Moisture retention and near saturated flow in Mechanically Biologically Treated (MBT) waste

Abstract

The aim of this research is to understand the interactions between liquid and gas flow at high degrees of saturation, as this could have a significant effect on the effectiveness of landfill remediation by flushing. Particular attention is paid to two key parametric functions that are believed to control the simultaneous flow of leachate and gas in waste materials. These are the relationship between capillary pressure or suction and the degree of leachate saturation or volumetric moisture content, known as the moisture retention characteristic; and the relationship between unsaturated hydraulic conductivity and moisture content.

The thesis starts with a review of previous work on moisture retention characteristic curves and relative permeability functions for waste materials. New data from a drainage column experiment, pressure plate apparatus and hanging water column tests on mechanically and biologically treated (MBT) waste specimens are then presented and compared. The results from the drainage experiments have been interpreted using the unsaturated flow model HYDRUS-1D (Šim?nek et al., 2005) and the University of Southampton Landfill Degradation and Transport model LDAT (White et. al 2004). These results give support to the modelling concepts and the integrity of the code for both models and highlight the capabilities of single and dual porosity models.

Use of the van Genuchten (1980) type curves to represent the moisture retention characteristics was found to be fruitful. The values of the parameters that control the shapes of these curves, and therefore the shape of the relationships between capillary pressure and moisture content and unsaturated hydraulic conductivity and moisture content, are reviewed using both results from the literature and results from the experimental work described in the thesis. The sensitivity of the shapes to the values of the parameters is examined as is the sensitivity of the results of numerical modelling that is based on the parameter values. The relationship of the parameter values to the material dry density is also explored. Whilst the flow in the gas phase is not central to the subject of the thesis, it has been possible to make observations on the relationship between unsaturated gas permeability and moisture content.

The thesis draws attention to the fact that different experimental techniques can lead to significantly different estimates of the moisture retention characteristics. Hanging column tests show an apparently sensible variation of moisture retention curve with density and are self-consistent. However a partial or full interruption of the liquid phase within the specimen, or between the specimen and the hanging column porous plate will inhibit the drainage of liquid from the specimen, resulting in an increase in the retained moisture content at a given applied external suction. This has significant implications for the study of liquid movement in unsaturated wastes. A key recommendation from the work is that the moisture retention characteristic curve for a waste is perhaps better determined from direct measurements of suction and moisture content, as in the drainage column apparatus for the suction range 0-10 kPa. For higher suctions carefully set up pressure plate tests are advisable.

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ORCID for William Powrie: orcid.org/0000-0002-2271-0826

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