Mathematical Modelling and Response Surface Methodology Approach of Electrocoagulation Hybrid Activated Sludge Process for an Efficient Removal of Selenium from Mining Wastewater

Selenium (Se) is an important nutritional element which exists at very low concentrations, easily accumulates via the food chain and creates adverse effects such as a deprived reproduction rate and diminutive growth in human and aquatic organisms. So, it has become a severe concern around the world. We explore electrocoagulation using Al and Fe electrodes and activated sludge process (ASP) in batch process and also in an integrated process to remove Se. The optimized parameters of the current density in the batch process were: 6.7 and 5.7 mA/cm² for Al and Fe, respectively. The mass transfer coefficient has been estimated through numerical modelling for batch and integrated processes using the equations K = \({\text{0}}{\text{.0146}}C_{{{\text{Se}}}}^{{{\text{0}}{\text{.3651}}}}{{I}^{{{\text{0}}{\text{.8916}}}}}\) and K = \({\text{295}}{\text{.387}}C_{{{\text{Se}}}}^{{{\text{6}}{\text{.607}}}}{{I}^{{{\text{3}}{\text{.587}}}}}\) ; the energy consumption and metal dissolution were 138240 and 384 MWh/m³, 60 and 3.58 g, respectively. The response surface methodology (RSM) was implemented in Box−Benken design to assess the parametric optimization, and the validation of experimental data was done using ANOVA and regression analysis. The obtained p-values and model F-values were 0.000 and 63.09 for Al and 0.000 and 79.98 for Fe, which indicated the significance of the model. The chemical oxygen demand (COD) reduction values estimated along with Se reduction in real effluent treatment were above 90 and 60% in electrolytic and 80% in an integrated ASP with very high-cost efficiency. The results assure that this proposed hybrid work will provide a higher reduction, improved energy and cost efficiency for the effluent with indeterminate influent Se and COD concentration. The proposed model also helps to make predictions of removal efficiency without requiring an extensive time and cost burden.