Abraham Chung Fai Chiu

Leachate Breakthrough Mechanism and Key Pollutant Indicator of Municipal Solid Waste Landfill Barrier Systems: Centrifuge and Numerical Modeling Approach

Groundwater pollution by leachate leakage is one of the most common environmental hazards associated with municipal solid waste (MSW) landfill sites. However, landfill leachate contains a large variety of pollutants with widely different concentrations and biotoxicity. Thus, selecting leachate pollutant indicators and levels for identifying breakthrough of barrier systems are key factors in assessing their breakthrough times. This study investigated the transport behavior of leachate pollutants through landfill barrier systems using centrifuge tests and numerical modeling. The overall objective of this study is to investigate breakthrough mechanism to facilitate the establishment of a consistent pollutant threshold concentration for use as a groundwater pollution alert. The specific objective of the study is to identify which pollutant and breakthrough threshold concentration should be used as an indicator in the transport of multiple pollutants through a landfill barrier system. The threshold concentration from the Chinese groundwater quality standards was used in the analysis of the properties of leachates from many landfill sites in China. The time for the chemical oxygen demand (COD) to reach the breakthrough threshold concentration at the bottom of a 2m compacted clay liner was 1.51years according to centrifuge tests, and 1.81years according to numerical modeling. The COD breakthrough times for single and double composite liners were within the range of 16 and 36.58years. Of all the pollutants, COD was found to consistently reach the breakthrough threshold first. Therefore, COD can be selected as the key indicator for pollution alerts and used to assess the environmental risk posed by MSW landfill sites.

State Boundary Surfaces for an Aged Compacted Clay

The yielding and the peak strength of an aged compacted clay were studied by conducting a series of suction-controlled triaxial tests. The test results were interpreted using the framework of intrinsic properties of reconstituted soil. The peak strength envelopes of undisturbed samples lie above those of reconstituted samples. The suction provides additional attractive forces to stabilize the soil structure, which result in the augmentation of the yield stress and peak strength envelope. The shear strength is normalized by the equivalent preconsolidation pressure ( pe′ ) and Hvorslev surfaces are identified from undisturbed samples which expand with suction. A single peak strength envelope and Hvorslev surface will be emerged from the saturated and unsaturated (degree of saturation >80% ) samples if the shear strength data are presented in terms of the average skeleton stress. The influence of the soil structure on the shear strength of the aged compacted clay may be measured by the ratio of normalized st...

A steady-state analytical profile method for determining methane oxidation in landfill cover

Gas concentration profiles of carbon dioxide (CO2), oxygen (O2), methane (CH4) and nitrogen (N2) are usually measured during tests investigating microbial aerobic methane oxidation in landfill cover. However, only qualitative/limited information can be obtained from gas concentration profiles by existing methods. A new method is proposed to determine methane oxidation in soil quantitatively and comprehensively, including methane oxidation efficiency, stoichiometry, gas transfer mechanism, methane generation rate and gas reaction rate distributions. Governing equations are established based on mass balance for O2, CO2, CH4 and N2 at one-dimensional and steady-state condition. Gas transfer mechanisms considered include gas diffusion, advection and gas reaction. The method utilizes gas concentration profiles to determine gas diffusion for each gas component according to Ficks law. Then gas advections and reactions can be determined by mass balance. The method is validated by (i) published soil column tests investigating methane oxidation and (ii) a calibrated numerical model based on a selected soil column test. The new method is capable of determining methane oxidation efficiency, stoichiometry, gas transfer mechanism, methane generation rate and gas reaction rate distributions for CH4, CO2 and O2.