Characterization of solid waste in a bioreactor landfill using seismic borehole methods

Abstract

The biodegradation of solid waste in bioreactor landfills is dependant on a number of factors including moisture content. Identification of relatively wet and dry areas throughout the landfill would help operators determine where moisture should be added or not. This study attempts to utilize seismic borehole techniques to identify changes in seismic wave velocity through the waste due to the addition of moisture. The study is based on the principle that P-wave velocity in a porous material increases with moisture content. The bioreactor landfill surveyed is located in Ste. Sophie, Quebec, Canada. Two vertical boreholes were installed 10 m apart and to a depth of 15 m, in order for vertical seismic profiles to be obtained. An initial characterization survey was conducted to understand seismic wave propagation through the waste. The P-wave velocity is on the order 290-370 m/s and the S-wave velocity is on the order of 140-175 m/s. Two-day surveys were conducted to note changes in P-wave velocity under relatively dry and wet conditions. There was no measurable change in P-wave velocity after the addition of approximately 7000 L of water between the boreholes. This is most likely due to that fact that it was not possible to sample the area between boreholes where change was most likely to occur and it was not possible to control exactly where the water was being added. Introduction The biodegradation of solid waste in bioreactor landfills is dependant on a number of factors including moisture content. The methane gas generated as a by-product is a valuable resource. Optimal moisture content promotes rapid waste degradation and maximizes methane production. Identification of relatively wet and dry areas throughout the landfill would help operators determine where moisture should be added or not. As a result, a more uniform biodegradation would occur throughout the landfill. If uniform biodegradation occurs, landfill mining also becomes a feasible option, in which valuable metal can be recovered and the degraded waste can be placed into a less engineered facility. Currently there is no efficient and accurate method to measure bulk moisture content in a bioreactor landfill (Catley et al., 2006). This study attempts to utilize seismic borehole techniques to identify changes in seismic wave velocity through the waste due to the addition of moisture. This study focuses on compressional waves (P-waves) and shear waves (S-waves). In porous media, P-wave velocity is dependant on a number of different factors including moisture content Yang (2002). Shear waves are mainly dependant on the density of the solid structure of the medium and are not affected by moisture content (Sheriff and Geldart, 1995). Figure 1, adapted from Yang (2002), shows the relationship between saturation and Pwave velocity for a clean sand. It illustrates that as the moisture content of the medium increases, the Pwave velocity will also increase. It is hypothesized that waste will behave in a similar fashion.