WO2006049494A1

WO2006049494A1 - Method of disinfecting water and/or oxidising compounds in water using ozone, with reduced bromate formation in the preparation of drinking water

Info

Publication number: WO2006049494A1
Application number: PCT/NL2005/000774
Authority: WO
Inventors: Alexander Wilhelmus Cornelis Van Der Helm; Patrick Willem Maria Hubertus Smeets; Eric Theodore Baars; Louis Cornelis Rietveld; Johannis Cornelis Van Dijk
Original assignee: Gemeente Amsterdam Waterleidingbedrijf
Priority date: 2004-11-02
Filing date: 2005-11-01
Publication date: 2006-05-11
Also published as: NL1027401C2

Abstract

A method of disinfecting water and/or oxidising compounds in water using ozone in the preparation of drinking water, wherein dissolved ozone is used, and a method of preparing drinking water by disinfecting water and/or oxidising compounds in water using ozone, wherein ozone is dissolved in a first water stream providing a first water stream containing dissolved ozone, which water stream is subsequently mixed with a second water stream to be disinfected and/or oxidised.

Description

Method of disinfecting water and/or oxidising compounds in water using ozone, with reduced bromate formation in the preparation of drinking water

The present invention relates to a method of disin¬ fecting water and/or oxidising compounds in water using ozone in the preparation of drinking water.

Such a method is known, for example, in the field of drinking water preparation.

Disinfecting water and/or oxidising compounds in water is a very important step in the preparation of drinking water. In general practice ozone, more specifically ozone in the gaseous form, is used for this purpose. When treating bromide-containing water such as surface water with ozone, bromate is formed. Bromate is detrimental to health because it is believed to be carcino¬ genic.

It is therefore an object of the invention to pro- vide a method of disinfecting water and/or oxidising com¬ pounds in water, with reduced bromate formation and effective disinfection.

To this end the method according to the invention is characterized, in that ozone as solution is used. In comparison to using the prior art method in which ozone in gaseous form is added to the water stream to be treated, using the method according to the invention advanta¬ geously reduces the formation of bromate considerably, while the disinfection and/or oxidation capacity is comparable to or better than the prior art method.

This is very surprising, especially considering the extensive research that has so far been done into reducing the formation of bromate. The approaches have been of an essentially chemical and hydrodynamic nature, with ozone being introduced into the water in gaseous form. Described are, among other things, the lowering of the pH, the addition of ammonium, the addition of ammonium preceded by chlorina- tion, the trapping of hydroxyl radicals and the trapping of hypobromic acid. Further hydraulic measures that have been examined include the application of phased instead of single- phase bubble columns, counterflow bubble columns instead of bubble columns going with the flow, and employing static mixers instead of bubble columns for dissolving ozone. Previ¬ ous research has lead to the conclusion that with respect to bromate formation, there is only a small difference between the various conventional hydraulic methods, and that the formation of bromate is largely controlled by chemical and not by hydraulic conditions.

Conspicuously, in the hydraulic approaches that have been described for reducing the formation of bromate, gaseous ozone is introduced into the stream to be treated. Even though the problem of bromate has been an issue for a long time, the addition of ozone in solution, as in the method of the present invention, has not been described before as method for reducing bromate formation in drinking water. Adding ozone in solution instead of gaseous ozone and subse¬ quently mixing thoroughly creates far better conditions for mass transfer. The result is that significantly less bromate is formed than with known hydraulic methods, while the disin¬ fection and oxidation capacity is maintained or even im¬ proved. Compared with a chemical approach, the further advan¬ tage is that no chemicals need to be added. It is generally accepted that the addition of chemicals in the preparation of drinking water should be avoided as much as possible.

In a following aspect, the present invention relates to a method of disinfecting water and/or oxidising components in water using ozone, which method is characterised in that ozone is dissolved in a first water stream providing a first water stream containing dissolved ozone, which water stream is subsequently mixed with a second water stream to be disin¬ fected and/or oxidised.

In another embodiment, the method according to the invention is characterised in that prior to dissolving the ozone in the first water stream, the first water stream is pre-treated by totally or partly removing the bromide and/or totally or partly removing organic material and/or lowering the pH.

This has the advantage that because of the reduced bromate formation, a suprisingly much larger amount of ozone can be added having a greater disinfection and/or oxidation capacity without the drawbacks of the (extra) formation of bromate.

In a preferred embodiment of the method according to the present invention, the pre-treatment involves ion ex- change, reverse osmosis, activated carbon filtration, coagu¬ lation, flocculation, or the addition of flocculators, acid or base or a combination of these.

The first water stream constitutes preferably less than 10% by volume of the second water stream. After mixing, the two water streams in the method according to the invention are further preferably introduced into a contact chamber. More preferably, the contact chamber is a pipe with a sluggish flow characteristic.

After mixing with the first water stream, the ozone concentration in the second water stream is preferably less than 10 mg/1.

This provides effective disinfection and/or oxida¬ tion.

In a following aspect, the method according to the invention is characterized in that the water stream in which ozone is dissolved to provide the first water stream, is obtained from the second water stream to be disinfected and/or oxidised.

This has the advantage that no external water stream is required.

In another preferred embodiment, the ozone is dis¬ solved in the first water stream at a pressure of 0-5 bars.

Advantageously, this results in higher ozone concen¬ trations in the first water stream. The present invention further relates to an appara¬ tus for disinfecting water and/or oxidising compounds in water for the preparation of drinking water, which apparatus is characterized by means for dissolving ozone 6, a mixer 8, and a contact chamber 10.

In a preferred embodiment of the apparatus according to the present invention, the means for dissolving ozone 6 is a bubble column.

In a further preferred embodiment of the apparatus according to the invention, the mixer 8 is a static mixer.

In another preferred embodiment of the apparatus according to the invention, the contact chamber 10 features a sluggish flow characteristic.

In still another preferred embodiment of the appara¬ tus according to the present invention, the apparatus is further provided with means for pre-treating water 4.

When using such an apparatus, the allowable ozone dose may, since less bromate is being formed, be surprisingly much higher so as to enhance the disinfection and/or oxidis¬ ing capacity without the drawbacks of (extra) bromate forma¬ tion.

The means for pre-treating water 4 preferably com- prise an ion exchanger, a reverse-osmosis apparatus, an activated-carbon filter, a coagulator flocculator or metering systems for flocculators, acids and bases or a combination of these.

Figures 1, 2 and 3 show schematic illustrations of preferred embodiments of the method according to the inven¬ tion using the apparatus according to the present invention.

The arrow 1 in the Figures 1, 2 and 3 represents the feed stream consisting of the water to be disinfected and/or the water containing the components to be oxidised (second water stream) , the arrow 2 represents the first water stream, the arrow 3 represents an ozonic gas stream, 4 represents means for the pre-treatment of water, the arrow 5 represents the pre-treated first water stream, 6 represents means for dissolving ozone, 7 represents the first water stream con- taining dissolved ozone, 8 represents a mixer, the arrow 9 represents the mixed first and second stream, 10 is a contact chamber and 11 represents an effluent stream. The first water stream 2 in Figure 1 is obtained from the second water stream 1 and is subsequently pre- treated at 4 in one of the ways mentioned above, whereafter ozone is added at 6, yielding a first water stream containing dissolved ozone 7 which, using mixer 8, is subsequently mixed with the second water stream 1 to be disinfected and/or oxidised, to yield a mixed stream represented by arrow 9 that is introduced into contact chamber 10, yielding effluent stream 11. The first water stream in Figure 2 is not obtained from the second water stream but apart from that, the same procedure is followed.

The first water stream in Figure 3 is not obtained from the second water stream and no pre-treatment is carried out but apart from that, the same procedure is followed. The feed stream 1 consisting of the water to be disinfected and/or the water containing the compounds to be oxidised may or may not be pre-treated.

The ozonic gas stream 3 may be ozone in air or ozone in oxygen.

The mixer 8 is preferably a static mixer ensuring immediate and rapid mixing of the stream 7 comprising ozone and the feed stream 1. In a static mixer, the streams are mixed by dividing them several times and rotating them. The mixer may also be a different type of mixer such as, for example, a venturi mixer, agitators or other suitable me¬ chanical or fluid-mechanical mixers.

During mixing, the ozone oxidises a portion of the dissolved organic material and other compounds, and kills microorganisms almost instantaneously.

Example 1

The preparation of drinking water using dissolved ozone, with reduced bromate formation.

The present example elucidates a method represented in Figure 2. The feed stream 1 consisted of naturally fil¬ tered surface water and had a flow rate of 90-95 1/h, a pH of 8, a temperature of 1O⁰C and a bromide content of 160 μg/1. The first stream 2 consisted of drinking water and had a flow rate of 5 to 10 1/h (5-10% of the feed stream 1) . The first stream 2 was pre-treated at 4 by means of reverse osmosis. During the pre-treatment 4, the bromide in the drinking water was removed so that no bromate was formed when ozone was dissolved in the first stream. The conditioned first stream 5 was introduced into a bubble column where gaseous ozone was added. The ozonic gas stream 3 was obtained from medicinal air with an ozone concentration in the gas that varied from 10-50 g/Nm³, depending on the desired ozone dose after mixing. The ozone was dissolved in the conditioned first stream 5 using an ozone bubble column 6, at a pressure of 0.5 bar. The ozone bubble column 6 was a counterflow column whose capacity varied from 70-90%, depending on the ozone concentration in the gas, so that the ozone concentration in the first stream containing dissolved ozone 7 was 10-30 mg/1. The residence time in the bubble column 6 was 20 minutes. The first stream 7 constituted 5-10% of the feed stream so that the ozone concentration in the feed stream after mixing in the static mixer 8 was 0.7-2.1 mg/1. After mixing, the mixed stream containing dissolved ozone 9 flowed directly into the contact chamber 10 with a residence time of approximately 2.5 minutes and featuring a virtually perfect sluggish flow characteris¬ tic. The effluent stream 11 left the contact chamber at least equally well disinfected as when using a conventional ozone installation in which the total main stream was treated in a bubble column, while containing 84% to 92% less bromate than when using the conventional ozone installation, see Table 1. Table 1 also shows that when using the method according to the present invention, an almost double dose of ozone pro¬ duces a bromate concentration that is only half as high as that produced with conventional methods. Table 1. Bromate reduction in method of present invention

0,5 μg/I is the detection limit for bromate

Claims

1. A method of disinfecting water and/or oxidising compounds in water using ozone in the preparation of drinking water, characterised in that dissolved ozone is used.

2. A method of claim 1, characterised in that ozone is dissolved in a first water stream providing a first water stream containing dissolved ozone, which water stream is subsequently mixed with a second water stream to be disin¬ fected and/or oxidised.

3. A method according to claim 2, characterised in that prior to dissolving the ozone in the first water stream, the first water stream is pre-treated to provide a first water stream that contains dissolved ozone.

4. A method according to claim 3, characterised in that the first water stream is pre-treated by totally or partly removing the bromide and/or totally or partly removing organic material and/or lowering the pH.

5. A method according to claim 4, characterised in that the pre-treatment involves ion exchange, reverse osmo¬ sis, activated carbon filtration, coagulation, flocculation, or the addition of flocculators, acid or base or a combina¬ tion of these.

6. A method according to one of the claims 2-5, characterised in that the first water stream constitutes less than 10% by volume of the second water stream.

7. A method according to one of the claims 2-6, characterised in that after mixing, the two water streams are introduced into a contact chamber.

8. A method according to claim 8, characterised in that the contact chamber is a pipe with a sluggish flow characteristic.

9. A method according to one of the preceding claims, characterised in that after mixing with the first water stream, the ozone concentration in the second water stream is less than 10 mg/1.

10. A method according to one of the preceding claims, characterised in that the water stream in which ozone is dissolved to provide the first water stream is obtained from the second water stream to be disinfected and/or oxi- dised.

11. A method according to one of the preceding claims, characterised in that ozone is dissolved at a pres¬ sure of 0-5 bars.

12. An apparatus for disinfecting water and/or oxi- dising compounds in water for the preparation of drinking water, which apparatus is characterized by means for dissolv¬ ing ozone (6), a mixer (8), and a contact chamber (10) .

13. An apparatus according to claim 12, character¬ ised in that the means for dissolving ozone (6) is a bubble column.

14. An apparatus according to claim 12 or 13, char¬ acterised in that the mixer (8) is a static mixer.

15. An apparatus according to one of the claims 12-

14, characterised in that the contact chamber (10) features a sluggish flow characteristic.

16. An apparatus according to one of the claims 12-

15, characterised in that the same, is further provided with means for pre-treating water (4) .

17. An apparatus according to claim 16, character- ised in that the means for pre-treating water preferably comprise an ion exchanger, a reverse-osmosis apparatus, an activated-carbon filter, a coagulator flocculator or metering systems for flocculators, acids and bases or a combination of these.