GB2380221A

GB2380221A - Interlocking fluid treatment tanks

Info

Publication number: GB2380221A
Application number: GB0123145A
Authority: GB
Inventors: Alan Brook
Original assignee: Hepworth Building Products Ltd
Current assignee: Hepworth Building Products Ltd
Priority date: 2001-09-26
Filing date: 2001-09-26
Publication date: 2003-04-02
Anticipated expiration: 2021-09-26
Also published as: GB2380221B; GB0123145D0; WO2003027030A1

Abstract

A tank (2), adapted for installation at least partially underground, includes a number of peripheral formations (12) which enable a number of like tanks to be connected together (Figures 8 and 9). Preferably each tank is hexagonal, hollow double walled and formed from a plastics material, furthermore the inside of the tank may be funnel shaped. The formations may be of tongue and groove type, such that a tongue fits into a groove and laterally restrains it therein. A fluid treatment apparatus comprising a number of said tanks may further include mobile or fixed media for use in a biofilm treatment process, a launder or a settler.

Description

APPARATUS FOR THE PURIFICATION OF WATER The present invention relates to apparatus for the purification of water. Particularly, but not exclusively, the invention relates to apparatus for the treatment of effluent.

Contaminated water may be purified using a number of well-known procedures. These include additive treatments, in which certain chemicals are added to the contaminated mix, and essentially mechanical operations, which typically involve filtration or gravitational effects.

A further class of water purification processes involves the use of various biological treatments. These normally take the form of exposing the contaminated mixture to bacterial cultures which will consume or convert contaminants.

Two basic biological treatment techniques are widely employed in various forms. The first of these is the'bioslurry'technique, in which micro-organisms suspended in the water aggregate to form a sludge. The second is the'biofilm'technique, in which surfaces are provided to act as a carrier medium and the micro-organisms collect on the surfaces to form a biofilm, which normally peels off after a particular thickness has been reached.

Biological techniques are employed in the purification and treatment of, for example, drinking water, along with a wide variety of other fluids, for a range of purposes. One of the main applications for biological purification methods is in the treatment of waste water and sewage. The mechanisms used for this latter class of methods are becoming more important, due to the perceived cost and disruption of pipelines and contamination risks.

EP 0 575 314 discloses a package sewage treatment plant which is suitable for use with small population areas having between 200 and 1,000 inhabitants. Such a plant is

suitable for replacing a small sewage works. The plant disclosed in EP 0 575 314 makes use of moving biological media which act as support surfaces in a biofilm process, where the initial carbonaceous reactions occur. The movement of the biological media is created by bubbling air into the bottom of tanks in which the biological media are suspended in the mixture. In a second stage of the process, reactors contain fixed media, which support slower growing nitrifying reactions.

The processes disclosed in EP 0 575 314 have been shown to be extremely effective.

However, construction of appropriate plant for use in the described process has hitherto been more costly and time-consuming than desired.

The present invention sets out to provide apparatus which can be employed in a purification process such as that described in EP 0 575 314, yet provide improved design flexibility and reduced design and installation costs. In addition, the invention sets out to provide apparatus which can be manufactured quickly and cost-effectively.

The invention also sets out to provide apparatus which can be installed discretely, preferably in subterranean dispositions.

According to a first aspect of the present invention there is provided fluid treatment apparatus for use in the purification of water, the said apparatus comprising a plurality of treatment tank units, each said tank unit comprising a tank adapted for installation at least partly below ground level and comprising peripheral formations that enable the said tank units to be connected together in a plurality of laterally interlocking formations.

Preferably, each said tank unit comprises a plurality of radially outer mating faces which are arranged to define a polygonal cross-sectional profile in at least one axial region of the said tank unit, wherein the said mating faces are each adapted for direct contact with a said mating face provided on another said tank unit. It is particularly preferred that the said cross-sectional profiles are generally hexagonal.

A plurality of the said mating faces situated on each tank unit may be provided with respective keying means for co-operation with a co-operatively configured portion of another said tank unit. Preferably, each said keying means comprises a generally radial projection and a generally radial recess, the arrangement being such that each said projection can be closely received in, and laterally constrained by, a said recess provided on another said tank unit. Each said projection may be in the form of a tongue and each said recess may be in the form of a groove. In such a case, each tongue may have a generally part-circular cross section and each groove have a generally part circular cross section.

It is particularly preferred that the tank of at least one said tank unit is formed from a plastics material.

The tank of at least one said tank unit may be double walled. The said double walled tank may then comprise at least one perforation in an inner one of its walls and an air flow path, communicating with the said perforation, extending between the said inner one of its walls and an outer one of its walls.

An inner surface of at least one said tank may be generally funnel-shaped in a region thereof that is adapted to be located towards the bottom of the said tank when in use Preferably, each said tank comprises an anchoring formation on an outer surface thereof.

Each said anchoring formation may take the form of a radial flange.

Each said tank may be provided with a plurality of buttresses.

Each said tank may comprise a plurality of apertures situated at regular circumferential intervals for connection with an inlet or outlet pipe. Each said tank may comprise a plurality of indicia, denoting respective locations at which an inlet or outlet pipe may communicate with the said tank.

Preferably, at least one said tank comprises at least one channel in a surface thereof for co-operation with a forklift truck or a lifting harness.

It is particularly preferred that each said tank unit comprises a lid having a convex surface that is adapted to be situated generally uppermost during use.

At least one said tank unit may be provided with mobile media for use in a biofilm treatment process. Furthermore, at least one said tank unit may be provided with fixed media for use in a biofilm treatment process. At least one said tank unit may be fitted with a launder and/or a tube settler. In such a case, the said tank unit fitted with the said launder and/or tube settler preferably further comprises a baffle wall for extending downwards from a region of the tank which is adapted for location uppermost during use, the said baffle wall being situated between an inlet region of the tank and an outlet region of the tank.

A second aspect of the invention provides a tank unit as set out in relation to the first aspect of the invention above.

Embodiments of the invention therefore enable a purification plant to be tailored to the specific needs of the environment with ease and speed. The invention also enables the plant to be installed below ground, thereby leading to aesthetic and environmental advantages. Furthermore, the invention, by use of modular units, vastly reduces design, manufacturing and installation times and costs.

Embodiments of the invention will now be described by way of example and with reference to the accompany drawings in which :- Figure 1 is a perspective view of a reactor tank in accordance with an embodiment of the invention; Figure 2 is a cut-away view of the tank of Figure 1 ;

Figure 3 is a plan view, showing the lid of the tank of Figure 1; Figure 4 is a perspective view of the lid and cover of the tank of Figure 1; Figure Sa is a cut-away view through the lid of Figures 3 and 4; Figure Sb is an underside view of the lid of Figures 3 and 4.

Figure 6 is a perspective view of a settlement tank in accordance with an embodiment of the invention; Figure 7 is a cut-away view of the settlement tank of Figure 6; Figure 8a is a perspective view showing a linear of three reactor tanks such as shown in Figure 1 in conjunction with a settlement tank such as shown in Figure 6; Figure 8b is a side view of the arrangement of Figure 8a; Figure 9a is a perspective view which shows a cluster formation comprising four reactor tanks such as shown in Figure 1 in conjunction with a settlement tank such as shown in Figure 6; Figure 9b is a side view of the arrangement shown in Figure 9a; Figure 9c is a schematic view of a sewage treatment system incorporating a plan view of the cluster formation shown in Figures 9a and 9b; Figure 1 osa is a cut-away view of the main body part of the reactor tank of Figure 1 ; Figure 1 Ob is a part-exploded view of the tank of Figure lOa ; Figure 11 a shows a feed pipe in accordance with the invention; Figure 11 b shows a series of reactor tanks in use, together;

Figure 12 is a perspective view of the settlement tank of Figure 6 fitted with a tube settler, a launder and feed pipes; Figure 13 is a plan view of the settlement tank of Figure 12; Figure 14 is a section through the settlement tank of Figures 12 and 13; and Figure 15 is a perspective view of a reactor tank in accordance with an alternative embodiment of the invention; Figure 16 is a side view of the reactor tank of Figure 15 ; Figure 17 is a plan view of the reactor tank of Figures 15 and 16; Figure 18 is a perspective view of a settlement tank in accordance with an alternative embodiment of the invention; Figure 19 is a side view of the settlement tank of Figure 18; and Figure 20 is a plan view of the settlement tank of Figures 18 and 19.

An embodiment of the invention will now be described in which a number of reactor modules 1, such as shown in Figure 1, are used in conjunction with a settlement module 20, such as shown in Figure 6. Each module can be considered to constitute a tank unit.

In practice, the numbers of modules 1,20 employed can be selected as required and this is in fact one of the primary advantages of the invention. For the treatment of waste water and sewage, at least one reactor module 1 and at least one settlement module 20 would normally be employed, although this is not absolutely necessary.

Referring first to Figure 1, it will be seen that the reactor module 1 comprises a reactor tank 2 fitted with a lid 4. As will be apparent from the figures, the reactor tank 2 is

double-skinned. The double-skinned structure allows the tank walls to be hollow, which leads to enormous weight advantages and significant manufacturing cost benefits.

The interior skin is generally circular in cross-section and tapers from a maximum diameter near the top of the tank 2 to a minimum diameter near the base of the tank 2.

The underside of the lid 4 is configured to correspond with the shape of the tank 2 and therefore has a depending seat 7 which is also circular in cross-section and of such a diameter as to sit comfortably within the open upper end of the tank 2. Both the reactor tank 2 and its lid 4 have a generally hexagonal outer perimeter in cross-section. The exact outer cross-sectional profile of the lid 4 and the reactor tank 2 can be seen in Figures 3 and 9c. From these, it will be appreciated that, when viewed from above, the hexagonal profile is embellished with a series of tangential projections 9 on the lid 4.

The corresponding formations, in fact, extend along the entire height of the tank 2, as can be seen in Figure 1 and therefore define buttresses 12 on the tank 2.

The cross-sectional profile of the buttresses 12 and projections 9 is such that they each extend laterally from a respective end of one of the sides of the hexagon defining the outer perimeter of the component. When viewed from above, each of the projections 9 or buttresses 12 would appear to trail from the edge from which it extends if the component were rotated in the anticlockwise direction. Of course the reverse configuration would be equally viable. Each projection 9 or buttress 12 terminates in an end face 13 which is followed by a return 19 having the same width as the end face 13.

The angle between each end face 13 and its respective return 19 is 120 . Furthermore, the angle between each end face 13 and the side 3 to which it is adjacent is also 120 .

Because the end face 13 has the same width as the return 19, the effect of the configuration is that the projections 9 and buttresses 12 are congruent and interlock when the tanks are arranged side by side. The six sides of each tank can therefore be considered mating faces 42. This can be seen particularly clearly in Figure 9c.

The advantage of having the buttresses 12 and projections 9 in this configuration is that the buttresses can be substantial without interfering with the close-packing characteristics of the modules, thereby enabling them to enhance the strength of each

individual tank significantly. In addition, the interlocking relationship between adjacent tanks leads to a particularly secure and stable structure.

It will be noted that the buttresses comprise a number of holes 11. These are to facilitate lifting of the tank.

Referring again to Figure 1, it will be seen that each lid 4 is provided with an access hole 6 with a cover 8. In this case, a rebate is provided around the perimeter of the access hole 6, so as to accommodate the cover 8. Other configurations are possible.

From Figure 5a, it will be appreciated that the lid 4 has a generally domed shape. This is primarily to provide strength, which is particularly important if the tank is to be set into the ground, (in such a case, it is quite possible that a person may wish to walk over the top of the installation). From Figure 5a it will also be noticed that the lid is doubleskinned and generally hollow. This is to keep the weight of the lid down to practical levels. However, the very fact that the lid is hollow would normally lead to a reduction in strength. For this reason, the lid is provided with a series of ribs 5. The ribs are effectively defined by forming the seat 7 as a corrugated structure. In this case, the corrugations have a profile of truncated saw-tooths, although other formations are equally viable.

It will be noted that each side 3 of the tank 2 is provided with a port aperture 10 in the upper quarter of its face. These are all positioned centrally, in lateral terms, thus ensuring that ports on opposed faces of immediately adjacent tanks can be directly aligned.

As will be seen particularly clearly in Figures 1 and 2, central portions of the sides 3 of the tank slope inwardly from the top to the bottom. However, peripheral portions of each side 3 remain parallel to the longitudinal axis of the tank. Due to the inclination of the sides in the non-peripheral portions, a radial flange 14 is defined in the base region of each side 3. These flanges 14 are employed to anchor the tank 2 in the ground when

it is installed below the surface. This has the benefit of countering the natural tendency for the tanks to"float" (i. e. rise towards the surface).

Referring to Figure 2, it will be seen that the inner skin of the base comprises a series of corrugations 16, which serve to define air channels. Due to the double-skinned nature of the structure, the walls of the tank are hollow and can communicate with these channels. A number of air feed apertures 21 are provided in the upper end face of the tank. The air feed apertures 21 define inlets to an air-flow path through the sides 3 and buttresses 12 and down to the flowpaths defined by the corrugations 16. The corrugations 16 are provided with a series of apertures 18, which effectively define outlets for the air, allowing it to enter the interior of the tank. Figures lOa and lOb illustrate the flow path of the air from the air feed apertures 21, between the inner and outer skins of the tank, beneath the base of the interior skin of the tank and up through the corrugations 16, exiting via the perforations 18. It has been found that the corrugations are particularly beneficial in preventing the perforations 18 from being blocked with sludge.

Figures 6 and 7 illustrate a settlement tank 20, also in accordance with the invention.

It will be noted from the figures that the general outer configuration of the settlement tank 20 is broadly similar to that of the reactor tank 2. In fact, the tank is used in conjunction with a lid 4 identical to that of the reactor tank 2. The primary difference between the settlement tank 20 and the reactor tank 2 lies in the shape of the internal skin. From Figure 7, which is a cut-away view of the settlement tank 20, it can be observed that the internal skin 23 is generally declivitous from one side to the other. In this particular embodiment, the declivity is not a simple curve, but instead has two distinct sections. These can be observed most clearly in Figure 14. As will be appreciated from the figure, there is an upper section, of greater diameter, which leads into a lower section, of smaller diameter. The slope of the lower section 27 is much steeper than that of the upper section 25. The resulting shape resembles an asymmetric, curved funnel.

Figures 12,13 and 14 show a settlement tank 20 in more detail. From the figures, it will be seen that the settlement tank 20 comprises an inlet pipe 36 for receiving flow from a reactor tank 2, a sludge removal pipe 40 and an outflow pipe 38. A tube settler 24 is located on one side of the tank 20 and takes the form of a number of inclined tubes of hexagonal cross-section arranged as an inclined array in the flowpath. A launder 26, which is situated above the tube settler 24 in the region of the outflow, takes the form of a shallow, perforated tray. A baffle 43 separates the inlet 36 and sludge removal pipe 40 from the region of the settlement tank that contains the tube settler 24 and launder 26.

In use, the baffle 43 and tube settler 24 promote settlement of sludge into the base of the settlement tank 20. The sludge is removed via the sludge removal pipe 40 by pumping.

The water passes through the tube settler 24 and launder 26 before exiting via the outflow pipe 38.

Both of the tanks comprise a number of base channels 17. These have the dual purpose of enhancing the strength of the base and also accommodating the tines of a forklift truck, should the tank need to be lifted. It will be noted, in the case of the reactor tank 2, that the channels 17 extend perpendicular to the corrugations 16. This enables the intersections of the lower regions of the corrugations 16 and the upper regions of the channel structure 17 to conjoin at so-called"kiss points". These enhance the strength of the structure.

As will be apparent from, inter alia, Figures 1 and 6, the upper surface of each tank comprises a rebate 15 around the whole of its perimeter. This serves to define a channel beneath the lid of the tanks for accommodating air pipes cables and the like.

As discussed above, both of the tanks and their corresponding lids are hollow doubleskin structures. Although complex, such a structure can be formed using rotational moulding, which is a particularly economical manufacturing technique employing relatively inexpensive tooling. Preferably, at least the lids 4 and covers 8 are moulded from plastics which is coloured to fit in with the surrounding environment.

Referring now to Figures 11a and 11 b, a method of connecting reactor modules 1 together in accordance with the invention will be described. To enable the tanks 2 to communicate with each other, a feed pipe 28 is employed. This comprises a horizontal run 30 and a vertical run 32, these being connected by a tee 33. A cylindrical sieve 34 is fitted to the lowermost end of the vertical run 32. The feed pipe 28 is located as shown in Figure 11 b. The vertical run 32, tee 33 and sieve 34 extend along the inside of a tank, proximate to a port 10. The horizontal run 30 extends through the port 10 and on into the adjacent tank via its communication port 10. Appropriate seals, taking the form of large rubber grommets, are provided in each port 10. The first tank in the series does not require a feed pipe 28 and can therefore be fed using an appropriate inlet pipe.

As will be appreciated from Figure lib, the sewage is fed into the first reactor tank 2 in the series and transfers from one to the next via consecutive feed pipes 28, until the outlet is reached. At each stage, air can be applied via the aeration paths described above.

By appropriate orientation of the lid, direct access to the feed pipe structure 28 can be obtained from above, without the need to remove the entire lid structure 4.

Figures 8a and 8b show a linear tank formation, in which three reactor tanks 2 are arranged in a line, terminating in a settlement tank 20. Figures 9a and 9b show an alternative configuration in which four reactor tanks 2 are clustered together and serve a single final settlement tank 20. It will be appreciated that by clustering the tanks in this manner, the length of the arrangement is considerably shorter than that of Figure 8, although it has greater width. These two arrangements are just examples of the almost limitless number of configurations that can be achieved by using appropriate numbers of modules in accordance with the invention.

In practice, the insides of the reactor tanks 2 are filled with moving or fixed media upon which a biofilm can grow. Air is fed into these tanks via the aeration system, so as to encourage the biological activity.

Although the tanks and associated components can be of any appropriate size, it has been found that tanks which are approximately 2.5 metres deep and have internal diameters of approximately 2 metres are particularly efficient.

A typical sewage treatment installation will now be described, making use of a cluster of four reactor modules 2 and one settlement module 20 such as shown in Figures 9a and 9b.

The cluster is, in this particular example, used for effecting a treatment stage in a complete treatment plant. The complete treatment plant has a number of stages as follows: 1. Incoming raw effluent flows (possibly through a coarse filter) into a large tank (not shown), which acts as a primary settlement and balancing tank. Dense particles and some sludge will be retained in this tank, which is periodically emptied by a tanker (every few months). Some control may be fitted to the outlet piping, in order to restrict the flow rate to the next stage. At times of high input flow (for example during the day) the level in this tank may rise and then fall later (for example, at night).

2. The output from the primary settlement tank flows (or is pumped) to the treatment stage, which is defined by a cluster of tanks such as shown in Figures 9a and 9b, where the effluent is converted into clean water and de-activated sludge. The sludge will normally be pumped back into the primary settlement tank.

3. The clean water output from the treatment stage may flow into a clean water storage tank, to be used for cleaning and wash-down purposes on the site. In an integrated scheme, the"grey water"feed would be taken from this tank.

4. The outlet from the final tank flows into the water course or river.

The tanks forming the treatment stage (Stage 2) are permanently full of liquid. Settled effluent is fed (either by gravity or pumped) into the top of the first reactor tank 2. The

overflow of effluent from the bottom of each tank to the next tank in the series occurs via the feed pipes 28. As more effluent is introduced into the first tank, the level in that tank rises slightly and so forces effluent from the bottom of the tank up the feed pipe 28 along the horizontal run 30 and into the next tank. This occurs between each successive tank until the final tank discharges to the outlet.

Each of the reactor tanks 2 contains plastics media, which provide a large surface area for the bacteria to reside on. In the first two reactor tanks 2, moving bed biofilm carriers (not shown) are used. Air bubbled in via the perforations 18 (as shown in Figure 1 Oa), causes the carriers to move vigorously within the tank 2, enabling rapid biological reactions between the effluent, bacteria and air to take place. The bacteria which breed best in these conditions are therefore those which are susceptible to rapid biological reactions (carbonaceous BOD removal). The later two reactors 2 contain fixed media (not shown), which are essentially large sheets of corrugated, interconnected plastics material fixed in place within the tank 2. Diffused air is supplied from the perforations 18, but at a much lower rate than in the first two tanks 2, as its function is merely to feed the reactions and not agitate the media. The bacteria which breed best in these conditions are those which start reacting more slowly (nitrification).

The final tank 20 of the treatment stage (Stage 2) is a settlement tank, as discussed above. The input fluid, which by now comprises clean water and sludge (dead bacteria which have dropped off the media) is directed by the inlet pipe 36 and baffle 43 towards the bottom of the tank 20. Some of the sludge settles to the bottom at this point. The water and remaining sludge then rises (with slow velocity since the cross-sectional area is large at this point) through the tube settler 24 to the top of the tank. Slow moving sludge particles settle on the plastic surfaces of the tube settler 24 and then slide down towards the bottom of the tank. The resulting clean water which rises to the top of the tank flows into the launder 26, which is connected to the outflow pipe 38. Because the perforations provided in the launder are large in number, the velocity of the water flowing through the launder is kept low, which ensures that the maximum amount of sludge is left behind.

The sludge which collects at the bottom of the tank is periodically (every few hours) pumped out to the primary tank.

It is also possible to use reactor tanks 2 for the clean water storage and for the primary settlement, if required. This would mean that an array of as many as twelve tanks would be required: five as discussed above; three settlement tanks; three balancing tanks and awash water tank. This is still very cost effective compared to a single large primary tank.

Figures 15 to 17 show an alternative embodiment of a reactor tank 2 according to the invention. Figures 18 to 20 are views of a corresponding alternative settlement tank 20.

The tanks 2,20 according to this second embodiment of the invention represent a simplified version of the first embodiment of the invention. However, the general operating principles are essentially the same: that is to say the reactor tank 2 can be used in exactly the same way as the reactor tank 2 of the first embodiment and the settlement tank 20 can be used in the same fashion as the settlement tank 20 of the first embodiment.

It will be noted that the tanks are of a single-walled construction, which reduces the amount of material required and simplifies the rotational moulding process, if this is how they are formed. Upon reference to the figures it would be immediately apparent that the walls are provided with horizontally orientated annular corrugations 58. These are provided for strength purposes. They also provide a tool keying formation and help to anchor the tank within the ground. Instead of ports 10, each of the tanks 2,20 is provided with a series of markers 52. These denote positions at which ports can be cut during installation. By providing such markers, correspondence of the ports from adjacent tanks can be guaranteed.

Once again, the upper portions of the tanks are hexagonal and designed to fit and key together in much the same way as the first embodiment. However, in this case, the keying is achieved by each of the six hexagonally arranged mating faces being provided

with a respective axially extending tongue 54 and groove 56, each of which is semicircular in cross-section. These are arranged so that, when two tanks are situated adjacent each other, the tongue 54 of one will mate with the groove 56 of the other and vice versa. This provides lateral support and strength. It will be noted that the reactor tank 2 retains a number of channels 17 formed in its base and these are provided for the same purposes as those provided in the first embodiment.

The settlement tank 20 differs from the settlement tank 20 of the first embodiment in that the base is conical and concentric with the upper cylindrical portion of the tank.

This provides greater flexibility in orientation of the tank.

Due to the fact that the tanks of the second embodiment are of a single-walled construction, the air can no longer be directed into the tanks via the wall structure.

Instead, the air is supplied to the tanks using an appropriate arrangement of pipes (not shown).

Each of the tanks is provided with a lid 4 which is essentially the same as that provided in the first embodiment of the invention, except for the fact that its hexagonal sides are shaped to correspond with the tongue and groove configuration of the hexagonal portions of the tanks.

Although the present invention has been discussed primarily with reference to the treatment of effluent, it is to be understood that the invention is not limited to apparatus which is for use in the treatment of effluent: the invention may be applied with equal effect in other purification processes.

For the avoidance of doubt, it should be understood that features from the two embodiments may be combined in any manner deemed to be appropriate. Furthermore, the embodiments are given merely by way of example and are not intended to limit the scope of the invention, that being determined by the appended claims.

Claims

CLAIMS:

1. Fluid treatment apparatus for use in the purification of water, the said apparatus comprising a plurality of treatment tank units, each said tank unit comprising a tank adapted for installation at least partly below ground level and comprising peripheral formations that enable the said tank units to be connected together in a plurality of laterally interlocking formations.

2. Fluid treatment apparatus according to Claim 1, wherein each said tank unit comprises a plurality of radially outer mating faces which are arranged to define a polygonal cross-sectional profile in at least one axial region of the said tank unit, wherein the said mating faces are each adapted for direct contact with a said mating face provided on another said tank unit.

3. Fluid treatment apparatus according to Claim 2, wherein the said cross-sectional profiles are generally hexagonal.

4. Fluid treatment apparatus according to Claim 2 or 3, wherein a plurality of said mating faces situated on each tank unit are provided with respective keying means for co-operation with a co-operatively configured portion of another said tank unit.

5. Fluid treatment apparatus according to Claim 4, wherein each said keying means comprises a generally radial projection and a generally radial recess, the arrangement being such that each said projection can be closely received in, and laterally constrained by, a said recess provided on another said tank unit.

6. Fluid treatment apparatus according to Claim 5, wherein each said projection is in the form of a tongue and each said recess is in the form of a groove.

7. Fluid treatment apparatus according to Claim 6, wherein each tongue has a generally part-circular cross section and each groove has a generally part circular cross section.

8. Fluid treatment apparatus according to any preceding claim, wherein the tank of at least one said tank unit is formed from a plastics material.

9. Fluid treatment apparatus according to Claim 8, wherein the tank of at least one said tank unit is double walled.

10. Fluid treatment apparatus according to Claim 9, wherein the said double walled tank comprises at least one perforation in an inner one of its walls and an air flow path, communicating with the said perforation, extending between the said inner one of its walls and an outer one of its walls.

11. Fluid treatment apparatus according to any preceding claim, wherein an inner surface of at least one said tank is generally funnel-shaped in a region thereof that is adapted to be located towards the bottom of the said tank when in use

12. Fluid treatment apparatus according to any preceding claim, wherein each said tank comprises an anchoring formation on an outer surface thereof.

13. Fluid treatment apparatus according to Claim 12, wherein each said anchoring formation takes the form of a radial flange.

14. Fluid treatment apparatus according to any preceding claim, wherein each said tank is provided with a plurality of buttresses.

15. Fluid treatment apparatus according to any preceding Claim, wherein each said tank comprises a plurality of apertures situated at regular circumferential intervals for connection with an inlet or outlet pipe.

16. Fluid treatment apparatus according to any preceding claim, wherein each said tank comprises a plurality of indicia, denoting respective locations at which an inlet or outlet pipe may communicate with the said tank.

17. Fluid treatment apparatus according to any preceding claim, wherein at least one said tank comprises at least one channel in a surface thereof for co-operation with a forklift truck or a lifting harness.

18. Fluid treatment apparatus according to any preceding claim, wherein each said tank unit comprises a lid having a convex surface that is adapted to be situated generally uppermost during use.

19. Fluid treatment apparatus according to any preceding claim wherein at least one said tank unit is provided with mobile media for use in a biofilm treatment process.

20. Fluid treatment apparatus according to any preceding claim, wherein at least one said tank unit is provided with fixed media for use in a biofilm treatment process.

21. Fluid treatment apparatus according to any preceding claim, wherein at least one said tank unit is fitted with a launder and/or a tube settler.

22. Fluid treatment apparatus according to Claim 21, wherein the said tank unit fitted with the said launder and/or tube settler further comprises a baffle wall for extending downwards from a region of the tank which is adapted for location uppermost during use, the said baffle wall being situated between an inlet region of the tank and an outlet region of the tank.

23. Fluid treatment apparatus substantially as hereinbefore described with reference to any of figures 1 to 14 or figures 15 to 20 of the accompanying drawings.

24. A tank as set out in any preceding claim.