a substantial portion of the width of upper portion 44 terminates in bottom edge 50 which should be between 6 inches and 2 feet below the liquid level the lower portion 43 of partition 42 extends outwardly from one side of the basin preferably about half the width and terminates in a vertical edge 52 such that a large open liquid flowpath 54 is provided between adjacent liquid zones or passes i through vi as further illustrated by the dotted lines 54 in fig3 alternatively open liquid flowpath 54 may comprise a plurality of smaller flowpaths where for example vertical columns and or horizontal cross pieces are required to extend through the illustrated flowpath 54 for purposes of structural integrity that is it has been determined by pilot plant studies using dye tracer techniques that the benefits of the present invention may be achieved so long as the total size of the open liquid flowpath 54 is within the operable range of 30 to 80 of the wetted cross sectional area of the basin as defined by w u00d7 h preferably the percent of the open flowpath should be within the range of 40 to 60 and for optimum results it should be within the range of 40 to 50 of the wetted cross sectional area of the basin it has also been discovered that the length to width ratio l 2 w of each liquid zone or pass should be at least within the range of 0 59 to 1 70 furthermore it has been determined that the preferred ratio of l 2 w should be in the range of 0 60 to 1 00 and that optimum results were obtained within the range of 0 65 to 0 85.