- Email: [email protected]
Treatment of wastewaters of paper industry by coagulation–electroflotation
Desalination 208 (2007) 34–41
Treatment of wastewaters of paper industry by coagulation–electroflotation L. Ben Mansour, I. Ksentini, B. Elleuch Laboratory of Water–Environment and Energy, Sciences Faculty of Sfax, B.P.802, 3018 Sfax, Tunisia Tel. +216 (98) 657 061; Fax: +216 (74) 451 346; email: [email protected] Received 18 November 2005; Accepted 15 April 2006
Abstract Treatment of paper effluent samples by coagulation–electroflotation was studied. The study aims to determine the best conditions of treatment which maximize the purification efficiency. For that, the methodology of experimental research with an orthogonal central composite design was adopted. Batch and continuous modes of treatment were studied. The batch mode aims to optimize coagulant concentration, pH and current density at fixed treatment duration. The current density is the most significant factor in the suspended solids elimination followed in decreasing order of importance by coagulant concentration and pH. Good agreement between theoretical analysis and experimental results was obtained. Continuous mode aims to optimize the residence time. Purification efficiency consisting in the elimination of suspended solids exceeded 95%. Physicochemical characterization of the effluent was done before and after the treatment in batch and continuous mode to improve the efficiency of this process. This characterization included pH, conductivity, salinity, BOD5, COD, suspended solids and chlorides. Keywords: Paper effluent; Optimisation; Coagulation; Electroflotation
1. Introduction The production of paper started two thousand years ago in China and was considered as a secret activity until XII century when it was developed. Nowadays, paper is produced from wood and the variation of the proportion of its components makes possible the obtaining of a large variety of *Corresponding author.
paper quality. The production of paper gets through several stages like slashing, boiling in a solution of sodium hydroxide, dilution and washing. Such stages operate with a large quantity of water and hence quantities of wastewater are generally important. This waste water contains generally large quantities of suspended solids; it also contains considerable amount of COD and BOD. Conventional physicochemical treatment of
0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.04.072
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
paper effluent consists of classic pretreatment such as coagulation /flocculation, sedimentation and sludge handling. Secondary and tertiary treatment such biological filters or activated sludge can be also practised. Unfortunately, the existing process is not very efficient if the effluent contains low density of colloidal suspended solids. In case of paper effluent, flotation can be efficient. In fact, separation by flotation is relatively simple and faster compared to coagulation and sedimentation. Matis and co-workers [1] have studied the effectiveness of different flotation techniques and they have noted that for wastewaters which contain colloidal particles, fine bubbles produced by electroflotation can very effectively float them. Janssen and Koene [2] have shown that electrochemical techniques are one of the competitive and interesting technologies in this field. Electroflotation techniques are highly versatile and competitive with settling tank techniques that require much land. Indeed, this process is also more competitive than other flotation techniques such as dissolved air flotation and dispersed air flotation [3,4]. So, electroflotation can be selected as a process for treatment of these effluents. In fact, electroflotation which supports the electrolysis of a light quantity of water due to a passage of electric current between an anode and a cathode is a process which produces fine oxygen and hydrogen bubbles as shown in the following reactions: C Anodic oxidation: 2 H2O
O2 + 4 H+ + 4e!
(1)
C Cathodic reduction: 4 H2O + 4e!
2 H2 + 4 OH! (2)
This process is complex because of its dependence on several factors. Indeed, the current density influences directly the number and size of
35
bubbles [5]. In fact, Chen [6] has demonstrated that current density and mass of bubbles produced are proportional. The pH is also a parameter which influences the mechanism of electroflotation owing to the fact that the hydrogen bubbles are the smallest with neutral pH and for the oxygen bubbles their sizes increase with pH [5]. Other than pH and current density there are several other parameters which affect this process such as the state, the arrangement of the electrodes [7], the nature of water to be treated and processing duration. The objective of this work is to optimize the treatment of hydrous effluents from paper industry by coagulation– electroflotation. The parameter of follow-up is the quantity of suspended solids. This study includes the two modes: continuous mode and batch mode. The parameters to be optimized in batch mode are: current density, pH and coagulant concentration. For the study in continuous mode, the parameter to be optimized is the processing time. In fact, these parameters are most influential on the process of electroflotation [8]. Considering the number of parameters, the scientific tool for analysis is the methodology of experiment planning which minimizes the number of experiments and makes possible the obtaining of a regression equation linking the purification efficiency with the parameters of study.
2. Materials and methods 2.1. Materials 2.1.1. Effluent characteristics Used waters, object of this treatment are hydrous effluents resulting from Tunisian company paper industry. These effluents are taken in a way to respect the standards of sampling and are stored in the cold storage during experiment days. A physicochemical characterization of the effluents is carried out before the treatment. Three types of effluent are treated:
36
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
C Effluent A: resulting from the unit of sodation, it is a unit in which a neutralization of the excess of chloride by a soda solution is carried out at a temperature of 50EC. C Effluent B: resulting from the unit of washing. C Effluent C: it is a mixture of the two effluents taken in a way which respects the standards of rejections. Table 1 presents the physicochemical characterization of these effluents in their crude states: Table 1 Characteristics of effluent samples before treatment Parameter
Effluent A Effluent B
Effluent C
Colour Decanting solids [ml/l]
Yellow 25
Dark yellow Yellow 30 30
pH Conductivity [mS/cm] Salinity [g/l] Chlorides Cl[mg/l] COD [mgO2/l] BOD5 [mgO2/l] Suspended solids [mg/l] Turbidity [FTU]
7.4 1.9
6.6 59
6.8 3.6
0.8 479
3.1 1917
1.8 1491
684 100 240
2575 300 880
1416 200 400
556
504
364
We note that these effluents present a high rate of COD, BOD5 and a considerable rate of suspended solids which makes effective an eventual treatment by coagulation–electroflotation. 2.1.2. Electroflotation cell The electroflotation cell, shown in Fig. 1, is a base rectangular column. Its surface is 11.25 cm2 (2.5 cm × 4.5 cm) and its height is 90 cm. It is provided with two electrodes: titanium coated with ruthenium oxide anode and a stainless steel
Fig. 1. Schematic diagram of the electroflotation cell in batch mode. A, anode; C, cathode; E, sludge evacuation; R, treated water.
cathode. These two electrodes are supplied by a generator of DC current which makes possible the variation of current density in the electrodes. It is also noted that these two electrodes are at a distance of 8 millimetres. The cathode compared to the anode is perforated and occupies the top position. This perforation allows the evacuation of bubbles produced at the anode. It is also noted that this column has two outputs: the first one is for the evacuation of sludge and it is at the top of the column and the second one is for water after treatment and it is at the bottom. In continuous mode, the electroflotation unit shown in Fig. 2 has a total volume of 4.2 liters. It is divided into two compartments. Each compartment is provided with two electrodes. The first compartment receives the effluent from a tank of coagulation using a piston pump. The effluent then undergoes a primary treatment
37
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
Fig. 2. Schematic diagram of the electroflotation unit in continuous mode.
which lowers the suspended solids rate more than 70%. The bubbles formed at electrodes and effluent are in co-current movement. After that, the effluent gets through a second compartment by overflow and undergoes its final treatment. The bubbles formed at electrodes and effluent are now in counter current. The treated effluent is evacuated and the sludge formed is eliminated at the top of the column. It is noted that the pump is equipped with a variator of flow and this ensures the obtaining of a sufficient range of supply rate. The agitation of the storing tank is ensured by a mechanical agitator necessary to avoid a possible decantation of sludge. A diagram of the process is given in Fig 2. 2.2. Methods In batch mode, a volume of representative effluent is taken. The parameters to be optimized
Table 2 Intervals variation of parameters studied Parameter
Ccoag (mg/l)
pH
D (A/m²)
Zimin Zimax Zi0 ΔZi
200 1000 600 400
5 8 6.5 1.5
100 250 175 75
are current density, pH and coagulant concentration. The coagulant used is aluminium sulphate (Al2(SO4)3.18H2O). Preliminary tests were done to fix the intervals of variations of these variables. Table 2 gives these intervals. We note that the treatment duration is fixed at 20 min. We note also that during all experiments the voltmeter has shown that the voltage varied in the interval of 7–9 Volts due to the variation of current density and conductivity of the medium.
38
L. Ben Mansour et al. / Desalination 208 (2007) 34–41 Y = a 0 + [ a1 × X 1 + a 2 × X 2 + a 3 × X 3 ]
Table 3 Experiments matrix
[
2
2
2
+ a11 × X 1 + a 22 × X 2 + a 33 × X 3
Experiment
C = X1
pH = X2
D = X3
Y
1 2 3 4 5 6 7 8 9* 10 11 12 13 14 15 16* 17* 18*
1000 200 1000 200 1000 200 1000 200 600 600 600 600 600 1086 114 600 600 600
8 8 5 5 8 8 5 5 6.5 6.5 6.5 8.3 4.7 6.5 6.5 6.5 6.5 6.5
250 250 250 250 100 100 100 100 175 266.1 83.9 175 175 175 175 175 175 175
-
The methodology of experiments planning, with an orthogonal central composite design, requires in this case 14 experiments and tests in center following this experiment matrix [9,10] (Table 3). Y is the rate of purification (i.e., elimination of suspended solids) obtained after treatment, 9*, 16*, 17 * and 18 * are the tests in the center. Y (%) = (Suspended solids before treatment ! Suspended solidsafter treatment)/ Suspended solidsbefore treatment The exploitation of this matrix leads to obtaining a second order regression equation which connects the parameters with the rate of purification Y. This equation takes account of the interactions which can exist between these variables [9,10].
]
(3)
+ [ a12 × X 1 × X 2 + ... + a13 × X 1 × X 3 + a 23 × X 2 × X 3 ]
a0: constant term of the regression equation. ai (i 0 {1,2,3}): linear effect coefficients. aii (i 0{1,2,3}): quadratic effect coefficients. aij (i,j 0 {1,2,3,}): interaction effect coefficients. It is also noted that these coefficients were obtained by matrix algebra and two tests of validity were made: a first test of Student which validates each coefficient and a second test of Fisher which validates the global model.
3. Results and discussion 3.1. Treatment in batch mode In this part, the matrix of experiments planning is the principal support of study. Indeed, 18 experiments were done for each type of effluent; the results are summarized in Table 4. The regression equations obtained are presented in Table 5. The equations were obviously validated by Student and Fisher tests. The optimization of these equations using the method of Newton gives us the values of the variables of studies (C, pH and D) which maximize the purification rate Y. The values of these parameters after optimization are given in Table 6. So, we note that: all the first order terms exist, this confirms that pH, coagulant concentration and current density act directly on the suspended solids reduction. The terms of the second order of the regression equations exist; this explains that the optimum is in the fixed intervals. Other interactions coefficients exist: C An interaction exists between the coagulant concentration and pH; this is well explained by the acid character of the coagulant. C An interaction exists between the coagulant
39
L. Ben Mansour et al. / Desalination 208 (2007) 34–41 Table 4 Experiment results Experiment
C = X1
pH = X2
D = X3
YA
YB
YC
1 2 3 4 5 6 7 8 9* 10 11 12 13 14 15 16* 17* 18*
1000 200 1000 200 1000 200 1000 200 600 600 600 600 600 1086 114 600 600 600
8 8 5 5 8 8 5 5 6.5 6.5 6.5 8.3 4.7 6.5 6.5 6.5 6.5 6.5
250 250 250 250 100 100 100 100 175 266.1 83.9 175 175 175 175 175 175 175
87 79 85 77 84 78 79 86 89 93 87 90 90 92 88 91 92 87
82 53 67 78 70 79 90 64 90 86 75 77 86 80 88 92 90 90
89 72 82 81 83 82 87 82 94 91 81 84 90 88 88 95 93 92
Table 5 Coefficients of regression equations obtained Parameter
Effluent A
Effluent B
Effluent C
Constant C pH D C² pH² D² C× pH C× D
!25.5 4.3 10!2 25.1 0.2 !2.5 10!5 !1.7 !7.1 10!4 !3.4 10!3 5.6 10!5
!60.9 3.5 10!2 34.7 0.5 !3.5 10!5 !3 !1.3 10!3 !3.1 10!4 2.6 10!5
!16.1 3.3 10!2 23.7 0.3 !2.2 10!5 !1.8 !8.1 10!4 !2.3 10!3 4.6 10!5
concentration and current density. Indeed, the current density acts directly on number and shape of the gas bubbles. In fact, the number of bubbles increases when current density increases. So the probability of collision between these bubbles
Table 6 Theoretical values of optimized parameters and a comparison between theoretical and experimental purification rate
Effluent A Effluent B Effluent C
Ccoag [mg/l]
pH
D [A/m²]
Ythe (%)
Yexp (%)
639 534 626
7 6 6
195 195 201
95 96 96
95 96 95
and optimal coagulated floats increase. As a result, more particles will be eliminated. A physicochemical characterization was made in the optimal conditions. This characterization is summarized in Table 7. We note that not only the suspended solids have been reduced, but the COD, BOD5 and [Cl!] have been also reduced. This is explained by the oxidation of dissolved
40
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
Table 7 Characteristics of effluent samples after batch mode treatment in optimal conditions Parameter
Effluent A Effluent B
Effluent C
Colour Decanting solids [ml/l] pH Conductivity [mS/cm] Salinity [g/l] Chlorides Cl[mg/l] COD [mgO2/l] BOD5 [mgO2/l] Suspended solids [mg/l] Turbidity [FTU]
Clear <0.2
Clear <0.2
Clear <0.2
7.1 2.3
6 5.8
6.7 4
1 372.7
3 1517
2 1029
204 85 7.5
1297 150 42.5
604 100 16
23
146
51
organic substances due to the presence of both the fine oxygen bubbles and the chlorine as a strong oxidizing agent in the medium. The chlorine was the oxidation product of the chloride in the secondary electrochemical anodic reaction during electrolysis. 3.2. Treatment in continuous mode The objective of the treatment in continuous mode is to optimize the supply rate of the effluent to be treated (i.e. to optimize the residence time) which can lead to a possible extrapolation of the process on an industrial scale. Current density, pH and coagulant concentration are optimized in the batch mode. We propose to vary the supply flow and to note the corresponding purification rate. The results are given in Fig. 3. For effluent A, it is noted that the optimal flow maximizing the elimination of suspended solids is 15.5 l/h, which corresponds to a residence time tr = 16.26 min. For the mixture of the two effluents i.e. the effluent C, the optimal flow is
Fig. 3. Effect of effluent supply flow on the purification efficiency.
reached in 18 l/h which corresponds to residence time tr =14min. The corresponding purification efficiencies are 95.6% and 95.4%. We also find that for lower supply flows than those of the optimum, the purification rate is almost the same i.e. the bubbles flow produced by the electrodes was able to eliminate the maximum of suspended solids. While for higher supply flows, the last is too high than the bubble flow. This means that the suspended solids partially reside in the treated water outflow. A complete analysis of water which was treated in the optimum conditions was made (Table 8). Table 8 Characteristics of effluent samples after the continuous mode treatment in optimal conditions Parameter
Effluent A
Effluent C
Colour Decanting solids [ml/l] pH Conductivity [mS/cm] Salinity [g/l] Chlorides Cl! [mg/l] COD [mgO2/l] BOD5 [mgO2/l] Suspended solids [mg/l] Turbidity [FTU]
Clear <0.2 6.7 2 0.8 319.5 114 70 4 10
Clear <0.2 6.5 4.6 1.9 1266.2 48.9 55 18 4
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
These results show that the suspended solids, COD, BOD5 and [Cl!] have been considerably reduced which prove that the treatment of these effluents by coagulation—electroflotation is effective. We also note that purification efficiency is better in continuous mode compared to the batch mode since the continuous mode treatment was operated in an electroflotation unit with two compartments where the effluents undergo twostage treatment. In the first stage gas bubbles and effluents are in co-current flow and in the second stage they are in the counter current flow.
4. Conclusion The coagulation–electroflotation process was used to treat paper industry effluents which contain an important rate of suspended solids. The regression equations obtained was optimized to find the best conditions of treatment in batch mode: coagulant concentration, pH and current density are the parameters of studies. Physicochemical characterization was done before and after treatment. Clearly, the coagulation– electroflotation technique can be successfully selected as an efficient process to treat this type of effluent since it leads to a high purification efficiency. Tests of treatment in continuous mode were carried out successfully by determining the optimal residence time of the effluents to be treated. Suspended solids were reduced more than 95%.
Acknowledgements The authors are thankful to Professor S. Venkatachalam, Indian institute of Technology,
41
Dept. of Metallurgical Engineering and Materials Science, Bombay 400076, India for his technical language support; and to Professor V.A Kolesnikov, Director of Electrochemical Process Laboratory, D.I. Mendeleev, Russian Chemical Technology University, for his guidance and encouragement.
References [1] A.I. Zouboulis, K.A. Matis and G.A. Stadilis, in: P. Mavros and K.A. Matis, eds., Innovations in Flotation Technology, Proc. NATO Science Series, Greece, Kluwer Academic, The Netherlands, 1991. [2] L.J. Janssen and L. Koene, The role of electrochemistry and electrochemical technology in environmental protection. Chem. Eng. J., 85 (2002) 137–146. [3] S.E. Bums, S. Yiacoumi and C. Tsouris, Microbubble generation for environmental and industrial separations. Sep. Purif. Technol., 11 (1997) 221–232. [4] X. Gu and S.H. Chiang, A novel flotation column for oily water cleanup. Sep. Purif. Technol., 16 (1999) 193–203. [5] Y. Fukui and S. Yuu, Removal of colloidal particles in electroflotation, AIChE J., 31(2) (1985) 201–208. [6] G. Chen, Electrochemical technologies in waste water treatment. Sep. Purif. Technol., 38 (2004) 11–41. [7] C. Llerena, J.C.K. Ho and D.L. Piron, Effect of pH on electroflotation of sphalerite, Chem. Eng. Commun., 155 (1996) 217–228. [8] L. Ben Mansour, Y. Ben Abdou and S. Gabsi, Effects of some parameters on removal process of nickel by electroflotation, Water, Waste Environment Res., 2 (2001) 51–58. [9] V. Kafarov, Méthodes Cybernétiques et Technologie Chimique, Mir, Moscow, 1974. [10] J. Goupy, Plans d’expériences, Technique de l’Ingénieur, 230 (1997) 1–24.
Treatment of wastewaters of paper industry by coagulation–electroflotation L. Ben Mansour, I. Ksentini, B. Elleuch Laboratory of Water–Environment and Energy, Sciences Faculty of Sfax, B.P.802, 3018 Sfax, Tunisia Tel. +216 (98) 657 061; Fax: +216 (74) 451 346; email: [email protected] Received 18 November 2005; Accepted 15 April 2006
Abstract Treatment of paper effluent samples by coagulation–electroflotation was studied. The study aims to determine the best conditions of treatment which maximize the purification efficiency. For that, the methodology of experimental research with an orthogonal central composite design was adopted. Batch and continuous modes of treatment were studied. The batch mode aims to optimize coagulant concentration, pH and current density at fixed treatment duration. The current density is the most significant factor in the suspended solids elimination followed in decreasing order of importance by coagulant concentration and pH. Good agreement between theoretical analysis and experimental results was obtained. Continuous mode aims to optimize the residence time. Purification efficiency consisting in the elimination of suspended solids exceeded 95%. Physicochemical characterization of the effluent was done before and after the treatment in batch and continuous mode to improve the efficiency of this process. This characterization included pH, conductivity, salinity, BOD5, COD, suspended solids and chlorides. Keywords: Paper effluent; Optimisation; Coagulation; Electroflotation
1. Introduction The production of paper started two thousand years ago in China and was considered as a secret activity until XII century when it was developed. Nowadays, paper is produced from wood and the variation of the proportion of its components makes possible the obtaining of a large variety of *Corresponding author.
paper quality. The production of paper gets through several stages like slashing, boiling in a solution of sodium hydroxide, dilution and washing. Such stages operate with a large quantity of water and hence quantities of wastewater are generally important. This waste water contains generally large quantities of suspended solids; it also contains considerable amount of COD and BOD. Conventional physicochemical treatment of
0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.04.072
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
paper effluent consists of classic pretreatment such as coagulation /flocculation, sedimentation and sludge handling. Secondary and tertiary treatment such biological filters or activated sludge can be also practised. Unfortunately, the existing process is not very efficient if the effluent contains low density of colloidal suspended solids. In case of paper effluent, flotation can be efficient. In fact, separation by flotation is relatively simple and faster compared to coagulation and sedimentation. Matis and co-workers [1] have studied the effectiveness of different flotation techniques and they have noted that for wastewaters which contain colloidal particles, fine bubbles produced by electroflotation can very effectively float them. Janssen and Koene [2] have shown that electrochemical techniques are one of the competitive and interesting technologies in this field. Electroflotation techniques are highly versatile and competitive with settling tank techniques that require much land. Indeed, this process is also more competitive than other flotation techniques such as dissolved air flotation and dispersed air flotation [3,4]. So, electroflotation can be selected as a process for treatment of these effluents. In fact, electroflotation which supports the electrolysis of a light quantity of water due to a passage of electric current between an anode and a cathode is a process which produces fine oxygen and hydrogen bubbles as shown in the following reactions: C Anodic oxidation: 2 H2O
O2 + 4 H+ + 4e!
(1)
C Cathodic reduction: 4 H2O + 4e!
2 H2 + 4 OH! (2)
This process is complex because of its dependence on several factors. Indeed, the current density influences directly the number and size of
35
bubbles [5]. In fact, Chen [6] has demonstrated that current density and mass of bubbles produced are proportional. The pH is also a parameter which influences the mechanism of electroflotation owing to the fact that the hydrogen bubbles are the smallest with neutral pH and for the oxygen bubbles their sizes increase with pH [5]. Other than pH and current density there are several other parameters which affect this process such as the state, the arrangement of the electrodes [7], the nature of water to be treated and processing duration. The objective of this work is to optimize the treatment of hydrous effluents from paper industry by coagulation– electroflotation. The parameter of follow-up is the quantity of suspended solids. This study includes the two modes: continuous mode and batch mode. The parameters to be optimized in batch mode are: current density, pH and coagulant concentration. For the study in continuous mode, the parameter to be optimized is the processing time. In fact, these parameters are most influential on the process of electroflotation [8]. Considering the number of parameters, the scientific tool for analysis is the methodology of experiment planning which minimizes the number of experiments and makes possible the obtaining of a regression equation linking the purification efficiency with the parameters of study.
2. Materials and methods 2.1. Materials 2.1.1. Effluent characteristics Used waters, object of this treatment are hydrous effluents resulting from Tunisian company paper industry. These effluents are taken in a way to respect the standards of sampling and are stored in the cold storage during experiment days. A physicochemical characterization of the effluents is carried out before the treatment. Three types of effluent are treated:
36
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
C Effluent A: resulting from the unit of sodation, it is a unit in which a neutralization of the excess of chloride by a soda solution is carried out at a temperature of 50EC. C Effluent B: resulting from the unit of washing. C Effluent C: it is a mixture of the two effluents taken in a way which respects the standards of rejections. Table 1 presents the physicochemical characterization of these effluents in their crude states: Table 1 Characteristics of effluent samples before treatment Parameter
Effluent A Effluent B
Effluent C
Colour Decanting solids [ml/l]
Yellow 25
Dark yellow Yellow 30 30
pH Conductivity [mS/cm] Salinity [g/l] Chlorides Cl[mg/l] COD [mgO2/l] BOD5 [mgO2/l] Suspended solids [mg/l] Turbidity [FTU]
7.4 1.9
6.6 59
6.8 3.6
0.8 479
3.1 1917
1.8 1491
684 100 240
2575 300 880
1416 200 400
556
504
364
We note that these effluents present a high rate of COD, BOD5 and a considerable rate of suspended solids which makes effective an eventual treatment by coagulation–electroflotation. 2.1.2. Electroflotation cell The electroflotation cell, shown in Fig. 1, is a base rectangular column. Its surface is 11.25 cm2 (2.5 cm × 4.5 cm) and its height is 90 cm. It is provided with two electrodes: titanium coated with ruthenium oxide anode and a stainless steel
Fig. 1. Schematic diagram of the electroflotation cell in batch mode. A, anode; C, cathode; E, sludge evacuation; R, treated water.
cathode. These two electrodes are supplied by a generator of DC current which makes possible the variation of current density in the electrodes. It is also noted that these two electrodes are at a distance of 8 millimetres. The cathode compared to the anode is perforated and occupies the top position. This perforation allows the evacuation of bubbles produced at the anode. It is also noted that this column has two outputs: the first one is for the evacuation of sludge and it is at the top of the column and the second one is for water after treatment and it is at the bottom. In continuous mode, the electroflotation unit shown in Fig. 2 has a total volume of 4.2 liters. It is divided into two compartments. Each compartment is provided with two electrodes. The first compartment receives the effluent from a tank of coagulation using a piston pump. The effluent then undergoes a primary treatment
37
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
Fig. 2. Schematic diagram of the electroflotation unit in continuous mode.
which lowers the suspended solids rate more than 70%. The bubbles formed at electrodes and effluent are in co-current movement. After that, the effluent gets through a second compartment by overflow and undergoes its final treatment. The bubbles formed at electrodes and effluent are now in counter current. The treated effluent is evacuated and the sludge formed is eliminated at the top of the column. It is noted that the pump is equipped with a variator of flow and this ensures the obtaining of a sufficient range of supply rate. The agitation of the storing tank is ensured by a mechanical agitator necessary to avoid a possible decantation of sludge. A diagram of the process is given in Fig 2. 2.2. Methods In batch mode, a volume of representative effluent is taken. The parameters to be optimized
Table 2 Intervals variation of parameters studied Parameter
Ccoag (mg/l)
pH
D (A/m²)
Zimin Zimax Zi0 ΔZi
200 1000 600 400
5 8 6.5 1.5
100 250 175 75
are current density, pH and coagulant concentration. The coagulant used is aluminium sulphate (Al2(SO4)3.18H2O). Preliminary tests were done to fix the intervals of variations of these variables. Table 2 gives these intervals. We note that the treatment duration is fixed at 20 min. We note also that during all experiments the voltmeter has shown that the voltage varied in the interval of 7–9 Volts due to the variation of current density and conductivity of the medium.
38
L. Ben Mansour et al. / Desalination 208 (2007) 34–41 Y = a 0 + [ a1 × X 1 + a 2 × X 2 + a 3 × X 3 ]
Table 3 Experiments matrix
[
2
2
2
+ a11 × X 1 + a 22 × X 2 + a 33 × X 3
Experiment
C = X1
pH = X2
D = X3
Y
1 2 3 4 5 6 7 8 9* 10 11 12 13 14 15 16* 17* 18*
1000 200 1000 200 1000 200 1000 200 600 600 600 600 600 1086 114 600 600 600
8 8 5 5 8 8 5 5 6.5 6.5 6.5 8.3 4.7 6.5 6.5 6.5 6.5 6.5
250 250 250 250 100 100 100 100 175 266.1 83.9 175 175 175 175 175 175 175
-
The methodology of experiments planning, with an orthogonal central composite design, requires in this case 14 experiments and tests in center following this experiment matrix [9,10] (Table 3). Y is the rate of purification (i.e., elimination of suspended solids) obtained after treatment, 9*, 16*, 17 * and 18 * are the tests in the center. Y (%) = (Suspended solids before treatment ! Suspended solidsafter treatment)/ Suspended solidsbefore treatment The exploitation of this matrix leads to obtaining a second order regression equation which connects the parameters with the rate of purification Y. This equation takes account of the interactions which can exist between these variables [9,10].
]
(3)
+ [ a12 × X 1 × X 2 + ... + a13 × X 1 × X 3 + a 23 × X 2 × X 3 ]
a0: constant term of the regression equation. ai (i 0 {1,2,3}): linear effect coefficients. aii (i 0{1,2,3}): quadratic effect coefficients. aij (i,j 0 {1,2,3,}): interaction effect coefficients. It is also noted that these coefficients were obtained by matrix algebra and two tests of validity were made: a first test of Student which validates each coefficient and a second test of Fisher which validates the global model.
3. Results and discussion 3.1. Treatment in batch mode In this part, the matrix of experiments planning is the principal support of study. Indeed, 18 experiments were done for each type of effluent; the results are summarized in Table 4. The regression equations obtained are presented in Table 5. The equations were obviously validated by Student and Fisher tests. The optimization of these equations using the method of Newton gives us the values of the variables of studies (C, pH and D) which maximize the purification rate Y. The values of these parameters after optimization are given in Table 6. So, we note that: all the first order terms exist, this confirms that pH, coagulant concentration and current density act directly on the suspended solids reduction. The terms of the second order of the regression equations exist; this explains that the optimum is in the fixed intervals. Other interactions coefficients exist: C An interaction exists between the coagulant concentration and pH; this is well explained by the acid character of the coagulant. C An interaction exists between the coagulant
39
L. Ben Mansour et al. / Desalination 208 (2007) 34–41 Table 4 Experiment results Experiment
C = X1
pH = X2
D = X3
YA
YB
YC
1 2 3 4 5 6 7 8 9* 10 11 12 13 14 15 16* 17* 18*
1000 200 1000 200 1000 200 1000 200 600 600 600 600 600 1086 114 600 600 600
8 8 5 5 8 8 5 5 6.5 6.5 6.5 8.3 4.7 6.5 6.5 6.5 6.5 6.5
250 250 250 250 100 100 100 100 175 266.1 83.9 175 175 175 175 175 175 175
87 79 85 77 84 78 79 86 89 93 87 90 90 92 88 91 92 87
82 53 67 78 70 79 90 64 90 86 75 77 86 80 88 92 90 90
89 72 82 81 83 82 87 82 94 91 81 84 90 88 88 95 93 92
Table 5 Coefficients of regression equations obtained Parameter
Effluent A
Effluent B
Effluent C
Constant C pH D C² pH² D² C× pH C× D
!25.5 4.3 10!2 25.1 0.2 !2.5 10!5 !1.7 !7.1 10!4 !3.4 10!3 5.6 10!5
!60.9 3.5 10!2 34.7 0.5 !3.5 10!5 !3 !1.3 10!3 !3.1 10!4 2.6 10!5
!16.1 3.3 10!2 23.7 0.3 !2.2 10!5 !1.8 !8.1 10!4 !2.3 10!3 4.6 10!5
concentration and current density. Indeed, the current density acts directly on number and shape of the gas bubbles. In fact, the number of bubbles increases when current density increases. So the probability of collision between these bubbles
Table 6 Theoretical values of optimized parameters and a comparison between theoretical and experimental purification rate
Effluent A Effluent B Effluent C
Ccoag [mg/l]
pH
D [A/m²]
Ythe (%)
Yexp (%)
639 534 626
7 6 6
195 195 201
95 96 96
95 96 95
and optimal coagulated floats increase. As a result, more particles will be eliminated. A physicochemical characterization was made in the optimal conditions. This characterization is summarized in Table 7. We note that not only the suspended solids have been reduced, but the COD, BOD5 and [Cl!] have been also reduced. This is explained by the oxidation of dissolved
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L. Ben Mansour et al. / Desalination 208 (2007) 34–41
Table 7 Characteristics of effluent samples after batch mode treatment in optimal conditions Parameter
Effluent A Effluent B
Effluent C
Colour Decanting solids [ml/l] pH Conductivity [mS/cm] Salinity [g/l] Chlorides Cl[mg/l] COD [mgO2/l] BOD5 [mgO2/l] Suspended solids [mg/l] Turbidity [FTU]
Clear <0.2
Clear <0.2
Clear <0.2
7.1 2.3
6 5.8
6.7 4
1 372.7
3 1517
2 1029
204 85 7.5
1297 150 42.5
604 100 16
23
146
51
organic substances due to the presence of both the fine oxygen bubbles and the chlorine as a strong oxidizing agent in the medium. The chlorine was the oxidation product of the chloride in the secondary electrochemical anodic reaction during electrolysis. 3.2. Treatment in continuous mode The objective of the treatment in continuous mode is to optimize the supply rate of the effluent to be treated (i.e. to optimize the residence time) which can lead to a possible extrapolation of the process on an industrial scale. Current density, pH and coagulant concentration are optimized in the batch mode. We propose to vary the supply flow and to note the corresponding purification rate. The results are given in Fig. 3. For effluent A, it is noted that the optimal flow maximizing the elimination of suspended solids is 15.5 l/h, which corresponds to a residence time tr = 16.26 min. For the mixture of the two effluents i.e. the effluent C, the optimal flow is
Fig. 3. Effect of effluent supply flow on the purification efficiency.
reached in 18 l/h which corresponds to residence time tr =14min. The corresponding purification efficiencies are 95.6% and 95.4%. We also find that for lower supply flows than those of the optimum, the purification rate is almost the same i.e. the bubbles flow produced by the electrodes was able to eliminate the maximum of suspended solids. While for higher supply flows, the last is too high than the bubble flow. This means that the suspended solids partially reside in the treated water outflow. A complete analysis of water which was treated in the optimum conditions was made (Table 8). Table 8 Characteristics of effluent samples after the continuous mode treatment in optimal conditions Parameter
Effluent A
Effluent C
Colour Decanting solids [ml/l] pH Conductivity [mS/cm] Salinity [g/l] Chlorides Cl! [mg/l] COD [mgO2/l] BOD5 [mgO2/l] Suspended solids [mg/l] Turbidity [FTU]
Clear <0.2 6.7 2 0.8 319.5 114 70 4 10
Clear <0.2 6.5 4.6 1.9 1266.2 48.9 55 18 4
L. Ben Mansour et al. / Desalination 208 (2007) 34–41
These results show that the suspended solids, COD, BOD5 and [Cl!] have been considerably reduced which prove that the treatment of these effluents by coagulation—electroflotation is effective. We also note that purification efficiency is better in continuous mode compared to the batch mode since the continuous mode treatment was operated in an electroflotation unit with two compartments where the effluents undergo twostage treatment. In the first stage gas bubbles and effluents are in co-current flow and in the second stage they are in the counter current flow.
4. Conclusion The coagulation–electroflotation process was used to treat paper industry effluents which contain an important rate of suspended solids. The regression equations obtained was optimized to find the best conditions of treatment in batch mode: coagulant concentration, pH and current density are the parameters of studies. Physicochemical characterization was done before and after treatment. Clearly, the coagulation– electroflotation technique can be successfully selected as an efficient process to treat this type of effluent since it leads to a high purification efficiency. Tests of treatment in continuous mode were carried out successfully by determining the optimal residence time of the effluents to be treated. Suspended solids were reduced more than 95%.
Acknowledgements The authors are thankful to Professor S. Venkatachalam, Indian institute of Technology,
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Dept. of Metallurgical Engineering and Materials Science, Bombay 400076, India for his technical language support; and to Professor V.A Kolesnikov, Director of Electrochemical Process Laboratory, D.I. Mendeleev, Russian Chemical Technology University, for his guidance and encouragement.
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