Air displacement tests of municipal solid waste in a large-scale model

Air displacement technology, which injects high-pressure air to drive pore liquid, is widely used in geoenvironmental engineering. This study explores its application to dewater landfilled waste to facilitate waste excavation and subsequent screening operations. Large-scale model tests were conducted by sequentially applying air pressure of 10 and 20 kPa by way of a vertical well into a waste pile initially at field capacity. The temporal and spatial distribution of pore gas pressure and volumetric water content were monitored. Results showed average volumetric water content decreased from 62 to 41% after 10 kPa air injection, and further to 27% after 20 kPa air injection. Higher air pressure significantly enhanced dewatering, increasing the specific dewatered area from 6·6 to 10·2 m2. Asymmetric variations in pore gas pressure and volumetric water content around the injection well revealed high heterogeneity of the waste pile. The decay rate of pore gas pressure along the horizontal distance in waste was 1·2 kPa/m. The proposed dual-porosity model for two-phase flow can better simulate transient drainage volume, pore gas pressure, volumetric water content and air flow rate compared to the single-porosity model. In addition, the effects of air pressure, heterogeneity and anisotropy on displacement efficiency were investigated.