GB2305427A

GB2305427A - Aeration by counterflow of fluid waste with gas

Info

Publication number: GB2305427A
Application number: GB9618209A
Authority: GB
Inventors: Wun Jern Ng; Guang Hao Chen; Hin Fatt Cheong
Original assignee: National University of Singapore
Current assignee: National University of Singapore
Priority date: 1995-09-14
Filing date: 1996-08-30
Publication date: 1997-04-09
Anticipated expiration: 2016-08-30
Also published as: GB9618209D0; SG54084A1; ID17210A; GB2305427B

Abstract

An aerator 2 for fluid waste, eg. a liquid/solid mixture, comprises an aeration chamber 14a and an impeller 7 at the top of the chamber, the impeller directing the fluid waste downwardly against the upward flow of an aeration medium, eg air or oxygen, the aeration medium being distributed from the base 3 of the chamber. The aerated fluid waste is subsequently discharged from the chamber via an outlet adjacent the impeller. Preferably the aerator also comprises a distributor in the form of a coiled pipe (3, Fig 6), below the impeller, for introducing the aeration medium into the aeration chamber and a discharge chamber 13a which receives the aerated fluid waste from the aeration chamber through an inlet adjacent the hub of the impeller, the discharge chamber being surrounded by the aeration and distributor chambers (Fig 6). The discharge chamber may have an outlet below the distributor.

Description

TITLE Aerator for Fluid Media This invention relates to a mechanical aerator primarily for the transfer of gaseous media, such as air or oxygen, into fluid media, such as a liquid/solid mixture or suspension comprising liquor or slurry.

This invention is particularly useful in the waste water treatment industry where there is a need to feed dissolved oxygen into biological waste water treatment systems. In general, this invention is useful in any application where an increased dissolved oxygen level would be beneficial in any water borne aerobic treatment process.

Aeration is necessary in aerobic biotreatment processes to supply the oxygen required for bioxidation of the organic contaminants in the waste water and maintain a minimum dissolved oxygen level in the bioreactor.

Aeration in such treatment systems can account for as much as 60% of the total energy consumption. In order to save energy, and hence reduce operating costs, efficient aeration systems play a key role.

Currently a number of aeration systems are commercially available.

These may be divided into three broad categories being air diffusers; mechanical aerators without an air supply; and mechanical aerators with an air supply. The air diffusers depend on an external air supply being forced through perforated discs or membranes. The latter break the air into small bubbles which enhance air-liquid contact and dissolution of oxygen into the water. Mechanical aerators without an air supply are often surface aerators which break up the water surface and hence also achieve better air-liquid contact than is possible naturally. The third group are often submerged aerators with air supplied from an outside source. Air and water are intimately mixed in the aerator which enhances transfer of oxygen into the water. The third group is also of increasing interest because, with the air supply cut off, such aerators can then serve as mixers.Where waste water treatment involves nitrification and denitrification, the capability of the aeration system to provide mixing without aeration would be a useful one. Diffuser systems are often not applicable where intermittent aeration is practised as biomass clogging of the diffuser pores would occur.

A number of aerator types belonging to the third type have been disclosed in'literature Tetsuo Ide (Water Treatment Engineering, 2nd Ed., Gihoto Publishers, 1990, pp 248-257) describes an aerator where air is released towards an impeller with flat blades. Water is simultaneously drawn in by the impeller downwards to the bottom of the aerator. Water and air are mixed in a mixing chamber where the impeller is located and then released into the main body of water.

U.S. Patent No. 4925598 (1990) describes an aerator where air is supplied to the rotor from whence it is discharged from openings. The airsewage mixture is forced down and out through stator ducts. U.S. Patent No. 5045202 (1991) describes an equipment where the impeller is used to create a highly agitated mixture of air and waste in a collector.

In the three examples described the impellers or rotor serve to mix the air-sewage intimately while also prolonging the contact time between the two. The size of the air bubbles so produced would obviously have an important contribution to the efficiency of the aerator system. While the processes disclosed by U.S. Patents 4925598 and 5045202 represent advancements in oxygen transfer in waste water treatment, they are relatively complex and cumbersome and may not be suited to communities which require simpler and cheaper equipment.

One object of this invention is to provide an aerator means, preferably submersible and preferably using recirculating aerator principles, for the main purpose of supplying oxygen to waste water, but not limited to such use.

Another object is to provide an aerator which can provide mixing of the contents of a bioreactor when aeration is not required.

A further object of this invention is to provide an aerator with a combination of aeration and mixing which will provide efficient oxygen transfer with water levels as low as I m. This will allow for installations with very limited headroom or use in shallow vessels or ponds or in retrofitting of existing waste water treatment plants to achieve greater aeration efficiency and hence treatment capacity. In this respect, the invention may provide a sufficiently compact and simple device such that only minor modifications need be made to the existing facilities. Simplicity in equipment construction also facilitates installation and maintenance in areas where skilled operators are lacking.

It is a further object of this invention to provide an aerator wherein mixing alleviates problems of odours in waste water collection systems such as pump sumps and buffer tanks by providing sufficient dissolved oxygen to inhibit anaerobic degradation.

According to this invention there is provided an apparatus for aeration of a fluid medium, the apparatus comprising an aeration chamber, a distributor for introducing an aeration medium into said chamber, an impeller positioned above said distributor, a discharge chamber positioned within the aeration chamber having an inlet adjacent the impeller and an outlet below the distributor, the distributor and aeration chamber being located around the discharge chamber, the impeller forcing fluid medium downwards into the aeration chamber against the upward flow of aeration medium, the aerated fluid medium passing through the inlet of the discharge chamber to the outlet thereof.

According to this invention there is also provided a method for the aeration of a fluid medium wherein the fluid medium is directed downwardly by an impeller against an upward counterflow of aeration medium distributed from the base of an aeration chamber, the aerated fluid medium in the chamber being discharged through an outlet adjacent the hub of the impeller and at the top of the aeration chamber.

This invention and advantages and preferred features thereof will be further described in the following more detailed description taken in conjunction with the accompanying drawings and showing embodiments by way of examples.

In the drawings: Figure 1 shows a fragmentary view of an aerator according to this invention Figure 2 shows a cross-section taken through the motor, rotor, impeller and discharge chamber of the aerator of Figure 1, Figure 3 shows a section taken substantially along line A-A of Figure 2, Figure 4 shows a section taken substantially along line B-B of Figure 2, Figure 5 shows a section taken substantially along line L-L of Figure 4, Figure 6 shows a section taken substantially along line C-C of Figure 2, Figure 7 shows a section taken substantially along line D-D of Figure 2, and Figure 8 shows a schematic view of an aerator according to this invention mounted in a waste water treatment reactor.

Referring to the drawings1 and more particularly Figures 1 and 2, the main components of the aerator comprise an outer aerator chamber casing 14 and an inner discharge chamber casing 13 coaxial therewith. The inner casing 13 defines a discharge chamber 1 3a whilst the upper annular space between the casings 13 and 14 defines an aeration chamber 14a. A flat coiled air distributor pipe 3 is located on a support plate 3a mounted between the casing 13 and the casing 14 towards the upper end thereof and closes off the aeration chamber in a fluid and air tight manner. The pipe 3 includes spaced air discharge apertures 3b along its length as more ciearly seen in Figure 6. The pipe 3 is connected to an air supply inlet 1 mounted on the side of casing 14.A connector 15 is provided in the casing wall 14 to serve as an aeration chamber pressure sampling or sensing point if required.

The upper end of casing 14 laterally widens and has a coupling flange 5 which is connected with flange 6 forming the upper portion of an impeller casing 6a. A bladed impeller 7 is located with the peripheral parts within the casing 6a and is connected through drive shaft 8 and coupling 9 with a drive motor 10.

The motor 10 is mounted on a top plate 11 which is coupled in vertically spaced relationship to the flange 6 by means of struts or legs 12.

An air supply pipe la is also mounted on the top part 11 and connects with the air inlet coupling 1. The aeration chamber is designated generally as 4.

A gasket 5a may be positioned between the flanges 5 and 6 which are secured by bolts Sb.

Motor 10 will preferably be controlled by means not shown to operate at variable speeds. The blades of the impeller 7 may include hardened and sharpened leading edges 7a to break up any solid matter contained in the slurry.

In operation the aerator unit 2 will be submerged in a fluid medium such as a slurry or liquor as indicated in Figure 8. Air pumped in through the inlet 1 is discharged through the apertures in the air distribution pipe 3 in an upward direction and the impeller 7 rotates in a direction which forces the fluid downwards against the air distribution pipe. Fluid which is drawn down into the aeration chamber 4,14a is forced against the air from pipe 3 producing a back pressure and thus vigorous aeration of the fluid is provided. By virtue of the gas pressure the aerated fluid is forced out and downwards through the discharge chamber 13 and the lower outlet 1 3a from this chamber may be associated with a dispersion means such as a box with tangentially mounted baffles to distribute the aerated fluid away from the base of the aerator 2.In some cases it may be preferable to omit a distributor so that the aerated fluid issues from the discharge chamber 1 3a as a narrow jet.

Although the drawings show the diameter of the impeller 7 to be greater than that of the aeration chamber base, this can be varied. The diameter of the top opening of the aeration chamber cover 6 may be the same as that of the aeration chamber base 5 or may be greater or smaller.

The ratio of the diameter of the aeration chamber base 5 to the discharge chamber 13 may be between 4:1 and 3:1. The clearance between the impeller 7 and the top of the chamber 13 may be about 3 to 5 cm. The ratio of the height of the aeration chamber 4 to the diameter may be between 1:1 and 2:1.

As shown in Figures 4 and 5, the impeller 7 has three blades with each aligned at 45" to the horizontal plane. The blades include sharp cutting edges 7a and serve to draw down liquid into the aeration chamber 4 and to mix same with air as well as breaking up by a cutting action any solid material in the liquid.

The air distribution pipe 3 may be positioned at any point within the aeration chamber 14a and the precise location may be adjusted according to requirements and performance desired. The unit may be located further towards the base of chamber 14a than the position shown in the drawing.

The discharge chamber wall 13 and the aeration chamber wall 14 may be secured together by laterally extending webs 17 as illustrated in Figure 7.

As shown in Figure 8, the aerator unit 2 may be located within a container or sump 18 either suspended from the top through the top plate 11 and struts 12 or alternatively may rest above the sump floor.

Compressed air is fed to the air inlet duct 1 by a pump 22 preferably feeding through a controlled flow meter 23. The impeller drive motor 10 may be controlled through a unit 22 and may be operated at constant or variable rotational speeds 275 to 1200 r.p.m. being typical. Speed may be controlled through a dissolved oxygen sensor 19 coupled to a meter and control unit 20. A recording means 21 may be provided. The aeration transfer efficiency of the unit is controlled by both the impeller rotational speed and the rate of supply of air. For top mounted units the aerated fluid is forced out of the aerator 2 in a downward direction and for units mounted on the floor of the sump, the air-fluid mixture may be forced through a dispersion box and out in a more horizontal direction.

Test results obtained from an apparatus constructed substantially in accordance with the foregoing description are illustrated in the following table and in comparison with a prior art construction using a ceramic diffuser.

Performance of the aerator in comparison with diffusers (air = 1 L min .1 water depth - 45 cm).

TABLE

Speed rpm IQ, h-1 OR , gO2h-1 OTE, % Remarks 510 17.20 3.64 21.13 Aerator 692 29.58 6.26 36.35 940 54.04 11.44 66.40 1043 72.76 15.41 89.41 3.96 0.84 4.86 Ceramic diffuser 29.74 6.61 36.65 Ceramic diffuser with 02 OR = Oxygen transfer rate Note: KL3 = Oxygen transfer coefficient OTE = Oxygen transfer efficiency

Claims

CLAIMS 1. An apparatus for aeration of a fluid medium, the apparatus comprising an aeration chamber, a distributor for introducing an aeration medium into said chamber, an impeller positioned above said distributor, a discharge chamber positioned within the aeration chamber having an inlet adjacent the impeller and an outlet below the distributor, the distributor and aeration chamber being located around the discharge chamber, the impeller forcing fluid medium downwards into the aeration chamber against the upward flow of aeration medium, the aerated fluid medium passing through the inlet of the discharge chamber to the outlet thereof.
2. An apparatus according to Claim 1, wherein the aeration chamber and discharge chamber are coaxially positioned with the distributor forming a substantially fluid tight barrier therebetween.
3. An apparatus according to Claim 1 or 2, wherein the diameter of the aeration chamber is between three and four times the diameter of the discharge chamber.
4. An apparatus according to Claim 1, 2 or 3, wherein the distributor comprises a spirally coiled pipe with a plurality of spaced apertures along the length thereof for discharge of the aeration medium.
5. An apparatus according to any preceding claim, wherein the aeration chamber and discharge chamber comprise coaxial cylinders the ends of which are positioned beneath the impeller, the distributor comprising a planar spirally coiled pipe with discharge apertures mounted on an annular support plate extending between the aeration chamber and discharge chamber below the ends thereof.
6. An apparatus according to any one of the preceding claims 2 to 5, wherein the aeration chamber and the discharge chamber are formed by casings extending below the distributor, the part of the casings extending below the distributor comprising preferably at least 50% of the overall length of the casings.
7. An apparatus according to any one of the preceding claims 2 to 6, wherein the aeration chamber and the discharge chamber comprise casings extending the same distance beneath the distributor.
8. An apparatus according to any one of the preceding claims, wherein the aeration chamber is formed by a casing extending below the distributor and serving to support the discharge chamber
9. An apparatus according to any preceding claim, wherein the height of the aeration chamber is twice the diameter.
10. An apparatus according to any preceding claim, wherein the impeller blades have cutting leading edges and the pitch angle thereof is between 30 and 60".
11. An apparatus according to any preceding claim wherein the impeller Is driven by a variable speed motor.
12. An apparatus according to any preceding claim, wherein the ends of the impeller blades are embraced by a casing integral with the aeration chamber casing
13. An apparatus according to Claim 12, wherein the impeller casing has a greater diameter than the aeration chamber casing.
14. An apparatus according to any preceding claim, wherein the height of the discharge chamber is between about three to four times the diameter thereof.
15. An apparatus according to any preceding claim, wherein the outlet of the discharge chamber includes baffles to produce lateral deflection of the aerated fluid medium discharged therefrom.
16. A method for the aeration of a fluid medium wherein the fluid medium is directed downwardly by an impeller against an upward counterflow of aeration medium distributed from the base of an aeration chamber, the aerated fluid medium in the chamber being discharged through an outlet adjacent the hub of the impeller and at the top of the aeration chamber.
17. An apparatus for the purposes herein disclosed constructed and arranged to function substantially as described herein and as shown in the accompanying drawings.
18. A method for aeration of a fluid medium carried out substantially as herein described and illustrated.