A Mechanical method of classifying the state of solid matter beneath a floating cover over an anaerobic lagoon

Abstract

Abstract. The formation and accumulation of scum—a layer formed from fats, oils and greases (FOG) and solid particles in the sewage—can occur under the geomembrane of a covered anaerobic wastewater treatment lagoon. Monitoring the state of scum is important for both operational reasons in the management of a wastewater treatment plant and for physical reasons related to the structural health assessment and monitoring of the covers. This task is challenging and currently involves an inspector physically assessing the scum through various sampling ports across the membrane. Another more subjective and less expensive method is where an experienced assessor classifies the state of the scum by walking on the cover and “feeling” its response to the impact of their footsteps. The development of a new objective, rather than the current subjective, approach to assessing the state and extent of the scum under the covers is proposed. In collaboration with Melbourne Water, we investigate an in-situ mechanical method to quantitatively assess the state of this scum. In this study, the frequency response functions of the membrane-scum system of different states of scum excited using a low-energy mechanical excitation were estimated. Our findings indicate that the high-gain frequency range of the cover-scum system is higher with the scum of a harder state ranging from 30-40 Hz for soft-to-fluffy scum up to 60-80 Hz for hard scum. Our findings also indicate that the response of the cover-scum system is highly damped. Differences between different states of scum have been observed in both the gain and the phase angle of the frequency response functions. Numerically, the area under the frequency response function curves decreases with the hardness of scum. This information paves the way for the next steps of this work, including improving the accuracy in modelling the scum and developing a mechanical means to quantitatively monitor the scum formation.