Volatile and semi-volatile organic compounds in landfill leachate: Concurrence, removal and the influencing factors

Volatile and semi-volatile organic compounds (VOCs and SVOCs) carried by landfilled wastes may enter leachate, and require appropriate treatment before discharge. However, the driving factors of the entry of VOCs and SOVCs into leachate, their removal characteristics during leachate treatment and the dominant factors remain unclear. A global survey of the VOCs and SOVCs in leachate from 103 landfill sites combined with 27 articles on leachate treatment was conducted to clarify the abovementioned question. The results showed that SVOCs such as polycyclic aromatic hydrocarbons (PAHs), phthalate acid esters (PAEs) and phenols were the most frequently detected in leachate on a global scale. However, four kinds of VOCs, i.e., toluene, ethylbenzene, xylenes and benzene, were frequently detected at high concentrations in landfill leachate as well. The concentrations of VOCs and SVOCs in leachate ranged from 1 x 10(0) to 1 x 10(8) ng/L. Solubility was a key factor driving the entry of VOCs and SOVCs into leachate, and higher solubility enables higher detectable concentrations in leachate (P<0.05). It was easiest to remove monocyclic aromatic hydrocarbons (MAHs) from leachate, followed by phenols and PAHs, and it was most difficult to remove PAEs. In terms of removing MAHs, the anoxic/oxic (A/ O) process and the sequential batch reactor (SBR) process were comparable to the advanced oxidization process and far superior to the ultrafiltration and nanofiltration processes, and the removal rate increased with an increase in the Henry's constant and/or the hydrophilicity of the contaminants during the A/O and SBR processes (P<0.05). There were no significant differences among biological, advanced oxidation and reverse osmosis processes in the removal of phenolic. In terms of removing PAHs, the A/O process was comparable to the advanced oxidization process and more efficient than the other treatment processes. As to removing PAEs, the membrane bioreactor process was almost the same efficient as the advanced oxidization process and far more efficient than the other biological treatment processes. Future research should focus on the pollution of atmospheric VOCs and SVOCs near aeration units in leachate treatment plants, as well as the health risk assessment of VOCs and SVOCs in the treated leachate effluent. To the best of our knowledge, this is the first review regarding the occurrence and removal of VOCs and SVOCs from landfill leachates worldwide.