Mass Transfer of Hydrogen Sulfide in Sewer Drop Structures
Creators: Sun, Letian
Publisher: name: University of Alberta Library
Year: 2022

<p>Hydrogen sulfide (H2S) is the primary cause of the widespread odor and corrosion issues in sanitary sewer systems. When wastewater falls in drop structures, the emission of H2S is expected to be significantly enhanced. This study focuses on the mass transfer coefficient, KL, for H2S and other surrogate gases from falling droplets, at turbulent water surface and in drop structures. <br>
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. In this study, laboratory experiments of mass transfer from falling liquid droplets to air were conducted with two gases: H2S and carbon dioxide (CO2). In the testing range of droplet diameter (3.02-4.68 mm) and free-falling height (0.1-1.5 m), the KL value at 20℃ was found to be 0.9-4.5 × 10-4 m/s, which increased with the falling height (or velocity) while decreased with the droplet size. 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. <br>
Laboratory experiments were conducted to study the mass transfer at the pool surface with falling water drops or a single jet. In the test range of falling flow rate of 49-223 mm/h and falling velocity of 3.1-5.2 m/s [kinetic energy flux KEF = 0.11-0.80 J/(m2s)], KL for the pool surface was found to be 2.6-14.8 × 10-5 m/s for H2S. KL was found to be 76% larger when the pool surface was impinged by water drops than by a single jet. In addition, KL was 27-47% larger in the half of the water surface directly receiving the drops or jet than the other half. The increase of water depth in the pool promoted the mass transfer, especially for the scenario of a falling jet. Equations were proposed to predict KL under drops or jet. Finally, KL for H2S and for O2 was found almost the same, indicating O2 can be a safe surrogate gas for H2S. <br>
Laboratory experiments were conducted using two forms of falling sewage in drop structures: free-falling and attached-falling jets. The results show that KL and concentration deficit ratio, r, increased with an increase of drop height (0.2 - 1.4 m) and a decrease of flow rate (0.9 - 2.0 L/min). Nonlinear correlations between r and the hydraulic parameters were proposed for both jet scenarios, with good agreements with experimental results. The difference between the two jet scenarios appeared to be related to the size of drop structures: in the large drop structure, r of the free-falling jet was larger than that of the attached-falling jet; while in the small drop structure, r was almost the same. Finally, the mass transfer of O2 in a prototype drop structure was estimated. If the drop height is &lt; 3 m, almost all the mass transfer happens at the bottom pool of drop structure; if the drop height is &gt; 6 m, falling droplets are the main (&gt; 80%) contributor. <br>
Field work was carried out in the west area of Edmonton, Alberta, Canada to investigate the wastewater quality and emission of H2S in sewer network. The wastewater samples were collected and analyzed for sulfide and other relevant parameters (e.g., sulfate and COD). Empirical models can predict sulfide generation. The operation of the upstream pump station could cause the sudden increase of H2S in the sewer air of the adjacent discharge manholes. Over 90% of H2S stayed in the liquid phase when wastewater flowed by gravity in the sewer pipes. In the drop structure of 8 m, the H2S concentration in the upstream was 2.6 times of that in downstream and r for H2S was 2.0, which proves the enhancement to the emission of H2S in drop structures.<br>
This research provides new insights on the physical processes and modeling of H2S emission in sewer drop structures. The research outcomes are useful for odor and corrosion control in municipal drainage systems.</p>