GB2416537A

GB2416537A - Water treatment for removal of fine particulates

Info

Publication number: GB2416537A
Application number: GB0416423A
Authority: GB
Inventors: Peter Ellenberger; Thomas Hofmann
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-07-23
Filing date: 2004-07-23
Publication date: 2006-02-01
Also published as: GB0416423D0

Abstract

A method of removing a substantial proportion of finely-divided solids from an aqueous suspension thereof by the steps of <SL> <LI>(a) placing the aqueous suspension in a settling tank; and <LI>(b) permitting suspended solids to settle to give water that is acceptable for <LI>recycling; </SL> in which there is added to the water at least one water-soluble poly(alkylene oxide) having a weight-average molecular weight of from 100,000 - 8,000,000, and, at a different time from this addition, at least one material that interacts with and causes the poly(alkylene oxide) to precipitate. The material that interacts with the poly(alkylene oxide) is preferably mineral, most preferably bentonite. The method is especially useful for the treatment of washwater from the washing of walls of road tunnels. It promotes the rapid settling of suspended dirt and the rapid turn-around of settling tanks.

Description

WATER TREATMENT

This invention relates to a method for removing fine particulate matter from water and to preparations for use in such a method. s

The result of many industrial processes is a fine aqueous dispersion of solid particles. The presence of this matter is often undesirable, because it means that the water either cannot be recycled for reuse or disposed of in the environment. One such example is the water collected from the washing of road tunnels. In service, road tunnel walls become lined with an adhesive mixture of road grit and dirt, exhaust fume residues, diesel and tyre particulates and oils and greases. This gradually darkens the tunnel interior (especially those with concrete linings and/or light, visibility-improving coatings). Tunnels therefore need to be washed periodically, to remove this layer. In a typical process, a tunnel cleaning agent is sprayed, brushed or rolled on to the tunnel surface, and allowed to soak in. After an appropriate time, the cleaning agent and the dirt are removed by washing with water, typically by means of a rotating brush with recycling water and rinse with tap water.

Because of its high concentration of environmentally undesirable substances, the resulting dirt-containing washwater cannot be disposed of directly into the environment or recycled, but it must first itself be cleaned. This involves its being transferred to a settling tank, so that the solid materials can be allowed to settle. The problem here is that, because of the extreme fineness of many of these solid materials, this can take a very long time and tie up settling tanks for an undesirable time.

It has now been found that it is possible to increase considerably the turn-around time of settling tanks by the use of a particular process. The invention therefore provides a method of removing a substantial proportion of finely-divided solids from an aqueous suspension thereof by the steps of (a) placing the aqueous suspension in a settling tank; and (b) permitting suspended solids to settle to give water that is acceptable for recycling; in which there is added to the water at least one water-soluble poly(alkylene oxide) having a weight-average molecular weight of from 100,000 - 8,000,000, and, at a different time from this addition, at least one material that interacts with and causes the poly(alkylene oxide) to precipitate.

In a particular embodiment of the invention, there is provided a method of cleaning accumulated dirt from the walls of road tunnels, comprising the steps of (a) washing the tunnel wall with washwater comprising an aqueous solution of a dirt removing agent; (b) collecting the used washwater from this process in a settling tank; and (c) permitting suspended solids to settle to give washwater that is acceptable for recycling; in which there is added to the washwater at least one water-soluble poly(alkylene oxide) having a weight-average molecular weight of from 100,000 - 8,000,000, and, at a different time from this addition, at least one material that interacts with and causes the poly(alkylene oxide) to precipitate, at least one of these additions being to the washwater during or after collection thereof.

The invention will be described with particular reference to the washing of road tunnels.

However, the skilled person will see that the principle is applicable to all aqueous suspensions of finely-divided solids, and that it is readily adaptable to any othe situation using only the ordinary skill of the art.

Poly(alkylene oxide) is one common name for a group of materials comprising a continuous chain of oxyalkylene units of the formula CHR'CHR2-O)-, in which R' and R2 may be the same or different and are selected from hydrogen or lower alkyl, typically C, or C2. The most commonly encountered cases are polyethylene oxide (R', R2 both H) and poly(ethylene oxide-propylene oxide) copolymers (in a minor proportion of units one of R' and R2 is methyl). The pure polyethylene oxides are water- soluble up to a very high molecular weight, whereas pure polypropylene oxides and poly(ethylene oxide-propylene oxide) copolymers with a high proportion of oxypropylene units are completely water-insoluble. There is therefore a limit as to how great a proportion of oxypropylene units can be tolerated, but the determination of such a limit is well within the skill of the art. It is possible and permissible to use two or more such poly(alkylene oxide)s, although this is generally unnecessary. For the purposes of this invention, use of the term "poly(alkylene oxide)" shall be considered also to include the group of materials known as poly(alkylene glycols). The difference between the two is the nature of the terminal group - the glycol is terminated with a glycol group -OR, where R is an alkyl group, usually a lower alkyl. However, the substances are essentially the same, having a long chain of alkylene oxide units, and for the purposes of this invention they are considered to be the same substance.

For the purposes of this invention, the preferred material is a polyethylene oxide of weight- average molecular weight from 2,000,000-5,000,000. There exist a considerable number of commercial materials that can be used, one example being the POLYGON range of Union Carbide Corp. It is well known in the art that poly(alkylene oxide) interacts with certain materials and is caused to precipitate (these materials shall hereinafter be referred to as 'precipitation materials"). This effect is used, for example, in EP (0233) to provide a dramatic decrease in the "slump" of cementitious compositions. However, the mechanism has never found any use

outside the field of cement and concrete.

In the abovementioned case, the precipitation materials included BNS (,Bnaphthalene sulphonate-formaldehyde condensate, a well-known cement 'plasticisei') and various cellulosic derivatives). While such precipitation materials (for example, the whole list of materials in EP (0233)) may be used in this invention, they are generally not completely satisfactory, and it has been found that, for the purposes of this invention, the preferred precipitation materials are mineral in nature, preferably finely-divided clay minerals. These have the advantange of higher density and therefore faster settling. The minerals also have the advantages of being very cheap and environmentally innocuous - the use of non-mineral materials may involve the use of materials that are either or both of environmentally undesirable and in need of some process to degrade or remove them. Further description of the invention shall refer only to this preferred embodiment, although the possibility of use of non- mineral precipitation materials is not excluded and should be borne in mind.

There is available commercially a large range of finely-divided clay minerals, used for a wide variety of purposes, particularly as innocuous fillers and extenders in a wide range of domestic and industrial products. A particularly preferred group of minerals are those of the smectite group, and among those the most preferred are the montmorillonite group A particularly preferred clay mineral is bentonite, a widely-available particulate clay used in a large variety of applications.A typical bentonite useful in this invention is IBECO (trade mark) HT-X commercially available from S&B Industrial Minerals GmbH, Mannheim, Germany, but any of the other common bentonite grades may be used. Again, it is possible to use two or more such materials, if desired.

Other clay minerals can also be used - one example is sepiolite, one commercially available version being Pangel_ S9 The important point of the invention is that the two materials must be added to the aqueous suspension at different times. In the case of tunnel washing, one of these is to the washwater during or after collection. By "during collection" is meant at any time from the flow of the washwater from the washed tunnel wall to its arrival in a settling tank. Thus, the second addition may be to the washwater in the tank, or it may be added to the washwater as it flows to the tank. Some tunnel drainage systems have an independent system of drainage for the collection of such water, and an addition point may be provided at some convenient point in this system. When the two are combined, the poly(alkylene oxide) precipitates around both clay particles and suspended dirt particles. The relatively dense clay particles cause the precipitated poly(alkylene oxide) with dirt particles to sink and thus cause sedimentation of the dirt much quicker than otherwise would be the case.

The order of addition is not critical, but it is preferable to add the poly(alkylene oxide) first and then the clay mineral. Both may be added to the settling tank, but in the case of tunnel washing, it is preferred to add the poly(alkylene oxide) to the washwater before it is applied to the tunnel wall. In other words, it is either dissolved in the aqueous solution of cleaning agent, or it is injected into the solution at the hose when this is being applied to the tunnel wall. It is possible to apply a poly(alkylene oxide) solution independently to the wall with or just after the application of the aqueous cleaning solution, but this is inconvenient.

The solution of the poly(alkylene oxide) in the aqueous cleaning solution is particularly preferred, as only one addition need then be made. In addition, it ensures that the poly(alkylene oxide) is dissolved and uniformly distributed through the washwater, such that, when the clay mineral is added, precipitation takes place throughout the bulk of the washwater. Poly(alkylene oxide) dissolves easily and does not interfere with the cleaning action of any known cleaning agent. The dirt-removing agent may therefore be any of the known types.

The proportions of the two materials to be added to the aqueous suspension will depend on the nature of the suspension to be cleaned - water with a high concentration of suspended solids will require a higher concentration. However, typical concentrations per 10001itres of aqueous suspension are (a) of poly(alkylene oxide), from 2 - 500g, preferably 3 50g, more preferably 3-20g; and (b) of clay mineral, from 2 - 500g, preferably 3 - 50g, more preferably 3- 20g.

It may sometimes be appropriate to use quantities that are higher or lower than these, but the skilled person can, by simple experimentation decide on appropriate quantities.

The invention allows the rapid removal from aqueous suspensions of finelydivided and dissolved undesirable substances, and thus the rapid turnaround of settling tanks. The invention therefore also includes the use in the removal of suspended solid matter from an aqueous suspension thereof in a settling tank of at least one poly(alkylene oxide) and at least one material that causes poly(alkylene oxide) to precipitate, these materials being added to the suspension at different times.

The invention additionally provides a settling composition for use with water containing suspended solids, consisting of two individual components adapted to be added separately to water, one component consisting essentially of at least one water-soluble poly(alkylene oxide), and the other component consisting essentially of at least one mineral material that causes dissolved poly(alkylene oxide) to precipitate from water.

It is naturally possible to use in conjunction with this invention other materials used for their known purposes in art-recognised quantities. Such materials include (but are not limited to) fungicides, surfactants, builders, and complexing agents.

The invention is further described with reference to the following nonlimiting examples.

Examplel s

A commercially-available tunnel cleaning agent LICOSIN (trade mark) CTB. 19 from Hydrior AG, Wettingen, Switzerland, was mixed with 0.65 M% of POLYGON WSR 301 a commercially-available poly(alkylene oxide) of Union Carbide.

0.2 g of this blend (tunnel cleaning agent + poly(alkylene oxide)) was added to a mixture of 0.2 g of activated carbon and 100 ml of tap water to simulate a washwater from a tunnel. To a sample of this "washwater" was then added 2 g of a 5% bentonite slurry in tap water (the bentonite used was IBECO (trade mark) MT-X. There was an immediate precipitation and settling of the carbon to give a clear liquid took less than 3 minutes. On the other hand, an untreated "washwater" sample left to settle took more than 6 hours to clear.

Example 2

A washwater sample from a highway tunnel was collected at the sedimentation container on site. To a sample of this washwater was added a slurry of 4 parts (by weight) TIXOTON (trade mark) bentonite in 96 parts tap water, at a rate of 1 g per 100 ml. This addition was followed by an addition of 1 g per 100 ml of a 1% aqueous solution of POLYGON WSR 301 poly(alkylene oxide). Sedimentation time was 5 minutes, compared to 3 days for the untreated washwater.

Claims

Claims: 1. A method of removing a substantial proportion of

finely-divided solids from an aqueous suspension thereof by the steps of (a) placing the aqueous suspension in a settling tank; and (b) permitting suspended solids to settle to give water that is acceptable for recycling; in which there is added to the water at least one water-soluble poly(alkylene oxide) having a weight-average molecular weight of from 100,000 - 8,000,000, and, at a different time from this addition, at least one material that interacts with and causes the poly(alkylene oxide) to precipitate.
2. A method of cleaning accumulated dirt from the walls of road tunnels, comprising the steps of (a) washing the tunnel wall with washwater comprising an aqueous solution of a dirt-removing agent; (b) collecting the used washwater from this process in a settling tank; and (c) permitting suspended solids to settle to give washwater that is acceptable for recycling; in which there is added to the washwater at least one water-soluble poly(alkylene oxide) and, at a different time from this addition, at least one material that interacts with and causes the poly(alkylene oxide) to precipitate, at least one of these additions being during or after collection thereof.
3. A method according to claim 1, in which the material that interacts with the poly(alkylene oxide) is a mineral, preferably a finely-divided clay mineral, most preferably bentonite.
4. Use in the removal of suspended solid matter from an aqueous suspension thereof in a settling tank of at least one poly(alkylene oxide) and at least one material that causes poly(alkylene oxide) to precipitate, these materials being added to the suspension at different times.
5. Use in the settling of suspended dirt in washwater from the washing of road tunnel walls in a settling tank of at least one poly(alkylene oxide) and at least one material that causes poly(alkylene oxide) to precipitate, these materials being added at different times, at least one to the washwater during or after collection.
6. A settling composition for use with water containing suspended solids, consisting of two individual components adapted to be added separately to water, one component consisting essentially of at least one water-soluble poly(alkylene oxide), and the other component consisting essentially of at least one mineral material that causes dissolved poly(alkylene oxide) to precipitate from water.