Air-Water Transport Processes

In addition to hydrogen sulfide used as an example, a number of other compounds in wastewater, for example, NH3 and VFAs, exist in both a molecular form and an ionic form. Therefore, conditions similar to those described for H2S exist for these substances. 

A number of approaches to the air-water mass transport exist. In relation to transport processes in sewer networks, the main developments have been directed toward the air-water oxygen transfer. The following are the main theoretical descriptions that are relevant in this respect  

Further details on air-water mass transfer phenomena may be found in Thibodeaux (1996) and Stumm and Morgan (1981). 

The two-film theory considering molecular diffusion through stagnant liquid and gas films is the traditional way of understanding mass transfer across the air-water boundary. As briefly described, other theories exist. However, the two-film theory gives an understanding of fundamental phenomena that may lead to simple empirical expressions for use in practice. 

According to the two-film theory, it is appropriate to consider the transport of volatile components between the water phase and the air phase in two steps from the bulk water phase to the interface and from the interface to the air, or vice versa. The driving force for the transfer of mass per unit surface area from the water phase to the interface and from the interface to the air phase is determined from the difference between the actual molar fractions, xA and yA, and the corresponding equilibrium values, xA and yA  

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