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SECTION II. APPLIED BIOTECHNOLOGY IN THE PHARMACEUTICAL INDUSTRY... 471equilibrium after 180 minutes. The adsorption capacity of the material decreased by 20.96 % and 37.10% after the second and third reuses, respectively. Next, the hydrogel bead was used to evaluate the treatment efficiency of ammonium-rich wastewater. The test ammonium concentration gradually increased from 10 to 200 mg/L. As a result, the chemical oxidation demand (COD) and ammonium treatment efficiencies reached 93.67%; and 95.47%, respectively.Keywords: Activated sludge immobilization, hydrogel material, Graphene oxide, adsorption capacity, ammonium-rich wastewater.1. INTRODUCTIONAmmonium is considered one of the main substances contributing to water pollution. Long-term exposure and absorption of ammonium compounds can cause adverse effects on the environment and living organisms. Therefore, controlling and minimizing ammonium emissions from human activities is crucial to protecting the natural environment. Many methods of treating ammonium in water and wastewater have been studied and applied worldwide, including Vietnam. In particular, biological technologies using activated sludge are widely applied due to their technical and economic advantages. Recently, the technology of immobilizing microbial biomass using hydrogelbead is getting a lot of attention thanks to its outstanding advantages compared to suspended growth processes.There have been many studies on the use of hydrogel-bead in wastewater treatment. Typically, the research group of Kazuichi Isaka (2017) [1] published results of the first full-scale system (580 m3/day) applying the partial nitrification-Anammox technologies with immobilizing biomass by hydrogel beads to treat wastewater