In this paper, the response surface methodology (RSM) approach using central composite design (CCD) was applied to develop mathematical model and to optimize process parameters for COD and TSS removal from tannery wastewater by electrocoagulation and electro-Fenton processes using iron electrodes. Analysis of variance (ANOVA) was used to analyze the data and to obtain the interaction between the process variables and the responses. ANOVA showed the relative significance of process parameters in the removal process. The second-order regression model was developed to predict the removal efficiency using Statgraphics Centurion XVI.I software program. The optimal conditions were determined as reaction time 5.0 min, pH 3.31, current density 53.72 mA/cm², and H₂O₂ dose 0.14 g/L for electro-Fenton process, and reaction time 40.4 min, pH 7.0, and current density 50.9 mA/cm² for electrocoagulation process. At optimum conditions 54.8 and 87.3% COD removal efficiencies, 86 and 88% TSS removal efficiencies were achieved in electrocoagulation and electro-Fenton processes, respectively. The operating costs for elecrocoagulation and electro-Fenton processes at optimized conditions were calculated to be 6.4 and 6.8 €/m³. Electro-Fenton process was found to be more effective for tannery wastewater treatment.

In this study, response surface methodology approach using Central Composite Design was applied to develop a mathematical model and to optimize process parameters for the COD, color, orthophosphate, TSS, and turbidity removal from dairy wastewater by electrocoagulation process using iron and aluminum electrodes. The second-order regression model was developed to predict the removal efficiencies using Statgraphics Centurion XVI.I software program. The optimum conditions for the COD removal were found to be 5.06 min for reaction time, 5.0 for pH, and 50.5 A/m² for current density with Al electrodes, whereas 5.21 min for reaction time, 5 for pH, and 65 A/m² for current density with Fe electrodes. High removal efficiencies (98.91% COD and 99.78% orthophosphate removal with Al electrodes, and 98.84% COD and 98.24% orthophosphate removal with Fe electrodes) were achieved under optimum conditions. The operating costs for the COD removal from dairy wastewater by electrocoagulation process using Fe and Al electrodes at optimized conditions were calculated to be 0.54 and 0.42 €/m³, respectively. The sludge formed under optimized conditions in EC process was characterized by Fourier Transform Infrared Spectroscopy (FT-IR) analysis. The FT-IR results showed that pollutants in dairy wastewater were linked with aluminum hydroxide and iron hydroxide complexes, precipitated at the bottom of the reactor containing milk components.

The main aim of this study was to investigate the effect of leachate recirculation and aeration on volatile fatty acid (VFA) concentrations in aerobic and anaerobic landfill leachate samples. In this study, two aerobic (A1, A2) and two anaerobic (AN1, AN2) reactors with (A1, AN1) and without (A2, AN2) leachate recirculation were used in order to determine the change of volatile fatty acids components in landfill leachate. VFA degradation rate was almost 100% in each reactor but the degradation rate show notable differences. In aerobic landfill reactors, total VFA concentrations decreased below 1000 mg L⁻¹ after 120 days of operation and only caproic and acetic acids were determined at this time. The stabilization of the VFA concentrations takes about 350 and 450 days for AN1 and AN2 reactors, respectively. VFA concentrations were higher than that of aerobic reactors because of the acidogenic phase occurred in anaerobic environment. According to the results of VFA components, the stabilization of the waste was achieved after 120 days of operation in aerobic landfills. At this time, anaerobic reactors were in the acidogenic phase which results with the high concentrations of VFA. The results also indicated that leachate recirculation does not affect the degradation rate in aerobic landfills as much as it does in anaerobic landfills