4 the component common to fig1 and 7 are denoted with the same indica following the mixing and settling steps in the two treatment zones the controller 30 activates both a decanting means pump 36 and an influent pump 24 simultaneously this results in a first predetermined volume of clarified liquid phase being removed from the last treatment zone 12 via a conduit 34 and a second predetermined volume of influent wastewater essentially equal to said first volume simultaneously added to said first treatment zone via a conduit 26 the influent wastewater enters the first treatment zone 10 via a plug flow producing means such as a pipe manifold 28 with multiple outlets which displaces clarified liquid phase in a plug flow mode from the upper portion 20 of the zone 10 through the connecting conduit 14 and into the last treatment zone 12 through an inlet conduit 26a and a pipe manifold 28a with multiple outlets perforated baffles may be substituted for the pipe manifolds 28 and 28a as described above the volume of wastewater added to the first treatment zone 10 is less than the volume of clarified liquid phase therein thus preventing flow of untreated influent wastewater into the last treatment zone this is easily monitored by the controller 30 the biological treatment systems of fig1 and 7 may be modified to a biophysical treatment system by operating.