Mass Transfer of ${\mathrm{H}}_2\mathrm{S}$ and ${\mathrm{O}}_2$ in Falling Sewage: Contributions via Droplets, Jet, and Bottom Pool

In urban drainage systems, falling sewage in drop structures expedites the mass transfer of hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$) and oxygen (${\mathrm{O}}_2$). This process is important for sewer odor and corrosion control; however, direct experiments and relevant knowledge are limited. This study conducted laboratory experiments using two typical forms of falling sewage: free-falling jet and attached-falling jet. The results show that mass transfer coefficient $K_L$ and concentration deficit ratio $r$ (the ratio of upstream-to-downstream gas concentration deficiency) increase with an increase of sewage drop height (0.2–1.4 m) and with a decrease of flow rate ($0.9–2.0\mathrm{L}/\min$). Nonlinear correlations between $r$ and the hydraulic parameters were proposed. The free-falling jet contributed about 40% more mass transfer than the attached-falling jet. The mass transfer rate in free-falling drop structures of this study was 3–13 times that in gravity sewers without drop structures. In addition, ${\mathrm{O}}_2$ is an appropriate surrogate gas for studying ${\mathrm{H}}_2\mathrm{S}$ mass transfer. Finally, the mass transfer of ${\mathrm{O}}_2$ in a prototype drop structure was estimated: if the drop height is $<3\mathrm{m}$, the jet may not break up, the mass transfer at the jet surface can be neglected, and almost all the mass transfer happens at the bottom pool of the drop structure; however, if the drop height is $>6\mathrm{m}$, falling droplets are the main ($>80\%$) contributor. This study provides a tool for estimating the mass transfer in drop structures, which can optimize the design of drop structures to control sewer odor and pipe corrosion.