Die invention relates to a device for the purification of waste water according to the active sludge technique, ocπprising a tank, one or none substantially vertically positioned partitions dividing said tank into two or more channels which are jjitercαnected by channel curves at the ends of said partitions and which constitute a closed flowing path for the waste water to be purified, wherein a surface aerator is placed in at least one curved channel section, at the respective partition end, said aerator having its axis positioned vertically and functioning also to propel the waste water through the channels, whereas one or more additional surface aerators are provided in recesses in the partition located between the ends of the partition.

Such a device is known - in a two-channel type - fro fig. 1 of NL-A-7606196. With this well-kncwn device the surface aerators placed in the recesses of the partition are evenly distributed across the length of the partition. As a consequence the nitrification and denitrificatiαn zones which are responsible for the deocB-positica of the contaminants in the waste water are divided into ---mall zones so that a relatively short denitrificatiαn zone will develop upstream of each aerator and a relatively short nitrification zone will be formed downstream of such aerator.

The invention aims at iπprσving this device by proposing measures to optimize both the nitrification process which takes place in a high-oxygen environment and the denitrificatiαn process which is taking place in a low-σxygen environment.

According to the invention this aim is achieved in that all of the surface aerators associated with a particular partition are positioned to be oαre-entrated along a part of the length of such partition so as to form together a ∞πtiπuσus nitrification zone in the respective part of the flowing path confined by said partition length, whereas a continuous denitrification zone will be formed in the flowing path part that is confined by the aerator-free section of the partition.

It may be concluded from test results that the σαnσeπtrated setting of the aerators enables a quicker purification due to the concentrated denitrification and nitrification zones forming a better climate for the denitrifying and nitrifying bacteria.

In a preferred e-πibc-diment the outlet for purified water is

located just downstream of a group of closely installed surface aerators, whereas an inlet for waste water to be treated is provided at a location downstream of said outlet.

According to a further feature of the invention there is a series of surface aerators placed one behind the other at the end and in recesses of a partition in which each, aerator is surrounded by a tubular casing and merges at its lower end into a guide wall which extends substantially parallel to the bottom of the tank.

Further characteristics of the invention will be hereinafter described by way of example with reference to the accompanying drawings.

Fig. 1A and IB shew a diagrammatic plan view and a diagrammatic longitudinal cross-section respectively of a well-known purification plant of the two-channel type; fig. 2 illustrates a diagrammatic plan view of a siπplest embodiment of the device according to the present invention; fig. 3 is a longitudinal section through the device of fig. 2; fig. 4 is an enlarged plan view of a surface aerator installed in a recess of a channel partition; fig. 5 is a partially sectional view along the line V-V of fig, 4; fig. 6 is a cross-sectional view along the line VE-VI of fig. 4; fig. 7 is an enlarged plan view of a surface aerator installed in a recess of a channel partition, illustrating a modification of the embodiment shewn in fig. 4; fig. 8 is a diagraimatic longitudinal section of the device of fig. 7; fig. 9 is a cross-sectional view along the line IX-IX of fig. 7; fig. 10 shows a plan view of a modification of the embodiment shewn in fig. 2; fig. 11 is a longitudinal section through the device of fig. 10 and fig. 12 is a cross-sectional view along the line XH-XII of fig. 10.

The well-known device shown in fig. 1A-B σcπprises an elongated tank 1 whi±i is divided by a vertical partition 2 into two channels 3 and 4 extending on either side of said partition, said channels being interconnected at the ends of the tank by curved sections 5 and 6

respectively. At each of the two ends of the partition 2 a so-called surface aerator 7 is placed in the respective curved channel sections 5 and 6. These surface aerators are of a well-known type and are driven by an electric motor (not shown) provided in the tank at a position above the weaste water lever therein. In the example shewn the direction of rotation is a-πti-clockwise. In operation, a portion of the waste water in the tank will be drawn by each rotating aerator upwardly and discharged at the surface in the form of a disc-shaped curtain of water drops that are slung through the air. At the same time the aerators establish a main flow of the waste water in the closed flow path that is constituted by the channels 3 and 4 and the curved channel sections 5 and 6, in a flowing direction that corres¬ ponds with the direction of rotation of the aerators (vide the arrow direction) . Just upstream of eacii aerator there is a denitrification zone, whereas downstream of it there is a nitrification zone.

In case the amount of waste water to be treated per time unit and therewith the amount of air to be supplied per time unit is so large that a third aerator is required, the well-known concept will lead to the embodiment of fig. IC, showing a four channel -system. It will be understood that 4 or 5 aerators will lead to a six channel system etc.

In the example shewn in fig. 2, the device according to the present invention is a two-channel system whicii ccπprises - like the well-kncwn device according to fig. 1A-B - an elongated tank 1, divided by a vertical partition 2 into two channels 3 and 4 extending on either of said partition and being iπt-ertxamected at the ends of the tank by curved sections 5 and 6 respectively.

With the device of the present invention, however, an aerator 7 is installed only at one of the ends of the partition 2, viz. in the curved channel section 5. Besides, in accordance to the present invention, two additional aerators 8 are installed at iπt-e-αnediary locations, in recesses 9 of the partition 2. These additional aerators have their (vertical) axis located in the plane of the partition 2 and rotate in the same direction as the aerator 7. A portion of the main waste water flew through the channels 3 and 4 is drawn by each of the additional aerators from the depth upwardly to the surface where it is fed back in an oxygen added state to the water body in the form of a curtain of water drops slinging through the air. The ---pacing between

the aerators is selected for an optimal utilization of the aerating capacity of the three aerators. At the partition end, at 10, and on either side of the recesses 9, at 11, the partition extends slightly upwardly. In terms of aerating capacity the device acxxπ-ding to fig. 2 may be considered to be the equivalent of the well-known 4-channel system of fig. IC. In fig. 2 the hatched area indicates a continuous denitrification zone, whereas the arrows 12 and 13 designate preferred locations for the discharge of the purified water and for the supply of the waste water to be treated respectively. In the embodiment according to figs. 4-6 the "feeding" of the aerators 8 with waste water from the main stream flowing throug the channels 3 and 4 is promoted by the baffles indicated at 14. In the example shewn these baffles extend on both sides of the partition in a plane at right angles to the partition and going through the axis of the aerator. The baffles 14 cover an overall width corresponding to the diameter of the rotor 8a of the aerator. As is shown in fig. 6 the upper edges 14a of the baffles extend obliquely and parallel to the lcwer ccnically shaped end face of the aerator rotor and the same applies for the lcwer edges 9a of the recesses 9 (vide fig. 5) . The baffles 14 need not to extend to the bottom of the bank 1 and they neither need to be vertical along the entire height. An e-π-bodimeπt in which the baffles extend from a location adjacent to the tank bottom in the downstream direction and gradually upwardly to a vertical position may even be advantageous. I the modified embodiment according to fig. 7-9 the baffles 14 of the exanple of fig. 4-6 are replaced by a tubular shell 15 that surrounds the aerator and extends downwardly to a certain distance from the bottcm of the tank. This promotes the suction of waste water from locations adjacent the tank bottcm where the oxygen content in the waste water is relatively poor. The aerating capacity of the aerators will thus be optimally utilized. In the of fig. 10-12 this effect will be enhanced yet. In this eπtoodimeπt the tubular shells 15 surrounding the surface aerators 7 and 8 merge into a guide wall 16 that extends at a certain d stance over the tank bottom. In fact this guide wall constitutes the bottcm of a trough which is turned upside down with its side walls 17 and 18 resting on the tank bottom so as to separate a part of the water from the main stream throug the tank and to feed this part to the successive aerators 7 and 8. This trough is closed at the end located in the curve section 5

and is divided by the partition 2 into an incoming passage 16a and an outgoing passage 16b. The inlet end of the passage 16a is located just downstream of the denitrification zone and will thus take in waste water having a low oxygen content. This secures that all of the aerators will be fed from the same partial stream of low oxygen waste water.

In the example -shewn the cross-section of the passages 16a and

16b in the flowing direction is constant. It will be understood, however, that a downstream decreasing cross-section may be used as well and that the guide wall 16 might extend across the entire width of one or both of the channels 3, 4 of the tank.