Emission of Hydrogen Sulfide from Falling Droplets in Sewage Drop Structures

Abstract

Hydrogen sulfide (H2S) is the primary cause of odor and corrosion in sewer systems. When wastewater breaks up into small droplets in drop structures, the emission of H2S 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, KL, 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: H2S and carbon dioxide (CO2). In the droplet diameter (3.02–4.68 mm) and free-falling height (0.1–1.5 m) testing ranges, the KL value at 20°C was found to be 0.9–4.5×10−4 m/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 KL and cannot accurately predict the experimental results. Therefore, a modified equation was proposed to better predict KL. In addition, CO2 was found to be a suitable surrogate for H2S in mass transfer due to the toxicity of H2S. Finally, the research results were applied in large and small sewage drop structures. More than 70% of H2S emission to equilibrium could be achieved after falling 15 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 H2S emission process. This study provides new insights into the physical process and modeling of H2S release in sewage drop structures, which are useful for odor and corrosion control in municipal drainage systems.