Effect of Changing Unit Weight and Decomposition on Unsaturated Hydraulics of Municipal Solid Waste in Bioreactor Landfills

Abstract

The effects of compression and decomposition on the water retention curve (WRC) and unsaturated hydraulic conductivity (Kθ) of municipal solid waste (MSW) were evaluated in the laboratory and field. Laboratory measurements of the WRC and Kθ were made using the multistep outflow (MSO) method. Numerical inversion of data collected during the Deer Track Bioreactor Experiment (DTBE) was used to estimate field-scale unsaturated hydraulic properties. For the MSW tested in this study, data indicated that increasing dry unit weight resulted in an increase of the air-entry suction. Increasing levels of decomposition had the opposite effect, reducing air-entry suction for MSW at constant dry unit weight. Multiple mechanisms likely resulted in the observed changes in air-entry suction, including reduction in pore size during compression and changes in overall MSW hydrophobicity during decomposition. At field-scale, air-entry suction tended to remain constant as MSW compressed and decomposed simultaneously indicating that the two processes, which had opposite effects in the laboratory, also may have opposing effects the field. For fresh MSW, van Genuchten’s n, which is inversely related to the variance of the pore size of a porous medium, ranged between 1.3 and 2.5 and decreased as the dry unit weight increased in laboratory-scale tests. For decomposed MSW, n ranged from 1.1 to 1.3 and did not vary systematically with dry unit weight. This indicated that for degraded MSW, the variance of the pore-size distribution did not change substantially with increasing dry unit weight. The value of Kθ decreased with increasing dry unit weight for MSW in this study, consistent with observations for saturated hydraulic conductivity in MSW. However, Kθ tended to increase in laboratory tests with increasing levels of decomposition at constant dry unit weight, consistent with observed increases in void ratio. At field scale, observed changes in Kθ varied consistently with changes in saturated hydraulic conductivity as decomposition and densification occurred simultaneously.