PRE-TREATMENT OF SUPERSATURATED WARM WATER

Abstract

Method for desalination of water by reverse osmosis, including a first desaturation step.

Claims

1. Method for desalination of water by reverse osmosis, comprising a desaturation step prior to the reverse osmosis treatment step.

2. Method according to claim 1, wherein said desaturation step is alternatively a decarbonatation step, a softening step or a combination of these two steps.

3. Method according to claim 1, wherein the decarbonatation step is carried out either with lime or with sodium hydroxide.

4. Method according to claim 1, wherein the softening step is carried out alternatively with lime or with sodium hydroxide or with sodium carbonate.

5. Method according to claim 1, wherein a desilication step is carried out simultaneously with said desaturation step.

6. Method according to claim 1, wherein said desaturation step is followed by a step of cooling the water for desalination to a temperature of 40-45° C. when the temperature of said water is greater than 40° C.

7. Method according to claim 1, further comprising, in this order, the following steps: a desaturation step, a filtration step including removal of suspended matter, and subsequently a step of desalination by reverse osmosis.

8. Method according to claim 1, wherein the water for desalination comprises compounds selected from calcium, magnesium, sodium, potassium, carbonates, bicarbonates, chlorides, sulfates or a mixture thereof.

9. Method according to claim 8, wherein the water for desalination comprises compounds selected from calcium, magnesium, sodium, potassium, carbonates, bicarbonates, chlorides, sulfates or a mixture thereof, with a total amount of at least 500 mg/l.

10. Method according to claim 1, wherein the water for desalination further comprises compounds selected from iron, manganese, silica, sulfur or a mixture thereof.

11. Method according to claim 2, wherein the permeate is cooled to a temperature at least less than 45° C.

12. Method according to claim 1, characterized in that the water for desalination comprises radionuclides.

13. Method according to claim 1, characterized in that the permeate of said water for desalination is cooled following its osmosis membrane traversal.

14. Method according to claim 2, wherein the decarbonatation step is carried out either with lime or with sodium hydroxide.

15. Method according to claim 2, wherein the softening step is carried out alternatively with lime or with sodium hydroxide or with sodium carbonate.

16. Method according to claim 3, wherein the softening step is carried out alternatively with lime or with sodium hydroxide or with sodium carbonate.

17. The method of claim 11, wherein the permeate is cooled to a temperature less than 40° C.

18. The method of claim 7, further comprising a desilication step performed simultaneously with the desaturation step.

19. The method of claim 7, further comprising a step of cooling the water to 40-45° C. between the desaturation and filtration steps.

20. The method of claim 18, further comprising a step of cooling the water to 40-45° C. between the desaturation and filtration steps.

Description

EXEMPLARY EMBODIMENT

[0082] A well water has the following characteristics: [0083] pH 6.7 [0084] bicarbonate 190 mg/l HCO.sub.3.sup.− [0085] calcium 130 mg/l Ca.sup.2+ [0086] CO.sub.2 60 mg/l [0087] temperature 60° C.

[0088] Direct passage of the raw water in the cooling tower could result in a loss of up to 60 mg/l of CO.sub.2 during traversal of the tower. Assuming a loss of only 55 mg/l of CO.sub.2, the precipitation potential of the CaCO.sub.3 is approximately 24 mg/l, a part of which will accumulate in the structure of the cooling tower and in the filtration step.

[0089] The degree of conversion in the reverse osmosis step will be limited to 75% (with use of a sequestrant).

[0090] On the other hand, if decarbonatation+partial softening with lime alone is carried out, upstream of the cooling tower, in accordance with the present invention, a dose of 214 mg/l of Ca(OH).sub.2 (lime) will cause the formation of 446 mg/l of CaCO.sub.3 sludge, which will be removed in the form of a suspension in the water.

[0091] The amounts in the water on exit from the clarifying unit will in this case be as follows: [0092] Calcium=75 mg/l in the form of Ca.sup.2+ [0093] Alkalinity of less than 0.6 meq/l (essentially in the form of HCO.sub.3.sup.−) [0094] pH between 8.5 and 9.0

[0095] The water thus treated no longer contains calcium carbonate capable of precipitating in the cooling tower or in the filtration.

[0096] The conversion capacity in the reverse osmosis will be increased to 90%, or even more, if there are no other limiting salts.

[0097] Accordingly, by implementing the desaturation step in accordance with the invention, decarbonatation is complete, but the softening is partial, since the level of calcium is greater than the level of bicarbonate (including the bicarbonate formed by the reaction of the lime with the CO.sub.2).

[0098] If, hypothetically, the reverse case were to occur, with calcium<bicarbonate, it would be possible to achieve complete softening and partial decarbonatation. In that case, it would be desirable to add sodium carbonate in order to continue the softening reaction and to obtain a lower final level.

[0099] By limiting the dose of lime, it is of course possible to carry out reduced diminishment both of calcium and of bicarbonate.

[0100] It is also possible to substitute sodium hydroxide for the lime. This reagent is more expensive, but generates less calcium carbonate sludge for the same result.

[0101] If, hypothetically, desaturation is carried out solely by softening, this softening is performed with a carbonate, generally sodium carbonate.

[0102] It is still possible to carry out dosage with acid upstream or downstream of the cooling tower, in order to adjust the alkalinity or the pH. Here again, the choice will depend on the treatment objectives, and the skilled person will be able to use his or her general knowledge in order to define the optimum conditions.

[0103] The present invention finds its primary application in the treatment of deep natural water which is hot and exhibits a calcium carbonate supersaturation potential. This treatment may be applied for production: [0104] of water intended for human consumption [0105] of water intended for supplying industrial processes, such as washing water, water involved in the production of manufactured products, water intended for feeding boilers, etc. [0106] of water intended for irrigation.

[0107] This invention may also be applied to the treatment of water resulting from an industrial manufacturing process which would bring a calcium carbonate supersaturation potential in a water of more than 40-45° C., if the aim is to recycle the water, recover components from it, or treat it prior to discharge.