Column filter using bundles of long fibers

Abstract: A column filter using bundles of long fibers (3) is disclosed. The bundles are fixed at their lower end portions but are free-standing at their upper end portions. The length of the fibers in the bundles may preferably be longer than 1,000 mm but shorter than 3,000 mm - especially longer than 2,000 mm but shorter than 3,000 mm. During operation, the bundles may be compressed to a height of 80-30%, generally 50-40% of their original height. A plurality of holders (2′) for the respective bundles, each of said holders being formed of a cap (21) and a tube (23) communicating to each other, said cap (21) defining orifices (20) and said tube (23) defining a slot (22) through a side wall (41) and opening at both upper and lower ends, are provided on a perforated plate (2) arranged transverse­ ly in a lower interior part of the shell (1) with the cap (21) being located above the perforated plate (2) and the tube (23) being positioned below the perforated plate (2). The holders (2′) cover substantially all perforations (24) of the perforated plate (2). The lower end portion of each of the bundles is fixed to a lower periphery of a side wall (42) of the associated cap (21).

Description: This invention relates to a column filter which uses bundles of long fibers as a filter medium to remove at a high rate suspended solids from raw liquids such as city water, industrial water, sewage, river water, lake or pond water, supernatant waters from coagulation and settling treatments, waters discharged intermediately during the practice of various processes, recovered waters such as those from pulp- and paper-making processes, various waste waters, processing waters or valuable-material-containing liquids from biological treatment apparatus, alcoholic beverages, oils and the like. A variety of filters have conventionally been used to remove suspended solids from a raw liquid. They may be classified into several different types, including filters adapted for the removal of relatively large suspended solids from a raw liquid by a screen such as a wire mesh, filters employed for the microfiltration of fine particles from a raw liquid by a filter membrane having micropores, and filters using filter elements, which carry a precoat layer of a filter aid coated thereon, for the removal of suspended solids. Various filters are thus used depending on the kinds of raw liquids and the purposes of their filtration. Among these, filters of the packed medium type are most common. A typical packed medium filter has a cylindrical upright shell with a packed layer of a granular filter medium such as filter sand or anthracite or a fibrous filter medium such as short fibers or fiber balls. A raw liquid is passed as a downflow or upflow through the packed medium layer so that solids suspended in the raw liquid may be trapped in the packed layer to obtain a filtered liquid, namely, filtrate. Filtration is stopped when the pressure drop has increased to a predetermined level or the quality of the filtered liquid has fallen to a prescribed level. The packed medium layer is then flushed with water or expanded or agitated by a water stream, and air or the like, so that the solids thus trapped may be flushed away to permit resumption of the filtration through the packed medium layer thus flushed. With packed medium filters it is desired that in the filtration step the pressure drop due to the filter medium resistance does not increase substantially and that the quality of the filtered liquid remains good even when filtration is at a high flow rate; and that in the washing step the solids trapped in the packed medium layer can be flushed completely with a minimum of both water and time. A filter using a conventional granular filter medium such as sand or anthracite is however accompanied by drawbacks - feed velocity of the liquid through the filter medium can be as low as 20 m/hr even at the maximum, and, depending on the kind of the suspended solids, the filter medium is prone to prematured clogging and results in an increased pressure drop at a relatively early stage. During washing, this filter develops further drawbacks - washing must be at a flow rate slow enough to prevent the granular filter medium from flowing out of the shell, and the washing thus tends to be insufficient to prevent the gradual accumulation of suspended solids and, in some instances, the formation of mud balls or the like may result. There are still further drawbacks in that a relatively large quantity of washing water is required and the level of concentration of suspended solids in the effluent of washing is small. A filter using a medium such as short fibers or fibrous balls allows free choice in size of the filter medium. It thus has an advantage that an optimal filter medium can be chosen depending on the kind of solids suspended in a raw liquid. Because the filter medium used has an extremely light weight, the filter is accompanied by drawbacks; for example, a screen or the like must be provided to prevent the filter medium from flowing out of the shell during the washing, the structure of the filter tends to become complex, and the screen becomes an obstacle to hamper the removal of trapped solids from the shell during washing, so that the washing cannot be effected sufficiently, resulting in the gradual accumulation of trapped solids. A high rate filter - which solves the above-described drawbacks of conventional packed medium filters, permits filtration at a high rate without any substantial increase in the pressure drop by filtration resistance, and can be washed completely with a small quantity of washing water in a short time, by using a filter with bundles of long fibers - has already been proposed by the present inventors in JP-A-315110/1988. The above-proposed high rate filter has a support provided transversely or packed within a shell and bundles of long fibers 400-1,000 mm long fixed at lower end portions thereof on an upper part of the support and free-standing at upper end portions thereof, whereby a filter medium is formed. A raw liquid is passed as a downflow from the upper end portions of the fibers toward the lower end portions thereof, so that suspended solids may be trapped in interstitial spaces among the long fibers. The use of a bundled long fiber filter has various advantages unavailable from the use of filters packed with conventional filter sand or anthracite; for example, a high filtration rate, pressure drop does not increase to any substantial extent, and complete washing is possible with a smaller quantity of wash water and in a shorter time. However, the filter has the problem that the operable filtration time is rather short, which is not satisfactory. SUMMARY OF THE INVENTION An object of this invention is to solve the above-described drawbacks of the filter proposed previously, which uses bundles of long fibers as a filter medium, and hence to provide a column filter featuring a longer operable filtration time while retaining the advantages of the filter proposed previously. According to the invention this object is solved with a column filter using bundles of long fibers, said filter being provided with an upright cylindrical shell, the bundles of long fibers being arranged substantially upright inside the shell with lower end portions thereof being fixed and upper end portions thereof being free-standing, a raw liquid feed line being provided at the top part of the shell and a filtered liquid outlet line being provided at the bottom part of the shell, characterized in that a supporting member is arranged transversely in a lower interior part of the shell, the bundles of long fibers have a length in a range longer than 1,000 mm, but shorter than 3,000 mm and are fixed at the lower end portions thereof on an upper surface of said supporting member, and the packing density of the bundles is in a range of 25 - 110 kg in terms of the dry weight of the bundles per bulk volume m³ of the bundles so that the bundles may retain 30 - 80% of the original height thereof while the raw liquid with suspended solids is being fed into the shell and compressing the bundles toward the supporting member. Other objects of the invention, which are no longer covered by the present application, are divided out and are prosecuted in the divisional European application EP-A-0 488 992 filed February 16, 1992 The fixing of the lower end portions of the bundles of long fibers can be effected, for example, by pinching the lower end portions, adhering the lower end portions, or fusion-bonding the lower end portions themselves. The column filter according to this invention allows to perform backwashing with backwashing water and/or compressed air of a high flow velocity in a short time. Compared to the conventional filters, the backwashing time can be shortened significantly and trapped solids can be surely discharged from the shell by backwashing. Different from the conventional filters, the column filter according to this invention is free from the accumulation of trapped solids and requires backwashing water in a far smaller quantity. Further, the column filter according to this invention allows to increase the flow velocity to a considerable extent compared to the conventional filters. It is hence possible to reduce the cross-sectional area of the column filter. This obviously leads to a considerable reduction in installation cost and space. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 through 3 illustrate a column filter according to a first embodiment of this invention, in which: FIG. 1 is a simplified vertical cross-section of the column filter in an initial stage of filtration, FIG. 2 is a simplified vertical cross-section of the column filter in a final stage of filtration, and FIG. 3 is a simplified vertical cross-section of the column filter under backwashing; DETAILED DESCRIPTION OF THE INVENTION Taking water with solids suspended therein as an exemplary raw liquid, the present invention will hereinafter be described in detail with reference to the drawings. Referring first to FIG. 1, a perforated plate 2 is arranged transversely in a lower interior part of an upright cylindrical shell 1. Bundles of fibers 3, which are longer than 1,000 mm but shorter than 3,000 mm, are fixed at lower end portions thereof on the plate 2, while their upper end portions are free-standing. A raw water feed line 4 is provided in communication with a top part of the shell 1 and a backwash water discharge line 5 is connected as a branch to the raw water feed line 4. Further, a filtered water outlet line 6 is provided in communication with a bottom part of the shell 1. Backwash water feed line 7 and air feed line 8 are also provided as branches in communication with the outlet line 6. Designated at numerals 9, 10, 11, 12 and 13 are valves. Preferably, the bundles of long fibers 3 are packed in such a quantity that even when a raw water is passed as a downflow at a relatively high flow rate, the bundled long fibers 3 may be slightly bent and compressed to a height from 80% to 30% of their original height. However, the bundled long fibers 3 should not be bent horizontally by the stream of the raw water and should remain substantially upright as a whole inside the shell 1. In other words, the height reduction of the bundled long fibers 3 ranges from 20% to 70%, generally about 40-50% while the raw liquid with suspended solids is being fed. This packing quantity can provide a filter member having an extremely large interstice volume. Filtration of the raw water through the filter according to the first embodiment of this invention can be conducted in the following manner. The valves 9,13 are opened, and the raw water containing suspended solids is fed as a downflow by way of the raw water feed line 4. By the stream of the raw water, the bundled long fibers 3 are somewhat bent at lower portions thereof and are reduced in height at an initial stage as shown in FIG. 1. The bundled long fibers 3 however remain upright as a whole within the shell 1, so that the raw water passes as a downflow from upper end portions of the bundled upright long fibers 3 toward their lower end portions. Suspended solids are thus trapped in interstices 40 between the bundled long fibers 3, whereby filtered water is withdrawn through a filtered water outlet line 6. Since the bundled long fibers 3 are somewhat bent or collapsed at lower parts thereof by the stream of the downflow, the lower parts of the long fibers 3 are packed at a somewhat higher density than their upper parts which extend vertically. As a result, no channeling of the raw water occurs so that the solids suspended in the raw water can be effectively trapped by the bundled long fibers. Where the solids suspended in the raw water are fine solid particles, a coagulant such as polyaluminum chloride, aluminum sulfate or a cationic, anionic or nonionic high molecular-weight material may be added as a filter aid to the raw water. As the feeding of the raw water is continued, more and more suspended solids are trapped as they are and/or as coagulated coarser solids in interstices between the long fibers in the bundles so that the pressure drop due to filtration resistance increases gradually. Along with the increase of the pressure drop, the extent of bending or collapse of the bundled long fibers at the lower parts thereof increases so that the vertical length of the bundled long fibers decreases little by little as depicted in FIG. 2. However, this bending or collapse does not advance to upper end portions of the bundled long fibers, and even at the end of the feeding of the raw water the upper end portions of the bundled long fibers are still extending vertically. The feeding of the raw water is continued in the above manner until the pressure drop reaches a predetermined value or the quality of the filtered water falls to a prescribed level. Backwashing is then carried out in the following manner. The valves 9,13 are closed, and the valves 10,12 are opened to allow compressed air to flow in through the air feed line 8. By the inflow of the compressed air, the water inside the shell 1 is agitated and at the same time the bundled long fibers are shaken. The interstices which have been defined between the fibers in the bundles are thus broken up, so that the solids and/or solid aggregates trapped in the interstices are disintegrated and the solids and/or solid aggregates deposited on the bundled long fibers are separated. While continuously feeding the compressed air or after closing the valve 12 to stop the inflow of the compressed air, the valve 11 is opened with the valve 10 maintained open. This allows backwash water to flow into the shell 1 via the backwash water feed line 7. Since the bundled long fibers 3 are fixed at the lower end portions thereof on the perforated plate 2 but are free-standing at the upper end portions thereof, the upflow of the backwash water flowed into the shell 1 causes the bundled long fibers 3 to extend as a streamer and also to shake with the lower end portions thereof fixed on the perforated plate 2 as illustrated in FIG. 3. The trapped solids are hence no longer allowed to remain among and on the bundled long fibers. The solids are hence released from the bundled long fibers and a backwash effluent containing a great deal of solids suspended therein is discharged through the backwash water discharge line 5. In addition to the above-mentioned backwashing method, the backwashing of the column filter according to the first embodiment of this invention can also be performed by feeding compressed air intermittently while feeding backwash water as an upflow at a constant rate. In fact, this backwashing method is more effective in many instances. Because the lower end portions of the bundled long fibers 3 are fixed on the perforated plate in the column filter according to the first embodiment of this invention, the bundled long fibers 3 are not washed out of the shell 1 even when backwash water and/or compressed air are fed at a high flow velocity. Compared to conventional filters using a filter medium such as sand or anthracite, the column filter according to the first embodiment of this invention allows to feed backwash water and/or compressed air at a higher flow velocity. Further, absolutely no flow obstacle such as a screen is provided in a top part of the shell. Therefore, the column filter according to the first embodiment of this invention can flush out trapped solids in a shorter time and effectively. Where the upright cylindrical shell 1 has a relatively large diameter, it is desirable to provide one or more upright partition walls substantially in parallel with the bundled long fibers 3 within the shell 1 to divide the interior of the shell 1 into vertically-extending sections so that the bundled long fibers 3 may be prevented from being bent horizontally or obliquely during the feeding of the raw water. A description will next be made of the bundled long fibers 3 useful in the practice of this invention. As already described above, the bundled long fibers 3 useful in this invention are somewhat bent or collapsed at lower parts thereof and are reduced in height at an initial stage even when a raw water of a relatively high flow velocity is fed as a downflow, and upon continuation of the feeding of the raw water, the lower parts of the bundled longer fibers 3 are progressively bent further and their height is reduced little by little. The bundled long fibers 3 are thus required to have stiffness and packing quantity or density sufficient to prevent their horizontal bending and to allow them to remain upright as a whole in the upright cylindrical shell 1 during the feeding of the raw water. Usable fibers include synthetic fibers such as acrylic fibers, polyester fibers and polyamide fibers as well as natural fibers such as cotton and wool. As these synthetic fibers or natural fibers, it is preferable to use units of non-twisted monofilaments having a diameter not greater than 80 µm, usually around 35 µm or so. However, units of twisted filaments may also be used so long as they take the form of bundled long fibers which are not bent horizontally and remain upright as a whole in the upright cylindrical shell 1 while the raw water is passed. As the packing density of the long fibers in the bundles employed in this invention becomes higher, finer suspended solids can be removed to provide filtered water of higher quality. However, the pressure drop increases. On the other hand, the quality of filtered water falls as the packing density of the long fibers becomes lower. In contrast, the pressure drop decreases. It is accordingly desirable to choose bundles of long fibers having a most suitable packing density depending on the nature or concentration of solids suspended in the raw water. When bundles of long fibers composed of units of non-twisted monofilaments 1,000-3,000 mm long are used by way of example, it is preferable to pack bundles to give a packing density of 25-110 kg in terms of their dry weight per bulk volume m³ of the bundles. The length of the bundled long fibers may be limited to the range exceeding 1,000 mm - especially longer than 2,000 mm - but shorter than 3,000 mm in this invention. This is for the following reasons. If the length of the bundled long fibers is 1,000 mm or shorter, the filtration time is so short than the column filter is not satisfactory as an industrial apparatus. Although the filtration time becomes longer as the length of the bundled long fibers increases, the filtration time reaches to saturation at the length exceeding about 3,000 mm. Then, the advantages available from the use of bundled fibers having a greater length are reduced and the use of bundled fibers of such a greater length results merely in a higher manufacturing cost. It is therefore not preferable to use bundled fibers having a length outside the above range. In other words, marked advantages in filtration time can be brought about where the bundled long fibers have a length in the range of 1,000-3,000 mm. The quality of filtered water is somewhat inferior where the length of the bundled long fibers is 1,000 mm or less. There is however no significant difference in the filtered water quality among the bundled fibers length range greater than 1,000 mm but shorter than 3,000 mm. To demonstrate the advantages of the present invention, the present invention will hereinafter be described specifically by the following example. Example: A perforated plate was arranged in a lower part of an upright cylindrical shell made of stainless steel. At a straight wall portion of the shell, the diameter and height were 650 mm and 4,000 mm, respectively. Seven bundles of long fibers composed of units of non-twisted acrylic monofilaments having a diameter of 35 µm, whose lengths ranged from 800 mm to 3,500 mm, were separately fixed at lower end portions thereof but left free-standing at upper end portions thereof, so that filters as illustrated in FIGS. 1 to 3 were constructed, respectively. The end point of feeding of a raw water was set at 0.5 kg/cm² in terms of pressure drop (initial pressure drop: 0.1 kg/cm²). After the end of the feeding of the raw water through each filter, the filter was backwashed first with air having a linear upflow velocity of 1,000 Nm³/m² for 1 minute, with 1,000 Nm³/m² of air and 100 m/hr of backwash water for 3 minutes, and then with 100 m/hr of backwash water alone for 1 minute. The pressure drop through the packed medium layer was reduced to the initial pressure drop, thereby indicating that the solids trapped by the bundled long fibers had been flushed completely and the the conditions before the filtration had been restored. The above experimental filtration was conducted for 6 months. The filtration times and the average turbidities of treated waters for the respective lengths of the bundled long fibers were as shown in the following table. Length of bundled long fibers, m Filtration time, hr Average turbidity of treated water, degree 0.8 4 11 1.0 5.3 8 1.5 8 7 2.0 10 7 2.5 12 7 3.0 13 7 3.5 13.5 7

Claims: A column filter using bundles of long fibers, said filter being provided with an upright cylindrical shell(1), the bundles of long fibers (3) being arranged substantially upright inside the shell with lower end portions thereof being fixed and upper end portions thereof being free-standing, a raw liquid feed line (4) being provided at the top part of the shell and a filtered liquid outlet line (6) being provided at the bottom part of the shell, characterized in that a supporting member (2) is arranged transversely in a lower interior part of the shell, the bundles of long fibers have a length in a range longer than 1,000 mm, but shorter than 3,000 mm and are fixed at the lower end portions thereof on an upper surface of said supporting member, and the packing density of the bundles is in a range of 25 - 110 kg in terms of the dry weight of the bundles per bulk volume m³ of the bundles so that the bundles may retain 30 - 80% of the original height thereof while the raw liquid with suspended solids is being fed into the shell and compressing the bundles toward the supporting member. The column filter as claimed in Claim 1, wherein the fibers in the bundles have a diamter up to 80 µm. The column filter as claimed in any one of the preceding claims, wherein at least one upright partition wall (43) is additionally provided substantially in parallel with the bundles of long fibers (3) within the shell (1) whereby the interior of the shell (1) is divided into vertically-extending sections.

Inventors: IWATSUKA TAKESHI C O JAPAN ORG, KASAI TOSHIO C O JAPAN ORGANO, IWATSUKA, TAKESHI C/O JAPAN ORGANO CO., LTD., KASAI, TOSHIO C/O JAPAN ORGANO CO., LTD.
Applicants: ORGANO KK, JAPAN ORGANO CO., LTD.
Application year: 1989
Main classification: B01D35
IPC codes: B01D35