Quasi-Active Thermal Imaging of Large Floating Covers Using Ambient Solar Energy

Abstract

Melbourne Water Corporation has two large anaerobic lagoons at the Western Treatment Plant (WTP), Werribee, Victoria, Australia. The lagoons are covered using numerous sheets of high-density polyethylene (HDPE) geomembranes to prevent the emission of odorous gases and to harness biogas as a source of renewable energy. Some of the content of raw sewage can accumulate and form into a solid mass (called “scum”). The development of a large body of solid scum that rises to the surface of the lagoon (called “scumbergs”) deforms the covers and may affect its structural integrity. Currently, there is no method able to effectively “see-through” the opaque covers to define the spread of the scum underneath the cover. Hence, this paper investigates a new quasi-active thermal imaging method that uses ambient solar radiation to determine the extent of the solid matter under the geomembrane. This method was devised by using infrared thermography and a pyranometer to constantly monitor the transient temperature response of the HDPE geomembrane using the time varying ambient solar radiation. Newton’s cooling law is implemented to define the resultant cooling constants. The results of laboratory-scale tests demonstrate the capability of the quasi-active thermography to identify the presence and the extent of solid matter under the cover. This paper demonstrates, experimentally, the importance of measuring the surface temperature of the cover and solar intensity profiles to obtain the cooling process when during variations in solar intensity during normal sunrise, sunset, daily transitioning from morning–afternoon–evening and cloud cover events. The timescale associated with these events are different and the results show that these daily transient temperature cycles of the geomembranes can be used to detect the extent of the accumulation of solid matter underneath the geomembrane. The conclusions from this work will be further developed for field trials to practically monitor the growth in the extent of the scum under the floating covers in WTP with the ambient solar energy.