In Situ Local Parameter Measurements for CFD Modeling to Optimize Aeration

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ABSTRACT
Measurement methods to determine in situ local parameters were developed, in order to optimize design and operating parameters impacting oxygen transfer in aeration tanks equipped with EDPM membrane diffusers and slow speed mixers. New tools to measure bubble sizes and gas hold-ups were coupled with the traditional ones used to determine liquid velocities and oxygen transfer coefficients. These methods have been developed and applied to an annular loop reactor (1493 m3). Using an immersed camera, 100 bubbles are sufficient to determine the local Sauter diameter by image analysis. Obtained results are reproducible and independent of the operator. The gas hold-up, deduced from water level measurements with the help of a magnetostrictif level meter, can be determined with a confidence interval of ± 5% using an integration time of 380 s. The obtained increase in the oxygen transfer coefficient (+ 29%) with the horizontal liquid velocity (from 0 to 0.42 m/s) is mainly due to the increase of the global gas hold-up, the bubble size varying only slightly (from 0.46 to 0.43 cm). These results will be used as input data and validation data to model hydrodynamics and mass transfer, in order to set up a simulation methodology for aeration tanks using computational fluid dynamics.

INTRODUCTION
Aeration can represent up to 70% of the energy expenditure of an activated sludge wastewater treatment plant. Optimizing this process is therefore required to reduce operating costs, in addition to guaranty a reliable and efficient treatment. Since the last 20 years, fine bubble aeration systems have extensively been implemented. They proved to give high oxygenation performances, and to be adaptive to oxygen requirements. For loop reactors commonly installed in Europe, aeration is separated from mixing, carried out by slow speed mixers. To optimize aeration capacities, numerous on site results pointed out the main parameters impacting oxygen transfer (Groves et al., 1992; Wagner et al., 1998; Gillot et al., 2000; Mueller et al., 2002; Gillot et al., 2005). Relationships resulting from a dimensional analysis (Gillot et al., 2005) pointed out the difficulty in predicting the oxygen transfer in deep aeration tanks. This may be due to parameters that were not taken into account in the analysis (bubble diameters or mixer pumping areas, for example). More precise information on the local hydrodynamics of aeration tanks is therefore necessary in order to better understand the impact of design and operating parameters on oxygen transfer. At the same time, computational fluid dynamics (CFD) is more and more used to optimize aeration systems (Simon, 2000; Cockx et al., 2001; Vermande et al., 2003; Vermande et al., 2005). However, these studies are still facing a lack of in situ measurements of the local physical parameters that govern oxygen mass transfer and hydrodynamics, i.e. bubble size, gas hold-up and local liquid velocities. Some of this local data are essential for a robust modeling as input data (i.e. bubble size), the other parameters allowing the validation process (i.e. oxygen transfer coefficient, gas hold-up, local liquid velocity). The objective of this work was therefore to develop measurement methods in order to characterize the hydrodynamics of full scale annular ditches equipped with fine bubble EPDM membrane diffusers and slow speed mixers. Methods to measure bubble sizes, gas hold-ups and horizontal liquid velocities have been developed and applied to an annular loop reactor (1 493 m3). Local parameter values are used to analyze the results obtained, concerning the impact of the horizontal velocity on oxygen transfer.

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