Author(s): Manuel Regueiro-Picallo; Jose Anta; Acacia Naves; Jorg Rieckermann

Linked Author(s): Jörg Rieckermann

Keywords: Temperature sensors; Sewer sediments; Sensing; Sediment accumulation; Urban drainage systems

Abstract:

Sediment accumulation in urban drainage systems (UDS) is a serious problem, which requires costly maintenance and cleaning programmes. These tasks are often linked to previous experience, but there is no control strategy, nor a complete knowledge of the sediment transport processes in UDS. Attempts so far to monitor these processes have been based on techniques that require exhaustive work in their installation or in taking punctual field measurements, such as acoustic devices (based on sonar sensors). Therefore, sediment monitoring in UDS remains challenging. Interestingly, recent work in rivers investigated streambed sediment transport with temperature measurements, based on considerations on heat transfer and ground-water flows. In this work, we present a new approach which uses high-resolution temperature data to identify sediments from changes in temperature dynamics between the water (wastewater or drainage water in UDS) and the sediment bed deposits. Since the heat transfer between both media can be predictably affected by the sediment thickness, we suggest a heat transfer model that relates the temperatures in the fluid and in the sediment layer to its thickness. A series of batch tests were carried out in isothermal boxes with different thicknesses (range: 2-8 cm) of real sewer sediment. The sediments were previously saturated and poured into their respective boxes, in which two PT100 temperature sensors were installed: one at the bottom of the boxes, and the other slightly above the sediment surface. This second temperature sensor was submerged in a freshwater layer of 2 cm above the sediment surface. Next to the sensor immersed into the water, a coil system was installed to heat-cool the water and thus simulate temperature oscillations in UDS. Batch tests consisted of a first phase of heating the fluid, followed by a subsequent cooling phase. The fluid temperature oscillations were set in the range of 2-3°C. As a result, the temperatures measured at the bottom of the sediment layers showed different responses depending on the sediment thickness. These tests were used to calibrate a 1D heat transfer model, whose variables were the temperature in the sediment layer and boundaries, its thickness, and its thermal properties. In a first approach, the thermal properties of the sediment were approximated with literature values for non-uniform sands and clays. Our positive results demonstrate the potential to monitoring sediment accumulation in UDS with passive temperature sensors. Further extensions to distributed temperature monitoring with fibre-optical sensors and active heating are currently being investigated.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221141

Year: 2022