6 Seawater Reverse Osmosis: The Real Experience R. BAKISH
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Director of Desalination Programs, Fairleigh Dickinson University, Teaneck, NJ 07070
Seawater RO today is a proven water conversion process with a respectable, though still most limited past and truly exciting future. While the total worldwide distillation based capacity has exceeded the 1 billion gallons-per-day mark, that of seawater RO has just about reached 10 million gallons per day. When one is to speak of experience, I believe that one should only talk about a process after it has become commercial. The most accurate statement as to the time for commercialization of seawater reverse osmosis plants is to say that it appears to be sometime between late 1974 and early 1975. As a criterion for commercialization, I consider the actual sale of a plant by a manufacturer to a user. One, of course, speaks here of relatively small plants, in fact, plants with capacity in the 2,500 to 20,000 gallons-per-day range. The difficulty in establishing the accurate date is the apparent fact that in the early installed plants, it is virtually impossible to establish with certainty whether the plant was being field tested or purchased on the open market as a plant for the purpose of water production. To me, at least, the commercialization is an important fact of a successful water conversion technique, because before it takes place, one cannot truly speak of it as part of the technology contributing to water conversion, or of experience with i t . In fact, it is even more complex than this as to some, real commercialization means the above stated definition, but applied to plants with minimum capacity of 1MGD. Pretreatment As you w e l l know seawater around the world v a r i e s extensively. This v a r i a b i l i t y i s f u r t h e r extended by the nature and l o c a t i o n of the plant intakes and introduces f a c t o r s beyond composition d i f f e r e n c e s , which are relevant to the q u a l i t y of the raw seawater to be converted. Is one withdrawing water from a sea well?
0097-6156/81/0153-0091$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
What k i n d of a w e l l i s i t ? Does the water o r i g i n a t e from a shallow bay or does i t come out o f deep c o a s t a l waters? What i s the nature of the bottom? Is i t withdrawn from deep open ocean, etc? Each o f these f a c t s a f f e c t f a c t o r s such as suspended s o l i d s , b i o - c o n t e n t , and l a s t but not l e a s t , depend on l o c a l atmospheric c o n d i t i o n s induced v a r i a b i l i t y . Table I shows the composition of some n a t u r a l seawaters around the world. T h i s wide seawater composition v a r i a t i o n a f f e c t s the q u a l i t y of the product, i . e. the product s a l i n i t y . T h i s , of course, i n the u n l i k e l y case that a l l we had to contend with i n the RO conversion were the composition v a r i a t i o n s . In f a c t , the other v a r i a b l e s such as: the number and the nature of microorganisms, the amount, s i z e , and nature of suspended s o l i d s and t h e i r v a r i a b i l i t y , presence or absence of p o l l u t a n t s , each and a l l of which can be a f f e c t e d by the p r e v a i l i n g atmos p h e r i c c o n d i t i o n s p r e s e n t , are much more consequential to t r o u b l e - f r e e operations than the seawater composition v a r i a t i o n s . TABLE I - SALINITY VARIATION AROUND THE WORLD Approximate S a l i n i t i e s B a l t i c Sea
ppm 7,000
Black Sea
13,000
A d r i a t i c Sea
25,000
P a c i f i c Ocean
33,600
Indian Ocean
33,800
Caribbean (W.I.L.)
38,500
A t l a n t i c Ocean
39,400
Arabian Gulf
43,000
Red Sea
43,000
1
The S D I of a water i s the accepted c r i t e r i o n of i t s q u a l i t y f o r RO conversion and some SDI's of water around the world are given on Table I I . The manufacturers u s u a l l y r e q u i r e values of waters reaching permeators to have an SDI below 3 i f they are to warranty t h e i r membrane design l i f e . This requirement of seawater f o r RO conversion i s accomplished through pretreatment. The q u a l i t y of the raw seawater determines the need f o r , and the s p e c i f i c type of pretreatment r e q u i r e d to produce the water q u a l i t y r e q u i s i t e to s a t i s f y a s p e c i f i c permeator manufacturer's requirements.
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.
In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. 6.0 2.5 45* 16* up to 6.2 4.0**
Open Ocean N a t u r a l Beach Sand F i l t e r Pipe Channel Pipe Channel - I n f i l t r a t i o n
New Providence, Bahamas
Cat Cay, Bahamas
Cat Cay, Bahamas
W r i g h t v i l l e Beach, U.S.A.
CADAFE, Venezuela
Jeddah, Red Sea, Saudi A r a b i a
Ral
**Iron i n System
* T o t a l Time
