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Monitoring storm water inflow in a foul sewer using distributed fibre optic temperature sensing

Severn trent graph

Image: The graphic shows the temperature behaviour of the 1.2km of sewer being monitored over a 24hr period, courtesy Dr Y Rui

The project

Surface water infiltration into sewers and illicit connections, most often unintended, of storm water to foul sewers and of foul sewage to storm sewers, is a major problem associated with separate sewer systems.

Unwanted infiltration can lead to sewer and treatment plant design capacity being exceeded. This can potentially result in over spilling and local flooding or the release of untreated sewage in surface water and the wider environment. Eradicating unwanted infiltration and removing illicit connections is beneficial to consumers, both in terms of hygiene and financial cost. However, effective remedial action requires precise knowledge of the location of infiltrations.

The innovation

Unwanted discharges are intermittent and their detection requires monitoring systems with good spatial and temporal resolution that can be deployed over kilometres of sewer networks. Distributed fibre optic temperature sensing meets these requirements. The system helps to accurately pinpoint anomalies in operation by detecting the sudden changes in temperature of the sewer liquid which is due to differences in foul and storm water temperature. The technology is autonomous and measures temperature continuously along the entire length of the optical fibre.

CSIC has demonstrated the system (initially pioneered by Delft University in the Netherlands) near Gloucester, by installing an armored fibre optic temperature sensor along a 1.5km-length of sewer. A pilot rope with a sensor cable and float attached was lowered down a manhole. While the float had travelled downstream, it was retrieved periodically at manholes in order to pull the cable into the sewer. This installation method ensured that the cable did not suffer excessive pulling forces.

The cable was connected to a fibre optic analyser enabling real time temperature monitoring of 1.5km of sewer, 24 hours per day, throughout the three-month survey period.

Whenever fluid entered the sewer, its native temperature caused the ambient temperature of the fluid in the sewer to change. The sensor detected this subtle difference in temperature along the entire length of sewer being monitored, and the data collected over the three-month period was plotted into waterfall charts.

This novel sensing method enabled the accurate detection of the time and location of discharges into the sewer, identifying whether discharges came from domestic connections, illicit connections, or surface rainwater infiltration.

Impact and value      

•          this project demonstrates the commercial viability of distributed fibre optic temperature sensing in detecting sewer operation and malfunction

•          data visualisation shows sewer operation patterns indicative of certain sewer events (illicit discharge, domestic sewer operational patterns, sewer blockages, manhole overflow events) of value to asset owners and managers

•          a network of fibre optic sensors can provide asset managers with a real-time view of the condition of their critical assets

•          this innovative monitoring provides information that informs planning to ensure asset integrity is maintained, and prevents the interruption of service due to failure.

This demonstration has proved the commercial viability of this sensing system in sewers. CSIC is investigating further demonstration and training opportunities with industry to commercialise this method and see it taken up by the supply chain.

Sensor & data collection