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Electrocoagulation of potable water
Water ICes. Vol. 18, No. 11, pp. 1355-1360, 1984 Printed in Great Britain. All rights reserved
0043-1354.84S3.00+ 0.00 Copyright ,C 1984Pergamon Press Ltd
ELECTROCOAGULATION OF POTABLE WATER ELLEN A. VIK~*, DALE A. CARLSON2, ARILD S. EIKUMI* and EGIL T. GJESSING~ ~Norwegian Institute for Water Research, P.O. Box 333, Blindern, Oslo 3, Norway and --Nalle Scandinavian Exchange Program, FX-10, University of Washington, Seattle, WA 98195, U.S.A. (Received April 1983) Abstract---Coagulation caused by electrolyticallyproduced ions (electrocoagnlation) followed by filtration has been studied as a possible alternative to the conventional coagulation process. Electrochemical processes have been used in water and wastewater treatment since 1887. Electrocoagulation is looked upon as an interesting process in use at small water treatment plants. There are several important design aspects of this process which deserve further study. This study presents results showing the correlation between the current density and the aluminium dosing and provides results showing the necessary overpotential. The process has proved efficient with regard to removal of aquatic humus. A comparison of this method with conventional coagulation shows that the aquatic humus is removed equally well with both methods. Conventionally coagulated water (using alum) contains higher concentrations of sulphate and thus has a higher specificconductivity than the electrochemicallytreated water. The electrochemicallytreated water contains higher residual aluminium concentrations than the conventionally treated water due to the higher pH values. Key words---electrocoagulation, potable water, humus removal, aluminium electrodes, small water works. electrochemical water treatment
INTRODUCTION
Approximately 95% of the Norwegian population use surface waters for potable water supply. 25~ of the population (1 million people) are served by small waterworks. Surface waters are characterized by high colour due to high humus concentrations, low specific conductivity and low alkalinity. Today, a number of small waterworks exist for the primary purpose of humus removal, disinfection and pH adjustment. The Norwegian Institute for Water Research (NIVA) decided in 1980 to concentrate the research activities on problems connected to small water treatment plants. Investigation of several small existing plants using coagulation showed that there are considerably operational difficulties involved in pH control, chemical mixing and Al-suiphate dosing. A simplification of the conventional coagulation process has been the major objective when studying the electrocoagulation process using aluminium electrodes. ELECTROCHEMICAL PROCESSES IN WATER AND WASTEWATER TREATMENT
Early development of electrochemical sewage treatment had as a primary interest the generation of chlorine for deodorizing and disinfecting wastewater. Eugene Hermite (Marson, 1965) received two British *Present address: Aquateam, Norwegian Water Technology Center A/S, P.O. Box 6593, 0501 Oslo 5, Norway.
and French patents in 1887 which described a method of treating sewage by mixing with a proportion of seawater and electrolysing. A treatment plant utilizing these patents was built in 1889 in London and operated for 10 years. Another plant for treating canal water was built the same year in Salford in England. Iron electrodes were used and seawater added as a chlorine source for disinfection. In 1909 J. T. Harries received patent (U.S. Patent, 1909) on a method for purification of wastewater by electrolysis. Both aluminium and iron were used together as anodes which corroded during electrolysis. Electrolytic sludge treatment plants were in operation as early as 1911 in Santa Monica, California and Oklahoma City, Oklahoma (Collier, 1912). Steel electrodes alternatively connected to the positive and negative terminals of a d.c. power supply with sewage flowing in demonstration plants were built and all were praised for their high quality effluent and lack of odor (Miller and Knipe, 1963). Operation costs were high since sludge from the settling tanks had to be hauled away. All plants were abandoned in 1930. Foyn (1963) described electrolytic treatment of wastewater using magnesium salt and alkalization in order to remove phosphorus at high pH. The chloride ions in seawater were oxidized to chlorine gas at the anode while hydrogen gas was formed at the cathode. The particles and colloids in water were adsorbed on the Mg(OH)2t~) and flotation occurred because of hydrogen gas formation. After a relatively short operational time, CaCO3t~ deposited at the cathodes
1355
1356
EILEN A. VIK et al.
and rendered ineffective the flotation process. The process was very effective in regard to phosphorus, nitrogen and organics removal, but was found to be expensive compared with biological treatment. Electrocoagulation has been studied for treatment of wastewater from food industry (Beck et al., 1974). The comparison was made between electrocoagulation and chemical treatment followed by dissolved air flotation. Floc formation for both processes was rapid. The electrocoagulation process, surfaced a floc in 2-3 min and compacted it in 3-10 min, while .the dissolved air flotation often required 10--20 min. In the Soviet U n i o n (Strokach, 1975), electrochemical water purification with a soluble iron anode was first used at the Shature Power Station about 1925. Stuart (1946) introduced the same process in U.S.A. in 1946 to remove coiour from drinking water. In this case aluminium electrodes were used. Holden (1956) treated water from the River Severin, using iron electrodes. Two water treatment plants were run in parallel, the only difference being the chemical dosing system. W a t e r quality measured as turbidity was the same in both systems. The electrolytic process was favoured as regards cleanliness and accuracy o f dosing. The capital cost was difficult to estimate while the operational costs were approximately the same as for the conventional process. EXPERIMENTAL
The research objective in this study was to elucidate the coagulation process caused by electrolytically produced ions (electrocoagulation) using aluminium electrodes for humus tmnoval from potable water. Experiments were done both in laboratory and in pilot scale. Three different raw waters with various humus concentrations were studied. Water quality attained using the electrocoagulation process was compared with that obtained using conventional alum coagulation. Electrocoagulation with aluminium electrodes In the electrochemical cell, raw water passes parallel aluminium plates connected to a battery. Aluminium dissolves from the anodes and hydrogen gas is developed at the cathodes. The mixing between water and dissolved aluminium occurring between the electrodes results in coagulation and floceulation. The hydrogen gas formation results in pH increase and flotation of the sludge formed. The basic principles involved in electrocoagulation are described in Fig. 1. The laboratory experiments were performed using a stabilized d.c. power supply (6--12 V), a resistance box to regulate the current density and a multimeter to read the current values. In the laboratory experiments water was treated in the electrochemical cell at a flowrate of 0.171rain -~ using four parallel aluminium electrodes (14 x 20 × 0.25 crn) 3 mm apart. A photo of the laboratory unit is presented in Fig. 2. The pilot scale experiments were performed with an electrochemical unit designed to treat 101 min- ~ water. Four parallel aluminium electrodes (40 x 30 x 0.3 cm) 10 mm apart were alternatively connected to positive and negative current from a 24 V battery power source. Water sources Three lakes in the vicinity at Oslo, Norway. with various concentrations of aquatic humus, were used as the raw
water sources. Water quality data for the three water sources. Lakes Tjernsmotjern, Hellerudmyra and Sm~iputten are summarized in Table 1. The specific conductivity and the pH of the water sources were in some of the experiments adjusted with table salt, sulphuric acid and sodium hydroxide, respectively. RESULTS AND DISCUSSION Aluminium dosing In electrocoagulation, the electrodes o f t b e electrochemical cell are connected to an electrical power source. Faraday's law can be used to describe the relationship between current density (Acrn -2) and the amount o f aluminium which goes into solution (g AI cm-2). itM ZF w i t M Z
= aluminium dissolving (g AI cm -2) = current density (A c m - : ) = time (s) = molecular weight of AI ( M = 27) =number of electrones involved in oxidation/reduction reaction (Z = 3) F = Faraday's constant, 96,500.
the
During the course of a series of experiments, the Al-electrodes were weighed. The theoretically calculated amount of AI dissolved was at various raw water temperatures compared with the weighed values of AI dissolved (Fig. 3). The correlation found was relatively good (r 2 = 0.94) and in the further experiments the Al-dose was calculated based on Faraday's law. In presentation o f the results from the coagulation experiments, the aluminium dosage is presented as the charge (Coulombs). A charge of 500 C at the figures equals an AI dose o f 6 mg AI 1- i. The necessary overpotential The measured potential necessary to get the desired current density is defined as a sum o f three components: r/Ae = r/x + r/M, + ~IR r/Ae = r/x = r/u, = r/t~ =
applied overpotential (V) kinetic overvoltage (V) mass-transfer overvoltage (V) potential caused by solution resistance (V), IR-drop.
The / R - d r o p can be reduced by decreasing the distance between the electrodes and by increasing the surface of the anodes and the specific conductivity o f the water: r/tR = I .
d
where 1= d = A = x =
current (A) distance between the electrodes (m) active anode surface (m 2) specific conductivity (10 3 mS m-~).
Electrocoagulation of potable water
1357
Anode J' J' Cathode
water
Influent water
s/
Fig. 1. The principles involved in electrocoagulation.
Fig. 2. Laboratory scale electrochemical reactor.
EZLEN A. VZK et al.
1358
Table I. Important water quality characteristics of water from Lakes Tjernsmotjern, Hellerudmyra and Sm/tputten Lakes
Parameters
Tjernsmotjem
Turbidity (NTU) Colour (mg Pt 1- I) u.v.-Absorptinn (cm -~) at 253.7 nm
0.90 110-140 0.53-0.65 12.2-15.6 29-55 6.0--6.5 0.08-0.14
TOC (rag C 1-t) Spec. conductivity (pS em-i) pH Alkalinity (mequiv 1- i)
Hellerudmyra
0.95 44-78 0.26-0.35 5.9-6,9 83-115 6.2-7,3 0.40
0.90 105-110 0.50-0.52 10.8-12.2 29-35 4.3-4.9 0
o•0.99x
- 2.05 (r2=0.94)
300" E o
~
Smtputten
200-
"/~'¢L
c
Temp ": 5°C
/ ,~
O
•
•
-'Y,"
5~c < T~mp~ 10°C
O10°C < lemp_< 19°C
•
.
~
lOO-
~
o
e
o
100
!
200
300
400
(x) Weighed amount AI dissolvedfrom anode (rag)
Fig. 3. Relationship between theoretically calculated and practical dissolution of aluminium for raw water
of various temperatures.
The mass-transfer overvoltage, ~/Mtcan be reduced by increasing the transport of aluminium ions from the anode surface to the bulk solution, i.e. by increased mixing between the electrodes. Increased mixing is also obtained when the water velocity is increased. In a series of experiments using the laboratory equipment, the distance between the electrodes was varied and so also the specific conductivity of the
12.0-
10.0-
• O • zx
solution. The measured potential as a function of the calculated /R-drop is presented in Fig. 4, shoging that when the/R-drop is going towards zero, there still exists a minimum overpotential for the laboratory unit, approximately equal to 1.3 V. This overpotential might be due to the mass transfer of ions.
Coagulation of aquatic humus The pH of the water to be treated is increasing
K= 53 uS/cm K=790 uS/cm K=1900 uS/cm K=5600 uS/cm
• A =A
8.0-
.-:!g"
6.0-
4.0.
o~o% °
2.00 0.01
o.~
d.1
o[5
~'.o
5'.0
~'o
sb
Fig. 4. The measured potential of the laboratory reactor as a function of the/R-drop. The distance (d)
between the electrodes and the specific conductivity (K) have been varied in the experiments.
Electrocoagulation of potable water
1359
Tjernsmotjern
s.o •
•
"%
•
.e.m_ _ _ ._e. . . .
.-'--
H=6.ge
oe
6.0
I
•
Natural water
•
5 . o ~ - - ' ~"t ~(3
pH = 2.8
3.0 N
2.0
~
~o
pH = 2.0
1~
,~oo
:~o'
Charge, I , t (Coulombs)
Fig. 5. Increase of pH during electrochemical treatment of raw water with different initial pH values.
Water source: Lake Tjernsmotjem (pH = 6.0). Tiernsrnotiern 160. x
14,O pH = 2,0
120
~
t0,O' ~
80.
pH = 6.9
6,0. 4,02,0Charge, I . t (Coulombs)
Fig. 6. The efficiency of the coagulation process in regard to total organic carbon removal (TOC) for various pH of the raw water.
during the electrocoagulation process due to the hydrogen gas formation at the cathode. The importance of pH in regard to coagulation is well known. In order to evaluate the effect of the pH of the raw water a series of experiments adjusting the raw water pH were performed using water from Lake Tjernsmotjern. The increase o f p H vs aluminium dose (charge) is illustrated in Fig. 5. For raw water an initial pH around 6.0 (natural water), the maximum pH value, 7.7, is reached at a charge of 500C (6 mg AI 1- '). The importance of raw water pH on the removal efficiency of aquatic humus, is illustrated in Fig. 6. Total organic carbon (TOC) is presented as a function of aluminium dose. The results show that the pH value of the raw water has no influence on the removal of organics at various chemical dosages as long as pH is kept between 3.9 and 6.0 (pH of natural water). As seen from Fig. 7, for a raw water o f p H 3.9 the pH of treated water is between 5.5 and 6.0 which is close to the optimum pH for AI(OH)~s~ formation. The effect of raw water pH on the coagulant demand for efficient removal was studied for another
water source, Lake SmAputten. The raw water pH varied between 6.8 and 7.3 during the experimental period. Another coagulation experiment was performed after adjusting the raw water pH from 7.3 to 0.4 ] -~
~
Smaputlen
Apr. 1. Ig62, ntial T O C = 9.2, pH = 7 3 ~ - - NOV. 12. 1982, initial TOC = 9.4. pH - 6.8 e - - Ap~ 1, 1982, initial TOC = 9.2, pH = 4.8 (pH adjusted)
E
0
o
~o
~o
e~o
lo~o
Charge, ] • t (Coulomll:]=)
Fig. 7. Residual u.v. absorption (at 253.'/nm) as a function of the coagulation dose (charge) for water from Lake SmAputten with various pH values.
1360
ELLEN A. VIK et aL Table 2. Water quality results from electrocoagulation of water from Hellerudmyra compared with conventional coagulation of the same raw water Water quality Water treated Water treated using using conventional Parameters Raw w a t e r electrocoagulation alum coagulation Coiour (rag Pt I-') 80 3 3 pH 4.8 7.0 5.9 Spec. cond. ~Scra -I) 35 20 114 Ca (rag 1- ') 1.08 0.76 1.09 Mg (mgl -I) 0.27 0.14 0.24 Fe ~ g I-') 130 10 20 AI (~g I-') 290 330 40 Na (rag 1-') 1.05 1.63 1.49 K (rag 1-') 0.32 0.41 1.87 Mn (/~g1-') 34 20 32 Cu (/zgI-') 37 16 23 Pb (/zg I-') 1.9 <0.5 0.5 NO3 (/~gl-I) < 10 < 10 < 10 SO4 (mg I- i) 4.6 2.4 32 CI (mg 1-j) 1.3 1.2 2.6 F (rag 1-')
<0.1
4.8. Figure 7 illustrates the increase o f the coagulant demand with increasing raw water pH.
Comparison o f electrocoagulation with conventional coagulation Water from Hellerudmyra was treated both with electrocoagulation and conventional alum coagulation. An aluminium dosage o f 6 m g A l l - ' was added to both. The conventionally treated water was p H adjusted with N a O H to a p H o f 6.0. The results are presented in Table 2. The residual colour is the same for both methods. The main differences between the results are the p H value, residual Alconcentration, the sulphate concentration and thus the specific conductivity. CONCLUSION There are several important aspects o f this process which need to be studied further before conventional production o f treatment plants using this process. Design studies are continuing. The design o f the unit has been changed several times and investigations are continuing to optimise the lifetime o f the electrodes and treatment of other water qualities. Electrocoagulation is a potential process in use at small water treatment plants and in developing countries. We think this process c o m p a r e d with conventional treatment is interesting due to the following points: The a m o u n t o f chemicals which has to be transported is lower than for conventional treatment (approx. 1/ 10 o f the amount). The electrochemical unit will be m a d e with electrodes enough for at least one year o f treatment. Approximately once a year new electrodes must be
<0.1
<0.1
installed. This reduces the time needed for handling of chemicals. A lesser a m o u n t o f sludge is formed, because of the higher dry solids content. The maintenance and operation o f this system will be simple. N o mixing o f chemicals for the operator and only once a year handling o f chemicals.
Acknowledgements--This work was originally sponsored by the Royal Norwegian Council for Scientific and Industrial Research, the Norwegian Institute for Water Research and the Vaile Scandinavian Exchange Program at the University of Washington, Seattle. The current phase of the work is sponsored by Aqua Care, a division of International Farvefabrik A/S.
REFERENCES
Beck E. C., Giannini A. P. and Ramircz E. R. (1974) Electrocoagulation clarifies food wastewater. Fd Technol. February. Collier W. R. (1912) Description of plants at Oklahoma City, Okla and Santa Monica, Calif. Engng Rec. 66, 55. Feyn E. (1963) Eletrolytisk kioakkrensing i teknisk mtlestokk. Fors~ksdriften p l Huk. Teknisk Ukeblad No. 19. Holden W. S. (1956) Electrolytic dosing of chemicals. Proc. Soc. War. Treat. Exam. 5, 120. Marson H. W. (1965) Electrolytic sewage treatment. The Engineer 4, 591. Miller H. C. and Knipe W. (1963) Electrochemical Treatment of Municipal Wastewater. Final Report. U.S. Public Health Service, Division of Water Supply and Pollution Control. Contract No. PH 86-62-113. Strokach P. P. (1975) The prospects of using anodic dissolution of metal for water purification. Electrochemistry in Industrial Processing and Biology, English translation No. 4, p. 55. Stuart F. E. (1946) Electronic principle of water purification. J. New Engl. War. Wks Ass. 60, 236. U.S. Patent (1909) 937, 210.
0043-1354.84S3.00+ 0.00 Copyright ,C 1984Pergamon Press Ltd
ELECTROCOAGULATION OF POTABLE WATER ELLEN A. VIK~*, DALE A. CARLSON2, ARILD S. EIKUMI* and EGIL T. GJESSING~ ~Norwegian Institute for Water Research, P.O. Box 333, Blindern, Oslo 3, Norway and --Nalle Scandinavian Exchange Program, FX-10, University of Washington, Seattle, WA 98195, U.S.A. (Received April 1983) Abstract---Coagulation caused by electrolyticallyproduced ions (electrocoagnlation) followed by filtration has been studied as a possible alternative to the conventional coagulation process. Electrochemical processes have been used in water and wastewater treatment since 1887. Electrocoagulation is looked upon as an interesting process in use at small water treatment plants. There are several important design aspects of this process which deserve further study. This study presents results showing the correlation between the current density and the aluminium dosing and provides results showing the necessary overpotential. The process has proved efficient with regard to removal of aquatic humus. A comparison of this method with conventional coagulation shows that the aquatic humus is removed equally well with both methods. Conventionally coagulated water (using alum) contains higher concentrations of sulphate and thus has a higher specificconductivity than the electrochemicallytreated water. The electrochemicallytreated water contains higher residual aluminium concentrations than the conventionally treated water due to the higher pH values. Key words---electrocoagulation, potable water, humus removal, aluminium electrodes, small water works. electrochemical water treatment
INTRODUCTION
Approximately 95% of the Norwegian population use surface waters for potable water supply. 25~ of the population (1 million people) are served by small waterworks. Surface waters are characterized by high colour due to high humus concentrations, low specific conductivity and low alkalinity. Today, a number of small waterworks exist for the primary purpose of humus removal, disinfection and pH adjustment. The Norwegian Institute for Water Research (NIVA) decided in 1980 to concentrate the research activities on problems connected to small water treatment plants. Investigation of several small existing plants using coagulation showed that there are considerably operational difficulties involved in pH control, chemical mixing and Al-suiphate dosing. A simplification of the conventional coagulation process has been the major objective when studying the electrocoagulation process using aluminium electrodes. ELECTROCHEMICAL PROCESSES IN WATER AND WASTEWATER TREATMENT
Early development of electrochemical sewage treatment had as a primary interest the generation of chlorine for deodorizing and disinfecting wastewater. Eugene Hermite (Marson, 1965) received two British *Present address: Aquateam, Norwegian Water Technology Center A/S, P.O. Box 6593, 0501 Oslo 5, Norway.
and French patents in 1887 which described a method of treating sewage by mixing with a proportion of seawater and electrolysing. A treatment plant utilizing these patents was built in 1889 in London and operated for 10 years. Another plant for treating canal water was built the same year in Salford in England. Iron electrodes were used and seawater added as a chlorine source for disinfection. In 1909 J. T. Harries received patent (U.S. Patent, 1909) on a method for purification of wastewater by electrolysis. Both aluminium and iron were used together as anodes which corroded during electrolysis. Electrolytic sludge treatment plants were in operation as early as 1911 in Santa Monica, California and Oklahoma City, Oklahoma (Collier, 1912). Steel electrodes alternatively connected to the positive and negative terminals of a d.c. power supply with sewage flowing in demonstration plants were built and all were praised for their high quality effluent and lack of odor (Miller and Knipe, 1963). Operation costs were high since sludge from the settling tanks had to be hauled away. All plants were abandoned in 1930. Foyn (1963) described electrolytic treatment of wastewater using magnesium salt and alkalization in order to remove phosphorus at high pH. The chloride ions in seawater were oxidized to chlorine gas at the anode while hydrogen gas was formed at the cathode. The particles and colloids in water were adsorbed on the Mg(OH)2t~) and flotation occurred because of hydrogen gas formation. After a relatively short operational time, CaCO3t~ deposited at the cathodes
1355
1356
EILEN A. VIK et al.
and rendered ineffective the flotation process. The process was very effective in regard to phosphorus, nitrogen and organics removal, but was found to be expensive compared with biological treatment. Electrocoagulation has been studied for treatment of wastewater from food industry (Beck et al., 1974). The comparison was made between electrocoagulation and chemical treatment followed by dissolved air flotation. Floc formation for both processes was rapid. The electrocoagulation process, surfaced a floc in 2-3 min and compacted it in 3-10 min, while .the dissolved air flotation often required 10--20 min. In the Soviet U n i o n (Strokach, 1975), electrochemical water purification with a soluble iron anode was first used at the Shature Power Station about 1925. Stuart (1946) introduced the same process in U.S.A. in 1946 to remove coiour from drinking water. In this case aluminium electrodes were used. Holden (1956) treated water from the River Severin, using iron electrodes. Two water treatment plants were run in parallel, the only difference being the chemical dosing system. W a t e r quality measured as turbidity was the same in both systems. The electrolytic process was favoured as regards cleanliness and accuracy o f dosing. The capital cost was difficult to estimate while the operational costs were approximately the same as for the conventional process. EXPERIMENTAL
The research objective in this study was to elucidate the coagulation process caused by electrolytically produced ions (electrocoagulation) using aluminium electrodes for humus tmnoval from potable water. Experiments were done both in laboratory and in pilot scale. Three different raw waters with various humus concentrations were studied. Water quality attained using the electrocoagulation process was compared with that obtained using conventional alum coagulation. Electrocoagulation with aluminium electrodes In the electrochemical cell, raw water passes parallel aluminium plates connected to a battery. Aluminium dissolves from the anodes and hydrogen gas is developed at the cathodes. The mixing between water and dissolved aluminium occurring between the electrodes results in coagulation and floceulation. The hydrogen gas formation results in pH increase and flotation of the sludge formed. The basic principles involved in electrocoagulation are described in Fig. 1. The laboratory experiments were performed using a stabilized d.c. power supply (6--12 V), a resistance box to regulate the current density and a multimeter to read the current values. In the laboratory experiments water was treated in the electrochemical cell at a flowrate of 0.171rain -~ using four parallel aluminium electrodes (14 x 20 × 0.25 crn) 3 mm apart. A photo of the laboratory unit is presented in Fig. 2. The pilot scale experiments were performed with an electrochemical unit designed to treat 101 min- ~ water. Four parallel aluminium electrodes (40 x 30 x 0.3 cm) 10 mm apart were alternatively connected to positive and negative current from a 24 V battery power source. Water sources Three lakes in the vicinity at Oslo, Norway. with various concentrations of aquatic humus, were used as the raw
water sources. Water quality data for the three water sources. Lakes Tjernsmotjern, Hellerudmyra and Sm~iputten are summarized in Table 1. The specific conductivity and the pH of the water sources were in some of the experiments adjusted with table salt, sulphuric acid and sodium hydroxide, respectively. RESULTS AND DISCUSSION Aluminium dosing In electrocoagulation, the electrodes o f t b e electrochemical cell are connected to an electrical power source. Faraday's law can be used to describe the relationship between current density (Acrn -2) and the amount o f aluminium which goes into solution (g AI cm-2). itM ZF w i t M Z
= aluminium dissolving (g AI cm -2) = current density (A c m - : ) = time (s) = molecular weight of AI ( M = 27) =number of electrones involved in oxidation/reduction reaction (Z = 3) F = Faraday's constant, 96,500.
the
During the course of a series of experiments, the Al-electrodes were weighed. The theoretically calculated amount of AI dissolved was at various raw water temperatures compared with the weighed values of AI dissolved (Fig. 3). The correlation found was relatively good (r 2 = 0.94) and in the further experiments the Al-dose was calculated based on Faraday's law. In presentation o f the results from the coagulation experiments, the aluminium dosage is presented as the charge (Coulombs). A charge of 500 C at the figures equals an AI dose o f 6 mg AI 1- i. The necessary overpotential The measured potential necessary to get the desired current density is defined as a sum o f three components: r/Ae = r/x + r/M, + ~IR r/Ae = r/x = r/u, = r/t~ =
applied overpotential (V) kinetic overvoltage (V) mass-transfer overvoltage (V) potential caused by solution resistance (V), IR-drop.
The / R - d r o p can be reduced by decreasing the distance between the electrodes and by increasing the surface of the anodes and the specific conductivity o f the water: r/tR = I .
d
where 1= d = A = x =
current (A) distance between the electrodes (m) active anode surface (m 2) specific conductivity (10 3 mS m-~).
Electrocoagulation of potable water
1357
Anode J' J' Cathode
water
Influent water
s/
Fig. 1. The principles involved in electrocoagulation.
Fig. 2. Laboratory scale electrochemical reactor.
EZLEN A. VZK et al.
1358
Table I. Important water quality characteristics of water from Lakes Tjernsmotjern, Hellerudmyra and Sm/tputten Lakes
Parameters
Tjernsmotjem
Turbidity (NTU) Colour (mg Pt 1- I) u.v.-Absorptinn (cm -~) at 253.7 nm
0.90 110-140 0.53-0.65 12.2-15.6 29-55 6.0--6.5 0.08-0.14
TOC (rag C 1-t) Spec. conductivity (pS em-i) pH Alkalinity (mequiv 1- i)
Hellerudmyra
0.95 44-78 0.26-0.35 5.9-6,9 83-115 6.2-7,3 0.40
0.90 105-110 0.50-0.52 10.8-12.2 29-35 4.3-4.9 0
o•0.99x
- 2.05 (r2=0.94)
300" E o
~
Smtputten
200-
"/~'¢L
c
Temp ": 5°C
/ ,~
O
•
•
-'Y,"
5~c < T~mp~ 10°C
O10°C < lemp_< 19°C
•
.
~
lOO-
~
o
e
o
100
!
200
300
400
(x) Weighed amount AI dissolvedfrom anode (rag)
Fig. 3. Relationship between theoretically calculated and practical dissolution of aluminium for raw water
of various temperatures.
The mass-transfer overvoltage, ~/Mtcan be reduced by increasing the transport of aluminium ions from the anode surface to the bulk solution, i.e. by increased mixing between the electrodes. Increased mixing is also obtained when the water velocity is increased. In a series of experiments using the laboratory equipment, the distance between the electrodes was varied and so also the specific conductivity of the
12.0-
10.0-
• O • zx
solution. The measured potential as a function of the calculated /R-drop is presented in Fig. 4, shoging that when the/R-drop is going towards zero, there still exists a minimum overpotential for the laboratory unit, approximately equal to 1.3 V. This overpotential might be due to the mass transfer of ions.
Coagulation of aquatic humus The pH of the water to be treated is increasing
K= 53 uS/cm K=790 uS/cm K=1900 uS/cm K=5600 uS/cm
• A =A
8.0-
.-:!g"
6.0-
4.0.
o~o% °
2.00 0.01
o.~
d.1
o[5
~'.o
5'.0
~'o
sb
Fig. 4. The measured potential of the laboratory reactor as a function of the/R-drop. The distance (d)
between the electrodes and the specific conductivity (K) have been varied in the experiments.
Electrocoagulation of potable water
1359
Tjernsmotjern
s.o •
•
"%
•
.e.m_ _ _ ._e. . . .
.-'--
H=6.ge
oe
6.0
I
•
Natural water
•
5 . o ~ - - ' ~"t ~(3
pH = 2.8
3.0 N
2.0
~
~o
pH = 2.0
1~
,~oo
:~o'
Charge, I , t (Coulombs)
Fig. 5. Increase of pH during electrochemical treatment of raw water with different initial pH values.
Water source: Lake Tjernsmotjem (pH = 6.0). Tiernsrnotiern 160. x
14,O pH = 2,0
120
~
t0,O' ~
80.
pH = 6.9
6,0. 4,02,0Charge, I . t (Coulombs)
Fig. 6. The efficiency of the coagulation process in regard to total organic carbon removal (TOC) for various pH of the raw water.
during the electrocoagulation process due to the hydrogen gas formation at the cathode. The importance of pH in regard to coagulation is well known. In order to evaluate the effect of the pH of the raw water a series of experiments adjusting the raw water pH were performed using water from Lake Tjernsmotjern. The increase o f p H vs aluminium dose (charge) is illustrated in Fig. 5. For raw water an initial pH around 6.0 (natural water), the maximum pH value, 7.7, is reached at a charge of 500C (6 mg AI 1- '). The importance of raw water pH on the removal efficiency of aquatic humus, is illustrated in Fig. 6. Total organic carbon (TOC) is presented as a function of aluminium dose. The results show that the pH value of the raw water has no influence on the removal of organics at various chemical dosages as long as pH is kept between 3.9 and 6.0 (pH of natural water). As seen from Fig. 7, for a raw water o f p H 3.9 the pH of treated water is between 5.5 and 6.0 which is close to the optimum pH for AI(OH)~s~ formation. The effect of raw water pH on the coagulant demand for efficient removal was studied for another
water source, Lake SmAputten. The raw water pH varied between 6.8 and 7.3 during the experimental period. Another coagulation experiment was performed after adjusting the raw water pH from 7.3 to 0.4 ] -~
~
Smaputlen
Apr. 1. Ig62, ntial T O C = 9.2, pH = 7 3 ~ - - NOV. 12. 1982, initial TOC = 9.4. pH - 6.8 e - - Ap~ 1, 1982, initial TOC = 9.2, pH = 4.8 (pH adjusted)
E
0
o
~o
~o
e~o
lo~o
Charge, ] • t (Coulomll:]=)
Fig. 7. Residual u.v. absorption (at 253.'/nm) as a function of the coagulation dose (charge) for water from Lake SmAputten with various pH values.
1360
ELLEN A. VIK et aL Table 2. Water quality results from electrocoagulation of water from Hellerudmyra compared with conventional coagulation of the same raw water Water quality Water treated Water treated using using conventional Parameters Raw w a t e r electrocoagulation alum coagulation Coiour (rag Pt I-') 80 3 3 pH 4.8 7.0 5.9 Spec. cond. ~Scra -I) 35 20 114 Ca (rag 1- ') 1.08 0.76 1.09 Mg (mgl -I) 0.27 0.14 0.24 Fe ~ g I-') 130 10 20 AI (~g I-') 290 330 40 Na (rag 1-') 1.05 1.63 1.49 K (rag 1-') 0.32 0.41 1.87 Mn (/~g1-') 34 20 32 Cu (/zgI-') 37 16 23 Pb (/zg I-') 1.9 <0.5 0.5 NO3 (/~gl-I) < 10 < 10 < 10 SO4 (mg I- i) 4.6 2.4 32 CI (mg 1-j) 1.3 1.2 2.6 F (rag 1-')
<0.1
4.8. Figure 7 illustrates the increase o f the coagulant demand with increasing raw water pH.
Comparison o f electrocoagulation with conventional coagulation Water from Hellerudmyra was treated both with electrocoagulation and conventional alum coagulation. An aluminium dosage o f 6 m g A l l - ' was added to both. The conventionally treated water was p H adjusted with N a O H to a p H o f 6.0. The results are presented in Table 2. The residual colour is the same for both methods. The main differences between the results are the p H value, residual Alconcentration, the sulphate concentration and thus the specific conductivity. CONCLUSION There are several important aspects o f this process which need to be studied further before conventional production o f treatment plants using this process. Design studies are continuing. The design o f the unit has been changed several times and investigations are continuing to optimise the lifetime o f the electrodes and treatment of other water qualities. Electrocoagulation is a potential process in use at small water treatment plants and in developing countries. We think this process c o m p a r e d with conventional treatment is interesting due to the following points: The a m o u n t o f chemicals which has to be transported is lower than for conventional treatment (approx. 1/ 10 o f the amount). The electrochemical unit will be m a d e with electrodes enough for at least one year o f treatment. Approximately once a year new electrodes must be
<0.1
<0.1
installed. This reduces the time needed for handling of chemicals. A lesser a m o u n t o f sludge is formed, because of the higher dry solids content. The maintenance and operation o f this system will be simple. N o mixing o f chemicals for the operator and only once a year handling o f chemicals.
Acknowledgements--This work was originally sponsored by the Royal Norwegian Council for Scientific and Industrial Research, the Norwegian Institute for Water Research and the Vaile Scandinavian Exchange Program at the University of Washington, Seattle. The current phase of the work is sponsored by Aqua Care, a division of International Farvefabrik A/S.
REFERENCES
Beck E. C., Giannini A. P. and Ramircz E. R. (1974) Electrocoagulation clarifies food wastewater. Fd Technol. February. Collier W. R. (1912) Description of plants at Oklahoma City, Okla and Santa Monica, Calif. Engng Rec. 66, 55. Feyn E. (1963) Eletrolytisk kioakkrensing i teknisk mtlestokk. Fors~ksdriften p l Huk. Teknisk Ukeblad No. 19. Holden W. S. (1956) Electrolytic dosing of chemicals. Proc. Soc. War. Treat. Exam. 5, 120. Marson H. W. (1965) Electrolytic sewage treatment. The Engineer 4, 591. Miller H. C. and Knipe W. (1963) Electrochemical Treatment of Municipal Wastewater. Final Report. U.S. Public Health Service, Division of Water Supply and Pollution Control. Contract No. PH 86-62-113. Strokach P. P. (1975) The prospects of using anodic dissolution of metal for water purification. Electrochemistry in Industrial Processing and Biology, English translation No. 4, p. 55. Stuart F. E. (1946) Electronic principle of water purification. J. New Engl. War. Wks Ass. 60, 236. U.S. Patent (1909) 937, 210.