Phosphate removal from wastewaters by batch and continuous electrocoagulation processes and optimization of the process

In the study, the effect of operating parameters such as pH, current density and initial phosphate concentration on removal of orthophosphates, pyrophosphates and metaphosphates from aqueous solutions by an EC model reactor operating under batch mode, where Al (aluminium) and Fe (iron) electrodes were used, was investigated. The Box-Behnken statistical design was utilized for optimization studies. According to the results obtained, in the EC model reactor where Al electrodes were used, it was found out that the optimum initial pH was 6.71, 6.96, 6.69, respectively, the optimum current density was 33.20, 36.51, 30.02 A/m2, respectively, and the optimum phosphate concentration for all of the phosphate fractions was 5.0 mg/L under the conditions of the highest removal efficiency for orthophosphates, pyrophosphates and metaphosphates. In the case of Fe electrode usage, the optimum values for the highest removal efficiency for orthophosphates, pyrophosphates and metaphosphates were obtained as 3.49, 3.00, 3.00 for the pH, respectively, 50.00, 50.00, 48.72 A/m2 for the current density, respectively, and 5.0 mg/L for orthophosphates and pyrophosphates and 11.48 mg/L for metaphosphates for the phosphate concentration. In order to determine the removal mechanism for the phosphate fractions, SEM, EDS and XRF analysis were carried out.The coefficient of determination (R2) value for removal of orthophosphates, pyrophosphates and metaphosphates in the EC model reactor, where Al electrodes were used, was found to be 0.95, 0.94, 0.91, respectively, under the optimum conditions whereas it was found to be 0.95, 0.91, 0.96, respectively in the EC model reactor where Fe electrodes were used. Being R2 and R2adj values close to 1 indicates that the experimental results and statistical inferences were compatible and the Box-Behnken statistical design was effective in order to determine the effects of process parameters.The continuous-flow EC model reactor was designed by considering the optimum conditions for the EC model reactor operating under the batch mode, and the continuous-flow reactor was used to investigate the effects of the time of electrolysis and current density on removal of phosphates from potato chips processing wastewaters. In the EC model reactor in which either Al or Fe electrodes were used, the optimum hydraulic retention time was found to be 40 and 60 min, respectively, while the final pH was found to be 7.85 and 8.15, respectively, and the current density was found to be 65 A/m2 for both electrode types. In case of using Al and Fe electrodes under these experimental conditions, the initial total phosphorus concentration of 54.4 mg/L decreased 10.4 and 5.0 mg/L, respectively, and the initial orthophosphate concentration of 45.3 mg/L decreased 5.9 and 2.9 mg/L, respectively. Considering the final pH value, it can be pointed out that the resultant effluent could be discharged without any pH adjustment.High removal efficiencies were obtained for low phosphorus concentrations in the whole studies carried out with the EC model reactor operating under batch mode whereas higher removal efficiencies were obtained even higher phosphorus concentrations in the studies carried out with the continuous-flow EC model reactor. Additionally, it was determined that the current density and initial phosphate concentration were more effective than the pH in the batch-mode EC model reactor, however, the pH was more effective than the others in the continuous-flow EC model reactor. Since the other contaminants out of the phosphate in the examined potato chips processing wastewaters had an effect on the removal efficiency of the continuous-flow EC process, the optimum conditions determined for phosphorus removal in the batch-mode EC model reactor were not able to be applied accurately although the conditions suggested a design for the continuous-flow EC reactor.It was found out that the primary parameters affecting operating costs for the EC model reactor, in which Al and Fe electrodes were used individually, are the current density and initial phosphorus concentration, and the secondary parameter is the pH.To remove orthophosphates, pyrophosphates and metaphosphates in the EC model reactor operating under the batch mode and the optimized conditions, operating costs were found out to be $1.04, 1.16 and 0.88 per cubic meter wastewater, respectively, when Al electrodes were used. In the case of using Fe electrodes under the same operational conditions, operating costs for the removal of orthophosphates and pyrophosphates decreased and found out to be $0.93 and 1.08 per cubic meter wastewater, respectively. However, operating costs for the removal of metaphosphates increased by 18% and found out to be $1.04 per cubic meter wastewater.Considering the costs for electrodes, energy and chemical agents, operating costs for the continuous-flow EC model reactor were found out to be $1.35 and 1.16 per cubic meter wastewater for Al and Fe electrodes, respectively. It was determined that Fe electrodes were more effective than Al electrodes for the removal of phosphates from potato chips processing wastewaters, and that operating costs were lower when Fe electrodes were used.