Synthetic Membranes - American Chemical Society

4 Durability Study of Cellulose Acetate ReverseOsmosis Membrane Under Adverse Circumstances for Desalting Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch004

Laboratory Investigation and Its Field Application Results HIROTOSHI MOTOMURA and YOSHIO TANIGUCHI Kurita Water Industries Ltd., 1723 Bukko-cho Hodogaya-ku, Yokohama 240, Japan In 1968 we started investigations of RO applications for desalting brackish water. In the course of the investigations, we have found the spirally wound module of asymmetric cellulose acetate RO membrane shows excellent durabilities against fouling materials and free chlorine. In 1971, we first installed a then—world—largest RO plant in KASHIMA industrial complex Ibaragi, Japan. The plant produced 3,000 m /day (0.8 MGPD) of fresh water from brackish water of the KITAURA Lake. This RO system has been expanded and now produces 13,400 m /day (3.54 MGPD). The success of the operation of the plant was reported in detail at the Mexico Conference in 1976 (1), and also at Niece in 1979 (2). Until now the RO system has been keeping the salt rejection well above 90 % with the module replacement rate of less than 5 % per year. The total capacity of RO production in Japan came up to 80,000 m /day (21.14 MGPD) in 1979, and KURITA's installations produce more than 70 % of the total production. Figure 1 shows the development of our installations. The rapid increases in the fields of ultrapure water polishing and the waste water reclamation are remarkable. This success of RO applications for the variety of fields are supported by 3

3

3

1)

the proper pretreatment system and the appropriate standard f o r feed water q u a l i t y which were e s t a b l i s h e d by KURITA WATER INDUSTRIES LTD., 2) the d u r a b i l i t y s t u d i e s of c e l l u l o s e acetate membranes under adverse c o n d i t i o n s . In 1976, the importance of pretreatment f o r s t a b l e RO operation was presented a t the Mexico Conference (1). This p r e s e n t a t i o n w i l l d i s c u s s the membrane performance and i t s p h y s i c a l and chemical changes under unfavourable c o n d i t i o n s . This kind of s t u d i e s w i l l g i v e us information on trouble-shooting counter-measures f o r unexpected membrane d e t e r i o r a t i o n s , and on the d u r a b i l i t y of a c e l l u l o s e acetate membrane under adverse conditions. 0097-6156/81/0153-0079$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

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SYNTHETIC MEMBRANES: DESALINATION

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch004

Process of Membrane D e t e r i o r a t i o n Table I shows processes of membrane d e t e r i o r a t i o n s . They can be c l a s s i f i e d i n t o the three c a t e g o r i e s ; p h y s i c a l , chemical and b i o l o g i c a l process. P h y s i c a l d e t e r i o r a t i o n i n c l u d e s compaction by creeping and surface d e t e r i o r a t i o n s by s c r a t c h i n g and v i b r a t i o n . Creeping i s a c c e l e r a t e d at higher temperatures and pressures, r e s u l t i n g i n the membrane compaction. This phenomenon i s w e l l analyzed and the membrane c h a r a c t e r i s t i c s of compaction can be estimated i n terms of m-value. Scratching and v i b r a t i o n can develop the microscopic defects i n the surface s t r u c t u r e of membranes, and give poor performances. We discussed t h i s type of d e t e r i o r a t i o n i n Mexico i n 1976 (1.) . The major chemical processes of membrane d e t e r i o r a t i o n s are h y d r o l y s i s and o x i d a t i o n . C e l l u l o s e acetate i s most s t a b l e at the l e v e l of around pH 4.7, and at the pHs lower or higher than that value, membrane h y d r o l y s i s i s a c c e l e r a t e d . In p r a c t i c a l a p p l i c a t i o n s of c e l l u l o s e acetate membranes, feed water pH i s u s u a l l y c o n t r o l l e d between 5 to 6. But i t i s impossible to c o n t r o l the pH of demineralized pure water f o r e l e c t r o n i c and pharmaceutical uses, i . e . f o r u l t r a p u r e water p o l i s h i n g . In such cases feed water pH 7 should be s u p p l i e d to c e l l u l o s e acetate m a t e r i a l . Studies of membrane behaviour under such c o n d i t i o n s w i l l give good informat i o n f o r estimating the membrane l i f e . Adverse o x i d a t i o n of membrane occurs at higher concentrations of o x i d i z e r s such as c h l o r i n e , ozone and hydrogen peroxide. The chemicals are important f o r slime c o n t r o l , and r a t h e r high concent r a t i o n s of the chemicals are dosed f o r s t e r i l i z a t i o n of RO feed system, e s p e c i a l l y i n cases of u l t r a p u r e water system, and of waste water treatment system. The e v a l u a t i o n of membrane d u r a b i l i t y against o x i d i z i n g chemicals informs us the proper procedures f o r RO maintenance. Any b i o l o g i c a l d e t e r i o r a t i o n of c e l l u l o s e acetate membranes i s always by " a c c i d e n t a l " . To prevent t h i s kind of d e t e r i o r a t i o n s , c h l o r i n e i n j e c t i o n to feed water i s common p r a c t i c e . Inadequate c o n t r o l of c h l o r i n e i n j e c t i o n may r e s u l t i n the enzymic d e t e r i o r a t i o n of c e l l u l o s e acetate membrane. Influences

of D e t e r i o r a t i o n on Membrane C h a r a c t e r i s t i c s

As membrane d e t e r i o r a t i o n s can be seen i n case of performance degradations or changes i n membrane s t r u c t u r e , we have i n v e s t i gated i n t o these two aspects. Information about the r e l a t i o n between membrane c h a r a c t e r i s t i c s and d e t e r i o r a t i o n processes i s u s e f u l f o r trouble-shooting. Even i f operation records showed no i m p l i c a t i o n of d e t e r i o r a t i n g process o f a membrane, the a n a l y s i s of the d e t e r i o r a t e d membrane w i l l r e v e a l i t s own h i s t o r y .



4.

MOTOMURA AND TANIGUCHI

Figure 1.

Cellulose Acetate

RO

Membrane

81

Development of RO installations by Kurita Water Industries Ltd

Table I.

Processes of Membrane Deteriorations

MEMBRANE DETERIORATIONS • PHYSICAL CREEPING SCRATCHING & VIBRATION

• CHEMICAL HYDROLYSIS OXIDATION

• BIOLOGICAL

Figure 2. Rejection-flux pattern of deteriorated cellulose acetate membrane: F , permeate flux of new membrane; F , permeate flux of deteriorated membrane. v


p


82


Performance. Figure 2 shows a r e j e c t i o n - f l u x p a t t e r n (R-F pattern). Compaction, as i t i s w e l l known, r e s u l t s i n the f l u x d e c l i n e with s a l t r e j e c t i o n i n c r e a s e . Contrary to t h i s , other types of membrane d e t e r i o r a t i o n give the f l u x i n c r e a s e with s a l t r e j e c t i o n d e c l i n e . In case of s c r a t c h i n g , v i b r a t i o n , or microb i o l o g i c a l d e t e r i o r a t i o n , small cracks or p i n h o l e s develop over membrane s u r f a c e s . I f the f l u x i n c r e a s e i s s o l e l y a t t r i b u t e d to the crack or p i n - h o l e s , and these s i t e s do not r e j e c t s a l t a t a l l , the r e l a t i o n between s a l t r e j e c t i o n and f l u x can be c a l c u l a t e d . The p a t t e r n i n F i g u r e 2 shows the r e s u l t of t h i s c a l c u l a t i o n , and agreed w e l l with the a c t u a l performance of the d e t e r i o r a t e d membrane. H y d r o l y s i s gives almost the same p a t t e r n as i n the case of p i n - h o l e s at higher s a l t r e j e c t i o n but l e s s permeate f l u x at lower s a l t r e j e c t i o n . Oxidation gives much higher water f l u x i n c r e a s e comparing with the cases of p i n - h o l e and h y d r o l y s i s . In both cases of h y d r o l y s i s and o x i d a t i o n , the R-F p a t t e r n v a r i e s somewhat with membrane types. P h y s i c a l and Chemical S t r u c t u r e . The analyses of p h y s i c a l and chemical s t r u c t u r e s i n c l u d e e l e c t r o n m i c r o s c o p i c a n a l y s i s , IR spectrophotometric a n a l y s i s , X-ray d i f f r a c t o m e t r y , and burst tests. F i g u r e 3 shows the s u r f a c e s t r u c t u r e of a p h y s i c a l l y d e t e r i o r a t e d membrane by scanning electronmicroscopes. Hard c r y s t a l s of an i n o r g a n i c s a l t might have scratched the membrane s u r f a c e and the rough s u r f a c e was developed. The s a l t r e j e c t i o n decreased to 60 % and the water f l u x doubled comparing that of normal membranes. F i g u r e 4 shows s u r f a c e h e a v i l y hydrolyzed by a concentrated a l k a l i n e s o l u t i o n . T h i s membrane could r e j e c t only 17 % of s a l t . I t looks l i k e the stormy sea s u r f a c e . T h i s s u r f a c e seems to have d i s s o l v e d , and then r e p r e c i p i t a t e d . F i g u r e 5 shows the o p t i c a l m i c r o s c o p i c view of a s t a i n e d membrane s u r f a c e which were b i o l o g i c a l l y d e t e r i o r a t e d . Microb i o l o g i c a l c o l o n i e s of 1 to 10/** s i z e can be seen spreading over the s u r f a c e . The d e n s i t y of microorganism over the s u r f a c e determined by u l t r a s o n i c d i s p e r s i o n technique was 2 x 1 0 c e l l s / cm . Figure 6 shows the e l e c t r o n m i c r o s c o p i c view of the same s u r f a c e . When the c o l o n i e s of microorganism were removed, the surface d e f e c t s were found. The p a t t e r n of the d e f e c t s i s similar to that of the o p t i c a l microscopic view. The enzymic h y d r o l y s i s occurred j u s t below the c o l o n i e s of microorganism. F i g u r e 7 shows the d e f e c t p e n e t r a t i n g the a c t i v e s u r f a c e . This s t r u c t u a l change gives the R-F p a t t e r n s i m i l a r to that of the membrane with p i n holes. T h i s membrane shows the s a l t r e j e c t i o n of 25 %, and permeate f l u x of 2.50 m/D. The X-ray d i f f r a c t i o n spectrum i n F i g u r e 8 shows the c r y s t a l l i n e s t r u c t u r e of a normal c e l l u l o s i c membrane. D i f f r a c t i o n peaks appeared around 10, 11, 16, and 21 degrees of 20. T h i s spectrum 6

2



4.


Cellulose Acetate RO Membrane

Figure 3.

83

Surface structure of physically deteriorated membrane

Figure 4.

Surface structure of heavily hydrolyzed membrane


SYNTHETIC MEMBRANES:


84

Figure 5.

DESALINATION

Optical microscopic view of membrane surfaces that were biologically deteriorated

Figure 6. Scanning electron microscopic view of membrane surfaces that were biologically deteriorated. Colonies of the microorganism on the surface were removed.



4.


Cellulose

Acetate

RO

Membrane

85

Figure 7. Cross section of biologically deteriorated membrane

.A 5

10

SR

95%

Fp

15

20

25

0.75m/0

30

35

40

20 (deg)

t

SR

17%

Fp

0.55 m/D

Figure 8. X-ray diffraction pattern of normal cellulose acetate membrane

>-

**

5

10

15

20

25

20 (deg)

30

35

40

Figure 9. X-ray diffraction pattern of hydrolyzed cellulose acetate membrane




WAVE

4000 3600

3200 2800

2400

2000 1800 1600 1400

WAVE

Figure 10.

2-5

1200 1000 800 650

NUMBER ( c m " ) 1

IR spectrum of normal cellulose acetate membrane

3

4000 3600 3200

4

2800 2400

WAVE 5

2000

WAVE

Figure 11.

L E N G T H (/on)

LENGTH U n ) 6

7

1800 1600

NUMBER

8

9

10 1112131415

1400 1200 1000 800

650

(cm" ) 1

IR spectrum of hydrolyzed cellulose acetate membrane




SR

Cellulose

Acetate

RO

Membrane

87

Z%% \

Fp 1.30m/D/

40

Figure 12. Burst strength of oxidized membrane that was soaked in NaCIO of 0.1% AsCl

60

TIME (HR)

2

Table II. Process of Membrane Deterioration and Its Influences on the Characteristics of Membrane DETERIORATION MEMBRANE CHARACTERISTICS • PHYSICAL Creeping

Fp

Scratching &

R-F pattern of "pin-hole"

. , SR

Rough Surface

• CHEMICAL Hydrolysis

Typical IR :

R-F pattern

C = 0 . , 0-H *

Rough

Surface

Typical R-F pattern Decrease of Burst Strength

• BIOLOGICAL

R-F pattern of "pin-hole" Pin-holes under colonies



88

can be assigned to that of c e l l u l o s e t r i a c e t a t e I I c r y s t a l . Figure 9 shows a spectrum of the hydrolyzed membrane, and peaks appeared a t 11, 20, and 22 degrees. The c r y s t a l s t r u c t u r e changed to that of c e l l u l o s e I I type. F i g u r e 10 shows the IR spectrum of a normal c e l l u l o s e acetate membrane. F i g u r e 11 shows the spectrum of the hydrolyzed membrane. The decrease o f a b s o r p t i o n around 1,720 c m , and the i n c r e a s e of a b s o r p t i o n around 3,200 t o 3,500 c m a r e shown. The f i r s t peak correspond to the C = 0 double bond, and the second t o the 0 - H s i n g l e bond. These s p e c t r a show the decrease of the a c e t y l content i n the membrane. X-ray and IR s p e c t r a of an o x i d i z e d membrane gave no informat i o n , but p h y s i c a l s t r e n g t h of the membrane was h i g h l y decreased. F i g u r e 12 shows the b u r s t s t r e n g t h of the o x i d i z e d membrane. The membrane was immersed i n sodium h y p o c h l o r i t e s o l u t i o n of 0.1 %, and the b u r s t s t r e n g t h was determined from time to time. The strength decreased g r a d u a l l y . Table I I shows the processes of membrane d e t e r i o r a t i o n s and i t s i n f l u e n c e s on the c h a r a c t e r i s t i c s o f membrane. -1


-1

Concluding

Remarks

Our i n v e s t i g a t i o n on d u r a b i l i t y and membrane c h a r a c t e r i s t i c s c h a n g e s u n d e r a d v e r s e c o n d i t i o n s have much c o n t r i b u t e d to development of RO a p p l i c a t i o n s . Among these a p p l i c a t i o n s are those f o r u l t r a - p u r e water i n e l e c t r o n i c and pharmaceutical industries. Even under the circumstance of pH 7 and with 2 t o 4 times per year o f s t e r i l i z a t i o n by H 0 of as high as 1 %, the c e l l u l o s e acetate membrane proved to show membrane l i f e of more than 3 years. Our i n v e s t i g a t i o n w i l l a l s o c o n t r i b u t e to improvement i n the system design, and techniques o f o p e r a t i o n and maintenance. We have t r i e d to r e l a t e the performance of a d e t e r i o r a t e d membrane t o i t s s t r u c t u r e by c l a s s i c a l methods. Recent advancement i n the techniques of morphological and physicochemical analyses i s remarkable, and i s much c o n t r i b u t i n g to b e t t e r understanding of the membrane behaviour. We have now v a r i o u s types of RO membranes made of s y n t h e t i c polymers a v a i l a b l e , and most these a n a l y t i c a l procedures a r e a p p l i c a b l e f o r the a n a l y s i s of these membranes. I n v e s t i g a t i o n s on the membrane s t r u c t u r e s are much more r e q u i r e d , and they w i l l r e v e a l the r e l a t i o n s between m a t e r i a l s and s t r u c t u r e , and s t r u c t u r e and performance. We b e l i e v e these i n v e s t i g a t i o n s w i l l c o n t r i b u t e to development not only i n the membrane i t s e l f , but i n the a p p l i c a t i o n of the membrane. We hope the progress of membrane s c i e n c e w i l l expand RO market. 2

2

Literature Cited

1. 2.

Taniguchi, Y. DESALINATION, 1977, 20, 353-364. Horio, K. DESALINATION, 1979, 32, 211-220.

RECEIVED December 4, 1980.


Synthetic Membranes - American Chemical Society

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