Landfill Settlement with Decomposition and Gas Generation

A one-dimensional multiphase numerical model is developed to simulate the vertical settlement involving liquid and gas flows in a deformable (settling) municipal solid waste (MSW) landfill. MSW is represented by a chemical composition, and a global stoichiometric reaction is used to estimate the maximum yield of gas generation. Following the general assumption accepted in the literature, the gas generated by waste decomposition is assumed to comprise of methane (CH4) and carbon dioxide (CO2). The gas generation rate follows an exponentially decaying function of time. The gas generation model developed based on a first-order kinetic single-bioreactor approach includes the governing equations of gas migration, liquid flow, and landfill deformation. The Galerkin finite element method is used to solve the resulting equations. The model developed can be used to estimate the transient and ultimate settlements due to waste decomposition and gas generation in MSW landfills. The proposed model can estimate the waste porosity, gas pressure, liquid pressure, gas saturation, liquid saturation, and stress distributions in settling landfills. The results obtained for a deformable landfill are compared with a landfill having a rigid solid skeleton. Due to settlement, the depth of waste is 27% smaller in deformable landfills than that of the rigid ones.     

Refuse Decomposition in the Presence and Absence of Leachate Recirculation

A side by side comparison of two 8,000 metric ton test cells was performed to evaluate the effects of leachate recirculation on refuse decomposition at Yolo County, CA. After about 3 years of operation, refuse was excavated in three borings from the enhanced cell (E1, E2, and E3) and two borings from the control cell (C1 and C2). Refuse moisture content data show that leachate recirculation resulted in an increase in refuse moisture content, but also show that the refuse in the enhanced cell was not uniformly wet. The average moisture content in E1, E2, and E3 was 38.8, 31.7, and 34.8%, respectively, while the average moisture content in C1 and C2 was 14.6 and 19.2%, respectively. Leachate recirculation resulted in both higher methane yields, (63.1 versus 27.9 L CH4/wet-kg over 1231 days) and increased settlement (15.5% versus 3% of the waste thickness). The extent of decomposition of excavated refuse samples was determined by the biochemical methane potential (BMP) and the ratio of cellulose plus hemicellulose to lignin [(C+H)/Li]. Solids analyses showed the average BMP in the enhanced and control cells to be 24.0 and 30.9 mL CH4/dry-g, respectively. The corresponding (C+H)/Li ratios were 1.09 and 1.44, respectively. These data correlate well with the increased methane production in the enhanced cell. Thus laboratory and field data show more decomposition in the enhanced cell relative to the control cell. The refuse sampling program conducted for the Yolo County test cells, in concert with data on settlement, methane production, and the volume of liquid actually recycled, represents perhaps the most complete set of data available to date on a field-scale leachate recirculation landfill.     

Relationship of Compressibility Parameters to Municipal Solid Waste Decomposition

Recently, there has been substantial interest in the enhancement of refuse decomposition in landfills, which results in increased settlement. In this paper, changes in waste compressibility as a function of the state of decomposition are reported. Samples representative of residential refuse were decomposed under conditions designed to simulate decomposition in both control and bioreactor landfills. Twenty four one-dimensional oedometer tests (63.5 mm cell) were performed on refuse prepared in laboratory-scale reactors for measurement of primary (Cc) and secondary (Cαi, representing creep, and Cβi, representing biological) compression indices. The state of decomposition was quantified by the methane yield and the cellulose (C) plus hemicellulose (H) to lignin (L) ratio. The magnitude of compressibility was shown to increase as refuse decomposed and compressibility parameters were correlated with the state of decomposition. Initial settlement increased with decreasing (C+H)/L ratio while the creep index was fairly independent of the state of decomposition. The coefficients of primary compression (Cc) for bioreactor samples showed an increasing trend with decreasing (C+H)/L ratios. Cc increased from 0.16 to 0.36 as (C+H)/L decreased from 1.29 to 0.25, and similar values of Cc were obtained with control samples at similar (C+H)/L ratios. The creep index range was estimated at 0.02–0.03 for control and bioreactor samples in various states of decomposition. The magnitude of the biological degradation index (Cβi) depended on the degradation phase with the highest value of 0.19 obtained during the phase of accelerated methane production. Proposing a single Cc for landfill settlement calculations may lead to inaccurate predictions. Properties of each waste sublayer will change as a function of the decomposition stage, and dominating processes with appropriate compressibility parameters should be applied to individual sublayers.     

Numerical Characterization of Advective Gas Flow through GM/GCL Composite Liners Having a Circular Defect in the Geomembrane

Numerical experiments were conducted to understand the effect of geometric and transport characteristics of a geomembrane-geosynthetic clay liner (GM/GCL) composite liner on gas leakage rate through a circular defect in the geomembrane (GM). The originality of the approach proposed in this paper rests on the use of a new conceptual two-layered system for modeling of GM/GCL composite liners where the interface zone between the GM and geosynthetic clay liner (GCL) has been merged with the GCL cover geotextile and handled as one layer; the GCL bentonite layer was considered the second layer. The role of the carrier geotextile layer was ignored since it can be considered as a no pressure loss layer. Analysis of numerical simulation results shows the existence of a constitutive leakage flow surface which enables evaluation of the leakage flow state for different geometric and transport properties of GM/GCL composite liners. Furthermore, the determined surface was also exploited to evaluate gas leakage rates under the framework of the Forchheimer’s analytical solution. The gas leakage rate predictions were found to be in good agreement with experimental results obtained at different GCL moisture content.     

Case Study of a Full-Scale Evapotranspiration Cover

The design, construction, and performance analyses of a 6.1?ha evapotranspiration (ET) landfill cover at the semiarid U.S. Army Fort Carson site, near Colorado Springs, Colo. are presented. Initial water-balance model simulations, using literature reported soil hydraulic data, aided selection of borrow-source soil type(s) that resulted in predictions of negligible annual drainage ( ? 1?mm/year). Final construction design was based on refined water-balance simulations using laboratory determined soil hydraulic values from borrow area natural soil horizons that were described with USDA soil classification methods. Cover design components included a 122?cm thick clay loam (USDA), compaction ? 80% of the standard Proctor maximum dry density (dry bulk density ～ 1.3?Mg/m3), erosion control measures, top soil amended with biosolids, and seeding with native grasses. Favorable hydrologic performance for a 5?year period was documented by lysimeter-measured and Richards’-based calculations of annual drainage that were all <0.4?mm/year. Water potential data suggest that ET removed water that infiltrated the cover and contributed to a persistent driving force for upward flow and removal of water from below the base of the cover.     

Numerical Examination of a Method for Reducing the Temperature of Municipal Solid Waste Landfill Liners

A method to control the increase in landfill liner temperature due to the heat generated by the waste is examined. The design involves installation of an array of cooling pipes beneath the waste. The feasibility of this system for cooling the liner was examined by performing a series of analyses for conditions based on the Tokyo Port Landfill. The results suggest that the introduction of a cooling system can substantially reduce liner temperature and consequently significantly increase the service life of a high-density polyethylene (HDPE) geomembrane liner in an engineered barrier system. The effects of pipe layout, pipe spacing, and coolant flow rate are examined. It is shown that a periodic pipe layout is the most efficient. Liner temperature decreases with increased coolant transfer flow rate     

Physical Characterization of Municipal Solid Waste for Geotechnical Purposes

A procedure to characterize municipal solid waste (MSW) for geotechnical engineering purposes is developed based on experience with waste characterization and testing. Existing MSW classification systems are reviewed briefly, and the field and laboratory waste characterization programs of two important projects are presented. Findings on the influence of the waste’s physical composition on its mechanical response from these projects and recent studies of MSW are integrated to develop a waste characterization procedure for efficient collection of the relevant information on landfill operation and waste physical characteristics that are most likely to affect the geotechnical properties of MSW. A phased approach to implementation of this procedure is proposed as a best practice for the physical characterization of MSW for geotechnical purposes. The scope of the phased procedure can be adjusted to optimize the effort required to collect relevant information on a project-specific basis. The procedure includes a systematic evaluation of the moisture and organic content of MSW, because they are important factors in the geotechnical characterization of MSW.     

Grid-Based Heuristic Method for Multifactor Landfill Siting

Siting a landfill requires the processing of a large amount of spatial data. However, the manual processing of spatial data is tedious. A geographical information system (GIS), although capable of handling spatial data in siting analyses, generally lacks an optimization function. Optimization models are available for use with a GIS, but they usually have difficulties finding the optimal site from a large area within an acceptable computational time, and not easily directly available with a raster-based GIS. To overcome this difficulty, this study developed a two stage heuristic method. Multiple factors for landfill siting are considered and a weighted sum is computed for evaluating the suitability of a candidate site. The method first finds areas with significantly high potentialities and then applies a previously developed mixed-integer programming model to locate the optimal site within the potential areas, and can significantly reduce the computational time required for resolving a large siting problem. A case study was implemented to demonstrate the effectiveness of the proposed method, and a comparison with the previously developed model was provided and discussed.     

Fugitive Methane Emissions from Landfills: Field Comparison of Five Methods on a French Landfill

The study presented in this paper has been initiated by the Veolia Environment research center for waste management and supported by the French Environmental Agency. A comparison of five field-scale measurement methods for measuring fugitive methane emissions has been conducted on a French landfill site. The five methods evaluated consisted of a tracer gas technique, laser radial plume mapping, inverse modeling technique, differential absorption light detection and ranging (LiDAR), and helicopter-borne spectroscopy. These methods are evaluated on their abilities to measure emissions from a practical user-oriented aspect (metrological, technical, and economical criteria). High disparities in terms of quantitative results and applicability were observed from all methods. Techniques that used Gaussian dispersion modeling appeared less applicable to landfill sites due to topographic complexity and did not provide high confidence in the results. However, the method using optical remote sensing (radial plume mapping) methods showed that a spatially detailed analysis is achievable (cell level), and the LiDAR method showed very promising approach and technical performances.     

Leachate from Land Disposed Residential Construction Waste

Solid waste from construction and demolition (C&D) activities is often disposed in unlined landfills. Leachate from unlined landfills poses a potential risk to groundwater quality. An understanding of the types of chemical constituents likely to be encountered in C&D waste landfill leachate and the concentrations at which they occur help assess this risk. An experiment was performed to characterize leachate from land-disposed residential construction waste. Four 54 m2 (580 ft2) test cells were excavated, lined, and filled with waste. Leachate samples were collected and analyzed for a number of water quality parameters over a 6 month period. No volatile or semivolatile organic compounds were detected at elevated constituent levels in the leachate. Inorganic ions were found to account for the bulk of the pollutant mass leached. Calcium and sulfate were the predominant ions in the leachate, resulting from the dissolution of gypsum drywall. The concentrations of several leachate constituents were found to exceed water quality standards. These constituents included aluminum, arsenic, copper, manganese, iron, sulfate, and total dissolved solids. Arsenic was the only primary water quality standard exceeded. The arsenic was concluded to result from chromated copper arsenate (CCA)-treated wood. The potential risk of impacting groundwater was examined by comparing the measured constituent concentrations with the water quality standards to assess the amount of dilution and attenuation needed in the groundwater so that a water quality standard would not be exceeded. The water quality standard exceeded by the greatest magnitude was manganese, followed by iron.