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Improved compact accelerated precipitation softening (CAPS)
DESALINATION ELSEVIER
Desalination 139 (2001) 155-159 www.elsevier.com/locate/desal
Improved compact acceleratedprecipitation softening (CAPS) Y. Oren*, V. Katz, N.C. Daltrophe Ben Gurion University, Institute for Applied Research, P.O. Box 653, Beer-Sheva 84105, Israel Tel. + 972 (7) 6477167; Fax + 972 (7) 6472960; e-mail: yoramo@,bgumail, bgu. ac. il
Received 6 March 2001; accepted 20 March 2001
Abstract
CAPS is a softening process in which adjusting water pH to the range 8-10.5 reduces calcium and carbonate alkalinity by accelerated CaCO3 nucleation and growth in 2 regions: a) in a pre-prepared slurry made of calcite small particles and, b) within a CaCO3 layer (cake) formed on the top of the filter through which water is pumped out. Whilst the largest degree of precipitation occurs within the slurry, the cake process is a polishing step in which calcium concentration is reduced further. Within the dense cake structure, the removal of the smaller calcium concentrations is possible within short contact times due to enhanced mass transfer rates made possible by large solution velocities within narrow pores and much larger surface to volume ratio. CAPS was first suggested for water soRening [1,2] and later tested for the possibility of simultaneous silica removal [3]. CAPS was also studied as a pretreatment for RO [4] with water taken fi'om fish ponds. The capability of reducing SDI, organics and hardness to levels satisfactory for prolonged RO treatment was demonstrated. In [4], water was mixed with CaCO3 particles, the slurry was circulated through a microfiltmtion module and the clear and softened permeate was then RO treated with a recovery rate above 80%. CAPS may be used as a stand-alone water treatment process or in conjunction with pressure and electrical driven membrane processes (UF, NF, RO, ED) as an effective pretrcalment routine for increasing recovery and decreasing fouling rates. In this work, a new concept for CAPS, which comprises in-tank mixing and filtration is presented. This makes the CAPS device more attractive due to compactness and the process more attractive technicaUy and in terms of cost. The advantages of in-tank filtration were appreciated in the past and it has been a subject for intensive investigation [5,6]. Laboratory CAPS units were run continuously (up to 250 h) and for shorter time periods in order to investigate tap water softening. The effect of the initial CaCO3 slurry concentration; residence time or pumping rate; pH; backwash frequency, duration and mode (dry or wet) and slurry mixing rate was investigated and analyzed in terms of Saturation Index (SO reduction, separated effects of the slurry and the cake on the softening action and filter cake load. Keywords: CAPS; Water softening; Membrane processes
*Corresponding author. Presented at the European Conference on Desalination and the Environment: Water Shortage, Lemesos, Cyprus, 28-31 May 2001.
0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All fights reserved P[I: S 0 0 1 1 - 9 1 6 4 ( 0 1 ) 0 0 3 0 5 - 8
156
Y Oren et al. /Desalination 139 (2001) 155-159
I. Introduction CAPS is a softening process in which water pH is adjusted to 8-10.5 to reduce calcium and carbonate alkalinity by accelerated CaCO3 nucleation and growth in two consecutive steps: a) in a pre-prepared slurry made of calcite small particles and, b) within a CaCO3 layer (cake) formed on the top of the filter through which water is pumped out. Whilst the largest degree of precipitation occurs within the slurry, the cake process is a polishing step in which calcium concentration is reduced further. Within the dense cake structure, the removal of the smaller calcium concentrations is possible within short contact times due to enhanced mass transfer rates made possible by large solution velocities within the narrow pores and much larger surface to volume ratio. CAPS was first suggested by Kedem and BenDror [I] and Kedem and Zalmon [2] for water softening and later tested by Massarwa et al. [3] for the possibility of simultaneous silica removal. Gilron et al. [4] studied the capability of reducing SDI, organics, and hardness to levels satisfactory for prolonged RO treatment by mixing and circulating a CaCO3 slurry through a microfiltration module. In this work the concept of submerged filtration is used for improving CAPS. This concept has been applied successfully to several water treating processes [5,6]. A CAPS device comprising mixing and filtering in the same tank is investigated. This makes CAPS more attractive for pretreatment of water for membrane processes due to compactness and simplicity and, therefore, more feasible in terms of cost.
.~~'-""~ pH Eleelrode
NaOH
Stirring
Feed
Fig. 1. The experimentalsetup. 251 vessel within which a filtering module made of microporous filtration elements having 1013 l~m nominal pore diameter is installed in a vertical position and submerged in the slurry. The total surface area is 197 or 394cm 2. The setup also contains a mixing unit capable of maintaining a stable and homogeneous suspension throughout an experiment. Water level in the tank is kept at 171 by a level switch controlled solenoid valve. Water is pumped past the filter module either at constant pump revolution speed or at a controlled flow rate. In the former, both the pressure drop across the filters and the flow rate varies while in the latter, only pressure drop changes within an experiment. A pH controller, metering pump and a glass electrode on the permeate line maintain the permeate pH on a preset value (in all eases discussed here pH was 9) by injecting NaOH solution. 2. 2. Experimental conditions
2. Experimental 2.1. C A P S ung
A schematic representation of the new laboratory CAPS setup is shown in Fig. 1. It consists of
In all experiments tap water with a typical composition listed in Table 1 is the feed. Prior to starting an experiment slurry of 0.15% CaCO3 in the form of calcite (as verified by XRD) is prepared within the tank. The average
157
Y. Oren et aL / Desalination 139 (2001) 155-159
Table 1 Feed composition (partial) Component Concentration
pH 7.5-8.5
Ca2+,ppm 65-80
particle diameter is 20 lxm. The powder is thoroughly mixed with the water contained within the vessel and the slurry homogeneity is verified by sampling it at different heights, Short (0.5-3h) and prolonged (up to 250h) experiments were conducted, aimed at investigating the effect permeate flow rate, pH, CaCO3 concentration and operation time on pressure drop, cake load, particle distribution and water quality. In the prolonged experiments, the filter module was air backwashed every 60min by applying 2-5 s pulses. The short experiments were followed by backwash at the end of each run.
Mg2÷,ppm 35-40
75 .70 o5 ¢~ .60 E
o
3. i. Short-term experiments
These experiments were made with constant pump revolution speed. It was found that pressure drop across the filter module increases and, accordingly, water flow rate decreases both with time and with increasing CaCO3 concentration within the vessel. However, at constant CaCO3 concentration, flows and pressures change rapidly within the first 20-40min and thereafter, variations slow down significantly. Referring to the results shown in Fig. 2, negligible changes can be observed in the cake load after 30min of operation. The implications are that the bulk of the cake layer is accumulated on the filters during the first 20-40min. The subsequent gradual changes in flow and pressure result from a further relief of supersaturation within the porous layer thereby increasing its hydraulic resistance. Fig. 2 also reveals that, as expected, cake load increases
CO32-, ppm 0
/
.55
I ~ .50
.-.
.45
~ .40 o ~ .3s ¢~ 30 25 .20
O.5%
.15 .10 ,05
3. R e s u l t s a n d d i s c u s s i o n
HCO3-, ppm 210-260
0.00
D " - " /
0
.5
1.0
0,125% 1.5
Run time, h Fig. 2. Cake load as a function of time for different CaCO3 concentrations.
with increasing the initial concentration of CaCO3. Assuming a close packed arrangement of hydrated CaCO3 in the cake and taking 1.77gem -3 for its specific gravity, cake average thickness of 0.144 mm are calculated for calcite concentrations of 0.125-5%. In Fig. 3, water quality is depicted in terms of saturation index (S o for CaCO3, corrected for feed pH, as a function of its initial concentration within the vessel. The effect of increasing calcite concentration is evident. However, the most striking result is that 70-80% of calcium reduction occurs in the slurry. Precipitation within the cake contributes another 10% to the concentration reduction.
158
Y. Oren et aL / Desalination 139 (2001) 155-159
il
"~
"
T| T
~ ~
Permeate In-tank
I
o:,
0.2
1i
~
-0.4 0
o.o~ 0:0
la ,2,+: • '='~
O,S
~1
~ 10
o i 20
•
~ 30
• •
i
i
40
50
60
70
CaO03oonaentration,g/I .5
1~0
1.5
2,0
2,5
3,0
CaCOs concentration, %
Fig. 4. Saturationindex as a function of CaCO3 concentration and flow velocityfor long experiments.
Fig. 3. Saturation index for short experiments.
Product quality is also reflected in the turbidity. It was found that at the beginning of the process just after backwash and for low calcite concentrations (e.g. 0.125%), turbidity is somewhat larger than that of the feed, decreasing quickly to much lower values as cake is built on the filters. At larger calcite concentrations, turbidity variations are insignificant and always less than that of the feed, indicating the good filtration capability of the cake.
3.2. Prolonged experiments
As mentioned above, in these experiments CAPS was run under controlled flow rate and up to 250h. All the experiments started with 1% of calcite concentration. However, since solids were not withdrawn during an experiment, CaCO3 was accumulated within the tank. This provides the possibility of tracking the influence of CaCO3 concentration changes under controlled conditions.
In Fig. 4 product quality is presented in term of corrected SI as a function of CaCO3 concentration within the tank, for different flow rates. It is evident that in most of the experiments, SI decreases with increasing CaCO3 concentration (and, in fact with experiment duration). SI values become roughly constant at concentration range of 1-2%, similar to the results obtained with the short experiments. It should be noted that in these experiments the improvement in product quality is not just a result of increasing CaCO3 concentration but also due to its changing morphology. SEM observations of the solid accumulated within the reaction tank show that CaCO3 precipitates in the form of small (number of pm) aragonite crystallites on the top of the calcite fine powder particles. This results in an increasing seeding surface area, thereby increasing supersaturation relief rate. It is also evident from Fig. 4 that SI values become larger (but yet, much smaller as compared to SI values for feed water) as flow rate increased as a result of decreasing residence time of the water within the reaction tank.
E Oren et al./Desalination 139 (2001) 155-159
4. Conclusions CAPS process investigated in this work is characterized by compactness of the device and high efficiency in removing scale-forming components. The factors leading to these characteristics are: a) fast precipitation kinetics occurring within two regions; the slurry and the cake, resulting in short residence times within the tank (the shortest applied in this work is 15min) and, b) submerged filtration modules that save the need to convey suspended particles through different parts of the device, which by itself is a complicated and costly procedure. With these, CAPS is more advantageous for pretreatment of water for membrane processes, as compared to conventional methods such as lime softening that requires very large residence times and, therefore, large facilities [7].
Acknowledgments The authors are indebted to the Magneton Program of the Israeli Ministry of Industry and
159
Commerce, Nitron Ltd. Israel, and the Charles Wolfson Charitable Foundation for supporting this work.
References [1] O. Kedem and J. Ben-Dror, Water SofteningProcess, US Patent 5152904,October 6, 1992. [2] O. Kedem and G. Zalmon, Desalination, 113 (1997) 65. [3] A. Massarwa, D. Meyrstein, N. Daltrophe and O. Kedem, Desalination, 113 (1997) 73. [4] J. Gikon, D. Chaikin and N. Daltrophe, Desalination, 127 (2000) 271. [5] H. Martyn, N. Graham, M. Day and P. Cooper, MBR2: Membrane Bioreactor Wastewater Treatment, 2nd International Meeting, 1999. [6] D. Mourato and G. Best, NSF International Conference, Washington DC, 1998. [7] R.A. Bergman, Desalination, 102 (1995) 11.
Desalination 139 (2001) 155-159 www.elsevier.com/locate/desal
Improved compact acceleratedprecipitation softening (CAPS) Y. Oren*, V. Katz, N.C. Daltrophe Ben Gurion University, Institute for Applied Research, P.O. Box 653, Beer-Sheva 84105, Israel Tel. + 972 (7) 6477167; Fax + 972 (7) 6472960; e-mail: yoramo@,bgumail, bgu. ac. il
Received 6 March 2001; accepted 20 March 2001
Abstract
CAPS is a softening process in which adjusting water pH to the range 8-10.5 reduces calcium and carbonate alkalinity by accelerated CaCO3 nucleation and growth in 2 regions: a) in a pre-prepared slurry made of calcite small particles and, b) within a CaCO3 layer (cake) formed on the top of the filter through which water is pumped out. Whilst the largest degree of precipitation occurs within the slurry, the cake process is a polishing step in which calcium concentration is reduced further. Within the dense cake structure, the removal of the smaller calcium concentrations is possible within short contact times due to enhanced mass transfer rates made possible by large solution velocities within narrow pores and much larger surface to volume ratio. CAPS was first suggested for water soRening [1,2] and later tested for the possibility of simultaneous silica removal [3]. CAPS was also studied as a pretreatment for RO [4] with water taken fi'om fish ponds. The capability of reducing SDI, organics and hardness to levels satisfactory for prolonged RO treatment was demonstrated. In [4], water was mixed with CaCO3 particles, the slurry was circulated through a microfiltmtion module and the clear and softened permeate was then RO treated with a recovery rate above 80%. CAPS may be used as a stand-alone water treatment process or in conjunction with pressure and electrical driven membrane processes (UF, NF, RO, ED) as an effective pretrcalment routine for increasing recovery and decreasing fouling rates. In this work, a new concept for CAPS, which comprises in-tank mixing and filtration is presented. This makes the CAPS device more attractive due to compactness and the process more attractive technicaUy and in terms of cost. The advantages of in-tank filtration were appreciated in the past and it has been a subject for intensive investigation [5,6]. Laboratory CAPS units were run continuously (up to 250 h) and for shorter time periods in order to investigate tap water softening. The effect of the initial CaCO3 slurry concentration; residence time or pumping rate; pH; backwash frequency, duration and mode (dry or wet) and slurry mixing rate was investigated and analyzed in terms of Saturation Index (SO reduction, separated effects of the slurry and the cake on the softening action and filter cake load. Keywords: CAPS; Water softening; Membrane processes
*Corresponding author. Presented at the European Conference on Desalination and the Environment: Water Shortage, Lemesos, Cyprus, 28-31 May 2001.
0011-9164/01/$- See front matter © 2001 Elsevier Science B.V. All fights reserved P[I: S 0 0 1 1 - 9 1 6 4 ( 0 1 ) 0 0 3 0 5 - 8
156
Y Oren et al. /Desalination 139 (2001) 155-159
I. Introduction CAPS is a softening process in which water pH is adjusted to 8-10.5 to reduce calcium and carbonate alkalinity by accelerated CaCO3 nucleation and growth in two consecutive steps: a) in a pre-prepared slurry made of calcite small particles and, b) within a CaCO3 layer (cake) formed on the top of the filter through which water is pumped out. Whilst the largest degree of precipitation occurs within the slurry, the cake process is a polishing step in which calcium concentration is reduced further. Within the dense cake structure, the removal of the smaller calcium concentrations is possible within short contact times due to enhanced mass transfer rates made possible by large solution velocities within the narrow pores and much larger surface to volume ratio. CAPS was first suggested by Kedem and BenDror [I] and Kedem and Zalmon [2] for water softening and later tested by Massarwa et al. [3] for the possibility of simultaneous silica removal. Gilron et al. [4] studied the capability of reducing SDI, organics, and hardness to levels satisfactory for prolonged RO treatment by mixing and circulating a CaCO3 slurry through a microfiltration module. In this work the concept of submerged filtration is used for improving CAPS. This concept has been applied successfully to several water treating processes [5,6]. A CAPS device comprising mixing and filtering in the same tank is investigated. This makes CAPS more attractive for pretreatment of water for membrane processes due to compactness and simplicity and, therefore, more feasible in terms of cost.
.~~'-""~ pH Eleelrode
NaOH
Stirring
Feed
Fig. 1. The experimentalsetup. 251 vessel within which a filtering module made of microporous filtration elements having 1013 l~m nominal pore diameter is installed in a vertical position and submerged in the slurry. The total surface area is 197 or 394cm 2. The setup also contains a mixing unit capable of maintaining a stable and homogeneous suspension throughout an experiment. Water level in the tank is kept at 171 by a level switch controlled solenoid valve. Water is pumped past the filter module either at constant pump revolution speed or at a controlled flow rate. In the former, both the pressure drop across the filters and the flow rate varies while in the latter, only pressure drop changes within an experiment. A pH controller, metering pump and a glass electrode on the permeate line maintain the permeate pH on a preset value (in all eases discussed here pH was 9) by injecting NaOH solution. 2. 2. Experimental conditions
2. Experimental 2.1. C A P S ung
A schematic representation of the new laboratory CAPS setup is shown in Fig. 1. It consists of
In all experiments tap water with a typical composition listed in Table 1 is the feed. Prior to starting an experiment slurry of 0.15% CaCO3 in the form of calcite (as verified by XRD) is prepared within the tank. The average
157
Y. Oren et aL / Desalination 139 (2001) 155-159
Table 1 Feed composition (partial) Component Concentration
pH 7.5-8.5
Ca2+,ppm 65-80
particle diameter is 20 lxm. The powder is thoroughly mixed with the water contained within the vessel and the slurry homogeneity is verified by sampling it at different heights, Short (0.5-3h) and prolonged (up to 250h) experiments were conducted, aimed at investigating the effect permeate flow rate, pH, CaCO3 concentration and operation time on pressure drop, cake load, particle distribution and water quality. In the prolonged experiments, the filter module was air backwashed every 60min by applying 2-5 s pulses. The short experiments were followed by backwash at the end of each run.
Mg2÷,ppm 35-40
75 .70 o5 ¢~ .60 E
o
3. i. Short-term experiments
These experiments were made with constant pump revolution speed. It was found that pressure drop across the filter module increases and, accordingly, water flow rate decreases both with time and with increasing CaCO3 concentration within the vessel. However, at constant CaCO3 concentration, flows and pressures change rapidly within the first 20-40min and thereafter, variations slow down significantly. Referring to the results shown in Fig. 2, negligible changes can be observed in the cake load after 30min of operation. The implications are that the bulk of the cake layer is accumulated on the filters during the first 20-40min. The subsequent gradual changes in flow and pressure result from a further relief of supersaturation within the porous layer thereby increasing its hydraulic resistance. Fig. 2 also reveals that, as expected, cake load increases
CO32-, ppm 0
/
.55
I ~ .50
.-.
.45
~ .40 o ~ .3s ¢~ 30 25 .20
O.5%
.15 .10 ,05
3. R e s u l t s a n d d i s c u s s i o n
HCO3-, ppm 210-260
0.00
D " - " /
0
.5
1.0
0,125% 1.5
Run time, h Fig. 2. Cake load as a function of time for different CaCO3 concentrations.
with increasing the initial concentration of CaCO3. Assuming a close packed arrangement of hydrated CaCO3 in the cake and taking 1.77gem -3 for its specific gravity, cake average thickness of 0.144 mm are calculated for calcite concentrations of 0.125-5%. In Fig. 3, water quality is depicted in terms of saturation index (S o for CaCO3, corrected for feed pH, as a function of its initial concentration within the vessel. The effect of increasing calcite concentration is evident. However, the most striking result is that 70-80% of calcium reduction occurs in the slurry. Precipitation within the cake contributes another 10% to the concentration reduction.
158
Y. Oren et aL / Desalination 139 (2001) 155-159
il
"~
"
T| T
~ ~
Permeate In-tank
I
o:,
0.2
1i
~
-0.4 0
o.o~ 0:0
la ,2,+: • '='~
O,S
~1
~ 10
o i 20
•
~ 30
• •
i
i
40
50
60
70
CaO03oonaentration,g/I .5
1~0
1.5
2,0
2,5
3,0
CaCOs concentration, %
Fig. 4. Saturationindex as a function of CaCO3 concentration and flow velocityfor long experiments.
Fig. 3. Saturation index for short experiments.
Product quality is also reflected in the turbidity. It was found that at the beginning of the process just after backwash and for low calcite concentrations (e.g. 0.125%), turbidity is somewhat larger than that of the feed, decreasing quickly to much lower values as cake is built on the filters. At larger calcite concentrations, turbidity variations are insignificant and always less than that of the feed, indicating the good filtration capability of the cake.
3.2. Prolonged experiments
As mentioned above, in these experiments CAPS was run under controlled flow rate and up to 250h. All the experiments started with 1% of calcite concentration. However, since solids were not withdrawn during an experiment, CaCO3 was accumulated within the tank. This provides the possibility of tracking the influence of CaCO3 concentration changes under controlled conditions.
In Fig. 4 product quality is presented in term of corrected SI as a function of CaCO3 concentration within the tank, for different flow rates. It is evident that in most of the experiments, SI decreases with increasing CaCO3 concentration (and, in fact with experiment duration). SI values become roughly constant at concentration range of 1-2%, similar to the results obtained with the short experiments. It should be noted that in these experiments the improvement in product quality is not just a result of increasing CaCO3 concentration but also due to its changing morphology. SEM observations of the solid accumulated within the reaction tank show that CaCO3 precipitates in the form of small (number of pm) aragonite crystallites on the top of the calcite fine powder particles. This results in an increasing seeding surface area, thereby increasing supersaturation relief rate. It is also evident from Fig. 4 that SI values become larger (but yet, much smaller as compared to SI values for feed water) as flow rate increased as a result of decreasing residence time of the water within the reaction tank.
E Oren et al./Desalination 139 (2001) 155-159
4. Conclusions CAPS process investigated in this work is characterized by compactness of the device and high efficiency in removing scale-forming components. The factors leading to these characteristics are: a) fast precipitation kinetics occurring within two regions; the slurry and the cake, resulting in short residence times within the tank (the shortest applied in this work is 15min) and, b) submerged filtration modules that save the need to convey suspended particles through different parts of the device, which by itself is a complicated and costly procedure. With these, CAPS is more advantageous for pretreatment of water for membrane processes, as compared to conventional methods such as lime softening that requires very large residence times and, therefore, large facilities [7].
Acknowledgments The authors are indebted to the Magneton Program of the Israeli Ministry of Industry and
159
Commerce, Nitron Ltd. Israel, and the Charles Wolfson Charitable Foundation for supporting this work.
References [1] O. Kedem and J. Ben-Dror, Water SofteningProcess, US Patent 5152904,October 6, 1992. [2] O. Kedem and G. Zalmon, Desalination, 113 (1997) 65. [3] A. Massarwa, D. Meyrstein, N. Daltrophe and O. Kedem, Desalination, 113 (1997) 73. [4] J. Gikon, D. Chaikin and N. Daltrophe, Desalination, 127 (2000) 271. [5] H. Martyn, N. Graham, M. Day and P. Cooper, MBR2: Membrane Bioreactor Wastewater Treatment, 2nd International Meeting, 1999. [6] D. Mourato and G. Best, NSF International Conference, Washington DC, 1998. [7] R.A. Bergman, Desalination, 102 (1995) 11.