Performances and Uncertainty of Temperature Methods for Illicit Infiltrations and Inflows Assessment in Stormwater Sewers

The term “illicit flows” refers to all those unexpected and unwanted waters that are drained by or discharged into the urban drainage systems. Unmanaged illicit flows may cause significant losses in functionality both to the sewer networks and to the wastewater treatment plants. This paper focuses on one of the main approaches for illicit flow individuation and estimation in sewers that is based on the joint use of flow probes and temperature sensors; such an approach is often chosen for practical aims, because of its simplicity, affordability and adaptability. In particular, this paper is meant to fulfill the still existing lack of a systematic method to assess a priori its performances in terms of reliability and accuracy, by rigorously implementing the general uncertainty analysis theory. Then, through some meaningful numerical case studies, it is shown how to minimize the uncertainty on the illicit flow estimation when just one flow probe is available for the field survey. But it also highlights the potential weakness of the field results when the illicit flow rates are too small and/or their temperatures are too close in comparison to the regular flow. This research has been carried out within the framework of the project “PerFORM WATER 2030”, funded by Regione Lombardia.