GB2537893A

GB2537893A - Submersible diffuser

Info

Publication number: GB2537893A
Application number: GB1507381.0A
Authority: GB
Inventors: O'hara Kenneth
Original assignee: FLOWTHROUGH Ltd
Current assignee: FLOWTHROUGH Ltd
Priority date: 2015-04-30
Filing date: 2015-04-30
Publication date: 2016-11-02
Also published as: GB201507381D0

Abstract

The invention relates to a submersible diffuser 30 for aeration of a body of water which may for example be a fresh water environment such as a pond, lake or river, or a sea water environment such as a coastal bay. The diffuser comprises a flexible gas diffusion membrane 42 having multiple openings 44 for release of gas. The diffusion membrane defines a plenum chamber (36, Fig. 3) having an inlet 38 for supply of pressurised gas and is carried upon a base 32. The gas diffusion membrane has a pre-formed non-flat shape by virtue of which it is able to adopt (i) a first configuration in which the diffusion membrane is outwardly convex and (ii) a second configuration in which the diffusion membrane is outwardly concave, and to make a reversible transition between the first and second configurations in response to variation of gas pressure in the plenum chamber. A method of aerating a body of water by submerging at least one diffuser in the body of water is also claimed.

Description

Submersible Diffuser The present invention is concerned with aeration of bodies of water, and more specifically with a submersible diffuser for use in that context and with its method of operation.

There are various reasons for carrying out aeration of a body of water.

Air (or sometimes another gas such as oxygen enriched air -and the term "aeration" as used throughout the present description and claims should be understood to refer to processes carried out not only with air but also with other gases) is introduced into wastewater during treatment. During the activated sludge treatment process in particular air is dissolved into mixed liquor (comprising wastewater and biological mass, typically sewage) promoting development of a biological floc which reduces the organic content. Aeration can also used for air stripping of volatile organic contaminants from water.

In open bodies of water such as ponds, lakes, lagoons and coastal bays, aeration can serve several purposes. One is to maintain or increase levels of dissolved oxygen, which is an important factor in the health of the aquatic environment. Human activities such as sewage discharge or agricultural run-off can deplete levels of dissolved oxygen. Fish and other aquatic animals are dependent on dissolved oxygen and so too are aerobic bacteria which help to maintain water quality by digesting organic matter. By raising the dissolved oxygen level, aeration can help to maintain the quality of the aquatic environment. Aeration can also promote circulation of water and consequent mixing through the water column.

A known process of aeration involves release of air into a body of water through a diffuser. The body of water in question may for example be a waste water treatment tank or lagoon for sewage or effluent treatment, or it may be a fresh water environment such as a pond, lake or river, or a sea water environment such as a coastal bay.

Diffused aeration systems require a supply of the relevant gas, such as from a compressor or air pump, at least one submerged diffuser to release the gas into the body of water in a controlled manner, and a conduit for conducting the gas from the supply to the diffuser. The properties of the diffuser itself are important. Since aeration is energy intensive, maximising efficiency is of major commercial and environmental importance. One relevant factor is bubble size and a distinction is sometimes drawn between "fine bubble" and "coarse bubble" diffusers. A stream of fine bubbles can have -for a given throughput of gas -a relatively large mixing effect, and is provided by using a diffuser with a large number of small openings or pores for release of gas.

Fine bubble diffusers produce a plethora of very small air bubbles which rise slowly through the water and provide substantial and efficient mass transfer of oxygen to it. In the case of sewage treatment, the oxygen, combined with the food source, sewage, allows the bacteria to produce enzymes which help break down the waste so that it can settle in secondary clarifiers or be filtered by membranes.

An example of an existing fine bubble diffuser 10 is represented, in somewhat simplified form, in Figure land comprises a flexible membrane 12 which provides a large number of fine outlet passages 14 for release of gas. The outlet passages 14 are formed by punching or cutting tiny slots or holes through the membrane. In the illustrated embodiment the outlet passages 14 are arranged in concentric circles, only a few of which are shown in Figure lb for the sake of representational simplicity although in practice the full area of the membrane may be perforated in this manner. The membrane in this example has an integral "0" ring 16 around its periphery which is received between annular upper and lower housing parts 18, 20, forming a seal with them. The membrane 12 lies over a flat base plate 22 of the lower housing part 16 and forms with it a plenum chamber 24 for receiving a supply of gas through an inlet 26. When the plenum chamber is pressurised by the gas supply the membrane 12 stretches and deforms into the somewhat domed shape seen in Figure lc. It is found that the outlet passages enlarge to some degree as gas flow increases.

Another example of a known fine bubble diffuser 410 is represented in somewhat stylised form in Figure 7 and comprises a flexible membrane 412 which in this example is of tubular form and has outlet passages 414 provided in an upper portion only. The flexible membrane 412 is carried on a base 422 formed as a relatively rigid tube. Band clamps 450, 452 secure the flexible membrane 412 to the base 422 at respective ends and define a plenum 424 between the outer face of the base 422 and the inner face of the flexible membrane 412. The depth of this plenum is somewhat exaggerated in the drawing for clarity. Air input through an end plug 454 passes through radial apertures in the base 422 to reach the plenum 424 whence it bubbles through the outlet passages 414.

Clogging and fouling of diffuser outlet passages is a long recognised technical challenge. Contaminants build up on the diffuser and can occlude or plug its outlets, impairing performance. In principle contaminants from both the gas side (within the plenum chamber 24/424, in the illustrated examples) and the water side (i.e. from the aquatic environment) can be involved. Forms of fouling from the water side are diverse and differ from one type of environment to another but include (a) microfouling such as biological slime, chemical precipitates including calcium carbonate, bacterial adhesion and biofilm formation and (b) macrofouling by larger organisms Obtaining access to diffusers for cleaning is typically troublesome due to the environments in which they are used so some means to cause fouling to be shed, without need of such access, is highly desirable. There are various approaches to this including for example in-situ acid cleaning, but one known method used with the membrane-type diffuser depicted in Figure 1 is to shut off air supply to the diffuser, allowing the membrane 12 to collapse onto its base plate 22. The membrane may be "bumped" by alternately applying and exhausting gas pressure to the diffuser. The gas pressure used in bumping may exceed the norm, and where the number of diffusers is large they may be connected in groups so that the members of one group at a time can be bumped, avoiding any need to over-specify the gas supply in order to obtain the pressure and flow rate required for bumping. The process causes flexure and movement of the membrane 12, 412 and can serve to reduce fouling of it.

Nonetheless fouling remains a technically and commercially significant problem in this field. Improvements in the capacity of the diffuser head to resist or shed fouling are highly desirable.

According to a first aspect of the present invention there is a submersible diffuser for aeration of a body of water, the diffuser comprising a flexible gas diffusion membrane having multiple through-going openings for release of gas, the diffusion membrane defining a plenum chamber having an inlet for supply of pressurised gas, being carried upon a base, and having a pre-formed non-flat shape by virtue of which it is able to adopt (a) a first configuration in which the diffusion membrane is outwardly convex and (b) a second configuration in which the diffusion membrane is outwardly concave, and to make a reversible transition between the first and second configurations in response to variation of gas pressure in the plenum chamber.

The transition between the first and second configurations can provide a gross change in shape of the gas diffusion membrane and hence promote shedding of fouling in a particularly effective manner.

The term "pre-formed" denotes that the membrane will adopt the non-flat shape when not subject to external force. Of course the membrane 12 seen in Figure 1 can be formed into a domed shape by gas pressure, but it is not pre-formed in this shape since it returns to a flat shape when the gas pressure is relieved. The domed shape seen In Figure 1c can only be formed due to the ability of the membrane 12 to stretch.

The non-flat shape adopted by the gas diffusion membrane according to the present invention when external forces are not applied to it may be that of the outwardly convex first configuration, or of the outwardly concave second configuration, or it may be able to rest stably in either. The transition between the first and second configurations is reversible in the sense that following the transition from the first state to the second, the membrane can undergo a reverse transition to return it to the first state.

The pre-formed non-flat shape of the gas diffusion membrane may be a curve. Specifically (but without limitation) it may be part-spherical, frusto-conical, bulbous or part-cylindrical. The term "membrane" does not imply that this part need be especially thin or insubstantial.

It may for example be formed with sufficient stiffness as to snap between its first and second configurations. It may be formed such that it will remain in its current configuration until acted upon by a force to cause a transition from one configuration to the other. It preferably comprises elastic material -natural or synthetic rubber may be used.

The gas diffusion membrane may be resiliently biased toward the second configuration, so that it adopts this configuration when pressure in the plenum chamber is relieved and is caused to make the transition to its first configuration by application of pressure in the plenum chamber. This resilient biasing of the gas diffusion membrane may be due to the formation of the membrane itself, or it may be provided by some additional component. For example an elastic tendon may act on the membrane, being stretched when the membrane is in its first configuration and so pulling the membrane toward the second configuration.

In some embodiments the gas diffusion membrane is an integral part of an envelope of flexible material which forms the entire plenum chamber. The envelope may be substantially spherical. This construction can be attractively simple and robust, without any need to provide the gas diffusion membrane with a peripheral seal.

Certain embodiments of the invention comprise a base which serves to mount the gas diffusion membrane, the base being shaped to accommodate the gas diffusion membrane in both its first and its second configurations. The base may have a concave surface facing toward the gas diffusion membrane which is shaped to accommodate the gas diffusion membrane in its second configuration. The plenum chamber may be defined between the base and the gas diffusion membrane. The gas diffusion membrane can be provided with a peripheral seal through which it seals with and is mounted upon the base.

Foreign material including condensed water may enter the plenum chamber. In some embodiments a lower region of the plenum chamber is provided with exhaust passages though which liquid in the plenum chamber is able to be exhausted.

The present invention further provides a method of aerating a body of water comprising submerging at least one diffuser as claimed in any preceding claim in the body of water and supplying pressurised gas to them, wherein at intervals gas pressure is relieved and then restored to cause the gas diffusion membrane of the diffuser to transition from its first state to its second state and then back to its first state.

Specific examples of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:-Figure 12 is a section in an axial plane through a membrane type gas diffuser belonging

to the prior art;

Figure lb is a view of the gas diffuser of Figure la from above; Figure lc corresponds to Figure la but shows the gas diffuser in use, its membrane being domed by virtue of internal gas pressure; Figure 2a shows a first gas diffuser embodying the present invention viewed from one side, a base part of the diffuser being shown in section in order to reveal a gas receiving envelope partly contained within it; Figure 2b is a view of the first gas diffuser of Figure 2a from above; Figure 3 corresponds to Figure 2a but shows a gas diffusion membrane in an outwardly concave configuration and in section; Figure 4 is a section in an axial plane through a second gas diffuser embodying the present invention; Figure 5 shows a third gas diffuser embodying the present invention viewed from one side, a base part of the diffuser being shown in section in order to reveal a gas receiving envelope partly contained within it; Figure 6 is a section in an axial plane through a fourth gas diffuser embodying the present invention; Figure 7 is a section in an axial plane through a further gas diffuser belonging to the prior art; Figure 8a is a section in an axial plane through a fifth gas diffuser embodying the present invention; Figure 8b is a section through the fifth gas diffuser in a radial plane indicated in Figure 8a by arrows A-A; and Figure 9 is a section in an axial plane through a sixth gas diffuser embodying the present invention.

Figures 2a, 2b and 3 depict a membrane type gas diffuser 30 which embodies the present invention and is for use in aerating a body of water. It comprises a base 32 and a gas receiving envelope 34 which in this embodiment is formed by a substantially spherical skin of flexible material. Within the envelope 34 is formed a plenum chamber 36 (see Figure 3 in particular) to which a flow of gas is supplied in operation through an inlet 38 at the bottom of the envelope 34. A lower portion 40 of the envelope 34 is secured in the base 32 (in the illustrated example this portion is the lower hemisphere of the envelope 34). An upper portion of the envelope 34 is exposed to the exterior and is penetrated by multiple gas diffusion passages 44 to form a gas diffusion membrane 42.

In accordance with an aspect of the present invention the gas diffusion membrane 42 has a pre-formed curvature. That is, when not subject to a net pressure difference or other external force, the shape it naturally adopts is curved rather than flat. In this respect the gas diffusion membrane 42 can be contrasted with the membrane 12 of the prior art diffuser depicted in Figure 1, which is naturally flat and becomes curved only when stretched due to gas pressure.

Further, the gas diffusion membrane 42 of the present embodiment has two configurations which can be referred to as "natural", in the sense that the membrane can rest in either without being crumpled or significantly stretched. The first configuration is depicted in Figure 2a, and will be referred to as "outwardly convex" since in this configuration the outer surface 46 of the gas diffusion membrane 42 has a convex curvature, which in this example is hemispherical. The second configuration is represented in Figure 3, where the gas diffusion membrane 42 has been inverted, forming an outwardly concave configuration, which in this example is once more generally hemispherical.

It will be apparent that gas pressure in the plenum chamber 36 urges the gas diffusion membrane 42 to adopt the outwardly convex configuration. External hydrostatic pressure or other factors (to be considered below) can cause it to make a transition to the outwardly concave configuration. Hence by control of the gas pressure supplied to the diffuser 30, the outer surface 46 of the gas diffusion membrane 42 can be caused to undergo a gross change in its shape, from convex to concave and vice versa. This shape change and the associated motion, as well as the corresponding changes of gas flow rate through the gas diffusion passages 44, can serve to shed fouling from the gas diffusion membrane 42 and so to maintain its performance.

The gas diffusion membrane 42 needs to be sufficiently flexible to make the transition between its convex and concave configurations. It may be formed with sufficient rigidity that this transition takes place rapidly, the membrane flipping or snapping elastically from one state to the other. Alternatively it may comprise more flexible material able to crumple somewhat during this transition, which thus takes place more slowly. In the illustrated example the gas diffusion membrane 42 comprises rubber or synthetic rubber and is thus able to stretch elastically, although the present invention makes it possible to use membranes which have little elasticity.

In Figure 2b the gas diffusion passages 44 are seen to be arranged in concentric circles. A large number of these is provided so that the diffuser emits a correspondingly large number of small bubbles. The passages are formed in this embodiment by punching through the gas diffusion membrane 42. Due to the elasticity of the material of the membrane, the gas diffusion passages 44 are able to enlarge somewhat in response to gas pressure increase in the plenum chamber 36, which can enable increased rates of flow when necessary and may also assist in alleviating fouling or plugging of the passages. The number, arrangement and formation of the gas diffusion passages may however vary greatly from one embodiment to another.

There is the possibility of condensation or other water fluid inside the plenum chamber 36.

Where the gas supplied is air, it will in some cases be drawn by a compressor from the atmosphere, and may consequently contain water vapour and be at a higher temperature the submerged diffuser 30. In this case cooling of the air in the aeration system can cause water to condense from it. To expel condensate and possibly other contaminants from the plenum 36, the envelope 34 is provided in a lower region with exhaust passages 48. The size of these is somewhat exaggerated in the drawings for the sake of representational clarity -they are in practice small enough that they do not prevent a required pressure from being attained in the plenum 36. Condensate can collect in the lower region of the envelope 34 and thus be driven out through the exhaust passages 48.

In operation, the illustrated gas diffuser 30 will normally be maintained in the outwardly convex configuration in which it is represented in Figure 2b, with a flow of gas being supplied to pressurise the plenum chamber 36 above the hydrostatic pressure of the surrounding water and cause emission of bubbles through the gas diffusion passages 44, to aerate and mix the surrounding water as previously described. Periodically the gas supply is cut off, causing gas pressure in the plenum 36 to be reduced and the gas diffuser membrane 42 to make the transition to its outwardly concave state, with fouling consequently being shed at least to some degree. Gas pressure may be repeatedly applied and cut off to cause the gas diffuser membrane 42 to repeatedly invert between its convex and concave states. A greater than normal gas pressure or gas volume may be applied during this process to enhance shedding of fouling.

When the gas supply is cut off and the gas diffusion membrane 42 is in its outwardly concave state it may, as seen in Figure 3, cover the inlet 38 and thus serve as a non-return valve for the inlet. Additionally or alternatively a non-return valve may be provided in the inlet 38. Any fluid present in the plenum chamber 36 when the gas supply is cut off can be vented through the exhaust passages 48.

In the embodiment of Figures 2a to 3 a single component -the envelope 34 -serves both to form the gas diffuser membrane 42 and to define the entire plenum chamber 36. This is advantageous in its constructional simplicity and in that it obviates any need for a seal at the periphery of the gas diffuser membrane 42. Nonetheless various other forms of construction are possible without departing from the scope of the present invention and one such is illustrated in Figure 4, where the plenum chamber 136 is formed between gas diffuser membrane 142 and base 132. In this embodiment the gas diffuser membrane 142 is provided with a peripheral seal 150, formed as a circular "0" ring which is integral with the membrane itself. The base 132 is hemispherical and its periphery forms a channel 152 in which the seal 150 is received. An annular collar 154 attached to the base secures the seal 150 in position. As in the first embodiment the gas diffusion membrane 142 (whose gas diffusion passages are omitted from Figure 4 in the interests of representational simplicity) can make a transition between the outwardly convex configuration in which it is represented in the drawing to an outwardly concave configuration in which it lies largely or wholly within the base 132.

The shape of the base 132 complements that of the gas diffusion membrane 142 and in particular it provides space for the gas diffusion membrane 142 to adopt its outwardly concave configuration, being hemispherical. It is not however vital that the shape of the base should precisely reflect that of the membrane -it could for example be flat bottomed.

Figure 5 represents a constructional variant of the diffuser of Figures 2a to 3. In this version envelope 234 is provided with an integral mounting flange 260 running around its equator. The flange is secured to an upper edge of base 232 -e.g. by means of bonding or by some mechanical means -and provides a convenient way to mount the envelope 234.

A further constructional variant is represented in Figure 6. Here both gas diffusion membrane 342 and base 332 are formed from a resilient material such as rubber. The base 332 is more rigid than the gas diffusion membrane 342-the membrane is intended to move between its two configurations while the base 332 is undergoes little or no change in shape -and to this end the base 332 is relatively thick-walled while the gas diffusion membrane 342 is thinner.

These two parts could be formed by a single moulding, but in the illustrated embodiment they are separate components joined to one another through flanges 360, which are shown slightly separated from one another in the drawing but would be in intimate contact in the assembled diffuser. The flanges 360 may be bonded to one another and/or they may be shaped to locate and engage with one another, e.g. through a projecting lip on one (not shown) to receive the other.

The embodiments described above all have a hemispherical gas diffusion membrane 42, 142, 242, 342 but other shapes are possible. For example this component could be part-spherical (i.e. a greater or lesser part of a sphere than a hemisphere) or it could be frusto-conical, semi-cylindrical or tetrahedral.

Examples of embodiments having a cylindrical gas diffusion membrane 542, 642 are provided in Figures 8 and 9. Figure 8 represents a gas diffuser 530 having an inflexible base 532 formed as a round tube. The gas diffusion membrane 542 is formed as a flexible round tube surrounding the base 532. An end plug 550 closes one end of the base 532 and band clamps 552 and 554 retain the gas diffusion membrane in position upon the base 532, and resist egress of gas at respective ends of the gas diffusion membrane 542. The base 532 is cut away at 556 to leave in that vicinity a substantially semi-circular portion 558 (see Figure 8b in particular). An inlet 538 is formed through a second end plug 560 and while gas is supplied through it the gas diffusion membrane 542 is maintained in the outwardly convex, cylindrical configuration seen in the drawings. When the gas supply is cut off, the gas diffusion membrane is able to collapse into the semi-circular portion 558 forming an outwardly concave configuration, shedding fouling in the process.

The gas diffuser 630 represented in Figure 9 is somewhat similar to that of Figure 8, having an inflexible semi-tubular base 632 which is semi-circular shaped in section, and a flexible tubular gas diffusion membrane 642. End plugs 650, 660 sit in the base 632 and straps or band clamps 662 secure the membrane 642 sealingly against the plugs and the end plugs 650, 660 to the base 632.

It was pointed out above that the transition of the gas diffusion membrane 42, 142, 242, 342, 542, 642 to its outwardly concave state may be caused by excess hydrostatic pressure, following exhaustion of gas pressure from the diffuser, but that other means may be provided to cause the transition. In some embodiments, the gas diffusion membrane is resiliently biased toward the concave configuration. This may be because the gas diffusion membrane itself is formed in such a way that it will adopt its concave configuration of its own volition, when not maintained in the convex configuration by gas pressure. Alternatively some elastic element may be provided to cause the membrane to invert.

Gas diffusers embodying the present invention may advantageously be supplied with gas via a flow regulator, which assists in ensuring distribution of gas to all diffusers in an installation despite variations in height between one diffuser and another. For details of a suitable flow regulator and of a diffuser installation using it, reference is made to published application US2005/0161409A1 (patent number US7074328).

Claims

CLAIMS1. A submersible diffuser for aeration of a body of water, the diffuser comprising a flexible gas diffusion membrane having multiple through-going openings for release of gas, the diffusion membrane defining a plenum chamber having an inlet for supply of pressurised gas, being carried upon a base, and having a pre-formed non-flat shape by virtue of which it is able to adopt (a) a first configuration in which the diffusion membrane is outwardly convex and (b) a second configuration in which the diffusion membrane is outwardly concave, and to make a reversible transition between the first and second configurations in response to variation of gas pressure in the plenum chamber.
2. A diffuser as claimed in claim 1 in which the pre-formed non-flat shape is curved.
3. A diffuser as claimed in claim 1 in which the gas diffusion membrane is pre-formed in a shape which is part-spherical, frusto-conical, bulbous, part-cylindrical or cylindrical.
4. A diffuser as claimed in claim 1 or claim 2 in which the gas diffusion membrane is formed such as to snap between its first and second configurations.
5. A diffuser as claimed in any preceding claim in which the gas diffusion membrane is formed such that it will remain in its current configuration until acted upon by a force to cause a transition from one configuration to the other.
6. A diffuser as claimed in any of claims 1 to 3 in which the gas diffusion membrane is resiliently biased toward the second configuration, so that it adopts this configuration when pressure in the plenum chamber is relieved and is caused to make the transition to its first configuration by application of pressure in the plenum chamber.
7. A diffuser as claimed in any preceding claim in which the gas diffusion membrane comprises natural or synthetic rubber.
8. A diffuser as claimed in any preceding claim in which the gas diffusion passages are slits cut in the membrane.
9. A diffuser as claimed in any preceding claim in which the gas diffusion membrane is an integral part of an envelope of flexible material which forms the entire plenum chamber.
10. A diffuser as claimed in claim 9 in which the envelope is substantially spherical.
11. A diffuser as claimed in claim 9 or claim 10 in which the base maintains a portion of the envelope in a hemispherical or part-spherical shape, the remainder of the envelope forming the gas diffusion membrane.
12. A diffuser as claimed in any of claims 1 to 8 in which the base is shaped to accommodate the gas diffusion membrane in both its first and its second configurations.
13. A diffuser as claimed in claim 11 in which the plenum chamber is defined between the base and the gas diffusion membrane.
14. A diffuser as claimed in any preceding claim in which the base has a concave surface facing toward the gas diffusion membrane which is shaped to accommodate the gas diffusion membrane in its second configuration.
15. A diffuser as claimed in any preceding claim in which the gas diffusion membrane is provided with a peripheral seal through which it seals with and is mounted upon the base.
16. A diffuser as claimed in any preceding claim in which a lower region of the plenum chamber is provided with exhaust passages though which liquid in the plenum chamber is able to be exhausted.
17. A diffuser as claimed in any of claims 1 to 9 in which the base is tubular and is cut away to enable the gas diffusion membrane to adopt the second configuration.
18. A diffuser as claimed in claim 17 in which the gas diffusion membrane is tubular.
19. A method of aerating a body of water comprising submerging at least one diffuser as claimed in any preceding claim in the body of water and supplying pressurised gas to them, wherein at intervals gas pressure is relieved and then restored to cause the gas diffusion membrane of the diffuser to transition from its first state to its second state and then back to its first state.
20. A submersible gas diffuser substantially as herein described with reference to, and as illustrated in, any of the following: Figures 2a, 2b and 3; Figure 4; Figure 5; Figure 6.