Desalination of sea water by direct osmosis

Desalination of sea water by direct osmosis

16 ( 1975) I 5 l-1 55 @ Elsevier ScientificPublishing Company. Amsterdam - Printed in The Netherlands Dedinurim. DESALINATTON RICHARD OF SEA WATER ...

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16 ( 1975) I 5 l-1 55 @ Elsevier ScientificPublishing Company. Amsterdam - Printed in The Netherlands Dedinurim.

DESALINATTON RICHARD

OF SEA WATER

E. KRAVATH

BY DIRECT

A>V JOANNA

A. DAVIS

Department of Pediatrics. Montefwre Hospirnl Medicine, Bronx, N. I’. 10467 ( U.S. A .I

(Rrxciwd

Septcmbrr

17. 1974: in rcvird

OSMOSlS

and Medical

Center.

Albert

Einstein

College

of

fonn January 3. 1975)

Sea water can be desalinated by direct osmosis across a cellulose acetate membrane by using the osmotic pressure of a hypertonic glucose solution as the energy source. The resulting glucose solution is potable. This may prove useful for an emeqency water supply in lifeboats. Recent developments in membrane science (I) have led to the commercial availability of stable membranes rhat are permeable to water but relatively impermeable to sugar and salt. These membranes are used for desalination of water by reverse osmosis. In reverse osmosis, high pressures are used to force water from the sea water across the membrane by counteracting the osmotic pressure of the solute in the sea water. This process requires equipment capable of withstanding the high pressures that are generated and an energy source for producing the pressure. One way of avoiding these problems posed by the production of a hydrostatic

pressure

differential

would

on the other side of the membrane

he to have a solution

of higher osmolality

from the sea water and to use the osmotic

pressure of the dissolved solute to move water by osmosis across the membrane from the sea water. If the solute that was used to produce the hypertonic solution is biologically useful, like glucose, it would then be possible to drink the glucose

solution without further preparation. While such a process may have little industrial or agricultural impact it may IX important in circumstances where drinking water is in very short supply, for example. as part of an emergency mtion at sea, or for recycling waste water in desert regions and in space, or for purifying contaminated water. To determine whether or not in practice the water flux would be sufficiently high and the salt flux sufficiently low to be useful, the following experiments were run. We initially used a flat cellulose acetate membrane of the kind used in reverse osmosis labelled Eastman KP 98. We placed the membrane without backing or further preparation between the two halves of an equilibrium type dialysis cell and placed a saline solution on one side and a glucose containing solution of higher osmolality on the other side. Osmotic water flow occurred as

R. E. KRAVAlH

152

AND

J. A. DAVIS

predict4 but the membrane used in this way was too permeable to the sodium and chloride ions to make it useful for desalinating sea water since the resulting solution had too high a sait concentration. We next used a device, the Dow Bio-Rad Beaker osmolyzer. incorporating hollow fibers made of cellulose acetate out!ined in Fig. 1. In this device a bundle of hollow fibers that comprises the semi-permeable membrane makes a U shaped loop through a plastic beaker. The two ends of the hollow fiber bundle have separate ports. There are also two ports that open into the beaker. A solution contained within the hollow fibers would therefore be separated from a solution in the beaker by the cellulose acetate membrane that forms the walls of the hollow fibers. This gives a large surface area. in a compact unit weighing I26 gm. said to have a nominal membrane area of IO00 square centimeters in the company literature.

Fig. 1. Outlim

of hollow fiber device.

Sea water was placed in the beaker and glucose solution of higher osmolality than sea water was pumped in one direction through the hollow fibers by an infusion pump. A continuous flow of sea water through the beaker was maintained by a gravity drip to hold the concentration of solute in the sea water relatively constant and was not stirred. The eflluent from the hollow fibers was collected, the volumes measured and then samples were analyzed for glucose, sodium, and chloride by usual laboratory techniques. Between runs the beaker and fibers were flushed with distiiled water and left filled with distilled water. Before runs the beaker and outer surfaces of the fibers were flushed three times with 50 ml of se-a water through the beaker ports. The beaker was left filled with 115 ml of sea water: then the inner surfaces of the hollow fibers were fiushed through the fiber

DESALINATION

OF SEA WATER BY DIRECT OSMOSIS

153

15 ml of the hypertonic glucose in sea water solution to be used in the run and then the first IO-15 ml of effluent during the run were discarded. Sea water from the Atlantic Ocean was obtained from a jetty at Atlantic Reach. Long Island. which had a concentration of sodium of 425 meq/l and a chloride concentration of 510 meq/l. This is equivalent to a salinity of 31. I yS and a chlorinity of 17.2~~. (_I) This sea water was run in the beaker exposed to the ports with

outer surface of the hollow

fibers and is the water that was desalinated_

In addition,

100 g of glucose was added to IO0 ml aliquots of the same sea water which produced a solution with a volume of I65 ml. a sodium concentration of 260 meq/l, a chloride concentration of 300 meq/I and a glucose concentration of 61 g per 100 ml of solution. This solution was pumped through the hollow fibers at varying rates and the cllluent was collected. The results of duplicate runs are presented in Table 1. At the faster rates of infusion there were smaller inc;emznts in volume and higher salt conccntlations. At the slowest rate of mfusion. &O-t7 ml/mitt, the increases in volume were greatest. from 5 ml to 15.7 ml and 15.2 ml and the concentrations of glucose were louest at 20 and 21 g/l00 ml but the salt concentrations were higher than at the more moderate rates of infusion. The lowest salt concentrations occurred at rates of 0.125 and 0.25 ml/min with sodium concenranging from 131-141 meq/l and effluent volumes were close to 3 times inflow volume. A small amount of glucose crossed the membrane ranging from 23-26 mp per 100 ml of sea water in the beaker. trations

TABLE I EFFLUEN-T A%D THE

\OLUWE

GLUCOSE HOLLO\V

FOR

WHEH FlflER5

EACH

lo& AGAIMT

OF

FIVE

ML

OF

GLUCOSE

I’iPUT PER

SEA \VATER

IS

A\VD A\cALYSIS

1c)o Ill! THE

OF

BEAliER

OF

SEA \\‘aTER AT THE

SAHPLES WAS

SPECIFIED

FOR USED

SODIUSI,

CHLORIDE

AS IS-PUT

THROUGH

RATES

(I ) and (2) represent individual xAes for duphcate runs. ._._. ______. ._._ .-_ .-.-. SOlfIIlIIl I’thmr in ml Role of in-flow

OF IhFLOB

Columns

ml/ min

per 5 ml input

c

0.25

D 0.115 E 0.047

_...... -.-.----

Gluco w g/l(x) ml

meq/l __.

(2)

(1)

(2)

(1)

(2)

(1)

(2)

7.3 9.5 11.3 13.5 15.2

191 153 132 131 180

176

214 176 149 144 188

204

40

172 156 159 203

30 26 21 21

38 30 ‘5 23 20

(1) A 1.22 B 0.50

_-_._... ..__. _....__ -__-

7.1 10.0

12 2

15.0 15.7

-.-- ---- -- ___

150 137 141 184

_ ..__..._...______. ___.

--_..--.-----

We added 20 g of glucose to 40 ml of ellluent obtained at an inflow rate of 0.125 ml/min as in the paragraph above yielding a solution with a sodium concentration of I05 meq/l, chloride of I I6 mcqfl, and glucose of 50 g per 100 ml and ran this solution against sea water in the beaker. Results are in Table II. This

R. E. KRAVATH

154 TABLE

II

SIHILAR

TO

FIBERS

WAS

TABLE

OF GLUCOSE Rare

-

of

FIKN

-

.-

ME

SOLUTIOS RUS

LSED

FQUIVALEhT

FOR TO

THE ROW

IS-FLOW

TtiROlJGtl

D OF T4HLE

I WITH

THE

HOLLOW

50 @‘[email protected])

ml

-.

in-fkm-

Volume in ml per 5 ml input

. (1) 9.2 10.3

0.5 0.23

. _..

THAT

A PREVIOUS

J. A. DAVIS

ADDED

ml;min _---. .--

__.

I EXCEPT

EFFLUENT

AND

._ _

._.

__

(2) 8.S 10 :!

.

_ _

ellluenr from a second run has roughly doubled in volume and has a lower salt concenrration: the sodium ranging from 74-81 mcq/l. We also tried a solution composed of 150 g of g:ucose per 100 ml of sea water. but there was considerable variability and less reliability in use. probably related to the high viscosity of the solution

that may have caused uneven flow or

clo_gging of the hollow fibers. A salt concentration below 150 meal in a hypertonic glucose solution is probably potable for a short tim e under emergency conditions given adequate volumes. healthy kidneys and minimal extrarenal water loss. and is almost certainly better than thirsting (3). Assuming, 3s seems reasonable from our data. that one could achieve in practice a tripling of the initial 165 ml volume of the 100 g glucose in 100 ml sea water. in one pass through the membrane system. then 100 g of glucose could yield 495 ml of a potable solution which would still contain practically all of the glucose” This volume of solution is close to the minimal daily water requirements for a man at rest in the shade at the sea surface (3) although the actual minimal requirements for this particular solution remain to be determined. If a lower salt concentration is needed. then the effluent from the first run could be collected and more glucose added to it and this solution could then be run through the membrane system to produce a solution with a lower salt concentration as in Table II. Repeated cycling through the system of the effluent should produce glucose solutions with even lower salt concentrations_ The resultant gIucose solution in any case is higher in osmolality than the initial sea water because of the added glucose, but the glucose need not be excreted in the urine and thus obligate a water loss, as is the case with salt, but can te metabolized to carbon dioxide and water. The carbon dioxide can be excreted as a gas in the lungs and the water of oxidation of the _elucose can be made available to the organism_ In addition to obtaining 60 ml of water from the oxidation of 100 g of glucose, 400 calories would be produced. This amount of glucose has other important effects during periods of restricted caloric intake in that it reduces the catabolism of protein, decreases the basal metabolic rate, prevents ketosis,

DESALINATION

OF SEA WATER

BY DIRECT

OSUOSIS

155

and so reduces the water and energy requirement (4. 5). Since 100 g of glucose per day is recommended for inclusion in an emergency ration anyway (4. 5). this in circumstances weight and volume need not be credited to the water requirement of a restricted weight allowance at sea that weight attributed to water requirements may be that of the membrane system itself, which is minimal. It is possible to supply not only minimal but adequate water and cner_ey requirements by drinking enough of such a solution. Other solutes that are relatively impermeable and can be metabolized could be used in addition to or combined with the glucose: e.g. sucrose. amino acids, fatty acids and vitamins. These substances could be stored in life boats, for example. combined as a dry powder along with a suitable membrane system.

nutritional minimal cunmple.

The solution

requirements.

by the method

400 .g of glucose and 50 g af amino

of the nutritional Before

including s)srems.

then obtained

described

could

supply

all the

requirements nccdcd to )ustain life at low cost 151th small bulk and external energy requirements as long as sea water was nlentiful. For while 100 g of glucose per day may be adequate for short term minimal requirements

this process

ev.duation

could

acids per day may supply enough

for moderate

actkity

for an indefinite

be applied.

further

studies

of the hypertonic

glucose in sea \\.acr solution

field trials of the process. and investigation

be more efficient

would

of other

time.

be necessary in biological

membranes

that may

(6).

REFERESCFS

1. S. LOEB Ah’D S. SO~~IURUAS. =If/tun. C-/rem. Srricr,

38 (1963)

117.

2. H. BARS=. J. &I-P. Rid., 31 (1954) 582. 3. 4. 5. 6.

A. v. WOLF, Thirst Ph_xsioIcqy uf the Urse tto DGnk und Pr~hlcr~u c$ IVazrr Lack, Charles C. Thomas. Springfield. 111.. 1958. pp. X7-317. J. L. GAMBLE. Hartc_v Leaf.. 42 (1937) 147. G. R. HERVEY ASD R. A. McC,xuct, Pruc. Roy. SM., Lud~n, S.B.. 139 (1951) 517. H. K. LOSSUALE. Desulinufion. 13 (1973) 317.