Thermographic Monitoring of Scum Accumulation beneath Floating Covers

Abstract

Large sheets of high-density polyethene geomembrane are used as floating covers on some of the wastewater treatment lagoons at the Melbourne Water Corporation’s Western Treatment Plant. These covers provide an airtight seal for the anaerobic digestion of sewage and allow for harvesting the methane-rich biogas, which is then used to generate electricity. There is a potential for scum to develop under the covers during the anaerobic digestion of the raw sewage by microorganisms. Due to the nature of the operating environment of the lagoons and the vast size (450 m × 170 m) of these covers, a safe non-contact method to monitor the development and movement of the scum is preferred. This paper explores the potential of using a new thermographic approach to identify and monitor the scum under the covers. The approach exploits naturally occurring variations in solar intensity as a trigger for generating a transient thermal response that is then fitted to an exponential decay law to determine a cooling constant. This approach is investigated experimentally using a laboratory-scale test rig. A finite element (FE) model is constructed and shown to reliably predict the experimentally observed thermal transients and cooling constants. This FE model is then set up to simulate progressive scum accumulation with time, using a specified scumberg geometry and a stepwise change in thermal properties. The results indicate a detectable change in the cooling constant at different locations on the cover, thereby providing a quantitative basis for characterising the scum accumulation beneath the cover. The practical application and limitations of these results are briefly discussed.

Structural health and performance monitoring of floating cover at the western treatment plant: Our chronology

“…Figure 6 shows the actual soil extent and the predicted region when analysing the thermal response over the membrane. A series of finite element analyses were presented in [16] to show how this technique can be extended to determine the state of scum formation beneath the cover.…”

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

“…Here, a cover-walk inspection is the current method to determine the extent and state of scum under the covers. In this method, an experienced operator qualitatively classifies the scum as "fluffy, soft, medium" or "hard" by walking on the cover and "feeling" its response to the impact of their footsteps [11]. A quantitative and objective method will provide a more useful, valuable, accurate, consistent, and automatable scum assessment procedure.…”

“…Recently, in the collaboration between MW Corporation and the research team from Monash University Acoustic Laboratory, efforts have been made to deal with the problem of how to quantitatively determine the extent and state of the scum under the covers. Several non-contact scum assessment methods with different approaches including thermography [11,12] and photogrammetry [8,13] have already been developed. This paper on the other hand demonstrates the potential additional benefits of a technique using mechanical vibrations to characterise the state of scum underneath the cover.…”