EP0956270A1

EP0956270A1 - Method of disinfecting water

Info

Publication number: EP0956270A1
Application number: EP96940206A
Authority: EP
Inventors: Lennart Olausson
Original assignee: Individual
Current assignee: Individual
Priority date: 1995-11-23
Filing date: 1996-11-20
Publication date: 1999-11-17
Also published as: JP2000500395A; AU702884B2; NO982317D0; NO982317L; AU7715096A; WO1997019025A1; CA2236375A1

Abstract

The invention relates to a method of disinfecting water which has a pH value >/=7.8, H2O2. The characteristic is that the pH value is adjusted to a value <7.8 and that hydrogen peroxide is added to the water at the said lower pH value in order to oxidize, without presence of free active chlorine, any micro-organisms and other organic matter existing in the water.

Description

METHOD OF DISINFECTING WATER

TECHNICAL FIELD The invenuon relates to the water treatment technique and especially to a method of disinfecting water intended for baths, particularly for public baths, at which the water has a pH value >_7.8 before the disinfection. However, in a wider perspective, it is possible that the invention can also be used to disinfect drinking water.

BACKGROUND OF THE INVENTION

In general the disinfectant that is used for bathwater and drinking water is chlorine or certain active chlorine compounds. The chlorination is also used to facilitate the purification ofthe water by oxidation. In general chlorine gas, sodium hypochlorite, NaCIO, or calcium hypochlorite, Ca(ClO)₂ , is used. Either chlorine gas, sodium hypochlorite or calcium hypochlorite is used, an equilibrium is reached in the water between the reactive species hypochlorous acid, HC1O, and hypochlorite ions, CIO^", that both are so called free active chlorine.

It is widely known that chlorination has a number of negative side effects: undesirable organic chlorine compounds are formed when free active chlorine reacts with organic compounds in the water, the chlorine can have an irritating effect on sensitive persons and chlorine has an unpleasant smell. Furthermore the handling of chlorine or active chlorine compounds is very dangerous for the staff who operate the purification plant. Despite these disadvantages chlorination has been considered to be irreplaceable for d sinfection of bathwater and drinking water.

However, compounds and techniques have been developed to reduce the amount of added chlorine or chlorine compounds. Chlorination has e. g. been combined with addition of oxidizing agents. Among other things ozone and hydrogen peroxide have been used in combination with chlorine dosage. Furthermore UV-radiation in combination with addition of chlorinating agents have been used.

Since at least ten years another method in which is used hydrogen peroxide in combina¬ tion with UV- radiation, is also known.This method, however, only has a limited disinfection potential, that among other things is shown in growth of organic matter on filters and in the boundary layer between water and air. Other inconveniences that contribute to the fact that this method has not come into general use, are its high need of labour input and its high costs for installation and operation.

BRIEF DISCLOSURE OF THE INVENTION The purpose ofthe invention is to offer a method to disinfect water that has enough capacity of disinfection, that does not have high installation and operating costs and that does not require any contribution of chlorine or active chlorine compounds. This can be attained by the invention by adjusting the pH value to a value < 7.8, suitably < 7.6 and preferably <7.5, and by adding hydrogen peroxide to the water in order to oxidize existing micro-organisms and other organic matter in the water at the mentioned lower pH value without presence of free active chlorine. Preferably the hydrogen peroxide is added in at least stoichiometric proportion to oxidize existing micro-organisms and other organic matter in the water. The pH value should not be lowered so much that the water gives an acid reaction. The lower pH limit is therefore set to 7.0, preferably to 7.1. The pH value should be adjusted to some value between 7.1 and 7.5, preferably between 7.3 and 7.4. The pH value is suitably adjusted by adding CO₂ΌΓ carbonic acid.

It is also suitable that the alkalinity ofthe water is kept at a value between 50 and 100, suitably at a value between 60 and 80 mg HCO₃" / 1 water, in which case the method of the invention is used for disinfection of water in a swirnming-pool.

The pH value and the alkalinity can be adjusted to the desired levels by adjusted addition of CO₂ (herein is in the following also included carbonic acid) and H₂O₂ , that preferably is added into a pipe parallel to the main pipe in a circular flowpath, that also includes the swimming-pool. If only CO₂ is added, the alkalinity expressed as the amount of HCO₃ ^" / 1 water decreases. On the other hand if both CO₂ and H₂O₂ are added at the same time into the pipe, HCO₃ ^" will be produced, i.e. the alkalinity increases. For the best control and regulation of both pH value and alkalinity it is appropriate that CO₂ , H₂O₂ and CO₂ + H₂O are added at different periods that do not overlap. A control sequence can thus comprise a first period when CO₂ but no H₂O₂ is added to the pipe, a second period when H₂O₂ but no CO₂ is added into the pipe and a third period when both CO₂ and H₂O₂ are added into the pipe.

In the case the invention refers to disinfection of water in baths, water is continuously lost by evaporation. This water, together with other water that can vanish or divert from the system, is compensated by fresh water. Normally fresh water contains metals which can be in a soluble state as metal ions or as complex ions. If these metals are not removed before fresh water is added into the water in the pool, the metals will gradually be accumulated in the water. Some metals, as aluminium, copper, iron and manganese, can have a great disturbing effect on the disinfecting process by stimulating the decomposition ofthe hydrogen peroxide, i.e. that it does not fulfil the purpose, or the amount of added H₂O₂ has to increase considerably to have the desired purification effect. To avoid these problems, incoming fresh water should be purified from such metals which can decompose the hydrogen peroxide. The mcoming fresh water should be cleaned so that the water in the pool will contain maximum 0.20 ppm Al, preferably maximum 0.09 ppm AI maximum 0.10 ppm and preferably maximum 0.005 ppm of each ofthe metals Cu, Fe and Mn. The purification ofthe incoming fresh water from the mentioned metals can be carried out by an ion exchanger.

BRIEF DESCRIPΗON OF DRAWING

The drawing shows schematically the water purification system for a swimming-pool that works in accordance with the method ofthe invention.

DESCRIPTION OF A PREFERRED EMBODIMENT

In the drawing a swimming-pool is designated 1. From the bottom ofthe pool an outlet waterpipe 2 leads to a coarse screen 3. From the pool 1 also surface water is directed through a pipe 4 to a compensation tank 5 and from there through a pipe 6 to the aheady mentioned outlet pipe 2, after which the joined flows are directed to the coarse screen 3. Fresh water can be directed through a pipe 7 to the compensation tank 5. In the pipe 7 there are two ion exchange columns 30 and 31 connected in series of which one is an anion exchange column and the other a cation exchange column, to remove aluminium, copper, iron and manganese from the water.

From the coarse screen 3 the water is circulated further by means ofa pump 10 to three filters 11, 12, 13 connected in parallel and through these. After the filters 11, 12, 13 the liquid flow is divided into two flows. One by-pass that represents 5-25% ofthe total circulating flow is led through a branch pipe 14 while the main part is directed through a mainpipe 15 which is parallel to the branch pipe 14. Both flows in the pipes 14 and 15 are joined in an inlet pipe 16, through which treated water is directed into the pool 1. For measurement ofthe content of hydrogen peroxide and the pH value ofthe water in mainpipe 15, a very little flow is at the measuring points directed through a pipe 17 for analysis of H₂O₂ in the measure equipment 18 and for analysis of pH in the measure equipment 19 and then back to outlet pipe 2. The branch pipe 14 goes through a safety cabin 21. In this safety cabin there is one or several containers 22 containing H₂O₂. From a container 22 hydrogen peroxide is dosed into the water in branch pipe 14 by means ofa pump 32 which pumps hydrogen peroxide from the container 22 and directs it into the branch pipe 14 at a point in the safety cabin 21. After, before and/or simultaneously with the addition of hydrogen peroxide, carbon dioxide is added to the water in the branch pipe 14 from one or several carbon dioxide bottles 24 via a pipe 25 with a valve 34.

The measurement test results ofthe H₂O₂ and the pH registered by the measuring equipment 18 and 19, are processed in a computer 33 in accordance with a control program that has been programmed into the computer for controlling the pump 32 for supply of H₂O₂, as well as the valve 34 for supply of CO₂ to the branch pipe 14. A control sequence can consist ofthe following parts. First the valve 34 is opened for adding of CO₂ and is kept open during a certain period of time, depending on pH value registered by the measuring equipment 19, after the vah/e 34 once again is closed at command from the computer 33. The pump 32 is during this first period inactive. Then the pump 32 starts and a certain amount of H₂O₂ is pumped into the pipe 14 from the bottle 22 depending on the content of H₂O₂ registered in the measuring equipment 18 and in accordance with the program in the computer 33, after which the pump 32 is stopped once again. During the third period the pump 32 is once again active and is pumping H₂O₂ from the container 22 into the pipe 14, simultaneously as the valve 34 is open, so that also CO₂ is led into the pipe 14 to rise the alkalinity to the desired level. Alternatively the second and third period can follow directly after each other, i. e. the pump 32 has to work continuously during the mentioned second and third periods. It should be realized that this control sequence can be varied, so during certain control sequences only some ofthe mentioned periods are active. It is also possible during each control sequence to lead in H₂O₂ first and then CO₂. The essential point is that CO₂ and H₂O₂ can be added separately as well as simultaneously for deliberate and controlled mixing of CO₂ and H₂O₂ , so that it is possible to regulate during a control sequence either the pH value, the value of H₂O₂ or the alkalinity.

ACHIEVED RESULTS

The invention has been developed primarily for disinfecting water for public baths. Normally such water has a pH value over 8 and there are pH values right up to about pH 9. In the experiments which the invention is based on, the above described equipment, was used, and the treatment was in the way that immediately is evident from the description ofthe equipment. The volume ofthe pool 1, see figure, was 30 m³. The pH value was lowered by addition of carbonic acid, CO₂ . Before the adjustment of pH the water had a pH value of 8.6. Hydrogen peroxide was added in varying amounts as a disinfectant. In each experiment was investigated how much hydrogen peroxide that had to be added, at different pH values, to keep the concentration of H₂O₂ in the water at 80 - 90 ppm, after it had been contaminated by 30 persons. It was presumed that 30 persons is a big enough population to give an acceptable statistic mean value ofthe whole contamination ofthe water in form of bacteria, other micro-organisms and other organic matter. The results are shown in the following table.

pH value after adding CO₂ Added amount of H₂O₂, litres

8.2 40

7.8 15

7.6 4.2

7.30-7.40 - 2.1 7.28 4.2

It is evident from the table that the capacity ofthe hydrogen peroxide to oxidize organic matter in water and thus its capacity to destroy micro-organisms, in other words its disinfecting capacity, was strongly dependent on the pH value. While 40 litres of H₂O₂ were required to maintain a concentration of H O of 80 - 90 ppm in the water at pH

8.2, which indicates a very high consumption of H₂O₂ to maintain the intended oxidation effect, only approximately 2.1 litres of H₂0₂were required to maintain the same effect at pH 7.30 to 7.40. At pH values under 7.30 the filters started to silt up with organic matter, which required increased addition of H₂O₂.

Successful operation tests with a lowering ofthe pH-value in bathwater to approximately 7.4 by adding CO₂ and using H₂O₂ as the only disinfectant, have verified the reliability of the method. The optimum pH in the plant in question was 7.37. The rehability ofthe method indicates that the concentration of H₂O₂ in water can be kept at a level lower than 80 ppnx According to this conceivable development H₂O₂ is added in such amount that the concentration of H₂O₂ in water is maintained within the interval 10 - 70 ppm, preferably within the interval 10 - 50 ppm.

It should be realized that the invention is not limited to the use of CO₂ or carbonic acid to lower the pH-value. Other acids may probably also be used. However, other oxidation agents, e.g. ozone, should not supplement the addition of hydrogen peroxide, since it can disturb the integrated process. On the other hand nothing obstructs that e.g. UV- radiation in a manner known per se, supplements the disinfection by means of hydrogen peroxide and eventually other chemical disinfectants, provided that chlorine or chlorine compounds are not used. It can also be suitable to add chemicals directly to the swimming-pool 1, e.g. sodium bicarbonate, to increase the alkalinity in the water in question, which does not contain active chlorine. This can simply be made by throwing the chemicals into the pool at the end ofthe day to let it mix and work during the night. It is suitable that the alkalinity is at alevel corresponding to 60 and 80 mg HCO₃71 water.

Claims

1. Method for disinfection of water that has a pH value > 7.8, characterized in that the pH value is adjusted to a value < 7.8 and that hydrogen peroxide is added to the water at the said lower pH value in order to oxidize, without presence of free active chlorine, any micro-organisms and other organic matter existing in the water.

2. Method according to claim 1, characterized in that the pH value is adjusted to <7.6.

3. Method according to claim 2, characterized in that the pH value is adjusted to

<7.5.

4. Method according to any ofthe claims 1-3, characterized in that the pH value is adjusted to values between 7.0 and 7.8, suitably to values between 7.0 and 7.6, better to values between 7.1 and 7.5 and preferably to values between 7.3 and 7.4.

5. Method according to any ofthe claims 1-4, characterized in that the hydrogen peroxide is added in at least such a stoichiometric proportion to the organic matter, including micro-organisms that exist in the water, to be able to oxidize this matter.

6. Method according to any ofthe claims 1-5. characterized in that the pH value is adjusted by means of carbon dioxide or carbonic acid.

7. Method according to any ofthe claims 1-6, characterized in that hydrogen peroxide, H₂O₂, is added in sufficient amount to maintain a level of H₂O₂ of 50-150 ppm in the water, preferably 80-90 ppm.

8. Method according to any ofthe claims 1-6, characterizedin that hydrogen peroxide, H₂O₂, is added in a sufficient amount, to maintain a level of H₂O₂ of 10-70 ppm in the water, preferably 10-50 ppm

9. Method according to any ofthe claims 1-8, to disinfect water that is circulated in a path in which is included a swimming-pool 1, characterizedin that the hydrogen peroxide is added to the water in a shunt or branch pipe (14) through which is led 5-25% ofthe total water flow to the pool.

10. Method according to claim 9, characterized in that the addition of CO₂ or carbonic acid to the water is done in the same shunt or branch pipe (25) to which the hydrogen peroxide is added.

11. Method according to claim 9, characterized in that the addition of CO₂ or carbonic acid to the water is done in another shunt or branch pipe than to which the hydrogen peroxide is added.

12. Method according to any ofthe claims 9- 11, characterized in that the pH value is measured in the water in the said path at a point after the pool but before the addition of hydrogen peroxide and, whenever appropriate, before the addition ofthe carbon dioxide or the carbonic acid.

13. Method according to any ofthe previous claims, characterized in that the alkalinity ofthe water in the pool is maintained at a value corresponding to a value between 50 and 150, suitably at a value corresponding to-a value between 60 and 80 mg HCO₃- / 1 water.

14. Method according to any ofthe claims 9- 10 or 12-13, characterized in that CO₂ or carbonic acid and hydrogen peroxide are added at the same time into the same pipe leading to the pool in order to increase the alkalinity ofthe water in the pool, so that it is kept at a level corresponding to at least 50, suitably at least 60 mg HCO₃ ^" / 1 water.

15. Method according to any ofthe claims 9-10 or 12-14, characterized in that CO₂ or carbonic acid is added without simultaneous addition of hydrogen peroxide into a pipe leading to the pool in order to decrease the alkalinity so that it is kept at a level corresponding to maximum 100, suitably to maximum 80 mg HCO₃71 water.

16. Method according to the claims 14 and 15, characterized in that CO₂ or carbonic acid, as well as hydrogen peroxide, and also CO₂ or carbonic acid together with hydrogen peroxide are added in said pipe during different periods, so that during each such period regulated amounts of said chemicals are added in order to keep the alkalinity ofthe water at a level corresponding to between 50 and 100, suitably between 60 and 80 mg HCO₃71 water.

17. Method according to the claims 9-16, characterized in that fresh water is added to said path to replace water that leaves the path, including water that evaporates, at which the fresh water, before it is led into the path, is cleaned with reference to at least some ofthe metals belonging to the group of metals consisting of aluminium, copper, iron and manganese.

18. Method according to claim 17, characterized in that mcoming fresh water is cleaned with reference to aluminium, so that the amount of aluminium in the water in the pool does not exceed 0.20 ppm, preferably does not exceed 0.09 ppm-

1 . Method according to claim 17, characterized in that mcoming fresh water is cleaned with reference to copper, so that the amount of copper in the water in the pool does not exceed 0,10 ppm, preferably does not exceed 0.05 ppm-

20. Method according to claim 17, characterized in that mcoming fresh water is cleaned with reference to iron, so that the amount of iron in the water in the pool does not exceed 0.10 ppm, preferably does not exceed 0.05 ppm

21. Method according to claim 17, characterized in that mcoming fresh water is cleaned with reference to manganese, so that the amount of manganese in the water in the pool does not exceed 0.10 pp , preferably does not exceed 0.05 ppm-