Enhanced monitoring and infrastructure management of sewers and anaerobic biodigesters using innovative fiber optic-based sensor systems

Purpose of the research and key message and knowledge transfer In summary, the key message from this work is two-fold: Close collaboration with industry enabled the design of a scientifically rigorous and highly focused experimental program in live sewers. The latter provided conclusive answers within four months and identified a way forward leading to technology implementation and identification of researchable gaps needed to make the technology more widely usable. The specific knowledge that this paper will transfer is that reliable humidity sensors to measure relative humidity >98% in gravity sewers can be achieved. This is a tool that will help the water industry to improve the management of its concrete gravity sewers and other infrastructure that is in contact with high concentrations of gaseous H2S in humid air. No commercially available technology can do this. In this work, the authors have demonstrated experimentally, building upon advanced design work, that optical fibre sensor systems represent an innovative, versatile and affordable technology that can be brought to bear very effectively on key monitoring problems in the water industry's infrastructure. Through the work reported and results demonstrated in this paper, the research has very successfully created a synergy of the skills of an academic group and a major water utility to identify, tackle and solve key problems in sewers -- problems that impact directly on consumers and are seen world-wide in the industry. In doing so, in an effective way like this, the paper demonstrates the benefits to be gained from a close collaboration between the two groups. Above all new knowledge has been transferred -- in both directions -- and through that new and more effective approaches to problem solving developed. Specifically, the project is based on new technology for humidity measurement -- this is an essential parameter that needs to be monitored to minimize corrosion of concrete gravity sewers. No humidity sensors that last longer than ~1 week in the aggressive gaseous atmosphere of gravity sewers and can reliably measure humidity above 98% relative humidity are available. Thus supporting better predictive maintenance, in situ monitoring of a sewer and an anaerobic digester is undertaken using innovative optical fiber-based sensor systems: these being chosen because of their inherent safety in the potentially methane rich environment, their ability to be protected against biofouling, their light weight, low power consumption (or battery powered), 4G compatible and ease of use over the long lengths (up to kilometers) needed for some applications. The multi-parameter sensors systems designed and evaluated are also well suited to use with the high levels of hydrogen sulfide, coupled with high humidity seen in sewers and biogas rich environments -- environments where conventional electronic-based sensor systems regularly fail due to acid attack. Benefits of Presentation and why this presentation should be selected and will provide a benefit to our industry Sydney Water's current management strategy relies on reducing gas phase H2S by the addition of chemicals (ferrous chloride, magnesium hydroxide and calcium nitrate) to the waste water. This strategy, implemented in a number of waste-water systems in Sydney, is costly (about $4M p.a.) and its effectiveness in reducing corrosion is unknown, due to an inability to directly measure concrete corrosion and deterioration of rehabilitation materials. No matter how effective this program is in slowing the corrosion rate, there will always be concrete corrosion and a method for prediction and detection of imminent pipe failure is needed because of the enormous costs of such failures. In particular, it is necessary to determine how much of the ubiquitous H2S and H2O can be left in the air in order to reduce corrosion rates to less than what Sydney Water defines as a sustainable 0.5 mm/year. Gravity sewers are buried infrastructures that undergo chemical deterioration because of microbiological induced corrosion (MIC). Microbiologically induced oxidation (MIC) of gaseous H2S into sulfuric acid results in concrete sewer corrosion. MIC costs billions of dollars annually to the water industry and has been identified as a main cause of global sewer deterioration. Sydney Water spends about A$60-80M annually repairing and protecting concrete gravity sewers and has a regular sewer inspection program. Currently, sewer condition is determined by man entry which is inconvenient and hazardous, has major safety issues and is expensive and so there is a need to minimize man entry by reducing damage from MIC, which can be reduced by controlling moisture in the gravity sewers to values approximately below 85% RH. No conventional humidity sensors that can last longer than one week exist under these aggressive conditions. Tackling this, this work describes the first and successful attempt to use optical fiber-based photonics sensors to monitor humidity in the range >98%RH in the overhead space in gravity sewers. This information is fundamental to develop reliable models that can predict the end-of-service life (EOSL) of concrete sewers. With the ageing of an asset, there is a point in its life, termed the End of Service Life (EOSL) and by doing so, it will be possible to save hundreds of millions of dollars that would have to be spent if collapses occur or premature repairs carried out to prevent a potential collapse. The approach developed together thus supports Sydney Water's corrosion management strategy, which has been created as an outcome of over $3M in research projects striving to better understand the processes that lead to the formation of H2S dissolved in the waste-water. Status of Completion The work done together by the authors has enabled the project to reach a high level of completion and results of the use of the systems in sewers and biodigesters have been reported in the major international media. Thus to date, three trials of tailor-made fiber optic-based sensor systems to monitor humidity in the range >98% RH in sewer air that contains high gaseous H2S concentrations of up to around 400 ppnv have been successfully completed. Based on this Sydney Water is implementing the photonic sensors technology in small catchments. For extensive implementation, Sydney Water is supporting extra research to develop 'low cost' interrogators that would make the technology more cost effective, well suited to long term, in-the-field research. Conclusion and take-away message The key 'take-away' message is that this is a significant innovation in optical fiber and photonic sensors that they can now be used in these 'dirty' environments. This has also allowed in-situ long-term monitoring under conditions where current technology fails. The major outcomes of the series of experiments carried out over the last five years is clear; fiber optic sensors systems can operate effectively, in the long term, in remote locations under battery power, saving of millions of dollars.