GB2167314A - Oxidation ditch in waste water treatment plant - Google Patents

Abstract

An oxidation ditch of an activated sludge waste water treatment plant defines an endless open channel round which the water flows. A separator 3 defines a quiescent zone within the channel and has overflows 9 for clarified water and a slotted floor 5 by which settled solids return to the channel. Upstream and downstream walls 7, 8 of the separator incorporate movable or removable gates 11, 12 which normally extend above the water surface (Fig. 3a). When scum 10 accumulates in the separator, the gates are opened, so that the water current carries the scum away to be broken up by an aerator or removed by a separate overflow 13. <IMAGE>

Description

SPECIFICATION An oxidation ditch of and activated sludge waste water treatment plant This invention relates to an oxidation ditch of an activated sludge waste water treatment plant.

British Patent No. GB 2075856 B describes a separation device for incorporation in an oxidation ditch type of activated sludge waste water treatment plant, the contents of which Patent are hereby expressly incorporated herein. The separation device comprises a compartment or series of compartments installed within the circuit of an oxidation ditch.

The or each compartment is provided with beams or shaped members which are fitted across the base to provide a slatted floor, under which the recirculating mixed liquor in the oxidation ditch is able to pass. The slatted floor isolates the contents of the compartment or compartments from the turbulent region underneath and permits solids to settle out, leaving a clear supernatant layer. The separated solids settle back through the slatted floor and maintain an equilibrium with the circulating mixed liquor beneath. The clarified water is drawn off from the upper region of the compartment. The device operates in a manner analogous to a fluidised bed. It has a particular advantage in that settlement is achieved within the structure of the oxidation ditch, eliminating the need for external settlement tanks.The device also eliminates the need for sludge return pumps which are necessary in a conventional oxidation ditch activated sludge system in order to maintain the mixed liquor solids concentration.

Claim 1 of British Patent No. GB 2075856 B reads as follows: "An intrachannel clarifier for clarifying waste liquid containing settlable solids for use in an aerobic wastewater treatment system of the type having means for retaining the waste liquid in a continuous substantially closed flow path, means for introducing wastewater influent into the flow path, means for removing waste sludge as mixed liquor from the flow path, and means for moving the waste liquor in a single direction in the flow path at sufficient velocity to prevent settling of the suspended solids, said clarifier comprising: tank means positionable within said retaining means for creating a zone of relatively quiescent waste liquid in the upper portion of the tank means to permit suspended solids to settle downwardly by gravity within the quiescent zone to produce clarified waste liquid; clarifier inlet means included in said tank means to permit entry of unclarified mixed liquor from the flow path into the quiescent zone at a relatively slow rate; and baffle means associated with said tank means for permitting the settled solids to return from the quiescent zone to the flowing mixed liquor within the retaining means but preventing movement of excessive quantities of flowing mixed Iquor into the quiescent zone, said baffle means comprising a plurality of spaced parallel slightly overlapping inverted V-shaped angle members configured to extend across the retaining means transversely of the waste liquid flow direction each of said angle members having a pair of angularly disposed legs configured to lie across the flow path when the clarifier is positioned within the retaining means, adjacent legs of adjoining angle members being spaced to permit the passage of settled solids therebetween, said angle members being configured such that settling of suspended solids occurs in the quiescent zone above said baffle means.

The device, as described and claimed in the above mentioned Patent, has however a disadvantage over conventional external sedimentation tanks in that floating materials present within the activated sludge mixed liquor tend to accumulate at the surface of the separation compartment and if allowed to accumulate unduly will cause blockage of the decanting system or may pass out with the treated effluent.

In a conventional sedimentation tank scum boards are fitted to prevent floating material from passing out with the effluent and a scraper blade to scrape the surface of the clarifier and propel floating materials into a scum box is fitted as an integral part of the floor scraping system.

According to the present invention, there is provided an oxidation ditch of an activated sludge waste water treatment plant, the oxidation ditch defining a trough-like channel for retaining the waste water in a continuous substantially closed flow path and having at least one separation device within the trough-like channel, the separation device being defined at upstream and downstream ends, as considered in relation to the direction of flow in the trough, by walls each of which has an openable or removable impermeable gate which is able to penetrate the water surface at all operating levels.

Preferably, each gate forms the upper portion of the respective wall and is either hinged to or vertically movable relative to a respective lower wall portion.

Side walls of the separation device may or may not comprise the side walls of the trough.

When it is desired to remove the scum the gates are opened, thus enabling the flow within the ditch itself to pass both over and under slatted floor beams of the separation device. The velocity in an oxidation ditch is typically in the region of 0.5 m/sec so that within a few seconds the carpet of floating scum is carried forward downstream into the oxidation ditch itself. The gates are then closed again and settlement resumes. During this flushing period and for a suitable time thereafter the decanting system is isolated and the water level within the ditch allowed to rise slightly. However, the time is such that the height of rise is small and well within the freeboard normally provided on such equipment.

The nature of the floating scum varies, usually much of it is aerated biomass which is redispersed by the action of the aeration rotor.

However, in cases where there is a persistent mass of floating material, such as plastics, a skimming trough may be placed across the oxidation ditch separately from the separation device. Floating material released from the separation device will be carried forward by the flowing stream to this descumming trough which may be placed conveniently at the leading edge or in front of the leading edge of the separation device. In such a situation it is furthermore convenient to discontinue discharge until the level within the oxidation ditch has risen to the rim of the descumming trough.

The invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of an oxidation ditch as described in GB 2075856 B, Figure 2 is a schematic vertical section through the separation device of the oxidation ditch of Figure 1, and Figures 3a and 3b are schematic vertical sections through one embodiment of a separation device of an oxidation ditch according to the present invention, with the gates shown in raised and lowered positions respectively.

Referring to the drawings, the oxidation ditch 1 shown therein typically comprises a pair of parallel channels connected by a "U" turn at either end. Various means can be employed to propel the mixed liquor around the continuous channel so formed and also to disperse air into the liquor to provide means of oxidising the waste matter. A brush type rotor 2 is one commonly used method which achieves both ends. In a conventional oxidation ditch the incoming effluent is fed directly into this ditch and the mixed liquor extracted through an external separation tank not shown in the drawings. Settled sludge from the base of the settlement tank is pumped back into the ditch. In the present arrangement the external settlement tank and sludge return pumps are replaced by the separation device 3.This is provided with a slatted floor 5 which may comprise triangular section beams or folded inverted V beams. The mixed liquor within the oxidation ditch passes underneath the floor of the separation device through the space 6. The device 3 is defined by an upstream wall 7 and a downstream wall 8. The mixed liquor may enter the separation device at a relatively slow rate either through the slatted floor or through the upstream wall 7.

The clarified effluent is decanted through perforated pipes 9.

After a period of operation floating matter collects on the surface to form a scum 10.

Outside the boundary of the separation device any scum tends to be dispersed and broken up by the action of the aerator. To remove the scum layer within the separation device gates 11 and 12 hinged to the upstream and downstream walls 7 and 8 respectively are lowered permitting a through flow of mixed liquor as previously described. Flow from the decanting tubes 9 is shut off until the mass of sludge is clear of the downstream gate. The gates are then raised to the vertical position and a period of time allowed to elapse for the liquid in the upper section of the device 3 to clarify.

A descumming trough 4 which may either comprise a circular rotating trough or alternatively a fixed horizontal trough 13 is fitted upstream of the separation device and preferably downstream of the aerator 2. Persistent floating material is removed, either by allowing the level to rise sufficiently to achieve decanting into this trough or alternatively the trough may be tilted or lowered into the circulating stream to achieve decanting.

The hinged gates shown in the figures are given by way of example only. Instead these could, for example, be rising or removable gates, or openable gates in the form of inflatable tubes.

The arrangement described provides a rapid and very effective means of removing considerable quantities of scum from the surface in the separation device without resorting to mechanical means such as traversing scrapers.

Claims (8)

1. An oxidation ditch of an activated sludge waste water treatment plant, the oxidation ditch defining a trough-like channel for retaining the waste water in a continuous substantially closed flow path and having at least one separation device within the trough-like channel, the separation device being defined at upstream and downstream ends, as considered in relation to the direction of flow in the trough, by walls each of which has an openable or removable impermeable gate which is able to penetrate the water surface at all operating levels.

2. The oxidation ditch of claim 1, wherein the separation device has a slatted floor and means therein for drawing off clarified water.

3. The oxidation ditch of claim 1 or claim 2, wherein each gate forms the upper portion of the respective wall.

4. The oxidation ditch of claim 3, wherein each gate is hinged to a respective lower wall portion.

5. The oxidation ditch of claim 3, wherein each gate is vertically movable relative to a respective lower wall portion.

6. The oxidation ditch of any one of the preceding claims, wherein a skimming trough is placed across the oxidation ditch separately from the separation device.

7. The oxidation ditch of claim 6, wherein the skimming trough is placed at the leading edge or adjacent to and upstream of the leading edge of the separation device.

8. An oxidation ditch of-an activated sludge waste water treatment plant substantially as hereinbefore described with reference to and as shown in Figures 3a and 3b of the accompanying drawings.