Startup of aerobic granulation technology: Troubleshooting scale-up issue

Abstract

Numerous laboratory-scale studies confirmed the effectiveness of aerobic granulation technology for diverse treatment purposes. However, few pilot scale investigations have been conducted so far, and these studies revealed that a large-scale module could frustrate the microbial granulation process. In this study, the effect of scale-up on granule formation was investigated. Upflow air velocity, a function of the aeration rate, is normally deemed to be the main source of hydrodynamic shear force in bubble column reactors. Shear force is known as one of the important inducers of aerobic granulation. Superficial upflow air velocity (SUAV) is defined as aeration rate per cross-section area of the reactor. However, the findings of this study proved that maintaining SUAV was not sufficient for successful granulation. In addition, the parameter SUAV could not well represent the hydrodynamics of bubble column reactors, especially during the scale-up procedure. This study proved that air bubble distribution was of greater importance, as opposed to the effect of aeration rate. Mean distance between bubbles should be maintained constant regardless of the size of reactor. This provided similar shearing activity in different reactors with different scales, which appeared to be the critical requirement for successful granulation process in a larger module. To ensure similar bubble distances in bioreactors of different sizes, diffusers with the same porosity should be used, and their surface areas and the aeration rate should be increased by the scale-up ratio. Air bubble distribution dictated the gas holdup which was eventually determined to be the crucial factor for the scale-up of aerobic granular bubble column reactors.