The inoculation of conventional activated sludge is a prospective option against transferring anaerobic ammonium oxidation (ANAMMOX) biomass from a currently existing ANAMMOX process in places where large amounts of ANAMMOX biomass are not available. In this study, poly(vinyl alcohol)/sodium alginate gel beads were utilized as an immobilization system to entrap activated sludge. Despite the longer start-up periods than those in previous examples using the ANAMMOX inoculum, the acclimation of ANAMMOX bacteria in the gel beads was successful. The maximum nitrogen removal rate of 1.12 kg N/m3-day was established at a nitrogen loading rate of 1.26 ± 0.04 kg N/m3-day with a total nitrogen removal efficiency of 88.9%. The exponential growth rate in the initial phase was enhanced by a higher nitrogen loading rate. The area-to-volume ratio of the gel beads was the significant control factor for the lag period, showing an inverse relationship due to the internal mass transport limitation. Based on the 16S rRNA gene, a sharp increase in the amount of ANAMMOX bacteria was identified along with an increase in the nitrogen removal rate. The responsible ANAMMOX bacteria were the species related to Candidatus ‘Brocadia sinica’.