This paper focuses on hybrid moving-bed biofilm reactors (MBBRs). For this research, we arranged a pilot-scale activated sludge plant with an anoxic tank (1 m(3)), an aerated tank (1 m(3)), and a settler (1 m(3)); it was fed with municipal wastewater (at a rate of 300 L/h). This plant removed only 11% of the NH4-N, so the aerated tank was divided into two reactors and polyethylene carriers were placed into each reactor. The upgraded plant removed 86% of the ammonium and 73% of the total nitrogen. The following step was a full-scale plant. A municipal activated sludge wastewater treatment plant, originally built for 3000 p.e., received a nitrogen load corresponding to 5800 p.e. (The term "p.e." represents the persons equivalent.) A sector of each aerated tank was transformed to a hybrid MBBR: the aeration equipment was changed and polyethylene carriers were placed in these sectors. With an incoming total Kjeldahl nitrogen (TKN) concentration of 79 mg/L, the outgoing NH4-N concentration was <1 mg/L.