A METHOD AND A DEVICE FOR AERATING WASTE WATER

Abstract: A method for aerating waste water, wherein the interface area between air and waste water is enlarged by splashing the water by means of blades (4) and wherein the sludge present in the waste water is kept in suspension by creating flows and/or turbulence in the water. The waste water is set in motion by moving a plurality of blades (4) arranged in a circle through the waste water, in such a manner that the connecting parts interconnecting the blades substantially do not extend into the waste water (13). The device for aerating waste water is provided with a container for waste water and with a rotor (1, 2) being rotatable about a substantially vertical axis, whose portion pointing downwards may extend into the waste water. The rotor comprises blades (4) having surfaces which are substantially radially and axially oriented, whereby said surfaces are at least partially located below the rotor parts interconnecting the blades, so that said connecting parts substantially do not extend into the waste water (13).

Description: A METHOD AND A DEVTCE FOR AERATING WASTE WATER The invention relates to a device, a rotor and a method for aerating waste water. The device is provided with a container for waste water and with a rotor being rotatable about a substantially vertical axis, whose portion pointing downwards may extend into the waste water. Such so-called surface aerators are known. They are often used in an aerating tank of an active sludge installation and their purpose is to dissolve oxygen in the water, thus providing the micro organisms in the aerating tank with oxygen. Transfer of oxygen mainly takes place at the interface between air and water, and an optimum transfer of oxygen of air to water is obtained by making the interface area as large as possible. Furthermore the transfer of oxygen is promoted by keeping near the interface the oxygen content in the water as low as possible, because the higher the oxygen content of the water at the interface the more difficult the dissolving of oxygen in water will be. Consequently it is important that the water near the interface is quickly renewed. The amount of energy required for aerating constitutes the larger portion of the energy consumption of a waste water purification plant. It is of paramount importance, therefore, that the amount of oxygen dissolved in the water per energy unit (the oxygen input efficiency) is as large as possible. The regulation of the oxygen input capacity of a surface aerator provided with a rotor which agitates the water takes place by changing the immersion depth and/or the speed of revolutions of the rotor. The immersion depth range of the surface aerator must be sufficiently large to cope with the normal variations in the water level of an f aerating tank, whilst maintaining a high oxygen input efficiency. The speed range within which a high oxygen input efficiency is achieved should likewise be as wide as possible, so that, both when a large amount of oxygen is put in and when less oxygen is put in, the highest possible oxygen input efficiency is achieved. The object of the invention is to aerate waste water with a high oxygen input efficiency. Furthermore the invention aims at obtaining a high and nearly constant oxygen input efficiency over a wide immersion depth range and a wide speed range. Another object of the invention is to develop sufficient flow velocity and turbulence in the waste water for keeping the sludge in suspension and in contact with the dissolved oxygen. Furthermore the invention aims at aerating waste water in such a manner, that the forces generated remain limited, so that it is possible to use a relatively light construction for the installation. Another object of the invention is to provide a device for aerating waste water, which is relatively insensitive to fouling, in particular to substances, materials and material waste present in the waste water becoming attached to the rotor. In order to accomplish these objectives the device for aerating waste water according to the invention comprises a rotor, which comprises blades having surfaces which are substantially radially and axially oriented, whereby said surfaces are at least partially located below the rotor parts interconnecting the blades, so that said connecting parts substantially do not extend into the waste water. According to a further aspect of the invention the blades may consist, at least partially so, of plate-shaped portions located at some distance from the axis of rotation of the rotor, which portions may have a width, seen in radial direction, of 0.07 - 0.3 times the outside diameter of the rotor. The width of the plate-shaped portions of the blades may decrease in downward direction thereby. According to a further aspect of the invention the blades may be interconnected by a substantially horizontal plate- shaped part, near the edge of which the blades are secured. In that case the blades may extend on either side of said horizontal plate-shaped part, whereby the height of the blades may be 0.05 - 0.3 times the outside diameter of the rotor. According to the invention the plate-shaped portion of the blade may blend at its upper side into a portion bent in the direction of rotation of the blade, which portion slopes upwards in outward direction. In order to stiffen the construction the bent portion may be connected at the radially inward side to the connecting parts interconnec¬ ting the blades, for example the horizontal plate-shaped part. The term "bent portion" is understood to mean a portion of the blade which is inclined at an angle to the plate-shaped portion of the blade. The bent portion may be a loose part, which is secured to the other portion of the blade. The invention furthermore relates to a method for aerating waste water, wherein the interface area between air and waste water is enlarged by splashing the water by means of blades and wherein the sludge present in the waste water is kept in suspension by creating flows and/or turbulence in the water, whereby the waste water is set in motion by moving a plurality of blades arranged in a circle through the waste water, in such a manner that the connecting parts interconnecting the blades substantially do not extend into the waste water. In order to more fully explain the invention an embodiment of the rotor for aerating waste water will be described below with reference to the drawing. Figure 1 is a plan view of a rotor; Figure 2 is a sectional view along the line II-II in Figure 1; and Figure 3 is a view along the line III-III in Figure 1. The Figures are only schematic illustrations of a rotor for aerating waste water. The rotor is provided with a central shaft 1, to which a plate 2 secured perpendicularly thereto is connected. The shaft 1 may be connected at its upper side to the rotor driving means. A plurality of recesses 3, 12 in the present embodiment, are provided in the outer edge of the plate 2, within which blades 4 are located. The blade 4 is plate-shaped and has a lowermost point 5, an edge 6 sloping upwards and outwards, which blends into a vertical edge 7 extending up to above the plate 2, an edge 8 / which extends inwards and downwards up to the plate 2, and edge 9, which extends vertically downwards from the plate 2, and an edge 10, which extends downwards and outwards up to the lowermost point 5. A plate 12 is secured along the entire upper edge 8 in the direction of rotation 11 of the rotor, said plate including a certain angle, 90° in the present embodiment, to the blade 4. The plate 12 may also be formed by bending the plate-shaped material, from which the blade 4 is made, in the direction of rotation of the rotor at the edge 8. In order to stiffen the attachment of the blade to the plate-shaped part 2 the plate 12 may be welded to the plate-shaped part 2 at the edge 14, or be secured thereto in any other manner, so that a stiff and solid connection between the blade and the plate-shaped part 2 is obtained. The lowermost portions of the rotor blades 4 extend into the waste water, and by rotating the rotor as indicated by the arrow 11 the waste water is agitated. Thus an upward flow against the blade 4 is created, whereby the flow is deflected at a certain angle by the plate 12. The water falls back into the waste water 13 at some distance from the blade. This creates a turbulence, whereby air bubbles are formed, resulting in a high amount of interface renewal of the water. The shape of the rotor with the blades 4 is such, that the lowermost portion of the blades extends into the waste water 13, whilst the plate-shaped part 2 connecting the blades 4 together and to the rotor is located at a level above the surface of the waste water 13, in such a manner that there is no or virtually no contact between said plate-shaped part and the waste water. This reduces the amount of energy required for the rotation, so that a higher oxygen input efficiency is obtained. Moreover it is prevented in this manner that additional forces are exerted on the rotor, is particular upward forces which might be produced if shape of the rotor would be such that the horizontal plate-shaped part interconnecting the blades is located on or in the water. Such undesirable forces would be transmitted via the rotor shaft to the gear box driving the rotor, in which case the gear box would have to be constructed extra solid. Since the plate-shaped part 2 is located above the water surface 13, the rotor drive may be constructed lighter and the drive will have a longer life. Preferably the distance between the water surface and the plate-shaped part 2 is at least 0.025 times the diameter of the rotor. The rotor is insensitive to fouling and is self-cleaning, due to the arrangement and the shape of the blades and due to the fact that the portion of the rotor extending into the waste water is kept as small as possible. Of course the invention is not limited to the illustrated embodiment. In addition to many other variations it is possible not to provide the plate 12 on every blade 4, but on for example every other blade 4. Thus the water will be splashed by the rotor over a larger area. A similar effect may be obtained by varying the angle which the edge 8 of the blade 4 includes with the plate 2 for all or for some of the blades 4. Also the orientation of the blades 4 with respect to the shaft 1 may be varied. The blades 4, which are radially oriented in the present embodiment, may also be somewhat tangentially oriented or have another orienta¬ tion deviating from the radial orientation. It is not necessary that all the blades have the same orientation thereby, but they may have different orientations in order to achieve an optimum effect.

Claims: CLAIMS 1. A device for aerating waste water, provided with a container for waste water and with a rotor being rotatable about a substantially vertical axis, whose portion pointing downwards extending into the waste water, said rotor comprising blades having plate- shaped portions which are substantially radially and axially oriented, whereby said plate-shape portions are at least partially located below the rotor parts interconnecting the blades, and that said parts, for example a plate shaped part, substantially do not extend into the waste water. 2. A device according to claim 1, characterized in that the width of said plate-shaped portions of the blades decreases in downward direction. 3. A device according to claim l or 2, characterized in that the plate-shaped portions of the blades extend on either side of said horizontal plate-shaped part. 4. A device according to any one of the preceding claims, characterized in that the plate-shaped portion of the blade blends at its upper side into a portion bent in the direction of rotation of the blade, which portion slopes upwards in outward direction. 5. A device according to claim 4, characterized in that said bent portion is connected to the said horizontal plate-shaped part. 6. A rotor for aerating waste water, which rotor may extend into the waste water in a first axial direction, and which rotor comprises blades having plate-shaped portions which are substantially radially and axially oriented, whereby said portions of the blades extend on either side of said plate- shaped part extending perpendicularly to the axis of rotation. 7. A rotor according to claim 6, characterized in that the width of said plate-shaped portion of the blades decreases in said first axial direction. 8. A rotor according to claim 6 or 7, characterized in that the plate-shaped portion of the blade, at the axial direction opposed to said first axial direction, blends into a portion bent in the direction of rotation of the blade, which portion slopes radially inwards in said former axial direction. 9. A rotor according to claim 8, characterized in that said bent portion is attached to the connecting parts interconnecting the blades, for example the said plate shaped part. 10. A method for aerating waste water, wherein the interface area between air and waste water is enlarged by splashing the water by means of blades and wherein the sludge present in the waste water is kept in suspension by creating flows and/or turbulence in the water, whereby the waste water is set in motion by moving a plurality of blades arranged in a circle through the waste water, whereby said blades are connected to a rotor rotating about a substantially vertical axis, in such a manner that the connecting parts interconnecting the blades, for example a plate- shaped part, substantially do not extend into the waste water. 11. A method according to claim 10, characterized in that the number of revolutions at which said rotor rotates is such that the velocity of the blades is 2 - 7 m/sec. 12. A method according to claim 10 or 11, characterized in that the water is brought up by the blade and subsequently flung sideways with respect to its direction of movement by an edge of the blade which slopes upwards and which is bent in forward direction. 13. A method according to any one of the claims 10 - 12, characterized in that the level of the waste water is at least 0.025 times the outside diameter of the rotor below the said plate-shape part.

Inventors: TEN HOVE DIRK, TEN HOVE, DIRK
Applicants: HUBERT BV, HUBERT B.V.
Application year: 1993
Main classification: C02F3
IPC codes: B01F3, B01F7, B01F15, C02F3