Development of a Model to Determine the Baseline Mass Transfer Coefficient in Bioreactors (Aeration Tanks)

Over 60 percent of all wastewater treatment plants in the developed countries use the activated sludge process as their secondary treatment system. About 50 to 85 percent of the total energy consumed in a biological wastewater treatment plant is in aeration. The activated sludge process, the most common such process, is performed in large aeration basins to provide air for microorganisms to remove nutrients and pollutants through biodegradation. Designs for the air supply often fail to meet sustained peak organic loading, which may lead to unsatisfactory treatment performance and even plant failure. On the other hand, excessive safety factors used in design for the air supply may similarly lead to unsatisfactory treatment performance and energy wastage. Improper sizing of the aeration system is primarily due to inability to estimate the mass transfer coefficient (K(L)a) correctly for different tank depths, leading to improper blower design and to inappropriate operation, among other things. In general, the efficiency of porous fine-bubble diffuser systems varies from 10 to 30 percent or more, depending on tank depth. The term K(L)a is used in ASCE/EWRI Standard 2-06 to define the apparent volumetric mass transfer coefficient in a non-steady-state clean water test in an aeration tank. This parameter is a function of tank depth, due to the variation of oxygen gas depletion with depth. The objective of this paper is to introduce a baseline oxygen mass transfer coefficient (K(L)a(0)), a hypothetical parameter defined as the oxygen transfer rate coefficient at zero depth, and to develop new models relating K(L)a to the baseline K(L)a(0) as a function of temperature, system characteristics (e.g., the gas flow rate, the diffuser depth Z(d)), and oxygen solubility (C-s). Results of this study on data extracted from the literature indicate that a uniform value of K(L)a(0) that is independent of tank depth can be obtained experimentally. Using the baseline, a family of rating curves for K(L)a(20) (the standardized K(L)a at 20 degrees C) can thus be constructed for various gas flow rates applied to various tank depths. The new model relating K(L)a to the baseline K(L)a(0) is an exponential function, and (K(L)a(0))(T) is found to be inversely proportional to oxygen solubility in water (C-s)(T) to a high degree of correlation. Using a predetermined baseline K(L)a(0), the new model predicts oxygen transfer coefficients K(L)a(20), for any tank depth to within 1-3% error compared to observed measurements, and similarly for the standard oxygen transfer efficiency. The discovery of a standard baseline (K(L)a(0))(20) determined from shop tests is important for predicting the K(L)a(20) value for any other aeration tank depth and gas flowrate, and this finding can be used in the development of energy optimization strategies for wastewater treatment plants. This work may also improve the accuracy of aeration models used for aeration system evaluations.