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Contact aeration for iron removal—A theoretical assessment
Wat. Res. Vol. 24, No. 3, pp. 329-331, 1990 Printed in Great Britain.All rights reserved
0043-1354/90$3.00+ 0.00 Copyright © 1990PergamonPress pie
CONTACT AERATION FOR IRON REMOVAL-A THEORETICAL ASSESSMENT HASAN Z. SARIKAYA Department of Civil Engineering, King Abdulaziz University, P.O. Box 9027, Jeddah 21413, Saudi Arabia (First received July 1988; accepted in revised form September 1989) Abstract--An iron removal process which makes use of the catalytic effect of ferric iron is proposed. Based upon the oxygenation rate equation, it has been theoretically demonstrated that the volumes of aeration tanks can be significantlyreduced by keeping high concentration of ferric iron in the reactor. Ferric iron is more effective in reducing the reactor volumes at lower pH values. Recyclingof ferric sludge is proposed to maintain the high ferric iron concentrations in the reactor. Key words--iron removal, oxygenation, water treatment, contact aeration
NOMENCLATURE
is available in the literature to relate the aeration tank volume to the ferric iron concentration. The aim of this technical note is to demonstrate the effect of ferric iron concentration on the aeration tank volume based upon a previously reported oxygenation rate equation (Tamura et al., 1976).
C = effluent ferrous iron concentration (mg/1) Co = influent ferrous iron concentration (mg/l) k = apparent homogeneous rate constant (s-~) k' = apparent heterogeneous rate constant [(mg/l)-~ s-~ ] Q = flow rate (m3/s) Qr = flow rate of recycled ferric sludge (m3/s) R = recirculation ratio = QJQ v = volume of contact aeration tank (m 3) V0 = volume of aeration tank for X ~- 0 X = effluent ferric iron concentration (mg/l) Xr = ferric iron concentration in the recycled sludge (mg/l).
PROCESS KINETICS
Laboratory studies have concluded that the ferric precipitate accelerated the reaction rate of ferrous oxygenation when pH is around 7 or more regardless of whether it was externally added or formed as a product of oxygenation (Tamura et aL, 1976; Sarikaya, 1980; Sung and Morgan, 1980; Davison and Seed, 1983). At constant pH and 02 concentration, the ferrous iron oxygenation rate has been expressed by Tamura et al. (1976) as:
INTRODUCTION
Aeration followed by solid/liquid separation is commonly applied for iron removal. The aeration process to oxidize ferrous iron is generally recommended for water with high concentrations ( > 5.0 mg/l) of iron to save chemical costs (Wong, 1984). Sedimentation and/or filtration are applied for the solid/liquid separation. If the total iron concentration is high, sedimentation tanks with sludge collection and removal facilities are used instead of a simple detention tank. The oxygenation of ferrous iron is catalyzed by the reaction product ferric hydroxide. This was evidenced by the improved efficiency of many treatment plants after a coating of oxidized iron has built up on the contact aerator, or multiple tray aerator, and filter media (Lerk, 1965; O'Connor, 1971; Anderson et al., 1973). Recently, Curry and Reynolds (1983) have reported that the recirculation of ferric hydroxide flocs (recirculation ratio, R = 0.1) to the reaction basin produced better settling flocs and improved the efficiency of oxygenation by reducing soluble effluent ferrous concentration of the reaction basin from 1.2 to 0.05 mg/l (a 24-fold decrease). Although the catalytic effect of ferric hydroxide has been observed in iron removal plants, no information
dC d-7 = - ( k + k ' X ) C
(l)
in which, C = ferrous iron concentrations (rag/l) X = ferric iron concentration (mg/l) k = apparent homogeneous rate constant (s -~) k ' = a p p a r e n t heterogeneous or catalytic rate constant [(mg/l)- i s - J].
THE EFFECT OF FERRIC IRON ON AERATION
TANK VOLUME It is assumed that the aeration tank is a completely mixed flow type reactor, the required volume of the tank is:
329
r = Q(Co - C) (k + k" X ) C
(2)
330
HmAN Z. SARIKAYA Table 1. Apparent rate constants ([02] = 0.25 x 10 -3 M, T = 25°C)
in which, V= Q = Co = C =
pH
volume of aeration tank (m 3) flow rate (m3/s) influent ferrous iron concentration (rag/l) effluent ferrous iron concentration (rag/I).
Apparent rate constant
For a very small concentrations of ferric iron (less than about 5 mg/l) the contribution of the heterogeneous oxygenation given in equation (1) can be neglected (Sarikaya, 1980). Then, the required aeration tank volume, V0 for small X values can be determined as: V0 =
Q ( C o - C) kC
(3)
6.5
7.0
7.5
k (s - I )
0.575 x 10 -4
0.575 x 10 -3
0.575 x 10 -2
k" [I/(mg.s)]
0.145 x 10 -4
0.458 x 10 -4
1.45 x 10 -4
experimentally verified range of Fig. 1 is considered. For example, the ratio of V/Vo is about 0.10 for X = 1 0 0 m g / 1 and p H = 7 . 0 . This means 90% reduction in volume. Ferric iron is more effective in reducing the volume at lower pH values. This is due to the dependence of k and k' on pH. k is proportional to [OH-]5 but k' is proportional to [OH-] (Tamura et al., 1976).
Dividing equation (2) by equation (3) V
k
(4)
Vo k + k ' X
It is clear from equation (4) that the aeration tank volume can be reduced by increasing the ferric iron concentration. In order to quantitatively illustrate the effectiveness of the ferric iron concentration in reducing the volume, k and k' were calculated using the real rate constants given by Tamura et al. (1976). The calculated k and k' values at 25°C and at oxygen concentration, [02] = 0.25 x 10-aM are shown in Table 1 for pH values of 6.5, 7.0 and 7.5. Using the k and k' values, the ratios of V/Vo are calculated from equation (4). The variations of V~ Iio with the ferric iron concentration are illustrated in Fig. 1. The validity of equation (1) has been tested up to X = 100mg/l (Tamura et al., 1976). Extrapolation of the rate equation beyond this value needs to be experimentally supported. Therefore, VIVa vs X curves are shown as dotted lines beyond X = 100 mg/l. The significant effect of X on the aeration tank volume is evident even if the
1.0 0.9 0.8 0.7 0.6
~~~,~d.
-0.25x 10"3 M T ,25"C
RECYCLING OF FERRIC PRECIPITATE
Ferrous iron concentrations in raw waters are usually not enough to maintain the high concentrations of ferric iron in the aeration tank unless the ferric precipitate is recycled back to the aeration tank. Therefore, an iron removal process with the recycling of ferric sludge is suggested. Assuming that the ferric iron concentration in raw water is negligible the recirculation ratio, R is given by: Qr
x
Q
Xr--X
R . . . .
(5)
in which, Q, = flow rate of recycled ferric sludge (m3/s) Xr = ferric iron concentration in the recycle line (mg/l). Although, it is shown that the high ferric concentrations reduce the aeration tank volume, maintenance of such high ferric concentrations is only possible with sludge recycling which will have additional cost. Therefore, total costs of the recycle treatment system versus that of a non-recycle system must be estimated to see the advantages of one over the other. The size of the reactor could also be reduced if oxidants that give faster oxidation were used. Thus, the final decision has to be a cost comparison considering different strategies.
[O e]
CONCLUSIONS
>0o. 5 0.4 0.3 0.2 0.1 0
I
I
I
I
I--'~--'7"
T
"l
;
20 40 60 80 100 120 140 160 180 200 Ferric iron concentration,X (mg/t)
Fig. 1. V / V o v s ferric h y d r o x i d e c o n c e n t r a t i o n , X.
It has been theoretically demonstrated that the volume of reactors for ferrous iron oxygenation can be significantly reduced by keeping high concentrations of ferric iron in the reactor. It has been suggested that high ferric iron concentrations can be maintained by recycling the ferric precipitates. Ferric iron is more effective in reducing the reactor volume at lower pH values. Cost comparison is essential in the final decision among the various strategies such as use of stronger oxidants, recycle and non-recycle treatment systems.
Contact aeration for iron removal REFERENCES
Anderson D. R., Row D. D. and Sindelar G. E. (1973) Iron and manganese studies of Nebraska water supplies. J. Am. War. Wks Ass. 65, 635-641. Curry M. D. and Reynolds M. (1983) Using by-products of an iron removal process to improve water treatment. J. Am. Wat. Wks Ass. 75, 246-248. Davison W. and Seed G. (1983) The kinetics of the oxidation of ferrous iron in synthetic and natural waters, Geochim. cosmochim, dcta 47, 67-79. Lerk C. F. (1965) Enkele aspecten van de ontijzering van groundwater. Thesis submitted to Delft University of Technology, The Netherlands in partial fulfillment of the degree of Doctor of Philosophy.
331
O'Connor J. T. (1971) Iron and manganese. In Water Quality and Treatment, Chap. 11, pp. 378-396. McGraw-Hill, New York. Sarikaya H. Z. (1980) Interactions between ferrous iron oxidation and phosphate. In Treatment and Disposal of Liquid and Solid Industrial Wastes (Edited by Kriton Cuff), pp. 143-160. Pergamon Press, New York. Sung W. and Morgan J. J. (1980) Kinetics and product of ferrous iron oxygenation in aqueous systems. J. envir. Sci. Technol. 14, 561-568. Tamura H., Goto K. and Nagayama M. (1976) The effect of ferric hydroxide on the oxygenation of ferrous ions in natural solutions. Corrosion Sci. 16, 197-207. Wong J. M. (1984) Chloffnation-filtration for iron and manganese removal. J. Am. Wat. Wks Ass. 76, 76-79.
0043-1354/90$3.00+ 0.00 Copyright © 1990PergamonPress pie
CONTACT AERATION FOR IRON REMOVAL-A THEORETICAL ASSESSMENT HASAN Z. SARIKAYA Department of Civil Engineering, King Abdulaziz University, P.O. Box 9027, Jeddah 21413, Saudi Arabia (First received July 1988; accepted in revised form September 1989) Abstract--An iron removal process which makes use of the catalytic effect of ferric iron is proposed. Based upon the oxygenation rate equation, it has been theoretically demonstrated that the volumes of aeration tanks can be significantlyreduced by keeping high concentration of ferric iron in the reactor. Ferric iron is more effective in reducing the reactor volumes at lower pH values. Recyclingof ferric sludge is proposed to maintain the high ferric iron concentrations in the reactor. Key words--iron removal, oxygenation, water treatment, contact aeration
NOMENCLATURE
is available in the literature to relate the aeration tank volume to the ferric iron concentration. The aim of this technical note is to demonstrate the effect of ferric iron concentration on the aeration tank volume based upon a previously reported oxygenation rate equation (Tamura et al., 1976).
C = effluent ferrous iron concentration (mg/1) Co = influent ferrous iron concentration (mg/l) k = apparent homogeneous rate constant (s-~) k' = apparent heterogeneous rate constant [(mg/l)-~ s-~ ] Q = flow rate (m3/s) Qr = flow rate of recycled ferric sludge (m3/s) R = recirculation ratio = QJQ v = volume of contact aeration tank (m 3) V0 = volume of aeration tank for X ~- 0 X = effluent ferric iron concentration (mg/l) Xr = ferric iron concentration in the recycled sludge (mg/l).
PROCESS KINETICS
Laboratory studies have concluded that the ferric precipitate accelerated the reaction rate of ferrous oxygenation when pH is around 7 or more regardless of whether it was externally added or formed as a product of oxygenation (Tamura et aL, 1976; Sarikaya, 1980; Sung and Morgan, 1980; Davison and Seed, 1983). At constant pH and 02 concentration, the ferrous iron oxygenation rate has been expressed by Tamura et al. (1976) as:
INTRODUCTION
Aeration followed by solid/liquid separation is commonly applied for iron removal. The aeration process to oxidize ferrous iron is generally recommended for water with high concentrations ( > 5.0 mg/l) of iron to save chemical costs (Wong, 1984). Sedimentation and/or filtration are applied for the solid/liquid separation. If the total iron concentration is high, sedimentation tanks with sludge collection and removal facilities are used instead of a simple detention tank. The oxygenation of ferrous iron is catalyzed by the reaction product ferric hydroxide. This was evidenced by the improved efficiency of many treatment plants after a coating of oxidized iron has built up on the contact aerator, or multiple tray aerator, and filter media (Lerk, 1965; O'Connor, 1971; Anderson et al., 1973). Recently, Curry and Reynolds (1983) have reported that the recirculation of ferric hydroxide flocs (recirculation ratio, R = 0.1) to the reaction basin produced better settling flocs and improved the efficiency of oxygenation by reducing soluble effluent ferrous concentration of the reaction basin from 1.2 to 0.05 mg/l (a 24-fold decrease). Although the catalytic effect of ferric hydroxide has been observed in iron removal plants, no information
dC d-7 = - ( k + k ' X ) C
(l)
in which, C = ferrous iron concentrations (rag/l) X = ferric iron concentration (mg/l) k = apparent homogeneous rate constant (s -~) k ' = a p p a r e n t heterogeneous or catalytic rate constant [(mg/l)- i s - J].
THE EFFECT OF FERRIC IRON ON AERATION
TANK VOLUME It is assumed that the aeration tank is a completely mixed flow type reactor, the required volume of the tank is:
329
r = Q(Co - C) (k + k" X ) C
(2)
330
HmAN Z. SARIKAYA Table 1. Apparent rate constants ([02] = 0.25 x 10 -3 M, T = 25°C)
in which, V= Q = Co = C =
pH
volume of aeration tank (m 3) flow rate (m3/s) influent ferrous iron concentration (rag/l) effluent ferrous iron concentration (rag/I).
Apparent rate constant
For a very small concentrations of ferric iron (less than about 5 mg/l) the contribution of the heterogeneous oxygenation given in equation (1) can be neglected (Sarikaya, 1980). Then, the required aeration tank volume, V0 for small X values can be determined as: V0 =
Q ( C o - C) kC
(3)
6.5
7.0
7.5
k (s - I )
0.575 x 10 -4
0.575 x 10 -3
0.575 x 10 -2
k" [I/(mg.s)]
0.145 x 10 -4
0.458 x 10 -4
1.45 x 10 -4
experimentally verified range of Fig. 1 is considered. For example, the ratio of V/Vo is about 0.10 for X = 1 0 0 m g / 1 and p H = 7 . 0 . This means 90% reduction in volume. Ferric iron is more effective in reducing the volume at lower pH values. This is due to the dependence of k and k' on pH. k is proportional to [OH-]5 but k' is proportional to [OH-] (Tamura et al., 1976).
Dividing equation (2) by equation (3) V
k
(4)
Vo k + k ' X
It is clear from equation (4) that the aeration tank volume can be reduced by increasing the ferric iron concentration. In order to quantitatively illustrate the effectiveness of the ferric iron concentration in reducing the volume, k and k' were calculated using the real rate constants given by Tamura et al. (1976). The calculated k and k' values at 25°C and at oxygen concentration, [02] = 0.25 x 10-aM are shown in Table 1 for pH values of 6.5, 7.0 and 7.5. Using the k and k' values, the ratios of V/Vo are calculated from equation (4). The variations of V~ Iio with the ferric iron concentration are illustrated in Fig. 1. The validity of equation (1) has been tested up to X = 100mg/l (Tamura et al., 1976). Extrapolation of the rate equation beyond this value needs to be experimentally supported. Therefore, VIVa vs X curves are shown as dotted lines beyond X = 100 mg/l. The significant effect of X on the aeration tank volume is evident even if the
1.0 0.9 0.8 0.7 0.6
~~~,~d.
-0.25x 10"3 M T ,25"C
RECYCLING OF FERRIC PRECIPITATE
Ferrous iron concentrations in raw waters are usually not enough to maintain the high concentrations of ferric iron in the aeration tank unless the ferric precipitate is recycled back to the aeration tank. Therefore, an iron removal process with the recycling of ferric sludge is suggested. Assuming that the ferric iron concentration in raw water is negligible the recirculation ratio, R is given by: Qr
x
Q
Xr--X
R . . . .
(5)
in which, Q, = flow rate of recycled ferric sludge (m3/s) Xr = ferric iron concentration in the recycle line (mg/l). Although, it is shown that the high ferric concentrations reduce the aeration tank volume, maintenance of such high ferric concentrations is only possible with sludge recycling which will have additional cost. Therefore, total costs of the recycle treatment system versus that of a non-recycle system must be estimated to see the advantages of one over the other. The size of the reactor could also be reduced if oxidants that give faster oxidation were used. Thus, the final decision has to be a cost comparison considering different strategies.
[O e]
CONCLUSIONS
>0o. 5 0.4 0.3 0.2 0.1 0
I
I
I
I
I--'~--'7"
T
"l
;
20 40 60 80 100 120 140 160 180 200 Ferric iron concentration,X (mg/t)
Fig. 1. V / V o v s ferric h y d r o x i d e c o n c e n t r a t i o n , X.
It has been theoretically demonstrated that the volume of reactors for ferrous iron oxygenation can be significantly reduced by keeping high concentrations of ferric iron in the reactor. It has been suggested that high ferric iron concentrations can be maintained by recycling the ferric precipitates. Ferric iron is more effective in reducing the reactor volume at lower pH values. Cost comparison is essential in the final decision among the various strategies such as use of stronger oxidants, recycle and non-recycle treatment systems.
Contact aeration for iron removal REFERENCES
Anderson D. R., Row D. D. and Sindelar G. E. (1973) Iron and manganese studies of Nebraska water supplies. J. Am. War. Wks Ass. 65, 635-641. Curry M. D. and Reynolds M. (1983) Using by-products of an iron removal process to improve water treatment. J. Am. Wat. Wks Ass. 75, 246-248. Davison W. and Seed G. (1983) The kinetics of the oxidation of ferrous iron in synthetic and natural waters, Geochim. cosmochim, dcta 47, 67-79. Lerk C. F. (1965) Enkele aspecten van de ontijzering van groundwater. Thesis submitted to Delft University of Technology, The Netherlands in partial fulfillment of the degree of Doctor of Philosophy.
331
O'Connor J. T. (1971) Iron and manganese. In Water Quality and Treatment, Chap. 11, pp. 378-396. McGraw-Hill, New York. Sarikaya H. Z. (1980) Interactions between ferrous iron oxidation and phosphate. In Treatment and Disposal of Liquid and Solid Industrial Wastes (Edited by Kriton Cuff), pp. 143-160. Pergamon Press, New York. Sung W. and Morgan J. J. (1980) Kinetics and product of ferrous iron oxygenation in aqueous systems. J. envir. Sci. Technol. 14, 561-568. Tamura H., Goto K. and Nagayama M. (1976) The effect of ferric hydroxide on the oxygenation of ferrous ions in natural solutions. Corrosion Sci. 16, 197-207. Wong J. M. (1984) Chloffnation-filtration for iron and manganese removal. J. Am. Wat. Wks Ass. 76, 76-79.