Beyond Surface Area: The Role of Carrier Media Structure on Nitrification Performance and Biomass Activity in MBBR...

This study evaluated the effects of increasing nitrogen loading rates and carrier type on nitrification performance and biomass characteristics in moving bed biofilm reactors (MBBRs) operated under autotrophic conditions. Two laboratory-scale reactors (R1 and R2), operated for over 1200 days and filled with distinct carrier media, non-porous Kaldnes K1 and porous Mutag Biochip, were subjected to stepwise nitrogen loading up to 11 g NH₄ ⁺-N/(m².d). Both systems maintained stable ammonium removal up to 3 g NH₄ ⁺-N/(m².d), but higher loading rates led to distinct performance patterns. R1, filled with Kaldnes K1 media, exhibited greater stability and higher nitrogen removal rates (exceeding 6 g NH₄ ⁺-N/(m².d)), whereas R2 (filled with Mutag Biochip carriers) showed reduced efficiency and higher nitrite accumulation above 6 g NH₄ ⁺-N/(m².d), likely associated with free ammonia inhibition and biomass detachment. Biomass accumulation was assessed through measurements of attached solids and microscopic examination of the carrier media. Although both reactors had the same theoretical surface area, R1 developed a thicker and more stable biofilm with higher biomass retention, whereas R2 showed limited immobilized biomass accumulation. The results suggest that nitrification efficiency under high nitrogen loads was influenced not only by available surface area and chemical inhibition, but also by the structural characteristics of the carrier, which affected biofilm accumulation and mass transfer. These findings provide insights that may support carrier selection and operational strategies to enhance the stability and performance of MBBRs treating high-strength nitrogen wastewaters.