GB1595408A - Activated sludge sewage treatment plant - Google Patents

Description

(54) ACTIVATED SLUDGE SEWAGE TREATMENT PLANT (71) We, MOLEX LIMITED, a British Company, of Farnham Trading Estate, Farnham, Surrey, GU9 9PH, 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:- The present invention relates to an activated sludge sewage treatment plant.

The activated sludge process for sewage treatment is well known. In this process sewage to be treated is collected in an aeration tank and aerated to encourage bacterial action in breaking down undesirable substances in the untreated sewage.

Generally, the undesirable substances in the untreated sewage, from an environmental viewpoint, are those having a high Biological Oxygen Demand (BOD) and it is these high BOD substances which are broken down by bacterial action consuming oxygen provided by the aeration of the liquid in the aeration tank. Normally, liquid from the aeration tank is drawn off and fed to a settling tank so that solids in suspension in the liquid can settle out. The clear liquid can then be discharged, for further treatment if necessary. The settled solids, commonly referred to as sludge, are usually recirculated into the aeration tank to undergo further bacteriological action.

The liquid in the aeration tank can be aerated either by vigorous mechanical agitation or by the introduction of bubbles of air or a combination of the two. In each case, it is necessary to ensure that the liquid in the aeration tank is thorough agitated so that solids remain in suspension and are thoroughly mixed up with the aerated liquid.

Known treatment plants are commonly relatively large scale installations suitable, for example, for the treatment of sewage from a centre of population such as a town or city.

Aeration tanks used hitherto are commonly of rectangular shape with a relatively even depth and the apparatus for aerating liquid in such tanks normally causes the liquids to circulate around the tank in a horizontal plane. This can leave dead spots in the liquid in the aeration tank at the corners of the tank where there may be insufficient agitation of the liquid. It is also known to agitate the liquid in the aeration tank using the stirring effect of a stream of bubbles emitted by an aerator located at the bottom of the tank.

The rising bubbles from the aerator tend to produce circulation in the tank in a vertical plane. However, again the aeration tanks normally have a rectangular cross section in the vertical plane so that this vertical circulation produces dead spots at the corners of the tank.

It is especially important to ensure that the liquid in the aeration tank is stirred evenly throughout its volume with a minimum consumption of energy in a small size plant, for example suitable for treating the sewage effluent of a single house.

According to the present invention, an activated sludge sewage treatment plant comprises a thin walled container for liquids, a wall dividing the container into an aeration tank on one side of the wall and a settling tank on the other side of the wall, the container being shaped to define side walls of the aeration tank converging downwards substantially to meet in a trough, an aerator located in the trough at the bottom of the aeration tank and arranged for discharging bubbles of air into liquid in the aeration tank, means to supply air to the aerator to aerate the liquid in the aerating tank, means for returning settled sludge from the settling tank to the aeration tank, inlet means for feeding sewage to the aeration tank for treatment, outlet means for discharging treated liquid from the settling tank when the liquid in the settling tank exceeds a predetermined level, and means communicating between the two tanks to permit passage of liquid and suspended sludge from the aeration tank to the settling tank when the liquid level in the aeration tank exceeds that in the settling tank, wherein the downwardly converging side walls of the aeration tank are planar with the trough extending in a horizontal line, when the plant is installed for use, and the aerator extends in the trough for discharging bubbles over substantially the whole length of the trough. It has been found that by locating a bubble aerator of this kind in the trough at the bottom of an aeration tank having the above shape. substantially the entire volume of liquid contained in the aeration tank can be evenly agitated and aerated very efficientlv with a low expenditure of energy. The bubbles rise from the aerator producing an upward current of liquid from the trough of the aeration tank.

Near the surface of the liquid this current tends to separate so that the liquid flows horizontally outwards towards the walls of the tank and then down again past the converging side walls to rejoin the upward flow from the aerator. Furthermore, as the individual bubbles from the aerator rise through the liquid. the reducing hydrostatic pressure allows the bubbles to increase in size as they rise between the upwardly divergent side walls of the tank. It is thought that this correspondence between the increasing bubble size and the increasing horizontal cross section of the liquid volume in the tank, as the bubbles rise through the liquid. further adds to the efficiency of aeration of the liquid.

Further the arrangement of the invention is constructionally most convenient and substantially prevents any dead spots from being produced in the liquid volume in the tank.

The present invention is especially convenient as a modular sewage treatment plant suitable for domestic installation, for example, instead of a cesspit or septic tank. The above reference to a "thin walled" container is intended to convey that the container is formed of a sheet material, such as fibreglass or metal and is thus portable before installation. Normally, the plant is installed by sinking the container, at least partially, in an excavation in the ground and is then, of course, connected up for sewage inlet and treated liquid outlet. The aerator conveniently comprises a perforated pipe extending along the trough and is connected to a supply of compressed air for discharging said bubbles.

The dividing wall is conveniently perpendicular to the line of the trough and the container is shaped to define an end wall of the aeration tank in a plane parallel to the dividing wall. Thus, the aeration tank has a generally rectangular horizontal cross section of reducing width down to the trough.

Preferably, the container has downwardly converging planar side walls defining said side walls of the aeration tank and which extend past the dividing wall to define downwardly converging planar walls of the settling tank. Further, an end wall of the container may define a wall of the settling tank and may converge downwardly towards the dividing wall so that said walls of the settling tank together with the dividing wall meet substantially in a point. Then, conveniently. said means for returning settled sludge may include a sludge lifting pipe having a suction end located in the settling tank adjacent said point. In this way, solids settling in the settling tank tend to congregate at the point where they can be readily lifted by the lifting pipe to be fed back into the aeration tank.

Preferably, said inlet means for feeding sewage to the aeration tank comprises an inlet into the aeration tank above said predetermined liquid level, and a screen located below the inlet to catch solids larger than a predetermined size entrained in the sewage to be treated and to prevent them entering the liquid in the aeration tank. Then the screen may be located substantially at said predetermined liquid level so that turbulence in the liquid in the aeration tank produced by the aerator causes the liquid surface continually to wash the screen agitating solids trapped thereon. In this way, the solids trapped on the screen are usually broken up by the agitation until they can pass through the screen for treatment.

Preferably, the outlet means includes an adjustable weir for adjusting said predetermined level above which liquid in the settling tank can flow over the weir to be discharged.

The adjustable weir may comprise a pipe extending in the settling tank and connected to a discharge aperture in a wall of the tank, the pipe having a side aperture in a cylindrical wall thereof through which liquid can flow into the pipe to be discharged from the tank, the pipe extending, when the plant is installed in a horizontal plane and being rotatably adjustable about its axis to set the height of said side aperture for adjusting said predetermined level. In one embodiment, the side aperture extends over the greater part of the length of the pipe in the settling tank defining upper and lower edges extending parallel with the axis of the pipe, the lower such edge defining the weir. With this arrangement, the pipe may be arranged to extend right across between opposed walls of the settling tank at a location spaced in the horizontal plane from a point of entry via the communicating means of liquid and suspended sludge from the aeration tank, the pipe being then arranged with said lower edge of the side aperture on the side of the pipe remote from said point of entry. With this arrangement, the pipe itself acts as a dam preventing any floating maerial, such as fats, or other solids from passing over the weir.

An example of the present invention will now be described with reference to the accompanying drawings in which Figure 1 is a perspective view of a modular sewage treatment plant embodying the present invention, Figure 2 is a top plan view of the plant of Figure 1 wih the air compressor removed for clarity, Figure 3 is a side elevation of the plant also with the compressor removed, Figure 4 is an elevation from one end, Figure 5 is an enlarged detailed view of the sewage inlet screen of the plant of Figures 1 to 4, Figure 6 is a cross-sectional view of the inlet arrangement taken along line 6 6 of Figure 5, Figure 7 is a cross-sectional view taken along line 7 7 of Figure 2 and illustrating the adjustble weir and dam of the plant, Figure 8 is an enlarged view of the sludge recirculation and discharge pipe, and Figure 9 is an enlarged perspective view of part of the plant illustrating the communication channel and the skimmer in the settling tank.

Referring initially to Figures 1-3, the illustrated sewage treatment plant is a modular unit comprising a water-tight container, indicated generally by the reference 10. In use, the container 10 is normally inserted, at least partially, in an excavation made in the ground. The container 10, when installed, is normally covered by a number of lids but most of these have been removed in Figure 1, and all of them removed in Figure 2 so that the interior of the container can be seen. As will be apparent later, the sewage treatment plant requires a supply of compressed air and this is provided by a small compressor mounted in a box 11 provided on top of the container 10. Any standard compressor of suitable capacity may be used and typically one powered by electricity. In Figures 2 and 3 the compressor 11 is removed for clarity.

The container 10 is made of a sheet material typically glass fibre reinforced plastics or metal and is substantially rectangular in section at the top. The sides 12 and 13 of the container 10 are formed of flat sheets converging downwardly to meet substantially in a trough 14 at the bottom of the container. The convergence of these side walls can be seen most clearly in Figure 4. In the present example, the side walls 12 and 13 are symmetrical about the trough 14 at corresponding angles to the vertical.

A dividing wall 15 divides the interior of the container 10 into an aeration tank 16 and a settling tank 17. When the plant is installed, the trough 14 extends in a horizontal line and the dividing wall 15 is orientated so as to be perpendicular to this line. One end wall 18 of the container 10, defining an end wall of the aeration tank 16, is also orientated so as to be perpendicular to the line of the trough 14. Thus, the aeration chamber 16 has a generally rectangular cross section of the same length throughout its height but of decreasing width towards the bottom of the tank. The other end wall 19 of the container 11 defines the settling tank of the plant together with the dividing wall 15 and portions of the side walls 12 and 13. The end wall 19 converges downwardly towards the dividing wall 15 so that the four walls of the settling tank 17 meet substantially at a point 20 at the bottom of the tank.

Located at the bottom of the aeration tank 16 along the trough 14, there is an aerator pipe 21. The pipe 21 extends substantially the length of the trough 14 and is connected at one end 22 to an air feeder pipe 23.

Compressed air is supplied to the aerator 21 by the air feeder pipe 23 from the compressor 11. The aerator pipe 21 is provided over substantially its entire length with perforations 24 so that compressed air fed to the pipe 21 bubbles through these perforations into liquid contained in the aeration tank 16 when the plant is in use. The bubbles produced by the aerator pipe 21 rising through the liquid in the aeration tank 16 tend to produce an upward current in the liquid in the region thereof above the trough 14. This current tends to separate towards the surface of the liquid body in the aeration tank 16 with the water moving substantially symmetrically outwards towards the side walls 12 and 13 and then down again parallel with the side walls towards the trough 14.

The shape of the aeration tank 16 together with the positioning of the aerator pipe ensures that all parts of the body of liquid in the aeration tank 16 are fully circulated and aerated. Liquid from the aeration tank 16 together with suspended solids can pass into the settling tank 17 through a communication channel 25.

As shown in Figure 9 the channel 25 comprises a U-section member secured to the dividing wall 15 forming a vertical channel open at both ends. Liquid from the aeration tank 16 enters the channel 25 through an aperture 50 in the dividing wall 15. The bulk of the liquid passing through the aperture 50 is directed by the channel 25 to emerge into the settling tank 17 from the bottom end 51 of the channel near the bottom of the settling tank 17.

A skimmer 52 is located in the settling tank 17 against the dividing wall 15 and next to the channel 25. The skimmer 52 comprises a box closed at the bottom and having an upper edge 53 which is arranged to be positioned accurately at the surface level of liquid in the settling tank, so that the surface layer of liquid tends to flow over the edge 53 into the box. In this way any floating solids, such as fats, in the settling chamber tend to be skimmed and collected in the skimmer 52.

Liquid with any skimmed solid matter is continuously drawn out of the skimmer 52 and returned to the aeration tank 16 by means of an air lift pump comprising a lifting pipe 54 extending to a point near the bottom of the skimmer. Compressed air is introduced to the lifting pipe 54 by an air line 55 to lift liquid and entrained solids up the pipe in the usual way.

The pipe 54 lifts the liquid and solids above the liquid level of the settling tank and then conveys them through the dividing wall 15 to deposit them in the aeration tank.

Generally the upper end of the channel 25 is open but extends above the level of the liquid in the settling tank 17. However, the edge 56 of the channel 25 adjacent the skimmer 52 is cut away to be at the same level as the top edge 53 of the skimmer. Thus, any solid material floating at the top of the channel 25 is also skimmed off by the skimmer.

As mentioned previously, the outlet of the channel 25 in the settling tank 17 is positioned near the bottom of the settling tank and oriented to direct liquid flowing from the aeration tank towards the bottom of the settling tank so that solids suspended therein tend to settle out more quickly. It can be appreciated that the shape of the settling chamber 19 causes settled sludge to congregate towards the bottom point 20 of the tank.

Clear liquid from which the solids have settled is discharged from the settling tank by means of a discharge device 26. This is illustrated in greater detail in Figure 7. The discharge device 26 comprises a pipe 27 mounted and extending right across between the dividing wall 15 and the end wall 19.

Liquid in the settling tank can enter the pipe 27 through an aperture 28 in the cylindrical side of the pipe. The aperture 28 extends substantially the entire length of the pipe 27 between the walls 15 and 19 and has upper and lower edges 29 and 30 respectively extending parallel with its axis. The lower edge 30 is formed with large serrations as illustrated best in Figure 7. Liquid flows into the pipe 27 from the settling tank 17 when the liquid level in the tank rises above the bottom most points of the serrations of the lower edge 30. Liquid entering the pipe 27 is discharged from the treatment plant through the aperture in the wall 19 via a connecting flange 31 which may be connected to an outlet pipe taking off the discharge liquid for appropriate disposal. The pipe 27 is mounted between the walls 15 and 19 so as to be rotatable about its axis. It can be seen, therefore, that the level of the lowermost points of the edge 30 can be adjusted by rotating the pipe 27 to a predetermined position. In this way the level of liquid in the settling tank 17 can be set accurately. Since liquid will flow from the aeration tank 16 into the settling tank 17 through the communication channel 25 if the level in the aeration tank 16 exceeds that in the tank 17, setting the position of the pipe 27 effectively controls the liquid level in both tanks of the plant.

As best shown in Figure 2, the pipe 27 is positioned horizontally spaced from the communication channel 25. Further, the lower edge 30 of the aperture 28 in the outlet pipe is on the side of the pipe 27 remote from the communication pipe 25. In this way, the pipe 27 acts as a dam preventing floating solids and fats from passing over the weir formed by the edge 30. It will be appreciated that any buoyant material in the liquid entering the settling tank 17 through the communication channel 25 be expected to come to the surface of the liquid in the settling tank 17 on the same side of the discharge device 26 as the communication channel 25. As a result, such floating material will be trapped behind the pipe 27 and prevented from passing over the edge 30.

Sewage to be treated by the plant is fed to the plant through an inlet pipe 32 which discharges the sewage into the aeration tank 16 at a point above the normal level of liquid in the tank as set by the outlet device 26. The inlet pipe 32 discharges the sewage onto a screen 33 which traps solids entrained in the incoming sewage which are too large to pass between bars 34 of the screen. The screen is shown in greater detail in Figures 5 and 6.

The screen is positioned so that the bars 34 are substantially at the same level as the liquid in the aeration tank 16. As a result, the liquid in the aeration tank 16 which is made turbulent by the bubble stream from the aerator pipe 21 tends to splash through the bars 34 of the screen continuously agitating any solids trapped thereon. The solids tend to become broken down as a result of this agitation until they can pass safely through the bars 34 to be digested by the bacteria in the aerated liquid in the tank 16. As can be seen in Figure 6, the bars 34 are tilted slightly downwards from a flat flange 35 extending across the end of the screen opposite the point of entry of the inlet pipe 32. Solids trapped above the bars 34 of the screen on entering by the inlet pipe 32 tend to be progressively moved up the bars 34 towards the flange 35 by the agitating action of the liquid in the tank 16. Any solids proving resistant to being broken up by this agitation are eventually driven right onto the flange 35 where they can collect without blocking the gaps between the bars 34. Holes 36 through the flange 35 ensure that liquids drain off the flange.

As explained previously, solids in the liquid entering the settling tank 17 tend to settle out collecting at point 20 of the tank.

This settled sludge is drawn up and supplied back to the aeration tank 16 by means of a lifting pipe 37 having a suction opening 38 adjacent the point 20. The lifting pipe 37 extends vertically upwards from the point of the settling tank to above the level of liquid in the plant. A length of horizontally extending pipe 39 connected to the upper end of the lifting pipe 38 carries sludge raised by the lifting pipe back over the liquid in the aeration tank 16 and discharges it into the aeration tank through an aperture 40 in the pipe 39 near the end wall 18. The location of aperture 40 is best seen from Figure 8. An air line 41 extends down the middle of the lifting pipe 37 from the compressor 11 to a point spaced above the bottom end 38 of the lifting pipe. A compressor 11 is connected to supply air to the air line 41 to produce bubbles in the lifting pipe 37 which rise up inside the pipe tending to suck sludge collected in the point 20 of the collecting tank into the bottom end 38 of the pipe 37.

During normal operation all the sludge collecting in the settling tank 17 is drawn back up the lifting pipe 37 and fed back into the aerating tank 16 through the aperture 40 in the pipe 39. However, as shown in Figure 8, the pipe 39 extends beyond the aperture 40 through an aperture in the end wall 18 of the container 10 to a valve 44, which is normally turned off. After a period of time, as continued batches of sewage for treatment enter the plant, although clear liquid can flow out of the plant via the outlet device 26, the amount of solids or sludge retained in the system may become excessive. Then sludge may be drawn off the system simply by opening the valve 44. Normally the aperture 40 is sufficiently restricted that opening the valve 44 causes the sludge flowing in the pipe 39 to flow preferentially past the aperture 40 and out of an outlet 42 where it can be collected either for disposal or use, e.g. as a fertiliser. It may be desirable to adjust the restriction to flow of the aperture 40 to ensure a good outflow of sludge from the outlet 42 when the valve 44 is opened. Thus, a sliding collar 43 is provided on the pipe 39 which can be positioned to occlude the aperture 40 by a desired amount until the desired restriction is achieved.

The described plant, is, as mentioned previously, of modular design so that it can be transported complete to a site for installation. This makes the described plant extremely convenient for domestic installation in place of a cesspit or septic tank. The shaping of the aeration tank 16 together with the location of the aerator pipe 21 permits a very effective and efficient aeration and agitation of the liquid to be treated at a relatively low cost in terms of energy input.

The slope of the side walls 12 and 13 is not especially critical but the walls are desirably at less than 45 to the vertical. In a preferred example, the walls are at 25 to the vertical.

WHAT WE CLAIM IS: 1. An activated sludge sewage treatment plant comprising a thin walled container for liquids, a wall dividing the container into an aeration tank on one side of the wall and a settling tank on the other side of the wall, the container being shaped to define side walls of the aeration tank converging downwards substantially to meet in a trough, an aerator located in the trough at the bottom of the aeration tank and arranged for discharging bubbles of air into liquid in the aeration tank, means to supply air to the aerator to aerate the liquid in the aerating tank, means for returning settled sludge from the settling tank to the aeration tank, inlet means for feeding sewage to the aeration tank for treatment, outlet means for discharging treated liquid from the settling tank when the liquid in the settling tank exceeds a predetermined level, and means communicating between the two tanks to permit passage of liquid and suspended sludge from the aeration tank to the settling tank when the liquid level in the aeration tank exceeds that in the settling tank, wherein the downwardly converging side walls of the aeration tank are planar with the trough extending in a horizontal line, when the plant is installed for use, and the aerator extends in the trough for discharging bubbles over substantially the whole length of the trough.

2. A plant as claimed in claim 1 wherein the aerator comprises a perforated pipe extending along the trough and is connected to a supply of compressed air for discharging said bubbles.

3. A plant as claimed in either of claims 1 or 2, wherein the dividing wall is perpendicular to the line of the trough, and the container is shaped to define an end wall of the aeration tank in a plane parallel to the dividing wall.

4. A plant as claimed in any preceding claim wherein the container has downwardly converging planar side walls defining said side walls of the aeration tank and which extend past the dividing wall to define downwardly converging planar walls of the settling tank.

5. A plant as claimed in claim 4 wherein an end wall of the container defines a wall of the settling tank and converges downwardly towards the dividing wall so that said walls of the settling tank together with the dividing wall meet substantially in a point.

6. A plant as claimed in claim 5 wherein said means for returning settled sludge includes a sludge lifting pipe having a suction end located in the settling tank adjacent said point.

7. A plant as claimed in any preceding claim, wherein said inlet means for feeding sewage to the aeration tank comprises an inlet into the aeration tank above said predetermined liquid level, and a screen located below the inlet to catch solids larger than a predetermined size entrained in the

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

Claims (16)

**WARNING** start of CLMS field may overlap end of DESC **. in the plant. A length of horizontally extending pipe 39 connected to the upper end of the lifting pipe 38 carries sludge raised by the lifting pipe back over the liquid in the aeration tank 16 and discharges it into the aeration tank through an aperture 40 in the pipe 39 near the end wall 18. The location of aperture 40 is best seen from Figure 8. An air line 41 extends down the middle of the lifting pipe 37 from the compressor 11 to a point spaced above the bottom end 38 of the lifting pipe. A compressor 11 is connected to supply air to the air line 41 to produce bubbles in the lifting pipe 37 which rise up inside the pipe tending to suck sludge collected in the point 20 of the collecting tank into the bottom end 38 of the pipe 37. During normal operation all the sludge collecting in the settling tank 17 is drawn back up the lifting pipe 37 and fed back into the aerating tank 16 through the aperture 40 in the pipe 39. However, as shown in Figure 8, the pipe 39 extends beyond the aperture 40 through an aperture in the end wall 18 of the container 10 to a valve 44, which is normally turned off. After a period of time, as continued batches of sewage for treatment enter the plant, although clear liquid can flow out of the plant via the outlet device 26, the amount of solids or sludge retained in the system may become excessive. Then sludge may be drawn off the system simply by opening the valve 44. Normally the aperture 40 is sufficiently restricted that opening the valve 44 causes the sludge flowing in the pipe 39 to flow preferentially past the aperture 40 and out of an outlet 42 where it can be collected either for disposal or use, e.g. as a fertiliser. It may be desirable to adjust the restriction to flow of the aperture 40 to ensure a good outflow of sludge from the outlet 42 when the valve 44 is opened. Thus, a sliding collar 43 is provided on the pipe 39 which can be positioned to occlude the aperture 40 by a desired amount until the desired restriction is achieved. The described plant, is, as mentioned previously, of modular design so that it can be transported complete to a site for installation. This makes the described plant extremely convenient for domestic installation in place of a cesspit or septic tank. The shaping of the aeration tank 16 together with the location of the aerator pipe 21 permits a very effective and efficient aeration and agitation of the liquid to be treated at a relatively low cost in terms of energy input. The slope of the side walls 12 and 13 is not especially critical but the walls are desirably at less than 45 to the vertical. In a preferred example, the walls are at 25 to the vertical. WHAT WE CLAIM IS:

1. An activated sludge sewage treatment plant comprising a thin walled container for liquids, a wall dividing the container into an aeration tank on one side of the wall and a settling tank on the other side of the wall, the container being shaped to define side walls of the aeration tank converging downwards substantially to meet in a trough, an aerator located in the trough at the bottom of the aeration tank and arranged for discharging bubbles of air into liquid in the aeration tank, means to supply air to the aerator to aerate the liquid in the aerating tank, means for returning settled sludge from the settling tank to the aeration tank, inlet means for feeding sewage to the aeration tank for treatment, outlet means for discharging treated liquid from the settling tank when the liquid in the settling tank exceeds a predetermined level, and means communicating between the two tanks to permit passage of liquid and suspended sludge from the aeration tank to the settling tank when the liquid level in the aeration tank exceeds that in the settling tank, wherein the downwardly converging side walls of the aeration tank are planar with the trough extending in a horizontal line, when the plant is installed for use, and the aerator extends in the trough for discharging bubbles over substantially the whole length of the trough.

2. A plant as claimed in claim 1 wherein the aerator comprises a perforated pipe extending along the trough and is connected to a supply of compressed air for discharging said bubbles.

3. A plant as claimed in either of claims 1 or 2, wherein the dividing wall is perpendicular to the line of the trough, and the container is shaped to define an end wall of the aeration tank in a plane parallel to the dividing wall.

4. A plant as claimed in any preceding claim wherein the container has downwardly converging planar side walls defining said side walls of the aeration tank and which extend past the dividing wall to define downwardly converging planar walls of the settling tank.

5. A plant as claimed in claim 4 wherein an end wall of the container defines a wall of the settling tank and converges downwardly towards the dividing wall so that said walls of the settling tank together with the dividing wall meet substantially in a point.

6. A plant as claimed in claim 5 wherein said means for returning settled sludge includes a sludge lifting pipe having a suction end located in the settling tank adjacent said point.

7. A plant as claimed in any preceding claim, wherein said inlet means for feeding sewage to the aeration tank comprises an inlet into the aeration tank above said predetermined liquid level, and a screen located below the inlet to catch solids larger than a predetermined size entrained in the

sewage to be treated and to prevent them entering the liquid in the aeration tank.

8. A plant as claimed in claim 7 wherein the screen is located substantially at said predetermined liquid level so that turbulence in the liquid in the aeration tank produced by the aerator causes the liquid surface continually to wash the screen agitating solids trapped thereon.

9. A plant as claimed in any preceding claim wherein the outlet means includes an adjustable weir for adjusting said predetermined level above which liquid in the settling tank can flow over the weir to be discharged.

10. A plant as claimed in claim 9 wherein the adjustable weir comprises a pipe extending in the settling tank and connected to a discharge aperture in a wall of the tank. the pipe having a side aperture in a cylindrical wall thereof through which liquid can flow into the pipe to be discharged from the tank, the pipe extending, when the plant is installed in a horizontal plane and being rotatably adjustable about its axis to set the height of said side aperture for adjusting said predetermined level.

II. A plant as claimed in claim 10 wherein the side aperture extends over the greater part of the length of the pipe in the settling tank defining upper and lower edges extending parallel with the axis of the pipe, the lower such edge defining the weir.

12. A plant as claimed in claim 11 wherein the pipe is arranged to extend right across between opposed walls of the settling tank at a location spaced in the horizontal plane from a point of entry via the communicating means of liquid and suspended sludge from the aeration tank, the pipe being then arranged with said lower edge of the side aperture on the side of the pipe remote from said point of entry.

13. A plant as claimed in any preceding claim wherein the container and the dividing wall are made of glass fibre reinforced plastics.

14. A plant as claimed in any preceding claim and including a skimmer comprising a box located in the settling tank and having an upper edge positioned to permit surface liquid in the settling tank to flow over the edge into the box carrying with it floating solid matter, and means for returning liquid and floating solid matter collected in the skimmer box to the aeration tank.

15. A plant as claimed in claim 14 wherein the means communicating between the two tanks comprises an aperture in the dividing wall communicating with an openended channel extending from above the level of liquid in the settling tank towards the bottom of the settling tank the skimmer box being located adjacent the channel and arranged to skim off also floating solids collecting therein.

16. An activated sludge sewage treatment plant substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.