WO2003043722A1

WO2003043722A1 - Device for aerating liquids

Info

Publication number: WO2003043722A1
Application number: PCT/AU2002/001576
Authority: WO
Inventors: Thomas Urie Lawson
Original assignee: Thomas Urie Lawson
Priority date: 2001-11-23
Filing date: 2002-11-22
Publication date: 2003-05-30
Also published as: AUPR907901A0

Abstract

A diffuser for aerating a liquid, and particularly for the aeration of domestic and industrial wastewater or liquid sewage contained in a treatment tank, each diffuser comprising an elongate rigid bottom plate or housing (2) having a curved or convex-profile and an upper multiply-perforated flexible membrane (3) sealingly secured to the bottom plate (2) along its periphery by sealing strips (4). Pressurised gas via inlet (5) is introduced into the air space formed between the bottom plate (2) and the membrane (3) to aerate the liquid contained in the treatment tank via the multiple perforations or pores in the membrane. Releasable means for securing the diffuser to a horizontal header pipe, and a method for aerating a liquid were also disclosed.

Description

DEVICE FOR AERATING LIQUIDS

FIELD OF THE INVENTION

The present invention relates to an apparatus for aerating a liquid, such as a liquid held in an aeration basin or tank, for instance wastewater or the like media. The invention also relates to a process for the intermittent aeration of such fluids or liquids by means of an aeration device arranged below the surface of the liquid.

BACKGROUND OF THE INVENTION

Large quantities of wastewater are produced as a result of daily domestic and industrial activity. Such wastewater must be treated in order that the water can be recycled or discharged safely into water courses such as rivers or the ocean. As part of such treatment, it is known that to introduce biologically active elements into a wastewater treatment system is beneficial in assisting the clean-up of the water. Such biological systems require the presence of oxygen to work efficiently. Oxygen is generally introduced by the use of aeration heads in an aeration tank. Such aeration heads are usually either diaphragm diffusers formed with a plurality of perforations or slits in a membrane diaphragm, or sintered or unglazed ceramic diffusers which are inherently porous. Aeration diffusers are widely used in wastewater treatment plants (most commonly municipal sewage treatment plants) to transfer oxygen efficiently into large tanks to support biological activity.

In the most commonly used wastewater treatment process, wastewater is passed through large tanks and brought into contact with an accumulation of naturally growing micro-organisms which feed on the impurities. The mixture of wastewater and biomass is referred to as "mixed liquor", and the process itself is referred to as the "activated sludge" process.

The mixed liquor has a high oxygen demand, so is vigorously aerated. Mixed liquor may then flow into separate settlement tanks, where it separates into settling sludge and clear effluent. Most of the settling sludge is pumped back to the aeration tanks to maintain the required solids concentrations, and the balance is wasted from the system as 'sludge' or 'biosolids' for which there is now a growing market as fertiliser. As an alternative to this, the aeration tank may be aerated only "intermittently", in which case solids separation occurs within one vessel. In intermittent aeration tanks, aeration is typically for 2 hours in 4. One hour is usually enough for solids separation, and one hour is typically allowed to decant the clear surface layer, though other separation and decant periods are commonly used.

Although process needs can be supplied with pure oxygen, or by "surface aerators" that bring air and water into contact by mechanical agitation, more than half of all activated sludge plants built in recent years have used "fine bubble" aeration, in which the aeration medium is air. The smaller the bubbles, the more efficient the oxygen transfer efficiency. This is important, since aeration systems often account for 60-70% of the energy needs of the entire treatment plant.

An aeration "diffuser" is a device for converting low pressure air, delivered to the tank bottom in pipes, into a stream of small bubbles - typically 2-3mm dia at their point of release.

In full-scale wastewater treatment plants, typical diffusers take the form of discs or domes of up to 500mm diameter, or horizontal tubes up to 100mm diameter and 1000mm long. They are usually fixed to grids of plastic or stainless steel pipework arranged over the whole tank floor.

For more than 80 years, the most common form of fine bubble diffuser has been of porous "ceramic" material. Many are of glass-bonded sintered silica (like Carborundum stone), some are made from resin-bonded round-grain sand, fired at lower temperatures. These diffusers are efficient in operation and are long-lasting, provided air supply is not interrupted frequently or for long periods. All fine-pore diffusers suffer from the disadvantage, however, of blocking up with back-flowing biomass if continuity of air supply cannot be maintained.

Starting in about 1980, a new type of fine bubble diffuser began to appear on the market in Germany, called a "membrane" diffuser. In essence, this consists of a sheet of elastomer (typically EPDM rubber 2-3mm thick) perforated with thousands of tiny knife-slits. The membranes are mounted on rigid bodies -usually discs or tubes of similar dimensions to those described above. When air pressure is applied to their underside, the slits open to emit streams of fine bubbles. When air supply stops, the slits close to prevent backflow of liquids. See, for example, U.S. Patents 4,288,394; 5,234,632 and 5,868,971.

Because there are many process advantages in being able to start and stop the air supply, membrane diffusers have been very widely used in recent years, and now out-sell porous diffusers. There are a number of disadvantages, however, in using cylindrical support bodies or discs. With cylindrical bodies, the underside of the membrane contributes poorly to bubble release, and membrane shrinkage problems can lead to excessive backpressures. Disc diameters are effectively limited to about 300mm, and this leads to use of excessive quantities of diffuser elements and pipework that is too closely spaced on the tank floor to be truly economic.

In recent years, attempts have been made to achieve more economic construction by adopting elongated plate designs. Usually, however, these units are small, and use complicated or expensive fixings. See, for example, PCT/AU97/00785 (WO 98/21151) in the name of Aquatec-Maxcon Pty Ltd.

There is a need to reduce installation and maintenance costs by utilising a smaller number of relatively large diffusers with comparatively simple sealing and fastening arrangements.

OBJECT OF THE INVENTION

It is an object of this invention to provide an improved apparatus for aerating a liquid, which goes at least some way towards overcoming or at least minimising the prior art problems or limitations outlined above, and for providing a clear alternative choice for use by wastewater processing or treatment authorities, or the like. It is a further object of this invention to provide an improved apparatus for aerating a liquid, such as domestic and industrial wastewater, said apparatus being relatively inexpensive to manufacture and simple to install, and which is reliable in operation and relatively simple to maintain.

These and other objects of this invention will become more apparent from the following descriptions and drawings.

SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is disclosed a diffuser for aerating a liquid, said diffuser comprising an elongate base and an elastomeric sheet extending over said base and surrounding the periphery of said base to form a compartment between said base and sheet, a sealing means to clamp said sheet to opposite sides of said periphery, and mounting means to permit said base to be mounted above, and substantially transverse to, an air supply feeder pipe, said mounting means permitting air to be introduced into said compartment from said pipe, and said membrane including a plurality of small openings to permit the diffuse escape of said air from said compartment.

According to a second aspect of the present invention there is disclosed system for aerating a liquid in a tank, said system comprising a plurality of parallel arranged feed pipes, each of said feed pipes having a plurality of the abovementioned transversely positioned diffusers, all of said diffusers being substantially co-planar.

According to a third aspect of the present invention there is disclosed a method of sealing an air receiving compartment of a diffuser, said compartment being formed between an elongate base and an elastomeric sheet extending over said base and surrounding the periphery of said base, said method comprising the step of clamping said membrane to opposite sides of said base around said periphery. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The invention will now be described by way of example only and with reference to the accompanying drawings, wherein:-

FIG. 1 is a top or plan elevational view of a portion of an aeration device according to the invention;

FIG. 2 is a side elevational, partly sectioned, view of the apparatus of Fig. 1 ;

FIG. 3 is a sectional elevation of the apparatus of Figs. 1 and 2, taken along the line ffl-πi of Fig. 1; FIG. 4 is an enlarged sectional view of the side clamping or sealing element of the main top and bottom members;

FIG. 5 is a cross-sectional side-elevation of the top elastomeric membrane member 3;

FIG. 6 is a cross-sectional side-elevation of the bottom main member 2;

FIG. 7 is a sectional elevation of the attachments to pipe element 1 illustrated in smaller scale in Fig 2;

FIG. 8 is an upward sectional elevation at line VIII- VIII through attachment spigot element 8 shown in Fig. 7;

FIG. 9 is a part sectional elevation showing alternative fixing details which may be used instead of the equivalent illustrated in Fig. 7; FIG. 10 Shows a side elevation of split plug fixing element 16;

FIG. 11 Shows a side elevation of plug fixing element 17;

FIG 12 is a side view similar to Fig. 7 but showing a further embodiment; and

FIG 13 is a cross-sectional view of the spring bellows seal used in Fig 12.

BEST MODE OF CARRYING OUT THE INVENTION

Referring to Fig. 1, there is shown a circular pipe 1 which conveys air to the diffusers (2,3), and on which the diffusers are mounted. There are grids of these pipes over the whole aeration tank floor, typically at 2.5-3.5m centres. They have their own mountings (not illustrated) which are common enough in the industry, and need no further description here. The pipes typically are of 150-200mm diameter, but could be larger or smaller. They are mounted with all their topsides in the same horizontal plane, typically 150-250mm above the tank floor. The diffusers (2, 3)are also all mounted in the same plane.

The diffuser (2,3) is mounted at right angles to the pipe 1, and only its top member 3 can be seen in the Plan View. Typical dimensions of the top member are 1500mm long by 120- 180mm wide, but these may vary.

Referring to Fig. 2, this sectional elevation shows the diffuser 2,3 mounted above the pipe 1.

As seen in Fig. 3, the flexible membrane 3 is located atop the main support member 2, and attached to it by its two side seals 4 as illustrated in Fig. 4. In the illustration shown, membrane element 3 is shown partly inflated by internal air pressure, to form an interior chamber 6, by air entering through inlet vents connecting the diffuser base 2 to the pipe 1 and hence separated from main support member 2. When air pressure is removed, element 3 ceases to stretch and the two main elements 2,3 are pressed together by external water pressure.

The edge seals and side seals 4 are illustrated in Fig. 4, and in this preferred embodiment are provided in the form of a U-shaped extrusion with internal ribbed teeth. The edge seals can be fabricated from a variety of materials, but preferred embodiments comprise anodized aluminium or a hard plastic such as ABS. The same seals can be used to seal the opposed ends of the diffuser, and the ends of edge and end seals can be mitred at 45° to join neatly with each other.

The rubber membrane 3 folds over the side edges of the diffuser body 2, as shown at 7 in Fig. 5, but can be extruded as a flat sheet and then folded over the edges in situ. The edge seals 4 are then forced over the outer edge of the membrane 3 folded over the outer perimeter edge of diffuser body 2. In use, the teeth gradually form compression grooves in the softer pliable membrane, which eventually become 'permanent' through a process referred to as compression set.

Preferably, the diffuser body is fabricated from stainless steel sheet, and can be rolled or crimped to a slight curve in section, as shown in Fig. 6, to provide some rigidity or strength.

Fig. 5 shows a cross-sectional elevation of membrane element 3, which will be extruded from EPDM, silicone or any other suitable elastomeric material. Its centre section will generally be of a uniform thickness of about 1.5 to 2.0mm, though other thicknesses can be used. It will be perforated over most of its width with thousands of tiny slits cut in it during the latter part of its manufacturing process.

Fig. 6 shows a cross-sectional elevation of the diffuser body or main body support element 2. It is preferably fabricated from stainless steel plate 1-1.5mm thick and rolled or pressed to a curved or similar shape to give stiffness. However, the diffuser body 2 may also be formed from extruded or drawn plastic, or from injection-moulded plastic - in which case it will ideally have a multiplicity of stiffening webs on its underside.

Fig. 7 illustrates a cross-sectional elevation of the attachment for fixing the diffuser 2,3 to the air supply pipe 1. Preferably, the main support element 2 has a dished (recessed) hole in it, through which fixing element 8 will pass. The recess is designed to enable the top flange of element 8 to lie flush with the rest of element 2. The hole may also have slots either side (see Fig. 8) into which small ribs in element 8 will fit to prevent it from rotating.

Fixing element 8 is preferably an injection-moulded plastic cylinder, flanged at its top, and externally threaded in its upper half. As mentioned above, it may also have shallow ribs on either side below the flange to fit the slots in element 2. Three slots 18 are preferably incorporated at equal spacings round the bottom edge, into which stub prongs 19 on element 13 fit to form a 'bayonet-style' fixing. The shape of these slots 18 is illustrated in Fig. 7. Note that in Fig. 7 - for illustrative purposes - the slots 18 are shown at right angles, rather than the intended 120 degree spacings. Elements 10 and 11 are internally threaded fixing nuts. Upper nut 10 is tightened in the upwards direction to clamp fixing element 8 to main body element 2. Thin rubber gasket element 9 is squeezed between elements 2 and 10 to form a seal. Lower nut 11 is tightened downwards to compress synthetic rubber O-ring 12 onto the inclined upper face of element 13. As well as effecting a seal on that face, this tightening action pulls fixing element 8 upwards to draw its slots 18 onto the stubs 19 on element 13.

Elements 10 and 11 may be fabricated from injection moulded plastic, but could be of die-cast metal alloy. Lower nut 11 may be made of larger diameter than upper nut 10 so that the tightening device (spanner or special tool) used on it will not loosen upper nut 10, which will most likely have been factory-tensioned off-site.

Fixing element or housing 13 is of injection moulded plastic, and may be glued directly onto the air supply pipe 1. Its upper part is cyhndrical to form a snug fit with (internal) element 8. Three stub prongs 19 are moulded at equal spacings round the inside of the upper part of element 13 to engage with the slots 18. A hole in the lower part of element 13 will convey air from supply pipe 1 to the diffuser.

Non-return valve fitting 14 is forced from above through the hole in element 13 and a similar sized hole drilled in air-supply pipe 1. The non-return element 15 preferably is in the form of a stemmed flap and spring, and has not been fully detailed here.

Fig. 8 shows an underside sectional view taken along the line VIII- VIII of Fig. 7 of the flange and ribs in fixing element 8.

Fig. 9 shows an alternative detail for fixing element 13 to air supply pipe 1, which may eliminate the need for gluing. Split plug element 16 is first pushed through the holes in element 13 and supply pipe 1. Plug 17 is then forced into element 16 from above, forcing it to expand and tightly grip both holes. Non-return valve 15 can also be fitted to element 17, but has been omitted from this figure for clarity. Fig. 10 shows a side elevation of expanding plug element 16, which has three or four slots in its side walls, and a split flange round its bottom ring. This element is preferably injection moulded from plastic, but could be of die-cast metal alloy.

Fig. 11 shows a side elevation of solid plug element 17, which has an air hole through its center, and a slightly tapered ribbed external surface. This element may be made either of injection moulded plastic or of die-cast metal alloy.

Turning now to Fig. 12, an alternative arrangement to that of Fig. 7 is illustrated for interconnecting the diffuser 2 to the pipe 1. The upper nut 10 and gasket 9 are as before as are the prongs 19 and slots 18. However, the lower nut 11 and O-ring 12 are replaced by a spring bellows 20 which is formed by a pair of annular washers 21, 22 (Fig 12), a helical spring 23 and a flexible cylindrical membrane 24 which extends between the washers 21, 22. As seen in Fig 12 the bellows 20 is compressed between the nut 10 of the diffuser 2 and the upper rim of the element 13. Yet irrespective of the degree of compression of the bellows 20 and its spring 23, an adequate seal is maintained between the diffuser 2 and pipe 1.

All of the above features combine to yield a diffused air aeration device that will be less expensive to manufacture and install than currently available diffused air aeration devices, and which uses considerably smaller quantities of diffuser elements and pipework for the same air flows and oxygen delivery. The diffusers are arranged in a tank in substantially the same way as illustrated in Fig. 8 of the abovementioned PCT specification WO98/21151, the contents of which are hereby incorporated herein by cross-reference.

Some of the advantages of the present invention include:-

The flat elongate shape of the diffusers gives a much more economic solution to aeration than current small disc type diffusers, with very much less supporting pipework. The relatively flat top surface gives better performance than cylindrical diffusers, the curved top surfaces of which lead to bubble interference and coalescence. The stainless steel body is relatively inexpensive to manufacture, and unlikely to suffer corrosion effects (anodised aluminium is suspect in this regard, especially if scratched). The plain membrane is less expensive to make than profiled extrusions or mouldings. The edge seal is simple and durable. Utilisation of compression-set grooves to retain the seal in place and prevent leakage is believed unique in this application.

The pipe attachment provisions allow very speedy (and therefore economical) yet secure installation and maintenance on site.

Although some exemplary embodiments of the present invention has been shown and described, it will be apparent to those having ordinary skill in the art that a number of changes, modifications or alterations to the invention described herein may be made, none of which depart from the spirit of the present invention. All such changes, modifications and alterations should therefore be seen as being within the scope of the present invention. For example, the spring 23 and membrane 24 can be replaced by concertina folded stainless steel thin-walled tube. Similarly, the spigot or element 8 can be on the pipe 1 and the recess or element 13 can be on the diffuser 13.

It should be appreciated that the present invention provides a substantial advance in diffusion apparatus for aerating a liquid, such as wastewater, providing all of the herein described advantages without incurring any relative disadvantage.

The term "comprising" (and its grammatical variations) as used herein is used in the inclusive sense of "having" or "including" and not in the exclusive sense of "consisting only of.

Claims

1. A diffuser for aerating a liquid, said diffuser comprising an elongate base and an elastomeric sheet extending over said base and surrounding the periphery of said base to form a compartment between said base and sheet, a sealing means to clamp said sheet to opposite sides of said periphery, and mounting means to permit said base to be mounted above, and substantially transverse to, an air supply feeder pipe, said mounting means permitting air to be introduced into said compartment from said pipe, and said membrane including a plurality of small openings to permit the diffuse escape of said air from said compartment.

2. The diffuser as claimed in claim 1 wherein said base is substantially planar.

3. The diffuser as claimed in claim 2 wherein said base slightly curved transverse to its longitudinal axis to increase the stiffness of said base.

4. The diffuser as claimed in any one of claims 1 to 3 wherein said clamping means comprises a strip of substantially U-shaped transverse cross-section which extends along said periphery.

5. The diffuser as claimed in claim 4 wherein said strip has interior ribs comprising teeth which, in use, form compressions grooves in said membrane.

6. The diffuser as claimed in any one of claims 1 to 5 wherein said mounting means comprises a spigot on one of said diffuser and pipe and a cylindrical recess on the other of said diffuser and pipe, said spigot and recess having a bayonet type releasable engagement means.

7. The diffuser as claimed in claim 6 wherein said spigot and recess are sealed by an O- ring and nut, said nut being threadably engaged with said spigot and in use compressing said O-ring between said nut and said cylindrical recess.

8. The diffuser as claimed in claim 6 wherein said spigot and recess are sealed by a spring bellows surrounding said spigot and compressed between said diffuser and said recess.

9. A system for aerating a liquid in a tank, said system comprising a plurality of parallel arranged feed pipes, each of said feed pipes having a plurality of transversely positioned diffusers as claimed in any one of claims 1 to 8, all of said diffusers being substantially co- planar.

10. A method of sealing an air receiving compartment of a diffuser, said compartment being formed between an elongate base and an elastomeric sheet extending over said base and surrounding the periphery of said base, said method comprising the step of clamping said membrane to opposite sides of said base around said periphery.

11. The method as claimed in claim 10 including the step of using a strip of substantially U-shaped transverse cross-section to clamp said membrane.

12. The method as claimed in claim 11 including the step of forming interior ribs comprising teeth on said strip.