Experimental determination of the H₂S mass transfer coefficient across the liquid-gas interface in gravity sewers

Hydrogen sulphide (H₂S) emissions in sewer systems lead to several problems such as corrosion, odour nuisances, and health damage to sewer workers. Although the gas is formed in the liquid phase, its effects are noticeable when released into the sewer atmosphere. Until recently, the lack of analytical procedures for continuous monitoring of H₂S in the liquid phase, as well as its toxicity, have challenged the quantification of the mass transfer coefficient under real conditions. Because of this, most studies have mainly focused on batch experiments with artificial wastewater and/or oxygen. The aim of this study was to experimentally determine the overall mass transfer coefficient for H₂S during intermittent pumping events common in actual sewer systems, using the two-film theory approach and employing online sensors for liquid and gas phase measurements. The mass transfer coefficient was quantified by carrying out 21 experiments with actual wastewater in a 25 m long gravity pipe of a sewer pilot plant located in Berlin (Germany). Results show that the corrected mass transfer coefficient (K_La₂₀) during a pumping event ranged between 0.1 and 8 h^-1 with a median value of 4.2 h^-1, within the range of the results obtained by the most common empirical models.