GB2476847A

GB2476847A - Flotation treatment of liquids

Info

Publication number: GB2476847A
Application number: GB1012035A
Authority: GB
Inventors: Mark Christopher Watts
Original assignee: WRC PLC
Current assignee: WRC PLC
Priority date: 2010-01-06
Filing date: 2010-07-16
Publication date: 2011-07-13
Also published as: GB201012035D0

Abstract

A system and method for continuous treatment of liquids by a flotation method, comprising at least one conduit 11 having a downward leg 11 a and an upward leg 11 b, the downward leg being in fluid communication with the upward leg, whereby the liquid can flow from the downward leg to the upward leg. The downward leg has an inlet at one end for receiving the liquid to be treated and the upward leg has an outlet at one end. There is a gas diffuser 12 for diffusing gas directly into the liquid in the downward leg of the conduit and a precipitation zone 13 in fluid communication with the outlet of the conduit where the gas dissolved in the liquid precipitates onto the suspended particles in the liquid to be treated, thereby increasing the buoyancy of the particles. The conduit may be U-shaped.

Description

Field of Invention

The present invention relates to an improved system and a method for treating liquids. More particularly, the invention relates to a system for treating liquids which may be potable water, seawater or any contaminated liquid, by diffusing gas and precipitating suspended particles in the liquid to be treated in a flotation method.

Background

In order to bring liquids such as water to a quality suitable for either industrial or potable use, or for discharge to the environment, small particles of solid matter (e.g. suspended clay, bacteria) may have to be removed. Such solids may be separated by one or more means including sedimentation (settlement under gravity), filtration and flotation. A flotation process commonly used in industrial and drinking water treatment is referred to as dissolved air flotation (DAF). It is particularly suitable for separation of solids that are of naturally low density (e.g. natural organic matter, algae) and that cannot therefore be separated readily by sedimentation.

The conventional DAF process comprises the following stages: -Coagulation by adding chemical coagulants or precipitants to precipitate dissolved material and to assist the removal of suspended particles by binding them to the coagulant and making them more liable to stick together subsequently; -Flocculation to increase the size of suspended particles by causing them to collide and stick together; -Flotation to bring particles from the flocculation stage into contact with micro bubbles of air such that the bubbles stick to the particles causing them to become buoyant and rise to the surface of the water in the flotation tank.

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The flotation stage comprises the following components (as shown in Fig 1): -A flotation tank through which the flocculated water passes and is contacted with bubbles, and from which treated water and separated solids are recovered as separate streams.

-A system for generating a large volume (typically 8 to 10% of the total process flow) of water at high pressure (typically 3 to 6 bar above atmospheric pressure) that is saturated with air. This system generally comprises pumps (often called "recycle" pumps), air compressors, air receivers and a means of contacting air and water under pressure such that the air dissolves rapidly to produce air-saturated water. The equipment is often referred to collectively as the saturator system.

Air-release nozzles located where flocculated water enters the flotation tank. Air-saturated water from the saturator system passes through the nozzles causing air bubbles to be released and blend with the flocculated water.

-A means of removing the floated solids that accumulate on the surface of the water in the flotation tank.

The conventional DAF process has the following disadvantages: high energy consumption, requiring an additional 20 to 25 kW.h ter MI of throughput to operate the saturator system; it has high maintenance costs associated with the air compressors, recycle pumps and pressure vessels needed to generate air-saturated water at high pressure; it uses air-release nozzles that are a source of unreliability since these nozzles can get easily blocked and require regular maintenance. A further source of unreliability in the air-release nozzles is erosion, due to the extreme turbulence in the nozzles.

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Canadian Pat No. 977078 discloses a complex apparatus for producing a gas/liquid mixture for use in purification of liquids by flotation.

US Pat No. 3799511 describes a method and an apparatus for dissolving gas in a body of liquid by forcing gas bubbles into liquid to form a gas/liquid mixture, supplying and maintaining the flow of the mixture to a treatment zone disposed at a depth below the surface of the body of liquid (e.g. a lake) as to expose the mixture to a pressure greater than that at the surface, collecting and removing undissolved gas bubbles from the gas-liquid solution formed in the treatment zone and, substantially without pressure change, distributing the solution into the body of liquid. The mixture is formed at surface level, pumped to the treatment zone; and caused to flow through the zone for at least three minutes. A gas cushion is maintained below the surface and above the treatment zone to collect undissolved gas bubbles, excess gas being removed from the cushion. The liquid used is either recycled from the main vessel or is separately introduced.

US Pat No. 4,735,750 describes a process and device for mixing and dissolving gas in liquid, this later being introduced under pressure through a nozzle plate into a reaction space, where a mixture of gas and solution flow out through outlets laterally at the bottom into a solution tank and the gas recirculates through inlets at the top near the nozzle plate. An embodiment of the nozzle plate (12) is disclosed with injector nozzles at the edge and mixing nozzles on the inside.

US Pat No. 4,279,754 discloses a long vertical shaft activated sludge bioreactor for the treatment of waste water and the flotation/separation of solids therefrom under high pressure, an improvement is provided whereby fluctuations in flow through the bioreactor are maintained at a level without the use of complex control mechanisms and whereby separation from the effluent of dispersed air bubbles occurs directly in the flotation/separation vessel rather than in the shaft or an auxiliary vessel. In addition, means are provided in the separation/flotation vessel to split off large dispersed gas bubbles which would otherwise disrupt sludge flotation. The bioreactor is configured to optimize oxygen transfer into a biochemical process.

US Pat No. 4,367,146 discloses an improved method and apparatus is provided for the treatment of sewage waste in a long vertical shaft bioreactor wherein waste sewage is caused to circulate under air injected oxidizing conditions around a ioop comprising adjacent riser and downcomer chambers connected at their upper and lower ends. The improved method comprises the steps of introducing influent waste at depth into the riser chamber at a point higher in the riser than the point of air injection and withdrawing treated effluent from the riser at a point lower in the riser than the point of air injection. A separation chamber may also be operatively connected to the riser chamber to recover sludge for return to the bioreactor. The method provides an improvement in oxygen uptake rate and reduces the anoxic zone in the reactor thus providing additional capacity for BOD removal.

US Pat No. 4,340,484 discloses a long vertical shaft bioreactor for the treatment of degradable wastes in a circulating loop, a process is provided for separating and treating the slowly biodegradable components of the waste. The process comprises the steps of aerating and oxidizing the waste liquor in the reactor to produce an effluent and a foam which foam contains the slowly biodegradable components, which have not been degraded in the oxidizing step, separating and collecting the foam, collapsing the foam and subjecting the collapsed foam to further aerationloxidation treatment to produce a treated effluent. The thus further treated effluent is returned to the bioreactor for reprocessing.

European Pat No. 0700 873 Al discloses a method for treating waste water involves mixing the waste water with an active slurry and water saturated with air under excess pressure. Following pressure reduction, air bubbles separate out from the liquid supersaturated with gas, and biological oxidation of impurities by means of the active slurry together with flotation are carried out at the same time in a single reaction chamber. A saturator is used for saturation purposes and consists of a pipe network made up of at least two loops having a common portion. The pipe network is vertical or almost vertical, has downward and upward stream portions, and two ejectors mounted in series on the upper portion of the pipe carrying the downward stream. The downward-flowing stream divides into two streams directed to each of the ejectors, thereby providing conimon recirculation of liquid and of still undissolved gas, whereby the liquid is totally saturated with gas. Excess undissolved gas is drawn off together with the gas-saturated liquid. The latter is drawn off at the top portion of the pipe carrying the upward-flowing stream.

US Pat No. 6,214,228 describes an effluent treatment plant and a method for treating effluent.

The plant comprises a pair of elongate vessels, one vessel being for aerobic and the other being for anoxic metabolisation of effluent by microorganisms, each vessel having a downcomer zone and a riser zone. The first riser zone of the vessel for anoxic metabolisation communicates with the downcomer zone of the vessel for aerobic metabolisation.

A Research paper on "Experiences in the treatment of domestic sewage by the microflotation process" by M. L. Hemming and W. R. T. Cottrell, ICI Pollution Control Systems, UK discloses a method and an apparatus for treatment of domestic sewage and industrial wastes based on flotation method. In this method prior to the treatment the liquid is mixed by aeration which raises the dissolved gas content. The air is injected at the bottom of the shaft and polyelectrolyte are added in down' corner of the shaft to aid floc agglomeration and to increase hydrophobicity of the solids. The undissolved air formed in the down' corner of the shaft moves upward against the in flow of the effluent. The effect of undissolved air rising in the down' corner of the shaft creates turbulence and mixing which in turn results in break down of the foci agglomerates in the effluent and thereby reducing the efficacy of separation of the solids from the liquid. The floc agglomeration and bubble generation occurs simultaneously in the up' corner of the shaft when the conditions are no longer turbulent. In this document, the process is a batch treatment process, carried out under standstill conditions to achieve reduction in BOD7 and phosphorus content. In order to achieve the desired results, water is recirculated during a period of time to make standstills shorter.

Accordingly there is a need to overcome the above mentioned disadvantages associated with the conventional DAF process.

It is the object of the present invention to provide a system and a method for treating liquids, which is economical and cost effective. The liquid may be, for example, potable water or seawater. The invention makes it possible to remove contaminants from the liquid.

It is the object of the present invention to provide a system and a method for treating potable water which operates continuously without the requirement of re-circulation.

It is the object of the present invention to provide a simple, less cumbersome and efficient system and a method for treating liquid by generating liquid supersaturated with gas.

It is also the object of the present invention to provide means for allowing maximum contact between liquid supersaturated with gas and the liquid to be treated such that suspended particles or solids become buoyant and move rapidly to the liquid surface ready to be removed as a sludge.

It is also the object of the present invention to produce liquid saturated with gas which is readily precipitated onto the suspended particles in the liquid to be treated by a flotation method.

It is also the object of the present invention to provide a system without the disadvantage of having nozzles that are easily blocked and prone to damage.

It is also the object of the present invention to provide a system and a method for treating seawater to produce potable water.

Summary of the invention

Accordingly the present invention provides a system for continuous treatment of liquids by a flotation method, comprising: -at least one conduit having a downward leg and an upward leg, the downward leg being in fluid communication with the upward leg, whereby the liquid can flow from the downward leg to the upward leg, wherein the downward leg has an inlet at one end for receiving the liquid to be treated, and the upward leg has an outlet at one end, -a gas diffuser for diffusing gas directly into the liquid in the downward leg of the conduit, -a precipitation zone in fluid communication with the outlet of the conduit where the gas dissolved in the liquid precipitates onto the suspended particles in the liquid to be treated, thereby increasing the buoyancy of the particles.

In another aspect of the present invention, the system further comprises: a flotation tank in which the buoyant particles precipitated with gas bubbles float on to the surface of the liquid and are removed.

In another aspect of the present invention, the system further comprises: a coagulation tank for coagulating the liquid to be treated with coagulant or precipitant chemicals to precipitate dissolved material and agglomerate the suspended particles by binding them together and making them more liable to stick together subsequently.

In another aspect of the present invention, the system further comprises: a flocculator for flocculating the suspended particles to increase their size by causing them to collide and stick together.

The present invention further provides a continuous process for treating liquids by flotation method, comprising the steps of: (a) passing the liquid to be treated through a conduit having a downward leg and an upward leg, the downward leg being in fluid communication with the upward leg, whereby the liquid can flow from the downward leg to the upward leg, wherein the downward leg has an inlet at one end for receiving the liquid to be treated, and the upward leg has an outlet at one end, (b) diffusing gas in the downward leg of the conduit directly into the liquid such that there is substantially no upward movement of undissolved gas in the downward leg of the conduit, and (c) allowing the gas diffused in the liquid to precipitate onto particles suspended in the liquid, thereby increasing the buoyancy of the particles.

In another aspect of the present invention, the method further comprises the step of passing the liquid comprising the buoyant suspended particles to a flotation tank, in which the floated particles and treated liquid free of contaminants are removed separately.

In another aspect of the present invention, prior to the treatment, the liquid is subjected to coagulation by adding coagulant or precipitant chemicals to precipitate dissolved material and agglomerate the suspended particles by binding them together and making them more liable to stick together subsequently.

In another aspect of the present invention, prior to the treatment, the liquid is subjected to flocculation to increase the size of the suspended particles by causing them to collide and stick together.

In another aspect of the present invention, the gas is diffused gently and directing into the liquid to be treated such that there are no disruptions of the agglomerates by mixing or turbulence in the downward leg of the conduit and at the same time the gas dissolves in the liquid and becomes supersaturated and forms gas bubbles in the upward leg of the conduit as the pressure drops.

In another embodiment of the present invention, a chemical such as polyelectrolyte is added which modifies the surface of the suspended particles such that precipitation of gas onto the surface of the suspended particles takes place more easily.

As discussed above, the conduit according to the present invention has a downward leg and an upward leg, the downward leg being in fluid communication with the upward leg such that the fluid can flow from the downward leg to the upward leg. The conduit may be of any suitable shape. The conduit may be straight, but it is preferred that the conduit is curved, as this provides the least turbulent flow. According to the present invention a U' shaped conduit is most preferred.

The gas is preferably diffused into the downward leg at a position which is above the lowermost part of the U-shaped conduit. In an embodiment, the U-shaped conduit has a curved portion extending from the downward leg to the upward leg, and the gas is preferably diffused into the downward leg above the curve portion. In an alternative embodiment, however, U-shaped conduit has a substantially straight portion extending from the downward leg to the upward leg, which may be arranged substantially perpendicular to the downward and upward legs.

In another aspect the present invention relates to an apparatus for treating liquids comprising: a system for generating liquid saturated or supersaturated with gas according to the present invention.

Brief description of the drawings

We refer to the accompanying drawings, in which: Fig 1: shows schematic representation of a conventional DAF process; Fig 2: shows schematic representation of a system for improving the flotation method in treatment of liquids according the present invention.

Detailed Description of the Invention

Figure 1 schematically depicts a conventional water treatment system which includes a coagulation zone 1, a flocculator 2, a system 3 for generating water dissolved with air bubbles and a flotation tank 4. The air saturated water from the system 3 is discharged into the flotation tank 4 through a nozzles 10 which is located at the junction where the liquid from the flocculator 2 enter the flotation tank 4. The system 3 includes a high pressure pump 5 which is capable of being used as a recycle pump, an air compressor 6, a pressure regulator 7, a level controller 8 and a saturator system 9.

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The saturator system 3 in the conventional system is complicated and it is costly to build and operate.

Figure 2 schematically depicts the system according to the present invention for continuous treatment of a liquid, preferably potable water. The system and method according to the present invention is capable of generating liquids supersaturated with gas for removing suspended particles by a flotation method.

The system includes a U-shaped conduit 11 having an inlet at one end of a downward leg of the conduit for receiving a liquid to be treated and an outlet at one end of an upward leg of the conduit, for discharging the liquid saturated with gas bubbles. The downward leg of the conduit is indicated by 1 la and the upward leg of the conduit is indicated by 1 lb. A gas diffuser 12 is disposed on the downward leg 11 a of the conduit and a precipitation zone 13 is disposed at the outlet of the conduit 11. The precipitation zone 13 is in fluid communication with the outlet of the conduit where the gas dissolved in the liquid is allowed to precipitate onto the suspended particles in the liquid to be treated, thereby increasing the buoyancy of the particles. The gas diffuser 12 receives compressed air from a compressor or blower 15 and discharges it as small bubbles which are subsequently pulled downwards by fluid motion in the downward leg. The gas diffuser 12 may include a microporous membrane or sintered material through which filtered and high quality compressed gas is passed continuously into the liquids in the conduit. A non-membrane diffuser (e.g. sintered ceramic material) operates such that gas bubbles are produced which dissolve subsequently, whereas a membrane diffuser operates such that the gas passes directly into solution without bubble formation.

The liquid to be treated is passed through the U-shaped conduit 11 and contacted with a gas from the integral gas diffuser 12 such that the gas dissolves in the downward leg and the liquid subsequently becomes supersaturated in the upward leg as the liquid rises and pressure falls. The gas, from the liquid that is supersaturated with gas, is allowed to precipitate onto the suspended particles or solids in the liquid to be treated at the precipitation zone 13 such that the particles or

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solids become buoyant. The precipitation zone 13 leads into a flotation tank 14 where the suspended particles precipitated with the gas bubbles float rapidly to the liquid surface in the flotation tank 14 to form a sludge, which is subsequently removed. The treated liquid free of contaminants is removed separately in a known manner.

The flotation tank 14 may include means for removing the floated solids that accumulate on the surface of the liquid. In a preferred embodiment, the means for removing is a hydraulic means or a mechanical means. In a preferred embodiment the mechanical means is a rotating cylinder to which rubber blades are attached, designed such that the blades periodically sweep floating solids away from the surface and the hydraulic means is carried out by overflow of liquid and solids.

The gas is preferably air, and the liquid is water, preferably potable water.

The liquid to be treated may be seawater.

In the process according to the present invention the gas is dissolved continuously into the liquid to be treated in the downward leg 11 a of the conduit 11 and the pressure then reduced such that the liquid becomes supersaturated in the upward leg 1 lb of the conduit 11 causing the gas to precipitate onto solid matter such that the solid matter becomes buoyant.

in a preferred embodiment a chemical such as polyelectrolyte is added which modifies the surface of suspended particles such that precipitation of gas onto the surface takes place more easily.

The gas diffuser device 12 may be compatible with the design of the U-shaped conduit 11 such that the size of bubbles is such that the bubbles will not rise in the downward leg. under normal operating conditions. Calculations using Stoke's Law, and assuming that bubbles will be of the order of I mm in diameter, suggest that the rise velocity of bubbles when the liquid is static is 0.3 to 0.5mIs.

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The gas diffuser 12 injects gas gently in the downward leg ha of the conduit 11 such that there is no turbulence or mixing in the downward leg of the conduit due to upward movement of undissolved gas. In this way, the suspended solids/agglomerates in the liquid are not disrupted or broken as the bubbles form and disperse through the liquid.

As liquid rises in the upward leg 1 lb of the U-shaped conduit 11 it becomes supersaturated with gas. It then passes to a precipitation zone 13 where the gas gradually precipitates onto the solid matter, such that the solids become buoyant and rise to the surface of the liquid in a flotation tank, ready for removal.

Passing the whole process flow through a U-shaped conduit in order to develop the pressure required for gas dissolution, eliminates the use of means such as recycle pumps and a saturator (as in the conventional system) which are costly and cumbersome. This markedly reduces the energy requirements and maintenance costs of this process relative to conventional DAF process.

The energy requirements are of the order of 2 to 3 kW.hIMl, representing a Ca. 90% reduction in energy relative to a conventional DAF saturator system.

The system according to the present invention is not prone to blockage, as in conventional DAF system where the nozzles used to discharge the liquid with gas bubbles are prone to blockage.

The system according to the present invention provides a continuous process of treatment of contaminated liquids and there is no requirement for re-circulation of liquids. The system is particularly suitable for treatment of potable water.

The system according to the present invention may also be used in the pre-treatment process for desalination of seawater.

In a preferred embodiment the U-shaped conduit 11 is sufficiently deep for the static pressure at the base to cause all gas bubbles to dissolve. The conduit may be of a tapered design such that at

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the design volumetric flowrate the downwards velocity is greatest at the top (where bubbles are largest) and least at the bottom (when bubbles have become smaller due to the increased pressure and dissolution of gas). This minimizes the loss of air due to bubbles rising in the downward leg and being lost, and it also increases the contact time between bubbles and water.

The conduit 11 (whether U-shaped or otherwise) is preferably made of a material selected from concrete, stainless steel, polyethylene (particularly high density polyethylene [HDPE]) or glass reinforced plastic (GRP). The conduit 11 may be extruded, especially when made of polyethylene. Alternatively, the conduit 11 may be of a stepped configuration, especially when made of concrete. Alternatively, the conduit may be made of plates of material designed to form an inverted pyramid.

In a preferred embodiment the water velocity through the conduit does not exceed 0.3 mIs. This helps to prevent floc disruption due to fluid shear.

The surface in the upward leg of the conduit 11 may be very smooth so as to reduce the number of sites where bubble nucleation occurs. This aids in reducing waste of air to bubbles formed at the conduit wall, rather than on particulates in the liquid.

In a preferred embodiment, the liquid to be treated is subjected to coagulation and br flocculation in a pre-treatment stage. The solids in the liquid to be treated are agglomerated in a pre-aeration stage.

Due to pre-treatment by coagulation/flocculation the liquid to be treated contains agglomerates/floc. Therefore, it is essential that these agglomerates in the liquid are not disrupted or broken in the downward leg of the conduit when the liquid passes through the conduit. This is achieved by providing the gas diffuser 12 at the side of the downward leg of the conduit 11 to gently diffuse gas into the liquid such that the agglomerates are not disrupted in the downward leg and at the same time dissolves gas into the liquid, which becomes supersaturated arid forms gas bubbles when the liquid rises in the upward leg.

In a preferred embodiment, the system according to the present invention comprising the U-shaped conduit 11 and an gas-solid precipitation zone 13, is situated between the mechanical flocculators and flotation tank of a typical conventional DAF process.

It will be appreciated that the invention may be modified within the scope of the claims.

Claims

SCLAIMS: 1. A system for Continuous treatment of liquids by a flotation method, comprising: -at least one conduit having a downward leg and an upward leg, the downward leg being in fluid communication with the upward leg, whereby the liquid can flow from the downward leg to the upward leg, wherein the downward leg has an inlet at one end for receiving the liquid to be treated, and the upward leg has an outlet at one end, -a gas diffuser for diffusing gas directly into the liquid in the downward leg of the conduit, -a precipitation zone in fluid communication with the outlet of the conduit where the gas dissolved in the liquid precipitates onto the suspended particles in the liquid to be treated, thereby increasing the buoyancy of the particles.
2. A system according to claim I wherein the gas is air.
3. A system according to claim 1 or 2 wherein the liquid is water, preferably potable water.
4. A system according to claim 1 or 2 wherein the liquid is seawater.
5. A system according to any preceding claim wherein the liquid is a contaminated liquid.
6. A system according to any preceding claim wherein a device for compressing gas is connected to the gas diffuser.
7 A system according to claim 6 wherein the device is a compressor or a blower.
8. A system according to any preceding claim wherein the gas diffuser is a microporous membrane or a sintered material.
9. A system according to any preceding claim wherein the gas is dissolved in the downwardSleg of the conduit to produce a saturated solution of the gas, and a supersaturated solution of the gas is produced in the upward leg of the conduit.
10. A system according to any preceding claim wherein the conduit has a tapered design such that the velocity of the liquid decreases at the bottom resulting in increase in the diffusion of gas in the liquid.
11. A system according to any preceding claim further comprises a flotation tank in which the buoyant particles precipitated with gas bubbles float onto the surface of the liquid and are subsequently removed.
12. A system according to claim 11 wherein the flotation tank is provided with a means for removing the floated buoyant particles from the surface of the liquid.
13. A system according to claim 12 wherein the means for removing the floated solids is either mechanical or hydraulic.
14. A system according to claim 12 wherein the mechanical means is a rotating cylinder with rubber blades.
15. A system according to claim 12 wherein the hydraulic means is carried out by overflow of water and solids.
16. A system according to claim 11 to 15 wherein the flotation tank is provided with means for removing the treated liquid free of suspended particles.
17. A system according to any preceding claim wherein the gas diffuser injects gas into the liquid to be treated such that there are no upward movement of undissolved gas in the downward leg of the conduit.S
18. A system according to any of the preceding claim further comprises: a coagulation tank for coagulating the liquid to be treated with coagulant or precipitant chemicals to precipitate the dissolved material and agglomerate the suspended particles by binding them together and making them more liable to stick together subsequently.
19. A system according to any of the preceding claim further comprises: a flocculator to increase the size of the suspended particles by causing them to collide and stick together.
20. A system according to any preceding claim wherein the gas diffuser injects gas such that the agglomerated particles or floc in the liquid to be treated are not broken or disrupted.
21. A system according to any preceding claim wherein the conduit is a U' shaped conduit.
22. A continuous process for treating liquids by a flotation method, comprising the steps of: (a) passing the liquid to be treated through a conduit having a downward leg and an upward leg, the downward leg being in fluid communication with the upward leg, whereby the liquid can flow from the downward leg to the upward leg, wherein the downward leg has an inlet at one end for receiving the liquid to be treated, and the upward leg has an outlet at one end, (b) diffusing gas in the downward leg of the conduit directly into the liquid such that there is substantially no upward movement of undissolved gas in the downward leg of the conduit, (c) allowing the gas diffused in the liquid to precipitate onto particles suspended in the liquid, thereby increasing the buoyancy of the particles.
23. A method according to claim 22 wherein the liquid comprising the buoyant suspended particles is passed into a flotation tank in which the floated particles and treated liquid free of contaminants are removed separately.
24. A method according to claim 23 wherein the floated particles are removed eitherSmechanically or hydraulically by overflowing the liquid and solids.
25. A method according to any of claims 22 to 24 wherein the gas is compressed prior to diffusing in the liquid in the downward leg of the conduit.
26. A method according to any of claims 22 to 25 wherein the pressure is reduced such that the gas dissolves in the downward leg of the conduit and becomes supersaturated with gas in the upward leg of the conduit as the liquid rises.
27. A method according to any of claims 22 to 26 wherein the gas is dissolved continuously into the liquid inside the conduit.
28. A method for according to any of claims 22 to 27 wherein the liquid is pre-treated by coagulating with coagulant or precipitant chemicals to precipitate dissolved material and agglomerate the suspended particles by binding them together and making them more liable to stick together subsequently.
29. A method for according to any of claims 22 to 28 wherein the liquid is pre-treated by flocculating the suspended particles to increase their size by causing them to collide and stick together.
30. A method according to any of claims 22 to 29 wherein the gas is injected such that the suspended particles/agglomerates in the liquid are not disrupted or broken in the downward leg of the conduit.
31. A method according to any of claims 22 to 30 wherein the gas is air.
32. A method according to any of claims 22 to 31 wherein the liquid is water, preferablypotable water.
33. A system according to any of claims claim 22 to 31 wherein the liquid is seawater.
34. A system according to any preceding claim 22 to 33 wherein the liquid is a contaminated liquid.
35. A method according to any of the preceding claims wherein a polyelectrolyte is added to the liquid to modify the surface of the suspended particles such that the gas is readily precipitated onto its surface.