Mingde Xie

The removal efficiency and mechanism of the ultraviolet/nanoscale Fe⁰/H₂O₂ (UV/nFe⁰/H₂O₂) system for refractory organics in membrane bioreactor effluent were investigated. The most effective removal of organics was achieved at initial pH = 3.0, H₂O₂ dosage = 50 mM, nFe⁰ dosage = 1.0 g/L, and UV power = 15 W, with a reaction time of 60 min. Under these conditions, the absorbance at 254 nm, chromaticity, and total organic carbon removal efficiencies were 65.13%, 79.67%, and 61.51%, respectively, and the aromaticity, humification, molecular weight, and polymerization of organics were all significantly reduced. The surface morphology and elemental valence analysis of nano zero‐valent iron (nFe⁰) before and after the reaction revealed the formation of iron‐based (hydrated) oxides, such as Fe₂O₃, Fe₃O₄, FeOOH, and Fe (OH)₃, on the surface of the nFe⁰. Refractory organics were removed by Fenton‐like reactions in the homogeneous and heterogeneous adsorption–precipitation of iron‐based colloids. At the same time, UV radiation accelerated the formation of Fe²⁺ on the nFe⁰ surface and promoted the Fe³⁺/Fe²⁺ redox cycle to a certain extent, enhancing the removal of refractory organics. The results provide a theoretical basis for the application of the UV/nFe⁰/H₂O₂ system to remove refractory organics in the effluent produced by the biological treatment of landfill leachate.Practitioner points: The UV/nFe⁰/H₂O₂ process is effective in refractory organics removal in leachate treatment. Humus in leachate was largely destroyed and mineralized by the UV/nFe⁰/H₂O₂ process. Active nFe⁰ material participated in the Fenton‐like process and was promoted by UV. The effects of nFe⁰ material and UV introduction were investigated.

Leachate and landfill gas are the main contaminants produced by modern sanitary landfills. The leachate easily leads to clogging in the leachate transportation pipe, and the landfill gas can be used as renewable energy after the removal of CO₂. The study aims to investigate the removal of the major scale forming ion of Ca²⁺ in leachate using raw landfill gas before pipe transportation. The research demonstrated that, under the given experimental conditions, the removal rate of Ca²⁺ in the leachate was positively correlated with the pH value of the leachate, and negatively correlated with the intake flow rate of the landfill gas; the highest removal rate of Ca²⁺ was achieved when the intake flow rate and volume were 0.05 L/min and 2.0 L, respectively, and the highest removal rate of Ca²⁺ from the leachate was about 90%. The maximum removal rate of CO₂ from landfill gas could reach 95%, and the CO₂ content of the post-reaction gas was as low as 1.74% (volume percentage). The scanning electron microscope (SEM) and X-ray diffraction (XRD) analysis showed that the precipitate was spherical and mainly contained inorganic substances such as CaCO₃, MgCO₃, Ca(OH)₂, Mg(OH)₂, and SiO₂. The study showed that, before the leachate was piped, the Ca²⁺ could be removed using the raw landfill gas, thereby reducing the potential for the formation of precipitation clogging in the pipeline. This study also provides new ideas for upgrading landfill gas to achieve a renewable-energy utilization plan, and reduces greenhouse gas emissions by reducing CO₂ emissions from landfills.

The preferential flow of leachate in landfill is unavoidable. Therefore, the mixed leachate formed by mixing young leachate and old leachate is the form of leachate mainly present in the gravel layer. In this study, static anaerobic culture experiments were used to investigate the changes in the properties of the mixed leachate. During the experiment, the chemical oxygen demand (COD) content of the three mixed leachates decreased greatly; CODs of 10%, 20%, and 30% were reduced by 12%, 38%, and 24%, respectively. The concentrations of Ca2+ and Mg2+ also decreased; Ca2+ was significantly reduced after a short time. The pH also dropped. The oxidation-reduction potential (ORP) of the three mixed leachates increased, indicating that the reducing organic matter in the leachate was gradually decomposed by microbial oxidation. Analysis by X-ray diffraction (XRD) showed that the main inorganic precipitate is magnesium-containing calcite, an isomorphous crystal. Studies have shown that if there is a mixed leachate in the gravel layer, severe clogging occurs therein. The clogging material is mainly a magnesium-containing calcite precipitate formed by the metabolic activity of organic matter in the microbial degradation of mixed leachate. Some suggestions were made for the operation and management of landfills.