5th ICLRS, Copper Mountain Colorado Session report on
LANDFILL METHANE EMISSIONS AND OXIDATION III Field Scale Investigation and Quantification of Methane Oxidation
M. Huber-Humer, P. Kjeldsen
The purpose of this session was to consider field scale studies and current approaches for the evaluation of the oxidation efficiency in landfill covers and in engineered bio-based oxidation systems, like biocovers, biowindows, and biofilters. Additionally, this session addressed the impact of environmental factors on the oxidation performance in the field, particularly how these factors can be controlled and their impact on the oxidation performance quantified. After a short introduction by the session chairs and the presentation of ongoing German (poster presentation by Sonja Bohn, presentation by Jan Streese-Kleeberg) and Danish case studies (poster presentation by Gitte Pedersen, presentation by Peter Kjeldsen), a first round of questions and short discussion took place. Afterwards, the second presentation block concerning Australian (presentation by Stuart Dever), Finish (presentation by Matti Ettala) and Canadian (presentation by Alex Cabral) field studies proceeded. Finally, small groups were formed and discussions ensued on the following questions: GROUP 1: What are the main impact factors on methane oxidation performance and efficiency in landfill covers and in engineered systems? How big is their influence under field conditions? GROUP 2: Which factors must be considered to optimize designs of oxidation systems? Impact of vegetation? How can these factors be controlled and evaluated, and their impact on the oxidation performance quantified? GROUP 3: How far are we in providing reliable (standard) approaches for measuring/ quantifying methane oxidation in the field? What are the limitations? How far do we need to develop such approaches with respect to getting reliable data for the design, operation and quality check of bio-based systems and to provide basics for scaling–up and inventory methodologies?
Following conclusions and statements from the group discussions were presented and can be summarized:
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GROUP 1 (group leader = Stuart Dever): Question: What are the main factors that affect methane oxidation performance in the field in engineered systems, and what is the significance of each factor? Main factors: Landfill gas loading, and its fluctuation, and its impact on oxygen supply within a biocover / biofilter. More significant in biofilters due to higher loading rates. Media characteristics also affects oxygen supply. Temperature, which is a function of landfill gas temperature, microbial activity, climate (temperature and rainfall) and biocover / biofilter profile. Probably more significant for biocovers – due to lower gas load and less effect of landfill gas temperature and greater effect of climatic conditions. Moisture content, which is a function of climate, media characteristics (porosity, water holding capacity, drainage), and biocover / biofilter design (stormwater management, subsurface drainage). Significance is dependent on climate and its affect on the moisture profile of the biocover / biofilter. Media characteristics, which affect the impact of the above (affect the movement of gas and water within the biofilter media), including (air filled porosity, gas conductivity, water holding capacity, drainage).
Note, all the above are inter-related! Design factors that may be used to control / manage above include: Flow controls for landfill gas flow / drainage system (for passive drainage and biofiltration systems). Note, load on biocovers cannot be controlled at sites where there is no active collection of landfill gas. Biofilter location – manage effect of climate on temperature and moisture content of biofilter media Biocover / biofilter profile, including media thickness and gas distribution / drainage layer Biofilter media characteristics (air filled porosity, gas conductivity, water holding capacity, drainage) Location of gas drainage pipes (for biofilters) – effect of climate Stormwater management measures, including drainage + covers / roof Irrigation
Note, temperature of the biocover / biofilter media cannot practically be controlled – system must be designed to suit likely temperatures that will occur in the media, and this has implications on the possible methane oxidation rates. Design of a biocover / biofilter must be designed to suit site conditions including landfill gas generation / fluctuation, climate, physical characteristics of the landfill (topography, landfill capping profile, final landform land use).
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Stuart Dever has summarized the interrelation of the different factors in the following flow chart:
GROUP 2 (group leader = Alex Cabral): Which factors must be considered to optimize designs of oxidation systems? Impact of vegetation? How can these factors be controlled and evaluated, and their impact on the oxidation performance quantified? Summarized statements from this group:
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Main important factors in the field that must be considered to optimize oxidation systems are the following. Spatial distribution of gas along the slope length of the gas oxidation layer o E.g.; Water saturation at the toe of the slope will lead to non-uniform distribution of gas along the base of the capillary barrier. - Don’t count on the gas oxidation at the toes! Thus: Improve the gas flow properties of the materials at the toe of the slopes o Try to direct gas toward biowindows – make these flatter so that gas flow is homogenous. o Remove water at toes using vegetation. Material selection o Optimize aeration o Consider penetration of roots o Consider Cracking o Check capillary barrier characteristics o Characteristics of using contaminated materials or un-recyclable material (e.g., plastics); How to develop material specifications, e.g. for Tires, waste brick, calcium carbonate, demolition materials (avoid calcium carbonate heavy materials like waste concrete, high pH leaching materials,…) o K-regulation for landfill covers; e.g. 10-5 cm/s – coarse as possible o Regional suitability of biocovers E.g., Finland – 50% average oxidation throughout the year Selection of vegetation (focusing on root depth and CO2 absorption) o Deep roots may cause preferential channels, especially when the roots die o Methane can go through the root channel itself o Selection of vegetation/roots is important, but depends on the water balance needs of the cover system design o Oxidation occurs near the surface/ oxygen competition with roots? o EPS formation was not observed when vegetation was used o Observation in the field: spontaneous vegetation in the field, no CO2 emissions => plants absorb CO2 applied by the landfill? => good growth conditions for plants (artificial “CO2-fertilization” on landfills like in greenhouses) o Day/night CO2 flow – day: plants absorb CO2 and produce O2; converse during night o Observation: more roots => more methanotrophs Factors that can be controlled in engineering design o Compaction vs. structure changes with time (consolidation) o E.g., decomposition in sewage sludge compost mix, led to a change in pore size distribution and grain size distribution
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Design for differential settlement o Don’t use materials with high cohesion (that cannot accommodate movement) o Avoid shear/desiccation-related cracks effects on gas flow o Can gas-distribution layers cope with sufficient gas distribution after differential settlements? (depends on dimension) Dimensions of covers o Oxygen penetration cannot go forever o Canada - Adapt designs to be consistent with existing regulations (achieve new goals with the same geometry) o Need top soil porous enough to permit ingress of oxygen while not letting too much water in o Below this, use a coarser layer (thicker gas distribution layer or courser material near the bottom of the oxidation layer) that permits good oxygen distribution o Deeper layer may be better for temperature control o 10 cm of landfill space can result in 1.5 million dollars – be aware of economic efficiency Control/evaluate impact of factors on oxidation performance – need research o Engineers need to figure out compatibility between water flow and gas flow by selecting proper materials, compaction conditions, and geometry o Need to study the optimal vegetation for root penetration, moisture removal, methanotrophic activity o Strategically plan study about materials – use a similar protocol – i.e., same materials but different climates
GROUP 3 (group leader = Julia Gebert): How far are we in providing reliable (standard) approaches for measuring/ quantifying methane oxidation in the field? What are the limitations? How far do we need to develop such approaches with respect to getting reliable data for the design, operation and quality check of bio-based systems and to provide basics for scaling–up and inventory methodologies? Participants from this group listed following techniques and approaches: Methods for process studies Stable Isotopes: issues: αox, αtrans, differential flowpaths oxidation Gas profiles: could be combined with the carbon balance method, but: respiration and limited possibility of extrapolation to the site Gas push-pull test: potential Carbon balances: respiration (determine CO2 flux by shutting of gas supply in test cells or by incubating field samples in the lab)
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We need the flux to the cover: tracer, subsurface chamber (only in settings with predominantly diffusive flux; sphere of influence during flushing is uncertain)
Methods for quantifying whole site oxidation Plume + stable isotopes o Stable weather condition, all seasons o Precipitation can pose problems, aim for a situation where the soil is well-drained or else o Go for high temporal resolution measurements (e.g. Eddy covariance) o Problem: differential flow paths oxidation, thus the bigger the scale, the least oxidized flow path becomes overrepresented (e. g. gas wells, cracks) Use of ratio CO2:CH4 in micromet measurements o Problem: are CO2 fluxes influenced by other processes? Vegetation influences: CO2 fluxes due to respiration and photosynthesis
Strategic approach: measure whole site emissions before and after implementation of mitigation measures (establish baseline)
What to do - future research needs: Combine Stable Isotope technique with Gas-Push-Pull-Test Investigate the influence of temperature, vmax, moisture on the fractionation factor in column studies Use radon to assess flux from the landfill and use another non-reactive gas that diffuses into the soil
The session was closed after the three presentations by the group leaders and short specific discussion of their conclusions and statements.
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