GB2409677A

GB2409677A - Treatment of wastewater

Info

Publication number: GB2409677A
Application number: GB0330267A
Authority: GB
Inventors: Barrie Christopher Woodbridge; Robert Yam Gwamfi Andoh
Original assignee: Hydro International Ltd
Current assignee: Hydro International Ltd
Priority date: 2003-12-31
Filing date: 2003-12-31
Publication date: 2005-07-06
Also published as: GB0330267D0; WO2005063630A1

Abstract

Waste water, for example municipal waste water including sewage, is treated by a method which comprises adding a particulate material to the waste water, and then performing a flotation process so that the particulate material rises to the surface of the waste water, entraining solids material in the waste water. The particulate material may comprise buoyant particles, or, if the flotation process includes the generation of gas bubbles within the waste water, particles which are slightly more dense than water but become buoyant as the gas bubbles agglomerate on the particles. The particles may comprise bubbles of soda-lime-borosilicate glass.

Description

TREATMENT OF WASTE WATER

This invention relates to the treatment of waste water, and particularly, although not exclusively, to the treatment of municipal waste water, including sewage.

Typically, municipal waste water undergoes a series of processes with the ultimate objective of delivering acceptably clean water that can be resumed to waterways. In general, such processes include primary treatment steps which physically separate solid materials from the waste water, and secondary treatment steps which biological separation of further matter from the waste water.

It is usual for the primary treatment steps to include one or more settling tanks in which grit and other heavy solids are removed from the bottom, while oil and large floatable solids are removed from the top. The effluent from such settling tanks still contains a significant proportion of suspended solids in the form of small organic and inorganic particles which will not separate under gravity in an acceptable time.

Various techniques have been developed to assist the removal of suspended solids from waste water. For example, it is known to add flocculants or coagulants to the waste water to cause the suspended particles to clump together so as to settle (or float to the surface) more quickly. It is also known, for example from EP 0767143, to add a ballast, for example in the form of sand, to the waste water to be treated. The ballast is more dense than water, so it sinks to the bottom of the treatment vessel, during which bme the suspended solids aggregate around the sand particles.

It is also known to use flotation techniques, for example employing dissolved air flotation (OAF). In such techniques, water under pressure, saturated with dissolved air, is injected into the treatment vessel. The reduced pressure within the treatment vessel causes the dissolved air to come out of solution and to form fine bubbles which rise through the waste water to be treated to form a froth at the surface. As the bubbles rise, suspended solids aggregate around them, and so are trapped in the froth enabling them to be removed by skimming.

While such techniques are effective in removing suspended solids from waste water, it is desirable to accelerate the separation process and to increase its efficiency (ie to increase the proportion of solids entering the treatment vessel which are removed by the separation process).

In its broadest scope, the present invention provides a method of treating waste water by a flotation process, in which method a buoyant particulate material is added to the waste water before the flotation process.

More specifically, the present invention provides a method of treating waste water comprising: adding a buoyant particulate material to the waste water; and generating gas bubbles within the waste water, whereby solids suspended in the waste water are entrained by the bubbles and the particulate material and are conveyed to the surface of the waste water.

A method in accordance with the present invention may be conducted in a treatment vessel, the bubble and the particulate material, with entrained solids, forming a layer at the surface of the waste water in the vessel, which layer is continuously or periodically removed, for example by skimming. The particulate material may be separated from the removed suspended solids for recycling into the inflowing waste water.

The gas bubbles may comprise air, and may be generated by injecting water saturated with dissolved air into the treatment vessel.

A flocculating agent may be added to the waste water before the waste water enters the treatment vessel. The flocculating agent may comprise a polymer, for example a polyelectrolyte, such as a cationic polyelectrolyte, having a molecular weight which is preferably greater than 2 x 1 Oe g/mol, and more preferably greater than 4 x 1 Oe g/mol.

The polyelectrolyte may, for example, be a polyacrylamide or a polyamine.

Although the present invention envisages using a buoyant particulate material, that is having a density less than 1 g/cc, a variant of the present invention envisages the use of a particulate material which is slightly heavier than water (ie has a density greater than 1 g/cc) with buoyancy being afforded by the agglomeration of gas or air bubbles around each particle. However, in preferred methods in accordance with the present invention, the particulate material has a density less than 0.75 g/cc, and more preferably less than 0.6 g/cc. For example, a preferred density range for the particulate material may be between 0.15 to 0.6 g/cc, and more preferably in the range 0.3 to 0.6 g/cc.

The particulate material may contain particles of different size. Preferably, at least 95% of the particles by volume have a size less than 150,u. A preferred particulate material has at least 95% of particles by volume less than 100,u, more preferably less than 90 10,u. Preferably, not more than 10% of the particles have a particle size less than 30,u, more preferably less than 20,u.

Preferably, the particles of the particulate material are resistant to crushing. In particular, it is preferable for them to have an isostatic strength such that at least 90% of the particles can survive a crushing force of 13,000 kPa. More preferably, at least 90% of the particles can survive a crushing force in excess of 20,000 kPa.

A suitable particulate material is available under the name 3M Scotchlite. This material comprises bubbles of soda-lime-borosilicate glass and is available in various grades, of which grades S32, K37, See, K46 and S60/10,000 may be suitable.

It will, of course, be appreciated that other forms of particulate material may be used in the practice of the present invention, for example solid or hollow plastics materials or natural mineral materials.

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, which diagrammatically represents part of a waste water treatment plant.

Waste water to be treated is fed by a pump 2 to a mixing chamber 4. This waste water may comprise raw municipal waste water but, preferably, comprises the outflow from one or more preliminary processes, such as grit removal and bulk solids removal. The waste water is thus likely to be relatively free of large solid particles, but contains very small suspended particles which are resistant to settling. These suspended solids may result in the waste water having high turbidity.

A flocculating agent, such as a high molecular weight cationic polyelectrolyte, is added to the mixing chamber 4 on a line 6. The flow rate of the flocculating agent is controlled by a valve 8. The mixing chamber 4 also receives, on a line 10, a buoyant particulate material, for example in the fomm of glass micro spheres. Mixing means is provided for agitating the waste water, the flocculating agent and the particulate material so as to achieve intimate mixing of these components before the mixture is fed, along a line 12, to a flotation vessel 14. The flotation chamber 14 is provided with upper baffles 16 and lower baffles 18 to assist in generating a desired flow pattern within the flotation vessel 14.

An injection pump 20 supplies water saturated with air from a pressurized container 22 at a pressure of, for example, 500 kPa to injectors 24 positioned around the base of the flotation vessel 14. Extraction means represented by a line 26 is provided at the top of the flotation vessel 14 for removing a froth layer 28 generated in operation. This layer is in the form of a sludge which is supplied to a separator 30, for example a cyclone separator, having an outlet feeding to the line 10 for particulate material taken from the layer 28, and a sludge outlet 32 for solids material separated from the waste water in the flotation vessel 14.

In operation, the mixture from the mixing vessel 4 enters the flotation vessel 14. The flocculating agent supplied through the line 6 causes the suspended solids in the waste water to agglomerate together, and to the individual particles of the particulate material supplied through the line 10. As the air-saturated water enters the flotation vessel 14 through the injectors 24, the reduced pressure causes the dissolved air to come out of solution and to form small bubbles which rise through the flotation vessel 14. These bubbles adhere to the flocculated suspended solids and to the individual particles of the particulate material, so enhancing the buoyancy of the particles and accelerating their progress to the surface of the mixture in the flotation vessel 14. The air bubbles, the particulate material and the entrained suspended solids, now separated from the bulk of the water of the incoming flow, form a froth layer 28 at the top of the flotation vessel 14, which has a relatively low water content. This froth is removed by any suitable mechanism, for example by skimming, and conveyed along the line 26 to the separator 30. In this separator, the remaining water and the solids formerly suspended in the incoming waste water, are stripped from the particles of the particulate material, which is then recycled along the line 10 to re- enter the mixing chamber 4. The remaining sludge exits the separator 30 on the line 32, for further processing or disposal.

The water remaining in the lower part of the flotation vessel 14 is substantially free of suspended solids, and is drawn off on a line 34. This water may be subjected to further process steps, for example UV sterilization, or chemical or biological treatment before either passing to natural waterways or being carried away for other uses, such as irrigation or industrial or domestic use.

It would be appreciated that a method in accordance with the present invention may be used for purposes other than waste water treatment (for example in the mineral extraction industry). Also, in some circumstances adequate removal of suspended solids may be achieved by the addition of the particulate material alone, or in conjunction with the addition of a flocculating agent, without the addition of dissolved 1 5 air.

Claims

1. A method of treating waste water by a flotation process, in which method a particulate material is added to the waste water before the flotation process, characterised in that the density of the particulate material is less than, or not substantially greater than, that of the waste water.

2. A method as claimed in claim 1, characterised in that the method comprises: adding the particulate material to the waste water; and generating gas bubbles within the waste water, whereby solids suspended in the waste water are entrained by the bubbles and the particulate material and are conveyed to the surface of the waste water.

3. A method as claimed in claim 2, characterised in that the gas bubbles comprise air.

4. A method as claimed in claim 2 or 3, characterised in that the gas bubbles are generated by injecting water saturated with dissolved gas into the waste water.

5. A method as claimed in any one of the preceding claims, characterised in that the method is conducted in a treatment vessel, the particulate material, with entrained solids, forming a layer at the surface of the waste water in the vessel, which layer is continuously or periodically removed.

6. A method as claimed in claim 5, characterised in that, after removal of the layer, the particulate material is be separated from the removed suspended solids and recycled into the inflowing waste water.

7. A method as claimed in claim 5 or 6, characterised in that a flocculating agent is added to the waste water before the waste water enters the treatment vessel.

8. A method as claimed in claim 7, characterised in that the flocculating agent is a polymer.

9. A method as claimed in claim 8, characterised in that the flocculating agent is a cationic polyelectrolyte, having a molecular weight which is greater than 2 x 1 on g/mol.

10. A method as claimed in any one of the preceding claims, characterised in that the particulate material is buoyant in the waste water.

11. A method as claimed in claim 10, characterised in that the particulate material has a density less than 0.75 g/cc.

12. A method as claimed in claim 11, characterised in that the particulate material has a density less than 0.6 g/cc

13. A method as claimed in claim 12, characterised in that the particulate material has a density between 0.15 to 0.6 g/cc.

14. A method as claimed in claim 13, characterised in that the particulate material has a density between 0.3 to 0.6 g/cc.

15. A method as claimed in claim 2 or in any one of claims 3 to 9 when appendant to claim 2, characterised in that the particulate material has a density slightly greater than that of the waste water, the particles becoming buoyant as a result of agglomeration of gas or air bubbles around each particle.

16. A method as claimed in any one of the preceding claims, characterised in that at least 95% by volume of the particles of the particulate material have a size less than 150,u.

17. A method as claimed in claim 16, characterised in that at least 95% by volume of the particles of the particulate material have a size less than 90,u.

18. A method as claimed in any one of the preceding claims, characterised in that not more than 10% by volume of the particles have a particle size less than 30,u.

19. A method as claimed in claim 18, characterized in that not more than 10% by volume of the particles have a particle size less than 30 flu.

20. A method as claimed in claim 19, characterized in that not more than 10% by volume of the particles have a particle size less than 20 p.

21. A method as claimed in any one of the preceding claims, characterized in that the particles of the particulate material have an isostatic strength such that at least 90% of the particles can survive a crushing force of 13,000 kPa.

22. A method as claimed in any one of the preceding claims, characterized in that the particles of the particulate material have an isostatic strength such that at least 90% of the particles can survive a crushing force of 20,000 kPa.

23. A method as claimed in any one of the preceding claims, characterized in that the particulate material comprises bubbles of soda-limeborosilicate glass.