The problems related to hydrogen sulfide in terms of deterioration of sewer networks, toxicity and odor nuisance have become very clear to the network stakeholders and the public. The hydraulic and (bio)chemical phenomena and parameters controlling sulfide formation, emission and their incidences in sewer networks are very complex. Recent research studies have been developed in gravity and pressure sewers and some transfer models have been published. Nevertheless, the models do not take into account all the physical phenomena influencing the emission process. After summing up the main scientific knowledge concerning the production, oxidation, transfer and emission processes, the present review includes: (i) a synthetic analysis of sulfide and hydrogen sulfide emission models in sewer networks, (ii) an estimation of their limit, (iii) perspectives to improve the modelling approach. It shows that sulfide formation and uptake models still need refinements especially for some phenomena such as liquid to gas mass transfer. Transfer models that have been published so far are purposely simplified and valid for simple systems. More efforts have to be undertaken in order to better understand the mechanisms and the dynamics of hydrogen sulfide production and emission in real conditions.
HS emission dynamics in sewers are conditioned by the mass transfer coefficient at the interface. This work aims at measuring the variation of the mass transfer coefficient with the hydraulic characteristics, with the objective of estimating HS emission in gravity pipes, and collecting data to establish models independent of the system geometry. The ratio between the HS and O mass transfer coefficient was assessed in an 8 L mixed reactor under different experimental conditions. Then, oxygen mass transfer measurements were performed in a 10 m long gravity pipe. The following ranges of experimental conditions were investigated: velocity flow [0-0.61 m.s], Reynolds number [0-23,333]. The hydrodynamic parameters at the liquid/gas interface were calculated by computational fluid dynamics (CFD). In the laboratory-scale reactor, the O mass transfer coefficient was found to depend on the stirring rate (rph) as follows: K = 0.016 + 0.025 N. A K/K ratio of 0.64 ± 0.24 was found, in accordance with previously published data. CFD results helped in refining this correlation: the mass transfer coefficient depends on the local interface velocity u (m.h): K = 0.016 + 1.02 × 10 u In the gravity pipe device, K also exponentially increased with the mean flow velocity. These trends were found to be consistent with the increasing level of turbulence.
Problems related to hydrogen sulfide may be serious for both network stakeholders and the public in terms of health, sustainability of the sewer structure and urban comfort. HS emission models are generally theoretical and simplified in terms of environmental conditions. Although air transport characteristics in sewers must play a role in the fate of hydrogen sulfide, only a limited number of studies have investigated this issue. The aim of this study was to better understand HS liquid to gas transfer by highlighting the link between the mass transfer coefficient and the turbulence in the air flow and the water flow. For experimental safety reasons, O was taken as a model compound. The oxygen mass transfer coefficients were obtained using a mass balance in plug flow. The mass transfer coefficient was not impacted by the range of the interface air-flow velocity values tested (0.55-2.28 m·s) or the water velocity values (0.06-0.55 m·s). Using the ratio between k to k, the HS mass transfer behavior in a gravity pipe in the same hydraulic conditions can be predicted.
Mass transfer coefficients at the gas-liquid interface were investigated for different flow configuration systems, a stirred tank reactor and a gravity pipe. Computational fluid dynamics (CFD) simulations were performed for all tested experimental conditions. Since a poorly soluble gas (oxygen) was used, the overall mass transfer coefficient was clearly correlated to the hydrodynamic conditions in the liquid phase. However, a generic correlation between averaged interfacial liquid velocity and mass transfer coefficients was not found for both geometries. Finally, the averaged turbulent kinetic energy (TKE) k i at the interface is the most relevant parameter that was correlated to the mass transfer coefficient for both systems. The same relationship between oxygen mass transfer coefficient K L,O2 and TKE (K L,O2 = 0:037 + 11:610 × 10 6 k i 2,46 ) can be applied for the two geometries investigated. K E Y W O R D S computational fluid dynamics, mass transfer, open channel, turbulent kinetic energy
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