Ammonia diffusivity in biofilm reactors: impact of polyurethane foam thickness and pore clogging on simultaneous...

Simultaneous nitrification and denitrification (SND) processes represent a promising approach for nitrogen removal from effluents characterized by a low COD/N ratio, especially when combined with fixed-bed reactors to ensure that slow-growing biomass (e.g., nitrifiers) is not washed out. In this reactor configuration, biofilms are formed, which promote mass transport of the substrates involved in SND. Therefore, understanding the effective diffusivity of ammonia through the biofilm is essential to improve nitrogen removal, as it is influenced by the thickness of the support media and biomass growth, particularly under counter-diffusion conditions. For this type of study, flow cells (units for study particularities of a bioreactor) are used, as they provide greater operational control of the process. To evaluate this issue, were operated three flow cells for 234 days, each one with different thicknesses of polyurethane foam (i.e., 2 mm, 5 mm and 10 mm) as a support media for SND adhered biomass. Within each flow cell, the foam serves to segregate the aerated and non-aerated zones, thereby inducing counter-diffusion. Throughout operation, tests were conducted to estimate the effective diffusivity factor (EDF) of ammonia in the biofilm using the AQUASIM software. Routine analyses demonstrated that the average removal of organic matter and ammoniacal nitrogen were 73%, 68%, 57%, and 66%, 54%, 34% in the 2, 5, and 10 flow cells, respectively. Furthermore, EDF estimation tests demonstrated a 95% reduction in ammonia diffusivity over operating time, attributable to pore clogging induced by heterotrophic biomass growth within the support media. The decline in EDF of ammonia exerted a substantial impact on the total nitrogen removal and, consequently, on the performance of the simultaneous nitrification and denitrification process. Thus, the importance of considering mass transport phenomena in reactor designs with support media and long operating times, i.e., with biofilm growth and establishment, becomes evident.