An Enhanced Oxygen Transfer Model for Water Treatment

Abstract

Johnny Lee and Yovanni A. Cataño-Lopera

The oxygen transfer efficiency of an aeration device based on a clean water test is defined by a standard specific baseline parameter for the mass transfer coefficient (K L a 0 ) that exemplifies the effect of gas depletion in oxygen transfer. The baseline is defined as the oxygen transfer rate coefficient at zero depth. The purpose of the work is to propose an enhanced oxygen transfer equation that would more accurately estimate the device performance in the field using the procedures as stipulated in the ASCE/EWRI standards. The enhanced model is applied to wastewater through an alpha factor (α) that pertains particularly to the wastewater characteristics. The paper argues that the alpha factor (the ratio of mass transfer in process water versus clean water) is independent of microbial activity. The theoretical baseline is a groundbreaking development in physics that can be used to derive the K L a for different heights and wastewater compositions. In this study, a new equation to estimate K L a is presented to unify all the standards applicable not only to clean water but also to most types of respiring systems. The proposed modified mass balance equation, where the consumption rate due to microbial respiration in the activated sludge should be considered twice as high as in conventional models to account for the impact of microbial resistance is a key finding. Based on case studies found in the literature, the parameter K L a is expected to be estimable with the help of computational fluid dynamics (CFD) and experimental testing in future studies. One key point is that the effect of gas-side oxygen depletion effects on C* (the saturation concentration) more than on the K L a coefficient, that without this understanding CFD or other modeling work cannot be successfully applied.

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