Desalination of brackish water by membrane separation techniques

Transcription

1 Indian Journal of Chemical Technology Vol. 3, January 1996, pp 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 , 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 availability 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 investigation; 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 industrial applications over traditional separation processes like distillation, evaporation and extraction. A comparison of the various membrane processes with respect to particle sizes which can be rejected 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 mechanism 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 force 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 pm pore radius Hydrostatic pressure 1-10 psi UltrafIltration Asymmetric microporous Hydrostatic pressure 1-20 nm pore radius psi Reverse Asymmetric skin type Hydrostatic pressure osmosis nm radius psi Dialysis Symmetric microporous Concentration nm radius gradient Electrodialysis Ion-exchange membrane Electrical potential. gradient Sieving due to pore radius and absorption Sieving Solute diffusion Diffusion in convection free layer; membrane permeation by molecules or ions Electrical charge and ionic mobility Suspensions colloidal particles of 300,000 MW Macrosolutes 300,000 to 300MW Microsolutes 300 MW Microsolutes Ions *Author to whom correspondence should be addressed

2 l~ INDIAN 1. CHEM. TECHNOL., JANUARY 11)1}6 to anions and the other to cations. The electrical energy required and the ionic movement are proportional to the concentration of the salt in the saline water3. The ionic movements and the resulting demineralisation in this process lead to the production of two streams, viz., brine which carries the concentrated ions ;-!ndthe product water. When the ED unit is in operation the feed water travels in parallel paths through all the cells providing 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 Nuchemweir 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 water (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 mounted. sizes (0.36, 0.72 and 2.0 m2) can be The other accessories include one double 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 larger 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 solution pipe leading to a high pressure feed pump which has a capacity of 10 Umin. The pump discharge side is connected to the heat exchanger at the module inlet which cools down the liquids before 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 water which has a IDS of less than 7000 ppm. Electrodialysis (ED) equipment-the equipment used is the Nuchemweir DeSAUN apparatus. Details of arrangement are similar to that of 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 into the stacks. The cartridge,filter helps in removing macroscopic impurities from the influent water; when the' d.c. supply unit is switched on and the water passed through the stacks, and further separates into a concentrate ate stream. stream and a perme Method of operation-water samples from five localities in the Madras Metropolis were acquired for treatment during May Sea water was also 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 experiment 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 collected and analysed when the steady state reached after five minutes. Electrodialysis- The experiments were conducted at 26 psi (1.79 bar) pressure for all the waters. For one of the influent water samples the experiment 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 reached after five minutes. after steady state has Orion ph and ion-electrode meter-analysis of raw water (feed) and permeate was carried out A-t power oouru Mrmbronr stack r----1 Product, water fig;-l-schematic diagram of the electrodialysis (ED) apparatus Concentrate disctlorge I I I. H II ""l~ '1" "I '~I Iii;1i ' ~Ij l' I U I II

3 ,. 4--' /""V- -:::1 -- '-'L-\ '\.1 MUTHUKRISHNAN et al; DESAUNATION OF BRACKISH WATER 19 employing this instrument. This has a facility for performing calibration, measurement and verification of results automatically providing improved speed and accuracy as well as an economically viable method of analysis. To measure a cation/anion, the relevant electrode is used: Standard salt solutions of the ion to be measured are used,to calibrate the equipment. A plot between the electric potential and the concentration is drawn internally by the instrument. It follow the Nernst equation. E= Eo+slogA Table 2 - Effect of operating pressure on permeate flux Operating Conversion Ultrafiltration (11m2 Permeate bar flux pressure Operating (30 (26 (28 ( h) flux psi) factor 1 psi = bar] Electrodialysis (11m2 h) 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 Nuchemweir 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 operating pressure; and the variation of permeate flux with respect to time (Table 4, Fig. 3) were initiallystudied3 Brackish ground water from various parts of Madras Metropolis, viz., lit Taramani Village, Velachery, Cosmopolitan club and Madras University 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 pm) Parameters Table (a)-Analysis 0Permeates Influent 7.7(feed) at 6pressure of lit water (bar) (UF) at 28 C.:.z; E ' ~ 10 (Concentrations of less than 0;0001 ppm appear as 0), Op.rating pr.~sur.,.bar Fig. 2-Effect of pressure on permeate flux Parameters ( (2.07 ( psi Influent bar) (feed) Table Permeates 3b- Analysis at pressure of lit (bar)(psi)i, water (ED) at 28 C 24 psi (Concentrations of less than ppm appear as 0)

4 20 INDIAN 1. CHEM. TECHNOL.,JANUARY 1996 feed water and the permeate obtained were analysed for some important ions and the results are given in Table 5. Employing the above results in each case an attempt was made to find a correlation between the feed water and the permeate quality with respect to total dissolved solids (IDS), and chloride content. Total dissolved solids ( TDS) -IDS in the permeate is plotted against IDS in the influent sample (Fig. 4) for each of the five samples processed by ultrafiltration and electrodialysis techniques. In both the techniques, the permeate IDS was found to be a linear function of the feed IDS. 500 ppm. Hence, to bring down the IDS of say, Taramani water which amongst the acquired samples in most saline to 500 ppm. By UF Yms= mj(uf)xms 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 = YTDS =(0.700) 1680 = YTDS =(0.700) = YTDS =(0.700) =411.8 $0 by UF, six cycles will be required to bring down the IDS of Taramani water to potable limits. YTDS = mj XTDS 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 40 UF.-1 'x Table 4- Effect of operating time on permeate flux Ultrafiltration Electrodialysis Time flux (11m Permeate (11m min Time h) flux h) o 40 Fig. 3 - Effect of operating time on permeate flow UF FED F UFED Cosmopolitan Seawater Madras I.I.T. I & O Taramani Velachery --or University Club Table 5-Analysis of brackish water from various parts of Madras City TOS c. "ljli>m) ljli>ml (Concentratijons Dissolved UF= Permeate oxygen from ofless was UF measured than (operating only pressure ppm for the appear = raw 8 bar) as waters. 0) I It t

5 MUTHUKRISHNAN et al: DESAUNATION OF BRACKISH WATER Go Go ~ 3000! IL E 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) b CI Final No. permeate, perme!lte, No. of46 53 passes IDS 500 of passes ppm 500 First cycle Second cycle Hence by ED, two cycles will be required to bring down the IDS of this brackish water to potable limits. Chloride content-fig. 5 shows a plot of chloride 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 Yms =(0.1"09) (4900)= Yms =(0.109) (534.1)= 58.2 It was observed from the plot that m2(uf) = m2(ed) = 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 pressure = 8 bar): 3000 kwh/m3 2. ED (membrane area = 11.2 m2, operating pressure = 26 psi or 1.79 bar): 12.2 kwh/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

6 22 Dissolved IDS NO; Mg+ 137: SO Cl- F- K Na Ca ph - INDIAN O2 J. CHEM. TECHNOL., JANUARY 1996 *Treated (ED) (All data in ppm) ble 7-Comparison of standards of potable water with saline and processed wat;trs study! is given in Figs 4 and 5). From the plots, the following can be inferred: (i) For IDS removal: ED is most effective followed 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 analysis 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 increases 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 linearly with pressure the chloride content in the permeate was found to be almost constant over the range of pressure studied. For ED, the permeate 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 correlated by the following equations. Mechanism Correlation for removal of (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 & Sastry C A, Indian J Environ Prot, 12(8) 1992, Sastry C A, Ch VI, 'Water' CHEMTECH-I(ChEEDC., lit, Madras) Harris R C, Elyanow D, Heshka D N & Fischer K L, De:r alination, 84 (1991) Kirk-Othmer, Membrane Technology, Encyclopaedia of Chemical Technology, Vol. 15, 3rd ed. (John Wiley & Sons), I "