GB2185905A - Moving screens - Google Patents

Abstract

Sewage flowing along a channel is screened to remove large solids by an array of parallel ropes 2, which are moved continuously or intermittently by rollers 20 to convey the solids out of the channel. As shown the ropes are non-endless, and they may be incinerated together with the solids after use. Alternatively they may be treated with disinfectant and/or sheathed in plastics to facilitate subsequent disposal. In Fig. 5 the screen is an endless band of wire mesh, and the solids are burned off before the band returns to the sewage channel. In Figs. 3 to 7a a non-endless net screen is used to monitor solids content downstream of a conventional fixed screen. In Figs. 9, 10 a mesh screen band is wound progressively from a supply roll to a take-up roll. <IMAGE>

Description

SPECI FICATION Improvements in or relating to methods of and apparatus for removing materials from a flow of fluid BACKGROUND TO THE INVENTION This invention relates to methods of and apparatus for removing floating and suspended materials from a flowofliquid,and moreparticularlyforscreening sewage.

Thefirststage of sewage treatment is the removal of paper, rags, plastics film, rubber and like materials from the waste liquid. This is accomplished by a screen madefrom rigid fixed iron bars with lateral spaces between adjacent bars varying from 1.25 to 12.5cm. A raking or combing mechanism is employed to remove the material (the screenings) which attaches itself to the screen bars. This raking mechanism,when it engages at the lower point ofthe bars, can force through the screen part of the material which had originally become attached to the screen bars.

This reduces the efficiency of the screen. In addition to this, damage can occur if stones or like material is jammed acrossthe bars. The raking mechanism besides bringing outthe rags etc., also brings out large pieces of excrement. This makes the subsequent handling and disposal of screenings difficult and unhygienic.

SUMMARIES OF THE INVENTION Acording to one aspect ofthe invention, a method of removing materials from a flow of liquid comprises providing a screen of flexible material and, when the screen becomes at least partly blocked, replacing at least part of the screen.

The screen may comprise a pluralityofflexible, elongate elements, offilamentaryform.

Alternatively, the screen may comprise a net, mesh or similar structure, (hereinafter collectively "net"), formed by such elements.

As used herein, the collective term "net" also includes a sheet of hessian, sack orthe like.

The screen may be moved progressively and/or from time to time, so as to bring fresh lengths of the screen into an operative position.

Movement of the screen may be controlled by means responsiveto fluid pressure and/or differential fluid pressure between locations upstream and downstream of the screen, and/or by timing means.

The invention also comprises screenings collected by the novel method.

The invention further consists in a method of removing materials from a flow of liquid, comprising providing a screen of flexible material and, when the screen becomes at least partly blocked, replacing at least part of the screen, and subjecting at least the screenings on the replaced portion of the screen to oneofthefollowing steps: (a) incineration; (b) disinfection; (c} sheathing; (d) disinfection followed by sheathing.

According to another aspect of the invention, apparatusforremoving materialsfrom a flow of liquid comprises a screen of flexible construction, and means operableto move the screen upwards so asto bring a fresh length of said screen into an operative position.

The screen may comprise a plurality offlexible, elongate elements offilamentaryform.

Alternatively, the screen may comprise a netformed by such elements. Net support structure may be provided.

The screen may extend round a transverse guide located adjacent the invert of a sewage flow channel.

There may be guides spaced along the channel, the channel being formed so as to receive the guides.

Means may be provided for applying tension to the screen. The tension may be adjustable. The tensioning means may, for example, comprise a roller adapted to engage the screen or a clutch mechanism attached to a reel of unused screen or a weight attached to an unused partofthe screen.

Control meansforthe upward movement ofthe screen may comprise means responsive to fluid pressure and/or differential fluid pressure between locations upstream and downstream ofthe screen, and/ortiming means.

Means may be provided for raising the screeen, for example, to an inspection position, in response to tension changes in the screen and/orto excess sewageflowand/orto powerfailure.

The screen may be of endless form.

The invention also comprises apparatusfor removing materials from a flow of liquid comprising a screen of flexible construction, means operableto move the screen upwards so asto bring a fresh length of screen into an operative position, and means for subjecting the replaced portion of the screen to one of the following:- (a) incineration; (b) disinfection; (c) sheathing, or (d) disinfection followed by sheathing.

The invention also comprises any novel subject matter or combination including novel subject matter herein disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS Some embodiments ofthe invention with possible modifications will now be described byway of example only, with reference to the accompanying drawings, wherein:- Figure 1 is a side view of one apparatus for screening sewage, Figure 2 is a view in perspective, and to an enlarged scale, of pa rt of th e a ppa ratus, Figure 3 is a side view of part of another apparatus for screening sewage, Figure4 is a section on the line 4-4 of Figure 3, Figure 5 is a section on the line 5-5 of Figure 3, Figure 6 is a schematic plan view of the apparatus of Figures3to S, with partomitted, Figure 7 is a detail, to an enlarged scale, of part of Figure 3, Figure 7A is a section on the line 7A-7A of Figure 7, Figure 8 is a side view offurther apparatusfor screening sewage, Figure 9 is a side view of yet further apparatus for screening sewage, and Figure 10 isa section on the line 10-10 of Figure 9.

In the figures, like reference numerals refer to like components and features.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS With reference to Figures 1 and 2, apparatus 1 for removing material from a flow of liquid is used to screen sewage and comprises a screen 2 of flexible material and construction and means 3 operable to move the screen 2 upwards so as to bring a fresh length of screen into an operative position.

The "bars" ofthe screen 2 comprise flexible, elongate elements 10 offilamentaryform.

The elements 10 are not of articulated form, i.e. they are not of chain-like form.

The screen elements 10 are spaced laterally apart in substantially parallel array. The elements 10 extend from respective supply reels (not shown) over a transverse guide roller 11, rotatable about a fixed axis, round anothertransverse guide roller 12 rotatable on a fixed axis, and then round a further roller 13. Each flexible element is ofcircularcross-section, and the roller 13 is formed with a plurality of "V"-shaped peripheral grooves for receiving the elements 10, so asto assist in gripping them. The roller 13 is arranged to applytension to the elements 10, which pass beneath a transverse tensioning element of a rotatable roller 14.The roller 14 can be displaced up and down so asto adjust tension in the elements 10, which pass over another transverse guide roller 15 rotatable on a fixed axis. The elements 10 then extend downwardlyand-awayfrom the roller 15 at an angle to the horizontal, into a sewage flow channel 16, and around a fixed guide tube 17 adjacentthe bottom 1 8 of the channel 16.The elements 10then extend upwardly at a steeper angle, over a guide roller 19 rotatable on a fixed axis, and then between two driven pinching rollers 20, before passing generally horizontally to an incinerator 21.

The rollers 20, which are driven by electric motors5, form the means 3 operableto movethescreen 2 upwa rds. The moto rs 5 a re b ro ug ht i nto a nd out of operation by a controller 6, connected to the motors by electrical signal lines 7.

Sewageflowalongthesewagechannel 16 is shown by arrow A.

The tension roller 14 is arranged to apply substantially equal and constant tension to the screen 2 formed by the flexible elements 10. As best shown in Figure 2, thetube 17 is welded to a side edge of a rigid plate 22 which has slots 23 to receive the elements 10 with side clearance, and help in keeping them at a desired spacing. The ends of the plate 22 are fixed to side supports 24which are secured in turn to a base plate 25.

A device 26 (Figure 1) is provided which is respon siveto tension in the elements 10. The device 26 is operatively connected to a lifting device 27 (Figure 2) operatively connected in turn to the base plate 25. The arrangement is such that if an element 10 breaks, the lifting device 27 may be operated so as to raise the plate 25, tube 17 and associated parts out of the sewageflowchannel 16, in orderto enable repairs to be effected. When the lifting device 27 is operated, the drives for the elements 10 are de-energised.

The rollers 20 are driven by the electric motors 5 so that the flexible elements 10 forming the screen 2 are drawn from their supply reels. In operation, the elements 10 are maintained substantiallytaut.

In use, material such as paper, rags, plastics film, and rubber is screened out of sewage flowing along the channel 16, and builds up on the front or leading face ofthe upwardly-extending section 32 ofthe screen 2. Pressure-sensing devices 30,31 disposed respectively upstream and downstream ofthe section 32 of the screen 2 are responsive to liquid pressure in the sewage channel 16. The devices 30,31 are connected to the controller 6 by signal lines 8,9.When the channel pressure difference across the apparatus 1 sensed bythe devices 30,31 reaches a predetermined value, the controller8 causes the motors Sto rotate the rollers 20 whereby the elements 10 are drawn upwards, say by cm. Thus fresh lengths of the elements 10 are moved to replace the clogged screening section 32which is then raised from the channel 16. When the pressure differential acrossthe screen falls to another predetermined value, upward movement is arrested.

Instead of, or in addition to, use of the pressure sensing devices30,31 to movethescreen section 32 intermittently, timing means are provided to draw up a length of screen 2 material from time to time. The timing means comprise an electronic timer 37 connectedtothecontroller6 by electrical signal lines 38.

The timer 37 may be set so as to cause a section of the screen 2to be drawn up every4 hoursforexample, regardless ofthe pressure differential sensed by the devices 30,31.

Timing means may be provided wherebythescreen is moved progressively.

The controller 6 also incorporates an override switch whereby the rollers 20 may be brought into operation atanytime.

The tube 17 is positioned so asto give small clearance between the elements 10 and the floor 18 of the channel 16.

During the passage of sewage along sewers and channels,grit and such like material bounces and rolls along the invert of the sewers and channels. To preventthis material from building up and obstructing the front section 32 ofthe screen 2, a rubber deflector 33 is provided in orderthatsuch materials can be deflected overtheguide tube 17. The deflector 33, which is replaceable, is secu red to the channel floor 18 by a removable clamping strip 36.

In addition to use of the rollers 20 to withdraw elements 10 out of an operative position, the rollers 20 are employed to remove excess water therefrom. The subsequent processes can comprise one or more of the following steps: a) INCINERATION This is the best means of disposing ofthe collected screenings, particularly as it is possible that the screenings may be contaminated by an AIDS virus.

(AIDS stands for Acquired immunity Deficiency Syn drome).

The screen is preferably incinerable. From the pinching rollers 20 the withdrawn elements 10, together with rags etc. attached thereto, pass into an incinerator 21. The incinerator 21 can be fired by methane gas as is sometimes produced in sewage works. Alternatively, natural gas as supplied commer ciallycan beutilised as can fuel oil for example. The temperature within the incinerator 21 is controlled such that substantially total combustion is accom plished. Washing oftheflue gasses can take place with subsequent disposal ofthe ash. The whole process may be made fully automatic so as to require little maintenance.

b) PLASTICS SHEA THING From the squeegy rollers 20, the withdrawn elements 10 move forward to where one continuous sheet of plastics film is passed underneath the elements and another sheet of the same material is passed along the tops ofthe elements. A set of rollers is then used to bring the sheets together with the elements sandwiched between. The edges of the plastics sheeting are then welded together so as to form an impervious seal whereby an element-enclosing sheath of plastics film is formed. The sheath is rolled into a continuous coil, and is subsequently removed for disposal.

c) SIMPLE REELING At the point where the pinching rollers 20 are presently disposed, a simple reeling device can be installed.Thiswould reel upthewithdrawn elements 10 and collected rags etc. attached thereto, and store the whole until such time as disposal became necessary.

d) DISINFECTION As an alternative to incineration, the screenings may be washed, for example, by passage through a bath or a spray booth wherein contact with a strong liquid disinfectant is caused to take place, or wherein disinfectant in powder form is applied. The disinfected screenings maythen be sheathed as described in b) above.

THE SCREEN 2.

Thefilementary elements 10 forming the flexible screen 2 may be in the form of ropes. Forexample, in man-made or natural fibres, in sizes and formations as are appropriate for a particular installation. Metallic ropes orwire in single or multi-strands can be used, or ropes orwires in which other material has been introduced, e.g. barbed wire, in order to retain the screenings. Tapes of rectangular or square section can also be used, such tapes being manufactured from a wide variety of suitable materials.

In some instances the filamentary elements 10 can be endless with screenings being stripped from withdrawn sections thereof by a variety of means.

The numbersofflexible elements 10 used on each apparatus 1 will be dependent upon the use to which the apparatus is to be applied.

Advantages of the apparatus 1 include:~ Increased efficiency As the screen 2 becomes blocked, the spaces between adjacent elements 10 are reduced, thus diminishing the chance of materials escaping through the spaces.

There is no need for raking mechanism to be provided in order to force materials through the screen, as at present.

Efficiency of incineration will be increased if synthetic rope is used as element 10 material, due to the high calorific value of such rope.

Incineration will also be assisted if the element 10 material is oil-absorbent.

"Tipping" ofthescreenings is possible, as the clogged screen elements 10 can be sheathed before disposal.

As substantially all screenings will be attached to the elements 10 from the point offirst attachment to final disposal, cleaning costs will be reduced.

Due to the higher efficiency of the screen 2, compared with presently known screens, further processing of the sewage will be less restricted by blockage by "escaped" screenings.

If safe to do so, the withdrawn sections of elements 10 can be laid on the ground, with their collected screenings, and covered with soil. Grass seed may then be sown. The result can be used to counter soil erosion. In a modification ofthisarrangement,the elements 10 could be laid on concrete instead of the ground. When grass has formed, the whole is then stripped away from the concrete, to form turves.

Reduced operational costs on secondary treatment processes.

Rags etc. which tend to block holes in distributor arms, will be reduced in number.

Cleaning costs for removing rags etc. from downstream filter beds reduced.

Mechanical maintenance costs reduced, as all parts used in the apparatus 1 comprise simple, slow rotating, mechanisms.

Reduced capital costs.

As the apparatus 1 can be installed in existing channels without substantial conversion work.

No bypass screen is required.

Automatic controls can be used to lift clogged screen elements 10 out of the channel 16, thus allowing thefree flow of sewage.

The need for smell nuisance control can be reduced.

As the apparatus can be totally enclosed.

If incineration is used as a disposal method, the stale airfrom the incoming sewage can be burnt in the incinerator.

Improved versatility As the screen elements 10 can be of any suitable thickness and disposed at any required spacing.

As the screen 2 can be used at any stage of the sewage treatment process.

As the screen 2 can be installed upstream of any sewage pumping equipment.

Instead ofthe guide rollers extending across the screen 2, individual pulleys (or rollers) could be used to guide individual elements 10.

A screen may comprise a net formed by filamentary elements. Reinforcement strips may extend across the net. The aperture size ofthe net may range, for example, from 0.01 mmto 150.0 mm.

"NETLON" (Trade Mark) is a suitable net material.

A screen may be of endless form.

A plurality of screens may be employed, one disposed behind the other, in spaced relationship.

The pinching rollers 20 may be replaced by a pair of endless belts, one disposed above and one disposed belowthe section of the screen drawn up, so as to apply substantial pressure thereto.

Another embodiment of apparatus for screening sewage is shown in Figures 3 to 7A. The figures show a semi-portable screening apparatus 40 used, for exam pie, to monitorthe performance ofan existing screen.

The existing screen may, or may not, comprise a screen as used in the present invention.

The apparatus 40 removes from sewage passing along the channel 44, almost all material to be screenedwhich has passed through the existing screen. The amount of screenings removed by the test apparatus 40 is measured and compared with those removed by the existing screen to determine the efficiency ofthe existing screen. Fortest purposes, several test screens can be used, at different times, with differing sizes of net or mesh.

The apparatus of Figures 3 to 7A could alternatively replace the existing screen or be installed initially in a sewage treatment plant The flexible screen 40a ofthe apparatus 40 comprises a net which can for long term trials be stored on a reel (not shown). For shortterm trials the net is stored between a weight 41 and a first fixed transverse rigid guide bar 42. The guide bar 42 is curved upwardly so asto provide a degree of tension to the centre ofthe net. The net is attached to the weight41 by a clamp 43.

The net is led downstream of the screen face to a position close to the invertofthe sewage channel 44.

The direction ofthe netforming the screen 40a is changed from sloping into horizontal at45 (Figure 7J andthence in an upwardly inclined direction.This change of direction is accomplished by means oftwo transverse fixed right guide bars 46 and 47 which curve downwardly as they extend across the channel 44. The sewage flow indicated by arrow A passes through the effective screen face 48 with screenings becoming enmeshed in the net. When the net has become almosttotally clogged, then an upward movement of the screen face 48 takes place. At the same time a new length of net is drawn from the store into the screen face position. Movement of the net is accomplished by means of a pulley/hoist unit 49. The unit49 can be manually operated oritcan be motorised by a power source.Activation of the unit 49 can be achieved by signals generated by a fluidpressure responsive sensor 50 on one side ofthe screen 40a either alone or in conjunction with a fluid-pressure responsive sensor 51 on the other side ofthe screen.

It may be advantageous, particularly if the net forming the screen 40a is of lightweight construction, to provide net support structure on the downstream side of the net. Such support structure could, for example, comprise a plurality of upwardly-extending, laterally-spaced, bars of metal.

The netforming the screen 40a is attached to the unit49 by means of a clamp 52.

Atthe invert of the sewage channel 44 is a sealing arrangementto prevent screenings passing between the invert and the two fixed guide bars 46,47.

The seal arrangement is best shown in Figure 7, where a base plate 53 is fixed at its ends to two side plates 54. Aclamp plate 56 is used to assist in the attachment. The rubber seal 55 effectively seals offthe space between the net forming the screen 40a and the invert of the channel 44.

The side plates 54 are attached to the base plate 53, thetransverseguidebars46,47andto a screen supporttube 57. Provision is madeforan effective seal 58 between the vertical edge ofthe screen 40a and the side plates 54. The screen 40a is supported by framework pivotable about a substantially horizontal shaft 60 securelyfixed to the side plates 54 and rotatable on bearing assemblies 61.

An alternative form of net supportstructure on the downstream side of the net could comprise a plurality oftubes, each similarto the tube 57. The additional tubes would be disposed substantially horizontally, in substantially parallel army.

A hoist 62 is provided to lift the screen 40a and supporting framework out of the sewage channel 44.

The hoist 62 may be manually operated or motorised.

Signals to operate the hoist 62 automatically may be generated by a liquid level responsive sensor 63. The tubular scaffolding 59 is a form offramework used principally as a temporary supporting structure. A similarform of structure can be used to support items 42,49,62,63 togetherwith all other items that may be necessaryto enable the screening process to function.

In some instances it may be desirable to sheath the screen 40a, when enmeshed with screenings, with a suitable material such as polythene. This can be accomplished by means of a tubular section of polythene, the length of wh ich wi il begreaterthanthe screen length to be removed.Thetubularsheath can be housed on a suitable receptacle encircling the width ofthe screen and placed at a position above the top water level (TWL) in the channel 44. The screen 40a on being removed from the sewage flow will thus pass through the tubularsheath and housing, drawing the sheath from the housing as it does so.

After a period of ti me, flow or length of screen consumed, the screen enmeshed with screenings will be removed. Aftersuitable analysis comparisons can be made with similar removed screenings.

In Figure 8 a screen 65 consists of laterally spaced, elongated elements of endless construction with transverse elements, also of elongate form, attached and spaced at suitable intervals. Such an arrangement forms a basic net. Screenings attached to the net 65 are stripped off by burning in the incinerator 21 at temperatures up to 9500C. The material of the screen 65 is chosen to be non-incinerable and non-distortion al at this temperature. Atypical material would be one ofthe Nimonicalloys. Such a metallic screen could be of articulated, i.e. chain-like form.

A pair of squeeze/pull rollers 20 are used to draw out the front or leading screen section 32, and, at the same time, press out as much water therefrom as possible.

Afterdewatering, the withdrawn section of the screen 65 passes to and through the incinerator 21 wherein substantially all material festooned on the screen section 32 is burnt off. The cleaned section of the screen 65 then leaves the incinerator 21 by way of rollers 70.

Otherdewatering means may be substituted.

Screen mesh-size could, forexample, be 50 mm square for crude sewaget 25 mm square forcrude sewage downstream of existing screens or comminutors and 4 mm square downstream of primary settlement tanks.

The front or leading screen section 32 can be kept stationary until a selected or predetermined condition exists, and then the section may be moved out of the flow of sewage in the channel 44 so asto bring a fresh screen section to replace section 32. Alternatively, the section 32 may be slowly withdrawn so as to bring fresh screen material into use.

Screeningswhich passthrough the leading section 32 may be collected by screen material downstream thereof and subsequently moved around in place of the section 32.

As the screen 65 becomes blocked it progressively becomes of fine mesh.

Figures 9 and 10 show a possiblefinescreen construction. The figures illustrate apparatus 75 comprising a screen 76 formed by a length of net 80 reeled onto a wooden drum 81 serving as a screen supply store.

Awooden drum 82 is provided on which the net 80 and screenings attached thereto comprise disposable screen/screening store.

The drum 81 is mounted on two square half shafts carried by bearing units between which is mounted a clutch unit provided to apply tension to the operative screen face. The tension applied by the slutch is adjustable and kept at a minimum consistent with maintaining a desired value of screen face tension.

From the drum 82 the net passes over a downwardly curved, transverse guide bar 83 of circular cross section, which causes tension to be applied to the centre of the net.

From the bar 83, the net passes to the invert ofthe sewage channel 44 adjacent which invert a transverse guide bar84ofsimilarform to the bar83is located. At the invert of the channel 44 a seal arrangement 77 is provided which is similarto the seal shown in Figure 7.

An overflow weir 86 is provided. The leading face of the screen 76, when clogged, is reeled up on the disposable drum 82. The disposable store, of which the drum 82 forms part, is ofsimilarform and construction as the supply store, having square half shafts and bearings. In place of a clutch unit however, a suitable drum driving device is provided which would rotate the drum 82 as required. Activation ofthe driving device is by means of a simple float-operated switch control 85 mounted upstream ofthe screen 76.

Replacement of the screen material is easily carried out by removal of one of the bearings and half shaft supporting the drum 82. The drum 82 can then be removed and replaced by another drum carrying net material. Replacement of the bearing and half shaft follows.

Replacement of the drum 81 is carried out in a similar manner.

Screens used by the invention may be employed by pumping stations.

The most difficult pumping station to apply such a screen would be one with a deep wet well with a sewer invert at the bottom thereof. This type of pumping station will now be considered.

It would be mostdesirableto have all machinery and moving parts outside of the wet well with only non-rotating parts within the station itself. It will also be preferablythat all maintenance be carried out outside the wet well. The only time it would be necessaryto enterthewell would beto install some basic fixed equipment.

Such equipmentwouldconsistofavertical box section,thetop of which would be open, as would also be one vertical side. This open vertical side would take the screen itself. The box would be designed and fixed so that the incoming sewer discharged into it. The opentopofthetankwould be at such a level astoform an emergency overflow weir. This would be needed if, for example, the screen was totally blinded then the tank side would be considered solid and the sewage level would rise and dischargeoverthetop of the tank.

Such conditions could occur ifthere was, for example, a power or mechanical failure.

On either side ofthe open side ofthetankwould be a channel section. This channel section would extend vertically from the bottom ofthetankto a point outside the wet well. This channel would form a slot designed to receivesectionalisedframeworkwhich would carry the screen. The sectionalised framework would be in easy-to-handle sections. Each would be fitted with a mating fixing device. The design would be such asto allow each frame to slide down the tank guide channels with each section being attached as sections were being lowered. When completed, there would bea rigidframeworkwithinthetankchannel section and extending from the bottom ofthetankto a pointoutsidethewetwell.

The framework would be designed to carry the screen feed, screen face and screenings.

Atthebottom section oftheframeworkwould be the two curved tubes and sealing device. On neither side would be a fairly simple screen guide to act as a seal between the frame and screen net. Each section would be suitable designed to mate with the bottom section and subsequent sections.

Fitting ofthe net would be done atthesametimeas the sections were lowered.

Where possible, a feature of one embodiment disclosed herein may be added to, or substituted for, a feature of another embodiment.

Claims (34)

1. A method of removing material from a flow of liquid, comprising providing a screen offlexible material, and, when the screen becomes at least partly blocked, replacing at least part of the screen.

2. A method as claimed in Claim 1, wherein the screen comprises a plurality offlexible, elongate elements offilamentaryform.

3. A method as claimed in Claim 1 or 2, wherein the screen comprises a net.

4. A method as claimed in Claim 1,2 or 3, wherein the screen is moved progressively to bring fresh lengths of screen into operative position.

5. A method as claimed in any one of Claims 1 to 4, wherein the screen is moved progressively and/or from time to time to bring fresh lengths of the screen into operative position.

6. A method as claimed in any one of Claims 1 to 5, wherein upward movement of the screen is controlled by means responsive to fluid pressure and/or differential fluid pressure between locations upstream and downstream ofthe screen.

7. A method as claimed in any one of Claims 1 to 6, wherein upward movement ofthe screen is controlled by timing means.

8. Screenings collected by the method claimed in any one of Claims 1 to 7.

9. A method of removing materials from a flow of liquid, comprising providing a screen of flexible material, and,when the screen becomes at least partly blocked, replacing at least part of the screen and then subjecting at leastthe screenings on the replaced portion ofthe screen to incineration.

10. A method of removing materials from a flow of liquid, comprising providing a screen of flexible material, and, when the screen becomes at least partly blocked, replacing at least part ofthe screen and then subjecting at leastthe screenings on the replaced portion of the screen to disinfection.

11. A method of removing materials from a flow of liqu id, comprising providing a screen of flexible material, and, when the screen becomes at least partly blocked, replacing at least part of the screen and then subjecting at leastthe screenings on the replaced portion of the screen to sheathing.

12. A method of removing materials from a flow of liquid, comprising providing a screen offlexible material, and,when the screen becomes at least partly blocked, replacing at least part of the screen and then subjecting at leastthe screenings on the replaced portion of the screen to disinfection, followed by sheathing.

13. Apparatusforremoving materialsfromaflow of liquid, comprising a screen offlexible material and means operableto move the screen upwards, so as to bring a fresh length of said screen into an operative position.

14. Apparatus as claimed in Claim 13, wherein the screen comprises a plurality offlexible, elongate elements offilamentaryform.

15. Apparatus as claimed in Claim 13 or 14, wherein the screen comprises a net.

16. Apparatus as claimed in Claim 13,14Or 15, wherein the moving means is adapted to move the screen progressively.

17. Apparatus as claimed in Claim 13,14Or 15, wherein the moving means is adapted to move the screen progressively and/orfrom timetotime.

18. Apparatus asclaimed in Claim 13, or 15, whereinthescreen extends round atransverse guide located adjacent the invert of a sewage flow channel.

19. Apparatus as claimed in Claim 18, wherein the screen extends overfirstand second transverse guides spaced along the sewage flow channel so that the screen extends downwards to the first guide and then upwards from the second guide.

20. Apparatus as claimed in any one of Claims 15 to 19, provided with means for applying tension to the screen.

21. Apparatus as claimed in Claim 20, wherein the tension applied is adjustable.

22. Apparatus as claimed in any one of Claims 13 to 21, provided with control means for upward movement ofthe screen, said control means compris ing means responsive to fluid pressure and/or dif ferential fluid pressure between locations upstream and downstream ofthe screen.

23. Apparatus as claimed in anyone of Claims 13 to 22, provided with meansfor incinerating lengths of screen withdrawn from operative positions.

24. The combination of apparatus as claimed in any one of Claims 13to 23, and a sewage flow channel.

25. Apparatus as claimed in anyoneofClaims 13 to 23, wherein the screen is of endless form.

26. Apparatus for removing materials from a flow of liquid, comprising a screen of flexible construction, means operable to move the screen upwards so asto bring a fresh length of screen into an operative position, and means for subjecting the replaced portion ofthescreento incineration.

27. Apparatus for removing materials from a flow of liquid, comprising a screen of flexible construction, means operable to move the screen upwards so as to bring afresh length of screen into an operative position, and means for subjecting the replaced portion of the screen to disinfection.

28. Apparatus for removing materials from a flow of liquid, comprising a screen of flexible construction, means operable to move the screen upwards so as to bring a fresh length of screen into an operative position, and meansfor subjecting the replaced portion of the screen to sheathing.

29. Apparatus for removing materials from a flow of liquid, comprising a screen offlexible construction, means operable to move the screen upwards so as to bring a fresh length of screen into an operative position, and means for subjecting the replaced portion ofthe screen to disinfection, followed by sheathing.

30. Apparatus for screening sewage substantially as hereinbefore described with reference to Figures 1 and 2 ofthe accompanying drawings.

31. Apparatusforscreening sewage substantially as hereinbeforedescribed with reference to Figures 3 to 7 of the accompanying drawings.

32. Apparatus for screening sewage substantially as hereinbefore described with reference to Figure 8 of the accompanying drawings.

33. Apparatus for screening sewage substantially as hereinbefore described with refrenceto Figures 9 and 10 of the accompanying drawings.

34. A method of screening sewage substantially as hereinbeforedescribed.