Treatment of Textile Effluents by Electrochemical Advanced Oxidation Processes

Abstract

Textile industries are associated with the generation of large volumes of wastewater containing recalcitrant dyes and other textile processing chemicals. The discharged effluents from these industries have been demonstrated bearing high pollution load (high dissolved solids, COD, color and chloride content) with poor biodegradability. Therefore, untreated textile wastewater causes sever damage to environment, if discharged without treatment. There are various traditional techniques have been used for the treatment of real textile wastewater, including biological processes, adsorption, membrane process and chemical coagulation. Treatment of real textile wastewater, which is of high strength and complex in nature, has become a real challenge with the traditional methods. Recently, Electro-oxidation (EO) and electro-Fenton (EF) is drawing attentions for the treatment of non-biodegradable wastewater such as textile wastewater. In the present study, the treatment performance of the EO and EF treatment processes for actual textile wastewater in batch and continuous mode of operation using RuO2 coated Ti electrode (Ti/RuO2) was studied. The effects of various selected process parameters of EO and EF processes (batch and continuous) parameters on percentage chemical oxygen demand removal (X1), percentage color removal (X2) and energy consumed (X3) were investigated. Parametric study and multiple response optimization for EO and EF (batch and continuous) was performed using RSM. Experimental data were then analyzed by multiple regression analysis of RSM. Simultaneous optimization of process parameters for the responses was performed with the desirability function approach. Box Behnken Design (BBD) under RSM was used for experimental design, data analysis, optimization, interaction analysis between the various batch EO parameters whereas Central Composite Design (CCD) under RSM was used for continuous EO process parameters and steady state time analysis. Quadratic model was suggested by exploiting sequential F-test and other adequacy measures for EO (batch and continuous). For the responses X1, X2 and X3, the adequate precision for EO (batch and continuous) indicated that model was efficient and significant. The model summary statistics for responses X1, X2 and X3 showed high value of coefficient of determination. It supports a satisfactory adjustment between the observed and predicted values for the selected responses for EO (batch and continuous) processes. Optimization was successfully performed using BBD for batch EO and CCD for continuous EO under RSM. The most appropriate optimization condition for batch EO was found to be i=1.66 A, t= 79.55 min and pH=5.49 with X1=80.00%, X2 = 97.25% and X3 =14.58 kWh/kg of COD removed, which showed highest overall desirability, D = 0.88. On the other hand, for continuous EO process the most appropriate optimization condition was found to be i=1.37 A, t= 124 min and pH=5.45 with X1=81.00%, X2 = 92.25% and X3 =10.88 kWh/kg of COD removed, with overall desirability, D = 0.89. Kinetic study of EO (batch and continuous) follows first order of kinetics for COD and color removal. During kinetic study of EO (batch and continuous) processes faster color removal was observed than COD. The revealing rate constant values for X1 and X2 were 0.025 min-1 and 0.015 min-1 respectively for batch EO whereas, for continuous EO with rate constant values to be 0.0127 min-1 and 0.0168 min-1, respectively. Furthermore, disposal study of treated wastewater by EO process was examined through spectrophotometric and GC-MS analysis by identifying eliminated organic compounds and transformation products formed during treatment. GC-MS analysis of the batch and continuous EO treated wastewater revealed that most of the organics were completely eliminated during EO process. The presence of chlorinated organic compounds and other transformation products with the already present compounds in untreated textile wastewater were also detected. Therefore, in view of disposal of treated wastewater, toxicity bioassay test was performed. 100% mortality rate approximately in one hr was observed for treated textile effluent. Acute toxicity bioassay proves that the treated textile wastewater is toxic and harmful to the environment. Moreover, operating cost (electrode and electricity cost) analysis was also performed to see the economic feasibility of the process. The total operating cost for batch and continuous EO was estimated as 8.97$/kg of COD removed and 8.66 $/kg of COD removed at optimum operational conditions, respectively. The parametric study of batch EF was performed by using BBD under RSM whereas CCD under RSM was used for continuous EF process parameters. Quadratic model was suggested by exploiting sequential F-test and other adequacy measures. For the responses X1, X2 and X3, the adequate precision indicated that model was efficient and significant. The model summary statistics for responses X1, X2 and X3 showed high value of coefficient of determination. It supports a satisfactory adjustment between the observed and predicted values for the selected responses. The EF (batch and continuous) performance of the treatment process was evaluated in terms of X1, X2, and X3 at different EF (batch and continuous) processes parameters. The most appropriate optimization condition for batch EF was found to be i= 0.32 A, t= 89.63 min and catalyst dose (CFe) =0.53 mM with X1=89.75, X2 = 99.49 and X3 =1.3 kWh/kg of COD removed, which showed highest overall desirability, D = 0.92. On the other hand, for continuous EF process the most appropriate optimization condition was found to be i=1.10 A, t= 137 min and CFe=0.55 mM with X1=84.16, X2 = 94.00 and X3 =15 kWh/kg of COD removed, which showed highest overall desirability, D = 0.80. Second order kinetic model for X1 and X2 was best fitted to the experimental data, at optimum conditions for batch and continuous EF processes. The values of rate constant for batch (X1 and X2) and continuous (X1 and X2) EF of second order reaction kinetics are 0.001 l/mg min, 2.756 l/mg min and 2x10-5 l/mg min, 9.5 x10-3 l/mg min, respectively. GC-MS and UV-visible spectrophotometric analysis of the untreated and treated wastewater were conducted to identify the oxidized and transformed/degraded compounds during the (batch and continuous) EF process. Spectrophotometric and GC-MS analysis showed that all the components of textile wastewater were totally eliminated after EF treatment of textile effluent. In view of disposal of treated wastewater, toxicity bioassay test was performed. 0% mortality rate in 96 hour was observed for treated textile effluent. Acute toxicity bioassay proves that the treated textile wastewater is not toxic and safe to the environment. To determine the economic feasibility of the EF treatment, the total operating cost was estimated. The total operating cost for batch and continuous EF was estimated as 7.86 $/kg of COD removed and 9.00 $/kg of COD removed, respectively. The comparative study of the EO and EF treatment processes shows that, EF was more effective for the treatment of textile wastewater as compare to the EO treatment. EO treatment process during the treatment of textile effluent generates chloro-compounds. These chloro-compounds are reported to be toxic but in case of EF treatment no chloro-compounds were generated.