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• Application of membrane separation process in China is comprehensively investigated. • Leachate treatment capacity of NF is the highest in landfill and incineration plant. • A combination of NF and RO exhibits the best efficiency for leachates treatment. • Membrane processes are more likely to be adopted in economically developed regions. • Operation costs of membrane process mainly depends on the pre-treatment process. To comprehensively investigate the application of membrane separation technology in the treatment of landfill leachate in China, the performance of nearly 200 waste management enterprises of different sizes in China were analyzed, with an emphasis on their scale, regional features, processes, and economic characteristics. It was found that membrane separation technologies, mainly nanofiltration (NF), reverse osmosis (RO), and NF + RO, have been used in China since 2004. The treatment capacity of the two most dominant membrane separation technologies, i.e., NF and RO, were both almost 60,000 m3/d in 2018, and both technologies are widely used in landfills and incineration plants. Their distribution is mainly concentrated in eastern and southwestern China, where the amount of municipal solid waste (MSW) is relatively high and the economy is developing rapidly. Membrane separation technology is the preferred technique for the advanced treatment of leachate because more contaminants can be effectively removed by the technology than by other advanced processes. However, the membrane retentate that is produced using this technology—commonly known as leachate concentrate—is heavily contaminated due to the enrichment of almost all the inorganic anions, heavy metals, and organic matter that remain after bioprocessing. An economic cost analysis revealed that the operating cost of membrane separation technology has stabilized and is between 1.77 USD/m3 and 4.90 USD/m3; electricity consumption is the most expensive cost component. This review describes the current problems with the use of membrane separation technology and recommends strategies and solutions for its future use. [ABSTRACT FROM AUTHOR]

• The performance differences of three pilot-scale landfills were investigated. • MSW stabilization in the SABL was greatly accelerated. • NH 4 +–N in SABL leachate was substantially lower than that from the other landfills. • SABL leachate exhibited the least chloride pollution risk. Many studies have sought to optimize operation parameters and enhance the treatment capacity of bioreactor landfills (BL) under ideal laboratory conditions. At pilot scale, conclusions drawn from laboratory-scale experiments will be different due to variations in actual landfill composition and changes in environmental conditions. However, comparative pilot-scale studies of traditional anaerobic landfills (AnL) and BLs are rare. In this study, three pilot-scale landfills, including an AnL, anaerobic BL (AnBL) and semi-aerobic BL (SABL), were monitored to examine the difference in performance at different scales and among types of landfills. Settlement amount followed the order SABL (25.45 cm) > AnBL (18.67 cm) > AnL (14.38 cm). Decomposition of organic matter (i.e., volatile fatty acids) was more rapid in SABL than in the other landfills and no hydrolytic acidification period was observed. Therefore, among the three landfills, SABL entered the methanogenic stage in a much shorter time and MSW stabilization was accelerated due to this landfill's unique combination of aerobic-anoxic-anaerobic ambient. In addition, NH 4 +–N concentration in leachate from the SABL (~19.96 mg/L) was substantially lower than from AnL (338.28 mg/L) and AnBL (233.22 mg/L), and SABL leachate exhibited the least chloride pollution risk. This study provides theoretical support and strong evidence for using SABLs to treat MSW in practical applications. [ABSTRACT FROM AUTHOR]

• Landfill leachate was effectively treated by SAARB combined Fenton method. • Fulvic- and humic-like substances were identified in SAARB effluent. • Two fluorescent substances were tracked with parameters varying in Fenton method. Semi-aerobic aged-refuse biofilter (SAARB) can achieve good treatment efficiency for organics and nitrogen ammonia, but the effluent of SAARB from treating landfill leachate (SAARB leachate) has high chemical oxygen demand (COD), compared to the discharge standard (100 mg/L), and high humification degree. In this study, the conventional Fenton method was used in the advanced treatment of SAARB leachate. Fluorescent components in SAARB leachate were identified and tracked with parallel factor (PARAFAC) analysis, and the influencing factors, i.e., Fe2+ dosage, n(H 2 O 2 /FeSO 4) ratio, initial pH, and reaction time, on the removal of fluorescent components were systematically investigated. UV–Vis spectra show the structural transformation of dissolved organic matter (DOM) in SAARB leachate. Results show that SAARB leachate mainly contains two fluorescent components: fulvic-like substances [C1; excitation (Ex)/emission (Em) (nm), (245)280/405] and humic-like substances [C2; Ex/Em (nm), (260)360/435]. A higher Fe2+ dosage, longer reaction time, and acidic conditions increased the degradation efficiency of organics, but excess H 2 O 2 inhibited the treatment process. Under optimum Fenton parameters (Fe2+ dosage of 5 mmol/L, n(H 2 O 2 /FeSO 4) ratio of 8, initial pH of 4, and reaction time of 60 min), removal rates of C1 and C2 were 52.97% and 77.81%, respectively. UV–Vis spectra indicate that the Fenton reaction can destroy unsaturated conjugated bonds, decreasing the humification degree, hydrophobicity, molecular weight, and condensation degree. Finally, the biodegradability (represented by BOD 5 (biochemical oxygen demand for 5-days)/COD) greatly increased (from 0.05 to 0.36), thus benefiting further biological treatment. The results provide suggestions and guidance for practical applications in landfill leachate treatment. [ABSTRACT FROM AUTHOR]

• O 3 /H 2 O 2 and MW/PS processes for the MBR effluent are systematically compared. • MW/PS process has a better treatment effect on refractory organics in MBR effluent. • Ozone plays a major role in the degradation of aromatic organic compounds in the O 3 /H 2 O 2 process. • Hydroxyl and sulfate radicals are the main reactive oxygen species in the MW/PS process. • Chemical oxidation (i.e. O 3 /H 2 O 2 , MW/PS) is a viable technology to eliminate organics in leachate. In this study, refractory organics in a membrane bioreactor (MBR) effluent were investigated following the treatment of landfill leachate by the ozone combined hydrogen peroxide (O 3 /H 2 O 2) and microwave-activated persulfate (MW/PS) processes. The treatment efficiency and the transformation characteristics of refractory organics and reactive oxygen species were determined. It was found that an acidic environment and an increase in the O 3 dosage improved the organic removal efficiency in the O 3 /H 2 O 2 process, and the use of H 2 O 2 improved the treatment efficiency, while excessive H 2 O 2 inhibited it. In the MW/PS process, an increase in the PS dosage and MW power greatly improved the treatment efficiency, while an alkaline environment inhibited it. Under the optimized reaction parameters, the O 3 /H 2 O 2 and MW/PS processes effectively degraded refractory organics (i.e., humic acid and fulvic acid) into components with a smaller molecular weight and simpler structure. The humification, aromaticity, and conjugation of organics in wastewater were greatly reduced. Compared to the O 3 /H 2 O 2 process, the MW/PS process had a better treatment effect on refractory organics, and there were more low molecular weight organics (<1 kDa) in the treated wastewater. Because O 3 is the main selective oxidant in the O 3 /H 2 O 2 process, a large amount of organic acids were accumulated. A large amount of hydroxyl radicals and sulfate radicals with strong oxidation ability were produced in the MW/PS process, and therefore the combined action of hydroxyl and sulfate radicals can efficiently decompose humus and intermediate organics. Overall, the MW/PS process was more effective in treating the MBR effluent than the O 3 /H 2 O 2 process. The results of this study provide a reference for the selection of an advanced oxidation process to eliminate refractory organics in landfill leachate. [ABSTRACT FROM AUTHOR]

Highlights • Coupling SAARB and ozonation was firstly applied to the treatment of leachates. • DOM in mature leachate is significantly different from that in young leachate. • Degradation and transformation mechanism were studied using 3D-EEM-PARAFAC model. • The combined process may provide suggestions and guidance for practical project. Abstract Dissolved organic matter (DOM) in mature landfill leachate is significantly different from that in young landfill leachate; the composition of DOM greatly influences both biological treatment and advanced treatment processes. In the present study, the transformation and degradation mechanisms of landfill leachates in a combined semi-aerobic aged refuse biofilter (SAARB) and ozonation process was investigated using organic removal analysis, molecular weight distribution (MWD), 3D-EEM-PARAFAC analysis, UV–Vis spectroscopy, and linear regression. Results revealed that the DOM in mature landfill leachate contained a greater amount of aromatic substances and had higher molecular weight than DOM in young landfill leachate. After the SAARB process, humus contained in the SAARB was discharged with effluent from both mature and young landfill leachate. Due to the differences in composition and structure of organic matter, the COD removal efficiency (17.89%) of SAARB effluent from treating mature landfill leachate (mature SAARB effluent) was much lower than that (45.91%) of SAARB effluent from treating young landfill leachate (young SAARB effluent) under the same operational parameters of the ozonation process. As indicated by PARAFAC analysis, better chemical stability of DOM in mature SAARB effluent resulted in inferior ozone treatment efficiency. Furthermore, the hydrophobicity and the concentration of benzene ring compounds in the mature and young SAARB effluent were reduced significantly by the ozonation process. Therefore, great improvements in the biodegradability of SAARB effluents were achieved in the ozonation process. Overall, the results of this study provide suggestions and guidance for practical applications of these technologies. [ABSTRACT FROM AUTHOR]

Highlights • The MW-Fe0/H 2 O 2 process was used to treat concentrated landfill leachate. • Reaction parameters and the degradation mechanism were systematically studied. • Refractory organics were degraded by the synergistic effects of MW, Fe0 and H 2 O 2. • Various analyses proved that refractory organics were efficiently degraded. • The reaction mechanism of the MW-Fe0/H 2 O 2 process has wide applicability. Abstract To study the treatment effect and mechanism of a combined microwave (MW)-Fe0/H 2 O 2 Fenton-like process on concentrated landfill leachate, the effects of initial pH, Fe0 dosage, H 2 O 2 dosage, MW power and reaction time on the removal of organic substances were investigated. The phase change of Fe0 before and after reaction and its catalytic mechanism were investigated using multiple analytical techniques. Results showed that the removal efficiencies of chemical oxygen demand, UV 254 and color number were 58.70%, 85.69% and 88.30%, respectively, at initial pH of 2.0, Fe0 dosage of 0.5 g/L, H 2 O 2 dosage of 20 mL/L, MW power of 400 W and reaction time of 14 min. Comparison of different Fenton-like processes indicated that the MW-Fe0/H 2 O 2 Fenton-like process was the most efficient and significantly decreased the aromaticity degree, molecular weight and condensation degree of organic substances in the leachate. The fluorescence peak of concentrated leachate exhibited a blue-shift in the MW-Fe0/H 2 O 2 process, further indicating that the condensation degree of humic substances declined and molecular weight remarkably decreased. The mechanism exhibited an advanced oxidation effect of a heterogeneous Fenton reaction between iron oxide and H 2 O 2 , as well as of adsorption and precipitation effects of iron-based colloids, on organic substances. Moreover, thermal and non-thermal effects of MW accelerated these reactions, achieving fast removal of organic pollutants in the concentrated landfill leachate. Overall, the results of this study showe that the MW-Fe0/H 2 O 2 process is an effective and promising method to handle concentrated landfill leachate. [ABSTRACT FROM AUTHOR]

Refractory organic matter in membrane bioreactor effluent resulting from landfill leachate treatment has a complex composition. This paper identified the transformation mechanism of organic matter in a flocculation-ultraviolet (UV)/peroxymonosulfate (PMS) system at the molecular level using electrospray ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry. The results showed that the flocculation system was able to remove a large amount of dissolved organic matter (DOM) with high oxidation and unsaturation/saturation. UV radiation displayed a relatively strong reactivity for DOM with an electron-rich structure, which it can transform into DOM with lower aromaticity through photolysis and photosensitivity, although the effectiveness of the transformation was poor. In comparison, due to the action of reactive oxygen species, the UV/PMS system can enable reactions such as demethylation, dehydrogenation, decarboxylation, dehydroxylation, ring cleavage, and decarbonylation. It can remove approximately 60% quantity of the total DOM and produce DOM featuring a higher degree of oxidation and saturation than that of the UV system alone. The results showed that the UV/PMS system was a complementary of flocculation in DOM removal from the membrane bioreactor effluent, while the system also resulted in a large number of sulfuric compounds; thus, requiring further evaluation of its ecological effects. [Display omitted] • Flocculation removed most oxidized DOM and newly-produced sulfuric and aliphatic DOM. • UV radiation decreased the aromaticity of electron-rich DOM. • The combination of UV/PMS and flocculation ahieved effective DOM removal. • Sulfuric compounds were produced in the treated leachate, needing further evaluation. [ABSTRACT FROM AUTHOR]

During leachate treatment, molecular information regarding the completely removed, partially removed, less-reactive, increased, and produced parts of dissolved organic matter (DOM) remains unknown. This study applied ESI FT-ICR MS to investigate the transformation characteristics of leachate nanofiltration concentrate (NFC) DOM during a combined flocculation–O 3 /H 2 O 2 process. The NFC contained 5069 compounds in four main classes (CHO, CHON, CHOS, and CHONS compounds). The DOM number decreased to 4489 during flocculation and to 2903 after the O 3 /H 2 O 2 process. During flocculation, the completely and partially removed DOM was mainly low-oxygen unsaturated and phenolic compounds. Saturated DOM was produced and remained in the flocculated effluent. During the O 3 /H 2 O 2 process, the completely and partially removed DOM were mainly low-oxygen unsaturated and phenolic compounds that were mainly in a reduced state. Flocculation can remove many (condensed) aromatic compounds, and methylation and hydrogenation reactions occurred during flocculation. In the O 3 /H 2 O 2 process, dearomatization, demethylation, carboxylation, and carbonylation reactions further achieved the degradation of DOM that was resistant to flocculation. Overall, the combined flocculation-O 3 /H 2 O 2 process collectively eliminated a broader range of DOM than the single processes could achieve. The results of this study provide an in-depth understanding of DOM transformation in an NFC treatment. [Display omitted] • Transformation of NFC DOM in flocculation-O 3 /H 2 O 2 process was studied by FT-ICR MS. • Removed, less-resistant, and produced parts of NFC DOM in flocculation-O 3 /H 2 O 2 were determined. • A Kendrick mass defect theory was used to study DOM transformation of NFC. • Flocculation-O 3 /H 2 O 2 collectively eliminated a broader range of DOM. [ABSTRACT FROM AUTHOR]

• Refractory organics are degraded by the synergistic effects of MW with PS and H 2 O 2. • The MW/PS process degrades organics better than the MW/H 2 O 2 process. • Differences in DOM degradation mechanisms were revealed at the molecular level. • Humus is better degraded in the MW/PS process due to the major contribution of SO 4 -. This study investigated the degradation of organics in mature leachate treated by microwave radiation-activated persulfate (MW/PS) and hydrogen peroxide (MW/H 2 O 2) processes. Obvious synergistic effects existed in both the MW/PS and MW/H 2 O 2 processes, but were significantly higher in the MW/PS process. Refractory organics were better degraded by the MW/PS process than the MW/H 2 O 2 process due to the major contribution of SO 4 –. Moreover, according to Fourier transform-ion cyclotron resonance mass spectrometry coupled with electrospray ionization analysis results, the refractory organics (e.g. polycyclic aromatics (AI > 0.66), polyphenols (0.66 ≥ AI > 0.50)) were greatly degraded by both the MW/H 2 O 2 and MW/PS processes, but the MW/PS process degraded dissolved organic matter (DOM) over a wider range than the MW/H 2 O 2 process due to the different dominant radicals in the two processes. In addition, after reaction in the MW/PS process, the O/C ratio of DOM in the treated effluent showed an obvious increase, which can be mainly attributed to the reaction of sulfate radicals with the N- and S- containing compounds via single electron transfer. [ABSTRACT FROM AUTHOR]

• A combined process to treated landfill leachate was studied at the molecular level. • Mature landfill leachate contained a variety of organics with low bioavailability. • A membrane bioreactor can remove some organics from mature landfill leachate. • A reverse osmosis system can almost completely remove organics from mature leachate. This study investigated the effectiveness of a combined membrane bioreactor (MBR) and reverse osmosis (RO) process for treating leachate produced by a large-scale anaerobic landfill. The MBR process had limited treatment efficiency for removing organic pollutants, but when combined with RO, the integrated system completely removed macromolecular compounds (i.e., humic- and fulvic-like substances) and produced effluent that satisfied the applicable discharge standard. The landfill leachate contained many types of DOM that had high molecular weight and were highly unsaturated. Although the MBR process removed some DOM that had a relatively low saturated degree (mainly aliphatic compounds (2.0 ≥ H/C ≥ 1.5) with relatively high bioavailability), many bio-refractory compounds were not removed. The RO system greatly reduced the content of residual DOM in MBR effluent and was effective for removing heteroatom DOM, especially polycyclic aromatics (AI > 0.66) and polyphenols (0.66 ≥ AI > 0.50). The effluent from the combined process of MBR and RO treatment mainly contained a small number of aliphatic compounds and phenolic compounds (AI ≤ 0.50 and H/C < 1.5) that had higher bioavailability than DOM in the raw leachate and posed little environmental risk. [ABSTRACT FROM AUTHOR]

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]