Supplemental materials
Design and optimization of a combined electrocoagulationelectroflotation reactor for wastewater treatment
C. Jiménez, C. Sáez, P. Cañizares, M.A. Rodrigoa
a
Department of Chemical Engineering. Universidad de Castilla La Mancha, Campus
Universitario 12. E13071 Ciudad Real. Spain; Fax: +34 926 29 52 56; Tel: +34 926 29 53 00;
E-mail: Manuel.Rodrigo@uclm.es
Experimental design
In this work, the effect of three operational inputs (current density (x1), residence time of
the reactor (x2) and pollutant concentration (x3)) has been studied using a 23 experimental
design combined with a star design using four replicates of the central point. For the star
design, half of the range of the experimental design has been taken. So, the experimental
design and the star design experiments can be shown in Table SM1 for kaolin, Table SM2
for EBT and Table SM3 for oil-in-water emulsions. According to literature (Box et al.
2002), all the experiments were randomly made.
Table SM1. Original and coded factors for kaolin suspensions treatment
Coded factor
Original
Factor
factor
-1
-0.5
0
+0.5
+1
Residence time/ h
Current density/ mAcm
-2
Kaolin concentration/
mg/L
X1
0.2
0.225
0.25
0.275
0.3
X2
1
1.375
1.75
2.125
2.5
X3
500
750
1000
1250
1500
Table SM2. Original and coded factors for EBT solution treatment
Coded Factor
Original
Factor
Factor
-1
-0.5
0
+0.5
+1
Residence Time/ h
X1
0.167
0.221
0.275
0.329
0.383
Current Density/ mAcm-2
X2
2.5
3.7
5
6.2
7.5
EBT concentration/ mg/L
X3
50
100
150
200
250
Table SM3. Original and coded factors for oil-in-water emulsion treatment
Coded Factor
Original
Factor
Factor
-1
-0.5
0
+0.5
+1
Residence Time/ h
X1
0.15
0.225
0.30
0.375
0.45
Current Density/ mAcm-2
X2
25.0
31.2
37.5
43.7
50.0
Oil concentration/ mg/L
X3
1000
2000
3000
4000
5000
Experimental data were fitted to a second order polynomial model if the results lead to a
non-linear system or to a first order polynomial model if they lead to a linear system, and
the regression coefficients a0, ai, aii and aij were obtained in each case (equations sm1 and
sm2).
3
3
i 1
i 1
3
2
2
3
Yi  a 0   a i x i   a ii x ii2   a ijx i x j
3
Yi  a 0   a i x i    a ijx i x j
i 1
(sm1)
i 1 ji 1
(sm2)
i 1 ji 1
where, a0, ai, aii and aij are the regression coefficients for intercept, linear, quadratic and
interaction terms, respectively and xi, xj are the independent variables. Yi represents
pollutant removal (Y1) and floated/settled solids ratio (Y2). Y1 indicates the efficiency of
the process for pollutants removal, and Y2 represents the efficiency of the flotation
process using gas bubbles generated in the cathode, related to sedimentation inside the
reactor. Both parameters are of great importance as the main objective of this work is to
remove pollutants from wastewater but trying to optimize the electroflotation process
inside the reactor.
Central experiments
First, four series of experiments with the central conditions of the experimental design
were made to calculate the experimental error (according to equation sm3) and the
standard deviation (eq sm3) where e is the experimental error, t is the t-Student function
value for a confidence level of 99% (taking into account 3 degrees of freedom this
function gets a value of 5.841), s is the standard deviation and r the number of replications.
e
ts
r
(sm3)
The experimental error has been calculated for both kaolin, EBT and emulsions assays
and taking into account pollutant removal and the floated-settled solids ratio. The results
for the four replications of the central conditions assays and the calculated experimental
error are shown in Table SM4.
Table SM4. Experimental error obtained from central conditions of the experimental
design. Confidence interval for calculate t-Student value: 99%.
Pollutant
Kaolin
EBT
O/W
Emulsions
*
Central conditions
Experimental
error
1
76.50
2
67.60
3
69.90
4
68.35
TFS/TSS*
0.68
0.88
0.86
0.92
COD removal/ %
52.69
56.41
55.88
49.95
8.79
Colour removal/
%
64.34
60.23
71.84
61.82
15.02
VFS/VSS*
14.34
12.64
13.25
13.52
2.06
Turbidity
removal/ %
95.16
99.01
97.15
97.61
4.66
COD removal/ %
89.64
83.30
94.21
84.60
14.59
TFS/TSS*
12.31
10.68
12.17
10.62
2.69
Turbidity
removal/ %
TFS=Total Floated Solids, TSS=Total Settled Solids, VFS=Volatile Floated Solids,
VSS=Volatile Settled Solids
7.46
0.10
As it can be seen, the higher values of the experimental error are obtained for colour
removal in the case of working with EBT solutions, and for COD removal in the case of
oil-in-water emulsions. Taking into account these results, the parameters selected for the
study have been COD for EBT solutions and turbidity removal for kaolin suspensions and
oil-in-water emulsions.
Experimental design
The experimental design has been made for two main parameters: pollutant removal
(COD has been considered in the case of EBT solutions and turbidity for oil-in-water
emulsions and kaolin suspensions) and floated-settled solids ratio. The experimental
conditions and the results obtained for both parameters in steady state conditions are
shown in Table SM5 for kaolin suspensions, in Table SM6 for EBT solutions and in Table
SM7 for oil-in-water emulsions. Although the experiments are shown in order in the
tables, although they were randomly made.
Table SM5. Experimental results obtained for the experimental design for Kaolin
suspensions. Pollutant: Kaolin; Temperature: 25 ºC; electrolyte: 5000 mg/L NaCl.
Kaolin
Turbidity
Residence Current density/
Assay
concentration/
removal/ TFS/TSS
time/ h
mAcm-2
mg/L
%
1
0.300
1.0
500
31.87
0.98
2
0.200
1.0
500
13.48
1.03
3
0.300
2.5
500
62.39
0.99
4
0.200
2.5
500
33.21
0.95
5
0.300
1.0
1500
55.09
0.77
6
0.200
1.0
1500
49.50
0.45
7
0.300
2.5
1500
55.30
0.55
8
0.200
2.5
1500
49.86
0.64
9
0.250
1.4
1000
34.79
0.94
10
0.250
2.1
1000
51.93
0.68
11
0.275
1.7
1000
50.22
0.56
12
0.225
1.7
1000
41.59
0.53
13
0.250
1.7
1250
53.18
0.48
14
0.250
1.7
750
39.64
0.84
Table SM6. Experimental results obtained for the experimental design for EBT
solutions. Pollutant: EBT; Temperature: 25 ºC; electrolyte: 5000 mg/L NaCl.
Current
EBT
COD
VFS/VSS
Assay Residence time/ h
density/
concentration/ removal/ %
mAcm-2
mg/L
1
0.167
2.5
50
25.24
9.74
2
0.383
2.5
50
40.00
15.84
3
0.167
7.5
50
42.86
4.27
4
0.383
7.5
50
43.14
6.99
5
0.167
2.5
250
7.14
10.03
6
0.383
2.5
250
18.61
5.44
7
0.167
7.5
250
40.29
13.12
8
0.383
7.5
250
67.23
12.77
9
0.275
3.7
150
5.45
4.16
10
0.275
6.3
150
70.75
12.22
11
0.221
5.0
150
18.63
18.24
12
0.329
5.0
150
69.98
10.49
13
0.275
5.0
100
57.22
16.25
14
0.275
5.0
200
38.66
10.13
Table SM7. Experimental results obtained for the experimental design for oil-in-water
emulsions. Pollutant: O/W emulsions; Temperature: 25 ºC; electrolyte: 10000 mg/L
NaCl.
Assay
Residence
time/ h
1
2
3
4
5
6
7
0.150
0.450
0.150
0.450
0.450
0.150
0.450
Current
density/
mAcm-2
25.0
25.0
50.0
50.0
50.0
50.0
25.0
Oil
concentration/
mg/L
1000
1000
1000
1000
5000
5000
5000
COD
removal/ %
TFS/TSS
97.92
99.67
99.31
99.42
99.66
92.69
97.93
8.91
1.96
6.71
3.30
3.38
9.62
4.22
8
9
10
11
12
13
14
76.12
0.150
25.0
5000
5.26
99.29
0.225
37.5
3000
6.05
99.54
0.375
37.5
3000
3.90
98.95
0.300
31.2
3000
3.35
99.32
0.300
43.7
3000
3.63
99.61
0.300
37.5
2000
6.10
99.14
0.300
37.5
4000
6.39
According to the response surface methodology, these results have been fitted to
a second order polynomial model and the regression coefficients have been calculated
according to equation sm4. The regression coefficients for the model in each case are
shown in Table SM8. The experimental results are compared with the results obtained
with the model in Figure SM1.
Y= a+b·J+c·θ+d·C+e·θJ+f·JC+g·θC+h·J2+i·θ2+j·C2+k·θJC
(sm4)
Where J is the current density, θ the resident time and C the pollutant concentration.
Table SM8. Regression coefficients for the second order polynomial model for turbidity
removal and TFS/TSS ratio (kaolin suspensions), COD removal and VFS/VSS ratio
(EBT solutions) and turbidity removal and TFS/TSS ratio (O/W emulsions)
Kaolin
Parameter
a
b
c
d
e
f
g
h
i
j
k
Turbidity
removal/ %
-59.845
65.560
-401.447
0.124
0.000
-0.019
-0.192
-10.368
1494.000
-1.158·10-5
0.000
EBT
TFS/TSS
-0.755
0.166
1.400
8.100·10-4
-0.465
-6.030·10-4
-4.170·10-3
2.000·10-3
COD
removal/ %
-66.413
26.782
300.356
-0.160
-18.740
-8.000·10-4
-0.336
-261.095
0.000
4.671·10-5
0.143
O/W emulsions
VFS/VSS
-3.393
9.858
-19.570
1.500·10-3
-3.769
8.600·10-3
-0.402
-1.137
117.872
4.480·10-5
0.037
Turbidity
removal/ %
107.748
-0.116
-15.283
-0.013
0.189
2.126·10-4
0.027
-4.277·10-4
TFS/TSS
6.750
0.175
0.500
-2.4·10-3
-0.414
3.000·10-5
8.000·10-4
-1.300·10-3
0.570
1.980·10-7
1.690·10-5
Values of adjusted r2 of 0.935, 0.7944 and 0.9999 are obtained for pollutant removal for
kaolin, EBT and oil-in-water emulsions and values of 0.927, 0.661 and 0.734 are obtained
for floated-settled solids ratio (for kaolin and emulsions, total solids have been measured,
as for EBT solutions volatile solids have been considered, as pollutant concentration is
much lower than electrolyte concentration) (Figure sm1). Additionally, Response
Surfaces have been obtained from these models for both pollutant removal and
floated/settled solids ratio.
Fig. SM1 Comparison of the modelled data with the experimental results. Left figures:
Pollutant removal; right figures: Floated/settled solids ratio; a) kaolin suspensions; b)
EBT solutions; c) oil-in-water emulsions. (TFS/TSS=Total Floated/Settled Solids;
VFS/VSS=Volatile Floated/Settled Solids)
References
Box GEP, Hunter WG, Hunter JS, de Pablos LA, Llabres JTM (2002): Estadística para
investigadores: introducción al diseño de experimentos, análisis de datos y
construcción de modelos. Reverté