GB2245267A - Aerator for activated sludge treatment - Google Patents

Abstract

Treatment apparatus 10 for activated sludge sewage has an aerator 13 driven by a motor 9 through gear 12. The aerator is moved up and down and/or the motor 9 raised in speed in response to the sensed dissolved oxygen in material being treated. The aerator and motor 9 are carried on plate 11 supported on screw jacks driven by motor 18 through transmission (15, 16, 17). Pillars (25) extend through apertures (26) in plate 11 to resist rotation of the plate 11. The aerator axis can be vertical as shown in fig. 2 or horizontal with plate 11 pivoted at one side (fig. 4). <IMAGE>

Description

TREATMENT APPARATUS AND AERATOR HANDLING MEANS This invention relates to treatment apparatus and in particular means for moving an aerator.

The largest item of cost in the operation of an activated sludge sewage treatment plant is almost always the cost of the energy needed to drive the aerators which introduce oxygen into the sludge. The cost may be kept to a minimum by using no more energy than is needed to transfer the amount of oxygen needed, strip carbon dioxide and keep the mixed liquor solids in suspension.

In most cases the limiting factor is the amount of oxygen needed, in which case the energy needed for a satisfactory performance will be a minimum if the concentration of dissolved oxygen is controlled efficiently at the minimum level needed for the process.

According to this invention apparatus for use with an aerator having means for driving the aerator comprises means for moving the aerator and the drive means to vary the extent to which the aerator is immersed in material to be aerated.

The invention includes treatment apparatus having an aerator, means for driving the aerator and said moving means.

The means for moving may comprise means mounting the aerator and driving means, and means for moving the mounting means.

The means for moving the mounting means may comprise a motor carried by the mounting means and arranged to drive jacks connected to move the mounting means.

The jacks may be screw-jacks.

The moving means may comprise control means having means for sensing the dissolved oxygen in the material to be treated, and means for operating the moving means in response to the sensing means.

The control means may also be connected for operating the aerator drive means in response to the sensing means.

The aerator drive means may comprise a motor having more than one speed, and the control means may be operative to change the motor speed.

The invention may be performed in various ways and one specific embodiment with possible modifications will now be described by way of example with reference to the accompanying schematic drawings, in which: Fig. 1 is a plan view of an aerator assembly for use in an activated sludge system; Fig. 2 is a side view in the direction of arrow A in Fig. 1; Fig. 3 is a plan view of another arrangement; Fig. 4 is a side view of Fig. 3; and Fig. 5 is a control diagram.

Described generally, the assembly 10 has a mechanical jacking system driven by a geared motor unit which raises or lowers a complete aeration unit in response to the requirements of a control unit receiving signals from a dissolved-oxygen sensor. The arrangement is suitable for any mechanical aeration system in which the power taken by the aerator drive motor and the oxygen dissolved is governed by the extent to which the aerator is immersed in the liquid in the aeration tank.

A typical arrangement of the jacking system for a vertical shaft mechanical aerator is illustrated diagrammatically in Figs. 1 and 2. The assembly 1Q has a U-shaped plate 11 to which is bolted a reduction gear 12 driving the aerator 13 and driven by electric motor 9.

The U-shaped plate 11 is carried by four inverted screw jacks 14 which are driven in unison through shafts 15 and angle gears 16 and couplings 17 by a reversible geared motor 18. The jack-screws 14 are supported in pairs by channel members 20 bolted to a concrete bridge 21 spanning the aeration tank 22 having an aperture 23 through which the aerator drive shaft 24 extends. The U-shaped plate 11 is prevented from rotating in response to the reaction torque from the aerator 13 by vertical pillars 25 on opposed sides extending upwardly from the bridge 21 through apertures 26 in the plate 11. Raising or lowering the aerator is achieved by applying power to the geared motor unit 18 for a period defined by â control unit 8, the period being repeated after a defined interval to provide the required rate of response.The total travel is limited to a set amount by limit switches 29. A linear potentiometer 7 is fitted for position indication. The dissolved-oxygen sensor is shown at 6.

The system is designed to enable installation beneath existing aeration units to be carried out relatively easily, using portable hand-operated equipment, without the need to dismantle the existing drive units.

A vertical shaft aerator drive unit is subject to a fluctuating torque. Provision is therefore made for adjustment to allow play in the jacking system to be reduced to a minimum, in order to avoid excessive vibration.

The system is set up in the first instance so that the aerator is fully immersed in the liquid, or sludge, i.e. the driving motor 18 is loaded to the design maximum when the jacking system is at its lowest position. If, during operation this results in an excess of dissolved oxygen in the liquid or sludge the control system causes the jacks 14 to be driven in the 'raise' direction, thus reducing the power taken by the aerator motor 9 and the amount of oxygen supplied.

Although the illustration is based on the use of a vertical shaft aerator the same approach may be used for a horizontal shaft aerator. In this case the arrangement shown diagrammatically in Figs. 3 and 4 may be used. In this case the aerator 30 rotates about a horizontal axis and is supported at one end by a motorised reduction gear 38 and at the other by a stub shaft 31. The bearings provided for the stub shaft and the reduction gear are mounted on beams 33, one end of each of which is supported by pivots 34. The other ends of the beams 33 are joined by member 32 to form a U shaped structure, comparable with the U shaped plate 11 of Fig. 1, and supported by screw jacks 36. The screw jacks are driven by the motorised reduction gear 35, mounted on member 32 by means of shafts 37.

The control system illustrated schematically in Fig. 5 compares the level of dissolved oxygen (DO) in the contents of the aeration tank, expressed as an average value over a short period, with the desired value. If the DO level is too high, the control unit 8 causes the jacking motor 18 to rotate in the 'raise' direction for a specified period determined by the divergence of the actual DO from the desired value. The system then rests for a period after which the process is repeated.

A similar action occurs if the DO is too low with the difference that in this case the motor is caused to rotate in the 'lower direction.

If the range of adjustment needed is greater than that which can be obtained efficiently by variation of immersion alone the range can be extended by the use of a two-speed aerator drive motor 9. In this case when the aerator has been raised to its top level, a demand for a further decrease in dissolved oxygen results a change to lower aerator speed. Because this will result in a large reduction in oxygen transfer, the DO after the resting period will probably be too low. The aerator will therefore be then driven downward under normal control action. Alternatively, reaching the top limit of movement may initiate a sub-control loop in the control which results in a change of motor speed and at the same time a lowering of the aerator unit almost to its lowest position. If the oxygen demand subsequently increases and the DO is too low when the aerator is in its lowest position a similar action is initiated; in this case the aerator speed being increased and the aerator raised. The speeds and range of immersion must be chosen so that there is a small degree of overlap, otherwise hunting would result at certain oxygen demand rates.

The arrangement provides automatic control of dissolved oxygen concentration in an activated sludge plant by movement of the whole aerator assembly, and also a possible change to a different aerator speed on reaching a limit of immersion adjustment.

The energy needed to move the aerator assembly is insignificant. The range of adjustment is governed by the range over which the oxygen transfer efficiency and the mixing characteristics of the aerator are acceptable.

This depends on the characteristics of the particular aerator and tank system but typically corresponds to a change in transfer rate of about two to one. If a wider range of adjustment is needed this method can be used in conjunction with a two speed aerator drive motor to give an effective range of about four to one.

Claims (11)

1. Apparatus for use with an aerator having means for driving the aerator comprising means for moving the aerator and the drive means to vary the extent to which the aerator is immersed in material to be aerated.

2. Apparatus as claimed in Claim 1, in which the means for moving comprises means mounting the aerator and the drive means, and means for moving the mounting means.

3. Apparatus as claimed in Claim 2, in which the means for moving the mounting means for moving the mounting means comprises a motor carried by the mounting means and arranged to drive jacks connected to move the mounting means.

4. Apparatus as claimed in Claim 3, in which the jacks are screw jacks.

5. Apparatus as claimed in any preceding claim, in which the moving means comprises control means having means for sensing dissolved oxygen in material to be treated, and means for operating the moving means in response to the sensing means.

6. Apparatus as claimed in Claim 5, in which the control means is connected for operating the aerator drive means in response to the sensing means.

7. Apparatus as claimed in Claim 5 or Claim 6, in which the aerator drive means comprises a motor having more than one speed, and the control means is operative to change the motor speed.

8. Apparatus as claimed in any of Claims 1 to 4, in which the aerator drive means comprises a motor and the moving means comprises means for sensing dissolved oxygen in material to be treated, and control means for varying the speed of the motor in response to the sensing means.

9. Apparatus as claimed in Claim 8, in which the control means includes means responsive to the position of the aerator.

10. Apparatus for use with an aerator having means for driving the aerator substantially as hereinbefore described with reference to and as shown in Figs. 1 and 2, or Figs. 3 and 4, of the accompanying drawings.

11. Treatment apparatus having an aerator, means for driving the aerator, and apparatus as claimed in any preceding claim.