Using photonics-based sensors to monitor the deterioration of concrete gravity sewers

In summary

Research undertaken at City St George’s, University of London has led to the development of a new class of photonics-based humidity sensors for use in Sydney’s sewers.  These have been developed by Professor Tong Sun and Professor Ken Grattan in collaboration with researchers at Edinburgh Napier University.

What did we explore and how?

Given the highly biofouling and corrosive conditions in Sydney’s sewers, optical fibre-based Bragg Grating sensors with specialised coatings needed to be developed to operate in those challenging conditions to monitor the deterioration of the concrete structures within the sewers.

The sensors which now operate in the bowels of one of Australia’s most important cities generate continuous valuable monitoring data on the state of the sewers in real time.

Benefits and influence of this research

After previous shorter evaluation studies and an extended 20-month period of monitoring using in situ humidity sensors, forming a fibre optic network into which a series of Bragg Grating-based devices were installed at two locations in Sydney Water’s sewer network (Eustace Street in Manly, Sydney and Old Toongabbie at Oakes Reserve, Western Sydney), was undertaken.

This study demonstrated that even after this period of use, the sensor is reliably recording humidity and temperature in this biofouling and highly corrosive, acidic environment.

The Bragg Grating-based sensors were shown to overcome the problems seen with conventional electrical sensors which typically fail within a couple of weeks of use in this continuous high acid/high humidity environment.

The data, recorded constantly from the sensor system, were stable throughout the full monitoring period and further, a comparison with the changing weather conditions was made over the different seasons during the study.

The sensor system developed was battery-operated and had 4G connectivity for data transfer and debugging. These features have enabled the system to be installed in situations where mains electrical power is unavailable with successful and minimal human operation, thus allowing for additional systems to be integrated into a future measurement system.

It is envisaged that the fiber optic sensor-based approach to sewer monitoring will lead to the wider integration of an effective suite of sensors on the whole infrastructure owned and operated by water companies, tackling a long-standing problem of lack of stability in conventional measurements and providing key data on the monitoring of wastewater structures and treatment plants.

Future work will focus on monitoring of additional parameters, such as the pH of the environment or the presence of other gases – all designed to allow a better understanding of the key corrosion issues and to enable the execution of preventative actions both a time-efficient and cost-effective manner.