Air-water transfer of hydrogen sulfide: An approach for application in sewer networks

The emissions process of hydrogen sulfide was studied to quantify air-water transfer of hydrogen sulfide in sewer networks. Hydrogen sulfide transfer across the air-water interface was investigated at different turbulence levels (expressed in terms of the Froude number) and pH using batch experiments. By means of the overall mass-transfer coefficient (K(L)a), the transfer coefficient of hydrogen sulfide (K(L)a(H2S)), referring to total sulfide, was correlated to that of oxygen (K(L)a(O2)) (i.e.. the reaeration coefficient). Results demonstrate that both turbulence and pH in the water phase play a significant role for K(L)a(H2S). An exponential expression is a suitable representation for the relationship between K(L)a(H2S) and the Froude number at all pH values studied (4.5 to 8.0). Because of the dissociation of hydrogen sulfide, K(L)a(H2S) increased with decreasing pH at a constant turbulence level. Relative differences in K(L)a(H2S) at pH values between 4.5 and 8.0 became larger as the turbulence level increased, whereas those at pH between 4.5 and 7.0 did not statistically show any change. At constant pH, K(L)a(H2S)/K(L)a(O2) was observed not to be dependent on the turbulence range studied. K(L)a(H2S)/ K(L)a(O2) ratio was 0.86, with a 95% confidence interval of 0.08 at low pH, where only molecular hydrogen sulfide was present. This ratio was found to be equal to the ratio between the diffusion coefficient of hydrogen sulfide and that of oxygen, which was determined by empirical equations proposed in the literature. This fact was observed at all the turbulence levels investigated. Based on the findings in this study, an empirical equation describing the pH dependence of K(L)a(H2S)/K(L)a(O2) at a pH range of 4.5 to 8.0 is proposed. Theoretical and experimental aspects of hydrogen sulfide emissions and an application are discussed related to in-sewer conditions.