A flexible pilot plant, fed with primary settled wastewater, was used to study (1) organic carbon and nitrification in a two(three)-stage continuous-flow aerobic process and (2) organic carbon and nitrogen removal in a heterotrophic moving-bed sequencing batch biofilm reactor (MBSBBR) for denitrification and in a continuous-flow autotrophic moving-bed biofilm reactor (MBBR) for nitrification. In both experiments the same polyethylene biofilm carriers were used. Filtered COD removal rates in aerobic conditions appear to be proportional to the corresponding loading rates up to 8 gCOD m−2 d−1. Nitrification tests, performed in oxygen limiting conditions and ammonia limiting conditions, showed that the reaction rate was nearly first order with respect to dissolved oxygen due to liquid film diffusion. Denitrification tests, performed without external carbon sources, showed that the denitrification rate never fell below 0.3 gNO3-N m−2 d−1 even at very low biodegradable filtered COD loading rates.