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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., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Abstract In this study, a ferrous-activated persulfate (PS) process induced by microwave (MW) energy was used to treat an industrial wastewater containing dinitrodiazophenol (DDNP). The effect of different operational parameters, such as PS dose, initial pH, MW power, and the mass ratio of ferrous to PS [i.e., n(Fe2+/PS)], on chemical oxygen demand (COD) removal and the pseudo first-order constant k was systematically studied. Results showed that the constant k and COD removal increased considerably with increasing PS dose, MW power, and n(Fe2+/PS); however, the constant k and COD removal decreased proportionally as the initial pH increased from 3 to 11. A base-activated PS system was formed when the initial pH further increased to 13, which resulted in a dramatic increase of k and COD removal. Controlled experiments demonstrated that the synergetic effect of MW, Fe2+, and PS were significant. The value of k was 0.2425 min−1, COD removal was 71.42%, and biodegradability remarkably increased from 0.056 to 0.6712 during treatment by MW-Fe2+/PS at a PS dose of 8 g/L, MW power of 600 W, initial pH of 3, n(Fe2+/PS) of 0.04, and reaction time of 14 min. Spectral analyses showed that the chromophores (including the diazo group and nitro group compounds) and benzene rings were greatly decomposed. Identification tests for reactive oxygen species showed that the predominant oxidizing species in the MW-Fe2+/PS process was the sulfate radical, and that the hydroxyl radical played a lesser role for color number elimination. From the perspective of both treatment efficiency energy consumption, the MW-Fe2+/PS process was confirmed to be an economical and promising pretreatment process for DDNP industrial wastewater. Graphical abstract Image 1 Highlights • MW-Fe2+/PS process was applied to the treatment of DDNP industrial wastewater. • A significant synergetic effect was observed among microwave, Fe2+, and persulfate in MW-Fe2+/PS process. • SO 4 – and OH were the dominant oxidizing species in the MW-Fe2+/PS process. [ABSTRACT FROM AUTHOR]