A wastewater treatment system including a biological reactor is disclosed. At least one controlled-reaction-volume module is provided to the biological reactor. The at least one controlled-reaction-volume module includes a fixed-film media for supporting a biological growth. At least one mixer, such as, for example, a high momentum mixer, for communicate a fluid to the at least one controlled-reaction-volume module so that wastewater and the biological growth communicate to thereby treat the wastewater.