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. From operating day 1 till 86, the residual wastewater was prepared by diluting a peptone mixture and meat extract (medium I).From day 87 on, medium II was used, and a mixture of sodium acetate and acetic acid were used as carbon source. The results obtained have show a high removal efficiency of both COD and N of 90% and 85%, respectively. With regard to P removal, best results were obtained when the system was fed with medium II, achieving P efficiencies above 95%. The use of the plastic support particles made feasible the development of nitrifying biofilm. This improved the stability of the system against the disturbances that took place in the reactor.