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Dehydration model for RO-membrane fouling: A preliminary approach
DESALINATION ELSEVIER
Desalination 153 (2002) 95-107 www.elsevieccom/locate/desal
Dehydration model for RO-membrane fouling: a preliminary approach Samir El-Manharawy”*, Azza Hafezb ONuclear Geochemistry Department, Nuclear Materials Corporation, Cairo, Egypt Tel. +20 (2) 3474822; Fax +20 (2) 34523 I; email: geo230@linknet hChemical Engineering and Pilot-Plant Department, National Research Center, Cairo, Egypt
Received 25 January 2002; accepted 16 March 2002
Abstract The proposed model is based on the hydration theory of dissolved chemical species in water. The molecular dehydration occurs when the solution system is subjected to critical water loss associated with re-displacement of ionic charges from attached water molecules towards the dissolved molecule. This mechanism allows the formation of solid phase, i.e. fouling. In order to estimate the true fouling load, it is necessary to define the type and molar concentration of the hard dissolved compound individually, then estimate the fraction available for deposition in a given chloride content. The mathematical mode1 is illustrated and tested for six RO plants, and the obtained results are in agreement with field observation. Keyword:
Dehydration model; Membrane fouling; Fouling fraction; Fouling flux
1. Introduction Several models are in use for the prediction of inorganic scale formation. The first model was introduced by Langelier [l]. It is based on the thermal behavior of water pH-value and hardness (as expressed empirically in CaCO,), the fouling *Corresponding author.
of calcium carbonate is determined on its saturation value at elevated temperature, which is known as Langelier Saturation Index (LSI). Ryznar [2] and Stiff and Davis 133 developed the earlier model to accommodate higher salinity effect. Later on, DuPont [4] introduced a mathematical model and monographs for the prediction of formation of other common scales (i.e. Ca, Ba, Sr-sulfates and
Presented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Alexandria University Desalination Studies and Technology Center, Sharm El Sheikh, Egypt, May 4-6, 2002. OOl l-9164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII:SOOll-9164(02)01109-8
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S. El-Manharawy, A. Hafez /Desalination
silica) based on their thermal specific solubility at higher temperatures. It should be noted that these thermal models were basically established on the concept of supersaturation of dissolved salts in heat exchangers at elevated temperature and sufficient contact time. In application of the above-mentioned thermal indices for prediction of scale formation in reverse osmosis membrane separation many serious errors happened. This is due to the basic difference between thermal desalination and membrane desalting, which is a pressure driven process under normal temperature. Most of the recorded scaled RO membranes were quite below the supersaturation level of their dissolved hard chemical species. The concentration polarization (CP) model has been recently introduced for the prediction of scale formation in membrane filtration. The basic concept of the CP model refers to Blatt et al. [5] who studied fouling of solid colloidal particles and macroorganic molecules on the ultrafiltration (UF) membrane surface used in food processing. Several unsuccessful trials have been done in order to apply the CP hydrodynamic model in the reverse osmosis separation. The difference between both applications is obvious, however, field investigation indicated that RO system is currently affected by concentration polarization or something alike. The missing link between both models back to the absence of a reliable chemical model is able to predict and estimate the type and amount of dissolved hard chemical species (or hard ionpairs) that turn into solid particulate under RO process. When it is possible, the CP-hydrodynamic model, and thermodynamic as well, can be applied. During the past 5 years, the authors have been involved deeply in the RO hydrochemistry, in order to investigate, study and try to solve the inorganic-scaling problems associated with desalination of brackish and seawater by the RO membrane process. The comprehensive chemical analyses, XRF, mineralogy and field investigation of waters and scales in more than 60 RO-plant cases indicated a strong relationship between the feedwater chemical characteristics and the generated
153 (2002) 95-107
scale type, as well as its potential [6]. It was clear that the molar ratio (SO,/HCO,) gradually and positively correlated with the chloride molar concentration for most of the natural waters. This natural phenomenon shows that the variation in most of natural waters is so tight that it ranges between -0 and 20. For example, the molar ratio (SOJHCO,) of the River Nile water is -0.12, brackish groundwaters 1-5, salty waters 5-10, oceanic water -12, and the Red Sea water -14. Some anomalous sulfate-enriched groundwater brines could be higher than 30. Furthermore, it was possible to conclude that two factors, i.e. water type and permeate recovery, mainly control inorganic scale formation in the RO membrane separation, and scale potential is directly proportional to the permeate recovery rate [7]. The authors have attributed the formation of inorganic scale to the dehydration of scale forming chemical species, which does not necessitate their concentration to be in the super-saturation zone. The aim of the present study is to present and discuss the chemical assessment of inorganic fouling that could happen on the RO-membrane surface in the light of the proposed hydration-dehydration model. 2. Basic approach 1. Mineral and salts dissolve in water according to the hydration (or solvation) theory [S], where the dissociated ions attract water molecules according to their charge, z to ionic radius, r. Low (z/r) ions (such as Na” and Cl’-) surrounded with a single water shell of 4 to 6 water molecules. Intermediate (z/r) ratio ions (such as Fe3’ and AP’) tend to extract OH- from water to form insoluble complexes. Ions of high (z/r) ratio can strongly extract O*- from water and form their soluble oxyanions, such as the scale-forming ions, i.e. PO 3- SOJ2-, C032-, HCO,- and SiO,*-. Due to the:r high (z/r) value, these hard ions tend to attract more than one water shell around to form hydrated ions. For example, the 4 oxygens bonded to sulfur, each of which carries an average charge of -l/2
S. El-Manharmuyt A. Hafez / Desalinaiion 153 (2002) 95-107
(2-+4 oxygen), accept 3 water molecules around each oxygen in the first shell (a total of 12H,O) and less attracted 6 in the second shell (a total of 24H,O), and so on. In low-salinity water, sulfate ions may accept up to 6 extra water shells, while in concentrated solution the attached water layers are normally lower than 4. Phosphate (Pod-) has the same configuration of SO,*- ion but with higher residual charge (-314 each) therefore it attracts up to 8 shells. Both SiO,*- and CO,*- ions have higher charges (-213 each), however, they are less branched. It is also the same for dissolved cations (e.g. Ca, Mg, Ba and Sr), where the positive charge is equally distributed around and attracts the negative pole of dipolar water molecules. 2. Under normal physical conditions, i.e. without chemical interference, hydrated ions exist in solution as the original compound dissolved from it (i.e. as ion-pairs or hydrated molecules) and can re-crystallize again when subjected to water loss (i.e. dehydration). It is interesting to mention that the hydrated molecules keep in memory the original crystal structure they dissolved from. 3. The hydrated ions are mostly present in less active ion-pairs associations, or relaxed hydrated molecules. The stability of these relaxed molecules in solution is increasing with the increase of water clusters around, consequently, under water loss conditions the highly hydrated molecules will be less stable than the lower ones and tend to precipitate, i.e. the dissolved phosphate and sulfate molecukes are most stabilized while carbonate and silicate are less stabilized and tend to foul earlier. Fig. 1 shows a sketch drawing illustrating the idea of the dehydration model. In the fully hydrated molecule (Fig. 1a), the ionic charges (+ve and -ve) are displaced outside in order to attract as much as possible of dipolar water shells around, and as a result, the attraction power between both cation and anion is minimized, and the ionic distance will be longer, which allows additional room for extra water molecules. This mechanism maximizes the dissolution stability in solution and overcome the effect of molecule density to keep it floating.
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Fig. 1. IllustraGon of a hydratedmolecule (a) and dehydration mechanism (b) on the RO membrane surface due to dewatering and change in charge displacement. Fig. 1b describes the situation when the hydrated molecule is subjected to serious water loss, either by RO extraction or evaporation. In this case, the ionic charges tend to move inside, i.e. inner displacement, and release some free water molecules to compensate the shortage in the system. At a critical point, the opposite inner charges become strong enough to attract both ions firmly to form a solid particle as started, and repel excess water molecules from its structure, i.e. forming a solid nucleus. The transformation of the dissolved compound into solid particles increases gradually with dehydration progress, i.e. fouling is proportional to pressure and recovery. 4. Dehydration of hydrated molecules is probably done in a random way on the RO membrane surface. The probability oftouching and dewatering is directly proportional to the concentration of molecules in water mass that equally distributed on the membrane surface area per time. The magnitude of molecular dehydration is directly proportional to the applied pressure and defusing rate of pure water through a given membrane type.
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5. It is assumed that all hydrated molecules could be subjected to dehydration, and only a fraction of hard molecules (e.g. CaSO, and CaCO,) will foul and exit the aqueous system as solid microparticles, while others (e.g. NaCI, Na,SO, and MgCl,) will redissolve again, i.e. regain their water shells or part of them. This depends on their specific solubility and the available time necessary for re-dissolution. There are several evidences in support of this assumption: l Field observation indicated that the RO concentrated stream (brine) got turbid after a short time from the start of operation. Generally, turbidity increases 2-3 times during the first few hours from daily flushing and operation. In some recorded cases in the Red Sea ROplants, the turbidity usually increases from less than 1 NTU to higher than 15 NTU in less than 1 hour. l The material chemical balance between the RO input and output is always disturbed because of the separation of some hard species from the aqueous system as a solid particulate, which cannot be determined by standard water analytical methods and may need strong acid digestion prior to the analysis. l The laboratory experiments on the closed recycling RO setup indicated a continuous and gradual change in the chemical composition of the circulating solution due to recycling progress. The acid-leach of the micron filtercartridge proved that considerable amount of hard suspended solids was retained and accumulated on the filter surface during the closed circulation. This must be taken into consideration when performing such experiments. These important field and laboratory observations support the idea of solid particulate formation due to reverse osmosis dehydration. 6. Scale deposition is another issue. It depends on the maturation of the fouling solid particle size to form the initial crystal nuclei under favorable conditions, i.e. enough time and suitable place. The applied mechanical high pressure plays an
important role in minimizing the time necessary for crystallization. It has been found that massive carbonate and sulfate scales are usually located in the exit of casing cavities and are stretched counter flow at different magnitude. The flow pattern of water inside the RO membrane module is highly turbulent and well mixed. This is due to the insertion of a complex mesh-spacer between membrane layers that delay the formation of the hard scale. Normally, water flows in a -l-mm thickness layer between the spiral wound RO membrane. 7. It has been observed that there is a considerable difference between the analytical molar ratio (CaSO,:CaCO,) found in brine solutions and that deposited in the formed scales. This phenomenon is common for all investigated cases but with different magnitude. This means that not all of the dissolved hardness molecules are readily to precipitate upon dehydration. This ratio is called the fouling fraction (F’. Our previous work has proven that the solubility, as well as deposition, of hard molecules changes widely under the influence of chloride ion concentration in the solution [9]. The statistical data processing of analytical results obtained from more than 60 RO feed, brine and scale samples indicated that there is a systematic relationship between the fouling fraction (F’ of scales and the Cl mmol/kg of the brine. Fig. 2 presents the graphical relationship between the brine chloride concentration and the fouling fraction (Ff. The CaS04 curve shows that the maximum scaling potential (100%) is at chloride concentration higher than 800 mmol/kg, and decreases at lower Cl. On the other hand, CaCO, almost has no chance at all (-0%) for deposition at Cl>800 mmol/kg, and its fouling potential increases to maximum (-100%) at Cl -10 mmovkg, then decreases again. Field observation and chemical analysis of collected scales generated from highly salty waters has proven that it is almost free of carbonate, in other words, not all hardness ions are participating in fouling. Three chloride categories were identified based on the geochemical affinity of dissolved minerals in natural water. The
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also be applied for Ba and Sr carbonate and sulfate. The fouling load (FL)obtained from multiplying the compound analytical molar concentration (in mmol/kg) by the estimated fouling fraction (Ffi at a given Cl concentration:
Fig.2.The relationship between brine chloride and FCaCO, and FCaSO, in feed water. obtained trend-line equations were used to predict fouling fraction (F’ of dissolved calcium carbonate and calcium sulfate that is expected to foul at a given chloride concentration level: (i) Cl- range in brine: O-10 mmol/kg F&K0
F-Ld
0.0791
[Cl] + 0.2945
= 0.0645
[Cl] + 0.0470
=
(1) (2)
(ii) Cl’ range in brine: lo-200 mmol/kg Ff,K03
FfCCaSO 4
=
0.0046 [Cl] + 0.9168
(3)
=
0.0013 [Cl] + 0.6126
(4)
(iii) Cl- range in brine: 200-800 mmol/kg Ffc,co, = 0.00009 [Cl] + 0.0946 = 0.00019 [Cl] + 0.8407 Ffcaso4
(5) (6)
(iv) Cl- range in brine: >800 mmol/kg FfcCao.3= 0.00
(7)
Ffca,,;= 1.OO
(8)
Because of possible mathematical extrapolation, it should be noted that Ffvalue should be between 0 and 1 (i.e. from 0% to 100% offoulant load), the results of values higher than 1 are to be considered as 1. In addition, there is no direct mathematical relation between Ffcaco and Ffc,,, . This means that it is not necessary that summatioh of both is 1, they are independent from each other. The above-mentioned emperical formulae could
Fouling load (FL,aco,), mmol/kg = ICaCO,l x Ffcaco3
(9)
Fouling load (FLcsso4), mmol/kg = [CaSO,lx t’fcaso4
(10)
For magnesium (Mg), the situation is different. The chemical analysis of collected scales proved that magnesium carbonate (MgCO,) follows exactly the above relations [Eqs. (1),(3),( 5)] as CaCO,. The unexpected difference comes from the behavior of MgSO,, which follows also the same relations of CaCO, with a deviation ranging between -1.5% and 5% lower than the real scale composition. This phenomenon means that the influence of chloride concentration is similar for both MgCO, and MgSO,, i.e. their solubility increases with the increase of chloride and vise versa. This may explain the anomalous capacity of seawater to dissolve excessive amount of magnesium salts, which is different from the geochemical behavior of calcium which is always found in seawater in limited quantity and easily precipitates. From the scale prediction point of view, it is safer to consider the maximum deviation as +5% for MgSO, estimation. In other words, the fouling fraction of both of MgCO, and MgSO, will be calculated according to Eqs. (l),(3),(5) at the given chloride level, then an empirical amount of 5% of brine [MgSO,] molar concentration will be added to the resulted fouling load of MgSO,, as folIows: (i)Cl- range in brine: O-l 0 mmol/kg Fouling load (FLMgco), mmol/kg = [MgCO,] x (0.0791. [Cl] + 0.2945)
(11)
Fouling load (FL,Bso), mmolikg = [MgSO,] x (0.0791 [Cl] + 0.2945) + (0.05 x [MgSO,]) (I 2,
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(ii)CI- range in brine: 1O-200 mmol/kg Fouling load (F,,MyCO ), mmol/kg = [MgCO,] x (0.0046 [Cl] f 0.9168)
(13)
Fouling load (F,,MgSO ), mmoi/kg = [MgSOJx (0.0046 [Cl] f 0.91&) + (0.05 x [MgSO,]) (14) (iii) Cl- range in brine: 200-800 mmol/kg Fouling load (F,,,sro ), mmol/kg = [MgCO;] x (O.OOOb9[Cl] + 0.0946)
(15)
Fouling load (F,,,B,, ), mmoVkg = [MgSO,] x (0.00009 [Cl] + 0.8946) + (0.05 x [MgS0,](‘6)
[C,,.= 1 +(I -R)] where R is the recovery 0.7, etc.).
(iv) Cl- range in brine: >800 mmol/kg Fouling load (F,,,scO,), mmol/kg = 0.00
(17)
Fouling load ((;I,MgsOq), mmol/kg = 0.05 x [MgSO,]
(18)
It is important to note that the summation of the calculated MgSO, foulant and the added part (5%) is never more than the original molar concentration. In the present chemical model, the total amount ofCaPO,, MgSiO,, Fe,O, and MnO, is considered as a full fouling load because the influence of the chloride ion on both is almost neglected. As a rule, iron and manganese oxides must be removed to less than 0.2 mg/l at the early stage of pretreatment.
3. Prediction
but we found several drawbacks in the application for waters of TDS> 10000 mg/l. In the meantime, it is better to follow the probable salt combination method manually calculated according to the following simplified model: (i) Convert feed-water ion concentration from (mg/l) to (mg/kg), multiplying by density. (ii) Convert feed water (ion-mg/kg) to (ionmmol/kg), dividing by molecular weight. (iii) Estimate brine-ion concentration (in mmol/ kg), multiplying by the concentration factor:
of inorganic fouling load (F,,)
In order to obtain the inorganic fouling load (F,,, mmol/kg), it is necessary to calculate the probable combination of dissolved hard salts in brine solution, i.e. the true concentration of [Ca, Mg, Ba and SrC03], [Ca, Mg, Ba and Sr(HCO,),], [Ca, Mg, Ba and SrSO,], [CaPO,] and [MgSiO,], in mmol/kg in the RO concentrated solution, i.e. the brine. These compounds usually constitute more than 99.99% of the RO inorganic scales, and its sum called the total fouling load (F,, mmol/kg). The USGS PHREEQC software version 2 [IO] can be used for aqueous geochemical calculations,
(19) decimal (e.g. 0.3, 0.5,
(iv) Combine (CO,‘-) with equal mmol of [Ca”, Mg2+, Ba2+ and Sr2+]. (v) Combine (HCO;) with 0.5 mmol of [Ca2+, MgZ+,Ba2+ and Sr2+]. (vi) Combine excess [Ca”, Mg2+, Ba*’ and S?+] with equal mmol of (S0,2-). (vii)Combine excess (S0,2-) with 2 mmol of ma+ + K’]. (viii) Combine excess [Na++ K’] with equal mmol of (Cl-). In case of seawater and highly salty waters as well, excess SO, is generally distributed between Mg and Na. We used the natural relative molar abundance of Na,SO, to MgSO, in oceanic water given by Miller0 [ 1I], where 48.33% of excess Mg combines with SO,, and the rest with Cl. In case of water containing considerable amount of PO,and SiO,, combine the first with equal amount of calcium [CaPO,] and silicate with magnesium [MgSiO,]. This step must be done at the start and before any ion distribution. This simplified ionpair stoichiometric calculation showed that the accuracy of distribution of ions for probable salt combination is better than 99%. The fouling flux (FX)of an individual foulant could be predicted from its concentration in brine that flows in a I-mm thickness layer spreading over 1 cm2 of the membrane surface per time, which is normally around 1 s:
S. EM4anharmvy, A. Hafez /Desalination FY, atom/O. I cm’s = F,, , Ff,
10m3X d,, , A,.
(20)
where F,r is the number of hard atoms found in 0.1 cd per unit time (s), F,, is the fouling load, in mmol/O. 1 cm’, Ffis the fouling fraction, A,. is the Avogadro constant [ 12](= 6.022 14199 x 1Oz3),and dh is the brine density, which could be estimated using the following formula: Brine density (d,) = 0.000759 [TDS], + 996.977739
(21)
where [TDS], is calculated from feedwater-TDS (in mg/kg) multiplying by C,: as in Eq. (19). Note: for magnesium fouling load calculation, refer to the given limitations in the basic approach.
4. Examples and short discussion In order to test the proposed dehydration mathematical model, six feedwater samples were collected for chemical analysis from 6 running RO plants used for water desalination for drinking and industrial purposes. These cases had been selected to cover a wide range of TDS (from -1500-59000 mg/l), and four cases ofthem (No. 1, 5 and 6) were subjected to heavy inorganic scaling during a short time from the start of operation. Table 1 presents the chemical characteristics of selected waters and molar ratios that differentiate between their chemical types clearly. The given analytical results (in mg/l) were corrected to correspond the water density to obtain the concentration as (mg/kg) before calculating molar ratios. The investigated cases show that water No. 1 (Red Sea groundwater brine) is rich in sulfate as compared with the Red Sea surface water (No. 2), and the molar ratio (SO,/Alk = 17.67) is the highest in the group. On the other hand, water sample No. 6 has the lowest ratio (SO,/Alk = 0.26) due to its high bicarbonate content. Table 2 gives the results of the probable salt combination in mmol per one kg of water, in addition to some simple mathematical indicators for individual chemical species groups. It is interesting to notice that the hardness chemical
153 (2002) 95-107
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species constitute a modest part (-4-9%) of the total dissolved species in the first five samples, while it reaches -5 1% in sample No. 6 which has the lowest TDS (I 539 mg/l). Generally, there is a reversible relationship between TDS and hardness, i.e. hardness decreases with the increase of natural TDS. It is also clear that the lower content of chloride allows generous dissolution of carbonate in water, as shown in sample No. 6 which contains bicarbonate salts of more than 44% of TDS. On the other hand, the Red Sea groundwater (No. 1, TDS = 59,088 mg/l), as well as the Red Sea surface water (No. 2, TDS = 43,553 mg/l) are quite depleted in carbonate and bicarbonate salts (0.23 and 0.42%). Only the Red Sea surface water (No. 2) acquires carbonate salts, while alkalinity in other samples is limited to bicarbonate. The sulfate salts are abundant in higher TDS waters (>90 mmol/kg) and less in lower TDS (-8 mmol/kg). However, the sulfate salt percentage to TDS increases remarkably from 3.6 to 5.9%, and does not decrease, which is confusing indeed. From the hardness point of view, sulfate hard salts constitute more than 90% of total hardness chemical species in seawater and higher salinity, and decrease rapidly to 11.5% in the lowest salinity. The carbonate salts composes more than 86% of total hard species in the lowest TDS sample (No. 6) and is minimized to 6% in the highest salinity sample (No. 1). Generally, hard sulfate salts increase and hard carbonate decrease with the increase of the chloride ion. This important information could help in prediction of chemical type(s) of possible foulant and scale. Table 3 illustrates the method of calculating the possible fouling load found in a thin layer (0.1 cm) of a brine solution covering 1 cm2 of membrane surface, the available fouling load is based on the fouling fraction as mentioned earlier. The carbonate and bicarbonate species estimated separately and then grouped numerically as listed. The obtained results indicated that the fouling load is independent from salinity, i.e. the fouling load is not proportional to the ionic strength. As shown, the highest fouling load (73.59 and 75.47 mmol/kg)
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Table 1 Chemical characteristics of feedwater samples used for calculation of possible RO membrane fouling load (A)
Water samples information
1 2 3
Case number Case location Water type
1 Quosir Brine GW
2 3 Hurghada Ataqua Red Sea SW Salty GW
4 Sokhna Salty GW
4 5
pH value Water density, gm/cc
7.2 1.0425
8.3 1.0299
6.9 1.0101
2 10,853 482 549 3105 0.68 10.43 23,479 176 39 0 4858 0.61 0.04 0 0.05 43,552.72 1.38207 0.01 1.03462
(B)
Initial water chemical analysis (mg/l)
ser. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Ion Sodium ion (Na) Potassium ion (K) Calcium ion (Ca ) Magnesium ion (Mg) Barium (Ba) Strontium (Sr) Chloride (Cl ) Bicarbonate (HC03) Carbonate (CO,) Hydroxyl (OH) Sulphate (SO,) Silica (Si03) Iron (Fe3+) Manganese (Mn4’) Phosphate (PO.,) TDS, mg/l TDS, mole/kg Cation-anion difference, % Ionic strength, eq/Kg
(C)
Molar ratios, based on molar concentration, mmole/kg
ser. 1 2 3 4 5 6 7 8 9 10 11 12
Molar ratio (SO4 /Alk*) (SO,/CI) (S04/Si03) (A&/Cl) (HCO$SiOJ [(Na+K)/(Ca+Mg)] (Na/Cl) (K/Cl) (Ca/Cl) (Mg/Cl) (Ca/Si03) (Ca/P04)
1
17,485 552 1652 1958 3.73 21.52 32,269 177 0 0 4923.78 21.55 0.36 0 18.24 59,088.18 2.76997 0.10 1.94018
1 17.669 0.056 180.961 0.003 10.242 6.450 0.836 0.016 0.043 0.089 139.596 205.871
*Alk: Alkalinity = OH + CO, f HCO,
2 14.308 0.076 6307.43 1 0.005 359.768 3.425 0.713 0.019 0.021 0.193 1708.552 26018.902
7.0 1.0057
5 Rawash Brackish GW 7.1 1.0019
6 Haram Brackish GW 7.3 0.9982
3 5308 143 341 521 8.92 5.31 9216 128 0 0 1570 14.64 0.74 0.06 12.79 17,269.52 0.54875 0.23 0.35124
4 2687 35 1343 138 1.06 2.17 6264 97 0 0 863 24.51 0.22 0 4.82 11,459.90 0.3464 0.39 0.24908
5 2067 11 193 65 3.04 0.72 2414 192 0 0 1634 19.14 0.45 0.16 6.81 6606.24 0.18663 0.60 0.13155
6 206 8 167 49 0.13 0 213 626 0 0 256 12.09 0.15 0 1.27 1538.73 0.0344 1.84 0.03078
3 7.791 0.063 84.939 0.008 10.902 7.833 0.888 0.014 0.033 0.082 44.218 63.179
4 5.652 0.05 1 27.891 0.009 4.935 3.005 0.662 0.005 0.19 0.032 104.02 660.26 1
5 5.405 0.25 67.611 0.046 12.508 12.042 1.32 0.004 0.071 0.039 19.143 67.158
6 0.260 0.444 16.776 1.708 64.564 1.482 1.491 0.034 0.694 0.336 26.223 311.601
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Table 2 The probable salt combination (mmoVkg)* of the studied concentrated brine samples at operating recovery levels (%) and concentration factors, C,.. ser. ii 2 3 4
9 10
11 12
13 14 15 16
17 18 19 20 21 22 23
Case number
1
2
3
4
5
6
Recovery % Concentration factor (CF) CaC03 MgC03 BaCO, SrC03 Total carbonates, mmole/kg Total carbonates, % Ca(HCQ)2 Mg(HCO,)z Ba(HCO& Sr(HCQ2 Total bicarbonates, mmole/kg Total bicarbonates, % CaS04 MgS04 BaS04 SrS04 Total sulfates, mmole/kg Total sulfates, % Fe203 Mn02 CaP04 MgSi03 Total others, mmole/kg Total others, % Na2S04 CaC12 MgC& BaC12 SrC12 NaCl KC1 Total non-hardness species, mmole/kg Total non-hardness species, % Total salt species, mmole/kg Hardness species, mrnoleikg Hardness species, % Non-hardness species, tnmole/kg Non-hardness species, % C03: hardness, % S04: hardness, % Others: hardness, %
30% 1.43 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 2.1254 4.3364 0.0015 0.0170 6.4803 0.2339 50.0730 49.3740 0.0346 0.3997 99.8813 3.6050 0.0137 0.0000 0.5721 0.8437 1.4295 0.0516 79.1790 99.4200 281.5300 0.0680 0.7868 2159.8000 42.0520 2662.8358 96.1095 2770.6269 107.7911 3.89 2662.8358 96.11 6.01 92.66 1.33
40% 1.67 0.2102 2.0189 0.0001 0.0019 2.2311 0.0969 0.6997 6.7205 0.0003 0.0063 7.4268 0.3226 16.3560 75.9250 0.0061 0.1464 92.4335 4.0152 0.0018 0.0000 0.0018 0.0275 0.03 11 0.0014 121.7600 42.9420 534.2200 0.0160 0.3843 1458.3000 42.3220 2199.9443 95.5639 2302.0668 102.1225 4.44 2199.9443 95.56 9.46 90.51 0.03
50% 2.00 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 1.7897 4.5403 0.0139 0.0130 6.3569 0.5788 18.5920 22.7950 0.1442 0.1346 41.6658 3.7939 0.0383 0.0035 0.5441 0.7775 1.3634 0.1241 36.5560 21.8890 91.1810 0.1635 0.1525 884.1300 14.7780 1048.8500 95.503 1 1098.2361 49.3861 4.50 1048.8500 95.50 12.87 84.37 2.76
60% 2.50 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 5.1638 0.8266 0.0012 0.0038 5.9954 0.6912 38.9140 3.0104 0.0090 0.0288 41.9622 4.8380 0.0142 0.0000 0.2552 1.6199 1.8893 0.2178 4.8277 189.2200 37.4840 0.0436 0.1398 581.2800 4.5014 817.4965 94.2529 867.3434 49.8469 5.75 817.4965 94.25 12.03 84.18 3.79
70% 3.33 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 10.3800 5.3011 0.0484 0.0180 15.7475 2.5295 25.2130 14.3110 0.1154 0.0428 39.6822 6.3742 0.0384 0.0154 0.4785 1.6786 2.2109 0.3551 110.7200 0.0000 0.0000 0.0000 0.0000 452.3100 1.8773 564.9073 90.7412 622.5479 57.6406 9.26 564.9073 90.74 27.32 68.84 3.84
75% 4.00 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 42.4770 18.9590 0.0097 0.0000 61 A457 44.4940 5.0167 3.1745 0.0011 0.0000 8.1923 5.9322 0.0153 0.0000 0.1068 1.2689 1.3910 1.0072 19.6440 0.0000 0.4288 0.0000 0.0000 45.3630 1.6340 67.0698 48.5665 138.0988 71.0290 51.43 67.0698 48.57 86.51 11.53 1.96
Concentration factor (C,J = l/( 1 - R), where R is recovery decimal (i.e. 0.3,0.4,0.5,0.6,0.7 *mmol/kg = (mg/l x density) + MW
and 0.75)
1355.5459 0.0000 1.0000 0.0000 0.0000 0.0000 0.0000 50.0735 2.4687 0.0346 0.3997 0.5721 0.8437 54.3922
Chloride in brine, mmole/kg Carbonate foulmg h-action (Fcoj) Sulfate fouling fraction (Fso4) CaC03 + Ca(HC03)2 MgC03 + Mg(HCQ)z BaCO, + Ba(HCO& SrCOj + Sr(HCO,), CaS04 MgS04 BaS04 SrS04 cap04 MgSi03
Total fouling load, mmolekg
1 2 3 4 5 6 7 8 9 10 11 12 13
107.7777
Total hard species, mmole/kg
Brine foulant species, mM/kg
CaCOj + Ca(JlCO& MgC03 + Mg(HCO& BaCOJ + Ba(HCO& St-CO3+ Sr(HCO& CaS04 MgS04 BaS04 SrS04 CaP04 MgSi03
1 2 3 4 5 6 7 8 9 10
(C)
2.1254 4.3364 0.0015 0.0170 50.0735 49.3740 0.0346 0.3997 0.5721 0.8437
Brine hard species, mmole/kg
(B)
59088 750 30% 1.4286 SWRO Blocked 34 d
1
Feedwater TDS, mg/l Capacity, m’/d Water recovery % Brine concentration factor (CF) RO membrane type Relative scaling condition Membrane life time
Case type
1 2 3 4 5 6 7
(A)
20.3343
1136.7739 0.0000 1.Oooo 0.0000 0.0000 0.0000 0.0000 16.3563 3.7963 0.0061 0.1464 0.0018 0.0275
102.1209
0.9099 8.7394 0.0003 0.0081 16.3563 75.925 1 0.0061 0.1464 0.0018 0.0275
43553 1000 40% 1.6667 SWRO None ‘2V
2
24.2296
525.1550 0.1419 0.9405 0.2520 0.6460 0.0020 0.0018 17.3571 4.3861 0.1360 0.1269 0.5441 0.7775
49.2742
1.7766 4.5534 0.0139 0.0130 18.4556 22.8606 0.1446 0.1349 0.5441 0.7775
17270 1200 50% 2.0000 SWRO None ‘2V
3
Table 3 Calculation sheet of hard chemical species in brine layer (1 mmx 1 cm*) of the investigated RO plant cases
39.2464
444.233 1 0.1346 0.925 1 0.6944 0.1118 0.0002 0.0005 35.9709 0.5584 0.0083 0.0268 0.2552 1.6199
49.8167
5.1598 0.8306 0.0012 0.0038 38.883 1 3.0252 0.0090 0.0289 0.2552 1.6199
11460 1500 60% 2.5000 SWRO None '2Y
4
73.5984
227.4003 0.1151 0.8839 1.1948 0.6114 0.0056 0.0021 66.1465 3.0549 0.3093 0.1148 0.4789 1.6803
111.7433
10.3834 5.3132 0.0485 0.0180 74.8343 18.5069 0.3499 0.1299 0.4789 1.6803
6606 500 70% 3.3333 BWRO Blocked 27d
5
75.4724
23.9888 1.oOOo 0.6438 42.4775 18.9585 0.0097 0.0000 9.4744 3.1745 0.0022 0.0000 0.1068 1.2689
80.7159
42.4775 18.9585 0.0097 0.0000 14.7167 3.1745 0.0033 0.0000 0.1068 1.2689
1539 500 75% 4.0000 BWRO Blocked 3d
6
1.3526E-04 1.4933E+18
1.2903E+18
3.4841E+18
0.1 1.02348+00 l.O234E-01 l.O234E-04 2.5794E-05 6.61 lOE-05 2.0214E-07 1.886OE-07 I .7764E-03 4.4889E-04 1.3921E-05 1.2988E-05 5.5689E-05 7.9568E-05 2.4798E-03 9.2295E-05 2.2522E-03
0.1 l.O537E+OO l.O637E+OO l.O637E-01 l.O537E-01 l.O637E-04 l.O537E-04 O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO 5.3262E-03 1.7234E-03 4.0000E-04 2.6259E-04 3.6767E-06 6.4164E-07 4.25 15E-05 1.5425E-05 6.0848E-05 1.9044E-07 8.9737E-05 2.90028-06 5.7856E-03 2.1426E-03 O.OOOOE+OO O.OOOOE+OO 5.635OE-03 2.1395E-03 1.5059E-04 3.0906E-06 0.1
*Number of hard atoms based on Avogadro constant (6.02214199 x 1O23atom/mole) given by NIST [l l]
15 16 17 18
MgSG BaS04 SrS04 CaP04 MgSi03 Total fouling load, mmole/O. 1cm3 Carbonates fouling load mmole/O. 1 cm3 Sulfates fouling load mmole/O. 1 cm3 Others fouling load mmofe/O. 1 cm3 No* of hard atoms/O.1 cm3 brine layer
MgC03 + Mg(HCWz BaC03 + Ba(HC03)* SrC03 + Sr(HCO& CaS04
Brine water layer volume, cm3 Brine water layer density, g/cm3 Brine water layer mass, g Brine water layer mass, kg CaC03 + Ca(HCO&
1 2 3 4 5 6 7 8 9 10 11 12 13 14
found in a brine water layer (1 mm/l cm*)
Foulant species @mole)
(D)
Table 3, continued
2.4080E+18
0.1 l.o188E+oo l.O188E-01 1.01888-04 7.0748E-05 l.l389E-05 1.6414E-08 5.2666E-08 3.6648E-03 5.6891E-05 8.5028E-07 2.7281E-06 2.6001E-05 1.6504E-04 3.99858-03 8.2206E-05 3.7253E-03 1.9104E-04
4.4930E+18
0.1 l.O137E+OO l.O137E-01 1.01378-04 1.2112E-04 6.19758-05 5.6627E-07 2.1020E-07 6.7054E-03 3.0968E-04 3.13508-05 l.l637E-05 4.8552E-05 1.7033E-04 7.4608E-03 1.8387E-04 7.05818-03 2.1888E-04
4.5525E+18
0.1 1.0016E+00 l.O016E-01 l.O016E-04 4.25478-03 1.8990E-03 9.6796E-07 O.OOOOE+OO 9.4899E-04 3.1797E-04 2.1590E-07 O.OOOOE+OO l.O696E-05 1.27108-04 7.5596E-03 6.1547E-03 1.2672E-03 1.3780E-04
106
S. El-Manharawy, A. Hafez /Desalination
belongs to the lowest TDS samples No. 5 and 6, while the Red Sea surface water recorded the lowest fouling load (20.33 mmol/kg). In addition, it is obvious that hard calcium fraction (i.e. CaCO,, CaHCO, and CaSO,) is responsible for most of inorganic fouling load and scaling of highly salty water, while hard magnesium fraction is participating differentially in fouling of medium and lower salinity water. In many recorded cases, where chloride is low and dissolved silica is high (>30 mg/l), MgSiO, fouling dominates others. The available carbonate fouling load is absent in cases of the Red Sea groundwater brine (No. 1) and Red Sea surface water (No. 2), and sulfate fouling is 100%. On the other hand, in sample No. 6, where chloride is low, available carbonate fouling is dominating over other types. In reality, it was true; cases 1, 5 and 6 had been blocked with heavy scales (2-3 times RO module dry weight) during a short time as noted from the membrane life. The XRD and chemical analysis results were in good agreement with the present model (*I 5% to 3% wtfwt). The proposed model points to the fact that not all hardness ions (i.e. Ca, Mg, Ba, Sr, CO,, HCO,, SO,, PO,, and SiOJ are participating in fouling as it sought, but follow selective preferential rules based on the associated chloride concentration in the RO brine solution. The available part for fouling is quite variable, it represents -20% of the total dissolved hardness species in brine solution of case 2, -49% in case 3, -79% in case 4, -66% in case 5 and -94% in case 6. It is not a salinity relationship as could be seen in case 1 (the highest TDS), where the fouling part is about 5 1% of the brine hard ions. The natural solubility of calcium and magnesium, as well as their deposition, under variable chloride concentrations are not yet fully understood and may need deeper investigation. The number of hard atoms/O. 1 cm3 in the brine layer is highly indicative. Generally, the concentration higher than 2.5~10’~ is critical indeed. Below this value, fouling was not sufficient to generate heavy solid scales (cases 2, 3 and 4),
153 (2002) 95-107
while a higher value destroyed the membrane modules. Field observation indicated that the deposition rate of amorphous loose or massive calcium carbonate scale is much faster than calcium sulfate that requires longer time to crystallize as coarse crystals of gypsum mineral (CaS0,.2H,O). As a guideline for the RO design, it is recommended to keep the number of hard atoms in a 1 mmx 1 cm2 brine layer to be lower than 2x 1O’* by using the proper pretreatment or/and lowering permeate recovery ratio. The economic importance of this limit lies in saving the cost of the total removal of hard ions from feedwater. Lowering of the brine fouling flux to between 1.5~ 1O’*and 2~10’~ atom/O.1 cm3.s could be considered satisfactory for either BWRO or SWRO desalination. 5. Conclusion Dehydration model provides a reasonable explanation of the fouling mechanism occurring during RO membrane dewatering. The model showed that not all hardness ions are participating in fouling and scaling, as it wa thought, but follow selective preferential rules based on the associated chloride concentration in the RO brine solution. The present model indicates that the available hard species ready for fouling and scaling are lower than expected. The mathematical correlation between the theoretical and experimental fouling results proved that the proposed model is an accurate and valuable tool for predicting the inorganic fouling load. As a guideline for the RO design, it is recommended to keep the number of hard atoms in 1 mmx 1 cm2 brine layer to be lower than 2x 10” by using the proper pretreatment or/and lowering the permeate recovery ratio. The possible source of error could arise from the interference with added chemicals, membrane type and wide variation in temperature. References [l]
W.F. Langelier, The analytical control of anti-corrosion water treatment, J. AWWA, 28 (1936) 1500-l 52 I.
S. El-Manharmvy, A. Hafez / Desalination 153 (2002) 95-107 [2]
[3]
[4] [5]
[6]
[7]
J.W. Ryznar, A new index for determining the amount of calcium carbonate scale formed by water, J. AWWA, 36 (1944) 472-483. H.A. Stiff and L.E. Davis, A method for predicting the tendency of oil field water to deposit calcium carbonate, AIME, 195 (1952) 213-219. DuPont Permasep, Pretreatment for Permeators, Eng. Man. Bulletin 4010, 1994. W.F. Blatt et al., Solute Polarization and Cake Formation in Membrane Ultrafiltration, Plenum Press, NY, 1970. S. El-Manharawy and A. Hafez, Molar ratios as useful tool for prediction of scaling potential inside ROsystem, Desalination, 136 (2000) 243-254. S. El-Manharawy and A. Hafez, Technical manage-
107
ment of RO system, Desalination, 131 (2000) 329344. [8] A. Geiger et al., Water and Aqueous Solutions, HilgerBristol, 1986. [9] S. El-Manharawy and A. Hafez, Water type and guidelines for RO system design, Desalination, 139 (2001) 97-113. [IO] L. David et al., PHREEQC-2 computer software for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, USGS, 2000. [ 1l] F.J. Millero, Chemical Oceanography, 2nd ed., CRC Press, 1996. [ 121 P.J. Mohr and B.N. Taylor, CODATA Recommended Values of the Fundamental Physical Constants, National Institute of Standards and Technology (NIST), Gaithersburg, 1998.
Desalination 153 (2002) 95-107 www.elsevieccom/locate/desal
Dehydration model for RO-membrane fouling: a preliminary approach Samir El-Manharawy”*, Azza Hafezb ONuclear Geochemistry Department, Nuclear Materials Corporation, Cairo, Egypt Tel. +20 (2) 3474822; Fax +20 (2) 34523 I; email: geo230@linknet hChemical Engineering and Pilot-Plant Department, National Research Center, Cairo, Egypt
Received 25 January 2002; accepted 16 March 2002
Abstract The proposed model is based on the hydration theory of dissolved chemical species in water. The molecular dehydration occurs when the solution system is subjected to critical water loss associated with re-displacement of ionic charges from attached water molecules towards the dissolved molecule. This mechanism allows the formation of solid phase, i.e. fouling. In order to estimate the true fouling load, it is necessary to define the type and molar concentration of the hard dissolved compound individually, then estimate the fraction available for deposition in a given chloride content. The mathematical mode1 is illustrated and tested for six RO plants, and the obtained results are in agreement with field observation. Keyword:
Dehydration model; Membrane fouling; Fouling fraction; Fouling flux
1. Introduction Several models are in use for the prediction of inorganic scale formation. The first model was introduced by Langelier [l]. It is based on the thermal behavior of water pH-value and hardness (as expressed empirically in CaCO,), the fouling *Corresponding author.
of calcium carbonate is determined on its saturation value at elevated temperature, which is known as Langelier Saturation Index (LSI). Ryznar [2] and Stiff and Davis 133 developed the earlier model to accommodate higher salinity effect. Later on, DuPont [4] introduced a mathematical model and monographs for the prediction of formation of other common scales (i.e. Ca, Ba, Sr-sulfates and
Presented at the EuroMed 2002 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and Alexandria University Desalination Studies and Technology Center, Sharm El Sheikh, Egypt, May 4-6, 2002. OOl l-9164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII:SOOll-9164(02)01109-8
96
S. El-Manharawy, A. Hafez /Desalination
silica) based on their thermal specific solubility at higher temperatures. It should be noted that these thermal models were basically established on the concept of supersaturation of dissolved salts in heat exchangers at elevated temperature and sufficient contact time. In application of the above-mentioned thermal indices for prediction of scale formation in reverse osmosis membrane separation many serious errors happened. This is due to the basic difference between thermal desalination and membrane desalting, which is a pressure driven process under normal temperature. Most of the recorded scaled RO membranes were quite below the supersaturation level of their dissolved hard chemical species. The concentration polarization (CP) model has been recently introduced for the prediction of scale formation in membrane filtration. The basic concept of the CP model refers to Blatt et al. [5] who studied fouling of solid colloidal particles and macroorganic molecules on the ultrafiltration (UF) membrane surface used in food processing. Several unsuccessful trials have been done in order to apply the CP hydrodynamic model in the reverse osmosis separation. The difference between both applications is obvious, however, field investigation indicated that RO system is currently affected by concentration polarization or something alike. The missing link between both models back to the absence of a reliable chemical model is able to predict and estimate the type and amount of dissolved hard chemical species (or hard ionpairs) that turn into solid particulate under RO process. When it is possible, the CP-hydrodynamic model, and thermodynamic as well, can be applied. During the past 5 years, the authors have been involved deeply in the RO hydrochemistry, in order to investigate, study and try to solve the inorganic-scaling problems associated with desalination of brackish and seawater by the RO membrane process. The comprehensive chemical analyses, XRF, mineralogy and field investigation of waters and scales in more than 60 RO-plant cases indicated a strong relationship between the feedwater chemical characteristics and the generated
153 (2002) 95-107
scale type, as well as its potential [6]. It was clear that the molar ratio (SO,/HCO,) gradually and positively correlated with the chloride molar concentration for most of the natural waters. This natural phenomenon shows that the variation in most of natural waters is so tight that it ranges between -0 and 20. For example, the molar ratio (SOJHCO,) of the River Nile water is -0.12, brackish groundwaters 1-5, salty waters 5-10, oceanic water -12, and the Red Sea water -14. Some anomalous sulfate-enriched groundwater brines could be higher than 30. Furthermore, it was possible to conclude that two factors, i.e. water type and permeate recovery, mainly control inorganic scale formation in the RO membrane separation, and scale potential is directly proportional to the permeate recovery rate [7]. The authors have attributed the formation of inorganic scale to the dehydration of scale forming chemical species, which does not necessitate their concentration to be in the super-saturation zone. The aim of the present study is to present and discuss the chemical assessment of inorganic fouling that could happen on the RO-membrane surface in the light of the proposed hydration-dehydration model. 2. Basic approach 1. Mineral and salts dissolve in water according to the hydration (or solvation) theory [S], where the dissociated ions attract water molecules according to their charge, z to ionic radius, r. Low (z/r) ions (such as Na” and Cl’-) surrounded with a single water shell of 4 to 6 water molecules. Intermediate (z/r) ratio ions (such as Fe3’ and AP’) tend to extract OH- from water to form insoluble complexes. Ions of high (z/r) ratio can strongly extract O*- from water and form their soluble oxyanions, such as the scale-forming ions, i.e. PO 3- SOJ2-, C032-, HCO,- and SiO,*-. Due to the:r high (z/r) value, these hard ions tend to attract more than one water shell around to form hydrated ions. For example, the 4 oxygens bonded to sulfur, each of which carries an average charge of -l/2
S. El-Manharmuyt A. Hafez / Desalinaiion 153 (2002) 95-107
(2-+4 oxygen), accept 3 water molecules around each oxygen in the first shell (a total of 12H,O) and less attracted 6 in the second shell (a total of 24H,O), and so on. In low-salinity water, sulfate ions may accept up to 6 extra water shells, while in concentrated solution the attached water layers are normally lower than 4. Phosphate (Pod-) has the same configuration of SO,*- ion but with higher residual charge (-314 each) therefore it attracts up to 8 shells. Both SiO,*- and CO,*- ions have higher charges (-213 each), however, they are less branched. It is also the same for dissolved cations (e.g. Ca, Mg, Ba and Sr), where the positive charge is equally distributed around and attracts the negative pole of dipolar water molecules. 2. Under normal physical conditions, i.e. without chemical interference, hydrated ions exist in solution as the original compound dissolved from it (i.e. as ion-pairs or hydrated molecules) and can re-crystallize again when subjected to water loss (i.e. dehydration). It is interesting to mention that the hydrated molecules keep in memory the original crystal structure they dissolved from. 3. The hydrated ions are mostly present in less active ion-pairs associations, or relaxed hydrated molecules. The stability of these relaxed molecules in solution is increasing with the increase of water clusters around, consequently, under water loss conditions the highly hydrated molecules will be less stable than the lower ones and tend to precipitate, i.e. the dissolved phosphate and sulfate molecukes are most stabilized while carbonate and silicate are less stabilized and tend to foul earlier. Fig. 1 shows a sketch drawing illustrating the idea of the dehydration model. In the fully hydrated molecule (Fig. 1a), the ionic charges (+ve and -ve) are displaced outside in order to attract as much as possible of dipolar water shells around, and as a result, the attraction power between both cation and anion is minimized, and the ionic distance will be longer, which allows additional room for extra water molecules. This mechanism maximizes the dissolution stability in solution and overcome the effect of molecule density to keep it floating.
97
Fig. 1. IllustraGon of a hydratedmolecule (a) and dehydration mechanism (b) on the RO membrane surface due to dewatering and change in charge displacement. Fig. 1b describes the situation when the hydrated molecule is subjected to serious water loss, either by RO extraction or evaporation. In this case, the ionic charges tend to move inside, i.e. inner displacement, and release some free water molecules to compensate the shortage in the system. At a critical point, the opposite inner charges become strong enough to attract both ions firmly to form a solid particle as started, and repel excess water molecules from its structure, i.e. forming a solid nucleus. The transformation of the dissolved compound into solid particles increases gradually with dehydration progress, i.e. fouling is proportional to pressure and recovery. 4. Dehydration of hydrated molecules is probably done in a random way on the RO membrane surface. The probability oftouching and dewatering is directly proportional to the concentration of molecules in water mass that equally distributed on the membrane surface area per time. The magnitude of molecular dehydration is directly proportional to the applied pressure and defusing rate of pure water through a given membrane type.
98
S. El-Manharawy, A. Hafez / Desalination 153 (2002) 95-107
5. It is assumed that all hydrated molecules could be subjected to dehydration, and only a fraction of hard molecules (e.g. CaSO, and CaCO,) will foul and exit the aqueous system as solid microparticles, while others (e.g. NaCI, Na,SO, and MgCl,) will redissolve again, i.e. regain their water shells or part of them. This depends on their specific solubility and the available time necessary for re-dissolution. There are several evidences in support of this assumption: l Field observation indicated that the RO concentrated stream (brine) got turbid after a short time from the start of operation. Generally, turbidity increases 2-3 times during the first few hours from daily flushing and operation. In some recorded cases in the Red Sea ROplants, the turbidity usually increases from less than 1 NTU to higher than 15 NTU in less than 1 hour. l The material chemical balance between the RO input and output is always disturbed because of the separation of some hard species from the aqueous system as a solid particulate, which cannot be determined by standard water analytical methods and may need strong acid digestion prior to the analysis. l The laboratory experiments on the closed recycling RO setup indicated a continuous and gradual change in the chemical composition of the circulating solution due to recycling progress. The acid-leach of the micron filtercartridge proved that considerable amount of hard suspended solids was retained and accumulated on the filter surface during the closed circulation. This must be taken into consideration when performing such experiments. These important field and laboratory observations support the idea of solid particulate formation due to reverse osmosis dehydration. 6. Scale deposition is another issue. It depends on the maturation of the fouling solid particle size to form the initial crystal nuclei under favorable conditions, i.e. enough time and suitable place. The applied mechanical high pressure plays an
important role in minimizing the time necessary for crystallization. It has been found that massive carbonate and sulfate scales are usually located in the exit of casing cavities and are stretched counter flow at different magnitude. The flow pattern of water inside the RO membrane module is highly turbulent and well mixed. This is due to the insertion of a complex mesh-spacer between membrane layers that delay the formation of the hard scale. Normally, water flows in a -l-mm thickness layer between the spiral wound RO membrane. 7. It has been observed that there is a considerable difference between the analytical molar ratio (CaSO,:CaCO,) found in brine solutions and that deposited in the formed scales. This phenomenon is common for all investigated cases but with different magnitude. This means that not all of the dissolved hardness molecules are readily to precipitate upon dehydration. This ratio is called the fouling fraction (F’. Our previous work has proven that the solubility, as well as deposition, of hard molecules changes widely under the influence of chloride ion concentration in the solution [9]. The statistical data processing of analytical results obtained from more than 60 RO feed, brine and scale samples indicated that there is a systematic relationship between the fouling fraction (F’ of scales and the Cl mmol/kg of the brine. Fig. 2 presents the graphical relationship between the brine chloride concentration and the fouling fraction (Ff. The CaS04 curve shows that the maximum scaling potential (100%) is at chloride concentration higher than 800 mmol/kg, and decreases at lower Cl. On the other hand, CaCO, almost has no chance at all (-0%) for deposition at Cl>800 mmol/kg, and its fouling potential increases to maximum (-100%) at Cl -10 mmovkg, then decreases again. Field observation and chemical analysis of collected scales generated from highly salty waters has proven that it is almost free of carbonate, in other words, not all hardness ions are participating in fouling. Three chloride categories were identified based on the geochemical affinity of dissolved minerals in natural water. The
S. El-Manharmvy,
A. Hafez /Desalination
153 (2002) 95-107
99
also be applied for Ba and Sr carbonate and sulfate. The fouling load (FL)obtained from multiplying the compound analytical molar concentration (in mmol/kg) by the estimated fouling fraction (Ffi at a given Cl concentration:
Fig.2.The relationship between brine chloride and FCaCO, and FCaSO, in feed water. obtained trend-line equations were used to predict fouling fraction (F’ of dissolved calcium carbonate and calcium sulfate that is expected to foul at a given chloride concentration level: (i) Cl- range in brine: O-10 mmol/kg F&K0
F-Ld
0.0791
[Cl] + 0.2945
= 0.0645
[Cl] + 0.0470
=
(1) (2)
(ii) Cl’ range in brine: lo-200 mmol/kg Ff,K03
FfCCaSO 4
=
0.0046 [Cl] + 0.9168
(3)
=
0.0013 [Cl] + 0.6126
(4)
(iii) Cl- range in brine: 200-800 mmol/kg Ffc,co, = 0.00009 [Cl] + 0.0946 = 0.00019 [Cl] + 0.8407 Ffcaso4
(5) (6)
(iv) Cl- range in brine: >800 mmol/kg FfcCao.3= 0.00
(7)
Ffca,,;= 1.OO
(8)
Because of possible mathematical extrapolation, it should be noted that Ffvalue should be between 0 and 1 (i.e. from 0% to 100% offoulant load), the results of values higher than 1 are to be considered as 1. In addition, there is no direct mathematical relation between Ffcaco and Ffc,,, . This means that it is not necessary that summatioh of both is 1, they are independent from each other. The above-mentioned emperical formulae could
Fouling load (FL,aco,), mmol/kg = ICaCO,l x Ffcaco3
(9)
Fouling load (FLcsso4), mmol/kg = [CaSO,lx t’fcaso4
(10)
For magnesium (Mg), the situation is different. The chemical analysis of collected scales proved that magnesium carbonate (MgCO,) follows exactly the above relations [Eqs. (1),(3),( 5)] as CaCO,. The unexpected difference comes from the behavior of MgSO,, which follows also the same relations of CaCO, with a deviation ranging between -1.5% and 5% lower than the real scale composition. This phenomenon means that the influence of chloride concentration is similar for both MgCO, and MgSO,, i.e. their solubility increases with the increase of chloride and vise versa. This may explain the anomalous capacity of seawater to dissolve excessive amount of magnesium salts, which is different from the geochemical behavior of calcium which is always found in seawater in limited quantity and easily precipitates. From the scale prediction point of view, it is safer to consider the maximum deviation as +5% for MgSO, estimation. In other words, the fouling fraction of both of MgCO, and MgSO, will be calculated according to Eqs. (l),(3),(5) at the given chloride level, then an empirical amount of 5% of brine [MgSO,] molar concentration will be added to the resulted fouling load of MgSO,, as folIows: (i)Cl- range in brine: O-l 0 mmol/kg Fouling load (FLMgco), mmol/kg = [MgCO,] x (0.0791. [Cl] + 0.2945)
(11)
Fouling load (FL,Bso), mmolikg = [MgSO,] x (0.0791 [Cl] + 0.2945) + (0.05 x [MgSO,]) (I 2,
100
S. El-Manharaljy, A. Hafeer/ Desalination I53 (2002) 95-107
(ii)CI- range in brine: 1O-200 mmol/kg Fouling load (F,,MyCO ), mmol/kg = [MgCO,] x (0.0046 [Cl] f 0.9168)
(13)
Fouling load (F,,MgSO ), mmoi/kg = [MgSOJx (0.0046 [Cl] f 0.91&) + (0.05 x [MgSO,]) (14) (iii) Cl- range in brine: 200-800 mmol/kg Fouling load (F,,,sro ), mmol/kg = [MgCO;] x (O.OOOb9[Cl] + 0.0946)
(15)
Fouling load (F,,,B,, ), mmoVkg = [MgSO,] x (0.00009 [Cl] + 0.8946) + (0.05 x [MgS0,](‘6)
[C,,.= 1 +(I -R)] where R is the recovery 0.7, etc.).
(iv) Cl- range in brine: >800 mmol/kg Fouling load (F,,,scO,), mmol/kg = 0.00
(17)
Fouling load ((;I,MgsOq), mmol/kg = 0.05 x [MgSO,]
(18)
It is important to note that the summation of the calculated MgSO, foulant and the added part (5%) is never more than the original molar concentration. In the present chemical model, the total amount ofCaPO,, MgSiO,, Fe,O, and MnO, is considered as a full fouling load because the influence of the chloride ion on both is almost neglected. As a rule, iron and manganese oxides must be removed to less than 0.2 mg/l at the early stage of pretreatment.
3. Prediction
but we found several drawbacks in the application for waters of TDS> 10000 mg/l. In the meantime, it is better to follow the probable salt combination method manually calculated according to the following simplified model: (i) Convert feed-water ion concentration from (mg/l) to (mg/kg), multiplying by density. (ii) Convert feed water (ion-mg/kg) to (ionmmol/kg), dividing by molecular weight. (iii) Estimate brine-ion concentration (in mmol/ kg), multiplying by the concentration factor:
of inorganic fouling load (F,,)
In order to obtain the inorganic fouling load (F,,, mmol/kg), it is necessary to calculate the probable combination of dissolved hard salts in brine solution, i.e. the true concentration of [Ca, Mg, Ba and SrC03], [Ca, Mg, Ba and Sr(HCO,),], [Ca, Mg, Ba and SrSO,], [CaPO,] and [MgSiO,], in mmol/kg in the RO concentrated solution, i.e. the brine. These compounds usually constitute more than 99.99% of the RO inorganic scales, and its sum called the total fouling load (F,, mmol/kg). The USGS PHREEQC software version 2 [IO] can be used for aqueous geochemical calculations,
(19) decimal (e.g. 0.3, 0.5,
(iv) Combine (CO,‘-) with equal mmol of [Ca”, Mg2+, Ba2+ and Sr2+]. (v) Combine (HCO;) with 0.5 mmol of [Ca2+, MgZ+,Ba2+ and Sr2+]. (vi) Combine excess [Ca”, Mg2+, Ba*’ and S?+] with equal mmol of (S0,2-). (vii)Combine excess (S0,2-) with 2 mmol of ma+ + K’]. (viii) Combine excess [Na++ K’] with equal mmol of (Cl-). In case of seawater and highly salty waters as well, excess SO, is generally distributed between Mg and Na. We used the natural relative molar abundance of Na,SO, to MgSO, in oceanic water given by Miller0 [ 1I], where 48.33% of excess Mg combines with SO,, and the rest with Cl. In case of water containing considerable amount of PO,and SiO,, combine the first with equal amount of calcium [CaPO,] and silicate with magnesium [MgSiO,]. This step must be done at the start and before any ion distribution. This simplified ionpair stoichiometric calculation showed that the accuracy of distribution of ions for probable salt combination is better than 99%. The fouling flux (FX)of an individual foulant could be predicted from its concentration in brine that flows in a I-mm thickness layer spreading over 1 cm2 of the membrane surface per time, which is normally around 1 s:
S. EM4anharmvy, A. Hafez /Desalination FY, atom/O. I cm’s = F,, , Ff,
10m3X d,, , A,.
(20)
where F,r is the number of hard atoms found in 0.1 cd per unit time (s), F,, is the fouling load, in mmol/O. 1 cm’, Ffis the fouling fraction, A,. is the Avogadro constant [ 12](= 6.022 14199 x 1Oz3),and dh is the brine density, which could be estimated using the following formula: Brine density (d,) = 0.000759 [TDS], + 996.977739
(21)
where [TDS], is calculated from feedwater-TDS (in mg/kg) multiplying by C,: as in Eq. (19). Note: for magnesium fouling load calculation, refer to the given limitations in the basic approach.
4. Examples and short discussion In order to test the proposed dehydration mathematical model, six feedwater samples were collected for chemical analysis from 6 running RO plants used for water desalination for drinking and industrial purposes. These cases had been selected to cover a wide range of TDS (from -1500-59000 mg/l), and four cases ofthem (No. 1, 5 and 6) were subjected to heavy inorganic scaling during a short time from the start of operation. Table 1 presents the chemical characteristics of selected waters and molar ratios that differentiate between their chemical types clearly. The given analytical results (in mg/l) were corrected to correspond the water density to obtain the concentration as (mg/kg) before calculating molar ratios. The investigated cases show that water No. 1 (Red Sea groundwater brine) is rich in sulfate as compared with the Red Sea surface water (No. 2), and the molar ratio (SO,/Alk = 17.67) is the highest in the group. On the other hand, water sample No. 6 has the lowest ratio (SO,/Alk = 0.26) due to its high bicarbonate content. Table 2 gives the results of the probable salt combination in mmol per one kg of water, in addition to some simple mathematical indicators for individual chemical species groups. It is interesting to notice that the hardness chemical
153 (2002) 95-107
101
species constitute a modest part (-4-9%) of the total dissolved species in the first five samples, while it reaches -5 1% in sample No. 6 which has the lowest TDS (I 539 mg/l). Generally, there is a reversible relationship between TDS and hardness, i.e. hardness decreases with the increase of natural TDS. It is also clear that the lower content of chloride allows generous dissolution of carbonate in water, as shown in sample No. 6 which contains bicarbonate salts of more than 44% of TDS. On the other hand, the Red Sea groundwater (No. 1, TDS = 59,088 mg/l), as well as the Red Sea surface water (No. 2, TDS = 43,553 mg/l) are quite depleted in carbonate and bicarbonate salts (0.23 and 0.42%). Only the Red Sea surface water (No. 2) acquires carbonate salts, while alkalinity in other samples is limited to bicarbonate. The sulfate salts are abundant in higher TDS waters (>90 mmol/kg) and less in lower TDS (-8 mmol/kg). However, the sulfate salt percentage to TDS increases remarkably from 3.6 to 5.9%, and does not decrease, which is confusing indeed. From the hardness point of view, sulfate hard salts constitute more than 90% of total hardness chemical species in seawater and higher salinity, and decrease rapidly to 11.5% in the lowest salinity. The carbonate salts composes more than 86% of total hard species in the lowest TDS sample (No. 6) and is minimized to 6% in the highest salinity sample (No. 1). Generally, hard sulfate salts increase and hard carbonate decrease with the increase of the chloride ion. This important information could help in prediction of chemical type(s) of possible foulant and scale. Table 3 illustrates the method of calculating the possible fouling load found in a thin layer (0.1 cm) of a brine solution covering 1 cm2 of membrane surface, the available fouling load is based on the fouling fraction as mentioned earlier. The carbonate and bicarbonate species estimated separately and then grouped numerically as listed. The obtained results indicated that the fouling load is independent from salinity, i.e. the fouling load is not proportional to the ionic strength. As shown, the highest fouling load (73.59 and 75.47 mmol/kg)
S. El-Manharawy, A. Hafez /Desalination
102
I53 (2002) 95-107
Table 1 Chemical characteristics of feedwater samples used for calculation of possible RO membrane fouling load (A)
Water samples information
1 2 3
Case number Case location Water type
1 Quosir Brine GW
2 3 Hurghada Ataqua Red Sea SW Salty GW
4 Sokhna Salty GW
4 5
pH value Water density, gm/cc
7.2 1.0425
8.3 1.0299
6.9 1.0101
2 10,853 482 549 3105 0.68 10.43 23,479 176 39 0 4858 0.61 0.04 0 0.05 43,552.72 1.38207 0.01 1.03462
(B)
Initial water chemical analysis (mg/l)
ser. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Ion Sodium ion (Na) Potassium ion (K) Calcium ion (Ca ) Magnesium ion (Mg) Barium (Ba) Strontium (Sr) Chloride (Cl ) Bicarbonate (HC03) Carbonate (CO,) Hydroxyl (OH) Sulphate (SO,) Silica (Si03) Iron (Fe3+) Manganese (Mn4’) Phosphate (PO.,) TDS, mg/l TDS, mole/kg Cation-anion difference, % Ionic strength, eq/Kg
(C)
Molar ratios, based on molar concentration, mmole/kg
ser. 1 2 3 4 5 6 7 8 9 10 11 12
Molar ratio (SO4 /Alk*) (SO,/CI) (S04/Si03) (A&/Cl) (HCO$SiOJ [(Na+K)/(Ca+Mg)] (Na/Cl) (K/Cl) (Ca/Cl) (Mg/Cl) (Ca/Si03) (Ca/P04)
1
17,485 552 1652 1958 3.73 21.52 32,269 177 0 0 4923.78 21.55 0.36 0 18.24 59,088.18 2.76997 0.10 1.94018
1 17.669 0.056 180.961 0.003 10.242 6.450 0.836 0.016 0.043 0.089 139.596 205.871
*Alk: Alkalinity = OH + CO, f HCO,
2 14.308 0.076 6307.43 1 0.005 359.768 3.425 0.713 0.019 0.021 0.193 1708.552 26018.902
7.0 1.0057
5 Rawash Brackish GW 7.1 1.0019
6 Haram Brackish GW 7.3 0.9982
3 5308 143 341 521 8.92 5.31 9216 128 0 0 1570 14.64 0.74 0.06 12.79 17,269.52 0.54875 0.23 0.35124
4 2687 35 1343 138 1.06 2.17 6264 97 0 0 863 24.51 0.22 0 4.82 11,459.90 0.3464 0.39 0.24908
5 2067 11 193 65 3.04 0.72 2414 192 0 0 1634 19.14 0.45 0.16 6.81 6606.24 0.18663 0.60 0.13155
6 206 8 167 49 0.13 0 213 626 0 0 256 12.09 0.15 0 1.27 1538.73 0.0344 1.84 0.03078
3 7.791 0.063 84.939 0.008 10.902 7.833 0.888 0.014 0.033 0.082 44.218 63.179
4 5.652 0.05 1 27.891 0.009 4.935 3.005 0.662 0.005 0.19 0.032 104.02 660.26 1
5 5.405 0.25 67.611 0.046 12.508 12.042 1.32 0.004 0.071 0.039 19.143 67.158
6 0.260 0.444 16.776 1.708 64.564 1.482 1.491 0.034 0.694 0.336 26.223 311.601
103
S. El-Manharmuy, A. Hafez / Desalination 153 (2002) 95-107
Table 2 The probable salt combination (mmoVkg)* of the studied concentrated brine samples at operating recovery levels (%) and concentration factors, C,.. ser. ii 2 3 4
9 10
11 12
13 14 15 16
17 18 19 20 21 22 23
Case number
1
2
3
4
5
6
Recovery % Concentration factor (CF) CaC03 MgC03 BaCO, SrC03 Total carbonates, mmole/kg Total carbonates, % Ca(HCQ)2 Mg(HCO,)z Ba(HCO& Sr(HCQ2 Total bicarbonates, mmole/kg Total bicarbonates, % CaS04 MgS04 BaS04 SrS04 Total sulfates, mmole/kg Total sulfates, % Fe203 Mn02 CaP04 MgSi03 Total others, mmole/kg Total others, % Na2S04 CaC12 MgC& BaC12 SrC12 NaCl KC1 Total non-hardness species, mmole/kg Total non-hardness species, % Total salt species, mmole/kg Hardness species, mrnoleikg Hardness species, % Non-hardness species, tnmole/kg Non-hardness species, % C03: hardness, % S04: hardness, % Others: hardness, %
30% 1.43 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 2.1254 4.3364 0.0015 0.0170 6.4803 0.2339 50.0730 49.3740 0.0346 0.3997 99.8813 3.6050 0.0137 0.0000 0.5721 0.8437 1.4295 0.0516 79.1790 99.4200 281.5300 0.0680 0.7868 2159.8000 42.0520 2662.8358 96.1095 2770.6269 107.7911 3.89 2662.8358 96.11 6.01 92.66 1.33
40% 1.67 0.2102 2.0189 0.0001 0.0019 2.2311 0.0969 0.6997 6.7205 0.0003 0.0063 7.4268 0.3226 16.3560 75.9250 0.0061 0.1464 92.4335 4.0152 0.0018 0.0000 0.0018 0.0275 0.03 11 0.0014 121.7600 42.9420 534.2200 0.0160 0.3843 1458.3000 42.3220 2199.9443 95.5639 2302.0668 102.1225 4.44 2199.9443 95.56 9.46 90.51 0.03
50% 2.00 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 1.7897 4.5403 0.0139 0.0130 6.3569 0.5788 18.5920 22.7950 0.1442 0.1346 41.6658 3.7939 0.0383 0.0035 0.5441 0.7775 1.3634 0.1241 36.5560 21.8890 91.1810 0.1635 0.1525 884.1300 14.7780 1048.8500 95.503 1 1098.2361 49.3861 4.50 1048.8500 95.50 12.87 84.37 2.76
60% 2.50 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 5.1638 0.8266 0.0012 0.0038 5.9954 0.6912 38.9140 3.0104 0.0090 0.0288 41.9622 4.8380 0.0142 0.0000 0.2552 1.6199 1.8893 0.2178 4.8277 189.2200 37.4840 0.0436 0.1398 581.2800 4.5014 817.4965 94.2529 867.3434 49.8469 5.75 817.4965 94.25 12.03 84.18 3.79
70% 3.33 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 10.3800 5.3011 0.0484 0.0180 15.7475 2.5295 25.2130 14.3110 0.1154 0.0428 39.6822 6.3742 0.0384 0.0154 0.4785 1.6786 2.2109 0.3551 110.7200 0.0000 0.0000 0.0000 0.0000 452.3100 1.8773 564.9073 90.7412 622.5479 57.6406 9.26 564.9073 90.74 27.32 68.84 3.84
75% 4.00 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 42.4770 18.9590 0.0097 0.0000 61 A457 44.4940 5.0167 3.1745 0.0011 0.0000 8.1923 5.9322 0.0153 0.0000 0.1068 1.2689 1.3910 1.0072 19.6440 0.0000 0.4288 0.0000 0.0000 45.3630 1.6340 67.0698 48.5665 138.0988 71.0290 51.43 67.0698 48.57 86.51 11.53 1.96
Concentration factor (C,J = l/( 1 - R), where R is recovery decimal (i.e. 0.3,0.4,0.5,0.6,0.7 *mmol/kg = (mg/l x density) + MW
and 0.75)
1355.5459 0.0000 1.0000 0.0000 0.0000 0.0000 0.0000 50.0735 2.4687 0.0346 0.3997 0.5721 0.8437 54.3922
Chloride in brine, mmole/kg Carbonate foulmg h-action (Fcoj) Sulfate fouling fraction (Fso4) CaC03 + Ca(HC03)2 MgC03 + Mg(HCQ)z BaCO, + Ba(HCO& SrCOj + Sr(HCO,), CaS04 MgS04 BaS04 SrS04 cap04 MgSi03
Total fouling load, mmolekg
1 2 3 4 5 6 7 8 9 10 11 12 13
107.7777
Total hard species, mmole/kg
Brine foulant species, mM/kg
CaCOj + Ca(JlCO& MgC03 + Mg(HCO& BaCOJ + Ba(HCO& St-CO3+ Sr(HCO& CaS04 MgS04 BaS04 SrS04 CaP04 MgSi03
1 2 3 4 5 6 7 8 9 10
(C)
2.1254 4.3364 0.0015 0.0170 50.0735 49.3740 0.0346 0.3997 0.5721 0.8437
Brine hard species, mmole/kg
(B)
59088 750 30% 1.4286 SWRO Blocked 34 d
1
Feedwater TDS, mg/l Capacity, m’/d Water recovery % Brine concentration factor (CF) RO membrane type Relative scaling condition Membrane life time
Case type
1 2 3 4 5 6 7
(A)
20.3343
1136.7739 0.0000 1.Oooo 0.0000 0.0000 0.0000 0.0000 16.3563 3.7963 0.0061 0.1464 0.0018 0.0275
102.1209
0.9099 8.7394 0.0003 0.0081 16.3563 75.925 1 0.0061 0.1464 0.0018 0.0275
43553 1000 40% 1.6667 SWRO None ‘2V
2
24.2296
525.1550 0.1419 0.9405 0.2520 0.6460 0.0020 0.0018 17.3571 4.3861 0.1360 0.1269 0.5441 0.7775
49.2742
1.7766 4.5534 0.0139 0.0130 18.4556 22.8606 0.1446 0.1349 0.5441 0.7775
17270 1200 50% 2.0000 SWRO None ‘2V
3
Table 3 Calculation sheet of hard chemical species in brine layer (1 mmx 1 cm*) of the investigated RO plant cases
39.2464
444.233 1 0.1346 0.925 1 0.6944 0.1118 0.0002 0.0005 35.9709 0.5584 0.0083 0.0268 0.2552 1.6199
49.8167
5.1598 0.8306 0.0012 0.0038 38.883 1 3.0252 0.0090 0.0289 0.2552 1.6199
11460 1500 60% 2.5000 SWRO None '2Y
4
73.5984
227.4003 0.1151 0.8839 1.1948 0.6114 0.0056 0.0021 66.1465 3.0549 0.3093 0.1148 0.4789 1.6803
111.7433
10.3834 5.3132 0.0485 0.0180 74.8343 18.5069 0.3499 0.1299 0.4789 1.6803
6606 500 70% 3.3333 BWRO Blocked 27d
5
75.4724
23.9888 1.oOOo 0.6438 42.4775 18.9585 0.0097 0.0000 9.4744 3.1745 0.0022 0.0000 0.1068 1.2689
80.7159
42.4775 18.9585 0.0097 0.0000 14.7167 3.1745 0.0033 0.0000 0.1068 1.2689
1539 500 75% 4.0000 BWRO Blocked 3d
6
1.3526E-04 1.4933E+18
1.2903E+18
3.4841E+18
0.1 1.02348+00 l.O234E-01 l.O234E-04 2.5794E-05 6.61 lOE-05 2.0214E-07 1.886OE-07 I .7764E-03 4.4889E-04 1.3921E-05 1.2988E-05 5.5689E-05 7.9568E-05 2.4798E-03 9.2295E-05 2.2522E-03
0.1 l.O537E+OO l.O637E+OO l.O637E-01 l.O537E-01 l.O637E-04 l.O537E-04 O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO O.OOOOE+OO 5.3262E-03 1.7234E-03 4.0000E-04 2.6259E-04 3.6767E-06 6.4164E-07 4.25 15E-05 1.5425E-05 6.0848E-05 1.9044E-07 8.9737E-05 2.90028-06 5.7856E-03 2.1426E-03 O.OOOOE+OO O.OOOOE+OO 5.635OE-03 2.1395E-03 1.5059E-04 3.0906E-06 0.1
*Number of hard atoms based on Avogadro constant (6.02214199 x 1O23atom/mole) given by NIST [l l]
15 16 17 18
MgSG BaS04 SrS04 CaP04 MgSi03 Total fouling load, mmole/O. 1cm3 Carbonates fouling load mmole/O. 1 cm3 Sulfates fouling load mmole/O. 1 cm3 Others fouling load mmofe/O. 1 cm3 No* of hard atoms/O.1 cm3 brine layer
MgC03 + Mg(HCWz BaC03 + Ba(HC03)* SrC03 + Sr(HCO& CaS04
Brine water layer volume, cm3 Brine water layer density, g/cm3 Brine water layer mass, g Brine water layer mass, kg CaC03 + Ca(HCO&
1 2 3 4 5 6 7 8 9 10 11 12 13 14
found in a brine water layer (1 mm/l cm*)
Foulant species @mole)
(D)
Table 3, continued
2.4080E+18
0.1 l.o188E+oo l.O188E-01 1.01888-04 7.0748E-05 l.l389E-05 1.6414E-08 5.2666E-08 3.6648E-03 5.6891E-05 8.5028E-07 2.7281E-06 2.6001E-05 1.6504E-04 3.99858-03 8.2206E-05 3.7253E-03 1.9104E-04
4.4930E+18
0.1 l.O137E+OO l.O137E-01 1.01378-04 1.2112E-04 6.19758-05 5.6627E-07 2.1020E-07 6.7054E-03 3.0968E-04 3.13508-05 l.l637E-05 4.8552E-05 1.7033E-04 7.4608E-03 1.8387E-04 7.05818-03 2.1888E-04
4.5525E+18
0.1 1.0016E+00 l.O016E-01 l.O016E-04 4.25478-03 1.8990E-03 9.6796E-07 O.OOOOE+OO 9.4899E-04 3.1797E-04 2.1590E-07 O.OOOOE+OO l.O696E-05 1.27108-04 7.5596E-03 6.1547E-03 1.2672E-03 1.3780E-04
106
S. El-Manharawy, A. Hafez /Desalination
belongs to the lowest TDS samples No. 5 and 6, while the Red Sea surface water recorded the lowest fouling load (20.33 mmol/kg). In addition, it is obvious that hard calcium fraction (i.e. CaCO,, CaHCO, and CaSO,) is responsible for most of inorganic fouling load and scaling of highly salty water, while hard magnesium fraction is participating differentially in fouling of medium and lower salinity water. In many recorded cases, where chloride is low and dissolved silica is high (>30 mg/l), MgSiO, fouling dominates others. The available carbonate fouling load is absent in cases of the Red Sea groundwater brine (No. 1) and Red Sea surface water (No. 2), and sulfate fouling is 100%. On the other hand, in sample No. 6, where chloride is low, available carbonate fouling is dominating over other types. In reality, it was true; cases 1, 5 and 6 had been blocked with heavy scales (2-3 times RO module dry weight) during a short time as noted from the membrane life. The XRD and chemical analysis results were in good agreement with the present model (*I 5% to 3% wtfwt). The proposed model points to the fact that not all hardness ions (i.e. Ca, Mg, Ba, Sr, CO,, HCO,, SO,, PO,, and SiOJ are participating in fouling as it sought, but follow selective preferential rules based on the associated chloride concentration in the RO brine solution. The available part for fouling is quite variable, it represents -20% of the total dissolved hardness species in brine solution of case 2, -49% in case 3, -79% in case 4, -66% in case 5 and -94% in case 6. It is not a salinity relationship as could be seen in case 1 (the highest TDS), where the fouling part is about 5 1% of the brine hard ions. The natural solubility of calcium and magnesium, as well as their deposition, under variable chloride concentrations are not yet fully understood and may need deeper investigation. The number of hard atoms/O. 1 cm3 in the brine layer is highly indicative. Generally, the concentration higher than 2.5~10’~ is critical indeed. Below this value, fouling was not sufficient to generate heavy solid scales (cases 2, 3 and 4),
153 (2002) 95-107
while a higher value destroyed the membrane modules. Field observation indicated that the deposition rate of amorphous loose or massive calcium carbonate scale is much faster than calcium sulfate that requires longer time to crystallize as coarse crystals of gypsum mineral (CaS0,.2H,O). As a guideline for the RO design, it is recommended to keep the number of hard atoms in a 1 mmx 1 cm2 brine layer to be lower than 2x 1O’* by using the proper pretreatment or/and lowering permeate recovery ratio. The economic importance of this limit lies in saving the cost of the total removal of hard ions from feedwater. Lowering of the brine fouling flux to between 1.5~ 1O’*and 2~10’~ atom/O.1 cm3.s could be considered satisfactory for either BWRO or SWRO desalination. 5. Conclusion Dehydration model provides a reasonable explanation of the fouling mechanism occurring during RO membrane dewatering. The model showed that not all hardness ions are participating in fouling and scaling, as it wa thought, but follow selective preferential rules based on the associated chloride concentration in the RO brine solution. The present model indicates that the available hard species ready for fouling and scaling are lower than expected. The mathematical correlation between the theoretical and experimental fouling results proved that the proposed model is an accurate and valuable tool for predicting the inorganic fouling load. As a guideline for the RO design, it is recommended to keep the number of hard atoms in 1 mmx 1 cm2 brine layer to be lower than 2x 10” by using the proper pretreatment or/and lowering the permeate recovery ratio. The possible source of error could arise from the interference with added chemicals, membrane type and wide variation in temperature. References [l]
W.F. Langelier, The analytical control of anti-corrosion water treatment, J. AWWA, 28 (1936) 1500-l 52 I.
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[3]
[4] [5]
[6]
[7]
J.W. Ryznar, A new index for determining the amount of calcium carbonate scale formed by water, J. AWWA, 36 (1944) 472-483. H.A. Stiff and L.E. Davis, A method for predicting the tendency of oil field water to deposit calcium carbonate, AIME, 195 (1952) 213-219. DuPont Permasep, Pretreatment for Permeators, Eng. Man. Bulletin 4010, 1994. W.F. Blatt et al., Solute Polarization and Cake Formation in Membrane Ultrafiltration, Plenum Press, NY, 1970. S. El-Manharawy and A. Hafez, Molar ratios as useful tool for prediction of scaling potential inside ROsystem, Desalination, 136 (2000) 243-254. S. El-Manharawy and A. Hafez, Technical manage-
107
ment of RO system, Desalination, 131 (2000) 329344. [8] A. Geiger et al., Water and Aqueous Solutions, HilgerBristol, 1986. [9] S. El-Manharawy and A. Hafez, Water type and guidelines for RO system design, Desalination, 139 (2001) 97-113. [IO] L. David et al., PHREEQC-2 computer software for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations, USGS, 2000. [ 1l] F.J. Millero, Chemical Oceanography, 2nd ed., CRC Press, 1996. [ 121 P.J. Mohr and B.N. Taylor, CODATA Recommended Values of the Fundamental Physical Constants, National Institute of Standards and Technology (NIST), Gaithersburg, 1998.