Saturated hydraulic conductivity of compacted steel slag-bentonite mixtures--A potential hydraulic barrier material of landfill cover.

The feasibility of using steel slag and bentonite mixtures to construct the hydraulic barrier of a landfill cover was explored in the present study. Fine-grained steel slag (SS; particle diameter < 1 mm) and sodium-activated calcium bentonite (SACB) were used to prepare compacted specimens, and the saturated hydraulic conductivity (ks) was measured using a flexible-wall permeameter. Influential factors including SACB content (BC), SS gradation, water-washing treatment of SS and compaction water content (ωcomp) were investigated. The hydraulic conductivity results were interpreted in microscopic scale through mercury intrusion porosimetry (MIP) and scanning electron microscope (SEM). It was found that when BC was below 10%, the ks value of the specimens prepared with well graded SS was about one order of magnitude lower than that of the specimens prepared with poorly graded SS. This was due to less macropores caused by better SS gradation. Yet, the effects of SS gradation on ks diminished as BC further increased to 15%, suggesting the dominant role of BC on ks at high BC. Water-washing treatment of SS helped reduce ks significantly to 1.2 × 10-10 m/s at BC of 10%, owing to less multivalent cations and hence lower osmotic swelling reduction caused by cations. Controlling ωcomp 1-2% wetter than the optimum water content (ωopt) also helped reduce ks significantly, owing to the reduction of macropores. Accordingly, it is suggested to use well-graded SS mixed with 10% SACB and then compact at ωcomp slightly wetter than ωopt to the degree of compaction greater than 90% in engineering practice.

Biochemical-thermal-hydro-mechanical coupling model for aerobic degradation of landfilled municipal solid waste.

Ventilating solid waste landfills with an oxygen supply can effectively accelerate the degradation of waste, achieve rapid stabilization, and realize the sustainable utilization of landfills. Aiming to understand and verify the aerobic degradation process in landfills, this paper proposed a biochemical-thermal-hydro-mechanical coupling model. The model considers aerobic biochemical reactions, dissolved solute migration, heat transport, two-phase flow, and skeleton deformation. The model was verified by comparison with an in-situ experiment at Jinkou landfill. The results showed the model could accurately represent the observed degradation phenomena during the experiment. The modelling results indicated that the rate of temperature increase and peak temperature of the upper layer, which were lower than those of the middle layer, were affected by heat exchange at the landfill surface. The lowest temperatures occurred near the bottom because of high water content and low oxygen concentrations. The high temperature zone migrated out from the injection well during degradation, reflecting the degradation of degradable organic matter associated with oxygen diffusion rates and aerobic degradation reactions. The initial accumulated settlement value was fast, but slowed and finally stabilized. The surface subsidence also developed from the center around the injection well to the surrounding area, and 70% of the total subsidence occurred within 150 days. This newly developed model provides a theoretical framework for analyzing the multi-field coupling of aerobic degradation of landfilled municipal solid waste (MSW).

Effects of thermal boundary condition on methane oxidation in landfill cover soil at different ambient temperatures.

Microbial aerobic methane oxidation (MAMO) has been considered as an environmental-friendly method for mitigating methane emission from municipal landfill sites. Soil column has in a landfill cover under one-dimensional (1-D) condition. However, most of the published soil column tests failed to simulate 1-D heat transfer due to the use of thermal conductive boundary at the sidewall. In the present study, a heavily instrumented soil column was developed to quantify the effects of thermal boundary condition on the methane oxidation efficiency under different ambient temperatures in landfill cover soil. The sidewall of the soil column was thermally insulated to ensure 1-D heat transport as would have been typically expected in the field condition. Two soil column tests with and without thermal insulation were conducted at a range of controlled ambient temperatures from 15 to 30°C, for studying how soil moisture, matric suction, gas pressure, soil temperature and gas concentration evolve with MAMO. The test results reveal that ignoring thermal insulation in a soil column test would result in a greater loss of soil heat generation by MAMO and hence oxidation efficiency by up to 100% for the range of temperature considered. When the ambient temperature increased to 30°C (but less than the optimum temperature for MAMO), the MAMO efficiency increased abruptly at first but then decreased substantially with time, and this is likely due to the accumulation of biomass generated by MAMO.