Production characteristics of N2O during stabilization of municipal solid waste in an intermittent aerated semi-aerobic bioreactor landfill

Estimating of gases emission from waste sites to generate electrical energy as a case study at Al-Hillah City in Iraq

Methane (CH4) is a greenhouse gas resulting from human activities, especially landfills, and it has many potential environmental issues, such as its major role in global warming. On the other hand, methane can be converted to liquid fuel or electricity using chemical conversion or gas turbine generators. Therefore, reusing such gases could be of great environmental and economic benefit. In this context, this study aims to estimate the emissions of methane gas from the landfills in Al-Hillah City, Iraq, from 2023 to 2070 and the producible electric energy from this amount. The estimating process was carried out using the Land GEM model and compared with traditional models. The obtained results demonstrated that the total estimated landfill methane emissions for 48 years are 875,217 tons, and the average annual methane emission is 18,234 tons based on a yearly waste accumulation rate of 1,046,413 tons and a total waste amount of 50,227,808 tons. The anticipated loads of methane gas can be utilized to generate about 287,442 MW/year of electricity from 2023 to 2070. In conclusion, the results obtained from this study could be evidence of the potential environmental and economic benefits of harvesting and reusing methane gas from landfills.

Intermittent aeration reducing N2O emissions from bioreactor landfills with gas-water joint regulation

Landfills are important emission sources of atmospheric N₂O, especially bioreactor landfills with leachate recirculation. In this study, N₂O emissions were characterized in four bioreactor landfills with different ventilation methods, including intermittent (2-h aeration per 12 h or 4 h/d in continuous) and continuous aeration (20 h/d), in comparison to a traditional landfill without aeration. During the experiment, the N₂O emissions from the landfill reactors with intermittent aeration were 7.48 and 7.15 mg, accounting for only 20.8% and 19.9% of those with continuous aeration, respectively. Continuous aeration was more favorable for the biodegradation of organic matter than intermittent aeration in the landfilled waste and leachate. Intermittent and continuous aeration could both effectively remove total nitrogen (TN) and NH₄⁺-N with removal efficiencies above 64% in the leachate. In the experimental landfill reactors with gas-water joint regulation, the proportion of N₂O-N to TN loss ranged from 0.02% to 0.75%. Luteimonas, Pseudomonas, Thauera, Pusillimonas and Comamonas were the dominant denitrifying bacteria in the landfill reactors. The denitrifying bacterial community in the landfilled waste was closely related to its degree of stabilization and nitrogenous compound concentrations in the landfilled waste and leachate. The NO₃^--N and NO₂^--N concentrations of leachate were the most important environmental factors affecting the succession of nirS-type and nirK-type denitrifying microbial communities in the landfilled waste. These findings indicated that intermittent aeration was an economical and effective way to accelerate the stabilization of landfilled waste and reduce the pollutants in leachate and N₂O emissions during landfill mining and reclamation.

Quantification and control of gaseous emissions from solid waste landfill surfaces

Landfilling is the most common option for solid waste disposal worldwide. Landfill sites can emit significant quantities of greenhouse gases (GHGs; e.g., methane, carbon dioxide, and nitrous oxide) and release toxic and odorous compounds (e.g., sulfides). Due to the complex composition and characteristics of landfill surface gas emissions, the quantification and control of landfill emissions are challenging. This review attempts to comprehensively understand landfill emission quantification and control options by primarily focusing on GHGs and odor compounds. Landfill emission quantification was highlighted by combining different emissions monitoring approaches to improve the quality of landfill emission data. Also, landfill emission control requires a specific approach that targets emission compounds or a systematic approach that reduces overall emissions by combining different control methods since the diverse factors dominate the emissions of various compounds and their transformation. This integrated knowledge of emission quantification and control options for GHGs and odor compounds is beneficial for establishing field monitoring campaigns and incorporating mitigation strategies to quantify and control multiple landfill emissions.

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.

Greenhouse gas emissions from semi-aerobic bioreactor landfills with different vent-pipe diameters

The emission patterns of three greenhouse gasses (GHGs), viz. CH₄, CO₂, and N₂O 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 CO₂ equivalents per ton of MSW from the SABL2, SABL1, and SABL3, respectively, during the operation of 465 days. Of the three GHGs, CH₄ influenced the GHE the most, contributing 72.53%, 79.17%, and 71.42% in SABL1, SABL2, and SABL3, respectively. In the same sequence, CO₂ (14.87%, 14.06%, and 21.9%) and N₂O (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 CH₄ collection and oxidation. The results provided a technical guidance for GHG mitigation in MSW management.

A systematic assessment of aeration rate effect on aerobic degradation of municipal solid waste based on leachate chemical oxygen demand removal

Aeration is one mainstream technique to accelerate municipal solid waste (MSW) degradation in landfills. The determination of an appropriate aeration rate is critical to the design and operation of a landfill aeration system. In this study, we analyze 132 waste degradation tests reported in forty one studies in the literature. We use L min^-1 kg^-1 dry organic matter (L min^-1 kg^-1 DOM) as the uniform unit to quantify the aeration rates in all tests. The first order rate coefficient for chemical oxygen demand (COD) removal in leachate (k_COD) is selected as the parameter to characterize MSW degradation process. We further divide aerobic tests into five aerobic groups base on the respective aeration rates, i.e., <0.02, 0.02-0.1, 0.1-0.3, 0.3-1, and >1 L min^-1 kg^-1 DOM. With an increase in the aeration rate, the k_COD increases first and then decreases. The aeration rate between 0.1 and 0.3 L min^-1 kg^-1 DOM has the best enhancement on the k_COD. The k_COD values are not much higher than the anaerobic and semi-aerobic tests when the aeration rates are <0.1 L min^-1 kg^-1 DOM, because such aeration rates may be lower than the actual oxygen consumption rates. An aeration rate >0.3 L min^-1 kg^-1 DOM reduces the k_COD likely due to excess water evaporation and ventilation cooling. Among the analyzed results, the aeration rate is the most related to the k_COD in principal component analysis than the other factors, including liquid recirculation and addition, waste total density, waste degradation level, and waste initial temperature.

Influence of the void fraction and vertical gas vents on the waste decomposition in semi-aerobic landfill: Lab-scale tests

The waste classification has not been carried out worldwide, especially in developing countries. The high content of water in the kitchen waste will definitely affect the amount of air flowing into waste layer from leachate collection pipe. In this experiment, three lab-scale landfill simulation reactors were established. Reactor A and B are semi-aerobic landfill modes, while reactor C has no vertical gas vent. The void fraction in the waste of reactor B was increased by adding gravel serve as part of the waste. The waste sample used in landfill reactor mainly included kitchen waste (58%). The waste decomposition conditions in the landfill were investigated using temperature sensors embedded in the waste, by determining the velocity and gas flow direction and by measuring the volume and composition of leachate produced. The results showed that the void fraction of waste in reactor A and C was 29.79% and 30.86% respectively, and that of waste in reactor B 34.96%. Compared with reactor A, reactor B had its temperature increase earlier. In addition, the temperature distribution in vertical gas vent of the reactor A and B is higher than in the leachate collection pipe. The gas flux of vertical gas vent increased with the temperature of gas vent. Reactor A and B had significantly lower concentration of chemical oxygen demand (COD) and ammonia nitrogen (NH₃⁺-N) than that of reactor C. It was concluded that landfill decomposition can be accelerated by increasing the void fraction in the waste and keeping a vertical gas vent open.

Impact of leachate recirculation frequency on the conversion of carbon and nitrogen in a semi-aerobic bioreactor landfill

To study the impact of leachate recirculation frequency on the transformation of carbon and nitrogen pollutants in a semi-aerobic bioreactor landfill (SABL), three laboratory-scale SABLs were investigated, each using a different leachate recirculation frequency (daily, once each 3 days, and once each 5 days). Results showed that degradation of total nitrogen (TN), ammonium nitrogen (NH₄⁺-N), chemical oxygen demand (COD), and total organic carbon (TOC) could be described using a quadratic polynomial-compound index model. Degradation rates of TN, NH₄⁺-N, COD, and TOC slightly increased from 0.01795, 0.01814, 0.01451, and 0.01166 day^-1 to 0.02054, 0.01903, 0.01488, and 0.01203 day^-1, respectively, when the recirculation frequency increased from once per 5 days to once per 3 days. When recirculation frequency was increased to daily, degradation rates of TN, NH₄⁺-N, COD, and TOC significantly increased to 0.03698, 0.02718, 0.02479, and 0.02872 day^-1, respectively. Moreover, when recirculation frequency increased from once per 5 days to once per 3 days, the gasification rate of nitrogenous and carbonaceous pollutants was enhanced between 20.38 and 8.17%, respectively. When the leachate recirculation rate further increased to daily, only a small amount of carbonaceous and nitrogenous pollutants was transformed to the liquid phase. Thus, increasing the leachate recirculation frequency in an SABL benefits the removal of carbonaceous and nitrogenous pollutants from the reactor. In addition, the greater is the recirculation frequency, the lower is the residual carbon and nitrogen in the solid phase, and the higher is the gasification rate. A proper recirculation frequency promotes the stabilization of landfill leachate. This study provides a theoretical reference and experimental evidence for accelerating the stabilization of MSW and contributes to the macro-control of landfills.

Greenhouse Gas Emissions from Landfills: A Review and Bibliometric Analysis

The landfill is an important method of disposal of municipal solid waste. In particular, the landfill is especially vital in many developing countries, with it being the main biodegradable waste disposal method due to its simple management and ability for mass manipulation. Landfills have recently been shown to be an important source of greenhouse gas (GHG) emissions by researchers in different countries. However, few reviews have been conducted within the related fields, which means that there is still a lack of comprehensive understanding related to relevant study achievements. In this study, a bibliometric analysis of articles published from 1999 to 2018 on landfill GHG emissions was presented to assess the current trends, using the Web of Science (WOS) database. The most productive countries/territories, authors and journals were analyzed. Moreover, the overall research structure was characterized based on co-cited references, emerging keywords and reference citations by means of bibliometric analysis. Due to the increasing amount of attention being paid to the GHG emissions and their mitigation methods, this study provided comprehensive bibliometric information on GHG emissions from landfills over the past two decades and highlighted the importance of the development and dissemination of updated knowledge frameworks.

Improving leachate quality and optimizing CH4 and N2O emissions from a pre-aerated semi-aerobic bioreactor landfill using different pre-aeration strategies

Landfill aeration efficiently accelerates municipal solid waste (MSW) stabilization. This method also impacts methane (CH₄) and nitrous oxide (N₂O) emissions during aeration. In this study, the effects of three pre-aeration strategies on leachate quality variations and CH₄ and N₂O emissions from three lab-scale pre-aerated semi-aerobic bioreactor landfills, which were filled with MSW, were investigated: low frequency and high frequency intermittent aeration (LIA and HIA) and continuous micro-aeration (CMA). Experimental results showed that these three strategies effectively reduced organic and N-based pollutants concentration in leachate. Compared with intermittent aeration (IA), CMA increased cumulative CH₄ emissions (9234.3 mg) and resulted in a longer emission period (95 days). HIA generated the least cumulative CH₄ emissions (4297.6 mg) and shortest emission period (65 days) due to organic matter loss during aeration. N₂O emissions were present at low levels in early stages for each bioreactor, and then, increased by 1-3 orders of magnitude in the later stages due to low influent carbon-nitrogen ratio. HIA resulted in maximum cumulative N₂O emissions (2884.6 mg) and experienced a longer emission period (179 days) compared to CMA (2281.6 mg; 151 days). LIA had the longest N₂O emission period (209 days), but had the lowest cumulative N₂O emissions (1486.3 mg). CH₄ and N₂O emissions mainly occurred in the early and later stages of landfill stabilization, respectively. Therefore, the study proposes an optimized pre-aeration strategy for practical landfill aeration management: early CMA may promote rapid organic matter removal and effective CH₄ recovery; and late LIA may reduce N₂O emissions.

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.

The S.An.A.® concept: Semi-aerobic, Anaerobic, Aerated bioreactor landfill

Hybrid Bioreactor Landfills are designed to enhance and speed up biological processes, aiming at reducing the duration of post operational phase until landfill completion. S.An.A.® (Semi-aerobic, Anaerobic, Aerated) concept consists in a Hybrid Bioreactor featuring a first semi-aerobic phase to enhance the methane production occurring in the following anaerobic step and a forced aeration for the abatement of the residual emissions. At the end of the last step, semi-aerobic conditions are restored and flushing applied for leaching residual non-biodegradable compounds. Results of the application of S.An.A.® concept to a lab scale bioreactor system showed that pre-aeration was effective in controlling the concentration of VFA, increasing pH and stimulating methane production during anaerobic phase; in particular with intermittent airflow the methane potential was 50% higher respect to control reactors. Forced aeration reduced organic compounds and nitrogen concentration in leachate of an order of magnitude, better performing in low airflow reactors. S.An.A.® Hybrid bioreactors proved to be an efficient system both for increasing methane production and reaching landfill completion in shorter time, suggesting that with proper landfill management, the duration of post-closure care might be reduced by 25-35%.

Characterization of temporal variations in landfill gas components inside an open solid waste dump site in Sri Lanka

A long-term monitoring of composition of landfill gases in the region with high rainfall was conducted using an argon assay in order to discuss air intrusion into the dump site. Gas samples were taken from vertical gas monitoring pipes installed along transects at two sections (called new and old) of an abandoned waste dump site in Sri Lanka. N₂O concentrations varied especially widely, by more than three orders of magnitude (0.046-140 ppmv). The nitrogen/argon ratio of landfill gas was normally higher than that of fresh air, implying that denitrification occurred in the dump site. Argon assays indicate that both N₂ and N₂O production occurred inside waste and more significantly in the old section. The Ar assay would help for evaluations of N₂O emission in developing countries.

IMPLICATIONS

A long-term monitoring of composition of landfill gases in the region with high rainfall was conducted using an argon assay in order to discuss air intrusion into the dump site. Argon assays indicate that both N₂ and N₂O production occurred inside waste and more significantly in the old section.

Recirculation of reverse osmosis concentrate in lab-scale anaerobic and aerobic landfill simulation reactors

Leachate treatment is a major issue in the context of landfill management, particularly in view of the consistent changes manifested over time in the quality and quantity of leachate produced, linked to both waste and landfill characteristics, which renders the procedure technically difficult and expensive. Leachate recirculation may afford a series of potential advantages, including improvement of leachate quality, enhancement of gas production, acceleration of biochemical processes, control of moisture content, as well as nutrients and microbe migration within the landfill. Recirculation of the products of leachate treatment, such as reverse osmosis (RO) concentrate, is a less common practice, with widespread controversy relating to its suitability, potential impacts on landfill management and future gaseous and leachable emissions. Scientific literature provides the results of only a few full-scale applications of concentrate recirculation. In some cases, an increase of COD and ammonium nitrogen in leachate was observed, coupled with an increase of salinity; which, additionally, might negatively affect performance of the RO plant itself. In other cases, not only did leachate production not increase significantly but the characteristics of leachate extracted from the well closest to the re-injection point also remained unchanged. This paper presents the results of lab-scale tests conducted in landfill simulation reactors, in which the effects of injection of municipal solid waste (MSW) landfill leachate RO concentrate were evaluated. Six reactors were managed with different weekly concentrate inputs, under both anaerobic and aerobic conditions, with the aim of investigating the short and long-term effects of this practice on landfill emissions. Lab-scale tests resulted in a more reliable identification of compound accumulation and kinetic changes than full-scale applications, further enhancing the development of a mass balance in which gaseous emissions and waste characteristics were also taken into consideration. Results showed that RO concentrate recirculation did not produce consistent changes in COD emissions and methane production. Simultaneously, ammonium ion showed a consistent increase in leachate (more than 25%) in anaerobic reactors, free ammonia gaseous emissions doubled with concentrate injection, while chloride resulted accumulated inside the reactor.

Biogas production enhancement using semi-aerobic pre-aeration in a hybrid bioreactor landfill

Landfilling continues to be one of the main methods used in managing Municipal Solid Waste (MSW) worldwide, particularly in developing countries. Although in many countries national legislation aims to reduce this practice as much as possible, landfill is a necessary and unavoidable step in closing the material cycle. The need for innovative waste management techniques to improve landfill management and minimize the adverse environmental impact produced has resulted in an increasing interest in innovative systems capable of accelerating waste stabilization. Landfill bioreactors allow decomposition kinetics to be increased and post-operational phase to be shortened; in particular, hybrid bioreactors combine the benefits afforded by both aerobic and anaerobic processes. Six bioreactor simulators were used in the present study: four managed as hybrid, with an initial semi-aerobic phase and a second anaerobic phase, and two as anaerobic control bioreactors. The main goal of the first aerated phase is to reduce Volatile Fatty Acids (VFA) in order to increase pH and enhance methane production during the anaerobic phase; for this reason, air injection was stopped only when these parameters reached the optimum range for methanogenic bacteria. Biogas and leachate were constantly monitored throughout the entire methanogenic phase with the aim of calibrating a Gompertz Model and evaluating the effects of pre-aeration on subsequent methane production. The results showed that moderate and intermittent pre-aeration produces a positive effect both on methane potential and in the kinetics of reaction.

Field study of nitrous oxide production with in situ aeration in a closed landfill site

Nitrous oxide (N(2)O) has gained considerable attention as a contributor to global warming and depilation of stratospheric ozone layer. Landfill is one of the high emitters of greenhouse gas such as methane and N(2)O during the biodegradation of solid waste. Landfill aeration has been attracted increasing attention worldwide for fast, controlled and sustainable conversion of landfills into a biological stabilized condition, however landfill aeration impel N(2)O emission with ammonia removal. N(2)O originates from the biodegradation, or the combustion of nitrogen-containing solid waste during the microbial process of nitrification and denitrification. During these two processes, formation of N(2)O as a by-product from nitrification, or as an intermediate product of denitrification. In this study, air was injected into a closed landfill site and investigated the major N(2)O production factors and correlations established between them. The in-situ aeration experiment was carried out by three sets of gas collection pipes along with temperature probes were installed at three different distances of one, two and three meter away from the aeration point; named points A-C, respectively. Each set of pipes consisted of three different pipes at three different depths of 0.0, 0.75 and 1.5 m from the bottom of the cover soil. Landfill gases composition was monitored weekly and gas samples were collected for analysis of nitrous oxide concentrations. It was evaluated that temperatures within the range of 30-40°C with high oxygen content led to higher generation of nitrous oxide with high aeration rate. Lower O(2) content can infuse N(2)O production during nitrification and high O(2) inhibit denitrification which would affect N(2)O production. The findings provide insights concerning the production potentials of N(2)O in an aerated landfill that may help to minimize with appropriate control of the operational parameters and biological reactions of N turnover.

IMPLICATIONS

Investigation of nitrous oxide production potential during in situ aeration in an old landfill site revealed that increased temperatures and oxygen content inside the landfill site are potential factors for nitrous oxide production. Temperatures within the range of optimum nitrification process (30-40°C) induce nitrous oxide formation with high oxygen concentration as a by-product of nitrogen turnover. Decrease of oxygen content during nitrification leads increase of nitrous oxide production, while temperatures above 40°C with moderate and/or low oxygen content inhibit nitrous oxide generation.

Epiphytic lichens as indicators of environmental quality around a municipal solid waste landfill (C Italy)

Epiphytic lichens have been used as indicators of environmental quality around a municipal solid waste landfill in C Italy. An integrated approach, using the diversity of epiphytic lichens, as well as element bioaccumulation and physiological parameters in the lichen Flavoparmelia caperata (L.) Hale was applied along a transect from the facility. The results highlighted the biological effects of air pollution around the landfill. The Index of Lichen Diversity (ILD) increased and the content of heavy metals (Cr, Cd, Cu, Fe, Ni and Zn) decreased with distance from the landfill. Clear stress signals were observed in lichens growing in front of the facility, i.e. discoloration, necrosis, membrane lipid peroxidation, lower ergosterol content, higher dehydrogenase activity. Decreased photosynthetic efficiency, altered chlorophyll integrity and production of secondary metabolites were also found. The results suggested that lichens can be profitably used as bioindicators of environmental quality around landfills.

Short tests to couple N₂O emission mitigation and nitrogen removal strategies for landfill leachate recirculation

Landfills implemented with onsite leachate recirculation can efficiently remove pollutants, but currently they are reckoned as N2O emission hot spots. In this project, we evaluated the relationship between N2O emission and nitrogen (N) removal efficiency with different types of leachate recirculated. Nitrate supplemented leachate showed low N2O emission rates with the highest N removal efficiency (~70%), which was equivalent to ~1% nitrogen emitted as N2O. Although in nitrite containing leachates' N removal efficiencies also reached to ~60%, their emitted N2O comprised ~40% of total removed nitrogen. Increasing nitrogen load promoted N2O emission and N removal efficiency, except in ammonia type leachate. When the ratio of BOD to total nitrogen increased from 0.2 to 0.4, the N2O emission flux from nitrate supplemented leachate decreased from ~25 to <0.5 μg N/kg-soil·h. We argue prior to leachate in situ recirculation, sufficient pre-aeration is critical to mitigate N2O surges and simultaneously enhance nitrogen removal efficiency.

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.