Desalination of brackish water by membrane separation techniques

Desalination of brackish water by membrane separation techniques

Shankar Muthukrishnan, Sasank Mohan Goli, M Babu Srinivas, S S Srinivas &T Venkatrarn*

Department of Chemical Engineering, Indian Institute of Technology, Madras 600036, India Received 3 August 1994; accepted 10 May 1995

The rapid increase in population and its gro~g needs has led to a sharp decrease in the availa- bility of potable water. Desalination of brackish water to obtain drinking water has become a very important option. One of the most important techniques is membrane separation process. In this in- vestigation; electrodialysis (ED) and ultrafiltration (UF) techniques have been experimented with a view to soften the brackish water to potable levels. Waters from five localities in the Madras Metro.- polis have been acquired and analysed for their constituents and are processed for softening through ED and UF equipments. These influent whters along with sea water have been analysed.for six ions, dissolved oxygen and total dissolved solids. The performance of both these techniques is compared with reverse osmosis (RO). Amongst the three, ED has been found to be the most effective method.

Membrane processes can be employed to recover valuable by-products, reduce pollutants in the permeate, concentrate bewrages and drugs and produce .potable water from brackish sources.

Some of the membrane processes used for the softening ami desalination of brackish water are ultrafiltration (UP); reverse osmosis (RO) and electrodialysis (ED).

Membrane technology has advanced in industri- al applications over traditional separation pro- cesses like distillation, evaporation and extraction.

A

comparison of the various membrane processes with respect to particle sizes which can be reject- ed is shown in Table 1.

UP applies primarily to dissolved or suspended macromolecular species (10-7 to 10-3 cm dia)

which often do generate a -small osmotic pressure.

Driving pressures are usually. several atmospheres

(-1-10

bar) though it is much higher in the case of RO (ranges from - 30 to 40 bar). The solvent transpbrt in UP is by a viscous flow like mechan- ism through the membrane pores. The schematic diagram of the apparatus is shown elsewhere1 •

In ED the diffusion of ions is accelerated by an electric current, ED differs2 from RO in that the salt is removed from the water rather than water from the salt as in RO and in that the driving for- ce is electric potential rather than pressure. This technique is most widely used for the .processing of brackish water.

ED utilises two different types of specially developed polymeric membranes, one permeable

Process Membrane

type and pore size

Table I-Salient features of membrane processes

Driving force Mechanism Rejected species and size

Microfiltration Symmetric microporous membrane 0.2-10 pm pore radius

UltrafIltration Asymmetric microporous 1-20 nm pore radius Reverse Asymmetric skin type osmosis 0.1-10 nm radius

Hydrostatic pressure 1-10 psi

Hydrostatic pressure 10-100 psi

Hydrostatic pressure 100-1000 psi

Sieving due to pore radius and absorption Sieving

Solute diffusion

Suspensions colloidal particles of 300,000 MW Macrosolutes 300,000 to 300MW

Microsolutes 300 MW Dialysis Symmetric microporous Concentration

0.1-10 nm radius gradient

Diffusion in convection Microsolutes free layer; membrane

permeation by molecules or ions

Electrodialysis Ion-exchange membrane Electrical potential . gradient

*Author to whom correspondence should be addressed

Electrical charge and ionic·mobility

Ions

l~

INDIAN 1. CHEM. TECHNOL., JANUARY 11)1}6

fig;-l-Schematic diagram ofthe electrodialysis (ED) apparatus

Kirk and Othmer4 and is shown in Fig. 1. The equipment comprises of stacks, d.c. supply unit, pump and regeneration system, cartridge filter and hydraulic system.

The sample water is pumped through a tube in- to the stacks. The cartridge ,filter helps in remov- ing macroscopic impurities from the influent wa- ter; when the' d.c. supply unit is switched on and the water passed through the stacks, and further separates into a concentrate stream and a perme- ate stream.

Method of operation-Water samples from five localities in the Madras Metropolis were acquired for treatment during May 1993. Sea water was al- so collected for analysis alone as the IDS was too high for the equipments to handle. This period was chosen to obtain the highest concentration of IDS in the influent saline water during the year.

Pretreatment- The suspended particulates were allowed to settle down by leaving the samples overnight in a container. This water was used as feed for the UF and ED operations.

Ultrafiltration-The module was compressed to a pressure of 150 kN using the hydraulic pump and the operating pressure was maintained at 8 bar. For one of the water samples the experi- ment was conducted for four pressures, viz., 6, 7, 8 and 9 bar. Ten sets of membranes consisting of 10 support plates with membrane on either side (totalling 20) were used. The permeate was col- lected and analysed when the steady state reached after five minutes.

Electrodialysis- The experiments were conduct- ed at 26 psi (1.79 bar) pressure for all the waters.

For one of the influent water samples the experi- ment was conducted for four pressures, viz., 24 psi (1.65 bar), 26 psi (1.79 bar), 28 psi (1.93 bar) and 30 psi (2.07 bar). The permeate was collected and analysed after steady state has reached after five minutes.

Orion pH and ion-electrode meter-Analysis of raw water (feed) and permeate was· carried out to anions and the other to cations. The electrical

energy required and the ionic movement are pro- portional to the concentration of the salt in the saline water3. The ionic movements and the re- sulting demineralisation in this process lead to the production of two streams, viz., brine which car- ries the concentrated ions ;-!nd the product water.

When the ED unit is in operation the feed water travels in parallel paths through all the cells pro- viding a continuous flow of product and brine.

In the present investigation, desalination and softening of brackish water employing ED and UF techniques has been attempted. It was planned to (a) Study the characteristics of the DDS lab unit UF/RO module M20 and the Nu- chemweir DeSAUN ED apparatus, by carrying out some experiments for softening the brackish water (b) Report the analysis of the influent and permeate obtained by desalination of ground wa- ter (brackish) acquired from various parts of Madras City.

Experimental Procedure

Ultrafiltration (UF) equipment-The DDS (De Danske Sukkerfabrikken) plate and frame lab unit module M20 employed consists of an operating table on which the membrane modules of three differrent sizes (0.36, 0.72 and 2.0 m2) can be mounted. The other accessories include one dou- ble tank consisting of a nine-litre tank adjacent to a three-litre tank. Both the tanks are connected to the pump solution side and can therefore be used as feed tanks as well. However, normally the larg- er tank is used as a feed tank and the smaller one for final concentrate/permeate. The outlet of each tank is provided with a shut off valve. The outlet from each shut off valve is led to a common solu- tion pipe leading to a high pressure feed pump which has a capacity of 10

Umin.

The pump dis- charge side is connected to the heat exchanger at the module inlet which cools down the liquids be- fore it enters the module. A bypass is provided on the pump suction side of the connecting pipe.

The bypass line is provided with a valve. When the bypass valve is closed the module inlet flow equals the pump capacity. Two gauges at relevant locations indicate the inlet and outlet pressures.

The operating pressure is the average of these two pressures. A hydraulic pump is provided to compress the module to the specified pressure, viz., 150 kN. The equipment can handle only wa- ter which has a IDS of less than 7000 ppm.

Electrodialysis (ED) equipment-The equip- ment used is the Nuchemweir DeSAUN appara- tus. Details of arrangement are similar to that of

A-tpower oouru

Mrmbronr stack

r ^----1

^{, Productwater}

Concentrate disctlorge

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MUTHUKRISHNAN et al; DESAUNATION OF BRACKISH WATER 19

employing this instrument. This has a facility for performing calibration, measurement and verifica- tion of results automatically providing improved speed and accuracy as well as an economically vi- able method of analysis. To measure a cation/an- ion, the relevant electrode is used: Standard salt solutions of the ion to be measured are used ,to calibrate the equipment. A plot between the elec- tric potential and the concentration is drawn in- ternally by the instrument. It follow§ the Nernst equation.

E= Eo+slogA

Table 2 - Effect of operating pressure on permeate flux Conversion factor 1 psi=0.06875 bar]

Ultrafiltration Electrodialysis Operating Permeate fluxPermeateOperating

pressure

(11m2 h)pressureflux (bar)

(11m2 h) 6

1.65 bar (24 psi)26.8 5.28 7

1.79 bar (26 psi)32.0 5.90 8

1.93 bar (28 psi)35.6 6.65 9

2.07 bar (30 psi)35.7 6.70

Table 3(a)-Analysis of lIT water (UF) at 28°C Parameters

Influent

Permeates at pressure (bar) (expressed as

(feed) ppm)

6 7

8 9 IDS

1150 920 900910900 Ca++

45.1 41.9 45.1 45.941.9 Na+

114 63 61.749.661 K+

8 4.9 5.35.7 4.1 Cl-

154161 149 153 144 F-

0.00090 000 NO;

7.7 5.0 5.3 4.94.7

(Concentrations of less than 0;0001 ppm appear as 0)

where A is activity of the ion, E is the electric potential developed, Eo is the standard electrical potential determined by the instrument and s is the slope of the curve.

Results and Discussion

The characteristics of the DDS lab unit (M20, UF/RO module) and the Electrodialysis Nuchem- weir DeSAIlN apparatus were first studied. For this, the variation of permeate flux (Table 2, Fig. 2) as well as solute (chloride) concentration [Table 3a,b] in the permeate with respect to op- erating pressure; and the variation of permeate flux with respect to time (Table 4, Fig. 3) were in- itiallystudied3•

Brackish ground water from various parts of Madras Metropolis, viz., lIT Taramani Village, Velachery, Cosmopolitan club and Madras Uni- versity were acquired. An attempt to desalinate these waters was made employing the techniques of ED at 1.79 bar (26 psi) and UF at 8 bar. The

50

.z; 40

..•^..••

- ^.:

^'0~^{....•~••}

.

^E302010

, 6 8 10

Op.rating pr.~sur., .bar Fig. 2-Effect of pressure on permeate flux Table 3b- Analysis of lIT water (ED) at 28°C

Parameters

Influent Permeates at pressure (bar)(psi)i, (expressed as ppm)

(feed) 24 psi

26 psi 28 psi30 psi (1.65 bar)

(1.79 bar)(1.93 bar)(2.07 bar) IDS

1150 145 145 150155 Ca++

45.15.3 5.87.4 11.8 Na+

114 13 23.225.638.8 K+

8 0.860.990.880.9 Cl-

161 16.1 19.424.932.3 F-

0.0009000 0 NO;

7.7 1.1 1.3 1.61.5

(Concentrations of less than 0.0001 ppm appear as 0)

20 INDIAN 1. CHEM. TECHNOL.,JANUARY 1996

$0 by UF, six cycles will be required to bring down the IDS of Taramani water to potable li- mits.

500 ppm. Hence, to bring down the IDS of say, Taramani water which amongst the acquired sam- ples in most saline to 500 ppm.

By UF Yms =^mj(uf)Xms feed water and the permeate obtained were ana-

lysed for some important ions and the results are given in Table 5.

Employing the above results in each case an at- tempt was made to find a correlation between the feed water and the permeate quality with respect to total dissolved solids (IDS), and chloride con- tent.

Total dissolved solids ( TDS)

-IDS

in the permeate is plotted against IDS in the influent sample (Fig. 4) for each of the five samples pro- cessed by ultrafiltration and electrodialysis tech- niques. In both the techniques, the permeate IDS was found to be a linear function of the feed IDS.

First cycle Second cycle Third cycle Fourth cycle Fifth cycle Sixth cycle

YTDS=(0.700) 4900 = 3430

YTDS=(0.700) 3430 = 2401

.YTDS=(0.700) 2401 = 1680.7

YTDS=(0.700) 1680 = 1176.50

YTDS=(0.700) 1176.5 = 588.2

YTDS=(0.700) 588.2 =411.8

YTDS= mjXTDS

where, Yms=IDS in permeate, Xms^"= IDS in feed sample and mj=slope of the plot.

H was observed from the plot that m1(uf)=O.700, m](ed)=O.109. As per ISI standards the potable limit for IDS is a maximum of

Table 4- Effect of operating time on permeate flux Ultrafiltration Electrodialysis

40

.-1

UF ^'x

Time Permeat fluxPermeateTime min

(11m2minh) flux(11m2 h) 5

34.2 5 6.25 10

35.210 6.30 15

35.015 6.34 20

35.120 6.30 25

34.825 6.34

o

Fig. 3 - Effect of operating time on permeate flow 40

Table 5-Analysis of brackish water from various parts of Madras City

t'arameters

I.I.T.TaramaniVelachery CosmopolitanSeawaterMadras UniversityClub F

ED UFFEDUF FED UF FED UFF ED UFF

pH

7.5 7.67.67.77.87.75 7.47.45 7.5 7.37.4 7.4 7.87.9 7.97.35

Temperature

28 28 28282828 2828 28 2828 28 2828 2828

rq TOS lJlI>ml

150 1150 91049003500550 1600 1250200 2150270 16002400275 175014300 Dtssolwd

--^6.59-- 7.97 7.82 -

7.03- -

7.63--

7.98

O,lppml c."lJlI>m)

5.8 45.1 45.795.771.47.2 39.29.6 36.7 18.710.3 18.6 30.47.1 30.1674 Na·(pptn)

25.6 144 49.626803741140 42927.4 190 72731.7 449 29349.8 25528100 K'!ppm!

8 0.865.7137.6122.314.6 18.42.5 11.112.32.1 9.229.64.7 19.9 720 enppm)

161 19.4 156132080783 25527.6 181 22018.6 20856230.7

3337160 --or

0.0005

I

F"'Ip"",1

0.0009 0 0 0.00240.00080 0.00070

O· 0

0 0.00040.005 00 No;"tppm)

7.7 1.3 4.93.20.41.9 5.30.7 3.6 10.71.2 7.54.00.38 2.30.00&

F= Feed; ED = Permeat from ED( operating pressure - 1.79 bar or 26 psi)

UF= Permeate from UF (operating pressure = 8 bar) (Concentratijons ofless than 0.0001 ppm appear as 0) Dissolved oxygen was measured only for the raw waters.

I It t 1 I I~ ij' t,

5000

~ 3000 E

800

Go

Go

. ^•

Go

^•

C;~

_• •

²⁰⁰⁰

!

0

^{• •}

Q. 400 IL

100e)

8000 fnflu.nt, ppm

Fig. 4- Effect of treatment on IDS Fig. 5 - Effect of treatment on chloride content

Sample

Table 6 -Number of passes required to produce potable water

UP (8 bar) ED (1.79 bar) (26 psi)

No. of passes Final permeate, ppmFinal perme!lte, ppmNo. of passes IDS

CI-CI- IDS

lIT

3 b94.5028.731

125.359.15 Taramani

6 411.8058.2042.044.302

Velachery

384.1625.621 4 174.4014.54 Cosmopolitan Club

5 361.4012.441

234.4012.54 Madras University

5 403.4031.801

261.6032.04 Potable limits

500

250 500

250

Hence by ED, two cycles will be required to bring down the

IDS

of this brackish water to po- table limits.

Chloride content-Fig. 5 shows a plot of chlo- ride concentration in the permeate as a function of chloride concentration in the influent sample for ED, UP and R01•

A similar procedure as cited earlier for

IDS

was employed and the chloride concentration in the permeate was correlated as

YC\=m2XCl

It was observed from the plot that

First cycle Second cycle

Yms=(0.1"09) (4900)= 534.1 Yms=(0.109) (534.1)= 58.2

m2(uf) =0.563

m2(ed) =0.057

The number of cycles required for each of the water samples to be brought to potable limits with respect to

IDS

and chloride ion concentration is given in Table 6.

Powerrequirements-'-From the power rating given for the equipment, to produce 1 m3 of treated water, and from computations for a single pass for the two equipments, the following power requirements were obtained:

1. UF (membrane area

=

0.28 ^m2, operating pres- sure

=

8 bar): 3000 kWh/m3•

2. ED (membrane area =11.2 m2, operating pressure

=

26 psi or· 1.79 bar): 12.2kWh/m3•

For a comparison of the performance of the different membrane techniques, viz., ED, UF and RO (the data on RO was taken from a previous

22 INDIAN J. CHEM. TECHNOL., JANUARY 1996

Table 7-Comparison of standards of potable water with saline and processed wat;trs (All data in ppm)

Source of

IDS pH DissolvedNO;Mg+SO.K+Cl-F-Na+Ca++

water/standards

O2 ISI Std.

6.5-8.5

-

500 75

- ^- -

250 0.6-1.245 WHO Std.

- -

-

-

-- -

200 0.7-1.550.100 Sea water

7.35 339803904.5

104301230193000.00523900.008350 Raw water

7.7 4900

6.5995.72680

137:6 1320

0.00243.2

(most brackish) *Treated (ED)

550 7.75

-

^7.2374

14.6 80

-

0.4

*Treated (UF)

3500

-

7.8

71.4122.31140 783 0.00081.9

*One cycle of treatment.

Correlation for removal of

study! is given in Figs 4 and 5).

From the plots, the following can be inferred:

(i) For IDS removal: ED is most effective fol- lowed by RO and UE (ii) For chloride removal:

ED is most effective followed by UF and RO.

Table 7 gives a comparison of the standards and requirements of potable water with the analy- sis obtained from sea water and most brackish water processed in this investigation.

Conclusions

The five brackish waters were treated by UF and ED and are analysed. The following are the conclusions from this study.

1 Varying ·of operating pressure- In both the techniques it was found that permeate flux in- creases linearly with operating pressure up to a threshold pressure (8 bar in case of UF and 1.79 bar (26 psi.) in case of ED), beyond which the pressure has little influence on the permeate flux.

2 For UF- Though permeate flux varied linear- ly with pressure the chloride content in the permeate was found to be almost constant over the range of pressure studied. For ED, the perme-

ate flux and chloride content varied with pressure.

3 For both UF and ED-At constant pressure, the permeate flux was found to be constant over the entire experimental period.

4 From an analysis of the feed and permeate from the five brackish water samples, it was found that IDS and chloride content could be correla- ted by the following equations.

Mechanism

(and operating _

pressure) IDS Chloride

UF (8 bar) YTDS = (0.700) XTDS YCI= (0.563) XCI

ED (1.79 bar ^YTDS =(0.109) XTDS YCI=(0.057) ^XCI or 26 psi)

References

1 Ajay Lakshmanan, Babu Srinivas M, Venkatnim T &Sas- try C A, Indian JEnviron Prot, 12(8) 1992, 572.

2 Sastry C A, Ch VI, 'Water' CHEMTECH-I(ChEEDC., lIT, Madras) 1975.

3 Harris R C, Elyanow D, Heshka D N& Fischer K L, De:r alination, 84 (1991) 109.

4 Kirk-Othmer, Membrane Technology, Encyclopaedia of Chemical Technology, Vol. 15, 3rd ed. (John Wiley &

Sons), 1981.

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