A parallel study of an-hybrid reactors with total media volume/total reactor volume ratio of 0.2, 0.5 and 0.8 was carried out under unsteady state start-up conditions. The results show that 20 to 50% media packings in an-hybrid reactors was evaluated to be sufficient to prevent the biological solids from escaping through effluent. It was also found that the hydraulic dead space and plug flow regime in the reactors trended to increase with the increase of packed media volume.