Patent Number: 047568755
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The filtering apparatus of this invention has a construction as shown in FIG. 2. As shown, the apparatus comprises a cylindrical casing 5 containing cylindrical partition walls 7 and a head plate 12 from which a plurality of U-shaped bundles 3 of hollow fibers having a sponge-like mesh structure are suspended. Cap 20 is located on the opposite side of head plate 12 from casing 5 and defines collection chamber 22 which is in communication with the hollow interior of the fibers in bundles 3. Water containing radioactive impurities (CRUD) is admitted into the casing 5 through a pipe 8 and filtered while passing through numerous pores extending in the transverse direction of each hollow fiber. Cleaned water passes from the hollow interior of the fibers into collection chamber 22, from which it is discharged to the outside through pipe 9. The casing 5 is supported by a plurality of pedestals 13. To effect backwashing, pressure is applied to the interior of the casing 5, by pressurized air, for example. Then clean water is forced to pass through the pores to remove small solid particles trapped therein or large solid particles closing the outside openings of the pores. The solid particles thus removed are discharged to the outside of the casing 5 through a drain pipe 14 at the bottom thereof. At the time of backwashing, it is advantageous to blow air bubbles into the water in the casing 5 through air nozzles 6 provided for an air inlet pipe 4 near the lower ends of respective U-shaped bundles to vibrate the hollow fibers in various directions as shown in FIG. 4 to facilitate removal of the solid particles. Air collected beneath the head plate 12 is discharged to the outside through a discharge pipe 11. According to this invention, the hollow fibers are made of flexible polyethylene. The following Table shows comparison of various mechanical characteristics of hollow fibers made of flexible polyethylene and polyvinyl alcohol. TABLE ______________________________________ Sample Polyvinyl Item Polyethylene alcohol ______________________________________ pull strength 336 253 (g/fiber) elongation 45.2 7.6 (%) knot strength 319 impossible (g/fiber) to measure elongation 48.3 because PVA (%) fiber is hang strength 568 brittle (g/fiber) elongation 32.0 (%) ______________________________________ Remarks: As diagrammatically shown in the Table, in the pull test one end of a fiber was secured and the other end is pulled; in the knot test a knot is formed at an intermediate point of the fiber; and in the hang test one end of one U-shaped fiber is fixed, the other U-shaped fiber is hung from the one U-shaped fiber. The tested flexible polyethylene had a pore size sufficient to pass solid particles having a molecular weight of about 300,000. As can be noted from this Table the flexible polyethylene hollow fibers utilized in this invention have higher mechanical strength than conventional polyvinyl alcohol hollow fibers. It should be particularly noted that the polyethylene hollow fibers utilized in this invention are pliable or flexible, so that there are such advantages that they would not be broken by external force so that they can be handled readily and that as shown in FIG. 4 they can bend in various directions during backwashing due to a turbulent flow caused by air bubbles thereby increasing removal of trapped solid particles. In contrast, since polyvinyl alcohol hollow fibers are solid and brittle they can not manifest these advantages. As the filtering operation proceeds, the pressure difference between the inlet pipe 8 and the discharge pipe 9 increases as a result of trapping or clogging of the solid particles in the pores, so that when the pressure difference reaches a predetermined value, backwashing is carried out in a manner described above. FIGS. 5 and 6 are graphs showing the relation between the number of backwashings and the variation in the pressure difference when hollow fibers having pore diameters of 1 micron and 0.1 micron respectively are used. Comparison of FIGS. 5 and 6 shows that, in the case of pore diameter of 1 micron, the rise in the pressure difference after backwashing is large (about 3.0 Kg/cm.sup.2 or more), whereas in the case of pore diameter of 0.1 micron, the rise in the pressure difference is not so large (only about 1 Kg/cm.sup.2). This means that with hollow fibers having a pore diameter of 1 micron, clogging is liable to occur but with hollow fibers having a pore diameter of 0.1 micron, the tendency of clogging is small with the result that the frequency of renewal of the hollow fibers can be reduced. As a consequence, hollow fibers having a pore diameter of about 0.1 micron are suitable for filtering water containing radioactive solid particles.