- Email: [email protected]
Use of simulated evaporation to assess the potential for scale formation during reverse osmosis desalination
Desalination 160 (2004) 285-292
Use of simulated evaporation to assessthe potential for scale formation during reverse osmosis desalination G.F. Huff U.S. Geological Survey,New Mexico State University, Dept. MRP, PO Box 30001, Las Cruces, NM88003 USA Tel. +I (505) 646-7950; Fax: i-1 (505) 646-7949; email: [email protected]
Received 16 April 2003; accepted 2 June 2003
Abstract The tendency of solutes in input water to precipitate efficiency lowering scale deposits on the membranes of reverse osmosis (RO) desalination systems is an important factor in determining the suitability of input water for desalination. Simulated input water evaporation can be used as a technique to quantitatively assessthe potential for scale formation in RO desalination systems. The technique was demonstrated by simulating the increase in solute concentrations required to form calcite, gypsum, and amorphous silica scales at 25°C and 40°C from 23 desalination input waters taken from the literature. Simulation results could be used to quantitatively assessthe potential of a given input water to form scale or to compare the potential of a number of input waters to form scale during RO desalination. Simulated evaporation of input waters cannot accurately predict the conditions under which scale will form owing to the effects of potentially stable supersaturated solutions, solution velocity, and residence time inside RO systems. However, the simulated scaleforming potential of proposed input waters could be compared with the simulated scale-forming potentials and actual scale-forming properties of input waters having documented operational histories in RO systems. This may provide a technique to estimate the actual performance and suitability of proposed input waters during RO. Keywords: Scaling potential; Simulated evaporation; Thermodynamic modeling; RO desalination
1. Introduction The tendency of solutes in input water to precipitate effkiency-lowering scale deposits on the membranes of reverse osmosis (RO) desalination systems is an important factor in determining the suitability of input water for desalination. Scale forms when solutes, concentrated during the RO process, precipitate from 001 l-9164/04/$PII: SO01
l-9
supersaturated solutions. Common scale-forming prkipitates in RO systems include calcite, gypsum, and amorphous silica, the formation of which are described by chemical reactions (l)(3), respectively: Ca*+ + CO:- = CaCO,
(1)
Ca*’ + SOi- + 2H,O = CaSO;2H,O
(2)
See front matter 8 2004 Elsevier Science B.V. All rights reserved
164(03)00668-4
286
H,SiO, = SiO, + 2H,O
G.F. Huff/ Desalination 160 (2004) 285-292
(3)
Pretreatmentof input water to minimize scale formation, andthe removal of scaleonceformed, can represent a significant cost in RO desalination.Detailed discussionsofthe theoreticaland applied aspectsof water purification using RO are available in the literature [ 1,2]. The main factors governingthe formation of scale inside RO systems include solute concentration, operatingtemperature,solution velocity, solution pH, and residence time [3]. The Langelier Saturation Index [4], the Ryznar Stability Index [5], and the Stiff-Davis Stability Index [6] describe the tendency of calcite to precipitatefrom aqueoussolution. Theseindices are limited in that they consideronly the values of pH, temperature, and the concentrationsof dissolved Ca and HCO, in solution [7]. Additional indicies, basedon high-temperature solubility data,areusedto describethe tendency of sulfate saltsand silica to precipitatefrom near roomtemperatureaqueoussolutions [7]. Thermodynamics-basedmodeling of the behavior of selected Ca salts and SiO, has been used to predict saturation concentrations of calcite, gypsum,andamorphoussilica in RO input water [3]. This thermodynamics-based approach accountedfor ionic interactions that affect the solubility of potential scale-formingprecipitates but did not account for the effects of solution modification by precipitation of solid phases duringRO. Alternative approachesto calculating a numerical value describing the tendency of individual scale-forming solids to precipitate in RO systemsuse classification schemesfor input waters based on Cl concentrations and molar ratios of selecteddissolvedconstituents[7,8] and graphicalinterpretationtechniques[9]. All ofthe aboveapproachesto calculating the tendencyof input waters to precipitate scale in RO systems are hamperedby the stability of aqueoussolutions supersaturatedwith respect to calcite,
gypsum, and amorphoussilica, particularly at or near room temperature. The purposeof this paper is to demonstrate the use of simulated evaporationto assessthe scale-forming potential of input waters for RO desalination systems. Simulated increases in concentrations of solutes needed to achieve saturation with respect to calcite, gypsum and amorphoussilica, along with the corresponding predicted solution ionic strengths,can be calculated for any RO system input water. Simulated input water evaporation also allows quantification of changesin solution composition during precipitation of scale-forming solids and allows simulation of the effects of these changeson precipitation of subsequentscale-formingsolids. The use of simulated evaporationto assessthe scale forming potential of RO input water is demonstratedusing 23 desalination operation input waterstaken from the literature. 2. Method and data Desalination of water using RO extracts fresherwater(permeate)from a more salineinput water, effectively increasing solute concentrations in the water remaining (concentrate) within the RO system.Flows of input water into a desalination system upstream of the RO membranearematchedby flows of permeateand concentrateout of the system during operation. Soluteconcentrationby simulatedevaporationof aqueoussolutions using PHREEQC [lo] can approximatethe chemical environment encountered during RO. PHREEQC is a widely used, publicly available, expandable,and well documented geochemical modeling code with an extensivethermodynamic data base.The capabilities of PHREEQC include simulating the chemical behavior of aqueoussolutions composed of all major and many minor solutes, including CO, and SO,,salts, as well as neutral speciesincluding SiO,. PHREEQC adequately
G.F. Hz@/ Desalination 160 (2004) 285-292
representsthe chemical behavior of solutes in aqueoussolutionshaving salinitiesof seawateror less [lo]. Seawaterof the composition given by Drever [ 1l] has an ionic strength of approximately 0.65. Simulated evaporationwas carriedout on RO input waters using PHREEQC in a series of sequentialstepsasdescribedbelow.All chemical analysis of RO input waters were screenedfor charge imbalances no greater than *3.0%. Simulated solution compositions were modified by evaporation and by reaction with plausible scale-formingphasesincludingCO, andSO, salts andamorphoussilica. Simulatedsolutioncompositions were not modified by reactionwith solid phasesthat typically do not form scaleinsideRO systems. As such, precipitation of dolomite, fluorite, sepiolite, talc, and crystalline forms of silica were not used to modify solution compositions eventhough supersaturationwith respect to these minerals was often reached during simulatedevaporation.Equilibration of simulated solutionswith a constantpartial pressureof CO, gas acts as a buffer preventing large changesin, and providing a reasonablerange of values for, pH during simulated evaporation.Buffering of pH is particularly important in calculating the solubility of carbonateminerals and amorphous silica, the solubility of which increasesrapidly at valuesof pH greaterthan about 8 [ 121. STEP 1: An initial pH of 7.5 is assumedfor input waters if no initial pH value is given. The composition of an RO input water is equilibrated at aselectedtemperaturewith CO, gasat a partial pressure of 10-2.5atmospheres.A speciation calculation is carried out and the results are inspectedfor minerals or amorphoussolids that are simulatedto be presentin aqueoussolution at concentrationsgreaterthanor equalto saturation. Step 1 is completed if no minerals or amorphous solids are simulated to be present at concentrations greaterthanor equalto saturationandthe
287
resultingsolution is usedin step2. Otherwise,the initially supersaturatedsolution composition is modified by precipitation of plausible scaleforming phasesto saturationconcentrations.The step-l equilibrium assemblageconsists of the final solution, the specified partial pressureof CO, gas,and any plausible scale-formingphases saturated with respect to the step-one final solution. STEP 2: Simulatedevaporationis carried out on the step-l equilibrium assemblageuntil the next plausible scale-forming phase in the PHREEQC data base reaches saturation with respectto the evaporatingsolution. All gas and solid components of the step-l equilibrium assemblagearepreservedat equilibrium with the aqueoussolution during step-2simulatedevaporation. The calculatedmass of water remaining after completion of step-2simulatedevaporation is recordedandusedto calculatea concentration factor (CF) for the newly-saturated plausible scale-formingphase.The CF valueis numerically equal to the mass of water initially present in step 1 divided by the mass of water remaining after simulated evaporation is completed in step 2. The step-2equilibrium assemblageconsists of all componentsidentified in step 1 plus any plausible scale-formingphasesidentified in step2. STEP 3-STEP n: The proceduredescribedin step 2 is sequentially repeatedon equilibrium assemblagesfrom the previous step until CF values for all plausible scale-forming phases present in the PHREEQC data base have been calculated.Values of the CF for plausible scaleforming phasesarecalculatedrelativeto the mass of water initially presentin step 1. Once identified, plausible scale-forming phasesarekept in equilibrium with the solution during simulated evaporation. This allows for
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simulated redissolution of any scale-forming phases that the solution may become undersaturated with respect to during progressive simulatedevaporation.Operatingtemperaturesin RO systemsrange from near room temperature up to approximately 50°C to take advantageof greatermembrane efficiency at elevated temperatures.Temperatureof 25OCand 40°C were selectedto representtypical operatingconditions for the purposeof this study. Input water analyses for samples shown in Tables 1 and 2 were taken from the following sources:Samples l-20 [9], samples21-22 [8], and sample 23 [3]. Samples I-10 and 11-20 representgroundwaterfrom southwesternKuwait withdrawn from the Kuwait Group aquifer and the Dammam limestone aquifer, respectively. Samples 21-22 were identified as part of the “available water analysis records that accumulated in Geochimica Laboratories(Cairo) during the last five years”. Sample 23 is brackish feed water for the Gabesdesalinationplant in South Tunisia. Data on the concentrations of major solutesincludingNa, K, Ca, Mg, SO,, HCO,, and Cl were presentfor all input water samples.Data on the concentrationof dissolvedSiO, andminor solutesincluding B, NO,, and F were presentfor samplesl-20.
3. Results and discussion Tables 1 and 2 list the results of simulated evaporationto assessthepotential of input waters to form calcite, gypsum, and amorphoussilica scale at 25°C and 40°C, respectively.Values of pH remained in the range of 7.05 to 7.52 and ionic strengthremainedlessthanthat of seawater at simulated equilibrium with potential scaleforming minerals. Values of the CF for calcite in input waters were at or near 1.00 for entries in Table 1 and equal to 1.00 for all entries in Table 2. Little or no solute concentrationwould be required to reach calcite saturationat 25OC.
Values of 1.OOwould be expectedfor theseinput waters at 40°C given the retrogradesolubility of calcite. Values of the CF for gypsum rangefrom 1.43-2.88 at 25°C and 1.52-3.07 at 40°C with the exceptionof sample2 1, which hasCF values of 4.93 at 25OCand 5.32 at 40°C. The apparent slight increase in gypsum solubility with increasing simulation temperature is primarily caused by the greater simulated mass of Ca removedfrom solution to achieveand maintain equilibrium with respectto calcite during simulated evaporation. Values of the CF for amorphoussilica show the greatesteffect of increased simulation temperaturerangingfrom 3.90-5.60at 25°C to 5.03-7.19 at 40°C. Conditions that would cause redissolution of plausible scaleforming phases were not observed in any simulation. Simulated evaporation indicates that input waters in samples l-20 will become saturated with respect to Ca scale minerals during RO desalination before reaching saturation with respectto amorphoussilica. Comparison of CF valuesfor samples l-10 and 1l-20 indicate that input water from the Kuwait Group aquifer has less potential to form gypsum scale than water from the Dammam limestone aquifer at both 25OCand 40°C. A large difference in the potential to form gypsum scale between samples 21 and22 at 25°C is illustrated by CF valuesof 4.93 and 1.43,respectively.A similar differencein the potential to form gypsum scale exists between these samples at 40°C. There is no large or systematic difference in the potential to form amorphoussilica scalebetweenwaters from the Kuwait Group and the Dammam limestone aquifersat 25OCor 40°C. Dataon major elementconcentrationsin input water used in this study appearto generally be adequate for the simple type of simulation described here. However, results of simulated evaporation(not shown in Tables 1 or 2) using unpublished analyses of input waters which containeddataon the concentrationsof Ba andSr
Solution properties at -saturation with respect to gypsum ..--- _____ Ionic CF PH strength 7.22 0.120 2.05 0.132 7.19 1.97 7.20 0.134 2.01 7.18 0.133 1.75 7.17 0.150 2.09 7.17 0.151 2.04 7.19 0.126 1.59 7.18 0.140 1.99 7.20 0.125 1.78 7.18 0.129 1.58 7.29 0.106 2.32 7.3 1 0.125 2.88 7.29 0.115 2.64 7.27 0.120 2.18 7.27 0.123 2.46 7.29 0.108 2.31 7.26 0.114 2.39 7.27 0.114 2.17 7.25 0.119 2.22 7.23 0.125 2.17 7.34 0.344 4.93 7.27 0.112 1.43 7.22 0.111 2.08
*A value of pH was not reported in the original data source material. A pH of 7.5 was assumed. -, Data needed to calculate values were not reported in the original source material.
Table 1 Results of simulated evaporation at 25°C on desalination input waters --.~~. Reported Solution pH in Solution properties at Sample Sample equilibrium with CO, saturation with respect number name PH gi;s at 1O-2.5 to calcite (this paper) (original data source) atmospheres Ionic CF PH strength 5K 7.40 7.34 7.32 0.0617 1.oo 1 7.18 7.26 0.0840 1.22 2 6K 7.95 7.30 0.0700 1.oo 3 8K 7.50 7.30 11K 7.80 7.23 0.0911 1.17 4 7.18 12K 7.45 7.24 7.26 0.0796 1.06 5 6 13K 7.60 7.21 7.25 0.0866 1.13 14K 7.45 7.23 7.25 0.0850 1.04 7 15K 7.50 7.25 7.27 0.0777 1.06 8 9 17K 7.85 7.25 7.27 0.0769 1.06 10 18K 7.50 7.24 7.24 1.oo 0.0839 2L 7.90 7.60 7.40 11 0.0502 1.00 I2 3L 8.00 7.59 7.43 0.0484 1.oo 13 5L 7.90 7.61 7.41 0.0479 I .oo 14 7L 7.20 7.77 7.37 0.0592 1.oo 8L 8.15 7.53 15 7.39 0.0546 1.00 16 9L 7.65 7.61 7.39 0.0511 1.00 17 12L 7.95 7.56 7.37 0.0515 1.00 18 17L 7.80 7.58 7.37 0.0565 1.00 19 19L 7.30 7.69 7.35 0.0578 1.00 20 2OL 7.35 7.84 7.34 0.0617 1.oo * 21 #40 7.99 7.52 0.0766 1.00 * 22 #206 8.02 7.32 1.00 0.0809 Gabes plant 7.40 23 7.46 7.32 0.0565 1.oo feed water .~ -7.22 7.17 7.19 7.15 7.12 7.14 7.16 7.16 7.18 7.14 7.33 7.33 7.32 7.30 7.30 7.33 7.29 7.30 7.27 7.24 -
-PH
__ ...~___ Ionic CF strength 0.196 4.20 0.215 3.91 0.222 4.05 0.245 4.05 0.322 5.60 4.34 0.269 4.05 0.249 0.229 3.92 0.218 3.93 0.250 3.90 0.154 4.09 0.163 4.23 0.167 4.63 0.215 5.02 0.185 4.42 0.173 4.66 0.186 4.95 0.180 4.29 0.198 4.62 0.190 3.97 -
Solution properties at saturation with respect to amorphous silica
5K 6K 8K 11K 12K 13K 14K 15K 17K 18K 2L 3L 5L 7L 8L 9L 12L 17L 19L 2OL #40 #206 Gabes plant feed water 7.40
7.40 7.95 7.50 7.80 7.45 7.60 7.45 7.50 7.85 7.50 7.90 8.00 7.90 7.20 8.15 7.65 7.95 7.80 7.30 7.35 * *
Solution properties at Solution pH in equilibrium with CO, saturation with respect to calcite gis at 1O-2.5 atmospheres Ionic CF PH strength 7.43 7.27 0.0599 1.oo 7.24 0.0682 1.00 7.27 7.25 0.0683 1.00 7.39 7.21 0.0767 1.oo 7.27 7.23 1.00 7.33 0.0734 7.30 7.23 0.0756 1.00 7.32 7.21 0.0800 1.00 7.34 7.23 0.0717 1.00 7.34 7.23 0.0710 1.oo 7.20 0.0819 7.33 1.00 7.69 7.36 0.0480 1.oo 7.68 7.40 0.0464 1.00 7.37 7.69 0.0459 1.oo 7.32 7.85 0.0568 1.00 7.62 7.35 0.0525 1.00 7.70 7.35 0.0490 1.00 7.65 7.33 0.0497 1.oo 7.67 7.33 0.0543 1.00 7.77 7.31 0.0556 1.oo 7.92 7.29 0.0594 1.oo 8.07 7.49 0.0746 1.00 8.09 7.28 0.0775 1.oo 7.55 7.28 0.0548 1.00 7.17 7.14 7.15 7.12 7.12 7.12 7.14 7.13 7.15 7.12 7.24 7.26 7.24 7.21 7.22 7.24 7.21 7.22 7.20 7.18 7.29 7.22 7.17
PH
Ionic strength 0.123 0.136 0.139 0.137 0.156 0.158 0.131 0.145 0.129 0.134 0.108 0.128 0.117 0.122 0.126 0.110 0.116 0.116 0.122 0.129 0.363 0.114 0.113 PH 7.16 7.11 7.13 7.08 7.05 7.07 7.09 7.09 7.12 7.07 7.29 7.29 7.27 7.25 7.26 7.29 7.24 7.25 7.22 7.19 -
2.18 2.10 2.13 1.86 2.22 2.18 1.69 2.11 1.89 1.68 2.46 3.07 2.81 2.32 2.62 2.45 2.53 2.31 2.36 2.32 5.32 1.52 2.20
-
Ionic strength 0.236 0.262 0.270 0.301 0.398 0.331 0.305 0.281 0.266 0.307 0.182 0.194 0.198 0.257 0.221 0.205 0.223 0.215 0.237 0.228
5.45 5.06 5.24 5.23 7.19 5.60 5.22 5.07 5.09 5.03 5.32 5.51 6.02 6.51 5.74 6.06 6.43 5.58 6.00 5.15 -
CF
Solution properties at saturation with respect to amorphous silica
CF
Solution properties at saturation with respect to gypsum
*A value of pH was not reported in the original data source material. A pH of 7.5 was assumed. -, Data needed to calculate values were not reported in the original source material.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Sample Sample Reported number name PH (this paper) (original data source)
Table 2 Results of simulated evaporation at 40°C on desalination input waters
G.F. Huff/ Desalination 160 (2004) 285-292
[ 131 indicate that potential scale-forming minerals such as celestite (SrSO,) and barite (BaSO,) may precipitate at CF values close to that of gypsum. This may explain the observed occurrenceof Ba and Sr in Ca-rich scales[S]. It is thereforerecommendedthat analysesof input waters used in the described simulated evaporation technique include information on concentrations of Ba and Sr as well as the major elementspreviously listed. A methodhasbeendescribedto this point that could be used to quantitatively assess the potential of a given input water to form scaleor to compare the potential of a number of input waters to form scale during RO desalination. However, the effects of stable supersaturated solutions, solution velocity, and residencetime inside RO systems do not allow calculation of actual CF vales at which scale will form. Simulated evaporationusing PHREEQC, or any purely thermodynamicmodel, cannotadequately addressthese effects. However, the simulated scale-forming potential of proposedinput waters could be compared with the simulated scaleforming potentials and actual scale-forming properties of input waters having documented operational histories in RO systems.This may provide a technique to estimate the actual performance of proposed input waters in RO systemsunder a specified set of conditions.The thermodynamic data base of PHREEQC is expandableandadditional chemicalreactionscan be defined within the program.Theseproperties of PHREEQC could allow the effects of antiscaling additives to be estimated provided the required information on their composition and thermodynamic propertiesare known. Experimental verification of the methodsand techniquesproposedin this study are needed.If validated by experimental data, the technique described in this study could provide a reasonableand systematic method for assessing the suitability of proposedinput waters for RO desalination.
291
4. Conclusions
Simulated evaporationof input water can be used as a technique to quantitatively assessthe potential for scaleformation in RO desalination systems.The useof PHREEQC to simulate input water evaporation allows quantification of changes in solution composition during precipitation of scale-forming solids and allows simulation of the effects of these changes on precipitation of subsequentscale-formingsolids. The techniquewas demonstratedby simulating the increasein soluteconcentrations,represented by CF values,requiredto form calcite, gypsum, and amorphoussilica scalesat 25°C and 40°C from 23 desalinationinput waterstaken from the literature. The resulting simulated CF values could be usedto quantitatively assessthe potential of a given input water to form scale or to compare the potential of a number of input waters to form scale during RO desalination. Simulated evaporation of input waters cannot accurately predict the conditions under which scalewill form owing to the effectsof potentially stablesupersaturated solutions,solutionvelocity, andresidencetime inside RO systems.However, thesimulatedscale-formingpotentialof proposed input waters could be compared with the simulated scale-forming potential and actual scale-formingpropertiesof input waters having documentedoperationalhistoriesin RO systems. This may provide a technique to estimate the actual performance of proposed input waters during RO desalinationunder a specified set of conditions.If validatedby experimentaldata,the techniquedescribedin this studycould provide a reasonableand systematicmethod for assessing the suitability of proposedinput waters for RO desalination. Acknowledgements
The contributionof information andtechnical expertiseby Michelle Chapmanofthe US Bureau
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G.F. Husf/ Desalination 160 (2004) 285-292
of Reclamation and Michael Hightower of Sandia National Laboratories is gratefully acknowledged.
References [l]
[2]
[3] [4] [5]
Z. Amjad, ReverseOsmosis:Membrane Technology, Water Chemistry, and Industrial Application, Van Nostrand Reinhold, New York, 1993. U.S. Environmental Protection Agency, Reverse osmosis process: Technology Transfer Capsule Report EPA-625-R-96-009, 1996. B. Hamrouni and M. Dhahbi, Desalination, 137 (2001) 275-284. W.F. Langelier, J. AWWA, 28 (1936) 1500-1521. J.W. Ryznar, J. AWWA, 36 (1944) 472-486.
[6] H.A. Stiff, Jr. and L.E. Davis, Pet. Trans. AIME, 195 (1952) 213-216. [7] S. El-Manharawy and A. Hafez, Desalination, 136 (2001) 243-254. [8] S. El-Manharawy and A. Hafez, Desalination, 139 (2001) 97-l 13. [9] K.M.B. Hadi, Desalination, 142 (2002) 209-219. [lo] D.L. Parkhurst and C.A.J. Appelo, User’s guide to PHREEQC (version 2) - A computer program for speciation,batch-reaction, one-dimensional transport, and inversegeochemical calculations. US Geological Survey, Water-Resources Investigations Report 994259,1999. [l l] J.I. Drever, The Geochemistry of Natural Waters, Prentice-Hall, Englewood Cliffs, 1982. [12] G.B. Alexander, W.M. Heston and R.K. Iler, J. Phys. Chem., 58 (1954) 453-455. [13] M. Chapman, U.S. Bureau of Reclamation, personal communication, 2002.
Use of simulated evaporation to assessthe potential for scale formation during reverse osmosis desalination G.F. Huff U.S. Geological Survey,New Mexico State University, Dept. MRP, PO Box 30001, Las Cruces, NM88003 USA Tel. +I (505) 646-7950; Fax: i-1 (505) 646-7949; email: [email protected]
Received 16 April 2003; accepted 2 June 2003
Abstract The tendency of solutes in input water to precipitate efficiency lowering scale deposits on the membranes of reverse osmosis (RO) desalination systems is an important factor in determining the suitability of input water for desalination. Simulated input water evaporation can be used as a technique to quantitatively assessthe potential for scale formation in RO desalination systems. The technique was demonstrated by simulating the increase in solute concentrations required to form calcite, gypsum, and amorphous silica scales at 25°C and 40°C from 23 desalination input waters taken from the literature. Simulation results could be used to quantitatively assessthe potential of a given input water to form scale or to compare the potential of a number of input waters to form scale during RO desalination. Simulated evaporation of input waters cannot accurately predict the conditions under which scale will form owing to the effects of potentially stable supersaturated solutions, solution velocity, and residence time inside RO systems. However, the simulated scaleforming potential of proposed input waters could be compared with the simulated scale-forming potentials and actual scale-forming properties of input waters having documented operational histories in RO systems. This may provide a technique to estimate the actual performance and suitability of proposed input waters during RO. Keywords: Scaling potential; Simulated evaporation; Thermodynamic modeling; RO desalination
1. Introduction The tendency of solutes in input water to precipitate effkiency-lowering scale deposits on the membranes of reverse osmosis (RO) desalination systems is an important factor in determining the suitability of input water for desalination. Scale forms when solutes, concentrated during the RO process, precipitate from 001 l-9164/04/$PII: SO01
l-9
supersaturated solutions. Common scale-forming prkipitates in RO systems include calcite, gypsum, and amorphous silica, the formation of which are described by chemical reactions (l)(3), respectively: Ca*+ + CO:- = CaCO,
(1)
Ca*’ + SOi- + 2H,O = CaSO;2H,O
(2)
See front matter 8 2004 Elsevier Science B.V. All rights reserved
164(03)00668-4
286
H,SiO, = SiO, + 2H,O
G.F. Huff/ Desalination 160 (2004) 285-292
(3)
Pretreatmentof input water to minimize scale formation, andthe removal of scaleonceformed, can represent a significant cost in RO desalination.Detailed discussionsofthe theoreticaland applied aspectsof water purification using RO are available in the literature [ 1,2]. The main factors governingthe formation of scale inside RO systems include solute concentration, operatingtemperature,solution velocity, solution pH, and residence time [3]. The Langelier Saturation Index [4], the Ryznar Stability Index [5], and the Stiff-Davis Stability Index [6] describe the tendency of calcite to precipitatefrom aqueoussolution. Theseindices are limited in that they consideronly the values of pH, temperature, and the concentrationsof dissolved Ca and HCO, in solution [7]. Additional indicies, basedon high-temperature solubility data,areusedto describethe tendency of sulfate saltsand silica to precipitatefrom near roomtemperatureaqueoussolutions [7]. Thermodynamics-basedmodeling of the behavior of selected Ca salts and SiO, has been used to predict saturation concentrations of calcite, gypsum,andamorphoussilica in RO input water [3]. This thermodynamics-based approach accountedfor ionic interactions that affect the solubility of potential scale-formingprecipitates but did not account for the effects of solution modification by precipitation of solid phases duringRO. Alternative approachesto calculating a numerical value describing the tendency of individual scale-forming solids to precipitate in RO systemsuse classification schemesfor input waters based on Cl concentrations and molar ratios of selecteddissolvedconstituents[7,8] and graphicalinterpretationtechniques[9]. All ofthe aboveapproachesto calculating the tendencyof input waters to precipitate scale in RO systems are hamperedby the stability of aqueoussolutions supersaturatedwith respect to calcite,
gypsum, and amorphoussilica, particularly at or near room temperature. The purposeof this paper is to demonstrate the use of simulated evaporationto assessthe scale-forming potential of input waters for RO desalination systems. Simulated increases in concentrations of solutes needed to achieve saturation with respect to calcite, gypsum and amorphoussilica, along with the corresponding predicted solution ionic strengths,can be calculated for any RO system input water. Simulated input water evaporation also allows quantification of changesin solution composition during precipitation of scale-forming solids and allows simulation of the effects of these changeson precipitation of subsequentscale-formingsolids. The use of simulated evaporationto assessthe scale forming potential of RO input water is demonstratedusing 23 desalination operation input waterstaken from the literature. 2. Method and data Desalination of water using RO extracts fresherwater(permeate)from a more salineinput water, effectively increasing solute concentrations in the water remaining (concentrate) within the RO system.Flows of input water into a desalination system upstream of the RO membranearematchedby flows of permeateand concentrateout of the system during operation. Soluteconcentrationby simulatedevaporationof aqueoussolutions using PHREEQC [lo] can approximatethe chemical environment encountered during RO. PHREEQC is a widely used, publicly available, expandable,and well documented geochemical modeling code with an extensivethermodynamic data base.The capabilities of PHREEQC include simulating the chemical behavior of aqueoussolutions composed of all major and many minor solutes, including CO, and SO,,salts, as well as neutral speciesincluding SiO,. PHREEQC adequately
G.F. Hz@/ Desalination 160 (2004) 285-292
representsthe chemical behavior of solutes in aqueoussolutionshaving salinitiesof seawateror less [lo]. Seawaterof the composition given by Drever [ 1l] has an ionic strength of approximately 0.65. Simulated evaporationwas carriedout on RO input waters using PHREEQC in a series of sequentialstepsasdescribedbelow.All chemical analysis of RO input waters were screenedfor charge imbalances no greater than *3.0%. Simulated solution compositions were modified by evaporation and by reaction with plausible scale-formingphasesincludingCO, andSO, salts andamorphoussilica. Simulatedsolutioncompositions were not modified by reactionwith solid phasesthat typically do not form scaleinsideRO systems. As such, precipitation of dolomite, fluorite, sepiolite, talc, and crystalline forms of silica were not used to modify solution compositions eventhough supersaturationwith respect to these minerals was often reached during simulatedevaporation.Equilibration of simulated solutionswith a constantpartial pressureof CO, gas acts as a buffer preventing large changesin, and providing a reasonablerange of values for, pH during simulated evaporation.Buffering of pH is particularly important in calculating the solubility of carbonateminerals and amorphous silica, the solubility of which increasesrapidly at valuesof pH greaterthan about 8 [ 121. STEP 1: An initial pH of 7.5 is assumedfor input waters if no initial pH value is given. The composition of an RO input water is equilibrated at aselectedtemperaturewith CO, gasat a partial pressure of 10-2.5atmospheres.A speciation calculation is carried out and the results are inspectedfor minerals or amorphoussolids that are simulatedto be presentin aqueoussolution at concentrationsgreaterthanor equalto saturation. Step 1 is completed if no minerals or amorphous solids are simulated to be present at concentrations greaterthanor equalto saturationandthe
287
resultingsolution is usedin step2. Otherwise,the initially supersaturatedsolution composition is modified by precipitation of plausible scaleforming phasesto saturationconcentrations.The step-l equilibrium assemblageconsists of the final solution, the specified partial pressureof CO, gas,and any plausible scale-formingphases saturated with respect to the step-one final solution. STEP 2: Simulatedevaporationis carried out on the step-l equilibrium assemblageuntil the next plausible scale-forming phase in the PHREEQC data base reaches saturation with respectto the evaporatingsolution. All gas and solid components of the step-l equilibrium assemblagearepreservedat equilibrium with the aqueoussolution during step-2simulatedevaporation. The calculatedmass of water remaining after completion of step-2simulatedevaporation is recordedandusedto calculatea concentration factor (CF) for the newly-saturated plausible scale-formingphase.The CF valueis numerically equal to the mass of water initially present in step 1 divided by the mass of water remaining after simulated evaporation is completed in step 2. The step-2equilibrium assemblageconsists of all componentsidentified in step 1 plus any plausible scale-formingphasesidentified in step2. STEP 3-STEP n: The proceduredescribedin step 2 is sequentially repeatedon equilibrium assemblagesfrom the previous step until CF values for all plausible scale-forming phases present in the PHREEQC data base have been calculated.Values of the CF for plausible scaleforming phasesarecalculatedrelativeto the mass of water initially presentin step 1. Once identified, plausible scale-forming phasesarekept in equilibrium with the solution during simulated evaporation. This allows for
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simulated redissolution of any scale-forming phases that the solution may become undersaturated with respect to during progressive simulatedevaporation.Operatingtemperaturesin RO systemsrange from near room temperature up to approximately 50°C to take advantageof greatermembrane efficiency at elevated temperatures.Temperatureof 25OCand 40°C were selectedto representtypical operatingconditions for the purposeof this study. Input water analyses for samples shown in Tables 1 and 2 were taken from the following sources:Samples l-20 [9], samples21-22 [8], and sample 23 [3]. Samples I-10 and 11-20 representgroundwaterfrom southwesternKuwait withdrawn from the Kuwait Group aquifer and the Dammam limestone aquifer, respectively. Samples 21-22 were identified as part of the “available water analysis records that accumulated in Geochimica Laboratories(Cairo) during the last five years”. Sample 23 is brackish feed water for the Gabesdesalinationplant in South Tunisia. Data on the concentrations of major solutesincludingNa, K, Ca, Mg, SO,, HCO,, and Cl were presentfor all input water samples.Data on the concentrationof dissolvedSiO, andminor solutesincluding B, NO,, and F were presentfor samplesl-20.
3. Results and discussion Tables 1 and 2 list the results of simulated evaporationto assessthepotential of input waters to form calcite, gypsum, and amorphoussilica scale at 25°C and 40°C, respectively.Values of pH remained in the range of 7.05 to 7.52 and ionic strengthremainedlessthanthat of seawater at simulated equilibrium with potential scaleforming minerals. Values of the CF for calcite in input waters were at or near 1.00 for entries in Table 1 and equal to 1.00 for all entries in Table 2. Little or no solute concentrationwould be required to reach calcite saturationat 25OC.
Values of 1.OOwould be expectedfor theseinput waters at 40°C given the retrogradesolubility of calcite. Values of the CF for gypsum rangefrom 1.43-2.88 at 25°C and 1.52-3.07 at 40°C with the exceptionof sample2 1, which hasCF values of 4.93 at 25OCand 5.32 at 40°C. The apparent slight increase in gypsum solubility with increasing simulation temperature is primarily caused by the greater simulated mass of Ca removedfrom solution to achieveand maintain equilibrium with respectto calcite during simulated evaporation. Values of the CF for amorphoussilica show the greatesteffect of increased simulation temperaturerangingfrom 3.90-5.60at 25°C to 5.03-7.19 at 40°C. Conditions that would cause redissolution of plausible scaleforming phases were not observed in any simulation. Simulated evaporation indicates that input waters in samples l-20 will become saturated with respect to Ca scale minerals during RO desalination before reaching saturation with respectto amorphoussilica. Comparison of CF valuesfor samples l-10 and 1l-20 indicate that input water from the Kuwait Group aquifer has less potential to form gypsum scale than water from the Dammam limestone aquifer at both 25OCand 40°C. A large difference in the potential to form gypsum scale between samples 21 and22 at 25°C is illustrated by CF valuesof 4.93 and 1.43,respectively.A similar differencein the potential to form gypsum scale exists between these samples at 40°C. There is no large or systematic difference in the potential to form amorphoussilica scalebetweenwaters from the Kuwait Group and the Dammam limestone aquifersat 25OCor 40°C. Dataon major elementconcentrationsin input water used in this study appearto generally be adequate for the simple type of simulation described here. However, results of simulated evaporation(not shown in Tables 1 or 2) using unpublished analyses of input waters which containeddataon the concentrationsof Ba andSr
Solution properties at -saturation with respect to gypsum ..--- _____ Ionic CF PH strength 7.22 0.120 2.05 0.132 7.19 1.97 7.20 0.134 2.01 7.18 0.133 1.75 7.17 0.150 2.09 7.17 0.151 2.04 7.19 0.126 1.59 7.18 0.140 1.99 7.20 0.125 1.78 7.18 0.129 1.58 7.29 0.106 2.32 7.3 1 0.125 2.88 7.29 0.115 2.64 7.27 0.120 2.18 7.27 0.123 2.46 7.29 0.108 2.31 7.26 0.114 2.39 7.27 0.114 2.17 7.25 0.119 2.22 7.23 0.125 2.17 7.34 0.344 4.93 7.27 0.112 1.43 7.22 0.111 2.08
*A value of pH was not reported in the original data source material. A pH of 7.5 was assumed. -, Data needed to calculate values were not reported in the original source material.
Table 1 Results of simulated evaporation at 25°C on desalination input waters --.~~. Reported Solution pH in Solution properties at Sample Sample equilibrium with CO, saturation with respect number name PH gi;s at 1O-2.5 to calcite (this paper) (original data source) atmospheres Ionic CF PH strength 5K 7.40 7.34 7.32 0.0617 1.oo 1 7.18 7.26 0.0840 1.22 2 6K 7.95 7.30 0.0700 1.oo 3 8K 7.50 7.30 11K 7.80 7.23 0.0911 1.17 4 7.18 12K 7.45 7.24 7.26 0.0796 1.06 5 6 13K 7.60 7.21 7.25 0.0866 1.13 14K 7.45 7.23 7.25 0.0850 1.04 7 15K 7.50 7.25 7.27 0.0777 1.06 8 9 17K 7.85 7.25 7.27 0.0769 1.06 10 18K 7.50 7.24 7.24 1.oo 0.0839 2L 7.90 7.60 7.40 11 0.0502 1.00 I2 3L 8.00 7.59 7.43 0.0484 1.oo 13 5L 7.90 7.61 7.41 0.0479 I .oo 14 7L 7.20 7.77 7.37 0.0592 1.oo 8L 8.15 7.53 15 7.39 0.0546 1.00 16 9L 7.65 7.61 7.39 0.0511 1.00 17 12L 7.95 7.56 7.37 0.0515 1.00 18 17L 7.80 7.58 7.37 0.0565 1.00 19 19L 7.30 7.69 7.35 0.0578 1.00 20 2OL 7.35 7.84 7.34 0.0617 1.oo * 21 #40 7.99 7.52 0.0766 1.00 * 22 #206 8.02 7.32 1.00 0.0809 Gabes plant 7.40 23 7.46 7.32 0.0565 1.oo feed water .~ -7.22 7.17 7.19 7.15 7.12 7.14 7.16 7.16 7.18 7.14 7.33 7.33 7.32 7.30 7.30 7.33 7.29 7.30 7.27 7.24 -
-PH
__ ...~___ Ionic CF strength 0.196 4.20 0.215 3.91 0.222 4.05 0.245 4.05 0.322 5.60 4.34 0.269 4.05 0.249 0.229 3.92 0.218 3.93 0.250 3.90 0.154 4.09 0.163 4.23 0.167 4.63 0.215 5.02 0.185 4.42 0.173 4.66 0.186 4.95 0.180 4.29 0.198 4.62 0.190 3.97 -
Solution properties at saturation with respect to amorphous silica
5K 6K 8K 11K 12K 13K 14K 15K 17K 18K 2L 3L 5L 7L 8L 9L 12L 17L 19L 2OL #40 #206 Gabes plant feed water 7.40
7.40 7.95 7.50 7.80 7.45 7.60 7.45 7.50 7.85 7.50 7.90 8.00 7.90 7.20 8.15 7.65 7.95 7.80 7.30 7.35 * *
Solution properties at Solution pH in equilibrium with CO, saturation with respect to calcite gis at 1O-2.5 atmospheres Ionic CF PH strength 7.43 7.27 0.0599 1.oo 7.24 0.0682 1.00 7.27 7.25 0.0683 1.00 7.39 7.21 0.0767 1.oo 7.27 7.23 1.00 7.33 0.0734 7.30 7.23 0.0756 1.00 7.32 7.21 0.0800 1.00 7.34 7.23 0.0717 1.00 7.34 7.23 0.0710 1.oo 7.20 0.0819 7.33 1.00 7.69 7.36 0.0480 1.oo 7.68 7.40 0.0464 1.00 7.37 7.69 0.0459 1.oo 7.32 7.85 0.0568 1.00 7.62 7.35 0.0525 1.00 7.70 7.35 0.0490 1.00 7.65 7.33 0.0497 1.oo 7.67 7.33 0.0543 1.00 7.77 7.31 0.0556 1.oo 7.92 7.29 0.0594 1.oo 8.07 7.49 0.0746 1.00 8.09 7.28 0.0775 1.oo 7.55 7.28 0.0548 1.00 7.17 7.14 7.15 7.12 7.12 7.12 7.14 7.13 7.15 7.12 7.24 7.26 7.24 7.21 7.22 7.24 7.21 7.22 7.20 7.18 7.29 7.22 7.17
PH
Ionic strength 0.123 0.136 0.139 0.137 0.156 0.158 0.131 0.145 0.129 0.134 0.108 0.128 0.117 0.122 0.126 0.110 0.116 0.116 0.122 0.129 0.363 0.114 0.113 PH 7.16 7.11 7.13 7.08 7.05 7.07 7.09 7.09 7.12 7.07 7.29 7.29 7.27 7.25 7.26 7.29 7.24 7.25 7.22 7.19 -
2.18 2.10 2.13 1.86 2.22 2.18 1.69 2.11 1.89 1.68 2.46 3.07 2.81 2.32 2.62 2.45 2.53 2.31 2.36 2.32 5.32 1.52 2.20
-
Ionic strength 0.236 0.262 0.270 0.301 0.398 0.331 0.305 0.281 0.266 0.307 0.182 0.194 0.198 0.257 0.221 0.205 0.223 0.215 0.237 0.228
5.45 5.06 5.24 5.23 7.19 5.60 5.22 5.07 5.09 5.03 5.32 5.51 6.02 6.51 5.74 6.06 6.43 5.58 6.00 5.15 -
CF
Solution properties at saturation with respect to amorphous silica
CF
Solution properties at saturation with respect to gypsum
*A value of pH was not reported in the original data source material. A pH of 7.5 was assumed. -, Data needed to calculate values were not reported in the original source material.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Sample Sample Reported number name PH (this paper) (original data source)
Table 2 Results of simulated evaporation at 40°C on desalination input waters
G.F. Huff/ Desalination 160 (2004) 285-292
[ 131 indicate that potential scale-forming minerals such as celestite (SrSO,) and barite (BaSO,) may precipitate at CF values close to that of gypsum. This may explain the observed occurrenceof Ba and Sr in Ca-rich scales[S]. It is thereforerecommendedthat analysesof input waters used in the described simulated evaporation technique include information on concentrations of Ba and Sr as well as the major elementspreviously listed. A methodhasbeendescribedto this point that could be used to quantitatively assess the potential of a given input water to form scaleor to compare the potential of a number of input waters to form scale during RO desalination. However, the effects of stable supersaturated solutions, solution velocity, and residencetime inside RO systems do not allow calculation of actual CF vales at which scale will form. Simulated evaporationusing PHREEQC, or any purely thermodynamicmodel, cannotadequately addressthese effects. However, the simulated scale-forming potential of proposedinput waters could be compared with the simulated scaleforming potentials and actual scale-forming properties of input waters having documented operational histories in RO systems.This may provide a technique to estimate the actual performance of proposed input waters in RO systemsunder a specified set of conditions.The thermodynamic data base of PHREEQC is expandableandadditional chemicalreactionscan be defined within the program.Theseproperties of PHREEQC could allow the effects of antiscaling additives to be estimated provided the required information on their composition and thermodynamic propertiesare known. Experimental verification of the methodsand techniquesproposedin this study are needed.If validated by experimental data, the technique described in this study could provide a reasonableand systematic method for assessing the suitability of proposedinput waters for RO desalination.
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4. Conclusions
Simulated evaporationof input water can be used as a technique to quantitatively assessthe potential for scaleformation in RO desalination systems.The useof PHREEQC to simulate input water evaporation allows quantification of changes in solution composition during precipitation of scale-forming solids and allows simulation of the effects of these changes on precipitation of subsequentscale-formingsolids. The techniquewas demonstratedby simulating the increasein soluteconcentrations,represented by CF values,requiredto form calcite, gypsum, and amorphoussilica scalesat 25°C and 40°C from 23 desalinationinput waterstaken from the literature. The resulting simulated CF values could be usedto quantitatively assessthe potential of a given input water to form scale or to compare the potential of a number of input waters to form scale during RO desalination. Simulated evaporation of input waters cannot accurately predict the conditions under which scalewill form owing to the effectsof potentially stablesupersaturated solutions,solutionvelocity, andresidencetime inside RO systems.However, thesimulatedscale-formingpotentialof proposed input waters could be compared with the simulated scale-forming potential and actual scale-formingpropertiesof input waters having documentedoperationalhistoriesin RO systems. This may provide a technique to estimate the actual performance of proposed input waters during RO desalinationunder a specified set of conditions.If validatedby experimentaldata,the techniquedescribedin this studycould provide a reasonableand systematicmethod for assessing the suitability of proposedinput waters for RO desalination. Acknowledgements
The contributionof information andtechnical expertiseby Michelle Chapmanofthe US Bureau
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of Reclamation and Michael Hightower of Sandia National Laboratories is gratefully acknowledged.
References [l]
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Z. Amjad, ReverseOsmosis:Membrane Technology, Water Chemistry, and Industrial Application, Van Nostrand Reinhold, New York, 1993. U.S. Environmental Protection Agency, Reverse osmosis process: Technology Transfer Capsule Report EPA-625-R-96-009, 1996. B. Hamrouni and M. Dhahbi, Desalination, 137 (2001) 275-284. W.F. Langelier, J. AWWA, 28 (1936) 1500-1521. J.W. Ryznar, J. AWWA, 36 (1944) 472-486.
[6] H.A. Stiff, Jr. and L.E. Davis, Pet. Trans. AIME, 195 (1952) 213-216. [7] S. El-Manharawy and A. Hafez, Desalination, 136 (2001) 243-254. [8] S. El-Manharawy and A. Hafez, Desalination, 139 (2001) 97-l 13. [9] K.M.B. Hadi, Desalination, 142 (2002) 209-219. [lo] D.L. Parkhurst and C.A.J. Appelo, User’s guide to PHREEQC (version 2) - A computer program for speciation,batch-reaction, one-dimensional transport, and inversegeochemical calculations. US Geological Survey, Water-Resources Investigations Report 994259,1999. [l l] J.I. Drever, The Geochemistry of Natural Waters, Prentice-Hall, Englewood Cliffs, 1982. [12] G.B. Alexander, W.M. Heston and R.K. Iler, J. Phys. Chem., 58 (1954) 453-455. [13] M. Chapman, U.S. Bureau of Reclamation, personal communication, 2002.