Advanced oxidation processes for treatment of leachate from hazardous waste landfill: A critical review

Leachate produced from hazardous waste landfills (HWLs) is toxic and contain high concentration of refractory organics, ammoniacal-nitrogen, heavy metals, inorganic salts and organo-chloro compounds. Various Advanced Oxidation Processes (AOPs) have been developed in last few decades for efficiently treating the landfill leachate and many are still under development. Researchers have reviewed the performance of conventional AOPs for treating leachate produced from municipal solid waste landfills (MSWLs) but as this leachate is entirely different in characteristics from HWL leachate, these studies are not very useful when it comes to management of HWL leachate; also, specific studies focused on HWL leachate are rarely available. Present study critically reviews various AOPs involving ozone, UV radiation, hydrogen peroxide, electrocoagulation and electrochemical oxidation for their mechanism, treatment efficacy, advantages and limitations with a focus on HWL leachate. A brief review of emerging AOPs like wet air oxidation, hydrodynamic cavitation and ultrasound assisted AOPs is also discussed. This study also aims to identify the AOP which is user friendly, capable to treat the HWL leachate efficiently, discourage the use of chemicals and can be operated at ambient temperature and pressure. After thorough review of different AOPs, Electrocoagulation appears to be a very promising and effective AOP involving in-situ generation of coagulants and converting the organic pollutants in simpler and noble compounds like carbon dioxide and water. Electrocoagulation can be considered as one of the greener and cleaner technology for treatment of HWL leachate, which on optimization can result in a reduction of COD up to 60% along with considerable decrease in metal content in range of 70%–90% and has further potential of improvement and research.