WO2021116724A1

WO2021116724A1 - Mixing unit for mixing liquids treated in wastewater treatment

Info

Publication number: WO2021116724A1
Application number: PCT/HU2020/050056
Authority: WO
Inventors: Blanc REMY; Andrea BOLGÁR; Ádám ZBOROVSZKY
Original assignee: Organica Zrt.
Priority date: 2019-12-12
Filing date: 2020-12-01
Publication date: 2021-06-17
Also published as: EP4072718A1; CN114786802A; EP4072718B1; HRP20240787T1; HU5199U; HUE066554T2; CN114786802B

Abstract

The object of the invention relates to a mixing unit for mixing liquids treated in wastewater tratment apparatuses using large gas bubbles, where the vessel for performing the timed introduction of the gas (1) is created with a completely unconventional geometrical form, and if the medium guiding body (30) of the vessel and the shape of the medium guiding passage (32) are created in a novel way, and, furthermore, if a unique bubble forming element different to those known of is placed above the outlet opening, then large sized air bubbles may be collected and introduced into the wastewater or sewage sludge in such a way that their introduction and temporary storage involve a minimal amount of energy use, and with respect to their shape and size the air bubbles created are able to perform efficient mixing in the liquid, i.e. in the wastewater and sewage sludge, while during their fast elevation there is no substantial dissolving of the oxygen content, and so the task may be solved.

Description

Mixing unit for mixing liquids treated in wastewater treatment apparatuses using large gas bubbles

The object of the utility model is a mixing unit for mixing liquids treated in wastewater treatment apparatuses using large gas bubbles, which has a collection bell closed from above and delimiting a gas space, a medium guiding body cooperating with the collection bell delimiting a medium inlet opening, a medium outlet opening and a medium guiding passage located between these two, as well as a dispensing pipe end for introducing the gas into the gas space of the collection bell, where the medium guiding body has a base member and a deflecting mantle located along the external edge of the base member at an angle of inclination to the plane of the base member of less than 180°, while the collection bell has a gas receipt member having a deflecting surface located at least partly above the deflecting mantle of the medium guiding body, a gas holding piece as a continuation of the gas receipt member at least partly running around the gas receipt member, and a discharge opening enclosed by the internal edge of the gas receipt member, the gas space of the collection bell is created by a trap space formed jointly by the gas receipt member and the gas holding piece and open from the direction of the dispensing pipe end but closed from the opposite direction, where the external edge of the deflecting mantle of the medium guiding body is positioned to extend into the trap space, and at least a part of the trap space is convex in shape viewed from the dispensing pipe end, the medium inlet opening of the medium guiding body is at least temporarily connected to the gas space of the collection bell, while the medium guiding passage of the medium guiding body leads to the discharge opening enclosed by the internal edge of the gas receipt member of the collection bell, and the gas holding piece is located to be in the path of the gas flowing out of the dispensing pipe end.

Bubble aeration devices of various designs are used in numerous cases for the treatment of liquids. The essence of a group of these is that a vessel suitable for collecting gas packages is positioned under the surface of the liquid to be treated into which a J-shaped pipe is installed. Where the lower free end of the pipe leads to the vessel while its upper free end leads to the liquid space outside of the vessel. Gas is introduced into the vessel via a pipe connected to an external gas tank. The gas is collected in the closed internal space of the vessel, and due to the increase in pressure it pushes the liquid from the upper free end of the J-shaped pipe. After some time the entire amount of liquid leaves the J-shaped pipe, and so the gas package leaves the vessel in the form of small gas bubbles.

Such a design may be seen in publication document number US 2011/0049047, among others. The purpose of the solution here is for the gas accumulated in the vessel to break upwards directly towards the liquid surface in the form of groups of smaller bubbles, and while the gas is moving for it to move the membranes hanging vertically down in the liquid and to detach a part of the biofilm accumulated on them.

However, this solution is not suitable for wastewater treatment apparatuses where the treated wastewater should be mixed with large bubbles in such a way that the oxygen comes into contact with the treated liquid for as short a time as possible. The small bubbles are not suitable for performing this. The oxygen content of the bubbles dissolves better into the wastewater thereby preventing efficient performance of the preferred treatment processes.

Devices are used for mixing in such wastewater treatment apparatuses in which an aeration pipe runs all along the base of the reactor, which pipe is connected to a high-pressure air tank. By opening and closing a control valve the high-pressure air is introduced from the air tank into the aeration pipe and the bubbles flowing out from there between plates mix the wastewater to be treated.

The disadvantage of these solutions is, however, that a significant amount of energy is required to operate the high- pressure air tank.

A further disadvantage is that the control valves and the other moving and wearing components wear out quickly, they require a high amount of maintenance and the probability of faults occurring is also greater.

Our objective with the arrangement according to the utility model was, by retaining the preferred characteristics of the known solutions and overcoming their deficiencies, to create a mixing unit that by using a small amount of energy and few moving, wearing components creates large sized bubbles regularly introduced in the liquid that rise quickly and ensure an efficient mixing effect, which bubbles only negligibly dissolve and, in this way, do not prevent the favourable treatment processes being performed in the various wastewater treatment apparatuses.

The recognition that led to the arrangement according to the utility model was that if the vessel for performing the timed introduction of the gas is created with a completely unconventional geometrical form, and if the medium guiding body of the vessel and the shape of the medium guiding passage are created in a novel way, and, furthermore, if a unique bubble forming element different to those known of is placed above the outlet opening, then large sized air bubbles may be collected and introduced into the wastewater or sewage sludge in such a way that their introduction and temporary storage involve a minimal amount of energy use, and with respect to their shape and size the air bubbles created are able to perform efficient mixing in the liquid, i.e. in the wastewater and sewage sludge, while during their fast elevation there is no substantial dissolving of the oxygen content, and so the task may be solved.

In accordance with the set objective the mixing unit according to the utility model for mixing liquids treated in wastewater treatment apparatuses using large gas bubbles, - which has a collection bell closed from above and delimiting a gas space, a medium guiding body cooperating with the collection bell delimiting a medium inlet opening, a medium outlet opening and a medium guiding passage located between these two, as well as a dispensing pipe end for introducing the gas into the gas space of the collection bell, where the medium guiding body has a base member and a deflecting mantle located along the external edge of the base member at an angle of inclination to the plane of the base member of less than 180°, while the collection bell has a gas receipt member having a deflecting surface located at least partly above the deflecting mantle of the medium guiding body, a gas holding piece as a continuation of the gas receipt member at least partly running around the gas receipt member, and a discharge opening enclosed by the internal edge of the gas receipt member, the gas space of the collection bell is created by a trap space formed jointly by the gas receipt member and the gas holding piece and open from the direction of the dispensing pipe end but closed from the opposite direction, where the external edge of the deflecting mantle of the medium guiding body is positioned to extend into the trap space, and at least a part of the trap space is convex in shape viewed from the dispensing pipe end, the medium inlet opening of the medium guiding body is at least temporarily connected to the gas space of the collection bell, while the medium guiding passage of the medium guiding body leads to the discharge opening enclosed by the internal edge of the gas receipt member of the collection bell, and the gas holding piece is located to be in the path of the gas flowing out of the dispensing pipe end, - is created in such a way that the base member of the medium guiding body is established to exclude the permeation of liquid, and that a gas bubble forming element is inserted above the discharge opening separated by a gap from the meeting line of the gas receipt member and the gas holding piece of the collection bell, where the gas bubble forming element is created as an at least partially curved rotation body shell, and the axis of rotation of the gas bubble forming element is coaxial with the main axis of the base member, and the gas bubble forming element is convex when viewed from the base member, furthermore, the deflecting mantle of the medium guiding body is a rotation body shell, and the axis of rotation of the deflecting mantle is coaxial with the main axis of the base member, and also the size of the cross-sections enclosed by the deflecting mantle, and perpendicular to its main axis decreases evenly from the external edge of the deflecting mantle in the direction of the base member, the deflecting surface of the gas receipt member of the collection bell is a rotation body shell, and the size of the cross-sections enclosed by the deflecting surface, and perpendicular to the main axis of the collection bell decreases evenly from the meeting line of the gas receipt member and the gas holding piece in the direction of the discharge opening at least at its part in the vicinity of the discharge opening.

A further feature of the mixing unit according to the utility model may be that the perpendicular projection of the external delimiting edge of the gas bubble forming element falling on the main plane perpendicular to the main axis of the base member is located outside of the internal edge of the gas receipt member delimiting the discharge opening.

In the case of another version of the mixing unit the combination of the gas receipt member and the gas holding piece of the collection bell has the shape of a rotation body shell, and the main axis of the collection bell is coaxial with the main axis of the base member of the medium guiding body.

In the case of yet another different embodiment of the utility model the deflecting surface of the gas receipt member of the collection bell is in the shape of a truncated cone shell and/or the deflecting mantle of the medium guiding body is in the shape of a truncated cone shell.

In the case of another different embodiment of the mixing unit the straight lines carrying the neighbouring sections of the gas receipt member and the gas holding piece falling in the intersection plane passing through the main axis of the collection bell are at an obtuse angle to each other, or the straight lines carrying the neighbouring sections of the gas receipt member and the gas holding piece falling in the intersection plane passing through the main axis of the collection bell are at an angle of greater than 30° to each other, but up to a maximum of 90°.

The mixing unit according to the utility model has numerous advantageous features. The most important of these is that as a consequence of the geometric form of the structural elements forming the gas collection vessel and their arrangement relative to each other, and the gas bubble shaping element placed above it, large gas bubbles may be reliably created using a small amount of energy that are capable of efficiently mixing various liquids, including, among others, the wastewater in anoxic and anaerobic wastewater treatment apparatuses, as well as the sewage sludge in wastewater treatment apparatuses, while the dissolving of the oxygen from the bubbles is negligible due to the quick elevation of the bubbles .

An advantage deriving from this is that the mixing unit according to the utility model does not have a disadvantageous influence on the desirable processes taking place in the wastewater treatment, however it performs mixing of the wastewater more effectively than the known solutions.

Another feature that may be listed among the advantages is that in order to create the gas package and periodically discharge it there is no need for the use of any moving components. Therefore, the mixing unit according to the utility model may be operated with a lower probability of faults occurring, more reliably, and with a low maintenance demand. A further advantage originating from this is that a wastewater treatment apparatus fitted with the mixing unit according to the utility model may be operated more efficiently, which also has a favourable effect on the operation costs.

It is also an advantage that due to the design of the gas collection bell, the mixing unit according to the utility model may also be installed in the wastewater treatment apparatuses of already operating wastewater treatment plants at a low investment cost. Therefore the operating costs and reliability of such plants may be quickly and effectively improved .

It is also an important advantage that the mixing unit according to the utility model may be connected to the low pressure air supply systems already used in wastewater treatment plants, and so it does not demand further independent air supply devices, which does not only facilitate installation, but also has a favourable effect on the investment costs.

Further details of the mixing unit according to the utility model will be explained by way of exemplary embodiments with reference to figures, wherein

Figure 1 depicts a cross-section view of a preferable embodiment of the mixing unit according to the utility model, Figure 2 depicts a schematic cross-section view of a possible embodiment of the gas collection bell of the mixing unit according to figure 1.

Figure 1 shows a version of the mixing unit according to the utility model which may be used to good effect for mixing liquids in wastewater treatment apparatuses, particularly for mixing wastewaters and sewage sludge in anoxic and anaerobic wastewater treatment apparatuses. It may be observed that the mixing unit consists of a gas collection bell 40, a medium guiding body 30 cooperating with it, and a gas bubble forming element 20 positioned above the gas collection bell 40.

In the case of the given embodiment, the medium guiding body 30 shaped as a rotation body has a base member 34 and a deflecting mantle 35 running around the external edge 34b of the base member 34. The base member 34 is preferably a planar disc, while the deflecting mantle 35 is in the shape of a truncated cone shell, and the base member 34 and the deflecting mantle 35 are produced from a single material, such as by pressing or injection moulding. The deflecting mantle 35 of the medium guiding body 30 is at an angle of inclination of less than 180° to the plane AS of the base member 34, preferably an obtuse angle.

The main axis 34a of the base member 34 of the medium guiding body 30 and the axis of rotation 35b of the deflecting mantle 35 are coaxial. The base member 34 is secured to the foundation 3 in the reactor space of the wastewater treatment apparatus receiving the liquid 2, not shown in figure 1, with the securing piece 4.

Taking figure 1 as the reference, the gas collection bell 40 is positioned above the medium guiding body 30, which is also in the shape of a rotation body shell. The gas collection shell 40 contains the gas receipt member 41, the gas holding piece 43, and the discharge opening 44 delimited by the internal edge 41a of the gas receipt member 41. The gas receipt member 41 and the gas holding piece 43 are also produced from a single piece, also by pressing or injection moulding, for example.

As figure 2 illustrates well, the straight line E2 carrying the section 41b of the gas receipt member 41 falling in the intersection plane S passing through the main axis 40a of the gas collection bell 40, and the straight line El carrying the section of the gas holding piece 43 are at an angle b to each other here of more than 30°, but a maximum of 90°.

It must be mentioned here, however, that depending on the nature of the task a gas collection bell 40 is conceivable in the case of which the section 41b of the gas receipt member 41 and the section 43b of the gas holding piece 43b are at an obtuse angle b to each other of greater than 90°.

The gas receipt member 41 and the gas holding piece 43 are formed in such a way that they together form a gas space 46 closed from above from the meeting line 45 of the gas receipt member 41 and the gas holding piece 43 and open from the direction of the medium guiding body 30. The upper part of this gas space 46 near to the meeting line 45 forms a trap space 47 from which the gas 1 accumulated there is only able to leave by passing into the medium inlet opening 31 created between the deflecting mantle 35 of the medium guiding body 30 and the gas receipt member 41 of the gas collection bell 40.

Returning now to figure 1, it illustrates well that a space- part is created between the medium guiding body 30 and the gas collection bell 40, the medium inlet opening 31 of which between the deflecting surface 42 of the gas receipt member 41 of the gas collection bell 40 and the external edge 35a of the deflecting mantle 35 of the medium guiding body 30 has a substantially circular ring shaped cross-section. While the medium outlet opening 33 essentially falls between the base member 34 of the medium guiding body 30 and the gas receipt member 41 of the gas collection bell 40 and leads to the discharge opening 44 delimited by the internal edge 41a. The medium guiding passage 32 may be found between this medium inlet opening 31 and medium outlet opening 33, which is delimited by the surface of the deflecting mantle 35 of the medium guiding body 30 facing towards the gas receipt member 41, and the deflecting surface 42 of the gas receipt member 41. It is preferable if the deflecting surface 42 of the gas receipt member 41 of the gas collection bell 40 forms a rotation body shell at least in the vicinity of the internal edge 41a that corresponds to the shape of the deflecting mantle 35 of the medium guiding body 30.

It is important to call attention to that the form of the gas holding piece 43 and its position as compared to the medium guiding body 30 are such that, on the one part, the external delimiting edge 43a of the gas holding piece 43 extends beyond the deflecting mantle 35 of the medium guiding body 30. On the other part, the external delimiting edge 43a of the gas holding piece 43 is closer to the foundation 3 than the discharge opening 44 of the gas collection bell 40. This arrangement ensures that the entire amount of the gas 1 getting into the gas space 46 reaches the discharge opening 44 via the route: medium inlet opening 31 - medium guiding passage 32 - medium outlet opening 33.

Incidentally, the main axis 40a of the gas collection bell 40 is also coaxial with the main axis 34a of the base member 34 of the medium guiding body 30 and with the axis of rotation 35b of the base member 34. But it must be noted that it is not obligatory for the base member 34 and deflecting mantle 35 of the medium guiding body 30, and the gas receipt member 41 and the gas holding piece 43 of the gas collection bell 40 to be rotation body shells. However, formation as a rotation body shell makes manufacturing substantially easier.

As illustrated in figure 1, the dispensing pipe end 10 ensures that the gas 1 gets into the gas space 46, which here is secured to the part of the gas holding piece 43 of the gas collection bell 40 near to the external delimiting edge 43a. Additionally, the gas outlet opening 11 of the dispensing pipe end 10 may be found between the gas holding piece 43 and the deflecting mantle 35, arranged so that as the gas 1 flowing through the gas outlet opening 11 of the dispensing pipe end 10 rises it arrives into the gas space 46 without obstruction between the gas holding piece 43 and the external edge 35a of the deflecting mantle 35.

It may also be seen that the gas bubble forming element 20 is located above the gas collection bell 40 separated by a gap T in such a way that the projection of the external delimiting edge 21 of the gas bubble forming element 20 perpendicular to the plane AS perpendicular to the main axis 34a of the base member 34 is outside the internal edge 41a of the gas receipt member 41 delimiting the discharge opening 44. This is a preferred arrangement because in this way the gas 1 passing through the discharge opening 44 is unable to escape beside any part of the external delimiting edge 21 of the gas bubble forming element 20, and in this way the gas bubble forming element 20 is able to create bubbles of the desired size that are suitable for the appropriate degree of mixing of the liquid 2. The retaining surface 23 of the gas bubble forming element 20, which faces towards the discharge opening 44 of the gas collection bell 40, is also preferably in the shape of an at least partly curved rotating body shell. It is preferably a spherical shell surface, the axis of rotation of which is coaxial with the main axis 34a of the base member 34 of the medium guiding body 30. It may be seen in the figure that when viewed from the base member 34 the gas bubble forming element 20 has a convex shape. The gas bubble forming element 20 may, naturally, be produced, for example, from metal sheeting by pressing or using plastic injection moulding as well. In such a case the entire gas bubble forming element 20 may be shaped as a rotation body, as is shown in figure 1.

The version of the mixing unit according to the utility model appearing in figure 1 works in the following way. At the start of the operation of the mixing unit the medium guiding passage 32 between the gas collection bell 40 and the medium guiding body 30 is filled with liquid 2. The gas transported to the dispensing pipe end 10, preferably air, for mixing the liquid 2 in the case of wastewater treatment apparatuses, passes through the gas outlet opening 11 to between the gas holding piece 43 of the gas collection bell 40 and the deflecting mantle 35 of the medium guiding body 30 in such a way that the gas 1, as a result of its density, rises upwards between the external delimiting edge 43a of the gas holding piece 43 and the external edge 35a of the deflecting mantle 35 to arrive in the gas space 46, and starts to accumulate in its trap space 47.

The more the gas 1 accumulating in the gas space 46, the more it endeavours to force out the liquid 2 in the gas space 46. However, as the external delimiting edge 43a of the gas holding piece 43 is lower than the discharge opening 44 of the gas collection bell 40, as a result of the hydrostatic pressure effect the gas 1 accumulated in the trap space 47 of the gas space 46 finally flows through the medium inlet opening 31 into the medium guiding passage 32 and forces the liquid 2 out of there.

As the liquid 2 in the medium guiding passage 32 leaves the medium guiding passage 32 through the discharge opening 44 of the gas receipt member 41 of the gas collection bell 40, and the gas 1 accumulating in the medium guiding passage 32 reaches the medium outlet opening 33 and arrives at the discharge opening 44 of the gas receipt member 41 delimited by the internal edge 41a, the gas 1, being released from the forcing effect of the deflecting surface 42 of the gas receipt member 41, rises through the discharge opening 44 above the gas collection bell 40.

When the gas 1 passes through the discharge opening 44 it exerts a suction effect on the gas 1 still in the medium guiding passage 32 and the emptying of the medium guiding passage accelerates. The gas 1 leaving the medium guiding passage 32 is once again replaced by liquid 2 for a time, while the gas 1 rising above the gas collection bell 40 of the mixing unit reaches the gas bubble forming element 20.

The gas 1 is distributed along the curved surface of the gas bubble forming element 20 and by going under the external delimiting edge 21 of the gas bubble forming element 20 the extended, initially torus shaped gas bubbles are formed that break off the external delimiting edge 21 of the gas bubble forming element 20 and rise quickly upwards in the liquid 2 and by creating turbulent flow around themselves they mix the liquid 2, then on rising to the surface they leave the surface of the liquid 2.

Meanwhile the gas 1 flowing in through the gas outlet opening 11 of the dispensing pipe end 10, in the same way as that presented previously, starts to fill up the trap space 47 of the gas space 46 once again, and the process is repeated as long as gas 1 flows through the gas outlet opening 11 of the dispensing pipe end 10 under the gas holding piece 43 of the gas collection bell 40 of the mixing unit.

It is obvious that by changing the flow rate of the gas 1 flowing in through the gas outlet opening 11 of the dispensing pipe end 10, the time required to fill up the trap space 47 may be changed. In this way the period of elevation of the individual gas packages may be precisely adjusted, in accordance with the requirements of the technology. This is performed in such a way that there is no need for the independent control, opening-closing, of the feed valves, nor for such valves either. A single valve may be used for the appropriate amount of gas 1 to be collected at given intervals in the trap space 47 of the mixing unit and to flow out of the mixing unit to mix the liquid 2.

The mixing unit according to the utility model may be used to good effect in places where liquids need to be simply and reliably mixed using gas and a small amount of energy, but it may be used to particularly good effect for mixing wastewater and sewage sludge treated in anoxic and anaerobic wastewater treatment apparatuses using large sized gas bubbles. In addition to this it may also be used beside aeration units to perform mixing tasks. List of reference signs

1 gas

2 liquid

3 foundation

4 securing piece

10 dispensing pipe end 11 gas outlet opening

20 gas bubble forming element 21 external delimiting edge

22 axis of rotation

23 retaining surface

30 medium guiding body 31 medium inlet opening

32 medium guiding passage

33 medium outlet opening

34 base member 34a main axis 34b external edge

35 deflecting mantle 35a external edge 35b axis of rotation

40 gas collection bell 40a main axis

41 gas receipt member 41a internal edge 41b section

42 deflecting surface

43 gas holding piece

43a external delimiting edge 43b section 44 discharge opening

45 meeting line

46 gas space

47 trap space

AS plane

El straight line E2 straight line

S intersection plane

T gap angle of inclination b angle

Claims

1. Mixing unit for mixing liquids treated in wastewater treatment apparatuses using large gas bubbles, which has a collection bell (40) closed from above and delimiting a gas space (46), a medium guiding body (30) cooperating with the collection bell (40) delimiting a medium inlet opening (31), a medium outlet opening (33) and a medium guiding passage (32) located between these two, as well as a dispensing pipe end (10) for introducing the gas (1) into the gas space (46) of the collection bell (40), where the medium guiding body (30) has a base member (34) and a deflecting mantle (35) located along the external edge (34b) of the base member (34) at an angle of inclination (a) to the plane (AS) of the base member (34) of less than 180°, while the collection bell (40) has a gas receipt member (41) having a deflecting surface (42) located at least partly above the deflecting mantle (35) of the medium guiding body (30), a gas holding piece (43) as a continuation of the gas receipt member (41) at least partly running around the gas receipt member (41), and a discharge opening (44) enclosed by the internal edge (41a) of the gas receipt member (41), the gas space (46) of the collection bell (40) is created by a trap space (47) formed jointly by the gas receipt member (41) and the gas holding piece (43) and open from the direction of the dispensing pipe end (10) but closed from the opposite direction, where the external edge (35a) of the deflecting mantle (35) of the medium guiding body (30) is positioned to extend into the trap space (47), and at least a part of the trap space (47) is convex in shape viewed from the dispensing pipe end (10), the medium inlet opening (31) of the medium guiding body (30) is at least temporarily connected to the gas space (46) of the collection bell (40), while the medium guiding passage (32) of the medium guiding body (30) leads to the discharge opening (44) enclosed by the internal edge (41a) of the gas receipt member (41) of the collection bell (40), and the gas holding piece (43) is located to be in the path of the gas (1) flowing out of the dispensing pipe end (10), characterised by that the base member (34) of the medium guiding body (30) is established to exclude the permeation of liquid, and that a gas bubble forming element (20) is inserted above the discharge opening (44) separated by a gap (T) from the meeting line (45) of the gas receipt member (41) and the gas holding piece (43) of the collection bell (40), where the gas bubble forming element (20) is created as an at least partially curved rotation body shell, the axis of rotation (22) of the gas bubble forming element (20) is coaxial with the main axis (34a) of the base member (34), and the gas bubble forming element (20) is convex when viewed from the base member (34), furthermore, the deflecting mantle (35) of the medium guiding body (30) is a rotation body shell, and the axis of rotation (35b) of the deflecting mantle (35) is coaxial with the main axis (34a) of the base member (34), and also the size of its cross-sections enclosed by the deflecting mantle (35), and perpendicular to its main axis (34a) decreases evenly from the external edge (35a) of the deflecting mantle (35) in the direction of the base member (34), the deflecting surface (42) of the gas receipt member (41) of the collection bell (40) is a rotation body shell, and the size of its cross-sections enclosed by the deflecting surface (42), and perpendicular to the main axis (40a) of the collection bell (40) decreases evenly from the meeting line (45) of the gas receipt member (41) and the gas holding piece (43) in the direction of the discharge opening (44) at least at its part in the vicinity of the discharge opening (44).

2. Mixing unit according to claim 1, characterised by that the perpendicular projection of the external delimiting edge (21) of the gas bubble forming element (20) falling on the plane (AS) perpendicular to the main axis (34a) of the base member (34) is located outside of the internal edge (41a) of the gas receipt member (41) delimiting the discharge opening (44).

3. Mixing unit according to claim 1 or 2, characterised by that the combination of the gas receipt member (41) and the gas holding piece (43) of the collection bell (40) has the shape of a rotation body shell, and the main axis (40a) of the collection bell (40) is coaxial with the main axis (34a) of the base member (34) of the medium guiding body (30).

4. Mixing unit according to any of claims 1 to 3, characterised by that the deflecting surface (42) of the gas receipt member (41) of the collection bell (40) is in the shape of a truncated cone shell.

5. Mixing unit according to any of claims 1 to 4, characterised by that the deflecting mantle (35) of the medium guiding body (30) is in the shape of a truncated cone shell.

6. Mixing unit according to any of claims 1 to 5, characterised by that the straight lines (El, E2) carrying the neighbouring sections (41b, 43b) of the gas receipt member (41) and the gas holding piece (43) falling in the intersection plane (S) passing through the main axis (40a) of the collection bell (40) are at an obtuse angle (b) to each other.

7. Mixing unit according to any of claims 1 to 5, characterised by that the straight lines (El, E2) carrying the neighbouring sections (41b, 43b) of the gas receipt member (41) and the gas holding piece (43) falling in the intersection plane (S) passing through the main axis (40a) of the collection bell (40) are at an angle of greater than 30° to each other, but up to a maximum of 90°.