CA1049162A

CA1049162A - Method for the biological purification of effluent and the plant for carrying out the process

Info

Publication number: CA1049162A
Application number: CA75239868A
Authority: CA
Inventors: Joseph R. Kaelin
Original assignee: Union Carbide Corp
Current assignee: Union Carbide Corp
Priority date: 1974-11-18
Filing date: 1975-11-18
Publication date: 1979-02-20
Also published as: DE2459634A1; IT1053080B; JPS5839596B2; FR2291157B1; ATA44575A; AU8670075A; DE2459634B2; JPS5173762A; GB1496069A; CH572003A5; CS190502B2; FR2291157A1; AT333676B

Abstract

METHOD FOR THE BIOLOGICAL PURIFICATION OF
METHOD FOR THE BIOLOGICAL PURIFICATION OF
EFFLUENT AND THE PLANT FOR CARRYING OUT THE
PROCESS

ABSTRACT OF DISCLOSURE
Effluent to be biologically purified is poured serially through three interconnected tanks with the first two tanks acting as activation tanks and the last tank acting as a post-clarification tank. After the concentra-tion of activated sludge in the first tank has dropped to a predetermined value, the flow through the tanks is re-versed so that the last tank, previously the post clarifi-cation tank, acts as one of the activation tanks, and the first tank, previously one of the activation tanks, acts as the post clarification tank, whilst the middle tank con-tinued to operate as the other of the activation tanks.

In one embodiment, an aerator is provided in each tank and the aerators in the first and last tanks are switched on or off depending upon the direction of flow of effluent. In another embodiment, two movable aerators are provided and are disposed in use, in those tanks which are acting for the time being as activation tanks.

This invention relates to a process for the bio-logical purification of effluent and a plant for carrying out the process.

S P E C I F I C A T I O N

Description

1049~1~2 DESCRIPTION OF PRIOR ART

There is a known construction for clarifying plant with preclarification, activation and post clarification tanks. In recent years however a further step forward has been made which allows preclarification for the equivalent of approximately lO,OOO inhabitants to be completely omitted.

It is also known that, in the case of a small clar-ifying plant, the effluent is purified in a single tank in which aeration, and after a given period of aeration, ~:
settling is allowed to take place in the same tank, where-: by above the settled activated sludge a zone of clarified water exists from which a purified effluent is withdrawn by means of a pump. In a larger clarifying plant which possesses no preclarification and in which purification is carried out by means of activation and postclarification, :~
there is the disadvantage in that either relatively deep sump has to be provided or expensive mechanical clearance equipment must collect the sludge and return it to the .: :

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: ' activating tank.
Attempts have been made to arrange two side-by-side tanks in such a way that they function alternately as aeration and post-clarification tanks. The aeration tank, which has the same dimensions as the post-clarification tank, in the next phase of the operation becomes the post-clarification tank and the post-clarification tank in a sub-sequent operation becomes the aeration tankO Both tanks have the same dimensions in order to permi~ reversal of the direction of operation. In practice, however, it so happens in most cases, that the volume of the post-clarifying tank should be about half ~he volume of the activating tank so that this technique using two tanks of similar size is not the optimim.
An object of the invention is to provide a method and a plant whereby, in the post-clarification tank, no mechanical sludge removal is necessary and in spite of this, no sludging occurs in the plant, and in which the volume of the aeration stage in both directions of operation of the plant is in the desired ratio to the substantially smaller volume of the post-clarifying stage.
BRIEF SUMMARY OF THE INVENTION
According to the present invention, there is provided a method or the biological purification of efflu-ent, comprising the steps of feeding effluent to be puri-fied successively to first and second tanks, acting as .

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activation tanks, in a set of at least three inter-connected tanks; introducing oxygen or an oxygen-containing mixture into the effluent in said first and second tanks; feeding, after a predetermined average residence time in said first and second tanks, the efflu-ent from the second tank to a third tank of the set, said third tank being a clarification tank, withdrawing clar-ified effluent from the third tank until the concentra-tion of activated sludge in the first tank is reduced to below a predetermined value; then reversing the flow of effluent through the set of tanks; and operating the third and second tanks as activation tanks and the first tank as a last post-clarification tank.
Where the tanks are particularly deep, for example, 6 to 12 metres deep, it ls an advantage if the pure oxygen or oxygen mixture is introduced by means of a rotary device disposed below the surface of the effluent, preferably by a double-sided centrifugal pump impeller into which the ëffluent in the aeration tank is intro-duced, and at least one supply pipe for supplying oxygen-containing mixture to an inlet zone of the device.
In order to avoid accumulation of sludge on the floor of each activating tank, it is an advantage if the oxygen or oxy$en-containing supply device causes an effluent flow rate directly above the bottom of the tank of at least 30 centimetres per second.

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-1~ 49~ 62 According to another aspect of the present in-vention, a plant for carrying out the method according to the invention, comprises a set of at least three interconnected ~anks; at least.two device for the set for introducing oxygen or an oxygen-containing mixture into the effluent present in the activating tanks; asso-ciated w~th the tanks at each end of the set,- controll able means for effluPnt supply and effluent removal; and a control arrangement for the controllable means and ~ .
: 10 said at least two devices for changing the direction of ; flow through the sets so that, in one direction, two of the tanks at one end of the set act as activation tanks and a tank at the other end o~ the set acts as a post- -clarif~cation tank, whilst, in the opposite direction, two tanks at said other end of ~he set act as ac~ivating ~ :
tanks a~d th~ tank at said one end of the set acts as a post-clarification tank.
In order to construct a compact plant, it is - expedient for the tanks to be interconnected ~n end-to-20 end relationship, with dividing walls therebetween having openings therethrough.
In the case of a larger plant, construc~ional costs can be reduced by building several such triple tanks so . that adjacent triple tanks have a common wall.

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; 25 In order to obtain the smoothest possible flow, .....

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despite the highly turbulent condition of the effluent, to achieve an optimum post-clarification effect, it is ex-pedient if the dividing wall be~ween an activating tank and a post-clarification tank is provided, on at least one side thereof facing the tank acting intermittently as the post-clarification tank with a bafflè which pro-jects from above the opening in the wall downwardly and inwardly of the respective tank so as to overlap said opening.
It is further advantage one of such baffles is provided on each side of the dividing wall. It is ~urther advantageous if an oxygen or oxygen-containing mixture introducing device is provided in each tank, the devices in the end tanks being arranged, in use, to be operative or inoperative depending upon the direction of effluent flow through the tanks, and the device in the middle tank being arranged to be operatèd, in use, in both directions of 10w of effluent through the tanks.
In order to reduce the number of the oxygen intro-ducing devices and to reduce the resultant costs it can be an advantage i the tanks are disposed adjacent to each other with dividing walls therebetween having at least partially closable openings therethrough and two introducing devices are movable 80 as to be capable o passing through said openings.

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~09~L62 BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-Figure 1 i6 a longitudinal section, on line I-I
of Figure 2 of one embodiment of a plant according to the present invention;
Figure 2 is a plan view of the plant illustrated in Figure l;
Figure 3 is a section on the line III-III in Figure 2; and ~igure 4 is a longitudinal section of ano~her form of plant in accordance with the present invention., DETAILED DESCRIPTION
With reference to Figures 1 to 3, effluent to be purifled is fed via a delivery pipe 1, a rake 2, a sand ~rap 3? a coarse material crusher 4? followed by a water metering device 5 and one of two remote controlled supply valves 6a and 6b into one of two outer tanks 7a and 7b of a unit consisking of three tanks 7a, 7b and 8 I~ for example, the ~upply valve 6a is ~pened, then at the same time the 6upply valve 6b is closed 80 that the effluent supplied for purification reaches only the righthand tank 7a of the three tank unit. Rotary surface aerators 9a and 9c in the tanks 7a and 8 arranged in ~his ~ L049~6Z
mode of plant operation to act as activating tanks are rotated simultaneously by respective drive motors 10.
Rotation of the aerators 10 causes air or air enriched by pure oxygen to be mixed with the effluent in tanks 7a and 8. The rotary surface aerator 9b in tank 7b acting as a post-clarification tank is at rest. A discharge arrangement llb associated with tank 7b is opened whilst a discharge arrangement lla associated with tank 7a is closed. Discharge pipes of the discharge arrangements lla and llb extending at right angles through the tanks 7a or 7b are so arranged below the level of the liquid that no floating material finds its way into these dis-charge openings.
Effluent being treated passes generally through tanks 7a and 8 where it is aerated by means of the aerators 9a and 9c and through tank 7b via respective ! openings 13 between the tanks 7a, 8 and 7b. In tank 7b, aerated sludge in the effluent settles out onto the floor of tank 7b over the area thereof. Thus, a post-clarifica-tion of the effluent takes place. Since~ in the tank 8, the effluent is in a turbulent condition and a smooth uniform flow through tank 7b is required to effect post-clarification on opposite sides of a wall 12 separating tanks 8 and 7b there are provided flow baffles 14 and 15 respectively. Each baffle 14, 15 is secured to the wall 12 above the openings 13 over the entlre width of the :' ,, , , .

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tank 8, 7b and projects downwardly in inclined relationship to the wall 12. This baffle arrangement produces a very smooth uniform flow in the post-clarification tank 7b.
The openings 13 between tank 7a and 8 are provided wlth similar baffles. Clarified and bio~ogically purified effluent liquor leaves tank 7b by way of the discharge arrangement llb and a discharge duct 16.
Since ~ the above described process, activated sludge is constantly displaced from both the first tanks 7a and 8 into the ~ank 7b and is not returned, the con-centration of activated sludge in the tanks 7a and 8 de~reasés as the process continues. Below a predetermined concentration of activated sludge, a heavy reduction in the purification capacity takes place. Furthermore, no more oxygen is added to the activated sludge deposited in tank 7b so that, in this tank also, after a predetermined operation time results would be noticeable.
In order to avoid the aforementioned results and to ensure an acceptable purification capacity9the direction of operation of the three tank unit is reversed after a predetermlned time andj before the concentration of the activated sludge in both the first tanks 7a and 8 has dropped to a predetermined value. In the course of this changeover, which takes place fully automatically, the rotary surface aerator 9a in the irst tank 7a is next switched off, the outlet arrangement llb is closed and . . .
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the remote controlled supply valve 6b is opened. The rotary surface aerator 9c in ~he middle tank 8 can remain in operation. After the rotary surface aerator 9a has stopped, the liquid in the tank 7a becomes ca~m and any activated sludge therein settles on the floor of tank 7a.
Settling of the activated sludge in tank 7a i6 monitored by a photo-electric cell device 17 situated at a pre-determined distance below the outlet arrangement lla The device 17 is arranged to pass a control pulse, when the sludge is sufficiently settled, to ~pen the outlet arrangement lla, whereby the draining of water from tank 7a is effected. At the same time, the rotary surface aerator 9b in tank 7b is switched on and consequently ~he sludge deposited in this tank 7b is brought into the required state of suspension and supplied with oxygen.
In this manner, effluent treatment occurs in the reverse direction through the three tank ùnit. The tank 7b is similarly provided with another device 17 for opening outlet arrangement llb to start effluent treatment in the forward direction.
For drainage of the excess sludge, there is provid-ed in ~he middle tank 8 an excess sludge pipeline 18 through which excess sludge is withdrawn and taken to a sludge concentrating and accumNlating tank 19. The sludge from tank 19 i6 removed by a sludge removing device 20.

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The plant can of course, as is indicated in Figure

2 by dotted lines, be constructed symmetrically to double the capacity.
The rotary surface aerators 9a and 9b arranged to operate intermittently in the tanks 7a and 7b depending upon the direction of effluent flow are so dimensloned and designed that, in their operation directly over the floors of these tanks, a flow rate of at least 30 centi-meters per second is produced in order to bring settled activated sludge back into a state of suspension.
It is, of course, also possible to seal the tops of the tanks 7a, 7b and 8 hermetically and, in operation, supply oxygen or oxygen enriched air into the space formed above the level of the effluent over pipelines.
To increase biological activation, it is possible for the temperature of the effluent in the activation tank, to be raised by at least 2C, preferably by 5 - 10 C, over the temperature of effluent entering the plant. Such heating may be effected by means of a heat exchanger through which hot water is circulated. Such hot water may be used cooling water from an atomic reactor.
Alternatively, waste heat from a sludge concentration plant may be employed.
In order to avoid the provision of rotary aerators in all three tanks 7a, 7b and 8, it is possible, as can , ' - ~ ' . ' ~ , ,. ' ;, ' ' . ' ~ ', '; . , ' ~O 49 be seen from Figure 4, to attach to a movable suppor~
above the tanks 7a, 7b and 8 ~wo rotary aerators 23 and 24, e.g~ two double-sided centrifugal pump impellers for introducing air, oxygenated air or pure air into the effluent to be cleared are used. Such centrifugal pump impellers 23 and 24 are especially useful in the case of deep tanks of, for example 6 to 12 metres deep, whereby in each of the tanks in which they are installed they form two main rotary flows superimposed on each other and in doing so thoroughly agitate the ent~re contents of the tank and rapidly bring the settled activated sludge into the required state of suspension. Pipelines ~indicated by dotted lines 25) for the supply of pure oxygen or oxygen mixture into the effluent to be clarified, extend as far as an inle~ zone of the aeration turbines 23 and 2~.
In order to obtain a vortex-free flow of effluent in both the inlet openings of the rotary aerators 23~ 24 ~iewed in the axial direction, in front of these inlet openings there are provided cruciformly arranged upper and lower flow baffles 26 and 27, respectively. The upper baffles 26 are attached to the movable support and the lower flow baffles 27 are attached to the floors~of the tank 7a, 8 and 7b.
If now the directionof opera~ion of the plant .

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shown in Figure 4 is changed so that the tank 7b operates as the activation tank, and tank 7a as the post-clarifica-tion tank, openings 28 in walls between the tanks 7a, 8 and 7b are exposed by laterally moving slides 29 which normally close openings 28. Then the rotary aerators 23 and 24 are moved by moving the movable support to the left as viewed in Fig. 4 until aerator 24 is disposed in the tank 7b and aerator 23 is disposed in tank 8. Finally, the openings 23 are closed by slides 29.
The control of the supply valve and the outlet arrangement for the operation of the plant takes place :
in a similar manner to that described with reference to Figures 1 to 3 and need not therefore be described more closely. .
If pure oxygen or oxygenated air is to be supplied, such supply is via a supply pipe 25 and the tops of tanks 7a, 7b and 8 are sealed so that the excess oxygen reaching the surface of the effluent can be removed from the spaze above the Liquid and recycled.

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Claims

WHAT IS CLAIMED IS:

1. A method for the biological purification of effluent, comprising the steps of feeding effluent to be purified successively to first and second tanks, acting as activation tanks, in a set of at least three interconnected tanks; introducing oxygen or an oxygen-containing mixture into the effluent in said first and second tanks; feeding, after a predetermined average residence time in said first and second tanks,the effluent from the second tank to a third tank of the set, said third tank being a clarification tank; withdrawing clarified effluent from the third tank until the concen-tration of activated sludge in the first tank is reduced to below a predetermined value; then reversing the flow of effluent through the set of tanks; and operating the third and second tanks as activation tanks and the first tank as a last post clarification tank.

2. The method according to Claim 1, wherein the oxygen or oxygen-containing mixture introduction step is effected by means of a rotary surface aerator.

3. The method according to Claim 1, wherein the oxygen or oxygen-containing mixture introduction step is effected by means of a rotary device situated below the surface of the effluent in each aeration tank, and by feeding said oxygen or oxygen-containing mixture via at least one supply pipe to an inlet region of each rotary device.

4. The method according to Claim 3, wherein each said rotary device is a double-sided centrifugal pump impeller.

5. The method according to Claim 1, wherein a flow rate of at least 30 cm/sec is maintained over the floor of the activation tank.

6. The method according to Claim 1, wherein the oxygen-containing mixture is air enriched with pure oxygen.

7. The method according to Claim 1, including the step of heating the effluent in at least one of the activating tanks to a temperature of at least 2°C above that of the effluent being fed into the set of tanks.

8. The method according to Claim 7, wherein said temperature is 5°C to 10°C above that of the effluent being fed into the tanks.

9. A plant for carrying out the process according to Claim 1, comprising a set of at least three interconnected tanks wherein an end tank and a next successive tank act as activation tanks; at least two devices for said set for introducing oxygen or an oxygen-containing mixture into the effluent present in said activation tanks; associated with the tanks at each end of the set, controllable means for effluent supply and effluent removal; and a control arrangement for said controllable means and said at least two devices fur changing the direction of flow through said set so that, in one direction, two of said tanks at one end of said set act as activation tanks and a tank at the other end of said set acts as a post-clarification tank, whilst, in the opposite direction, two tanks at said other end of said set act as activating tanks and the tank at said one end of the set acts as a post-clarification tank.

10. The plant according to Claim 9, wherein said tanks are interconnected in end-to-end relationship, with dividing walls therebetween having openings there-through.

11. The plant according to Claim 10, wherein the dividing wall between an activating tank and a post-clarification tank is provided, on at least one side thereof facing the tank acting intermittently as the post-clarification tank with a baffle which projects from above said opening in said wall downwardly and inwardly of the respective tank so as to overlap said opening.

12. The plant according to Claim 11, wherein one of said baffles is provided on each side of said dividing wall.

13. The plant according to Claim 9, wherein three tanks are provided, and an oxygen or oxygen-containing mixture introducing device is provided in each tank, said devices in the end tanks being arranged, in use, to be operative or inoperative depending upon the direction of effluent flow through said tanks, and said device in the middle tank being arranged to be operated, in use, in both directions of flow of effluent through said tanks.

14. The plant according to Claim 9, wherein said tanks are disposed adjacent to each other, with dividing walls therebetween having at least partically closeable openings therethrough and two introducing devices are movable so as to be capable of passing through said openings.

15. The plant according to Claim 9, wherein each of said introducing devices is a rotary surface aerator.

16. The plant according to Claim 9, wherein each of said introducing devices is a submerged rotary aerator.

17. The plant according to Clam 16, wherein each said aerator is a double-sided centrifugal pump impeller.

18. The plant according to Claim 17, wherein a pipe is provided for the supply of pure oxygen or an oxygen-containing mixture to each aerator.