WO2004085026A1

WO2004085026A1 - Method for treating a filter and a control method therefore

Info

Publication number: WO2004085026A1
Application number: PCT/SE2004/000465
Authority: WO
Inventors: Lennart Olausson
Original assignee: Lennart Olausson
Priority date: 2003-03-27
Filing date: 2004-03-26
Publication date: 2004-10-07
Also published as: SE0300853D0; SE526517C2; SE0300853L

Abstract

A method for treating a purification filter (30a, 30b, 30c) for bathwater in a baths establishment, which purification filter has a water supply conduit (31a, 31b, 31c) for water to be purified and a water discharge conduit (35a, 35b, 35c) that leads purified water to a swimming pool in the baths establishment, in which method a disinfecting agent is supplied to said purification filter. According to the invention, said disinfecting agent is supplied.(93a, 93b, 93c) to said water supply conduit while the water supply conduit and the water discharge conduit are both open for water running through the purification filter, until said disinfecting agent has penetrated essentially all the way through the purification filter, where after the supply of disinfecting agent is stopped and the water supply conduit and the water discharge conduit are closed of water running through and the disinfecting agent is allowed to act on the purification filter. The invention also relates to a control method for the method.

Description

METHOD FOR TREATING A FILTER AND A CONTROL METHOD THEREFORE

TECHNICAL FIELD

The present invention relates to a method for treating a purification filter for bathwater in a baths establishment, which purification filter has a water supply conduit for water to be purified and a water discharge conduit that leads purified water to a swimming- pool in the baths establishment, in which method a disinfecting agent is supplied to the purification filter.

PRIOR ART

Filters and chemicals are used for purification of water in a baths establishments e.g., in order to meet hygienic requirements. Often, the chemicals used for disinfection are chlorine-containing chemicals. The used filters are mostly sand filters of a certain bed height. As chlorine-containing chemicals for the purification of the bathwater cause troubles both to bathers and employees at baths establishments, there has been a strive to avoid the use of chlorine. An alternative to chlorine-containing chemicals is to use hydrogen peroxide as a chemical added for disinfection of the bathwater. This chemical is considered by the bathers and the employees to be very positive, i.e. it has not the disadvantages of the chlorine-containing chemicals. The hydrogen peroxide is less aggressive.

As hydrogen peroxide is a less aggressive chemical, purification problems will however arise in an establishment that solely uses hydrogen peroxide to purify the water. In time, the consumption of hydrogen peroxide will increase in order to purify a constant amount of water in the establishment. Also, the sand filters will accumulate organic material that the hydrogen peroxide cannot handle fully.

The sand filters in a water purification plant are cleaned by back-flushing. In the back- flushing, a large flow of water is used. As the hydrogen peroxide cannot split all the organic substances, the back-flushing cannot completely remove them from the sand filters. This leads to accumulation of the organic substances in the sand filters, and then bacteria may grow on these organic substances. This will become a problem firstly since the amount of hydrogen peroxide required to purify the water increases, and secondly since there is a risk that the purification will not meet the strict requirements put by the authorities on water quality in the baths establishment. In order to manage an establishment in which accumulation of organic material is anticipated, so called shock-chlorination is used. The Public Health Authorities prescribe shock-chlorination, and it should be performed in all baths establishments independent of which chemical is used to purify the water, i.e. establishments using chlorine should also undergo shock-chlorination regularly.

In establishments using hydrogen peroxide, shock-chlorination of the establishment has been the method used in order to manage the growth of organic material in the filters and to keep down consumption of hydrogen peroxide in the establishment.

Shock-chlorination is performed during the time of day when the baths establishment is closed for bathing and it involves a considerable increase of chlorine content in the water. Having to shock-chlorinate an establishment leads to an increased consumption of chlorine, which means increased costs, especially if it has to been done more often than prescribed by the authorities.

DE 100 10255 discloses a method for disinfecting a filter particle bed, in which a chlorine oxide solution and/or halogen and/or peroxide is allowed to act on the filter for a certain time period, where after reaction products are removed. In the patent application, the method is exemplified by the filter first being flushed by water and then emptied. Thereafter, it is filled from below by the disinfecting agent and water, until the filter is full, and the agent is allowed to act, preferably for at least one hour. Water- flushing of the filter then takes place by a flow of 40 m³/h in case of a filter with the diameter of 2.1 m.

Hitherto, no method is known that efficiently and specifically cleans the sand filters in a baths establishment that uses hydrogen peroxide for the purification of the bathwater, so that shock-chlorination need not be done more often than prescribed by law.

BRIEF ACCOUNT OF THE INVENTION

The object of the present invention is to eliminate, or at least minimize, the above problems, which is done by way of a method according to the introduction, in which during a period of time a disinfecting solution is supplied to an inlet for said purification filter, so that the water in said filter is largely replaced by said disinfecting solution. More specifically, the disinfecting agent is supplied to the water supply conduit while the water supply conduit and the water discharge conduit both are open for water running through the purification filter, until said disinfecting agent has penetrated essentially all the way through the purification filter, where after the supply of disinfecting agent is stopped and the water supply conduit and the water discharge conduit are closed for water running through and the disinfecting agent is allowed to act on the purification filter.

One of the major advantages of the present invention is that the sand filters are thoroughly cleaned, so that the hydrogen peroxide consumption for the purification of bathwater in the baths establishment may be kept at a low and stable level, and that the bathers will not get in contact with the chlorine used for cleaning. The thorough disinfection is enabled by the filter never being emptied of liquid, since emptying of the filter followed by filling with new liquid (comprising disinfecting agent), according to DE 100 10255, will lead to up-come of air pockets in the particle bed, in which air pockets no disinfecting is achieved. In the inventive method however, a displacement of the normal water content in the filter, i.e. the water content that is in the filter during normal water purification operation, by water comprising disinfecting agent, takes place, which secures that the disinfecting agent is well distributed over the entire filter volume and thereby is brought to act in all spaces between the particles.

Another major advantage of the present invention is that the invention enables use of hydrogen peroxide to purify the bathwater in establishments having larger water volumes than hitherto has been managed by hydrogen peroxide. By the invention, the total cost for purification of a baths establishment is lowered, which baths establishment makes use of hydrogen peroxide for the purification of the bathwater.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in greater detail, with reference to the drawings, of which:

Fig. 1 schematically shows a flowchart for a baths establishment,

Fig. 2 specifically shows inlets and outlets for the sand filters in the baths establishment.

DETAILED DESCRIPTION

Fig. 1 schematically shows a flowchart for a baths establishment. From a pool 10, the water is led via a conduit 11, to an equalizing tank 20. From the equalizing tank 20, the water is conveyed in a conduit 22 by aid of a pump 21, to a number of sand filters 30a, 30b, 30c, connected in parallel. From the sand filters 30a, 30b, 30c, the filtered water is conveyed in conduit 40, to a branch point 41. In the branch point 41, a part flow of the filtered water is led to a conduit 42, and in this conduit 42 hydrogen peroxide is added at a point 43 in the conduit 42, and a pH-adjuster, such as acid, is added at a point 44 of the water conduit 42. Thereafter, the water in conduit 42 flows to the reunion point 45 in which the water, comprising pH-adjuster and hydrogen peroxide, is blended with the water in conduit 0. Thereafter, a conduit 6 leads from the reunion point 5 to the pool 10, and supplies the filtered water including added pH-adjuster and hydrogen peroxide, to the pool.

If the pH- value is to be lowered, the pH-adjuster may for example be an acid or carbon dioxide. If the pH-value is to be increased, the pH-adjuster may for example be sodium hydroxide or soda.

From the conduit 40, between the branch point 41 and the reunion point 45, a conduit 51 conveys a part flow of the water to a heat exchanger 50. The water from the heat exchanger re-enters the conduit 40, via a conduit 52.

In the point 53, a conduit 54 exits from conduit 40, and in this conduit 54 a small amount of water is led to analysis. In the analysing unit 60, an analyzer and measuring instruments operate, and send their analysis data to a computer in which the analysis data is treated (not shown).

At all or some of the inlets and outlets for the filters a water flow can be taken out, so that measuring instruments (not shown) can register and analyse substances entering and leaving the filters, which substances are used for disinfecting the bathwater and filters.

At points 43 and 44, hydrogen peroxide and pH-adjuster, respectively, are supplied to conduit 42. The hydrogen peroxide is kept in a container 70 and is conveyed via a conduit 71 to the point 43, where the hydrogen peroxide enters the part flow in the conduit 42. In the point 44 a pH-adjuster is added to control the pH-value of the water. The agent is kept in a container 80, from which it is conveyed in a conduit 81 to point 44, where the pH-adjuster enters the part flow in conduit 42. The added pH-adjuster may for example be hydrochloric acid. It is also conceivable to use carbon dioxide, i.e. carbonic acid, to lower the pH-value of the bathwater.

Suitably, a container 90, in which a chlorine-containing compound is kept, is arranged in connection with the container 80 for the pH-adjuster and the container 70 for hydrogen peroxide. When shock-chlorination is to be done, as prescribed by the authorities, the chlorine-containing substance is supplied via a conduit 93 to the points 94a, 94b, 94c on the water supply conduits 3 la, 3 lb, 3 lc, and this is done while the baths establishment is closed for visitors. As this is done, there is naturally no supply of hydrogen peroxide from the container 70. Supply of the chlorine compound continues until a desired concentration of chlorine is reached in the bathwater. For example, a surplus of chlorine of 5-20 ppm can be attained in the bathwater. After the chlorine having been allowed to act for a night e.g., for 6 to 12 hours e.g., hydrogen peroxide is again supplied from tank 70, via the conduit 71, to the part flow in conduit 42. When an adequate amount of hydrogen peroxide has been supplied to the water, the chlorine and smell of chlorine finally disappears from the bathwater, and the baths establishment is again purified by hydrogen peroxide only.

When, according to the invention, a chlorine-containing compound is to be supplied to a sand filter, the conduit 93 that goes from the container 90 to the points 94a, 94b, 94c on the water supply conduits 3 la, 3 lb, 3 lc is used again.

In Fig. 2, the sand filters are shown in detail, and the method of supplying the chlorine- containing compound to the sand filters will be described in greater detail with reference to this figure.

Fig. 2 shows a detail enlargement of the three filters 30a, 30b, 30c. Liquid leaves the equalizing tank 20 in conduit 22 that branches into the three water supply conduits 31a, 3 lb, 3 lc that lead to the respective sand filter. Filter 30a has two supply conduits 32a, 33a and a first 35 and a second 34 discharge conduit. A water supply valve la and a back-flushing valve 3a are arranged on inlet conduits 32 and 33, respectively. A first water discharge valve 2a and a second discharge valve 4a are arranged on the discharge conduits 35a and 34a, respectively. When the filters are in normal operation, water is pumped in conduit 22, via water supply conduits 3 la, 3 lb, 3 lc, to the filters 30a, 30b, 30c. At this time, valves la and 2a are open and valves 3a and 4a are closed. The water discharge conduits 35a, 35a, 35c are reunited and the water from the filters flows on in conduit 40. Valves 5a, 5b, 5c are arranged on conduits 93a, 93b, 93c. These valves are closed during normal operation. Sand beds is arranged inside filters 30a, 30b, 30c, through which sand beds the water entering in conduits 31a, 32b, 32c is filtered. The area of a sand bed in a filter may vary depending on the volume of the swimming-pool, or -bath, and the number of filters. Usually the sand has a height of between 0.7 and 1.2 m. The filters operate in parallel when they are in filtering phase. When for example an analysis of the bathwater shows that the filters need cleaning, a back-flushing takes place. Back-flushing or so called backwashing takes place for one filter at a time. If for example filter 30a is to be back-flushed, valves la and 2a are closed. Valve 3a and valve 4a are opened. Then, water in water supply conduit 3 la is led into the sand filter via valve 3 a, at a flow of 40 m³/h/m² filter area. As this water flows into the lower part of the filter, the sand bed will be exposed to a through water- flow that is opposed to the flow at normal operation, whereby the sand filter is raised and particles and organic substances can escape the sand and follow the water out into the second discharge conduit 34a and into the sewer. The back-flushing is done for between 3 and 6 minutes. The length of the time period for the back-flushing is determined from the flow through valve 3 a among other things. Thereafter, valves 3 a and 4a are closed and valves la and 2a are opened, where after the filter 30a can operate again. Thereafter, the same back-flushing process can be repeated for filters 30b and 30c, one at a time.

If a baths establishment only uses hydrogen peroxide and the back-flushing of the filters is performed regularly, organic material will accumulate in the filters. Bacteria may grow in the organic material, which means that an establishment operating in this way will get an increased hydrogen peroxide consumption over time. By adding a step according to the invention in which chlorine is allowed to act on the filter, the problem of organic material accumulation in the filters is managed.

A chlorination step is added before the back-flushing, which chlorination step takes place in the following way. When the filter 30a is in normal operation, i.e. valves la and 2a are kept open, valve 5a is opened, whereby chlorine flows into water supply conduit 31a and into filter 30a.

By precise measuring of flows (volume/time unit) and times for the filters 30a, 30b, 30c at trimming of the establishment, values can be calculated for the time periods that the valves need to be open in order to supply or discharge a certain volume to/from the filters. This is also true for the supply of chlorine-containing compound via conduit 93, to the water supply conduits 3 la, 3 lb, 3 lc. By then varying the parameters time and flow, the process according to the present invention can be very accurately controlled. A time period for supply of chlorine is between 1 and 4 minutes, preferably between 1.5 and 3 minutes. A normal filter velocity is 15 m³/h/m² filter area. By the flow of chlorine in conduit 93 being known and also the liquid flow in the water supply conduit 31a and 33 a, the process can be precisely adjusted so that the entire filter 30a during a certain time period is filled by chlorine-containing water. When a time required for filling of the filter has elapsed, valves la and 2a are closed. Also valve 5a is closed. Valve 3a, that in normal operation is kept closed, will remain closed. Valve 4a is however opened.

It is also possible to control the process according to the present invention by measuring concentrations by aid of measuring equipment. By for example positioning measuring equipment in the water discharge conduit 35a, at valve 2a, the process can be precisely controlled so that the entire filter 30a is filled by this chlorine-containing water. When the measuring equipment at valve 2a indicates that chlorine-containing water starts to flow out from conduit 35a, valves la and 2a are closed. Also valve 5a is closed. Valve 3a, that in normal operation is kept closed, will remain closed. Valve 4a is however opened.

The chlorine that has entered filter 30a will now start to act on the organic material. A certain gas development takes place, why valve 4a must be open in this step. The chlorine is allowed to act in filter 30a for between 5 and 25 min. Preferably between 10 and 20 min. The length of this time period is adjusted by testing, for an optimal result.

When the chlorination step is finalised, the back-flushing valve 3a is opened and water in water supply conduit 31a flows into filter 30a via valve 3a. As the second discharge valve 4a is open, all chlorine in the filter and killed organic material and killed bacteria that has come loose from the sand, will exit via the second discharge conduit 34a and the second discharge valve 4a and out to the sewer. Hence, this step is identical with an ordinary back-flushing step. The back-flushing step following the chlorination step takes place for an appropriate time period for all chlorine to exit into the sewer. When the appropriate time period has elapsed, valve 4a and 3 a are closed. The back-flushing takes place for at least 3 minutes, preferably between 3 and 6 minutes. A guideline value is that the back-flushing takes place at a filter velocity of at least 30 m³/h/m² filter area, preferably at least 40 m³/h/m² filter area and even more preferred at least 50 m³/h/m² filter area.

Alternatively measuring equipment can be positioned for example at conduit 34a, which equipment detects the content of chlorine in the liquid. When this measuring equipment indicates that all chlorine has passed out from the filter, the back-flushing step can be ended. Valve 4a and valve 3 a are closed.

Thereafter, valves la and 2a and filter 30a can once again operate normally. Now, this cleaned filter will not consume any hydrogen peroxide due to collection of organic material, but will filter the water in an optimal way. Thereafter, chlorination steps can be performed for filters 30b and 30c, one at a time.

The chlorine that is added in the chlorination step for a sand filter according to the present invention, may for example originate from a concentrated solution of sodium hypochlorite. Usually, it is delivered at a concentration of between 10 and 15 %. The amount of sodium hypochlorite added per m² of filter area can be between 1 and 7 L/m² for a time of between 1 and 6 minutes, preferably between 2 and 4 minutes, at a filter velocity of about 15 m³/h/m² filter area.

The water that after the chlorination step exits via conduit 35a and further into conduit 40, will not contain any significant amount of chlorine, since all chlorine has exited via conduit 34a and into the sewer. The bathers in this baths establishment, who potentially are sensitive to chlorine, will accordingly not get in contact with any chlorine, but the water that enters the pool is continuously purified by hydrogen peroxide.

This chlorination step according to the present invention can be compared with e.g. a shock-chlorination. At shock-chlorination, the system operates at a chlorine concentration of e.g. 5-10 ppm and the actual shock-chlorination process continues between 6 and 12 h for example. In the chlorination step according to the invention, for the sand filter, the time of action is certainly shorter, but this is compensated by a higher concentration of chlorine in the blend of bathwater and chlorine solution. Accordingly, the chlorination step according to the present invention is efficient. This is due to the addition and chlorination taking place in the filter itself, in which the organic material is accumulated.

The organic growth, that is accumulated in the sand filters, is e.g. composed of humus substances. The chlorination process according to the present invention, preceding the backwashing, may e.g. be performed as often as a backwashing of a sand filter, i.e. about 1-3 times a week depending on the size of the establishment and the number of filters. This is considered as a conventional value for a conventional establishment. The hydrogen peroxide consumption can be seen as a quality measure for the cleaning process. As organic material starts to accumulate, the hydrogen peroxide consumption increases. A conventional consumption for a conventional establishment is about 3-5 L hydrogen peroxide per day. If an increasing trend of consumption is seen, additional chlorination steps followed by back-flushes can be used. Then, the increasing trend of hydrogen peroxide consumption is stopped and the consumption goes back to normal.

If, for any reason, the consumption increases despite the performed chlorination steps followed by back-flushing, a shock-chlorination of the entire system can be undertaken. This is done at night-time, when no bathers are present in the establishment. In such a shock-chlorination, 190 L of chlorine may for example be added during 2-3 h, which will give a surplus of chlorine of 5-20 ppm in the entire system. This will then run through the system the entire night, so that it is all cleaned through. A few hours before the opening time for the baths, hydrogen peroxide is added. When employees and bathers come to the baths establishment, the water is again free from any irritating smell of chlorine and has a hydrogen peroxide concentration of for example 90 ppm.

In the back-flushing following the chlorination step, the flow in relation to the filter area should be large. The pump that supplies the water into conduit 33a and valve 3a, should accordingly be as powerful as possible. Thanks to the chlorination step according to the present invention, baths establishments of considerably larger pool volumes can be operated with hydrogen peroxide as sole purification chemical, than has hitherto been possible. A previous guideline value has been about 100 m³ limiting water volume of the pool. A volume that is at least 10 times larger is now conceivable for an establishment that uses hydrogen peroxide.

As chlorine is used for the chlorination process according to the present invention, there will of course be a certain cost for that chlorine. However, this cost should be taken in view of the saving that can be done by keeping the consumption of hydrogen peroxide at a low and even level over time. Due to the higher price of the hydrogen peroxide as compared to chlorine, it may be the case that the overall operative expenses for the establishment will not be higher when using the filter chlorination process according to the present invention, but instead be lower.

It is realised that the invention is not limited to the embodiment described above, but can be varied in many different ways. The time of action of the chlorination step of the sand filter can naturally be extended or shortened depending on the design and size of the sand filter. From 5 minutes and up to half an hour is conceivable for a chlorination step. Furthermore, it is conceivable to use different concentrations of the added chlorine solution. A solution containing from 1 % of chlorine and up to 15 % of chlorine, can be added.

In the shown embodiment of the present invention hypochlorite, containing hypochlorite ion CIO^", is used. It is also conceivable to use other oxohalogenic acids, such as sodium chlorite, containing the chlorite ion CIO2^"1. If appropriate, it is also conceivable to use other chlorine-containing compounds in the present invention. It is also conceivable to use other halogens, i.e. bromide and iodide in suitable compounds, such as hypobromite ion (BrO^") or hypoiodite ion (IO^") or other bromide or iodide compounds, if economically and chemically feasible.

It has proven practicable to supply the chlorine-containing solution to the conduit that exits the pool via the equalizing tank, since the pressure in conduit 3 la very efficiently supplies the chlorine to the filter 30a. It is however and naturally conceivable to add the chlorine-containing liquid at some other point than the one in the shown embodiment.

In the embodiment shown above, the chlorine is added in a chlorination step and thereafter a back-flushing takes place. It is of course conceivable also to shift this sequence in order e.g. to save chlorine. It is then conceivable first to back-flush, to remove the organic material that can come loose from the sand filter without addition of chlorine. As this first back-flushing is completed, water is once again taken in and is then blended with a chlorine-containing solution and the chlorination step is fully executed. Thereafter, a new back-flushing takes place that removes the organic material having become loose and liberated by the chlorination step. By doing in this way, i.e. by positioning the chlorination step between two back-flushes, it may be possible to lower the concentration of the chlorine-containing solution and thereby to lower the consumption of chlorine.

The chlorination step makes use of an ejector, whereby the water at a pressure of about 4-5 bar in the conduit brings the chlorine-containing solution with it. Other arrangements to add the chlorine-containing solution to the water in the conduit, are of course also conceivable. The chlorine-containing solution can be pumped in, or a mixing device can be constructed, in which the two solutions are thoroughly mixed. If a filter should prove to be very hard to clean, a conceivable alternative is that also a first part of the back-flushing takes place by water in the conduit, mixed with the chlorine-containing solution. Before the filter is put in operation, conduit water without chlorine must however be back-flushed through the and filter in order to remove all chlorine via conduit 34a.

The pump used for the back-flushing is also used to pump the liquid in conduit 22. It is also conceivable to use a particular pump only for the back-flushing, in order to get as large a flow as possible and thereby to remove as much organic material as possible at the back-flushing.

It is furthermore conceivable to use a pump that enables the chlorination step, i.e. the step in which the chlorine-containing solution is mixed into the water in the conduit and is supplied to the filter, to be performed at elevated pressure. Thereby, it is ensured that the chlorine is efficiently supplied to the sand filter.

Claims

1. Method for treating a purification filter (30a, 30b, 30c) for bathwater in a baths establishment, which purification filter has a water supply conduit (3 la, 3 lb, 3 lc) for water to be purified and a water discharge conduit (35a, 35b, 35c) leading purified water to a swimming-pool in the baths establishment, in which method a disinfecting agent is supplied to said purification filter, ch aracteri s ed in that said disinfecting agent is supplied (93 a, 93b, 93 c) to said water supply conduit while the water supply conduit and the water discharge conduit both are open for water running through the purification filter, until said disinfecting agent has penetrated essentially all the way through the purification filter, where after the supply of disinfecting agent is stopped and the water supply conduit and the water discharge conduit are closed for water running through and the disinfecting agent is allowed to act on the purification filter.

2. A method according to claim 1, chara ct eri s e d i n that hydrogen peroxide H₂O₂) is used to purify the bathwater in the swimming-pool.

3. A method according to claim ^ ch aracteri s e d i n that the disinfecting agent is a chlorine-containing compound.

4. A method according to claim 3, chara cteri s e d i n that the chlorine- containing compound is a hypochlorite solution (CIO^").

5. A method according to claim 4, characteri s ed in that the hypochlorite solution has a concentration of between 1 and 15 %, preferably between 2 and 14 %.

6. A method according to any one of the preceding claims, ch ara ct eri s ed i n that the disinfecting agent is allowed to act on the filter for 2-25 min, preferably 10-20 min.

7. A method according to any one of the preceding claims, ch ara ct eri s ed i n that a gas that is formed as the disinfecting, preferably chlorine-containing, agent acts on the purification filter, is allowed to leave via a second discharge conduit (34a) of the purification filter, which second discharge conduit does not lead to said swimming-pool.

8. A method according to any one of the preceding claims, ch aract eri s e d in that the disinfecting agent, after its time of action, is removed from the purification filter by back-flushing the purification filter.

9. A method according to claim 8, characteri s e d in that a back-flushing flow of at least 30 m³/h/m² filter area is used, preferably at least 40 m³/h/m² filter area and even more preferred at least 50 m³/h/m² filter area.

10. A method according to claim 8 or 9, ch aracteri se d i n that the disinfecting, preferably chlorine-containing, agent is removed from the purification filter via a second discharge conduit (34a), which second discharge conduit does not lead to said swimming-pool.

11. A method according to any one of claims 8-10, ch aracteri s ed i n that the disinfecting agent is removed from the purification filter in such a way that essentially no disinfecting agent enters the swimming-pool.

12. A control method for a method according to any one of the preceding claims, characteri s e d in that

- said disinfecting agent is added to the water supply conduit (3 la, 3 lb, 3 lc) of the purification filter (30a, 30b, 30c), while a water supply valve (la) and a water discharge valve (2a) are kept open for a first time period,

- the supply of the disinfecting agent is stopped and the water supply valve (la) and the water discharge valve (2a) are closed as the first time period has come to an end and said disinfecting agent has penetrated essentially all the way through the purification filter, - a second discharge valve (4a) is opened and is kept open for degassing during a second time period while the disinfecting agent is allowed to act on the purification filter,

- where after, as said second time period has come to an end, a back-flushing valve (3 a) is opened for flushing the purification filter by a flushing liquid during a third time period, while said second discharge valve (4a) is open for discharge of said flushing liquid, and that

- said back-flushing valve (3a) and said second discharge valve (4a) are closed as the third time period has come to an end,

- where after the water supply valve (la) and the water discharge valve (2a) are reopened.

13. A control method according to claim 11, characteri se d in that said first and third time periods are determined based on liquid flows through the water supply valve (la) and back-flushing valve (3a), respectively, and based on the volume of the purification filter.

14. A control method according to claim 11, characteri se d in that said first time period is determined by detecting appearance of said disinfecting agent in the water discharge conduit (35 a, 35b, 35 c), preferably by measuring concentration.

15. A control method according to any one of claims 11-13, characteri sed in that said second time period is determined empirically.