GB2293988A - Sedimentation tanks - Google Patents

Abstract

A sedimentation tank comprises a stilling box for use in reducing the momentum of water entering the tank. Specifically, there is a stilling box (11) which is arranged to fit over a conduit means (3) in order to interrupt the flow of a stream of liquid passing therethrough, wherein exit means (16) is provided in said box to allow liquid to escape from the box in a direction which is substantially perpendicular to the direction of liquid flow through the conduit means. When such a stilling box is placed in a sedimentation tank with water entering in a generally upwards direction, this will mean that the stilling box directs the liquid in a lateral manner and thereby avoids scouring the bottom of the tank. In a particularly preferred embodiment, the water is directed in a rotational manner in the tank. <IMAGE>

Description

SEDIMENTATION TANKS The present invention relates to sedimentation tanks, particularly, but not exclusively, those used in the water industry to treat domestic and industrial effluents.

One of the principle separation processes used to treat domestic and industrial effluents in the water industry is the sedimentation of solid particulates from the carrier fluid by gravitational forces. In order to make the process economic and consistent with the throughput demand by the water industry, a continuous sedimentation method is required. This has historically been provided by introducing raw effluent at one point into a large tank whilst removing "decanted clean" and "precipitated dirty" fluid streams from other points. Separation is achieved by gravitational forces acting on solids; denser-than-water particles will tend to sink, while less dense particles will tend to float. However, fluid motion tends to keep the particulates well mixed in the carrier fluid.There are therefore competing forces at work; fluid motion tending to hold the particulates in suspension, while buoyancy forces tend to separate them out. These general principles have been well understood for many years (Hazen, A. On sedimentation. Trans. Am. Soc Civ. Engrs, 1904, 53, pp 45-71; Dobbins, W.E. Effects of Turbulence on Sedimentation. Trans. ASCE, 1944, 109 (2218), 629-656).

Traditional sedimentation tanks were designed with the aim of engineering a situation where the effluent spent a sufficiently long residence time in a region where the bulk flow velocities were low enough to allow separation to occur. Sediment may then be removed from the floor of the tank and relatively clean water taken from the surface at the periphery of the tank. In an ideal sedimentation tank, the fluid entering the tank should move radially outward with an even velocity across the whole of the circumferential area of the tank.

A recent survey (Innes, H., Smith, M., and Wise, D., Research into New Equipment for the Sedimentation Process in Waste Water Treatment. M Eng Thesis, 1994, Mechanical Engineering Department, Bristol University, UK) suggests that 20% of the land area in typical sewage treatment plants in the UK is taken up by sedimentation tanks. Since land usage affects profitability, there is a strong economic pressure to construct and operate sedimentation tanks in the most efficient manner possible.

A typical sedimentation tank is substantially cylindrical, with floor which slopes downwardly towards the centre.

Water flows into the tank through an inflow pipe arranged centrally in the tank and leaves by way of outlet weirs arranged around the periphery of the tank. During the time the water remains in the tank, particulate matter separates from it and drops to the floor of the tank. The opening of the inflow pipe is arranged so that in use it lies below the water level in the tank. A "momentum breaker" or "stilling box" is provided around the opening of the inflow pipe. The purpose of the stilling box is to reduce excessive radial velocities, and to attempt to achieve a uniform flow with minimal azimuthal components. Without it there is a danger that the effluent would flow along the surface and straight to the outlet weir, with much reduced residence time. This would have a detrimental effect to the settling out phenomenon.

However the applicants have found that the design of conventional stilling boxes is such that they produce a significant scouring or shearing effect at the tank floor, particularly in the central region, which tend to inhibit the settling of particulate matter there.

According to the present invention, there is provided a stilling box for a sedimentation tank, said box being arranged to fit over a conduit means in order to interrupt the flow of a stream of liquid passing therethrough, wherein exit means are provided in said box to allow liquid to escape from the box in a direction which is substantially perpendicular to the direction of liquid flow through the conduit means.

In a preferred embodiment, the stilling box comprises facing upper and lower wall means interconnected by side walls, the lower wall having an opening to accommodate an inflow pipe, and said side walls including at least one opening to allow exit of water from the stilling box.

The openings are suitably arranged at the lower edge of the stilling box and/or the stilling box is arranged at a suitable depth within the tank to ensure that water leaving the stilling box does not 'short circuit' across the surface of the tank.

By arranging the stilling box so that water flows outward through the side walls rather than downwardly, the shearing effect which interferes with the sedimentation process is avoided.

In a further preferred embodiment, a conical base is provided in the stilling box and arranged to direct sediment to the side openings. This inhibits the build up of sediment on the lower surface of the stilling box.

Furthermore, in a conventional sedimentation tank, expensive mechanical power (in the form of excess momentum) is destroyed by the 'stilling box'. Indeed, as mentioned above, this destruction is ineffective, and leads to scouring or shearing of the floor of the sedimentation tank.

In a preferred embodiment of the invention, guide means are provided at openings in the side walls of the stilling box, said guide means being arranged so that water exiting through the openings is directed in a rotational flow within the tank.

In such an arrangement, the momentum from the input is converted to weak rotational flow within the tank. This rotational flow produces a weak central vortex into which particulate matter is drawn and which therefore enhances the collection of particulates on the floor at the centre of the tank.

The invention further provides a sedimentation tank which comprises a generally cylindrical tank, fluid inlet means, means for interrupting the flow of liquid through said inlet means and for directing said liquid in a generally radial direction within the tank, and liquid outlet means.

Preferably the said generally radial direction includes a rotational element.

The invention will now be particularly described by way of example with reference to the accompanying diagrammatic drawings in which: Figure A is a cross section through a conventional sedimentation tank including a known stilling box; Figure 1 is a cross section through a sedimentation tank including a stilling box according to the invention; Figure 2 is a section on line X-X in Figure 1; Figure 3 is a schemmatic side view of a further embodiment of the stilling box of the invention, showing the inlet pipe; Figure 4 is a plan diagram of a sedimentation tank incorporating the stilling box of Figures 1 and 2 showing the direction of flow in a segment thereof; Figure 5 is a plan section through an alternative embodiment of the stilling box of the invention; Figure 6 is section through a further alternative embodiment of the invention; ; Figure 7 is a schematic diagram of an arrangement used to test the stilling boxes; Figure 8 shows the results of flow visualisations experiments using a conventional stilling box (8A) and stilling boxes in accordance with the invention (8B and 8C); and Figure 9 shows the results of retention time experiments using the same stilling boxes.

A conventional sedimentation tank (Figure A) comprises a generally cylindrical tank 1 having a sloping floor 2 which forms a conical shape. An inflow pipe 3 to bring effluent into the tank projects upwardly through the centre of the floor 2 and is arranged so that an open end 4 is positioned below the level of the water 5 when the tank is in use.

Around the periphery of the tank is an outlet weir 6 which controls the level of the water 5 and collects the overflow surface water. The weir may comprise a plate having a large number of relatively small 'V' shaped notches (not shown) which help to ensure a uniform azimuthal distribution of flow.

Mounted above the opening 4 of the inflow pipe 3 is a stilling box 7. Mounting devices are well known in the art Conventional stilling boxes comprise a generally cylindrical portion 8 and a closed upper surface 9 against which effluent entering through the inflow pipe abuts. The upward momentum of the incoming effluent is thus destroyed but instead the effluent flow is directed generally downward substantially in the direction of the arrows.

This flow may be quite vigorous and applies a scouring or shearing effect on the floor 2 of the tank, thereby impeding the settlement of sediment 10 which should accumulate there.

Figure 1 shows a similar sedimentation tank to that shown in Figure A but one which includes a stilling box 11 in accordance with the invention. In this case the stilling box comprises is generally cylindrical in shape and has an upper surface 12 and a lower surface 13. An aperture 14 in the lower surface 13 is arranged to accommodate the inflow pipe 3 in a substantially watertight fit. Side wall 15 (Figure 2) of the box 11 is provided with a plurality of circumferentially spaced openings 16 to allow effluent to flow into the tank in a generally radial direction as indicated by the arrows (Figure 4). This means that the scouring of the bottom of the tank where the sediment 10 is located is generally avoided.

The openings 16 are suitably arranged at the lower edge of the stilling box as illustrated in Figure 1. This, combined with the depth at which the openings are arranged within the tank will reduce the incidence of water passing directly across the surface of the water.

Suitably there will be from 6 to 20 openings, preferably evenly spaced circumferentially around the box.

In order to prevent accumulation of sediment on the lower surface of the stilling box, it is, in a preferred embodiment, provided with a conical base 17 with the apex in the centre as illustrated in Figure 3.

In further preferred embodiment of the stilling box (Figure 5), the side surfaces 18 of openings 16 are inclined so as to act as guide means to direct effluent leaving the box in a direction tangential to the side wall 15. This means that a weak rotational flow will be established within the tank 1 as effluent passes from the stilling box 11 towards the weir 6 as a result of the momentum of the effluent.

This rotational movement will have the effect of concentrating particulate matter in the central region of the floor of the tank form where it may be removed by conventional methods.

A similar rotational flow may be established using the embodiment of the stilling box illustrated in Figure 6. In this case, the side wall 15 of the stilling box 11 comprises circumferentially spaced inner and outer concentric cylindrical wall sections (19, 20 respectively).

One end of each outer wall section 20 is connected by a transverse connect or 21 to an end of an inner wall section 19.

In this way, each opening 16 defines a passage 22 through which water will flow in a substantially tangential manner as indicated by the arrows. The surfaces of the outlets may have rounded corners (as illustrated) or smooth curves.

Although the latter may be preferable from the point of view of the resulting flow patterns, the former is an easier construction to produce and may therefore be less costly.

This embodiment of the stilling box has the advantage that it will be easy to construct.

Sedimentation tanks used by the water industry are categorised as primary and secondary tanks, the primary tank being used initially to remove the larger particulates and the secondary tanks being used to remove smaller particulates. Stilling boxes of the invention may be employed in both primary or secondary sedimentation tanks although they are principally envisaged as being of use in primary sedimentation tanks.

The stilling boxes of the invention are constructed of any suitable material such as metal, plastics or wood.

The following Examples illustrate the invention.

Example 1 Equipment Design Experiments with different designs of stilling box were carried out using a 1:20 scale model of a sedimentation as illustrated in Figure 7. The sedimentation tank 1 was made of a transparent plastics material and was connected to a header tank 24 by way of standard central heating pump 25 which was capable of producing a maximum flow of 1 1/s.

The tank 1 incorporated a weir (not shown) with 16 Vnotches of 15 mm depth and 90" internal angle leading to an outlet tank 26, and four outlet pipes (not shown) leading back to the header tank.

Lighting facilities were also provided and arranged to produce velocity flow photographs.

Various stilling boxes were installed at the centre of the sedimentation tank and the flow patterns within the sedimentation tank and the retention time within the tank was studied for each design as described below.

Three designs were employed. These were Design 1 :- a conventional stilling box as illustrated in Figure A; Design 2 :- a stilling box as shown in Figures 1 and 2, having eight equally spaced holes in the side wall.

The total area of the outlet was the same as that of Design 1. It extended slightly deeper into the tank so as to ensure that water did not "short circuit" along the surface and was provided with a 60mm diameter hole which accommodated the inlet pipe, which releases water at surface height, near the top of the stilling box; and Design 3 :- a stilling box as shown in Figure 6 which had rounded corners and a 60mm diameter hole for the inlet pipe.

Each stilling box had an uppermost diameter of 150mm.

Example 2 Flow Visualisation Flow visualisation was achieved using potassium permanganate crystals. The crystals were first adhered to a fine wire using cyanoacrylate adhesive. The flow in any part of the tank could then be examined by placing the wire in the appropriate place, whereupon the flow was observed using a video camera. To ensure that all areas of the tank were covered, the video camera was set up and a grid was superimposed on the screen, showing a cross section through the tank. The wire was then manipulated to place the crystal at the intersections of the grid and the image was recorded.

In practice, it was found to be easier to use a relatively long wire with crystals adhered at regular intervals along the length. The flow at various heights within the tank could then be measured at the same time.

The results of the flow visualisation experiments are shown in Figures 8A, 8B and 8C which are cross sections through on half of the tank. In these Figures, the velocity of the flow was estimated and is indicated by the length of the arrows.

It can be seen that in the case of Figure 8A, the fluid exits the box and moves outward in a radial direction along the base of the tank. An eddy can be identified in the centre of the cross section. One of the possible reasons for this eddy is the downward velocity of the water leaving the stilling box which entrains the water, dragging it down. The eddy is essentially undesirable because it reduces the effective volume of the tank. The outward radial velocity below the eddy is high due to the movement of both the water entering the tank from the stilling box and the water recycled through the eddy flowing through a reduced flow area. The reduction in effective volume of the tank greatly reduces the retention time of the tank.

In contrast, the flow visualisation patterns obtained with Design 2 of the stilling box (Figure 8B) show that although water leaving the stilling box will entrain neighbouring water it is carryied outward in a radial direction. Some of the water travelling in the radial direction takes a circumferential motion when it reaches the edge of the tank.

Finally the flow visualisation results for the stilling box of Design 3 (Figure 8C) show clear circumferential flow.

The water at the base of the tank can be seen to travel towards the centre, which will have the effect of directing sludge to this point.

Example 3 Retention Time Experiments Retention time tests were carried out by injecting a known concentration of a tracer dye (potassium permanganate) into the outlet of the header tank. Water samples were then taken from one point at the weir at regular intervals and compared to a control sample of tank water taken at the start of each experiment using a Micam UV2- 100 Ultraviolet/Visual Light Spectrometer.

These concentration values were then plotted against time to produce comparable graphs for all three stilling boxes.

The time scales on the graphs were then normalised with respect to the ideal retention time for the flow rate at which the tests were carried out, assuming plug flow. The ideal retention times for the sedimentation tank was 1800 seconds at 0.1 l/s and 900 seconds at 0.2 1/s.

Representative examples of the results are illustrated in Figure 9. It is clear from these graphs that the stilling boxes of the invention, and particularly those of Design 3 produce substantially better retention times.

Computer modelling techniques were also employed and substantiate the results as discussed above.

Claims (13)

Claims

1. A stilling box for a sedimentation tank, said box being arranged to fit over a conduit means in order to interrupt the flow of a stream of liquid passing therethrough, wherein exit means are provided in said box to allow liquid to escape from the box in a direction which is substantially perpendicular to the direction of liquid flow through the conduit means.

2. A stilling box according to claim 1 which comprises facing upper and lower wall means interconnected by side wall means, the lower wall means having an opening arranged to accommodate an inflow pipe, the said exit means comprising at least one opening in said side wall means.

3. A stilling box according to claim 1 or claim 2 wherein the stilling box is generally cylindrical.

4. A stilling box according to claim 2 or claim 3 wherein the said exit means comprises a plurality of openings arranged in the side wall means in the region of the lower wall means.

5. A stilling box according to any one of claims 2 to 4 wherein the lower wall means is substantially conical in shape with the apex arranged in a central region.

6. A stilling box according to any one of the preceding claims wherein guide means are provided at said exit means in order to direct liquid exiting through said exit means in a rotational flow within the tank.

7. A stilling box according to claim 6 wherein said guide means comprises inclined surfaces at the openings.

8. A stilling box according to claim 6 wherein said guide means comprises circumferentially spaced inner and outer wall sections which are interconnected.

9. A sedimentation tank which includes a stilling box according to any one of the preceding claims.

10. A sedimentation tank which comprises an generally cylindrical tank, fluid inlet means, means for interrupting the flow of liquid through said inlet means and for directing said liquid in a generally radial direction within the tank, and liquid outlet means.

11. A sedimentation tank according to claim 8 wherein the said generally radial direction includes a rotational element.

12. A sedimentation tank substantially as hereinbefore described with reference to the accompanying diagrammatic drawings.

13. A stilling box substantially as hereinbefore described with reference to the accompanying diagrammatic drawings.