem -

brane cu to f f ) = 300-1000 Da l ton . Th i s co r re l a -

t i on can . g ive a reasonab le p red i c t i on o f f l ux

dec l ine changes wi th re spec t t o an i n i ti a l se t o f

condi t ions:

J / J o = 0 .6 5 e x p ( - 0 . 1 3 t ) e x p ( - 0 . 0 3 1 I )

1 )

exp (131.896 TO C) ex p(1 .23 P) ex p( -0 .34 d)

5 . S e p a r a t io n m e c h a n i s m w i t h N F m e m b r a n e s

N F c o m b i n e s r e m o v a l o f u n c h a r g e d c o m -

ponen t s a t nanosca l e w i th cha rge e f fec t s be tween

so lu t ion and the su r face o f t he membrane . The

r e m o v a l o f u n c h a r g e d c o m p o n e n t s r e s u lt s f r o m

size exc lus ion o r m ay be a re su l t f rom d i f fe rences

in d i f fus ion ra te s i n a no n-porou s s t ruc tu re , wh ich

depend a l so on molecu la r s i ze [2 ] . The cha rge

e f fec t , on t he o the r hand , re su l t s i n remova l o f

(ma in ly mul t i va l en t ) ions ; t he fo rm er e f fec t re -

su i ts i n t he remov a l o f unchar ged o rgan ic spec i es .

T h u s , t h e s e p a ra t io n p e r f o r m a n c e o f N F m e m -

branes can be i den t i f i ed i n to t he s i ev ing ( s te r i c-

h ind rance ) e f fec t and Donnan (e l ec t ros t a t i c )

effect [103].

Kosu t i c and Kuns t [104] showed tha t

t h e

r e j e c t i o n o f o r g a n i c s d e p e n d e d o n t h e s i e v i n g

paramete rs ( so lu t e and po re s i ze) . In add i t i on to

tha t fac to r , Van de r Bruggen e t a l . [105] con -

c luded tha t t he po l a r i ty and the cha rge o f o rgan ics

migh t i n f luence the re j ec t i on p rocess , e spec i a l l y

w h e n t h e p o r e s i z es o f m e m b r a n e s a r e l a rg e . T h e

mo lecu la r d i f fus ion i s ano the r fac to r a f fec t i ng the

r e j e ct i o n a s m e n t i o n e d b y C h e l l a m a n d T a y l o r

[106]. The re j ec t i on o f i on i c comp onen t s i n NF

occur red as a re su l t o f cha rge i n t e rac t ions

b e t w e e n t h e m e m b r a n e s u r f a c e a n d t h e i o n s

(Donn an exc lus ion ) . H igh re t en t ions were found

fo r Na2SO4 and fo r M gC12 wh i l e t he re t en t ion fo r

NaC1 was m uch low er [ 107] . Lhassan i e t a l [ 108]

sugges t ed tha t i ons a re t rans fe r red by two

mec han i sm s in NF : (1 ) convec t ion : t he l a rge r i ons

a re more re t a ined (phys i ca l pa ramete rs ) ; (2 ) so lu -

b i l i za t i on -d i f fus ion : a fun c t ion o f t he so lva t ion

energ i es and the pa r t i ti on coe f f i c ien t . The l a rge r

the i on , t he l e ss we l l i t i s re t a ined (chemica l

pa ramete rs ) . A p i lo t p l an t was b u i l t to rem ove the

ions o f t he same va l ence s ( f l uo r ide , bromide~ and

ch lo r - ide i ons ) . The re su l t s i nd i ca t ed tha t t h e

smal l e r t he i on , t he be t t e r i t i s re ta ined , ma in ly a t

l o w p r e s s u r e , w h e n c h e m i c a l s e l e c t i v i t y

p redomina t es . Th i s i s de r ived f rom the so lva t ion

energy o f t he i ons by wa te r . S ince f l uo r ide i ons

a re more so lva t ed , t hey a re be t t e r re t a ined than

ch lo r ide and iod ide i ons .

Schaep and Vandecas t ee l e [109] u sed th ree

m e t h o d s t o d e t e r m i n e t h e c h a r g e o f N F m e m


12/28

292 N. Hilal et aL / Desalination 170 2004) 281-308

branes . The me thods were : t i t ra t i on , s t reaming

po ten t ia l , and m em brane po ten t i a l. Th e re su l ts o f

the membrane po ten t i a l measu remen t s , where a

membrane sepa ra t e s two e l ec t ro ly t e so lu t ions ,

were i n qua l i t a ti ve acco rdanc e wi th t he re su l t s o f

t i tra t ion . I t wa s sho wn tha t ion d i ffusio n i s inf lu-

e n c e d b y t h e m e m b r a n e c h a r g e , a n d a n i o n

d i ff u s io n t h r o u g h n e g a t i v e m e m b r a n e s i s m o r e

h indered than ca t ion d i f fus ion , wh i l e t he op pos i t e

i s found fo r pos i t i ve ly cha rg ed mem branes . The

e n h a n c e d D o n n a n e f f e c t o n r e j e c t i o n o f m i x t u r e

o f po lye l ec t ro ly t e and monova len t sa l t (NaC1)

us ing two NF m emb ranes was s tud i ed [110].

T h e r e te n t io n o f N a + and M g 2÷ ions in thei r

n i t ra t e so lu tions wa s s tud i ed in t he pH range o f

0 .5 t o 7 .0 us ing a NF membrane [111] . I t was

s h o w n t h a t t h e M g2+ ions w ere v e ry we l l re t a ined

(97 .5 -99 .5 ) w i th in tha t pH range , wh i l e t he

re t en t ion o f t he N a + ion va r i ed f rom 80 to 40 .

T h e s m a l l N a + re t en t ion was mea su red in neu t ra l

cond i t ions . In add i t i on , i t seems tha t a t ve ry l ow

p H t h e m e m b r a n e l o se s m o s t o f i ts c h a r g e d u e t o

a h igh concen t ra t i on o f p ro tons . Th i s enab les t he

separa ti on o f ca t i ons , ma in ly b y the i r s i ze cha rac -

ter is t ics . Afo nso e t a l . [ 112] sug geste d a re la t ion-

sh ip be tween the e f fec t i ve su r face cha rg e (CM) o f

tWO NF mem branes (Desa l G -10 and D esa l G-20)

th rough s t reaming po ten t i a l measu remen t s and

the feed so lu t ion concen t ra t i on (Cf) , o f t he t ype o f

a Freun dl ich i so therm [Eq. (2)] .

In CM = a + b In Cf (2)

su r face . W ang e t a l. [103] conc lud ed tha t NF45

membrane was expec t ed to be su i t ab l e fo r t he

sepa ra t i on o f t he mix tu re so lu t ions o f b iva l en t

an ion e l ec t ro ly t es o r neu t ra l o rgan ic so lu tes w i th

a f e w h u n d r e d s o f M W a n d u n i v a l e n t a n i o n

e l ec t ro ly t es o r neu t ra l o rgan ic so lu t es w i th

seve ra l 10 - fo lds o f M W . T he exper im en ta l re su l ts

ind i ca t ed tha t t he re j ec t i on o f o rgan ic so lu t es and

ino rgan ic e l ec t ro ly t es by the NF45 membrane

f r o m h i g h t o l o w w e r e r a n k e d a s :

Raff inose Na2SO 4> MgSO4 > suc rose >

g lucose >> KC1 = NaC1 > M gCI 2 > g lyce r in

Recen t ly , Mat suura [113] p rov ided seve ra l

d i rec ti ons i n wh ich p rog ress i n sepa ra t ion can be

improved in t he fu tu re . Fo r NF membranes , i t

w a s s h o w n t h a t i m p r o v e m e n t o f p r o d u ct i v it y b y

h i g h e r f lu x m e m b r a n e s w a s a c h i e v e d t h ro u g h t h e

d e v e l o p m e n t o f t h i n - f il m c o m p o s i t e s w i t h a v e r y

th in se l ec t i v i ty sk in l aye r . In ad d i t ion , K ur iha ra e t

a l. [114] a t t r ibutes the h ighe r prod uct iv i ty of cur-

ren t memb ranes t o an i nc rease i n su r face rough-

n e s s. O t h e r m e t h o d s u s e d t o i m p r o v e m e m b r a n e

p e r f o r m a n c e i n c l u d i n g d e v e l o p m e n t o f m i x e d -

m a t r i x m e m b r a n e m a t e r i a l s i n c l u d i n g h y b r i d

o rgan ic - ino rgan ic ma te r i a l s and the su r face mod i -

f i ca t io n o f m e m b r a n e s . T h e s e i m p r o v e m e n t s a r e

no t on ly l im i t ed t o h ighe r f l ux a t l ow er ope ra t i ng

pressu res bu t a l so i n t e rms o f l e ss fou l ing

propens i ty , h ighe r ch lo r ine t o l e rance and

increa sed so lvent resis ta nce [ 115].

T h e D e s a l G - 2 0 m e m b r a n e , t h e o n e b e a r i n g a

h igher su r face cha rge , appea red to d i sp l ay a l og -

ari thmic re la t ionship for NaC1 [Eq. (3)] :

In CM m o l / m3) = 3.57

+ 0 .475 In Cf (mol /m 3) 3 )

Fur the rmore , t he su r face cha rge o f Desa l G-20

was l i ke ly t o be pa r t i a l ly due to t he adso rp t ion o f

an ions f rom the so lu t ion on to t he membrane

6 M o d e l li n g o f N F m e m b r a n e s

A good p red i c t i ve mo de l w i l l a l l ow use rs to

ob ta in mem brane cha rac t e r i st i c s , p red i c t p rocess

pe r fo rm ance as we l l a s op t imize t he p rocess . The

ab i l i t y t o deve lop such mode l l i ng t echn iques

success fu l ly w i l l re su l t i n a sma l l e r number o f

exper imen t s and subsequen t ly save t ime and

m o n e y i n t h e d e v e l o p m e n t s t a g e o f a p r o c e s s

[116].


13/28

N. Hilal e t al . / Desal inat ion 170 (2004) 281 -30 8 293

6 .1 . M o d e l s b a s e d o n t h e e x t e n d e d N e r n s t - P l a n c k

e q u a t i o n

F o r N F m e m b r a n e s , t h e m o s t c o m m o n l y

a d a p te d m o d e l s w e r e t h o s e b a s e d o n t h e e x t e n d e d

Nernst -Plank equat ion [Eq. (4)] :

d c i z i ci D i p F d ~

J i : - D i , p d x R T ~ + K i e

ci v

(4)

Tsuru e t a l. [117] f i rs t pro pos ed su ch a mod el for

NF membranes , desc r ib ing the t ranspor t o f i ons

in t e rms o f an e f fec t ive membrane th i ckness :

po ros i t y ra t io (m) A x / A k , a n d a n e f f e c t iv e m e m -

b r a n e c h a r g e d e n s i t y ( m o l . m -3) Xa. T h e m o d e l

was success fu l i n desc r ib ing the re j ec t i on o f

mixed sal t so lu t ions. In a la ter work [118], a

s imi l a r mode l , bu t w i th t he i nc lus ion o f s t e r i c

e f fec t s , was used to mode l t he t ranspor t o f

o rgan ic e l ec t ro ly t es i n t he p resence o f Na ÷ a n d

C1- ions.

The e l ec t ros t a t i c and s t e r i c -h ind rance (ES)

m o d e l w a s d e v e l o p e d f r o m t h e s t e ri c - h in d r a n c e

pore mode l and the Teore l l -M eyer -S ieve rs mode l .

The new mode l i s u sed to p red i c t t he t ranspor t

p e r f o r m a n c e o f c h a r g e d s ol u te s t h r o u g h N F m e m -

branes based on the i r cha rg ed po re s t ruc tu re : an

aqueous so lu t ion o f d i f fe ren t t race r cha rged

so lu tes ( sod ium benzene su l fona t e , sod ium naph-

tha l enesu l fona t e and s od ium t e t rapheny l -bo ra t e )

and a suppor t i ng sa l t ( sod ium ch lo r ide ) t o ve r i fy

t h e E S m o d e l . T h e p r e d i c t i o n b a s e d o n t h e E S

m o d e l w a s i n g o o d a g r e e m e n t w i t h t h e e x p e r i-

me ntal resul t s .

B o w e n a n d M u k h t a r [ 1 19 ] s u g g e s t e d a h y b r i d

mod e l i n re j ec t ion o f a s ing l e e l ec t ro ly t e a t s i x NF

m e m b r a n e s . T h e h y b r i d m o d e l i s b a s e d o n t h e

e x t e n d e d N e r n s t - P l a n c k e q u a t i o n w i t h a D o n n a n

cond i t ion a t t he mem brane / so lu t ion in t e r faces and

the h indered na tu re o f t ranspor t i n t he mem -

branes . Suc h an in t e rp re t a t i on a l l ows a cha rac t e r i -

z a t io n o f th e m e m b r a n e s i n t e r m s o f t w o

paramete rs , e f fec t i ve membrane th i ckness (Ax)

and e f fec t ive membrane cha rge dens i ty (X) . A

know ledge o f t he e f fec t i ve mem brane thi ckaaess ,

e f fec t i ve cha rg e dens i ty and e f fec t ive po re s i ze

a l lows use o f t he m ode l t o p red i c t t he sepa ra t i on

o f m i x t u r e s o f e l e c t r o ly t e s a t a m e m b r a n e . V e r y

g o o d a g r e e m e n t b e t w e e n s u c h a p r e d i c t io n a n d

exper imen ta l da t a has been ob ta ined .

Bowen et a l . [120] were the f i rs t to coin the

t e r m D o n n a n - s t e r i c- p o r e m o d e l ( D S P M ) t h a t

w a s a m o d i f i e d f o r m o f a h y b r i d m o d e l . T h e

mo de l t akes i n to accoun t t he h ind ranc e e f fec ts fo r

d i f fus ion and con vec t ions t o a l l ow fo r transpor t

o f i ons / cha rged so lu t es t ak ing p l ace wi th in a con -

f ined space i n s ide t he mem branes . Th i s i nc lus ion

a l lows the mem bran e to be cha rac t e r i zed in t e rms

of an e f fec t ive po re rad ius , re , in addi t ion to

A x / A k

a n d Xd. Sub seque nt s tudies [ 121,122] sho w ed that

t h e m o d e l c a n p r e d i c t t h e r e j e c ti o n p e r fo r m a n c e

very w e l l o f s i ng l e sa l t so lu t ions .

Recen t ly , Bowen and Wel l foo t [123] sug -

g e s t ed m o d i f i c a ti o n s to t h e D S P M m o d e l s f o r N F

membranes . The f i r s t mod i f i ca t i on was used to

t ake i n to accoun t t he e f fec t s o f l og -norma l po re

s i ze d i s tr i bu t ions on the re j ec t i on o f uncharged

so lu tes and NaC1 a t hypo the t i ca l NF mem branes .

The re su l t s show ed tha t t he e f fec t o f po re -wi se

va r i a t ion in v i scos i ty i s l e ss apparen t w hen con-

s ide red a t a de f ined app l i ed p ressu re ra the r than

a t a de f ined f l ux , show ing a fu r the r advan tage o f

bas ing theo re t i ca l ana lys i s o f NF in te rms o f

app l i ed p ressu re . Trunca t ed po re s i ze d i s t r i bu -

t i ons gave b e t t e r ag reem en t t han fu l l d is t r ibu t ions

wi th exper imen ta l re j ec t i on da t a t he fo r NF

m e m b r a n e . T h e s e c o n d m o d i f ic a t io n w a s d e v e l -

oped to ca l cu l a t e t he unch arged so lu t e re j ec t i on

o n t h e b a s i s o f a s i n g le m e m b r a n e p a r a m e t e r

(po re rad ius ). T he theo re t i ca l desc r ip ti on i s based

o n a h y d r o d y n a m i c m o d e l o f h i n d e r e d s o l u t e

t ransport i n po res . The g ood ag reem en t be twe en

th i s mode l and exper imen ta l da t a con f i rms tha t

u n c h a r g e d s o l u te r e j e c t io n i n N F m e m b r a n e s m a y

b e w e l l - d e s c ri b e d b y s u c h a c o n t i n u u m m o d e l .

T h e s u r f a c e p r o p e r t i e s o f m e m b r a n e s h a v e

been cha rac t e r i zed in t e rms o f po t en t i a l i n t he

Oute r He lmho l t z P l ane ( ~d ) [ 124] . A theo re t i ca l


14/28


ana lys is o f e l ec t r i ca l and e l ec t rok ine t i c pheno-

m ena (e lect ro ly te c ond uct iv i ty inside pores Xpor~,

mem brane po ten t i a l , Era , and s t reaming po ten t ia l

(SP) occu r r ing in cha rge d cap i l la r i e s was dev e l -

o p e d i n t h e f r a m e w o r k o f t h e l i n e a r t h e r m o -

dynam ics o f i r reve rs ib l e p rocesses w i th t he a im

of s tudyin g the varia t ion of SP, E, , an d ~por~ as a

func t ion o f Wd fo r va r ious po re s i zes and

e l ec t ro ly t e concen t ra t i ons . The ana lys i s e s t ab -

l ishe d the var iat io n o f )~poro, Em and SP vs. the

Oute r He lmho l t z P l ane po ten t i a l (Wd) . On the

o the r hand , a comprehens ive semi -empi r i ca l

m a t h e m a t i c a l m o d e l b a s e d o n t h e P o i s s o n -

Bo l t zmann equa t ion , membrane su r face cha rge

mode l , was deve loped [125] t o i nves t i ga t e t he

e f fec ts o f so lu t ion phys i co -che mis t ry on the ze t a

po ten t ia l o f NF mem branes . T he co r re l a t i ons

be tween the ze t a po t en t i a l, su r face ch a rge dens i ty

and so lu t ion chemis t ry were we l l de l i nea t ed by

th is new mode l .

6.2. Models based on the Spiegler-Kedem model

A n o t h e r a p p r o a ch f o r N F m o d e l i n g is t h r o u g h

t h e S p i e g l e r - K e d e m m o d e l [ 1 2 6 - 1 2 9 ] . T h i s

b l ack -box approach a l l ows the mem branes t o be

charac t e r i zed in t e rms o f sa l t pe rmeab i l i ty , P s,

and the re f l ec t ion coe f f i c i en t o . Th i s mo de l i s i n

the f i rs t instance l imi ted to b inary sa l t systems,

and in t he l im i t i ng case t o a b ina ry sa l t sys t em in

the p resence o f a com ple t e ly re j ec t ed o rgan ic i on

[127,129].

A re j ec t i on o fpe rch lo ra t e an ion (C104) us ing

N F a n d U F m e m b r a n e s w a s p er f o r m e d b y Y o o n

e t a l. [130] . The da t a ob t a ined wer e mod e l l ed by

app l i cat i on o f a non-equ i l i b r ium the rmo dyna mic

m o d e l . T h e m o d e l h a s f i v e p a r a m e t er s : m o l e c u l a r

t ranspor t coe f f i c i en t (w) , o smot i c p ressu re g rad -

i en t (AII ) , mo lec u la r re f l ec t i on coe f f i c i en t (o ) ,

t he ave rage bu lk f l u id i n t e r fac i a l concen t ra t i on

be tween the feed an d p e rm ea te s ide (Cavg), and

the solvent f lux (Jr) . Th e resul t s ind icated that , in

a pu re com ponen t sys t em, t a rge t i ons ( i n t h is ease

C104) can be exc lud ed f rom (neg a t ive ly ) cha rged

mem branes wi th l a rge r po res w i th re spec t to t he

size of the ion , but th is re ject ion capabi l i ty i s

qu ick ly l o s t in t he p resen ce o f a su f f i c ien t amoun t

o f o the r i ons t ha t can sc reen the apparen t e l ec t ro -

s ta t ic force f ie ld . As in tu i t ively expected , the

pe rch lo ra t e f l ux i s governed by convec t ion in

l a r g e - p o r e m e m b r a n e s . A n o t h e r m o d e l w a s

developed by Pontal ier e t a l . [131]; i t describes

m a s s t r an s f e r th r o u g h t h e c o m b i n a ti o n o f c o n -

vect ive--d i ffusive f lux in the pores and di ffusive

f lux in t he mem bran e ma te r i a l. Th i s m ode l sup -

posed so lu t e t rans fe r on ly occu rs i n t he po res

whi l e so lven t can a l so pass t h rough the mem-

brane ma te r i a l . So lu t e and so lven t f l uxes t hen

dep end on s ie ving effect , f r ic t ion and e lect rosta t ic

fo rces , pe rm eab i l i t y and po re rad ius o f t he mem-

brane . A l l t hese cha rac t e r i s t i c s were i n t roduc ed in

the mod e l v i a f i ve pa ramete rs wh ich can be e s t i-

ma ted f rom ex per imen ta l da t a . Th i s mode l cou ld

be us ed fo r t he op t imiza t ion o f t he NF p rocess .

D iaw ara e t a l . (132) u sed an NF45 mem brane

to t rea t h igh ly f l uo r ina t ed b rack i sh d r ink ing

w a t e r. T h e i r a p p r o a c h w a s b a s e d o n t h e p h e n o -

m e n o l o g i c a l m o d e l o f S p i e g l e r , K e d e m a n d

Katcha l sky (SKK) under d i l u t e so lu t ion cond i -

t i ons bu t neg lec t ed the cha rges ca r r i ed by the

m e m b r a n e . E x p e r i m e n t a l r e s u lt s s h o w e d f o r t h e

f i rs t t ime tha t se l ec t i ve de f luo r ina t ion can occu r

u s i n g a N F m e m b r a n e . T h e r e j e c ti o n w a s a r o u n d

9 6 . T h e p h e n o m e n o l o g i c a l p a r a m e t er s o ( th e

membrane re f l ec t i on coe f f i c i en t ) and P8 ( t he

so lu t e pe rme ab i l i t y ) we re ca l cu l a t ed , and i t was

obse rved tha t o cou ld be l i nked l i nea r ly t o t he

h y d r a t io n e n e r g y o f t h e h a l id e i o n s a s s h o w n i n

Eq . (5 ) . Fu r the rmore , P s appea red to be l i nke d to

the t ype o f e l ec t ro ly t e .

o (kJ /mo l ) = 0 .01397 + 0 .00212 Ehy (kJ /mol )

5 )

T h e S K K m o d e l w a s a ls o u s ed b y V a n d e r

Bruggen and Vandecas t ee l e [133] t o deve lop a

m o d e l f o r t h e r e t e n ti o n o f o r g a n i c m o l e c u l e s w i t h

a g iven NF me mb rane a t d i f fe ren t p ressu res a s a


15/28

N. Hilal et aL / Desalination 170 200 4)2 81- 308 295

f u n c ti o n o f t h e m o l e c u l a r w e i g h t . I t w a s s h o w n

t h a t fo r th e U T C - 2 0 m e m b r a n e , a t r a n s m e m b r a n e

p r e s s u r e o f 8 - 1 0 b a r i s o p t i m a l i n v i e w o f t h e

re t en t ions . Th i s i s he lp fu l i n t he cho ice o f t he

opera t ing p ressu re fo r p rac t i ca l app l i ca ti ons , i n

add i t i on to economica l cons ide ra t i ons such as

ene rgy requ i remen t s , work ing cos t s and inves t -

m ent costs .

6 . 3 . S o p h i s t i c a t e d m o d i f i c a t i o n s t o t h e m o d e l s

H a g m e y e r an d G i m b e l [ 13 4] p r o p o s e d t o

inco rpora t e t he cha nge o f t he d i e l ec t r i c cons t an t

be tween bu lk and po re so lu t ion to ca l cu l a t e t he

ion d i s tr i bu tion be twe en bu lk and po re so lu tion .

Based on the ex t ended Nerns t -P l anck equa t ion ,

t h e d e v e l o p e d m o d e l w a s u s e d t o m o d e l t h e d a t a

ob ta ined f rom the re j ec t i on o f s ing l e sa l t so lu -

t i ons a t d i f fe ren t concen t ra t i ons , t he re j ec t i on o f

t e rna ry i on mix tu res a s we l l a s t he re j ec t i on a t

d i f f e r e n t p H v a l u e s b y N F m e m b r a n e s . T h e

dec rease o f NaC1 re j ec t i on as we l l a s t he i nc rease

o f CaC12 re j ec t i on was we l l desc r ibed by the

m o d e l . T h e r e j e c t io n s o f t e r n a r y i o n m i x t u re s o f

NaC I/Na2SO4 and NaC1/CaC 12 w ere d eterm ined

e x p e r im e n t a l ly a n d w e r e p r e d i c t e d b y t h e m o d e l

excep t fo r t he NaC1/CaC12 mix tu re i n a NF

m e m b r a n e w h e r e t h e n e g a t i v e r e j e c t io n s o f N a +

we re obse rved bu t no t p red i c t ed .

Lefebvre e t a l . [135] s tud i ed the re j ec t i on o f

single sa lt s and a m ul t i -e le ct ro ly te mix ture

exper imen ta l l y and theo re t i ca l l y u s ing a l oose

ce ramic

TiO

m e m b r a n e o v e r a c e r t a in r a n g e o f

opera t i ng cond i t i ons pH and feed sa l t concen -

t ra t i on ) . The re su l t s were compared wi th t hose

o b t a in e d b y a n o rg a n i c N F m e m b r a n e N F 2 0 0 ) .

Theore t i ca l i on re j ec t i on p red i c t i ons fo r mu l t i -

e l ec t ro ly te so lu t ions a re o b t a ined by num er i ca l ly

so lv ing the h inde red t ranspor t ex t ende d Nerns t -

P l anck ENP ) ion f l ux equa t ions us ing the com-

p u t e r s i m u l at io n p r o g r a m , N A N O F L U X , w h i c h

incorporates e lect rosta t ic , s ter ic , and hydro-

dynamic i n t e rac t ions and a new cho ice fo r i on

size bare crysta l , or Paul ing, radius) . Th e resul ts

showed tha t t he l oose r ce ramic nanof i l t e r does

no t necessa r i l y pe r fo rm l e ss we l l than the o rgan ic

one . In a l l ope ra t i ng cond i t i ons , t he re j ec t i on o f

C a2+ i s ve ry h igh com pare d to t he re j ec t i on o f

N a +. Th i s w as due to t he s ign o f a pos i t i ve

m e m b r a n e c h a r g e . T h e h i g h r e j e c t i o n o f C a 2+ fo r

pos i t i ve ly cha rge d mem brane s is ma in ly due to a

s t rong Donnan / s t e r i c exc lus ion a t t he feed /

m e m b r a n e i n t e rf a c e . T h e l o w r e j e c t i o n o f N a + f o r

pos i t i ve ly cha rged m em bran es i s due to i t s re l a -

t i v el y w e a k D o n n a n / s t e ri c e x c l u s i o n c o m b i n e d

wi th i t s e l ec t r i ca l m ig ra t i on in t he s t reaming

po ten t ia l , wh ich t ends t o fav o r t he t ransmiss ion o f

co - ions . The au tho rs con c lude d tha t t he i r me tho d

cou ld l ead to a cos t -e f fec t i ve way fo r choos ing

appropr ia t e NF mem branes , op t imiz ing the imp le -

men ta t i on o f i ndus tr i a l NF p rocesses , and f i na l ly

d imen s ion ing indus t r i a l -sca l e NF p l an ts .

B a s e d o n t h e M a x w e l l - S t e f a n t r a ns p o r t e q ua -

t ions, S t raatsma et a l . [136] developed a NF

me mb rane f i l t ra t i on mo de l t o ca l cu l a t e t he pe rm-

ea t e f l ux and re j ec t i ons o f mu l t i -componen t

l i qu id feeds as a func t ion o f mem bran e p ropert i e s

mean po re s i ze , po ros i t y , t h i ckness , su r face

charge cha rac t e r i s t i c ) and feed p ressu re . A

Freund l i ch equa t ion was used to desc r ibe t he

m e m b r a n e c h a r g e b y m e a n s o f a d s o r p ti o n o f i o n s.

Rece n t ly Band in i and Vezzan i [ 13 7 ] p resen t ed

a genera l mode l ca l l ed t he Donnan s t e r i c po re

m o d e l D S P M ) a n d d i e le c t r ic e x c lu s i o n m o d e l t o

desc r ibe mass t rans fe r o f e l ec t ro ly t es and neu t ra l

so lu t es t h rough NF membranes . In t h i s mode l ,

i on i c pa r t i t i on ing a t t he i n t e r faces be tween the

membrane and the ex t e rna l phases t akes i n to

accoun t t h ree sepa ra t i on mechan i sms: s t e r i c

h ind rance , Donnan equ i l i b r ium and d i e l ec t r i c

exc lus ion . The membrane i s cha rac t e r i zed

th rough the use o f ad jus t ab l e pa ramete rs such as

ave rage po re rad ius , e f fec t i ve membrane th i ck -

ness and vo lum et r i c cha rg e dens i ty . The sepa ra -

t ion effect re la ted to the d ie lect r ic exclusion i s

re l evan t w i th re spec t t o t he Donna n equ i l i b r ium

in de t e rmin ing b iva l en t coun te r - ion re j ec t i on ,

such as CaC12 as w e l l a s M gSO4, w hereas


16/28

296 N. Hila l et a l . / Desalination 170 2004) 281-30 8

d ie l ect r ic e f fec ts a re no t so rema rkab le i n t he case

o f mix tu res con ta in ing va r ious co - ions , such as

NaC1

Na2SO 4.

van de L i sdonk e t a l . [138] p resen t ed a m ode l ,

ca l l ed t he Supersa tu ra t i on P red i c t i on Mode l ,

wh ich i s ab l e o f ca l cu l a t i ng the supersa tu ra ti on

ra t i o s o f spa r ing ly so lub le compounds a t t he

m e m b r a n e s u rf a ce . T h e m o d e l i s a c o m b i n a ti o n o f

a concen t ra t i on po la r i za t ion m ode l and a mo de l

fo r ca l cu l a t i on o f t he supersa tu ra t i on ra t i o o f

spa r ing ly so lub le com pound s , e spec i a l l y ca l c ium

carbona te and ba r ium su lpha t e .

6 . 4 . I n c o r p o r a t i n g c o n c e n t r a t i o n p o l a r i z a t i o n

e f f e c t

Concen t ra t i on po la r i za t i on CP) i s a se r ious

p rob lem tha t has re s t r ic t ed t he use o f membranes .

Bha t t acha r j ee e t a l. [139] deve loped a coup led

m o d e l o f C P a n d p o r e t r a n s p o r t o f m u l t i c o m -

ponen t sa l t m ix tu res i n c ross f low NF . T h i s mode l

p red i c ts l oca l va r i a t i ons o f i on i c concen t ra t i ons ,

f l ux and ind iv idua l i on re j ec t i ons a long a rec -

t angu la r c ross f low f i l t ra t i on channe l fo r t h ree -

comp onen t sa lt m ix tu res by a coup led so lu t ion o f

the convec t ive -d i f fus ion equa tion , wh ic h governs

the t ranspor t o f sa l t i ons i n t he boundary l aye r

ad j acen t t o t he membrane , and the ex t ended

Nerns t -P l anck equa t ion , wh ich governs t he i on

m i g r a t io n t h r o u g h t h e m e m b r a n e p o r e s . T h e t w o

t ra n s p or t m o d e l s a r e c o u p l e d t h r o u g h b o u n d a r y

cond i t i ons a t the me mb rane fe ed -so lu t ion in t e r-

face . The in f luence o f a w ide va r i e ty o f funda-

men ta l phys i co -chem ica l pa ram ete rs and opera t -

i ng cond i t i ons on the i n t r i n s i c and obse rved

re j ec t ion we re p red i c t ed by th is m ode l .

B o w e n a nd M o h a m m a d [ 1 40 ] a n d M o h a m m a d

[141] mod i f i ed t he DSPM mode l t o t ake i n to

accoun t t he CP e f fec t fo r m ix tu re o f cha rged ions /

so lu t es . Wi th t h i s app roach , t he pe rmea te f l ux

was ca l cu l a t ed based on the concen t ra t i on o f i ons /

cha rged so lu tes a t the me mb rane su r face . The CP

effec t was t aken in to accoun t by desc r ib ing the

t ranspor t o f mu l t i -com pone n t i ons and so lu t e i n

the mass t rans fe r f i lm l aye r u s ing the ex t ended

Nemst -P l anck equa t ion . In such a manner , t he

pe rmea te f l uxes were p red i c t ed based on the

o s m o t i c p r e s s u r e d e t e r m i n e d a t t h e m e m b r a n e

in t er face . A com par i son o f t he mode l ca lcu l a t i on

wi th pub l i shed exper im en ta l da t a show s tha t t he

mo de l can p red i c t t he pa t t e rn o f re j ec t ion and f l ux

reduc t ion behav io r qu i t e we l l fo r sys t ems con-

t a in i n g N a C l - d y e - H 2 0 .

Hilan N., Al-Zoubi H., Darwish N.a.-a Comprehensive Review of Nanofiltration Membranes_ Treatment,...

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