AREA OF INVENTION The present invention relates to a method and an ap¬ paratus for purification of ground water and surface water to the quality of drinking water.

BACKGROUND OF INVENTION In today' s drinking water purification plants for pu¬ rification of ground water, large aeration tanks are used. Contaminations such as iron and manganese, which are of frequent occurrence, are oxidized out from the ground water in the large aeration tanks, possibly with the addition of a strong oxidizing agent, such as potassium permanganate, followed by a filtering process. As to surface water, generally some sort of floccu- lants is added to be able to precipitate the contamina- tions, whereupon the floccules are removed through some sort of filtering process. Since contaminations such as toxins, fluorides and heavy metals have become more and more frequent, new types of flocculants have been developed, the floccules of which have to be removed effectively by the following filtering process. The technique has been increasingly complicated, since it is desirable e.g. at reduction of fluoride, to have a high contents of aluminium in the water before fil¬ tering, because the fluoride forms complex compounds with aluminium, which thereafter may be removed through filter¬ ing. This implies that the filtering construction has to work as an effective barrier when it comes to aluminium particles. To lower the load on the filter, sometimes flotation is used as a first step of removing the contaminations of the coarse particulate matter in the water. In this way it is possible to prolong the operating lifetime of the fil¬ ter, i.e. reflushing of the filter may be performed less often. However, problems with this previously known tech- nique may occur if the water has not been properly de- airated before passing through the subsequent filtering ma¬ terial. The remaining air bubbles may be caught in the sand in the filter mass and there is a large risk of canaliza¬ tion in the filter mass, which may result in the passage of unfiltered water as a result of the formation of preferen¬ tial paths. Furthermore, a filter of sand is not a sufficiently effective barrier when purifying water with high amounts of fluoride or heavy metals. The sand filter has to be re- placed with a filter membrane, which allows particles with sizes less than 0.05 microns to pass.

DISCLOSURE OF INVENTION The object of the present invention is to provide a method for purification of water, in which the problems with for example reduction of fluorides has been mitigated. In a first aspect, there is provided a method of pu¬ rifying raw water, comprising: supplying raw water to be purified under pressure from a source of water; adding pressurized air and possible further agents into the pres¬ surized raw water, to obtain a mixture with the raw water for oxidizing contaminants of the water and for forming bubbles of air in the mixture; passing the mixture to a flotation tank under releaved pressure, whereby the air bubbles expand and flotate possible floccules and particles to the surface of the flotation tank for removal, and pass¬ ing the water through a filter means to obtain purified wa¬ ter. The method may further comprise one or several of the following steps: adding a surplus of pressurized air; mix- ing said pressurized water and pressurized air in a agita¬ tion chamber, such as a cyclone device, in order to form a large amount of micro bubbles; maintaining said pressurized mixture in a pressure tank for a predetermined time period in order to promote oxidation of possible contaminants; re- leaving the pressure before entering the mixture in the flotation tank, such as by means of valves; passing air bubbles along an outer membrane surface of said filters, said membrane surface being arranged vertically, for remov- ing contaminants from the outer surface of the membrane; back flushing said filters intermittently; adjusting the pH in the flotation tank; and removing surface water and/or bottom water from the flotation tank, for adjusting the concentration of contaminants and possible further agents in the flotation tank. The raw water may be surface water or ground water. The further agents may be selected from the group comprising: chemical oxidizing agents, floccula- tion agents, aluminium particles and combinations thereof. In another aspect, there is provided an apparatus for purifying raw water, comprising: a source for raw water to be purified under pressure; a source for pressurized air and possible further agents to be added into the pressur¬ ized raw water, to obtain a mixture with the raw water for oxidizing contaminants of the water and for forming bubbles of air in the mixture; a valve means for releaving or de¬ creasing the pressure when passing the mixture to a flota¬ tion tank, whereby the air bubbles expand and flotate pos¬ sible floccules and particles to the surface of the flota¬ tion tank for removal, and a filter means for passing the water to obtain purified water. The apparatus may further comprise one or several of the following: an agitation chamber, such as cyclone device, in which added surplus of pressurized air and said pressurized water are mixed in or¬ der to form a large amount of micro bubbles; a pressure tank for maintaining said pressurized mixture during a pre- determined time period in order to promote oxidation of possible contaminants; an air supply for passing air bub¬ bles along an outer membrane surface of said filters, said membrane surface being arranged vertically, for removing contaminants from the outer surface of the membrane; a pump for back flushing said filters intermittently; a pH meter and a control device for adjusting the pH in the flotation tank; water removal means for removing surface water and/or bottom water from the flotation tank, for adjusting the concentration of contaminants and possible further agents in the flotation tank. The raw water may be surface water or ground water. The further agents may be selected from the group comprising: chemical oxidizing agents, potassium permanganate, flocculation agents, aluminium particles and combinations thereof.

BRIEF DESCRIPTION OF DRAWINGS Further objects, features and advantages of the in¬ vention will become apparent from the following description of several embodiments of the invention with reference to the drawing, in which: Fig. 1 is a schematic view of an arrangement for pu¬ rification of water according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Fig. 1 discloses an apparatus for purification of raw water, such as surface water or ground water. The raw water may come from a lake, river, wetland, and a dug well or a deep drilled well, alternatively. The water is pumped by a first pump Pl, with a pressure of at least 4 bars, to a pressure tank or chamber A, in which oxidation and maturing may take place. A surplus or excess of air is supplied by an air com¬ pressor Ll from an inlet of air. Further agents may be added, such as a possible complementing oxidizing agent Ml and possible flocculants for e.g. heavy metals, fluorides and humus M2 and M3. These agents and the air are supplied under pressure to the raw water. These components are mixes heavily and monumentally and turbulently in a mixer B, such as a cyclone device, be¬ fore entering the pressure chamber A. The heavy mixing en¬ tails that the water will contain a large amount of air, comprising millions of microscopically small air bubbles. Part of the oxygen in these air bubbles is consumed during the fast oxidation of iron and manganese into the pressur¬ ized oxidation and maturing chamber A. Because the oxida¬ tion takes place under elevated pressure, the required pre¬ determined dwell time in the pressure chamber A may nor¬ mally be less than 3 minutes. If an air cushion is formed at the top of, the pres¬ sure chamber A, such air is vented or released through a ventilating valve 0 of the chamber A. From the pressure chamber the oxidized water is passed to a flotation chamber C. The water enters the flo- tation tank through nozzles E also comprising releave valves, which lowers the pressure to substantially atmos¬ pheric pressure. Since the water is supersaturated with air under a high pressure, the microscopically small air bubbles will rapidly expand when the pressure is releaved. The expanded air bubbles will attach to the surface of any particulate matter or floccules present in the water and rise to the surface of the flotation chamber C bringing with it the contaminants. In this way, an effective flotation is pro- vided. The floccules and other particulate contaminations present in the water are lifted by the expanding air bub¬ bles to the surface from where they are simply removed by scraping or overflow over an edge. Alternatively, the water level in the flotation chamber C is periodic raised, where- upon the surface water with the contaminations spills over and flows via over flow flutes D and pipes out through the drain. In the manner described above, the same amount of added air used for oxidation in the pressure chamber A and is also used for obtaining flotation when the water is passing through the flotation chamber C. The pressure chamber further comprises several large filter devices F having filter membranes with a small cut¬ off size, in the order of 0.05 microns. A pump P2 is con- nected to the filtration side of the membranes and sucks the water through the membranes to obtain highly purified water free from contaminants of all types. The filters are arranged vertically, with an entrance opening at the bottom. The water passes through the mem- branes of the filters and is collected at the top of the filters. The pressure at the top of the filters is con¬ trolled by the pump and the flow of purified water re¬ quired, but may be 0.3 bar below atmospheric pressure or even lower. The flotation nozzles E are located higher than the entrance openings G of the filter elements, which means that the filtering elements F will be protected from being reached by most of the floccules and other particulate con¬ taminations. In addition, the surface of filtering elements F is kept free from any contaminant by means of an air diffuser H, which is located immediate beneath each filtering ele¬ ment F and emits a continuous curtain of air bubbles, which touch the wall of the filtering element and dislocate par- tides and contaminants to the surface of the water in the flotation chamber C, which counteract and prevents clogging of the filter elements F. The pump P2, which is self-priming, sucks water from the combined flotation and filtering chamber C through the filtering elements F, whereby particles, which are larger than 0.05 microns and any bacteria and virus stay in the flotation chamber. The filter has an exclusion size of about 0.05 microns, meaning that only particles and matter smaller than 0.05 microns can pass the filter membranes. This excludes all bacteria and most viruses. The surface area or size of the filter is dimensioned so that a suffi¬ cient operation time is obtained, whereupon the filters are reconditioned by passing pure water in the opposite direc¬ tion. The purified water, which passes through the filter¬ ing membrane F, is pumped to a reflushing reservoir J, from where it is delivered to some sort of water reservoir and further on to a drinking water supply. The reflushing chamber is always immediately filled with water when the pump P2 is running, since the purified water spills over from the reflushing reservoir J. The amount of aluminium and other contaminants at the outside of the filtering barrier F may be held at a suit¬ able level to provide a maximum binding effect of e.g. fluoride to aluminium particles, which are floating in the flotation chamber C, by controlling the drain from the flo¬ tation chamber C of surface water, trough the flow flutes D, as well as bottom water, through the drain valve I. Moreover, the pH is controlled and adjusted in the flota- tion chamber by a pH probe Nl, which transmit a signal to the control and regulating centre N2, which in turn con¬ trols the dosage pump N3. The pH-adjusting agent is added before the flotation chamber C, but may as well be added directly to the flotation chamber C at a low pressure. The filtering elements F are reflushed regularly by passing pure water from the reflushing reservoir J the op¬ posite way through the filtering elements F by means of the backflushing pump P3. The different further agents are shown to be added in a pipe or tube connecting the pump Pl with the cyclone de- vice B. However, the further agents may as well be added before the pump Pl or alternatively after the cyclone de¬ vice B. The flocculation agent may be added still later in the process, such as shortly before the flotation chamber C. Since the pump P2 cannot generate a very low pres¬ sure, the flow will decrease as the filters are blocked by contaminants. An increased pressure inside the flotation chamber will counteract this. The pressure in the flotation chamber will adjust itself to the flow conditions set by pump Pl, pump P2, valves E and valve I. If the pump P2 can¬ not pump a sufficient amount, more water will be given off to the drain. It is desired to keep the pressure in the flotation chamber close to the atmospheric pressure, so that the mi¬ cro bubbles are able to expand rapidly. The addition of air to the lower end of the filters is adjusted by the valve L2. The valve may provide air con¬ tinuously or intermittently. A gaseous medium such as air is supplied to the cy¬ clone chamber B. However, in order to increase the oxida¬ tion, oxygen gas may be added or air enriched of oxygen. Carbon dioxide may be added as well. The gaseous medium is supplied by an air compressor or by a narrow injector pipe. The pH may as well be adjusted in the pressure cham¬ ber, by the addition of a pH-adjusting agent. In order to summarize, several advantages are ob¬ tained by the present invention, such as: - The use of the same type of equipment for ground water as well as surface water - A decrease of the required building size with a factor of up to 80 % compared to known technique. - Lower total operation and maintenance costs com¬ pared to traditional water plants, - The possibility to reduce for example fluorides by the addition of a flocculation agent - The use of the supplied air both for the oxidizing reaction and for the subsequent flotation in the flotation chamber. Oxidation occurs under pressure in the pressure tank A and filtering occurs through several reflushable filter membranes F, as mentioned above. Because of this, the re¬ quired building volume of the equipment may be decreased by up to 70 - 80% compared to prior art equipment. This has been made possible since oxidation as well as flocculation occurs much faster under pressure. In addition, the space required for the filter devices has been decreased since filtration occurs in the flotation chamber C. The equipment is very compact in structure and is also very useable as catastrophic water plants, since it delivers water, which is completely void of bacteria. Hereinabove has been described an apparatus for puri¬ fication of water comprising a pressure chamber for oxida- tion under pressure, followed by a flotation chamber com¬ prising an integrated filter barrier. Concentration of the contaminations in the flotation chamber is controlled by adjustment of pH and removal of surface and bottom water in the flotation chamber. The apparatus can be used for sur- face water and ground water treatment and different floccu¬ lation agents can be added. A water plant built according to the invention reduces different types of contaminations, such as iron, manganese, sulphur hydrogen, carbon acid, hu¬ mus, radon, fluorides, arsenic, toxins, heavy metals etc. By performing the oxidation under pressure and by using the supplied air for facilitating flotation of floccules in the following flotation chamber under reduced pressure, an ef¬ ficient and synergistic use of the supplied air is ob¬ tained. By controlling the removal of surface and bottom water from the flotation chamber and by control of the pH, formation of complex substances between the fluorides and aluminium can be controlled and separated. The need for ad¬ dition of chemical oxidating agents is reduced and can in many cases be dispensed with. An embodiment has been described above, including a number of devices and method steps. These method steps and devices may be combined in other manners than those de¬ scribed, and any such combination is within the scope of the invention, which is only limited by the appended patent claims.