GB2300130A - Screening apparatus - Google Patents

Abstract

Apparatus for removing coarser, in particular fibrous, solids from a solids-liquid mixture comprises a perforated screen plate 15, and parallel screen bars 17 on the upstream surface of the plate to catch large solids before these reach the perforated screen plate. Solids are removed by an upwardly sweeping scraper 35 which lifts deposited solids to a cleaning device 53 by which they are transferred to a screenings trough 55. The apparatus is particularly suitable for preliminary screening of material at the entry of sewage treatment plant.

Description

SCREENING APPARATUS This invention relates to apparatus for the separation of liquids and solids, with particular reference to the preliminary separation of liquids and solids at the point of entry of sewage into a sewage treatment works. The invention is however not restricted to this application.

Standards and regulations for the protection of the environment are becoming progressively stricter. The treatment of sewage is a field of particular importance. In this field, as in others, it is necessary to separate liquids and solids before or at an intermediate stage of treatment, in order to remove solids above a specified size so that these will not impair subsequent treatment.

By way of example, recent European Union legislation requires that all screenings that are above 6mm in size should be removed from sewage to be treated prior to its discharge from the treatment plant.

Most screens that are supplied to comply with the E.U. requirements are bar screens which have 6mm wide slots. The slots allow any material which has one minimum dimension which is 6mm or less to pass through. This allows fibrous materials to pass through which are greater than 6mm in size in other dimensions. In recent years this problem has been exacerbated by the products of modem hygiene technology, which has developed a large variety of disposable paper and cotton items which are resistant to breakdown in water.

Thus, although bar screens have the advantages of simplicity, robustness and reliability, they do not adequately screen out long thin solids.

A perforated flat screen can screen such solids more effectively, but is more susceptible to blockage. Thus, flat screens with 6mm perforations will comply with the E.U.

requirements, but thin paper items may blind the screen temporarily and reduce the flow velocity of the incoming sewage, and can be difficult to remove. This may result in settling of heavier materials in the incoming launder prior to it reaching the screen.

According to the present invention, solids-liquids separation apparatus comprises a perforated surface-type screen member, a plurality of spaced-apart screen bars adjacent the upstream surface of the screen member, and scraping means arranged to scrape solids from the said upstream surface. Usually, the screen member and bars are essentially stationary, and the scraping means is arranged to sweep over the screen surface.

In a particularly convenient arrangement the screen surface and bars are arcuate, in cross section, concave towards the upstream side, and the scraping means moves pivotally along a circular or otherwise arcuate path in contact with the screen surface.

It is particularly advantageous if the scraping means traverses the screen surface from a lower towards an upper boundary of the latter, so that scraped-off solids are conveyed upwards, and can then be removed from the scraper above the normal liquid level in the apparatus.

Apparatus in accordance with the invention can combine the advantages of a bar-type screen and a flat perforated surface-type screen, while eliminating the disadvantages of both types of screen.

In the apparatus of the invention, the perforated screen prevents the passage of solids above a size corresponding to the size of the screen perforations, while the adjacent bars hold back larger solids, for example pieces of paper or rag, and prevent these from resting against the screen surface. Consequently these larger solids are prevented from blinding the perforated screen because the bars hold them substantially or entirely spaced upstream of the screen surface. Furthermore, because such solids are prevented by the bars from lying flat against the screen surface, they are easily removed by the scraping means, whereas sheet-like solids which lie flat against a conventional flat perforated screen can significantly resist removal by scraper mechanisms.

The apparatus of the invention can therefore provide a screening performance significantly greater than that of either a bar-type screen or a perforated screen, and is much easier to clean than a perforated screen so that its high performance is maintained reliably in use.

The invention will be further described, by way of example only, with reference to the accompanying drawings. In the drawings: Fig. 1 is a side view of apparatus embodying the invention, installed in a launder shown in longitudinal section; Fig. 2 is a perspective view of the screen and scraper components of the apparatus of figure 1; Fig. 3 is a partial plan view in section in the line III-III of figure 2; Fig. 4 is a cross section in a vertical plane through a preferred form of scraper, on section line B-B shown in figure 3; Figs. 5 and 6 show details of means for removing solids from the scraper; and Fig. 7 is a view similar to Fig. 3, illustrating a modified construction.

Figure 1 shows an incoming launder 1 of a sewage treatment or other waste treatment plant, through which a solidsfliquids fluid mixture flows in the direction of the arrow 3.

The launder is occluded by a screen assembly 5, extending from a lower edge region 7 anchored and sealed in the bottom wall 9 of the launder, obliquely in an upwards and downstream direction to an upper edge region 11 mounted to an upper wall or cover 13 of the launder.

The screen assembly is composed of an arcuate plate 15, and arcuate screen bars 17 integrated with or in contact with the plate 15 on the upstream surface of the latter.

Structural support is provided by side bars 19 secured to opposite arcuate edges of the plate 15. The bars 17, 19 are parallel to one another, and the bars define intervening regions or channels of equal width.

The bars 17 are to collect the larger rags, pieces of paper, etc. The spacing in practice will be closer than shown on the diagrams which were shown spaced for clarity. The bars hold rags, paper and the like away from the perforated screen so that the latter does not become blocked by such materials. Even if such materials accumulate on the bars, there will normally be significant space between them and the screen to permit adequate flow through the screen. Furthermore the presence of such a spacing prevents such solids from becoming stuck on the screen, and they can therefore easily be removed by the scraping means (to be described below).

The screen plate 15 has a perforated lower plate region 21 and a plain or unperforated upper region 23.

The maximum level on the upstream side is calculated according to the design flow, and a further 100mum (approx) is added.

The perforations in the plate region 21 are of such a size as to screen solids above a predetermined lower size limit. Specifically, to comply with the above-mentioned E.U.

requirements, perforations of such a size that solids of 6mm and over will not pass through, are provided. As a result, when a liquid-solids mixture flows along the launder, solids above the specified size limit (6mm) are retained and accumulate on the upstream concave surface of the screen plate, while the liquid, and suspended smaller particles, cascades through the perforated screen plate as shown at 25 in figure 1, and thence to a subsequent treatment means. Other size perforations can be used for other applications or requirements.

It will be understood that, although the described screening apparatus is intended primarily for the initial screening of coarse solids, preparatory to subsequent treatment of waste liquid containing finer solids, the screened mixture of liquid and fine solids may be discharged without further treatment, depending on the nature of the liquid and solids and the required application of the screening apparatus.

A typical arrangement comprises scraping means 31 mounted upstream of the screen assembly 5, for scraping the upstream surface of the screen plate on which solids accumulate. This comprises an oscillating pivoted rake arm 33 carrying at its free end a scraper head 35 comprising a spring-loaded scraper 37. In its upper end region, the rake arm is pivotally mounted on a rocker arm 41 pivoted on a stationary support 43, and is pivotally connected to a counterweighted crank 45 which is driven by a motor through a reduction gear unit 47, so as to rotate. The rocker arm or link, the support 43, the crank 45 and the rake arm form a four-bar linkage, such that when the crank rotates, the scraper head follows a path in the form of a loop, with an arcuate ascending limb adjacent the plate 15, and a descending limb spaced from the plate.

The ascending limb of the scraper head path is not strictly a circular arc. To ensure that the scraper is in contact with the plate surface throughout the ascending movement of the scraper head, the scraper may be spring-loaded into contact with the screen plate, and/or the screen plate and bars may have a non-circular profile conforming to the path of the scraper head.

At the bottom of the scraping stroke the scraper moves in towards the screen surface so that the fine screen scraper 37 engages on the face of the screen on each side of the screen bars, which are equi-spaced across the upstream face of the screen surface.

As the scraper continues its travel up the face of the screen it lifts the screenings from the bars and the fine screen surface and carries them to the top of its stroke.

At the top of its stroke, the spring loaded scraper rises above the top of a rocking apron 51 and picks up a pivotally suspended scraper head cleaning device 53. At this point in its travel the scraper head is moving away from the screen surface. As it does so the screenings are wiped off the head by the cleaner 53 and discharged into a screenings trough 55 on the downstream side of the rocking apron.

The screen scraper is now disengaged from the screen bars. It continues the remainder of its path to start its next stroke at the bottom of the screen.

Fig. Sa shows the rake head cleaner 53 and the rocking apron 51 in their rest position.

An apron return weight 81 is set so that it is free of the ground. This will ensure that the rocking apron is held against a rubber buffer 83. The rake head cleaner is set to pick up on the scraper head just clear of the collected solids.

At the top of its travel up the screen surface, the spring loaded scraper head 35 engages with the front face 51a ofthe rocking apron. The rocking apron rotates on its pivot 85 but is held against the tip of the spring loaded scraper by the apron return weights.

The head 35 continues to rise and as it does so the rake head cleaner 53 contacts the fixed shield plate on the spring loaded scraper head. The rake head cleaner will pivot at its support 87 and will slide towards the tip of the scraper head (fig. 5b).

Eventually, in its upward travel the spring loaded scraper head will clear the top of the rocking apron, which will be impelled by the apron return weights to return to its rest position against the rubber buffers. The rake head cleaner will continue to move collected solids off the end of the spring loaded scraper head. These will fall by gravity onto the back face 5 lib of the rocking apron and hence into the screenings trough.

The rake arm will continue its motion with the rake head 35 engaging a guide bar 89 on the rake head cleaner. Being at the top of its stroke, the rake head will move away from the rake head cleaner which will stop at its parked position waiting for the next scraping cycle as shown in Fig. 5a.

The buffers 83 are mounted on horizontally adjustable arms 91 secured on the same support posts 93 as the pivot 87 of the scraper cleaner 53. The latter is associated with an adjusting screw 95 for setting the rest or parked position of the pivoted arms 97 carrying the cleaner 53.

The rocking apron is of inverted V-section and has side plates 99 between which the rake head 35 passes.

The rake head cleaner 53 can be adjusted on the arms 97 by adjusting bolts 98.

The apron return weight 81 is suspended from the post 93 by a chain 82. Any other convenient means for returning the apron to its parked position, for example springs, can be used instead of weights.

The scraper cleaner 53 may engage the scraper under its own weight, or under the influence of additional weights and/or springs.

The scraper head is shown in detail in Figure 4. It comprises a rigid steel scraper head member 35 fitting over and secured to the end ofthe rake arm 31, carrying a steel support plate 61 on which the scraper 37 is slidably secured by a bolt or stud 63 in a slot 65, with laterally spaced parallel compression springs 67 acting between the scraper head 35 and the scraper 37 so as to urge the scraper against the screen surface.

It will be understood that the head and the scraper extend continuously across the width of the screen, as can be seen in Figure 2. A protective flexible plastics cover strip 69 secured to the scraper head 35 rests against the upper surface of the scraper.

The scraper 37 comprises an assembly of a polycarbonate scraping member 64 and a nylon brush 60.

The scraper assembly comprises an upper steel plate 62 parallel to and spaced above the plate 61 and welded to a guide tube 66 accommodating the bolt 63. A top hat spacer 68 is placed between the bolt head and the tube 66, with a spring washer 70 between the flange of the top hat spacer 68 and a rigid washer or perforated metal strip 72 which rests against the underside of the plate 61. This construction ensures that the scraper assembly 37 is held together while the spring washer 70 allows it enough freedom of movement for the springs 67 to hold it firmly against the perforated screen 21. It will be understood that the scraper head comprises a plurality of slots 65 and bolts 63 spaced along the length of the scraper head parallel to the screen surface.

Each compression spring 67 is located by means of a steel push rod 73 located slidably in an aperture in the head member 35. The forward end of each push rod is provided with a head 74 which locates the forward end of the spring and transmits the spring pressure to the rear end of the scraper 37.

The brush 60 consists of an array of nylon bristles, held together by a back member 75 for example an aluminium extrusion. The later is mounted in a further holder 76 of rectangular external profile, which may be an aluminium extrusion.

Polycarbonate filler strips 77, 78 are placed above and below the brush to form an assembly of the same thickness as the holder 76. Behind the brush holder 76, front and rear polycarbonate spacerstrips 79, 80 fill the space between the brush holder and the spring head 74 thereby transmitting the force of the compression springs 67 to the brush.

Below the described brush assembly is a polycarbonate strip 82, placed between the brush assembly and the steel plate 61. The forward edge ofthis strip is close to but spaced from the screen surface, the rear edge abuts the compression spring head 74.

Above the brush assembly is a polycarbonate strip forming the primary scraping member 64. Its forward edge is in contact with the screen surface, its rear edge abuts the compression spring head 74 so that its forward edge is urged into contact with the screen surface.

In the new condition of the scraper head, only the polycarbonate scraper strip 64 contacts the screen surface. Its forward edge, and those of the other strips 77, 78, 82, are chamfered to acute angles at the upper side as shown in figure 4. As the apparatus is used, the polycarbonate wears so that the scraping member 64 beds in and conforms very closely to the screen surface.

The construction of the scraper head is of course subject to modification and/or to the use of different materials, adapted to particular applications. The metal parts may be made of mild steel or stainless steel as conditions require. The scraping member or members can be of other materials than polycarbonate, preferably plastics, although polycarbonate has been found to be particularly suitable because it combines toughness, resistance to most materials found in effluents, the ability to wear into conformity with the screen surface, and quietness in operation.

As can be seen in Figure 3 the cover strip 69 terminates short of the screen bars 17, whereas the scraper 37 projects between these bars and is provided with laterally spaced slots 71 in its components in order to accommodate the screen bars. The support plate 61 is provided with similar slots (not shown).

In the embodiment illustrated by figure 3, the screen plate is rectilinear in cross-section.

Alternatively, to increase the effective surface area of the screen, its profile in a horizontal cross-section may be other than rectilinear, for example comprising an arc, a V-section, or a plurality of arcuate regions or corrugations, or a saw tooth profile.

Such profiles can be used to provide an adequate screen surface within a limited width.

If such profiles are used, the profile of the scraper head and in particular of the scraping member and brush must be correspondingly adapted. For example, if the screen has a corrugated or saw tooth profile, it may be convenient to provide an individual scraper assembly of scraping member, brush and compression spring, for each corrugation or other element of the screen profile, or for a group of profile elements.

Figure 7 shows such a modification, in a view corresponding to figure 3. This differs from the previously described embodiment only in that the horizontal profile of the perforated screen 22 is corrugated or scalloped in such a way that the part of the screen between each two adjacent bars 17 is arcuate, concave on the upstream side, instead of rectilinear. Correspondingly, all the components of the scraper head assembly 37 have a forward edge profile which is series of respective a convex arcs, between each pair of bars 17. In particular these comprise the components 60, 62, 64, 77, 78 and 82 of figure 4, or the equivalent of these. In the case illustrated in figure 7 these components are continuous across the width of the screen; alternatively a separate set of such components can be provided for each inter-bar space.The plate 61 may also have a forward edge profile conforming to the screen but this is not essential. The cover 69 terminates short of the screen bars 17 and has a straight edge.

However such a construction does not lend itself readily to removal of the screen scrapings by the mechanisms described. Therefore, in a further development, the upper region of the screen surface may be so shaped and placed, relative to the path of the scraper head, that those parts of the scraping member and brush which conform to the profile of the perforated screen are made to retract relative to a scraper head component presenting a rectilinear forward edge which then conveys the scraped material upwards over the unperforated upper part of the screen to the cleaner 53.

Other possible uses for the equipment include:1) Tertiary treatment separation of solids from liquids.

These screens may be situated near the discharge end of sewage treatment works to ensure that no + 6mm material is discharged.

2) Removal of river water-borne debris from water intakes to industrial plants.

3) Final polishing of discharge water to sea and river outfalls.

4) Recovery of fruit and vegetable waste from food processing industry.

Claims (11)

1. Solids-liquids separation apparatus comprising a perforated surface-type screen member, a plurality of spaced-apart screen bars adjacent the upstream surface of the screen member, and scraping means arranged to scrape solids from the said upstream surface.

2. Apparatus according to claim 1, in which the screen member and bars are essentially stationary, and the scraping means is arranged to sweep over the screen surface.

3. Apparatus according to claim 2, in which the screen surface and bars are arcuate, in cross section, concave towards the upstream side, and the scraping means moves pivotally along a circular or otherwise arcuate path in contact with the screen surface.

4. Apparatus according to claim 2 or 3, in which the scraping means traverses the screen surface from a lower towards an upper boundary of the latter, so that scrapedoff solids are conveyed upwards.

5. Apparatus according to claim 4, further comprising means for removing the scraped-off solids from the scraper above the normal liquid level in the apparatus.

6. Apparatus according to claim 4 or 5, further comprising driving means adapted to move the scraping means in a closed loop comprising an ascending limb in contact with the screen surface and a descending limb spaced from the screen surface.

7. Apparatus according to claim 5, further comprising a receptacle arranged to receive the solids removed from the scraper.

8. Apparatus according to any of claims 1 to 7, in which the screen has perforations of a size adapted to permit passage only of solids not exceeding 6mm.

9. Apparatus according to any of claims 1 to 8, in which the scraping means comprise at least one scraper member and means resiliently urging the said at least one scraper member into contact with the said surface.

10. Apparatus according to claim 9, in which the scraper member comprises at least one scraping member and at least one brush member.

11. Solids-liquids separation apparatus substantially as herein described with reference to the accompanying drawings.