GB2034188A - Liquid aeration device - Google Patents

Abstract

Circulation of a liquid in a closed path defined by a partition wall 4 in a basin is effected by at least one paddle wheel 12 mounted for rotation about a vertical axis lying in the plane of the wall. Air is introduced into the liquid flow displaced by the paddle wheel through apertures or nozzles. <IMAGE>

Description

SPECIFICATION Liquid aeration device From Netherlands patent (NL-C) 131 423 a device is known for aerating a liquid, in particular for biologically purifying waste water, said device comprising a basin in which a partition wall is arranged in such a manner that a closed circulation flow path for the liquid is obtained. At one end of said partition wall a surface aerator is mounted, the vertical axis of which being situated substantially in the plane of the partition wall.

During rotation of this aerator the liquid surface is disturbed, so that air is introduced into the liquid and is mixed therewith, and at the same time the liquid is put in motion so that it will flow through the closed flow path. The introduced air is used in the biological purification for maintaining the activity of the active silt, and the flow of the liquid will prevent sedimentable substances from settling. The quantity of introduced air can be controlled by varying the immersion depth or the rotation speed of the aerator.

A disadvantage of this. known device is that the propulsion effect on the liquid in the direction of circulation depends on the immersion depth or the rotation speed of the aerator, which is unfavourable if, at a lower load, less air introducion is required, since, then, also the circulation velocity will decrease, and can become too low for keeping the silt in suspension. It may be necessary then to use more energy than is required for the air demand.

Another disadvantage of this known device is the rather unfavourable propulsion effect of the surface aerator, since the propulsion takes mainly place near the surface, and, moreover, the onflowing water can be rather strongly decelerated by the water thrown off by the aerator. Still another disadvantage is that the splashing water pollutes the surroundings of the aerator, and noise produced by the aerator can be disturbing. Measures to remove these objections are very expensive. A final disadvantage of such surface aerators is that they consume rather much energy in proportion to the air introduction and propulsion effects.

From the British patent 1533 621 of the present applicant a similar device is known, in which the aeration and propulsion effects are improved by arranging additional surface aerators in the circulation path. Although, in that manner, a better adaptation of the aeration to the load can be obtained, the other disadvantages of using surface aerators remain.

Also other aeration devices are known, e.g.

from the Netherlands patent application laid open to public inspection (NL-A) 77 11 01 7 of the present applicant, in which the air supply and the propulsion are mutually separated, and this in that the air is introduced under pressure by means of nozzles submerged into the liquid, and, in particular, through bottoms grids extending over substantially the full width of the bottom, whereas the propulsion takes place by means of one or more separate driving screws. In this manner an effective aeration can take place, and the introduction of air and the propulsion can be controlled independently from one another. To that end, however, driving screws arranged in the liquid and provided with water-tight driving means are required, and such a driving screw can produce a uniform liquid displacement only within a restricted cross-sectional area, and has only a limited efficiency.

The object of the invention is to provide an aeration device not having these disadvantages, which device is characterised in that a rotatable propelling means in the form of at least one paddle wheel with a vertical axis of rotation is used, which is provided with paddles extending over at least part of the height of the liquid, which rotation axis is situated substantially in the plane of a wall defining the circulation path, the aeration means being formed by air introduction apertures or nozzles opening into the liquid displaced by these paddles. Such a paddle wheel can be rotated relatively slowly for obtaining an effective liquid displacement at the normally required low velocity, the efficiency being high, and because of the low speed no splashing of the liquid will take place.Thus pollution of the surroundings, and particularly of the driving means situated above the liquid surface, will be avoided. Because of the low driving power and the absence of the noise of splashing water, the noise disturbance is considerably reduced. Since the driving means can be arranged above the liquid surface, special sealings are not required, and maintenance is simple. These driving means can be mounted, furthermore, on a relatively light bridge. The paddle wheel can be supported, if necessary, on a bottom bearing. Since the liquid is propelled also near the bottom, it is possible to use deeper basins.

The invention will be elucidated below by reference to a drawing, showing in: Fig. 1 a diagrammatical representation in perspective, with parts broken away, of an aeration device according to the invention; Fig. 2 a corresponding representation of a part of a modified embodiment of this device; Fig. 3 a simplified plan view at a smaller scale of the device of Fig. 1 with a modified basin; Figs. 4A..C cross-sections according to the line IV-IV of Fig. 1 with differently shaped paddle wheels; and Fig. 5 a plan view corresponding to Fig. 1 with a plural circulation path.

In Fig. 1 an elongated basin with straight sidewalls 2 and curved end walls 3 is shown, and in the longitudinal median plane thereof a partition wall 4 is arranged, extending only over a part of the length of the basin, so that a closed circulation path 5 is obtained. In one of the walls 3 a liquid supply 6, and in a lateral wall 2 near this end wall an overflow 7 is provided, connnecting with a discharge chamber 8 with a discharge duct 9.

It is the intention that the liquid supplied at 6 flows in the sense indicated by arrows through the circulation path 5, and flows off after at least one turn via the overflow 7. The liquid remains, then, in the path 5 during a minimal circulation duration which is determined by the distance between the points 6 and 7 and by the circulation velocity of the liquid.

For obtaining a biological purification by means of micro-organisms (active silt), oxygen, and generally air, is supplied to the liquid, e.g. by means of bottom grids according to NL-A 77 11 017, or by means of equivalent nozzles, the gas bubbles 1 1 rising in a uniform flow over substantially the full width and height of the liquid, so that these bubbles, also because of the forward velocity of the liquid, are protractedly and intimately contacted therewith, and the oxygen will be dissolved effectively into the liquid. It is, of course, possible to introduce pure oxygen, but then the basin should be at least partly covered in order to avoid the pure oxygen leaving the liquid from being lost.

For circulating the liquid a paddle wheel 12 with a vertical shaft 13 is used, which shaft is situated in the plane of the partition wall 4, and its paddles 14 remain at a small distance from the terminal edge of the wall 4. Above the liquid surface a driving unit 16 is situated, consisting of an electric motor and a reducing gear, by means of which the paddles can be driven with a speed which is sufficient for circulating the liquid in the indicated sense and with the required velocity in the circulation path 5.

The rotational velocity is relatively low, so that the liquid is not splashed up, and the driving unit 16 and the surroundings are not exposed to the eroding and polluting effects of the liquid.

The support for the driving unit 1 6, not shown, can be relatively light, and in particular the paddle wheel 12 can be additionally supported in a bottom bearing 17. The distance 1-5 should be as small as possible in order to restrict the circular flow around the paddle wheel as much as possible.

The paddles 14 shown in Fig. 1 extend over the full height of the liquid, so that the liquid will be uniformly propelled over the full height, and is contacted with the air bubbleflow 11 in a corresponding uniform manner, and also the suspended silt is uniformly entrained.

If the basin 1 is long, it may be favourable to use more than one paddle wheel for maintaining a uniform flow. A second paddle wheel can, for instance, be arranged at the other end of the wall 4, and it also possible, in the manner of Fig. 2, to interrupt the wall 4 between its extremities, and to arrange, there, a paddle wheel which has the same effect on both branches of the path 5. In the same manner it is possible to interrupt the side wall 2, and to provide it with an adapted recess in which a paddle wheel is arranged, in which case the shaft 13 should be situated substantially in the plane of this wall.

in order to obtain a gradual deflection of the flow, it may be favourable, in the manner shown in Fig. 1 , to arrange a curved, and in particular semicylindrical, guiding partition 18. Also at the other end it may be advantageous to provide such a guiding partition. Fig. 3 shows a simplified plan view of a basin of Fig. 1, having, however, straight end walls 3', and similar guiding partitions 18 are provided as well. As a matter of fact two or more guiding partitions might be used as well. It is, then, not necessary to arrange, as shown, the guiding partitions co-axially with the paddle wheels. If a coaxial guiding partition 18 is used which is situated at a very small distance from the extremities of the paddles, the operation of the paddle-wheel can be improved in some cases.

Fig. 4 shows other embodiments of the paddles 14 of such a paddle wheel 12. At A the paddles extend from the bottom of the basin 1 only over a part of the height, so that the liquid is propelled at the lower side only. In the vicinity of the air introduction point, a higher liquid flow velocity than the average flow velocity will be obtained, so that the contact between air and liquid, and thus the air introduction effect, can be improved. The partition 4 is extended, above the paddles 14, up to the shaft 13, so that a narrow gap 15 is present everywhere.

At B trapezoidal paddles 14 are used, and the operation substantially corresponds to that according to A, but now also the upper layer of the liquid will be propelled, although to a smaller extend, so that a gradually upwardly decreasing propulsion is obtained. The terminal edge of the wall 4 is correspondingly tapered. It is also possible to use a stepped paddle wheel, and then a comparable distribution of the propulsion effect is obtained.

The paddle wheel shown at C has more or less helicoidally curved paddles 14, by means of which, on rotation, the upper liquid layers will be forced downwards or upwards, so that the mixing is improved. The paddles can also be made covergent or divergent towards an extremity. The terminal edge of the wall should, of course, be adapted to the shape of the cylindrical or conical surface generated by the paddle edges.

The paddles shown in Figs. 1 and 4A and B are lying in an axial plane. It is, however, also possible to mount the paddles substantially tangentially to the shaft 13. Instead of plane paddles also curved ones may be used.

Fig. 5 shows a plural circulation path obtained by arranging an additional U-shaped partition 4', one end wall consisting of two curved wall parts 3". Two paddle wheels 12 are positoned near these walls 3", and if, necessary, near the other end of the intermediary wall 4 a third paddle wheel can be arranged, as indicated with interrupted lines. Furthermore two aeration grids 10 are shown, but it will be clear that also more or less grids may be used. Moreover the number of bends in the circulation path can be extended at will.

Besides with the bottom grids shown or with equivalent injection nozzles, the air introduction can also take place near the paddle wheels, e.g. by means of grids or nozzles arranged therebelow, or by means of air injection nozzles arranged in the shaft and/or the paddles themselves.

Furthermore the paddle wheels described above can be advantageously used in the case of aeration by means of surface aerators, and then the propulsion of the liquid can also take place if the surface aerators are switched off partially or completely, e.g. during a period of little or no supply, so that settling of active silt is prevented thereby.

The use of the paddle wheels according to the invention, which can operate independently of the air introduction means, has still another special advantage, viz. that an aeration installation can be made too large for the actual needs, in order to take into account a future larger demand, since the air supply can be adjusted independently of the desired circulation velocity.

It has appeared in practice that it may be favourable to use paddle wheels which remain at some distance from the bottom of the basin, and the adjacent partition wall will, then, be extended below the paddle wheel so as to suppress liquid whirls below the paddle wheel.

In the case of paddles extending more or less tangentially in respect of the shaft 13, it may be advisable to displace the shaft 13 laterally so that the paddle nearest to partition 4 extends substantially in the plane of the latter.

Generally the outer diameter of the paddle wheel will not be substantially larger than the width of the flow path 5, and it is not always necessary to use a guiding partition 18, not even in the case of a non-cylindrical end wall 3.

Also basins shaped in another manner can be used, e.g. banana or pear shaped basins, and in the latter case the more or less pointed part of the pear shaped inner wall will play the role of the partition 4 near which a paddle wheel is mounted.

In use the rotational velocity of the paddle wheel will be chosen so as to maintain, in the flow path, a linear flow velocity of not more than about 0.5 m/s, and this velocity should not be so low that the active silt is no longer kept in suspension sufficiently. The velocity should, therefore, generally be higher than 0,2 m/s. When increasing the rotational speed of the paddle wheel, the required power will steeply increase because of friction losses, and the air absorption will not increase in the same manner, so that higher speeds should be avoided. For the same reason the dimensions of the paddles should not be made larger than is required for obtaining the desired liquid displacement The invention in not restricted to the described and shown embodiments, and can be applied in all cases in which a circulating liquid flow is to be maintained.

Claims (12)

1. A device for aerating a liquid, in particular waste water to be biologically purified, comprising an elongate basin with a supply for the liquid to be treated and a discharge for the treated liquid, in which basin a partition wall is provided extending substantially parallel to the longer sides of the basin up to some distance from the extremities thereof, so that a closed circulation path is obtained, and a rotatable propelling means is arranged and is to be driven in such a manner that the liquid is circulated through the closed path, means being, furthermore, present for introducing air or oxygen into the liquid, characterised in that the propelling means is formed by at least one paddle wheel with a substantially vertical axis of rotation, which is provided with paddles extending over at least a part of the liquid height, said axis of rotation being situated substantially in the plane of a wall defining the circulation path, the aeration means being constituted by air introduction openings or nozzles opening into the liquid displaced by the paddles.

2. The device of claim 1, characterised in that the rotation axis is situated substantially in the plane of the partition wall.

3. The device of claim 1 or 2, characterised in that the paddles have a transversal dimension which, in the upward direction, varies in conformity with the locally desired propulsion velocity.

4. The device of any one of claims 1 . .3, characterised in that the paddles are helicoidaily curved.

5. The device of any one of claims 1. .4, characterised in that the paddles are curved in respect of an axial plane.

6. The device of any one of claims 1 .. 5, characterised in that the adjacent boundary wall extends closely to the surface generated by the terminal edges of the paddles, or, as the case may be, up to the shaft of the paddle wheel.

7. The device of any one of claims 1 .. 6, characterised in that the boundary wall extends at both sides of the paddle wheel.

8. The device of any one of claims 1 . .7, characterised in that a curved guiding partition is provided beyond one or each extremity of the partition wall and outside the circumference of a paddle wheel, if present.

9. The device of claim 8, characterised in that the guiding partition is situated at a small distance from the terminal edges of the paddles.

10. The device of any one of claims 1 to 9, characterised in that the paddles are extending more or less tangentially in respect of the shaft of the paddle wheel, the paddle wheel axis being displaced in respect of the adjacent wall so that a paddle, when nearest to the extremity of said wall, is situated substantially in the plane of said wall.

11. The device of any one claims 1 to 10, characterised in that the driving means for a paddle wheel and the dimensions of the paddles thereof are chosen so that a linear flow velocity of less than about 0.5 m/s can be maintained in the circulating flow.

12. A device for aerating liquid substantially as hereinbefore described with reference to Figure 1 or to Figure 1 as modified by Figure 2 or Figure 3 or Figure 4 or to Figure 5 or to Figure 5 as modified by Figure 4 of the accompanying drawings.