INNOVATION IN CORROSION MONITORING IN SEWERS
Use of novel photonic sensors for humidity measurements in gravity sewers
H Bustamante, L S. M. Alwis, K T. V. Grattan, T Sun, L Vorreiter, J Gonzalez
Publication Date (Web): 14 November 2017
Sydney Water’s current Sewer Rehabilitation Program costs about A$50m annually. The program relies on chemical addition to minimise hydrogen sulphide (H2S) transfer from the wastewater to the sewer air, and as well the use of ventilation. Humidity plays a key role in microbiologically-induced corrosion of concrete gravity sewers and minor reductions in humidity are known to reduce corrosion rates. However, no reliable long-lived (>1 week) humidity sensors are available, thus limiting the development of useful models to better manage corrosion – our experience using commercially available electrical sensors has demonstrated that they typically fail after around one week.
Accurate and long term humidity measurements will enable a quantitative correlation between allowable H2S in sewer air and acceptable corrosion rates of ≤ 0.5 mm/year. This paper describes collaborative research between Sydney Water, City, University of London and Edinburgh Napier University on the use of photonic sensors to measure humidity. The photosensitive sensors used in this study were designed and manufactured specifically for this project by City, University of London (CUL) and their collaborators, to meet the requirements for high performance photonic sensors with specialised, tailored coatings, designed to operate under both highly biofouling and corrosive conditions in headspace of gravity sewers.
The experimental program was carried out in the head space of a balancing tank that receives the building’s wastewater upstream of the on-site wastewater treatment plant at Sydney Water’s Parramatta office. The gaseous hydrogen sulphide concentration typically varied during the day between 5 and 130 ppm by volume with relative humidity between 97 and 100%. The temperature of the gas phase was around 20-23 degrees during the day and night. The results indicated that the sensors produced strong dynamic responses and accurately recorded the high humidity levels (between 97 and 100% RH). Furthermore, in two cases where the humidity was rapidly lowered by removing them for exposure to the ambient air, the sensors rapidly responded and measured ~100% humidity when replaced in the overhead tank. Thus, the sensors showed their dynamic capability as they could respond to drastic increases and decreases in humidity.
The results with various sensor designs showed that using a thinner coating takes less than 15 minutes to saturate while thicker coatings take nearly an hour for saturation. However, the sensitivity of the thicker coating is approximately more than 2.5 times that of the thinner coating. Depending on the application, it should therefore be possible to select the coating thickness to achieve faster response or higher sensitivity. Thinner coatings for rapid response are more suitable for this study.
Furthermore, visual observations of the photonic sensors indicated that the aggressive and biofouling sewer conditions had only minor impact on the physical integrity of the photonic sensors after five months.
This study demonstrated that photonic sensors can be used to monitor humidity in the range where hydrogen sulphide is converted into sulphuric acid.
The use of photonic sensors in Sydney Water will both improve the monitoring of humidity and help optimise ventilation. It will also provide information for the design and management of future sewer systems.
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