| Mature landfill leachate is a kind of high-strength and complicated organic waste-water. If not properly treated or directly discharged, it may cause serious contamination to aqueous environment. Due to the main characteristics of mature landfill leachate such as maladjustment of nutrient, high concentration of ammonia nitrogen and low bio-degradability, the performance of biological processes is not ideal when it was directly applied to degradation of mature landfill leachate. Combination of physico-chemical process and biological process is a preferred alternative for the treatment of mature landfill leachate.Electrochemical oxidation, which is a kind of environmental-friendly advanced oxidation process, exhibits several advantages including versatility, simplicity of operation, no or little need for adding chemical reagents, easy to realize automation and without secondary pollution. In recent years, electrochemical oxidation of nonbiodegra-dable wastewater has caused wide concern. Electrochemical oxidation was mainly proposed as the polishing treatment of landfill leachate. Few researches on electro-chemical oxidation of mature landfill leachate were reported.Based on the improvement of biodegradability and enhancement of electrolysis performance, electrochemical oxidation of mature landfill leachate was conducted in this study. The influence of anode materials on the removal rate of contaminants was investigated during the electrolysis of mature landfill leachate. The removal efficiency of organic compounds and ammonia was analyzed in the anodic oxidation of mature landfill leachate on boron-doped diamond(BDD) anode. Meanwhile reaction kinetics models of TOC, NH3-N and nitrate were established. The optimal operating conditions for the improvement of biodegradability were studied during the electrolysis of mature landfill leachate on BDD anode. During the electrochemical oxidation of mature landfill leachate on BDD anode, the effects of the interactions between the operating parameters on removal efficiency of contaminants were analyzed using response surface method-ology(RSM). Based on the fore-mentioned research contents, the conclusions were listed as follow.(1) Uner the different operating conditions with different current density, initial p H and area-to-volume(A/V) ratio, electrochemical oxidation of mature landfill leachate was carried out on different anode such as titanium dioxide(Ti O2), mixed metal oxide(MMO) and boron-doped diamond(BDD), and the influence of anode materials on the anodic oxidation performance was analyzed. Experimental results indicated that the sequence of anode materials was BDD>Ti O2>MMO in terms of TOC, COD and NH3-N removal rate. Increment of current density can improve the removal efficiency of TOC and NH3-N during the electrolysis on the entire applied anodes. Neutral condition favored the removal rate of TOC on BDD and Ti O2 anode, and alkaline condition was helpful to TOC removal on MMO anode. Removal efficiency of NH3-N was enhanced in alkaline condition for the entire applied anodes.(2) During the electrochemical oxidation of mature landfill leachate on BDD anode, current density and A/V ratio were the crucial operating parameters which had srong effect on the oxidation performance. As the increment of current density and A/V ratio, the removal rate of organic contaminants and ammonia was improved. When current density, initial p H, inter-electrode gap and A/V ratio was 100 m A/cm2, 8.35, 10 mm and 5.85 m-1, respectively, the removal rate of TOC was 82% after electrolysis of 6 h, meanwhile NH3-N was entirely removed. Initial p H had a little effect on the removal rate of organic compounds and ammonia.TOC removal rate was enhanced in alkaline condition. The removal rate of NH3-N was 86.6% when initial p H was 8.35. The dependence of inter-electrode gap on the removal efficiency of organic compounds and ammonia was little. The variation of TOC and NH3-N could be well predicted by first-order and zero-order kinetics models, respectively. The relationship between apparent removal rate constant of TOC and the operating parameters including current density, A/V ratio and initial p H could be well explained by polynomial fitting equation. It is the same as NH3-N. The variation of TOC and NH3-N could be well predicted by the multivariate nonlinear regression equation in the electrochemical oxidation process.During the electrochemical oxidation of mature landfill leachate on BDD anode, current density, initial p H and A/V ratio had strong effect on nitrate concentration existed in the treated leachate. The nitrate concentration increased as the increment of current density and A/V ratio. The concentration of nitrate was lower in the alkaline condition.The nitrate concentration was 400 mg/L after the electrolysis time of 6 h in the condition with current density of 100 m A/cm2, initial p H of 8.35, A/V ratio of 3.66 m-1 and inter-electrode gap of 10 mm. The relationship between accumulation rate of nitrate and current density fitted to linear fitting function, it is the same to A/V ratio. And quadratic polynomial function was introduced to explicate the relationship between accumulation rate of nitrate and initial p H. The multivariate nonlinear regression equation was established to predict the evolution of nitrate during the electrochemical oxidation process.(3) Increment of current density and A/V ratio could improve the biodegradability of mature landfill leachate during the electrolysis process on BDD anode. As initial p H increased, the biodegradability increased firstly and then decreased. Inter-electrode gap had little influence on the biodegradability of the treated leachate. The optimal operating condition for the improvement of biodegradability was that current density was 100 m A/cm2, initial p H was 8.35, A/V ratio was 3.66 m-1 and inter-electrode gap was 10 mm. The biodegradability of the treated lanfill leachate was improved up to 0.66 after the electrolysis time of 6 h under the optimal operating condition.(4) Under the applied operating conditions with current density of 100~120 m A/cm2, initial p H of 6.5~7.5, A/V ratio of 3.91~5.85 m-1 and electrolysis time of 4~6 h, the denpendence of interactions between current density, A/V ratio, initial p H and electrolysis time on the electrochemical oxidation performance of mature landfill leachate was analyzed using RSM. Experimental results indicated that interactions of the operating parameters, except the interaction between current density and initial p H, had significant influence on TOC removal rate. The interaction between current density and A/V ratio had a significant influence on COD removal rate. The interactions between current density and electrolysis time, A/V ratio and electrolysis time both had significant effects on NH3-N removal rate. The interactions of current density and initial p H, A/V ratio and electrolysis time both had significant influences on nitrate concentration. |
PDF Full Text Request