Evaluating the dual porosity of landfilled municipal solid waste

The objective of this paper was to study the pore-size distribution of municipal solid waste (MSW) and provide a basis for understanding the mechanism of preferential flow in MSW. Two methods were used to investigate the pore-size distribution in MSW. The first was an indirect method based on the soil-water characteristic curve (SWCC) and the second was a direct method using nuclear magnetic resonance (NMR). Samples taken from different depths of a landfill were used. In the SWCC method, SWCCs of the matrix region were obtained by a pressure plate extractor and SWCCs of the macropore region were determined by water breakthrough tests. A SWCC equation is proposed by modifying the Van Genuchten equation to consider the dual-porosity feature of MSW and the pore-size distribution was obtained based on the Young-Laplace equation. In the NMR method, the pore-size distribution of MSW was obtained by analyzing the T₂ curves. The results of the two methods were qualitatively similar and both showed a dual-porosity characteristic of MSW. The average macropore radii of shallow, middle, and deep MSWs obtained from the SWCC method are 0.193, 0.184, and 0.173 mm, and those obtained from the NMR method are 0.213, 0.138, and 0.145 mm, respectively. The proportion of macropores decreases with depth. The average micropore radii given by the SWCC method are 0.022, 0.011, and 0.008 mm, and those given by the NMR method are 0.013, 0.011, and 0.008 mm, respectively. As the depth and fill age increase, the average micropore size becomes smaller and the proportion of micropores increases. The volume ratios of macropores obtained by the two methods are quantitatively quite different. The discrepancy is mainly caused by the different test principles adopted by the two methods.