Efficiency of a Gas‐Liquid Contactor for the Oxygenation of Activated Sludge: Assessment of Mass Transfer Coefficient

Abstract

The aim of this work is to investigate a co-current air-liquid downward flow bubble column with air entrainment by liquid injection nozzle in order to use it as an aerator in activated sludge treatment plants. The study concerns the determination of mass transfer efficiency by measuring the mass transfer coefficient, k L a, both in clean water and in activated sludge. In clean water, this parameter is determined by three methods, i.e., gassing out method, absorption with chemical reaction and off-gas method. In activated sludge medium, k L a values are measured by two methods, i.e., sludge reoxygenation and the hydrogen-peroxide method. The values of k L a obtained in clean water are compared to those obtained in sludge, enabling the assessment of the a factor, i.e., ratio of oxygen transfer coefficient sludge/clean water. The results are in good agreement with those reported previously in the literature. IntroductionOxygen is required for oxidation of organic matter in aerobic biological systems. It is also needed for biological nitrification of ammonia-nitrogen. The microorganisms involved in these reactions need oxygen for growth, product formation, and cell maintenance [1]. Thus, an adequate amount of oxygen must be transferred to the activated sludge with minimal energy consumption. Gas-liquid contactors enable this double aim to be achieved, and an interesting option is the co-current, gasliquid, down flow contactor with entrainment by a liquid jet. This apparatus requires just one pump to circulate both the liquid and the gas simultaneously. The air is aspirated by the circulation of the liquid through a nozzle, which generates a liquid jet. In a recent study [2], the current authors have reported, through the residence time distribution (RTD) method, that this contactor behaves like a perfectly mixed system under the operating conditions used in this current study. This work is the logical result of studies concerning this contactor. The question addressed here involves the testing of the system as a tool for gas-liquid mass transfer, and more particularly as an aerator for activated sludge systems. The main parameter used to characterize the oxygen transfer in aeration processes is the liquid mass transfer coefficient, k L a, where k L represents the mass transfer coefficient based on the liquid film resistance and a is the interfacial area. The authors have tested three methods to determine the k L a value, i.e., clean water reoxygenation method, off-gas method and the sulfite oxidation method 1) . The reoxygenation (or gassing out) method and the method based on absorption with chemical reaction are often used in the literature for the determination of k L a [3]. The first method supposes an ideally mixed liquid reactor, which is the case for the current reactor. Recently, Gillot [4] has proposed an alternative method to measure oxygen transfer coefficients, k L a, in biofilters in clean water. The methodology in this instance is based on the oxidation of sulfite in excess, and is adapted from the off...

Activated Sludge and other Aerobic Suspended Culture Processes

“…A new composite variable (the Equivalent Contact Time, ECT) was defined which makes it possible for a specific aeration tank to predict the α-factor knowing the SRT, the clean water oxygen transfer coefficient, and the supplied air flow. Almoustafa et al [42] discussed the use of a down-flow, gas-liquid contactor for oxygen transfer into activated sludge. The mass transfer efficiency was evaluated by measuring k La in clean water and in activated sludge.…”