Long-term moisture measurements in large-scale bioreactor cells using TDR and neutron probes

Use of electrical resistivity tomography for detecting the distribution of leachate and gas in a large-scale MSW landfill cell

In this study, integrate electrical resistivity tomography (ERT) tests were carried out in a large-scale (5.0 × 4.0 × 7.5 m) MSW landfill cell to investigate the possibility of detecting perched leachate mounds, leachate level, and gas accumulation zones at wet landfills. The resistivity of both bulk waste and waste components at different moisture states were measured and the three-phase volumetric relationships of the waste pile were analyzed to better interpret the ERT test results in the large-scale cell. The following observations were given: (1) The relationship between resistivity and volumetric moisture content (VMC) of waste sample can be reasonably fitted by Archie's law. The resistivity of waste components at a saturated state was all lower than 21 Ω m. (2) A significant amount of void gas was entrapped in the underwater waste, being 30.4-34.8% of the whole waste pile in volume. (3) Low-resistivity zones (< 5.0 Ω m) were observed in the waste pile being fully drained under a gravity condition, which was believed to be related to a perched leachate. (4) The average VMC values of the waste layer below and above the leachate level were in the ranges of 46.5-53.1% and 28.1-41.3%, respectively. (5) Irregular variations of high-resistivity zones (> 40 Ω m) observed in the underwater waste were associated with the accumulation and dissipation of gas pressure. It was found that the "gas-breaking value" in the gas accumulation zone was up to 10.5 kPa greater than the pore liquid pressure in the stable methanogenesis stage. These findings shone a light on the possibility of using the ERT method as an efficient tool for mapping the gas/leachate distribution and improving operations at wet landfills.

Influence of dynamic coupled hydro-bio-mechanical processes on response of municipal solid waste and liner system in bioreactor landfills

A two-dimensional (2-D) mathematical model is presented to predict the response of municipal solid waste (MSW) of conventional as well as bioreactor landfills undergoing coupled hydro-bio-mechanical processes. The newly developed and validated 2-D coupled mathematical modeling framework combines and simultaneously solves a two-phase flow model based on the unsaturated Richard's equation, a plain-strain formulation of Mohr-Coulomb mechanical model and first-order decay kinetics biodegradation model. The performance of both conventional and bioreactor landfill was investigated holistically, by evaluating the mechanical settlement, extent of waste degradation with subsequent changes in geotechnical properties, landfill slope stability, and in-plane shear behavior (shear stress-displacement) of composite liner system and final cover system. It is concluded that for the given specific conditions considered, bioreactor landfill attained an overall stabilization after a continuous leachate injection of 16years, whereas the stabilization was observed after around 50years of post-closure in conventional landfills, with a total vertical strain of 36% and 37% for bioreactor and conventional landfills, respectively. The significant changes in landfill settlement, the extent of MSW degradation, MSW geotechnical properties, along with their influence on the in-plane shear response of composite liner and final cover system, between the conventional and bioreactor landfills, observed using the mathematical model proposed in this study, corroborates the importance of considering coupled hydro-bio-mechanical processes while designing and predicting the performance of engineered bioreactor landfills. The study underscores the importance of considering the effect of coupled processes while examining the stability and integrity of the liner and cover systems, which form the integral components of a landfill. Moreover, the spatial and temporal variations in the landfill settlement, the stability of landfill slope under pressurized leachate injection conditions and the rapid changes in the MSW properties with degradation emphasizes the complexity of the bioreactor landfill system and the need for understanding the interrelated processes to design and operate stable and effective bioreactor landfills. A detailed discussion on the results obtained from the numerical simulations along with limitations and key challenges in this study are also presented.

Quantification of parameters influencing methane generation due to biodegradation of municipal solid waste in landfills and laboratory experiments

The energy conversion potential of municipal solid waste (MSW) disposed of in landfills remains largely untapped because of the slow and variable rate of biogas generation, delayed and inefficient biogas collection, leakage of biogas, and landfill practices and infrastructure that are not geared toward energy recovery. A database consisting of methane (CH4) generation data, the major constituent of biogas, from 49 laboratory experiments and field monitoring data from 57 landfills was developed. Three CH4 generation parameters, i.e., waste decay rate (k), CH4 generation potential (L0), and time until maximum CH4 generation rate (tmax), were calculated for each dataset using U.S. EPA's Landfill Gas Emission Model (LandGEM). Factors influencing the derived parameters in laboratory experiments and landfills were investigated using multi-linear regression analysis. Total weight of waste (W) was correlated with biodegradation conditions through a ranked classification scheme. k increased with increasing percentage of readily biodegradable waste (Br0 (%)) and waste temperature, and reduced with increasing W, an indicator of less favorable biodegradation conditions. The values of k obtained in the laboratory were commonly significantly higher than those in landfills and those recommended by LandGEM. The mean value of L0 was 98 and 88L CH4/kg waste for laboratory and field studies, respectively, but was significantly affected by waste composition with ranges from 10 to 300L CH4/kg. tmax increased with increasing percentage of biodegradable waste (B0) and W. The values of tmax in landfills were higher than those in laboratory experiments or those based on LandGEM's recommended parameters. Enhancing biodegradation conditions in landfill cells has a greater impact on improving k and tmax than increasing B0. Optimizing the B0 and Br0 values of landfilled waste increases L0 and reduces tmax.

Impact assessment of intermediate soil cover on landfill stabilization by characterizing landfilled municipal solid waste

Waste samples at different depths of a covered municipal solid waste (MSW) landfill in Beijing, China, were excavated and characterized to investigate the impact of intermediate soil cover on waste stabilization. A comparatively high amount of unstable organic matter with 83.3 g kg(-1) dry weight (dw) total organic carbon was detected in the 6-year-old MSW, where toxic inorganic elements containing As, Cd, Cr, Cu, Mn, Ni, Pb, and Zn of 10.1, 0.98, 85.49, 259.7, 530.4, 30.5, 84.0, and 981.7 mg kg(-1) dw, respectively, largely accumulated because of the barrier effect of intermediate soil cover. This accumulation resulted in decreased microbial activities. The intermediate soil cover also caused significant reduction in moisture in MSW under the soil layer, which was as low as 25.9%, and led to inefficient biodegradation of 8- and 10-year-old MSW. Therefore, intermediate soil cover with low permeability seems to act as a barrier that divides a landfill into two landfill cells with different degradation processes by restraining water flow and hazardous matter.

Experimental and theoretical assessment of the multi-domain flow behaviour in a waste body during leachate infiltration

The optimisation of landfill operation is a key challenge for the upcoming years. A promising solution to improve municipal solid waste (MSW) management is the bioreactor technology. A meso-scale (around 1m(3)) experimental set-up was performed to study the effect of moisture control in low density conditions with different leachate injection operations and bioreactor monitoring including the use of a neutron probe. The moisture content distribution evolution demonstrates a multi-domain flow behaviour. A classic van Genuchten-Mualem description of the connected porosity proved insufficient to correctly describe the observed phenomena. A bimodal description of the connected porosity is proposed as solution and a connected/non-connected porosities numerical model was applied to the results. The model explains the experimental results reasonably well.

Contribution of 3-D time-lapse ERT to the study of leachate recirculation in a landfill

Leachate recirculation is a key process in the operation of municipal waste landfills as bioreactors. It aims at increasing the moisture content to optimise the biodegradation. Because waste is a very heterogeneous and anisotropic porous media, the geometry of the leachate plume recirculation is difficult to delineate from the surface at the scale of the bioreactor site. In this study, 3-D time-lapse electrical resistivity tomography (ERT) was used to obtain useful information for understanding leachate recirculation hydrodynamics. The ERT inversion methodology and the electrode arrays were optimised using numerical modelling simulating a 3-D leachate injection scenario. Time-lapse ERT was subsequently applied at the field scale during an experimental injection. We compared ERT images with injected volumes to evaluate the sensitivity of time-lapse ERT to delineate the plume migration. The results show that time-lapse ERT can accomplish the following: (i) accurately locate the injection plume, delineating its depth and lateral extension; (ii) be used to estimate some hydraulic properties of waste.