Effect of increasing organic loading rates on the performance of moving-bed biofilm reactors filled with different support media: Assessing the activity of suspended and attached biomass fractions

• Two MBBRs with different media were subjected to increasing organic loading rates.
• Complete COD and NH4 removal were achieved at 3.2 kgCOD/(m3 d) in both reactors.
• The distribution of biomass in the media affected the functionality of the MBBRs.
• The suspended biomass fraction greatly contributed to the overall nitrification.
• At constant COD load, nitrification performance was governed by the applied HRT.

In this study, two moving-bed biofilm reactors (MBBR1 and MBBR2) filled with different carrier media (Kaldnes K1 and Mutag Biochip, respectively) were subjected to increasing organic loading rates for 700 days. Regardless of the carrier used, both systems could withstand high organic loads up to 3.2 kgCOD/(m3 d), condition under which complete ammonium removal was achieved. However, the type of media influenced the quantity and distribution of attached biomass in the support, which in turn affected the activity of specific microbial functional groups in the biofilm. As the chemical oxygen demand (COD) input was gradually increased, the biofilm got thicker and the surface detachment rates were enhanced. Consequently, the amount of suspended solids has increased considerably to levels commonly found in hybrid bioreactors. Activity batch tests have shown that the contribution of the bulk phase biomass to the overall nitrification was very significant, being more relevant as the biofilm sloughing events became more intense. At constant organic loading rate, the hydraulic retention time (HRT) had a noticeable impact on the nitrification process, as it directly influenced the fraction of ammonium oxidized either by the attached or suspended biomass. Total nitrogen removal amounted up to 86 and 73% in MBBR1 and MBBR2, respectively.