The invention relates to an aeration diffuser for small bubble aeration comprising a gas supply means and a membrane.

Background of the invention.

Aeration diffusers convert a gaseous flow into a stream of small bubbles in a fluid. Diffusers of this type are for example used to aerate water, for instance in biological waste water treatment plants, or for artificial respiration, for instance to improve living conditions in lakes.

In order to generate bubbles, an air flow can be passed through a ceramic material. This material has however the disadvantage that it is quickly fouled and clogged up, for example by grit or biomass which enters the pores of the ceramic material.

Interruption in the air flow has a detrimental effect since it increases the risk of clogging up the diffusers.

Moreover, ceramic membranes are quite brittle and difficult to clean, which adversely influences the durability of these diffusers.

An alternative way to generate bubbles is by passing an air flow through an air supply structure covered by a perforated elastomeric membrane, for example a rubber membrane, as described in W098/21151. The membrane is lifted by the pressure of the air and the air flow is thereby forced through fine slits in the membrane.

When the air supply is turned off, the membrane collapses back onto its support. During the collapsing of the membrane, there is a high risk that the membrane clogs up.

In order to reduce the clogging up effect, the pores may be made smaller, but as a consequence thereof, the energy loss and inefficiency in forcing the air through the pores increases.

A further drawback of an elastomeric membrane is that cleaning is at the risk of tearing the membrane.

Since cleaning and replacing of the membranes is a time-consuming and an expensive operation, the frequency of cleaning has to be as low as possible.

On the other hand, if fouled membranes are not cleaned or replaced by new membranes in time, the efficiency decreases considerably.

Maintenance is thus even more intensive if efficiency must be kept within reasonable limits.

US 3,490,902 describes the use of metal fibers for diffuser membranes.

Since the membrane of metal fibers has only a low strength, it has to be reinforced by means of at least one pervious metal sheet.

However, this kind of reinforcing layers induces the effect of coalescence of fine bubbles. Consequently, the use of this type reinforced diffuser membranes avoids as such that small bubbles are released.

Summary of the invention.

It is an object of the present invention to provide an aeration diffuser with a high efficiency and with a decreased risk of clogging up.

It is another object to provide a membrane having a sufficient strength so that reinforcing layers at the flow out side can be avoided.

It is another object to provide an aeration diffuser with a mechanical and chemical inert membrane, which can easily and repeatedly be cleaned and which therefore has a high durability and a long life time.

It is still a further object to provide an aeration diffuser which generates small bubbles, whereby all bubbles generated over the entire surface of the diffuser membrane are homogenous in diameter and whereby the diameter of the bubbles is constant during the whole period of aeration.

According to a first aspect of the invention, an aeration diffuser for small bubble aeration is provided. The diffuser comprises a gas supply means, a porous membrane and a frame for fastening and sealing the membrane.

The membrane is non-reinforced at its flow out side and comprises at least one layer of metal fibers.

The frame holds the membrane in a watertight and an airtight manner.

The frame is preferably welded to the membrane.

When air or another gas is blown via the gas supply means, which can be provided with a nozzle, no air passes through the seam at the periphery of the membrane, but all air passes through the porous membrane. Small bubbles are thereby generated at the other side of the membrane.

The membrane comprises at least one layer. Each layer comprises a web of fibers, preferably metal fibers.

The porosity of the membrane is higher than 50 %, more preferably the porosity is higher than 60 %, and most preferably higher than 75 %. The membrane has at the flow out side pores with a size lower than 20 pm.

Preferably the pores have a size lower than 10 um, for instance 5 pm. The pore size of the layer at the flow out side, has a direct influence on the diameter of the bubbles. Small pore sizes are thus desired in order to generate small bubbles.

Furthermore, small pore sizes prevent the growth of organisms in the pores.

The bubbles generated by the aeration device according to the present invention have a diameter preferably between about 0.2 mm and about 5 mm.

The porous membrane as such has already a certain strength. This strength is further improved by the way in which this membrane is fastened by means of the frame. Reinforcing layers as described in US 3,490,902 are not necessary.

This is of particular importance, since-as described above-the pore size of the layer at the flow out side has a direct influence on the size of

the bubbles. A reinforcing layer at the flow out side as described in US 3,490,902 does not allow to obtain small bubbles.

In one embodiment, the porous membrane comprises one layer of metal fibers which has been sintered and compacted. The diameter of the fibers is between 2 and 22 pm and is preferably lower than 12 um, for instance 6.5 pm.

In another embodiment, the membrane comprises at least two layers of metal fibers. The metal fibers of the first layer, at the flow in side of the air have a diameter between 4 and 12 pm, for example between 4 and 6.5 pm, whereas the fibers of the second layer have a diameter between 2 and 4 um. The second layer is brought into contact with the first layer at the flow out side of this first layer to form a layered membrane.

The layered membrane, comprising the layers of metal fibers, is sintered and compacted.

The compacting is preferably done by a cold isostatic pressing operation, since this allows obtaining a homogeneous bubbling of the membrane. Such a membrane has uniform pore size over the entire surface and features the advantage that all bubbles generated over the entire surface have substantially the same size.

The membrane may further comprise a third layer, in contact with the second layer. The fibers in this third layer have preferably a diameter between 4 and 12 pm, and more preferably between 6.5 and 12 pm.

Possibly, the membrane may be supported at the flow in side of the gas.

This can for example be achieved by fixing a mesh to the membrane as support layer at the flow in side or, in an alternative way, between two consecutive layers of the membrane.

The membrane is preferably mounted, by means of the frame, in such a way that it is not substantially lifted when the air supply is turned on.

This means that the air flow is almost not inflating the membrane away from its original position, the position before the air flow was switched on.

'Almost not inflating'means that preferably there is no variance between the position before and after the switching on of the air supply. In some cases, a variance of a few millimeter, for example 2 mm has been observed.

Since the membrane is almost not inflating away from its original position, the membrane does not collapse back onto the support when the air flow is switched off.

The variance between the position before and after the switching on of the air supply is preferably as low as possibly ; in some embodiments a variance of a few millimeter, for example 2 mm has been observed.

In one embodiment, the membrane is fixed in such a way as to form a flat surface. Even if the air supply is turned on, the membrane maintains its flat, plane surface. Occasionally, there can be some smaller irregularities in the plane surface of the membrane.

In another embodiment the membrane is mounted in such a way that it forms a curved surface, for example by placing it onto a curved base plate.

Also in the latter embodiment, the membrane remains in the same position, either if the air supply is switched on or off.

The membrane may be coated, for example at the flow out side, either with an organic or an inorganic coating layer. The inorganic coating layer may comprise oxide and/or carbide particles, such as Al203, ZrO2 and/or TiOz particies or SiC deposited on the membrane and sintered thereto.

By depositing such a coating, the sizes of the pores are further reduced, which has a positive influence on the size of the generated bubbles.

The efficiency of an aerator diffuser can be defined as the mass of oxygen transferred per unit of energy needed for the generation of the bubbles.

Therefore, the oxygen transfer is preferably as high as possible, while the utilise energy is preferably as low as possible.

To obtain a high oxygen transfer, small bubbles are required.

Smaller bubbles have a larger specific surface area for oxygen transfer into the liquid. Moreover, since smaller bubbles rise more slowly through the liquid than bigger ones, the time during which oxygen transfer can occur is much longer.

Since the membrane is characterised by a high permeability, and as a consequence thereof by a low pressure drop, the required energy is low.

As the membrane remains unchanged and preferably plane from the switching on of the air supply until the switching off, the diameter of the bubbles is constant during the whole period of aeration.

The plane shape of the membrane further has the advantage that bubbles generated over the whole surface of the membrane all have the same diameter.

By switching the air supply off the membrane does not collapse.

In contrast with the conventional elastomeric membranes, interruptions in the air supply have no detrimental effect on the clogging up of the membrane. If occasionally some dirt particles enter the pores, they are blown out when the air is switched on again.

Accordingly, membranes according to the present invention require only low maintenance. The frequency of cleaning can be kept low.

In some cases it can be desirable that the aeration occurs periodically, this means that air is blown through the membrane during a certain time period in order to generate bubbles, followed by an interval during which the air supply is turned off.

This can for example be desirable in systems where the desired amount of oxygen to be introduced is low, so that a continuous aeration is not necessary.

The aeration membranes can have any shape, they can for example be rectanguiar or circular.

According to a second aspect of the invention a method of manufacturing an aeration diffuser is provided. The method comprises the following steps: -providing a layer of metal fibers or a layered structure comprising at least two layers, each layer comprising a web of metal fibers; -sintering and compacting said layer or said layered structure; -fastening and sealing the membrane by means of a frame.

The fastening and sealing step can be performed by welding the frame to the membrane.

Preferably, the membrane is fastened in such a way that it is almost not inflating when the air supply is turned on.

The compacting is preferably done by a cold isostatic pressing operation.

According to a third aspect, a tank comprising at least one aeration diffuser according to the present invention is provided.

This can for example be a tank used in an activated sludge process, where air is introduced in the water tank to allow microbial oxidation to proceed.

The number of aerators may vary from 1 up to 50, or even more.

The aeration diffusers are placed in the water tank so that the membrane is under the water level.

Preferably, aeration diffusers are situated in the lower part of the tank and most preferably at the bottom of the tank. In this way, the bottom can be covered partially or completely with the diffusers.

Agitating currents in the tank may help to keep the membrane clean.

Agitating currents are realised by the bubbles of the aeration diffuser or diffusers. In some cases, it can be desirable to create additional motion in the tank. This can for example be realised in a mechanical way, it is by mixing or stirring the water in the tank.

An alternative way to realise these movements is by blowing air into the tank. The air is preferably blown low in the tank.

Brief description of the drawings.

The invention will now be described into more detail with reference to the accompanying drawing wherein Figure 1 shows an aeration diffuser according to the invention.

Description of the preferred embodiments of the invention.

Referring to Figure 1, an aeration diffuser 10 according to the invention comprises an aeration membrane 12. The membrane comprises a first layer, at the flow in side of the gas, of a sintered and compacted fiber web. The fibers in the first layer have a diameter of 6.5 um. The first layer has a weight of 300 g/m2.

A second layer of a sintered metal web is in contact with the first layer.

The fibers in the second layer have a diameter of 2 pm. The weight of

the second layer is 225 g/m2.

A third layer functioning as support layer comprises fibers with a diameter of 6.5 pm and has a weight of 75 g/m2. This third layer is brought into contact with the second layer.

The membrane 12 comprising the first, second and third layer has a weight of 600 g/m2, a thickness of 0.36 mm and a porosity of about 79 %.

Air is blown through the air supply means 14. The gas supply means may terminate in one gas outlet or alternatively in a plurality of gas outlets.

The air is dispersed over the membrane to create small bubbles at the flow out side of the membrane.

An aeration diffuser as shown in figure 1 can be made in the following way.

A non-woven web of metal fibers with a diameter of 6.5 um is provided as the first layer. Metal fibers are obtained by means of bundled drawing as for example described in US 3,379,000.

Metal fibers used for the porous membrane according to the invention may be conventional compositions such as stainless steel 316L, AlloyHR or Inconel@.

Metal fibers with a diameter of 2 um are obtained with the above- mentioned technique of bundled drawing and are used to provide a second non-woven web, which forms the second layer.

In a similar way a third web of metal fibers with a diameter of 6.5 pm is provided to form the third layer.

The first, second and third layer are brought into contact with each other.

The thus obtained layered structure is sintered and compacted to obtain the porous membrane. This compacting is done by a cold isostatic pressing operation.

The frame is welded or glued to the membrane.

The membrane is thereby fastened in such a way that is almost not inflating when the air supply is switched on.

Care should be taken to avoid a gas leakage in the system during the fixing of the membrane.