Method and Device for Biological Waste Water Purification

Abstract

The present invention relates to a method for carrying out biological purification of wastewater with the aid of activated sludge in a sewage treatment plant, the sewage treatment plant comprising: an activated sludge tank that can be ventilated (B tank), at least two sedimentation and recirculation tanks (SU tanks), and a tank for biological phosphor elimination (P tank), wherein the P tank is hydraulically connected with the B tank via one or more openings, wherein the B tank is divided into two tanks B.sub.1 and B.sub.2 (B.sub.1 tank and B.sub.2 tank) which are hydraulically connectable via the P tank, wherein each of the B.sub.1 tank and the B.sub.2 tank is continuously connected hydraulically to at least one SU tank, wherein the P tank comprises closure means to cut off the hydraulic connection between the P tank and the B.sub.1 tank and/or the B.sub.2 tank, and wherein each of the SU tanks comprises an overflow unit for draining the excess water in the sewage treatment plant, wherein in the event of an emergency, the hydraulic connection between the P tank and either the B.sub.1 tank or the B.sub.2 tank is cut off, and the waste water is then accumulated and lifted up in the tanks that are not cut off, and the treated wastewater can effluent via the overflow unit of the respective SU tank(s). The present invention also relates to a sewage treatment plant for carrying out said method.

Claims

1. A method for carrying out biological purification of wastewater with the aid of activated sludge in a sewage treatment plant, which method allows for an emergency operation of the sewage treatment plant, the sewage treatment plant comprising: an activated sludge tank that can be ventilated (hereinafter referred to as the B tank), at least two sedimentation and recirculation tanks (hereinafter referred to as SU tanks), wherein the at least two SU tanks include at least one first sedimentation and recirculation tank (hereinafter referred to as SU1 tank) and at least one second sedimentation and recirculation tank (hereinafter referred to as SU2 tank), wherein the at least one SU1 tank and the at least one SU2 tank are continuously connected hydraulically to the B tank, wherein in the at least one SU1 tank and in the at least SU2 tank a number of operating cycles are carried out over the course of a day, including a sludge return phase, a recirculation phase, a pre-sedimentation phase and a draw-off phase (hereinafter referred to as the S phase, U phase, V phase, and A phase, respectively), and a tank for biological phosphor elimination (hereinafter referred to as P tank), wherein the P tank is hydraulically connected with the B tank via one or more openings, and wherein the volume of the P tank is mixed permanently or intermittently, wherein in said method the wastewater is first introduced into the P tank and subsequently into the B tank, and then from the B tank, in alternation, into the at least one SU1 tank and into the at least one the SU2 tank, wherein consecutively, in the S phase at least part of the thickened activated sludge is introduced from the at least one SU1 tank and the at least one SU2 tank, respectively, into the P tank, in the U phase the activated sludge is again mixed with the water, in the V phase the activated sludge is sedimented, and in the A phase treated water is drawn off, wherein in the at least one SU1 tank and the at least one SU2 tank said operating cycles are phase-shifted in relation to one another, the A phases in the at least one SU1 tank and the at least one SU2 tank border one another, a flow passes through the at least one SU1 tank and the at least one SU2 tank, respectively, merely in the A phases, an approximately constant water level is provided and therefore a wastewater treatment system discharge corresponding to the wastewater treatment system supply develops (continuous flow principle), wherein: the B tank is divided into two tanks B1 and B2 (hereinafter referred to as B1 tank and B2 tank) which are hydraulically connectable via the P tank, wherein each of the B1 tank and the B2 tank is continuously connected hydraulically to at least one SU tank in order to build up a one-line sewage treatment plant, wherein the P tank comprises closure means to cut off the hydraulic connection between the P tank and the B1 tank and/or the B2 tank, and wherein each of the SU tanks comprises an overflow unit for draining the excess water in the sewage treatment plant, wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B1 tank or the B2 tank is cut off, and the waste water is then accumulated and lifted up in the tanks that are not cut off, and the treated wastewater can effluent via the overflow unit of the respective SU tank(s).

2. The method according to claim 1, wherein the P tank is positioned in the middle of the B tank and adjacent to the at least two SU tanks, and the P tank divides the B tank into the B1 tank and the B2 tank, wherein each of the B1 tank and the B2 tank is hydraulically connectable with the P tank via at least one closable opening, wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B1 tank or the B2 tank is cut off by closing the respective closable opening(s).

3. The method according to claim 1, wherein the B tank is located between the P tank and the SU tanks, the B tank is divided into the B1 tank and the B2 tank by a wall, wherein each of the B1 tank and the B2 tank is hydraulically connectable with the P tank via at least one closable opening, wherein, in the S phase, the thickened activated sludge is transferred via one or more pipes from the at least one SU1 tank and the at least one SU2 tank, respectively, into the P tank, wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B1 tank or the B2 tank is cut off by closing the respective closable opening(s).

4. The method according to claim 2, wherein the B1 tank is continuously connected hydraulically to one SU1 tank and wherein the B2 tank is continuously connected hydraulically to one SU2 tank, wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B1 tank or the B2 tank is cut off by closing the respective closable opening(s), in order to shut down either both the B1 tank and SU1 tank or both the B2 tank and SU2 tank, and the waste water is then accumulated and lifted up in the tanks that are not shut down, and the treated wastewater can effluent via the overflow unit of the respective SU tank that is not shut down.

5. The method according to claim 1, wherein the P tank is positioned in the middle of the B tank and divides the B tank into the B1 tank and the B2 tank, wherein each of the B1 tank and the B2 tank is hydraulically connectable with the P tank via at least one closable opening, wherein the B1 tank is positioned between the P tank and at least one SU tank and wherein the B2 tank is positioned between the P tank and at least one SU tank, wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B1 tank or the B.sub.2 tank is cut off by closing the respective closable opening(s).

6. The method according to claim 5, wherein the B1 tank is continuously connected hydraulically to one SU1 tank, and the B2 tank is continuously connected hydraulically to one SU2 tank, wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B1 tank or the B2 tank is cut off by closing the respective closable opening(s), in order to shut down either both the B1 tank and SU1 tank or both the B2 tank and SU2 tank, and the waste water is then accumulated and lifted up in the tanks that are not shut down, and the treated wastewater can effluent via the overflow unit of the respective SU tank that is not shut down.

7. The method according to claim 5, wherein the B1 tank is continuously connected hydraulically to one SU1 tank and one SU2 tank (hereinafter referred to as tanks B1-SU1-SU2), and the B2 tank is continuously connected hydraulically to one SU1 tank and one SU2 tank (hereinafter referred to as tanks B2-SU1-SU2), wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B1 tank or the B2 tank is cut off by closing the respective closable opening(s), which leads to a shut-down of either tanks B1-SU1-SU2 or tanks B2-SU1-SU2, and the waste water is then accumulated and lifted up in the tanks that are not shut down, and the treated wastewater level can ascent up to the upper edge of the overflow unit of the respective SU tanks that are not shut down.

8. The method according to claim 5, wherein the excess sludge is pumped from the SU tanks into the P tanks via airlifts and at least two pipes.

9. The method according claim 3, wherein, during the full operation phase, in the S-phase, the thickened activated sludge is largely channeled into the B1 tank and the B2 tank, respectively and the P tank primarily has the task to divide the incoming wastewater to B1 and B2.

10. The method according to claim 1, wherein the P tank is aerated and, optionally, also the B1 tank and the B2 tank.

11. The method according to claim 1, wherein the P tank comprises aeration and/or stirring units that are removable for repairs.

12. The method according to claim 1, wherein the tanks that are cut off in the case of an emergency, are emptied for a short time, e.g. for repair, while at the same time the biological purification of wastewater is operated with the tanks that are not shut down.

13. The method according to claim 1, wherein in full operation of all tanks the aeration in the P tank is activated and a biological phosphorus elimination is dispensed with.

14. The method according to claim 1, wherein in full operation of all tanks the aeration in the P tank is switched off and the biological phosphorus elimination goes into operation.

15. The method according to claim 1, wherein the contents of the P tank are mixed permanently or intermittently with a stirring system.

16. The method according to claim 1, wherein the P tank is constructed in a form of a circulation tank.

17. The method according to claim 1, wherein, in the event of an emergency, an agent for enhancing sludge sedimentation, preferably a flocculant, is added to one or more of the tanks that are not cut off.

18. The method according to claim 1, wherein, in the event of an emergency, the excess sludge is removed from the SU tank(s) that is/are not cut off.

19. A sewage treatment plant for carrying out biological purification of wastewater with the aid of activated sludge (4), wherein the sewage treatment plant comprises: an activated sludge tank that can be ventilated (hereinafter referred to as the B tank), at least two sedimentation and recirculation tanks (hereinafter referred to as SU tanks), wherein the at least two SU tanks include at least one first sedimentation and recirculation tank (hereinafter referred to as SU1 tank) and at least one second sedimentation and recirculation tank (hereinafter referred to as SU2 tank), wherein the at least one SU1 tank and the at least one SU2 tank are continuously connected hydraulically to the B tank, wherein the sewage treatment plant is configured such that in the at least one SU1 tank and in the at least SU2 tank a number of operating cycles are carried out over the course of a day, including a sludge return phase, a recirculation phase, a pre-sedimentation phase and a draw-off phase (hereinafter referred to as the S phase, U phase, V phase, and A phase, respectively), and a tank for biological phosphor elimination (hereinafter referred to as P tank), wherein the P tank is hydraulically connected with the B tank via one or more openings (2), and wherein the volume of the P tank is mixed permanently or intermittently, wherein: the B tank is divided into two tanks B1 and B2 (hereinafter referred to as B1 tank and B2 tank) which are hydraulically connectable via the P tank, wherein each of the B1 tank and the B2 tank is continuously connected hydraulically to at least one SU tank, in order to build up a one-line sewage treatment plant, wherein the P tank comprises closure means to cut off the hydraulic connection between the P tank and the B1 tank and/or the B2 tank in events of emergency, and wherein each of the SU tanks comprises an overflow unit for draining the excess water in the sewage treatment plant.

20. The sewage treatment plant according to claim 19, wherein the P tank is positioned in the middle of the B tank and adjacent to the at least two SU tanks, and the P tank divides the B tank into the B1 tank and the B2 tank, wherein each of the B1 tank and the B2 tank are hydraulically connectable with the P tank via at least one closable opening (2).

21. The sewage treatment plant according to claim 19, wherein the B tank is located between the P tank and the SU tanks, the B tank is divided into the B1 tank and the B2 tank by a wall (11), wherein each of the B1 tank and the B2 tank is hydraulically connectable with the P tank via at least one closable opening, wherein each SU tank is connected with one or more pipes (12) which pipes are adapted to transfer the thickened activated sludge (4) from the respective SU tank into the P tank.

22. The sewage treatment plant according to claim 20, wherein the B1 tank is continuously connected hydraulically to one SU1 tank and wherein the B2 tank is continuously connected hydraulically to one SU2 tank.

23. The sewage treatment plant according to claim 19, wherein the P tank is positioned in the middle of the B tank and divides the B tank into the B1 tank and the B2 tank, wherein each of the B1 tank and the B2 tank is hydraulically connectable with the P tank via at least one closable opening (2), wherein the B1 tank is positioned between the P tank and at least one SU tank and wherein the B2 tank is positioned between the P tank and at least one SU tank.

24. The sewage treatment plant according to claim 23, wherein the B1 tank is continuously connected hydraulically to one SU1 tank, and the B2 tank is continuously connected hydraulically to one SU2 tank.

25. The sewage treatment plant according to claim 23, wherein the B1 tank is continuously connected hydraulically to one SU1 tank and one SU2 tank (hereinafter referred to as tanks B1-SU1-SU2), and the B2 tank is continuously connected hydraulically to one SU1 tank and one SU2 tank (hereinafter referred to as tanks B2-SU1-SU2).

26. The sewage treatment plant according to claim 19, wherein the P tank comprises aeration and/or stirring units (7) that can be removed for repairs.

27. The sewage treatment plant according to claim 19, wherein the P tank is constructed in a form of a circulation tank.

Description

[0041] Further details of the present invention will emerge from the following drawings, which illustrate exemplary, non-limiting embodiments of the invention. In the drawings, two operating cycles (FIG. 1) and different exemplary embodiments of wastewater treatment systems (FIG. 2-FIG. 7) according to the present invention are shown.

[0042] FIG. 1 shows two operating cycles (full operation cycle and emergency operation cycle) for the SU tanks SU.sub.1 and SU.sub.2 shown in the exemplary embodiments of FIGS. 2-6 and a possible operation of the overflow 3;

[0043] FIG. 2 shows a schematic illustration of a first embodiment of the present invention (top view);

[0044] FIG. 3 shows a vertical sectional view of the embodiment of FIG. 2;

[0045] FIG. 4 shows a schematic illustration of a second embodiment of the present invention (top view), with a P tank in form of a circulation tank (top view);

[0046] FIG. 5 shows a schematic illustration of a third embodiment of the present invention (top view);

[0047] FIG. 6 shows a schematic illustration of a fourth embodiment of the present invention (top view);

[0048] FIG. 7 shows a schematic illustration of a fifth embodiment of the present invention (top view).

[0049] As mentioned above, the activity of microorganisms in a sewage treatment plant depends on the water temperature. The efficiency of a sewage treatment plant is therefore higher in summer than in winter. A similar effect is given by not yet full utilization of the treatment plant. In order to save space and costs, it is useful to take advantage of this effect. In summer (or with incomplete utilization), fewer wastewater treatment volumes are needed than in winter. In the summer, therefore, this volume can be used for a biological phosphorus elimination. For this purpose, the P tank described in FIGS. 2-7, whichequipped with an aeration devicemay act as a biological phosphor elimination in summer and may act as a B tank during winter. The P tank also serves to optimally distribute the incoming raw sewage and the recycled sludge into the B tank system (i.e. B.sub.1 and B.sub.2 tanks). The aerated P tank also allows cascade operation. Thus, the P tank as used in this invention and described in the embodiments of accompanying FIGS. 2-7 is multifunctional.

[0050] FIG. 1 shows the operating cycle for the SU tanks SU.sub.1 and SU.sub.2 shown as per the exemplary embodiments of FIGS. 2-6, wherein time extends in horizontal direction from left to right. The course and function of the individual phases, i.e. S phase, U phase, V phase and A phase, taking place in the respective SU tanks, have already been discussed above in greater detail. In this figure a full operation cycle and an emergency operation cycle and a possible operation of the overflow unit (see FIGS. 2-6, overflow unit 3 of the SU tanks) are shown. The emergency operation cycle illustrates the course and function of the phases, when the B.sub.2 tank and SU.sub.2 tank of the embodiments shown in FIGS. 2-6 have been shut down and emptied due to an event of emergency (e.g. if the B.sub.2 or SU.sub.2 tank needs repair or maintenance work).

[0051] FIG. 2 shows a schematic illustration of a wastewater treatment system, in which two SU tanks SU.sub.1 and SU.sub.2 are arranged side by side on one side of the B tank and adjacent to the P tank. The B tank is divided into a B.sub.1 tank and B.sub.2 tank by the P tank. In this illustration, currently an A-phase takes place in the SU.sub.1 tank, wherein in the SU.sub.2 tank, a V-phase takes place (description of the different phases, see above). The flow of the wastewater in the P tank is signified with 1 and the flow out of the system with 6. The thickened activated sludge 4 is transferred from the SU tanks to the P tank with airlifts 5 (during the S-phase). The hydraulic connection and water flow from the P tank to the B.sub.1 and B.sub.2 tanks is realized by means of closable openings 2. The closable openings comprise flat slides to cut off the hydraulic connection between the P tank and the B.sub.1 tank or the B.sub.2 tank in events of emergency. The contents of the P tank can also be homogenized with a stirring device 7. Both the B-tank and the P tank are supplied with aeration 9.

[0052] In the full operation the closable openings 2 between the P tank and the B.sub.1 tank and B.sub.2 tank, respectively, are open and all tanks are in operation with an approximate constant water level (throughflow-principle). This one-line system can be in operation with or without a biological phosphor elimination. In this case, the overflow units 3 which are part of the SU tanks is not needed. If the aeration 9 in the P tank is in action, a cascade method is achieved.

[0053] In the emergency operation part of the tanks, i.e. either B.sub.1 and SU.sub.1 or B.sub.2 and SU.sub.2, can be taken out of service and emptied by closing the respective closable opening 2 (either between P tank and B.sub.1 tank or between P tank and B.sub.2 tank) by means of the slide. In other words, the hydraulic connection between the P tank and the B.sub.1 tank or the B.sub.2 tank is closed, leading to a cut-off/shut-down of either the B.sub.1 and SU.sub.1 tanks or B.sub.2 and SU.sub.2 tanks. In this case, the water level rises in the S-, U- and first half of the V-phase. As the water level rises, the sludge settles in the SU-tank that is not shut down. After approximately 30 min, the water level reaches the level of the emergency overflow unit 3 of the SU tank that is not taken out of service, treated wastewater without the sludge can drain off and a maximum height of the water level is not exceeded. In the subsequent A-phase, an effluent device 10 opens, whereby the water level assumes a lower level. In this case we speak of filling up principle.

[0054] FIG. 3 shows a vertical sectional view of the system of FIG. 2 (along a line which, in FIG. 2, extends between A-A). Q.sub.m signifies the flow of the wastewater introduced into the P tank, wherein Q.sub.out is the flow of the treated water flowing off from the water treatment system. The thickened activated and aerated sludge 4 is transferred from the SU tanks SU.sub.1 and SU.sub.2 into the P tank via e.g. a pipe. In order to mix the waste water in the P tank efficiently with the thickened activated sludge 4, the volume of the P tank is mixed permanently or intermittently. The mixture of waste water and sludge is then transferred into the B tank and further to the SU tanks SU.sub.1 and SU.sub.2 via the one or more closable openings 2 connecting the P tank with the B.sub.1 tank and B.sub.2 tank as described above. For transferring the waste water from the B.sub.1 tank and the B.sub.2 tank to the SU tanks SU.sub.1 and SU.sub.2, respectively, one or more closable openings are also provided between the B.sub.1 tank and the SU.sub.1 tank as well as between the B.sub.2 tank and the SU.sub.2 tank; see FIG. 2. 5 signifies the airlift operation unit for operation of the S phase. A maximum height of the water level 8 is not exceeded.

[0055] FIG. 4 shows a schematic illustration of a second embodiment of the present invention (top view). The second embodiment fully corresponds to the embodiment as shown in FIGS. 2 and 3, with the only exception that the P tank is in form of a circulation tank. As in FIGS. 2 and 3, also in this illustration, currently an A-phase takes place in the SU.sub.1 tank, wherein in the SU.sub.2 tank, a V-phase takes place (description of the different phases, see above).

[0056] FIG. 5 shows a schematic illustration of a third embodiment of the present invention (top view). The arrangement of the P, B and SU tanks in this embodiment is slightly different from the arrangement shown in FIGS. 2-4 as described above, but the applied full operation mode and emergency operation mode as described above in relation to FIGS. 2-4 apply, mutatis mutandis, to the embodiment of FIG. 5. Features in FIG. 5 are provided with the same reference signs as the corresponding features already described above in relation to the embodiments shown in FIGS. 2-4.

[0057] In the embodiment shown in FIG. 5, the B tank is located between the P tank and the SU tanks, the B tank is divided into the B.sub.1 tank and the B.sub.2 tank by a wall 11, wherein each of the B.sub.1 tank and the B.sub.2 tank is hydraulically connectable with the P tank via at least one closable opening 2. In the S phase, the thickened activated sludge is transferred via one or more pipes 12 from the SU.sub.1 tank and the SU.sub.2 tank, respectively, into the P tank, wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B.sub.1 tank or the B.sub.2 tank is cut off by closing the respective closable opening(s) 2.

[0058] As in the embodiments shown in FIGS. 2-4, also in the embodiment of FIG. 5, the B.sub.1 tank is continuously connected hydraulically to the SU.sub.1 tank and the B.sub.2 tank is continuously connected hydraulically to the SU.sub.2 tank, wherein in the event of an emergency, the hydraulic connection between the P tank and either the B.sub.1 tank or the B.sub.2 tank is cut off by closing the respective closable opening(s) 2, in order to shut down either both the B.sub.1 tank and SU.sub.1 tank or both the B.sub.2 tank and SU.sub.2 tank, and the waste water is then accumulated and lifted up in the tanks that are not shut down, and the treated wastewater can effluent via the overflow unit 3 of the respective SU tank that is not shut down.

[0059] FIG. 6 shows a schematic illustration of a fourth embodiment of the present invention (top view). The arrangement of the P, B and SU tanks in this embodiment is different from the arrangements as described above, but the applied full operation mode and emergency operation mode as described above in relation to FIGS. 2-4 apply, mutatis mutandis, for the arrangement of FIG. 6. Features in FIG. 6 are provided with the same reference signs as the corresponding features already described above in relation to the embodiments shown in FIGS. 2-4.

[0060] In the embodiment shown in FIG. 6 the P tank is positioned in the middle of the B tank and divides the B tank into the B.sub.1 tank and the B.sub.2 tank, wherein each of the B.sub.1 tank and the B.sub.2 tank is hydraulically connectable with the P tank via at least one closable opening 2, wherein the B.sub.1 tank is positioned between the P tank and the SU.sub.1 tank and wherein the B.sub.2 tank is positioned between the P tank and the SU.sub.2 tank, wherein in said method, in the event of an emergency, the hydraulic connection between the P tank and either the B.sub.1 tank or the B.sub.2 tank is cut off by closing the respective closable opening(s) 2. In the embodiment of FIG. 6, the B.sub.1 tank is continuously connected hydraulically to the SU.sub.1 tank, and the B.sub.2 tank is continuously connected hydraulically to the SU.sub.2 tank, wherein, in the event of an emergency, the hydraulic connection between the P tank and either the B.sub.1 tank or the B.sub.2 tank is cut off by closing the respective closable opening(s) 2, in order to shut down either both the B.sub.1 tank and SU.sub.1 tank or both the B.sub.2 tank and SU.sub.2 tank, and the waste water is then accumulated and lifted up in the tanks that are not shut down, and the treated wastewater can effluent via the overflow unit 3 of the respective SU tank that is not shut down.

[0061] The P tank of FIG. 6 has the form of a circulation tank. The flow of the wastewater in the P tank is signified with 1 and the flow of the treated water out of the system with 6. The thickened activated sludge 4 is pumped from the SU.sub.1 and SU.sub.2 tanks into the P tank by means of airlifts 5 and pipes 12.

[0062] FIG. 7 shows a schematic illustration of a fifth embodiment of the present invention (top view). The arrangement of the P, B and SU tanks in this embodiment is different from the arrangements as described above, but the applied full operation mode and emergency operation mode as described above in relation to FIGS. 2-4 apply, mutatis mutandis, for the arrangement of FIG. 7. Features in FIG. 7 are provided with the same reference signs as the corresponding features already described above in relation to the embodiments shown in FIGS. 2-4.

[0063] In the embodiment shown in FIG. 7 the P tank is positioned in the middle of the B tank and divides the B tank into the B.sub.1 tank and the B.sub.2 tank, wherein each of the B.sub.1 tank and the B.sub.2 tank is hydraulically connectable with the P tank via at least one closable opening 2, wherein the B.sub.1 tank is positioned between the P tank and one SU.sub.1 tank and one SU.sub.2 tank and wherein the B.sub.2 tank is positioned between the P tank and one SU.sub.1 tank and one SU.sub.2 tank, wherein, in the event of an emergency, the hydraulic connection between the P tank and either the B.sub.1 tank or the B.sub.2 tank is cut off by closing the respective closable opening(s) 2. Accordingly, in the embodiment shown in FIG. 7, the B.sub.1 tank is continuously connected hydraulically to one SU.sub.1 tank and one SU.sub.2 tank (hereinafter referred to as tanks B.sub.1-SU.sub.1-SU.sub.2), and the B.sub.2 tank is continuously connected hydraulically to one SU.sub.1 tank and one SU.sub.2 tank (hereinafter referred to as tanks B.sub.2-SU.sub.1-SU.sub.2), wherein, in the event of an emergency, the hydraulic connection between the P tank and either the B.sub.1 tank or the B.sub.2 tank is cut off by closing the respective closable opening(s) 2, which leads to a shut-down of either tanks B.sub.1-SU.sub.1-SU.sub.2 or tanks B.sub.2-SU.sub.1-SU.sub.2, and the waste water is then accumulated and lifted up in the tanks that are not shut down, and the treated wastewater level can ascent up to the upper edge of the overflow unit of the respective SU tanks that are not shut down.

[0064] The P tank of FIG. 7 has the form of a circulation tank. The flow of the wastewater in the P tank is signified with 1 and the flow of the treated water out of the system with 6. The thickened activated sludge 4 is pumped from the SU.sub.1 and SU.sub.2 tanks (i.e. a total of four SU tanks) into the P tank by means of airlifts 5 and pipes 12.

[0065] In the embodiments illustrated in FIGS. 2-7, only one closable opening 2 that hydraulically connects the P tank with the B.sub.1 tank as well as the P tank with the B.sub.2 tank is shown in the drawings. It will, however, be clear to the skilled person, that more than one closable opening 2 between the P tank and the B.sub.1 tank as well as between the P tank and the B.sub.2 tank may be foreseen.