GB2080276A

GB2080276A - The treatment of effluent by aeration

Info

Publication number: GB2080276A
Application number: GB8024392A
Authority: GB
Original assignee: Simon Hartley Ltd
Current assignee: Simon Hartley Ltd
Priority date: 1980-07-25
Filing date: 1980-07-25
Publication date: 1982-02-03

Abstract

Apparatus for the treatment of effluent by the activated sludge treatment process, comprises a vessel (10) in which a wall (11) defines a path (12) in the form of a closed loop around which effluent may flow, there being an aerator (15) rotatable in said path (12), a rotary paddle (17) driven by a variable speed motor (19), and probes (20) at spaced positions around said path to compare the dissolved oxygen levels at said positions, signals from the probes (20) being fed to a comparator (21) which produces a resultant signal used to control the speed of operation of motor (19) and paddle (17), thus to control the circulation time for the effluent around the path (12) and thereby optimise the biological utilisation of the carbonaceous and nitrogenous pollutants with the effluent. <IMAGE>

Description

SPECIFICATION The treatment of effluent by aeration THIS INVENTION concerns apparatus for the treatment of sewage or other effluent by the socalled activated sludge treatment process wherein the effluent is subjected to aeration in the presence of a biological population which metabolises on the impurities to produce so-called secondary sludge which can be separated from the effluent by sedimentation or other technique.

During recent years more and more interest has been shown in symstems incorporating a path in the form of a closed loop around which the effluent is caused to flow, the effluent being subjected to aeration at one position in such closed path. In one wellknown arrangement the aeration is effected by a rotary surface aerator mounted on a vertical drive shaft, which aerator also causes the liquid to circulate around the path.

Experieince has shown that the dissolved oxygen level in the effluent should fall to a low level, ideally approaching zero, over a substantial portion of the length of the path before the effluent re-enters the aeration zone. In this way denitrification processes take place as well as the normal oxidation processes (which are mainly responsible for the destruction of carbonaceous poliutants) in the aerobic portion of the path.

One problem is that the biochemical oxygen demand (B.O.D.) can vary substantially, particularly with municipal sewage effluent where the B.O.D. is influenced by the time of day, climatic and numerous other factors.

The present invention is based upon an appreciation of the possibility of adjusting the length of time it takes the effluent to travel around the closed loop path whilst maintaining a desired degree of oxygen transfer in the aeration zone, thus to optimise the treatment process by controlling the ratio between the lengths of the paths in which the oxidation and denitrification processes take place respectively.

According to the present invention apparatus for the treatment of effluent by the so-called activated sludge treatment process comprises a path in the form of a loop around which effluent may flow, aeration means at one position in said path and flow control means at another position in said path for varying the length of time taken for effluent to flow around said path, means for sensing the rate of utilisation of dissolved oxygen in the effluent in the path and means for adjusting the flow control means in such a manner in response to the measurement of the rate of utilisation of dissolved oxygen so as to give the effluent a circulation time around the path which optimises the biological utilisation of the carbonaceous and nitrogenous pollutants within the effluent In one preferred form of apparatus embodying the invention the flow control means is comprised by a motor driven circulation inducing or retarding means and in another preferred form of apparatus by a system of gates or baffles located in the path for controlling the rate of admission of effluent to the path.In yet another form of apparatus the flow control means is adapted to control the direction of admittance of liquid, such as the waste water to be treated and/or activated sludge returned to the circulating path so as to assist the circulation or retard it.

The invention will be further apparent from the following description with reference to the several figures of the accompanying drawings which show, by way of example only, a number of forms of apparatus embodying the invention.

Of the drawings: Fig. 1 shows a diagrammatic plan view of a first form of apparatus; Fig. 2 shows a diagrammatic plan view of a second form of apparatus; Fig. 3 shows a diagrammatic plan view of a third form of apparatus; and Fig. 4 shows a vertical cross-section on the line IV-IV of the apparatus of Fig. 3.

Referring firstly to Fig. 1 of the drawings, it will be seen that the first form of apparatus is comprised by an elongated vessel ortank 10 having awall or baffle 11 extending longitudinally thereof at a position halfway across its width between positions spaced from each of the opposed ends of the tank thus to define a channel 12 in the form of a closed loop around which effluent may circulate.

Effluent to be treated is supplied to the channel 12 through a duct 13 and treated effluent is removed through a duct 14. It will be understood that the volume of the channel 12 is such that the effluent supplied has the required residence time therein.

At one end of the tank 10 is located a rotary surface aerator 15 of the well-known kind mounted on the lower end of a vertical drive shaft 16.

A paddle device 17 is provided in one of the longitudinally extending legs of the channel 12 and this paddle device is mounted on a horizontal drive shaft 18 extending from a variable speed drive motor 19.

Probes 20 adapted to measure the dissolved oxygen level in the effluent in the channel 12 at spaced positions downstream from the rotary surface aerator 15 are provided. The signals from the probes 20 are fed to a control unit 21 which serves to compute the rate of utilisation of dissolved oxygen and to control the motor 19 in response thereto to vary its speed and perhaps even its direction of rotation (to retard the circulation induced by the aerator 15) to adjust the time taken for the effluent to make one complete circuit of the channel 12 whereby optimum biological utilisation of the carbonaceous and nitrogenous pollutants within the effluent is achieved.

Generally the control unit 21 will operate in accordance with a model determined by empirical means.

A number of variations are possible. Thus, for example, instead of a variable speed paddle device, a propeller running at a constant speed but have The drawing(s) originally filed was/were informal and the print here reproduced is taken from a later filed formal copy.

variable pitched blades may be used.

Referring now to Fig. 2 it will be seen that the second form of apparatus is generally similar to the first, like parts being indicated by like reference numerals.

In this form of apparatus the aerator 15 is located within a tank-like enclosure 30 which communicates with the downstream and upstream legs of the closed loop path by way of ports 31 and 32 respectively in which are mounted motor-driven gate or valve members 33 and 34. The controller 21 serves to adjust the positions of the members 33 and 34 so to control the rate at which effluent is admitted to the closed loop path as to achieve the desired time of circulation around such path.

Additionally a gate may be provided in a further port located within the baffle 11 adjacent the enclosure 30 to permit a certain quantity of effluent to be circulated without passing the aerator 15.

If desired the apparatus of Fig. 2 may itself incorporate a paddle such as that 17 described in connection with the apparatus of Fig. 1. If the paddle is provided it would itself be controlled by the unit 21 as well as the members 33 and 34.

The apparatus of Figs. 3 and 4 is somewhat different and comprised by a circular vessel 40 divided into inner and outer zones 41 and 42 respectively by means of an annular wall 43. The annular wall 43 extends upwardly through the surface of liquid contained within the vessel 40 but terminates at a position upwardly spaced from the base 44 of the vessel 40. A rotary surface aerator 15 is mounted at the centre of the inner zone 41 and serves to pump liquid outwardly through ports 45 located at circumferentially spaced intervals in the wall 43 at the level of the liquid surface.

In use, as the aerator operates, liquid is pumped outwardly and a portion thereof flows through the ports 45 to follow a downward helical path through the outer zone 42 before returning to the inner zone 41 by passage beneath the lower edge of the annular wall 43.

As in the previously described embodiments probes 20 are provided to measure the dissolved oxygen level at spaced positions in the helical path downstream from the aerator 15. These probes are connected with a control unit 21 which controls motor driven circulation inducing means 46 operative within the outer zone 42. The means 46 may take the form of a paddle similar to that described in relation to the embodiment of Fig. 1 or the form if a pump returning activated sludge to the outer zone under pressure and in a direction to induce circulation along the downward helical path referred to earlier.

Additionally, or alternatively, the control unit 21 may serve to operate on pivotally mounted baffle members 47 associated with each of the ports 45 to control the rate at which effluent is passed from the inner zone 41 to the outer zone 42 and hence the time taken for such effluent to pass through the outer zone 42 following the downward helical path referred to before returning to the inner zone 41.

It will be appreciated that it is not intended to limit the invention to the above examples only, many variations, such as might readily occur to one skilled in the art, being possible without departing from the scope thereof.

Thus, for example, the control unit 21 may additionally be programmed to affect the operation of the aeration means by, for example in the case of a rotary surface aerator, varying its speed of rotation and/or its dept of immersion, thus to influence the rate of oxygen transfer and providing an additional control possibility.

Claims

1. Apparatus for the treatment of effluent by the so-called activated sludge treatment process, comprising a path in the form of a loop around which effluent may flow, aeration means at one position in said path and flow control means at another position in said path for varying the length of time taken for effluent to flow around the path, means for sensing the rate of utilisation of dissolved oxygen in the effluent in the path and means for adjusting operation of the flow control means accordingly so as to give the effluent a circulation time around the path which optimises the biological utilisation of the carbonaceous and nitrogenous pollutants within the effluent.

2. Apparatus according to Claim 1, wherein said flow control means is comprised by a driven, circulation-inducing or retarding means whose speed of operation is adjustable thus to select the flow rate of effluent around the path.

3. Apparatus according to Claim 1, wherein said flow control means comprises at least one gate or baffle located in the path and adjustable to control the rate of flow of effluent around the path.

4. Apparatus according to Claim 1, wherein said flow control means is adapted to control the direction of admittance of liquid to said path so as to assist the circulation therein, orto retard it.

5. Apparatus according to any preceding claim, wherein said path is defined by an elongate vessel having an internal wall or baffle extending part way along the length thereof thus to define a channel around said wall or baffle.

6. Apparatus according to any preceding claim, including a rotary surface aerator, said flow control means comprising a rotating paddle disposed at a position remote from said aerator, said paddle, in use, being driven in rotation at a variable speed.

7. Apparatus according to any preceding claim, wherein said means for sensing the rate of utilisation of dissolved oxygen comprises a pair of probes disposed at spaced positions around said path thus to measure the dissolved oxygen level at said two positions, means for comparing the dissolved oxygen levels sensed by said two probes thus to commute the rate of utilisation of dissolved oxygen, said comparator means producing a resultant signal adapted to control the speed of operation of said flow control means.

8. Apparatus according to any preceding claim, wherein said flow control device comprises a pair of gates or baffles disposed one upstream and one downstream of said aeration means with respect to the direction offlowofeffluentaroundsaid path, said gates or baffles being movable selectively to restrict the flow of effluent around said path.

9. Apparatus according to Claim 1, including a circular vessel divided into inner and outer concentric zones by an annular wall having passage means for the effluent to pass between the zones in the upper and lower regions of the vessel, said aeration means being adapted to cause circulation of the effluent in the vessel between said zones thus to rise in one zone and descend in the other, said flow control means comprising a driven paddle providing a substantially helical flow path for said effluent, said sensing means being adapted to detect the dissolved oxygen content in the effluent in the upper and lower regions of one of said zones, said paddle being driven at a speed dependent upon the relative levels of dissolved oxygen in said regions.

10. Apparatus according to Claim 9, wherein said flow control means is comprised by pivotally mounted gates or baffles adapted to control the flow of liquid between said zones.