Sewer networks are essential sanitary structures that collect wastewater produced by communities, as well as precipitation runoff from urban areas, transporting them for further treatment and/or disposal. Some major problems associated to sewers arise from the formation of the gaseous compound hydrogen sulphide (H2S), which corrodes concrete sewer structures, generates malodours in populated neighbourhoods and, last but not least, threatens public health due to its toxicity. The associated maintenance, renovation or sewer network renewal operations highly affect the operational cost of water utilities all over the world, in the order of milliards of euros. These problems are expected to be aggravated in the near future due to the global warming trend, as higher temperatures foster the production and emission of hydrogen sulphide. Therefore, H2S monitoring is a compulsory task in order to identify problematic hotspots in servicing and maintenance of sewer installations. Hydrogen sulphide is formed under anaerobic conditions in the biofilm in the liquid phase,but its severe effects become noticeable when released into the sewer atmosphere, from where it can escape into the open air. This is usually the case in gravity operated sewers transporting municipal wastewater, as they are partially filled with air. Sewers are highly dynamic systems, so there are noticeable daily, weekly or seasonal variations in the sulphide production. Until now, monitoring of the liquid phase has been a challenging task, since laboratory standard methods are time-consuming and manually complex, restraining sampling frequency and complicating data acquisition. However, the development of online sensors over the last few years for wastewater applications has notably simplified the operations in liquid phase monitoring, and as the process is automated, they provide a large number of measures, and chemical reagents are no longer required. This thesis provides insight on the application of online sensors in sewer monitoring, as well as on the H2S mass transfer processes occurring across the liquid-gas interface. In a first instance, the focus of the thesis was set on the local calibration of a UV/Vis spectrometer for monitoring the sulphide species -among other wastewater parameters-, because of their key role in odour generation and corrosion episodes in sewers. The obtained results showed that the spectrometer is a suitable device for predicting sulphide concentration trends associated to sewer networks. Second, two comparisons of H2S sensors for sewer monitoring were performed: one for the liquid phase, and another one for the gas phase. The main achieved goal was to provide an overview of the available commercial sensors, as well as to evaluate their performance and measurement accuracy under different operational conditions. In addition, the limitations, challenges, and strengths of the devices were assessed. Third, the knowledge acquired during the evaluation of the sensors was applied for continuous monitoring of both phases. As a result, an empirical determination of the mass transfer coefficient (KLa) across the liquid-gas interface was obtained. All experiments were performed in a sewer pilot plant with actual wastewater. Summing up, results obtained in this thesis highlight a) the importance of sewer monitoring with online sensors in order to identify and control corrosion and odour hotspots, and b) their role in the experimental determination of the H2S mass transfer coefficient under real conditions. This is still a challenging research topic due to the difficulties of working with the highly toxic hydrogen sulphide, both in real and in laboratory setups.
