GB2508401B

GB2508401B - Apparatus and method for treating alkaline water containing solids

Info

Publication number: GB2508401B
Application number: GB1221592.7A
Authority: GB
Inventors: Coulton Richard
Original assignee: Siltbuster Ltd
Current assignee: Siltbuster Ltd
Priority date: 2012-11-30
Filing date: 2012-11-30
Publication date: 2019-09-04
Anticipated expiration: 2032-11-30
Also published as: NZ709364A; AU2013350913B2; WO2014083346A1; AU2013350913A1; GB2508401A

Description

- 1 -

Apparatus and method for treating alkaline water containing solids

This invention relates to an apparatus and method for 5 treating alkaline water. Such water may be generated by a variety of industrial processes; the present invention is particularly, but not exclusively, concerned with treatment of water resulting from hydrodemolition of concrete structures . .0 In our earlier patent application No. GB 2464141 a treatment unit for treating various waste water products of the construction industry is described. The treatment described in that application comprises the steps of introducing water to be treated into a first compartment, .5 bubbling carbon dioxide into the water in the first compartment, causing solids to precipitate, transferring water with solids that have precipitated into a second compartment, separating, in the second compartment, solids that have precipitated from the water, transferring water JO from which solids have been separated to a third compartment, bubbling carbon dioxide into the water in the third compartment to reduce further the pH of the water, and removing water from the third compartment, the pH of the water removed from the third compartment being lower !5 than the pH of the water in the second compartment. The compartments are provided in a single portable unit.

The apparatus described in GB 2464141 is useful in a variety of applications but is not suited to treating water with a significant amount of solids as may be found in JO water that has been used in hydrodemolition.

In our earlier patent application No. GB 2467005, we describe an apparatus suitable for treating water with a particularly high concentration of solids, such as may be present in washout from cleaning a concrete skip. That J5 apparatus is well suited to such applications but is unnecessarily large for treatment of alkaline water with - 2 - only a modest proportion of solids, such as is typically found in waste water from hydrodemolition work.

It is an object of the invention to provide an improved method and apparatus suitable for treating 5 alkaline water, and in particular, but not exclusively, alkaline water produced from hydrodemolition.

According to a first aspect of the invention, there is provided a method of treating alkaline water, the method including the following steps .0 i) providing a treatment unit that comprises first and second treatment tanks contained within a common structure; ii) placing a flexible bag into the first treatment tank; iii) inserting a gas introduction device into the top of the bag with one or more outlets of the device in a lower .5 region of the bag; iv) passing alkaline water containing some solids into the flexible bag; v) introducing carbon dioxide through the gas introduction device, the carbon dioxide passing into the water through JO the one or more outlets of the device; wherein water in an upper region of the first treatment tank passes into the second treatment tank and flows between a multiplicity of inclined plates in the second treatment tank, solids separating out and collecting in a !5 bottom region of the second treatment tank; and vi) subsequently withdrawing the gas introduction device from the bag and thereafter removing the bag from the first treatment tank with solids retained in the bottom of the bag. JO By providing a flexible bag in the first treatment tank, solids that settle in that tank can be easily removed. At the same time the first treatment tank is arranged to reduce the pH in the water in that tank and cause solids to be precipitated, at least some of those J5 precipitated solids being carried over into the second treatment tank, where they are separated. In this way an especially effective treatment of alkaline water can be - 3 - carried out in a compact unit. Since the first and second treatment tanks are contained within a common structure, the apparatus can readily be moved from one site to another. In an embodiment of the invention described 5 below, the overall length of the apparatus is 2.75 m and the overall width is 1.36m. Whilst it may appear that the need to remove a flexible bag from time to time is not appropriate where it is also intended to have arrangements for feeding both water and gas into the bag, we have found .0 that these apparently conflicting requirements can be met simply in practice.

Where reference is made in this specification to carbon dioxide, it should be understood that the gas referred to will contain a substantial amount of carbon .5 dioxide, but may also have other components. Preferably at least 90 per cent by volume of the gas would be carbon dioxide .

The method preferably further includes the subsequent steps of placing a flexible bag into the first treatment JO tank and thereafter inserting the gas introduction device into the top of the bag with one or more outlets of the device in a lower region of the bag; and subsequently repeating steps iv) to vi). The flexible bag placed into the first treatment tank in this subsequent step may be the !5 same bag as is removed in step vi) above, preferably after removing solids from the bag, or may be a different bag. Typically, the steps of removing and replacing the bag, together with their associated steps involving the feeding of water and gas, would be carried out many times. JO The step of passing alkaline water containing some solids into the flexible bag may comprise the steps of placing an open end portion of a pipe in or over the first treatment tank with the pipe removably supported on the common structure, and passing the alkaline water containing J5 some solids through the pipe.

Preferably water in an upper region of the first treatment tank passes over a weir into the second treatment - 4 - tank. The weir may be formed by the upper edge of a plate.

The position of the weir may be adjustable, for example, by adjusting the position of the upper edge of the plate in the case where that edge provides the weir. That allows 5 for the apparatus to operate effectively on ground that is not level.

Whilst it is at least possible in theory for the flexible bag simply to rest freely in the first treatment tank, it is preferred that upper portions of the flexible .0 bag are held in the upper region of the treatment tank by clamps. The flexible bag may be arranged in the treatment tank with all of its upper edge portions at approximately the same level, or with a portion of an upper edge of the bag at a lower level than other portions of the upper edge .5 of the bag, water flowing over the portion at the lower level as it passes from the first treatment tank into the second treatment tank. In that way water is prevented from flowing out of the bag other than over the portion of the upper edge of the bag that is at a lower level. !0 In step iv) carbon dioxide may be introduced through a single inlet, in which case steps are preferably taken to disperse bubbles of carbon dioxide throughout the water in the first treatment tank. Preferably, however, carbon dioxide is introduced through a multiplicity of !5 inlets extending across at least part of the compartment.

Preferably the pH of the water in the first treatment tank is monitored and the rate at which carbon dioxide is introduced into the water is controlled by the monitoring.

Preferably, the height at which the carbon dioxide is JO introduced into the first treatment tank is adjustable.

The height at which the carbon dioxide is introduced into the first treatment tank preferably increases as solids collect in the bottom of the first treatment tank. In that way the smallest height at which carbon dioxide is J5 introduced into the first treatment tank may be lower than the greatest height to which solids collect in the first treatment tank. - 5 -

The multiplicity of inclined plates preferably define a multiplicity of channels therebetween, the water flowing upwardly along the channels and solids passing down the channels towards the bottom of the compartment. Such a 5 separation technique is known per se and is a particularly effective way of separating solids, which may be calcium carbonate precipitate, cement particles and/or fine aggregate particles, from the water.

Water passing into the second treatment tank .0 preferably flows downwardly into a region below the multiplicity of inclined plates before flowing upwardly along the channels between the plates. In an embodiment of the invention described below, water flows downwardly into opposite lateral regions of the unit and then into the .5 region below the multiplicity of inclined plates.

Some of the solids removed from the second treatment tank may be introduced into the first treatment tank, preferably into a lower region of the tank. Such solids can promote the precipitation that is desired to take place JO in the first treatment tank.

Water is preferably removed from the second treatment tank through an outlet in an upper region of the second treatment tank. Such water is preferably suitable for ordinary discharge, but, if desired or necessary, !5 further treatments may of course be carried out.

As already indicated the invention is of particular advantage in the case where the alkaline water is waste water produced from hydrodemolition. Such water typically has a solids content and a degree of alkalinity that JO renders it particularly suitable for treatment by the method of the invention.

According to the first aspect of the invention there is also provided an apparatus for treating alkaline water, the apparatus comprising: J5 a first treatment tank and a second treatment tank contained within a common structure, the first treatment tank having a water outlet in an upper region of the tank; - 6 - a gas introduction device inserted into a flexible bag placed in the first treatment tank with one or more gas outlets of the device in a lower region of the first treatment tank, the gas introduction device also being 5 suitable for removal from the flexible bag; the second treatment tank being arranged to receive water passing from the first treatment tank through its outlet, the second treatment tank containing a multiplicity of inclined plates defining a multiplicity of channels .0 therebetween for separating solids in the water received from the first treatment tank.

As will be understood, the apparatus of the invention is particularly suited for carrying out the method of the invention. Many of the preferred features of the apparatus .5 described herein reflect the preferred features of the method.

Preferably the multiplicity of the inclined plates are substantially parallel to one another. A water flow path is preferably provided between an !0 upper region of the first treatment tank and an upper region of the second treatment tank. Also, one or more water flow channels are preferably defined in the second treatment tank providing a flow path for water entering the second treatment tank from the first treatment tank down !5 from the upper region of the second treatment tank to a lower region of the second treatment tank below the multiplicity of inclined plates. A water outlet is preferably provided in an upper region of the second treatment tank downstream of the JO multiplicity of inclined channels defined by the inclined plates . A solids outlet for removing solids from the second tank is preferably provided in a bottom region of the second treatment tank. As will be understood, solids that J5 precipitate from the water as it flows up through the multiplicity of inclined channels collect, in use, on the bottom of the second treatment tank. The solids outlet can - 7 - provide a simple way of removing those solids from the second treatment tank. A further outlet may be provided in a lower region of the second treatment tank above the solids outlet. Such an 5 outlet can be useful if it is desired to drain much of the water from the tank, whilst retaining the solids in the tank. If desired a plurality of such outlets can be provided at different levels.

The gas introduction device may have a multiplicity of .0 gas outlets for introducing gas into a lower region of the first treatment tank. The heights of the gas outlets may be adjustable.

The apparatus may further include a pH sensor which may be suitable for insertion into a flexible bag placed in .5 the first treatment tank. The pH sensor assembly may be physically separate from the gas introduction device or may be mounted thereon.

The apparatus preferably further includes a clamping arrangement for holding upper portions of a flexible bag !0 inserted into the first treatment tank. Preferably a spring clamping arrangement is provided with the arrangement biased into a clamping condition but being movable into a released position for allowing portions of a bag to be inserted or removed. A plurality of clamping !5 arrangements are preferably provided, each comprising a member, which may be an elongate plate, that is able to be resiliently biased towards an abutment face in an upper region of the first treatment tank for clamping upper portions of a flexible bag inserted into the first JO treatment tank between the elongate plate and the abutment face. The abutment face is preferably approximately horizontal. The first treatment tank may have an open top and the abutment face in a region close to the second treatment tank is preferably at a lower level than one or J5 more abutment faces in other regions further from the second treatment tank. For example, the open top of the first treatment tank may be of generally rectangular - 8 - (including square) shape and separate clamping arrangements may be provided in each of the four side regions of the opening with the abutment face in the side region next to the second treatment tank at the lower level. 5 The apparatus further comprises a flexible bag placed in the first treatment tank, the gas introduction device being removably inserted into the flexible bag with one or more gas outlets of the device in a lower region of the first treatment tank. This is a common state of the .0 apparatus during use. The flexible bag may be a standard flexible intermediate bulk container. Whilst it is within the scope of the invention for it to be porous, it is preferred that it is impermeable. In an embodiment of the invention it is of generally cubic shape with a side length .5 in the range of 0.9 to 1.0 m.

The height of the gas introduction device within the flexible bag is preferably adjustable. That, for example, enables the height of the device to be adjusted upwardly as solids collect in the bottom of the flexible bag. !0 The apparatus may further comprise a pipe for feeding alkaline water to the first treatment tank, the pipe including an open end portion removably supported on the common structure and positioned over a lower region of the first treatment tank. Again the pipe is likely to be so !5 positioned during use of the apparatus.

The apparatus may further comprise a gas store for storing gas for feeding to the gas introduction device. The apparatus may still further comprise one or more cylinders of carbon dioxide. JO Whilst it is within the scope of the invention to provide more than two treatment tanks, it is an especially preferred feature of the invention that only two treatment tanks are provided, enabling an especially compact unit to be constructed. J5 The apparatus may further comprise an arrangement for adding flocculant to the first treatment tank. The arrangement may comprise a flocculant feed pipe connected - 9 - to the pipe for feeding alkaline water to the first treatment tank. The apparatus may further comprise a pump for pumping the flocculant along the flocculant feed pipe.

Preferably a storage enclosure, which is preferably 5 lockable, is also provided in the unit. The enclosure may contain control equipment, a store of flocculant and/or the pump for pumping the flocculant. The store may further contain one or more of the gas introduction device and the pH sensor, for example during transport of the unit. .0 The apparatus is provided in a single portable unit.

The unit preferably has a length of less than 3.5 m and a width of less than 1.5 m. Such a unit, although of significant size, is portable and can be transported on a vehicle from one construction site to another. .5 In the first aspect of the invention a method and apparatus for introducing gas into the first treatment tank is provided. Another possibility, in accordance with a second apect of the invention, is to provide a method and apparatus for introducing gas into the second treatment !0 tank upstream of the inclined plates. Thus, according to a second aspect of the invention there is provided a method of treating alkaline water, the method including the following steps i) providing a treatment unit that comprises first and !5 second treatment tanks contained within a common structure; ii) placing a flexible bag into the first treatment tank; iii) providing a gas introduction device; iv) passing alkaline water containing some solids into the flexible bag; and JO v) introducing carbon dioxide into the water through the gas introduction device, the carbon dioxide passing into the water through the one or more outlets of the device; wherein water in an upper region of the first treatment tank passes into the second treatment tank and flows J5 between a multiplicity of inclined plates in the second treatment tank, solids separating out and collecting in a bottom region of the second treatment tank, and carbon - 10 - dioxide is introduced into the water in the second treatment tank upstream of the multiplicity of inclined plates .

The carbon dioxide may be introduced into the water in 5 a chamber situated between the first treatment tank and the inclined plates. The chamber may be part of the second treatment tank.

According to the second aspect of the invention there is also provided an apparatus for treating alkaline water, .0 the apparatus comprising: a first treatment tank and a second treatment tank contained within a common structure, the first treatment tank having a water outlet in an upper region of the tank; and the first treatment tank having a flexible bag placed .5 therein; a gas introduction device suitable for introducing carbon dioxide into the water; the second treatment tank being arranged to receive water passing from the first treatment tank through its outlet, !0 the second treatment tank containing a multiplicity of inclined plates defining a multiplicity of channels therebetween for separating solids in the water received from the first treatment tank, the gas introduction device being disposed downstream of the first treatment tank and !5 upstream of the multiplicity of inclined plates.

The gas introduction device may be in a chamber situated between the first treatment tank and the inclined plates. The chamber may be part of the second treatment tank. JO As should be understood, apart from the different apparatus and method for introducing gas, the apparatus and method of the second aspect of the invention may have any of the features of the apparatus and method of the first aspect of the invention. 55 - 11 -

Certain features of the invention have been described above only with reference to the method of the invention whilst other features have been described only with - 12 - reference to the apparatus of the invention. It should be understood that a feature described in respect of the method may be represented also in a feature of the apparatus and vice versa. 5 By way of example, an apparatus and method for treating alkaline water will now be described with reference to the accompanying drawings of which:

Fig. 1 is an isometric view of a portable unit for treating alkaline water; .0 Fig. 2 is a plan view of the unit of Fig. 1;

Fig. 3 is an end view from one end of the unit;

Fig. 4 is an end view from an opposite end of the unit;

Fig. 5 is a side view of the unit; .5 Fig. 6 is an isometric view of the unit of Fig.l from the opposite side to Fig. 1, with one side of the unit omitted and with a gas outlet module shown in the unit; and

Fig. 7 is an isometric view of the head of the gas JO outlet module shown in Fig.6.

The portable unit shown in the drawings and generally indicated by reference numeral 4 generally comprises a first treatment tank 1 and a second treatment tank 2 placed in end to end relationship and contained within a common !5 structure which also defines an enclosure 3 at the end of the first tank 1 that is away from the second tank 2. A dividing wall 5 is provided between tanks 1 and 2 and a dividing wall 6 is provided between the tank 1 and the enclosure 3. The unit also has exterior end walls 7 and 8 JO and side walls 9 and 10. The bottom of the unit 4 extending between the end and side walls is closed while the top of the unit is generally open apart from the top of the enclosure 3. The unit is mounted on a pair of skids 11 which rest on the ground on which it is placed. The unit J5 is of generally cuboidal shape and in one particular example has a length of about 2.75 m, a height of about 1.2 m and a width of about 0.96 m, those dimensions not - 13 - including a cage 12 mounted on the sidewall 10. That cage has a height of about 1.8m a width of about 0.4 m and a length of about 0.7 m and is provided for housing a pair of carbon dioxide cylinders as will be described below. 5 As shown in the drawings the first tank 1 is of square shape when viewed in plan and is suitable for receiving a flexible bag (not shown) such as a flexible intermediate bulk container which may for example be made of polypropylene. In a particular example of the invention .0 the bag has a square cross-section of 0.9 m along each side. Around the top of the first tank 1 four spring clamping arrangements 14 are provided. Each arrangement, which is of a kind known per se, includes an elongate plate which can be pressed against a horizontal abutment face .5 bordering the open top of the tank. Three of the abutment faces are of the same height whilst a fourth 14A, at an upper region of the dividing wall 5 is at a lower height.

As may now be understood, a flexible bag (not shown) may be placed in the tank 1 and its upper edges clamped against !0 the abutment faces 14, 14A, holding the bag securely in the tank 1. Handles 15 on the clamps facilitate their movement between clamping and released positions.

The top of the dividing wall 5 is higher along both of its edge portions bordering the sidewalls 9, 10 than in a !5 middle portion 16. A weir plate may be adjustably secured to the top of the middle portion 16 of the dividing wall 5 and define the height of that middle portion which provides an outlet from the tank 1 into the tank 2.

Within the second tank 2 an inclined partition 20 50 (Fig. 6) is provided which prevents water entering the tank 2 from passing anywhere other than downwards and then through channels on opposite sides of the bottom of the tank 2, and then into a middle bottom region of the tank 2.

As shown in Figs.1 and 6 (but omitted from Fig. 2) the 55 inclined partition 20 defines the first of a multiplicity of parallel inclined plates 22 which end in the end wall 8. Below the inclined plates 22, an outlet 23 for removing - 14 - solids from the tank 2 is provided. As can be seen from the drawings, two further outlets 24 and 25 are provided in the lower portion of the tank 2 at different heights and a drain 34 is provided in the tank 1. During normal use of 5 the unit all of the outlets 23, 24, and 25 are closed.

At the top of the inclined wall 8, a scum board 26 is provided. That defines a water flow path into an end region of the tank 2 where a treated water outlet 27, which is open, is provided. .0 The unit 4 is provided with padeyes 28 by which it can be lifted, for example to remove it from a vehicle and place it in a desired position on the ground.

Also shown in the drawings is an end portion of an inlet pipe 29 which extends upwardly along the outside of .5 the tank 1 and has a U-shaped open end for feeding water to be treated into the tank 1. The pipe 29 is supported on the side wall 9 of the unit and has an upstream end 30 connectible to the source of alkaline water to be treated.

In use of the apparatus, a bag is placed in the first !0 tank 1 and its upper ends are clamped around the top of the tank 1 using the clamping arrangements 14, 14A. If desired, the inlet pipe 29 may be removed during the installation of the bag. A carbon dioxide dispersion unit 31, the head of which is shown in Fig.7, is then !5 lowered on chains (not shown) connected to lifting eyes 32 into the bag to a position in a lower region of the tank 1 but spaced from the bottom of the tank as shown in Fig. 6, and kept suspended in that position on the chains. In the example shown the dispersion unit comprises a pair of JO dispersers 33A and 33B of a kind known per se but it should be understood that any suitable gas dispersion unit may be used. The dispersion unit 31 is connected to a flexible pipe (not shown). The flexible pipe extends upwardly through the open top of the bag and the tank 1 and is J5 connected to a cylinder of carbon dioxide gas stored in the cage 12. A pH sensor (not shown) is also inserted into the bag 1 through its open top. - 15 -

Alkaline water, typically being waste water from a hydrodemolition process is fed into the unit along the pipe 29 and carbon dioxide is fed into the dispersion unit 31; bubbles of carbon dioxide rise from the unit 31 5 and dissolve in the water. The carbon dioxide reacts with the alkaline water to create calcium carbonate, which is insoluble and precipitates out of the water. The pH of the alkaline water is reduced by this treatment and the amount of carbon dioxide that is added is controlled by a control .0 unit in the enclosure 3, that control unit acting in dependence upon a signal it receives from the pH sensor.

If desired an anionic polymer may be added to the alkaline water at an injection point 36 (Fig. 1) as it passes through the inlet pipe 29, to promote the flocculation of .5 the precipitate. A store of the flocculant can be provided in a drum in the enclosure 3, together with a pump for feeding the flocculant via a small bore pipe to the injection point 36.

Solids in the alkaline water entering the bag 1 !0 through the pipe 29 settle in the bottom of the bag.

Once the bag is almost full of water, the feeding of further water into the bag causes water at the top of the bag to flow over the middle portion 16 of the dividing wall 5 into the second tank 2. Water entering the tank 2 !5 travels downwardly to the lower region of the tank, along the side channels referred to above and into the region immediately below the inclined plates 22. The water then flows upwardly between the inclined plates 22. The speed of the water flow reduces considerably as it travels up 50 between the inclined plates because of the cross sectional area provided between all the plates. Solids in the water fall slowly down the inclined plates and collect at the bottom of the second tank. The water flows upwardly between the plates, over the scum board 26 and leaves the J5 apparatus through the outlet 27 which can be connected to a pipe taking the treated water to a desired location. - 16 -

Solids that collect at the bottom of the second tank 2 are removed at intervals through the outlet 23. If it is desired to remove water from the tank 2 without removing solids, then one of the outlets 24 or 25 can be opened. 5 Over time solids accumulate in the bottom of the bag in the first tank 1. As they do so, the dispersion unit is raised so that it is above the accumulated solids and the apparatus can continue to operate, but, as will be understood, there will come a time when the dispersion unit .0 cannot be raised further without severely compromising its effectiveness. At that stage, feeding of water to the unit is stopped, the dispersion unit 31, the pH sensor and, preferably, the inlet pipe 29 are lifted out of the tank 1, and the bag in the tank 1 is then released from the clamps .5 14,14A. The bag can then be lifted out of the apparatus; usually that would be done with the bag full of water but if desired the excess water can be removed before the bag is lifted out. A new bag is then inserted into the tank 1, and its upper portions held by actuating the clamps 14, !0 14A. The dispersion unit 31, the pH sensor and the inlet pipe 29 can then be put back in position and alkaline water again fed to the treatment tank 1.

Whilst a particular example of the invention has been described above with reference to the drawings, it should !5 be understood that many modifications may be made to the method and apparatus described. Just as one example, a different device for introducing carbon dioxide into the bag could be adopted; for example a device similar to that shown in GB 2464141 could be employed. 50 In the first aspect of the invention described above, the gas introduction device is inserted into the first treatment tank. Another possibility, in accordance with a second aspect of the invention is to provide the gas introduction device in the second tank 2 but upstream of 55 the inclined plates 22. More particularly the device may be positioned in the chamber of triangular cross-section in the second tank 2 defined between the wall 5 and the - 17 - inclined partition 20. In that case the gas introduction device may be permanently fixed in that position, preferably towards the bottom of the chamber, and connected to a gas pipe leading to the cage 12 for connection to a 5 carbon dioxide cylinder. As will be understood, other parts of the apparatus can be as previously described.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein .0 incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or .5 the like are optional and do not limit the scope of the independent claims.

Claims

- 18 - Claims :

1. A method of treating alkaline water, the method including the following steps 5 i) providing a treatment unit that comprises first and second treatment tanks contained within a common structure; ii) placing a flexible bag into the first treatment tank; iii) inserting a gas introduction device into the top of the bag with one or more outlets of the device in a lower .0 region of the bag; iv) passing alkaline water containing some solids into the flexible bag; v) introducing carbon dioxide through the gas introduction device, the carbon dioxide passing into the water through .5 the one or more outlets of the device; wherein water in an upper region of the first treatment tank passes into the second treatment tank and flows between a multiplicity of inclined plates in the second treatment tank, solids separating out and collecting in a !0 bottom region of the second treatment tank; and vi) subsequently withdrawing the gas introduction device from the bag and thereafter removing the bag from the first treatment tank with solids retained in the bottom of the bag. !5

2. A method according to claim 1, further including the subsequent steps of placing a flexible bag into the first treatment tank and thereafter inserting the gas introduction device into the top of the bag with one or more outlets of the device in a lower region of the bag; JO and subsequently repeating steps iv) to vi).

3. A method according to claim 1 or 2, in which the step of passing alkaline water containing some solids into the flexible bag comprises the steps of placing an open end portion of a pipe in or over the first treatment tank with J5 the pipe removably supported on the common structure, and passing the alkaline water containing some solids through the pipe. - 19 -

4. A method according to any preceding claim, wherein water in an upper region of the first treatment tank passes over a weir into the second treatment tank.

5. A method according to any preceding claim, wherein 5 upper portions of the flexible bag are held in the upper region of the treatment tank by clamps.

6. A method according to any preceding claim, wherein the flexible bag is arranged in the treatment tank with a portion of an upper edge of the bag at a lower level than .0 other portions of the upper edge of the bag, water flowing over the portion at the lower level as it passes from the first treatment tank into the second treatment tank.

7. A method according to any preceding claim, in which in step iv) carbon dioxide is introduced through a .5 multiplicity of inlets extending across at least part of the first treatment tank.

8. A method according to any preceding claim, in which the multiplicity of inclined plates defines a multiplicity of channels therebetween, the water flowing upwardly along !0 the channels and solids passing down the channels towards the bottom of the compartment.

9. A method according to claim 8, in which water passing into the second treatment tank flows downwardly in to a region below the multiplicity of inclined plates before !5 flowing upwardly along the channels.

10. A method according to any preceding claim, in which water is removed from the second treatment tank through an outlet in an upper region of the second treatment tank.

11. A method according to any preceding claim, in which 50 the alkaline water is waste water produced from hydrodemolition.

12. An apparatus for treating alkaline water, the apparatus comprising: a first treatment tank and a second treatment tank J5 contained within a common structure, the first treatment tank having a water outlet in an upper region of the tank; a gas introduction device inserted into a flexible bag - 20 - placed in the first treatment tank with one or more gas outlets of the device in a lower region of the first treatment tank, the gas introduction device also being suitable for removal from the flexible bag; 5 the second treatment tank being arranged to receive water passing from the first treatment tank through its outlet, the second treatment tank containing a multiplicity of inclined plates defining a multiplicity of channels therebetween for separating solids in the water received .0 from the first treatment tank.

13. An apparatus according to claim 12, in which a water flow path is provided between an upper region of the first tank and an upper region of the second treatment tank and one or more water flow channels are defined in the second .5 treatment tank providing a flow path for water entering the second treatment tank from the first treatment tank down from the upper region of the second treatment tank to a lower region of the second treatment tank below the multiplicity of inclined plates. !0

14. An apparatus according to claim 12 or 13, in which a water outlet is provided in an upper region of the second tank downstream of the multiplicity of inclined channels defined by the inclined plates.

15. An apparatus according to any of claims 12 to 14, in !5 which a solids outlet for removing solids from the second treatment tank is provided in a bottom region of the second treatment tank.

16. An apparatus according to claim 15, in which a further outlet is provided in a lower region of the second JO treatment tank above the solids outlet.

17. An apparatus according to any of claims 12 to 16, in which the gas introduction device has a multiplicity of gas outlets for introducing gas into a lower region of the first treatment tank. J5

18. An apparatus according to any of claims 12 to 17, further including a clamping arrangement for holding upper portions of the flexible bag inserted into the first - 21 - treatment tank.

19. An apparatus according to claim 18, in which a plurality of clamping arrangements are provided each comprising a member that is able to be resiliently biased 5 towards an abutment face in an upper region of the first treatment tank for clamping upper portions of the flexible bag inserted into the first treatment tank between the elongate plate and the abutment face.

20. An apparatus according to claim 19, in which the .0 abutment face is approximately horizontal.

21. An apparatus according to claim 20, in which the first treatment tank has an open top and in which the abutment face in a region close to the second treatment tank is at a lower level than one or more abutment faces in other .5 regions further from the second treatment tank.

22. An apparatus according to claims 12 to 21, in which the height of the gas introduction device within the flexible bag is adjustable.

23. An apparatus according to any of claims 12 to 22, !0 further comprising a pipe for feeding alkaline water to the first treatment tank, the pipe including an open end portion removably supported on the common structure and positioned over a lower region of the first treatment tank.

24. An apparatus according to claim 23, further comprising !5 an arrangement for adding flocculant to the first treatment tank, the arrangement comprising a flocculant feed pipe connected to the pipe for feeding alkaline water to the first treatment tank.

25. An apparatus according to any of claims 12 to 24, JO further comprising a store for storing gas for feeding to the gas introduction device.

26. An apparatus according to claim 25, further comprising one or more cylinders of carbon dioxide.

27. An apparatus according to any of claims 12 to 26, in J5 which the apparatus is provided in a single portable unit.

28. A method of treating alkaline water, the method - 22 - including the following steps i) providing a treatment unit that comprises first and second treatment tanks contained within a common structure; ii) placing a flexible bag into the first treatment tank; 5 iii) providing a gas introduction device; iv) passing alkaline water containing some solids into the flexible bag; and v) introducing carbon dioxide into the water through the gas introduction device, the carbon dioxide passing into .0 the water through the one or more outlets of the device; wherein water in an upper region of the first treatment tank passes into the second treatment tank and flows between a multiplicity of inclined plates in the second treatment tank, solids separating out and collecting in a .5 bottom region of the second treatment tank, and carbon dioxide is introduced into the water in the second treatment tank upstream of the multiplicity of inclined plates .

29. An apparatus for treating alkaline water, the !0 apparatus comprising: a first treatment tank and a second treatment tank contained within a common structure, the first treatment tank having a water outlet in an upper region of the tank; and the first tank having a flexible bag placed therein; !5 a gas introduction device suitable for introducing carbon dioxide into the water; the second treatment tank being arranged to receive water passing from the first treatment tank through its outlet, the second treatment tank containing a multiplicity of JO inclined plates defining a multiplicity of channels therebetween for separating solids in the water received from the first treatment tank, the gas introduction device being disposed downstream of the first treatment tank and upstream of the multiplicity of inclined plates. J5

30. An apparatus according to claim 29, in which the gas introduction device is in a chamber situated between the first treatment tank and the inclined plates. - 23 -

31. An apparatus according to claim 30, in which the chamber is part of the second treatment tank.