Testbed assessment of wave propagation analysis: Towards scum characterisation in covered anaerobic lagoon

Abstract. The assessment of the state and extent of scum conditions under floating covers is crucial for operational management and structural health monitoring of wastewater treatment plants. Advancing beyond our previous research that employed a single accelerometer to examine the covered scum’s frequency response, we propose a more comprehensive approach using wave propagation analysis with an array of sensors to gain a deeper understanding of the complex cover-scum system. Traditional wave propagation analysis applications typically target either large length scale, low-frequency scenarios as seen in seismic surface waves analysis, or small length scale, high-frequency scenarios as observed in ultrasonic guided waves analysis. However, in anticipation of a mid-scale of sub-metre depth and sub-hundred hertz frequency range for early-stage scum in covered anaerobic lagoons, we established a water bladder testbed to evaluate the adaptability of the multi-sensor approach. In this study, we deployed 16 accelerometers on the bladder filled with water to three distinct water levels and used a durometer to gauge the shore hardness. We applied impulse excitations, recorded the wavefields and examined dispersion diagrams. Our findings showed that the wave was dispersive across all tested shore hardness levels, with the phase velocity decreasing as the frequency increased. Notably, a higher shore hardness corresponded to a higher phase velocity which can be clearly differentiated on the dispersion diagrams. By selecting different subsets of accelerometers, we further explore the impact of sensor array length and source offset on the quality of the analysis. Our results confirmed that the sensor array should be sufficiently close to the excitation source to capture high-frequency component information, which attenuates rapidly over distance. For a given number of sensors, optimising the sensor pitch is important to maintain an optimal balance between wavenumber resolution and maximum resolvable wavenumber. These insights could inform the design and deployment of sensor arrays to characterise varying hardness levels of scum. This research offers potential for monitoring and assessing the state of this scum and enhancing our understanding and management of large-scale wastewater treatment lagoons covered with floating covers.