Computer simulation of gas generation and transport in landfills II: Dynamic conditions

Extensive simulations have been carried out to study generation of landfill gases by biodegradation of wastes and their transport in a model landfill, using a comprehensive three-dimensional model developed in Part I of this series. The model has been developed for a four-component gas mixture, and takes into account the effect of heterogeneity in the landfill's morphology, as well as that of the surrounding soil. It accounts for the presence of gas extraction wells with an arbitrary spatial distribution. The model is utilized for investigating the dynamic behavior of a landfill, and in particular pressure build-up, under a variety of conditions. These include, for example, the cases in which some of the extraction wells are shut down, or new wells are drilled. It is shown that the spatial distribution of the permeability has a very strong effect on the dynamic behavior of a landfill, whereas mechanical dispersion, manifested by effective dispersion coefficients that depend on the flow velocities, has virtually no effect. Thus, development of an accurate data base for the spatial distributions of the permeability, porosity, and other morphological characteristics of a landfill, in addition to its transport and reaction properties, is essential for accurate forecasting of the landfill's behavior, including identifying the areas with large amounts of methane and a potential for explosion. Limited comparison of the model's predictions with experimental data for a particular landfill indicates the potential of the model for predicting the dynamic behavior of large landfills.     

武汉市垃圾卫生填埋场沉降计算及意义

生活垃圾卫生填埋场的沉降是目前卫生填埋法所涉及的主要的岩土工程问题之一。深人开展生活垃城,卫生填埋场沉降特性的研究意义重大。选取武汉市的一处卫生填埋场,研究了填埋过程中和填埋场封顶后垃圾土的长期沉降变形,结果表明填埋沉降百分率高达100％左右。     

Mitigation of methane emissions from sanitary landfills and sewage treatment plants in Jordan

Mitigation of Greenhouse gases deals with measures to reduce the vulnerability of a certain sector to climate change through minimizing net emissions. In this paper, mitigation scenarios aimed at reducing Jordan methane emissions from sewage treatment plants and sanitary landfill sites were proposed and investigated. In the case of sewage treatment plants, As-Samra plant (the largest in Jordan) was selected for this mitigation study. Two scenarios (I and II) were proposed, the first was to expand the plant by the year 2005 using waste stabilization ponds the current treatment technology, and the second scenario involved switching the treatment technology to activated sludge type when the expansion starts in the year 2005. For sanitary landfills, the proposed mitigation scenario was the construction of two biogas plants, each with a processing capacity of 1,000 tons of solid waste per day at Rusaifeh and Akaider—the two largest landfills in Jordan at the beginning of the year 2005. For As-Samra plant, the cumulative reduction in methane emissions by the year 2030 was calculated to be 49 and 146 Gg under mitigation scenarios I and II, respectively. On the other hand, the biogas plant scenario reduces the methane emissions at each landfill by 28.1 Gg annually. The total emission reduction from both landfills in the life span (25 years) of the biogas plants will be about 1,406 Gg CH_4. In addition, this scenario generates electricity at a cost of 4.6 cents per kWh, which is less than the Jordan electric long-run marginal cost of generation at 5.5 cents/kWh. Moreover, annual savings of US$4.65 million will be achieved by the replacement of fuel oil with the generated biogas. The mitigation scenarios presented in this paper include measures that positively contribute to the national development of Jordan in addition to considerable reduction in methane emission.. This forms a win–win situation that favors the adoption of investigated mitigation scenarios by the decision-makers of the waste sector in Jordan.     

Estimation of Soil–Water Characteristic Curve of Clayey Till Using Measured Pore-Size Distributions

The soil–water characteristic curve (SWCC) of fine-grained soils is usually determined experimentally. In the design of mine waste covers and landfill liners, the unsaturated hydraulic conductivity function, k(h), is often derived theoretically from the measured SWCC. Implicit in these derivations is the transformation of the SWCC to a pore-size distribution (PSD), typically assumed to be constant and monomodal. However, PSD measurements of a clayey till compacted at various water contents after compaction, after flexible-wall permeability testing and before and after SWCC tests show that the PSD of the same material varies significantly under the stated physical conditions. Predictions of the SWCCs using PSDs measured both before and after the SWCC tests significantly underpredicted the values measured. By applying a simple transformation to the PSD to account for the scaling effect from the porosimetry samples (approximately 1 g dry weight) to the SWCC test samples (approximately 200 g dry weight), the predicted SWCCs were found to envelop the measured values. A simple model that simulates the change in PSD during the SWCC test predicted water contents close (1% root mean square error) to the measured SWCCs.     

Assessing the Performance of Evapotranspiration Covers for Municipal Solid Waste Landfills in Northwestern Ohio

Evapotranspiration (ET) covers have gained considerable interest as an alternative to conventional covers for the final closure of municipal solid waste (MSW) landfills, but often produce higher rates of percolation in regions that receive more than 32??cm?year-1 of precipitation. The goal of this project is to design ET covers for MSW landfills in northwestern Ohio (long-term annual rate of precipitation of 83??cm?year-1) that produce rates of percolation <32??cm?year-1, the rate considered acceptable by the Ohio Environmental Protection Agency (OEPA), and promote habitat restoration. To attain this goal, an adequate soil water-storage capacity was provided using dredged sediment amended with organic material. Two plant mixtures were tested to evaluate the performance of ET covers immediately following construction (immature plants seeded onto the soil) and in the future (mature plants transplanted from a restored tall-grass prairie that is more than 10?years old). ET covers were constructed in drainage lysimeters (1.52-m diameter, 1.52-m depth) and watered at a rate of 91.12 to 95.72??cm?year-1, which included simulated 100-year rain events (11.7?cm over 24?h) in July and October. During the 1-year monitoring period, the ET covers using the mature plant mixture produced considerably less percolation (0.12 to 11.44??cm?year-1) than the covers with the immature plant mixture (6.71 to 24.16??cm?year-1). Thus far, all ET covers have produced rates of percolation less than the maximum standard by the OEPA, and they will continue to be monitored.     

Analytical Solution for Diffusion of VOCs through Composite Landfill Liners

Analytical solutions are presented for analyzing volatile organic compound (VOC) diffusion through intact composite landfill liners for two scenarios with boundary conditions at the base of either a VOC concentration of zero or a VOC mass flux of zero. A time-dependent concentration top boundary condition is included in the presented analytical solutions to model typical variations of VOC concentration in the leachate over time. The presented solutions are verified against alternative numerical solutions and applied to analyze dichloromethane diffusion through a composite liner. The analytical solutions are found to provide useful predictions of VOC concentration and mass flux for the design of composite liners. VOC concentrations and fluxes at the base of the composite liner at 30?years predicted by consideration of representative transient variation in leachate concentration, for an example problem, are nearly half of those when a constant leachate concentration assumed.     

Long Term Landfill Primary and Secondary Leachate Production

This study evaluated long term primary and secondary leachate production from operating landfills in the northeastern United States. The data were obtained for periods when the landfills were in use (open) and were covered (closed). During the open period of operation, primary leachate production was 64% of the precipitation that fell on the site. After landfill cell closure, the primary leachate production declined and, four years after closure, such production reached quasi-equilibrium and averaged 2% of the precipitation. For secondary leachate production during the open and closed period, such production was 1 and 0.25% of the precipitation, respectively.     

Sepiolite as an Alternative Liner Material in Municipal Solid Waste Landfills

Compacted clay has traditionally been used as a lining material in municipal solid waste landfills. However, natural clays may not always provide good contaminant sorption properties. One alternative material that is abundant in some parts of Europe and Turkey as well as Western United States is sepiolite. A laboratory study was undertaken to investigate the feasibility of sepiolite as a liner material. Two clays, one rich in sepiolite and the other one rich in kaolinite mineral, as well as their mixtures were subjected to geomechanical, hydraulic, and environmental tests. The same soils were also subjected to strength and hydraulic conductivity tests after a series of freeze and thaw cycles. The results of the study indicated that relatively high hydraulic conductivity and shrinkage capacity of sepiolite necessitates addition of kaolinite before being used as a landfill material. The valence of the salt solutions affected the swell and hydraulic conductivity characteristics of the clays tested. Retardation factors for sepiolite for metal solutions are 1.2–2.2 times higher than those calculated for the clay that is rich in kaolinite, and the inorganic contaminant adsorption capacity of the clay can be improved by addition of sepiolite. The results indicated that the clay mixtures utilized in this study provide good geomechanical, hydraulic, and metal adsorption properties which may justify their potential use as a liner material in solid waste landfills.     

Composition of Municipal Solid Waste in the United States and Implications for Carbon Sequestration and Methane Yield

Eleven statewide waste characterization studies were compared to assess variation in the quantity and composition of waste after separation of recyclable and compostable materials, i.e., discarded waste. These data were also used to assess the impact of varying composition on sequestered carbon and methane yield. Inconsistencies in the designation of waste component categories and definitions were the primary differences between study methodologies; however, sampling methodologies were consistent with recommended protocols. The average municipal solid waste (MSW) discard rate based on the statewide studies was 1.90?kg?MSW?person?1?day?1, which was within the range of two national estimates: 2.35 and 1.46?kg?MSW?person?1?day?1. Dominant components in MSW discards were similar between studies. Organics (food waste, yard trimmings), paper, and plastic components averaged 23.6±4.9%, 28.5±6.5%, and 10.6±3.0% of discarded MSW, respectively. Construction and demolition (C&D) waste was 20.2±9.7% of total solid waste discards (i.e., MSW plus C&D). Based on average statewide waste composition data, a carbon sequestration factor (CSF) for MSW of 0.13 kg C dry kg?MSW?1 was calculated. For C&D waste, a CSF of 0.14 kg C dry kg C and D waste?1 was estimated. Ultimate methane yields (Lo) of 59.1 and 63.9?m3 CH4 wet Mg refuse?1 were computed using EPA and state characterization study data, respectively, and were lower than AP-42 guidelines. Recycling, combustion, and other management practices at the local level could significantly impact CSF and (Lo) estimates, which are sensitive to the relative fraction of organic components in discarded MSW and C&D waste.     

One-Dimensional Gas Flow Model for Horizontal Gas Collection Systems at Municipal Solid Waste Landfills

Extraction of biogas from horizontal layers above, below, and within municipal solid waste landfills is becoming more commonplace. A steady-state one-dimensional analytical landfill gas model was developed to assist in the assessment and design of such collection systems. The model simulates the distribution of gas pressure within a layer of landfill waste under a variety of operating conditions that include upper and lower boundaries specified at given fluxes or pressures. The model can be used to predict where maximum pressures will build up within the landfill and what vacuum pressures must be applied to achieve specific gas collection efficiency in a horizontal collection layer. The utility of the model was illustrated for several scenarios of interest. In the absence of gas collection from a landfill’s leachate collection system, considerable gas pressures can build up at the bottom of the landfill. The design of leachate collection systems for landfill gas removal should be considered from the outset. An evaluation of the parameters that impact vacuum requirements—waste depth, gas generation rate, and waste permeability—suggests that it may not be feasible to rely solely upon the leachate collection system for the removal of landfill gas. The model was thus used to illustrate cases where a horizontal collection layer underneath the landfill cap is used in conjunction with gas extraction from the bottom of the landfill. Several recommendations are proposed to improve the gas collection efficiencies for landfills utilizing horizontal gas collection layers.