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Abstract Landfill leachate is a typical refractory wastewater for which research into rapid and efficient treatment methods has become very topical. In this study, a coagulation-ozonation process was developed to treat the concentrate arising from membrane treatment of landfill leachate. The effect of coagulant type and initial pH on treatment efficiencies was investigated. Results showed that many of organics were effectively removed in the coagulation process. Thereafter, ozone was applied to further treat the coagulation-resistant organic substances. Our results revealed that the degradation rate of these coagulation-resistant substances followed the trend (color number) CN > (light absorbance at 254 nm) UV 254 > (chemical oxygen demand) COD, and the residual coagulation-resistant substances were oxidized rapidly in the ozone process. Ozone first destroyed the molecular structure of fulvic acid and the by-products generated, such as protein-like substances. In addition, the molecular weight, organic condensation degree, and concentration of benzene ring compounds were considerably decreased. Moreover, the macro molecular organics (i.e., humic acid and fulvic acid) within the size range 1–100 kDa were effectively degraded and partially transformed into bicarbonate. Overall, the combined coagulation-ozonation process reduced COD, UV 254 , and CN in the landfill leachate concentrate by 88.32%, 94.37%, and 98.83%, respectively, and thus the biodegradability of the treated leachate also was significantly improved. This excellent performance proved the feasibility of the combined coagulation-ozonation process for the removal of recalcitrant organic substances contained in landfill leachate concentrate, benefiting subsequent biological treatment. Highlights • Ozonation was applied to treat residual coagulation-resistant substances. • Residual coagulation-resistant substances were oxidized rapidly in the ozone process. • Degradation mechanism of coagulation-resistant organics was investigated using multiple ways. • The combined process significantly improved the biodegradability of the concentrates. [ABSTRACT FROM AUTHOR]

Ultraviolet-quenching substances (UVQS), recently identified pollutants in landfill leachate, can interfere with ultraviolet disinfection when landfill leachate is co-treated with municipal sewage. This study investigated the elimination of UVQS in mature landfill leachates through a membrane bioreactor (MBR) and a semi-aerobic aged refuse biofilter (SAARB). Humus (i.e., fulvic and humic acids) was the main component of organic matter in both MBR- and SAARB-treated landfill leachates, while there was a more stable chemical structure of humus in the MBR-treated leachate. The concentration of UVQS in MBR-treated mature landfill leachate was higher than that of SAARB-treated leachate. Ozonation can degrade UVQS effectively, especially for landfill leachate containing a high concentration UVQS (i.e., MBR-treated landfill leachate). However, a large accumulation of small molecule acid might be caused by ozonation for highly concentrated UVQS in landfill leachate, leading to the delayed degradation of total organic carbon. Moreover, ozonation degraded both fulvic acid and humic acid; and degraded humic acid more effectively. For instance, 88.0% removal (MBR-CP2) and 96.0% removal (SAARB-CP2) of humic acid was higher than those (83.3% for MBR-CP1 and 92.3% for SAARB-CP1) of fulvic acid. The destruction of UV-quenching functional groups of organics (such as C C) by ozone was the main UVQS degradation mechanism of ozonation applied to MBR- and SAARB-treated landfill leachates. Therefore, the ozonation process can efficiently decrease UV absorption intensity in both MBR- and SAARB-treated landfill leachates. • More leachate UVQS can be reduced by SAARB (6.35 cm−1) than MBR (29.85 cm−1). • Humus is the main UVQS in MBR- and SAARB-treated landfill leachates. • Humic can be degraded more efficient than fulvic in ozonation process. • Ozonation process can destruct UV-quenching polar functional groups. [ABSTRACT FROM AUTHOR]