Characterisation of Water Infrastructure using Impedance Spectroscopy

Sediment in urban drainage systems can lead to a reduction in flow capacity and potentially to surcharge and overflow, but the ability to monitor sedimentation is limited. This study aimed to develop conductance-based sensing systems to characterise the multiphase depths and hydraulic properties of sediments in sewer pipes.
Firstly, a novel sensor based on conductance measurement has been developed and tested under a laboratory environment and validated by a finite-element model. The relative conductance is measured between pairs of adjacent electrodes to provide a conductance profile along the sensor length. A piecewise linear relationship between conductance and electrode length was derived and the interface positions between sediment, water, and air can be determined from the profile. The results demonstrated that the root mean square error of the model and the measured interface level are within 1.4% and 2.6% of sensor’s measurement range of 172 mm depth. An error distribution of interface height shows that all anticipated errors are within the resolution of the electrode length increments. Furthermore, it was found that the conductivity of the measured medium is proportional to the gradient of the linear relationship of conductance and electrode length.
Secondly, a sensing system based on electrical impedance spectroscopy (EIS) has been developed as a method for characterising the grain size, porosity and hydraulic permeability of various sizes of glass beads and mixtures composed of sand or glass beads. The EIS involves applying an AC electrical signal to the sample and measuring the resulting voltage response, which is represented by the Bode plot and Nyquist plot. The Nyquist plot is fitted with the modified Randles circuit, a model that can be used to extract the electrical conductance of the test samples. The relationship between the electrical conductance and the hydraulic properties has been investigated, including a linear relationship between electrical conductance and grain size, and power law relationships between electrical conductance and porosity and permeability. Then a combined model is derived from the Kozeny-Carman equation, Archie’s Law and fitted parameters from this study, to predict the hydraulic permeabilities, which demonstrates a good agreement with the measured permeability. Considering the actual applications, the temperature dependence has been analysed so that temperature changes can be taken into account when interpreting the measured conductance. In addition, the relationship between the electrode spacing and measured electrical conductance is also characterised by the experiments and a recommendation has been proposed to help optimise the design of the sensor for future applications.
These findings demonstrate the feasibility of two valuable sensing systems for the accurate quantification of multiphase depths and hydraulic properties in sewer systems. The low-cost and real-time measurements can improve the understanding of the sedimentation process and optimise its monitoring in the water industry. Further work should validate the sensing systems with real sediment samples, and field tests would be desirable.