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Ozone assisted electrocoagulation for the treatment of distillery effluent
Desalination 297 (2012) 1–7
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Ozone assisted electrocoagulation for the treatment of distillery effluent P. Asaithambi, Modepalli Susree, R. Saravanathamizhan, Manickam Matheswaran ⁎ Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620 015, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 31 January 2012 Received in revised form 28 March 2012 Accepted 14 April 2012 Available online 18 May 2012 Keywords: Electrocoagulation Distillery effluent Ozonation Colour removal COD removal
a b s t r a c t The hybrid technique of ozone assisted electrocoagulation for the removal of colour and COD in the industrial effluent treatment was investigated. The synergistic effect of the combined process was tested with conventional processes of electrocoagulation and ozonation. The result showed that the hybrid technique was more effective than electrocoagulation and ozonation alone. The influence of operating parameters such as initial COD concentration, initial pH, current density, inter-electrode distance and electrode combination were studied on the percentage colour and COD removal and also on the power consumption for the treatment of distillery effluent. Iron and aluminum electrodes in the different combination were investigated. The percentage COD removal increased from 45% to 92% with increase in current density from 1 Adm − 2 to 5 Adm − 2. The maximum removal of COD efficiency was found to be 83% at current density of 3 Adm − 2, initial COD concentration 2500 ppm, initial pH 6 and ozone gas mixture flow rate 15 L min − 1 requiring an energy consumption of 5.1 kW h m − 3. The complete colour removal was observed within 2 h of process time in all experimental conditions. The pseudo‐first‐order kinetic was studied based on COD removal in the hybrid technique. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Industries are producing huge quantity of pollutants which contains biorefractory organic compounds that causes toxicity, colour and odor problems to the aquatic environment and ecosystem. The organic compounds in the discharged effluents are difficult to treat using various combinations of physical, chemical and biological techniques. The treatment of effluents having various compositions of the organic compounds using these methods is becoming inadequate and insufficient. The ever-increasing generation of effluent from different industries is on one side while the stringent legislative regulations of its disposal are on the other side. Hence, it is mandatory to treat the effluent before discharging it into the environment. There are many processes such as adsorption [1–3], precipitation [4], chemical oxidation [5], photo-degradation [6–8], biological processes [9], chemical coagulation [10], electrochemical oxidation [11] and electrocoagulation [12–17] for the removal of pollutant, colour and odor from the industrial effluents. The electrochemical technology can be applied for the treatment of wastewater due to its safe and environment-friendly nature. The electrochemical oxidation, electrocoagulation are employed for the various industrial effluents depending on the nature of composition. Electrocoagulation involves the generation of coagulants in-situ by the dissolution of metal ions electrochemically from the anode with the simultaneous formation of hydroxyl ions and also involves the
⁎ Corresponding author. Tel.:/fax: + 91 4312503120. E-mail address: [email protected] (M. Matheswaran). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.04.011
production of hydrogen gas at the cathode. The metal ions form flocculates which traps the contaminants while the hydrogen gas floats these particles. This has been successfully employed for the treatment of various industrial effluents like textile [18], paper [19,20], distillery [21], paint manufacturing [13], trivalent chromium removal [14], mechanical polishing [15], cyanide removal [17], almond industry [22], vegetable oil refinery [23], and indium (III) ion removal [24]. However, electrocoagulation process generates considerable amount of sludge as a secondary pollutant during the treatment. Hence, an alternative treatment process is needed for effective sludge reduction by combining electrocoagulation with ozone. In the electrocoagulation, iron as an anode produces Fe2+ ions. In the presence of O2, Fe 2+ will be converted into Fe(OH)3. The ozone coupled electrocoagulation system involves direct attack of Fe2+ with O3 to generate the intermediate (FeO) 2+. The intermediate species produce •OH radicals. Therefore, ozone assisted electrocoagulation system can accelerate the removal efficiency of colour and COD. Some of the researchers have investigated the ozone assisted electrocoagulation process for the treatment of synthetic effluent. Song et al., [25,26] and Bernal-Martíneza et al., [27] studied the colour and COD removal of dye solution and municipal wastewater using ozone assisted electrocoagulation. Hernández-Ortega et al., [28] studied the pretreatment of industrial wastewater with the process of electrocoagulation and ozone treatment before proceeding to biological oxidation. They found that the effluent was completely treated before its discharge into the environment. Hence, the hybrid treatment process needs detail investigation for the various real industrial effluent treatments. The distillery industries are producing the densely coloured melanoidin during the processing of sugarcane molasses for alcohol
2
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production. Few investigators have studied the treatment of distillery effluent for the removal of colour and COD using different methods like biological and electrochemical methods [29–31]. Krishna Prasad et al., have used electrochemical degradation and electrocoagulation for the treatment of spent wash from distilleries and observed the removal of COD and colour using ruthenium oxide coated titanium anode [32]. However, nobody has so far attempted the hybrid technique for the treatment of real distillery effluent, so in the present investigation it has been chosen to study the ozone assisted electrocoagulation process. Further, the synergetic effect of ozone assisted electrocoagulation is tested with electrocoagulation and ozonation alone. Influence of operating parameters such as effluent concentration, initial effluent pH, current density, distance between the electrodes and electrode combinations has been studied on colour and COD removal. The kinetic model is studied for the removal of COD in the distillery effluent by hybrid process and is compared with experimental value. 2. Theory of ozone assisted electrocoagulation process The electrocoagulation process electrochemically generates metallic ions from the electrode, a coagulant of hydroxide flocs which aggregates suspended particles and dissolved pollutants and absorbs precipitates. The electrochemical reactions involved in the reactor for iron as an anode can be written as [28,33]. Anodic reaction − FeðsÞ →Fe2þ ðaqÞ þ 2e
Eo ¼ −0:44 V vs SHE
ð1Þ
Cathodic reaction 2H2 OðlÞ þ 2e− →H 2 ðgÞ þ 2OH− ðaqÞ
Eo ¼ 0:83 V vs SHE
ð2Þ
Overall reaction Fe þ 2H 2 O→FeðOHÞ2ðsÞ þ H 2ðgÞ
ð3Þ
ozone decomposition to generate hydroxyl radical. The reaction of ozone addition with electrocoagulation system is given below [27] 2þ
O3 þ Fe
2þ
→FeO
þ O2
ð12Þ
2þ
þ H2 O→Fe3þ þ HO þ HO
−
ð13Þ
2þ
þ Fe
þ
þ H2 O
ð14Þ
FeO FeO
•
2þ
3þ
þ 2H →2Fe
3. Material and methods The effluents were collected from nearby distillery industries. The main characteristics of the effluent are: pH: 4.1–4.3, COD: 80,000– 90,000 mg L − 1, BOD: 7000–8000 mg L − 1, TSS: 15.44 g L − 1, TDS: 5550–5750 mg L − 1, colour — dark brown, odor — burnt sugar. All the chemicals were of analytical reagent (AR) grade obtained from Merck and used without further purification. The pH of the effluent was measured using a pH-meter (Elico; Model LI120) and was adjusted by the addition of appropriate amounts of 0.1 M NaOH or H2SO4 solutions. 4. Experimental technique A schematic diagram of the lab-scale experimental setup is shown in Fig. 1. Electrocoagulation experiments were carried out in a batch type electrochemical reactor of 1000 mL capacity made of Plexiglass. In this process, iron (grade MS 104) was used as an anode as well as cathode with the dimensions of 9 cm × 5 cm. The effective surface area of the electrode was 45 cm 2. The inter-electrode distance was maintained as 1 cm. The electrodes were cleaned with 15% HCl followed by distilled water prior to each experiment. The volume of the effluent taken was 500 mL for each experimental run. The electrodes were connected to a DC power supply [APLAB Ltd; Model L1606] with galvanostatic operation for applying constant current density. All the experiments were performed at constant temperature of 30 ± 2 °C. In the hybrid technique, ozone was continuously purged into the bottom of electrochemical reactor with constant gas mixture
If O2 is present, dissolved Fe 2+ is oxidized to insoluble Fe(OH)3 þ
2þ
4FeðaqÞ þ 10H2 OðlÞ þ O2ðgÞ →4FeðOHÞ3ðSÞ þ 8HðaqÞ
5
ð4Þ
Volt
þ
−
O3 þ 2H þ 2e →O2 þ H 2 O •
O3 þ H 2 O→2HO þ O2 −
•−
•
O3 þ OH →O2 þ HO2 O3 þ OH
•
•
− →O2
þ
• HO2
•−
O2 þ HO2 ↔2O2 þ H •
•
O3 þ HO2 →2O2 þ HO •
2HO2 →O2 þ H2 O2
-
8
9 11
ð5Þ ð6Þ ð7Þ ð8Þ
þ
+
4
In case of ozone, O3 can react in aqueous solution with various organic and inorganic compounds either by a direct ozone attack and/or indirect free radical reaction involving the hydroxyl radical induced by the ozone decomposition in water [28].
Amp 10
2 1
7 6
ð9Þ ð10Þ ð11Þ
When ozone is purged in the electrocoagulation system, catalytic reaction of Fe 2+ with O3 generates the intermediate FeO 2+, a species that produce the •OH. Fe 2+ also acts as a catalyst for the
Fig. 1. Experimental setup for ozone assisted electrocoagulation process to the treatment of distillery effluent. (1. Ozone generator, 2. Ozone outlet, 3. Control valve, 4. 2–5% KI solution, 5. vent, 6. Gas flow rotameter, 7. reactor, 8. Anode, 9. Cathode, 10. DC power supply, and 11. Sample port).
P. Asaithambi et al. / Desalination 297 (2012) 1–7
Colour removal ¼
Absi −Abst 100 Absi
ð15Þ
where Absi and Abst are absorbance of initial and at time t samples for corresponding wavelength λmax. The percentage COD removal was calculated by: COD removal ¼
CODi −CODt 100 CODi
ð16Þ
where CODi and CODt are the COD value (ppm) initial and at time t. The total power consumption of ozone assisted electrocoagulation can be calculated as Total power consumption = EEC + EOzone EEC ¼
VIt VR
ð17Þ
where V is cell voltage (V), I is applied current (A), t is electrolysis time (h) and VR is the volume of effluent (L). The total power consumption was expressed in terms of kW h m − 3 for process based on the volume of effluent treated and also power consumption of ozone generation was included.
100
Colour removal (%)
flow rate of 15 L min − 1. Ozone was generated using an ozone generator (Ozonetek limited, Chennai). The generation capacity of ozone generator was around 2 g h − 1. Ozone concentrations were determined using an iodimetric method [34]. During the process, the samples were collected at regular interval of time and centrifuged using REMI Model: R-24 (10,000 rpm, 15 min) and were analyzed for colour and COD removal. The COD of the samples were determined using the dichromatic closed reflux method, strictly following the APHA [35]. The colour was measured at the wavelength corresponding to maximum absorbance λmax (300 nm) using UV/Vis spectrophotometer (Jasco, V-570). To study the synergistic effect, electrocoagulation, ozonation, ozone assisted electrocoagulation experiments were carried out. To investigate the effect of operating parameters on the percentage colour and COD removal in the effluent, current density (1–5 Adm − 2), initial pH (2–10), initial concentration of effluent (1250–5000 ppm) and inter-electrode distance (1–3 cm) were varied. The performance of electrocoagulation system was checked by varying the electrode material as iron and aluminum and changing the combination of anode and cathode. The percentage colour removal was calculated by:
3
80 60 Ozone assisted EC EC Ozonation
40 20 0 0
1
2
3
4
Time (h) Fig. 2. Comparison of ozonation, electrocoagulation and ozone assisted electrocoagulation for the percentage colour removal of the distillery effluent (condition: initial COD concentration: 2500 ppm, current density: 3 Adm− 2, initial pH: 6, ozone flow rate: 15 L min− 1).
5.2. Effect of initial pH The effect of effluent initial pH on the percentage COD removal and power consumption for the treatment of distillery effluent by ozone assisted electrocoagulation process is shown in Fig. 4. The results in the figure show that the percentage COD removal increases from 24 to 83% with increasing initial solution pH from 2 to 6. With further increase of initial pH from 6 to 10 the percentage COD removal decreases to 68%. The power consumption increases when pH is increased from 2 to 6 and decreases when pH is increased from 6 to 10. This may be due to the decrease in the conductivity of the electrolyte when pH changes from acidic to neutral and its increase when pH changes from neutral to alkali condition in the hybrid system. The maximum COD removal was observed at pH 6. When pH is increased above 6, the amount of hydroxide ions was increased in solution. Consequently, some of the hydroxide ions may be oxidized at the anode. This reduces the production of iron ions, thereby decreasing the percentage COD removal in the effluent. Moreover, Fe(OH)63 and Fe(OH)4− ions may be present at high pH, which is reducing the decolourization and degradation capacity [36]. At lower pH, the protons in the solution are reduces to H2 at the cathode and same proportion of hydroxide ions cannot be produced. The generation of Fe(OH) 2+, Fe(OH)2+ are disadvantages for colourant precipitation [37]. Hydroxyl radicals are formed from ozone decomposition at high pH values, while the molecular ozone remains as the main oxidant at low pH values. The value of pH is a very important operating condition in
5. Results and discussion
100
The comparison of the three processes, ozonation, electrocoagulation and ozone assisted electrocoagulation on the percentage of colour and COD removal is shown in Figs. 2. and 3. The colour removal efficiency was observed to be 7% for ozonation and 100% for electrocoagulation and ozone assisted electrocoagulation system within 2 h of the process. Similarly, the percentage of COD removal was found to be 13, 62 and 83% in the 4 h process time for ozonation, electrocoagulation, and ozone assisted electrocoagulation respectively, shown in Fig. 3. These results show that ozone accelerated the electrocoagulation process which generates free radicals according to Eqs. (6) and (10). For the 83% COD removal from 500 mL of an effluent, an energy consumption of 5.1 kW h m - 3 was observed for the hybrid system. Song et al., also observed similar results for decolourization and COD reduction of C.I. Reactive Black 5 in aqueous solution by ozone assisted electrocoagulation [25].
COD removal (%)
5.1. Comparison of ozone, electrocoagulation and ozone assisted electrocoagulation process
Ozone assisted EC EC Ozonation
80 60 40 20 0 0
1
2
3
4
Time (h) Fig. 3. Comparison of ozonation, electrocoagulation and ozone assisted electrocoagulation for the percentage COD removal of the distillery effluent (condition: initial COD concentration: 2500 ppm, current density: 3 Adm− 2, initial pH: 6, ozone flow rate: 15 L min− 1).
P. Asaithambi et al. / Desalination 297 (2012) 1–7
60 3 40 2
COD removal
20
1
Power Consumption 0
COD removal (%)
COD removal (%)
4
Power Consumption (kWhm-3)
5
80
80
2
4
6
8
4 40 2
COD removal
20
Power Consumptiion 0
0 0
12
10
6
60
0 0
8
100
6
100
Power Consumption (kWhm-3)
4
1
2
3
4
5
6
pH
Current Density (Adm-2)
Fig. 4. Effect of initial pH on the percentage COD removal and power consumption (condition: current density: 3 Adm− 2, initial COD concentration : 2500 ppm, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
Fig. 6. Effect of current density on the percentage COD removal and power consumption (condition: initial COD concentration: 2500 ppm, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
the ozone assisted electrocoagulation to enhance the percentage colour and COD removal. The space time yield can be represented as the mass of a product (mp) formed per volume (m 3) of the reactor and time (h). YSTY (kg m − 3 h − 1) can be calculated using the following equation.
generated heat and did not significantly affect the COD removal efficiency. The pH and specific electrical conductivity were increased as a function of electrolysis time for increasing current density as a consequence of continuous OH- formation at the cathode. This is due to the redox reaction of water which being predominant than the anodic water oxidation. The percentage colour removal of the distillery effluent increased from 25% to 100% with increasing current density of 1 to 5 Adm− 2 respectively. The cell potential increased with increase in current density which directly affects the electrical power consumption of the process which increases from 2.33 to 6.78 kW h m − 3. Hence, this process needs to operate at limiting current to prevent the excess oxygen evolution and to eliminate the heat generation.
YSTY ¼
mp V t
ð18Þ
The effect of initial pH on space time yield is shown in Fig. 5. It is observed from the figure that increasing the pH value from 2 to 6, increases the space time yield from 8.46 to 28.71 kg m − 3 h − 1, but further increase in pH value from 6 to 10 decreases the space time yield from 28.71 to 23.49 kg m − 3 h − 1. The maximum space time yield is at initial pH 6 while charge loading is minimum. 5.3. Effect of current density
The effect of effluent initial COD concentration on the percentage colour and COD removal are shown in Figs. 7 and 8. Increase in the effluent COD concentration decreases the percentage colour and COD removals from 100% to 59% and 96% to 44% respectively. According to Faraday's law, a constant amount of Fe 2+ is passed to the solution for the increasing initial effluent concentration at a constant galvanostatic value of current density and time. Consequently, the same amount of ferrous hydroxyl ions could be produced in the solution. As per the results, the ferrous hydroxyl ions produced at high COD concentration were insufficient to absorb all of the effluent concentration in the solution which decreases the percentage COD removal. However, the quantity of COD removal is higher under
30
100
25
80
Colour removal (%)
YSTY(kgm–3 h–1)
Current density is one of the significant operating parameters influencing the percentage COD removal and power consumption in the hybrid process. Experiments were carried out by varying the current density from 1 to 5 Adm − 2 and the results are shown in Fig. 6. According to Faraday's law, the current density and electrolysis time are directly proportional to the rate of coagulant dosage. So the rate of dissolution of iron electrode increased with increasing the current density. The percentage COD removal sharply increased up to the current density of 4 Adm − 2. Further increase of current density
5.4. Effect of initial COD concentration
20 15 10
60 40
1250 ppm 2500 ppm
20
3750 ppm 5000 ppm
0
5 0
2
4
6
8
10
12
pH Fig. 5. Space time yield at various initial pH (condition: current density: 3 Adm− 2, initial COD concentration: 2500 ppm, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
0
1
2
3
4
Time (h) Fig. 7. Effect of initial concentration on the percentage colour removal (condition: current density: 3 Adm− 2, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
P. Asaithambi et al. / Desalination 297 (2012) 1–7
80
6
60 4 40 2
20
COD removal Power Consumption
0 0
1250
2500
3750
Power Consumption (kWhm-3)
8
100
COD removal (%)
5
Fig. 10. Decolourization of the distillery effluent at various time intervals during the ozone assisted electrocoagulation process.
0 6250
5000
5.6. Effect of electrode combination
Initial COD Concentration (ppm) Fig. 8. Effect of initial concentration on the percentage COD removal and power consumption (condition: current density: 3 Adm− 2, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
constant current density which decreases the power consumption with increasing initial COD concentration of effluent. The UV–Vis absorbance spectra for the distillery effluent with initial concentration of 3750 ppm for initial and various time treatment of ozone assisted electrocoagulation process are shown in Fig. 9. It can be ascertained from the figure that the absorption peak diminishes with progressive interval of time under the wavelength of 300 nm. The decrease in absorption peaks at λmax in this figure shows the decolourization of distillery effluent. Fig. 10 shows the colour removal of the effluent at various time intervals. It is observed that complete colour removal is achieved with in 2 h of process time.
The different electrode material and electrode combinations are very important factors affecting the performance of the ozone assisted electrocoagulation process. The electrode combinations of Fe/Fe, Fe/Al, Al/Fe and Al/Al (anode/cathode) have been selected to investigate the percentage colour and COD removal which is shown in Fig. 12. The percentage COD removal of distillery effluent in the different electrode combination of Fe/Fe, Fe/Al, Al/Fe and Al/Al is 83, 78, 68 and 64%, respectively. The colour of the effluent is completely removed within 2 h of electrolysis time. However, the removal of COD mainly depends on the combination of electrode. The electrochemical reactions take place during electrocoagulation at Al anode and Fe anode: For Al anode: 3þ
Al→Al
þ 3e
−
ð19Þ
For Fe anode 5.5. Effect of inter-electrode distance
2þ
Fe→Fe The effect of inter-electrode distance on the percentage COD removal is shown in Fig. 11. The percentage COD removal decreases from 83% to 66% with increasing inter-electrode distance between anode and cathode from 1 to 3 cm. This is due to the fact that increase in inter-electrode distance increases IR resistances which increase the cell voltage and adversely affects the percentage colour and COD removal. Therefore, the increase in IR drop is not recommended for ozone assisted electrocoagulation process for the effluent treatment in order to have acceptable power consumption as well as desired effective removal of pollutants. Hence, in order to achieve high removal of COD in the effluent concentration the inter-electrode distance between anode and cathode should be minimized.
ð20Þ
The Fe electrode material shows higher COD removal compared to the Al electrode. This is due to the higher oxidation potential of Fe (−0.447 V) than Al (−1.662 V). The generation of Fe coagulant is three times greater than Al which causes the higher COD removal efficiency in the iron electrode as anode. 5.7. Kinetic modeling The kinetics model has been proposed for the ozone assisted electrocoagulation process. Olad et al., have chosen the pseudo‐first‐order kinetics for the electrocoagulation process for removal of the
100
1.5 initial
COD removal (%)
1.2 0.5 h 1h
Abs
0.9
2h
0.6
3h 4h
0.3 0 200
−
þ 2e
1 cm
80
2 cm 3 cm
60 40 20 0
300
400
500
600
700
800
0
1
2
3
4
Time (h)
Wavelength (nm) Fig. 9. Absorbance spectra of the distillery effluent solution as a different electrolysis time in the ozone assisted electrocoagulation.
Fig. 11. Effect of inter-electrode distance on the percentage COD removal (condition: current density: 3 Adm− 2, initial COD concentration: 2500 ppm, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
6
P. Asaithambi et al. / Desalination 297 (2012) 1–7
1
100 Fe - Fe Fe - Al Al - Fe Al - Al
60
Model
0.8
Experimental 0.6
C/Co
COD Removal (%)
80
40
0.4
20
0.2 0
0
1
2
3
4
0 0
Time (h)
1
2
3
4
Time (h) Fig. 12. Effect of electrode combination on the percentage COD removal (condition: current density: 3 Adm− 2, initial effluent concentration: 2500 ppm, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
Fig. 13. Comparison of experimental and model values for ozone assisted electrocoagulation process to the treatment of distillery effluent.
Alphazurine FG dye from simulated solution [38]. Hence, the present kinetic analysis of COD removal for the real distillery effluent using ozone assisted electrocoagulation is based on pseudo-first-order kinetics. The experimental results exhibited pseudo-first-order kinetics with respect to the effluent COD concentration and current density in the hybrid process. A mathematical relation has been derived to study the system design easily for further potential applications. Based on the experimental results, it is assumed that the rate constant, kobs, is affected by the key factors of initial COD concentration and current density. An empirical form is then used to present their relationships,
study has been optimized to minimize electrical power consumption and maximize the percentage colour and COD removal of the effluent. The maximum percentage colour and COD removal of 100% and 83% has been observed for an initial COD concentration of 2500 ppm, initial pH 6 and current density 3 Adm − 2. The inter-electrode distance of 1 cm shows better removal efficiency. The pseudo‐first‐order kinetic model satisfactorily matches with the experimental results and also the mathematical model was developed as a function of initial COD concentration and current density. This technology will continue to work in the water and wastewater treatment process due to the significant improvement over the conventional system.
kobs ¼ f ðinitial COD concentrationðCA Þ; current density ðCDÞÞ a b kobs ¼ m½CA ½CD
References
ð21Þ
where m, a, and b are constants. The constants a and b can be evaluated from the time vs concentration plot and time vs current density plot. After combining these two values the constant, m can be calculated. From the experimental value the above equation can be written as C=C 0 ¼ e
−kobs t
− 0:5656½C A −0:656 ½CD0:3265 Þt ¼e ð
ð22Þ
The theoretical COD removal can be calculated using the following equation ð23Þ Percentage COD removal ¼ ð1−C=C 0 Þ 100 −ð0:5656½C A −0:656 ½CD0:3265 Þt 100 ¼ 1−e In order to validate the model, model simulations are compared with experimental values as shown in Fig. 13. It is observed from the figure that the kinetic model satisfactorily matches with the experimental observation. 6. Conclusion The ozone assisted electrocoagulation process was successfully tested for the treatment of real distillery industry effluent. The wide range of operating parameters such as effluent initial COD concentration, initial pH, current density, inter-electrode distance and electrode combinations on the percentage of colour and COD removal, power consumption were investigated. The percentages of colour and COD removal for ozonation, electrocoagulation and hybrid processes were compared. The result showed that hybrid technique was more effective than electrocoagulation and ozonation alone. The present
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Ozone assisted electrocoagulation for the treatment of distillery effluent P. Asaithambi, Modepalli Susree, R. Saravanathamizhan, Manickam Matheswaran ⁎ Department of Chemical Engineering, National Institute of Technology, Tiruchirappalli 620 015, Tamilnadu, India
a r t i c l e
i n f o
Article history: Received 31 January 2012 Received in revised form 28 March 2012 Accepted 14 April 2012 Available online 18 May 2012 Keywords: Electrocoagulation Distillery effluent Ozonation Colour removal COD removal
a b s t r a c t The hybrid technique of ozone assisted electrocoagulation for the removal of colour and COD in the industrial effluent treatment was investigated. The synergistic effect of the combined process was tested with conventional processes of electrocoagulation and ozonation. The result showed that the hybrid technique was more effective than electrocoagulation and ozonation alone. The influence of operating parameters such as initial COD concentration, initial pH, current density, inter-electrode distance and electrode combination were studied on the percentage colour and COD removal and also on the power consumption for the treatment of distillery effluent. Iron and aluminum electrodes in the different combination were investigated. The percentage COD removal increased from 45% to 92% with increase in current density from 1 Adm − 2 to 5 Adm − 2. The maximum removal of COD efficiency was found to be 83% at current density of 3 Adm − 2, initial COD concentration 2500 ppm, initial pH 6 and ozone gas mixture flow rate 15 L min − 1 requiring an energy consumption of 5.1 kW h m − 3. The complete colour removal was observed within 2 h of process time in all experimental conditions. The pseudo‐first‐order kinetic was studied based on COD removal in the hybrid technique. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Industries are producing huge quantity of pollutants which contains biorefractory organic compounds that causes toxicity, colour and odor problems to the aquatic environment and ecosystem. The organic compounds in the discharged effluents are difficult to treat using various combinations of physical, chemical and biological techniques. The treatment of effluents having various compositions of the organic compounds using these methods is becoming inadequate and insufficient. The ever-increasing generation of effluent from different industries is on one side while the stringent legislative regulations of its disposal are on the other side. Hence, it is mandatory to treat the effluent before discharging it into the environment. There are many processes such as adsorption [1–3], precipitation [4], chemical oxidation [5], photo-degradation [6–8], biological processes [9], chemical coagulation [10], electrochemical oxidation [11] and electrocoagulation [12–17] for the removal of pollutant, colour and odor from the industrial effluents. The electrochemical technology can be applied for the treatment of wastewater due to its safe and environment-friendly nature. The electrochemical oxidation, electrocoagulation are employed for the various industrial effluents depending on the nature of composition. Electrocoagulation involves the generation of coagulants in-situ by the dissolution of metal ions electrochemically from the anode with the simultaneous formation of hydroxyl ions and also involves the
⁎ Corresponding author. Tel.:/fax: + 91 4312503120. E-mail address: [email protected] (M. Matheswaran). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.04.011
production of hydrogen gas at the cathode. The metal ions form flocculates which traps the contaminants while the hydrogen gas floats these particles. This has been successfully employed for the treatment of various industrial effluents like textile [18], paper [19,20], distillery [21], paint manufacturing [13], trivalent chromium removal [14], mechanical polishing [15], cyanide removal [17], almond industry [22], vegetable oil refinery [23], and indium (III) ion removal [24]. However, electrocoagulation process generates considerable amount of sludge as a secondary pollutant during the treatment. Hence, an alternative treatment process is needed for effective sludge reduction by combining electrocoagulation with ozone. In the electrocoagulation, iron as an anode produces Fe2+ ions. In the presence of O2, Fe 2+ will be converted into Fe(OH)3. The ozone coupled electrocoagulation system involves direct attack of Fe2+ with O3 to generate the intermediate (FeO) 2+. The intermediate species produce •OH radicals. Therefore, ozone assisted electrocoagulation system can accelerate the removal efficiency of colour and COD. Some of the researchers have investigated the ozone assisted electrocoagulation process for the treatment of synthetic effluent. Song et al., [25,26] and Bernal-Martíneza et al., [27] studied the colour and COD removal of dye solution and municipal wastewater using ozone assisted electrocoagulation. Hernández-Ortega et al., [28] studied the pretreatment of industrial wastewater with the process of electrocoagulation and ozone treatment before proceeding to biological oxidation. They found that the effluent was completely treated before its discharge into the environment. Hence, the hybrid treatment process needs detail investigation for the various real industrial effluent treatments. The distillery industries are producing the densely coloured melanoidin during the processing of sugarcane molasses for alcohol
2
P. Asaithambi et al. / Desalination 297 (2012) 1–7
production. Few investigators have studied the treatment of distillery effluent for the removal of colour and COD using different methods like biological and electrochemical methods [29–31]. Krishna Prasad et al., have used electrochemical degradation and electrocoagulation for the treatment of spent wash from distilleries and observed the removal of COD and colour using ruthenium oxide coated titanium anode [32]. However, nobody has so far attempted the hybrid technique for the treatment of real distillery effluent, so in the present investigation it has been chosen to study the ozone assisted electrocoagulation process. Further, the synergetic effect of ozone assisted electrocoagulation is tested with electrocoagulation and ozonation alone. Influence of operating parameters such as effluent concentration, initial effluent pH, current density, distance between the electrodes and electrode combinations has been studied on colour and COD removal. The kinetic model is studied for the removal of COD in the distillery effluent by hybrid process and is compared with experimental value. 2. Theory of ozone assisted electrocoagulation process The electrocoagulation process electrochemically generates metallic ions from the electrode, a coagulant of hydroxide flocs which aggregates suspended particles and dissolved pollutants and absorbs precipitates. The electrochemical reactions involved in the reactor for iron as an anode can be written as [28,33]. Anodic reaction − FeðsÞ →Fe2þ ðaqÞ þ 2e
Eo ¼ −0:44 V vs SHE
ð1Þ
Cathodic reaction 2H2 OðlÞ þ 2e− →H 2 ðgÞ þ 2OH− ðaqÞ
Eo ¼ 0:83 V vs SHE
ð2Þ
Overall reaction Fe þ 2H 2 O→FeðOHÞ2ðsÞ þ H 2ðgÞ
ð3Þ
ozone decomposition to generate hydroxyl radical. The reaction of ozone addition with electrocoagulation system is given below [27] 2þ
O3 þ Fe
2þ
→FeO
þ O2
ð12Þ
2þ
þ H2 O→Fe3þ þ HO þ HO
−
ð13Þ
2þ
þ Fe
þ
þ H2 O
ð14Þ
FeO FeO
•
2þ
3þ
þ 2H →2Fe
3. Material and methods The effluents were collected from nearby distillery industries. The main characteristics of the effluent are: pH: 4.1–4.3, COD: 80,000– 90,000 mg L − 1, BOD: 7000–8000 mg L − 1, TSS: 15.44 g L − 1, TDS: 5550–5750 mg L − 1, colour — dark brown, odor — burnt sugar. All the chemicals were of analytical reagent (AR) grade obtained from Merck and used without further purification. The pH of the effluent was measured using a pH-meter (Elico; Model LI120) and was adjusted by the addition of appropriate amounts of 0.1 M NaOH or H2SO4 solutions. 4. Experimental technique A schematic diagram of the lab-scale experimental setup is shown in Fig. 1. Electrocoagulation experiments were carried out in a batch type electrochemical reactor of 1000 mL capacity made of Plexiglass. In this process, iron (grade MS 104) was used as an anode as well as cathode with the dimensions of 9 cm × 5 cm. The effective surface area of the electrode was 45 cm 2. The inter-electrode distance was maintained as 1 cm. The electrodes were cleaned with 15% HCl followed by distilled water prior to each experiment. The volume of the effluent taken was 500 mL for each experimental run. The electrodes were connected to a DC power supply [APLAB Ltd; Model L1606] with galvanostatic operation for applying constant current density. All the experiments were performed at constant temperature of 30 ± 2 °C. In the hybrid technique, ozone was continuously purged into the bottom of electrochemical reactor with constant gas mixture
If O2 is present, dissolved Fe 2+ is oxidized to insoluble Fe(OH)3 þ
2þ
4FeðaqÞ þ 10H2 OðlÞ þ O2ðgÞ →4FeðOHÞ3ðSÞ þ 8HðaqÞ
5
ð4Þ
Volt
þ
−
O3 þ 2H þ 2e →O2 þ H 2 O •
O3 þ H 2 O→2HO þ O2 −
•−
•
O3 þ OH →O2 þ HO2 O3 þ OH
•
•
− →O2
þ
• HO2
•−
O2 þ HO2 ↔2O2 þ H •
•
O3 þ HO2 →2O2 þ HO •
2HO2 →O2 þ H2 O2
-
8
9 11
ð5Þ ð6Þ ð7Þ ð8Þ
þ
+
4
In case of ozone, O3 can react in aqueous solution with various organic and inorganic compounds either by a direct ozone attack and/or indirect free radical reaction involving the hydroxyl radical induced by the ozone decomposition in water [28].
Amp 10
2 1
7 6
ð9Þ ð10Þ ð11Þ
When ozone is purged in the electrocoagulation system, catalytic reaction of Fe 2+ with O3 generates the intermediate FeO 2+, a species that produce the •OH. Fe 2+ also acts as a catalyst for the
Fig. 1. Experimental setup for ozone assisted electrocoagulation process to the treatment of distillery effluent. (1. Ozone generator, 2. Ozone outlet, 3. Control valve, 4. 2–5% KI solution, 5. vent, 6. Gas flow rotameter, 7. reactor, 8. Anode, 9. Cathode, 10. DC power supply, and 11. Sample port).
P. Asaithambi et al. / Desalination 297 (2012) 1–7
Colour removal ¼
Absi −Abst 100 Absi
ð15Þ
where Absi and Abst are absorbance of initial and at time t samples for corresponding wavelength λmax. The percentage COD removal was calculated by: COD removal ¼
CODi −CODt 100 CODi
ð16Þ
where CODi and CODt are the COD value (ppm) initial and at time t. The total power consumption of ozone assisted electrocoagulation can be calculated as Total power consumption = EEC + EOzone EEC ¼
VIt VR
ð17Þ
where V is cell voltage (V), I is applied current (A), t is electrolysis time (h) and VR is the volume of effluent (L). The total power consumption was expressed in terms of kW h m − 3 for process based on the volume of effluent treated and also power consumption of ozone generation was included.
100
Colour removal (%)
flow rate of 15 L min − 1. Ozone was generated using an ozone generator (Ozonetek limited, Chennai). The generation capacity of ozone generator was around 2 g h − 1. Ozone concentrations were determined using an iodimetric method [34]. During the process, the samples were collected at regular interval of time and centrifuged using REMI Model: R-24 (10,000 rpm, 15 min) and were analyzed for colour and COD removal. The COD of the samples were determined using the dichromatic closed reflux method, strictly following the APHA [35]. The colour was measured at the wavelength corresponding to maximum absorbance λmax (300 nm) using UV/Vis spectrophotometer (Jasco, V-570). To study the synergistic effect, electrocoagulation, ozonation, ozone assisted electrocoagulation experiments were carried out. To investigate the effect of operating parameters on the percentage colour and COD removal in the effluent, current density (1–5 Adm − 2), initial pH (2–10), initial concentration of effluent (1250–5000 ppm) and inter-electrode distance (1–3 cm) were varied. The performance of electrocoagulation system was checked by varying the electrode material as iron and aluminum and changing the combination of anode and cathode. The percentage colour removal was calculated by:
3
80 60 Ozone assisted EC EC Ozonation
40 20 0 0
1
2
3
4
Time (h) Fig. 2. Comparison of ozonation, electrocoagulation and ozone assisted electrocoagulation for the percentage colour removal of the distillery effluent (condition: initial COD concentration: 2500 ppm, current density: 3 Adm− 2, initial pH: 6, ozone flow rate: 15 L min− 1).
5.2. Effect of initial pH The effect of effluent initial pH on the percentage COD removal and power consumption for the treatment of distillery effluent by ozone assisted electrocoagulation process is shown in Fig. 4. The results in the figure show that the percentage COD removal increases from 24 to 83% with increasing initial solution pH from 2 to 6. With further increase of initial pH from 6 to 10 the percentage COD removal decreases to 68%. The power consumption increases when pH is increased from 2 to 6 and decreases when pH is increased from 6 to 10. This may be due to the decrease in the conductivity of the electrolyte when pH changes from acidic to neutral and its increase when pH changes from neutral to alkali condition in the hybrid system. The maximum COD removal was observed at pH 6. When pH is increased above 6, the amount of hydroxide ions was increased in solution. Consequently, some of the hydroxide ions may be oxidized at the anode. This reduces the production of iron ions, thereby decreasing the percentage COD removal in the effluent. Moreover, Fe(OH)63 and Fe(OH)4− ions may be present at high pH, which is reducing the decolourization and degradation capacity [36]. At lower pH, the protons in the solution are reduces to H2 at the cathode and same proportion of hydroxide ions cannot be produced. The generation of Fe(OH) 2+, Fe(OH)2+ are disadvantages for colourant precipitation [37]. Hydroxyl radicals are formed from ozone decomposition at high pH values, while the molecular ozone remains as the main oxidant at low pH values. The value of pH is a very important operating condition in
5. Results and discussion
100
The comparison of the three processes, ozonation, electrocoagulation and ozone assisted electrocoagulation on the percentage of colour and COD removal is shown in Figs. 2. and 3. The colour removal efficiency was observed to be 7% for ozonation and 100% for electrocoagulation and ozone assisted electrocoagulation system within 2 h of the process. Similarly, the percentage of COD removal was found to be 13, 62 and 83% in the 4 h process time for ozonation, electrocoagulation, and ozone assisted electrocoagulation respectively, shown in Fig. 3. These results show that ozone accelerated the electrocoagulation process which generates free radicals according to Eqs. (6) and (10). For the 83% COD removal from 500 mL of an effluent, an energy consumption of 5.1 kW h m - 3 was observed for the hybrid system. Song et al., also observed similar results for decolourization and COD reduction of C.I. Reactive Black 5 in aqueous solution by ozone assisted electrocoagulation [25].
COD removal (%)
5.1. Comparison of ozone, electrocoagulation and ozone assisted electrocoagulation process
Ozone assisted EC EC Ozonation
80 60 40 20 0 0
1
2
3
4
Time (h) Fig. 3. Comparison of ozonation, electrocoagulation and ozone assisted electrocoagulation for the percentage COD removal of the distillery effluent (condition: initial COD concentration: 2500 ppm, current density: 3 Adm− 2, initial pH: 6, ozone flow rate: 15 L min− 1).
P. Asaithambi et al. / Desalination 297 (2012) 1–7
60 3 40 2
COD removal
20
1
Power Consumption 0
COD removal (%)
COD removal (%)
4
Power Consumption (kWhm-3)
5
80
80
2
4
6
8
4 40 2
COD removal
20
Power Consumptiion 0
0 0
12
10
6
60
0 0
8
100
6
100
Power Consumption (kWhm-3)
4
1
2
3
4
5
6
pH
Current Density (Adm-2)
Fig. 4. Effect of initial pH on the percentage COD removal and power consumption (condition: current density: 3 Adm− 2, initial COD concentration : 2500 ppm, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
Fig. 6. Effect of current density on the percentage COD removal and power consumption (condition: initial COD concentration: 2500 ppm, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
the ozone assisted electrocoagulation to enhance the percentage colour and COD removal. The space time yield can be represented as the mass of a product (mp) formed per volume (m 3) of the reactor and time (h). YSTY (kg m − 3 h − 1) can be calculated using the following equation.
generated heat and did not significantly affect the COD removal efficiency. The pH and specific electrical conductivity were increased as a function of electrolysis time for increasing current density as a consequence of continuous OH- formation at the cathode. This is due to the redox reaction of water which being predominant than the anodic water oxidation. The percentage colour removal of the distillery effluent increased from 25% to 100% with increasing current density of 1 to 5 Adm− 2 respectively. The cell potential increased with increase in current density which directly affects the electrical power consumption of the process which increases from 2.33 to 6.78 kW h m − 3. Hence, this process needs to operate at limiting current to prevent the excess oxygen evolution and to eliminate the heat generation.
YSTY ¼
mp V t
ð18Þ
The effect of initial pH on space time yield is shown in Fig. 5. It is observed from the figure that increasing the pH value from 2 to 6, increases the space time yield from 8.46 to 28.71 kg m − 3 h − 1, but further increase in pH value from 6 to 10 decreases the space time yield from 28.71 to 23.49 kg m − 3 h − 1. The maximum space time yield is at initial pH 6 while charge loading is minimum. 5.3. Effect of current density
The effect of effluent initial COD concentration on the percentage colour and COD removal are shown in Figs. 7 and 8. Increase in the effluent COD concentration decreases the percentage colour and COD removals from 100% to 59% and 96% to 44% respectively. According to Faraday's law, a constant amount of Fe 2+ is passed to the solution for the increasing initial effluent concentration at a constant galvanostatic value of current density and time. Consequently, the same amount of ferrous hydroxyl ions could be produced in the solution. As per the results, the ferrous hydroxyl ions produced at high COD concentration were insufficient to absorb all of the effluent concentration in the solution which decreases the percentage COD removal. However, the quantity of COD removal is higher under
30
100
25
80
Colour removal (%)
YSTY(kgm–3 h–1)
Current density is one of the significant operating parameters influencing the percentage COD removal and power consumption in the hybrid process. Experiments were carried out by varying the current density from 1 to 5 Adm − 2 and the results are shown in Fig. 6. According to Faraday's law, the current density and electrolysis time are directly proportional to the rate of coagulant dosage. So the rate of dissolution of iron electrode increased with increasing the current density. The percentage COD removal sharply increased up to the current density of 4 Adm − 2. Further increase of current density
5.4. Effect of initial COD concentration
20 15 10
60 40
1250 ppm 2500 ppm
20
3750 ppm 5000 ppm
0
5 0
2
4
6
8
10
12
pH Fig. 5. Space time yield at various initial pH (condition: current density: 3 Adm− 2, initial COD concentration: 2500 ppm, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
0
1
2
3
4
Time (h) Fig. 7. Effect of initial concentration on the percentage colour removal (condition: current density: 3 Adm− 2, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
P. Asaithambi et al. / Desalination 297 (2012) 1–7
80
6
60 4 40 2
20
COD removal Power Consumption
0 0
1250
2500
3750
Power Consumption (kWhm-3)
8
100
COD removal (%)
5
Fig. 10. Decolourization of the distillery effluent at various time intervals during the ozone assisted electrocoagulation process.
0 6250
5000
5.6. Effect of electrode combination
Initial COD Concentration (ppm) Fig. 8. Effect of initial concentration on the percentage COD removal and power consumption (condition: current density: 3 Adm− 2, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
constant current density which decreases the power consumption with increasing initial COD concentration of effluent. The UV–Vis absorbance spectra for the distillery effluent with initial concentration of 3750 ppm for initial and various time treatment of ozone assisted electrocoagulation process are shown in Fig. 9. It can be ascertained from the figure that the absorption peak diminishes with progressive interval of time under the wavelength of 300 nm. The decrease in absorption peaks at λmax in this figure shows the decolourization of distillery effluent. Fig. 10 shows the colour removal of the effluent at various time intervals. It is observed that complete colour removal is achieved with in 2 h of process time.
The different electrode material and electrode combinations are very important factors affecting the performance of the ozone assisted electrocoagulation process. The electrode combinations of Fe/Fe, Fe/Al, Al/Fe and Al/Al (anode/cathode) have been selected to investigate the percentage colour and COD removal which is shown in Fig. 12. The percentage COD removal of distillery effluent in the different electrode combination of Fe/Fe, Fe/Al, Al/Fe and Al/Al is 83, 78, 68 and 64%, respectively. The colour of the effluent is completely removed within 2 h of electrolysis time. However, the removal of COD mainly depends on the combination of electrode. The electrochemical reactions take place during electrocoagulation at Al anode and Fe anode: For Al anode: 3þ
Al→Al
þ 3e
−
ð19Þ
For Fe anode 5.5. Effect of inter-electrode distance
2þ
Fe→Fe The effect of inter-electrode distance on the percentage COD removal is shown in Fig. 11. The percentage COD removal decreases from 83% to 66% with increasing inter-electrode distance between anode and cathode from 1 to 3 cm. This is due to the fact that increase in inter-electrode distance increases IR resistances which increase the cell voltage and adversely affects the percentage colour and COD removal. Therefore, the increase in IR drop is not recommended for ozone assisted electrocoagulation process for the effluent treatment in order to have acceptable power consumption as well as desired effective removal of pollutants. Hence, in order to achieve high removal of COD in the effluent concentration the inter-electrode distance between anode and cathode should be minimized.
ð20Þ
The Fe electrode material shows higher COD removal compared to the Al electrode. This is due to the higher oxidation potential of Fe (−0.447 V) than Al (−1.662 V). The generation of Fe coagulant is three times greater than Al which causes the higher COD removal efficiency in the iron electrode as anode. 5.7. Kinetic modeling The kinetics model has been proposed for the ozone assisted electrocoagulation process. Olad et al., have chosen the pseudo‐first‐order kinetics for the electrocoagulation process for removal of the
100
1.5 initial
COD removal (%)
1.2 0.5 h 1h
Abs
0.9
2h
0.6
3h 4h
0.3 0 200
−
þ 2e
1 cm
80
2 cm 3 cm
60 40 20 0
300
400
500
600
700
800
0
1
2
3
4
Time (h)
Wavelength (nm) Fig. 9. Absorbance spectra of the distillery effluent solution as a different electrolysis time in the ozone assisted electrocoagulation.
Fig. 11. Effect of inter-electrode distance on the percentage COD removal (condition: current density: 3 Adm− 2, initial COD concentration: 2500 ppm, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
6
P. Asaithambi et al. / Desalination 297 (2012) 1–7
1
100 Fe - Fe Fe - Al Al - Fe Al - Al
60
Model
0.8
Experimental 0.6
C/Co
COD Removal (%)
80
40
0.4
20
0.2 0
0
1
2
3
4
0 0
Time (h)
1
2
3
4
Time (h) Fig. 12. Effect of electrode combination on the percentage COD removal (condition: current density: 3 Adm− 2, initial effluent concentration: 2500 ppm, initial pH: 6, electrolysis time: 4 h, ozone flow rate: 15 L min− 1).
Fig. 13. Comparison of experimental and model values for ozone assisted electrocoagulation process to the treatment of distillery effluent.
Alphazurine FG dye from simulated solution [38]. Hence, the present kinetic analysis of COD removal for the real distillery effluent using ozone assisted electrocoagulation is based on pseudo-first-order kinetics. The experimental results exhibited pseudo-first-order kinetics with respect to the effluent COD concentration and current density in the hybrid process. A mathematical relation has been derived to study the system design easily for further potential applications. Based on the experimental results, it is assumed that the rate constant, kobs, is affected by the key factors of initial COD concentration and current density. An empirical form is then used to present their relationships,
study has been optimized to minimize electrical power consumption and maximize the percentage colour and COD removal of the effluent. The maximum percentage colour and COD removal of 100% and 83% has been observed for an initial COD concentration of 2500 ppm, initial pH 6 and current density 3 Adm − 2. The inter-electrode distance of 1 cm shows better removal efficiency. The pseudo‐first‐order kinetic model satisfactorily matches with the experimental results and also the mathematical model was developed as a function of initial COD concentration and current density. This technology will continue to work in the water and wastewater treatment process due to the significant improvement over the conventional system.
kobs ¼ f ðinitial COD concentrationðCA Þ; current density ðCDÞÞ a b kobs ¼ m½CA ½CD
References
ð21Þ
where m, a, and b are constants. The constants a and b can be evaluated from the time vs concentration plot and time vs current density plot. After combining these two values the constant, m can be calculated. From the experimental value the above equation can be written as C=C 0 ¼ e
−kobs t
− 0:5656½C A −0:656 ½CD0:3265 Þt ¼e ð
ð22Þ
The theoretical COD removal can be calculated using the following equation ð23Þ Percentage COD removal ¼ ð1−C=C 0 Þ 100 −ð0:5656½C A −0:656 ½CD0:3265 Þt 100 ¼ 1−e In order to validate the model, model simulations are compared with experimental values as shown in Fig. 13. It is observed from the figure that the kinetic model satisfactorily matches with the experimental observation. 6. Conclusion The ozone assisted electrocoagulation process was successfully tested for the treatment of real distillery industry effluent. The wide range of operating parameters such as effluent initial COD concentration, initial pH, current density, inter-electrode distance and electrode combinations on the percentage of colour and COD removal, power consumption were investigated. The percentages of colour and COD removal for ozonation, electrocoagulation and hybrid processes were compared. The result showed that hybrid technique was more effective than electrocoagulation and ozonation alone. The present
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