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Improved electroflotation for the removal of suspended solids from algal pond effluents
~,l, ar~,, R c ~ , a r c h ',oL ~. pp. 5~7 to 592. Persamon Press 1 9 " 1 Printed in Great Britain.
IMPROVED ELECTROFLOTATION FOR THE REMOVAL OF SUSPENDED SOLIDS FROM ALGAL POND EFFLUENTS E. SAXlmA~. G. SHEL~Fand A. M. WACHS Environmental Engineering Laboratories, Technion-lsrael Institute of Technology Israel (Receired 13 February 1973: i, ret'ised tbrm 13 Noremher 1973)
Abstract--Experiments with a patented modification of an electroflotation cell showed that the power requirements for the treatment of pond effluents were less than for other electroflotation methods. The relationship between current densities and removal efflcienciesof algae in suspension was fo~.nd in laboratory and pilot-scale experiments. It was found that simultaneous flocculation-electroflotation gives better results than consecutive operation. The results obtained with electroflotation even with low current densities were almost equal to dissolved air flotation. It is believed that better adherence of electrolytically produced gases to the floc is responsible for the superiority of electroflotation.
In the treatment of wastewater in stabilization ponds bacteria and organic constituents by means of the elecor in high rate ponds, where the effluent contains con- trolysis alone. This method required a large power incenmttions of algae var.ving from ItX) to 400 mg I.- t. put. Unlike these applications of electrochemical treatthe removal of algae is essential in order to reduce the concentration of nutrients and of suspended solids ment. the Objective of the work described in this paper was to produce the maximum amount of homogenous whenever high quality effluent is required. Separation of suspended solids is most common by bubbles by electrochemical means for the flotation of flocculation followed by sedimentation or flotation. flocs formed by chemical means. Disinfection, though Flotation may be advantageous due to the low specific not the principal objective, was a desirable by-product. Due to the potential reuse of was~.ewater effluent in gravity of the solids. Although the most common flotation process is by the use of dissolved air, electroflo- Israel. mixture of the effluent with seawater was not tation, which is based on electrolysis of the solute to considered feasible, in contrast to some previous work. produce gas bubbles and induce some chemical reacMATERIALS AND METHODS tion, has attracted considerable interest in recent years. Electrochemical methods for the treatment of wasElectroflotation studies were carried out o n a labortewater have been proposed. Foyn (1964) used electro- atory and on a pilot-plant scale. chemical treatment to treat raw sewage by mixing it with seawater, passing the mixture through an electro- ( 1) Electroflotation studies on a labora tory scale An electrode assembly was built w~th graphite and lytic cell. The floeculation was attained by the formation of magnesium hydroxide at the cathode while flo- expanded iron, with a surface of 60 cm 2 and a distance tation of the flocculated matter was attained by the ris- between electrodes of 0.6 cm. The electrodes were connected to a rectifier, which produces a 12 V d.c. In ing of hydrogen bubbles produced during the process. Mendia (1958) used electrochemical treatment to pro- order to improve the current efficiency even in wasteduce chlorine and hypochlorite in a mixture of sea- water of low conductivity, a patented device was designed, and incorporated in the cell. (Since a patent water and sewage in order to disinfect the sewage before disposal into the sea. Marson (1967} described a simi- by the Technion Res. & Dev. Foundation is pending, lar method which was applied for treating the sewage its disclosure is limited). The complete cell was placed with ;1 60° angle in of the city of Guernsey (20,000 p). Electrolysis was performed on seawater in order to increase the electrical beakers containing 10 I. of: (a) tap water, (b) stabilizaconductance between the electrodes. The seawater was tion pond effluent. The experiments with tap water involved the deterlater mixed with sewage and then disposed of to the sea. Miller and Knipe (1965) treated secondary effluent mination of the rate of bubble formation and of chlorby an electrochemical method to reduce the number of ine production. These experiments were designed to 587 WR. ~ 9 - - ^
588
E.S.~-,to~,,'-/K. G SHLL~Fand V M. ~,~,~ ~ts
obtain intbrmation on the effect of changing the current density in the ele,atrodes on the production of gas bubbles and of chlorine in the v~ater. The ~,as bubbles were collected at the ,~urface of the liquid o~er the celt by immersing a funnd at a depth of 5 cm, and collecting the gases in a displacement syringe. The amperage and time needed for producing 10 cm 3 of gas were mea~;ured. The po~ver requirement per unit of volume of ga:~ were calculated from the data. In order to have an approximation of the amount of chlorine introduced into the liquid during the electroflotation process at different current densities, chlorine was determined bv the orthotolidine method on samples taken every. 2 rain. At the same time. chlorides were determined by potentiometric titration with Ag AgCI electrodes. Electroflotation experiments with stabilization pond elltuent were :timed at evaluating the possibility of using this process for the separation of unicellular algae from the pond effluents. Electrochemical flocculation of algae usin, g low Coulomb investment pro,,ed unsuccessful. Therefore in all further experiments, the algae were first flocculated with 200 mg 1- ~ aluminium sulphate at pH 6, and the flocs then flotated by electrolytically formed gases. Batch laboratc.ry experiments with 10 1. e(fluent were carried Otzt using two flocculation-flotation proccdures: (a) flocculation followed by elcctroflotation
Ib~ chemical flocculation and electroilotation performed simuhancousI~. In both experiments floccula. tion has been perlbrmed m t~vo steps: one minute retention at SI}re,, rain a rotor ~elocitx followed b3 10 rain at rotor velocity Of 30 re~ rain ~. The removal of algae ~as measured in terms of the percentage of transmittance in a photocolorimeter at 420 **m. using samples of algae taken at l-min inter,,als. Concentrations of total nitrogen, total phosphate, suspended solids and chlorides ~ere also determined.
I21 Pilot pkmr ~c~tlv c.'cpt'ril~eplt~ Continuous electroflotation was performed at the site of the stabilization ponds at Tirat Hacarmel (see Fig. 11 and the algal ponds at the Technion Environmental Engineering Laboratories (TEELJ. using two electroflotation units o f a capacit? of 600 and 1000 I. respectively. Both electroflotation units had a slow mixing zone where the Itocs were Ibrmed and a nonturbulcnt zone for flotation. The electrodes were composed of a graphite anode and an exp;,nded iron cathode spaced 0.5 cm apart, with an effective area of 600 cm'. A continuous flow of ettluent was maintained by pumping directly from the algal pond. The unit operating at the Tirat Hacarmel pond was connected to a generator supplying direct current of 8 A and t2 V, with energy input of 0'2 k w h m -3 and
Fig. I. Pilot plant electroflotation unit.
Improved eleetroflotation methods
589
Table 1. Effects of changing electrode current density on the production of gas bubbles and on power requirements
Current lrm,~)
Voltage (V)
Current density (mA cm- ")
140 400 600 3200
11"5 11'5 11'5 9"2
2'2 6'3 9"5 50"0
Time needed for producing 10 cm 3 gas (min)
Volume of gas produced per cm" of electrode (era3 cm-: rain- ')
55 5 3"5 0'75
0-003 0'041 0-047 0-263
current density of 16-6 m A c m - - ' . Since oxygen concentrations in the pond during daytime were at supersaturation, experiments were performed at 3 p.m. and another at 9 p.m. The latter flotation was due exclusively to electroflotation, since no gas bubbles resulting from photosynthesis would be produced at night-time. Volatile suspended solids, total N and Total P were determined on each sample using Standard Methods (1964) procedures. The percentage of transmittance was measured with a photocolorimeter at 420/~m. In the unit at the TEEL pond, a rectifier working at 9 A and 15 V was used, with current density of 15 mA cm--" and energy input of 135 Wh m -~.
RESULTS AND DISCUSSION Results of the laboratory scale experiments with the improved electroflotation cell treating tap water are shown in Table 1 which summarizes the effect of ehaning the electrodes current density on the production of gas bubbles. The results show that the time required to produce a given volume of gas may be reduced from 55 to 3.5 rain and even to 0.75 rain, according to the current density applied. The power requirement could be reduced from 462 to 84 C 1-1 for the same gas production. By increasing the current density, the power requirement could be reduced from 146 to 30--40 Wh I- z of gas produced. Using tap-water for electrofiotation created some difficulties such as scaling by carbonates on the cathode and the disintegration of the graphite anode at high current densities. The first problem was solved in the experiments with wastewater effluents as shown below, and the latter problem required the introduction of lead dioxide electrodes in further experimental runs. Part of the gas given offat the electrodes was chlorine. Although the purpose of electroflotation was to produce gas bubbles for flotation, the amount of chlorine produced at different current densities was also of interest. The concentration of chlorine determined in
Power requirements expressed as: C l I l of gas Wh I- t ofgas 46,200 9600 12,600 14,400
146 38"3 40-3 38"0
samples drawn at intervals from the electroflotation cell at various current densities are given in Fig. 2. In order to obtain concentrations of 3--4 ppm chlorine in the water the electrodes had to be operated for 6, 8 and 12 rain with current densities of 50, 10 and 6 mA cm-2 respectively. At 10 mA cm--', 30,800 C were used per 1. The amount of chlorine produced (3-4 mg 1- t) at a current density of 10 mA cm--" can provide only partial disinfection of the effluent. The Coulomb requirement to produce adequate flotation of algal flocs is only one third of that needed to produce 3-4 mg I- t of chlorine. Nevertheless, the value of chlorine produced during electroflotation should be taken into consideration since it reduces chlorine demand at later stages. The concentrations of chlorides in the treated tap water, never exceeded 250 mg 1- t starting from an initial figure of approx. 200 mg 1-t, thus allowing the reuse of the final effluent for irrigation purposes. The effect of increasing the electrodes current density on the removal of algal flocs from pond effluent by
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Fig. 2. Effect of current density on the concentration of chlorine in tap water.
590
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20
22
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Fig. 4. Effect of performing flocculation and etectrotlotation simultaneously or consecutively.
60
0 ~l 4 6 8 iO I :l 14 16 Time of sampling after beginning of electrolysis, rnin
Fig. 3. Effect of current density on the removal of algae from pond effluent. electroflotation, is shown in Fig. 3. The electroflotation cell had to be operated for either I l, 8 or 4 min at 6, 10 and 15 mA cm--' respectively in order to obtain the same removal of algae. Removal of nitrogen at a current density of 15 mA cm--" was 61 per cent compared with the untreated pond eMuent. The st, linity measured by electric conductivity increased from 500 to 600-630 ti T3c m - t No carbonate deposition occurred at the cathode. This is due to the low pH levels maintained during the floccukttion with aluminium sulphate. The results of flocculation followed consecutively by electroflotation compared with a simultaneous floccuhition-electroflotation are given in Fig. 4. Simultaneous flocculation and electroftotation was completed in 10 min with a power requirement of 33 C 1-
and an energy requirement of I00 Wh m-3. Consecutive flocculation and flotation required a larger retention time, thus increasing the size of the flotator. It required I I min for the flocculation step and I1 additional minutes for the flotation step. The quality of the effluent obtained with both treatments is given in Table 2. shows that the removal efficiency of algae was the same with both methods of treatment. In order to explain the action that occurs during electroflotation it is suggested that the bubbles produced by electrolysis are entrapped inside the floc while it is growing. This is due to the small size of the bubble and the fact that no turbulence is produced during bubble formation. It is also suggested that part of the bubbles adheres to the surface of the flocculation. In the case of dissolved air flotation, the release of a liquid-air mixture which then releases the bubbles is followed by the creation of turbulence which might interfere with the formation of the floc. The results of the pilot experiments at Tirat Hacarreel stabilization ponds are given in Table 3. Experiments were carried out twice a day, one under aerobic conditions at the pond accompanied with oxygen
Table 2. Effluent quality treated by separate flocculation followed by electroflotation as compared toa simultaneous floceulation-electroflotation Stabilization pond effluent Percentage of transmittance Volatile suspended solids (rag I- ~) Total N (rag l- t) Total PO.~ (rag I- ~) Chlorides (rag [- 1)
33°,,0 306 133 41.7 230
Effluent obtained by floceulation followed by electroflotation
Effluent obtained by simultaneous flocculation and electroflotation
82~
83°0
-56 7 255
146 58 8.8 230
Improved electroflotation methods
591
Table 3. Effluent quality obtained by electroflotation of pond effluent at two levels of dissolved oxygen
Stabilization pond effluent (mg 1- t) Volatile susp. solids Total N Total PO, oo Transmittance
277 57.5 23.2 15%
Daytime electroflotation of pond effluent containing oxygen at supersaturation (mg 1- t) o; Removal 31 28.5 2-5 8133o
supersaturation levels and the other with no detectable dissolved oxygen at the pond. The effect of the bubbles produced photosynthetically on electroflotation of algae is shown by the difference in the removal efficiencies. The removal efflciencies were lower at night, and the effluent obtained was far from satisfactory. This shows that the power input would have to be increased at night or whenever photosynthetic activity of algae was insufficient to produce oxygen supersaturation. A comparison of the results of the experiments with pilot plant scale electroflotation and dissolved air flotation using the TEEL pond effluents is shown in Table 4. It is seen that the removal percentage and the removal efftciencies were higher with the dissolved air flotation. This can be attributed to the fact the applied power input of 135 Wh m -3 in the electroflotation experiment was too small and should have been increased. The recirculation of about 30 per cent of clear effluent back in the dissolved air flotation unit also contributed to the higher removal efficiencies. The removal efficiencies of the electroflotation could have been much higher had the power input been increased.
Night-time electroflotation of pond effluent ~vith no detectable dissolved oxygen (mg 1- ~1 "o Removal
88 50-3 89-2 --
56 31.7 66 76° o
79 44.9 7 i" i --
Calculation shows that the volume of the gas needed to attain the same removal efflciencies was much higher with dissolved air flotation than with electroflotation. Expressed as the power equivalent, electroflotation required between 200-600 Wh m-3 compared with 886 Wh m-3 which would have been needed in order to supply electrochemically the amount of gas needed for dissolved air flotation. This is due to the fact that at 30 per cent recirculation of the flow in dissolved air flotation, at 3 atm pressure, the gas requirement are 22 1. ofair m -3 flow Vablik (1959). This indicates that the bubbles of gas produced by electrofloration have better adherence characteristics than those produced by dissolved air flotation. Recirculation of 30 per cent of the clear effluent, pressurized at 3 atm, which is needed to carry the airliquid mixture whenever dissolved air flotation is used, contributed also to the increased power requirements as than those needed for electroflotation. The deposition of carbonates at the cathode which was observed in the experiments with tap-water was not observed in the flocculation-flotatio.n process since the pH was maintained at levels below 6. The short life
Table 4. Treatment efflciencies obtained by electroflotation as compared to those obtained by dissolved air flotation Raw sewage pH BOD (mg 1- t) COD (Mg I- tj Total N (mg 1- t) NH3-N (mg 1" t) Total phosphate (mg 1- t) Volatile suspended solids (rag 1- t) Fixed suspended solids (mg I- ') Optical density (at 420/am)
Effluent of TEEL high rate pond
Effluent from dissolved air flotation unit
Effluent from electroflotation unit
7.8 380 970 69 33
8.4 99.5 628'0 59.9 21-4
6.5 19.5 181.0 30.3 23.t
6-9 21"2 202'0 34.2 22"1
50
43.6
6.6
9.1
410
309-0
68.0
8 I-0
--
46-0
22"0
30"0
--
0.580
0-144
0.170
592
E, SANDBANK.G. SHELEFand A. M. WACHS
of the g a p h i t e anode can be overcome by using a lead dioxide-titanium anode. The lack of information concerning the life of the various components made it difficult to accurately cost electroflotation and so as compare costs with other flotation methods. The patented improvements in the electroflotation cell introduced in the Technion required much less power than the cells used by Foyn (19641. Mendia et al. (1958) and Miller and Knipe ( 19641.
Acknowledgement--This work was partially supported by a Research Grant of the Israeli National Council for Research and Development.
REFERENCES
Foyn E. (19641 Removal of sewage nutrients b? electrol? tic treatment. V'erh. lnterat. Verein. Limnol IX 569-579. Mendia L. and Duonincontro E. I I958~ Stufio preliminare nell trattamento etecttrochimico die liquami. Sixth European Seminary on Sanitary Eng. Nice, (19581. Marson H. W. {1967) Electrolytic methods in modern sev,age treatment. Effl. War Treat. J. 7, 7i-73, 7~77 (I967). Miller H. C. and Knipe W. (1964) Electrochemical treatment of municipal wastewater. U.S. Pub. Health Service. Pub. No. 999. WP-19. End'. Health Sercice A N T R 13 (1965). Standard methods for the examination of water and wastewater. A.P.A., A.U.W.A,. WP.C.F. 13th Edition. American Public Health Ass. (1971~. Vrablik E. R. (1959) Fundamental principles of dissolved air flotation of industrial waters. Proc, 14th Ind. Wa,stes Cm!f Purdue Univ. Ext. Serv. 104, 743 (19591.
IMPROVED ELECTROFLOTATION FOR THE REMOVAL OF SUSPENDED SOLIDS FROM ALGAL POND EFFLUENTS E. SAXlmA~. G. SHEL~Fand A. M. WACHS Environmental Engineering Laboratories, Technion-lsrael Institute of Technology Israel (Receired 13 February 1973: i, ret'ised tbrm 13 Noremher 1973)
Abstract--Experiments with a patented modification of an electroflotation cell showed that the power requirements for the treatment of pond effluents were less than for other electroflotation methods. The relationship between current densities and removal efflcienciesof algae in suspension was fo~.nd in laboratory and pilot-scale experiments. It was found that simultaneous flocculation-electroflotation gives better results than consecutive operation. The results obtained with electroflotation even with low current densities were almost equal to dissolved air flotation. It is believed that better adherence of electrolytically produced gases to the floc is responsible for the superiority of electroflotation.
In the treatment of wastewater in stabilization ponds bacteria and organic constituents by means of the elecor in high rate ponds, where the effluent contains con- trolysis alone. This method required a large power incenmttions of algae var.ving from ItX) to 400 mg I.- t. put. Unlike these applications of electrochemical treatthe removal of algae is essential in order to reduce the concentration of nutrients and of suspended solids ment. the Objective of the work described in this paper was to produce the maximum amount of homogenous whenever high quality effluent is required. Separation of suspended solids is most common by bubbles by electrochemical means for the flotation of flocculation followed by sedimentation or flotation. flocs formed by chemical means. Disinfection, though Flotation may be advantageous due to the low specific not the principal objective, was a desirable by-product. Due to the potential reuse of was~.ewater effluent in gravity of the solids. Although the most common flotation process is by the use of dissolved air, electroflo- Israel. mixture of the effluent with seawater was not tation, which is based on electrolysis of the solute to considered feasible, in contrast to some previous work. produce gas bubbles and induce some chemical reacMATERIALS AND METHODS tion, has attracted considerable interest in recent years. Electrochemical methods for the treatment of wasElectroflotation studies were carried out o n a labortewater have been proposed. Foyn (1964) used electro- atory and on a pilot-plant scale. chemical treatment to treat raw sewage by mixing it with seawater, passing the mixture through an electro- ( 1) Electroflotation studies on a labora tory scale An electrode assembly was built w~th graphite and lytic cell. The floeculation was attained by the formation of magnesium hydroxide at the cathode while flo- expanded iron, with a surface of 60 cm 2 and a distance tation of the flocculated matter was attained by the ris- between electrodes of 0.6 cm. The electrodes were connected to a rectifier, which produces a 12 V d.c. In ing of hydrogen bubbles produced during the process. Mendia (1958) used electrochemical treatment to pro- order to improve the current efficiency even in wasteduce chlorine and hypochlorite in a mixture of sea- water of low conductivity, a patented device was designed, and incorporated in the cell. (Since a patent water and sewage in order to disinfect the sewage before disposal into the sea. Marson (1967} described a simi- by the Technion Res. & Dev. Foundation is pending, lar method which was applied for treating the sewage its disclosure is limited). The complete cell was placed with ;1 60° angle in of the city of Guernsey (20,000 p). Electrolysis was performed on seawater in order to increase the electrical beakers containing 10 I. of: (a) tap water, (b) stabilizaconductance between the electrodes. The seawater was tion pond effluent. The experiments with tap water involved the deterlater mixed with sewage and then disposed of to the sea. Miller and Knipe (1965) treated secondary effluent mination of the rate of bubble formation and of chlorby an electrochemical method to reduce the number of ine production. These experiments were designed to 587 WR. ~ 9 - - ^
588
E.S.~-,to~,,'-/K. G SHLL~Fand V M. ~,~,~ ~ts
obtain intbrmation on the effect of changing the current density in the ele,atrodes on the production of gas bubbles and of chlorine in the v~ater. The ~,as bubbles were collected at the ,~urface of the liquid o~er the celt by immersing a funnd at a depth of 5 cm, and collecting the gases in a displacement syringe. The amperage and time needed for producing 10 cm 3 of gas were mea~;ured. The po~ver requirement per unit of volume of ga:~ were calculated from the data. In order to have an approximation of the amount of chlorine introduced into the liquid during the electroflotation process at different current densities, chlorine was determined bv the orthotolidine method on samples taken every. 2 rain. At the same time. chlorides were determined by potentiometric titration with Ag AgCI electrodes. Electroflotation experiments with stabilization pond elltuent were :timed at evaluating the possibility of using this process for the separation of unicellular algae from the pond effluents. Electrochemical flocculation of algae usin, g low Coulomb investment pro,,ed unsuccessful. Therefore in all further experiments, the algae were first flocculated with 200 mg 1- ~ aluminium sulphate at pH 6, and the flocs then flotated by electrolytically formed gases. Batch laboratc.ry experiments with 10 1. e(fluent were carried Otzt using two flocculation-flotation proccdures: (a) flocculation followed by elcctroflotation
Ib~ chemical flocculation and electroilotation performed simuhancousI~. In both experiments floccula. tion has been perlbrmed m t~vo steps: one minute retention at SI}re,, rain a rotor ~elocitx followed b3 10 rain at rotor velocity Of 30 re~ rain ~. The removal of algae ~as measured in terms of the percentage of transmittance in a photocolorimeter at 420 **m. using samples of algae taken at l-min inter,,als. Concentrations of total nitrogen, total phosphate, suspended solids and chlorides ~ere also determined.
I21 Pilot pkmr ~c~tlv c.'cpt'ril~eplt~ Continuous electroflotation was performed at the site of the stabilization ponds at Tirat Hacarmel (see Fig. 11 and the algal ponds at the Technion Environmental Engineering Laboratories (TEELJ. using two electroflotation units o f a capacit? of 600 and 1000 I. respectively. Both electroflotation units had a slow mixing zone where the Itocs were Ibrmed and a nonturbulcnt zone for flotation. The electrodes were composed of a graphite anode and an exp;,nded iron cathode spaced 0.5 cm apart, with an effective area of 600 cm'. A continuous flow of ettluent was maintained by pumping directly from the algal pond. The unit operating at the Tirat Hacarmel pond was connected to a generator supplying direct current of 8 A and t2 V, with energy input of 0'2 k w h m -3 and
Fig. I. Pilot plant electroflotation unit.
Improved eleetroflotation methods
589
Table 1. Effects of changing electrode current density on the production of gas bubbles and on power requirements
Current lrm,~)
Voltage (V)
Current density (mA cm- ")
140 400 600 3200
11"5 11'5 11'5 9"2
2'2 6'3 9"5 50"0
Time needed for producing 10 cm 3 gas (min)
Volume of gas produced per cm" of electrode (era3 cm-: rain- ')
55 5 3"5 0'75
0-003 0'041 0-047 0-263
current density of 16-6 m A c m - - ' . Since oxygen concentrations in the pond during daytime were at supersaturation, experiments were performed at 3 p.m. and another at 9 p.m. The latter flotation was due exclusively to electroflotation, since no gas bubbles resulting from photosynthesis would be produced at night-time. Volatile suspended solids, total N and Total P were determined on each sample using Standard Methods (1964) procedures. The percentage of transmittance was measured with a photocolorimeter at 420/~m. In the unit at the TEEL pond, a rectifier working at 9 A and 15 V was used, with current density of 15 mA cm--" and energy input of 135 Wh m -~.
RESULTS AND DISCUSSION Results of the laboratory scale experiments with the improved electroflotation cell treating tap water are shown in Table 1 which summarizes the effect of ehaning the electrodes current density on the production of gas bubbles. The results show that the time required to produce a given volume of gas may be reduced from 55 to 3.5 rain and even to 0.75 rain, according to the current density applied. The power requirement could be reduced from 462 to 84 C 1-1 for the same gas production. By increasing the current density, the power requirement could be reduced from 146 to 30--40 Wh I- z of gas produced. Using tap-water for electrofiotation created some difficulties such as scaling by carbonates on the cathode and the disintegration of the graphite anode at high current densities. The first problem was solved in the experiments with wastewater effluents as shown below, and the latter problem required the introduction of lead dioxide electrodes in further experimental runs. Part of the gas given offat the electrodes was chlorine. Although the purpose of electroflotation was to produce gas bubbles for flotation, the amount of chlorine produced at different current densities was also of interest. The concentration of chlorine determined in
Power requirements expressed as: C l I l of gas Wh I- t ofgas 46,200 9600 12,600 14,400
146 38"3 40-3 38"0
samples drawn at intervals from the electroflotation cell at various current densities are given in Fig. 2. In order to obtain concentrations of 3--4 ppm chlorine in the water the electrodes had to be operated for 6, 8 and 12 rain with current densities of 50, 10 and 6 mA cm-2 respectively. At 10 mA cm--', 30,800 C were used per 1. The amount of chlorine produced (3-4 mg 1- t) at a current density of 10 mA cm--" can provide only partial disinfection of the effluent. The Coulomb requirement to produce adequate flotation of algal flocs is only one third of that needed to produce 3-4 mg I- t of chlorine. Nevertheless, the value of chlorine produced during electroflotation should be taken into consideration since it reduces chlorine demand at later stages. The concentrations of chlorides in the treated tap water, never exceeded 250 mg 1- t starting from an initial figure of approx. 200 mg 1-t, thus allowing the reuse of the final effluent for irrigation purposes. The effect of increasing the electrodes current density on the removal of algal flocs from pond effluent by
l/
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/
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Tim
I 4
I 6
I e
I io
I t2
I la
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18
of sampiinq after beginninq of etectr~ysis, rain
Fig. 2. Effect of current density on the concentration of chlorine in tap water.
590
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rnin
Fig. 4. Effect of performing flocculation and etectrotlotation simultaneously or consecutively.
60
0 ~l 4 6 8 iO I :l 14 16 Time of sampling after beginning of electrolysis, rnin
Fig. 3. Effect of current density on the removal of algae from pond effluent. electroflotation, is shown in Fig. 3. The electroflotation cell had to be operated for either I l, 8 or 4 min at 6, 10 and 15 mA cm--' respectively in order to obtain the same removal of algae. Removal of nitrogen at a current density of 15 mA cm--" was 61 per cent compared with the untreated pond eMuent. The st, linity measured by electric conductivity increased from 500 to 600-630 ti T3c m - t No carbonate deposition occurred at the cathode. This is due to the low pH levels maintained during the floccukttion with aluminium sulphate. The results of flocculation followed consecutively by electroflotation compared with a simultaneous floccuhition-electroflotation are given in Fig. 4. Simultaneous flocculation and electroftotation was completed in 10 min with a power requirement of 33 C 1-
and an energy requirement of I00 Wh m-3. Consecutive flocculation and flotation required a larger retention time, thus increasing the size of the flotator. It required I I min for the flocculation step and I1 additional minutes for the flotation step. The quality of the effluent obtained with both treatments is given in Table 2. shows that the removal efficiency of algae was the same with both methods of treatment. In order to explain the action that occurs during electroflotation it is suggested that the bubbles produced by electrolysis are entrapped inside the floc while it is growing. This is due to the small size of the bubble and the fact that no turbulence is produced during bubble formation. It is also suggested that part of the bubbles adheres to the surface of the flocculation. In the case of dissolved air flotation, the release of a liquid-air mixture which then releases the bubbles is followed by the creation of turbulence which might interfere with the formation of the floc. The results of the pilot experiments at Tirat Hacarreel stabilization ponds are given in Table 3. Experiments were carried out twice a day, one under aerobic conditions at the pond accompanied with oxygen
Table 2. Effluent quality treated by separate flocculation followed by electroflotation as compared toa simultaneous floceulation-electroflotation Stabilization pond effluent Percentage of transmittance Volatile suspended solids (rag I- ~) Total N (rag l- t) Total PO.~ (rag I- ~) Chlorides (rag [- 1)
33°,,0 306 133 41.7 230
Effluent obtained by floceulation followed by electroflotation
Effluent obtained by simultaneous flocculation and electroflotation
82~
83°0
-56 7 255
146 58 8.8 230
Improved electroflotation methods
591
Table 3. Effluent quality obtained by electroflotation of pond effluent at two levels of dissolved oxygen
Stabilization pond effluent (mg 1- t) Volatile susp. solids Total N Total PO, oo Transmittance
277 57.5 23.2 15%
Daytime electroflotation of pond effluent containing oxygen at supersaturation (mg 1- t) o; Removal 31 28.5 2-5 8133o
supersaturation levels and the other with no detectable dissolved oxygen at the pond. The effect of the bubbles produced photosynthetically on electroflotation of algae is shown by the difference in the removal efficiencies. The removal efflciencies were lower at night, and the effluent obtained was far from satisfactory. This shows that the power input would have to be increased at night or whenever photosynthetic activity of algae was insufficient to produce oxygen supersaturation. A comparison of the results of the experiments with pilot plant scale electroflotation and dissolved air flotation using the TEEL pond effluents is shown in Table 4. It is seen that the removal percentage and the removal efftciencies were higher with the dissolved air flotation. This can be attributed to the fact the applied power input of 135 Wh m -3 in the electroflotation experiment was too small and should have been increased. The recirculation of about 30 per cent of clear effluent back in the dissolved air flotation unit also contributed to the higher removal efficiencies. The removal efficiencies of the electroflotation could have been much higher had the power input been increased.
Night-time electroflotation of pond effluent ~vith no detectable dissolved oxygen (mg 1- ~1 "o Removal
88 50-3 89-2 --
56 31.7 66 76° o
79 44.9 7 i" i --
Calculation shows that the volume of the gas needed to attain the same removal efflciencies was much higher with dissolved air flotation than with electroflotation. Expressed as the power equivalent, electroflotation required between 200-600 Wh m-3 compared with 886 Wh m-3 which would have been needed in order to supply electrochemically the amount of gas needed for dissolved air flotation. This is due to the fact that at 30 per cent recirculation of the flow in dissolved air flotation, at 3 atm pressure, the gas requirement are 22 1. ofair m -3 flow Vablik (1959). This indicates that the bubbles of gas produced by electrofloration have better adherence characteristics than those produced by dissolved air flotation. Recirculation of 30 per cent of the clear effluent, pressurized at 3 atm, which is needed to carry the airliquid mixture whenever dissolved air flotation is used, contributed also to the increased power requirements as than those needed for electroflotation. The deposition of carbonates at the cathode which was observed in the experiments with tap-water was not observed in the flocculation-flotatio.n process since the pH was maintained at levels below 6. The short life
Table 4. Treatment efflciencies obtained by electroflotation as compared to those obtained by dissolved air flotation Raw sewage pH BOD (mg 1- t) COD (Mg I- tj Total N (mg 1- t) NH3-N (mg 1" t) Total phosphate (mg 1- t) Volatile suspended solids (rag 1- t) Fixed suspended solids (mg I- ') Optical density (at 420/am)
Effluent of TEEL high rate pond
Effluent from dissolved air flotation unit
Effluent from electroflotation unit
7.8 380 970 69 33
8.4 99.5 628'0 59.9 21-4
6.5 19.5 181.0 30.3 23.t
6-9 21"2 202'0 34.2 22"1
50
43.6
6.6
9.1
410
309-0
68.0
8 I-0
--
46-0
22"0
30"0
--
0.580
0-144
0.170
592
E, SANDBANK.G. SHELEFand A. M. WACHS
of the g a p h i t e anode can be overcome by using a lead dioxide-titanium anode. The lack of information concerning the life of the various components made it difficult to accurately cost electroflotation and so as compare costs with other flotation methods. The patented improvements in the electroflotation cell introduced in the Technion required much less power than the cells used by Foyn (19641. Mendia et al. (1958) and Miller and Knipe ( 19641.
Acknowledgement--This work was partially supported by a Research Grant of the Israeli National Council for Research and Development.
REFERENCES
Foyn E. (19641 Removal of sewage nutrients b? electrol? tic treatment. V'erh. lnterat. Verein. Limnol IX 569-579. Mendia L. and Duonincontro E. I I958~ Stufio preliminare nell trattamento etecttrochimico die liquami. Sixth European Seminary on Sanitary Eng. Nice, (19581. Marson H. W. {1967) Electrolytic methods in modern sev,age treatment. Effl. War Treat. J. 7, 7i-73, 7~77 (I967). Miller H. C. and Knipe W. (1964) Electrochemical treatment of municipal wastewater. U.S. Pub. Health Service. Pub. No. 999. WP-19. End'. Health Sercice A N T R 13 (1965). Standard methods for the examination of water and wastewater. A.P.A., A.U.W.A,. WP.C.F. 13th Edition. American Public Health Ass. (1971~. Vrablik E. R. (1959) Fundamental principles of dissolved air flotation of industrial waters. Proc, 14th Ind. Wa,stes Cm!f Purdue Univ. Ext. Serv. 104, 743 (19591.