WO1996027422A1

WO1996027422A1 - Rotating screen and drive unit

Info

Publication number: WO1996027422A1
Application number: PCT/GB1996/000473
Authority: WO
Inventors: Anthony Paul Buckley; Nicholas Jason John Preston
Original assignee: United Utilities Plc
Priority date: 1995-03-04
Filing date: 1996-03-04
Publication date: 1996-09-12
Also published as: GB9504364D0; GB2314033B; GB9718595D0; AU4839996A; GB2314033A

Abstract

A liquid filtering apparatus for use in, for example, storm water overflows of sewage systems. A cylindrical liquid permeable screen is supported in a housing so as to obstruct the passage of solids of greater than a predetermined size from an inlet of the housing to an outlet of the housing. The screen is rotable about a horizontal axis and its interior is drained by the outlet from the housing. The exterior of the body is at least partially submerged by the liquid to be filtered, and the body supports internal vanes arranged such that it is driven in rotation by the filtered liquid. The interior of the body is partitioned by the vanes into a series of sections arranged relative to the outlet such that for any given rotational position of the body liquid is retained in at least one of the submerged sections as a result of one or more of the vanes preventing liquid flowing from that section to the outlet. The weight of liquid retained by the vanes applies a torque to the body to cause it to rotate.

Description

Rotating screen and drive unit

The present invention relates to a liquid filter apparatus, and in particular

to such an apparatus which is capable of filtering a liquid such as water to remove

solids.

Rotating drum screens are known for removing solid waste material from

flows of, for example, water. Such screens are generally used for removing floating

debris such as leaves or small pieces of wood from river water at the intake to

industrial plant, although some rotating screens have been installed in fish farms

to prevent small fish being lost at the overflow of ponds. In a rotating drum

screen, a perforated cylindrical drum is supported in a channel so as to rotate

about a horizontal axis, the drum being partially submerged in water flowing

through the channel. Water enters the interior of the perforated drum and exits

from one end of the drum through a simple circular opening in a side wall of the

channel in which the drum is supported.

The drum rotates and as a result any solid matter in the main water course

which becomes temporarily attached to the outside surface of the drum is washed

off by the flow of water as the drum rotates. The flow of water beneath the drum

assists in causing its rotation in a manner equivalent to a conventional undershot

water wheel but further torque is applied to the drum as a result of the positioning

of an impeller in the end of the drum from which water issues and the provision of

helical vanes on the interior of the drum. Water entering the drum flows axialiy towards one end of the drum and in so doing interacts with the vanes and impeller

to cause the drum to rotate.

The known rotating drum screens are particularly advantageous in that

they do not require an external power source. The screen does not blind, and there

is no need for regular maintenance. As a result the known rotating drum screens

have proved successful in circumstances where there is a continuous relatively high

volume of relatively clean water.

One of the problems confronted by the water industry is that of handling

storm water overflows from sewerage systems. In some circumstances, in the event

of very heavy rain surface water enters sewerage systems and mixes with the waste

water being transported to a treatment plant. The combined flow of waste water

and rain water can be greater than the maximum flow which can be carried by the

sewerage system. To accommodate such temporary conditions, it is conventional

practice to provide overflow chambers in sewerage systems to temporarily store

excess flows. Unfortunately, unless such chambers are very large indeed, in

exceptional weather conditions raw sewage can overflow from such chambers into,

for example, surface waterways. In circumstances where overflows are likely to

occur, for example in periods of exceptionally heavy rain, it is usually arranged

that any waste water which does escape from a sewerage system will be very

rapidly diluted and therefore not lead to major problems. It is not acceptable,

however, if any overflow from a sewerage system results in solid materials

reaching surface water courses. In addressing the above problem, and also the problem of finding an

efficient mechanism for dealing with primary separation of liquid from solids in

sewage treatment works, the possibility of using a conventional rotating drum

screen was considered. It was found, however, that the conventional rotating

drum screens were not adequate because when the screens are only rotated on

fairly rare occasions, for example in the event of flooding, the relatively low torque

generated by axial flow along the vanes within the drum was insufficient to reliably

start the rotation of the drum. Furthermore, given the nature of the solid material

included in flows of sewage it was difficult to avoid the screens becoming clogged

with solid material without providing some additional cleaning of the screen

surface and this required more power than that which was available from the

conventional rotating drum screen assembly.

It is an object of the present invention to obviate or mitigate the problems

outlined above.

According to the present invention, there is provided a liquid filtering

apparatus comprising a housing defining an inlet and an outlet, and a liquid

permeable screen supported in the housing so as to obstruct the passage of solids of

greater than a predetermined size from the inlet to the outlet, the screen being in

the form of a rotatable body, the interior of which is drained by the outlet and the

exterior of which is in use at least partially submerged by the liquid to be filtered,

and the body supporting internal vanes arranged such that the body is driven in

rotation by the filtered liquid, wherein the interior of the body is partitioned by the vanes into a series of sections arranged relative to the outlet such that for any given

rotational position of the body liquid is retained in at least one of the submerged

sections as a result of one or more of the vanes preventing liquid flowing from that

section to the outlet, whereby the weight of liquid retained by the vanes applies a

torque to the body to cause it to rotate.

Thus, in embodiments of the present invention, the rotatable body is

rotated by a force generated by the weight of water in a section of the body rather

than by the flow of water as in the case of conventional rotating drum screens.

Preferably the body is cylindrical and is rotatable about a horizontal axis

coincident with the cylinder axis. The outlet may be defined by an opening in an

end wall of the housing, the opening being located beneath and to one side of the

axis of rotation of the body. The vanes and outlet may be arranged such that

liquid is released from a section of the body progressively as the body rotates, for

example by arranging that the outlet is defined by a vertical edge adjacent the axis

of rotation of the body and an arcuate edge adjacent the periphery of the body,

and that the vanes are swept back relative to the direction of rotation of the body.

The vanes may extend parallel to the axis of rotation of the body such that the

body and vanes has a uniform cross section along its length.

The housing may define a chamber into which sewage flows through the

inlet and from which filtered water flows through the outlet to an overflow, solids

in the sewage being retained in the chamber and flushed from the chamber

through an outlet. A brush may be provided driven by rotation of the body to sweep the external surface of the body. A device may be arranged to spray

washing water onto the internal or external surface of the body, that device also

being powered by rotation of the body, or alternatively being powered by a motor

driven pump.

Embodiments of the present invention will now be described, by way of

example, with reference to the accompanying drawings, in which:-

Fig. 1 is a schematic perspective view of an embodiment of the present

invention;

Figs. 2 and 3 are respectively illustrations of components of the

arrangement shown in Fig. 1;

Fig. 4 is a part sectional side view of the arrangement of Fig. 1;

Figs. 5 and 6 illustrate the operation of the arrangement of Fig. 1;

Fig. 7 illustrates an alternative embodiment to that of Figs. 1 to 6 intended

to smooth out the torque applied to the rotating component; and

Fig. 8 shows an alternative arrangement to that of Fig. 7 which also

smooths out the torque applied to the rotating component.

Referring to Figs. 1 to 4, the illustrated filter apparatus comprises a

rotatable drum 1 having a perforated cylindrical outer surface supported on

axiaily extending planar vanes 2 extending radially outwards from a hub 3. The

vanes 2 are not perforated and thus divide the interior of the drum into eight

mutually isolated sections. The hub 3 is supported on a shaft (not shown) hich is

received in bearings 4 in a housing defined by reinforced concrete walls 5, 6, 7 and 8. As shown in Fig. 1, a sewage inlet 9 carries a flow of sewage which in normal

circumstances leaves the housing immediately via sewage outlet 10. In the event of

the outlet 10 being incapable of receiving the flow entering through inlet 9 the

chamber defined between walls 5, 6, 7 and 8 fills up until the lower half of the

drum 1 is submerged.

As shown in Fig. 4, the end of the drum 1 adjacent the wall 5 is closed and the end

of the drum 1 adjacent the wall 8 supports a brush 9 which sweeps across the face

of the wall 8 so as to prevent any direct leakage of sewage between the end of the

drum 1 and the wall 8. The brush 9 could be replaced by an alternative seal, for

example an annular seal of L-shaped cross-section with one limb of the cross-

section parallel to the drum axis and the other perpendicular thereto. Such a seal

could have grooves being arranged such that water which does leak is directed so

as to assist the desired rotation of the drum. A portion of the wall 8 is cut out so as

to define an arcuate edge 11 and a vertical edge 12. Any liquid inside the drum

flows out over the lower part of the edge 11 and is discharged as appropriate, for

example to a surface water drain or water course.

The shaft on which the drum 1 is rotatable extends through the end wall 5

and supports a gear 13. That gear meshes with a gear 14 which in turn meshes

with a gear 15 mounted on a shaft 16 carrying a brush 17. Of course, any

convenient number of gears could be provided to transmit torque from the drum

shaft to the brush, for example two gears rather than 3 as in the described

embodiment. The brush supports bristles which bear against the external surface of the drum and accordingly rotation of the drum causes rotation of the brush

which sweeps off any material adhered to the external surface of the drum 1.

Figs. 5 and 6 illustrate how the flow of water through the perforated

cylindrical wall of the drum 1 causes the drum to rotate. The water level within

the chamber defined by the housing walls is indicated by the line 18. Water flows

through the perforations in the drum and assumes a level substantially the same as

that outside the drum in sections 19 and 20 defined between adjacent pairs of

vanes. Those vanes are so located relative to the cut out portion of the end wall 8

that the liquid within them is trapped and cannot flow out through the cut out

portion of the end wall 8. In contrast, the water level in the adjacent section 21 of

the drum assumes a level corresponding to the level of the water pouring over the

edge 11 defined by the end wall 8. Thus the vane defined between the sections 20

and 21 of the drum has a water level on one side much higher than the water level

on the other and therefore a torque in the direction of arrow 22 is applied to the

drum. The drum therefore starts to rotate in the direction of arrow 22. As soon as

the vane between sections 20 and 21 has past the edge 12 defined by the cut out

portion of the end wall 8 the water within section 20 can flow out through the cut

out in wall 8 and thus, after the drum has rotated through approximately 30°, the

water levels are as shown in Fig. 6, that is there is almost no water in section 21, the

water level in section 20 has fallen to the level of the water flowing over the edge 11,

the water level in section 19 has not changed although that section has no taken

in an increased volume of water through the wall of the drum 1, and water has just started to enter the section 23. Thus torque is still applied to the drum 1 as a result

of the difference in water levels on the opposite sides of the vane separating

sections 19 and 20.

It will be appreciated from Figs. 5 and 6 that regardless of the orientation of

the drum relative to the end wall 8 as soon as the water level in the housing has

increased sufficiently to submerge a lower portion of the drum hydrostatic forces

will be applied to the drum so as to cause it to rotate. The greater the depth to

which the drum is submerged the greater will be the torque applied to it. The

torque will not increase with depth in a linear manner however as with increased

depth there will be an increase in the flow of water into the side of the drum that is

drained through the outlet. Thus, sufficient torque can be generated to start drum

rotation even when that drum has been stationary for a long period of time.

Rotation of the drum causes the rotation of the brush 17. The perforated drum

surface is also backwashed as a result of its motion relative to the water in which it

is partially immersed.

With the arrangement illustrated in Figs. 1 to 6, as a vane sweeps past the

edge 12 defined by the cut-out in the end wall 8 there will be a sudden rush of

water out of the drum. The torque applied to the drum will therefore vary

cyclically as the drum rotates and it would be desirable to have an arrangement in

which the torque is less variable. This can be achieved by appropriate design of

the vanes and the edges of the cut out in the end wall of the housing. For example,

in Fig. 7 the cut out in the end wall is the same as in Figs. 1 to 6 but the vanes are swept back relative to the direction of rotation of the drum. Thus, assuming that

the line 24 shows the water level outside the drum, the line 25 will represent the

water level in section 26 of the drum and the line 27 will represent the water level

in section 28 of the drum. The level of the water in section 27 will fall progressively

as the drum rotates. One disadvantage with such an arrangement would be that if

the water level was very close to the top of the end wall 8 vanes rotating

downwards into the water surface would not immediately allow water to penetrate

behind them and this would act against the forces causing rotation of the device.

This disadvantage could, however, be avoided, for example by retaining radial

vanes as shown in Fig. 8 but adjusting the shape of the edge 12 defined in the end

wall 8. In Fig. 8 the water level within the housing is represented by line 29, the

water level in section 30 is represented by line 31, and the water level in section 32

is represented by line 33.

The embodiment of Fig. 8 also incorporates a shield 34 arranged outside the

drum in the area joining the portion of the drum which is drained through the

opening formed in the end wall 8. Such a shield reduces the amount of water

flowing into that section of the drum through the drum wall and thereby increases

the differential head that causes the drum to rotate.

It will of course be appreciated that the three alternative arrangements

illustrated in Figs. 1 to 8 are no more than examples of many configurations which

would be possible in order to select an appropriate control of the torque as the

drum rotates. Although in the illustrated arrangements the rotatable drum cannot be

much more than half submerged before water flows over the cut out in the end

wall 8, it will be appreciated that the end wall could extend to above the height of

the uppermost part of the drum. Providing the opening in the end wall 8 is also

increased in height, for example by extending the edge 12 vertically upwards and

continuing the circular arc 11 to meet the edge 12, extra water will be retained in

one half only of the drum, thereby increasing the torque applied to the drum.

Although it is not shown in the drawings, just as the brush 17 in Fig. 4 can

be driven by the body 1 via an appropriate set of gears, a pump could also be

powered in a like manner so as to spray water onto the inner or outer surface of

the drum 1, thereby improving the ability of the device to avoid blinding of the

perforated surface of the drum. For example, water could be pumped through

pipes mounted on the vanes inside the drum from pressure sets mounted inside the

drum.

The described embodiment of the invention comprises a drum closed at one

end and drained from the other end. It will be appreciated that the drum could be

drained at both ends by arranging for each end of the drum to be open and

disposed joining an end wall defining an outlet such as the wall 8 of the attached

drawings. This would allow the use of a longer drum given an increased rate of

discharge from the drum and therefore a greater differential head to cause the

drum to rotate. The greater differential head arises because there is less backing

up of water inside the drum. Of course, where very large volumes of water must be handled, two or more drums could be supported in a common chamber. A

series of such drums could be linked mechanicallv.

Claims

1. A liquid filtering apparatus comprising a housing defining an inlet and

an outlet, and a liquid permeable screen supported in the housing so as to obstruct

the passage of solids of greater than a predetermined size from the inlet to the

outlet, the screen being in the form of a rotatable body, the interior of which is

drained by the outlet and the exterior of which is in use at least partially

submerged by the liquid to be filtered, and the body supporting internal vanes

arranged such that the body is driven in rotation by the filtered liquid, wherein the

interior of the body is partitioned by the vanes into a series of sections arranged

relative to the outlet such that for any given rotational position of the body liquid is

retained in at least one of the submerged sections as a result of one or more of the

vanes preventing liquid flowing from that section to the outlet, whereby the weight

of liquid retained by the vanes applies a torque to the body to cause it to rotate.

2. An apparatus according to claim 1, wherein the body is cylindrical and

is rotatable about a horizontal axis coincident with the cylinder axis.

3. An apparatus according to any preceding claim, wherein the outlet is

defined by an opening in an end wall of the housing, the opening being located

beneath and to one side of the axis of rotation of the body.

4. An apparatus according to claim 1, 2 or 3, wherein the vanes extend

parallel to the axis of rotation of the body.

5. An apparatus according to claim 4, wherein the vanes and outlet are

arranged such that liquid is released from a section of the body progressively as

the body rotates.

6. An apparatus according to claim 5, wherein the outlet is defined by a

vertical edge adjacent the axis of rotation of the body, and an arcuate edge

adjacent the periphery of the body, and the vanes are swept back relative to the

direction of rotation of the body.

7. An apparatus according to any preceding claim, wherein the housing

defines a chamber into which sewage flows through the inlet and from which

filtered water flows through the outlet to an overflow, solids and the sewage

retained in the chamber being flushed from the chamber through a sewage outlet.

8. An apparatus according to any preceding claim, comprising means

powered by rotation of the body to brush the external surface of the body.

9. An apparatus according to any preceding claim, comprising means

powered by rotation of the body to spray water onto a surface of the body.

10. A liquid filter apparatus substantially as hereinbefore described with

reference to the accompanying drawings.