GB2173818A - Method of and apparatus for treating water - Google Patents

Abstract

A method of treating water, particularly potable water, is provided which is characterised by filling chambers 13,14 separated by a semi-permeable membrane 12 with water 17 containing dissolved impurities and passing electrical current through the water in order to reduce the concentration of undesirable anions (e.g. F or No3) in the water in at least one of the chambers. The water can then be collected from one of the chambers for use. Water treatment apparatus for carrying out the method comprises receptacles 10 and 11, which are preferably made of material which does not conduct electricity. <IMAGE>

Description

SPECIFICATION Method and apparatus for treating water This invention relates to a method of and apparatus for treating water, particularly potable water.

The water supplied through the mains by municipal water authorities can be derived from a variety of natural water sources. Thus, depending on the nature and location of the source in question the untreated water may contain inter alia impurities such as sediment, particulate material, dissolved mineral salts such as nitrates, sulphates and phosphates, dissolved metals such as arsenic, barium, cadmium and lead, and organic contaminants such as detergents and pesticides. For instance, if the water is taken from a river which flows through an agricultural area, the untreated water will contain higher than average amounts of dissolved nitrates and phosphates and pesticides resulting from fertilisers and pesticides used on the agricultural land being washed by rain into the river.

Many of these impurities will be either completely removed or substantially reduced in quantity when the raw water undergoes treatment prior to entering the mains system.

However, some undesirable impurities may remain. Moreover, the actual treatment of the raw water often involves the addition of substances which do not occur naturally in water in any great quantity, such as chlorine or fluoride compounds, and the presence of such substances may in itself be regarded as undesirable. Thus, the water which reaches the consumer through the mains may be unacceptable to the consumer due to the presence of one or more such natural impurities or substances added during water treatment.

One example of a naturally occurring impurity which it may be desirable to remove completely or at least reduce in quantity is dissolved nitrate compounds. As mentioned above, a high concentration of dissolved nitrate compounds is often found in raw water drawn from source in agricultural areas. Moreover, the concentration of dissolved nitrate compounds is substantially unaffected by treatment of the water prior to its entry into the mains system. However, it has been shown that a high concentration of dissolved nitrates in drinking water can be harmful, particularly to babies.

It is also becoming more and more common for water to be fluoridated when it is being treated prior to entering the mains system.

Such fluoridation is carried out on the basis that small quantities of fluoride can strengthen the teeth and bones of an individual. However, fluoridated water generally contains a much higher concentration of fluoride ions than is necessary to achieve the benefit referred to above. Furthermore, it has been shown that such concentrations of fluoride ions in drinking water can, in fact, be harmful in that the presence of fluoride ions can inhibit the activity of enzymes which are essential to the healthy functioning of the body. Also, many people experience allergies which can be traced to the presence of. fluoride in their drinking water.

Water is probably the most essential ingredient for maintaining human life. However, for most people, the only source of potable water available is the mains water supply which may be unacceptable due to the presence of substances mentioned above. Clearly, it is desirable that a person should be able to remove or reduce the quantity of undesirable or potentially harmful substances in at least his drinking water supply if he so wishes.

Whilst there are many different types of water softening system currently available on the market which are capable of reducing the quantity of metal salts which cause hardness in water, there are very few types of water purification system available which can remove or reduce the quantity of undesirable anions, such as nitrates and fluorides, present in water. One water purification system which has this capability is the "Aqua-Cleer" system manufactured by Culligan International Company. This system uses a combination of reverse osmosis and filtration through a fine filter and a carbon adsorption filter to purify the mains water. Other types of water purification system which are capable of removing inter alia undesirable anions generally involve the use of special crystal columns or ion-exchange resins.

However, the above water purification systems are generally expensive to buy because they include specially designed components, such as special membranes, filters and resins, which are expensive to produce. Moreover, since these special components must generally be replaced at regular intervals if the efficiency of the system is to be retained, such systems are often expensive to maintain. A further factor influencing the cost of such systems is the fact that all these systems must be plumbed into the mains water supply of the building where they are to be used thus increasing installation costs. This also restricts the area of use of such systems.It should also be noted that such systems are designed to remove or reduce the quantity of a wide range of impurities in mains water, such as heavy metals, dissolved mineral salts, organic contaminants and materials causing unpleasant odours, whereas the consumer may only be concerned about removing or reducing the quantity of potentially harmful anions, such as nitrates and fluorides. Thus, the consumer may be paying for the provision of purification processes which he does not specifically desire.

It is thus an object of the present invention to provide a method of treating water to reduce the concentration of undesirable anions therein which can be accomplished by means of simple inexpensive apparatus and which can be used in any location.

According to the invention there is provided a method of treating water characterised by filling chambers separated by a semi-permeable membrane with water containing dissolved impurities and passing electrical current through said water in order to reduce the concentration of undesirable anions in the water in at least one of the chambers.

Most commonly the water to be treated will be mains water and the undesirable anions will be nitrate and fluoride ions.

It is preferred that the method also includes the step of collecting water from at least one of the chambers. This may be accomplished, for instance, by siphoning or by removing one of the chambers from the vicinity of the other chamber.

According to a further aspect of the invention there is provided water treatment apparatus for use in the method of the invention comprising first and second chambers separated from one another by a semi-permeable membrane and electrode means associated with each chamber.

The chambers may comprise first and second receptacles in which case it is preferred that one receptacle is located inside the other receptacle and that one receptacle is removable from the apparatus. Alternatively, the chambers may be located in a single receptacle in which case it is preferred that the semipermeable membrane is located so as to divide the receptacle into two portions comprising the chambers. Preferably, the or each receptacle is made of a material which does not conduct electricity.

It is preferred that the electrode means comprises two inert electrodes such as carbon electrodes.

Preferably, the apparatus is portable.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a schematic drawing illustrating one embodiment of the invention; and Figure 2 is a schematic drawing illustrating another embodiment of the invention.

Referring to Fig. 1 of the drawings, water treatment apparatus is shown comprising a receptacle 1 divided by a semi-permeable membrane 2 into two chambers 3 and 4. A carbon electrode 5 is located in chamber 3 and a carbon electrode 6 is located in chamber 4.

In order to carry out the method of the invention, which utilises the principles of electrolysis, water 7 to be treated is first poured into chambers 3 and 4 until a desired level has been reached. Electrodes 5 and 6 are then connected to a d.c. electrical source (not shown) so that electrode 5 becomes a cathode and electrode 6 becomes an anode and the electrical current produced by the electrical source is allowed to flow through the water for a chosen length of time. The current is then turned off and the water in chamber 3 is collected for use, for instance, by siphoning into a storage container (not shown). The water in chamber 4 is then discarded and the apparatus is rinsed out ready for re-use.

Fig. 2 illustrates a second embodiment of the invention which utilises two receptacles 10 and 11, receptacle 10 being larger than receptacle 11. One end of receptacle 11 comprises a semi-permeable membrane 12 and receptacle 11 is located inside receptacle 10 so that the semi-permeable membrane 12 lies nearest to the base of receptacle 10 thus defining two chambers 13 and 14. A carbon electrode 15 is located in chamber 13 and a carbon electrode 16 is located in chamber 14.

In a manner similar to that described with reference to Fig. 1, the method of the invention is carried out by pouring water 17 to be treated into chambers 13 and 14 until a desired level has been reached. Electrodes 15 and 16 are then connected to a d.c. electrical source (not shown) so that electrode 15 becomes a cathode and electrode 16 becomes an anode. The electrical current produced by the electrical source is allowed to flow through the water for a selected period of time and the current is then turned off. The water in chamber 13 is then collected for use by removing receptacle 11 and, if desired, pouring this water into a storage container.

The water in chamber 14 is then discarded and the apparatus rinsed out ready for re-use.

The invention utilises the feature of electrolysis that, when an electrical current is passed through a solution, positive ions (i.e. cations) will migrate towards the negative electrode (cathode) and negative ions (i.e. anions) will migrate towards the positive electrode (anode). In the absence of any barrier, the anions and cations will re-distribute themselves throughout the solution as soon as the current is switched off. However, the provision of a semi-permeable membrane between the two water-containing areas allows the anions and cations to migrate towards the anode and cathode respectively whilst the current is flowing through the solution but inhibits re-distribution of the ions throughout the solution once the current is switched off. Thus, it is possible to harvest water having a reduced concentration of anions from the chamber in which the cathode is located and water having a reduced concentration of cations from the chamber in which the anode is located. Since a reduction in the concentration of potentially harmful anions such as fluoride and nitrate ions is normally desired, the water around the cathode will normally be collected.

Chemical analysis of test samples has shown that a significant reduction in the con centration of fluoride ions in a given sample can be achieved using the method and apparatus of the invention. The tests were conducted in the apparatus of Fig. 2 using a standard solution containing 1.00 mg per litre of fluoride ions as the water sample with the current flowing for periods of 1 hour and 3 hours. The water in the chamber containing the cathode was then collected after the current had been switched off and analysed to determine the concentration of fluoride ions present. The sample taken after 1 hour of operation contained 0.55 mg per litre of fluoride ions and the sample taken after 3 hours of operation contained 0.40 mg per litre of fluoride ions. Thus, a considerable reduction in the fluoride ion concentration was achieved in both cases. Although the concentration of nitrate ions was not determined, it is expected that this would show a similar reduction.

It will of course be understood that the present invention has been described above purely by way of example, and modifications of detail can be made within the scope of the invention. For instance, it is envisaged that at least one of the receptacles could be connected to the electrical source and used as an electrode provided that the or each receptacle is made of a suitable inert material. It is also envisaged that at least one of the electrodes could be combined with the semi-permeable membrane. Moreover, it is envisaged tha a semi-permeable membrane could be used which is designed to selectively allow the passage of certain anions, e.g. fluoride and/or nitrate ions, whilst preventing the passage of other anions.

Claims (18)

1. A method of treating water characterised by: (a) filling chambers separated by a semi-permeable membrane with water containing dissolved impurities; and (b) passing electrical current through said water in order to reduce the concentration of undesirable anions in the water in at least one of the chambers.

2. A method according to claim 1 in which the water is mains water.

3. A method according to claim 1 or claim 2 in which the undesirable anions are fluoride ions.

4. A method according to claim 1 or claim 2 in which the undesirable anions are nitrate ions.

5. A method according to any preceding claim further characterised by collecting the water from at least one of the chambers.

6. A method according to claim 5 in which the water is collected by siphoning.

7. A method according to claim 5 in which the water is collected by removing one of the chambers from the vicinity of the other chamber.

8. Water treatment apparatus for use in a method according to any preceding claim comprising first and second chambers separated from one another by a semi-permeable membrane and electrode means associated with each chamber.

9. Water treatment apparatus according to claim 8 in which the first and second chambers comprise first and second receptacles.

10. Water treatment apparatus according to claim 9 in which one receptable is located inside the other receptacle.

11. Water treatment apparatus according to claim 9 or claim 10 in which at least one receptacle is removable from the apparatus.

12. Water treatment apparatus according to claim 8 in which the first and second chambers are located in a single receptacle.

13. Water treatment apparatus according to claim 12 in which the semi-permeable membrane is located so as to divide the receptacle into two portions comprising the first and second chambers.

14. Water treatment apparatus according to any one of claims 9 to 13 in which the or each receptacle is made of a material which does not conduct electricity.

15. Water treatment apparatus according to any one of claims 8 to 14 in which the electrode means comprises two carbon electrodes.

16. Water treatment apparatus according to any one of claims 8 to 15 which is portable.

17. A method of treating water substantially as hereinbefore described with reference to the accompanying drawings.

18. Water treatment apparatus substantially as hereinbefore described with reference to the accompanying drawings.