Greenhouse gas emissions from landfill leachate treatment plants: a comparison of young and aged landfill

Characteristics and key driving factors of nitrous oxide emissions from a full-scale landfill leachate treatment system

The characteristics of N2O emission from a full-scale landfill leachate treatment system were investigated by in-situ monitoring over 1.4 years and driving factors responsible for these emissions were identified by statistical analysis of multidimensional environmental variables. The results showed that the maximum N2O emission flux of 2.21 × 107 mg N·h-1 occurred in the nitrification tanks, where 98.5 % of the total N2O was released, with only 1.5 % of the total N2O emitted from the denitrification tanks. Limited oxygen in nitrification tank was responsible for N2O hotspot. The N2O emissions from the parallel lines A and B (both comprising the primary biochemical system) accounted for 52.6 % and 46.6 %, respectively, while the secondary biochemical system contributed only 0.8 % to the total emissions. Higher nitrite concentration in line A and lower nitrogen loading in the secondary biochemical system caused these discrepancies. We found that during the steady state of leachate treatment, intensive N2O emissions of 253.4-1270.5 kg N·d-1 were measured. The corresponding N2O emission factor (EF) ranged from 8.86 to 49.6 %, much higher than those of municipal wastewater treatment. But N2O EF was inconceivably as low as 0.42 % averagely after system maintenance. Influent with low salinity was the key reason, followed by the high MLSS and varying microbial community after maintenance. The dominant genus shifted from Lentimicrobium and Thauera to Norank-F-Anaerolineaceae and Unclassified-F-Rhodocyclaceae. This study underscores the significance of landfill leachate treatment in urban nitrogen management and provides valuable insights into the characteristics and driving factors of N2O emissions from such systems. The findings offer important references for greenhouse gas emission inventories and strategies for N2O control in full-scale wastewater treatment plants.

The key role of denitrification and dissimilatory nitrate reduction in nitrogen pollution along vertical landfill profiles from metagenomic perspective

Landfill are persistent sources of nitrogen (N) pollution even in the decades after closure. However, the biological pathways of N-pollution, particularly N₂O and NH₄⁺, at different landfill depths have received little attention. In this study, metagenomic analysis was conducted on landfill refuse from vertical reservoir profiles in two closed landfills named XT and MT. NH₄⁺ concentrations were found to be higher in deeper layers of MT, while greater potential for N₂O emissions occurred in XT and the shallow layers of MT. Furthermore, the community structure and function of N-metabolizing microbes were more strongly defined by landfill depth than landfill type. Denitrification, involving abundant nirK and norB genes, was identified as the major pathway for N₂O production in both XT and MT-shallow, while dissimilatory nitrate reduction with abundant nirBD genes was identified as the major pathway for NH₄⁺ accumulation. Microbes of norB-type and nirBD-type were positively affected by NO₃^- in XT, whereas negatively affected by contents of organic material and moisture in MT-shallow. The mechanism by which nitrogen fixation, with abundant nifH genes, contributes to NH₄⁺ accumulation in MT-deep should be further elucidated. These findings can provide a theoretical basis for governing scientific N-pollution control strategies throughout the entire landfill process.

Identification of key water parameters and microbiological compositions triggering intensive N2O emissions during landfill leachate treatment process

Landfill leachate treatment processes tend to emit more N₂O compared to domestic wastewater treatment. This discrepancy may be ascribed to leachate water characteristics such as high refractory COD, ammonium (NH₄⁺) content, and salinity. In this work, the leachate influent was varied to examine the N₂O emission scenarios. NH₄⁺-N, COD, and Cl^- concentrations ranged between 1000-2500, 1000-10,000, and 500-3000 mg L^-1, respectively. Simultaneously, we attempted to combine statistical analysis with high-throughput sequencing to understand the microbial mechanism with regards to N₂O emission. Results show that the strong N₂O emissions occur in the nitrifying tank due to the intensive aeration. The system receiving the lowest COD shows the maximum N₂O emission factor of 42.7% of the removed nitrogen. Both redundancy analysis and a structural equation model verify that insufficient degradable organics are the key water parameter triggering intensive N₂O emission within the designed influent limits. Furthermore, two essential but non-abundant functional bacteria, Flavobacterium (acting as a denitrifier) and Nitrosomonas (acting as a nitrifier), are identified as the core functional species that dramatically influence N₂O emissions. An increase in influent COD promotes the proliferation of Flavobacterium and inhibits Nitrosomonas, which in turn reduce N₂O release. Meanwhile, two keystone species of Castellaniella and Saprospiraceae unclassified are recognized. They may supply a suitable niche and integrity of the microbial community for N-cycle functional bacteria. These findings reveal the essential role of non-abundant species in microbial community, and expand the current understanding of microbial interactions underlying N₂O dynamics in leachate treatment systems.

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.

Time-resolved characteristics and production pathways of simulated landfilling N2O emission under different oxygen concentrations

Nitrous oxide (N₂O), an important greenhouse gas, is emitted from landfill reservoirs, especially in the working face, where nitrification and denitrification occur under different O₂ concentrations. In order to explore the effects of O₂ concentration on N₂O emissions and production pathways, the production of N₂O from simulated fresh waste landfilling under 0%, 5%, 10%, and 21% (vol/vol) O₂ concentrations were examined, and ¹⁵N isotopes were used as tracers to determine the contributions of nitrification (NF), heterotrophic denitrification (HD), and nitrification-coupled denitrification (NCD) to N₂O production over a 72-h incubation period. Equal amounts of total nitrogen consumption occurred for all studied O₂ concentration and the simulated waste tended to release more N₂O under 0% and 21% O₂. Heterotrophic denitrification was the main source of N₂O release at the studied oxygen concentrations, contributing 90.51%, 69.04%, 80.75%, and 57.51% of N₂O under O₂ concentrations of 0%, 5%, 10%, and 21%, respectively. Only denitrification was observed in the simulated fresh waste when the oxygen concentration of the bulk atmosphere was 0%. The nitrate reductase (nirS)-encoding denitrifiers in the simulated landfill were also studied and significant differences were observed in the richness and diversity of the denitrifying community at different taxonomic levels. It was determined that optimising the O₂ content is a crucial factor in N₂O production that may allow greenhouse gas emissions and N turnover during landfill aeration to be minimised.

Temporal and spatial variation of greenhouse gas emissions from a limited-controlled landfill site

Landfilling biodegradable waste is an important source of global greenhouse gas (GHG) emissions. Among the several types of landfill, limited-controlled landfill is a common method used to dispose of domestic solid waste, especially in developing countries. However, information about GHG emissions from limited-controlled landfill sites has rarely been reported. In this study, the GHG emissions from a typical limited-controlled landfill site were investigated under a regular period for one year. The number and positions of static chambers were arranged according to the guidance on Monitoring Landfill Gas Surface Emissions by the UK Environment Agency to obtain representative data from the heterogeneous surface of the landfill. Inverse distance weighting (IDW) was applied to evaluate and visualise the GHG emissions from the whole landfill surface based on the measurements of distributed static chambers. As an important GHG source of the landfill site, the emissions from the landfill leachate treatment plant were also measured. The results revealed that CH₄ and N₂O emission fluxes from the landfill area were 1324.73 ± 2005.17 mg C m^-2 d^-1 and 2.16 ± 2.33 mg N m^-2 d^-1, respectively, and the fluxes from the leachate treatment plants were 23.92 ± 29.20 mg C m^-2 d^-1 and 16.40 ± 16.89 mg N m^-2 d^-1, respectively. CH₄ and N₂O releases preferred to present spatial heterogeneity, while temporal heterogeneity was expected to exist in CH₄ and CO₂ emissions. The annual GHG emissions from the limited-controlled landfill was calculated to be 1.078 Gg CO₂-eq yr^-1, which was the least among all types of landfill sites. In addition, the GHG emission factor was 0.042 t CO₂-eq t^-1 waste yr^-1 which could not be ignored compared to the sanitary landfills. Therefore, it is advisable to give more attention and determine a potential solution for reducing GHG emissions from limited-controlled landfill sites.

Treatment of concentrated leachate with low greenhouse gas emission in two-stage membrane bioreactor bio-augmented with Alcaligenes faecalis no. 4

Methane (CH₄) and nitrous oxide (N₂O) emissions from two-stage membrane bioreactor (MBR) bio-augmented by Alcaligenes faecalis no. 4 during municipal solid waste leachate treatment were investigated. The system was operated at hydraulic retention time (HRT) of 2.5 and 1 days in each reactor under the presence and absence of sludge recirculation. Alcaligenes faecalis no. 4 bio-augmentation helped improving organic carbon and nitrogen removals while reducing CH₄ and N₂O emissions. CH₄ and N₂O emissions were decreased by 46% and 85% when A. faecalis no. 4 was introduced at HRT of 2.5 days. Under the presence of A. faecalis no. 4, the operation of two-stage MBR with sludge recirculation could reduce CH₄ and N₂O emissions by 51% and 54% as compared to its operation without sludge recirculation. An operation under short HRT of 1 day also yielded high organic carbon and nitrogen removals of more than 85% while emitting lower CH₄ and N₂O emission of 6.7% C and 0.04% N when operated with sludge recirculation. Implications: A two-stage membrane bioreactor was effectively applied to the treatment of concentrated leachate (BOD~20,000 mg/L) at a short hydraulic retention time of 2.5 days and 1 day. About 80% of CH₄ and N₂O was emitted from the anaerobic and aerobic reactors, respectively. Introduction of Alcaligenes faecalis no. 4 reduced CH₄ and N₂O emissions in both reactors as it became the predominant microorganism under an elevated pH condition. Lower CH₄ and N₂O emissions were achieved under a sludge recirculation operation, as Alcaligenes faecalis no. 4 could suppress methanogenic activities in the anaerobic reactor and converted a majority of nitrogen into its cell mass, thus reducing N₂O production through a biological nitrification-denitrification pathway.

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.

Leachate treatment in landfills is a significant N2O source

The importance of methane (CH₄) emissions from landfills has been extensively documented, while the nitrous oxide (N₂O) emissions from landfills are considered negligible. In this study, three landfills were selected to measure CH₄ and N₂O emissions using the static chamber method. Dongbu (DB) and Dongfu (DF) landfills, both located in Xiamen city, Fujian Province, were classified as sanitary. The former started to receive solid waste from Xiamen city in 2009, and the latter was closed in 2009. Nanjing (NJ) landfill, located in Nanjing county, Fujian Province, was classified as managed. Results showed that for the landfill reservoirs, CH₄ emissions were significant, while N₂O emissions occurred mainly in operating areas (on average, 16.3 and 19.0mgN₂Om^-2h^-1 for DB and NJ landfills, respectively) and made a negligible contribution to the total greenhouse gas emissions in term of CO₂ equivalent. However, significant N₂O emissions were observed in the leachate treatment systems of sanitary landfills and contributed 72.8% and 45.6% of total emissions in term of CO₂ equivalent in DB and DF landfills, respectively. The N₂O emission factor (EF) of the leachate treatment systems was in the range of 8.9-11.9% of the removed nitrogen. The total N₂O emissions from the leachate treatment systems of landfills in Xiamen city were estimated to be as high as 8.55gN₂O-Ncapita^-1yr^-1. These results indicated that N₂O emissions from leachate treatment systems of sanitary landfills were not negligible and should be included in national and/or local inventories of greenhouse gas emissions.

Contributions of nitrification and denitrification to N2O emissions from aged refuse bioreactor at different feeding loads of ammonia substrates

Nitrous oxide (N₂O) is a strong greenhouse gas, and its emissions from microbial nitrification (NF) and denitrification (DNF) are a threat to the environment. In the present study, a combined approach consisting of ¹⁵N stable isotope and molecular biology (qPCR) was used to determine the contributions of autotrophic nitrification (ANF), heterotrophic nitrification (HNF), and DNF to N₂O emissions in laboratory incubations of aged refuse for different ammonia (NH₄⁺-N) loads (200, 400, and 800mg·NH₄⁺-N/kg·aged refuse) and incubation times (2-144h). Experimental results showed that the N₂O emissions increased with the increase in applied amount of NH₄⁺-N substrates. Simultaneous nitrification and denitrification (SND) were demonstrated to be present in the incubations of aged refuse. The results of ¹⁵N stable isotope labelling experiment indicated that NF (54.60%-68.8%) and DNF (83.38%-85.90%) contributed to majority of N₂O emissions in the incubations of 24h and 72h, respectively. The results of functional genes (amoA and nosZ) quantification experiments indicated that the high gene copies of amoA and nosZ were present at 24h and 72h, respectively. The study also demonstrated the utility of a combined stable isotope and molecular biology approach. The approaches not only provide similar inferences about the N₂O emissions, but also enable the determination of relative contributions of ANF, HNF, and DNF to N₂O emissions. The results of the study are important in providing guidance to artificially optimize the operating conditions for alleviating N₂O emissions in aged refuse bioreactors.

Quantifying N2O emissions and production pathways from fresh waste during the initial stage of disposal to a landfill

Intensive nitrous oxide (N₂O) emissions usually occur at the working face of landfills. However, the specific amounts and contributions of the multiple pathways to N₂O emissions are poorly understood. N₂O emissions and the mutual conversions of N-species in both open and sealed simulated landfill reactors filled with fresh refuse were examined during a 100-h incubation period, and N₂O sources were calculated using ¹⁵N isotope labelling. N₂O peak fluxes were above 70μgNkg^-1 waste h^-1 for both treatments. The sealed incubation reactors became a N₂O sink when N₂O in the ambient environment was sufficient. The total amount of N₂O emissions under sealed conditions was 2.15±0.56mgNkg^-1 waste, which was higher than that under open conditions (1.91±0.34mgNkg^-1 waste). The NO₂^- peak appeared prior to the peak in N₂O flux. The degree and duration of total nitrogen reduction in open incubations were larger and longer than those of sealed incubations and could possibly be due to oxygen supplementation. Denitrification (DF) was a major source of N₂O generation during these incubations. The contribution of the DF pathway decreased from 89.2% to 61.3% during the open incubations. The effects of nitrification (NF) and nitrification-coupled denitrification (NCD) increased during the increasing phase and the decreasing phase of N₂O flux, contributing 24.1-37.4% and 31.7-34.4% of total N₂O emissions, respectively. In sealed treatments, the DF pathway accounted for more than 90% of the total N₂O emission during the entire incubation.

A comparison of CH4, N2O and CO2 emissions from three different cover types in a municipal solid waste landfill

High-density polyethylene (HDPE) membranes are commonly used as a cover component in sanitary landfills, although only limited evaluations of its effect on greenhouse gas (GHG) emissions have been completed. In this study, field GHG emission were investigated at the Dongbu landfill, using three different cover systems: HDPE covering; no covering, on the working face; and a novel material-Oreezyme Waste Cover (OWC) material as a trial material. Results showed that the HDPE membrane achieved a high CH₄ retention, 99.8% (CH₄ mean flux of 12 mg C m^-2 h^-1) compared with the air-permeable OWC surface (CH4 mean flux of 5933 mg C m^-2 h^-1) of the same landfill age. Fresh waste at the working face emitted a large fraction of N₂O, with average fluxes of 10 mg N m^-2 h^-2, while N₂O emissions were small at both the HDPE and the OWC sections. At the OWC section, CH₄ emissions were elevated under high air temperatures but decreased as landfill age increased. N₂O emissions from the working face had a significant negative correlation with air temperature, with peak values in winter. A massive presence of CO₂ was observed at both the working face and the OWC sections. Most importantly, the annual GHG emissions were 4.9 Gg yr^-1 in CO₂ equivalents for the landfill site, of which the OWC-covered section contributed the most CH₄ (41.9%), while the working face contributed the most N₂O (97.2%). HDPE membrane is therefore, a recommended cover material for GHG control.

IMPLICATIONS

Monitoring of GHG emissions at three different cover types in a municipal solid waste landfill during a 1-year period showed that the working face was a hotspot of N₂O, which should draw attention. High CH₄ fluxes occurred on the permeable surface covering a 1- to 2-year-old landfill. In contrast, the high-density polyethylene (HDPE) membrane achieved high CH₄ retention, and therefore is a recommended cover material for GHG control.

Effect of hydraulic retention time and sludge recirculation on greenhouse gas emission and related microbial communities in two-stage membrane bioreactor treating solid waste leachate

Methane (CH4) and nitrous oxide (N2O) emissions and responsible microorganisms during the treatment of municipal solid waste leachate in two-stage membrane bioreactor (MBR) was investigated. The MBR system, consisting of anaerobic and aerobic stages, were operated at hydraulic retention time (HRT) of 5 and 2.5days in each reactor under the presence and absence of sludge recirculation. Organic and nitrogen removals were more than 80% under all operating conditions during which CH4 emission were found highest under no sludge recirculation condition at HRT of 5days. An increase in hydraulic loading resulted in a reduction in CH4 emission from anaerobic reactor but an increase from the aerobic reactor. N2O emission rates were found relatively constant from anaerobic and aerobic reactors under different operating conditions. Diversity of CH4 and N2O producing microorganisms were found decreasing when hydraulic loading rate to the reactors was increased.

Temperature-induced changes in treatment efficiency and microbial structure of aerobic granules treating landfill leachate

This paper investigates the effect of temperature on nitrogen and carbon removal by aerobic granules from landfill leachate with a high ammonium concentration and low concentration of biodegradable organics. The study was conducted in three stages; firstly the operating temperature of the batch reactor with aerobic granules was maintained at 29 °C, then at 25 °C, and finally at 20 °C. It was found that a gradual decrease in operational temperature allowed the nitrogen-converting community in the granules to acclimate, ensuring efficient nitrification even at ambient temperature (20 °C). Ammonium was fully removed from leachate regardless of the temperature, but higher operational temperatures resulted in higher ammonium removal rates [up to 44.2 mg/(L h) at 29 °C]. Lowering the operational temperature from 29 to 20 °C decreased nitrite accumulation in the GSBR cycle. The highest efficiency of total nitrogen removal was achieved at 25 °C (36.8 ± 10.9 %). The COD removal efficiency did not exceed 50 %. Granules constituted 77, 80 and 83 % of the biomass at 29, 25 and 20 °C, respectively. Ammonium was oxidized by both aerobic and anaerobic ammonium-oxidizing bacteria. Accumulibacter sp., Thauera sp., cultured Tetrasphaera PAO and Azoarcus-Thauera cluster occurred in granules independent of the temperature. Lower temperatures favored the occurrence of denitrifiers of Zooglea lineage (not Z. resiniphila), bacteria related to Comamonadaceae, Curvibacter sp., Azoarcus cluster, Rhodobacter sp., Roseobacter sp. and Acidovorax spp. At lower temperatures, the increased abundance of denitrifiers compensated for the lowered enzymatic activity of the biomass and ensured that nitrogen removal at 20 °C was similar to that at 25 °C and significantly higher than removal at 29 °C.

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.