The air we breathe and the water we drink are essential ingredients for our well-being and a healthy life. Water pollution is any contamination of water with chemicals or other foreign substances that are detrimental to human, plant, or animal health. Rapid industrialization and urbanization resulted in the discharge of large amount of wastewater to the environment, which in turn creates more pollution. These pollutants include fertilizers and pesticides from agricultural runoff, sewage and food processing waste, lead, mercury, and other heavy metals, chemical wastes, chemical contamination and radioactive chemical toxic materials from industrial discharges. Many industries like textiles, refineries and chemical, plastic, pharmaceutical and food processing plants produce wastewater characterized by a perceptible content of organics with strong color, high Chemical Oxygen Demand (COD) and with wide variation in pH values. Hazardous wastes are a continuous problem in today's world, increasing in both quantity and toxicity.
Among the organic pollutants in industrial effluents, the common hazardous pollutants are Synthetic organic resin effluent, Hydroquinone, Chlorophenols and Aromatic Nitrocompounds. Synthetic ion exchange resin contaminations are found in many industries in radioactive nuclides and nuclear power plants in the form of spent ion exchange resin waste and in other phenol based chemical industries. Hydroquinone occurs in the environment because of manmade processes and from plants and animals. Chlorophenols are present in chemical industry wastewaters such as pulp and paper, pesticides, petrochemicals and plastics. 2, 4-Dicholoropnenols has been extensively used as a wood preservative, pesticide and as a precursor for the synthesis of herbicides (Rape, 1980). Aromatic nitro compounds are among the most toxic substances used commonly in the manufacture of explosives, pesticides, dyes, plasticizers and pharmaceuticals (Hallas et al., 1983; Spain et al., 1991 and Hanne et al., 1993). Lypczynska-Kochany, (1992) in his study detected these compounds detected not only in industrial wastewaters but also in freshwater and marine environment. In particular, para-nitrophenol is a toxic derivative of the parathion insecticide and is present in wastewaters from industries such as refineries.
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Many treatment technologies are in use and researchers have proposed quite a few for the recovery or destruction of these pollutants. The treatment methods include activated carbon adsorption, solvent extraction for recovery of chemicals, electrochemical oxidation for destruction, direct incineration, chemical destruction and even direct immobilization in matrix like cement, polymer. Microorganisms process most organic based effluents. In addition, industries employ techniques like wet oxidation, photochemical oxidation and electrochemical oxidation for the management of hazardous organic mixed wastes. Among the above methods, Electrochemical Oxidation offers an attractive way of treating solid or liquid organic waste as it uses electron as a reactant. Research studies report the application of electrochemistry for the protection of the environment (Pletcher, and Walsh 1990; Sequeira, 1994 and Rajeshwar and Ibanez, 1997).
Depending on the nature of pollutant, with due considerations for the toxicity of the reactants and products, an appropriate choice of the treatment method has to be made. Rajeshwar and Ibanez (1997) suggested that it might be even necessary to use more than one method for getting effective treatment of effluent. Rajeshwar et al., (1994) in their study proposed the advantages of using electrochemical techniques (electro-oxidation and electro-coagulation) as environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability to automation, and cost effectiveness. The use of electrochemical technology has been widely studied as a method for the removal of organic substances (Juttner et al., 2000 and Chen, 2004). Vlyssides et al., (2004) observed good removal rates and suggested usage of electrochemical method as a pre-treatment step in pesticide waste disposal.
Research studies reveal that Mediated Electrochemical Oxidation is one of the most promising technologies extensively used for the destruction of organics since it is capable of mineralizing the organics into carbon dioxide and water completely, without the emission of any toxic materials like dioxins (Farmer et al., 1992; Chiba et al., 1995 and Nelson, 2001). Raju and Basha (2005) in their study identified that the mediated metal ions have a strong potential to oxidize and a high temperature is not required for organic oxidation and therefore produces less volatile and off gases.
Electro-coagulation method has turned out to be the most preferred method for the removal of Hydroquinone from water. Research studies reveal the ability of electro-coagulation to eliminate a variety of pollutants from wastewaters; metal and arsenic (Ratna Kumar et al., 2004; Gao et al., 2005 and Hunsom et al., 2005) clay minerals (Matteson et al., 1995 and Holt et al., 2004) oil (Xu and Zhu, 2004) chemical oxygen demand (Murugananthan et al., 2004 and Xu and Zhu, 2004) and color and organic substances (Jiang et al., 2002).
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Removal of 2,4 Dichlorophenol and para-nitrophenol are carried out by electrochemical and biological methods. Biological treatment of chlorophenols attracts more attention than physical and chemical methods, because a variety of microorganisms such as Pseudomonas pickettii, Alcalilgenes eutrophus, Desulfomonile tiedjei and Phanerochaete chrysosporium utilize chlorophenols as the sole carbon or energy source (Mohn et al., 1992; Fava et al., 1995; Hill et al., 1996 and Perez et al., 1997).
Electrochemical treatments of effluents are carried out using Batch and Batch recirculation processes by varying a number of parameters to find out the optimum COD removal rate. The optimum COD removal rate could be determined using Response Surface Methodology, which is a powerful tool for understanding complex processes for describing factor interactions in multifactor system. Response Surface Methodology is a collection of statistical and mathematical methods that are useful for the modeling and analyzing of engineering problems. The main objective of the technique is to optimize the various process parameters and quantify the relationship between the controllable input parameters and the obtained response surfaces (Gunaraj and Murugan, 1999 and Kwak, 2005). Researchers use Response Surface Methodology to determine the relation between the percentage of COD removal, specific energy consumption and important operating parameters such as current density, flow rate, concentration of effluent, supporting electrolyte concentration, time of electrolysis and volume of electrolyte.
The main objective of the study is to investigate the electrochemical approaches for the degradation of selected refractory organics. This study considers electrochemical parameters for the treatment of synthetic resins, Hydroquinone, 2,4 Dichlorophenol and para-nitrophenol effluents.
To study the effect of TOC reduction by electrochemical oxidation on synthetic resin effluent using RuO2/Ti by batch and batch recirculation processes.
To study the effect of COD reduction by electro coagulation on Hydroquinone removal from water by using flow electrolyser in Monopolar and Bipolar configuration in a batch recirculation mode of operation.
To study the effect of COD reduction by combined electrochemical oxidation and biological process in wastewater containing 2,4 Dichlorophenol.
To study the effect of COD reduction by combined electrochemical oxidation and biological process in wastewater containing para-nitrophenol.
To study the optimum COD reduction and specific power consumption of Hydroquinone, 2, 4 Dichlorophenol and para-nitrophenol by varying various operational parameters like current density, supporting electrolyte concentration, flow rate, volume of electrolyte, time of electrolysis, and concentration of effluent using Response Surface Methodology (Box-Behnken method).
MATERIALS AND METHODS
The current study focused on the treatment of resin effluent using batch process with recirculation and batch process. For Batch process with recirculation the synthetic resin effluent was prepared by dissolving an appropriate amount of cationic resin (Amberlite strong acid styrene based cation exchange resin - functional group - SO3H) in water in the presence of ferrous sulphate (Fe2-/Fe3-) as catalyst, with drop wise addition of H2O2 by maintaining temperature of the reaction mixture at 95-100°C. Experiments were carried out under galvanostatic condition using RuO2 coated Titanium expanded mesh anodes and stainless cathodes. The study used NaCl as a supporting electrolyte with various concentrations. The electrolysis was carried out at different flow rates of 20, 40, 60, 80, 100 l/h and at different current densities such as 1.25, 2.50, 3.75, 5.00, 6.25, 7.50, 8.75, 10.00 A/dm2. The study used a similar procedure for Batch process at different current densities of 1.25, 2.50, 3.75, 5.00 A/dm2 and conducted for 7 hours. One ml of sample was collected every one hour and the temperature, pH, cell voltage and electrode potentials were measured. The experiments were conducted using 200, 300 and 400 ml of effluent.
The second study involved the treatment of Hydroquinone by electro-coagulation method in Monopolar and Bipolar configuration. Monopolar configuration used two Aluminum electrodes as anode and cathode and used PVC frame as a middle compartment. The effective surface area of anode and cathode were 7cm - 7cm. The electrodes were positioned vertically and parallel to each other with an inter electrode gap of 10mm and electrolysis was carried out under Batch recirculation mode by adjusting the specified flow rates. The study passed the required current using regulated power supply and the cell voltages were noted. The experiments were carried out for six hours at different operating parameters. One ml of sample drawn from the reservoir was analyzed for the estimation of COD every hour. The Bipolar configuration had two compartments and four aluminum electrodes were used with a dimension of 9 cm - 9 cm - 0.1 cm. The working area of each of the electrode surface area was the same as that of Monopolar configuration. The volume of the electrolytic cell was 50 ml whereas the reservoir capacity was one litre and electrolysis was carried out under Batch recirculation mode. The experiments were carried out for six hours with different operating parameters. Every hour one ml sample was collected from the reservoir and COD was analyzed.
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The third part of the study involved the treatment of effluent containing 2,4 Dichlorophenol using electrochemical and biological treatment methods sequentially. The electrochemical oxidation of the effluent was carried out using a batch electrolytic cell with recirculation. The experimental setup consists of an undivided electrolytic cell of 300 ml working capacity closed with a PVC lid having provisions to fix a cathode and an anode keeping 2.5 cm inter-electrode distance. The electrode used was a noble metal oxide MOX (RuO2) coated on Ti as anode, an expanded mesh of area 39.2 cm2 was employed and a stainless steel plate of dimension 8.0cm-8.0cm-0.2 cm was used as the cathode. The optimum COD removal was determined using Response Surface Methodology. The biological treatment of effluent was carried out using the bacterial culture of Pseudomonas aeruginosa isolated from paper mill wastewater obtained from Microbiology laboratory of Bharathiyar University, Coimbatore. Nutrient medium (5 g/l peptone and 3 g/l Beef extract) was taken and it was dissolved in distilled water followed by sterilization of medium in autoclave at 1210C for 15minutes at 15lb/inch2. The final study was carried out using para-nirtophenol with the experimental setup similar to that of 2,4 Dichlorophenol.
The study choose the Box-Behnken experimental design of Response Surface Methodology to find the relationship between the response functions and variables using the statistical software package Design Expert Software-7.1.2, (Stat-Ease, Inc., Minneapolis, USA). The Box-Behnken design is a highly fractionalized three-level factorial design where the treatment combinations are the midpoints of edges of factor levels and the center point. These designs are rotatable (or nearly rotatable) and require three levels of each factor under study. Box-Behnken designs can fit full quadratic response surface models and offer advantages over other designs. Oliveira et al., (2007) lists the advantages of the Box-Behnken design over other response surface designs (a) needs fewer experiments than central composite design and similar ones used for Doehlert designs (b) in contrast to central composite and Doehlert designs, it has only three levels (c) easier to arrange and interpret than other designs (d) can be expanded, contracted or even translated and (e) it avoids combined factor extremes since midpoints of edges of factors are always used.
RESULTS AND DISCUSSION
The study using Batch process with recirculation for resin effluent highlighted that in batch reactor set up the best effect of TOC reduction was found to occur at 3.75 A/dm2 with a flow rate of 20 l/h. From the results it was found that TOC reduction is almost independent of NaCl and has only very little effect on TOC elimination. NaCl concentration should be kept minimum since anodic oxidation of organics in the presence of NaCl leads to the chlorination of some organics. The percentage reduction of TOC was found to be high for slow flow rates since the residence time of the electrolyte solution inside the electrolytic cell was more than at higher flow rates.
Figure 1: Percentage reduction of TOC Vs Flow rate for a Current density of 10 A/dm2
Figure 2: Current density Vs Percentage reduction of TOC for Flow of rate 20 l/h
Simulated studies were carried out for different volumes of effluent and current densities. The electrochemical treatment of resin effluent using batch process highlighted that in batch reactor set up the maximum TOC reduction was found to occur at 3.75 A/dm2 with flow rate of 20 l/h. The simulated studies carried out for different volumes of effluent and current density reveals that the experimental and simulated values are within the limits.
Experiments were carried out on the removal of Hydroquinone in flow eletrolyzer by electro-coagulation technique covering wide range of operating conditions in Monopolar and Bipolar configurations. Investigations revealed that the percentage of COD reduction was significantly influenced by the Hydroquinone concentration, supporting electrolyte concentration, flow rate and current density. Current density and supporting electrolyte concentration are the important factors for the degradation of Hydroquinone.
The experimental data were analyzed using Response Surface Methodology and the individual and combined parameter effects on COD reduction were analyzed. Three levels and four factors Box-Behnken experimental design was applied. The study developed a regression equation for COD removal and energy consumption using sets of experimental data and solved by using the Design Expert software.
Figure 3:Response surface plot showing the effect of concentration of HQ and SE on % of COD removal (Conditions: Q 35 mL.min−1; CD 0.6 A.dm−2; configuration: Monopolar).
Figure 4:Response surface plot showing the effect of concentration of HQ and SE on % of COD removal (Conditions: Q 35 mL.min−1; CD 0.6 A.dm−2; configuration: Bipolar).
Results revealed that the model predictions of COD removal and energy consumption were in good agreement with the experimental observations. Further, the study optimized the parameters for effective degradation of Hydroquinone in flow electrolyzer using Response Surface Method. The optimized values for 80.95% of COD removal through electro-coagulation were; supporting electrolyte concentration 2.67 g/l, flow rate 27 ml/min, current density 0.7 A/dm2 and energy consumption of 2.36 kWh per kg of COD for the 1000 mg/l of Hydroquinone concentration in Monopolar configuration. In Bipolar configuration for 87.13 % of COD removal through electro-coagulation the optimized values were; supporting electrolyte concentration 4 g/l, flow rate 29.15 ml/min, current density 1 A/dm2 and energy consumption of 8.495 kWh per kg of COD for the 1000 mg/l of Hydroquinone concentration.
The combination of electrochemical and biological oxidation treatment of synthetic effluent containing 2,4 Dichlorophenol revealed that the percentage of COD removal and specific energy consumption were significantly influenced by the current density and time of electrolysis. It was found from the simulation produced by Response Surface Methodology that 37.21% of COD reduction was the highest amount of COD removal and the optimum conditions were satisfied at 100% effluent concentration, current density 3 A/dm2, Time two hours, flow rate 35 l/hour and volume of 2 litre. This optimized condition was taken for pretreatment before all biological processes. Aerobic process and Anoxic process were carried out over nine different proportions of inoculums of pseudomonas sp 50, 75 and 100 ml/l and nutrients of 0, 25 and 50 ml/l.
Figure 5: Time of Aerobic process Vs % COD Removal (50 mg/l)
Figure 6: Time of Aerobic process Vs % COD Removal (75 mg/l)
Figure 7: Time of Aerobic process Vs % COD Removal(100 mg/l)
Results revealed that as the amount of inoculum and nutrient increases, the percentage of COD reduction and biodegradability index increases. Here, the percentage of COD reduction and biodegradability index was found to be maximum of 83.10 % and 0.609 respectively for aerobic and for anoxic it was found to be 86.45 % and 0.789 respectively.
The combination of electrochemical and biological oxidation treatment of synthetic effluent containing para-nitrophenol revealed that the percentage of COD reduction and specific energy consumption were significantly influenced by the current density and time of electrolysis. The simulation produced by Response Surface Methodology revealed that 39.3% of COD reduction was the highest amount of COD removal i.e. 1584 mg/l. This simulated value was taken for the biological process. Specific energy consumption was found to be 32.34 kWh/kg COD. It was also found that as the current density and time increases, the amount of specific energy consumption also increases, while other factor did not have much effect on Specific energy consumption. Aerobic and anoxic processes were carried out over nine different proportions of inoculums of pseudomonas sp 50, 75 and 100 ml/l and nutrients of 0, 25 and 50 ml/l. From the study, it was observed that as the amount of inoculum and nutrient increases the percentage of COD reduction and biodegradability index increases. Here, the percentage of COD reduction and biodegradability index was found to be maximum of 78.92 % and 0.521 respectively for aerobic and for anoxic it was found to be 97.30 % and 0.710 respectively.
Figure 8: Time of Aerobic Process Vs Biodegradability Index (50mg/l)
Figure 9: Time of Aerobic Process Vs Biodegradability Index (75mg/l)
Figure 10: Time of Aerobic process Vs Biodegradability Index (100mg/l)
The present study discusses the diverse methods that could be applied for the treatment of industrial organic effluents. Among the industrial effluents, the study investigated optimum removal of TOC and COD in resin effluent, Hydroquinone from water, 2, 4 Dichlorophenol and para-nitrophenol by taking into account parameters namely current density, flow rate, concentration, supporting electrolyte concentration and time.
Experiments revealed that for resin effluent the optimum COD reduction of 80.17% was achieved with Batch recirculation process compared to Batch process of 52.2%. The results of the study reported that TOC reduction is almost independent of NaCl and has only very little effect on TOC elimination. NaCl concentration should be kept minimum since anodic oxidation of organics in presence of NaCl leads to the chlorination of some organics. The study further revealed that the TOC removal was very low; about 25 % only in the absence of Fenton's mediator, which implied the catalytic action of Fenton's mediator. The study aligned with the earlier studies of Beauchesne et al., (2005); Chen and Lim, (2005); Hwang et al., (1987) that the electro chemical oxidation method is advantageous and efficient for the removal of resin effluents which predominantly contain metal ions.
In the electro-coagulation method for the removal of Hydroquinone from water the model predictions of COD reduction and energy consumption using Response Surface Methodology are in good agreement with experimental observations. The optimized values for COD reduction was 80.95% in Monopolar configuration and 87.13% in Bipolar configuration and the corresponding energy consumption values were 2.36kWh per kg of COD and 8.495kWh per kg of COD respectively.
Experiments carried out with effluent containing 2,4 Dichlorophenol and para-nitrophenol revealed that as the amount of inoculum and nutrient increases the % of COD reduction and Biodegradability index increases. For 2,4 Dichlorophenol the % of COD reduction and biodegradability index was found to be maximum of 83.10 % and 0.609 respectively for aerobic and for anoxic it was found to be 86.45 % and 0.789 respectively. For para-nitrophenol the % of COD reduction and biodegradability index was found to be maximum of 78.92 % and 0.521 respectively for aerobic and for anoxic it was found to be 97.30 % and 0.710 respectively. Outcome of the study reveals the applicability of combined electrochemical and biological treatment of synthetic wastewater containing 2,4-Dichlorophenol and para-nitrophenol as an alternative method to previous conventional solutions and it is more economical than the other existing processes.