Quantifying the effect of settlement and gas on solute flow and transport through treated municipal solid waste

The effect of degradation and settlement on transport properties of mechanically and biologically treated (MBT) waste was examined by applying three different tracers to two waste columns (~0.5 m diameter) in a series of closed-loop experiments. One column was allowed to biodegrade and the other was bio-suppressed. Permeability and drainable porosity were reduced by settlement, in line with previous results. A dual-porosity model performed well against the data and suggested that more preferential flow occurred early on in the un-degraded column. Diffusion timescales were found to be between 0.8 and 6 days. Volumetric water contents of the mobile region were found to be small in the bio-suppressed cell (~0.01) and even smaller values were found in the degrading waste, possibly due to displacement by gas. Once either settlement or gas production had disrupted this pattern into a more even flow, subsequent compression made little difference to the diffusion time-scale. This may indicate that transport was thereafter dominated by other aspects of the waste structure such as the distribution of low-permeability objects. The presence of gas in the degrading waste reduced the volumetric water content through displacement. The model indicated that the gas was primarily located in the more mobile porosity fraction. Primary compression of the degrading waste tended to squeeze this gas out of the waste in preference to water.