Department Urban Water Management
SedTemp: Identifying sediment deposits from temperature signals
Approach to sedimentation in sewer and urban drainage systems based on temperature measurements
Sediment accumulation in natural and engineered systems, such as urban drainage systems, is a challenging problem. In urban drainage systems, for example, sediments are an important source of pollution, which may lead to the decrease of the discharge capacity of these systems and the subsequent sediment mobilization and release of pollutants during rain. Removing the sediments from sewers is very costly and currently no established method exists to continuously monitor sediment deposits (Figure 1, left). In this project, simple temperature sensors will be used to identify and assess sediment accumulation from variations in temperature dynamics. For this purpose, a portable compact logging and control unit was developed at Eawag (Figure 1, right).
The idea lays on characterizing thermodynamics in sewer and urban drainage systems. These systems present a marked temperature daily pattern, such as in combined sewers during dry weather flow conditions. The presence of sediments introduces a heat transfer between the wastewater and sediments that attenuates and time-lags the values of the temperature time series. Thus, if we measure and analyse the temperatures in both the wastewater and the sediment bed we can estimate the accumulated sediment thickness, as the attenuation will be more pronounced the greater the sediment thickness (Figure 2, left). Therefore, monitoring sediment accumulation will improve cleaning frequency strategies and support the development of better sediment transport models for sewer and urban drainage systems.
Thermal properties play a main role in heat transfer process: i) thermal conductivity, ii) thermal diffusivity and iii) heat transfer capacity. To obtain the value of these parameters the monitoring system includes a cartridge heater that supplies a heat pulse to the sediment. Thus, by analysing the temperature response between the heater and the temperature sensor placed at the bottom, thermal properties can be estimated (Figure 2, right). This measurement provides a better approach of the heat transfer processes and, therefore, a better simulation and prediction of the sediment accumulation. Furthermore, thermal parameters are also related to the mechanical properties of the sediments. Hence, we can roughly predict the type of sediment deposited or, at least, differentiate between organic/inorganic and coarse/fine sediments by using this system. This would help in the operation and cleaning strategies, such as, selecting the type of cleaning system.