Spatial variability of nitrous oxide and methane emissions from an MBT landfill in operation: strong N2O hotspots at the working face

Direct Methane Removal from Air by Aerobic Methanotrophs

The rapid pace of climate change has created great urgency for short-term mitigation strategies. Appropriately, the long-term target for intervening in global warming is CO2, but experts suggest that methane should be a key short-term target. Methane has a warming impact 34 times greater than CO2 on a 100-year timescale, and 86 times greater on a 20-year timescale, and its short half-life in the atmosphere provides the opportunity for near-term positive climate impacts. One approach to removing methane is the use of bacteria for which methane is their sole carbon and energy source (methanotrophs). Such bacteria convert methane to CO2 and biomass, a potentially value-added product and co-benefit. If air above emissions sites with elevated methane is targeted, technology harnessing the aerobic methanotrophs has the potential to become economically viable and environmentally sound. This article discusses challenges and opportunities for using aerobic methanotrophs for methane removal from air, including the avoidance of increased N2O emissions.

Gas transport in landfill cover system: A critical appraisal

Landfill gas (LFG) emission is gaining more attention from the scientific fraternity and policymakers recently due to its threat to the atmosphere and human health of the populace living in surrounding premises. Though landfill cover (LFC) (viz., daily, intermittent and final cover) is widely used by landfill operators to mitigate or reduce these emissions, their overall performance is still under question. A critical analysis of available literature, primarily pertaining to (i) the composition of the landfill gases and their migration in the LFC system, (ii) experimental and mathematical investigations of the transport mechanism of gas and (iii) the impact of additives to cover soils on transport and fate of gas, has been conducted and presented in this manuscript. Investigation of the efficiency of modified soil was mainly focused on laboratory test. More field tests and application of amended cover soils should be conducted and promoted further. Studies on nitrous oxide and emerging pollutants, including poly-fluoroalkyl substances transport in landfill cover system are limited and need further research. The transport mechanisms of these unconventional contaminants should be considered regarding the selection of LFC materials including geomembrane and geosynthetic clay liners. The existing analytical and numerical models can provide a basic understanding of LFG transport mechanisms and are able to predict the migration behaviour of LFG; however, there are still knowledge gaps concerning the interaction between different species of the gas molecule when modeling multi-component gas transport. Gas transport through fractured cover should also be considered when evaluating LFG emission in the future. Simplified design method for landfill cover system regarding LFG emission based on analytical models should be proposed. Overall, mathematical models combined with experiments can facilitate more visualized and intensive insights, which would be instrumental in devising climate adaptive landfill covers.

Recycling agricultural, municipal and industrial pollutant wastes into fertilizers for a sustainable healthy food production

This work was focused on recycling different typology of pollutant wastes (olive pomace and orange residues; municipal wastes and sulphur residue of hydrocarbon refining processes) with the triple objectives of limiting wastes in landfill, reducing greenhouse gas emission and producing organic-mineral fertilizers. The environmental risks and benefits of the whole process have been considered. The specific objectives were: 1) innovation in waste management techniques by reducing the accumulation of different typology of wastes using a unique process 2) verifying efficiency of the obtained organic-mineral fertilizers on soil and plant growth 3) improving soil and crop quality relating wastes to food, economy and environment. Sulphur-based pads improved soil quality mostly when contained orange residues. Onion and Garlic grew better in presence of sulphur-based pads (+20%), and mostly when pads contained orange residues (+45%). Onion and Garlic quality, in terms of antioxidant compounds and antioxidant capacity, increased in presence of sulphur-based pads (+30%) mostly when orange residues were present in the pads (+90%). In short, in addition to the environmental advantages, numerous economic benefits coming from the decrease in the production and use of chemical fertilizers, the reduction of costs for landfilling and the gain rising from the sale of the new fertilizers produced, emerged.

Emission of volatile organic compounds during aerobic decomposition of banana peel

Emissions of volatile organic compounds (VOCs) were continuously measured during the aerobic decomposition of banana peel in a laboratory-scale landfill simulator over 25 d. Using direct membrane inlet single-photon ionisation time-of-flight mass spectrometry (MI-SPI-ToF-MS), 18 VOCs belonging to 10 functional groups were detected in the air samples, and their VOC emission profiles were established using cluster analysis on time-resolved data. Three emission stages were clearly identified, with the major release for most VOC compounds occurring during the first 14 d. The emission patterns of the individual compounds were quite similar despite the different release mechanisms. In addition, no apparent increase in temperature was observed inside the simulator during the entire experimental period. We suggest that the volatilisation of the constituents in the waste pile contributed equally to VOC emissions as did the degradation of banana peel via microbial activity. The average emission rate of total VOCs reached 44.3 × 10^-3 mg VOC kg^-1 of dry banana peel, with more than half belonging to malodourous substances. The malodourous emissions of the decaying banana peel in an aerobic environment mainly originated from styrene, dimethyl sulphide, and diethyl sulphide, the most common contributors to offensive odourants during food waste biodegradation.

Methane and nitrous oxide emissions from shallow windrow piles for biostabilisation of municipal solid waste

Shallow windrow piles were applied as a low-cost option for biostabilisation of municipal solid wastes (MSW) prior to their utilization as refuse-derived fuel (RDF). A considerable amount of greenhouse gas (GHG) emissions can be emitted during the biostabilisation of MSW, especially when in operation under high moisture conditions such as there are in tropical Asia. This study investigated the emission of methane (CH₄) and nitrous oxide (N₂O) from shallow windrow piles - with heights of 0.5-1.0 m - for the stabilization of MSW at a full-scale facility in Thailand. Measurements of CH₄, CO₂, and N₂O emissions using the static-chamber method revealed high spatial heterogeneity characteristics in all zones with different waste ages. Peak methane emissions were observed after four months of biostabilisation. The average spatial methane emissions from the waste piles ranged from 7.33 to 26.88 g m^-2 d^-1 (14.86 g m^-2 d^-1, on average). The CH₄ generation-rate constant was within the range of 3.3 to 4.0 yr^-1, which is higher than that reported - about 2.20-3.50 yr^-1 - from a deep windrow pile (3.5-4.0 m height). The spatial distribution of N₂O emissions was in the range of 4.51-199.14 mg N₂O t^-1dry wt.d^-1 (6.6-111.7 mg N₂O m^-2 d^-1), similar to those previously studied from landfill operations. This shallow windrow pile technique can be applied as low-cost technology for biostabilisation of MSW in developing countries, where land area is available.Implications: Shallow windrow pile was applied as a low-cost option for biological treatment of municipal solid waste in developing countries where land area is available. This study evaluated the greenhouse gas emission characteristics during the operation of windrow pile. The findings suggest that the emission rates were varied spatially with waste ages in different zones. Higher methane generation rate constant was derived from shallow window pile as compared to deep windrow pile. The methane and nitrous oxide emission factors were derived.

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.

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.

Mathematical Modeling of the Biogas Production in MSW Landfills. Impact of the Implementation of Organic Matter and Food Waste Selective Collection Systems

Municipal solid waste (MSW) landfills are one of the main sources of greenhouse gas emissions. Biogas is formed under anaerobic conditions by decomposition of the organic matter present in waste. The estimation of biogas production, which depends fundamentally on the type of waste deposited in the landfill, is essential when designing the gas capture system and the possible generation of energy. BIOLEACH, a mathematical model for the real-time management of MSW landfills, enables the estimation of biogas generation based on the waste mix characteristics and the local meteorological conditions. This work studies the impact of installing selective organic matter collection systems on landfill biogas production. These systems reduce the content of food waste that will eventually be deposited in the landfill. Results obtained using BIOLEACH on a set of scenarios under real climate conditions in a real landfill located in the Region of Murcia (Spain) are shown. Results demonstrate that actual CH4 and CO2 production depends fundamentally on the monthly amount of waste stored in the landfill, its chemical composition and the availability and distribution of water inside the landfill mass.

Screening methane-oxidizing bacteria from municipal solid waste landfills and simulating their effects on methane and ammonia reduction

Municipal solid waste landfills are not only a crucial source of global greenhouse gas emissions; they also produce large amounts of ammonia (NH₃), hydrogen sulfide, and other odorous gases that negatively affect the regional environment. Several types of methane-oxidizing bacteria (MOB) were proved to be effective in mitigating methane emission from landfills. Nevertheless, more MOB species and their technical parameters for best mitigating methane still need to be explored. In landfills, methane is simultaneously generated with ammonia, which may impede the CH₄ bio-oxidizing process of MOB. However, very limited studies examined the enhancement of methane reduction by introducing ammonia-oxidizing bacteria (AOB) in landfills. In this study, two enriched MOB cultures were gained from a typical municipal solid waste landfill, and then were cultured with three strains of ammonia-oxidizing bacteria (AOB). The MOB enrichment culture used in this work includes Methylocaldum, Methylocystaceae, and Methyloversatilis, with a methane oxidation capacity of 43.6-65.0%, and the AOB includes Candida ethanolica, Bacillus cereus, and Alcaligenes faecalis. The effects on the emission reduction of both NH₃ and CH₄ were measured using self-made landfill-simulating equipment, as MOB, AOB, and a MOB-AOB mixture were added to the soil cover of the simulation equipment. The concentrations of CH₄ and NH₃ in the MOB-AOB mixture group decreased sharply, and the CH₄ and NH₃ concentration was 76.4% and 83.7% of the control group level. We also found that addition of AOB can help MOB oxidize CH₄ and improve the emission reduction effect.

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.

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.

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.

Assessment of biogas production from MBT waste under different operating conditions

In this work, the influence of different operating conditions on the biogas production from mechanically-biologically treated (MBT) wastes is investigated. Specifically, different lab-scale anaerobic tests varying the water content (26-43% w/w up to 75% w/w), the temperature (from 20 to 25°C up to 55°C) and the amount of inoculum have been performed on waste samples collected from a full-scale Italian MBT plant. For each test, the gas generation yield and, where applicable, the first-order gas generation rates were determined. Nearly all tests were characterised by a quite long lag-phase. This result was mainly ascribed to the inhibition effects resulting from the high concentrations of volatile fatty acids (VFAs) and ammonia detected in the different stages of the experiments. Furthermore, water content was found as one of the key factor limiting the anaerobic biological process. Indeed, the experimental results showed that when the moisture was lower than 32% w/w, the methanogenic microbial activity was completely inhibited. For the higher water content tested (75% w/w), high values of accumulated gas volume (up to 150Nl/kgTS) and a relatively short time period to deplete the MBT waste gas generation capacity were observed. At these test conditions, the effect of temperature became evident, leading to gas generation rates of 0.007d(-1) at room temperature that increased to 0.03-0.05d(-1) at 37°C and to 0.04-0.11d(-1) at 55°C. Overall, the obtained results highlighted that the operative conditions can drastically affect the gas production from MBT wastes. This suggests that particular caution should be paid when using the results of lab-scale tests for the evaluation of long-term behaviour expected in the field where the boundary conditions change continuously and vary significantly depending on the climate, the landfill operative management strategies in place (e.g. leachate recirculation, waste disposal methods), the hydraulic characteristics of disposed waste, the presence and type of temporary and final cover systems.