GB1594767A

GB1594767A - Purification of sewage

Info

Publication number: GB1594767A
Application number: GB19975/78A
Authority: GB
Original assignee: Mahon & Mcphillips Ltd
Current assignee: Mahon & Mcphillips Ltd
Priority date: 1978-05-16
Filing date: 1978-05-16
Publication date: 1981-08-05

Description

(54) PURIFICATION OF SEWAGE (71) We, MAHON AND MCPHILLIPS LIMITED, a Company incorporated under the laws of the Republic of Ireland of Dublin Road, Kilkenny, County Kilkenny, Republic of Ireland, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the purification of sewage by the activated sludge method, wherein oxygen is added to the sewage by means of an aerator which also circulates the sewage.

Recent developments in this field have led to the production of a so-called surface aerator adapted to rotate about a vertical axis. Such aerators can have a higher efficiency that the previously used aerators which rotated about a horizontal axis, thus adding more oxygen per kWh to a sewage, and also have the advantage that the majority of the mechanical parts are kept dry.

Initially, single vertical surface aerators were only used in activated sludge installations with aerarting tanks which were square in plan view. Such an arrangement is shown in Figure 1 of the accompanying drawings, which is a cross-sectional view of a prior art aerating tank and surface aerator. The tank 10 is of substantially square shape in crosssection and contains the sewage 11 to be treated. The aerator 12 of conventional design is supported at the surface of the sewage and is rotatably driven about a vertical axis by an electric motor 13. The movement of the sewage 11 in operation of the equipment is indicated by the arrows 14, whereupon it can be seen that the sewage is thrown outwardly by the aerator 12, flows along the surface of the sewage, down the wall of the tank 10, along the bottom of the tank, and then upwardly in the centre of the tank back to the aerator.

The aerator 12 adds oxygen to the sewage by two mechanisms. The first is the splashing in the immediate vicinity of the aerator. The second mechanism is by the trapping of air bubbles in the liquid flowing outwardly from the aerator. When the liquid meets the sidewalls of the tank the liquid is diverted downwards towards the bottom of the tank, taking the air bubbles with it and thus increasing the oxygen transferred to the liquid by prolonging the amount of time the bubbles are in contact with the liquid.

The oxygen transfer efficiency of surface aerators is very dependent upon this second mechanism and as the distance from the aerator to the sidewalls increase there is a marked drop is performance. Thus there is a limit to the size of the tank 10 beyond which the efficiency of the aerator drops to below an acceptable amount.

UK Patent Specification 1,212,457 discloses a sewage purification installation using a surface aerator wherein this problem is reduced and sewage can be treated in large tanks with acceptable efficiency. This is achieved by locating the aerator at one end of an elongated trough and causing the sewage to circulate in a closed circuit along the length of the trough and back again around a central partition extending longitudinally of the trough. The advantage of this arrangement is that the surface aerator is closely surrounded on three sides by the walls of the trough, thus providing efficient oxygenation by the second mechanism described above, while the volume of the trough as a whole allows a large quantity of sewage to be treated.

This prior installation has two main disadvantages, however. Firstly, the aerator is discharging towards only three walls and thus is not operating at full efficiency as would be the case if the aerator were discharging to walls substantially completely surrounding it.

Secondly, the horizontal velocity of the liquid circulating in the circuit is not uniform with depth. The velocity is much greater at the surface than at the bottom of the trough.

This often results in "sludge" settling on the floor of the trough which is unacceptable.

Accordingly, the present invention enables the provision of a design of installation which avoids or reduces these disadvantages.

According to a first aspect of the present invention there is provided an installation for the purification of sewage by the activated sludge method, comprising an enclosure for containing sewage to be purified, the enclosure comprising a tank portion and a further portion external of the tank portion defining flow and return paths for sewage from the tank portion, the flow and return paths communicating with the interior of the tank portion via openings in the latter below the level of the sewage therein, and a surface aerator adapted to rotate about a vertical axis within the tank portion for agitating sewage therein and thereby adding oxygen to the sewage, the aerator being surrounded substantially on all sides by the tank portion, the arrangement being such that rotation of the aerator effects continuous circulation of sewage from the tank portion around the flow and return paths of the further portion and thence back to the tank portion.

According to a second aspect of the present invention, there is provided an installation for the purification of sewage by the activated sludge method, the installation comprising sidewalls defining an enclosure for containing sewage to be purified, partition means defining a closed circuit for the flow of sewage within the enclosure, and a surface aerator adapted to rotate about a vertical axis for circulating sewage around the circuit and adding oxygen to the sewage, the partition means comprising a first partition separating portions of the circuit upstream and downstream of the aerator, the aerator being located in a portion of the circuit communicating between the upstream and downstream portions, a second partition generally transverse the first partition and located between the aerator and the said upstream and downstream portions of the circuit, the second partition and certain portions of the enclosure sidewalls serving to form a tank surrounding the aerator substantially on all sides, and openings in the second partition below the level of sewage in the enclosure for allowing passage of sewage from the tank into the downstream portion of the circuit and from the upstream portion of the circuit into the tank.

The advantages of the invention are that by substantially completely surrounding the surface aerator by the tank the efficiency of oxygenation is improved compared to the case where the aerator is surrounded on only three sides, and that by providing the openings from the tank to the external portion of the flow circuit below the surface of the sewage an adequate velocity of sewage can be obtained near the bottom of the flow circuit.

For a better understanding of the present invention, and to show how the same may be put into effect, reference will now be made to the accompanying drawings, in which: Figure 1, previously described, illustrates a prior art installation for sewage treatment; Figure 2 is a schematic diagram of an installation according to the present invention seen in plan view; and Figure 3 is a schematic diagram of the installation of Figure 2 in side view.

Referring to Figures 2 and 3, the installation comprises sidewalls 20 and a base 21 defining a rectangular trough-like enclosure for containing sewage 22 to be treated. The enclosure contains a first partition 23 extending longitudinally along the centre of the enclosure, and a second partition 24 transverse the first partition 23. The second partition 24 together with the portions of the sidewalls 20 to the right thereof as viewed in Figure 2 form a tank 31 of square shape in plan view. A surface aerator 25 of known type is located centrally within the tank 31 and is adapted for rotation about a vertical axis by an electric motor 26. The second partition 24 comprises respective openings 27' and 27" on either side of the first partition 23, each opening being below the surface 28 of the sewage near the bottom of the enclosure.

The first partition 23 divides the portion of the enclosure to the left of the partition 24 into flow and return paths 29 and 30 respectively for sewage leaving or entering the tank through the openings 27' and 27" upon rotation of the aerator in an anti-clockwise direction. The portion of the partition 24 on the flow path side of the partition 23 is sloped downwardly in the direction of the sewage flow. Thus upon rotation of the aerator 25 sewage is discharged towards the four walls of the tank 31 and is then deflected downwardly taking air bubbles with it. Due to the portion 32 owt the partition 23 which extends in the tank to a position adjacent to the aerator 25, the sewage is first deflected through the opening 27' leading to the downstream portion or go path 29 of the troughlike enclosure. The sewage passes along the portion 29, around the portion 33 communicating with the downstream and upstream portions 29 and 30, back along the upstream portion or return path 30, and into the tank 31 through the other opening 27".

The sewage thus flows around a closed circuit continuously while the aerator 25 is rotating.

As mentioned before, the presence of the sloped portion 24' of the partition 24 at the commencement of the flow path 29 promotes efficient oxygenation of the sewage by the aerator, and the location of the openings 27' and 27" below the level of the sewage surface ensures an adequate flow velocity at the bottm of the enclosure. The slope and size of the partition 24 and the size of the openings 27' and 27", both upstream and downstream of the aerator 25, must be carefully designed to provide adequate circulation and scouring velocity in the aeration tank. In order to increase the scouring veloc ity, without additional energy inputs, it is necessary to transfer momentum from the high velocity upper layers to the lower layers near the tank floor. In this respect the aerator discharge is unaffected by the portion of the partition 24 placed upstream, however the downstream portion 24' of the partition 24 unless correctly designed could effectively minimise the momentum transfer from the high velocity discharge with a resultant reduction in the said scouring velocity and the circulation velocity in the installation.

The downstream portion 24' of the partition is therefore suitably sloped so as to provide effective oxygen transfer and allowing sufficient momentum transfer to provide the required scouring velocity and an adequate circulation velocity.

WHAT WE CLAIM IS: 1. An installation for the purification of sewage by the activated sludge method, comprising an enclosure for containing sewage to be purified, the enclosure comprising a tank portion and a further portion external of the tank portion defining flow and return paths for sewage from the tank portion, the flow and return paths communicating with the interior of the tank portion via openings in the latter below the level of the sewage therein, and a surface aerator adapted to rotate about a vertical axis within the tank portion for agitating sewage therein and thereby adding oxygen to the sewage, the aerator being surrounded substantially on all sides by the tank portion, the arrangement being such that rotation of the aerator effects continuous circulation of sewage from the tank portion around the flow and return paths of the further portion and thence back to the tank portion.

2. An installation according to Claim 1 in which the aerator is disposed centrally within the tank portion.

3. An installation for the purification of sewage by the activated sludge method, the installation comprising sidewalls defining an enclosure for containing sewage to be purified, partition means defining a closed circuit for the flow of sewage within the enclosure, and a surface aerator adapted to rotate about a vertical axis for circulating sewage around the circuit and adding oxygen to the sewage, the partition means comprising a first partition separating portions of the circuit upstream and downstream of the aerator, the aerator being located in a portion of the circuit communicating between the upstream and downstream portions, a second partition generally transverse the first partition and located between the aerator and the said upstream and downstream portions of the circuit, the second partition and certain portions of the enclosure sidewalls serving to form a tank surrounding the aerator substantially on all sides, and openings in the second partition below the level of sewage in the enclosure for allowing passage of sewage from the tank into the downstream portion of the circuit and from the upstream portion of the circuit into the tank.

4. An installation according to claim 3, in which the tank formed by the second partition and the sidewall portions is substantially square in plan view, and the aerator is located substantially centrally within the square.

5. An installation according to claim 3 in which a portion of the second partition, which is located between the aerator and the upstream portion of the circuit, is a partition sloped downwardly in the direction of the sewage flow.

6. An installation according to claim 3, in which the first partition extends into the tank to adjacent the aerator.

7. An installation for the purification of sewage substantially as described with reference to Figures 2 and 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. ity, without additional energy inputs, it is necessary to transfer momentum from the high velocity upper layers to the lower layers near the tank floor. In this respect the aerator discharge is unaffected by the portion of the partition 24 placed upstream, however the downstream portion 24' of the partition 24 unless correctly designed could effectively minimise the momentum transfer from the high velocity discharge with a resultant reduction in the said scouring velocity and the circulation velocity in the installation. The downstream portion 24' of the partition is therefore suitably sloped so as to provide effective oxygen transfer and allowing sufficient momentum transfer to provide the required scouring velocity and an adequate circulation velocity. WHAT WE CLAIM IS:

1. An installation for the purification of sewage by the activated sludge method, comprising an enclosure for containing sewage to be purified, the enclosure comprising a tank portion and a further portion external of the tank portion defining flow and return paths for sewage from the tank portion, the flow and return paths communicating with the interior of the tank portion via openings in the latter below the level of the sewage therein, and a surface aerator adapted to rotate about a vertical axis within the tank portion for agitating sewage therein and thereby adding oxygen to the sewage, the aerator being surrounded substantially on all sides by the tank portion, the arrangement being such that rotation of the aerator effects continuous circulation of sewage from the tank portion around the flow and return paths of the further portion and thence back to the tank portion.