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The influence of the addition of drag reducing polymers and fibers on two transport processes in mechanically agitated liquid systems is examined. The first one concerns the determination of mixing times, which were found to be insignificantly to moderately increased in the drag reducing systems. The second one concerns the determination of the critical RPM at which complete suspension of solid particles occurs. It was shown that this was unaffected in the drag reducing systems. The results of this work can be applied to fermentation processes, biological reactors with flocs, and metallurgical processes. The theoretical implications of this work are to be seen in the association of polymeric and fibrous drag reducing additives with different parts of the turbulent spectrum. SCOPEAlthough a considerable body of knowledge exists on the reduction of turbulent skin friction in liquids by polymers, surfactants, and fibers, most of this work as reviewed recently by Hoyt (1972a, b), Virk (1975), and White (1976) refers either to internal flows in conduits or to external flows past regularly shaped bodies. Recently, a paper written by Quraishi et al. (1976) reports on the torque suppression by polymeric and fibrous drag reducing additives in mechanically stirred tanks. I t is suggested that this phenomenon, apart from having theoretical implications, could be applied to those technologies where extremely small quantities of drag reducing additives are not harmful and where the presence of a second phase (gaseous or solid) actually enhances the suppression of torque.The question of mass transfer and of the mixing performance was, however, in the work of Quraishi et al. (1976), left open. Since agitation is never done for its own sake, it must be highly desirable to find out what influence the drag reducing additives might exercise upon the transport processes in stirred tanks. Then and only then can the significance of using drag reducing additives in agitated tanks be properly assessed.For aerobic wastewater treatment and for fermentation processes, the gas-liquid mass transfer of oxygen is of crucial importance. The work on these aspects is still in progress, and the results will be reported shortly. The scope of this paper is to report on the liquid-phase mixing and on the suspending of solid particles in stirred tanks with drag reducing additives. CONCLUSIONS A N D SIGNIFICANCEPolyacrylamid and polyethylenoxide on their own and in combination with glass fibers were used to suppress the torque of standard six-blade turbines operating in two acrylic-glass tanks (0.24 and 0.30 m). The liquidphase mixing was assessed by measuring mixing times, that is, the times required for an iodine-sodium thiosulfate reaction to reach completion. It was found that up to the concentration of 250 p.p.m. of a polymer, the increase of mixing time is insignificant. In 250 p.p.m. polyacrylamide solutions, the mixing times could be, however, increased by as much as a factor of 2. Still larger increases were found when glass fibers were used (...

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