APPARATUS FOR CONTACTING A GAS WITH A LIQUID, IN PARTICULAR FOR AERATING WASTE WATER

Abstract: An apparatus for aerating waste water comprising a surface aerator (2) adapted to rotate about a vertical axis (3). A number of circumferentially spaced baffles (13) are provided at the inner side of the cap (5). These baffles (13) extend upwardly from the lower side of the cap (5) to the top wall 6 and extend over a distance radially. The cap (5) may have a side wall (7) with a lower edge extending vertically downwards or inwards. Other embodiments of the cap (5) are specifically adapted for use in an installation for treating waste water, in which the apparatus serves also for bringing the liquid into circulation.

Description: The invention relates to an apparatus for contacting a gas with a liquid, in particular for aerating waste water. More in particular it relates to an apparatus comprising a surface aerator adapted to rotate about a vertical axis and a driving means coupled with said aerator. Such apparatuses are being used successfully for a long time in installations for treating waste water. In further developments of these apparatuses it was proposed according to the British patent specification 1,547,115 of applicant to dispose a rigid cap over the surface aerator, the cap having a downwardly directed circumferential side wall which terminates, during operating, below the liquid surface, and having a gas inlet opening with an adjustable cross-section. By the use of said cap, the nuisance due to the splashing of the surface aerator and to the noise generated by the same was effectively reduced and in addition, if a stench was produced, this was arrested to a very considerable extent by the cap. Although the cap, of course, impedes the gas suppy to the surface aerator, it had been surprisingly found that the cap causes a very significant improvement of the oxygen introduction efficiency. This improvement appeared to be due to a different operation of the surface aerator in the closed space at a reduced gas supply. By the presence of the cap the outflow of the gas bubbles produced by the aerator is slowed down. Furthermore, throwing out liquid by the surface aerator against the inner side of the cap causes a high rate of turbulence, which improves the transfer of oxygen of the gas bubbles to the liquid. This optimal condition can be recognized in the appearance of foam along the inner side of the side wall of the cap as a consequence of the accumulation of gas bubbles. It is a main object of the present invention to further improve the cap of this known apparatus. One of the secondary objects is to provide a new aerator cap, with which the oxygen introduction efficiency is further increased. Another object of the invention is to provide an aerator cap, with which a high oxygen introduction efficiency can be obtained for a wide range of aerator speeds. For this purpose the apparatus according to the invention is characterized in that the side wall of the aerator cap has a lower edge portion lying at least partially below the liquid surface during operation, and extending vertically downwardly or inwardly from the the remaining portion of the side wall of the cap. Such a lower edge portion causes gas bubbles supplied to the liquid by the aerator, which would escape outwardly along the lower side of the side wall of the cap and therefore would arrive outside of the cap without such lower edge portion, to be deflected inwardly by the edge portion and thereby remaining under the cap. As a result the number of gas bubbles under the cap increases, whereby the range of aerator speeds, in which the fenomenon of the foam formation occurs and in which consequently a high efficiency is obtained, is increased substantially. In case the side wall of the cap is provided with a vertical edge portion it is advantageously when this edge portion is adjustable in the vertical direction with respect to the remaining portion of the cap. In this way the immersion depth of the cap can be changed in a simple manner and independently of the aerator. According to a further embodiment of the apparatus according to the invention the cap is provided not only with a gas inlet opening but also with a gas outlet opening in one of the walls of the cap, an exhauster mounted in the cap serving as means for exhausting gas from under the cap. In this way it is possible to increase the efficiency of the apparatus. The explanation for this is as follows. Tests have proved that in the known cap a certain degree of oxygen depletion occurs in the oxygen containing gas, mostly air, which is inside the cap. This is related to the fact that a substantial part of the gas bubbles which are smashed into the liquid by the aerator return again under the cap (necessary for improving the efficiency). These gas bubbles have delivered a part of their oxygen to the liquid, whereby the oxygen concentration in these gas bubbles has decreased. Consequently the oxygen concentration inside the cap decreases. As a result of the underpressure prevailing inside the cap fresh gas is indeed continuously sucked in through the gas inlet opening, but this is insufficient to compensate the oxygen depletion mentioned. Since, according to the invention, gas is sucked out from under the cap, which gas is poor in oxygen, additional gas can be sucked in through the gas inlet opening as a result of the growing underpressure, which inflowing gas is more rich in oxygen than the gas under the cap. Consequently the oxygen concentration of the gas under the cap increases, whereby the oxygen introduction efficiency of the aerator increases. According to another embodiment of the apparatus according to the invention circumferentially spaced baffles are disposed at the inner side of the cap, the baffles extending over a distance radially inwardly from the side wall of the cap. These baffles cause the horizontal circulation of the liquid along the inner side of the cap to be interrupted and to be deflected into a more vertically directed flow. In this way the exchange between the liquid in the reservoir is improved, whereby the oxygen introduction efficiency increases. Herein it is possible that the baffles extend over a relatively small distance in radial direction, so that some horizontal circulation of the liquid is still possible and the gas bubbles are spread uniformly over the circumferential spacing between the baffles. If the baffles extend over a larger distance inwardly for instance nearly up to the circumference of the aerator at the level thereof, an accumulation of gas bubbles will occur at the side of the baffles which is the front side when viewed in the direction of rotation of the aerator. The invention will hereafter be elucidated with reference to the drawings, which very schematically show a number of embodiments of the apparatus for contacting a gas with a liquid by way of example. Fig. 1 is transverse section of a first embodiment of said apparatus. Fig. 2 is a plan view of the apparatus of fig. 1. Fig. 3. is a transverse section of a second embodiment of said apparatus. Fig. 4 is a plan view of the apparatus of fig. 3. Fig. 5 shows an alternative of the apparatus of fig. 3 in transverse section. Fig. 6 is a plan view of the apparatus of fig. 5. Fig. 7-10 show half of transverse sections of embodiments of the apparatus according to the invention. Fig. 11 is a diagram, in which the oxygen introduction efficiency is illustrated as function of the rotational speed of the aerator, wherein the influence of the aerator cap thereon is demonstrated. The drawings show a number of embodiments of an apparatus 1 for contacting a gas with a liquid, and in particular for aerating waste water. In the several figures corresponding parts have been indicated with the same reference numerals. The embodiments of Fig. 1-6 per sé do not constitute part of the present invention, but the features of these embodiments may be incorporated into the embodiments of Fig. 7-10 which are illustrative to the present invention. In all the embodiments the apparatus 1 is equipped with a surface aerator 2, lying at the level of the water surface during its operation; in the embodiment as shown this aerator has a conical aerating body. Of course, other kinds of surface aerators may be used instead. The surface aerator 2 is carried by a vertical shaft 3, which is coupled with a driving means 4, by which it is brought into rotation during operation. A cap 5 disposed over the surface aerator 2 consists of a substantially horizontal top wall 6 having a circular circumference, and an adjoining downwardly directed circumferential side wall 7 terminating below the liquid surface during operation. Generally the side wall 7 is inclined at 45° to the horizontal. In the top wall 6 of the cap 5 an air inlet opening 8 is formed, whose cross-section is adjustable by means of a valve 9. Of course, the cap 5 may also be provided with a plurality of air inlet openings, each having its own valve. In a first embodiment according to fig. 1 and 2 an air outlet opening 10 is provided in the top wall 6 of the cap 5. An outflow pipe 11 extends through this air outlet opening 10, a driveable exhauster 12 being arranged in said outflow pipe 11, which serves for exhausting air from under the cap 5. The fig. 3 and 4 show an embodiment of the apparatus, wherein a number of circumferentially spaced baffles 13 are provided at the inner side of the cap 5. These baffles 13 extend upwardly from the lower side of the cap 5 to the top wall 6, and also extend over a distance radially inwardly. The radial dimension of the baffles 13 is constant along the whole length. It is also possible that the baffles terminate in upward direction at a distance below the top wall 6. As an alternative fig. 5 and 6 show an embodiment, wherein the baffles 13 terminate short before the circumference of the aerator 2 at the height thereof, and the inner sides of the baffles 13 extend vertically upwardly, so that the radial dimension of the baffles 13 decreases in the upward direction. Fig. 7-10 show a series of embodiments of the apparatus 1 for aerating waste water according to the invention, wherein the side wall 7 of the cap 5 comprises a lower edge portion 14. This edge portion 14 is lying below the liquid surface during operation. As can be seen in fig. 7 the lower edge portion 14 may extend vertically downwardly from the remaining portion of the side wall 7 of the cap 5, so that the lower edge 14 has a cylindrical configuration. Fig. 8 illustrates an embodiment, wherein the lower edge portion 14 tapers conically from the remaining portion of the side wall 7, and according to fig. 9 the lower edge portion 14 extends inwardly in a plane which is substantially perpendicular to the axis of rotation 3 of the aerator 2. The length of the lower edge portion 14 transverse to the circumference thereof amounts to about 0.05 to 0.4 times the diameter of the aerator 2. Fig. 10 shows another embodiment, wherein the lower edge portion 14 which is cylindrical around the axis of rotation 3 of the aerator 2 is adjustable in the vertical direction with respect to the remaining portion of the side wall 7 of the cap 5. Fig. 11 shows a diagram, which illustrates the oxygen introduction efficiency as a function of the rotational speed of the aerator 2, wherein the influence of the cap 5 thereon is demonstrated. Line a indicates the curve of the efficiency for an aerator without a cap, line b indicates this curve if the known cap according to British patent specification 1,547,115 is used, and line c indicates the curve of the efficiency in case the cap of fig. 8 is provided for. In this diagram it can be seen, that in the latter case the maximum efficiency is almost not increased with respect to the situation in which the known cap is used, that, however, the range of rotational speeds of the aerator, in which an increased efficiency is obtained is enlarged substantially.

Claims: 1. An apparatus (1) for contacting a gas with a liquid, in particular for aerating waste water, comprising a surface aerator (2) adapted to rotate about a vertical axis of rotation (3), a driving means (4) coupled to the surface aerator (2), and a rigid cap (5) disposed over the aerator (2) and including a top wall (6) an adjoining side wall (7) inclining downwardly and outwardly from the top wall (6), and a gas inlet opening (8) provided in one of said walls (6,7) of the cap (5), the cross-section of the gas inlet opening (8) being adjustable, characterized in that the side wall (7) of the rigid cap (5) has a lower edge portion (14) lying at least partially below the liquid level during operation and extending vertically downwardly or inwardly from the remaining portion of the side wall (7). 2. An apparatus as claimed in claim 1, wherein the lower edge portion (14) has a cylindrical configuration around the axis of rotation of the aerator. 3. An apparatus as claimed in claim 2, wherein the lower edge portion (14) is adjustable in vertical direction with respect to the remaining portion of the side wall (7) of the cap (5). 4. An apparatus as claimed in claim 1, wherein the lower edge (14) tapers conically from the remaining portion of the side wall (7) of the cap (5). 5. An apparatus as claimed in claim 1, wherein the lower edge portion (14) extends inwardly from the remaining portion of the side wall (7) of the cap (5) in a plane substantially perpendicular to the axis of rotation (3) of the aerator (2). 6. An apparatus as claimed in one of the preceding claims, wherein a gas outlet opening (18) is provided in one of the walls (6,7) of the cap (5) and a means (12) for exhausting gas from under the cap (5) is mounted. 8. An apparatus as claimed in one of the preceding claims, wherein a number of circumferentially spaced baffles (13) are provided at the inner side of the cap (5), the baffles (13) extending upwardly from the lower side of the cap (5) and over a distance inwardly from the side wall (7) of the cap (5).

Inventors: TEN HOVE, DIRK
Applicants: DHV RAADGEVEND INGENIEURSBUREAU BV
Application year: 1987
Main classification: C02F3
IPC codes: C02F3, B01F3