SLUDGE TREATMENT

Abstract: A gas bubble generator (1) for mixing or promoting turbulence in a liquid medium (2), particularly sewage or effluent, comprises a collector (3) for gas which collector is submerged in the medium (2) and has an exit (5) from which leads a pipe (4) with a curved part (9) between the exit (5) and a free end (8) of the pipe from which gas escapes as a bubble to surrounding liquid medium (2). The free end (8) is spaced laterally from the exit (5), and the curva­ ture of the curved part (9) is 135° so that there is no obstruction of the pipe (4) by the medium (2).

Description: The invention relates to a gas bubble generator for example of the kind used in sludge treatment in a digestion tank or digester, though it will be understood that a gas bubble generator as described herein may be used in other applications. It is often necessary to create turbulence in a liquid or liquid containing solid matter such as sludge, such liquid or sludge being hereinafter referred to as 'liquid medium'. One way of creating turbulence is to pass a steady stream of bubbles through the liquid medium. Such bubbles as they pass upwardly through the liquid medium promote mixing in the medium and also act to break up or disperse scum on the surface when they reach the surface and burst. A bubble generator of this kind is shown in EP-A-033407. Bubble generators of this kind used in digesters are immersed in the liquid medium and systematically create a bubble by forcing a gas out of a tube by a hydrostatic effect as liquid medium passes through the generator forcing a trapped pocket of gas therethrough prior to expulsion as a bubble. As the liquid medium contains solid matter which is often fibrous, known as 'rag', the rag can and usually does become caught in the generator as it has a U-shaped tube forming a tight bend between the generator and an exit thereof. This has the disadvantage of either reducing the efficiency of bubble formation or preventing it altogether, while the construction of the generator with a tight bend makes it virtually impossible to clean by purging or mechanically by rodding. It is accordingly an object of the invention to seek to mitigate these disadvantages of the prior art. According to one aspect of the invention there is provided a gas bubble generator for mixing or promoting turbulence in a liquid medium, comprising a collector for gas having a bottom open to surrounding liquid medium, and an exit for gas from the collector the pipe having an opening from which gas can escape as a bubble, the opening being spaced laterally from the exit from the collector at an end of the pipe remote from the exit of the collector, there being a curved part of the pipe between the end and the exit, characterized by the radius of curvature of the curved part being about 135°. Using the invention it is possible to provide a smoothly curved or opened out gas bubble generator which enables purging by gas or liquid for cleaning, or cleaning by 'rodding'. The gas collector may comprise a device adapted to close off the opening to ingress in use of surrounding medium. The device may comprise an expansible member adapted to be expanded into the collector to obturate the cross-sectional area thereof against ingress of liquid medium. This expansible member may be a sleeve enclosed by a metal cover and extending over an aperture in a boundary wall of the collector whereby the sleeve expands into the collector to obturate it when fluid under pressure is passed into a space between the sleeve and the cover therefor. There may be two sleeves each with its own cover and spaced opposite one another in the boundary wall of the collector. The or each resilient sleeve may comprise a rubber sleeve. The or each cover may comprise an aluminium cover. The generator may be in combination with a riser tuber into which the opening of the pipe may be directed. The generator may include an adjustable support structure whereby the collector and pipe are supported. The adjustable support structure may comprise a plurality of spaced quart support legs connected by support beams, and there may be a captive rotatable screw threaded jack passing through a screw threaded member adjacent a beam, to adjust the position of the beam on rotation of the jack. The legs may each have an angled foot member for mounting the support structure on a sloping floor. According to a second aspect of the invention there is provided a sludge digester, including a gas bubble generator as hereinbefore defined. There may be a plurality of gas bubble generators in the digester. The digester may be adapted to pass cleaning means through the gas bubble generator. The cleaning means may, in use, be the liquid medium. The means may be a flexible rod. A gas bubble generator and a sludge digester embodying the invention are hereinafter described, by way of example, with reference to the accompanying drawings. Figs. 1 to 4 are schematic side elevational view of a gas bubble generator according to the invention in operation to form a bubble; Figs. 5 to 8 are schematic side elevational views of the gas bubble generator of Figs.1 to 4, showing 'in service' cleaning thereof; Fig. 9 shows to an enlarged scale a side elevational view of a pipe of the gas bubble generator of Figs. 1-8; Fig. 9A shows an enlarged detail of Fig. 9; Figs. 10-12 show respectively side elevational, end elevational and plan view of the gas bubble generator for Figs. 1 to 9, and a support structure therefor; Fig. 13 shows a schematic elevational view of an adjustment means of the support structure of Fig. 10-12; and Fig. 14 shows a perspective schematic view of a sludge digester according to the invention. Referring to the drawings there is shown a gas bubble generator 1 for mixing or promoting turbulence in a liquid medium 2, comprising a collector 3 for gas and a pipe 4, leading from an exit 5 from the collector 3, having an opening 6 from which gas can escape in use as a bubble 7 to surrounding liquid medium 2, wherein the opening 6 is spaced laterally from the exit 5 from the collector 3. The opening 6 is in the embodiment shown at an end of the pipe 4 remote from the collector 3 and is in use horizontally displaced therefrom as shown. The end 8 of the pipe 4 is vertically, as viewed, higher than the exit 5. The pipe 4 has a curved part 9, between the end 8 and the exit 5, which curved part 9 is as shown a lower part of the pipe 4 and which has a radius of curvature of about 135° in the embodiment. The collector 3 is essentially a tube closed at one end (the top 10 in use) and an open bottom 11, there being a gas inlet 12 opposite the exit 5, which has a chamfered transition from the interior of the collector 3 to seek to avoid corners or edges where solids may collect (Fig. 9A). The collector 3 also has means whereby the interior may be obturated against ingress of surrounding liquid medium 2. The collector 3 has two facing holes or slots 14 in its boundary wall and in these holes or slots 14 there are expansible members in the form of rubber sleeves or diaphragms 15 held in place by metal, for example cast aluminium, covers 16. The covers 16 are secured to flanges of the collector 3 by complementary flanges with the sleeves or diaphragms 15 in between the flanges to be secured in position. The covers 16 form a manifold with the sleeves, and there is a connection 17 whereby a line 18 from a source 19 of gas under pressure like a cylinder of nitrogen is connected with each manifold 15, 16 via a T-piece connector 20 (Fig. 14). The gas bubble generator 1 is supported on a structure 21 comprising three upright support legs 22, 23, 24 each with an angled foot 25 for mounting on a sloping surface such as the angled floor of a sludge digester 26. The three support legs 22, 23 and 24 are arranged in the form of a triangle as considered in plan (Fig. 12) and are rigidly connected together by a main beam 27 and an extension beam 28. The main beam 27 comprises spaced apart girders 27′ which are doubly convergent and supports the collector 3 while the extension beam 28 comprises two girders 28′ which converge towards a leg 22 and which provide a support for plate 29 (Fig. 9) which is shaped and welded to the pipe 4. The support structure 21 is adjusted by an adjusting device at each leg (Fig. 13) comprising an elongate externally threaded stud 30 which is received at one end on a captive cap 31 to locate the stud 30 and which is secured to the respective leg 22, 23 or 24 adjacent its foot 25 by a U-bolt 32. The stud 30 passes through a nut with a torque arm 33 and through a girder 27′ or 28′ to terminate in a nut 34 secured to it as by welding. In use, the stud 30 is rotatable freely in the cap 31 when a turning tool is applied to the nut 34 and thereby jacks the beams 27, 28 upwards by reaction against the nut and torque arm 33. In this way the correct attitude of the support structure is achieved by approprate adjustment at each leg 22, 23, 24. A bubble 7 generating sequence is shown in Figs. 1 to 4, assuming the gas bubble generator 1 is completely immersed in sludge 2 in a digester 26 like the one in Fig. 14. In this case, the pipe opening 6 is directed into the flared lower (as viewed) end of a riser tube 35 out of the top of which the bubble 7 formed by the generator 1 emerges after rising through the tube 35. The bubble then rises through the liquid medium (sludge) effecting turbulence therein. It will be understood that there are four gas bubble generators 1 in combination with respective riser tubes 35 used in Fig. 14 but only one is shown completely for clarity. Methane gas from the digester 26 is passed down a conduit 36 to the gas entry 12 to the colletor 3, the rubber sleeves being relaxed or unexpanded so that the interior of the collector 3 is completely free, and open for ingress of the surrounding liquid medium 2. The gas displaces the sludge (liquid medium) forming menisci 37, 38 as shown in Figs. 1-3. Eventually a siphon effect is achieved, and liquid medium below the meniscus 37 has a hydraulic effect forcing the gas pocket trapped between the menisci 37 and 38 out of the collector 3 through the exit 5 and along the pipe 4 (Fig. 2). The siphon effect ensures that the gas continues along the pipe 4 until it exists the opening 6, and forms a large gas bubble 7 at that opening (Fig. 3). The bubble 7 then breaks free, rising through the riser tube 35 to the surface of the sludge (Fig. 4) as a further bubble begins to form as the gas continues to enter (Fig. 4). Using the gas bubble generator described herein, 6 bubbles of 18″ diameter can be formed per minute in a steady and repeatable cycle. This provides for even mixing of the fluid medium 2 and smooth and efficient operation of the digester 26. The liquid medium 2 contains solids, often of a fibrous nature, which can catch or snag on corners of the generator such as at the inlet 12 and exit 5, in the curved part 9 of the pipe 4, and at the outlet opening 6. To clean the generator in service or in situ , without shutting down the generator 1, using a gas or effluent purge, nitrogen under pressure is passed in to the manifolds from source 19 to expand the sleeves or diaphragms 15 to close off or obturate the interior of the collector 3 againt ingress of surrounding liquid medium 2. The methane gas as cleaning means is then forced to flow in a continuous stream through the exit 5 into and through the pipe 4 and this action removes blockages and scours grit from the curved part 9 of the pipe 4 (Fig. 6). Alternatively, (Fig. 7) the methane gas flow can be turned off by operation of suitable valving 39, and effluent (liquid medium) under high pressure is passed as cleaning means through collector 3 and pipe 4, removing blockages and scouring the pipe as in Fig. 6. Finally, instead of effluent, the methane gas supply is again isolated as before and cleaning means in the form of rods such as helical spring steel and polythene sewer rods 40 are fed down the gas supply conduit and out through the inlet 12, exit 5, pipe 4 and opening 6 (as shown in Fig.8). In Fig.8, the devices 13 need not be utilised. It will be understood that when the supply of nitrogen to the manifolds is turned off at 19, the natural resilience of the rubber sleeves 15 returns them to their inoperative position (Figs.1-4). After cleaning, the methane supply is resumed by suitable operation of the valving 39. The digester 26 shown in Fig.14 will not be described in further detail, though it is to be understood that it can be a heating and mixing installation, water being heated by return flow through the riser tubes 35 which have a double wall.

Claims: A gas bubble generator for mixing or promoting turbulence in a liquid medium, comprising a collector for gas having a bottom open to surrounding liquid medium, and an exit for gas from the collector, a pipe leading from the exit from the collector, the pipe having an opening from which gas can escape as a bubble, the opening being spaced laterally from the exit from the collector at an end of the pipe remote from the exit of the collector, there being a curved part of the pipe between the end and the exit, characterised by the radius of curvature of the curved part (9) being about 135°. A gas bubble generator according to Claim 1, characterised by a device (13) adapted to close off the opening (11) to ingress of surrounding medium. A gas bubble generator according to Claim 2, characterised by the device (13) comprising an expansible member (15) adapted to be expanded into the collector (3) to obturate the cross-sectional area thereof against ingress of liquid medium. A gas bubble generator according to Claim 3, characterised by the expansible member (15) being a sleeve enclosed by a metal cover (16) and extending over an aperture (14) in a boundary wall of the collector (3) whereby the sleeve (15) can expand into the collector (3) to obturate it when fluid under pressure is passed into a space between the sleeve (15) and the cover (16) therefor. A gas bubble generator according to any preceding claim, characterised by the generator (1) being in combination with a riser tube (35) into which the opening of the pipe (4) is directed. A gas bubble generator according to Claim 5, characterised by an adjustable support structure (21) whereby the collector (3) and tube (35) are supported. A gas bubble generator according to Claim 6, characterised in that the adjustable support structure (21) comprises a plurality of spaced apart support legs (22,23,24) connected by support beams (27,28), and in that there is a captive rotatable screw-threaded jack (20) passing through a screw-threaded member (31) adjacent a beam, to adjust the position of the beam (27,28) on rotation of the jack (30). A gas bubble generator according to Claim 7, characterised in that the legs (22,23,24) each have an angled foot member (25) for mounting the support structure (21) on a sloping floor. A sludge digester, characterised by a gas bubble generator (1) according to any preceding claim. A sludge digester according to Claim 9, characterised by being adapted to pass cleaning means (19,40) through the gas bubble generator (1). A sludge digester according to Claim 10, characterised in that the cleaning means is, in use, the liquid medium. A sludge digester according to Claim 11, characterised in that the means is a flexible rod (40).

Inventors: CLARK, KENNETH GEORGE
Applicants: THAMES WATER UTILITIES LIMITED
Application year: 1989
Main classification: C02F3
IPC codes: F16K7, C02F3