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

Highlights

• Necessity of AOPs for treating leachate from hazardous waste landfill.

• Mechanism of various AOPs along with their limitations critically reviewed.

• Extensive datasheet compiled for leachate treatment efficiency of various AOPs.

• AOPs: Research hot spot to treat hazardous waste landfill leachate.

• Electrocoagulation: A cleaner process comparative to chemical based AOPs.

Abstract

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.

Introduction

Rapid industrialization in the last few decades has resulted in the generation of huge amount of industrial solid waste, a major portion of which unfortunately ends up in the engineered landfills. Heterogenous waste (produced from multiple sources) grounded in landfills (common hazardous waste landfills) undergo several physicochemical changes as it is exposed to different environmental conditions and results in a highly concentrated liquid extract (Yao, 2017) called as leachate. The characteristics of leachate depend on many operational and environmental factors (Reinhart and Townsend, 1998) including characteristics of grounded parent waste material, the moisture content and the age of landfill (Farrokhi et al., 2015) and often characterized by the low BOD, high COD, high NH₃-N, high COD/BOD₅ ratio, blackish or dense brown color and high redox potential. Leachate generation can also be understood as transformation of pollutants from solid to liquid phase.

As this leachate has huge potential to contaminate the surface and subsurface water which further leads to mortality of flora and fauna, its environmentally sound treatment is a challenging task for sustainable environment (Sil and Kumar, 2017). Literature studies have revealed information about characterization and treatment of leachate produced from municipal solid waste landfills (MSWLs) to a greater extent but studies characterizing the leachate produced from Hazardous waste landfills (HWLs) are very limited. Conventional biological treatment processes have been extensively used for treating MSWL leachate but such conventional biological treatments are not effective for HWL leachate as it contains toxic and bio-refractory compounds in the matrix and has high variability in organic loading. This led to the evolution of advance oxidation process for treatment of both type of leachates. Researchers have used Potassium permanganate, calcium hypochlorite, chlorine, ozone and hydrogen peroxide as oxidizing agents during their experiments to treat landfill leachate by converting organic pollutants into less complex compounds (Forgie, 1988). The main concern with conventional treatment processes and many of chemical-based advanced oxidation processes is that they require chemical addition that majorly converts water pollutants into solid waste and involves a cycle of sludge generation and dumping as shown in Fig. 1. Such processes cannot be considered as a sustainable solution for leachate management as they leave a huge environmental footprint.

Need of the hour is to opt for greener technologies/AOPs for leachate treatment which eliminates or minimize the chemical consumption, produce lesser sludge, involves lesser mechanical operations and easy to handle. With advancement in research, other treatment techniques like Fenton's process, ozonation, UV assisted processes; electrochemical oxidation, photo-assisted Fenton and electrocoagulation were developed for treating leachate. The ‘later’ introduced processes are perceived as cleaner technologies as they discourage the use of chemicals that ultimately result in the generation of a lesser quantity of sludge comparative to other chemical based or biological processes.

Current study is focused on the performance of various AOPs for treating HWL leachate with their advantages and limitations so that it can be taken as a reckoner for current research work going on for developing new technologies for leachate treatment. The ongoing research hotspot in AOPs like hydrodynamic cavitation, electro fenton and electrocoagulation and others are also discussed in detail. The review is aimed to identify a suitable AOP from all available AOPs that has following benefits:

• User friendly

• Capable to treat the HWL leachate efficiently

• Discourage the use of chemicals

• Operated at ambient temperature and pressure

• One-pot technology