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Urban wastewater treatment by electrocoagulation and flotation
e
Pergam on
War. Sci. Tech . Vol. 3 1, No . 3-4 , pp, 275- 283, 199 5. Cop yright © 19951A WQ Printed in Greal Britain. AUrights reserved. 0273-1223195S9·50 + 0'00
0273- 1223(95)00230-8
URBAN WASTEWATER TREATMENT BY ELECTROCOAGULATION AND FLOTATION M.-F. Pouet* and A. Grasmick** * Laboratoire de Genie de l'Environnement Industriel, Ecole des Mines d'Ales, 30319 Ales Cedex, France ** Laboratoire des Materiaux et Precedes Membranair es, case 024. UM Il, 34095 Montp ellier Cedex 5. France
ABSTRACf In waste water treatment, the use of a physico-chemical process by flotation wo uld present some advantages compared to a separa tion by settling. However like each physico-chem ical process, a separa tion by flota tion needs a chemi cal destabrlization. We have studied the use of an electrochemical destab ilizati on coupled to a process of flotation (OAF). Th is paper presents the results obtained on an urban was te wa ter treated by electroco agulation and dissolved air flotation (DAF). To show the intere st of coupling flotati on and electrocoagulation. we have studied each process separate ly. Then we have combined the two processes. The role of each operation on pollution removal is presented. An effect of synergism betw een the two processes on the polluti on abatem en t is shown. A reducti on of 75% of the global COD is obtained. The resul ts of the coupling are compared (0 the performance of an inten sive treatment by flocculation-lamellar settler.
KEYWORDS Flotation; electrocoagulation; urban waste water; coagulation; lamellar settling; s ludge. INTROD UCTION In waste water treatment, the use of a physico-chemical process by flotation would present some advantages compared to separation by settling: - higher upflow velociti es i.e. shorter hydraulic retention times - better removal of smaller particles However like each physico -chemical processes. separati on by flotation needs chemical destabilization. Classical coagulation-floccu lation needs chemicals added in a mixing chamber where energy shares does not allow an optimal coagulat ion. According to the results of some auth ors (Vik et al 1984, Gard ias, 199 1), the process of electr ocoagulation is a good alternativ e because it can remove the disadvantages of the classical chemical destabilization. Electrocoagulation is a process which consists of creating a floc of metallic hydroxides within the effluent to be purified , by electrodissolution of soluble anode s. Comp ared with traditional flocculation coagulation, electrocoagulation has. in theory, the advantage of removing the smallest colloidal particles: the smallest charged particles have a greater probability of being coagulated because of the electric field that sets them in motion (Persin and Rumeau, 1989). According to Vik et at 275
M.-F. POUET and A. GRASMICK
276
(l984) it has also the advantage of producing a relatively low amount of sludge. This paper presents the results obtained on an urban waste water treated by electrocoagulation and dissolved air flotation (DAF). To show the interest of coupling flotation and electrocoagulation, we have studied each process separately. Them we combined the two processes. The role of each operation on pollution removal is presented. The results of the coupling are compared to those obtained by chemical coagulation-lamellar settling. MATERIALS AND METHODS Description of the pilot The suspension is pre-treated with an aeroflotation unit combined with electrocoagulation. The a.3l8m 3 pilot can treat a flow of Im 3lhour. Its characteristics are given in Table 1. Table l. Characteristics of Coagulation-Flotation Pilot
Aeroflotation umt
Electrocoagulation
Parameters Height Horizontal section Recirculation rate Pressurization Retention time Electrode type Number Anode surface Voltage Amperage
Characteristics 1,45 m 0,25 m 2 1OA30% 4 bars 19rnn Alurnnuum 21 0,15 m 2 0-80V 0-40V
[S) (4)
(3)
(7)
Treated waltr Q
(2) Tr eat ed waler . q r=IUJO% Q.
(9)
[1)
Higb pressure pump
Raw-water
Air
I. Strainer 2. Diffusion chamber 3. Aluminium two - poles electrodes 4. Stabilized power supply (40 A et 80 V) 5. Separation chamber 7. Treated water 8. Recirculation of treated water
9.DAF Fig. I. Diagram of flow of the pilot
(8)
277
Electrocoagulation and flotation
Raw sewage is pumped [1] after the grit settling chamber and is introduced into the electrocoagulation cell above the diffusion chamber [2]. Raw waste water undergoes electrochemical destabilization thanks to an electrocoagulation device [3] placed at the entrance of the cell. A direct current imposed by a stabilized power supply (40 A-80 V) is applied to the terminal electrodes, so we worked with a series of two-pole electrodes [4]. The separation of treated water and sludge is operated in the separation chamber [5]. Sludges are scraped on the top of the flotator [6]. The effluent is evacuated at the base of the pilot [7]. A part of the treated water is recirculated [8], pressurized and saturated with air [9] and introduced into the cell [2]. The decompression it undergoes there releases fine gas bubbles that adhere to the particles and cause them to float: it is the process of Dissolved Air flotation (OAF). The OAF process is improved by the release of gas (oxygen and hydrogen) caused by electrolysis of the water and principally by the generation of flocculates of aluminium hydroxides that trap the colloidal and supracolloidal pollution and form a floc. Analytical analyses The water is characterized before and after treatment. The characterization consists of determining the granulometric size distribution of the organic pollution. The suspension will be described also using global pollution criteria, namely: - The Chemical Oxygen Demand (COD), in accordance with the AFNOR NF 90-101 standard; - Suspended Solids (SS), measured in accordance with the AFNOR NF 90-105 standard. Values are given with a 10% margin of error. There parameters will be determined for samples that have undergone: - no treatment; - Two-hours settling (decantation) in an Imhoff tanks (ad 2 sample); - electrocoagulation-flotation treatment; - filtration on I urn, 0.05 urn and 0.01 urn calibrated cellulose membranes. Table 2. Characteristics of Water C.O.D (mg02/1)
(mg/!)
Raw wastewater
900
j40
Settleable fraction
2lS.0
70.0
~.M
(%)
Fraction ad 2 > 1 urn ...
28.2
(%) = supracolloidal fraction
Fraction < 0.2 urn
43.8
(%)= colloidal + soluble
fraction ... ad 2 - after a decantation of 2 hours RESULTS Characteristics of water The tests were carried out on municipal waste water from the town of Meze. The average characteristics of the water are shown in Table 2.
278
M.-F. POVET and A. GRASMICK
Characteristics of raw sewage are comparable to those given in the literature (Levine et al., 1985; Degrern ont , 1989). PQllutiQn abatement by DAF Figure 2 presents the role of DAF alone on the removal of supracolloidal and colloidal < I urn) particles. We observe that the fraction rem oved by flotation is increased by the transfer of the soluble and colloidal fractio ns which are remo ved with the settleable fraction during flotation. %/CODof raw water
Treatment by notation: R
=20 %
100 80
60
•
o
Raw water Treated water
40
COD < \11Jll
COD ad2 > \ I1Jll
Seu.COD
Fig. 2. Waste water treatm ent by DAF witho ut elec trocoagulation.
The removal of colloids by DAF is not negligible: indeed about 40% of the fine co lloids and more than 20% of the supracolloidal fraction are removed. Flotation has thus an impact on the very fine particles. In practice these colloids are not destabilized but would be floated or trapped with the removed particles. As has been observed by Janssens and Buekens (1993), flotation in water treatment plants is not effective on the removal of the finest colloids. We could explain the effici ency observed by the presence of surfacta nts in sewage. Surfactants have a structure which can change the surface charge of colloids and bubbles and thus improve the attachment of bubbles particles. The detergents are molecule s composed of a hydrophile or polar part and a hydrophobe or apolar part. They can adsorb on the liqu id-gas, liquid-liquid or liquid-solid interface s and they decrease the interface energy (Falletti, 1990). Hyd rophobicity of particles is an impo rtant point of flotation (Janssens and Buekens, 1993). The presence of detergents whose concentrations can reach 10 mg/I in urban waste water (Brunner et al, 1988) could increa se the hydrophibicity of colloid s and expla in the results observed. Influence Qf electrocoa~ulatiQn Electrocoagulation causes a destabilization of suspension and a s hift in granulometrix distribution toward s larger diameters is observed (figure 3). There is a reduction of the non-settle able fraction. This redu ction produces a corresponding increase of the settleable fraction. The se paration, without DAF between liquid and sludges is obtained preferentially by settling, but a part of the sludge is floated by the bubbles produced by a electrolysis of the water.
Electrocoagulation and flotation
%/COD of raw water
279
Treatment by electrocoagulation : I = 8 A
100
80 •
60
o
Raw water Treated water
40
20
o
COD < l
um
Fractions CODad2 > 1 IlJIl
SetLCOD
Fig. 3. Granulometric size distribution of COD before and after electrocoagulation treatment (without DAF).
By comparison between figures 2 and 3, we can notice that electrocoagulation increases the removal of the supracolloidal fraction; 65% of the colloidal, supracolloidal and soluble pollution is removed. Couplint; electrocoat;ulation - DAF: influence of the NS ratio and the current intensity I. Role of the AlS ratio It is shown that the NS ratio (defined by the quantity of air introduced the quantity of particles caused to float) is one of the main factors of flotation; the efficiency of the flotation pilot is an increasing function of the NS ratio. To control the role of the ratio NS, the recycle rate R of the water treated is tested, with a constant flow and a current intensity of 6A. It is very difficult to measure the value of S, so this parameter is taken close to the SS value of the raw water. The value A is calculated according to Henry's law and considering that only 60% is dissolved in this kind of process (Degrernont, 1989). The Table 3 gives the percentage of pressurized water, average values of SS in raw sewage, and the corresponding NS ratio. Tests have been done with an intensity of 6A, with a flow of I m 3/h. Table 3. NS Ratio Versus the Recycle Rate R (%Qe)
S.S (mg/l) A/S
0 214 0
15 255 26,4
20 278 31,5
(~p
= 4 Bars)
30 240 51,6
Granulometric distribution of COD have been measured by determining the settleable and non-settleable fractions. The non-settleable fraction is thus composed of the colloidal, supracolloidal and soluble fractions. Figure 4 represents the percentage of the non-settleable fractions in raw waste water and in water treated according to the NS ratio.
280
M.-F. POUET and A. GRASMICK
No - settleable fraction (% I COD of raw water) }oo
------0-
Raw water Treated water
60 40
20
Fig. 4. Evolution of the non-settleable fraction versus the NS ration.
For low amounts of air, electrocoagulation does not allow a good separation of the organic fractions by flotation. The flow is settled and is evacuated with the water treated. From an NS ratio of 31.5 cm 3/g (i.e. a recycle rate of 20%), we observe a better separation between the floc and the effluent, in this case the nonsettleable (ad2) decreases and the settleable fraction increased. Increasing the NS ratio to 51.6 cm 3/g does not improve significantly the separation. We thus notice that: - for NS ::;; 31.5 cm 3/g: the separation of liquid and solid is preferent ially obtained by settling; - for NS ~ 3 1.5 cm3/g: the separation of liquid and solid is preferentially obtained by flotation; In our operating conditi ons, for an initial concentration of SS of 250 mg/l, the optimal air flow corresponds to an NS ratio of 31.5 cm 3/g i.e. recycle rate of 20%. No settleable organic fraction in treated water (% I COD of raw water) 100 - ~
V 80 -V 60 / 40
" 20
-
ff-
~
o
'-
Applied current intensity (A) Fig. 5. Non -settl eable frac tion in treated water for eac h intensity tested (qr = 20% Qe)
281
Electrocoagulation and flotation
Role of the current intensity
To demonstrate the influence of the current intensity applied, we have measured the transfer of the nonsettleable fraction (ad2) into a settleable fraction for current intensity values varying from 0 to 10 A. The formation of microscopic bubbles of gas on electrodes and the formation of aluminium hydroxides are directly proportional of the strength of the current applied according to Faraday's Law (Glasstone, 1962). Figure 5 shows that the transfer of colloidal and supracolloidal particles into settleable fractions is an increasing function of the current intensity applied up to an optimal value of 8 A (Pouet et al, 1993). The results show that there is an optimum current strength of approximately 8 A. this is the value at which maximum conversion of the colloidal fraction into a decantable fraction occurs. Study of couDling Figure (, presents the evolution of the granulometric size distributions during the coupling by electrocoagulation-flotation during optimal conditions. %/CODof raw water
Electrocoagulation. flotation Operating conditions 1=8 A q air=20% Qe
100 80
•
60
o
Raw water Treated water
40
20
o
Fractions COD < I um
COD ad2 >11UJl
CODseu.
Fig. 6. Granulometric size distribution before and after treatment by electrocoagulation - OAF
This illustration allows us to note the following. - 30% of the soluble fraction smaller than I 11m is removed. This pollution abatement corresponds to that observed with DAF alone. - 80% of the supra colloidal fraction is obtained, this value can be compared to the 20% removed by flotation alone and to the 65% by electrocoagulation alone. The coupling allows us to increase significantly the abatement of this fraction. CONCLUSIONS These results are compared to the performance of an intensive treatment by flocculation-lamellar settler. The tests were carried on sewage. A precoagulation with FeCl3 (10 to 30 mg/l) occurred at the entrance of the plant. Then, the water was treated by flocculation to aluminium sulphate coupled with a lamellar settler. Comparative results are obtained in relation to:
M.-F. POUET and A. GRASMICK
282
- quality of water treated ; - rate of treatment applied and in particular in relation to sludge production. Ouality of the water treated The quality of water treated is presented in table 4. Table 4. Quality of the Water Treated by Chemical or Electrochem ical Destabilization Waters of
Montpellier
waters of
Meze
Water treated
coagulation • lamellar setttln II
Waler treated
electrocoagulation • notation
Water treated t,3 2U- 5FfU 20A Ome:/1 162 mg/! max = 165 min = 130 53m10. max =90 min = 12
Raw sewage 7,7 254 NTU 364 620mg/!
109m~
325 mg/!
by
Parameters
COD raw
Raw sewage 8,1 321 t'TU 385 765 mg/!
COD> 1 j.UI1
357 mgll
COD < 1 j.UI1
120 mgll
pH Turbidity
S.S
by
Water treated lS,U 20NTU 37 mz/l 179 mg/! max = 312 min =91 23 mg/! max=6O min e O 160 mg/! max = 265 min =70
152 mgll
max = 155 min = 65 6O-7U
NH4-N orthoP.
-
< detection limit
-
Concerning the global parameters of pollution, COD, SS and Turbidity, the qualit y of the two waters were the same. 100% of orthoph osph ates which represent about 80% of total phosphorus were removed. Soluble nitrogen pollution was not rem oved. We have observed an increase of pH for the water of Meze which can be explained by the production OH-ions during the electrolysis of the cathode. On the other hand during classical coagulation-flocculatio n by aluminium sulphate. an acidification of the water treated was noted. The value of the supracolloi dal fraction remains higher in the case of the water of Montpellier, electrocoagulation could allow a better removal of this fraction. T he abatement of the finest colloids is better with the process of flocculation-lamellar settling. This can be explained by: - the chemical destabilization in two steps (feCl 3 at the entrance of the Montpellier sewage treatment plant then with the aluminium sulphate during our test); - the presence of a sludge bed at the base of the settler that can improve the catching of the finest colloids, Table 5 gives the average value s of pollution abatement accordin g to the pollution loads measured: Table 5. Pollution Abatement by Each Treatment Tested Pollution abatement (%) suspen solids lUI ltV C
water treated by electrocoagulation • notation 85 9U 75
~
Water treated by coagulation • lamellar settling '3 5
Il
Il Il
~O
o
Il
Electrocoagulation and flotation
283
Pollution abatement obtained with treatment by coagulation-lamellar settling was higher than obtained by electrocoagulation-flotation. We can explain this result by the fact that the value of the pollution load of Montpellier sewage was more important. We can note that the pollution abatement obtained with electrocoagulation is comparable to that given in the literature (Gardias, 1991; Deltour, 1988). Aluminium dose in solution In our coagulation-flocculation experiments, the optimal dose of coagulant is obtained for concentrations of aluminium sulphate of 500-700 mg/l i.e. about 45-63 mg/l of Aluminium AI. In electrocoagulation tests, the optimal intensity is 8 A which corresponds to a theoretical aluminium quantity of 54 mgAl/1. But it has been shown that the faradic yield does not exceed 50% (Daniel, 1992). So, in practice, the aluminium dose put in solution during electrolysis run would by only 27 mg/l, i.e. half as much as the dose necessary in chemical coagulation. Characteristics of the sludges produced The production of sludges is directly proportional to: - the characteristics of raw sewage, settleable solids and matters destabilized by coagulation; - the concentrations of flocculants. Chemical reagent consumption was twice as low in the process of electrocoagulation, so dry solids production was also slightly smaller in this process. But it was shown that sludges produced by OAF are twice as concentrated (2 to 4%, Zabel, 1985) as those obtained by gravity settling. So the process of electrocoagulation allows us to reduce the dry solids production but above all the sludge flow rate. In all cases, the sludges were not stabilized, they contained about 75% of organic matter. ACKNOWLEDGEMENT We acknowledge Mr Jeremy Josephs who has kindly read this paper and corrected our English. REFERENCES Brunner, P. H., Capri, S., Marcomini, A. and Giger, W. (1988). Occurence and behaviour of linear alkyl benzene sulfonates. nonylphenyl, nonylphenol mono and nonylphenol diethooxylates in sewage and sewage sludge treatment. Wat. Res. 22(3), 1465-1472. Daniel, B. (1992). Etude de l'electrocoagulation sur des eaux usees urbaines, Rapport d'avant, project d'Ingeniorat, ISIM, Universite de Montpellier II. Degremont (1989). Memento technique de l'eau", Edite par Lavoisier. FalIethi, F. (1990). Etude de Ia regeneration par microfiltration tangentielle sur membranes minerales des fluides de coupe ageux et des solutions aqueuses de degraissage. These de Doctorat, Universitede Montpellier, 25 Octobre. Gardais, D. (1991). Electricite et Environnernent, Electra DOPEE. Glasstone, S. (1962). An introduction to Electrochemistry, Tenth printing Edition Revolucionaria. Janssens, J. G. and Buekens, A. (1993). Assessment of process selection for particle removal in surface water treatment. J. Water Supply Research and Technology, AQUA, 42(5), 279-288. Levine, A. D., Tchobanoglous, G. and Asano, T. (1985). Characterization of the size distribition of contaminants in wastewater: treatment and reuse implications. Journal WPCF, 57(7). Persin, F. and Rumeau, M. (1989). Le traitement electrochirnique des eaux et des effluents. Tribune de l'eau, 3(42),45-46. Pouet, M.-F., Grasmick, A. and Persin, F. (1993). Combined electrocoagulation and flotation: a pretreatment for tangential flow microfiltration. European Water Filtration Congress. 13-16 March. in Proc, European Water Filtration Congress. Ostend, Vol. 17. pp. 3.27-3.37, ed. Royal Flemish Society of Engineers, ISBN. 90-5204-020-6. Vik, E.l., Carlson, D. A., Eikum, A. S. and Gjessing, E. T. (1984). Electrocoagulation of potable water. Wat. Res., 18, 1355-1360. Zabel, T. F. (1985). The advantages of Dissolved Air Flotation for water treatment. JA WWA, May. pp 42-46.
Pergam on
War. Sci. Tech . Vol. 3 1, No . 3-4 , pp, 275- 283, 199 5. Cop yright © 19951A WQ Printed in Greal Britain. AUrights reserved. 0273-1223195S9·50 + 0'00
0273- 1223(95)00230-8
URBAN WASTEWATER TREATMENT BY ELECTROCOAGULATION AND FLOTATION M.-F. Pouet* and A. Grasmick** * Laboratoire de Genie de l'Environnement Industriel, Ecole des Mines d'Ales, 30319 Ales Cedex, France ** Laboratoire des Materiaux et Precedes Membranair es, case 024. UM Il, 34095 Montp ellier Cedex 5. France
ABSTRACf In waste water treatment, the use of a physico-chemical process by flotation wo uld present some advantages compared to a separa tion by settling. However like each physico-chem ical process, a separa tion by flota tion needs a chemi cal destabrlization. We have studied the use of an electrochemical destab ilizati on coupled to a process of flotation (OAF). Th is paper presents the results obtained on an urban was te wa ter treated by electroco agulation and dissolved air flotation (DAF). To show the intere st of coupling flotati on and electrocoagulation. we have studied each process separate ly. Then we have combined the two processes. The role of each operation on pollution removal is presented. An effect of synergism betw een the two processes on the polluti on abatem en t is shown. A reducti on of 75% of the global COD is obtained. The resul ts of the coupling are compared (0 the performance of an inten sive treatment by flocculation-lamellar settler.
KEYWORDS Flotation; electrocoagulation; urban waste water; coagulation; lamellar settling; s ludge. INTROD UCTION In waste water treatment, the use of a physico-chemical process by flotation would present some advantages compared to separation by settling: - higher upflow velociti es i.e. shorter hydraulic retention times - better removal of smaller particles However like each physico -chemical processes. separati on by flotation needs chemical destabilization. Classical coagulation-floccu lation needs chemicals added in a mixing chamber where energy shares does not allow an optimal coagulat ion. According to the results of some auth ors (Vik et al 1984, Gard ias, 199 1), the process of electr ocoagulation is a good alternativ e because it can remove the disadvantages of the classical chemical destabilization. Electrocoagulation is a process which consists of creating a floc of metallic hydroxides within the effluent to be purified , by electrodissolution of soluble anode s. Comp ared with traditional flocculation coagulation, electrocoagulation has. in theory, the advantage of removing the smallest colloidal particles: the smallest charged particles have a greater probability of being coagulated because of the electric field that sets them in motion (Persin and Rumeau, 1989). According to Vik et at 275
M.-F. POUET and A. GRASMICK
276
(l984) it has also the advantage of producing a relatively low amount of sludge. This paper presents the results obtained on an urban waste water treated by electrocoagulation and dissolved air flotation (DAF). To show the interest of coupling flotation and electrocoagulation, we have studied each process separately. Them we combined the two processes. The role of each operation on pollution removal is presented. The results of the coupling are compared to those obtained by chemical coagulation-lamellar settling. MATERIALS AND METHODS Description of the pilot The suspension is pre-treated with an aeroflotation unit combined with electrocoagulation. The a.3l8m 3 pilot can treat a flow of Im 3lhour. Its characteristics are given in Table 1. Table l. Characteristics of Coagulation-Flotation Pilot
Aeroflotation umt
Electrocoagulation
Parameters Height Horizontal section Recirculation rate Pressurization Retention time Electrode type Number Anode surface Voltage Amperage
Characteristics 1,45 m 0,25 m 2 1OA30% 4 bars 19rnn Alurnnuum 21 0,15 m 2 0-80V 0-40V
[S) (4)
(3)
(7)
Treated waltr Q
(2) Tr eat ed waler . q r=IUJO% Q.
(9)
[1)
Higb pressure pump
Raw-water
Air
I. Strainer 2. Diffusion chamber 3. Aluminium two - poles electrodes 4. Stabilized power supply (40 A et 80 V) 5. Separation chamber 7. Treated water 8. Recirculation of treated water
9.DAF Fig. I. Diagram of flow of the pilot
(8)
277
Electrocoagulation and flotation
Raw sewage is pumped [1] after the grit settling chamber and is introduced into the electrocoagulation cell above the diffusion chamber [2]. Raw waste water undergoes electrochemical destabilization thanks to an electrocoagulation device [3] placed at the entrance of the cell. A direct current imposed by a stabilized power supply (40 A-80 V) is applied to the terminal electrodes, so we worked with a series of two-pole electrodes [4]. The separation of treated water and sludge is operated in the separation chamber [5]. Sludges are scraped on the top of the flotator [6]. The effluent is evacuated at the base of the pilot [7]. A part of the treated water is recirculated [8], pressurized and saturated with air [9] and introduced into the cell [2]. The decompression it undergoes there releases fine gas bubbles that adhere to the particles and cause them to float: it is the process of Dissolved Air flotation (OAF). The OAF process is improved by the release of gas (oxygen and hydrogen) caused by electrolysis of the water and principally by the generation of flocculates of aluminium hydroxides that trap the colloidal and supracolloidal pollution and form a floc. Analytical analyses The water is characterized before and after treatment. The characterization consists of determining the granulometric size distribution of the organic pollution. The suspension will be described also using global pollution criteria, namely: - The Chemical Oxygen Demand (COD), in accordance with the AFNOR NF 90-101 standard; - Suspended Solids (SS), measured in accordance with the AFNOR NF 90-105 standard. Values are given with a 10% margin of error. There parameters will be determined for samples that have undergone: - no treatment; - Two-hours settling (decantation) in an Imhoff tanks (ad 2 sample); - electrocoagulation-flotation treatment; - filtration on I urn, 0.05 urn and 0.01 urn calibrated cellulose membranes. Table 2. Characteristics of Water C.O.D (mg02/1)
(mg/!)
Raw wastewater
900
j40
Settleable fraction
2lS.0
70.0
~.M
(%)
Fraction ad 2 > 1 urn ...
28.2
(%) = supracolloidal fraction
Fraction < 0.2 urn
43.8
(%)= colloidal + soluble
fraction ... ad 2 - after a decantation of 2 hours RESULTS Characteristics of water The tests were carried out on municipal waste water from the town of Meze. The average characteristics of the water are shown in Table 2.
278
M.-F. POVET and A. GRASMICK
Characteristics of raw sewage are comparable to those given in the literature (Levine et al., 1985; Degrern ont , 1989). PQllutiQn abatement by DAF Figure 2 presents the role of DAF alone on the removal of supracolloidal and colloidal < I urn) particles. We observe that the fraction rem oved by flotation is increased by the transfer of the soluble and colloidal fractio ns which are remo ved with the settleable fraction during flotation. %/CODof raw water
Treatment by notation: R
=20 %
100 80
60
•
o
Raw water Treated water
40
COD < \11Jll
COD ad2 > \ I1Jll
Seu.COD
Fig. 2. Waste water treatm ent by DAF witho ut elec trocoagulation.
The removal of colloids by DAF is not negligible: indeed about 40% of the fine co lloids and more than 20% of the supracolloidal fraction are removed. Flotation has thus an impact on the very fine particles. In practice these colloids are not destabilized but would be floated or trapped with the removed particles. As has been observed by Janssens and Buekens (1993), flotation in water treatment plants is not effective on the removal of the finest colloids. We could explain the effici ency observed by the presence of surfacta nts in sewage. Surfactants have a structure which can change the surface charge of colloids and bubbles and thus improve the attachment of bubbles particles. The detergents are molecule s composed of a hydrophile or polar part and a hydrophobe or apolar part. They can adsorb on the liqu id-gas, liquid-liquid or liquid-solid interface s and they decrease the interface energy (Falletti, 1990). Hyd rophobicity of particles is an impo rtant point of flotation (Janssens and Buekens, 1993). The presence of detergents whose concentrations can reach 10 mg/I in urban waste water (Brunner et al, 1988) could increa se the hydrophibicity of colloid s and expla in the results observed. Influence Qf electrocoa~ulatiQn Electrocoagulation causes a destabilization of suspension and a s hift in granulometrix distribution toward s larger diameters is observed (figure 3). There is a reduction of the non-settle able fraction. This redu ction produces a corresponding increase of the settleable fraction. The se paration, without DAF between liquid and sludges is obtained preferentially by settling, but a part of the sludge is floated by the bubbles produced by a electrolysis of the water.
Electrocoagulation and flotation
%/COD of raw water
279
Treatment by electrocoagulation : I = 8 A
100
80 •
60
o
Raw water Treated water
40
20
o
COD < l
um
Fractions CODad2 > 1 IlJIl
SetLCOD
Fig. 3. Granulometric size distribution of COD before and after electrocoagulation treatment (without DAF).
By comparison between figures 2 and 3, we can notice that electrocoagulation increases the removal of the supracolloidal fraction; 65% of the colloidal, supracolloidal and soluble pollution is removed. Couplint; electrocoat;ulation - DAF: influence of the NS ratio and the current intensity I. Role of the AlS ratio It is shown that the NS ratio (defined by the quantity of air introduced the quantity of particles caused to float) is one of the main factors of flotation; the efficiency of the flotation pilot is an increasing function of the NS ratio. To control the role of the ratio NS, the recycle rate R of the water treated is tested, with a constant flow and a current intensity of 6A. It is very difficult to measure the value of S, so this parameter is taken close to the SS value of the raw water. The value A is calculated according to Henry's law and considering that only 60% is dissolved in this kind of process (Degrernont, 1989). The Table 3 gives the percentage of pressurized water, average values of SS in raw sewage, and the corresponding NS ratio. Tests have been done with an intensity of 6A, with a flow of I m 3/h. Table 3. NS Ratio Versus the Recycle Rate R (%Qe)
S.S (mg/l) A/S
0 214 0
15 255 26,4
20 278 31,5
(~p
= 4 Bars)
30 240 51,6
Granulometric distribution of COD have been measured by determining the settleable and non-settleable fractions. The non-settleable fraction is thus composed of the colloidal, supracolloidal and soluble fractions. Figure 4 represents the percentage of the non-settleable fractions in raw waste water and in water treated according to the NS ratio.
280
M.-F. POUET and A. GRASMICK
No - settleable fraction (% I COD of raw water) }oo
------0-
Raw water Treated water
60 40
20
Fig. 4. Evolution of the non-settleable fraction versus the NS ration.
For low amounts of air, electrocoagulation does not allow a good separation of the organic fractions by flotation. The flow is settled and is evacuated with the water treated. From an NS ratio of 31.5 cm 3/g (i.e. a recycle rate of 20%), we observe a better separation between the floc and the effluent, in this case the nonsettleable (ad2) decreases and the settleable fraction increased. Increasing the NS ratio to 51.6 cm 3/g does not improve significantly the separation. We thus notice that: - for NS ::;; 31.5 cm 3/g: the separation of liquid and solid is preferent ially obtained by settling; - for NS ~ 3 1.5 cm3/g: the separation of liquid and solid is preferentially obtained by flotation; In our operating conditi ons, for an initial concentration of SS of 250 mg/l, the optimal air flow corresponds to an NS ratio of 31.5 cm 3/g i.e. recycle rate of 20%. No settleable organic fraction in treated water (% I COD of raw water) 100 - ~
V 80 -V 60 / 40
" 20
-
ff-
~
o
'-
Applied current intensity (A) Fig. 5. Non -settl eable frac tion in treated water for eac h intensity tested (qr = 20% Qe)
281
Electrocoagulation and flotation
Role of the current intensity
To demonstrate the influence of the current intensity applied, we have measured the transfer of the nonsettleable fraction (ad2) into a settleable fraction for current intensity values varying from 0 to 10 A. The formation of microscopic bubbles of gas on electrodes and the formation of aluminium hydroxides are directly proportional of the strength of the current applied according to Faraday's Law (Glasstone, 1962). Figure 5 shows that the transfer of colloidal and supracolloidal particles into settleable fractions is an increasing function of the current intensity applied up to an optimal value of 8 A (Pouet et al, 1993). The results show that there is an optimum current strength of approximately 8 A. this is the value at which maximum conversion of the colloidal fraction into a decantable fraction occurs. Study of couDling Figure (, presents the evolution of the granulometric size distributions during the coupling by electrocoagulation-flotation during optimal conditions. %/CODof raw water
Electrocoagulation. flotation Operating conditions 1=8 A q air=20% Qe
100 80
•
60
o
Raw water Treated water
40
20
o
Fractions COD < I um
COD ad2 >11UJl
CODseu.
Fig. 6. Granulometric size distribution before and after treatment by electrocoagulation - OAF
This illustration allows us to note the following. - 30% of the soluble fraction smaller than I 11m is removed. This pollution abatement corresponds to that observed with DAF alone. - 80% of the supra colloidal fraction is obtained, this value can be compared to the 20% removed by flotation alone and to the 65% by electrocoagulation alone. The coupling allows us to increase significantly the abatement of this fraction. CONCLUSIONS These results are compared to the performance of an intensive treatment by flocculation-lamellar settler. The tests were carried on sewage. A precoagulation with FeCl3 (10 to 30 mg/l) occurred at the entrance of the plant. Then, the water was treated by flocculation to aluminium sulphate coupled with a lamellar settler. Comparative results are obtained in relation to:
M.-F. POUET and A. GRASMICK
282
- quality of water treated ; - rate of treatment applied and in particular in relation to sludge production. Ouality of the water treated The quality of water treated is presented in table 4. Table 4. Quality of the Water Treated by Chemical or Electrochem ical Destabilization Waters of
Montpellier
waters of
Meze
Water treated
coagulation • lamellar setttln II
Waler treated
electrocoagulation • notation
Water treated t,3 2U- 5FfU 20A Ome:/1 162 mg/! max = 165 min = 130 53m10. max =90 min = 12
Raw sewage 7,7 254 NTU 364 620mg/!
109m~
325 mg/!
by
Parameters
COD raw
Raw sewage 8,1 321 t'TU 385 765 mg/!
COD> 1 j.UI1
357 mgll
COD < 1 j.UI1
120 mgll
pH Turbidity
S.S
by
Water treated lS,U 20NTU 37 mz/l 179 mg/! max = 312 min =91 23 mg/! max=6O min e O 160 mg/! max = 265 min =70
152 mgll
max = 155 min = 65 6O-7U
NH4-N orthoP.
-
< detection limit
-
Concerning the global parameters of pollution, COD, SS and Turbidity, the qualit y of the two waters were the same. 100% of orthoph osph ates which represent about 80% of total phosphorus were removed. Soluble nitrogen pollution was not rem oved. We have observed an increase of pH for the water of Meze which can be explained by the production OH-ions during the electrolysis of the cathode. On the other hand during classical coagulation-flocculatio n by aluminium sulphate. an acidification of the water treated was noted. The value of the supracolloi dal fraction remains higher in the case of the water of Montpellier, electrocoagulation could allow a better removal of this fraction. T he abatement of the finest colloids is better with the process of flocculation-lamellar settling. This can be explained by: - the chemical destabilization in two steps (feCl 3 at the entrance of the Montpellier sewage treatment plant then with the aluminium sulphate during our test); - the presence of a sludge bed at the base of the settler that can improve the catching of the finest colloids, Table 5 gives the average value s of pollution abatement accordin g to the pollution loads measured: Table 5. Pollution Abatement by Each Treatment Tested Pollution abatement (%) suspen solids lUI ltV C
water treated by electrocoagulation • notation 85 9U 75
~
Water treated by coagulation • lamellar settling '3 5
Il
Il Il
~O
o
Il
Electrocoagulation and flotation
283
Pollution abatement obtained with treatment by coagulation-lamellar settling was higher than obtained by electrocoagulation-flotation. We can explain this result by the fact that the value of the pollution load of Montpellier sewage was more important. We can note that the pollution abatement obtained with electrocoagulation is comparable to that given in the literature (Gardias, 1991; Deltour, 1988). Aluminium dose in solution In our coagulation-flocculation experiments, the optimal dose of coagulant is obtained for concentrations of aluminium sulphate of 500-700 mg/l i.e. about 45-63 mg/l of Aluminium AI. In electrocoagulation tests, the optimal intensity is 8 A which corresponds to a theoretical aluminium quantity of 54 mgAl/1. But it has been shown that the faradic yield does not exceed 50% (Daniel, 1992). So, in practice, the aluminium dose put in solution during electrolysis run would by only 27 mg/l, i.e. half as much as the dose necessary in chemical coagulation. Characteristics of the sludges produced The production of sludges is directly proportional to: - the characteristics of raw sewage, settleable solids and matters destabilized by coagulation; - the concentrations of flocculants. Chemical reagent consumption was twice as low in the process of electrocoagulation, so dry solids production was also slightly smaller in this process. But it was shown that sludges produced by OAF are twice as concentrated (2 to 4%, Zabel, 1985) as those obtained by gravity settling. So the process of electrocoagulation allows us to reduce the dry solids production but above all the sludge flow rate. In all cases, the sludges were not stabilized, they contained about 75% of organic matter. ACKNOWLEDGEMENT We acknowledge Mr Jeremy Josephs who has kindly read this paper and corrected our English. REFERENCES Brunner, P. H., Capri, S., Marcomini, A. and Giger, W. (1988). Occurence and behaviour of linear alkyl benzene sulfonates. nonylphenyl, nonylphenol mono and nonylphenol diethooxylates in sewage and sewage sludge treatment. Wat. Res. 22(3), 1465-1472. Daniel, B. (1992). Etude de l'electrocoagulation sur des eaux usees urbaines, Rapport d'avant, project d'Ingeniorat, ISIM, Universite de Montpellier II. Degremont (1989). Memento technique de l'eau", Edite par Lavoisier. FalIethi, F. (1990). Etude de Ia regeneration par microfiltration tangentielle sur membranes minerales des fluides de coupe ageux et des solutions aqueuses de degraissage. These de Doctorat, Universitede Montpellier, 25 Octobre. Gardais, D. (1991). Electricite et Environnernent, Electra DOPEE. Glasstone, S. (1962). An introduction to Electrochemistry, Tenth printing Edition Revolucionaria. Janssens, J. G. and Buekens, A. (1993). Assessment of process selection for particle removal in surface water treatment. J. Water Supply Research and Technology, AQUA, 42(5), 279-288. Levine, A. D., Tchobanoglous, G. and Asano, T. (1985). Characterization of the size distribition of contaminants in wastewater: treatment and reuse implications. Journal WPCF, 57(7). Persin, F. and Rumeau, M. (1989). Le traitement electrochirnique des eaux et des effluents. Tribune de l'eau, 3(42),45-46. Pouet, M.-F., Grasmick, A. and Persin, F. (1993). Combined electrocoagulation and flotation: a pretreatment for tangential flow microfiltration. European Water Filtration Congress. 13-16 March. in Proc, European Water Filtration Congress. Ostend, Vol. 17. pp. 3.27-3.37, ed. Royal Flemish Society of Engineers, ISBN. 90-5204-020-6. Vik, E.l., Carlson, D. A., Eikum, A. S. and Gjessing, E. T. (1984). Electrocoagulation of potable water. Wat. Res., 18, 1355-1360. Zabel, T. F. (1985). The advantages of Dissolved Air Flotation for water treatment. JA WWA, May. pp 42-46.