fig1 through 4 show one embodiment of the filtration unit of the present invention . fig5 shows one embodiment of the filtration apparatus of the present invention in which the filtration unit 1 of this embodiment is installed inside a water storage tank 11 such as a settling tank in a water cleaning plant , etc . fig6 through 14 illustrate a filtration method for raw water w that uses the filtration unit 1 . in the filtration unit 1 of the shown embodiment , filtration modules 3 and agitation means 4 are accommodated inside a casing 2 . substantially the casing 2 is formed in the shape of a tube that has a bottom , and this casing 2 is provided with introduction ports 2 a into which raw water w is introduced . the casing 2 is further provided with an active carbon supply means 5 that adds active carbon c to the raw water w that is introduced into this casing 2 , a back flow prevention means 6 that prevents the back flow of raw water w inside the casing 2 to the introduction ports 2 a , and a discharge means 7 that discharges the raw water w from the casing 2 . the casing 2 is formed in an oblong rectangular shape ( when seen from above or in a top view ) from a metal material , etc ., that has a high corrosion resistance such as stainless steel . furthermore , the casing 2 is formed in the shape of a rectangular tube that is long in the vertical direction , and it has a bottom . the height of the casing 2 is greater than the lengths of the longer and shorter sides of the rectangular shape top and bottom . in other words , the casing 2 is in a shape in which the bottom of a rectangular tube is closed off . the casing 2 is disposed so that the height direction of the casing is oriented in the vertical direction with its bottom facing down . the bottom of this casing 2 is substantially in the shape of a downward - protruding truncated square pyramid which has pyramid faces that are respectively continuous to the four side surfaces of the casing 2 and which shows a gradually decreasing cross section in the downward direction . the substantial shape of a downward - protruding truncated square pyramid , in this embodiment , is a shape that is employed to reduce the residual amount of drain water and does not place any limitation on the bottom shape used in the present invention . the introduction ports 2 a are formed in the upper part of the casing 2 , and control means 2 b that control the flow of the raw water w through the introduction ports 2 a are provided in the introduction ports 2 a . the introduction ports 2 a are formed by the inner circumferential portions of a plurality of tubular members that are caused to protrude from the upper wall surfaces of the casing 2 at intervals around the circumference of the casing 2 so that the introduction ports pass through the wall surfaces . the control means 2 b are formed by opening and closing valves such as butterfly valves and are installed in the root portions of the tubular members . raw water w is introduced into the casing 2 via the introduction ports 2 a when the introduction ports 2 a are opened by the opening and closing valves via a driving means ( not shown in the drawings ); and when the introduction ports 2 a are closed , the inside of the casing 2 is isolated from the outside raw water w , and the inflow of raw water w into the casing 2 and the outflow of raw water w from the casing 2 via the introduction ports 2 a are , as a result , prevented . the through - flow of raw water w is thus controlled . as shown in fig6 through 14 , the introduction ports 2 a are disposed at a height that opens into the water surface h of the raw water w held in the water storage tank 11 , so that the water introduction ports 2 a open into this water surface h . however , this does not limit the manner of the water inflow control of the present invention . a partition plate 2 c is provided around the outer circumference of the casing 2 so that the partition plate 2 c surrounds the introduction ports 2 a . the partition plate 2 c is comprised of four rectangular plate members that are respectively parallel to the upper wall surfaces of the casing 2 and are formed into the shape of a frame wall with no gaps in the circumferential direction so that the plate members form a surrounding rectangular shape ( when seen from above or in a top view ) that is slightly larger than the above - described rectangular shape ( when seen from above or in a top view ) of the casing 2 . the top and bottom of the partition plate 2 c are left open . furthermore , as seen from fig5 through 14 , the partition plate 2 c is provided so that there is a gap between the partition plate 2 c and the introduction ports 2 a and so that the partition plate 2 c is positioned at the height of the water surface h of the raw water w inside the water storage tank 11 . accordingly , the raw water w flows via beneath this partition plate 2 c into the space between this partition plate 2 c and the casing 2 and is then introduced into the casing 2 via the introduction ports 2 a . dirt and other matters floating on the water surface h is thus prevented from entering “ as is ” into the interior of the casing 2 . the filtration modules 3 that are accommodated inside the casing 2 are modules in which , for example , numerous elements that have hollow filament membranes stretched in screen form or numerous elements that have flat - film - form separating membranes are laminated , or in which numerous tubular ceramic film elements are formed into bundles and put together by means of headers ( not shown in the drawings ) so that modules are formed . in the shown embodiment , the individual filtration modules 3 have the shape of substantially a rectangular block that are long in the horizontal direction . furthermore , a plurality of ( four ) such filtration modules 3 that have the same size and shape are installed inside the casing 2 so that the pair of side surfaces extend in the direction of length of each module oriented in the horizontal direction , the longer sides of the side surfaces are oriented parallel to the longer sides of the rectangular shape formed by the casing 2 ( when seen from above or in a top view ), and the filtration modules 3 stacked in a row so that the modules 3 are lined up in the vertical direction with the side surfaces that face each other . the plurality of filtration modules 3 that are thus stacked are arranged so that the filtration module 3 at the uppermost end of the stack is positioned below the introduction ports 2 a of the casing 2 as shown in the figures . the filtration modules 3 that are adjacent to each other in the vertical direction may adhere tightly to each other or may be installed with an appropriate gap left between such adjacent modules . the headers of the respective filtration modules 3 are connected to a filtration suction pump 3 b via a membrane filtered water pipe 3 a , and the filtered water t that is filtered from the raw water w by the filtration modules 3 is discharged by this filtration suction pump 3 b via the membrane filtered water pipe 3 a . in the shown embodiment , the filtration modules 3 of a rectangular block shape are installed in a horizontal attitude and lined up in the vertical direction . however , as seen from fig4 , for example , flat - plate - form flat - membrane - type filtration modules 3 can be installed in the casing 2 so that these modules are placed in a vertical attitude and lined up in the horizontal direction ; and in addition , a plurality of rows of such modules can be installed . furthermore , instead of flat - plate - form modules , the modules can be those that have hollow filaments bundled in the vertical direction . thus , there are no restrictions on the modules used in the present invention . in the shown embodiment , the agitation means 4 also serves as a cleaning means for the filtration modules 3 , so that the agitation means 4 agitates the raw water w and cleans the filtration modules 3 using a fluid that is supplied to the interior of the casing 2 . in particular , an aeration means that spreads air a used as a fluid for agitation and cleaning inside the casing 2 is used in this embodiment . more specifically , as shown in fig2 , the aeration means ( which is an agitation means 4 and also is a cleaning means ) has a structure in which , for example , a plurality of aeration devices 4 a , which are pipes or sintered tubes that have numerous air holes opened downward and are disposed beneath the filtration modules 3 , are connected to an air supply source 4 c such as a blower via an air pipe 4 b that is used as an agitation fluid supply pipe or a cleaning fluid supply pipe . as a result , air a ( see fig8 and 9 ) that is supplied from this air supply source 4 c passes through the air pipe 4 b and is blown out from air holes opened in the pipes of the aeration devices 4 a or from fine pores formed in the sintered tubes , thus agitating the raw water w inside the casing 2 and cleaning the filtration modules 3 located above the aeration devices 4 a by way of air - scrubbing made by the aeration . the active carbon supply means 5 supplies a suspension containing 2 to 5 % active carbon c to the interior of the casing 2 from a tank 5 a via an active carbon supply pipe 5 b , so that this active carbon c is added to the raw water w inside the casing 2 . as shown in fig6 through 14 , the position of the lower end of this active carbon supply pipe 5 b which opens into the upper part of the inside of the casing 2 is set at a point lower than the upper end edge of the casing 2 . it is desirable that the mean particle size of the active carbon c that is added be approximately 10 to 200 μm and that the amount of active carbon c that is added be approximately 10 to 100 mg per liter of raw water w . instead of supplying a suspension of active carbon c as describe above , it is also possible to inject the active carbon c itself from above the casing 2 , so that this active carbon c is added to the raw water w . in the shown embodiment , furthermore , descending pipes 6 a that extend downward from the introduction ports 2 a are installed inside the casing 2 . the lower - end openings 6 b of the descending pipes 6 a are positioned below the aeration devices 4 a of the above - described aeration means used as agitation means 4 and cleaning means . with the descending pipes 6 a , the above - described back flow prevention means 6 that prevents raw water w that has been introduced into the casing 2 from flowing back and being discharged into the water storage tank 11 from the introduction ports 2 a is constructed . here , in the descending pipes 6 a , angle - type strip members that have a flattened “ c ” shape cross section are joined to the inside walls of the casing 2 in an area extending from the openings of the introduction ports 2 a on the inside of the casing 2 to points located immediately above the bottom of the casing 2 ( below the aeration devices 4 a ), and the open part of this cross section faces the corresponding inside wall of the casing 2 as shown in fig3 . the upper end portions of the descending pipes 6 a are closed so that the introduction ports 2 a communicate with the interiors of the descending pipes 6 a as shown in fig6 through 14 . the cross section of the descending pipes 6 a in the water introduction part may also be semi - circular , or the descending pipes 6 a may be pipes that run along the wall surfaces . there are no restrictions on the shape of the descending pipes 6 a . the discharge means 7 comprises a discharge pipe 7 b that opens at one end thereof in the bottom of the casing 2 and is connected at another end thereof to a discharge suction pump 7 a provided outside the casing 2 . the discharge pipe 7 b is provided so that the pipe extends from the discharge suction pump 7 a side to a point directly above the casing 2 of the filtration unit 1 and then passes through the interior of the casing 2 in the above - described height direction from the opening in the upper part of this casing 2 , with one end of the pipe opening in the bottom of this casing 2 . the end portion of the discharge pipe 7 b opens downward near the lower end of the inside bottom of the casing 2 , which is in the shape of a truncated square pyramid protruding downward as described above . the membrane filtered water pipe 3 a of the filtration modules 3 and the air pipe 4 b of the aeration means ( agitation means 4 and cleaning means ) also pass through the interior of the casing 2 from the opening in the upper part of this casing 2 in the same manner as the discharge pipe 7 b , and they are connected to the headers of the filtration modules 3 or the aeration devices 4 a . an ultraviolet lamp may be installed inside the casing 2 so as to face the filtration modules 3 . when an ultraviolet lamp is employed , a plurality of waterproofed fluorescent tube - form lamps , for instance , are supported in the gaps between the filtration modules 3 and the inside wall surfaces of the casing 2 or descending pipes 6 a , or between adjacent descending pipes 6 a , etc ., along substantially the entire length of the filtration modules 3 in the above - described height direction of the casing 2 so that the direction of length of the respective lamps is oriented parallel to this height direction and so that the lamps are installed at substantially equal intervals in the above - described circumferential direction . the lamps are connected to a power supply and lit so that the filtration modules 3 are irradiated with ultraviolet light . next , one embodiment of the filtration apparatus of the present invention in which such filtration units 1 as described above are installed inside a water storage tank 11 will be described with reference to fig5 . an embodiment of the filtration unit installation method of the present invention that is used to install the filtration units 1 in this water storage tank 11 will also be described . the water storage tank 11 in the filtration apparatus of this embodiment is , for example , a settling tank installed in an existing water cleaning plant ; and this tank is formed so that the tank shows an oblong rectangular shape when seen from above or in a top view , and its direction of length is oriented in the left - right direction in fig5 . this water storage tank 11 is comprised of a settling tank main body 12 and an overflow trough tank 13 . the settling tank main body 12 has a shape of a pool which is formed so that the oblong shape extends downward “ as is ” to make the depth , and the overflow trough tank 13 is provided adjacent to one end ( first end ), which is the right side end in fig5 , of the settling tank main body 12 with respect to the above - described direction of length . in fig5 , the filtration units 1 are installed in both the settling tank main body 12 and the overflow trough tank 13 . however , the filtration unit ( s ) 1 can be installed only in the settling tank main body 12 or only in the overflow trough tank 13 . on the opposite end ( or a second end ) of the settling tank main body 12 ( with respect to the direction of length ) from the overflow trough tank 13 ( i . e ., the left side end in fig5 ), a flocculant mixing tank ( not shown in the drawings ) and a floc - forming tank 14 to which the raw water w that is introduced into the settling tank main body 12 is supplied are installed , so that these tanks are adjacent to the settling tank main body 12 with the end wall 12 a on this second end of the settling tank main body 12 in between . the above - described flocculant mixing tank can be supplied with flocculent , and a mixer is installed in this tank . the raw water w that is supplied to the flocculant mixing tank is agitated at a high speed by the mixer , so that this water is mixed with the flocculant . the raw water w mixed with flocculant then flows into the floc - forming tank 14 . the floc - forming tank 14 is provided therein with a flocculator ( not shown in the drawings ) so that this flocculator is rotatable . the raw water w that flows into the floc - forming tank 14 after being mixed with flocculant in the flocculant mixing tank is agitated at a slower speed by the flocculator so that flocs that will readily precipitate are sufficiently formed , and this raw water w is then introduced into the settling tank main body 12 from the end wall 12 a at the second end . in the end wall 12 a which is at the second end ( with respect to the direction of length ) of the settling tank main body 12 and is between the settling tank main body 12 and the floc - forming tank 14 , numerous introduction holes 12 b are formed in a lattice configuration or staggered configuration in the vertical and horizontal directions . the end wall 12 a thus makes a flow adjusting wall . the raw water w that is supplied to the floc - forming tank 14 is introduced into the settling tank main body 12 through the introduction holes 12 b . the water level of the raw water w that is supplied to the settling tank main body 12 through the floc - forming tank 14 from the flocculant mixing tank is higher than the introduction holes 12 b that are located at the highest positions in the end wall 12 a . thus , when the raw water w is held in the settling tank main body 12 , the raw water w does not overflow from the floc - forming tank 14 into in the settling tank main body 12 and is supplied to the settling tank main body 12 from the second end with respect to the direction of length . the introduction holes 12 b located at the lowest positions in the end wall 12 a are sufficiently higher than the bottom 12 c of the settling tank main body 12 . the settling tank main body 12 is formed in a substantially rectangular shape when seen from the side ( or in a side view ) as well ; and it has longer sides in the above - described direction of length and is open at the top , as seen from fig5 . however , the bottom 12 c , which is of the settling tank main body 12 and is at the second end , is formed so as to be lowered by one step , as shown in fig5 , to make a mud discharge pit 15 . a mud discharge pipe ( not shown in the drawings ) which discharges mud m via a mud discharge valve ( likewise not shown in the drawings ) is connected to this mud discharge pit 15 . in addition , a mud scraping means 16 is installed in the bottom 12 c of the settling tank main body 12 . the mud scraping means 16 is movable in a direction of the longer sides of the rectangular shape of the settling tank main body 12 when seen from above or in a top view , especially in the movement direction f in the present embodiment , where the direction of the longer sides of the above - described rectangular shape ( the above - described direction of length , i . e ., the left - right direction in fig5 ) is taken as this movement direction f . rails 16 a are respectively provided so as to be parallel to the movement direction f ( along the longer sides of the rectangular shape of the settling tank main body 12 when seen from above or in a top view ) in the bottom 12 c of the settling tank main body 12 , and an underwater car 17 is provided so that the underwater car 17 car is movable on the rails 16 a in the movement direction f . the underwater car 17 and rails 16 a thus constitute the mud scraping means 16 . on both sides and both ends in the above - described direction of length , the underwater car 17 has wheels 17 a that are placed on the rails 16 a ; and a scraper 17 b , which extends in the direction of the shorter sides of the settling tank main body 12 so that this scraper does not interfere with the rails 16 a , is attached to the bottom of the underwater car 17 a . as seen from fig5 , a wire rope 16 b is disposed in the direction of length of the settling tank main body 12 ( i . e ., the movement direction f ) directly above the running path of the underwater car 17 , and the wire rope 16 b is connected to the underwater car 17 . the wire rope 16 b is wound around pulleys 16 c , which are attached to the bottom 12 c of the settling tank main body 12 at both ends in the direction of length , and is mounted on a rope winding means ( not shown in the drawings ) disposed on a slab on the second end of the settling tank main body 12 . by way of winding up and releasing the wire rope 16 b by the rope winding means , the wire ripe 16 b installed in the direction of length of the bottom 12 c of the settling tank main body 12 is caused to advance and retract in the movement direction f , so that the underwater car 17 makes a reciprocating motion in the movement direction f . in the shown embodiment , a plurality of sets of the mud scraping means 16 and underwater cars 17 are provided in the bottom 12 c of the settling tank main body 12 so that the shorter sides of the bottom in the rectangular shape seen in a top view are divided into equal parts ; and a plurality of sets of the rope winding means that respectively drive ( winds up and releases ) the wire ropes 16 b of the mud scraping means 16 are installed on the slab . in this case , the driving units that drive the rope winding means may be provided for each rope winding means so that the underwater cars 17 are moved independently . a plurality of sets ( e . g ., two sets ) of rope winding means can be thus driven by a single driving unit so that when one underwater car 17 is moved in one direction of the reciprocating motion in the movement direction f , the other underwater car 17 is moved in the other ( opposite ) direction of this reciprocating motion . as described above , the underwater cars 17 are movable in the direction of the longer sides of the oblong rectangular shape of the settling tank main body 12 . however , in cases where the length of the shorter sides of the rectangular shape of this settling tank main body 12 is long enough , then it is possible to install a plurality of pairs of rails 16 a in the direction of the shorter sides , install a plurality of underwater cars 17 , and install mud scraping means 16 , so that the direction of the shorter sides is taken as the movement direction f . the underwater cars 17 are connected to the wire ropes 16 b so that the scrapers 17 b of the underwater cars 17 move in linkage with the reciprocating motion that accompanies the advancing and retraction motion of the wire ropes 16 b in the movement direction f . more specifically , when the portion of each wire rope 16 b that is connected to the corresponding underwater car 17 advances toward the second end ( left side end in fig5 ) of the settling tank main body 12 with respect to the direction of length ( in the movement direction f ), and the underwater car 17 is accordingly moved in one direction of the reciprocating motion toward the mud discharge pit 15 at this second end in the direction of length ( in the movement direction f ), the corresponding scraper 17 b is lowered from the underwater car 17 and is placed so that the scraper makes sliding contact with the bottom 12 c of the settling tank main body 12 or is positioned in close proximity to this bottom 12 c . as a result , the mud that has accumulated in the bottom 12 c is scraped by the moving underwater car 17 in the above - described direction and dropped into the mud discharge pit 15 . conversely , when the portion of each wire rope 16 b that is connected to the corresponding underwater car 17 is retracted toward the first end , and the underwater car 17 is moved in the other direction of the reciprocating motion that is opposite to the above - described first direction ( in the movement direction f ), the scraper 17 b is raised to be separated from the bottom 12 c of the settling tank main body 12 , so that the accumulated mud m is not taken up . furthermore , numerous inclined plates 18 , which are inclined with respect to the horizontal plane and are stacked with an appropriate spacing , are provided in the upper part of the interior of the settling tank main body 12 that has the mud scraping means 16 in the bottom 12 c . the inclined plates 18 are for promoting the precipitation of flocs . inclined pipes can be employed instead . in cases where there is a sufficient length in the direction of flow in the settling tank , and a sufficient precipitation time is obtained , then such inclined plates are not required . the overflow trough tank 13 is in a rectangular shape ( oblong shape ) that extends along the shorter side at the first end of the rectangular shape of the settling tank main body 12 when viewed from above . the width of the overflow trough tank 13 in the direction of length is set at a smaller value than the settling tank main body 12 . the overflow trough tank 13 is smaller in depth ( thus shallower ) than the settling tank main body 12 . the overflow trough tank 13 is provided next to the settling tank main body 12 with the partition wall 12 d in between at the first end ( with respect to the direction of length ) of the settling tank main body 12 . numerous introduction holes 12 e are formed in a lattice configuration or staggered configuration in the vertical and horizontal directions ( in the same manner as in the end wall 12 a at the second end ) in the partition wall 12 d . relatively clean raw water w , which has been introduced into the settling tank main body 12 of the water storage tank 11 from the floc - forming tank 14 and from which the major portion of the mud m has been separated by precipitation as described above , is introduced into the overflow trough tank 13 through the introduction holes 12 e . between the partition wall 12 d and the inclined plates 18 inside the settling tank main body 12 , a space is provided . an overflow trough ( not shown in the drawings ) is provided in the overflow trough tank 13 , and the supernatant of the inflowing water flows out from this overflow trough . in the shown embodiment , a plurality of the filtration units 1 which have the same size , same shape and same structure are provided in the settling tank main body 12 and overflow trough tank 13 , so that the bottoms of the casings 2 of the filtration units 1 face downward , and the height direction of the casings 2 is oriented in the vertical direction and the filtration units 1 are immersed in the raw water w that is introduced into the settling tank main body 12 and overflow trough tank 13 . in the settling tank main body 12 , the filtration units 1 are disposed between the inclined plates 18 and partition wall 12 d ; and such filtration units 1 that are in a rectangular shape when viewed from above ( in a top view ) are disposed so that their sides are set parallel to each other . the filtration units 1 are installed also with gaps formed between the filtration units 1 and the inside walls of both the settling tank main body 12 ( including the partition wall 12 d ) and the overflow trough tank 13 . when necessary , the filtration units 1 are lined up in the ( width ) direction in which the overflow trough tank 13 extends ( i . e ., in the direction perpendicular to the drawing sheet of fig5 ). accordingly , when seen from above or in a top view , the filtration units 1 are disposed on one side ( first side ) of the settling tank main body 12 which is the opposite side from the end wall 12 a where the raw water w is introduced into the water storage tank 111 of the filtration apparatus . the inclined plates 18 of the settling tank main body 12 are disposed between the filtration units 1 and the end wall 12 a which is on the second end where the raw water w is introduced into the water storage tank 11 . the above - described overflow trough is not used when the filtration units are installed in the overflow trough tank 13 . the filtration units 1 are provided at the same height in their vertical direction . in other words , with gaps being formed between the filtration units 1 and the bottom 12 c of the settling tank main body 12 and also between the filtration units 1 and the bottom 13 a of the overflow trough tank 13 , the casings 2 and partition plates 2 c of the filtration units 1 are suspended by , for example , wire ropes that extend from a frame installed over the settling tank main body 12 and overflow trough tank 13 . it is also possible that the casings 2 and partition plates 2 c of the filtration units 1 be suspended by being directly attached to a frame consisting of angle members , etc ., which are disposed inside the settling tank main body 12 and overflow trough tank 13 . for each one of the filtration units 1 , it is possible that the casing 2 is supported by a wire rope or a frame as described above and the partition plate 2 c is supported by the casing 2 or vice versa . the filtration modules 3 and cleaning means 4 that are accommodated inside the casings 2 can be likewise supported by being suspended by wire ropes , etc ., from a frame that is installed over the settling tank main body 12 and over the overflow trough tank 13 or by being directly attached to brackets , etc ., that are disposed inside the casings 2 . as to the upper portions of the casings 2 of the filtration units 1 that are thus supported , the upper end edges of the casings 2 and partition plates 2 c are positioned so that they are above the water level , i . e ., above the water surface h , of the raw water w that is introduced into the settling tank main body 12 and overflow trough tank 13 as described above ( as shown in fig6 through 14 ), and so that the lower end edges of the partition plates 2 c are positioned below the water surface h . the lower end edges of the introduction ports 2 a of the casings 2 are positioned below the water surface h , and the introduction ports 2 a open into this water surface h . in particular , in the shown embodiment , the introduction ports 2 a are positioned and opened at substantially the same height as the water surface h of the raw water w as shown in fig6 . in the filtration apparatus in which the filtration units 1 are thus installed inside the water storage tank 11 , raw water w passing through the floc - forming tank 14 from the flocculant mixing tank is introduced into the settling tank main body 12 of the filtration apparatus through the introduction holes 12 b of the end wall 12 a on the second end as described above . while the raw water w moves toward the first end ( with respect to the above - described direction of length ) of the rectangular shape settling tank main body 12 , most of the flocs of mud m , etc . are precipitated and separated by the inclined plates 18 so that the flocs are deposited in the bottom 12 c , thus producing relatively clear raw water w . moreover , the mud m that has thus been deposited in the bottom 12 c is scraped into the mud discharge pit 15 at the second end by the underwater car ( s ) 17 of the mud scraping means 16 and discharged . the raw water w that has thus been clarified is then filtered by the filtration units 1 disposed at the first end of the settling tank main body 12 . furthermore , the raw water w that is introduced into the overflow trough tank 13 through the introduction holes 12 e of the partition wall 12 d at this first end is filtered by the filtration unit 1 installed in this overflow trough tank 13 . more specifically , during the filtration operation that is performed in the filtration units 1 , the opening - and - closing valves used as the control means 2 b are first opened so that the introduction ports 2 a are opened as shown in fig6 , and the raw water w inside the water storage tank 11 passes through the introduction ports 2 a and is introduced into the casings 2 since the introduction ports 2 a are formed so that the introduction ports 2 a can open into the water surface h of the raw water w . however , the openings of the introduction ports 2 a inside the casings 2 are covered by the descending pipes 6 a of the back flow prevention means 6 . thus , the raw water w that is introduced through the introduction ports 2 a passes through the interiors of the descending pipes 6 a and then flows into the interior of the casings 2 ( located further to the inside than the descending pipes 6 a ) from the lower - end openings 6 b of the descending pipes 6 a located in the bottoms of the casings 2 , and the interiors of the casings 2 are filled with this raw water w . the introduction of raw water w reaches equilibrium and is completed when the water level of the raw water w inside the casings 2 becomes equal to the water level of the water surface h of the raw water w in the water storage tank 11 as shown in fig7 . next , as seen from fig8 , active carbon c ( a suspension of active carbon c in the shown embodiment ) is supplied by the active carbon supply means 5 to the raw water w that has thus been introduced in the casings 2 . in this case , the introduction ports 2 a are closed by the control means 2 b , and a control is made so that the flow of raw water w into or out of the casings 2 through the introduction ports 2 a is blocked . afterward , as seen from fig9 , the raw water w inside the casings 2 to which active carbon c has been added is agitated by the aeration means used as the agitation means 4 , as a result of air a being caused to jet from the aeration devices 4 a as an agitating fluid , so that the active carbon c is uniformly dispersed in the raw water w . the introduction ports 2 a are still kept closed by the control means 2 b during this period as well . when the raw water w inside the casings 2 has thus been agitated , the filtration suction pump 3 b connected to the filtration modules 3 is driven , so that the filtration modules 3 filters the raw water w is filtered as shown in fig1 . in this case , organic matter , etc ., that is mixed with the raw water w is adsorbed by the active carbon c that has been added to this raw water w ; and as a result of filtration by suction , the active carbon c itself also adheres to the surfaces of the hollow filament membranes , etc ., that constitute the filtration modules 3 , and forms a layer on the surfaces as shown in fig1 . when a layer of active carbon c is thus formed and filtration is performed , turbid matter such as mud is filtered by this layer , so that the load on the membranes is reduced . small particles of such turbid matter that pass through the active carbon layer are filtered by the membranes ; and as a result , clear raw water w alone is discharged and recovered as filtered water t . during the above filtration of the raw water w by the filtration modules 3 , as shown in fig1 and 11 , the introduction ports 2 a are opened by the control means 2 b , so that the flow of raw water w through the introduction ports 2 a is allowed . consequently , when the raw water w is filtered by the filtration modules 3 and clear water t is discharged , a pressure difference is generated in the raw water w between the insides and outsides of the casings 2 as the water level of the raw water t inside the casings 2 tends to fall below the water level of the raw water w in the water storage tank 11 . as a result , the raw water w flows into the interiors of the casings 2 through the introduction ports 2 a as shown by white arrows , so that a continuous filtration operation of the raw water w is performed . when such continuous filtration is performed , the surfaces of the layers of active carbon c on the surfaces of the filtration modules 3 would be covered by organic matter , etc ., and the filtration efficiency of the layers of active carbon c in newly introduced raw water w would drop . in this situation , such a filtration operation is performed that agitation is performed by the agitation means 4 after temporarily closing the introduction ports by the control means 2 b , thus returning the filtering cycle to the state shown in fig9 , and then the active carbon c is again dispersed , so that the process shown in fig9 through 11 is repeated . during the agitation , the suction in the membranes created by the filtration suction pump 3 b is stopped . in cases where the adsorption capacity of the active carbon is surpassed by the mud or organic matter in the raw water w while such a filtration operation is being performed , then the active carbon is discharged after agitation is performed by the agitation means 4 installed in the filtration units 1 , and fresh active carbon is introduced . in other words , as shown in fig9 , the suction and discharge of the filtered water t by the filtration suction pump 3 b is stopped , and the flow of raw water w through the introduction ports 2 a is cut off by closing the opening - and - closing valves of the control means 2 b , air a is next spread from the aeration devices 4 a of the aeration means , so that adhering matter such as the active carbon c , mud and microscopic particles of turbid matter adhering to the filtration modules 3 is stripped by air scrubbing . the raw water w is , along with this stripped adhering matter , discharged by the discharge means 7 . the water level of the raw water w inside the casings 2 is gradually lowered as shown in fig1 , until this water level is eventually caused to fall below the lower ends of the discharge pipes 7 b as shown in fig1 , whereupon the discharge finishes . air scrubbing can be continued during the discharge of the raw water . subsequently , raw water and active carbon are again introduced , and operation is started . when such a filtration operation is further performed , a filtration efficiency drop would occur by the microscopic particles of turbid matter in the raw water w that adheres to the surfaces of the hollow filament membranes , etc ., of the filtration modules 3 , and by the load increase on the filtration modules . in this situations cleaning of the filtration modules 3 is performed by the cleaning means that is also used as the agitation means 4 installed in the filtration units 1 . in other words , as shown in fig1 , the suction and discharge of the filtered water t by the filtration suction pump 3 b is stopped , and the flow of raw water w through the introduction ports 2 a is cut off by the closing of the opening - and - closing valves of the control means 2 b , air a is next spread from the aeration devices 4 a of the aeration means , and the adhering matter such as the active carbon c , mud and microscopic particles of turbid matter adhering to the filtration modules 3 is stripped by air scrubbing . furthermore , reverse washing is applied by supplying air or filtered water produced by the filtration modules 3 via the membrane filtered water pipes 3 a . the water inside the casings 2 increases by this reverse washing . thus , the raw water w inside the casings 2 is appropriately sucked in by the discharge means 7 and discharged so that water does not overflow from the casings 2 . after the filtration modules 3 have thus been cleaned by the cleaning means and the adhering matter is stripped away , reverse washing is stopped . the raw water w is , together with this stripped adhering matter , discharged by the discharge means 7 . the water level of the raw water w inside the casings 2 gradually decreases as shown in fig1 , until this water level is eventually caused to fall below the lower ends of the discharge pipes 7 b as shown in fig1 , whereupon the discharge finishes . air scrubbing may be continued during the discharge of the raw water . subsequently , raw water and active carbon are again introduced , and operation is started . the discharged raw water w is recovered in a wastewater treatment facility . in some cases , however , this raw water may be supplied to the flocculant mixing tank on the opposite side from the filtration units 1 in the water storage tank 11 , and solid matter may again be precipitated as flocs . as seen from the above , the filtration units 1 constructed as described above are installed in a manner that such casings 2 are immersed in the settling tank main body 12 and overflow trough tank 13 in the water storage tank 11 such as a settling tank in a water cleaning plant . accordingly , the required space does not exceed the size of the water storage tank 11 , and space can be saved . furthermore , since the filtration modules 3 and agitation means ( cleaning means ) 4 are accommodated inside the casings 2 that are substantially tubular in shape and have bottoms , even if , for example , the aeration means is used as the agitation means ( cleaning means ) 4 , and agitation and cleaning are accomplished by spreading air a as in the above embodiment , the raw water w held inside the water storage tank 11 which is outside the casings 2 is not agitated . accordingly , mud deposited in the bottoms 12 c and 13 a of this water storage tank 11 is prevented from rolling up ; and there is likewise no introduction of the rolling up mud , etc ., into the casings 2 along with the raw water w following the agitation or following the cleaning . as a result , a high - speed filtration operation can be performed , greatly improving the working efficiency . in addition , since the active carbon supply means 5 is provided in the filtration units 1 , and active carbon c is added to the raw water w that is introduced into the casings 2 , organic matter , etc ., contained in the raw water w can be adsorbed by this active carbon c . furthermore , mud , etc ., can be separated as a result of the active carbon c that forms a layer on the surfaces of the filtration modules 3 , so that only microscopic particles of turbid matter are separated by the hollow filament membranes , etc ., of the filtration modules 3 . efficient filtration can be thus achieved . moreover , in the above - described installation method and filtration apparatus , since the filtration units 1 are installed inside the water storage tank 11 such as a settling tank , the casing 2 of each filtration unit 1 is small ; and since relatively clear raw water w is introduced into the casings 2 , the amount of active carbon c that is added can be conspicuously smaller than the amount required in cases where such active carbon is added to tanks ( the water storage tank 11 in the above embodiment ) into which the raw water is introduced as in japanese patent application laid - open ( kokai ) no . h9 - 24250 and japanese patent no . 3440402 . moreover , since the amount of mud , etc ., contained in the raw water w that is discharged by the discharge means 7 is small , the raw water w can be recycled into the pre - stage of the filtration units 1 , and the active carbon c can be re - utilized . accordingly , the system is economical , and the cost of filtration is low . in the filtration unit 1 , the back flow prevention means 6 , which prevents the raw water w that has been introduced into the casings 2 from flowing back to the introduction ports 2 a and being discharged into the water storage tank 11 , is provided . accordingly , when the introduction ports 2 a are opened after the raw water w to which active carbon c was added has been agitated by the agitation means 4 , and filtration is continuously performed by the filtration modules 3 while introducing raw water w into the water storage tank 11 , it is possible to prevent such a situation that efficient filtration by the active carbon c is hindered as a result of raw water w that contains active carbon c inside the casings 2 being discharged into the water storage tank 11 from the introduction ports 2 a , and it is also possible to prevent such a situation that the reduction of the cost of filtration is diminished as a result of the need to supply fresh active carbon c from the active carbon supply means 5 in order to make up for the active carbon c that is discharged . in other words , in the filtration unit 1 as described above , and the installation method and filtration apparatus that uses this filtration unit , the effects of improving the filtration efficiency and reducing the cost that are obtained by adding such active carbon c can be manifested much more certainly as a result of the use of the back flow prevention means 6 . in the above embodiment , the descending pipes 6 , which extend downward inside the casings 2 from the introduction ports 2 a and whose lower - end openings 6 b are positioned below the aeration devices 4 a of the agitation means 4 , are used as the back flow prevention means 6 , so that the raw water w is agitated mainly in the area above the agitation means 4 , and the back flow of this raw water w toward the introduction ports 2 a is thus more securely prevented . furthermore , the tubular members such as common pipes may be disposed as the descending pipes 6 a so that the tubular members are oriented downward from the openings of the introduction ports 2 a on the insides of the casings 2 and open below the agitation means 4 . however , if it is devised so that strip members with a c - shaped cross - sectional shape that are open on one side are joined to the inside walls of the casings 2 so that descending pipes 6 a are formed in a tubular shape together with the casings 2 as in the present embodiment , the following advantages are obtainable . namely , the formation of a dead space in which an agitating effect is not sufficiently exerted in the spaces between the tubular members and the inside walls of the casings 2 , etc ., in cases where such tubular members are installed can be prevented , and a large space used for filtration can be ensured inside the casings 2 . in the above embodiment , an aeration means that agitates the raw water w by jetting and spreading air a from the aeration devices 4 a are used as the agitation means 4 ; and this aeration means is used as a cleaning means during the cleaning of the filtration modules 3 so as to strip away that adhering matter , etc ., adhering to the filtration modules 3 by air scrubbing . thus , much more effective operation is accomplished . it is also possible to employ a structure in which the agitation means 4 is not also used as the cleaning means in this way and in which , for example , the agitation means disperses the added active carbon c by agitating the raw water w by means of rotating agitating vanes . furthermore , it is also possible in the present invention to install pipes having holes or sintered pipes similar to such aeration pipes above the lower - end openings 6 b of the descending pipes 6 a or on the filtration modules 3 side within the casings 2 and to supply raw water w or clear water that is pressurized to the pipes , thus agitating the raw water inside the casings 2 or cleaning the filtration modules 3 by a water jet flow . furthermore , in the shown embodiment , the flow of raw water w through the introduction ports 2 a of the casings 2 is controlled by the control means 2 b . accordingly , even if the raw water w inside the casings 2 to which active carbon c has been added should flow back toward the introduction ports 2 a during agitation in spite of the back flow prevention means 6 , the discharge of this raw water w into the water storage tank 11 is prevented . furthermore , the introduction ports 2 a are also closed off by closing the opening - and - closing valves of the control means 2 b during the cleaning of the filtration modules 3 . accordingly , a state in which the raw water w inside the casings 2 and the raw water w inside the water storage tank 11 outside are isolated from each other can be obtained . accordingly , even if the water level of the raw water w inside the casings 2 rises as a result of the air scrubbing performed by the aeration means as shown in fig1 , a situation can be avoided in which adhering matter stripped from the filtration modules 3 by cleaning is discharged into the water storage tank 11 from the introduction ports 2 a , and this matter is again introduced into the casings 2 from the introduction ports 2 a when filtration work is re - started , so that such matter adheres to the filtration modules 3 . accordingly , in the shown embodiment , a drop in the filtration efficiency following cleaning caused by the re - adhesion of the such stripped adhering matter is prevented , and filtration work can be quickly re - started following cleaning , so that much more efficient filtration of the raw water w can be achieved . the partition plates 2 c are installed around the introduction ports 2 a of the casings 2 ; and when raw water w inside the water storage tank 11 is introduced into the casings 2 via the opened introduction ports 2 a , the introduction of raw water w in the water surface h into which the introduction ports 2 a open is limited . more specifically , since the partition plates 2 c are provided so that the plates surround the outer circumferences of the casings 2 at substantially the same height as the introduction ports 2 a , the raw water w in the water surface h located to the outside of the partition plates 2 c does not flow into the introduction ports 2 a “ as is ”; instead , only raw water w that is below the water surface h is introduced into the interiors of the casings 2 via the introduction ports 2 a from the spaces between the partition plates 2 c and casings 2 as indicated by the white arrows in fig6 and 11 . accordingly , even if dust or dirt , etc ., is floating in the water surface h of the raw water w held inside the water storage tank 11 , there is no loss of filtration efficiency as a result of such dust or dirt entering the interiors of the casings and adhering to the surfaces of the filtration modules 3 , and even more efficient filtration can be promoted . meanwhile , the active carbon c that has adsorbed mud or organic matter , etc ., contained in the raw water w introduced into the interiors of the casings 2 , or adhering matter that has been stripped from the filtration modules 3 by cleaning , settles to the bottoms inside the casings 2 . however , in the above filtration units 1 , one end of the discharge pipe 7 b connected to the discharge suction pump 7 a used as the discharge means 7 is caused to open in the bottom of each of the casings 2 , so that adhering matter , etc ., that has thus settled into the bottoms of the casings 2 can be quickly sucked up and discharged together with the raw water w inside the casings 2 . accordingly , adhering matter that has thus settled is prevented from accumulating as a deposit in the bottoms inside the casings 2 and being taken up inside the casings 2 by the raw water w that is introduced following cleaning , so that much more efficient filtration is achieved , and filtration work is quickly re - started following cleaning . furthermore , in the above embodiment , the bottom of each one of the casings 2 is in a pointed shape that protrudes downward , so that the adhering matter , etc ., that has settled is collected at the lower end of the pointed bottom ; and the lower ends of the discharge pipes 7 b open near such a pointed bottom . accordingly , the settled adhering matter is discharged through the discharge pipes 7 b more quickly and more efficiently . furthermore , the discharge pipes 7 b of the shown embodiment are provided so that they pass through the interiors of the casings 2 from the upper openings of the casings 2 and so that the lower ends of the discharge pipes 7 b open in the bottoms of the casings 2 . consequently , the portions of the discharge pipes that are immersed in the raw water w contained inside the water storage tank 11 are not outside of the casings 2 . accordingly , a problem such as the need to make the gap between the discharge pipes 7 b and the walls of the water storage tank 11 larger than would otherwise be necessary in order to avoid interference between the discharge pipes 7 b and the walls of the water storage tank 11 is prevented . in addition a problem such as the need to make the gaps between the filtration units 1 larger than would otherwise be necessary in order to avoid interference between the discharge pipes 7 b of the respective filtration units 1 in cases where a plurality of filtration units 1 are provided inside the water storage tank 11 as in the present embodiment , i . e ., a problem which arises in cases where the discharge pipes 7 b are disposed outside the casings 2 , is prevented . as a result , filtration units 1 of a sufficient size and number can be securely installed even in a water storage tank 11 of limited size . furthermore , as a result of the discharge pipes 7 b that is provide so as to run through the interiors of the casings 2 , there is no need to form through - holes in the walls of the casings 2 or to attach the discharge pipes 7 b so that the discharge pipes 7 b pass through the holes in a liquid - tight manner , as there is in cases where , for example , such discharge pipes 7 b are passed through into the interiors of the casings 2 from the outside parts of the casings 2 . accordingly , designing and manufacturing of the filtration units 1 can be facilitated . moreover , in the above embodiment , the membrane filtered water pipes 3 a of the filtration modules 3 and the air pipe 4 b of the aeration means which functions as the agitation means 4 and cleaning means are also likewise caused to extend through the interiors of the casings 2 from the upper openings of the casings 2 . accordingly , an even more secure disposition of the filtration units 1 and a simplification of the design and manufacture of the filtration units 1 are achieved . furthermore , the filtration suction pump 3 b of the above embodiment is connected to the filtration modules 3 via the membrane filtered water pipe 3 a , and the filtered watery that is filtered from the raw water w by the filtration modules 3 is forcibly sucked in and discharged by this filtration suction pump 3 b . accordingly , much more reliable and efficient filtration is achieved . however , if sufficient suction of the filtered water t can be achieved by some other system , e . g ., a method in which the filtered water is discharged by a gravity system or siphon system , there is no problem to use such a system . by way of installing ultraviolet lamps inside the casings 2 so that the lamps face the filtration modules 3 as described above , even if microorganisms contained in the raw water w that is introduced into the interiors of the casings 2 should adhere to the surfaces of the filtration modules 3 , such microorganisms can be killed and eradicated by ultraviolet light emitted from the ultraviolet lamps 6 . it is thus possible to avoid the problem of sliminess being generated by the propagation of such microorganisms adhering to the surfaces of the filtration modules 3 so that it becomes difficult to strip away the adhering matter on the surfaces of the filtration modules 3 even by cleaning that uses the cleaning means , thus making it possible to maintain a high filtration efficiency over a long period of time . furthermore , as a result of microorganisms thus being killed and eradicated by the ultraviolet lamps , there is little chemical residue in the filtered water t as there is in cases where , for example , a chemical solution is injected into the raw water w . therefore , the time and cost required for the disposal of such residual chemicals can be reduced . meanwhile , in the filtration apparatus that is provided with the above filtration units 1 installed in a water storage tank 11 , an increase in filtration efficiency and a reduction of costs are achieved in the filtration units 1 as described above . accordingly , efficient and economical filtration of the raw water w is achieved in terms of the filtration apparatus as well . moreover , in the installation method of such a filtration apparatus and the filtration units 1 that are used to construct this filtration apparatus , since the filtration units 1 are positively employed , there is no need for large - scale modifications even in the case of , for example , a water storage tank 11 such as the final settling pond in the existing water cleaning plant , so that the present invention is applicable quickly and easily , and at a low cost ., and so that the installation of the filtration units 1 is accomplished even while the water cleaning plant is being operated . furthermore , in the installation method for the filtration unit 1 of the above embodiment , and in the filtration apparatus of the above embodiment in which the filtration units 1 are installed using this installation method , the filtration units 1 are installed inside the water storage tank 11 on the opposite side from the side where the raw water w is introduced when seen from above or in a top view . accordingly , relatively clear raw water w from which the major portion of the mud , etc ., has been separated by precipitation following introduction into the water storage tank 11 can be introduced into the casings 2 of the filtration units 1 and filtered . accordingly , the load on the filtration modules 3 of the filtration units 1 is alleviated , and a high filtration efficiency is maintained over a long period of time . however , in the case where , for example , the mud discharge pit 15 is installed on the long - side end side of the rectangular settling tank main body 12 when seen from above or in a top view or the like , the underwater car 17 of the mud scraping means 16 can be made movable with the short - side direction of the above - described rectangular shape taken as the movement direction f . furthermore , in the above - described embodiment , the filtration units 1 are installed in a water storage tank 11 ( settling tank main body 12 and overflow trough tank 13 ) that has a rectangular shape in a top view . however , the filtration units 1 of the embodiment may also be applied to a water storage tank such as a cylindrical filtration tank that has a cylindrical shape in a top view as in , for example , the above - described japanese patent application laid - open ( kokai ) no . h8 - 57273 . here , in cases where the cylindrical water storage tank , etc ., is divided into a central cylindrical section and an outer circumferential doughnut - shaped section , and raw water w is introduced into the cylindrical section ( in the same manner as in the filtration apparatus described in patent reference 1 ), the filtration units 1 can be installed on the opposite side from the side where this raw water w is introduced , i . e ., in the doughnut - shaped section on the outer circumferential side of the water storage tank , etc . furthermore , the introduction ports 2 a of the above embodiment are tubular members that pass through the upper wall surfaces of the casings 2 , and the control means 2 b are opening - and - closing valves that are installed in the tubular members . however , for example , the introduction ports 2 a may also be formed as simple openings with a window frame configuration , etc ., that are formed in the upper wall surfaces of the casings 2 , and the control means 2 b may be formed as doors with a flat plate configuration , etc ., that open and close the openings . it is also possible that openings are not formed in the wall surfaces of the casings 2 , and the upper openings of the rectangular tubular casings 2 themselves instead open into the water surface h of the raw water w . in more concrete terms , it is also possible to design so that the casings 2 be immersed so as to be positioned slightly below the water surface of the raw water w , thus causing the upper openings to function as introduction ports 2 a , and so that rectangular tubular blocking walls , etc ., whose dimensions are slightly greater than the peripheral dimensions of the casings 2 are installed around the peripheries of the upper parts of the casings 2 so that the blocking walls can be moved upward and downward by driving means , with the introduction of raw water w from the upper openings being controlled by way of raising and lowering the blocking walls as control means 2 b . furthermore , for example , in cases where openings are formed in the wall surfaces of the casings 2 and taken as introduction ports 2 a as described above , and doors that can be opened and closed are installed in the introduction ports 2 a as control means 2 b , if the introduction ports 2 a are provided so as to be immersed and open into the raw water w at a point slightly below the water surface h of the raw water w in the water storage tank 11 , then the introduction ports 2 a open at a point lower than the water surface h , and the inflow of the raw water w in this water surface h together with floating matter such as dirt can be restricted even if partition plates 2 c are not installed on the outer circumferences of the upper parts of the casings 2 . accordingly , the external dimensions of the filtration units 1 are reduced by an amount that corresponds to the omission of the partition plates 2 c , and a larger number of filtration units 1 can be installed in the water storage tank , etc .