Kinetic modeling of the electrochemical removal of ammonium and COD from landfill leachates

Abstract

Leachates generated from municipal landfills are complex effluents that contain high concentrations of organic pollutants, ammonium, chloride and many other soluble compounds. The aim of this study is to develop a generalized mathematical model for the description of the kinetics of the electro-oxidation of COD and NH₄ ⁺ contained in landfill leachates at boron doped diamond anodes. This complex scenario has been structured by defining two regimes for COD oxidation kinetics: (i) for j _lim,COD < j _appl ≤ (j _lim,COD + $j_{{{ \lim },{\text{NH}}_{4}^{ + } }}$) only direct and ^•OH mediated oxidation reactions close to the anode surface occur, and the process is mass transfer controlled and described by the mass transfer coefficient k _m, a parameter affected by the hydrodynamics of the cell; ii) for j _appl > (j _lim,COD + $j_{{{ \lim },{\text{NH}}_{4}^{ + } }}$) indirect oxidation in the bulk takes place which is quantified by an adjustable parameter k, for which a correlation with the applied current and the chloride concentration has been obtained. Ammonium oxidation occurred by electrogenerated chlorine, for which a second adjustable parameter k′ has been defined. k′ has been correlated to the initial COD concentration and the applied current, expressed as (A/V) · (j _appl/j _lim,COD). The robustness of this model was corroborated by its good description of the experimental results obtained with leachates from two landfill sites located at Chania (Greece) and Meruelo (Spain), and with variable degrees of pretreatment, therefore with a wide range of compositions, and for different operating conditions. The model developed from laboratory scale data was validated at pilot scale with a total BDD anodic area of 1.05 m².