A stormwater treatment unit comprising a containment vault having an inlet and an outlet separates debris from a flow of stormwater through the unit. The water flow is controlled by a wall which directs flow from the inlet to the outlet through a reservoir of fluid in the unit. Debris separation by density occurs whereby lighter elements such as oil float on top of the fluid and heavier elements such as sediment collect at the bottom of the unit or in collectors in the reservoir. A wire mesh, deflector, and/or ripple boards placed beneath the inlet further increase efficiency of the separation, and a vent pipe is placed above the outlet to release pressure built up in the outlet during operation. A stormwater treatment system and method of retrofitting an existing stormwater trunk line locates the stormwater treatment unit in an off-line position from an existing drainage trunk line.