WO2009130397A1

WO2009130397A1 - Process and composition for purification of household water

Info

Publication number: WO2009130397A1
Application number: PCT/FI2009/050329
Authority: WO
Inventors: Jani Talasma; Teuvo Kekko; Tomi Korhonen; Pekka LÖNNQVIST
Original assignee: Pac-Solution Oy
Priority date: 2008-04-24
Filing date: 2009-04-24
Publication date: 2009-10-29
Also published as: BRPI0911684A2; CN102046010A; EP2282635A1; RU2010146302A; FI20085364A0

Abstract

A composition for use in purification of household water, such as tap water, drinking water, or bottled water. The composition contains peracid, such as peracetic acid. The composition typically comprises peracetic acid, acetic acid, and hydrogen peroxide. Also a process for purification of household water. In the process, the composition is added to water, e.g., in a water tank, piping, or a drinking vessel. The process may also comprise sedimentation, filtration, or UV disinfection.

Description

PROCESS AND COMPOSITION FOR PURIFICATION OF HOUSEHOLD WATER

FIELD OF THE INVENTION Household water refers herein to all water intended for human use, in particular for human consumption as drinking water.

At present, the use, and thereby the need, of house- hold water is growing rapidly around the world. Efforts are being made to produce household water from less pure water than before using less chemicals, without, however, considerably raising the cost of the purification process. In addition, there is a need for the use of biodegradable or otherwise less harmful chemicals, and also chemicals which would not form compounds having detrimental health effects. In addition, efforts are being made to find chemicals with the least possible amount of taste problems and which would not cause damage in the water system, such as water pipes, water tanks and water towers. In bottled water, partly the same problems and in particular those relating to taste limit the use of chemicals. Purification of raw water to produce drinking water normally includes mechanical filtration, usually biological processing, and adding the water purifying chemicals in different steps of the purification process to enhance the purification and/or to oxidize the organic and inorganic matter and organisms.

When raw water is disinfected to produce household water, new impurities are formed in the water during the purification process, in particular if the raw water contains suitable organic matter to produce the impu- rities, such as humus. Generally, surface water (river and lake water) contains more of such substances than groundwater. However, as the use of household water increases, the limits of using groundwater are met and the need to use the surface, lake and river waters increases. The purer the raw water, the less impurities are produced by use of the purifying chemicals. When household water is disinfected in the conventional manner, i.e. by chlorination, various chlorinated compounds are formed in the water as the chlorine reacts with the organic matter. Drinking water that has been chlorinated from humous surface water may contain hundreds of different chlorinated compounds. If the water contains bromide, also bromated compounds are formed.

Generally in the purification of household water, chlorine and/or its derivatives are used to disinfect the water. However, the use of chlorine as a water purifying chemical does not provide the desired result if the solids content is high. For example mutagenic trihalomethanes and chlorinated furanones are formed in connection with chlorination. These are suspected to cause health risks, for example an increased risk of cancer. The more mutagenic the drinking water, the higher the risk of cancer. Mutagenic chlorinated drinking water has been detected in many countries where the raw water contains suitable organic matter.

As a chemical, chlorine is a toxic and powerful oxidizer and detrimental to the environment. Processing the chlorine/chlorammonia, which are generally used in the purification of household water, is hazardous, and for example their reaction with hydrogen is intense and entails the risk of explosion. Chlorine/chlorammonia contain liquid chorine which evaporates immediately as the pressure is reduced to normal pressure, which in turn irritates the skin and the eyes. Chlorammonia is also harmful when inhaled be- cause liquid chlorine evaporates quickly. At temperatures below 9.6 degrees Celsius chlorine forms crystalline hydrates with water, which is why a low temperature of water restricts considerably the dissolu- tion of chlorine. Another problem is caused by the purification efficiency of chlorine/chlorammonia, and although chlorine reduces the total amount of microbes in the water, it does not, however, destroy all organisms from the water - for example, the giardia para- site has been found to remain in household water after chlorination.

There are also other methods of disinfecting household water, for instance ozonization. The use of ozone in disinfecting household water has increased due to its strong oxidizing power. By the use of ozone it has been possible to reduce the use of chlorine and, at the same time, the formation of the chlorination byproducts. Despite many advantages, ozone also has downsides. Gaseous ozone is a reactive gas, so its use requires precautions in view of occupational and environmental safety. When inhaled, ozone may disable operation of the lungs. Purification by ozone is expensive in terms of investments in the equipment and is therefore not applicable for all uses. The use of ozone is problematic if the raw water contains bromide. As by-product of the purification, ozone oxidizes bromide to form bromate which is a carcinogenic cancer inducing agent.

Hydrogen peroxide is relatively inexpensive in investments and multifunctional when used for disinfecting household water. It provides quickly large amounts of soluble oxygen and has a disinfective effect where needed. Hydrogen peroxide breaks the surface membranes of the bacteria, destroys their attachment points and consumes the protective polysaccharide layer. The organic iron, copper and zinc compounds generally present in the bacteria contribute to this operation through catalyzation. The bacteria are most affected by shock dosing the hydrogen peroxide, but viruses and protozoa normally stay unharmed. However, the use of hydrogen peroxide is restricted by the quickly developing catalase activity and therefore resistance of the microbes to hydrogen peroxide.

ADVANTAGES OF THE INVENTION

The process and composition according to the invention provide a new method of disinfecting and/or purifying water to produce household water, such as tap water, drinking water or bottled water. By the process and composition according to the invention, it is in some cases possible to reduce the employed amount of chlorine and/or ozone and/or hydrogen peroxide, and the amount of the employed chemicals, in particular those which are detrimental to nature. The process and composition according to the invention do not form detrimental compounds to the same extent as some of the presently used processes and chemicals for purifica- tion of water. In some cases, the composition according to the invention purifies the water pipes. When added to water, the composition according to the invention is nearly tasteless and colourless. The composition according to the invention has not been associ- ated with serious health risks, and it is therefore well suited for purifying drinking water, including bottled water. The composition according to the invention is biodegradable.

DESCRIPTION OF THE INVENTION According to the invention, the composition for use in the purification of water contains peracetic acid. The invention is based on the combined activity of peracetic acid and preferably hydrogen peroxide and acetic acid in the purification of drinking water. In the process according to the invention, peracetic acid or a solution thereof is added to water to oxidize and/or remove or neutralize the organic and/or inorganic matter or the matter that is being formed in the water. Peracetic acid is an extremely oxidizing microbicide and in certain conditions more oxidizing than chlorine and bromine. The disinfecting power of peracetic acid is based on quick oxidization of the organic matter, such as the cell membrane and the enzymes of the mi- crobes, whereupon the peracetic acid is able to enter the cell, making it non-viable. The active metabolism of the microbes and hydrogen peroxide facilitate the conveyance of the peracetic acid inside the cell. Inside the cell, peracetic acid oxidizes the vital amino acids, so that the form of the enzymes changes and they loose their activity. Peracetic acid is a hydrogen peroxide derivative and a multifunctional disinfectant; it acts rapidly on the bacteria, yeasts, fungal growth, spores and viruses. The mode of action is oxidative and rapid, so the microorganisms do not have much time to develop resistance to peracetic acid. Peracetic acid is a biodegradable substance and nearly harmless to the environment and, when diluted, it is almost odourless, colourless and tasteless. It is safe to use, because it does not irritate the skin, eyes or respiration in diluted solutions. Peracetic acid has a pH-lowering effect which is one reason for improving the activity of the agent. It is 100% soluble in water, irrespective of the temperature, and it kills the Giardia parasite and the E. coli, Salmonella, and Listeria bacteria. According to a preferred embodiment of the invention, the composition used for the purification of water contains peracetic acid in an amount of 5 to 17% by weight, preferably 5 to 15% by weight, in aqueous solution. This aqueous solution is relatively stable and therefore the peracetic acid may be stored even for long periods of times. In addition, it is relatively easy to transport and process, because this type of a quite diluted aqueous solution does not require the use of transportation containers made from special steel .

According to one embodiment of the invention, the com- position contains hydrogen peroxide. The liquid stabilized peracetic acid is an equilibrium product between peracetic acid, hydrogen peroxide, acetic acid and water. During production of the peracetic acid, acetic acid obtains in the carboxylic acid group one addi- tional oxygen molecule from hydrogen peroxide. As a result, a peroxide group in which the additional oxygen molecule is at a reactive position is formed in the acetic acid. The equilibrium reaction of the product is provided according to the concentration equa- tion.

3 * [hydrogen peroxide] * [acetic acid] = [peracetic acid] * [water] .

Having reached the equilibrium, the mixture remains stable. Among the ingredients in the equilibrium mix- ture, peracetic acid has, besides water, the highest vapour pressure of 1.1...1.2kPa, so it concentrates in an open space as the water evaporates. In compensation, the chemical is diluted as a result of slow degradation. The degradation products of peracetic acid include hydrogen peroxide and acetic acid. Hydrogen peroxide is ^■ further transformed into water and oxygen. Generally known stabilizers may be used to slow down the degradation reactions. Peracetic acid also has an inhibiting effect on the enzyme which disintegrates hydrogen peroxide. The added hydrogen peroxide shifts the equilibrium reaction towards the peracetic acid and thereby slows down degradation of the peracetic acid. In addition, in some conditions hydrogen peroxide is preserved longer than peracetic acid without disintegrating and, as their combined effect, perace- tic acid first purifies the water together with the hydrogen peroxide after which the hydrogen peroxide operates in the water as a preservative. This kind of composition may be particularly advantageously used in the purification of bottled water which is often stored even for longer periods of time and, in this case, degradation of the peracetic acid in the bottle would cause problems.

According to one embodiment of the invention, the com- position contains a solid agent to which the liquid agents and the optional gaseous agents have been bound. Such composition is preferably solid. The bound liquid agents may include for example peracetic acid, acetic acid, hydrogen peroxide, water or any other agents included in the composition. According to one embodiment of the invention, the composition contains some other peracid instead of peracetic acid. The binding may include chemical binding, absorbing or any other type of binding wherein the liquid and/or gas is held within the solid agent during transportation and storage. According to the invention, the liquid agents dissolve in water as the solid agent is introduced into the water. Processing of such composition is easy, as is also the distribution of the composition because it does not require any containers or measuring the amount of liquid. Further, the solid composi- tion may be used easily to purify water in emergency situations or without using a water purification plant .

According to one embodiment of the invention, the composition according to the invention is in the form of a solid tablet. Thus, dosing the composition according to the invention to water is easy, straightforward and easily instructable. Further, the tablet according to the embodiment of the invention is easy to be stored, processed and transported.

In the process according to the invention, the composition according to the invention, preferably perace- tic acid, is added to water. Even a dose of 2mg/l of peracetic acid is most often sufficient to kill half of the microbes contained in the water to be purified. Only a few times larger dose of active peracetic acid, i.e. 5 to 15mg/l, typically reduces the amount of mi- crobes for 90 to 99.9%. In the process according to the invention, a sufficient amount of peracetic acid or aqueous solution of peracetic acid in added to water, depending on the degree of impurity of the starting water. Peracetic acid reacts with water, forming an equilibrium mixture that contains peracetic acid, water, hydrogen peroxide and acetic acid. The mixture of peracetic acid, hydrogen peroxide and acetic acid is preferably added in an amount of about 0.15ppm to 1500ppm, or more preferably 0.05ppm to 500ppm. Perace- tic acid, hydrogen peroxide and acetic acid are common industrially prepared and used chemicals, and their preparation or processing is therefore not discussed in any more detail in this context. Peracetic acid is a chemical which is harmless to the environment. It disintegrates quickly and is not as toxic as the other microbicides. Peracetic acid disintegrates already in the environment of use into hydrogen peroxide and ac- etum as it reacts with the organic impurities. It does not accumulate or concentrate in any part of the ecosystem. Halogen-based oxidizing microbicides may form hazardous and carcinogenic degradation products and entail the risk of corrosion. Peracetic acid entails a considerably smaller corrosion risk and less detrimental degradation products.

According to one embodiment of the invention, the process according to the invention for purification of water is used for purifying bottled water. In bottled water, preservation of the water during long periods of storage causes problems. A further problem is that for example chlorine cannot be used in sufficient concentrations, because bottled chlorine evaporates in part into the air pocket of the bottle, exposing the user to bad odour when opening the bottle. In using the process according to the invention, the peracetic acid and hydrogen peroxide which remain in the bottle preserve the bottled water even for longer periods of time. Further, in using the process according to the invention, the bottled water is not associated with the odour problems which are present in the use of chlorine, because peracetic acid and hydrogen peroxide are nearly odourless and tasteless.

According to one embodiment of the invention, the process according to the invention is used for purifying tap water, according to one embodiment of the invention, peracetic acid may be used in any prior art water purifying process to partly or entirely substitute for or in addition to the previously used water purifying chemicals; in particular chlorine, chlorammonia, ozone, sodium hypochlorite, ammonia and ferrisulfite. Further according to the invention, UV radiation used in the purification of water may be reduced by using peracetic acid. Further, in using peracetic acid, some of the previously applied steps of purification of water may be preferably speeded up or even left out com- pletely. Such steps may include for example ozoniza- tion, chlorination, filtration, activated carbon filtration and UV disinfection. The use of gaseous chlorine and ozone at the waterworks is much more hazardous than the use of peracetic acid. A diluted perace- tic acid solution of 5 to 17% by weight, preferably 5 to 15% by weight, comprises the same risk rating as hydrogen peroxide which is used without problems in more than a million tons per year. However, as the active agent, hydrogen peroxide is needed in an amount which is nearly triple compared to peracetic acid to stop the growth immediately. To prevent long-term growth, hydrogen peroxide is needed in an amount which is nearly hundred-fold compared to peracetic acid.

According to one embodiment of the invention, the composition according to the invention is added in the early stage of the purification of water during or before sedimentation, so that the amount of organic matter in the water would be as small as possible as the process moves on, and thereby the amount of impurities would be considerably smaller. The result is purer and safer household water. In this embodiment of the invention, the removal of the pathogens takes place either entirely or mostly as early as in the initial stage, and the effect of the agent remains throughout the process. Further, the optional UV/disinfection applied later in the process reactivates the purification power of peracetic acid, so the period of action of peracetic acid becomes very long. Since water is purer virtually from the beginning of the process, almost immediately after addition of peracetic acid, it is possible to speed up the other steps of the process. In the other steps of the process, for example the time of exposure to ozonization may be shortened, and in UV/disinfection it is possible to use lower wattage lamps and therefore save time, energy, costs. Also the amount of chlorine/chloramine may be reduced, because they are now needed mainly for maintaining the water pure as it flows through the pipework.

According to one embodiment of the invention, the composition according to the invention is added in a necessary amount, measuring at the same time the purification of the water. The composition according to the invention may be added to the water to be purified in unlimited amounts according to need. The process according to the invention preferably comprises a circulation in which the composition according to the invention is added to the water to be purified, which is then measured for impurities and to which more of the composition according to the invention is added until the water is sufficiently pure as measured and can be conveyed to the next water purification step. According to another embodiment of the invention, a number of points for adding the composition according to the invention have been arranged sequentially to the water purification line, with measurement points arranged between them, so that the composition according to the invention is added in an amount calculated on the base of information obtained from the previous measurement point. Since peracetic acid, hydrogen peroxide and acetic acid are all completely water soluble, and since peracetic acid and hydrogen peroxide are consumed during the purification process and transform into water and acetic acid, it is possible to add in each case the necessary amount of the composition according to the invention, depending on the purity of the starting water. For example the solubility of chlorine in water is limited, and, furthermore, because chlorine itself is not consumed but remains in the water, taste, odour as well as health problems arise when it is used in large quantities.

According to one embodiment of the invention, the process comprises adding the composition according to the invention, and halogen, such as chlorine, or a de- rivative thereof, such as chlorammonia, is added only after UV disinfection to ensure that the water remains pure during its passage in the water system.

According to a preferred embodiment of the invention, the composition according to the invention is used to substitute for the use of halogens prior to UV disinfection. In this manner, halogens and derivatives thereof may be used in very small amounts, and the formation of carcinogenic halogenated hydrocarbons and the taste, odour and health problems associated with chlorine may be minimized.

According to one embodiment of the invention, halogens or derivatives thereof are not used at all for purify- ing the water. According to a preferred embodiment of the invention, the composition according to the invention is used to entirely substitute for the use of halogens, so that the formation of carcinogenic halogenated hydrocarbons and the taste, odour and health problems associated with chlorine are avoided.

According to one embodiment of the invention, ozone is not used at all for purifying the water. According to a preferred embodiment of the invention, the composi- tion of the invention is used to entirely substitute for the use of ozone, so that the expensive ozone equipment is not needed and processing of toxic and hazardous ozone in the water treatment plant can be avoided.

According to one embodiment of the invention, perace- tic acid is added to the water in an amount of 0.1 to 5mg/l and hydrogen peroxide is added in an amount of 0.1 to 50mg/l. According to a preferred embodiment of the invention, hydrogen peroxide is added to the water in an amount of 5 to 15mg/l. In particular in purification of bottled water, peracetic acid is added in the process according to the invention in an amount of 0.1 to 5mg/l, depending on the degree of impurity of the starting water, and hydrogen peroxide is added in an amount of 0.1 to 50mg/l, the dosage being preferably 5 to 15mg/l, and acetic acid is added in a necessary amount. Most of the peracetic acid is consumed right at the beginning of the dosing, after which the degradation of peracetic acid slows down. Peracetic acid is halved in tens of minutes. At smaller dosage concentrations, peracetic acid is halved in hours, which increases the effective period of action. A sufficient period of action is met with the peracetic acid residual concentration of 5 to lOmg/1 in 5 to 10 minutes. Normally, the residual concentrations of peracetic acid constitute about half of the dosage concentrations .

If the water is pure or almost pure, residual concentrations and stability of peracetic acid are higher. Peracetic acid is consumed faster than hydrogen peroxide, because the ratio of the residual chemicals is different from the originally dosed peracetic acid. In a pure water system, peracetic acid lowers the pH value considerably. The recovery of acidity to the normal level usually takes M. an hour after the dose has run out, but in slow water systems it may even take hours. Dosing of peracetic acid has also a slightly electroconductivity increasing effect.

According to one embodiment of the invention, the com- position according to the invention is added in different parts of the water supply network or to the water tower. Peracetic acid functions as a chemical removing and destroying slime formations. In addition, it prevents new slime formations from being produced. Peracetic acid impedes binding of the microbes to the equipment and to each other, which improves the quality of the water. A water-soluble chemical, peracetic acid penetrates into the intermediate space between cells in a slime formation. As hydrogen peroxide and peracetic acid disintegrate to gaseous oxygen, small gas bubbles are formed, which deteriorate the biofilm structure. As the cell membranes become damaged, binding of the microbes becomes more difficult in the presence of peracetic acid. Owing to the biofilm- degrading effect of peracetic acid, the composition according to the invention functions anticorrosively, protecting the water pipes, water towers and water tanks. It is not corrosive as such in suitable concentrations. When added according to the embodiment of the invention, peracetic acid is added in different parts of the water supply network, so that recontami- nation of the water after degradation of peracetic acid is avoided.

In one embodiment of the invention, the solid or liquid composition is added to the end user's pipework and/or in conjunction with the tap. Preferably in areas where the quality of tap water varies or where it is not potable, it is possible to add, according to the invention, the composition according to the invention to the water pipes of individual households to improve the quality of the water. Furthermore, this embodiment of the invention makes it possible to draw water for example from a river or other equivalent environment and to purify it to be potable without large investments in the equipment at the water treatment plant .

According to a preferred embodiment of the invention, the composition according to the invention is added in conjunction with the tap when water is drawn from the pipework, so that the quickly purifying effect of per- acid, preferably peracetic acid, allows immediate purification and use of the water, and at the same time possible recontamination of water is avoided when the peracid, preferably peracetic acid, disintegrates.

According to one preferred embodiment of the invention, the composition according to the invention is added to a drinking vessel or a water container, so that the quickly purifying effect of peracid, preferably peracetic acid, allows immediate purification and use of the water.

According to one embodiment of the invention, the com- position according to the invention is used for purifying artificial groundwater. According to one preferred embodiment of the invention, the composition according to the invention is used to substitute partly or entirely for the use of chlorine or the de- rivatives thereof in the purification of artificial groundwater. According to one embodiment of the invention, the composition according to the invention is used for purifying artificial groundwater basins and/or sand by adding the composition according to the invention to the water from which artificial groundwater is produced. In the following section, the invention will be described in detail with reference to the accompanying figures .

Fig. 1 presents a typical process for purifying tap water in which water is disinfected by means of for instance ozone, UV light and chlorine. The process and composition according to the invention are not limited to the purification process presented herein, but instead the composition according to the invention may be used by modification of any known water purification process. In the process for purifying water presented in Fig. 1, acidity of the water is adjusted by means of lime (2) and lime water (3) , and humus is precipitated by supplying ferric sulphate (4) into the water. After precipitation, the water is conducted to horizontal sedimentation where most of the precipitate deposits on the bottom of the sedimentary basins, whence it is conducted to the waste-water purification plant. After sedimentation the water is filtered by sand filters prior to disinfection with ozone (5) . After ozonization, carbon dioxide (1) is supplied to the water, raising the alkalinity of the water and thereby reducing water-induced corrosion in the water supply network. Then the water is conducted to activated carbon filtration and UV disinfection. After UV disinfection that serves as a back-up disinfection step, a small amount of chlorine in the form of sodium hy- pochlorite (6) and ammonia (7) which binds the chlorine to form chloramine are supplied to the water. In the last step, carbon dioxide and lime water are added to the water to adjust the pH of the water to a desired value. After purification of the water, it is pumped into the water supply network. Fig. 2 presents an example of the purification process according to the invention in which peracetic acid is added in two steps; prior to sedimentation in a prestage (8) and after sedimentation during filtration (9). In the process according to the preferred embodiment of the invention, peracetic acid is added prior to sedimentation, in the beginning of the process, so that the amount of the organic matter in the water would be as small as possible as the process moves on and the amount of the impurities can be reduced considerably. The result is purer and safer household water. Thus, the removal of the pathogens takes place already in the beginning, and the effect of the composition according to the invention lasts throughout the process. Further, UV/disinfection reactivates the purifying power of the peracetic acid, so the effect of peracetic acid is very long-lasting. Since the water is purer virtually from the beginning of the process, almost immediately after having added the peracetic acid, it is possible to speed up the other steps of the process. In the other steps of the process, for example the time of exposure to ozonization may be shortened and lower wattage lamps may be used in UV/disinfection to save time, energy and costs. Also the amount of chlorine/chloramine may be reduced.

In one process according to the invention, peracetic acid is dosed in the water in the prestage of the raw water treatment, preferably (8) and/or (9), as it comes to the water supply plant. In this embodiment of the invention, peracetic acid is dosed prior to biological filtration, activated carbon filtration and/or sand filtration of the water. Preferably, the dosage amount in continuous dosing is 0.05 to 5mg/l, more preferably 0.1 to 2mg/l, even more preferably 0.1 to lmg/1. As peracetic acid is dosed in the prestage of the water treatment, the water is disinfected at an early stage, preferably accompanied by improvement of taste and odour and preferably precipitation of iron and manganese. By the process according to the inven- tion it is possible to substitute partly or preferably entirely for the traditional aeration and/or adding of iron and manganese / potassium permanganate. Furthermore, elimination of most of the microbes as early as in the prestage or after it by the effect of peracetic acid enhances the effect of the subsequent sand or other biological filtration in the process, because in this case the microbiota of the filter is able to utilize the dead biological matter. Furthermore, preferably the subsequent sand or other biological filtra- tion in the process enhances the effect of peracetic acid and allows smaller dosage of peracetic acid to provide the same disinfective power. Typically, it is possible to reduce the dosage from lOOOppm to 1 - 5ppm, preferably to 1 - 2ppm. A similar predisinfec- tion cannot be performed favourably by chlorine due to formation of detrimental organic chlorine compounds.

In this process of the invention, the water that has been disinfected with peracetic acid in the prestage is then filtered by sand or other biological filtration, preferably removing larger organisms. Furthermore, in this filtration the amount of peracetic acid is preferably reduced to about 1/100, preferably to 1/1000 of the initially added amount. Preferably, in this filtration most of the largest sewage pathogens such as cysts, amoebas, parasites and protozoa are filtered. Preferably, the filter is sized so as to filter most of all organic material.

In this process of the invention, the sand or other biological filtration operates as a mechanical and biological filter. The amount of the residuals of per- acetic acid is reduced in the filter and the residual acetic acid can be utilized as nutrient by the biofilm of the filter. Hydrogen peroxide which is provided from peracetic acid or added operates as an oxygen source. In the prestage, peracetic acid is preferably added only in a concentration such that during the sand or other biological filtration the concentrations of peracetic acid and its residuals are sublethal to the normal biofilm but preferably lethal to the pathogenic microorganisms which are more sensitive to peracetic acid than the normal iron and manganese precipitating microbes and the biofilm in general. The discussed sand or other biological filter is a process which is known per se in the removal of iron and manganese. In other words, the sand or other biological filter is a pre-existing device supplementing this process. A sand or other biological filter alone would be too coarse to remove the microorganisms, allowing them to pass through. In this embodiment of the invention, peracetic acid is preferably dosed in the prestage in an amount which is sufficient to kill most of the harmful microbes in the water prior to the filter or during filtration, the death rate being, how- ever,, small enough to leave the biofilm of the filter intact. Preferably, peracetic acid is added in a' concentration of 1 to 43% by weight, more preferably 5 to 17% by weight. The dosage of peracetic acid may be adjusted for example by measuring the microbiota and/or the concentration of peracetic acid in the water when it flows into and/or out of the filter and adjusting the dosage by the feedback to have a desired result. The invention is not based on adjusting the dosage; instead, the adjustment may be made by any suitable process known in the process technology. Thus in this embodiment of the invention, the chemical, mechanical and biological aspects are combined by adding the per- acetic acid prior to biological filtration. In this manner, the effect of the biological and/or mechanical filtration and/or peracetic acid is improved. In addi- tion, it is possible to preferably substitute entirely or partly for the conventional precipitation chemicals of iron and/or manganese by peracetic acid.

It is further possible to combine in the filtration the normal precipitation chemistries, such as the aluminium, polymer and iron chemistries. Use of the iron chemistry enhances the activity of peracetic acid and hydrogen peroxide. This is known for the hydrogen peroxide as the Fenton process. Again, the iron chemicals speed up the removal of the peracetic acid residuals. The filter is preferably cleaned once a week by periodic larger-scale backwashing, preferably with adding peracetic acid which thus removes the biofilm from the filter. This facilitates the operation of the filter and prevents the growth of pathogenic microbes.

UV light is known to be used in the disinfection of water. However, ageing and staining of the lamps put constraints on the process. In this embodiment of the invention, peracetic acid is preferably dosed in the prestage relative to the age of the lamps so that the costs of the UV treatment are reduced. The UV treatment is not an essential part of the invention, and the process works without it. In dosing the peracetic acid as described in the invention, a pre-existing UV treatment may be utilized or a constructable UV treatment may be replaced by dosing the peracetic acid.

Peracetic acid disintegrates in time and the residuals of peracetic acid are reduced to the desired level either in time or, according to one embodiment of the invention, the acetic acid residual provided by dosing the peracetic acid and/or other residuals are preferably partly or entirely removed by sand filter. Preferably, the content of peracetic acid in the end prod- uct is less than O.lmg/1. According to one embodiment of the invention, the residuals may be removed by carbon filter or carbon filtration directly after the dosing of peracetic acid with sufficient delay.

In this process of the invention, the optional ozoni- zation step becomes preferably unnecessary. Alternatively, the effect of the ozonization step may be improved by the residuals of peracetic acid. Ozonization also reduces the residuals of peracetic acid and speeds up the reduction of the concentrations.

The water produced by the process according to the invention may be bottled or conducted to the water supply network. In one embodiment of the invention, chlorine is added to the water purified by the above- described process to prevent proliferation of any unwanted microbes possibly remaining in and/or brought into the water. In other words, chlorine is used in this embodiment of the invention as a preservative in the water. Preferably, in this embodiment of the in- vention, the dosed chlorine content is much smaller compared to the conventional purification of water because there is no need to redisinfect the water, and the content of the organic matter in the water is preferably lower as well compared to the conventional purification of water, and because the purpose of chlorine is merely to preserve the water and not to purify or disinfect it. Reduction of the chlorine content according to this embodiment of the invention preferably reduces health risks and improves taste and odour of the water. The smaller amount of the organic matter in the water produced by the process according to the invention during chlorination reduces the formation of organic chlorine compounds, many of which may be carcinogenic or unwanted in other respects.

Fig. 3 presents a comparison between the effect of chlorine and peracetic acid in purification of water for removing three common microbes. The table shows that in order to remove listeria, staphylococcus and enterococcus from water, only a fraction of peracetic acid is required compared to chlorine. In particular, this difference is apparent for a water of 5 degrees . Typically, temperature of the groundwater as well as raw water drawn from a river or a lake is low, in many parts of the world even close to five degrees. Thus, peracetic acid is needed in much smaller amounts compared to chlorine, reducing the costs of transportation of the chemicals. As chlorine is a much more hazardous agent than peracetic acid when released in the environment, the environmental risks are smaller with the use of peracetic acid. Furthermore, when used for purifying water, peracetic acid is much less harmful to the user than chlorine and forms less toxic and/or carcinogenic residuals in the service water.

Fig. 4 and 5 present the effect of peracetic acid in removing salmonella and listeria from water. The tables show that even an exposure of 30 seconds to the composition according to the invention is sufficient to remove these bacteria nearly completely from water. Thanks to the quick effect of peracetic acid in purification of water, it is possible to continuously monitor the purification result and add more peracetic acid if needed. This provides for a more efficient and faster purification process, saving costs and allowing better results as only the optimum amount of peracetic acid is used in the dosing. The biocidic effect of peracetic acid on organisms is greater in an order of magnitude than that of hydrogen peroxide, due to its faculty to resist the catalase enzyme that cleaves hydrogen peroxide. The amounts of use of peracetic acid in microbe control are lower than those of hydrogen peroxide, which makes the overall environmental effect of peracetic acid more favourable than that of hydrogen peroxide. The long-term environmental effects of peracetic acid can be described as equal to those of acetic acid due to the small increase in the BOD load.

Aquatic organisms are approximately ten times more sensitive to peracetic acid than terrestrial organisms. The acute toxicity in the organisms of saline water is smaller than in the freshwater organisms. Peracetic acid is more toxic to smaller organisms or during embryogenesis . It is nearly equally harmful to algae as it is to fish, the algaecidic effect of peracetic acid being mostly provided by hydrogen peroxide. As for the microbes, peracetic acid functions as a biocidic and -static agent. Peracetic acid is able to resist catalase, so it does not disintegrate biologically particularly quickly as does hydrogen peroxide. The biological half life of peracetic acid at low concentrations and in a clean and completely closed system lasts for a few weeks. The immediate environmental effects of peracetic acid are equal to those of hydrogen peroxide.

With appropriate dosage models, it is possible to use peracetic acid longer than other chemicals because microbes are not easily adjusted to it. Dosing of the peracetic acid may be continuous or periodic. To achieve the strongest effect, peracetic acid is dosed in shocks.

Peracetic acid acts more quickly than many other mi- crobicides, so its use reduces the purifying time. The disinfective concentrations are safe, non-irritating to skin and they do not leave toxic residuals in the product. Completely water-soluble, peracetic acid does not produce precipitations and is easily washable. Peracetic acid does not foam or cause corrosion in stainless steel.

Peracetic acid is effective against most microbes - bacteria, fungi, spores and viruses - and resistant microbial strains cannot be formed due to quick effect of the peracetic acid. Being an oxidizing microbicide, peracetic acid disintegrates more quickly into harmless substances and is less toxic to the environment than the traditional common biocides. Most of the per- acetic acid disintegrates as early as in the environment of use when reacting with the organic impurities . When sufficiently diluted or during degradation, peracetic acid only leaves hydrogen peroxide and acetic acid as residuals.

Fig. 6 presents degradation of peracetum and hydrogen peroxide in a closed bottle as a function of time. The figure shows that adding about 40mg/l of hydrogen peroxide and 15mg/l of peracetic acid to water results after 10 hours in an equilibrium containing about 2mg/l of peracetic acid and lOmg/1 of hydrogen peroxide. This amount of these compounds is ideally sufficient to prevent in the bottle the growth of the microbes which may have survived the purification or brought into the bottle, even for longer periods of storage.

Claims

I. A composition for use in purification of household water, c h a r a c t e r i z e d in that it contains peracid.

2. The composition according to claim 1, c h a r a c t e r i z e d in that it contains peracetic acid.

3. The composition according to claim 1 or 2, c h a r a c t e r i z e d in that the peracetic acid is a 5 to 17% by weight aqueous solution.

4. The composition according to claim 1, 2 or 3, c h a r a c t e r i z e d in that it contains hydrogen peroxide .

5. The composition according to any one of claims 1 to 4, c h a r a c t e r i z e d in that it contains acetic acid.

6. The composition according to any one of claims 1 to 5, c h a r a c t e r i z e d in that it contains a solid agent to which the liquid agents have been bound.

7. The composition according to claim 6, c h a r a c t e r i z e d in that it is in tablet form.

8. A process for purification of household water, c h a r a c t e r i z e d in that the composition according to any one of claims 1 to 7 is added to water.

9. The process according to claim 8, c h a r a c t e r i z e d in that the water is bottled.

10. The process according to claim 8, c h a r - a c t e r i z e d in that the water is conducted to a water supply network.

II. The process according to any one of claims 8 to 10 wherein the water is subjected to sedimentation in the beginning of purification, c h a r - a c t e r i z e d in that the composition according to any one of claims 1 to 7 is added during or before sedimentation.

12. The process according to any one of claims 8 to 11 wherein the water is filtered, c h a r a c - t e r i z e d in that the composition according to any one of claims 1 to 7 is added during or after filtration.

13. The process according to any one of claims 8 to 12, ch a r a c t e r i z e d in that a necessary amount of the composition according to any one of claims 1 to 7 is added, measuring at the same time purification of the water.

14. The process according to any one of claims 8 to 13 wherein the water is UV disinfected, c h a r a c t e r i z e d in that halogens and/or derivatives thereof are only added in the process after UV disinfection.

15. The process according to any one of claims 8 to 14, c h a r a c t e r i z e d in that halogens and/or derivatives thereof are not used in the process.

16. The process according to any one of claims 8 to 15, c h a r a c t e r i z e d in that ozone is not used in the process.

17. The process according to any one of claims 8 to 16, c h a r a c t e r i z e d in that 0.1 to 5mg/l of peracetic acid and 0.1 to 50mg/l of hydrogen peroxide is added to water.

18. The process according to any one of claims 8 to 17, c h a r a c t e r i z e d in that hydrogen peroxide is added in an amount of 5 to 15mg/l.

19. The process according to any one of claims 8 to 18, c h a r a c t e r i z e d in that the composition according to any one of claims 1 to 6 is added in different parts of the water supply network.

20. The process according to any one of claims 8 to 19, c h a r a c t e r i z e d in that the composition according to any one of claims 1 to 7 is added to a water tank.

21. The process according to any one of claims 8 to 19, c h a r a c t e r i z e d in that the composition according to any one of claims 1 to 6 is added to the piping of the end user and/or in conjunction with the tap.

22. The process according to any one of claims 8 to 18, c h a r a c t e r i z e d in that the composition according to any one of claims 1 to 6 is added to a drinking vessel and/or container.