Conventional methods for immobilization of microorganisms in hydrogel are not very efficient and cost-effective. Ultraviolet (UV) technology, a rapid method for hydrogel preparation in ambient environment, was applied to immobilize activated sludge in poly(ethylene glycol) (PEG) gel carriers synthesized from PEG pre-polymer. Conditions for cell immobilization were optimized by selecting suitable UV photoinitiators, crosslinker concentrations, and protection gas. The optimum conditions are the following: 12% PEG pre-polymer with 1% MBA and 0.1% DEAP. More than 90% of ammonia was reduced, and 75% of the total organic compound was removed after treating 40 and 10 mg/L synthetic ammonia wastewater with different C/N ratios in 5 h under aeration conditions. Ammonia degradation increased with increasing C/N ratios, and the scanning electron microscopy images revealed that the immobilized beads had porous structures dominantly filled with nitrifiers. The bioactivity of the immobilized activated sludge was maintained during immobilization by UV technology. (C) 2010 Elsevier Ltd. All rights reserved.