GB2316071A

GB2316071A - Small scale waste water treatment apparatus

Info

Publication number: GB2316071A
Application number: GB9616567A
Authority: GB
Inventors: Ian Hinckley Rees; Philip Graeme Morgan
Original assignee: ALBION ECOTEC Ltd
Current assignee: ALBION ECOTEC Ltd
Priority date: 1996-08-07
Filing date: 1996-08-07
Publication date: 1998-02-18
Anticipated expiration: 2016-08-07
Also published as: GB9616567D0; GB2316071B

Abstract

A sewage treatment plant 1 comprises a first settlement chamber 8, an aerobic treatment chamber 14 and a second settlement chamber 15, the treatment chamber 14 having aeration means 25 and mobile media which support microorganisms and are lifted in use by an upward flow within the chamber, the second settlement chamber 15 receiving liquid from the treatment chamber 14 through an opening 16 which connects the bases of these chambers. Preferably the second settlement chamber has an air-lift pump 27 which periodically transfers sludge back to the first chamber 8 and/or the treatment chamber 14. Preferably the treatment chamber and second settlement chamber are separated by a sloping wall 13. The mobile media may consist of plastic beads.

Description

Small-scale Waste Water Treatment This invention relates to a waste water treatment plant, and to a method of treating waste water for domestic and other small-scale use. The invention is especially concerned with plants suitable for treating up to between 1,000 and 25,000 litres per day of waste water. It is generally considered that a treatment capacity of about 250 litres per day is required per person. Thus the invention relates to plants suitable for a single dwelling or a small group of dwellings.

The design of waste water treatment plants for domestic and other small-scale use involves several very different considerations from the design of such a plant for municipal waste water treatment. With a treatment plant for domestic and other small-scale use, it is especially advantageous for the plant to be easy to transport and install and for it to be compact. Also, the plant must be able to cope with wide variations in the amount of waste water to be treated each day and with periods of a week or more during which there is no waste water to be treated.

Waste water plants for small-scale use employ treatment techniques that can be divided into three broad categories: firstly and most simply there are cesspools which merely store waste water; secondly there are septic tanks in which waste water is subjected to an anaerobic treatment; thirdly there are aerobic treatment systems.

Cesspools have the advantage that they are very simple to construct and install but they must be relatively large and require frequent emptying which severely limits their usefulness.

Septic tanks can again be relatively simple but rely on an anaerobic reaction, which results in much slower waste water treatment. It has also been found difficult to achieve a consistent effluent discharge quality from a septic tank.

Aerobic treatment systems are the most sophisticated. In the correct conditions aerobic systems will provide consistent effluent discharge quality and a relatively small unit should be able to treat a relatively large amount of liquid. Since the aerobic reaction rate cannot itself be altered significantly, higher treatment rates for a given size of plant can be obtained only by providing an increased population density of micro-organisms in the plant.

In a typical aerobic treatment system a plurality of micro-organism carrying elements are mounted, either fixedly or with limited freedom of movement, in a vessel and the contents of the vessel are aerated by suitable active means including rotating discs, fine and coarse diffusers and/or by passive diffusion of air at airliquid interfaces. Whilst such a system undoubtedly has potential advantages over cesspools and septic tanks, a number of problems are found to arise with its use as a result of the fixed mounting of the micro-organism carrying elements. Firstly, the micro-organism population is widely variable resulting in a wide variation in the treatment efficiency of the system; the variation in population is due to a number of factors including the time of year (excess micro-organisms tend to build up in the winter as a result of new spring growth), the amount of waste water treated (during a period of low treatment rate, the population of microorganisms will tend to reduce), and discharge of the micro-organisms with the effluent as a result of them being removed in a shear action from their carrying elements. Another disadvantage of such a system is that the excessive growth of the micro-organism population can lead to development of a biofilm that blocks intended voids in the media leading to poor aeration in certain regions and/or to certain regions becoming ineffective in terms of treatment; as a result there can be a marked deterioration in effluent quality.

An alternative aerobic treatment system employs mobile, micro-organism carrying, media, thereby avoiding at least some of the disadvantages referred to in the paragraph immediately above. Such a system makes it possible to treat a greater volume of waste water in a given size of plant so that, for a given treatment capacity, the size of the plant can be reduced. Also, whereas it might be thought that employing mobile, microorganism carrying, media would exacerbate problems caused by a variation in the micro-organism population, we have found that in fact those problems are reduced. Because the media are mobile, they are not liable to form aggregated masses of little or no effectiveness; rather the movement of the media and rubbing of one element against another serves to maintain a consistent microorganism population even during extended periods of little or no use of the plant.

Especially in the case where mobile media are employed for aerobic treatment and in the case where the treatment plant is to be compact, it is important that the various components of the plant are arranged in a manner that provides a physically compact design and also provides good and consistent treatment of the water. In a known plant, waste water is treated in an aerobic treatment chamber employing mobile media and is then passed to a clarifier where final settlement of the liquid takes place. In order for liquid to pass from the aerobic treatment chamber into the clarifier it must pass over the top of a wall in the treatment chamber, that wall serving to retain the sludge from the aerobic treatment in that chamber and prevent it reaching the clarifier unit.

In order to provide a compact design for a domestic water treatment plant it is known to provide a plant employing an aerobic treatment in the form of a generally cylindrical housing of circular cross-section divided by vertical walls into, typically, three treatment zones: a primary settlement chamber, a secondary treatment chamber in which the aerobic treatment is carried out and a tertiary settlement chamber. In a known plant, the primary settlement chamber is semi-cylindrical and the secondary and tertiary chambers occupy respective quadrants of the remaining semi-cylindrical space. The minimum size of the plant for a given treatment capacity is of course determined by the need for each of the treatment chambers to be sufficiently large to fulfil its role in the treatment of the waste water. For example, even at maximum liquid flow rate, there must be quiescent conditions in the top of the tertiary settlement chamber.

It is an object of the invention to provide a waste water treatment plant and a method of treating waste water which overcomes or mitigates disadvantages of the systems described above.

According to a first aspect of the invention there is provided a waste water treatment plant including: a treatment means including a chamber connected to receive liquid for treatment, means for aerating liquid in the chamber and mobile, micro-organism carrying, media lifted, in use, by an upward flow within the chamber; and a settlement chamber connected to receive liquid from the treatment chamber, wherein the settlement chamber is connected to the treatment chamber via an opening at the bottom of each of the chambers.

Conventionally a settlement chamber (referred to below as a tertiary settlement chamber) would be connected to a treatment chamber (referred to below as a secondary treatment chamber) at the top of each chamber; for example, in some cases liquid at the top of the treatment chamber flows over a weir into a settlement chamber. Such an arrangement has obvious apparent advantages because the liquid at the top of a chamber is likely to be relatively free of solid material. In accordance with the first aspect of the invention, however, the chambers are connected at their bottoms. In use, sludge will collect in the bottom of the settlement chamber and therefore liquid enters the settlement chamber by passing through sludge in the bottom of the chamber. Such an arrangement is believed to be advantageous in that the sludge will act as a filter for the liquid entering the settlement chamber so that the number of particles entering the liquid containing portion of the settlement chamber is reduced. Also, because there will be low oxygen levels within the sludge, the amount of nitrates present in the liquid leaving the settlement chamber may be reduced.

The treatment chamber and the settlement chamber are preferably disposed alongside one another and the opening connecting the settlement chamber to the treatment chamber is preferably provided by a gap in a dividing wall between the chambers. A compact arrangement is readily obtained by placing the chambers side-by-side.

The cross-sectional area of the settlement chamber preferably increases from the bottom of the chamber towards the top. Complementarily, the cross-sectional area of the treatment chamber preferably decreases from the bottom of the chamber towards the top. Furthermore, the cross-sectional area of the combination of the settlement chamber and the treatment chamber is substantially constant throughout the height of the chambers.

The dividing wall between the treatment chamber and the settlement chamber is preferably inclined to the vertical; the other side walls of the chambers may be vertical. Arrangements of the kind just defined are especially advantageous in providing a compact yet efficient system. The increasing cross-sectional area of the settlement chamber, taken together with its inlet arrangement, serves to reduce horizontal and vertical flows of liquid within the chamber to provide highly quiescent conditions which, furthermore, are increasingly quiescent towards the top of the chamber.

Preferably the volume of the (secondary) treatment chamber combined with the volume of the (tertiary) settlement chamber is not more than one third, preferably not more than about one quarter, of the total volume of the plant. By providing a compact and efficient design for the secondary and tertiary treatments, the amount of space that can be allocated to other purposes, in particular primary treatment, is increased.

Preferably the waste water treatment plant includes a housing of upright cylindrical shape. The housing may contain substantially all the water treatment parts of the plant and, by providing all the parts within a single housing, transport handling and installation of the plant can be very much simplified.

The housing is preferably divided into a plurality of respective "segments" each containing one or more treatment chambers extending substantially throughout the height of the housing. The term "segment" as used herein is to be interpreted as referring to a solid having a cross-section of any portion of a circular cross-section bounded by one or more radii of the circle and/or one or more chords of the circle and/or the circumference of the circle. In one embodiment of the invention described with reference to the drawings there are three "segments" each of which has a cross-section of a sector of a circle. Preferably the treatment chamber and the settlement chamber are incorporated in the same segment of the housing.

The housing preferably has an internal volume of less than 15m3. If the housing is bigger than this, then it may become difficult to transport and install. At least the exterior side walls and the base of the housing are preferably made of concrete.

It will be appreciated that the mobile, microorganism carrying, media, that in use is disposed in the secondary treatment chamber, is not present in the plant when supplied in dry form from a factory. The media is added as part of the process of installing the plant.

The media may comprise plastic beads, advantageously of a density slightly less than that of pure water and preferably with the average radius of each bead lying in the range of 2 mm to 5 mm.

Preferably, the plant includes a primary settlement chamber occupying one of the segments of the housing, the primary settlement chamber having an inlet at or adjacent to one circumferential extremity of the chamber and an outlet at or adjacent to the opposite circumferential extremity of the chamber. By spacing the inlet and outlet of the primary settlement chamber as far apart in a circumferential direction as possible, maximum utilisation of the primary settlement chamber is achieved and good conditions for sedimentation provided. If desired, more than one primary settlement chamber may be provided; in an embodiment of the invention described below a first stage of primary settlement is followed by a second stage in a septic tank.

Preferably an air lift pump is provided for periodically lifting sludge from the bottom of the (tertiary) settlement chamber and transferring it into a primary settlement chamber and/or the treatment chamber.

Preferably the pump removes only a part of the sludge so that some sludge remains to provide the beneficial filtering action referred to above.

In the description above various advantageous features of the plant have been described in the context of the first aspect of the invention. Many of those features can also, however, be employed independently of some of the features of the first aspect of the invention.

According to a second aspect of the invention there is provided a waste water treatment plant including: a treatment means including a chamber connected to receive liquid for treatment, means for aerating liquid in the chamber and mobile, micro-organism carrying, media lifted, in use, by an upward flow within the chamber, and a settlement chamber connected to receive liquid from the treatment chamber, wherein the chamber of the treatment means and the settlement chamber are disposed adjacent to one another being separated by a dividing wall that is inclined to the vertical in a direction extending upwardly towards the treatment chamber and away from the settlement chamber.

According to a third aspect of the invention there is provided a waste water treatment plant including a housing containing: a primary settlement chamber, a secondary treatment means including a chamber connected to receive liquid directly or indirectly from the primary settlement chamber and means for aerating liquid in the chamber of the secondary treatment means, a tertiary settlement chamber connected to receive liquid from the chamber of the secondary treatment means, wherein the combined volume of the secondary treatment chamber and the tertiary settlement chamber is not more than one third of the total volume of the housing.

According to a fourth aspect of the invention there is provided a waste water treatment plant including a housing of upright cylindrical shape, a primary settlement chamber, and at least one other chamber in which further treatment is carried out, the primary settlement chamber extending around a part only of a peripheral portion of the interior of the housing and having an inlet at or adjacent to one circumferential extremity of the chamber and an outlet at or adjacent to the opposite circumferential extremity of the chamber.

According to a fifth aspect of the invention there is provided a waste water treatment plant including: a primary settlement chamber, a secondary treatment means including a chamber connected to receive liquid directly or indirectly from the primary settlement chamber and means for aerating liquid in the chamber of the secondary treatment means, a tertiary settlement chamber connected to receive liquid from the chamber of the secondary treatment means, and an air lift pump for periodically lifting sludge from the bottom of the tertiary settlement chamber and transferring it into a primary settlement chamber and/or the secondary treatment chamber.

It should be appreciated that the various features described above as advantageous features of the first aspect of the invention may also advantageously be incorporated as features of the second, third, fourth or fifth aspect of the invention.

The invention further provides a method of treating waste water employing a waste water treatment plant as defined above.

According to a first aspect of the invention there is provided a method of treating waste water, including: passing waste water to be treated into a treatment chamber, providing micro-organism carrying media in the chamber, aerating the water in the chamber and thereby lifting the micro-organism carrying media in the chamber, passing the water from the treatment chamber into the bottom of a settlement chamber, the water passing through sludge that has settled in the bottom of the settlement chamber as it enters the settlement chamber.

According to a fifth aspect of the invention there is provided a method of treating waste water, including: passing waste water to be treated into a primary settlement chamber, passing the water from the primary settlement chamber into a secondary treatment chamber, providing micro-organism carrying media in the secondary treatment chamber, aerating the water in the chamber, passing water from the secondary treatment chamber into a tertiary settlement chamber, and periodically lifting sludge from the bottom of the tertiary settlement chamber using an air lift pump and transferring it into the primary settlement chamber and/or the secondary treatment chamber.

It should be appreciated that the various features described above as features of the water treatment plant of any aspect of the invention may also be employed in a corresponding method of water treatment according to the invention.

By way of example, an embodiment of the invention will now be described with reference to the accompanying drawings, of which: Fig. 1 is a schematic side sectional view of a waste water treatment plant, Fig. 2 is a schematic plan view of the plant with the cover removed, and Fig. 3 is a block diagram showing the treatment steps that are carried out by the plant.

The plant shown in Figs. 1 and 2 includes an upright cylindrical housing 1 of circular cross-section having a base 2, a circumferential wall 3 and a cover 4. The base 2 and the circumferential wall are integrally formed and made of concrete; the cover 4 is also made of concrete.

The interior of the housing is divided by radial partitions 5, 6 and 7 that extend substantially throughout the entire height of the housing into three separate sector-shaped compartments 8, 9 and 10 of varying size.

The compartment 8 defines a first primary settlement chamber which occupies a little more than one half of the internal volume of the housing; upper and lower radial supports 12 extend across the circumferential mid-point of the compartment to provide structural strength to the arrangement of the partitions 5, 6 and 7.

The compartment 9 defines a second primary settlement chamber defining a septic tank which occupies a little more than one quarter of the internal volume of the housing.

The compartment 10 is divided into two chambers by a curved inclined wall 13 that extends from a position just above the base 2 and just inside the circumferential wall 3 upwardly and inwardly to a height equal to that of the partitions 5, 6 and 7; at its top, the wall 13 is close to, but spaced from, the centre of the housing. Thus the wall 13 divides the compartment 10 into an inner aerobic secondary treatment chamber 14 and an outer tertiary treatment or clarifier chamber 15. The wall 13 extends between the partitions 5 and 7 and therefore keeps the chambers 14 and 15 entirely separate except at the bottom where they are interconnected by an opening 16 defined between the wall 13, the base 2 and the housing wall 3.

The plant has an inlet 20 for raw sewage, the inlet being provided in an upper portion of the housing and entering the primary settlement chamber 8 adjacent to the partition 5. An opening 21 is provided in an upper portion of the partition 6, approximately midway between the centre and the outside of the housing, to provide a flow path from the chamber 8 into the septic tank 9. An opening 22 is provided in the upper portion of the partition 7 adjacent to the inner edge of the partition to provide a flow path from the septic tank 9 into the aerobic secondary treatment chamber 14 which, as described above, communicates with the clarifier chamber 15 via the opening 16. The clarifier chamber 15 has an outlet 23 for treated water in the wall 3 of the housing below the inlet 20 and positioned in a central circumferential position as shown in Fig. 2. A box weir 24 is positioned in the chamber 15 around the outlet 23.

An air diffuser 25 is provided in the bottom of the aerobic secondary treatment chamber 14 adjacent to the partition 7 and is connected by a pipe 26 to an air compressor or air blower (not shown) provided outside the housing 1. If desired, for increased efficiency oxygen may be supplied along the pipe 26 as well as, or instead of, air. An air lift pump is also provided having an inlet 27 positioned in the clarifier chamber 15 towards the bottom of the chamber but above the opening 16. The pump is powered by the same air compressor as the air diffuser 25 and has an outlet in the top of the primary settlement chamber 8.

Fig. 3 shows the treatment steps carried out by the plant. In use the plant is; typically, installed with the cover 4 of the housing about 0.6m below ground level.

Raw sewage enters the primary settlement chamber 8 through the inlet 20 and initial sedimentation of solids from the water takes place in chamber 8. Water passes from the chamber 8 through the opening 21 into the septic tank 9 where anaerobic treatment conditions exist. The septic tank treatment is of a conventional kind and will not be described in further detail. From the septic tank 9, water passes through the opening 22 into the aerobic secondary treatment chamber 14. In use, the treatment chamber 14 is provided with an activated bed comprising a plurality of beads of plastics material which are continuously circulated in the chamber 14.

Micro-organisms on the beads provide a biomass which is maintained automatically at a desired level, little affected by changes in climatic conditions or amount of effluent treated. The concentration of biomass in the chamber 14 can be maintained at a level of 15 - 40 g 1-1 which is a high level for a small-scale system.

Air is continuously pumped into the chamber 14 through the air diffuser 25. The liquid flow within the chamber 14 is maintained by micro-organisms being carried to the surface on the beads by rising bubbles from the air diffuser and gravitating back to the base of the vessel. Arrows in Fig. 1 show the liquid flow through the chamber 14.

After the aerobic treatment water flows under the wall 13 through the opening 16 into the clarifier chamber 15 where final settling of solids takes place. Sludge settling in the chamber 15 rests in and above the opening 16 forming a bed of sludge through which water entering the chamber 15 flows. The sludge bed serves to capture particles in the water entering the chamber 15 and smooths the flow of the water promoting quiescent conditions in the chamber 15, especially towards the top of the chamber where the cross-sectional area of the chamber is increased. Water at the top of the chamber 15 flows over the box weir 24 and through the outlet 23 in the housing.

The box weir serves to inhibit flocks formed during the waste treatment process from leaving the plant and also to promote quiescent conditions in the chamber 15.

The top of the box weir is approximately at a height below the inlet 20 and the precise position may be adjustable and/or may be selected to maximise the performance of the plant. The top lip of the box weir over which the liquid in the chamber 15 flows may be serrated or saw-tooth shape as viewed from the side to promote retention of flocks in the chamber 15. Also the outer upright wall of the weir against which the liquid in the chamber 15 rests may be of corrugated form, with the ridges and valleys of the corrugations extending approximately vertically; in that way the surface area of the upright surface presented to the liquid in the chamber 15 by the weir is increased.

Periodically (for example, once or twice every 24 hours) the air lift pump is automatically operated to transfer the excess of sludge that collects above the opening 16 from the clarifier chamber 15 into the primary settlement chamber 8. The inlet 27 of the pump is, however, positioned sufficiently high above the opening 16 that a part of the sludge collected at the opening 16 is retained so that the filtering action of the sludge bed is maintained. Some sludge is also washed back from the bottom of the chamber 15 into the chamber 14.

At infrequent intervals, for example once every six or twelve months, waste may be removed from the bottom of the primary settlement chambers 8, 9 and from the bottom of the chamber 14 as part of a regular maintenance programme.

In a particular example of the invention, the principal dimensions of the plant are as follows: Internal diameter of housing: 1500mm External height of housing and cover: 2200mm Arcuate spacing of partitions 5 and 6: 200 Arcuate spacing of partitions 6 and 7: 100 Arcuate spacing of partitions 7 and 5: 600 Spacing of bottom of wall 13 from base of housing: 100mm Spacing of bottom of wall 13 from side wall of housing: 150mm Spacing of top of wall 13 from centre of housing: 100mm A plant having the dimensions indicated above is capable of treating about 1000 litres per day of raw sewage whilst providing treated water of good quality.

Claims

1. A waste water treatment plant including: a treatment means including a chamber connected to receive liquid for treatment, means for aerating liquid in the chamber and mobile, micro-organism carrying, media lifted, in use, by an upward flow within the chamber; and a settlement chamber connected to receive liquid from the treatment chamber, wherein the settlement chamber is connected to the treatment chamber via an opening at the bottom of each of the chambers.

2. A plant according to claim 1, in which the chamber of the treatment means and the settlement chamber are disposed alongside one another and the opening connecting the settlement chamber to the treatment chamber is provided by a gap in a dividing wall between the chambers.

3. A plant according to claim 1 or 2, in which the cross-sectional area of the settlement chamber, when viewed in plan, increases from the bottom of the chamber towards the top.

4. A plant according to claim 3, in which the crosssectional area of the treatment chamber, when viewed in plan, decreases from the bottom of the chamber towards the top.

5. A plant according to claim 4, in which the crosssectional area of the combination of the settlement chamber and the treatment chamber is substantially constant throughout the height of the chambers.

6. A plant according to claim 2, or any of claims 3 to 5 when dependent on claim 2, in which the dividing wall between the treatment chamber and the settlement chamber is inclined to the vertical.

7. A plant according to any preceding claim, in which the volume of the treatment chamber combined with the volume of the settlement chamber is not more than about one quarter of the total volume of the plant.

8. A plant according to any preceding claim, including a housing of upright cylindrical shape.

9. A plant according to claim 8, in which the housing is divided into a plurality of respective segments each containing one or more treatment chambers extending substantially throughout the height of the housing.

10. A plant according to claim 9, in which the treatment chamber and the settlement chamber are incorporated in the same segment of the housing.

11. A plant according to claim 9 or 10, including a primary settlement chamber occupying one of the segments of the housing, the primary settlement chamber having an inlet at or adjacent to one circumferential extremity of the chamber and an outlet at or adjacent to the opposite circumferential extremity of the chamber.

12. A plant according to any preceding claim, including an air lift pump for periodically lifting sludge from the bottom of the settlement chamber that is connected to receive liquid from the treatment chamber and transferring it into a primary settlement chamber and/or the treatment chamber.

13. A waste water treatment plant including: a treatment means including a chamber connected to receive liquid for treatment, means for aerating liquid in the chamber and mobile, micro-organism carrying, media lifted, in use, by an upward flow within the chamber, and a settlement chamber connected to receive liquid from the secondary treatment chamber, wherein the chamber of the treatment means and the settlement chamber are disposed adjacent to one another being separated by a dividing wall that is inclined to the vertical in a direction extending upwardly towards the treatment chamber and away from the settlement chamber.

14. A waste water treatment plant including a housing containing: a primary settlement chamber, a secondary treatment means including a chamber connected to receive liquid directly or indirectly from the primary settlement chamber and means for aerating liquid in the chamber of the secondary treatment means, a tertiary settlement chamber connected to receive liquid from the chamber of the secondary treatment means, wherein the combined volume of the secondary treatment chamber and the tertiary settlement chamber is not more than one third of the total volume of the housing.

15. A waste water treatment plant including a housing of upright cylindrical shape, a primary settlement chamber, and at least one other chamber in which further treatment is carried out, the primary settlement chamber extending around a part only of a peripheral portion of the interior of the housing and having an inlet at or adjacent to one circumferential extremity of the chamber and an outlet at or adjacent to the opposite circumferential extremity of the chamber.

16. A waste water treatment plant including: a primary settlement chamber, a secondary treatment means including a chamber connected to receive liquid directly or indirectly from the primary settlement chamber and means for aerating liquid in the chamber of the secondary treatment means, a tertiary settlement chamber connected to receive liquid from the chamber of the secondary treatment means, and an air lift pump for periodically lifting sludge from the bottom of the tertiary settlement chamber and transferring it into a primary settlement chamber and/or the secondary treatment chamber.

17. A waste water treatment plant substantially as herein described with reference to and as illustrated by the accompanying drawings.

18. A method of treating waste water employing a waste water treatment plant according to any preceding claim.

19. A method of treating waste water, including: passing waste water to be treated into a treatment chamber, providing micro-organism carrying media in the chamber, aerating the water in the chamber and thereby lifting the micro-organism carrying media in the chamber, passing the water from the treatment chamber into the bottom of a settlement chamber, the water passing through sludge that has settled in the bottom of the settlement chamber as it enters the settlement chamber.

20. A method of treating waste water, including: passing waste water to be treated into a primary settlement chamber, passing the water from the primary settlement chamber into a secondary treatment chamber, providing micro-organism carrying media in the secondary treatment chamber, aerating the water in the chamber, passing water from the secondary treatment chamber into a tertiary settlement chamber, and periodically lifting sludge from the bottom of the tertiary settlement chamber using an air lift pump and transferring it into the primary settlement chamber and/or the secondary treatment chamber.

21. A method as claimed in claim 19 or 20 employing a waste water treatment plant according to any one of claims 1 to 17.

22. A method of treating waste water, the method being substantially as herein described with reference to and as illustrated by the accompanying drawings.