WO2003031349A1

WO2003031349A1 - Device and method for purifying wastewater

Info

Publication number: WO2003031349A1
Application number: PCT/NL2002/000643
Authority: WO
Inventors: Boele De Jong
Original assignee: Boele De Jong
Priority date: 2001-10-11
Filing date: 2002-10-10
Publication date: 2003-04-17
Also published as: EP1440042A1; NL1019162C2

Abstract

A device for purifying wastewater, with a conversion space (16; 116), aeration and agitation means (10, 12, 18; 110) for aerating and agitating a water-sludge suspension in the conversion space (16; 116), a deposition space (20; 120; 320) for depositing sludge in the deposition space (20; 120; 320), and a water drain (21) with an inlet (25; 325) in the deposition space (20; 120; 320) for draining purified water from the deposition space (20; 120; 320). Because the device is further provided with a rising substance drain (10, 12, 18, 26; 226; 326) with an inlet in the deposition space (20; 120; 320) at a level above the inlet (25; 325) of the water drain (21) for draining rising substance from the deposition space (20; 120; 320), the formation of a blanket or cake of the rising substance in the deposition space (20; 120; 320) is prevented. A method for achieving this effect is also described.

Description

Title: Deviee and method for purifying wastewater

The invention relates to a device according to the preamble of claim device 1 as well as to a method according to the preamble of claim 20.

Such a device and method is known from practice in the form of a so- called "Bever" wastewater purifying system marketed by Afmitech of Katlijk, The Netherlands, or in the form of the operation of that system.

This known system has a compact structure and has a conversion space between a first vessel with a downwardly conical point and a second vessel, suspended in the first vessel, with an upwardly directed closed side and a downwardly open side. The conversion space contains water to be purified and activated sludge. The first vessel and the second vessel bound an intermediate deposition space, in which sludge can deposit and from which purified water is drained.

In the conversion space the water-sludge suspension is agitated to keep the sludge in suspension and aerated to feed oxygen to bacteria in the suspension. In operation, deposited sludge (the wastewater/sludge ratio is selected such that practically only growth occurs of bacteria that can properly deposit) collects in the area of the transition between the deposition space and the conversion space.

A problem of this known device and method is that in Λhe dβrøsition space rising gas entrains sludge particles to the water surface, which particles form a floating layer of sludge, which eventually dries to form a sludge cake. This dirty layer on the purified water needs to be removed with some regularity to prevent blockage of the device and, in particular, the draining openings, and causes unpleasant odor emissions, in particular during removal. Furthermore, in the warm seasons the dirty layer attracts huge amounts of insects, which also causes nuisance. It is an object of the invention to provide a solution to the problem of rising sludge in a wastewater purifying device with a conversion space and a deposition space communicating therewith.

This object is achieved according to the invention by designing a device for purifying wastewater according to claim 1 as well as by carrying out a method for purifying waste according to claim 20.

Because rising or risen substance is drained at a level above the water drain from the deposition space, the formation of a cake of risen material in the deposition space is prevented. Very advantageous elaborations of the invention are laid down in the subclaims.

The invention will hereinbelow be illustrated and explained in more detail on the basis of an exemplary embodiment and with reference to the drawing, in which: Fig. 1 is a cross-sectional side view on the line I-I in Fig. 2 of an example of a wastewater purifying device according to the invention,

Fig. 2 is a cross-sectional top view on the line II-II in Fig. 1,

Fig. 3 is a diagrammatic top view of a second example of a wastewater purifying device according to the invention, Fig. 4 is a side view according to Fig. 1 of a third example of a wastewater purifying device according to the invention,

Fig. 5 is a side view according to Fig. 1 of a fourth example of a wastewater purifying device according to the invention, and

Fig. 6 is a cross-sectional top view on the line II-II in Fig. 5. The invention will first be explained on the basis of an exemplary embodiment of the invention as shown in Figs. 1 and 2.

The device according to the example shown in Figs. 1 and 2 has an outer vessel 1 with a conical lower part 2 and a closed upper side 3 with an opening 5 bounded by a flange 4. The upper side 3 may optionally be detachable to enable easy access to the inner space of the outer vessel 1 for maintenance purposes. The outer vessel 1 is preferably provided with an insulation and may therefore also have a multiple wall. For the sake of convenience, such measures are not shown.

In the outer vessel 1 is suspended an inner vessel 6, which has a closed upper side 7 and an open lower side 8. Through a hole 9 in the upper side 7 of the inner vessel 6 a tube 10 extends, which is attached on the upper side to a driven shaft 11 of a motor 12. This motor 12, for its part, is attached to a motor support 13, which is suspended in the hole 5 in the upper side of the vessel 1. At a distance below the motor 12 the tube 10 is bearing- mounted in a frame 14, which at the transition from a cylindrical part 15 to the conical part 2 of the outer vessel 1 is supported on the outer wall of the vessel 1. This frame 14 also carries the inner vessel 6 and extends through openings in the wall thereof.

In the outer vessel 1 is located a conversion space or chamber 16 to receive an aqueous suspension 17 of activated sludge. The tube 10, which can be rotated by the motor 12 about its central axis, is provided with outflow pipes 18, which are oriented radially away from the tube 10. The outflow pipes contain channels in the longitudinal direction of those pipes, which channels communicate with a channel 19 extending through the tube 10. The channels of the outflow pipes discharge above the surface 17 of the suspension in the conversion space 16. Through this orientation of the outflow pipes 18 and through the discharge of the channels above the surface of the suspension, which channels extend through the outflow pipes, water-sludge suspension, during rotation of the tube 10, is swung out and sucked in from the channel 19 in the tube 10. This, for its part, has the effect that water-sludge suspension is sucked in via the free end of the tube 10 in a subjacent part of the conversion space. Thus, the water-sludge suspension 17 in the conversion space 16 is kept moving and aerated in a very efficient manner, so that the sludge is kept in suspension and sufficient oxygen is fed to the bacteria in the suspension. The aeration is very effective, because the outflow pipes are inclined upward to the outside, so that water-sludge suspension is swung along in a curved path above the surface of the water-sludge suspension 17.

The inner vessel 6 shields a deposition space 20 for depositing sludge upward and sideward from the conversion space 16. As a result, water in the deposition space 20 is practically still and sludge particles in the deposition space 20 are given the opportunity to deposit. Since the deposition space 20 is open toward the bottom, these particles deposit to the conversion space 16. The deposition space, for that matter, need not be open over its entire underside, but may, for instance, gradually taper downward at the underside or over the entire height to an opening with a smaller surface than the maximum horizontal cross-sectional surface of the deposition space. This offers the advantage that penetration of possible eddies from the conversion space to the deposition space is prevented. From the deposition space 20 a water drain 21 extends to drain purified water from the deposition space 20. This water drain 21 comprises a number of receiving lines 22, which extend from the deposition space, an overflow 23 connected to the receiving lines 22, which overflow, depending on the level of the surface of the water-sludge suspension 17 in the conversion space 1, controls the drainage of water through the overflow to the draining pipe 24, so that this level is kept within a specific range, also when the feed of wastewater via the pipe 29 fluctuates. From the overflow 23 a draining pipe 24 extends, via which draining pipe 24 purified water can be drained, for instance by draining into surface water, for further purification or for reuse.

For letting in wastewater to be purified, the device has an inlet 29 discharging into the conversion space 16. Material fed thus directly comes into contact with activated sludge in the conversion space 16. Moreover, the inlet 29 discharges high in the conversion space 16, so that components of the wastewater must cover a relatively long distance through the conversion space 16, before they can reach the deposition space 20, so that there is little probability that they will be drained untreated.

At a level above the level of the inlets 25 of the water drain 21 the tube 10 is provided with an inlet 26. The rotating tube 10 with the outflow pipes 18 extending radially away therefrom thus also forms a rising substance drain with an inlet in the deposition space 20 at a level above the level of the inlet 25 of the water drain 21, to drain rising substance from the deposition space 20.

By draining rising substance from the deposition space from a level above the level where water is drained from the deposition space 20, the possible formation of a cake of rising substance in the deposition space 20 is prevented. Thus, the upper area of the deposition space 20 always remains clean and the necessity of periodically removing the cake of rising substance is eliminated. The outlet of the rising substance drain formed by the rotating tube

10 with the inlet 26 and the outflow pipes 18 is located in the conversion space 16 to recirculate rising substance drained from the deposition space 20 to the conversion space 16. As a result, the rising substance, which, in operation, is drained from the deposition space 20, is recirculated to the conversion space 16. This renders it possible to further treat the rising substance, while a suitable sludge/water ratio ensures a high probability that it will still be deposited by adhesion of properly depositing bacteria.

The rising substance drain also forms a mixer for mixing rising substance from the deposition space 20 with water-sludge suspension in and/or from the conversion space 16. Since the rising substance drained from the deposition space 20 is mixed with water-sludge suspension in and/or from the conversion space 16, the (further) conversion thereof by bacteria in the conversion space 16 is promoted. The intensive mixing, which occurs because the rising substance is ejected in a jet together with water-sludge suspension above the surface of the water-sludge suspension, further ensures a fine distribution of returned rising substance and removal of any air or gas adhering thereto or enclosed therein.

Since the aeration and agitation means are provided with the inlet 26 in an upper part of the deposition space 20 and also form the rising substance drain, no separate drainage provision is needed for draining the rising substance and all this can be realized in a structurally simple manner.

Moreover, since the lifting channel 19 of the aeration and agitation means extending through the tube 10 extends through the deposition space 20 and the deposition space communicates with the lifting channel 19 via the inlet 26, the connection between the lifting channel 19 and the deposition space 20 is realized in a very simple manner.

To reliably drain the rising substance, it is further advantageous that the inlet is movable and moves, in operation, at least at intervals. As a result, rising substance is sucked in from different places and the risk of blockage of the inlet 26 is further reduced. Optionally, the inlet 26 can move along a cutting edge to cut away any material remaining in the inlet 26.

The water drain 21 is arranged for such controlled drainage of water that, in operation, a water level is maintained in the device within a specific level range. The deposition space is located in a deposition chamber communicating with the conversion space 16, which deposition chamber has an upper bounding surface 7 located below the water level. Since the level reached by the water-sludge suspension in the device is positioned above the upper bounding surface 7 of the deposition space 20, rising substance in the deposition space 20 is pushed against the upper bounding surface 7, from which position it can be easily drained. To this end, the inlet 26 of the rising substance drain is located close to the upper bounding surface 7. It could also be included in the upper bounding surface.

The upper part 7 of the vessel 6, which bounds the deposition space 20, has a cross-section gradually decreasing toward an upper end. As a result, rising substance is collected in a relatively small area, from where it can be drained in a high concentration. This, for its part, is advantageous, because the drainage of rising substance thus causes relatively little, deposition impeding agitation of water in the deposition space 20. Furthermore, since the upper part 7 of the vessel 6, which bounds the deposition space 20, is inclined upward from the outside to the inside, the formation, in operation, of a sludge package on top of the inner vessel 6 is prevented. To prevent such an effect, it is further advantageous that the outflow pipes 18, which, in operation, at least periodically, rotate, are located partly below the surface of the water-sludge suspension and thus also function as agitators.

Since the upper bounding surface 7 of the deposition space 20 is located below the surface of the water-sludge suspension 17 in the conversion space 16, it is advantageous that via the inlet 26 and the tubes 10, 18 gas is also removed from the deposition space 20. As a result, no bubble can be formed in the deposition space 20, as a result of which rising substance could possibly no longer reach the inlet 26.

The deposition space 20, at its lower side, passes into the conversion space 16 and is located above a deepest part of the conversion space 16. As a result, a large free space is located below the deposition space 20, in which sinking sludge particles can deposit.

Moreover, because agitation of the water-sludge suspension by the aeration and agitation means 10, 12, 18 substantially takes place in an upper part of the conversion space, the part of the conversion space 16 below the deposition space 20 is agitated relatively little. This effect is enhanced, because the inner vessel 6 also forms a screen between the part of the conversion space 16 located below it and the agitation by the movement of the outflow pipes 18 and the water-sludge jets leaving them. The open lower side of the deposition space 20, which passes into the conversion space 16, is further located above an inlet 31 of the aeration and agitation means — formed by the subjacent end of the tube 10 - for recirculating the water-sludge suspension. Sludge depositing from the deposition space 20 to the conversion space 16 is thus recirculated from the bottom of the deposition space to a higher level in the conversion space 16. The formation of a sludge package below the deposition space 20, which, as a result of gases formed therein, can completely or partly rise in the deposition space 20 and can thus disturb the sludge deposition process in the deposition space, is prevented by the recirculation from a position below the deposition space 20. It is thereby advantageous that the deposition space 20, at least considered in at least a side view, is located centrally in the conversion space 16, because the downward flow through the conversion space 16 thereby runs along the outer walls of the conversion space and thus, in particular in the narrowing lower part 2 of the vessel 1, deposition of sludge is prevented.

In operation, however, solid substances and sludge-forming components are entrained with the wastewater, so that the amount of sludge in the conversion space 16 gradually increases. In the device shown in Figs. 1 and 2, sludge is collected and thickened in a very simple manner, for instance to a content of 3-5% dry substance, so that it can be removed from the device in a relatively compact form. To this end, the device is provided with a sludge collector 27 communicating with the conversion space 16 for collecting and thickening sludge from the conversion space 16 and provided with a passage 28 for removing thickened sludge from the sludge collector 27. The sludge collector 28 has passages 30, via which the inner space of the sludge collector 28 communicates with the conversion space 16. Via these passages 30, suspension leaving the outflow pipes 18 and reaching the passages can enter the sludge collector, where sludge contained therein can deposit. Simultaneously, excess suspension can overflow from the sludge collector 27 via the passages 30. As a result of deposition occurring in the sludge collector, however, overflowing suspension will contain less sludge than entering suspension, so that the amount of water in the sludge collector gradually increases. At a frequency depending on the amount of wastewater and the nature of the pollution thereof, thickened sludge can be withdrawn from the sludge collector 27 via the closable passage 28.

Since the sludge collector 27 comprises a deposition chamber with a closed lower part, a very effective deposition at the bottom of the sludge collector 27 is obtained. Withdrawal of sludge can further be simply carried out, because the sludge collector 27 is located at the outside of the conversion space 16.

In Fig. 3, a second example of a device according to the invention is diagrammatieally shown. In this device, three deposition spaces 120 are arranged in the conversion space 116, while through each of the deposition spaces 120 a vertical, motor-driven rotatable tube 110 extends. The water drains from the deposition spaces 120 are coupled together to form a common water drain 121.

A subjacent part 102 of the outer vessel 101 is, seen in cross-section,

V-shaped and has, at the ends, upwardly inclined parts. Optionally, the conversion space 116 may be divided by partitions, so that the device can also partially be placed into service. This may, for instance, be advantageous for uses in which the supply of wastewater strongly varies, such as in holiday regions.

The device according to the example shown in Fig. 4 is largely identical with the device according to the example shown in Figs. 1 and 2.

Corresponding parts of both devices are denoted by the same reference numerals.

On the driven shaft 11 of the motor 12 is mounted, according to this example, an upper tube 234, which extends coaxially and in line with the driven shaft 11. The free end of this upper tube 234 is bearing- mounted in a superjacent end of an inner vessel 210, which, for its part, is stationarily suspended in an outer tube 232. The outer tube 232 is, according to this example, non-rotatably suspended in the vessel 1 and is supported on the frame 14. The suspension of the inner tube 210 relative to the outer tube 232 is, according to this example, realized by anchors 236 and parts of the frame 14 extending through the outer tube 232. Located between a superjacent end of the upper side 207 of the inner vessel 206 and the outer tube 232 are passages 226, via which passages 226 rising sludge and gas can reach the conversion space 16 from the deposition space 20. The passages may also be designed differently, for instance as holes or grids in the upper side of the screen of the deposition space.

The channel 19 extends in the longitudinal direction through the inner tube 210 and the upper tube 234 connected thereto as well as through the bearing of the inner tube 210 relative to the upper tube 234. When rising substance is formed in the deposition space 20, it will lift via the draining openings 226 to the conversion space 16. The dimensions of the passages 226 are sufficiently large to pass rising substance, but small enough to prevent penetration of sludge from the conversion space 16 via the passages 22 to the deposition space 20. Since the rising substance and gases recirculate from the deposition space 20 to the conversion space 16 under the influence of their own rising power, some agitation of water in the deposition space occurs only if substance actually rises. If no gases or rising substance are contained in the deposition space, the flow through the passages 226 is not driven by rising matter, so that unnecessary recirculation of water from which sludge is deposited is avoided.

Moreover, by designing the rising substance drain as a direct connection between the deposition space 20 and a superjacent part of the conversion space 16, no drive is needed for moving rising substance from the deposition space 20 to the conversion space 16. This embodiment can therefore also be realized in a structurally simple manner.

In operation, when the motor 12 is active, a circulation is formed in the conversion space, while water-sludge suspension 17 moves down via the intermediate space between the inner vessel 6 and the wall of the outer vessel 1 and is passed up via the channel 19 through the middle of the conversion space 16. When the motor 12 is switched off, a push is caused by the kinetic energy of the sinking substance at the bottom of the deposition space 20, which push would push water from the deposition space 20 via the passages 226 to the conversion space 16 and, moreover, would cause agitation of water in the deposition space 20. Since a second channel 235 and passages 237 form an open connection between a lower part of the conversion space 16 and an upper part of the conversion space 16 near the passages 226, water-sludge suspension pushed up by sudden stoppage of flow through the lifting channel 19 can escape in a subjacent part of the conversion space 16 via the further channel 235. A flow through the deposition space 20 and through the passages 226 in reaction to stoppage of the motor 12 is thereby prevented. Conversely, through the channel 235, which connects a subjacent part of the conversion space 16 with a superjacent part of the conversion space 16, a downward flow through the passages 226 when starting the motor 12 is prevented as well. Also, any other causes of a pressure drop over the passages 226 and thus an undesirable flow through these passages 226 are thereby prevented.

The device according to the example shown in Figs. 5 and 6, which is mostly preferred at this moment, is largely identical with the device according to the example shown in Figs. 1 and 2. Corresponding parts of both devices are denoted by the same reference numerals.

On the driven shaft 11 of the motor 12 is mounted, according to this example, an upper tube 334, which extends coaxially and in line with the driven shaft 11. This upper tube 334 can be rotated about its central axis by the motor 12. The free end of this upper tube 334 is bearing-mounted in a superjacent end of a tube 310. The tube 310 is, according to this example, non-rotatably suspended in the vessel 1 and is supported on a frame 314, which is supported on the conical part 2 of the vessel 1. The channel 19 extends in the longitudinal direction through the tube 310 and the upper tube 334 connected thereto.

Between a superjacent end of an upper side 307 of an inner vessel 306 and the tube 310 are located passages 326, via which passages 326 rising sludge and gas can reach the conversion space 16 from the deposition space 320. These passages 326 can be closed by closing means 336, according to this, a vertically movable rubber disk that functions as a nonreturn valve. The closing means 336 have the advantage that they offer the possibility of preventing, after the drainage of rising substance from the deposition space 320 to the conversion space 16, by means of closure of the passages 326, the penetration of sludge from the conversion space 16 via the passages 326 to the upper part of the deposition space 320. In operation, this water, which is practically ready for being drained, must remain practically free from sludge. Also, when the passages 326 are closed, fewer undesirable flows occur in the deposition space 320 than when the passages 326 are open, which is an advantage, because disturbance of the deposition of sludge in the deposition space 320 is thus prevented.

The opening and closure of the valve 336 occurs, according to this example, because rising substance or rising gases collecting below the valve 336 push up the valve 336, so that it opens. After, accordingly, the rising substance or gases have passed, the valve closes again, because it, or at least an assembly of which it forms a substantially fixed part, sinks down again. To reliably perform this function, the valve 336, which can rise relative to the opening to release it, and which can sink down again on the opening to close it, preferably has a specific weight between 1.05 and 2 g/cm³. Optionally, ballast or floating bodies, springs and/or counterweights may be added to influence the opening of the valve 336 under the influence of rising material or gas and subsequent closing of the valve. Furthermore, the lower side of the valve 336, at least in the open condition, may be inclined upward to the edge, so as to promote the passage of rising material and gas along the valve 336. The fact that at least the lower side of the valve 336 where it lies against the edge of the passages 326 is made of a relatively soft material, according to this example rubber, is advantageous to obtain an effective closure.

Compared to the example shown in Figs. 1 and 2, in the example of Figs. 5 and 6, a contribution to the reduction of undesirable flows in the deposition space 320 is also made, because the tube 310 does not rotate in the deposition space 320 and therefore does not cause entrainment of suspension. Because the deposition space 320 gradually narrows down to an opening 308, disturbance of the deposition process is inhibited as well. Furthermore, in the example of Figs. 5 and 6, the inlets 325 of the water drain 21 are located at a higher level than the inlets 25 of the water drain 21 in the example of Figs. 1 and 2. As a result, a further reduction of the sludge content in the drained water can be obtained.

Claims

1. A device for purifying wastewater, comprising: a conversion space (16; 116); aeration and agitation means (10, 12, 18; 110; 210; 310) for aerating and agitating a water-sludge suspension in the conversion space (16; 116); a deposition space (20; 120; 320) for depositing sludge in the deposition space (20; 120; 320); and a water drain (21) with an inlet in the deposition space (20; 120; 320) for draining purified water from the deposition space (20; 120; 320); characterized by a rising substance drain (10, 12, 18, 26; 226; 326) with an inlet in the deposition space (20; 120; 320) at a level above the inlet (25; 325) of the water drain (21) for draining rising substance from the deposition space (20; 120; 320).

2. A device according to claim 1, in which the rising substance drain (26; 226; 326) has an outlet in the conversion space (16; 116) for recirculating rising substance from the deposition space (20; 120; 320) to the conversion space (16; 116).

3. A device according to claim 2, further comprising a valve (336) in the rising substance drain (26; 226; 326).

4. A device according to claim 3, in which the valve is a non-return valve for passing flow from the deposition space (320) to the conversion space (16).

5. A device according to any one of claims 2-4, in which the rising substance drain (26) forms a mixer for mixing rising substance from the deposition space (20; 120) with water-sludge suspension in and/or from the conversion space (16; 116).

6. A device according to any one of the preceding claims, in which the water drain (21) is arranged for such controlled drainage of water that, in operation, a water-sludge suspension level is maintained in the device within a specific level range, and in which the deposition space (20; 120; 320) forms a deposition chamber (20; 120; 320) communicating with the conversion space (16; 116), which deposition chamber (20; 120; 320) has an upper bounding surface (7; 207; 307), which is located entirely below said water-sludge suspension level.

7. A device according to claim 6, in which the rising substance drain (26; 226; 326) has an inlet (26; 226; 326) in or close to said upper bounding surface (7; 207; 307).

8. A device according to claim 7, in which at least an upper part of said deposition chamber (20; 120; 320) has a cross-section gradually decreasing toward an upper end.

9. A device according to any one of the preceding claims, in which the deposition space (20; 120; 320), at its lower side, passes into the conversion space (16; 116) and is located above a deepest part of the conversion space (16; 116).

10. A device according to any one of the preceding claims, in which the deposition space (20; 120; 320), at its lower side, passes into the conversion space (16; 116) and is located above an inlet (31) of the aeration and agitation means (10, 12, 18; 110; 310) for recirculating the water-sludge suspension.

11. A device according to claims 9 and 10, in which the deposition space (20; 120; 320), at least considered in at least a side view, is located centrally in the conversion space (16; 116).

12. A device according to any one of the preceding claims, further comprising a sludge collector (27) communicating with the conversion space (16; 116) for collecting and thickening sludge from the conversion space (16; 116) and provided with a passage (28) for removing thickened sludge from the sludge collector.

13. A device according to claim 12, in which the sludge collector (27) comprises a deposition chamber with a closed lower part.

14. A device according to any one of the preceding claims, in which the aeration and agitation means (10, 12, 18; 110; 310) are designed as a pump for lifting water-sludge suspension from a lower part of the conversion space (16; 116) and ejecting it above the surface of the water-sludge suspension.

15. A device according to any one of the preceding claims, in which the aeration and agitation means (10, 12, 18; 110; 310) are provided with an inlet (26) in an upper part of the deposition space (20; 120; 320) and also form the rising substance drain (26).

16. A device according to claims 14 and 15, in which a lifting channel (19) of the aeration and agitation means (10, 12, 18; 110) extends through the deposition space (20; 120), and in which the deposition space (20; 120) communicates with the lifting channel (19) via said inlet (26).

17. A device according to any one of the preceding claims, in which the rising substance drain (26) has a movable inlet (26) in an upper part of the deposition space (20; 120).

18. A device according to any one of the preceding claims, further comprising an inlet (29) for letting in wastewater to be purified, which inlet (29) discharges into the conversion space (16; 116).

19. A device according to any one of the preceding claims, in which the rising substance drain (226; 326) at least comprises a passage (226; 326), via which passage the deposition space (226; 326) directly communicates with the conversion space (16).

20. A method for purifying wastewater, comprising: aerating and agitating a water-sludge suspension (17) in a conversion space (16; 116); depositing sludge in a deposition space (20; 120; 320); draining purified water from the deposition space (20; 120; 320) via an inlet (25; 325) of a water drain (21) from a specific level in the deposition space (20; 120; 320); characterized by draining rising substance from the deposition space (20; 120; 320) at a level above the inlet (25; 325) of the water drain (21).

21. A method according to claim 20, in which the rising substance drained from the deposition space (20; 120; 320) is recirculated to the conversion space (16; 116).

22. A method according to claim 21, in which the rising substance drained from the deposition space (20; 120; 320) is mixed with water-sludge suspension in and/or from the conversion space (16; 116).

23. A method according to any one of claims 20-22, in which the deposition space (20; 120; 320) forms a deposition chamber (20; 120; 320) communicating with the conversion space (16; 116), and in which purified water is drained in such controlled condition that in the device a water level is maintained within a specific level range above an upper bounding surface (7; 207; 307) of the deposition space (20; 120; 320).

24. A method according to any one of claims 20-23, in which sludge from the deposition space (20; 120; 320) above a deepest part of the conversion space (16; 116) deposits from the deposition space (20; 120; 320) to the conversion space (16; 116).

25. A method according to any one of claims 20-24, in which sludge from the deposition space (20; 120; 320) deposits to the conversion space (16; 116) and is recirculated from below the deposition space (20; 120; 320) to a higher level in the conversion space (16; 116).

26. A method according to claims 24 and 25, in which the deposition space (20; 120; 320), at least considered in at least a side view, is located centrally in the conversion space (16; 116).

27. A method according to any one of claims 20-26, in which sludge is collected in a sludge collector (27) communicating with the conversion space (16; 116), is thickened and then removed from the sludge collector (27).

28. A method according to any one of claims 20-27, in which the aeration and agitation means (10, 12, 18; 110; 310) lift water-sludge suspension from a lower part of the conversion space (16; 116) and ejects it above the surface of the water-sludge suspension (17).

29. A method according to any one of claims 20-28, in which the aeration and agitation means (10, 12, 18; 110; 310) also drain gas and rising substance from the deposition space (20; 120).

30. A method according to any one of claims 20-29, in which the rising substance drain (10, 12, 18, 26) has a moving inlet (26) in an upper part of the deposition space (20; 120).

31. A method according to any one of claims 20-30, in which rising substance driven by its own rising power rises from the deposition space

(20; 320) to the conversion space (16).