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Globally, the production of vast volumes of municipal solid waste impacts public health and the climate. Greening the solid waste sector could contribute to the achievement of the Sustainable Development Goals (SDGs). Therefore, this study assessed how the potential electricity and environmental benefits of recycling paper and plastic wastes could contribute to the achievement of the SDGs. The study used model equations methods to estimate the electricity consumption, diesel consumption, and greenhouse gas (GHG) emissions saved due to recycling. The key findings show that the 132.4 thousand toe/day of electricity saved globally in 2012 soared to 182.3 thousand toe/day in 2025. In 2012 and 2025, an average of 63.6% of electricity was saved. Globally, recycling saved a record of 1.4 million ktons CO 2 eq/day of GHG emissions in 2012 and 1.9 million ktons CO 2 eq/day in 2025. It was further realized that the overall volume of diesel fuel saved in 2012 grew from 511,146 to 703,887 million Liters/day in 2025, representing a rise of 37.7%. The sensitivity analysis shows an increase in the waste collection rate, the fraction of paper waste, the fraction of plastic waste, and the recycling rate in 2025 will boost recyclable resources' energy and environmental benefits. The findings of this study could offer scientific guidance for the achievement of the SDGs related to solid waste recycling and management., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The emission patterns of three greenhouse gasses (GHGs), viz. CH 4 , CO 2 , and N 2 O from landfills, were examined on a lab scale. Three simulated semi-aerobic bioreactor landfills (SABL1, SABL2, SABL3), with respective vent-pipe inner diameters (φ) of 25, 50, and 75 mm, were used to investigate their effect on the greenhouse effect (GHE) during the municipal solid waste (MSW) stabilization process. We found that the vent-pipe φ influenced both MSW degradation and GHG emissions, increasing the vent-pipe φ which improved the removal of carbon and nitrogen-based pollutants. The GHG emissions were 364, 356, and 309 kg CO 2 equivalents per ton of MSW from the SABL2, SABL1, and SABL3, respectively, during the operation of 465 days. Of the three GHGs, CH 4 influenced the GHE the most, contributing 72.53%, 79.17%, and 71.42% in SABL1, SABL2, and SABL3, respectively. In the same sequence, CO 2 (14.87%, 14.06%, and 21.9%) and N 2 O (12.6%, 6.77%, and 6.69%) were the second and third contributors to the GHE, respectively. Considering the rapidly MSW stabilization and the mitigation of GHG emissions, a vent pipe with φ of 75 mm in the SABL column (φ of 800 mm) was suggested. Moreover, the GHG mitigation in the SABL should be implemented by prioritizing CH 4 collection and oxidation. The results provided a technical guidance for GHG mitigation in MSW management.

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 m 3 /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/m 3 and 4.90 USD/m 3 ; 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., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

Landfill leachate contains high concentrations of complex organic matter (OM) that can severely impact the ecological environment. If landfill leachate is to be treated using a combined "biological + advanced treatment" process, the molecular information of OM must be investigated to optimize the operation parameters of the combined process and maximize the removal of organic pollutants. This study applied ultra-high resolution mass spectroscopy to investigate the degradation and transformation characteristics of refractory OM in mature landfill leachate at the molecular level (m/z = 150-800) during biological treatment (i.e., semi-aerobic aged refuse biofilter, SAARB) and subsequent chemical oxidation (i.e., the Fenton process and ozonation). After SAARB treatment, the polycyclic aromatics (aromatic index, AI > 0.66) and polyphenol (0.66 ≥ AI > 0.50) contents increased, and the highly unsaturated phenolic compounds (AI ≤ 0.50 and H/C < 1.5), which have a high bioavailability, were mostly removed. Compared with raw leachate, SAARB effluent (i.e., SAARB leachate) contained fewer organics with short carbon chains, more organics with long carbon chains, an elevated condensation degree for organics and, thus, a considerably reduced biodegradability. Although both the Fenton and ozonation processes could remove many of the polycyclic aromatics and polyphenols, ozone produced considerable amounts of aliphatic compounds with high bioavailability. Compared to ozonation, the Fenton process utilized the hydroxyl radical to non-selectively react with OM and produced better mineralization results., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

Fly ash is a hazardous material that is produced from municipal solid waste incineration. It contains heavy metals and should be properly treated to meet landfill entry requirements. In this study, under the precondition that the leachable concentration of lead (Pb) exceeded the limit value for landfill disposal, the effects of cement solidification, chemical stabilization, and their combination on the leachable Pb concentration and the chemical state of Pb were systematically investigated. In addition, the reaction conditions were optimized by response surface methodology (RSM) in terms of leachable Pb concentration, volume change ratio, and treatment cost. The results indicated that the leachable Pb concentration decreased at lower cement or sodium dihydrogen phosphate (NaH 2 PO 4 ) dosages in cement solidification or NaH 2 PO 4 stabilization, and the liquid-to-solid ratio had a significant influence on cement solidification. The leachable Pb concentration met the limit value for landfill disposal in the individual processes with 20% cement or 5% NaH 2 PO 4 , and in the combined process with 10% cement + 2% NaH 2 PO 4 . The combined process achieved the best treatment efficiency by enabling Pb to transform to a stable residual state. According to the RSM, a combined cement content of 11.64%, NaH 2 PO 4 content of 2.79%, and liquid-to-solid ratio of 0.48 were the optimal parameters, resulting in substantial decreases in the volume change ratio and treatment costs, while satisfying the preconditions for landfill disposal. In conclusion, the combined process can reduce the pollution risk to the environment, and is an efficient and cost-effective pre-treatment method for incinerator fly ash., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

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.)

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., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing finacial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

To achieve a uniform water distribution and to promote the stabilization of bioreactor landfills, the effects of different density on the transport and hydraulic characteristics of recirculated leachate were studied in a Li + tracer experiment. The landfill density was related negatively to the maximum production rate of leachate, the percentage of Li + tracer solution and the cumulative amount of Li + that washed out in the first recirculation round. The cumulative amount of Li + that was washed out accounted for 92.61%, 82.07% and 75.25% of the injection volume in three waste columns (1#, 2# and 3#) with landfill densities of 497.5, 726.4 and 970.1 kg m -3 at the thirtieth recirculation, respectively. The results revealed that a greater landfill density yielded a smaller amount of Li + that was washed out per recirculation cycle and the cumulative amount of Li + washed out, and a longer contact time between the waste and leachate. Moreover, waste columns 1#, 2# and 3# are reactors with a high dispersion and obvious retention zones, and their Morrill Dispersion Index (MDI) values were 25.00, 34.51 and 24.55, respectively. A short-flow phenomenon is obvious in the reactor with a low landfill density. Furthermore, the plug-flow effect is relatively better in the reactor with a high landfill density. To achieve a uniform distribution of recirculated leachate in the landfill, the MDI of the reactor should be reduced combined with other measures, such as pre-crushing and different recirculation schemes.

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., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

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., (Copyright © 2018 Elsevier Ltd. All rights reserved.)