Jean E. Bogner

Microbial methane oxidation processes and technologies for mitigation of landfill gas emissions

Landfill gas containing methane is produced by anaerobic degradation of organic waste. Methane is a strong greenhouse gas and landfills are one of the major anthropogenic sources of atmospheric methane. Landfill methane may be oxidized by methanotrophic microorganisms in soils or waste materials utilizing oxygen that diffuses into the cover layer from the atmosphere. The methane oxidation process, which is governed by several environmental factors, can be exploited in engineered systems developed for methane emission mitigation. Mathematical models that account for methane oxidation can be used to predict methane emissions from landfills. Additional research and technology development is needed before methane mitigation technologies utilizing microbial methane oxidation processes can become commercially viable and widely deployed.

Mitigation of global greenhouse gas emissions from waste: conclusions and strategies from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. Working Group III (Mitigation):

Greenhouse gas (GHG) emissions from post-consumer waste and wastewater are a small contributor (about 3%) to total global anthropogenic GHG emissions. Emissions for 2004-2005 totalled 1.4 Gt CO2-eq year—1 relative to total emissions from all sectors of 49 Gt CO2-eq year— 1 [including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and F-gases normalized according to their 100-year global warming potentials (GWP)]. The CH4 from landfills and wastewater collectively accounted for about 90% of waste sector emissions, or about 18% of global anthropogenic methane emissions (which were about 14% of the global total in 2004). Wastewater N2O and CO2 from the incineration of waste containing fossil carbon (plastics; synthetic textiles) are minor sources. Due to the wide range of mature technologies that can mitigate GHG emissions from waste and provide public health, environmental protection, and sustainable development co-benefits, existing waste management practices can provide effective mitigation of GHG emissions from this sector. Current mitigation technologies include landfill gas recovery, improved landfill practices, and engineered wastewater management. In addition, significant GHG generation is avoided through controlled composting, state-of-the-art incineration, and expanded sanitation coverage. Reduced waste generation and the exploitation of energy from waste (landfill gas, incineration, anaerobic digester biogas) produce an indirect reduction of GHG emissions through the conservation of raw materials, improved energy and resource efficiency, and fossil fuel avoidance. Flexible strategies and financial incentives can expand waste management options to achieve GHG mitigation goals; local technology decisions are influenced by a variety of factors such as waste quantity and characteristics, cost and financing issues, infrastructure requirements including available land area, collection and transport considerations, and regulatory constraints. Existing studies on mitigation potentials and costs for the waste sector tend to focus on landfill CH4 as the baseline. The commercial recovery of landfill CH4 as a source of renewable energy has been practised at full scale since 1975 and currently exceeds 105 Mt CO2 -eq year—1. Although landfill CH 4 emissions from developed countries have been largely stabilized, emissions from developing countries are increasing as more controlled (anaerobic) landfilling practices are implemented; these emissions could be reduced by accelerating the introduction of engineered gas recovery, increasing rates of waste minimization and recycling, and implementing alternative waste management strategies provided they are affordable, effective, and sustainable. Aided by Kyoto mechanisms such as the Clean Development Mechanism (CDM) and Joint Implementation (JI), the total global economic mitigation potential for reducing waste sector emissions in 2030 is estimated to be > 1000 Mt CO2-eq (or 70% of estimated emissions) at costs below 100 US

LANDFILLS AS ATMOSPHERIC METHANE SOURCES AND SINKS

t— 1 CO2-eq year—1. An estimated 20—30% of projected emissions for 2030 can be reduced at negative cost and 30—50% at costs < 20 US

Landfill CH4: Rates, fates, and role in global carbon cycle

t—1 CO 2-eq year—1. As landfills produce CH 4 for several decades, incineration and composting are complementary mitigation measures to landfill gas recovery in the short- to medium-term — at the present time, there are > 130 Mt waste year— 1 incinerated at more than 600 plants. Current uncertainties with respect to emissions and mitigation potentials could be reduced by more consistent national definitions, coordinated international data collection, standardized data analysis, field validation of models, and consistent application of life-cycle assessment tools inclusive of fossil fuel offsets.

Limits and dynamics of methane oxidation in landfill cover soils.

Abstract Sanitary landfills are recognized as globally significant sources of atmospheric methane, but field measurements are rare. Existing country-specific landfill emissions have been estimated from solid waste statistics and a series of assumptions regarding methane generation and emission rates. There has been no attempt to reconcile the national and global estimates with limited field data on landfill methane emissions which range over six orders of magnitude (Bogner and Scott, 1995). This paper addresses controlled field measurements of methane emissions at sites in Illinois and California (USA) using a closed chamber technique. Overall, observed rates from various controlled monitoring experiments during 1988–1994 ranged from 0.003 to more than 1000 g CH 4 m −2 d −1 . Rates were related to the presence or absence of gas recovery wells, physical properties of cover soils (texture, moisture, and temperature) relating to their aeration status for diffusional flux, and rates of methane oxidation by indigenous methanotrophs. Surprisingly, at the Illinois site during spring, 1994, the landfill surface was consuming atmospheric methane rather than emitting landfill methane. This was attributed to high capacities for methane oxidation in well-aerated soils which had reduced landfill methane compared to 1993, the result of an effective pumped gas recovery system. Three independent methods confirmed that the landfill cover soils were functioning as a methane sink: (a) static closed chamber measurements yielding negative flux rates (uptake of atmospheric methane); (b) rates of methane oxidation similar to chamber results from in vitro field incubation studies using ambient methane; and (c) a reversal in the soil gas methane concentration gradient at the 25 cm depth. Field verification of landfill cover soils functioning as methane sinks has profound implications for revision of landfill contributions to global methane budgets; furthermore, it should be feasible to develop mitigation strategies incorporating a combination of engineered and natural methanotrophic controls.

Compressibility and shear strength of municipal solid waste under short-term leachate recirculation operations

Published estimates for worldwide landfill methane emissions range from 9 to 70 Tg yr−1. Field and laboratory studies suggest that maximum methane yields from landfilled refuse are about 0.06 to 0.09 m3 (dry kg)−1 refuse, depending on moisture content and other variables, such as organic loading, buffering capacity, and nutrients in landfill microenvironments. Methane yields may vary by more than an order of magnitude within a given site. Fates for landfill methane include (1) direct or delayed emission to the atmosphere through landfill cover materials or surface soils; (2) oxidation by methanotrophs in cover soils, with resulting emission of carbon dioxide; or (3) recovery of methane followed by combustion to produce carbon dioxide. The percent methane assigned to each pathway will vary among field sites and, for individual sites, through time. Nevertheless, a general framework for a landfill methane balance can be developed by consideration of landfill age, engineering and management practices, cover soil characteristics, and water balance. Direct measurements of landfill methane emissions are sparse, with rates between 10−6 and 10−8 g cm−2 s−1; very high rates of 400 kg m−2 yr−1 have been measured at a semiarid unvegetated site. The proportion of landfill carbon that is ultimately converted to methane and carbon dioxide is problematical; the literature suggests that, at best, 25% to 40% of refuse carbon can be converted to biogas carbon. Cellulose contributes the major portion of the methane potential. Routine excavation of nondecomposed cellulosic materials after one or two decades of landfill burial suggests that uniformly high conversion rates are rarely attained at field sites. For a longer-term viewpoint, considering archaeologic and geologic preservation of organic carbon through anaerobic burial, one can speculate that widespread landfilling practices in developed and developing countries may be providing a measurable sink for organic carbon, as well as increasing the atmospheric methane burden.

Hydraulic conductivity of MSW in landfills

In order to understand the limits and dynamics of methane (CH(4)) oxidation in landfill cover soils, we investigated CH(4) oxidation in daily, intermediate, and final cover soils from two California landfills as a function of temperature, soil moisture and CO(2) concentration. The results indicate a significant difference between the observed soil CH(4) oxidation at field sampled conditions compared to optimum conditions achieved through pre-incubation (60 days) in the presence of CH(4) (50 ml l(-1)) and soil moisture optimization. This pre-incubation period normalized CH(4) oxidation rates to within the same order of magnitude (112-644 μg CH(4) g(-1) day(-1)) for all the cover soils samples examined, as opposed to the four orders of magnitude variation in the soil CH(4) oxidation rates without this pre-incubation (0.9-277 μg CH(4) g(-1) day(-1)). Using pre-incubated soils, a minimum soil moisture potential threshold for CH(4) oxidation activity was estimated at 1500 kPa, which is the soil wilting point. From the laboratory incubations, 50% of the oxidation capacity was inhibited at soil moisture potential drier than 700 kPa and optimum oxidation activity was typical observed at 50 kPa, which is just slightly drier than field capacity (33 kPa). At the extreme temperatures for CH(4) oxidation activity, this minimum moisture potential threshold decreased (300 kPa for temperatures <5°C and 50 kPa for temperatures >40°C), indicating the requirement for more easily available soil water. However, oxidation rates at these extreme temperatures were less than 10% of the rate observed at more optimum temperatures (∼ 30°C). For temperatures from 5 to 40°C, the rate of CH(4) oxidation was not limited by moisture potentials between 0 (saturated) and 50 kPa. The use of soil moisture potential normalizes soil variability (e.g. soil texture and organic matter content) with respect to the effect of soil moisture on methanotroph activity. The results of this study indicate that the wilting point is the lower moisture threshold for CH(4) oxidation activity and optimum moisture potential is close to field capacity. No inhibitory effects of elevated CO(2) soil gas concentrations were observed on CH(4) oxidation rates. However, significant differences were observed for diurnal temperature fluctuations compared to thermally equivalent daily isothermal incubations.

Geotechnical properties of municipal solid waste at different phases of biodegradation

This paper describes a comprehensive laboratory study performed to investigate the compressibility and shear strength properties of 1.5-year-old municipal solid waste (MSW) exhumed from a landfill cell where low amounts of leachate were recirculated. The study results are compared with results from a previous study on fresh MSW collected from the same landfill and data from previous studies with known MSW age to assess the variation in properties due to degradation. Laboratory testing was conducted on shredded landfilled and fresh MSW that consisted of similar particle-size distribution, with maximum particle size less than 40 mm and approximately 80% of the waste consisting of particles ranging from 10 to 20 mm. Standard Proctor, compressibility, direct shear, and triaxial consolidated undrained (CU) shear tests were conducted in general accordance with the American Society of Testing and Materials Standard Procedures. These tests were conducted with samples at an in-situ moisture content of 44% (dry weight basis) as well as elevated moisture contents of 60, 80 and 100% (dry weight basis). Standard Proctor compaction tests yielded a maximum dry density of 600 kg/m3 at 77% optimum moisture content for landfilled MSW compared to the 420 kg/m3 maximum dry density at 70% optimum moisture content for fresh MSW. Compression ratio values for landfilled MSW varied in a close range of 0.19—0.24 with a slight increasing trend with increase in moisture content; however, for fresh waste they were in the close range of 0.24—0.33 with no definitive correlation with moisture content. Based on direct shear tests, drained cohesion and friction angle were varied in the range of 12—64 kPa and 31—35° for landfilled MSW and 31—64 kPa and 26—30° for fresh MSW. Neither cohesion nor friction angle demonstrated any correlation with the moisture content. Based on triaxial CU tests, the average total strength parameters (TSP) were found to be 39 kPa and 12° for landfilled MSW and 32 kPa and 12° for fresh MSW, while effective strength parameters (ESP) were 34 kPa and 23° for landfilled MSW and 32 kPa and 16° for fresh MSW. This study was limited to small-scale laboratory testing using MSW samples with the specimen size relative to the maximum particle size in the range of 1.6 to 2.6; therefore, large-scale laboratory and field studies are recommended to systematically assess the influence of composition, particle size distribution and specimen size on the geotechnical properties of MSW.

Seasonal greenhouse gas emissions (methane, carbon dioxide, nitrous oxide) from engineered landfills: daily, intermediate, and final California cover soils.

This paper presents a laboratory investigation of hydraulic conductivity of municipal solid waste (MSW) in landfills and provides a comparative assessment of measured hydraulic conductivity values with those reported in the literature based on laboratory and field studies. A series of laboratory tests was conducted using shredded fresh and landfilled MSW from the Orchard Hills landfill (Illinois, United States) using two different small-scale and large-scale rigid-wall permeameters and a small-scale triaxial permeameter. Fresh waste was collected from the working phase, while the landfilled waste was exhumed from a borehole in a landfill cell subjected to leachate recirculation for approximately 1.5 years. The hydraulic conductivity tests conducted on fresh MSW using small-scale rigid-wall permeameter resulted in a range of hydraulic conductivity 2.8× 10−3 –11.8× 10−3  cm/s with dry unit weight varied in a narrow range between 3.9–5.1 kN/ m3 . The landfilled MSW tested using the same permeameter produced ...

Controlled study of landfill biodegradation rates using modified BMP assays

This paper presents the results of laboratory investigation conducted to determine the variation of geotechnical properties of synthetic municipal solid waste (MSW) at different phases of degradation. Synthetic MSW samples were prepared based on the composition of MSW generated in the United States and were degraded in bioreactors with leachate recirculation. Degradation of the synthetic MSW was quantified based on the gas composition and organic content, and the samples exhumed from the bioreactor cells at different phases of degradation were tested for the geotechnical properties. Hydraulic conductivity, compressibility and shear strength of initial and degraded synthetic MSW were all determined at constant initial moisture content of 50% on wet weight basis. Hydraulic conductivity of synthetic MSW was reduced by two orders of magnitude due to degradation. Compression ratio was reduced from 0.34 for initial fresh waste to 0.15 for the mostly degraded waste. Direct shear tests showed that the fresh and degraded synthetic MSW exhibited continuous strength gain with increase in horizontal deformation, with the cohesion increased from 1 kPa for fresh MSW to 16-40 kPa for degraded MSW and the friction angle decreased from 35° for fresh MSW to 28° for degraded MSW. During the triaxial tests under CU condition, the total strength parameters, cohesion and friction angle, were found to vary from 21 to 57 kPa and 1° to 9°, respectively, while the effective strength parameters, cohesion and friction angle varied from 18 to 56 kPa and from 1° to 11°, respectively. Similar to direct shear test results, as the waste degrades an increase in cohesion and slight decrease in friction angle was observed. Decreased friction angle and increased cohesion with increased degradation is believed to be due to the highly cohesive nature of the synthetic MSW. Variation of synthetic MSW properties from this study also suggests that significant changes in geotechnical properties of MSW can occur due to enhanced degradation induced by leachate recirculation.