Transfer rate reaeration

We must next formulate an expression for the mass transfer rate N g and here we encounter the same difficulty we had seen in Illustration 2.2 dealing with the reaeration of rivers. In both cases the interfacial area is unknown and we must therefore resort again to the use of a volumetric mass transfer coefficient Kji where the unknown interfacial area a (m /m column volume) is lumped together with If the gas phase is assumed to be controlling, we can write... 

Under equiUbrium or near-equiUbrium conditions, the distribution of volatile species between gas and water phases can be described in terms of Henry s law. The rate of transfer of a compound across the water-gas phase boundary can be characterized by a mass-transfer coefficient and the activity gradient at the air—water interface. In addition, these substance-specific coefficients depend on the turbulence, interfacial area, and other conditions of the aquatic systems. They may be related to the exchange constant of oxygen as a reference substance for a system-independent parameter reaeration coefficients are often known for individual rivers and lakes. 

A major problem in the quantification of air-water transport phenomena in terms of the rate expression [Equation (4.18)] is to find appropriate values for Kl. As far as sewer systems are concerned, the most well-established knowledge concerning air-water mass transfer is on reaeration (Section 4.4). 

Several empirical relationships for reaeration coefficients were recently reviewed and tested by Moog and Jirka (1995), who found that the form of the relationship given in Eq. (20.15) best characterized stream reaeration rates. A liquid mass transfer coefficient for each compound, ki can then be determined from a ratio of the Schmidt numbers (Mackay and Yven, 1983) ... 

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