A UCT modified hybrid system composed by an anaerobic, anoxic and aerobic chambers connected in series, was operated at laboratory scale. Biomass growth in the system was promoted both in suspension and adhered onto small granular plastic particles. The granular particles were maintained in suspension in the aerobic chamber. During the operation the Hydraulic Retention Time was varied from 18 to 10 hours. Solids Retention Time was 15 d. Two different synthetic wastewaters (medium I and II) with a similar composition than urban wastewater, were fed to the system. The reactor was operated to promote the removal of C, N and P in the wastewaters. The system was operated continuously during 150 days.