Gas holdup variations driven by porosity changes in a biological aerated filter under field operation

The spatial variability of gas-liquid hydrodynamics in Biological Aerated Filters (BAFs) is critical for improving the accuracy of process models, yet it remains poorly documented under representative full-scale operating conditions. This study presents the first bed-resolved assessment (bottom and top zones) of gas holdup and porosity in a large pilot-scale BAF operated under field conditions. Both parameters were derived from pressure-drop measurements and related to local fouling induced by suspended-solids filtration and biofilm growth. Porosity ranged from 12 to 30% between the operating biofilter and clean-bed reference conditions, with a difference between top and bottom zones under biofiltration: 12-22% in the bottom zone and 16-25% in the top zone. Porosity was negatively correlated with local fouling. Gas holdup varied from 0.8-8.6% depending on fouling, and the superficial velocities of the mobile phases. The gas-to-liquid volume ratio exhibited a non-monotonic trend with porosity, increasing as porosity decreased above 28%, remaining stable between 28 and 15%, and dropping sharply below 15%, reflecting shifts in bubble hydrodynamics driven by porosity changes. Including fouling as an indicator of bed-structure changes greatly improved gas holdup predictions compared with correlations that omit this effect.