A driven decanter assembly for a wastewater primary treatment system, comprising a longitudinal decanter disposable in the wastewater and having a longitudinal trough and a horizontal entrance weir, a polygonal or arcuate screen attached to the decanter and having at least two screen surfaces disposable below the surface of the wastewater outside of the trough, and a floating baffle pivotably mounted to the decanter and disposable in the wastewater outside of the screen. The driven decanter assembly can directly replace a prior art driven decanter in any waste water primary treatment system.