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.
Highlights
Geomembranes have been extensively used in several applications, such as landfill liner systems [1,2], water reservoirs [3] and mining facilities [4], to assist with the degradation of wastes, protect groundwater, provide a barrier to stop the diffusion of contaminants, contain or harvest gaseous emissions, or a combination of any of these
It seems reasonable to expect that the change in the cooling constant that is predicted by the current modelling assumptions is representative of what would be measured in practice and, that monitoring the cooling constant should constitute a promising approach for a quantitative characterisation of scum accumulation beneath the floating covers at the Western Treatment PlantPlant (WTP)
The physical size of the floating geomembrane covers at the anaerobic sewage treatment lagoons operthe floating High-density polyethylene (HDPE) geomembrane covers at the anaerobic sewage treatment lagoons operthebycover m) and the hazardous operating environment demand an area ated the
Summary
Geomembranes have been extensively used in several applications, such as landfill liner systems [1,2], water reservoirs [3] and mining facilities [4], to assist with the degradation of wastes, protect groundwater, provide a barrier to stop the diffusion of contaminants, contain or harvest gaseous emissions, or a combination of any of these. In contrast frame of a thermal image, this work explores the use of a cooling constant derived from to conventional thermography, which relies on the temperature contrast within a single. Newton’s law of cooling for the purpose of monitoring the state and extent of scum accuframe of a thermal image, this work explores the use of a cooling constant derived from mulation. Newton’s law of cooling for the purpose of monitoring the state and extent of scum higher temperature than ambient. It is shown that the FE model predicvalidating In thewell second this validated measurements, FE methodologythereby is used tions of thethe thermal transients agree withpart, the experimental to investigate the variation in the cooling constant due to a progressive change in the thervalidating the FE methodology. Thermal properties of simulated scum, and the implications of the results for practical implementation are discussed
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