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Semi-aerobic aged refuse biofilters (SAARB) are known to efficiently remove organic matter, nitrogenous substances, and anions from landfill leachate. However, long-term recirculation of mature landfill leachate inevitably leads to accumulation of pollutants and decreases treatment capacity. In this study, the washing action provided by domestic sewage was used to recover and even enhance the treatment performance of SAARBs treating mature landfill leachate. Three SAARB columns were operated for 300 d after which a "Recirculation-Washing-Recirculation" sequence was followed. In the first recirculation period (22 d), removal of chemical oxygen demand (COD) and total nitrogen (TN) decreased from ca. 90% and 60%, respectively, initially to about 75% and less than 20%, respectively. Thereafter, washing (20 d) of the SAARBs was accomplished by applying domestic sewage. In the subsequent second recirculation period (30 d), the SAARBs were operated at the same hydraulic loading as used initially, but achieved high (ca. 90%) COD and relatively high (ca. 59%–76%) TN removal, including degradation of refractory organic matter such as humic- and fulvic-like substances. Overall, the mechanisms of the treatment performance recovery (including organics degradation and nitrification-denitrification) using domestic sewage can be attributed to three main effects: (1) some accumulated pollutants were washed out, thereby leading to recovery of the adsorption ability of aged refuse; (2) the inhibition of bio-refractory organics stress on microbial activities was mitigated by domestic sewage washing; and (3) the wash out of some accumulated salts (e.g., chloride and sulfate ions) probably helped the microbial activity recover. • A "Recirculation-Washing-Recirculation" sequence was operated in SAARBs. • SAARBs treated landfill leachate and were washed with domestic sewage. • Pollutant removal from landfill leachate was improved by washing action. • Microbial activity was improved after washing action. • Washing action greatly promote nitrification-denitrification within SAARBs. [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]