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Optimization of nitrate removal operation from ground water by electrodialysis
Separation and Purification Technology 29 (2002) 235– 244 www.elsevier.com/locate/seppur
Optimization of nitrate removal operation from ground water by electrodialysis Azzeddine El Midaoui a,*, Fatima Elhannouni a, Mohamed Taky a, Laaroussi Chay a, Mohamed Amine Menkouchi Sahli a, Lhoucine Echihabi b, Mahmoud Hafsi b a
Laboratory of Separation Processes, Department of Chemistry, Faculty of Sciences, B.P. 1246 Kenitra, Morocco b Laboratory of Water Quality, Office National de l’Eau Potable, Rabat, Morocco Received 29 November 2001; received in revised form 3 May 2002; accepted 9 May 2002
Abstract An electrodialysis operation have been performed to remove nitrate from a ground water containing 800 ppm of total dissolved solids (TDS) and 90 ppm of nitrate. The influence of electrodialysis parameters of nitrate removal was studied in order to determine the optimum conditions to prevent the possible precipitation of bivalent salt and to minimize the use of acid and antiscalant in the concentrate compartment. The optimal conditions to remove nitrate with a blockade of the bivalent anions especially sulfate in the dilute compartment have been proposed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrodialysis; Nitrate removal; Optimisation; Bivalent anions
1. Introduction Nitrate concentration in surface and ground water have increased in almost all areas of the world to such an extent that the admitted world health organization norm of 50 ppm has largely been exceeded in many regions. This contamination have caused shutdown of wells and rendered many aquifers unusable as drinking water sources. In Morocco, the concentration of nitrate in ground water in some regions exceeds 250 ppm * Corresponding author. Tel./fax: +212-7-373-033 E-mail address: [email protected] (A. El Midaoui).
[1,2]. This rise relates to an increase in industrial and agricultural nitrates wastes and especially to the heavy utilization of artificial fertilizers. Contamination of drinking water by nitrate can be endangering health especially for infants. NO− 3 can be reduced to NO− 2 , which combines with haemoglobin in the blood to form methaemoglobin, and leads to a condition commonly known as ‘blue baby syndrome’ [3,4]. A survey of literature yielded an abundance of information on the treatment technique to remove nitrate from water. These process include ion exchange, biological denitration, chemical denitration, reverse osmosis, electrodialysis and catalytic denitration [4–8].
1383-5866/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 0 9 2 - 8
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Table 1 Stack design Description of equipment Ion-exchange membranes (Neosepta, Tokuyama Soda)
Separator frame
Electrode
Table 2 The mean characteristics of the untreated water
Cation exchange membrane
CMX
Anion exchange membrane Membrane active area (cm2) Number of cell pairs Gasket Separator and distributor Anode Cathode
ACS
Parameters of functioning Dilute compartment (l/h) Concentrated compartment (l/h) Electrode compartment (l/h) Anode Cathode Current max (A) Voltage max (V/cell)
200 10 EPDM PE+PP
Ions
C (ppm)
NO− 3 Cl− HCO− 3 SO2− 4 Na+ K+ Mg2+ Ca2+ TDS Hardness PH
82.2 232.6 221.2 57.9 105.7 7.0 42.6 72.6 821.8 7.1 meq/l 8.1
DSE DSE 180–200 180–200 150 150 9 1.5
The ONEP Co. (National Office of Drinking Water) in Morocco, and Eurodia Co. (an affiliate of the Tokuyama Soda, Japan) has an interest in
the application of the electrodialysis process to remove nitrate from Moroccan ground water. The objective of this work is to study the influence of the electrodialysis parameters (flow rate, temperature and applied voltage) on nitrate removal from ground water using an anion exchange membrane with a good selectivity toward nitrate. This in order to determine the optimum conditions to prevent the possible precipitation of bivalent salt in the concentrate compartment.
Fig. 1. Principle of the denitration by electrodialysis.
A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
2. Experimental The electrodialysis of the nitrated water was carried out with a nitrate selective membrane to avoid a complete demineralization which is not really suitable for drinking water. The electrodialysis stack was equipped with a NEOSEPTA ACS and CMX membranes manufactured by Tokuyama Co. The ACS membrane is a mono-anion permselective membrane. The electrodialysis apparatus was supplied by the Eurodia Co. It is a TS-2-10 pilot which was a batch type dialysis unit. The stack design characteristics are given in Table 1. The two electrodes compartments are separated from the others to prevent a modification in the composition of the concentration sides, which could be caused by electrode reactions. About 3 l solution of 0.1 M Na2SO4 was used as an electrode rinse. For the concentrate compartment, 3 l of 0.01 M NaCl solution was used. All solutions circulate in a closed loop. To prevent scaling and fouling of the membranes, the polarity and the flow were reversed periodically. The concentration of each ion was determined analytically. The concentration of nitrates and chlorides were determined by specific Ion Method using Orion nitrate, chloride referTable 3 Characteristics of the used membranes Grade
ACS
Type
Strongly basic Strongly acidic Anion permeable Cation permeable Mono-anion – Pemselective 2.3 3
Properties Electric resistancea (V cm2) Transport numberb Exchange capacityc (meq/g) Thickness (mm)d a
CMX
Cl− 0.98B SO2− 0.005\ 4 1.8
Na+ 0.7B Ca2+ 0.28B 1.7
0.18
0.18
Equilibrated with 0.5 N NaCl solution at 25 °C. Measured by electrophoresis with seawater. c Na-form dry membrane(or Cl-form). d Current density: 2 (A/dm2) at 25 °C. b
237
ence electrodes, and specific Ion Meter. Cations were estimated by Atomic Absorption Spectroscopy (UNICAM 926 AA spectrometer) and 2− the other parameters (pH, TDS, HCO− 3 , SO4 ) were determined following standard methods. The principle of the denitration is given in Fig. 1.
3. Results and discussion In electrodialysis, a precipitation of carbonate or sulfate salt in the concentrate compartment limits the performances of the operation by decreasing the recovery rate of water and the overall rejection of ions and by causing scaling and fouling of the membranes. To avoid these risks, usually, acid and antiscalant were used. In this work, the influence of flow rate, temperature and applied voltage on the nitrate removal operation was studied in order to determine the optimum conditions to prevent the possible precipitation and to minimize the use of acid and antiscalant in the concentrate compartment. The approach consists in determining the conditions allowing a good denitration and a blockade of the bivalent anions especially sulfate in the dilute compartment. The mean characteristics of the untreated water and of the used membranes are collected in Tables 2 and 3. Fig. 2 shows the variation with time of the − − 2− removal rate of NO− and of 3 , Cl , HCO3 , SO4 TDS in the treated water and for several temperatures: 15, 25 and 40 °C. Table 4 gives the analytical results of the treated water after 6 and 10 min of the electrodialysis time. The increase in the removal rate with temperature can be attributed to the increase in the ion mobilities and to the dilation of the membrane network which facilates the membrane swelling and the diffusion of ions into the membrane. For all temperatures the different anions are removed − − in the following order NO− 3 \ Cl \ HCO3 \ 2− SO4 . The kinetic of the sulfate removal is very slow at the lower temperature. At this temperature (15 °C) and after 10 min only 20% of SO24 − have been removed. These results show that the transport of the bivalent anions was more controlled at the lower temperature.
89.69 219.58 202.06 58.20 102.33 6.73 45.37 73.33 797.29 7.39 8.25
NO3− Cl− HCO3− SO42− Na+ K+ Mg2+ Ca2+ TDS Hardness (meq/l) pH
Flow rate: 180 l/h; voltage, 15 V.
Untreated water (ppm)
Analyzed parameters
38.29 65.90 152.70 50.15 55.00 2.60 18.23 24.00 406.87 2.70 7.73 57.31 69.99 24.43 13.83 46.25 61.37 59.82 67.27 48.97 63.46 –
24.24 43.25 99.49 46.15 43.60 1.90 13.82 16.00 287.84 1.93 7.34
72.97 80.30 50.76 20.70 57.39 80.83 69.54 78.18 63.90 73.88 –
26.43 57.46 124.94 46.15 51.00 2.30 12.54 16.00 338.65 1.83 7.58
C (ppm) 70.53 73.83 38.17 20.70 50.16 65.82 72.36 78.18 57.52 75.24 –
% Removal
C (ppm)
C (ppm)
% Removal
6 min
10 min
6 min % Removal
Treated water (25 °C)
Treated water (15 °C)
Table 4 Analytical results of the treated water after 5 and 10 min of electrodialysis for different temperatures
21.00 35.91 72.88 38.34 40.00 1.50 9.20 10.00 228.83 1.25 7.13
C (ppm)
10 min
76.59 83.65 63.93 34.12 60.91 77.71 79.72 86.36 71.29 83.08 –
% Removal 19.14 39.50 83.29 42.22 45.00 1.70 7.38 13.00 251.23 1.26 7.59
C (ppm)
6 min
78.66 82.01 58.78 27.46 56.02 74.74 83.73 82.27 68.48 82.95 –
% Removal
Treated water (40 °C)
15.57 27.56 54.37 34.50 36.60 1.20 6.05 8.00 183.85 0.90 7.05
C (ppm)
10 min
82.64 87.45 73.09 40.72 64.23 82.17 86.66 89.09 76.94 87.82 –
% Removal
238 A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
239
Fig. 2. Variation with time of the removal rate of anions and of TDS at different temperatures. (Flow rate: 180 l/h, Voltage: 15 V).
The evolution with time of removal rate of anions and of TDS in the treated water and for two flow rate (100 and 180 l/h) are shown in Fig.
3. Table 5 gives the performances of the electrodialysis after 5 and 10 min. The increase in the demineralization with flow rate may be attributed
78.05 242.02 230.34 57.70 107.25 7.15 43.09 73.23 838.83 7.34 8.13
NO− 3 Cl− HCO− 3 SO2− 4 Na+ K+ Mg2+ Ca2+ TDS Hardness (meq/l) pH
Voltage: 10 V, Temperature: 20 °C.
Untreated water (ppm)
Analyzed parameters
28.17 132.94 206.30 55.69 96.60 5.74 30.00 44.00 601.44 4.76 8.10
63.91 45.07 10.44 3.48 9.93 19.72 30.38 39.91 28.30 35.15 –
16.67 79.15 152.50 52.20 77.40 4.27 22.00 32.00 436.19 3.41 8.20
78.64 67.30 33.79 9.53 27.83 40.28 48.94 56.30 48.00 53.54 –
19.98 121.01 201.30 49.50 90.70 4.58 19.44 40.50 547.51 3.62 7.84
C (ppm) 74.40 50.00 12.61 14.21 15.43 35.94 54.89 44.69 34.73 40.68 –
% Removal
C (ppm)
C (ppm)
% Removal
5 min
10 min
5 min % Removal
Treated water (180 l/h)
Treated water (100 l/h)
Table 5 Analytical results of the treated water after 5 and 10 min of electrodialysis for the two flow rates
10.53 65.67 146.40 43.40 71.50 3.00 14.58 22.50 377.58 2.32 7.80
C (ppm)
10 min
86.51 72.86 36.44 24.78 33.33 58.04 66.16 69.27 54.99 68.39 –
% Removal
240 A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
241
Fig. 3. Influence of the flow rate on the removal rate of anions and of TDS during electrodialysis (Voltage: 10 V, Temperature: 20 °C).
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A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
Fig. 4. Variation of removal rate of ions and of TDS vs. time at different voltages. (Flow rate: 180 l/h, Temperature: 20 °C).
78.86 236.38 231.26 57.70 107.47 7.17 39.23 71.16 829.23 6.78 7.95
NO− 3 −
Flow rate: 180 l/h, Temperature: 20 °C.
Cl HCO− 3 SO2− 4 + Na K+ Mg2+ Ca2+ TDS Hardness (meq/l) PH
Untreated water (ppm)
Analyzed parameters
54.00 193.30 222.00 56.00 106.00 6.30 32.00 58.00 727.30 5.52 7.84
31.52 18.22 4.00 2.95 1.36 12.13 18.43 18.49 12.26 18.58 –
30.15 153.1 219.60 54.00 105.10 5.37 27.50 43.00 637.82 4.41 7.80
61.77 35.23 5.04 6.41 2.21 25.11 29.90 39.57 23.08 34.95 –
19.98 121.01 201.30 49.50 90.70 4.58 30.00 44.50 561.57 4.68 7.84
C (ppm) 74.66 48.81 12.95 14.21 15.60 36.12 23.53 37.46 32.28 30.97 –
% Removal
C(ppm)
C (ppm)
% Removal
5 min
10 min
5 min % Removal
Treated water (10 V)
Treated water (5 V)
Table 6 Analytical results of the treated water after 5 and 10 min of electrodialysis for various voltages
10.53 65.67 146.40 43.40 71.50 3.00 22.00 22.50 385.00 2.93 7.80
C (ppm)
10 min
86.65 72.22 36.69 24.78 33.47 58.16 43.92 68.38 53.57 56.78 –
% Removal 14.57 68.83 144.37 42.50 62.60 4.13 22.50. 23.00 382.50 3.00 7.68
C (ppm)
5 min
81.52 70.88 37.57 26.34 41.75 42.40 42.65 67.68 53.87 55.75 –
% Removal
Treated water (15 V)
5.90 26.88 73.20 30.10 30.80 1.29 11.20 13.34 192.71 1.59 7.56
C (ppm)
10 min
92.52 88.63 68.35 47.83 71.34 82.01 71.45 81.25 76.76 76.55 –
% Removal
A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244 243
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A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
to the decrease in the thickness of the boundary layers adjacent to the membrane surfaces (static zones of solutions) with increasing solution velocity. The applied voltage dependencies of the denitration parameters were shown in Fig. 4. The operation was carried out for three applied voltages: 5, 10 and 15 V. − increase The removal rates of NO− 3 and Cl rapidly with time at various voltages. Those of bicarbonate and sulfate increase rapidly at the higher voltage (15 V) and slowly at the lower one (5 V). However, at 5 V, the amounted of 2− ions in the diluate remained HCO− 3 and SO4 practically stopped with increasing electrodialysis duration up to 10 min and then decreased and SO24 − at slowly. The transport of HCO− 3 the lower voltage becomes significant after and Cl−. These nearly 90% removal of NO− 3 results can be attributed among others to the strong preference of the anion exchange membrane to transport monovalent ions and to the difference in the anionic contents in the untreated water. Table 6 gives the analytical results of the treated water after 5 and 10 min of electrodialysis operation at various applied voltages. Percentage of NO− 3 removal is always greater than the percentage removal of the other anions and of TDS. For the higher applied voltages (15 V), the total hardness and the TDS reach a limit level, the removal rate of SO24 − reaches 47%. The best results to control the denitration operation were obtained at the lower voltage and especially for 5 V: the nitrate content reaches the recommended guide level, the sulfate is practically stopped, the TDS and the total hardness were satisfactory. As it was indicated by Oldani et al. [8], practically the low applied voltages can be compensated by a large membrane areas. The higher costs of larger membrane areas well be partially balanced by lower electricity consumption better quality of the concentrate and diluate and by the higher water recovery.
4. Conclusion The electrodialysis operation not only removes nitrate but also the other anions. After a long time one thus obtains a deminiralized water which is not really suitable for drinking purposes. It is, therefore, necessary to stop the operation after a predetermined time. On the other hand to minimize the eventual precipitation of salt especially the divalent salt of sulfate in the concentrate compartment it is preferable to operate in condition which allow to blockade the sulfate (or the divalent cation) in the treated water. It is, therefore, necessary to find a compromise between time, temperature, flow rate and the applied voltage.
References [1] A. Elacheb, L. Bahi ‘les nitrates dans les eaux souterraines de Doukkala (Maroc)’ Rencontre Internationale sur les Fluorures Nitrates et les Pesticides dans les eaux du bassin Mediteraneen. Proble`mes et traitements 24 – 25 Avril 97, Kenitra, Morocco. [2] L. Berrada, T Chefadi ‘Etat de la qualite´ des ressources en eaux au Maroc et pollution par les nitrates’. Rencontre Internationale sur les fluorures, Nitrates et Pesticides dans les eaux du Bassin Mediteraneen: proble`mes et traitements 24 – 25 Avril 97 Kenitra, Morocco. [3] ‘Nitrates, Nitrites and N, Nitroso compounds’ Environmental Health Criterion —5 World health organization, Geneva, 1987. [4] A. Kapoor, T. Virapaghavan, Nitrate removal from drinking water, J. Environ. Eng. 123 (1997) 371 – 379. [5] F. Lutin, G. Guerif, ‘Electrodialysis applied to denitration of drinking water’ EURODIA Co. 1994, private communication. [6] R. Rautenbach, W. Kopp, R. Hellekes, R. Peter, G. Vanopbergen, Separation of nitrate from well water by membrane processes (Reverse Osmosis/Electrodialysis Reversal), Aqua 5 (1986) 279 – 282. [7] J.P. Van Der Hoek, A.B. Griffioen, A. Klapwijk, Biological regeneration of nitrate loaded anion exchange resins by denitrifing bacteria, J. Chem. Tech. Biotechnol. 43 (1988) 213 – 222. [8] M. Oldani, E. Killer, A. Miquel, G. Schock, On the nitrate and monovalent cation selectiorty of ion exchange membranes used in drinking water purification, J. Membr. Sci. 75 (1992) 265 – 267.
Optimization of nitrate removal operation from ground water by electrodialysis Azzeddine El Midaoui a,*, Fatima Elhannouni a, Mohamed Taky a, Laaroussi Chay a, Mohamed Amine Menkouchi Sahli a, Lhoucine Echihabi b, Mahmoud Hafsi b a
Laboratory of Separation Processes, Department of Chemistry, Faculty of Sciences, B.P. 1246 Kenitra, Morocco b Laboratory of Water Quality, Office National de l’Eau Potable, Rabat, Morocco Received 29 November 2001; received in revised form 3 May 2002; accepted 9 May 2002
Abstract An electrodialysis operation have been performed to remove nitrate from a ground water containing 800 ppm of total dissolved solids (TDS) and 90 ppm of nitrate. The influence of electrodialysis parameters of nitrate removal was studied in order to determine the optimum conditions to prevent the possible precipitation of bivalent salt and to minimize the use of acid and antiscalant in the concentrate compartment. The optimal conditions to remove nitrate with a blockade of the bivalent anions especially sulfate in the dilute compartment have been proposed. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrodialysis; Nitrate removal; Optimisation; Bivalent anions
1. Introduction Nitrate concentration in surface and ground water have increased in almost all areas of the world to such an extent that the admitted world health organization norm of 50 ppm has largely been exceeded in many regions. This contamination have caused shutdown of wells and rendered many aquifers unusable as drinking water sources. In Morocco, the concentration of nitrate in ground water in some regions exceeds 250 ppm * Corresponding author. Tel./fax: +212-7-373-033 E-mail address: [email protected] (A. El Midaoui).
[1,2]. This rise relates to an increase in industrial and agricultural nitrates wastes and especially to the heavy utilization of artificial fertilizers. Contamination of drinking water by nitrate can be endangering health especially for infants. NO− 3 can be reduced to NO− 2 , which combines with haemoglobin in the blood to form methaemoglobin, and leads to a condition commonly known as ‘blue baby syndrome’ [3,4]. A survey of literature yielded an abundance of information on the treatment technique to remove nitrate from water. These process include ion exchange, biological denitration, chemical denitration, reverse osmosis, electrodialysis and catalytic denitration [4–8].
1383-5866/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 0 9 2 - 8
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A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
Table 1 Stack design Description of equipment Ion-exchange membranes (Neosepta, Tokuyama Soda)
Separator frame
Electrode
Table 2 The mean characteristics of the untreated water
Cation exchange membrane
CMX
Anion exchange membrane Membrane active area (cm2) Number of cell pairs Gasket Separator and distributor Anode Cathode
ACS
Parameters of functioning Dilute compartment (l/h) Concentrated compartment (l/h) Electrode compartment (l/h) Anode Cathode Current max (A) Voltage max (V/cell)
200 10 EPDM PE+PP
Ions
C (ppm)
NO− 3 Cl− HCO− 3 SO2− 4 Na+ K+ Mg2+ Ca2+ TDS Hardness PH
82.2 232.6 221.2 57.9 105.7 7.0 42.6 72.6 821.8 7.1 meq/l 8.1
DSE DSE 180–200 180–200 150 150 9 1.5
The ONEP Co. (National Office of Drinking Water) in Morocco, and Eurodia Co. (an affiliate of the Tokuyama Soda, Japan) has an interest in
the application of the electrodialysis process to remove nitrate from Moroccan ground water. The objective of this work is to study the influence of the electrodialysis parameters (flow rate, temperature and applied voltage) on nitrate removal from ground water using an anion exchange membrane with a good selectivity toward nitrate. This in order to determine the optimum conditions to prevent the possible precipitation of bivalent salt in the concentrate compartment.
Fig. 1. Principle of the denitration by electrodialysis.
A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
2. Experimental The electrodialysis of the nitrated water was carried out with a nitrate selective membrane to avoid a complete demineralization which is not really suitable for drinking water. The electrodialysis stack was equipped with a NEOSEPTA ACS and CMX membranes manufactured by Tokuyama Co. The ACS membrane is a mono-anion permselective membrane. The electrodialysis apparatus was supplied by the Eurodia Co. It is a TS-2-10 pilot which was a batch type dialysis unit. The stack design characteristics are given in Table 1. The two electrodes compartments are separated from the others to prevent a modification in the composition of the concentration sides, which could be caused by electrode reactions. About 3 l solution of 0.1 M Na2SO4 was used as an electrode rinse. For the concentrate compartment, 3 l of 0.01 M NaCl solution was used. All solutions circulate in a closed loop. To prevent scaling and fouling of the membranes, the polarity and the flow were reversed periodically. The concentration of each ion was determined analytically. The concentration of nitrates and chlorides were determined by specific Ion Method using Orion nitrate, chloride referTable 3 Characteristics of the used membranes Grade
ACS
Type
Strongly basic Strongly acidic Anion permeable Cation permeable Mono-anion – Pemselective 2.3 3
Properties Electric resistancea (V cm2) Transport numberb Exchange capacityc (meq/g) Thickness (mm)d a
CMX
Cl− 0.98B SO2− 0.005\ 4 1.8
Na+ 0.7B Ca2+ 0.28B 1.7
0.18
0.18
Equilibrated with 0.5 N NaCl solution at 25 °C. Measured by electrophoresis with seawater. c Na-form dry membrane(or Cl-form). d Current density: 2 (A/dm2) at 25 °C. b
237
ence electrodes, and specific Ion Meter. Cations were estimated by Atomic Absorption Spectroscopy (UNICAM 926 AA spectrometer) and 2− the other parameters (pH, TDS, HCO− 3 , SO4 ) were determined following standard methods. The principle of the denitration is given in Fig. 1.
3. Results and discussion In electrodialysis, a precipitation of carbonate or sulfate salt in the concentrate compartment limits the performances of the operation by decreasing the recovery rate of water and the overall rejection of ions and by causing scaling and fouling of the membranes. To avoid these risks, usually, acid and antiscalant were used. In this work, the influence of flow rate, temperature and applied voltage on the nitrate removal operation was studied in order to determine the optimum conditions to prevent the possible precipitation and to minimize the use of acid and antiscalant in the concentrate compartment. The approach consists in determining the conditions allowing a good denitration and a blockade of the bivalent anions especially sulfate in the dilute compartment. The mean characteristics of the untreated water and of the used membranes are collected in Tables 2 and 3. Fig. 2 shows the variation with time of the − − 2− removal rate of NO− and of 3 , Cl , HCO3 , SO4 TDS in the treated water and for several temperatures: 15, 25 and 40 °C. Table 4 gives the analytical results of the treated water after 6 and 10 min of the electrodialysis time. The increase in the removal rate with temperature can be attributed to the increase in the ion mobilities and to the dilation of the membrane network which facilates the membrane swelling and the diffusion of ions into the membrane. For all temperatures the different anions are removed − − in the following order NO− 3 \ Cl \ HCO3 \ 2− SO4 . The kinetic of the sulfate removal is very slow at the lower temperature. At this temperature (15 °C) and after 10 min only 20% of SO24 − have been removed. These results show that the transport of the bivalent anions was more controlled at the lower temperature.
89.69 219.58 202.06 58.20 102.33 6.73 45.37 73.33 797.29 7.39 8.25
NO3− Cl− HCO3− SO42− Na+ K+ Mg2+ Ca2+ TDS Hardness (meq/l) pH
Flow rate: 180 l/h; voltage, 15 V.
Untreated water (ppm)
Analyzed parameters
38.29 65.90 152.70 50.15 55.00 2.60 18.23 24.00 406.87 2.70 7.73 57.31 69.99 24.43 13.83 46.25 61.37 59.82 67.27 48.97 63.46 –
24.24 43.25 99.49 46.15 43.60 1.90 13.82 16.00 287.84 1.93 7.34
72.97 80.30 50.76 20.70 57.39 80.83 69.54 78.18 63.90 73.88 –
26.43 57.46 124.94 46.15 51.00 2.30 12.54 16.00 338.65 1.83 7.58
C (ppm) 70.53 73.83 38.17 20.70 50.16 65.82 72.36 78.18 57.52 75.24 –
% Removal
C (ppm)
C (ppm)
% Removal
6 min
10 min
6 min % Removal
Treated water (25 °C)
Treated water (15 °C)
Table 4 Analytical results of the treated water after 5 and 10 min of electrodialysis for different temperatures
21.00 35.91 72.88 38.34 40.00 1.50 9.20 10.00 228.83 1.25 7.13
C (ppm)
10 min
76.59 83.65 63.93 34.12 60.91 77.71 79.72 86.36 71.29 83.08 –
% Removal 19.14 39.50 83.29 42.22 45.00 1.70 7.38 13.00 251.23 1.26 7.59
C (ppm)
6 min
78.66 82.01 58.78 27.46 56.02 74.74 83.73 82.27 68.48 82.95 –
% Removal
Treated water (40 °C)
15.57 27.56 54.37 34.50 36.60 1.20 6.05 8.00 183.85 0.90 7.05
C (ppm)
10 min
82.64 87.45 73.09 40.72 64.23 82.17 86.66 89.09 76.94 87.82 –
% Removal
238 A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
239
Fig. 2. Variation with time of the removal rate of anions and of TDS at different temperatures. (Flow rate: 180 l/h, Voltage: 15 V).
The evolution with time of removal rate of anions and of TDS in the treated water and for two flow rate (100 and 180 l/h) are shown in Fig.
3. Table 5 gives the performances of the electrodialysis after 5 and 10 min. The increase in the demineralization with flow rate may be attributed
78.05 242.02 230.34 57.70 107.25 7.15 43.09 73.23 838.83 7.34 8.13
NO− 3 Cl− HCO− 3 SO2− 4 Na+ K+ Mg2+ Ca2+ TDS Hardness (meq/l) pH
Voltage: 10 V, Temperature: 20 °C.
Untreated water (ppm)
Analyzed parameters
28.17 132.94 206.30 55.69 96.60 5.74 30.00 44.00 601.44 4.76 8.10
63.91 45.07 10.44 3.48 9.93 19.72 30.38 39.91 28.30 35.15 –
16.67 79.15 152.50 52.20 77.40 4.27 22.00 32.00 436.19 3.41 8.20
78.64 67.30 33.79 9.53 27.83 40.28 48.94 56.30 48.00 53.54 –
19.98 121.01 201.30 49.50 90.70 4.58 19.44 40.50 547.51 3.62 7.84
C (ppm) 74.40 50.00 12.61 14.21 15.43 35.94 54.89 44.69 34.73 40.68 –
% Removal
C (ppm)
C (ppm)
% Removal
5 min
10 min
5 min % Removal
Treated water (180 l/h)
Treated water (100 l/h)
Table 5 Analytical results of the treated water after 5 and 10 min of electrodialysis for the two flow rates
10.53 65.67 146.40 43.40 71.50 3.00 14.58 22.50 377.58 2.32 7.80
C (ppm)
10 min
86.51 72.86 36.44 24.78 33.33 58.04 66.16 69.27 54.99 68.39 –
% Removal
240 A. El Midaoui et al. / Separation and Purification Technology 29 (2002) 235–244
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241
Fig. 3. Influence of the flow rate on the removal rate of anions and of TDS during electrodialysis (Voltage: 10 V, Temperature: 20 °C).
242
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Fig. 4. Variation of removal rate of ions and of TDS vs. time at different voltages. (Flow rate: 180 l/h, Temperature: 20 °C).
78.86 236.38 231.26 57.70 107.47 7.17 39.23 71.16 829.23 6.78 7.95
NO− 3 −
Flow rate: 180 l/h, Temperature: 20 °C.
Cl HCO− 3 SO2− 4 + Na K+ Mg2+ Ca2+ TDS Hardness (meq/l) PH
Untreated water (ppm)
Analyzed parameters
54.00 193.30 222.00 56.00 106.00 6.30 32.00 58.00 727.30 5.52 7.84
31.52 18.22 4.00 2.95 1.36 12.13 18.43 18.49 12.26 18.58 –
30.15 153.1 219.60 54.00 105.10 5.37 27.50 43.00 637.82 4.41 7.80
61.77 35.23 5.04 6.41 2.21 25.11 29.90 39.57 23.08 34.95 –
19.98 121.01 201.30 49.50 90.70 4.58 30.00 44.50 561.57 4.68 7.84
C (ppm) 74.66 48.81 12.95 14.21 15.60 36.12 23.53 37.46 32.28 30.97 –
% Removal
C(ppm)
C (ppm)
% Removal
5 min
10 min
5 min % Removal
Treated water (10 V)
Treated water (5 V)
Table 6 Analytical results of the treated water after 5 and 10 min of electrodialysis for various voltages
10.53 65.67 146.40 43.40 71.50 3.00 22.00 22.50 385.00 2.93 7.80
C (ppm)
10 min
86.65 72.22 36.69 24.78 33.47 58.16 43.92 68.38 53.57 56.78 –
% Removal 14.57 68.83 144.37 42.50 62.60 4.13 22.50. 23.00 382.50 3.00 7.68
C (ppm)
5 min
81.52 70.88 37.57 26.34 41.75 42.40 42.65 67.68 53.87 55.75 –
% Removal
Treated water (15 V)
5.90 26.88 73.20 30.10 30.80 1.29 11.20 13.34 192.71 1.59 7.56
C (ppm)
10 min
92.52 88.63 68.35 47.83 71.34 82.01 71.45 81.25 76.76 76.55 –
% Removal
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to the decrease in the thickness of the boundary layers adjacent to the membrane surfaces (static zones of solutions) with increasing solution velocity. The applied voltage dependencies of the denitration parameters were shown in Fig. 4. The operation was carried out for three applied voltages: 5, 10 and 15 V. − increase The removal rates of NO− 3 and Cl rapidly with time at various voltages. Those of bicarbonate and sulfate increase rapidly at the higher voltage (15 V) and slowly at the lower one (5 V). However, at 5 V, the amounted of 2− ions in the diluate remained HCO− 3 and SO4 practically stopped with increasing electrodialysis duration up to 10 min and then decreased and SO24 − at slowly. The transport of HCO− 3 the lower voltage becomes significant after and Cl−. These nearly 90% removal of NO− 3 results can be attributed among others to the strong preference of the anion exchange membrane to transport monovalent ions and to the difference in the anionic contents in the untreated water. Table 6 gives the analytical results of the treated water after 5 and 10 min of electrodialysis operation at various applied voltages. Percentage of NO− 3 removal is always greater than the percentage removal of the other anions and of TDS. For the higher applied voltages (15 V), the total hardness and the TDS reach a limit level, the removal rate of SO24 − reaches 47%. The best results to control the denitration operation were obtained at the lower voltage and especially for 5 V: the nitrate content reaches the recommended guide level, the sulfate is practically stopped, the TDS and the total hardness were satisfactory. As it was indicated by Oldani et al. [8], practically the low applied voltages can be compensated by a large membrane areas. The higher costs of larger membrane areas well be partially balanced by lower electricity consumption better quality of the concentrate and diluate and by the higher water recovery.
4. Conclusion The electrodialysis operation not only removes nitrate but also the other anions. After a long time one thus obtains a deminiralized water which is not really suitable for drinking purposes. It is, therefore, necessary to stop the operation after a predetermined time. On the other hand to minimize the eventual precipitation of salt especially the divalent salt of sulfate in the concentrate compartment it is preferable to operate in condition which allow to blockade the sulfate (or the divalent cation) in the treated water. It is, therefore, necessary to find a compromise between time, temperature, flow rate and the applied voltage.
References [1] A. Elacheb, L. Bahi ‘les nitrates dans les eaux souterraines de Doukkala (Maroc)’ Rencontre Internationale sur les Fluorures Nitrates et les Pesticides dans les eaux du bassin Mediteraneen. Proble`mes et traitements 24 – 25 Avril 97, Kenitra, Morocco. [2] L. Berrada, T Chefadi ‘Etat de la qualite´ des ressources en eaux au Maroc et pollution par les nitrates’. Rencontre Internationale sur les fluorures, Nitrates et Pesticides dans les eaux du Bassin Mediteraneen: proble`mes et traitements 24 – 25 Avril 97 Kenitra, Morocco. [3] ‘Nitrates, Nitrites and N, Nitroso compounds’ Environmental Health Criterion —5 World health organization, Geneva, 1987. [4] A. Kapoor, T. Virapaghavan, Nitrate removal from drinking water, J. Environ. Eng. 123 (1997) 371 – 379. [5] F. Lutin, G. Guerif, ‘Electrodialysis applied to denitration of drinking water’ EURODIA Co. 1994, private communication. [6] R. Rautenbach, W. Kopp, R. Hellekes, R. Peter, G. Vanopbergen, Separation of nitrate from well water by membrane processes (Reverse Osmosis/Electrodialysis Reversal), Aqua 5 (1986) 279 – 282. [7] J.P. Van Der Hoek, A.B. Griffioen, A. Klapwijk, Biological regeneration of nitrate loaded anion exchange resins by denitrifing bacteria, J. Chem. Tech. Biotechnol. 43 (1988) 213 – 222. [8] M. Oldani, E. Killer, A. Miquel, G. Schock, On the nitrate and monovalent cation selectiorty of ion exchange membranes used in drinking water purification, J. Membr. Sci. 75 (1992) 265 – 267.