A radial-flow filter (120) employing nonbonded granular particles (56) for filtering impurities from an influent, and that can be backwashed by fluidizing the granular particles (56) to free the impurities therefrom. During a backwashing operation, a backwash liquid applies an upwardly-directed drag force on an upper portion of the granular particles (56) to lift the same into a backwash chamber (62) for fluidization. Once the top portion of the granular particles (56) of the filtration bed is fluidized, the backwash liquid applies an upward force on a subsequent portion of the granular bed, thereby fluidizing the granular particles (56) in the backwash chamber (62). Subsequent sections of the granular bed are fluidized in a similar manner to thereby completely clean the granular particles (56). After the backwash operation, the granular particles (56) fall back to the filtration chamber (156) and form a filter bed for carrying out a filtration operation.