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.     

Secondary Compression of Municipal Solid Wastes and a Compression Model for Predicting Settlement of Municipal Solid Waste Landfills

The estimation of the capacity and settlement of landfills is critical to successful site operation and future development of a landfill. This paper reports the results of a study on biodegradation behavior and the compression of municipal solid wastes. An experimental apparatus was developed which had a temperature-control system, a leachate recycling system, a loading system, and a gas and liquid collection system. Experiments were performed both with and without optimal biodegradation for comparative purposes. Test results indicated that settlement resulting from creep was relatively insignificant when the biodegradation process was inhibited. Compression due to decomposition under optimal biodegradation conditions was found to be much larger than compression associated with creep. The biodegradation process was significantly influenced by the operational temperature. A one-dimensional model is proposed for calculating settlement and estimating the capacity of the landfill under relatively optimal biodegradation conditions. The model was developed to accommodate the calculation of settlement in landfills when a multistep filling procedure was used. The calculation method is relatively simple and convenient for design purposes. Simulations of the physical processes showed that enhancing solid waste biodegradation during the filling stage can considerably increase the capacity of the landfill and reduce postclosure settlements.     

Evaluation of Decomposition Effect on Long-Term Settlement Prediction for Fresh Municipal Solid Waste Landfills

A considerable amount of settlement occurs due to the decomposition of municipal solid waste (MSW) in landfills over a period of years. Therefore, the effect of biological decomposition governs the long-term settlement characteristics of municipal solid waste landfills. In this study, we investigated the long-term settlement characteristics by applying a number of prediction methods to fresh MSW sites and predicting the settlement curves. Most proposed methods, excluding the power creep law, successfully predicted long-term settlement only if accelerated logarithmic compression due to decomposition of biodegradable MSW was included in the settlement prediction.     

Compaction Characteristics of Municipal Solid Waste

Compaction characteristics of municipal solid waste (MSW) were determined in the laboratory and in the field as a function of moisture content, compactive effort, and seasonal effects. Laboratory tests were conducted on manufactured wastes using modified and 4X modified efforts. Field tests were conducted at a MSW landfill in Michigan on incoming wastes without modifications to size, shape, or composition, using typical operational compaction equipment and procedures. Field tests generally included higher efforts and resulted in higher unit weights at higher water contents than the laboratory tests. Moisture addition to wastes in the field was more effective in winter than in summer due to dry initial conditions and potential thawing and softening of wastes. The measured parameters in the laboratory were γdmax-mod = 5.2?kN/m3, wopt-mod = 65%, γdmax-4×mod = 6.0?kN/m3, and wopt-4×mod = 56%; in the field with effort were γdmax-low = 5.7?kN/m3, wopt-low = 70%; γdmax-high = 8.2?kN/m3, and wopt-high = 73%; and in the field with season were γdmax-cold = 8.2?kN/m3, wcold = 79.5%, γdmax-warm = 6.1?kN/m3, and wwarm = 70.5%. Soil compaction theory was reasonably applicable to wastes with the exception that the Gs of waste solids increased with compactive effort resulting in steep degree of saturation curves and low change in wopt between efforts. Moisture addition to wastes during compaction increased the workability, the unit weight, and the amount of incoming wastes disposed, and reduced the compaction time. The combined effects have significant environmental and economic implications for landfill operations.     

Municipal Solid Waste Landfill Settlement: Postclosure Perspectives

This paper presents settlement mechanisms and the methods for estimating settlements of municipal solid waste landfills, including bioreactor landfills. Based on results of field monitoring and data in published literature, coefficients of secondary compression for solid waste due to self-weight and external load are estimated. Special considerations are given to bioreactor landfills. Uses of these coefficients for long-term settlement estimation and their application to postclosure maintenance and development plans are discussed. Four case histories illustrating the use of these coefficients are presented. Methods of landfill treatment to reduce settlements are also presented.     

Air Permeability of Waste in a Municipal Solid Waste Landfill

The permeability of compacted municipal solid waste in a landfill with respect to air (or gas) flow was estimated using a short-term air injection test. Air was added to 134 vertical wells installed at three different depths at flow rates in the range of 0.14?–1.4?m3?min?1 and the corresponding steady state pressures were recorded. The permeability of the waste with respect to airflow (described here as the air permeability) was estimated for different anisotropy ratios (kr/kz = 1, 10, and 100) using a steady state, two-dimensional, axisymmetric analytical fluid flow model in conjunction with the measured flow and pressure data. The air permeability of landfilled municipal solid waste modeled as an isotropic medium was found to range from 1.6×10?13 to 3.2×10?11?m2. The estimated air permeability results were on the low end of values previously applied to model landfill gas flow. Estimated air permeability decreased significantly with increasing waste depth. The lower permeability encountered in the deeper layers was primarily attributed to the lower porosity of the waste caused by higher overburden pressures and higher moisture content of waste in deeper layers of the landfill than in shallow layers. The results suggest that multiple wells screened at different depths provide greater control of air distribution within the landfill. Leachate recirculation was documented to impact the ability to add air. In addition to limitations posed by standing water in many of the deeper wells, waste exposed to leachate recirculation was found to be significantly less permeable to air when compared to original conditions.     

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.     

Constitutive Model for Municipal Solid Waste

Municipal solid waste (MSW) is a refuse composed of various materials with different properties. Some of the components are stable while others degrade as a result of biological and chemical processes. These aspects impart to MSW a complex behavior that has been modeled, with many limitations, within the concepts of soil mechanics. In this paper, a framework to model the MSW mechanical behavior is proposed based on results from laboratory tests, such as triaxial compression and confined compression of large samples. It is suggested that two different effects command MSW mechanical behavior: (a) the reinforcement of MSW by synthetic fibers (composed by many types of polymers) and (b) the behavior of the MSW paste, without fibers. Accordingly, two distinct frameworks were used to represent the main MSW characteristics: (a) a critical state framework for MSW paste and (b) an elastic perfectly plastic framework for waste fibers, with a time lag for fiber loading (function fm). The proposed model is capable of reproducing quite well the results obtained from triaxial and confined compression tests performed in the laboratory as well as the settlement recorded in a sanitary landfill.     

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.     

Thermal Analysis of Cover Systems in Municipal Solid Waste Landfills

Cover temperature variations were determined at four municipal solid waste landfills located in different climatic regions in North America: Michigan, New Mexico, Alaska, and British Columbia. Cover temperatures varied seasonally similarly to air temperatures and demonstrated amplitude decrement and phase lag with depth. Elevated temperatures in the underlying wastes resulted in warmer temperatures and low frost penetration in the covers compared to surrounding subgrade soils. The ranges of measured temperatures decreased and average temperatures generally increased (approximately 2°C/m) with depth. The ranges of measured temperatures (Tmax?Tmin) were 18–30°C and 13–21°C and the average temperatures were 13–18°C and 14–23°C at 1 and 2?m depths, respectively. For soil and geosynthetic barrier materials around 1?m depth, the maximum and minimum temperatures were 22–25°C and 3–4°C, respectively. Frost depths were determined to be approximately 50% of those for soils at ambient conditions. The main direction of heat flow in the covers was upward (negative gradients). The cover gradients varied between ?18 and 14°C/m, with averages of ?7?to?1°C/m. The gradients for soil and geosynthetic barrier materials around 1?m depth varied between ?11 and 9°C/m with an average of ?2°C/m. Cover thawing n-factors ranged between 1.0 and 1.4 and the cover freezing n-factor was 0.6. Design charts and guidelines are provided for cover thermal analyses for variable climatic conditions.     

Shear Strength of Municipal Solid Waste

A comprehensive large-scale laboratory testing program using direct shear (DS), triaxial (TX), and simple shear tests was performed on municipal solid waste (MSW) retrieved from a landfill in the San Francisco Bay area to develop insights about and a framework for interpretation of the shear strength of MSW. Stability analyses of MSW landfills require characterization of the shear strength of MSW. Although MSW is variable and a difficult material to test, its shear strength can be evaluated rationally to develop reasonable estimates. The effects of waste composition, fibrous particle orientation, confining stress, rate of loading, stress path, stress-strain compatibility, and unit weight on the shear strength of MSW were evaluated in the testing program described herein. The results of this testing program indicate that the DS test is appropriate to evaluate the shear strength of MSW along its weakest orientation (i.e., on a plane parallel to the preferred orientation of the larger fibrous particles within MSW). These laboratory results and the results of more than 100 large-scale laboratory tests from other studies indicate that the DS static shear strength of MSW is best characterized by a cohesion of 15?kPa and a friction angle of 36° at normal stress of 1?atm with the friction angle decreasing by 5° for every log cycle increase in normal stress. Other shearing modes that engage the fibrous materials within MSW (e.g., TX) produce higher friction angles. The dynamic shear strength of MSW can be estimated conservatively to be 20% greater than its static strength. These recommendations are based on tests of MSW with a moisture content below its field capacity; therefore, cyclic degradation due to pore pressure generation has not been considered in its development.     

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.     

Spatial and Temporal Temperature Distributions in Municipal Solid Waste Landfills

Long-term spatial and temporal variations in temperatures have been investigated in covers, wastes, and liners at four municipal solid waste landfills located in different climatic regions: Alaska, British Columbia, Michigan, and New Mexico. Temperatures were measured in wastes with a broad range of ages from newly placed to old (up to 40 years). The characteristic shape of waste temperature versus depth relationships consisted of a convex temperature profile with maximum temperatures observed at central locations within the middle third fraction of the depth of the waste mass. Lower temperatures were observed above and below this central zone, with seasonal fluctuations occurring near the surface and steady and elevated values (above mean annual earth temperature) near the base of the landfills. Heat gain and long-term temperatures were directly affected by placement temperatures. Sustained concave temperature profiles were observed for winter waste placement. The highest heat gain and resulting high temperatures were observed in Michigan followed by British Columbia, New Mexico, and Alaska. The high heat gain in Michigan was attributed to coupled precipitation/moisture content and waste density. The time-averaged waste temperature ranges were 0.9–33.0, 14.4–49.2, 14.8–55.6, and 20.5–33.6°C in Alaska, British Columbia, Michigan, and New Mexico, respectively. Temperature increases occurred rapidly (over multiple years) in British Columbia and then dissipated for tens of years. Longer periods of temperature increase were observed at the other sites. Temperatures, temperature increases, and heat gain were higher during anaerobic decomposition of wastes than aerobic decomposition. A parametric study indicated that use of insulating materials over covers decreased temperature variations compared to uninsulated conditions for prevention of frost penetration or desiccation and for optimum methane oxidation. Overall, thermal regime of landfills is controlled by climatic and operational conditions.     

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.     

Unit Weight of Municipal Solid Waste

The unit weight of municipal solid waste (MSW) is an important parameter in engineering analyses of landfill performance, but significant uncertainty currently exists regarding its value. A careful review of reliable field data shows that individual landfills have a characteristic MSW unit weight profile. Based on in situ unit weight data and trends observed in large-scale laboratory tests, a hyperbolic relationship was developed to represent this characteristic MSW unit weight profile. Within the context of this characteristic profile, landfill-specific values of MSW unit weight depend primarily on waste composition, operational practices (i.e., compaction, cover soil placement, and liquids management), and confining stress. Guidance is provided for developing landfill-specific MSW unit weight profiles, including procedures for performing large-scale tests for in situ measurement of MSW unit weight at a landfill.     

Estimating the Hydraulic Conductivity of Landfilled Municipal Solid Waste Using the Borehole Permeameter Test

This paper reports the in situ field saturated hydraulic conductivity of municipal solid waste at a landfill in Florida. The saturated hydraulic conductivity (Ks) was estimated at 23 locations using the borehole permeameter test, a method commonly used for determination of the Ks of unsaturated soil. The Ks of the landfilled waste was found to range from 5.4×10?6 to 6.1×10?5?cm/s. The Ks was found to be on the lower end of the range of Ks reported by previous studies. The hydraulic conductivity of the waste decreased with depth, the likely result of greater overburden pressures associated with deep locations of the landfill. Permeability values (kw) of the landfilled waste calculated based on Ks were compared with permeability values estimated using air as the fluid (air permeability, ka). Values of ka were found to be approximately three orders of magnitude greater than those of kw. The lower permeability of the waste to water was primarily attributed to entrapped gas. Other factors such as potential clogging of media and short-circuiting of air along the well may also have contributed to the differences in ka and kw.     

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.     

Shear Strength Parameters of Municipal Solid Waste with Leachate Recirculation

The objective of this study was to characterize relative changes in waste shear strength parameters during waste decomposition. Twelve direct shear tests (100?mm diameter by 50?mm thickness) were performed on waste specimens ranging from fresh to well-decomposed residential refuse. In addition, nine direct shear tests were performed on selected waste components including fresh paper, partially decomposed refuse, and plastics. Results indicate that the friction angle of refuse decreased with decomposition. As refuse decomposed, the plastic content increased, which contributed to a decrease in friction angle as the friction angle of plastics is 18–19° as compared to 33° for fresh shredded waste. The extent of refuse decomposition was characterized by the cellulose plus hemicellulose to lignin ratio [(C+H)/L]. The measured friction angle decreased from 32 to 24° as (C+H)/L decreased from 1.29 to 0.25. The shearing pattern for decomposed refuse showed a peak, followed by residual, which was then followed by a steady increase in shear stresses with displacement; the final rate of increase was similar to that observed in fresh paper specimens. Results from this work were comparable to data from previous reports, though it is important to characterize the extent of solids decomposition for a valid comparison with published studies.     

Integrated Modeling for Optimal Municipal Solid Waste Management Strategies under Uncertainty

In this study, an inexact fuzzy-probabilistic programming (IFPP) method is advanced for developing optimal municipal solid waste (MSW) management strategy with uncertain information. The IFPP can support the assessment of risk of violating constraints associated with fuzzy and random features. The developed method is applied to a case study of long-term MSW management planning in the city of Changchun, China. Violations for transfer-station capacity constraints are allowed under a range of probability and possibility levels, which are related to trade-offs between the system cost and the constraint-violation risk. The results indicate that useful solutions for planning the MSW management practices have been generated. They are valuable for supporting the identification of efficient waste-flow allocation patterns, the long-term capacity planning of the city’s waste management system, and the formulation of local policies regarding waste management under uncertainty.     

Pilot-Scale Simulation of Landfill Bioreactor and Controlled Dumping of Fresh and Partially Stabilized Municipal Solid Waste in a Tropical Developing Country

Four pilot-scale lysimeters were used to study the benefits of landfill operation with and without leachate recirculation in tropical weather conditions. Young and old landfills were simulated by filling lysimeters with a segregated fraction of fresh municipal solid waste (MSW) and MSW mined from an open dump site, respectively, and periodically monitoring leachate quantity and quality and biogas quality. For each substrate, one lysimeter was operated as a bioreactor with leachate recirculation and another lysimeter was operated as a controlled dump, for a period of 10 months. Densities between 652 and 825??kg/m3 could be achieved with fresh and mined MSW. Despite such compaction during waste placement, bioreactor technology helps in leachate management, especially in the case of the young landfill lysimeter operated in tropical weather. The benefits of leachate recirculation in the young landfill lysimeter were evident from the significant decrease in chemical oxygen demand (COD) (86%), biochemical oxygen demand (BOD) (82%), dissolved organic carbon (DOC) (85%), and volatile solids (75%) in leachates. However, ammonia nitrogen (amm-N) and chlorides in the leachates accumulated in bioreactor landfills. Operating an old landfill lysimeter as a bioreactor seemed to have no exceptional advantage in the context of leachate management, although leachate recirculation enhanced the methane potential of both fresh and mined MSW.