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 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.