Emission of Hydrogen Sulfide from Falling Droplets in Sewage Drop Structures

Hydrogen sulfide (${\mathrm{H}}_2\mathrm{S}$) is the primary cause of odor and corrosion in sewer systems. When wastewater breaks up into small droplets in drop structures, the emission of ${\mathrm{H}}_2\mathrm{S}$ is expected to be significantly enhanced relative to that of the continuously falling sewage. However, limited studies have been conducted on the mass transfer coefficient, $K_L$, of dissolved gas in falling water droplet emissions into air. In this study, laboratory experiments of falling liquid droplets in air were conducted with two gases: ${\mathrm{H}}_2\mathrm{S}$ and carbon dioxide (${\mathrm{CO}}_2$). In the droplet diameter ($3.02–4.68\mathrm{mm}$) and free-falling height ($0.1–1.5\mathrm{m}$) testing ranges, the $K_L$ value at 20°C was found to be $0.9–4.5\times {10}^{-4}\mathrm{m}/\mathrm{s}$, which increased with the falling height (or velocity) and decreased with the droplet size. Existing prediction equations show a great range (up to 10 times) for $K_L$ and cannot accurately predict the experimental results. Therefore, a modified equation was proposed to better predict $K_L$. In addition, ${\mathrm{CO}}_2$ was found to be a suitable surrogate for ${\mathrm{H}}_2\mathrm{S}$ in mass transfer due to the toxicity of ${\mathrm{H}}_2\mathrm{S}$. Finally, the research results were applied in large and small sewage drop structures. More than 70% of ${\mathrm{H}}_2\mathrm{S}$ emission to equilibrium could be achieved after falling $15\mathrm{m}$ in large drop structures depending on droplet sizes. In small drop structures, the results of free-falling water droplets were found to approximately model the complex ${\mathrm{H}}_2\mathrm{S}$ emission process. This study provides new insights into the physical process and modeling of ${\mathrm{H}}_2\mathrm{S}$ release in sewage drop structures, which are useful for odor and corrosion control in municipal drainage systems.