GB2551385A

GB2551385A - Aerated wastewater treatment

Info

Publication number: GB2551385A
Application number: GB1610574.4A
Authority: GB
Inventors: Baird Andrew
Original assignee: WPL Ltd
Current assignee: WPL Ltd
Priority date: 2016-06-17
Filing date: 2016-06-17
Publication date: 2017-12-20
Anticipated expiration: 2036-06-17
Also published as: GB2551385B; GB201610574D0

Abstract

A treatment chamber for use in an aerated wastewater treatment system comprises a hollow container 13 having a fluid inlet 16, a fluid outlet 17, and a first fluid permeable barrier 22 in an upper region of the container below the fluid inlet and the fluid outlet. The barrier forms an upper boundary of a biozone 15 within the container. A plurality of independently mobile inert substrates 23 are located within the biozone, each substrate having a high surface area to volume ratio and being neutrally buoyant in wastewater. The chamber comprises nozzles 19 below the biozone, the nozzles having a connection to a supply of oxygen. The chamber also comprises one or more ducts 20 in fluid communication with the biozone, wherein the one or more ducts are isolated from the substrates. A second barrier 21 can form a lower boundary of the biozone and may be provided in the bottom region of the container above the nozzles. A biozone cage for use in a wastewater treatment tank is also claimed, wherein the cage has upper and lower fluid permeable barriers, and a fluid impermeable peripheral barrier extending between the upper and lower fluid permeable barriers.

Description

Aerated Wastewater Treatment

Field of the invention [0001] The present invention reiates to aerated wastewater treatment systems and methods, in particuiar, but not exclusively, the present invention is suited for use as part of a submerged aerated filter (SAF) package treatment plant.

BACKGROUND OF THE INVENTION

[0002] Package treatment plants (PTP) for wastewater are smaller-scale wastewater treatment systems particularly suited for providing wastewater treatment to smaller communities of people such as, but not limited to, small villages, leisure facilities, schools, hotels, industrial sites, offices, camping sites etc. PTPs are also suitable for providing emergency wastewater treatment. PTP tanks tend to be constructed and assembled at a factory using steel, precast concrete or Glass Reinforced Plastic (GRP) and are subsequently installed as a pre-packaged unit at the wastewater treatment site.

[0003] Typically a PTP includes an aeration tank (also commonly referred to as a biological reactor) for aerobic treatment of the wastewater and a settling tank, often referred to as a clarifier, for sludge removal. In aerobic treatment microorganisms, such as bacteria which naturally occur in sewage, are used to decompose the sewage in the wastewater producing as by-products carbon dioxide and water. Oxygen is the energy source for the microorganisms to metabolize the sewage and so an air feed is submerged in the aeration tank enabling air to be bubbled through the wastewater in the aeration tank.

[0004] Also, biologically inert media may be added to the wastewater in the aerobic tank to support growth of the microorganisms. Such biologically inert media may be manufactured using a plastics material such as, but not limited to, polypropylene, polyethylene, polyvinylchloride and other similar polymers and are designed to have a large surface area for the microorganisms to colonize.

[0005] An example of a conventional aerated wastewater treatment plant 1 is illustrated in FIG. 1. The wastewater treatment plant comprises a first mechanical filter 2 which receives the raw wastewater and removes large solids by passing the raw wastewater through, for example, a screen. A scraper may be used to remove the larger solids captured by the mechanical filter. The wastewater passes from the first mechanical filter 2 to a first clarifier or primary tank 3 in which slower settling particles are allowed to fall to the tank bottom. Here a bottom scraper can be used to remove the settled particles from the primary tank or left for removal by vacuum. The wastewater then flows from the primary tank 3 to a biological reactor 4 containing inert media. The biological reactor 4 and includes an aerator 5 which pumps air containing oxygen into the wastewater within the biological reactor 4 and a mechanical mixer 6 which induces movement of the effluent over denitrifying media when denitrification is required. A scraper (not illustrated) can be used to remove sludge which forms at the bottom of the biological reactor 4. Downstream of the biological reactor 4 the treated wastewater may optionally flow through a second clarifier 3, a second filter 7, a disinfector 8 in which disease-causing organisms are removed / destroyed through the use of, for example, ultra-violet light or chlorine and a reaerator 9 in which air is bubbled through the treated wastewater to increase the amount of dissolved oxygen in the treated wastewater. Outflow from the aerated wastewater treatment plant may be to a sewer or to a natural body of water such as the sea.

[0006] An example of a wastewater treatment plant of the type described above which uses inert media in a biological reactor is described in US2007/0102354 the contents of which is incorporated herein by reference. However, with such conventional wastewater treatment plants sludge and solids build up within the biological reactor which reduces the efficiency of the aerobic treatment as a result of reduced oxygen transfer rates. Therefore regular scouring is required to maintain the efficacy of the aerobic treatment of the wastewater. Where inert media is used, scouring is commonly performed by significantly increasing the rate of air flow from the aerator into the biological reactor, the size and/or the number of air bubbles generated by the aerator. However this approach is wasteful of the air used to aerate the wastewater in the biological reactor and significantly increases the power required to operate the plant. In the case of membrane wastewater treatment plants scouring is believe to account for up to 75% of the overall energy requirements of the plant (see US2010/0264080)..

SUMMARY OF THE INVENTION

[0007] The present invention seeks to address the problems discussed above. In particular, the present invention seeks to provide an aerobic wastewater treatment system and method which requires less power than conventional aerobic wastewater treatment plants. The present invention further seeks to provide an aerobic wastewater treatment system and method which does not require additional equipment or adaptation of existing apparatus to provide a scouring function within the biological reactor.

[0008] The present invention therefore provides a treatment chamber for use in an aerated wastewater treatment system, the treatment chamber comprising a hollow container having a fluid inlet, a fluid outlet; a first fluid permeable barrier in an upper region of the container below the fluid inlet and the fluid outlet, the barrier forming an upper boundary of a biozone within the container, a plurality of independently mobile biologically inert substrates within the biozone, each biologically inert substrate having a high surface area to volume ratio and being substantially neutrally buoyant in wastewater; a plurality of nozzles below the biozone, the plurality of nozzles having a connection to a supply of oxygen; and one or more ducts in fluid communication with the biozone wherein the one or more ducts are isolated from the plurality of inert substrates. .

[0009] In a preferred embodiment the treatment chamber the first barrier isolates an uppermost opening of the one or more ducts from the plurality of biologically inert substrates in the biozone.

[0010] More preferably the treatment chamber includes a second barrier in the bottom region of the container above the plurality of nozzles, the second barrier forming a lower boundary of the biozone and separating the plurality of biologically inert substrates from the plurality of nozzles and from a lowermost opening of the one or more ducts.

[0011] Preferably the plurality of biologically insert substrates have a surface area to volume ratio of at least 250m^/m^, more preferably at least 310m^/ml [0012] Preferably the specific gravity of the biologically inert substrates is between 0.99 and 1.06, more preferably between 1.01 and 1.05 and ideally 1.04.

[0013] In a particularly preferred embodiment the plurality of biologically inert substrates fills 80% or more of the volume of the biozone, more preferably 90% or more of the volume of the biozone.

[0014] Ideally the connection to a supply of oxygen comprises a connection to a supply of air whereby the plurality of nozzles supply oxygen in air to the biozone within the container.

[0015] In a second aspect the present invention provides an aerated wastewater treatment system comprising a treatment chamber as described above and at least one filter adapted to separate dead bacteria from the treated wastewater.

[0016] Preferably the aerated wastewater treatment system further comprising a filter consisting of a planar mesh and/or a filter containing sand or other fine particulate material.

[0017] Also the aerated wastewater treatment system may include one or more clarifiers at least one of which may be a settling tank.

[0018] The aerated wastewater treatment system may additionally include a disinfection station for removing biologically active contaminants in the wastewater which may include an ultraviolet light source.

[0019] Also the aerated wastewater treatment system may include an aeration tank comprising one or more nozzles for supplying oxygen or air to wastewater in the aeration tank.

[0020] In a still further aspect the present invention provides a biozone cage for use in a wastewater tank comprising a fluid inlet, a fluid outlet and a plurality of nozzles supplying bubbles containing oxygen, the biozone cage being adapted for mounting within the wastewater tank in a position above the plurality of nozzles which permits wastewater to circulate within the wastewater tank outside of the biozone cage, the biozone cage comprising upper and lower fluid permeable barriers; a fluid impermeable peripheral barrier extending between the upper and lower fluid permeable barriers, and a plurality of independently mobile biologically inert substrates located within the peripheral barrier between the upper and lower fluid permeable barriers, each biologically inert substrate having a high surface area to volume ratio and being substantially neutrally buoyant in wastewater wherein wastewater tank outside of the biozone cage is isolated from the plurality of inert substrates.

[0021] In a yet further aspect the present invention provides a method of aerated wastewater treatment using a treatment chamber comprising a hollow container containing a first fluid permeable barrier in an upper region of the container, the barrier forming an upper boundary of a biozone within which is confined a plurality of independently mobile biologically inert substrates with each biologically inert substrate having a high surface area to volume ratio and being substantially neutrally buoyant in wastewater, and one or more ducts in fluid communication with the biozone, the method comprising: feeding a supply of wastewater to the treatment chamber; exposing the plurality of biologically inert substrates and any biomass formed on the substrates to bubbles containing oxygen rising in the wastewater within the biozone whereby the rising bubbles induce circulatory movement of the plurality of biologically inert substrates within the biozone and the wastewater flows between the biozone and the one or more ducts; and emitting treated wastewater including dead bacteria as scum from the treatment chamber.

[0022] Thus the present invention is an hydraulic redesign of conventional aerobic wastewater treatment systems to provide a natural, relatively slow but controlled circulation of the inert media within the biological reactor. This hydraulic redesign minimizes the buildup of biomass on the inert media; minimizes the accumulation of biomass at the bottom of the biological reactor; and reduces the power requirements of the biological reactor in comparison to conventional aerobic wastewater treatment plants.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: FIG. 1 illustrates schematically a conventional aerated wastewater treatment plant; FIG. 2 illustrates schematically an aerated wastewater treatment system in accordance with the present invention; FIG.3 illustrate schematically internal features of the biological reactor and the movement of inert media within the biological reactor in accordance with the present invention; and FIG. 4 illustrates a preferred shape for the inert media used in the aerated wastewater treatment system in accordance with the present invention.

[0024] In the figures like reference numerals are used to identify the same components.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0025] An embodiment of an aerobic wastewater treatment system is shown in FIG. 2. In overview the aerobic wastewater treatment system of FIG. 2 includes first and second filters 2, 18; a first clarifier or primary tank 3 and a second clarifier or secondary tank 7; a disinfector 9 and a re-aerator 10 all of which are conventional in design and which perform the same function as the equivalent components in the conventional aerobic wastewater treatment plant of FIG. 1. A detailed discussion of these components is, therefore, omitted. The aerobic wastewater treatment system additionally includes a biological reactor 12 which is described in greater detail below with reference to FIGS. 3 and 4.

[0026] The biological reactor 12 generally comprises a hollow reactor container or tank 13 for containing wastewater to be treated and a cover 14 to prevent leaf and sunlight entering the biological reactor 12. As the reactor tank 13 is hollow it contains a cavity which includes a biozone 15 (described in greater detail below). The hollow interior of the reactor tank 13 is in fluid communication with a wastewater inlet 16 and a treated wastewater outlet 17. Both the inlet 16 and the outlet 17 are in the form of pipes located close to the top of the reactor tank 13 and preferably at different distances below the top of the reactor tank 13 with the outlet 17 further from the top of the reactor tank 13 than the inlet 16. The reactor tank 13 may be formed of steel, precast concrete, Glass Reinforced Fiber (GRP) or a simiiar strong material whereas the cover 14 is preferably formed of thin GRP for ease of removal from the reactor tank 13.

[0027] The reactor tank cavity is dimensioned to hold a sufficient volume of wastewater to ensure a retention time within the biological reactor 12 necessary to treat the effluent to the relevant wastewater treatment standard. Typical retention times are between 15 minutes and 2 hours. So as to be accommodated within existing and currently available tanks, the biozone 15 preferably does not exceed a width of 2.8 meters and a depth not exceeding 5 metres. Where the reactor tank 13 exceeds 14m^ baffle walls to direct flow (not shown in the drawings) may be added.

[0028] An air distribution pipework 18 is provided in the reactor tank cavity adjacent the bottom of the cavity. The air distribution pipework 18 includes a connection (not iiiustrated) extending through the reactor tank for attachment to a forced air suppiy (such as a biower or air pump) externai to the reactor tank 13. The air distribution pipework 18 shown in FiG. 3 comprises a piurality of pipes arranged in parallel each with a plurality of air apertures or nozzies 19 which release air into the reactor tank cavity . Each air aperture 19 preferably has a diameter of 5 mm, although other nozzle diameters are aiso envisaged, in an aiternative embodiment the air distribution pipework 18 may comprise an intersecting network of pipes with nozzles 19 distributed across the intersecting pipe network in a 2D array parallel to the bottom of the cavity.

[0029] One or more open ducts 20 are provided around the periphery of the reactor tank cavity extending from an upper region of the reactor cavity (but beiow the inlet and outlet pipes 16, 17) to adjacent the bottom of the cavity. The walls of the ducts 20 are fluid impermeable and the open bottom ends of the one or more ducts 20 are positioned outside of the periphery of the array of air nozzles 19 so that air from the air bubbles from the nozzles 19 do not rise within the ducts 20. In this way, the ducts 20 provide an unimpeded fluid circulation path for the wastewater which is described in greater detail below. Also the uppermost edges of the ducts 20 preferably function as baffle walls forming calm areas behind the baffle walls at the top of the ducts 20.

[0030] Above the air distribution pipework 18 and the open bottom of the ducts 20 is a fluid permeable barrier in the form of a first open mesh or grid 21 which permits free movement of water and air bubbles through the grid 21. A similar fluid permeable barrier in the form of an open mesh or grid 22 is provided close to the top of the reactor tank cavity but below the wastewater inlet and outlet 16, 17 and below the top edge of the ducts 20. The region of the cavity between the two grids 21, 22 and the walls of the ducts 20 constitutes the biozone 15 and biologically inert substrates or media 23 are confined between the two grids 21, 22 and the walls of the ducts 20. The biologically inert media 23 fill at least 80%, more preferably 90% or more, of the biozone volume.

[0031] A biologically inert media 23 used in the aerobic wastewater treatment system described herein is illustrated in FIG. 4. The biologically inert media 23 has a high surface area to volume of 250m^/m^ or more, ideally 310m^/m^ or more and is substantially neutrally buoyant in wastewater with a specific gravity ranging between 0.99 and 1.06, more preferably 1.01 and 1.05 and ideally 1.04. The media 23 is formed of polypropylene or a similar plastics material having the characteristics of being biologically inert and with a specific gravity within the range 0.99 and 1.06.

[0032] During operation of the biological reactor 12, as illustrated in FIG. 3, wastewater is fed into the biological reactor 12 via the inlet 16 and filled up to the top wastewater level 24. When the biological reactor 12 is full in the absence of aeration the surface of the wastewater is below the inlet and outlet 16, 17 and at approximately the same height as the uppermost edges of the ducts 20. When the biozone 15 is aerated, the surface of the wastewater 24 above the biozone 15 is slightly higher than the level of the surface of unaerated wastewater. This difference in the surface heights of the aerated and unaerated wastewater induces a flow of wastewater from the region above the biozone 15 towards the open upper ends of the ducts 20 with the uppermost edges of the ducts 20 acting as baffle walls. Within the ducts 20, the wastewater has unimpeded downward flow towards the bottom of the reactor 12 and upward circulation of the wastewater is induced or entrained by the rising air bubbles from the air distribution pipes 18.

[0033] With the biological reactor 12 described herein the rate of air flow from the air distribution pipes 18 is less than the minimum required with conventional aerobic reactors. The biological reactor 12 is hydraulically designed to enable the blowers or air pumps used to force air through the air distribution pipes 18 and out through the nozzles 19 to be switched on and off to meet with diurnal peaks and troughs. This can result in a reduction in power consumption of up to 50% in comparison to conventional wastewater treatment systems. Furthermore, with the aerobic biological reactor 12 described herein the required airflow rate does not exceed the airflow rate required for biological processes alone. The current UK industry standard for aerobic biological reactors is an air flow rate of 12 m^/m^ per hr. In contrast, the wastewater treatment system described herein requires an airflow rate of less than 12 m^/m^, preferably below 10 m^/m^, and more preferably below 6 m^/m^. As a result of lower airflow rates being required, when switched on the blowers or air pumps of the wastewater treatment system described herein consume up to 33% less power than an equivalent conventional wastewater treatment system.

[0034] The circulation of the wastewater within the biological reactor 12 induces an associated slow and gentle circulatory movement of the inert media 23 within the biozone 15. As the media 23 has substantially neutral buoyancy the media become entrained in the fluid circulation induced by the rising air bubbles and follow a generally circular or elliptical path within the biozone 15. As the media 23 become coated with biomass the weight of the media is increased and the media sinks towards the bottom of the reactor. Only a small amount of biomass is sufficient to cause the media to sink because the media is neutrally buoyant. This has the effect of reducing the buildup of biomass on the media in comparison to conventional aerobic chambers. Once the media has sunk to the bottom of the cavity 15, the media encounter rising air bubbles from the air distribution nozzles 18 and the small amount of biomass sloughs off from the surface of the inert media even in the low rates of air flow from the nozzles 19.

As the biomass is prevented from building up on the media, once the biomass has soughed off from the surface of the media, both the media and the biomass become entrained in the bubble induced fluid circulation within the biozone 15 and flow to the surface 24 of the wastewater as scum. A cross current at the surface of the wastewater 24 induced, for example, by the introduction of more wastewater into the biological reactor 12, carries the biomass as scum away from above the biozone 15 and out via the outlet 17. In this way the amount of biomass which collects as sludge in the bottom of the biological reactor 12 is minimized.

[0035] As mentioned above, this continuous but slow movement of the media 23 within the reactor 12 prevents sludge or solids building up on the inert media and avoids the need for scouring, which in an equivalent conventional biological reactor involves the use of much higher rates of air flow. Moreover, with the wastewater treatment system described herein this is possible even with the high fill percentages for the inert media 23 of 80% or more. Furthermore, the circulatory movement of the inert media 23 continues even where there is a low rate of incoming wastewater flow or no incoming wastewater flow. This inertia of the inert media circulatory flow permits the air supply to be intermittently switched off or pulsed during low diurnal flow. Also, the self-cleaning effect of the inert media 23 enables fewer and smaller air nozzles to be used.

[0036] In contrast to conventional aerobic tanks which collect dead bacteria in the form of sludge from the bottom of the tank, the wastewater treatment system described herein allows dead bacteria in the form of scum floating on the surface of the wastewater to exit with the treated wastewater via the outlet 17. If desired, the dead bacteria can then be extracted from the treated wastewater by means of a downstream filter or clarifier.

[0037] It will, of course, be apparent that the biological reactor 12 described above may be integrated into an existing aerobic wastewater treatment plant through substitution of the biological reactor 12 for an existing aerobic chamber. Also the method of aerobic wastewater treatment described above can be implemented with existing aerobic wastewater treatment systems by retrofitting one or more circulation ducts 20 to the internal walls of the existing aerobic chamber; positioning the grids 21, 22 above the existing air distribution nozzles at different heights within the aerobic chamber and filling 80% or more of the wastewater space bounded by the ducts 20 and the grids 21, 22 with the biologically inert media 23 described above. When the method of aerobic wastewater treatment in accordance with the present invention is retro-fitted to an existing biological reactor, the rate of air flow from the existing air distribution nozzles will need to be lowered by 10% or more relative to the rate of air flow used prior to retro-fitting. Also scheduled scraping of the bottom of the reactor tank to remove sludge may be either halted or performed less regularly than was the case prior to retro-fitting.

[0038] For larger installations and for retro-fits it is possible to preconstruct the circulation ducts 20 and the grids 21, 22 in a cage type arrangement within which the inert media is contained. In the cage type arrangement the circulation ducts 20 extend through the girds 21, 22. As described above the inert media 23 must have a surface area to volume of at least 250 m^/m^ and be substantially neutrally buoyant with a specific gravity between 0.99 and 1.06. The cage type arrangement can then be installed as a unit into an existing reactor tank above existing air distribution nozzles. It will be immediately apparent that use of a pre-constructed cage type arrangement can greatly improve the ease of installation and the associated cost of installation.

[0039] With the aerobic wastewater treatment system and method described herein energy usage can be reduced up to around 66% in comparison to conventional aerobic wastewater treatment plants bringing through-life costs of aerobic treatment plants in line with through-life costs for rotating biological contactors (RBCs).

[0040] It is to be understood that the aerobic wastewater treatment system and method described above is only one embodiment of the aerobic wastewater treatment system and method of the present invention. Changes may be made to the components and the arrangement of those components without departing from the scope of the invention as defined in the appended claims. In particular, the filters, clarifiers, disinfector and re-aerator described above are optional and may be omitted as desired. Also a wastewater treatment system may Include a plurality of biological reactors which may be operated In series or in parallel.

Also, the illustrated biological reactor Is generally box-shaped but may take alternative shapes including but not limited to cylindrical or barrel shaped. Furthermore, whilst reference is made herein to air being supplied to the biozone, as it is the oxygen in the air that is required it is possible for pure oxygen or other gas mixes containing oxygen to be used instead of air. The aerated wastewater treatment system and method described herein is suitable for both above and below ground installation.

Claims

CLAIMS 1 A treatment chamber for use in an aerated wastewater treatment system, the treatment chamber comprising a hollow container having a fluid inlet, a fluid outlet; a first fluid permeable barrier in an upper region of the container below the fluid inlet and the fluid outlet, the barrier forming an upper boundary of a biozone within the container, a plurality of independently mobile biologically inert substrates within the biozone, each biologically inert substrate having a high surface area to volume ratio and being substantially neutrally buoyant in wastewater; a plurality of nozzles below the biozone, the plurality of nozzles having a connection to a supply of oxygen; and one or more ducts in fluid communication with the biozone wherein the one or more ducts are isolated from the plurality of inert substrates. 2 The treatment chamber as claimed in claim 1, wherein the first barrier separates an uppermost opening of the one or more ducts from the plurality of biologically inert substrates in the biozone. 3 The treatment chamber as claimed in either of claims 1 or 2, comprising a second barrier in the bottom region of the container above the plurality of nozzles, the second barrier forming a lower boundary of the biozone and separating the plurality of biologically inert substrates from the plurality of nozzles and from a lowermost opening of the one or more ducts. 4 The treatment chamber as claimed in any one of the preceding claims, wherein the plurality of biologically insert substrates have a surface area to volume ratio of at least 250m^/m^. 5 The treatment chamber as claimed in claim 4, wherein the plurality of biologically insert substrates have a surface area to volume ratio of at least 310m^/m^ 6 The treatment chamber as claimed in any one of the preceding claims, wherein the specific gravity of the biologically inert substrates is between 0.99 and 1.06. 7 The treatment chamber as claimed in claim 6, wherein the specific gravity of the biologically inert substrates is between 1.01 and 1.05. 8 The treatment chamber as claimed in any one of the preceding claims, wherein the plurality of biologically inert substrates fills 80% or more of the volume of the biozone. 9 The treatment chamber as claimed in claim 8, wherein the plurality of biologically inert substrates fills 90% or more of the volume of the biozone. 10 The treatment chamber as claimed in any one of the preceding claims, wherein the connection to a supply of oxygen comprises a connection to a supply of air whereby the plurality of nozzles supply oxygen in air to the biozone within the container. 11 An aerated wastewater treatment system comprising a treatment chamber as claimed in any one of claims 1 to 10, and at least one filter adapted to separate dead bacteria from the treated wastewater. 12 An aerated wastewater treatment system as claimed in claim 11, further comprising a filter consisting of a planar mesh. 13 An aerated wastewater treatment system as claimed in claim 11, further comprising a filter containing sand or other fine particulate material. 14 An aerated wastewater treatment system as claimed in any one of claims 11 to 13, further comprising one or more clarifiers. 15 An aerated wastewater treatment system as claimed in claim 15, wherein at least one of the one or more clarifiers is a settling tank. 16 An aerated wastewater treatment system as claimed in any one of claims 11 to 15, further comprising a disinfection station for removing biologically active contaminants in the wastewater. 17 An aerated wastewater treatment system as claimed in claim 16, wherein the disinfection station includes an ultraviolet light source. 18 An aerated wastewater treatment system as claimed in any one of claims 11 to 17, further comprising an aeration tank including one or more nozzles for supplying oxygen or air to wastewater in the aeration tank. 19 A biozone cage for use in a wastewater tank comprising a fluid inlet, a fluid outlet and a plurality of nozzles supplying bubbles containing oxygen, the biozone cage being adapted for mounting within the wastewater tank in a position above the plurality of nozzles which permits wastewater to circulate within the wastewater tank outside of the biozone cage, the biozone cage comprising upper and lower fluid permeable barriers; a fluid impermeable peripheral barrier extending between the upper and lower fluid permeable barriers, and a plurality of independently mobile biologically inert substrates located within the peripheral barrier between the upper and lower fluid permeable barriers, each biologically inert substrate having a high surface area to volume ratio and being substantially neutrally buoyant in wastewater wherein wastewater tank outside of the biozone cage is isolated from the plurality of inert substrates. 20 A method of aerated wastewater treatment using a treatment chamber comprising a hollow container containing a first fluid permeable barrier in an upper region of the container, the barrier forming an upper boundary of a biozone within which is confined a plurality of independently mobile biologically inert substrates with each biologically inert substrate having a high surface area to volume ratio and being substantially neutrally buoyant in wastewater, and one or more ducts in fluid communication with the biozone, the method comprising: feeding a supply of wastewater to the treatment chamber; exposing the plurality of biologically inert substrates and any biomass formed on the substrates to bubbles containing oxygen rising in the wastewater within the biozone whereby the rising bubbles induce circulatory movement of the plurality of biologically inert substrates within the biozone and the wastewater flows between the biozone and the one or more ducts; and emitting treated wastewater including dead bacteria as scum from the treatment chamber. 21 An aerated wastewater treatment system substantially as hereinbefore described with reference to FIGS. 2 to 4. 22 A treatment chamber for use in aerated wastewater treatment system substantially as hereinbefore described with reference to FIGS. 2 to 4. 23 A method of aerated wastewater treatment substantially as hereinbefore described with reference to FIGS. 2 to 4.