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Compact accelerated precipitation softening (CAPS) as a pretreatment for membrane desalination I. Softening by NaOH
DESALINATION Desalination
113 (1997) 65-71
Compact accelerated precipitation softening (CAPS) as a pretreatment for membrane desalination I. Softening by NaOH 0. Kedema*, G. Zalmonb a Weizmann Institute of Science, Rehovot, Israel bEcosoft Company, Israel Received 2 April 1997; accepted 3 1 July 1997
Abstract This study examines the application of compact accelerated precipitation softening (CAPS)-a previously developed process-as a pretreatment for membrane desalination. In this CAPS process, rapid precipitation of calcium carbonate is achieved by cake filtration. The importance of this pretreatment step lies in the fact that softening (by NaOH in this case) facilitates an increase of the recovery ratio in membrane desalination by decreasing the potential for scaling. In addition, CAPS serves as an effective filtration step, reducing both turbidity and the silt density index and hence the potential for membrane fouling. Keywords: Water softening; Pretreatment;
Cake filtration; SD1
1. Introduction In the desalination reverse osmosis (RO)
of
brackish
water
by
or electrodialysis (ED), recovery is usually limited by scaling and fouling of the membranes, since osmotic pressure of the concentrate becomes the limiting factor only at
*Corresponding author. Present address: The Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. Fax: +972 (7) 647-2969; E-mail: ok.bgu.mail.bgu.ac.il. 001 l-9164/97/$09.50 PIZ 001 l-9164(97)001
relatively high salinities. Similarly, in marginal desalination and in demineralization of potable water for power stations and industry, scaling and/or fouling constitute the limiting factors. Scale formation is thus detrimental in a variety of processes [ 1,2]. The challenge to be faced in the desalting of low-salinity water is to increase the recovery by the suppression of scaling, without increasing the burden of chemicals to be added. In such a desalting process, the precipitation of calcium carbonate is prevented by acidification,
0 1997 Elsevier Science B.V. All rights reserved 15-X
66
0. Kedem, G. Zalmon /Desalination
leaving calcium sulfate as the main potentially scale-forming mineral. Thus, if the feed water could be partially softened before the desalting step, the feasible concentration factor and hence the recovery ratio could be increased. Such a pretreatment step can be viable only if a compact and environmentally acceptable softening process is available. Neither of the traditional softening processeslime softening and ion exchangeanswers these requirements: Lime softening is slow, requires extensive space, and generates a cumbersome “sludge”. Ion exchange, which has largely replaced lime softening because of its clean and convenient operation, requires a salt solution for regeneration, and it is currently felt that the disposal of these regenerating solutions may contribute significantly to salination of groundwater. Membrane softening, i.e., the use of nanotiltration membranes to retain calcium in preference to sodium, was not shown to lead to very high recovery. These considerations led us to develop a softening process [3], which we designated compact accelerated precipitation softening (CAPS). This process is a compact variation of softening by lime or sodium hydroxide, applied in cake filtration technology. During the development of the process, it became evident that co-precipitation and filtration also may eliminate various contaminants. The results presented here and in the companion paper [4] suggest that CAPS used as a pretreatment for RO may reduce both fouling and scaling of RO membranes.
2. Accelerated
precipitation
of CaCO,
As is the case of all crystallization processes, precipitation of CaC03 from a supersaturated basic solution is determined by the rate of nucleation or by the rate of crystal growth. Clean supersaturated solutions may remain metastable for prolonged periods. The addition of a solid and presence of various influences solutes
I13 (I 997) 65-7/
precipitation. The kinetics of all relevant steps of the crystallization of CaCO, have been studied in detail by the group of Nanchollas and a number of other workers [ 5-81, For any softening process used for practical applications, it is desirable to avoid metastability and to approach equilibrium as fast as possible so as to achieve, within practically reasonable residence times, concentrations close to their equilibrium values. After the addition of a base to a solution containing Ca(HCO&, the addition of CaCO, would appear to be the natural way to provide sites for crystal growth and thus to accelerate precipitation. It was indeed found that the precipitation rate was considerably enhanced by the presence of a suspended CaCO, precipitate. However, if the solid were separated out after a contact time of minutes, a substantial part of the Ca was removed, but the clear solution still remained supersaturated. It was shown by Graveland [9] that nucleation by other minerals is more effective and that, with this nucleation, the subsequent crystallization takes place along the axis of fast crystal growth. This phenomenon has been applied for the construction of large water softening installations [IO]. Kedem and Ben-Dror have found that very fast equilibration is obtained when the supersaturated solution is filtered through a cake of CaCO, [3]. Apparently, this fast process includes secondary nucleation as well as crystal growth [l 11, since the size distribution of the particles is not drastically changed upon repeated tiltrationwith-crystallization through the same cake. In the experiments described below, accelerated precipitation was investigated on a laboratory scale. A description of an industrial plant, which is currently under testing, will be published elsewhere.
3. Methods of precipitation Three systems were tested: (1) A loop for circulating and separating the suspended solid;
67
0. Kedem, G. Zalmon /Desalination 113 (1997) 65-71 (2) cake filtration under vacuum in a conventional Buchner funnel; and (3) circulating slurry and cake filtration in a filter tube.
3.1. Circulating slurry A microfiltration membrane was clamped into a flow-through cell (Fig. 1). A suspension of CaCOs (3 wt%) was circulated along one side of the membrane by means of a pump, and a vacuum was applied to the other, i.e., downstream, surface of the membrane. The solution removed from the downstream side of the membrane was continuously replaced by the same volume of supersaturated solution from a separating funnel. The supersaturated solution prepared immediately before each was experiment by adding NaOH (various amounts) to tap water.
I
I
Fig. 1. Supersaturated solution is prepared in the separating funnel (1). A slurry containing suspended CaCO, is circulated by a pump (2) along the membrane (3). Softened water is drawn off by vacuum into a vessel (4).
3.2. Buchnerfiltration A suspension containing 25 g of precipitated CaCO, was filtered through Whatman No. 50 filter paper in a 90-mm Buchner funnel to form a cake of a few millimeters. Supersaturated solutions were then filtered through this cake.
3.3. Cake$ltration
in filter tubes
For CAPS systems, filter tubes are prepared from filter fabric by thermal sealing. A suspension of small CaCO, crystals is circulated under pressure. Hard water and NaOH solution are continuously added to the reservoir in the cycle. In this system, part of the precipitation takes place in the suspension, and the remainder during passage through the cake. In the current experiments, a single tube of 700 cm2 filter area was mounted in a shell. A 3% suspension of CaCO, was circulated through a suitable reservoir and the addition of base was controlled to give the desired pH of the product.
4. Softening parameters For control of the softening process, both calcium and magnesium concentrations have to be considered, since routine determinations are carried out for the sum of calcium plus magnesium. The magnesium concentration is only slightly changed in the process by coprecipitation with calcium carbonate. Thus, for a given type of water, we have to know the necessary reduction of “total hardness”, i.e., calcium + magnesium, and the final calcium concentration required. This information is readily translated into the required quantity of base or, given the concentration of bicarbonate in the water, the pH of the product if equilibrium is reached. In practice, the product will always be at least slightly oversaturated. The customary measure for oversaturation is the saturation index (SI), i.e., the difference between the measured pH of the solution and the equilibrium pH, corresponding to the given ion concentrations [ 11.
68
0. Kedem, G. Zalmon /Desalination
SI=pH-
pH,
SD1 = lOO(t,- t,)lt,T
pHs =pK, - pK, +p[Ca2’] +p[HCO,-] + 5pfm where pH, is the equilibrium pH, K ,the second dissociation constant of carbonic acid, KS the solubility constant of calcium carbonate, and jn represents the activity coefficients. Under the conditions studied here: pfm=A
[JIl(l+-Jl)]
I13 (I 997) 65-71
- 0.31
(forZc0.5)
where I is the ionic strength. At 20°C
where q is the time required for filtration of a given volume at the end of the testing period, t, is the initial time, and T is the total testing period. Since it is essential to perform this test on line to avoid distortion of results by bacterial growth during transportation or storage of filtered water, we required a large enough continuous source of product water. Therefore, an SD1 unit of sufficient size was constructed and attached to our CAPS pilot plant so that water could be tested before and after softening.
pK, = 8.45; pK2 = 10.38;pK2 - pK, = 1.93 6. Results and discussion A = 0.506 6. I. Softening by NaOH In practice, the relationship between [Ca2’] and pH is determined empirically in the process itself, and SI is calculated. The value of SI characterizes the process performance. Process control is thus based on feedback from the pH of the product rather than on the calculated amount of alkali to be added to the feed water.
5. Silt density index The turbidity of the softened water was monitored and low values were obtained. Low turbidity reflects removal of suspended solids and possibly some colloids. However, practical experience has shown that turbidity is not the best measure for the fouling tendencies of a given water source. The widely used silt density index (SDI) [ 121 is a better means of evaluating foiling potential. To obtain the SDI, the feed water is filtered through a standard 0.45-mm microfiltration membrane, and the water flux is determined at zero time and after 15 min. The relative increase in time required for the passage of a given volume per minute indicates the clogging tendency of the tested source.
6.1.1. Circulating slurry The results of this series of measurements are given in Table 1. Comparison of results for Ca2+ plus M$+ before and after the process shows the decrease of total hardness. The time of contact, t,, between the supersaturated solution and the suspended precipitate (Table 1) is related to the total filtration time, f/, and the ratio between circulating volume V, and total volume V, by the following equation: t, = tf v/v,
In our system, V, = 140m1, and V, = 1.5 1;thus t, = 0.093 9. 6.1.2. Cake filtration in a Buchner funnel Table 2 shows results of softening with cake filtration. The most striking finding was the low residual concentration of calcium at a given pH, in comparison with the results given in Table 1, despite the much shorter residence time. Although there was no way for direct measurement of this time interval, the order of magnitude, which is given by the filtration rate
0. Kedem, G. Zalmon /Desalination
69
113 (1997) 65-71
Table 1 Circulating sluny of CaCO,. Composition of tap water and filtrate after the addition of NaOH [NaOH], meq/l
Tap water [Ca*+ +Mti+, meq/l
PH
5.90 5.90 6.58 6.7 6.58 6.58
7.36 7.36
1.0 1.2 1.4 2.0 2.2 2.83
7.2
Contact time, s
Filtrate [Ca*‘], meq/l
meq/l
4.47 3.90 4.47 3.41 3.26 2.05
3.11 2.61 3.03 1.81 0.61
1.36 1.37 1.44 1.45 1.44
Table 2 Cake filtration in a Buchner funnel. Composition of softened water (filtrate) after addition of NaOH and cake filtration [Ca2+], meq/l
[M$tl, meq/l
PH
5.21 3.73 3.06 2.15 1.50 1.12 1.07 0.73 0.54
1.78 1.67 1.58 1.63 1.58 1.60 1.64 1.57 1.54
7.4 8.1 8.0 8.2 8.2 8.3 8.4 8.5 8.7
P@+l>
[Ca2’ + M$+], meqll
and the thickness of the cake layer, is limited to a few seconds. Clearly, the contact between the supersaturated solution and the crystals in the cake is very effective. Thus, cake filtration is more effective than crystallization on a circulating slurry. 61.3. Cake filtration in aJilter tube In contrast to the experiments described above, water treatment in the single tube was carried out as a continuous process. The pH of the filtered product was monitored, and the addition of base to the hard water was regulated by feedback from the pH of the filtered solution.
PH
8.1 8.20 8.20 8.5 9.20
17.3 15.4 32 25.2 31.7
In the experiments described in Tables 1 and 2, partial softening was carried out, as is desirable for the treatment of drinking water. The upper limit of the pH was kept in the limits required for this purpose. Table 3 shows the parameters for continuous softening to a similar residual hardness. For the application of CAPS as pretreatment for membrane desalting, the calcium concentration has to be reduced to much lower values. The relationship between the product pH and residual calcium in the product water is shown in Table 4. We chose a final Ca2+ concentration of ~0.3 meq/l, and hence a pH of 9.0. Table 5 gives the parameters for softening at this pH. Tap water is normally slightly supersaturated. As pointed out above, in principle, the softened water must also be slightly oversaturated, as is any supernatant after precipitation. The supersaturation is, in fact, low, although the overall residence time of the solution in the plant is short, and the contact time between solution and cake at the approach to equilibration is only a few seconds. Table 5 shows that the saturation index of the hard water and the softened water are quite similar. However, the amount of precipitate that would be created upon equilibration is much smaller in the softened water. For desalting following the softening, the important parameter is the concentration of calcium. Before concentration by RO can be
70
0. Kedem, G. Zalmon /Desalination
113 (1997) 65-71
Table 3 Parameters of CAPS-treated tap water, pH 8.3
Table 5 Parameters of CAPS-treated tap water, pH 9.00
Parameter
Parameter
Experiment 1
D2+l,
Experiment 2
Tap water
Product
Tap water
Product
5.42
1.52
5.4
1.72
Experiment 2
Tap water
Product
Tap water
Product
CCa2+l,
5.88
0.26
5.68
0.3
7.68
1.86
7.62
1.82
4.05
1.63
4.06
1.68
-
0.29
986
817
996
832
7.39 24.9 -
9.02 31.2 0.05
7.25 25.3
9.00 30.2 0.11
0.36
0.28
0.26
0.32
meq/l [Ca”] +M82+], meq/l
6.88
3.00
6.98
3.00
meq/l [Ca2’] + MC], meq/l
WO,l>
3.936
2.282
3.85
2.312
WOjl> meqil
meq/l
[WI,
-
0.144
meq/l Conductivity, @/cm
926.6
806.4
924.6
812.2
PH SI
7.58 0.58
8.29 0.49
7.4 0.28
8.31 0.22
-
0.156
Table 4 Influence of pH on water hardness. Feed water alkalinity: 4.06 meq/l PH
[Ca2’ + M?‘],
7.39 8.3 8.49 8.82 9.00 9.27 9.5 9.76 10.01
7.65 2.98 2.4 2.12 1.82 1.61 1.54 1.44 1.34
meq/l
out,
the
5.8 1.34 0.28 0.17 0.11 0.03 ND
water
w:ml> meq/l Conductivity, j_Slcm PH Temp., “C Turbidity, NTU SI
0.32
Flow Vmin: Recycle Product
8.7 0.6
9.3 0.8
[Ca2+], meq/l
has, of course, to be acidified, and then the concentrations of sulfate and calcium become decisive for the maximal concentration factor. The concentrations of bicarbonate and carbonate after softening given in Table 5 require less acid than hard water for stoichiometric acidification. If ED is applied for the desalting, even less acid is consumed, since for this process only the brine is acidified to a pH that prevents precipitation of carbonate.
carried
Experiment 1
Table 6 SD1 of tap water and softened water Tap water
Treated water
SD1
PH
[Ca2+], meq/l
SD1
1.65 1.95 3.04 3.48 3.18 2.05 2.45
8.30 8.29 8.31 8.30 8.28 9.01 8.98
1.56 1.72 1.64 1.66 1.64 0.26 0.30
0.38 0.42 1.05 1.07 1.10 0.61 0.70
6.2. Silt density index The tap water used in our experiments was obtained from wells with very clear water. Table 6 shows the change in SD1 effected by the CAPS softening. Even though the values for tap
0. Kedem, G. Zalmon /Desalination
water fluctuated, and the correlation between the degree of softening and the decrease of SD1 was not clear cut, it was consistently found, as is evident from the table, that the softening decreased the SDI, and hence the clogging tendency of the product water. 7. Conclusions CAPS combines compact softening with effective filtration. Cake filtration facilitates fast equilibration between solution and precipitate. In addition, co-precipitation and/or adsorption remove other suspended and dissolved substances. This is shown by decreased turbidity and SDI. It is predicted that as a pretreatment for RO or ED, CAPS should lead to increased recovery, since both scaling and fouling will be suppressed. CAPS will be economically feasible if it can replace more extensive pretreatment technologies. This can be evaluated only in field trials In this study, we concentrated on the treatment of potable water used in the production of process water. Water from other sources will be studied in the future.
113 (I 997) 65-71
71
References Cl1 J.W. Clark, W. Viessman, Jr. and M.J. Hammer, Water Supply and Pollution Control. International Textbook, Scranton, Toronto, London, 197 1. PI A.A. Delyannis and E.A. Delyannis, Gmelin Handbuch der Anorganischen Chemie, 8th. ed., Sauerstoff, Water Desalting; Springer Verlag, Berlin, Heidelberg, New York, 1974. [31 0. Kedem and J. Ben-Dror, Water softening process, US Patent 5,152,904. [41 A. Masarwa, D. Meyerstein, N. Daltrophe and 0. Kedem, Desalination, 113 (1997) 73. Interactions in Electrolyte PI G.H. Nanchollas, Solutions, Elsevier, Amsterdam, 1966. [61 G.H. Nancollas and M.M. Reddy, J. Colloid Interface Sci., 36 (1971) 166. [71 M.M. Reddy and W.D. Gaillard, J. Colloid Interface Sci., 80 (1981) 171. PI L. Benjamin, R.E. Loewenthal and G.v.R. Marais, Water SA, 3 (1977) 155. A. Graveland, Aqua, 2 (1980) 3. [91 PO1 C. van der Veen and A. Graveland, J. AWWA, 80 (1988) 51. [ill R.W. Peters, P.-H. Chen and T.-K. Chang, in: Industrial Crystallization 84, S.J. Jancic and E.J. de Jong, eds., Elsevier, Amsterdam, 1984. P21 J.C. Schippers and J. Verdouw, Desalination, 32 (1980) 137, and references therein.
113 (1997) 65-71
Compact accelerated precipitation softening (CAPS) as a pretreatment for membrane desalination I. Softening by NaOH 0. Kedema*, G. Zalmonb a Weizmann Institute of Science, Rehovot, Israel bEcosoft Company, Israel Received 2 April 1997; accepted 3 1 July 1997
Abstract This study examines the application of compact accelerated precipitation softening (CAPS)-a previously developed process-as a pretreatment for membrane desalination. In this CAPS process, rapid precipitation of calcium carbonate is achieved by cake filtration. The importance of this pretreatment step lies in the fact that softening (by NaOH in this case) facilitates an increase of the recovery ratio in membrane desalination by decreasing the potential for scaling. In addition, CAPS serves as an effective filtration step, reducing both turbidity and the silt density index and hence the potential for membrane fouling. Keywords: Water softening; Pretreatment;
Cake filtration; SD1
1. Introduction In the desalination reverse osmosis (RO)
of
brackish
water
by
or electrodialysis (ED), recovery is usually limited by scaling and fouling of the membranes, since osmotic pressure of the concentrate becomes the limiting factor only at
*Corresponding author. Present address: The Institutes for Applied Research, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel. Fax: +972 (7) 647-2969; E-mail: ok.bgu.mail.bgu.ac.il. 001 l-9164/97/$09.50 PIZ 001 l-9164(97)001
relatively high salinities. Similarly, in marginal desalination and in demineralization of potable water for power stations and industry, scaling and/or fouling constitute the limiting factors. Scale formation is thus detrimental in a variety of processes [ 1,2]. The challenge to be faced in the desalting of low-salinity water is to increase the recovery by the suppression of scaling, without increasing the burden of chemicals to be added. In such a desalting process, the precipitation of calcium carbonate is prevented by acidification,
0 1997 Elsevier Science B.V. All rights reserved 15-X
66
0. Kedem, G. Zalmon /Desalination
leaving calcium sulfate as the main potentially scale-forming mineral. Thus, if the feed water could be partially softened before the desalting step, the feasible concentration factor and hence the recovery ratio could be increased. Such a pretreatment step can be viable only if a compact and environmentally acceptable softening process is available. Neither of the traditional softening processeslime softening and ion exchangeanswers these requirements: Lime softening is slow, requires extensive space, and generates a cumbersome “sludge”. Ion exchange, which has largely replaced lime softening because of its clean and convenient operation, requires a salt solution for regeneration, and it is currently felt that the disposal of these regenerating solutions may contribute significantly to salination of groundwater. Membrane softening, i.e., the use of nanotiltration membranes to retain calcium in preference to sodium, was not shown to lead to very high recovery. These considerations led us to develop a softening process [3], which we designated compact accelerated precipitation softening (CAPS). This process is a compact variation of softening by lime or sodium hydroxide, applied in cake filtration technology. During the development of the process, it became evident that co-precipitation and filtration also may eliminate various contaminants. The results presented here and in the companion paper [4] suggest that CAPS used as a pretreatment for RO may reduce both fouling and scaling of RO membranes.
2. Accelerated
precipitation
of CaCO,
As is the case of all crystallization processes, precipitation of CaC03 from a supersaturated basic solution is determined by the rate of nucleation or by the rate of crystal growth. Clean supersaturated solutions may remain metastable for prolonged periods. The addition of a solid and presence of various influences solutes
I13 (I 997) 65-7/
precipitation. The kinetics of all relevant steps of the crystallization of CaCO, have been studied in detail by the group of Nanchollas and a number of other workers [ 5-81, For any softening process used for practical applications, it is desirable to avoid metastability and to approach equilibrium as fast as possible so as to achieve, within practically reasonable residence times, concentrations close to their equilibrium values. After the addition of a base to a solution containing Ca(HCO&, the addition of CaCO, would appear to be the natural way to provide sites for crystal growth and thus to accelerate precipitation. It was indeed found that the precipitation rate was considerably enhanced by the presence of a suspended CaCO, precipitate. However, if the solid were separated out after a contact time of minutes, a substantial part of the Ca was removed, but the clear solution still remained supersaturated. It was shown by Graveland [9] that nucleation by other minerals is more effective and that, with this nucleation, the subsequent crystallization takes place along the axis of fast crystal growth. This phenomenon has been applied for the construction of large water softening installations [IO]. Kedem and Ben-Dror have found that very fast equilibration is obtained when the supersaturated solution is filtered through a cake of CaCO, [3]. Apparently, this fast process includes secondary nucleation as well as crystal growth [l 11, since the size distribution of the particles is not drastically changed upon repeated tiltrationwith-crystallization through the same cake. In the experiments described below, accelerated precipitation was investigated on a laboratory scale. A description of an industrial plant, which is currently under testing, will be published elsewhere.
3. Methods of precipitation Three systems were tested: (1) A loop for circulating and separating the suspended solid;
67
0. Kedem, G. Zalmon /Desalination 113 (1997) 65-71 (2) cake filtration under vacuum in a conventional Buchner funnel; and (3) circulating slurry and cake filtration in a filter tube.
3.1. Circulating slurry A microfiltration membrane was clamped into a flow-through cell (Fig. 1). A suspension of CaCOs (3 wt%) was circulated along one side of the membrane by means of a pump, and a vacuum was applied to the other, i.e., downstream, surface of the membrane. The solution removed from the downstream side of the membrane was continuously replaced by the same volume of supersaturated solution from a separating funnel. The supersaturated solution prepared immediately before each was experiment by adding NaOH (various amounts) to tap water.
I
I
Fig. 1. Supersaturated solution is prepared in the separating funnel (1). A slurry containing suspended CaCO, is circulated by a pump (2) along the membrane (3). Softened water is drawn off by vacuum into a vessel (4).
3.2. Buchnerfiltration A suspension containing 25 g of precipitated CaCO, was filtered through Whatman No. 50 filter paper in a 90-mm Buchner funnel to form a cake of a few millimeters. Supersaturated solutions were then filtered through this cake.
3.3. Cake$ltration
in filter tubes
For CAPS systems, filter tubes are prepared from filter fabric by thermal sealing. A suspension of small CaCO, crystals is circulated under pressure. Hard water and NaOH solution are continuously added to the reservoir in the cycle. In this system, part of the precipitation takes place in the suspension, and the remainder during passage through the cake. In the current experiments, a single tube of 700 cm2 filter area was mounted in a shell. A 3% suspension of CaCO, was circulated through a suitable reservoir and the addition of base was controlled to give the desired pH of the product.
4. Softening parameters For control of the softening process, both calcium and magnesium concentrations have to be considered, since routine determinations are carried out for the sum of calcium plus magnesium. The magnesium concentration is only slightly changed in the process by coprecipitation with calcium carbonate. Thus, for a given type of water, we have to know the necessary reduction of “total hardness”, i.e., calcium + magnesium, and the final calcium concentration required. This information is readily translated into the required quantity of base or, given the concentration of bicarbonate in the water, the pH of the product if equilibrium is reached. In practice, the product will always be at least slightly oversaturated. The customary measure for oversaturation is the saturation index (SI), i.e., the difference between the measured pH of the solution and the equilibrium pH, corresponding to the given ion concentrations [ 11.
68
0. Kedem, G. Zalmon /Desalination
SI=pH-
pH,
SD1 = lOO(t,- t,)lt,T
pHs =pK, - pK, +p[Ca2’] +p[HCO,-] + 5pfm where pH, is the equilibrium pH, K ,the second dissociation constant of carbonic acid, KS the solubility constant of calcium carbonate, and jn represents the activity coefficients. Under the conditions studied here: pfm=A
[JIl(l+-Jl)]
I13 (I 997) 65-71
- 0.31
(forZc0.5)
where I is the ionic strength. At 20°C
where q is the time required for filtration of a given volume at the end of the testing period, t, is the initial time, and T is the total testing period. Since it is essential to perform this test on line to avoid distortion of results by bacterial growth during transportation or storage of filtered water, we required a large enough continuous source of product water. Therefore, an SD1 unit of sufficient size was constructed and attached to our CAPS pilot plant so that water could be tested before and after softening.
pK, = 8.45; pK2 = 10.38;pK2 - pK, = 1.93 6. Results and discussion A = 0.506 6. I. Softening by NaOH In practice, the relationship between [Ca2’] and pH is determined empirically in the process itself, and SI is calculated. The value of SI characterizes the process performance. Process control is thus based on feedback from the pH of the product rather than on the calculated amount of alkali to be added to the feed water.
5. Silt density index The turbidity of the softened water was monitored and low values were obtained. Low turbidity reflects removal of suspended solids and possibly some colloids. However, practical experience has shown that turbidity is not the best measure for the fouling tendencies of a given water source. The widely used silt density index (SDI) [ 121 is a better means of evaluating foiling potential. To obtain the SDI, the feed water is filtered through a standard 0.45-mm microfiltration membrane, and the water flux is determined at zero time and after 15 min. The relative increase in time required for the passage of a given volume per minute indicates the clogging tendency of the tested source.
6.1.1. Circulating slurry The results of this series of measurements are given in Table 1. Comparison of results for Ca2+ plus M$+ before and after the process shows the decrease of total hardness. The time of contact, t,, between the supersaturated solution and the suspended precipitate (Table 1) is related to the total filtration time, f/, and the ratio between circulating volume V, and total volume V, by the following equation: t, = tf v/v,
In our system, V, = 140m1, and V, = 1.5 1;thus t, = 0.093 9. 6.1.2. Cake filtration in a Buchner funnel Table 2 shows results of softening with cake filtration. The most striking finding was the low residual concentration of calcium at a given pH, in comparison with the results given in Table 1, despite the much shorter residence time. Although there was no way for direct measurement of this time interval, the order of magnitude, which is given by the filtration rate
0. Kedem, G. Zalmon /Desalination
69
113 (1997) 65-71
Table 1 Circulating sluny of CaCO,. Composition of tap water and filtrate after the addition of NaOH [NaOH], meq/l
Tap water [Ca*+ +Mti+, meq/l
PH
5.90 5.90 6.58 6.7 6.58 6.58
7.36 7.36
1.0 1.2 1.4 2.0 2.2 2.83
7.2
Contact time, s
Filtrate [Ca*‘], meq/l
meq/l
4.47 3.90 4.47 3.41 3.26 2.05
3.11 2.61 3.03 1.81 0.61
1.36 1.37 1.44 1.45 1.44
Table 2 Cake filtration in a Buchner funnel. Composition of softened water (filtrate) after addition of NaOH and cake filtration [Ca2+], meq/l
[M$tl, meq/l
PH
5.21 3.73 3.06 2.15 1.50 1.12 1.07 0.73 0.54
1.78 1.67 1.58 1.63 1.58 1.60 1.64 1.57 1.54
7.4 8.1 8.0 8.2 8.2 8.3 8.4 8.5 8.7
P@+l>
[Ca2’ + M$+], meqll
and the thickness of the cake layer, is limited to a few seconds. Clearly, the contact between the supersaturated solution and the crystals in the cake is very effective. Thus, cake filtration is more effective than crystallization on a circulating slurry. 61.3. Cake filtration in aJilter tube In contrast to the experiments described above, water treatment in the single tube was carried out as a continuous process. The pH of the filtered product was monitored, and the addition of base to the hard water was regulated by feedback from the pH of the filtered solution.
PH
8.1 8.20 8.20 8.5 9.20
17.3 15.4 32 25.2 31.7
In the experiments described in Tables 1 and 2, partial softening was carried out, as is desirable for the treatment of drinking water. The upper limit of the pH was kept in the limits required for this purpose. Table 3 shows the parameters for continuous softening to a similar residual hardness. For the application of CAPS as pretreatment for membrane desalting, the calcium concentration has to be reduced to much lower values. The relationship between the product pH and residual calcium in the product water is shown in Table 4. We chose a final Ca2+ concentration of ~0.3 meq/l, and hence a pH of 9.0. Table 5 gives the parameters for softening at this pH. Tap water is normally slightly supersaturated. As pointed out above, in principle, the softened water must also be slightly oversaturated, as is any supernatant after precipitation. The supersaturation is, in fact, low, although the overall residence time of the solution in the plant is short, and the contact time between solution and cake at the approach to equilibration is only a few seconds. Table 5 shows that the saturation index of the hard water and the softened water are quite similar. However, the amount of precipitate that would be created upon equilibration is much smaller in the softened water. For desalting following the softening, the important parameter is the concentration of calcium. Before concentration by RO can be
70
0. Kedem, G. Zalmon /Desalination
113 (1997) 65-71
Table 3 Parameters of CAPS-treated tap water, pH 8.3
Table 5 Parameters of CAPS-treated tap water, pH 9.00
Parameter
Parameter
Experiment 1
D2+l,
Experiment 2
Tap water
Product
Tap water
Product
5.42
1.52
5.4
1.72
Experiment 2
Tap water
Product
Tap water
Product
CCa2+l,
5.88
0.26
5.68
0.3
7.68
1.86
7.62
1.82
4.05
1.63
4.06
1.68
-
0.29
986
817
996
832
7.39 24.9 -
9.02 31.2 0.05
7.25 25.3
9.00 30.2 0.11
0.36
0.28
0.26
0.32
meq/l [Ca”] +M82+], meq/l
6.88
3.00
6.98
3.00
meq/l [Ca2’] + MC], meq/l
WO,l>
3.936
2.282
3.85
2.312
WOjl> meqil
meq/l
[WI,
-
0.144
meq/l Conductivity, @/cm
926.6
806.4
924.6
812.2
PH SI
7.58 0.58
8.29 0.49
7.4 0.28
8.31 0.22
-
0.156
Table 4 Influence of pH on water hardness. Feed water alkalinity: 4.06 meq/l PH
[Ca2’ + M?‘],
7.39 8.3 8.49 8.82 9.00 9.27 9.5 9.76 10.01
7.65 2.98 2.4 2.12 1.82 1.61 1.54 1.44 1.34
meq/l
out,
the
5.8 1.34 0.28 0.17 0.11 0.03 ND
water
w:ml> meq/l Conductivity, j_Slcm PH Temp., “C Turbidity, NTU SI
0.32
Flow Vmin: Recycle Product
8.7 0.6
9.3 0.8
[Ca2+], meq/l
has, of course, to be acidified, and then the concentrations of sulfate and calcium become decisive for the maximal concentration factor. The concentrations of bicarbonate and carbonate after softening given in Table 5 require less acid than hard water for stoichiometric acidification. If ED is applied for the desalting, even less acid is consumed, since for this process only the brine is acidified to a pH that prevents precipitation of carbonate.
carried
Experiment 1
Table 6 SD1 of tap water and softened water Tap water
Treated water
SD1
PH
[Ca2+], meq/l
SD1
1.65 1.95 3.04 3.48 3.18 2.05 2.45
8.30 8.29 8.31 8.30 8.28 9.01 8.98
1.56 1.72 1.64 1.66 1.64 0.26 0.30
0.38 0.42 1.05 1.07 1.10 0.61 0.70
6.2. Silt density index The tap water used in our experiments was obtained from wells with very clear water. Table 6 shows the change in SD1 effected by the CAPS softening. Even though the values for tap
0. Kedem, G. Zalmon /Desalination
water fluctuated, and the correlation between the degree of softening and the decrease of SD1 was not clear cut, it was consistently found, as is evident from the table, that the softening decreased the SDI, and hence the clogging tendency of the product water. 7. Conclusions CAPS combines compact softening with effective filtration. Cake filtration facilitates fast equilibration between solution and precipitate. In addition, co-precipitation and/or adsorption remove other suspended and dissolved substances. This is shown by decreased turbidity and SDI. It is predicted that as a pretreatment for RO or ED, CAPS should lead to increased recovery, since both scaling and fouling will be suppressed. CAPS will be economically feasible if it can replace more extensive pretreatment technologies. This can be evaluated only in field trials In this study, we concentrated on the treatment of potable water used in the production of process water. Water from other sources will be studied in the future.
113 (I 997) 65-71
71
References Cl1 J.W. Clark, W. Viessman, Jr. and M.J. Hammer, Water Supply and Pollution Control. International Textbook, Scranton, Toronto, London, 197 1. PI A.A. Delyannis and E.A. Delyannis, Gmelin Handbuch der Anorganischen Chemie, 8th. ed., Sauerstoff, Water Desalting; Springer Verlag, Berlin, Heidelberg, New York, 1974. [31 0. Kedem and J. Ben-Dror, Water softening process, US Patent 5,152,904. [41 A. Masarwa, D. Meyerstein, N. Daltrophe and 0. Kedem, Desalination, 113 (1997) 73. Interactions in Electrolyte PI G.H. Nanchollas, Solutions, Elsevier, Amsterdam, 1966. [61 G.H. Nancollas and M.M. Reddy, J. Colloid Interface Sci., 36 (1971) 166. [71 M.M. Reddy and W.D. Gaillard, J. Colloid Interface Sci., 80 (1981) 171. PI L. Benjamin, R.E. Loewenthal and G.v.R. Marais, Water SA, 3 (1977) 155. A. Graveland, Aqua, 2 (1980) 3. [91 PO1 C. van der Veen and A. Graveland, J. AWWA, 80 (1988) 51. [ill R.W. Peters, P.-H. Chen and T.-K. Chang, in: Industrial Crystallization 84, S.J. Jancic and E.J. de Jong, eds., Elsevier, Amsterdam, 1984. P21 J.C. Schippers and J. Verdouw, Desalination, 32 (1980) 137, and references therein.