Characterization of Waste Density in a Bioreactor Landfill via Microgravity and Settlement Analysis
Optimizing the utilization of landfill airspace and production of biogas for use as an energy source relies on an understanding of the compaction, settlement, and stabilization of waste over time. A higher degree of compaction reduces the landfill footprint; however, it may not provide the optimal conditions for bacteria development and waste stabilization. This article reports on a research project piloting the use of repeated microgravity surveys to characterize variations in waste density in space and time in a bioreactor landfill. Over the duration of three years, four microgravity surveys were conducted on a new cell at a bioreactor landfill in Sainte-Sophie, Quebec, Canada, as it was gradually filled with waste up to a height of 25 m. After each survey, waste density values were extracted by comparing current gravity data with previous data from lower elevations. For accurate density computations, a settlement model was applied to estimate the degree to which waste had settled over time. This model was derived from settlement data recorded by two gauges buried in the waste column. The density values obtained from the microgravity surveys were 1.35 ± 0.31 g/cm 3 in the first waste slab (between surveys 1 and 2), 1.44 ± 0.22 g/cm 3 and 1.20 ± 0.25 g/cm 3 in the second waste slab in the northeast and southwest ends of the cell, respectively (between surveys 1 and 3). In these cases, the microgravity and conventional air utilization factor (AUF) waste densities agreed within 3% when settlement gauges were within 90 m from the survey area. For the third waste slab (between surveys 1 and 4), the densities derived from microgravity decreased to 1.15 ± 0.28 g/cm 3 and 1.12 ± 0.15 g/cm 3 in the northeast and southwest ends of the cell, respectively. This decrease might be associated with water drainage.