Metagenomics insights into the effect of co-landfill of incineration fly ash and refuse for bacterial community succession and metabolism pathway of VFAs production

With the development of incineration technologies, incineration has become the most common treatment method of municipal solid waste in China. However, stabilized fly ash may enter landfills during the transition from landfill to incineration, which caused uncertain impact on landfill waste stabilization. Two simulated co-landfill columns were constructed based on different co-landfill methods (layer co-landfill and mixed co-landfill) to investigate the effect of stabilized fly ash co-landfilled municipal solid waste for bacterial community succession and change in metabolic pathways during hydrolysis-acidogenesis stage. The mixed co-landfill method resulted in higher degree of organic matter degradation, and the concentrations of volatile fatty acids (VFA) and chemical oxygen demand (COD) in leachate were higher. The dominant phyla were Firmicutes in the layered co-landfill column and Bacteroidetes in mixed co-landfill column. The dominant genera for the total bacterial composition and VFA production were different, Pseudomonas and Propionibacterium, Proteiniphilum and unclassified Bacteroides were the dominant genera responsible for VFA generation in the layered and mixed co-landfill columns. The genes for butyrate production were enriched in the layered co-landfill column, whereas those related to acetate production were enriched in mixed co-landfill column. However, the layered co-landfill inhibited the microbial metabolic activity at the end of the co-landfill process.

Impact of incineration slag co-disposed with municipal solid waste on methane production and methanogens ecology in landfills

Co-landfill of incineration slag and municipal solid waste (MSW) is a main method for disposal of slag, and it has the potential of promoting methane (CH₄) production and accelerating landfill stabilization. Four simulated MSW landfill columns loaded with different amount of slag (A, 0%; B, 5%; C, 10%; D, 20%) were established, and the CH₄ production characteristics and methanogenic mechanisms were investigated. The maximum CH₄ concentration in columns A, B, C and D was 10.8%, 23.3%, 36.3% and 34.3%, respectively. Leachate pH and refuse pH were positively correlated with CH₄ concentration. Methanosarcina was the dominant genus with abundance of 35.1%∼75.2% and it was positively correlated with CH₄ concentration. CO₂-reducing and acetoclastic methanogenesis were the main types of methanogenesis pathway, and the methanogenesis functional abundance increased with slag proportion during stable methanogenesis process. This research can help understanding the impact of slag on CH₄ production characteristics and microbiological mechanisms in landfills.

Seasonal occurrence of multiple classes of antibiotics in East China rivers and their association with suspended particulate matter

Understanding the occurrence and fate of antibiotics from different categories is vital to predict their environmental exposure and risks. This study presents the spatiotemporal occurrence of 45 multi-class antibiotics and their associations with suspended particulate matter (SPM) in Xiaoqing River (XRB) and Yellow River (YRB) via 10-month monitoring in East China. Thirty-five and 31 antibiotics were detected in XRB and YRB, respectively. Among them, fluoroquinolones (FQs) had the highest total mean concentration (up to 24.8 μg/L in XRB and 15.4 μg/L in YRB), followed by sulfonamides (SAs) (14.0 μg/L and 15.4 μg/L) and macrolides (MLs) (1.1 μg/L and 1.6 μg/L). Significant spatial-temporal variations were found in both rivers where higher concentrations of antibiotics were observed in urban and densely populated areas during winter and spring. Hydrological factors such as river flow and water volume, instream attenuation and antibiotic usage may cause the observed variabilities in the seasonal patterns of antibiotic pollution. Using linear regression analysis, for the first time, this study confirmed that the total concentrations of MLs (p < 0.05), FQs (p < 0.001) and SAs (p < 0.001) were strongly correlated with the turbidity/total suspended solids in the studied rivers (except MLs in YRB). It is thus suggested that partitioning processes onto SPM might affect the distribution of detected antibiotics in rivers, which are largely dependent on SPM composition and characteristics. The risk quotient (RQ) determined for up to 87 % of individual compound was below 0.1 in both rivers; however, the high joint toxicity reflected by the mixed RQs of detected antibiotics may rise risk alarm for aquatic species. Further aspects regarding active mechanisms of SPM-antibiotic interactions and ecological risks of coexistence of multiple antibiotics need to be investigated.

Insights into the landfill leachate properties and bacterial structure succession resulting from the colandfilling of municipal solid waste and incineration bottom ash

Four simulated bioreactors were loaded with only MSW, 5 % BA + MSW, 10 % BA + MSW and 20 % BA + MSW to investigate the leachate property and bacterial community change trends during the colandfilling process. The results showed that with increasing BA addition proportion (5 %∼20 %), the leachate oxidation-reduction potential (ORP) was lower, the leachate pH quickly entered the neutral stage, and the chemical oxygen demand (COD), volatile fatty acids (VFA), NH₄⁺-N, Ca²⁺ and SO₄^2- presented faster downward trends. The leachate SUVA₂₅₄ and E_300/400 confirmed that BA can accelerate the leachate humification process. BA can quickly increase bacterial diversity, and the higher the addition proportion of BA, the more significant the change in microbial community structure during the landfilling process. The leachate pH and COD greatly influenced the bacterial community structure. A low BA proportion can increase metabolism pathway abundance during the initial stage, but a high BA proportion had an inhibitory effect on the metabolism pathway.

Remediation of Cr(VI)-contaminated soil by combined chemical reduction and microbial stabilization: The role of biogas solid residue (BSR)

In this work, the use of chemical reduction combined with microbial stabilization to remediate Cr(VI) in contaminated soil was systematically investigated. The effectiveness, phytotoxicity and microbial diversity resulting from the combination of ferrous sulfate with microbial stabilization by biogas solid residue (BSR) were determined. The stabilization experiments showed that the optimum Cr(VI) conversion rate of 99.92% was achieved with an Fe (II)/Cr(VI) molar ratio of 3:1, a BSR dose of 5.2% (wt), and a water content of 40%. Under these conditions, the residual Cr(VI) content was 0.80 mg/kg, which satisfied the risk screening value (≤ 5.7 mg/kg) for soil contamination of land for general development in China. The remaining Cr(VI) level was stable for 90 days during the chemical reduction and biogenic stabilization process. Moreover, Zucconi test analysis suggested that the soil phytotoxicity to Brassica campestris L. disappeared. The results of microbial diversity analysis indicated that the bacterial community changed significantly during chemical reduction and microbial stabilization processes, and Bacillus, Pseudomonas and Psychrobacter may participate in the reduction of Cr(VI) into Cr(III).

Stability evaluation of potentially toxic elements in MSWI fly ash during carbonation in view of two leaching scenarios

Carbonation treatment (CT) by alkaline fly ash (FA) affects the stability of potentially toxic elements (PTEs). This study investigated the leachability and environmental risk of six PTEs contained in FA during natural and accelerated carbonation (NC, AC) using two typical leaching scenarios with distilled water (DW) and acetic acid (AA). The leaching of Pb/Cu/Cr/Ni in solidified/stabilized FA decreased due to CT in DW leaching, but the leaching of Pb/Zn/Cu/Cd increased due to CT in AA leaching. The leaching of the six PTEs (especially Pb/Cd) in AA leaching was significantly higher than that in DW leaching. CT was a promoting factor to increase the environmental risk level of PTEs in FA leachate, especially in AA leaching with H⁺ input. In the early stage of NC, under DW leaching tests, the environmental risk level of PTEs in FA leachate can be weakened due to the formation of carbonate minerals in the FA matrix. However, excessive NC increases the environmental risk of leached PTEs due to the decalcification of carbonate minerals. Both NC and AC increased the potential environmental risk of PTEs contained in the carbonated FA matrix. The nucleation and dissolution of carbonate minerals were interdependent with the immobilization and leaching of PTEs, which played a dominant role in the CT and leaching tests respectively. They jointly affected the occurrence behavior of PTEs in the FA matrix in CT tests and the leachability of PTEs in leaching tests. This study demonstrates that it is more scientific to evaluate the leachability of PTEs in carbonated FA according to the actual disposal scenarios.

Mechanisms of emerging pollutant Dechlorane Plus on the production of short-chain fatty acids from sludge anaerobic fermentation

The effect of emerging pollutant Dechlorane Plus (DPs), an organochlorine aliphatic flame retardant, on waste-activated sludge anaerobic fermentation was investigated, and the related mechanisms were revealed for the first time. The results of this experiment suggested that the presence of DPs had a significant inhibitory effect on sludge anaerobic fermentation to generate the intermediate valuable product short-chain fatty acids (SCFA), and when the DP content was 3034.1±101.7 mg/kg total suspended solids (TSS), the maximal output of SCFA was only 215.04 mg/g, which was 0.47 times of that in the blank. The underlying mechanism investigation indicated DPs promoted the disintegration of sludge, but inhibited the process of hydrolysis and acidification. DPs inhibited the release of soluble bound extracellular polymers (SB-EPS) in sludge. The analysis of microbial community characteristics indicated that DPs reduced the level of Firmicutes and Actinobacteriathe, which were the key acid producing bacteria. At the genus level, DPs reduced the relative abundance of Proteiniclasticum and Mycobacteriumwas.

Insights into the stabilization of landfill by assessing the diversity and dynamic succession of bacterial community and its associated bio-metabolic process

The distribution of bacterial community in an actual landfill was analyzed and the bioprocess involved in refuse degradation was clarified. The results showed that the degradation degree of refuse showed great differences with the landfill age, in which the contents of organic matter (OM) and total Kjeldahl nitrogen (TKN) in refuse as well as the chemical oxygen demand (COD) in leachate presented decreasing trends with increasing landfill age. The diversity of bacterial community increased first and then decreased with increasing landfill age. The main bacterial phyla involved in refuse degradation were Proteobacteria, Firmicutes and Bacteroidetes, among which, Proteobacteria had an absolute advantage with a relative abundance ranging of 66-78%. With increasing landfill age, the abundance of Firmicutes decreased gradually, while that of Bacteroidetes increased. Pseudomonas, Thiopseudomonas, Psychrobacter and Desemzia were the main genera. The distribution of bacterial community in samples with landfill ages of 0-1 and 1-3 years were greatly influenced by TKN and pH, respectively. Amino acid and carbohydrate metabolism were the main biological pathways according to the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, and the biodegradation of xenobiotics as well as terpenoids and polyketides also accounted relatively high frequencies in the landfill. These results provide a better understanding of landfill microbiology and bioprocesses for landfill stabilization.

Leaching behavior and environmental risk assessment of toxic metals in municipal solid waste incineration fly ash exposed to mature landfill leachate environment

Solidification/stabilization pretreatment + landfill disposal in municipal solid waste (MSW) landfill sites is a widely accepted MSW incineration (MSWI) fly ash (FA) management strategy in China. However, in reality, the stability of FA disposed in MSW landfill sites may be affected by the organic landfill leachate environment. The purpose of this study was to explore the mobility and environmental risks of six toxic metals (Mⁿ⁺, Pb/Zn/Cu/Cd/Cr/Ni), from raw and solidified/stabilized FA, by simulating a leaching environment with mature landfill leachate (MLL). The leaching of Mⁿ⁺ mainly occurred in the early leaching stage, and their leaching behavior was controlled by the diffusion of surface Mⁿ⁺ in the FA matrix. The destructive effect of dissolved organic matter (DOM) on the local precipitation-dissolution equilibrium of FA-leachate interface, the formation of non-adsorptive DOM-Mⁿ⁺ complex (easy to migrate), and the competitive effect of DOM on the binding sites of Mⁿ⁺ on the surface of the FA matrix may play an important role in increasing the leaching level of most Mⁿ⁺. By contrast, the potential of solidified FA in reducing the environmental risk level of leached Mⁿ⁺ was better than that of stabilized FA. However, the immobilization capability of solidification/stabilization pretreatment on various types of Mⁿ⁺ in FA should be judged according to their practical disposal environment. Compared to MLL leaching tests, Acetic Acid Buffer Solution Method (HJ/T300-2007) can effectively strengthen the exposure environment and provide a reliable reference level of environmental risk for MSWI FA disposed in MSW landfill sites.

Effect of fluoxetine on enhanced biological phosphorus removal using a sequencing batch reactor

In this work, the potential impact of emerging pollutant Fluoxetine (FLX) on enhanced biological phosphorus removal (EBPR) was systematically investigated using the sequencing batch reactor. The experimental results showed that even 200 μg/L FLX had no significant effect on EBPR during the short-term exposure. However, in the long-term exposure test, high dosage of FLX inhibited EBPR. 200 μg/L FLX induced biological phosphorus removal efficiency dropped to 71.3 ± 2.1%, significantly lower than that of the blank. The mechanism investigation showed that high concentration of FLX reduced anaerobic phosphorus release and oxic phosphorus absorption, and the consumption of organic matter during the anaerobic period. In addition, FLX decreased the synthesis of intracellular polymer polyhydroxyalkanoates (PHA), but promoted the metabolism of glycogen and polyhydroxyvalerate. FLX reduced the activity of key enzymes in EBPR and the relative abundance of Accumulibacter, but improved the relative abundance of Candidatus Competibacter.

Municipal solid waste incineration fly ash exposed to carbonation and acid rain corrosion scenarios: Release behavior, environmental risk, and dissolution mechanism of toxic metals

This study investigated the leaching behavior, environmental risk, and dissolution mechanism of toxic metals (TMs) in solidified/stabilized municipal solid waste incineration fly ash (MSWI FA) exposed to alternative "carbonation + acid rain corrosion" disposal scenarios. The content of TMs (mg/kg) showed a trend of Zn (12,187.10 ± 168.60) > Pb (3374.43 ± 66.12) > Cu (1055.14 ± 32.52) > Cr (127.95 ± 8.12) > Cd (119.05 ± 6.26) > Ni (49.50 ± 3.20). Initial leaching of CO₂-saturated water (CSW) and replacement of simulated acid rain (SAR) increased the environmental risk of leached TMs. The results of "average release rate" (mg/(kg·d)) of TMs indicated that Zn (0.8307)/Cu (0.0278)/Cd (0.0109) and Cu (0.0581)/Cr (0.001176)/Ni (0.004339) in phosphoric acid stabilized FA and Pb (0.0753)/Cr (0.001921)/Ni (0.00111) and Pb (0.0656)/Zn (1.0560)/Cd (0.0050) in Portland cement solidified FA were the key "problem TMs" during carbonation and acid rain corrosion, respectively. CSW leaching increased the independent environmental risk of most TMs in residual FA (especially Zn/Cd) due to the increased carbonate-bound fraction. Compared with independent carbonation, alternative "carbonation + acid rain corrosion" contributed to a higher comprehensive environmental risk for TMs in residual FA. CSW leaching system was an indirect carbonation based on CO₂-water and FA matrix, in which "nucleation and dissolution" of carbonates and "immobilization and dissolution" of TMs coexisted. The dissolution mechanism of TMs was mainly controlled by reaction equilibrium of nucleation and dissolution of carbonates containing TMs. Dissolution and nucleation were the dominant mechanism in the early and later periods of CSW leaching, respectively. Carbonate layer dissolution, H⁺ corrosion/displacement, and counter-ion effect (SO₄^2- > NO₃^- > Cl^-) were the main mechanisms affecting TM dissolution during SAR leaching.

Effects of plant radial oxygen loss on methane oxidation in landfill cover soil: A simulative study

Radial oxygen loss (ROL) by the spreading root systems of vegetation can improve soil aeration for subsequent oxidation of methane (CH₄) by microbes in landfill cover soils. This study proposes a theoretical model that elucidated the effects of ROL on microbial oxidation of CH₄ to understand landfill gas transportation and oxidation in landfill cover soils. Parametric analyses were conducted to investigate the effects of root depth, root architecture, and ROL rate on the CH₄ oxidation efficiency of landfill cover soils. The simulation results suggested that disregarding O₂ emissions by plants root systems could underestimate the CH₄ oxidation efficiency, especially when the water content ranged from 20% to 35%. Additionally, plants with a parabolic root architecture indicated 7-13% higher CH₄ oxidation efficiency than other root architectures, i.e., uniform, triangular, and exponential. The CH₄ oxidation efficiency increased rapidly at root depths less than 0.25 m. Therefore, plants characterized by a parabolic root architecture, longer root length, and higher ROL capacity should be selected as the preferred species for mitigating CH₄ emissions from landfills in humid areas.

Methane emissions from landfill: influence of vegetation and weather conditions

Vegetation plays an important role in CH₄ transport and oxidation in landfill cover soil. This study investigated CH₄ emission fluxes in two landfills with different surface coverage conditions and it found that the CH₄ emission fluxes presented spatial and temporal disparities. A significant discrepancy in CH₄ emission flux between day and night in areas covered with Kochia sieversiana indicated that enhanced diffusion induced by rising temperature was the main mechanism for CH₄ transport during daytime. A significant increase of CH₄ emission flux after the K. sieversiana and Suaeda glauca plants were cut indicated that these plants provide greater contributions to CH₄ oxidation than to CH₄ transport. Diel CH₄ emission flux was found closely correlated with the climatic conditions. Diffusion was determined as the main mechanism for CH₄ transport at daytime in bare area, mediated by solar radiation and air temperature. Diffusion and plant-mediated transport by convection was established as the main transport mechanism in areas covered with K. sieversiana. Our results further the understanding of both the CH₄ emission mechanism and the impact of vegetation on CH₄ oxidation, transport, and emission, which will benefit the development of a reliable model for landfill CH₄ emissions.

Effect of soil types and ammonia concentrations on the contribution of ammonia-oxidizing bacteria to CH4 oxidation

Irrigation of stabilized landfill leachate to landfill cover soil is a cost-effective operation for leachate treatment. The contribution of ammonia-oxidizing bacteria (AOB) in the cover soil to CH₄ oxidation, however, is unclear, because AOB and methane-oxidizing bacteria (MOB) can co-oxidize CH₄ and NH₄⁺-N. Thus, the contribution of AOB and the inhibitory effect of NH₄⁺-N to CH₄ oxidation were determined by using an acetylene pretreatment discrimination method. The results showed that the contributions of AOB to CH₄ oxidation varied with the soil type and the concentration of NH₄⁺-N addition. The relative contribution of AOB to CH₄ oxidation for compost without NH₄⁺-N addition was the highest (65.0%), and was 2.5 and 3.4 times higher than the corresponding values for aged refuse and landfill cover soil, respectively. The inhibitory effect of NH₄⁺-N was enhanced by increasing the concentration of NH₄⁺-N addition for all the soil samples. At equal NH₄⁺-N addition concentrations, the inhibitory effect was always the lowest for the compost sample. The abundances of particulate methane monooxygenase (pmoA) and ammonia monooxygenase (amoA) genes were key factors influencing the CH₄ oxidation rate and contribution of AOB to CH₄ oxidation. The higher abundance of pmoA and lower abundance of amoA in landfill cover soil could explain the higher CH₄ oxidation rate and lower contribution of AOB to CH₄ oxidation in this soil type. Meanwhile, the higher contribution of AOB to CH₄ oxidation for compost could be attributed to the higher abundance of the amoA gene and lower abundance of pmoA.

Methylmercury levels in cover soils from two landfills in Xi'an and Shanghai, China: Implications for mercury methylation potentials

Landfills are considered important sources of mercury for surrounding ecosystems. Methylmercury (MeHg) levels in waste layers have been studied extensively; however, the levels of MeHg in cover soils remain undefined. Here, total mercury (THg) and MeHg concentrations in surface cover soils and soil cores from two landfills in China and possible factors affecting Hg methylation were studied. The mean MeHg concentration in surface cover soils from both landfills was 0.048 ng g^-1, suggesting that cover soil layers are not active sites of MeHg production. Soil MeHg concentrations in both landfills were affected little by closure time. In the Jiangcungou landfill, no correlations between MeHg concentration and the measured environmental factors (e.g., THg, soil pH, organic matter (OM), and S) were observed that indicated that these parameters might have indirect effects on MeHg concentration. However, in the Laogang landfill, significant correlations were found between MeHg concentration and the measured environmental factors. The results showed that MeHg concentration in the surface cover soil from area D of the Laogang landfill is regulated mainly by soil pH, OM, and S, and that its vertical distribution in areas C and D is regulated mainly by soil pH and soil OM, respectively. These findings fill a knowledge gap regarding MeHg levels in cover soils and they advance our understanding of Hg cycling in landfills, presenting positive implications for landfill management and risk assessment of MeHg.

A Simulation model for estimating methane oxidation and emission from landfill cover soils

Quantification of methane (CH₄) oxidation and emission from landfill cover soils is important for evaluating measures to mitigate anthropogenic greenhouse gas emissions. In this study, a model that combines the multicomponent diffusive equation and Darcy's law, coupled with the dual Monod kinetic equation, was established to simulate CH₄ transport, oxidation and emission in landfill cover soils. Sensitivity analysis was performed to illustrate the influence of model parameters on CH₄ transport, oxidation and emission. The model was then applied to predict CH₄ emissions from several column experiments. The results of the sensitivity analysis showed that a high CH₄ oxidation rate can be obtained with a high V_max of cover soil, even for a low cover soil thickness, and that oxidation efficiency is constant when the thickness of the cover soil becomes greater than a threshold value. The simulated results fitted well with the measured values, confirming that the new model provides a reliable method for estimating CH₄ emissions from landfills.

A simulation model for methane emissions from landfills with interaction of vegetation and cover soil

Global climate change and ecological problems brought about by greenhouse gas effect have become a severe threat to humanity in the 21st century. Vegetation plays an important role in methane (CH₄) transport, oxidation and emissions from municipal solid waste (MSW) landfills as it modifies the physical and chemical properties of the cover soil, and transports CH₄ to the atmosphere directly via their conduits, which are mainly aerenchymatous structures. In this study, a novel 2-D simulation CH₄ emission model was established, based on an interactive mechanism of cover soil and vegetation, to model CH₄ transport, oxidation and emissions in landfill cover soil. Results of the simulation model showed that the distribution of CH₄ concentration and emission fluxes displayed a significant difference between vegetated and non-vegetated areas. CH₄ emission flux was 1-2 orders of magnitude higher than bare areas in simulation conditions. Vegetation play a negative role in CH₄ emissions from landfill cover soil due to the strong CH₄ transport capacity even though vegetation also promotes CH₄ oxidation via changing properties of cover soil and emitting O₂ via root system. The model will be proposed to allow decision makers to reconsider the actual CH₄ emission from vegetated and non-vegetated covered landfills.

Co-composting of municipal solid waste mixed with matured sewage sludge: The relationship between N2O emissions and denitrifying gene abundance

Aerobic composting is an alternative measure to the disposal of municipal solid waste (MSW). However, it produces nitrous oxide (N₂O), a highly potent greenhouse via microbial nitrification and denitrification. In this study, the effects of matured sewage sludge (MSS) amendment on N₂O emissions and the inter-relationships between N₂O emissions and the abundance of denitrifying bacteria were investigated during aerobic composting of MSW. The results demonstrated that MSW composting with MSS amendments (C1, and C2, with a MSW to MSS ratio of 2:1 and 4:1, (v/v), respectively) significantly increased N₂O emissions during the initial stage, yet contributed to the mitigation of N₂O emissions during the cooling and maturation stage. MSS amended composting emitted a total of 18.4%-25.7% less N₂O than the control treatment without MSS amendment (CK). Matured sewage sludge amendment also significantly altered the abundance of denitrifying bacteria. The quantification of denitrifying functional genes revealed that the N₂O emission rate had a significant positive correlation with the abundance of the nirS, nirK genes in both treatments with MSS amendment. The nosZ/(nirS + nirK) ratio could be a good indicator for predicting N₂O emissions. The higher N₂O emission rate during the initial stage of composting mixed with MSS was characterized by lower nosZ/(nirS + nirK) ratios, compared to CK treatment. Higher ratios of nosZ/(nirS + nirK) were measured during the cooling and maturation stage in treatments with MSS which resulted in a reduction of the N₂O emissions. These results demonstrated that MSS amendment could be a valid strategy for mitigating N₂O emissions during MSW composting.

N2O emissions from an intermittently aerated semi-aerobic aged refuse bioreactor: Combined effect of COD and NH4+-N in influent leachate

The carbon-nitrogen ratio (COD/NH₄⁺-N) is an important factor affecting nitrification and denitrification in wastewater treatment; this factor also influences nitrous oxide (N₂O) emissions. This study investigated two simulated intermittently aerated semi-aerobic aged refuse bioreactors (SAARB) filled with 8-year old aged refuse (AR). The research analyzed how differences in and the combination of influent COD and NH₄⁺-N impact N₂O emissions in leachate treatment. Experimental results showed that N₂O emissions increased as the influent COD/NH₄⁺-N decreased. The influent COD had a greater effect on N₂O emissions than NH₄⁺-N at the same influent ratios of COD/NH₄⁺-N (2.7 and 8.0, respectively). The maximum N₂O emission accounted for 8.82±2.65% of the total nitrogen removed from the influent leachate; the maximum level occurred when the COD was 2000mg/L. An analysis of differences in influent carbon sources at the same COD/NH₄⁺-N ratios concluded that the availability of biodegradable carbon substrates (i.e. glucose) is an important factor affecting N₂O emissions. At a low influent COD/NH₄⁺-N ratio (2.7), the N₂O conversion rate was greater when there were more biodegradable carbon substrates. Although the SAARB included the N₂O generation and reduction processes, N₂O reduction mainly occurred later in the process, after leachate recirculation. The maximum N₂O emission rate occurred in the first hour of single-period (24h) experiments, as leachate contacted the surface AR. In practical SAARB applications, N₂O emissions may be reduced by measures such as reducing the initial recirculation loading of NH₄⁺-N substrates, adding a later supplement of biodegradable carbon substrates, and/or prolonging hydraulic retention time (HRT) of influent leachate.

N₂O emission from a combined ex-situ nitrification and in-situ denitrification bioreactor landfill

A combined process comprised of ex-situ nitrification in an aged refuse bioreactor (designated as A bioreactor) and in-situ denitrification in a fresh refuse bioreactor (designated as F bioreactor) was constructed for investigating N2O emission during the stabilization of municipal solid waste (MSW). The results showed that N2O concentration in the F bioreactor varied from undetectable to about 130 ppm, while it was much higher in the A bioreactor with the concentration varying from undetectable to about 900 ppm. The greatly differences of continuous monitoring of N2O emission after leachate cross recirculation in each period were primarily attributed to the stabilization degree of MSW. Moreover, the variation of N2O concentration was closely related to the leachate quality in both bioreactors and it was mainly affected by the COD and COD/TN ratio of leachate from the F bioreactor, as well as the DO, ORP, and NO3(-)-N of leachate from the A bioreactor.