Sulfide emissions in sewer networks: focus on liquid to gas mass transfer coefficient

Abstract

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.

Using computational fluid dynamics to describe H2S mass transfer across the water–air interface in sewers

“…where λ is the interface length and ξ the curvilinear coordinate associated to the interface. Following Carrera et al (2017), the mass transfer can also be obtained by fitting the decrease of concentration in the liquid phase to the following function:…”

“…Furthermore, Carrera et al (2017) propose a calculation of the mass transfer coefficient based on the degassing technique, which has been used in this publication to cross validate the simulated mass transfer coefficients. In the degassing technique, the measured decrease of the H 2 S concentration in the water phase has been fitted to Equation 7.21:…”

“…On the one hand, conceptual model approaches have been developed to describe the occurrence of odour and corrosion (Hvitved-Jacobsen et al, 2013). On the other hand, numerous experiments have been conducted to better understand driving factors such as the influence of turbulence on H 2 S mass transfer across the water surface (Carrera et al (2017), Matias et al (2017), Wu (1995)). Being able to model and directly quantify these emissions cannot only improve existing model approaches by the possibility of investigating a wider range of cases but also to analyse details of sewer construction such as the design of H 2 S hotspots and to find ways to improve the design.…”

“…Wu (1995) analysed the influence of different factors such as the stirrer inundation (h/R -ratio between water level above stirrer h and stirrer diameter R) and the stirring rate on O 2 mass transfer. Carrera et al (2017) investigated the influence of stirring rates on H 2 S mass transfer and used CFD results from FLUENT TM 1.4 to extract hydrodynamic parameters such as flow velocities. Being able to describe H 2 S emissions in a stirring tank with a CFD model cannot only enable us to investigate more complex phenomena on a larger scale but could also help avoiding lab experiments that come with a health risk and decrease the number of experiments necessary.…”

“…First, a graphic analysis is performed, then a sensitivity study on parameters such as the grid size, the influence of the turbulent diffusivity, the choice of turbulence model and the inundation of the stirrer is performed. In a quantitative analysis, the simulated results are then compared to experimental results by Carrera et al (2017) and Wu (1995).…”