Radicals-catalyzed oxidation reactions for degradation of recalcitrant compounds from landfill leachate

This article reviews the technical applicability and the treatment performance of various advanced oxidation technologies (AOTs) for landfill leachate. A particular focus is given to ozonation, homogeneous systems without irradiation (Fe2+/H2O2, O3/H2O2) and homogeneous systems with irradiation (UV/O3, UV/H2O2, UV/O3/H2O2, photo-Fenton oxidation). Their advantages and limitations in application are evaluated. The formation of {radical dot}OH radicals in enhancing oxidation reactions is also elaborated. Selected information such as the pH, the dose of oxidants required, the characteristics of leachate in terms of COD and NH3-N concentration and treatment performance is presented. It is evident from a review of 167 papers (1976-2005) that none of the individual AOTs is universally applicable or highly effective for leachate treatment. Among the individual AOTs reviewed, ozonation and/or Fenton oxidation are the most frequently studied and widely applied for leachate treatment. Both treatments are effective for 40-89% of COD removal with its concentrations ranging from 560 to 8894 mg/L. By combining the Fenton oxidation and coagulation-flocculation process, about 69-90% of COD removal with its concentrations ranging from 417 to 7400 mg/L was achieved. An almost complete COD removal (98%) was attained by combining the activated sludge (AS) and the Fenton oxidation (COD: 7000 mg/L) and/or the AS and wet air oxidation (WAO) (COD: 4140 mg/L). In general, the selection of the most suitable AOT for leachate treatment depends on the leachate characteristics, technical applicability and potential constraints, effluent discharge standard, cost-effectiveness, regulatory requirements and long-term environmental impacts.