WO2004018370A1

WO2004018370A1 - Device for removing manganese in raw water

Info

Publication number: WO2004018370A1
Application number: PCT/JP2003/010120
Authority: WO
Inventors: Tadayoshi Nagaoka
Original assignee: Tadayoshi Nagaoka
Priority date: 2002-08-20
Filing date: 2003-08-08
Publication date: 2004-03-04
Also published as: AU2003256070A1; JP2004136268A

Abstract

A device for removing manganese in raw water is connected at a posterior stage of a filtering device for filtering out iron in raw water by oxidizing it and comprises a raw water supply tube (52), aeration means (5) for performing aeration for increasing dissolved oxygen concentration of the raw water supplied from the raw water supply tube (52), a filter layer container (3) containing a filter layer (2) which has a catalytic function for enhancing oxidation of manganese in the raw water, and a filtered water outlet (9) provided in the filter layer container (3).

Description

DESCRIPTION

Device for Removing Manganese in Raw Water

Technical Field

This invention relates to a device for removing manganese in raw water and, more particularly, to a device for removing manganese in raw water such as underground water at a high water treating speed of 120m/day with a relatively compact device without employing an oxidizing agent or flocculant.

Background Art

Underground water is utilized as raw water for tap water and also utilized in industries which require a large quantity of water such as food, soft drink, beverage, dyestuff industries and also in public baths. In these industries, iron and manganese contents contained in underground water have raised a problem. While iron and manganese are substances which are necessary for a human body, contents of these substances exceeding a certain amount give a metal taste to water and turn water to red or black water, thereby making the water unsuitable for drinking and causing various difficulties in these industries. Further, in a foundation work in building industry, it is indispensable to remove underground water from a foundation site before the foundation work starts. If a large quantity of iron and manganese is contained in the underground water, the iron and manganese must be removed from the underground water before the underground water is discharged to the sewerage because it is legally prohibited to discharge such underground water the sewerage without any treatment.

In a currently prevailing conventional water treatment device for removing iron or manganese, an oxidizing agent such, for example, as sodium hypochlorite or a flocculant such, for example, as poly-aluminum chloride (PAC) is added to raw water to oxidize iron or manganese which is dissolved in the raw water and thereby convert it to iron oxide or manganese oxide which is insoluble in water and the iron oxide or manganese oxide is filtered out by filtering the raw water through filtering sand. In the conventional water treatment device requiring addition of an oxidizing agent or a flocculant, however, a relatively large amount of oxidizing agent or flocculant is consumed in the device and, therefore, the cost of such oxidizing agent or flocculant is tremendous.

Further, since hypochlorous acid which is used as the oxidizing agent remains in water after the treatment for oxidizing iron and manganese, trihalomethane which is a carcinogen is generated and, for removing trihalomethane, the water must further be filtered through an activated carbon layer which adds to the cost of the water treatment. If provision of such activated carbon layer is omitted for economic reason, water after filtering must be constantly analyzed for preventing generation of trihalomethane caused by addition of an excessive amount of oxidizing agent and, if necessary, the amount of addition of the oxidizing agent must be adjusted. This method requires a high cost of maintenance in addition to the cost of purchasing the oxidizing agent. Further, the conventional water treatment device generally is a complex and large-scale system including an aeration tank, a flocculation tank, a precipitation tank, a sand filter tower, an iron and manganese removing tower and a chemical agent tank and this system requires a large space for installation. It is impossible to install such a large device in a site of a limited space such in a town.

Furthermore, filtering sand which is used in this water treatment device requiring addition of an oxidizing agent is blocked by accumulated impurities and therefore must be replaced from time to time. The used sand to be abandoned must be treated as industrial waste because it contains a chemical agent and a place where it can be abandoned is extremely restricted by laws and regulations.

For eliminating the disadvantages of the prior art methods, the applicant of the present invention has proposed inW02/34677 a water treatment device capable of treating water soluble substance such as iron and manganese in underground water by oxidizing them and thereby making them insoluble with a simple and compact device without using an oxidizing agent or a flocculant.

According to the water treatment device, by turning raw water to a jet water stream by means of the jet nozzle which is at one end portion thereof in communication with the raw water supply tube and introducing air into the jet nozzle from the air introducing tube which is opened to the inside of the jet nozzle, air is drawn into the jet water stream and turned to a multiplicity of small air bubbles. The jet water stream containing the multiplicity of air bubbles is blown out of the raw water jetting outlet and is struck against the water surface above the filter layer disposed below the raw water jetting outlet thereby causing vehement aeration both in the water above the filter layer and on the surface of the filter layer. By virtue of this aeration, soluble substances such as iron and manganese contained in the water are oxidized and thereby are turned to insoluble substances which form flocks or precipitates and are caught on the surfaces of grains of the filtering material such as filtering sand which constitutes the filter layer.

The water quality standard of Japan stipulates that the allowable concentration of iron in tap water is 0.3mg/l or belwo and the allowable concentration of manganese is 0.05mg/l or below. As a result of experiments, it has been found that the water treatment device made according to the above-mentioned WO 02/34677 can reduce the concentration of iron in underground of water sufficiently to a level below the allowed concentration. As to manganese, it has been found that the device can reduce the concentration of manganese but it is difficult to reduce the concentration to a level below the allowable concentration.

Known in the art of removing manganese in underground water are a contact filtering method and a method utilizing iron bacteria besides the above described methods using chemical preparations.

According to the contact method, raw water is pre-treated and then is subjected to quick filtering using manganese sand as a filtering material. Manganese in the raw water comes into contact with the surface of the manganese sand and thereby is oxidized and removed. The method enables quick filtering at a speed of 120m per day and, therefore, raw water can be treated at a high efficiency. In this method, however, the film (MnOa ^• H2O) of the manganese sand which has contacted manganese ion in the raw water becomes Mnθ2 'MnO Η2O which is inactive and therefore loses its function of oxidizing manganese upon contact. It is therefore necessary to add chlorine previously in the raw water to revive the oxidizing function of the manganese sand. This method, therefore, is a kind of method which requires a chemical preparation and is not exempt from the disadvantages of the methods utilizing chemcial preparations.

The method using iron bacteria utilizes the ability of iron bacteria for oxidizing iron and manganese dissolved in water to iron and manganese oxides which are water-insoluble and settle to the surface and body of the iron bacteria. After manganese is absorbed by the iron bacteria, the iron bacteria is separated from water by the sand filter layer. This method does not require addition of a chemical preparation but its filtering speed is at best 10m per day to 30m per day and, therefore, this method cannot achieve removal of manganese at a high efficiency at the filtering speed of 120m per day or more.

It is, therefore, an object of the present invention to provide a device which is of a simple and compact design and can efficiently remove manganese in raw water such as underground water by quick filtering with a filtering speed of 120m per day or over without using an oxidizing agent or flocculant.

Disclosure of the Invention

For achieving the above described object of the invention, there is provided a device for removing manganese in raw water connected at a posterior stage of a filtering device for filtering out iron in raw water by oxidizing it, comprising: a raw water supply tube; aeration means for performing aeration for increasing dissolved oxygen concentration of the raw water supplied from the raw water supply tube; a filter layer container containing a filter layer which has a catalytic function for enhancing oxidation of manganese in the raw water; and a filtered water outlet provided in the filter layer container.

According to the invention, since the device for removing manganese is connected at a post stage of a filtering device for filtering out iron in raw water by oxidizing it, iron in the raw water has mostly been trapped by the filtering device of the prior stage and the concentration of iron in the raw water supplied to the device of the invention has significantly reduced. Competition for oxidation between iron and manganese in the filter layer thereby is substantially eliminated with the result that an environment which is very suitable for oxidation of manganese is formed. The aeration means increases the dissolved oxygen concentration of the raw water and oxidation of manganese in the filter layer having a catalytic function for enhancing oxidation of manganese thereby is enhanced. The oxidized manganese settles on the filter layer and filtered water is taken out of the filtered water outlet. According to the invention, manganese in raw water such as underground water can be removed efficiently by the quick filtering at a filtering speed of 120m per day or over with a simple and compact device without using chemical preparations such as oxidizing agent and flocculant.

In one aspect of the invention, the filter layer comprises colonies of iron bacteria. According to this aspect of the invention, the dissolved oxygen increased by aeration is considered to enhance growth of the iron bacteria.

In another aspect of the invention, the filter layer contains manganese sand as a principal filter material. According to this aspect of the invention, the dissolved oxygen increased by aeration is considered to enhance the oxidation of manganese by the manganese sand upon contact with manganese.

In another aspect of the invention, the raw water supply tube is located in the upper portion of the device and said aeration means comprises a liquid- as contact device having a flow-path forming structure which causes raw water supplied from the raw water supply tube to flow in a plurality of divided flow-paths, said hquid-gas contact device having a column packing and a plurality of adaptors for supplying the raw water from the raw water supply tube to the column packing, said column packing consisting of a plurality of column packing constituting elements in the form of line elements or plate elements, each of the column packing constituting elements being extending vertically in parallel to adjacent column packing constituting elements in a non-contact state, and each of said adaptors being formed integrally with a corresponding one of the column packing constituting elements and being connected directly to the raw water supply tube without branching off from another of the adaptors. According to this aspect of the invention, the raw water flowing from the raw water supply tube to the respective column packing constituting elements flow down without being transferred to adjacent column packing constituting elements and, therefore, a uniform contact between liquid and air is realized and, as a result, uniform aeration is performed and removal of manganese can be achieved with the highest efficiency.

In another aspect of the invention, the device further comprises : a clogging-removing bar holder to which a plurality of clogging-removing bars are fixed, each of said clogging-removing bars being provided in such a manner that a tip end portion thereof is inserted in a surface portion of the filter layer,^" and means for reciprocating the clogging-removing bar holder in a plane parallel to the surface of the filter layer.

According to this aspect of the invention, by moving the clogging-removing bar holder in reciprocating motion in a plane parallel to the surface of the filter layer, the cloggin-removing bars are caused to reciprocate with their tip end portion inserted in the surface portion of the filter layer. As a result, the surface portion of the filter layer is turned over by the clogging-removing bars and clogging in the surface portion of the filter layer is effectively removed.

In another aspect of the invention, the device comprises filtered water circulating means for stopping supply of raw water from the raw water supply tube during stoppage of supply of filtered water and returning the filtered water to the raw water supply tube thereby to circulate the filtered water through the filter layer.

According to this aspect of the invention, even during a period in which filtered water is unnecessary and supply of the filtered water is stopped, the filtered water is returned to the raw water supply tube and caused to circulate through the filter layer while aeration is continued and, therefore, dissolved oxygen is sufficiently supplied to iron bacteria and other mircroorganism in the filter layer even during stoppage of supply of the filtered water and, as a result, dying or reduction of the iron bacteria and other microorganism due to shortage of oxygen can be prevented and decrease in the manganese removal effect at the initial period of resuming supply of the filtered water can be prevented.

The filtered water circulating means may comprise-^" a filtered water circulating tube for connecting the filtered water outlet with the raw water supply tube; a water supply pump connected to the filtered water circulating tube; a changeover valve provided in the filtered water circulating tube for passing filtered water and stopping supply of water to the filtered water circulating tube during supply of the filtered water and stopping supply of the filtered water and returning the filtered water to the raw water supply tube through the filtered water circulating tube during stoppage of supply of the filtered water; and valve means provided in a raw water supply channel for passing raw water and stopping passage of the filtered water from the filtered water circulating tube during supply of the filtered water and stopping passage of the raw water and passing the filtered water from the filtered water circulating tube during stopping of supply of the filtered water.

The filtered water circulating means may also comprise •^" a filtered water circulation outlet provided separately from the filtered water outlet; a filtered water circulating tube for connecting the filtered water outlet with the raw water supply tube,^" a water supply pump connected to the filtered water circulating tube; a valve provided in a tube provided on the side of the filtered water outlet for opening during supply of the filtered water and closing during stoppage of supply of the filtered water; and valve means provided in a raw water supply channel for passing raw water and stopping passage of the filtered water from the filtered water circulating tube during supply of the filtered water and stopping passage of the raw water and passing the filtered water from the filtered water circulating tube during stoppage of supply of the filtered water.

In another aspect of the invention, the device further comprises a reverse washing tube disposed in the filter layer at a depth which is sufficinet for removing foreign matters which have been accumulated in the upper portion of the filter layer by reverse washing and also is sufficient for allowing a substantially thick portion of the filter layer to exist under the reverse washing tube.

According to this aspect of the invention, foreign matters which have been accumulated in the upper portion of the filter layer are removed by reverse washing whereas the portion of the filter layer under the reverse washing tube is not substantially affected by reverse washing and, therefore, colonies of iron bacteria and other microorganism existing in this portion of the filter layer are not destroyed and, when the filtering operation of the device is resumed after reverse washing, the manganese removal effect by the microorganism existing in this portion of the filter layer is maintained and, therefore, the manganese removal effect of the device as a whole is improved.

In still another aspect of the invention, there is provided a method for removing manganese in raw water after filtering out iron in the raw water by a filtering material comprising steps of: increasing dissolved oxygen concentration of the raw water supplied from a raw water supply tube by aeration; causing the raw water to pass through a filter layer which has a catalytic function for enhancing oxidation of manganese,^" taking out filtered water which has passed through the filter layer,^" and stopping supply of the raw water from the raw water supply tube during stoppage of the filtered water and returning the filtered water to the raw water supply tube for circulating the filtered water through the filter layer, while maintaining the dissolved oxygen concentration of the filtered water at a predetermined level by continuing aeration. Brief Description of the Drawings

In the accompanying drawings,

FIG. 1 is a sectional view schematically showing an embodiment of the device for removing manganese according to the invention,^"

FIG. 2 is a perspective view schematically showing a gas-liquid contact mechanis,^"

FIG. 3 is a perspective view showing a part of a column packing in an enlarged scale; FIG. 4 is a corss-sectional view of a packing forming element assembly!

FIG. 5 is a cross -sectional view of a raw water distribution tube;

FIG. 6 is a perspective view schematically showing a filter layer container and a clogging-removing bar holder;

FIG. 7 is a perspective view showing an example of an iron removing device connected at a prior stage of the device of the invention;

FIG. 8 is a sectional view similar to FIG. 1 schematically showing another embodiment of the invention,^"

FIG. 9 is a perspective view schematically showing a filter layer container and a clogging-removing bar holder of this embodiment; and FIG. 10 is a sectional view similar to FIG. 8 showing a modified example of a filtered water circulating means.

Best Mode for Carrying Out the Invention

The device for removing manganese made according to the invention is connected at a posterior stage of a filtering device for filtering out iron in raw water by oxidizing it. Underground water containing manganese usually contains iron also. When iron and manganese coexist in the underground water, iron is oxidized sooner than manganese. Since iron which has been oxidized sooner than manganese settles between grains of the filter material such as sand constituting the filter layer and thereby reduces an effective surface area of the filter material functioning as an oxidation catalysis, this is disadvantageous for subsequent oxidation reaction of manganese. In the present invention, a filter layer which has a catalytic function for enhancing oxidation of manganese in raw water is used but, when iron and manganese coexist in the raw water, the catalytic function of the filter layer for oxidizing manganese is impaired for the reason stated above.

According to the invention, removal of iron in the raw water and removal of manganese in the raw water are made by two-stage devices, namely iron is removed first by the device disposed in the prior stage to reduce concentration of iron sufficiently and then oxidation of manganese and trapping of oxidized manganese by the filter layer are performed by the device of the invention which is connected at a posterior stage. By this arrangement, competition by iron occurring in oxidation of manganese is ehminated and an environment which is optimum for oxidation of manganese is created.

As a filter layer having a catalytic function for enhancing oxidation of manganese in the raw water, the present invention utilizes a filter layer comprising colonies of iron bacteria or a filter layer containing manganese sand as a principal filter material.

As the filter layer containing colonies of iron bacteria, filter sand called black sand may be used. This filter sand as a whole appears black because oxidized manganese settles on the surface and in the body of iron bacteria as a result of a manganese removal operation continued over several months in a underground water filter device. Alternatively, filter sand made by transferring colonies of iron bacteria in a suitable amount to ordinary filter sand may be used. Filter sand which is a mixture of these two types of filter sand may also be used.

Among many known types of iron bacteria, Leptothrix and Clonothrix may preferably be used for the purpose of the invention.

Manganese sand used as a principal filter material of a filter layer consists of sand grains each of which is covered with a manganese oxide

(M11O2 • H₂O) over the surface of the sand grain. The filter layer may consist only of the manganese sand or may comprise the manganese sand as a principal filter material in the form of a mixture with one or more of ordinary sand, ceramic balls and textile balls etc.

For achieving the quick filtering of 120m per day or over, the filter material such as filter sand used in the filter layer should preferably have a grain diameter of 2mm to 6mm but it has been found as a result of experiments that the quick filtering of 300m per day can be achieved with a grain diameter of 0.6mm.

According to the invention, notwithstanding that the filter layer which traps oxidized manganese is a known filter layer in which colonies of iron bacteria exist or a known filter layer containing manganese sand, removal of manganese in the raw water by the quick filtering of 120m per day or over is realized without relying upon the conventional method of using chemical preparations, which was never achieved by the conventional method using the filter layer containing iron bacteria. The result of the invention can be achieved by increase i the concentration of dissolved oxygen in the raw water by aeration combined with adoption of the two-stage devices of oxidizing iron first and then oxidizing manganese.

In the conventional contact filtering method using manganese sand, addition of chlorine to the raw water was an essential condition for restoring the catalytic function of the manganese sand for oxidation. It was quite unexpected by those skilled in the art to replace chlorine by increase in the concentration of dissolved oxygen in the raw water for enhancing oxidation of manganese and thereby achieving a manganese removal effect which is equivalent to the conventional method using chlorine. The conventional method using iron bacteria was limited to the filtering speed of 10m per day to 30m per day as described above and it was totally impossible to achieve the quick filtering of 120m per day or over with this conventional method. According to the invention, it has been surprisingly found that the quick filtering of 120m per day or over can be achieved with a simple method of increasing the concentration of dissolved oxygen in the raw water. Detailed mechanism of enhancement of oxidation of manganese by iron bacteria by increase in the concentration of dissolved oxygen in the raw water is not known. It is supposed that increase in the concentration of dissolved oxygen enhances growth of iron bacteria in the filter layer and this enhanced growth of iron bacteria in turn enhances oxidation of manganese in the raw water by iron bacteria and settling of manganese on iron bacteria.

As a result of experiments, it has been found that, for reducing the concentration of manganese in purified water to the statutory standard of O.Oδmgtl or below by the quick filtering of 120m per day or over, the concentration of dissolved oxygen in the raw water should be 7mg/l or over and, preferably, be 8mg/l or over.

There is no limitation in the specific method for causing aeration for increasing dissolved oxygen in the raw water. For example, water to be treated may be jetted out of a nozzle or hose on water contained in a container containing a filter layer for causing aeration in the water. Alternatively, one of various types of column packing may be used for causing water to be treated to flow down through the column packing while causing air to be blown up through the column packing, thereby causing aeration by contact between water and air. One preferred method for causing aeration is to utilize a Hquid-gas contact device having a flow-path forming structure which causes raw water supplied from a raw water supply tube to flow in a plurality of divided flow-paths and having a column packing and a plurality of adaptors for supplying the raw water from the raw water supply tube to the column packing. The column packing consists of a plurality of column packing constituting elements in the form of line elements or plate elements. Each of the column packing constituting elements extends vertically in parallel to adjacent column packing constituting elements in a non-contact state, and each of said adaptors is formed integrally with a corresponding one of the column packing constituting elements and is connected directly to the raw water supply tube without branching off from another of the adaptors.

By this arrangement, the raw water flowing from the raw water supply tube to the respective column packing constituting elements flows down without being transferred to an adjacent column packing constituting element and, therefore, a uniform contact between water and air is realized and, as a result, uniform aeration is performed and removal of manganese can be achieved with the highest efficiency.

Referring now to the drawings, preferred embodiments of the manganese removing device according to the invention will be described.

Referring first to FIG. 7, an example of a device for removing iron which is connected at a prior stage of the manganese removing device of the invention will be described.

FIG. 7 is a perspective view with a part of side walls of the filter container being removed.

A water treatment device 103 comprises, as its principal elements, a filter container 103 containing a filter layer 102, a raw water supply tube 104, jet nozzles 105 and an air introducing tubes 106.

The raw water supply tube 104 made of, e.g., a steel pipe, for supplying raw water to be filtered such as underground water or river water to the filter container 103 is connected to a water supply pump 107 by means of a rubber hose 108. The water supply pump 107 is provided with raw water from a water source of the raw water and supplies the raw water to the raw water supply tube 104 at a predetermined flow rate. The raw water supply tube 104 is disposed above one end portion of the filter container 103 in such a manner that the raw water supply tube 104 extends in a plane parallel to the surface of the filter layer 102.

One or more (six in the illustrated example) jet nozzles 105 are provided in a manner to branch off downward from the raw water supply tube 104. The upstream side end portion of each jet nozzle 105 is fitted in the raw water supply tube 104 in such a manner that the inside of the jet nozzle 105 communicates with the raw water supply tube 104. In the downstream side end portion of the jet nozzle 105 is formed a raw water jetting outlet which blows out raw water in a jet water stream.

One or more (six in the illustrated example) air introducing tubes 106 are provided, one for each, in the respective jet nozzles 105. The upstream side end portion of each air introducing tube 106 projects obliquely upwardly from the jet nozzle 105 with its inside being opened to the atmosphere. The downstream side end portion is opened to the inside of the jet nozzle 105 upstream of the raw water jetting outlet.

In the filter container 103, the filter layer 102 is provided with its upper surface being disposed below the jet nozzles 105 with a predetermined interval from the raw water jetting outlets. The filter layer 102 is made of filtering sand of a uniform grain size and functions to filter the raw water by catching flocks of oxidized substances and other foreign matters in the raw water supplied as a jet stream from the jet nozzles 105.

A baffle plate 113 made of, e.g., a steel plate, is provided between the raw water jetting outlets of the jet nozzles 105 and the surface of the filter layer 102 in such a manner that the baffle plate 113 extends in parallel to the raw water supply tube 104 and are disposed beneath the raw water jetting outlets.

A filter layer support 114 made of a plate screen is provided at a predetermined height from the bottom of the filter container 103 for supporting the filter layer 102 in its entirety. A reverse washing tube 115 is provided in a lower space 116 of the filter container 103 under the filter layer support 114 for reverse washing the filter layer 102.

A filtered water outlet 117 for taking out water which has been filtered through the filter layer 102 is provided in a side wall 103c of the filter container 103. In one end portion of the filter container 103 (in the right end portion in the illustrated example) is provided an overflow trough 118 with its upper edges being located slightly above the surface of the filter layer 102. One end of the overflow trough 118 is closed with a side wall 103b of the filter container 103 and the other end of the overflow trough 118 is closed with a cover 119.

In the present embodiment, a raw water supply tube reciprocating mechanism 120 for reciprocating the raw water supply tube 104 in a plane parallel to the surface of the filter layer 102 is comprised of a screw box 121, a feed screw 122, a feed screw drive device 123 including an electric motor and reduction gear for driving the feed screw 122.

For performing the reciprocating motion of the raw water supply tube

104 in a smooth and stable manner, in the present embodiment, rollers (not shown) are secured to the raw water supply tube 104 in locations on the side walls 103a and 103b of the filter container 103 and the side walls 103a and 103b are formed in their upper end portion with guide grooves 129 in which the rotating rollers are guided.

During filtering of raw water, raw water is supplied to the jet nozzles

105 while the depth of water above the surface of the filter layer 102 is maintained at a predetermined level. The raw water is caused to flow in a jet water stream by setting the flow rate of water in the jet nozzles 105 at, e.g., 1.5 1/min. to 3 1/min. while air is drawn into the jet nozzles 105 from the air introducing tubes 106 opening to the inside of the jet nozzles 105 at a flow rate of , e.g., 0.5 1/min. to 1 1/min. The air is drawn into the jet water stream in a multiphcity of small air bubbles and the jet water stream containing the air bubbles is blown out of the raw water jetting outlets of the jet nozzles 105 and is struck against the water surface of the filter layer 102 thereby causing vehement aeration on the water surface and on the filter layer 102. The baffle plate 113 enhances this aeration. By virtue of this aeration, soluble substances such as iron are oxidized and thereby are turned to insoluble substances which form flocks or precipitates and are caught on the surfaces of the grains of the filtering sand which constitutes the filter layer 102. The filtered water from which the insoluble substances and other foreign matters have been removed by the filter layer 102 is taken out of the filtered water outlet 117.

As the above described filtering operation is continued, flocks of oxidized substances and other foreign matters are accumulated on the surface of the filter layer 102 as time elapses. The surface of the filter layer 102 is covered and closed with these flocks and foreign matters and, as a result, the filtering function of the filter layer 102 is reduced.

In this case, while the raw water and air are supplied, the raw water supply tube reciprocating mechanism 120 is actuated to reciprocate the raw water supply tube 104 in a plane parallel to the surface of the filter layer 102. Since, by this operation, a jet water stream containing a multiphcity of air bubbles which is blown out of the raw water jetting outlets is struck vehemently against the closed surface of the filter layer 102, the entire surface of the filter layer 102 is turned over whereby the closed state of the surface of the filter layer 102 is ehminated and the filter layer 102 restores its filtering function.

An embodiment of the manganese removing device according to the invention which is connected at a posterior stage of the above described iron removing device 100 will now be described with reference to FIGs. 1 to 6 in which FIG. 1 is a sectional view schematically showing an embodiment of the manganese removing device of the invention, FIG. 2 is a perspective view schematically showing the Hquid-gas contact device, and FIG. 6 is a perspective view showing a filter container and a clogging-removing bar holder. In FIGs. 2 and 6, the device is shown with a part of the housing and side walls being removed.

In the present embodiment, a manganese removing device 1 comprises, as principal constituting elements, a filter container 3 containing a filter layer 2and a Hquid-gas contact device 4 which constitutes the aeration means of the present invention. Description will be made first with respect to the Hquid-gas contact device 4.

The Hquid-gas contact device 4 is disposed above the filter container 3 and has a housing 50 which is of an oblong box-Hke configuration with an opened bottom. The housing 50 contains a column packing 5 which consists of a plurahty (20 in the illustrated embodiment) of column packing constituting element assembHes 64.

A raw water supply main tube 52 connected to the filtered water outlet 117 (FIG. 7) of the iron removing device 100 by means of a hose (not shown) is disposed above the top surface of the housing 50 and this raw water supply main tube 52 is branched off to a plurahty (5 in the illustrated embodiment) of raw water supply branch tubes 54. A plurahty (4 in the fllustrated embodiment) of downward projecting raw water distribution tubes 56 are provided at a predetermined interval on the lower surface of each of the raw water supply branch tubes 54. These raw water distribution tubes 56 communicate with the raw water supply branch tubes 54.

In the housing 50, there are provided a plurahty of adaptors 58 each being made of a line element and connected to the lower end portion of the raw water distribution tube 56 and the column packing 5 consisting of a plurahty of column packing constituting elements 60 in the form of line elements.

In this embodiment, as shown in FIG. 3 which is a perspective view showing a part of the column packing 5 in an enlarged scale, the Hquid-air contact device 4 comprises 20 vertically oblong box-Hke column packing constituting element assembHes 64 and adaptors 58. Each raw water distribution tube 56 supphes raw water, i.e., water to be treated in which the concentration of iron has been significantly reduced by the prior stage iron removing device 100, to corresponding ones of the assembHes 64 ("raw water" hereinafter means this water to be treated in which the concentration of iron has been significantly reduced).

In each column packing constituting element assembly 64, each column packing constituting element 60 in the form of a line element extends verticaHy in parallel to each other in a non-contact state, i.e., maintaining a predetermined interval with each other in such a manner that the column packing constituting elements 60 form lines and columns in a horizontal section. Each column packing constituting element 60 is disposed in a manner to maintain a non-contact state with the inner wall of the housing 50.

As shown in the cross-sectional_^ view of FIG. 4, spacers 66 made of elongated members such as steel bars are provided in such a manner that each of the spacers 66 extends in a horizontal plane and crosses the column packing constituting elements 60 of each column of the column packing constituting element assembly 64. The spacers 66 are also provided in a manner to extend in a horizontal plane and cross the column packing constituting elements 60 of Hnes at the two ends of the column packing constituting element assembly 64. The spacers 66 may be provided in a manner to cross the column packing constituting elements 60 of all Hnes.

In the present embodiment, each adaptor 58 consisting of a line element and connecting the raw water distribution tube 56 with the column packing 5 for supplying raw water from the raw water distribution tube 56 to the column packing 5 is formed integraHy with one of the column packing constituting element 60 in one-to-one relation. Therefore, for each of the column packing constituting element assembly 64, the same number of adaptors 58 as the column packing constituting elements 60 constituting the column packing constituting element assembly 64 are provided. Each adaptor 58 is connected directly to the raw water distribution tube 56 corresponding to each column packing constituting element assembly 64 without branching off from another adaptor and without causing another adaptor to branch off from the adaptor. Accordingly, a plurahty of adaptors 58 which are integraHy formed with column packing constituting elements 60 of one column packing constituting element assembly 64 are inserted and held in a bundle in the lower end portion of the raw water distribution tube 56 corresponding to the assembly 64. FIG. 5 is a cross -sectional view showing the state of the lower end portion of the raw water distribution tube 56 in which the adaptors 58 are inserted in a bundle. The adaptors 58 are inserted somewhat loosely in the raw water distribution tube 56 in such a manner that a shght gap is formed between adjacent adaptors so that raw water flows down along the entire peripheral surface of each adaptor 58.

As the Hne elements used for forming the column packing constituting elements 60 and the adaptors 58, a metal line or any type of fiber including plastic fiber, carbon fiber, ceramic fiber, plant fiber such as cotton fiber and animal fiber such as wool may be used. The Hne element may be made of a monofilament or a single piece of wire but a Hne element made of twine which is made by twisting thin steel Hnes or plastic Hnes is preferable because Hquid flows along the space between the lines which constitute the wire or twine due to the capillary action and thereby enhances transfer of the Hquid. In the present embodiment, seven steel Hnes each having a diameter of 0.1mm are twisted together to a single wire and two of these wires are twisted together to a single wire and this wire is used as the column packing constituting element 60 and the adaptor 58.

In this embodiment, a column packing constituting element in the form of a line is used but a column packing constituting element in the form of a plate or belt may also be used. Referring to FIGs. 1 and 6, a clogging-removing bar holder 28 is provided in such a manner that it extends in a plane paraUel to the surface of the filter layer 2 and can reciprocate in the plane paraUel to the surface of the filter layer 2 above the filter container 3. FIGs. 1 and 6 show a state in which the clogging-removing bar holder 28 is reciprocating in the central portion of the filter container 3 during reverse washing.

A plurahty of verticaHy extending clogging-removing bars 42 are fixed to the lower surface of the clogging-removing bar holder 28. The clogging-removing bars 42 are made of a rigid material such as steel bars and their tip end portions 42a are inserted in the surface portion of the filter layer 2.

As the filter layer 2, a filter layer comprising manganese sand which has a catalytic function for enhancing oxidation of manganese in the raw water or a filter layer comprising colonies of iron bacteria is used.

Flow speed of the raw water in the filter container 3, i.e., filtering speed, should preferably be about the same speed as the filtering speed of the quick filtering, i.e., 120m per day or over.

In the filter container 3, the filter layer 2 is supported by a filter support

8 made of wedge- wires. There is defined a lower space 16 below the filter support 8 from which filtered water is taken out. Facing the lower space 16, a filtered water outlet tube 9 is fixed to a side waH of the filter container 3 in a manner to communicate with the lower space 16.

In the lower space 16 are provided one or more reverse washing tubes 70 in a direction crossing the raw water supply branch tube 54. The reverse washing tubes 70 are formed with a plurahty of reverse washing water jetting openings at a predetermined interval in the longitudinal direction.

Reverse washing water overflow troughs 18 are provided on the side waUs 3a and 3b in such a manner that their upper edges are located above the surface of the raw water on the sand filter layer 2. One end 18a of each trough 18 is closed and the other end 18b is opened to drain out soiled reverse washing water. .

In the present embodiment, a clogging-removing bar holder reciprocating mechanism 20 for reciprocating the clogging-removing bar holder 28 in a plane paraUel to the surface of the sand filter layer 2 is comprised of a running box 22 which is fixed to the end portion of the clogging-removing bar holder 28 and has a running wheel 21 on its bottom, a drive device 23 including an electric motor for driving the running wheel 21 and a reduction gear, a running plate 25 which is fixed to the other end portion of the clogging-removing bar holder 28 and has a running wheel 24 on its bottom, and a pair of raUs 26 which are fixed to a frame 3e of the filter container 3 in a manner to engage with the running wheels 21 and 24. By rotating the electric motor in the drive device 23 in one direction, the clogging-removing bar holder 28 is moved in one direction and, by rotating the electric motor in the reverse direction, the clogging-removing bar holder 28 is moved in the reverse direction.

The reciprocating mechanism is not Hmited to the above described mechanism but other mechanism such as one using a feed screw and one using a chain drive may be used.

The operation of this manganese removing device wiU now be described. During filtering raw water, the raw water which is supphed from the iron removing device 100 and in which the concentration of iron has been reduced is supphed through the raw water supply main tube 52, raw water supply branch tubes 54, raw water distiribution tubes 56 and adaptors 58 of the Hquid-gas contact device 4 to the respective column packing constituting elements 60 of the column packing 5. In the meanwhile, air is blown up from below of the column packing 5 by an uniUustrated blower. Contact of the raw water with air is thereby performed in the column packing 5 whereby aeration is performed. The raw water which has been subjected to aeration while flowing down through the column packing 5 falls on the filter layer 2 of the filter container 3 and oxidation of manganese is performed in the filter layer 2.

As the above described filtering operation is continued, manganese oxide and other foreign matters are accumulated on the surface of the filter layer 2 as time elapses. The surface of the filter layer 2 is covered and closed with these foreign matters and, as a result, the filtering function of the filter layer 2 is reduced.

In this case, while the raw water is supphed with the water level being adjusted at a level of, e.g., 10cm above the surface of the filter layer 2, the clogging-removing bar holder reciprocating mechanism 20 is actuated to reciprocate the clogging-removing bar holder 28 in a plane paraUel to the surface of the sand filter layer 2 in directions shown by arrows in FIG. 6. Since, by this operation, the clogging-removing bars 42 are reciprocated with their tip end portions inserted in the surface portion of the filter layer 2, the entire surface of the filter layer 2 is turned over by the cloggign-removing bars 42 and clogging of the surface of the filter layer 2 by foreign matters is removed whereby the closed state of the surface of the filter layer 2 is ehminated and the filter layer 2 restores its filtering function.

After removing clogging of the surface of the filter layer 2, reverse washing water is supphed from the reverse washing tube 70 through the filter support 8 toward the upper surface of the filter layer 2 thereby to drain soiled reverse washing water to outside from the reverse washing water overflow troughs 18.

FIG. 8 is a perspective view similar to FIG. 1 showing another embodiment of the invention and FIG. 9 is a sectional view similar to FIG. 6 of the same embodiment. In this embodiment, the same components as those in FIG. 1 are designated by the same reference characters and description thereof will be omitted. In the embodiment of FIG. 8, one or more reverse washing tubes 72

(three in the iUustrated example) having upward opening reverse washing water jetting holes are disposed in the filter layer 2 at a depth which is sufficient for removing foreign matters which have been accumulated in the upper portion of the filter layer 2 by reverse washing and also is sufficient for aUowing a substantiaUy thick portion of the filter layer 2 to exist under the reverse washing tubes 72. This depth is determined having regard to factors including the types and amount of foreign matters which close the surface of the filter layer 2, grain diameter of filtering sand which forms the filter layer 2, flow speed of reverse washing water and frequency of reverse washing during a predetermined period of time.

In the filter layer 2, colonies of iron bacteria and other microorganisms are naturaUy formed as time elapses not only in a case where the filter layer comprising colonies of iron bacteria is used but also the filter layer comprising manganese sand as a principal filter material is used. As a result of experiments, it has been found that these colonies of microorganisms are formed not only in the surface portion of the filter layer 2 but also in the middle portion and lower portion of the filter layer 2 and these colonies of microorganisms in the middle and lower portions of the filter layer 2 perform the function of removing manganese significantly, though the effect of the removal of manganese is not so great as that in the surface portion of the filter layer 2. Accordingly, by disposing the reverse washing tubes 72 at such depth in the filter layer 2, foreign matters which have been accumulated in the upper portion of the filter layer 2 are washed away by reverse washing whereas, in the portion of the filter layer 2 under the reverse washing tubes 72 where influence of the reverse washing does not reach, the colonies of iron bacteria and other microorganisms are not destroyed whereby, when operation of the device is resumed after completion of reverse washing, the effect of removing manganese by the microorganisms surviving in this portion of the filter layer 2 is maintained continuously and, therefore, the effect of removing manganese of the device as a whole is improved.

This embodiment of the manganese removing device further comprises filtered water circulating means 80 for stopping supply of raw water from the raw water supply main tube 52 during stoppage of supply of filtered water, i.e., when supply of the filtered water is not needed, and returning the filtered water to the raw water supply main tube 52 thereby to circulate the filtered water through the filter layer 2.

The filtered water circulating means 80 comprise a filtered water circulating tube 74 for connecting the filtered water outlet tube 9 with the raw water supply main tube 52, a water supply pump 82 connected to the filtered water circulating tube 74, a changeover valve 76 provided at a connecting portion of the filtered water circulating tube 74 and the filtered water outlet tube 9 for passing filtered water and stopping supply of water to the filtered water circulating tube 74 during supply of the filtered water and stopping supply of the filtered water and returning the filtered water to the raw water supply main tube 52 through the filtered water circulating tube 74 during stoppage of supply of the filtered water, and a changeover valve 78 which is valve means provided at a connecting portion of the raw water supply main tube 52 and the filtered water circulating tube 74 for passing raw water and stopping passage of the filtered water from the filtered water circulating tube 74 during supply of the filtered water and stopping passage of the raw water and passing the filtered water from the filtered water circulating tube 74 during stopping of supply of the filtered water.

During stopping of supply of the filtered water, the changeover valve 78 is changed over to stop passing of the raw water to to the raw water supply main tube 52 and aUow passing of the filtered water from the filtered water circulating tube 74 and the changeover valve 76 is changed over to stop supply of the filtered water and aUow passing of the filtered water to the filtered water circulating tube 74. In this state, by actuating the water supply pump 82, the filtered water is returned from the filtered water outlet tube 9 to the raw water supply main tube 52 through the filtered water circulating tube 74 and faUs on the filter layer 2 after being subjected to the aeration through the column packing 5. Accordingly, even during the period during which supply of the filtered water is stopped because it is not needed, by returning the filtered water to the raw water supply main tube 52 to circulate the filtered water through the filter layer 2 while continuing aeration, dissolved oxygen is supphed abundantly to iron bacteria and other microorganism in the filter layer 2 even during stoppage of supply of the filtered water and, as a result, dying or reduction of the iron bacteria and other microorganism due to shortage of oxygen can be prevented and decrease in the manganese removal effect in the filter layer 2 at the initial period of resuming supply of the filtered water can be prevented.

FIG. 10 is a sectional view similar to FIG. 8 showing a modified example of the filtered water circulating means. In this example, filtered water circulating means 90 comprises a filtered water circulation outlet 92 provided separately from a filtered water outlet tube 9, a filtered water circulating tube 94 for connecting the filtered water outlet 92 with a raw water supply main tube 52, a water supply pump 96 connected to the filtered water circulating tube 94, a valve 98 provided in the filtered water outlet tube 9 for opening during supply of the filtered water and closing during stoppage of supply of the filtered water, and a changeover valve 78 which is valve means provided at a connecting portion of the raw water supply main tube 52 and the filtered water circulating tube 94 for passing raw water and stopping passage of the filtered water from the filtered water circulating tube 94 during supply of the filtered water and stopping passage of the raw water and passing the filtered water from the filtered water circulating tube 94 during stoppage of supply of the filtered water. Various other modifications of the filtered water circulation means may be conceived within the scope of the claimed invention.

As a result of experiments conducted by using black sand as a filter material in the device of FIG. 8, it has been found that in a case where supply of the filtered water was stopped for a week and then supply of the filtered water was resumed, the ratio of manganese removal by the device during one week after resumption of supply of the filtered water was about 50% in average when the filtered water circulation means 80 was not used during stoppage of supply of the filtered water whereas the ratio of manganese removal during the same period of time was about 93% in average when the filtered water circulation means 80 was used. Thus, utiHty of the filtered water circulation means was proved.

Examples Example 1 A test for removing manganese by the device shown in FIG. 1 was conducted by using, as water to be treated, underground water which was coUected at Haibaramachi, Hyogo-ken, Japan and in which the concentration of iron was reduced by treating the underground water by the iron removing device shown in FIG. 1. As the filter layer, manganese sand having a grain diameter of 2mm was used. The filtering speed was 120m per day. Concentration of manganese and iron and concentration of dissolved oxygen of both water to be treated and purified water after treatment, i.e., treated water, were measured once a day at a preset time. The concentration of dissolved oxygen of treated water was measured by coUecting water immediately after completion of aeration by the Hquid-gas contact device (column packing 5). Results of the test are shown in Table 1. In the table, "untreated" means water to be treated and "treated" means treated water.

Table 1 Results of manganese removal test Day Concentration of Concentration of Concentration of manganese mg/1 iron mg/1 dissolved oxygen mg/1 untreated treated untreated treated untreated treated

1^st day 0.516 0.041 0.06 0.01 5.8 8.7 2^nd day 0.464 0.017 0.10 0.03 4.9 8.8 3^rd day 0.402 0.012 0.02 0.00 5.0 8.7

4^ day 0.333 0.018 0.02 0.02 7.0 9.5 5^th day 0.385 0.008 0.01 0.01 6.6 10.1

6^a day 0.314 0.010 0.00 0.00 4.8 9.9 7^th day 0.319 0.012 0.01 0.03 7.0 9.7

8^th day 0.404 0.011 0.05 0.04 5.5 8.4

9^Λ day 0.411 0.011 0.03 0.00 5.0 8.1 10^th day 0.281 0.011 0.03 0.02 not measured

For comparison, a test was conducted by removing manganese under the same condition as described in the above test excepting that aeration by the Hquid-gas contact device was not conducted but water to be treated was supphed directly from the iron removing device 100 to the filter container 3. Results of the test are shonw in Table 2.

Table 2 Results of manganese removal test without aeration Day Concentration of Concentration of Concentration of manganese mg/1 iron mg/1 dissolved oxygen mg/1 untreated treated untreated treated untreated treated

1^st day 0.392 0.248 0.02 0.03 6.1

2^nd day 0.468 0.234 0.00 0.04 4.8

3^rd day 0.405 0.225 0.36 0.00 3.5

4^th day 0.288 0.201 0.03 0.04 5.3

5^th- day 0.366 0.198 0.03 0.03 5.4

6^th day 0.308 0.164 0.04 0.01 not measured

From the results of Tables 1 and 2, it wiU be understood that the example of the present invention enables the concentration of manganese in the treated water to be reduced to below 0.05mg/l which is aUowed by the statutory water quaHty standard whereas the concentration of manganese by the comparative example which does not use aeration is within a range from 0.164mg/l to 0.248mg/l and, therefore, the comparative example cannot reduce the concentration of manganese to an aUowable value. Example 2

A test for removing manganese by a modified device of the device shown in FIG. 1 was conducted by using, as water to be treated, underground water which was coUected at Haibaramachi, Hyogo-ken, Japan and in which the concentration of iron was reduced by treating the underground water by the iron removing device shown in FIG. 1. In this test, the device of FIG. 1 was modified so that aeration was performed by using a hose instead of using the Hquid- gas-contact device. More specificaUy, aeration was performed by causing water to be treated from the iron removing device 100 to faU down at a flow speed of 1 Htre per minute onto the surface of water contained in the filter container 3 from a hose having an inner diameter of 10mm located at a height of l above the surface of water contained in the filter container 3. As the filter layer, black sand (a filter layer comprising colonies of iron bacteria) having a grain diameter of 0.6mm was used. The filteing speed was 120m per day. Concentration of manganese and iron and concentration of dissolved oxygen of both water to be treated and purified water after treatment, i.e., treated water, were measured once a day at a preset time. Average concentration of dissolved oxygen of water to be treated was 5 mg/1 and concentration of dissolved oxygen in water immediately after completion of aeration by the hose was 7.2 mg/1. Results of the test are shown in Table 3. In the table, "untreated" means water to be treated and "treated" means treated water.

Table 3 Results of manganese removal test

Day Concentration of Concentration of

Iron mg/1 manganese mg/1 untreated untreated treated

1^st day 0.02 0.547 0.017

2^nd day 0.05 0.427 0.018

3^rd day 0.07 0.358 0.040

4^th day 0.03 0.402 0.028

5^th day not measured 0.425 0.011

7^7h day not measured 0.385 0.011

8^th day not measured 0.300 0.011

9^th day not measured 0.389 0.005

10^th day not measured 0.293 0.005

11^th day not measured 0.257 0.013

12^th day not measured 0.172 0.010

13^th day not measured 0.221 0.014

14^th day not measured 0.238 0.007

15^th day not measured 0.278 0.009 16^th day not measured 0.202 0.007

17^th day not measured 0.152 0.009

18^th day not measured 0.222 0.007

Example 3

A test for removing manganese was conducted under the same conditions as described above in Example 2 excepting that a filter layer made by transferring iron bacteria Leptothrix on filtering sand having a grain diameter of 2mm was used. Results of the test are shown in Table 4. Table 4

Results of manganese removal test Day Concentration of manganese mgϊ untreated treated

1^st day 0.300 . 0.038

2^nd day 0.389 0.027

3^rd day 0.293 0.018

4^th day 0.257 0.016

5^th day 0.172 0.017

7^th day 0.221 0.008

8^th day 0.236 0.032

For comparison, a test was conducted under the same conditions as described in Example 3 excepting that ordinary filtering sand having a grain diameter of 2mm to which iron bacteria was not transferred was used as the filter layer. Results of the test are shown in Table 5. Table 5

Results of manganese removal test (comparative example) Day Concentration of manganese mg/1 untreated treated 1^st day 0.547 0.486

2^nd day 0.427 0.279

3^rd day 0.358 0.233

4^th day 0.402 0.229 From the results of Tables 4 and 5, it wiU be understood that even if aeration is conducted, a sand filter layer comprising no colonies of iron bacteria cannot achieve a sufficient manganese removal effect.

Industrial AppHcabiHty The device for removing manganese in raw water of the present invention can be utUized for filtering manganese in raw water such as underground water, river water, lake water, water flowing from farm and mountain.

Claims

1. A device for removing manganese in raw water connected at a posterior stage of a filtering device for filtering out iron in raw water by oxidizing it, comprising: a raw water supply tube; aeration means for performing aeration for increasing dissolved oxygen concentration of the raw water supphed from the raw water supply tube; a filter layer container containing a filter layer which has a catalytic function for enhancing oxidation of manganese in the raw water; and a filtered water outlet provided in the filter layer container.

2. A device as defined in claim 1 wherein said filter layer comprises colonies of iron bacteria.

3. A device as defined in claim 1 wherein said filter layer contains manganese sand as a principal filter material.

4. A device as defined in claim 1 wherein the raw water supply tube is located in the upper portion of the device and said aeration means comprises a

Hquid-gas contact device having a flow-path forming structure which causes raw water supphed from the raw water supply tube to flow in a plurahty of divided flow-paths, said Hquid-gas contact device having a column packing and a plurahty of adaptors for supplying the raw water from the raw water supply tube to the column packing, said column packing consisting of a plurahty of column packing constituting elements in the form of line elements or plate elements, each of the column packing constituting elements being extending verticaUy in paraUel to adjacent column packing constituting elements in a non-contact state, and each of said adaptors being formed integraUy with a corresponding one of the column packing constituting elements and being connected directly to the raw water supply tube without branching off from another of the adaptors.

5. A device as defined in claim 1 further comprising: a clogging-removing bar holder to which a plurahty of clogging-removing bars are fixed, each of said clogging-removing bars being provided in such a manner that a tip end portion thereof is inserted in a surface portion of the filter layer,^" and means for reciprocating the clogging-removing bar holder in a plane paraUel to the surface of the filter layer.

6. A device as defined in claim 1 further comprising filtered water circulating means for stopping supply of raw water from the raw water supply tube during stoppage of supply of filtered water and returning the filtered water to the raw water supply tube thereby to circulate the filtered water through the filter layer.

7. A device as defined in claim 6 wherein the filtered water circulating means comprises- a filtered water circulating tube for connecting the filtered water outlet with the raw water supply tube; a water supply pump connected to the filtered water circulating tube,^" a changeover valve provided in the filtered water circulating tube for passing filtered water and stopping supply of water to the filtered water circulating tube during supply of the filtered water and stopping supply of the filtered water and returning the filtered water to the raw water supply tube through the filtered water circulating tube during stoppage of supply of the filtered water; and valve means provided in a raw water supply channel for passing raw water and stopping passage of the filtered water from the filtered water circulating tube during supply of the filtered water and stopping passage of the raw water and passing the filtered water from the filtered water circulating tube during stopping of supply of the filtered water.

8. A device as defined in claim 6 wherein the filtered water circulating means comprises: a filtered water circulation outlet provided separately from the filtered water outlet; a filtered water circulating tube for connecting the filtered water outlet with the raw water supply tube,^" a water supply pump connected to the filtered water circulating tube; a valve provided in a tube provided on the side of the filtered water outlet for opening during supply of the filtered water and closing during stoppage of supply of the filtered water; and valve means provided in a raw water supply channel for passing raw water and stopping passage of the filtered water from the filtered water circulating tube during supply of the filtered water and stopping passage of the raw water and passing the filtered water from the filtered water circulating tube during stoppage of supply of the filtered water.

9. A device as defined in claim 1 further comprising a reverse washing tube disposed in the filter layer at a depth which is sufficinet for removing foreign matters which have been accumulated in the upper portion of the filter layer by reverse washing and also is sufficient for aUowing a substantiaUy thick portion of the filter layer to exist under the reverse washing tube.

10. A method for removing manganese in raw water after filtering out iron in the raw water by a filtering material comprising steps of: increasing dissolved oxygen concentration of the raw water supphed from a raw water supply tube by aeration; causing the raw water to pass through a filter layer which has a catalytic function for enhancing oxidation of manganese; taking out filtered water which has passed through the filter layer; and stopping supply of the raw water from the raw water supply tube during stoppage of the filtered water and returning the filtered water to the raw water supply tube for circulating the filtered water through the filter layer, while maintaining the dissolved oxygen concentration of the filtered water at a predetermined level by continuing aeration.