High-density polyethylene (HDPE) is commonly the material of choice for covered anaerobic lagoons (CAL) at wastewater treatment plants. The membrane floats on the wastewater, and hence is called a “floating cover”, and is used for odour control and to harvest the methane-rich biogas as a renewable resource to generate electricity. The floating cover is an expensive and high-value asset that demands an efficient methodology for the determination of a set of engineering quantities for structural integrity assessment. Given the dynamics of the anaerobic activities under the floating cover, the state of deformation of the floating cover is an engineering measurand that is useful for its structural health assessment. A non-contact measurement strategy is preferred as it offers practical and safety-related benefits over other methods. In collaboration with Melbourne Water Corporation (MWC), an unmanned aerial vehicle (UAV) assisted photogrammetry approach was developed to address this need. Following the definition of the appropriate flight parameters required to quantify the state of deformation of the cover, a series of periodic flights were operated over the very large floating covers at MWC’s Western Treatment Plant (WTP) at Werribee, Victoria, Australia. This paper aims to demonstrate the effectiveness and practicality of this inspection technique to determine the state of deformation of the floating covers measured over a ten-month period.

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.

High-density polyethylene geomembranes are employed as covers for the sewage treatment lagoons at Melbourne Water Corporation’s Western Treatment Plant, to harvest the biogas produced during anaerobic degradation, which is then used to generate electricity. Due to its size, inspecting the cover for defects, particularly subsurface defects, can be challenging, as well as the potential for the underside of the membrane to come into contact with different substrates, viz. liquid sewage, scum (consolidated solid matter), and biogas. This paper presents the application of a novel quasi-active thermography inspection method for subsurface defect detection in the geomembrane. The proposed approach utilises ambient sunlight as the input thermal energy and cloud shading as the trigger for thermal transients. Outdoor laboratory-scale experiments were conducted to study the proposed inspection technique. A pyranometer was used to measure the intensity of solar radiation, and an infrared thermal camera was used to measure the surface temperature of the geomembrane. The measured temperature profile was analysed using three different algorithms for thermal transient analysis, based on (i) the cooling constant from Newton’s law of cooling, (ii) the peak value of the logarithmic second derivative, and (iii) a frame subtraction method. The outcomes from each algorithm were examined and compared. The results show that, while each algorithm has some limitations, when used in combination the three algorithms could be used to distinguish between different substrates and to determine the presence of subsurface defects.

High-density polyphenylene geomembranes are utilized as large-scale floating covers in anaerobic water treatment plants for the purpose of collecting renewable biogas energy and preventing the emission of odorous by-products. During degradation, the accumulation of scum, the presence of biogas pockets and sewage can negatively impact the structural integrity of the covers and hinder degradation efficiency. Traditional methods are insufficient in identifying the substances under the covers. In this study, a thermography technique was developed and tested in lab-scale experiments using ambient weather information to inspect the geomembranes. The technique was then applied to a remote monitoring investigation of large-scale floating covers located at a sewage treatment plant. Long-term monitoring of two regions on the covers was conducted using a thermal camera, with ambient weather information recorded to help analyze temperature changes. The acquired thermal image sequences were analyzed, resulting in the creation of a segmented cover map indicating the profiles of substances. The results of the on-site thermal imaging field trials verify the effectiveness of the developed remote sensing thermography technique, which has the potential to assist in future asset maintenance, remove scum efficiently and enhance anaerobic degradation efficiency at sewage water treatment plants.

Floating covers are examples of a large membrane structure used at sewage treatment plants. At the Western Treatment Plant (WTP), Werribee, Melbourne, Australia, floating covers are used in the anaerobic lagoons. They are deployed to assist with the anaerobic treatment of the raw sewage beneath, to harness the methane-rich biogas generated, and for odor control. In this respect, these floating covers are important assets for harnessing a sustainable and renewable energy source, as well as protecting the environment from the release of the damaging greenhouse methane-rich biogas from the treatment plant. Given the continuous nature of the biological process beneath the cover, the forces imposed on the floating cover will change with time. Hence, the monitoring and the assessment of the structural integrity of the floating cover are of paramount importance. These floating covers are made from high-density polyethylene (HDPE), a polymeric material. The size of these covers, the hazardous environment, and the expected life span demand a novel, remotely piloted, unmanned aerial vehicle based noncontact technique for the structural health assessment. This assessment methodology will utilize photogrammetry as the basis for determining the surface deformation of the membrane. This paper reports on an experimental study to determine the flight parameters and to assess the accuracy of the measurement technique. It was conducted over an area having similar dimensions to the large covers at the WTP. There are also features in this area, which are of similar scale to those expected in the floating cover. A total of nine test flights were used to investigate the parameters for optimal definition of the significant features to describe the deformation of the floating cover. The findings inform the selection of the unmanned aerial vehicle assisted photogrammetry parameters for optimal flight altitude, photogrammetry image overlap, and flight grid path for future integrity assessment of the floating covers. Two trial flights at WTP are also discussed to demonstrate the effectiveness of this noncontact technique for the future structural health assessment and in assisting with the operation of this large high-value asset.

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.