GB2278070A

GB2278070A - Device, installation, method for sewage treatment

Info

Publication number: GB2278070A
Application number: GB9310394A
Authority: GB
Inventors: George Francis Gilbert Clough
Original assignee: Individual
Current assignee: Individual
Priority date: 1993-05-20
Filing date: 1993-05-20
Publication date: 1994-11-23
Also published as: GB9310394D0

Abstract

A device for applying a shearing force to faecal matter entrained within a sewage stream whereby the matter is broken into small particles comprises a rotatable shaft 6 and a plurality of spaced-apart arcuate arms 8 mounted thereon in a substantially helical (e.g. stepped helical) pattern. Arms 8 may be in form of wires or blades. Shaft is rotatable by a motor (e.g. at 100 - 200 r.p.m.); a torque limiting device may be provided. <IMAGE>

Description

SHEAR DEVICE FOR SEWAGE TREATMENT The invention relates to devices, installations and methods for the treatment of sewage, and in particular for breaking up faecal matter entrained within sewage streams prior to screening.

In known sewage treatment processes, the raw sewage is often screened prior to a series of downstream biological treatment steps. This offers a number of advantages, and in particular removes coarse solids which would otherwise disrupt the further downstream processing.

However, in some installations the sewage is not pumped or macerated prior to. screening, and the faecal matter therefore remains in the form of relatively large pieces.

In these circumstances, screening has the disadvantage of removing the faecal matter which is best retained for treatment in downstream biological treatment processes.

The breaking up of the faecal matter requires relatively small forces, and there is therefore a need for a device which can apply such forces but which is not significantly affected by the presence in the sewage of more robust (refractory) solids which cannot (and indeed need not) be so-broken (such as rags and plastics).

Accordingly, it is an object of the present invention to provide devices, installations and methods for breaking up faecal matter into particles small enough to pass through a sewage screen during subsequent screening while tolerating the presence of other refractory material which may be of a relatively large size. It is a further object of the invention to provide devices for the aforementioned purpose which can be readily installed in existing sewage systems, use relatively little energy, are easily maintained and require infrequent cleaning.

In a first aspect, the present invention provides a device for applying a shearing force to faecal matter entrained within a sewage stream whereby the matter is broken into small particles, the device comprising a rotatable shaft and a plurality of spaced-apart arcuate arms mounted thereon in a substantially helical (e.g.

stepped helical) pattern.

The cross-sectional profile of the arms arising from their substantially helical arrangement is such that all parts of the channel cross section are swept at each revolution but at any time during each revolution there is a space sufficient to allow large unsheared items to pass through the channel. The arrangement of the arms (i.e. the helical or similar pattern) is therefore such that in operation the device is not unduly affected by the occasional passage of relatively large items, such as packaging, now found in sewage.

In relatively small installations, the arms may be in the form of wires. In larger installations requiring longer arms they may be in the form of blades having a similar thickness to those in the form of wires but a greater width. In a typical case, when the sewage has passed through screen apertures of about 5 mm2, the arms are preferably spaced apart at an axial distance of about 3 mm and at an equal angular displacement such that at least two of the arms (which would normally, but not necessarily, be those at the ends of the stack) occupy similar angular positions. Thus, two axially outer arms which bound the other arms may each occupy the same angular position as the other.

Typically, the arms are about 3 mm thick irrespective of their form (e.g. wires or blades). The arms are preferably spaced apart at an angular distance corresponding to 360 degrees divided by the number of arms in one turn of the helix (e.g. about 15 degrees).

The device according to the invention may further comprise a motor operably connected thereto for rotating the shaft in a direction such that the convex faces of the arms are the leading faces. The motor may be geared to drive the shaft at speeds in the low hundreds of r.p.m., e.g. 100 to 200 r.p.m. A torque limiting device (e.g. a slipping clutch) may also be provided, which reduces the risk of damage to the device and/or any motor connected thereto when unusually large items are encountered.

The device according to the invention may form part of a sewage treatment installation for applying a shearing force to faecal matter entrained within a sewage stream whereby the matter is broken down into small particles, the installation comprising; (a) a channel along which the sewage stream flows and (b) a device according to the invention located in the channel and oriented such that the shaft is substantially vertical, wherein in use the arms are rotated in a direction such that the convex faces of the arms are leading and the arms sweep substantially all of the cross-sectional area of the channel. The clearance between the blades and the sides of the channel is preferably about 3mm.

In another aspect, the invention provides a method of treating sewage containing faecal matter and other more refractory solids, comprising the step of applying a shear force to the faecal matter to break it up into small particles, the shear force being applied with the device of the invention, e.g. in an installation according to the invention as described above. The device is preferably rotated at a speed in the low hundreds of r.p.m., e.g. 100 to 200 r.p.m., but the optimum speed depends inter alia on the geometry of the installation, the speed of the sewage flow and the physical properties of the sewage. It is readily determined emprically when the device is in situ. After treatment according to the invention, the sewage may then be screened to remove the other solids but not the broken faecal matter and then biologically treated in a downstream biological treatment plant.

In a further aspect, the invention provides a sewage treatment plant comprising the device or installation of the invention.

A sewage treatment plant according to the invention may advantageously comprise filtration apparatus as described in my co-pending U.K. Patent Application Serial No.

2,247,186 and co-pending International Publication No.

W092/03208, the entire disclosure of which is incorporated herein by reference.

The invention will now be described in greater detail with reference to a particular embodiment. The embodiment is described by way of exemplification and is in no way intended to limit the scope of the invention. The embodiment is described with reference to Figures 1 and 2, in which; Fig. 1 is a diagrammatic cross-sectional view through an installation embodying the invention.

Fig. 2 shows in plan view the cross section at AA' of Fig. 1.

Referring to the Figures, the installation comprises a channel (2) about 13 cm wide along which a sewage stream (not shown) flows and in which is mounted at bearings (3) the device of the invention (4). The device comprises a shaft (6) rotated in the direction shown by the arrows by a motor (not shown). Mounted on the shaft are a number of spaced-apart arcuate arms (8). The arms are mounted in a helical pattern, as can be clearly seen in Fig. 1. In the embodiment shown the shaft bears 24 arms each about 3 mm thick and each separated by an axial distance of 3 mm and an angular distance of 15 degrees. Two axially outer arms (10,12) each occupy the same angular position as the other. The total depth of the arm array is therefore about 72 mm, although the number of arms may be increased to provide a greater depth for installations having deeper channels. The blades are separated from the walls of the channel by a clearance distance (14) of about 3 mm.

In use, the motor (a 0.37 kW motor - not shown) drives the shaft at about 130 r.p.m. as the sewage containing faecal matter and other non-faecal solids flows through the channel and past the rotating arms. The direction of rotation and arcuate shape of the arms are such that material which cannot be sheared by the arms (mainly the other non-faecal solids and rag) slides along the arms and off the end. Moreover, the cross-sectional profile of the arms arising from their substantially helical arrangement is such that all parts of the channel cross section are swept at each revolution but at any time during each revolution there is a space sufficient to allow large unsheared items to pass through the channel.

In this way, non-faecal solids and rag are passed through the device without becoming entrained on or between the arms (which would effectively block the channel and increase the resistance to rotation of the shaft). The energy required to operate the device is thereby minimised.

The shear force exerted by the rotating arms on the faecal solids in the sewage flow efficiently breaks them into particles small enough to pass through a downstream screening device (such as that described in W092/03208), while most of the non-faecal solids and rag pass through the device in a form which allows collection during screening. Thus the faecal matter passes through the screen to the biological treatment plant and the nonfaecal solids and rag are screened out and removed prior to the biological treatment stage.

Claims

CLAIMS:

1. A device for applying a shearing force to faecal matter entrained within a sewage stream whereby the matter is broken into small particles, the device comprising a rotatable shaft and a plurality of spacedapart arcuate arms mounted thereon in a substantially helical (e.g. stepped helical) pattern.

2. A device according to claim 1 wherein the arms are wires.

3. A device according to claim 1 wherein the arms are blades.

4. A device according to any one of the preceding claims wherein the arms are about 3 mm thick.

5. A device according to any one of the preceding claims wherein the arms are spaced apart at an axial distance of about 3mm and at an angular distance of e.g. about 15 degrees.

6. A device according to any one of the preceding claims wherein the arms comprise two axially outer arms which bound the other arms and each of which occupy the same angular position as the other.

7. A device according to any one of the preceding claims, further comprising a motor operably connected thereto for rotating the shaft in a direction such that the convex faces of the arms are the leading faces.

8. A device according to claim 7 wherein the motor is geared to drive the shaft at speeds in the low hundreds of r.p.m., e.g. 100 to 200 r.p.m.

9. An installation for applying a shearing force to faecal matter entrained within a sewage stream whereby the matter is broken down into small particles, the installation comprising; (a) a channel along which the sewage stream flows and (b) a device according to any one of the preceding claims located in the channel and oriented such that the shaft is substantially vertical, wherein in use the arms are rotated in a direction such that the convex faces of the arms are leading and the arms sweep substantially all of the cross-sectional area of the channel.

10. An installation according to claim 9 wherein the clearance between the blades and the sides of the channel is about 3mm.

11. A method of treating sewage containing faecal matter and other more refractory solids, comprising the step of applying a shear force to the faecal matter to break it up into small particles, the shear force being applied with the device of any one of claims 1 to 8, e.g. in an installation according to claim 9 or claim 10.

12. A method according to claim 11 wherein the device is rotated at a speed in the low hundreds of r.p.m., e.g.

100 to 200 r.p.m.

13. A method according to claim 11 or claim 12, further comprising the steps of screening the sewage to remove the other solids but not the broken faecal matter and then biologically treating the screened sewage.

14. A sewage treatment plant comprising the device of any one of claims 1 to 8 or the installation of claim 9 or claim 10.

15. A device or installation substantially as hereinbefore described with reference to the accompanying drawings.