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.