Estimate of Landfill Settlements Due to Mechanical and Decompositional Processes

Approximate closed form solutions are developed for estimating the mechanical and decomposition settlement of landfills. The mechanical settlements are the immediate (primary) settlement and long term creep. The decomposition settlements are generated by mass loss and gas generation. Based on the approximate solution, the decomposition settlement can constitute a significant portion of the total settlement and can be in the same order as the mechanical settlements. Settlement remaining after landfill closure depends on the decomposition environment during filling. For an average decomposition environment, postoperation settlement can still be significant 20?years after completion of filling.     

Stochastic Model for Landfill Gas Transport and Energy Recovery

Predicting the amount of landfill gas (LFG) that will be recovered at a sanitary landfill is generally associated with a high level of uncertainty, which is primarily due to the uncertainty in the definition of the parameters that control the LFG generation rate and LFG transport. To quantify these uncertainties, a three-dimensional stochastic model for the generation and transport of LFG is proposed. Using Monte Carlo simulations, multiple realizations of key input parameters are generated. For each realization, LFG transport is simulated and then used to evaluate probabilistically the rates and efficiency of energy recovery. For demonstration, the stochastic model is applied to the Kemerburgaz landfill in Istanbul, Turkey. Uncertainty in the definition of three key parameters, namely: the LFG production rate, the waste gas permeability and the soil cover gas permeability were accounted for. Modeling results suggest that the collection system is sufficient to capture most of the generated gas, but that uncertainty in the factors controlling LFG production is the main source of uncertainty in the amount of energy that will be recovered.     

Influence of High-Permeability Layers for Enhancing Landfill Gas Capture and Reducing Fugitive Methane Emissions from Landfills

Gas collection systems of various designs have been used to control landfill gas emissions, which can be problematic, particularly before installation of final landfill covers. In this work, an innovative gas collection system that includes a permeable layer near the top surface of landfills was evaluated for enhancing capture of landfill gas and reducing fugitive methane emissions. A computational model that accounts for advective and diffusive fluxes of multiple gas components was used to evaluate the efficiency of this new design for intermediate landfill covers. The utility of the high-permeability gas-conductive layer was illustrated for several conditions of interest including varying refuse permeability, varying degrees of permeability anisotropy, and temporal atmospheric pressure changes. Simulations showed that the permeable layer decreased methane emissions by 43% when the horizontal to vertical permeability ratio for refuse was kh/kv = 3 and the domain average kh = 3×10?12?m2, while reductions in methane emissions decreased to 17% for the same anisotropy but with kh = 10?11?m2. With this design, barometric pressure changes did not significantly affect oxygen intrusion or methane emission rates.     

Ageing of HDPE geomembrane exposed to air,water and leachate at different temperatures

The latest findings regarding the long-term performance and service life of HDPE geomembrane (GM) samples exposed to air, water and leachate are presented based on data from samples that have been ageing for 8–10 years. Some of the GM samples are in Stage II, some in Stage III and some have completed all three stages of the service life. The paper provides: (1) improved data on antioxidant depletion rates for GMs immersed in air, water and leachate; (2) estimates of antioxidant depletion time (Stage I) at typical liner temperatures in air, water and leachate and, based on this data, an estimate for a composite liner at typical liner temperatures; (3) data regarding the changes in the physical and mechanical properties of the GM samples with time; (4) a surface analysis of virgin and aged GMs; (5) an initial estimate of the induction time (Stage II) and polymer degradation time (Stage III) and service lives of GM in laboratory immersion tests; and (6) predictions of the service life of leachate immersed GM at typical landfill temperatures. Based on these predictions, it appears likely that the service life of the specific GM tested immersed in leachate is likely to exceed 700 years and will probably be of the order of 1000 years (or longer) at 20 °C, more than 150 years and likely 225–375 years at 35 °C and more than 40 years and likely 50–90 years at 50 °C. The service life in a liner configuration may be expected to be longer than predicted here for immersion in leachate.     

Gas Transport Parameters for Compacted Reddish-Brown Soil in Sri Lankan Landfill Final Cover

Gas exchange through the compacted final cover soil at landfill sites plays a vital role for emission, fate, and transport of toxic landfill gases. This study involved measuring the soil-gas diffusivity (Dp/Do, the ratio of gas diffusion coefficients in soil and free air) and air permeability (ka) for differently compacted soil samples (reddish-brown soil) from the final cover at the Maharagama landfill in Sri Lanka. The samples were prepared by either standard Proctor compaction or hand compaction to dry bulk densities of 1.60–1.94??g?cm-3. Existing and modified models for predicting Dp/Do and ka were tested against the measured data. The simple, single-parameter Buckingham model predicted measured Dp/Do values across compaction levels equally well or better than a dry bulk density (DBD) dependent model and a soil-water retention (SWR) dependent model. The measured ka values for differently compacted samples were highly affected by the compaction level and the sample moisture preparation method. Also, for air permeability, a single-parameter Buckingham-type ka model was most accurate in predicting ka in the differently compacted soil samples. Equivalent air-filled pore diameters (the effective diameter of the drained pores active in leading air through the sample) for gas flow, deq, were calculated from the measured Dp/D0 and ka values. The deq increased with compaction level, suggesting that a very high compaction level creates well-connected macropores in the reduced total pore space of the cover soil. This is an important consideration when designing cover soils for optimally low water and high oxygen exchange while minimizing climate and toxic gas emissions from the waste layer to the atmosphere.     

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.     

Installation of Horizontal Wells in Landfilled Waste Using Directional Drilling

Leachate levels within a landfill must often be controlled for environmental and/or regulatory reasons by means of pumping from wells. Conventional vertical wells are usually used for this purpose, but there is a perception that they are inefficient. In this paper, the feasibility of using directional drilling to install horizontal wells for leachate control in landfills is investigated with reference to pilot and full scale field trials at Rainham, U.K. The difficulties of well-screen design and installation in a landfilled waste are discussed; the insights gained during trial installation are described; and the effectiveness of three trial wells is assessed with reference to the leachate flow rates and drawdowns achieved, in comparison with conventional vertical wells. It is concluded that the drilling rig used must be sufficiently powerful to cope with the likelihood of at least partial borehole collapse around the well-screen during installation; that the screen slot size can be based on at least the D30 particle size of the waste and a natural filter allowed to develop around the well (provided that the resulting well screen is strong enough); and that as experience with the technology grows, directionally drilled horizontal wells could represent a viable, cost effective alternative to conventional vertical wells for leachate control in landfills.     

Application of Fuzzy Logic to Estimate Flow of Methane for Energy Generation at a Sanitary Landfill

This paper proposes the use of a multicriteria assessment technique to evaluate the methane flow during gas extraction from a sanitary landfill. A number of parameters determine the gas generation and the feasibility for its extraction from a landfill. These parameters form a complex set of information with unknown mathematical interrelationships making potential gas flow evaluations difficult and elusive. In addition, the data available for a particular landfill are very often imprecise, uncertain, or subjective, making it even more difficult to evaluate the potential for gas extraction without conducting pilot tests. The method proposed in this paper uses fuzzy composite programming that allows for the use of imprecise information. A landfill gas potential index has been defined, which can be determined by easily obtainable climatological, geological, and landfill parameters. The landfill gas (LFG) potential index is related to the landfill gas flow using an empirical equation. The LFG potential model was calibrated and verified using data obtained from 61 landfills where gas extraction is being conducted. A sensitivity analysis was done to study the impact of variations in the input data on model output.     

Leachate Recirculation in Bioreactor Landfills Using Geocomposite Drainage Material

The key purpose of this study was to test the use of a permeable blanket made up of a geocomposite drainage layer (GDL) for leachate recirculation in municipal solid waste (MSW) landfills and to predict the observed leachate travel in the blanket using a numerical model. A 34?m long by 12?m wide permeable blanket made up of GDL was constructed at an active MSW landfill located in Michigan. Leachate was injected in the GDL using a perforated pipe placed centrally above the GDL along its length. Moisture content sensors, pressure transducers, thermistors, thermocouple sensors, and a vertical load sensor were embedded immediately below the GDL blanket to monitor the flow of injected leachate. After the blanket was covered with waste, leachate was injected into the blanket at rates ranging from 0.9 to 2.6?m3/h per meter length of the blanket. Data collected from the embedded sensors indicated that the injected leachate traveled at rates ranging from 5 to 18?m/h through the blanket depending upon the leachate injection rate. Only pressure transducers and thermistors were consistently able to detect migration of injected leachate once the blanket got saturated. Moisture content sensors could not register any change in readings once the blanket became saturated. Leachate injection pressure monitored over a period of about 12 months indicated no signs of clogging of the blanket. The leachate pressures measured immediately below the blanket were less than the net leachate injection pressure indicting that there was a head loss in the GDL blanket. Numerical modeling of liquid flow in the blanket indicated that predicted leachate travel in the blanket was consistent with the field data for assumed values of the waste hydraulic conductivity. In the absence of measured representative hydraulic properties of the waste, absolute verification of the field data was not possible.     

Depletion of Antioxidants from a HDPE Geomembrane in a Composite Liner

The results of two series of accelerated aging tests are reported. Both series of tests were conducted at temperatures of 85, 70, 55, and 26°C over a period of about 3?years. In the simulated liner series, the top of the geomembrane was covered with a geotextile (protection) layer that was exposed to simulated municipal solid waste (MSW) landfill leachate while the bottom of the geomembrane was in contact with a hydrated geosynthetic clay liner. In the immersion series, the geomembrane was immersed in the simulated MSW leachate, and hence, both sides were exposed to leachate. The results from oxidative induction time tests indicate that the antioxidant depletion is about 2.2–4.8 times faster for the leachate immersed geomembrane than for geomembrane in a composite liner. The higher rates are attributed to the higher extraction of antioxidants from two sides of the geomembrane immersed in leachate. The measured antioxidant depletion rates are extrapolated to a range of temperatures (0–60°C) using Arrhenius modeling. At a liner temperature of 35°C, the calculated time for the depletion of antioxidants is about 40?years for a geomembrane in a composite liner compared to 10?years if it is simply immersed in leachate. These tests suggest that to obtain realistic estimates of geomembrane service life one needs data from tests that simulate the expected field conditions and that prediction based on immersion tests may underestimate the service life.