This work focuses on the removal of ammonia nitrogen pollution from wastewaters in a two-stage laboratory model based on a combination of the nitritation and anammox processes with the biomass immobilized in a polyvinyl alcohol (PVA) matrix. Owing to the immobilization approach inside the PVA pellets, the bacterial activity remained nearly unchanged on an abrupt change in the environmental conditions. The nitritation kinetics were significantly dependent on the dissolved oxygen concentration. The critical dissolved oxygen concentration at which the nitritation process using the immobilized bacterial culture stops is 0.6 mg/L. The volumetric rate of nitrogen removal by the anammox bacteria was 158 mg/(L·d). The technology presented is well-suited for removing high ammonia nitrogen concentrations (≥300 mg/L).