TECHNICAL FIELD OF THE DESCRIPTION The invention refers to a process and an installation for the obtaining the deuterium depleted water preferably for food use. BACKGROUND OF THE INVENTION Continuous processes are known for the obtaining the deuterium depleted water with a concentration of 2 to 80 ppm D/(D+H), which comprise the isotopic distillation of natural water with a concentration of 145-150 ppm D/(D+H) or of the residue from the manufacture of heavy water with a concentration of 120 ppm D/(D+H) under a 100 mm Hg vacuum, water which is passed through a pelicular boiler in order to achieve total evaporation of a controlled and regulated constant flow of water, which determines also the continuous feed of water, the vapor flow having an ascendent flow in a 15 m high and 0.1 m diameter distillation column, provided with packing, after which the extracted vapor flow is converted in liquid phase, part of it being recycled as a reflux at the top zone of the column at a temperature as close to the actual temperature of the zone as possible, and the other part is collected at a constant extraction flow separately from the collection of a liquid phase which comes from the bottom of the column, in both cases the collection being made under a 100 mm Hg vacuum. The disadvantages of these processes consist in the pseudo stationary regimen of these ones, which occur with perturbations of the isotopic separation considering there is only one processing cycle, for the feed deionized water is necessary, and the feed water consumption is rather large because it must provide, by total evaporation, the vapor flow necessary for the isotopic transfer with the liquid flow. Installations are known for the manufacture of deuterium depleted water in which the processes presented above are applied which comprise a thermally insulated distillation column, in which there is an ordered packing disposed above a liquid collector, at the bottom of the column being connected a peiicular boiler heated with steam, linked to a constant level feeding tank, through a pipe fitted with a flow meter, a regulating tap and a heat exchanger respectively the liquid collecting tank, being connected through another pipe provided with a tap, the heat exchanger and another flow meter, to a collecting tank, the column being connected at its upper side to a vacuum pump, through another pipe fitted with two serial condensers and a tap respectively, these condensers being connected through another pipe fitted with a flow meter and a heat exchanger, heated with steam, with the upper part of the column, this last pipe being connected to another pipe fitted with a tap and a flow meter, which communicates to another collecting tank, which in turn, together with the other collecting tank are connected to the vacuum pump. The disadvantages of these installations are that they have a relatively low capacity and the manufacturing scale-up determines an amplification of the perturbations, with unfavorable effects on the production capacity or on the quality of the product. The deuterium depleted water obtained by applying the process in the installation described in patent RO 112422B1 has a deuterium concentration of 2 - 80 ppm D/(D+H) with the following characteristics: pH= 6,5.7,5 ; Consumption KMnO4 < 1 ,5 mg/l ; CP <0,1 mg/l ; Conductivity < 1 ,5 μS/cm ; SO42" <0,1 mg/l ; PO43" < 0,1 mg/l ; Ca2+ <0,1 mg/l ; Mg2+ < 0,1 mg/l ; Fe2+ <0,1 mg/l ; Cu2+ < 0,1 mg/l ; Bacteriologic exam: steril. The disadvantages of these deuterium depleted waters are that having similar physico-chemical characteristics to distilled water, they are not accepted in the category of potable waters for food use. Other processes for obtaining the deuterium depleted water are known, patent RO 115148 B1 , which comprise continuous feeding with natural water or with residue from the manufacturing of heavy water having a concentration of 120 ppm D/(D+H), of a distillation zone in feeding points situated at different heights depending on the required degree of deuterium depletion, after which the ascendent vapor flow is intimately contacted with the descending liquid flow achieving thus the deuterium enrichment of the liquid and the deuterium depletion of the vapors, obtaining continuously deuterium depleted water, named light water and by batches heavy water. The disadvantages of these processes consist in the permanent danger of impurification due to the crossing of the light water with the heavy water, which is very toxic, at any accidental hydrodynamic perturbation, due for example to the pressure variations on the columns or to the variations of liquid flows. Installations for the manufacture of deuterium depleted water are known which apply the processes described in the patent RO 115148 B1 , comprising three distillation columns connected serially with equal heights, each of them having an ordered packing, the first column which has a larger diameter than the other two which have the same diameter, being provided with several feeding points at different heights, at the bottom of each of the first and the last column being connected a boiler which provides the ascending vapor flow at constant flow rates, at the top of the first column being connected two condensers, two buffer tanks, a reflux tank and a vacuum pump, providing thus the reflux introduced in the first column, a part of the reflux being extracted as product as light water, while in the other columns the liquid flow is enriched in deuterium, a part of it being extracted from the outlet of a pump for the heavy water. The disadvantages of these installations consist in their relatively large dimensions, the control and regulation of the different parameters characteristic to an optimal regimen for obtaining the two products simultaneously at the two ends of the column are relatively hard to achieve, due to the sensitivity of the system to the perturbations determined by the variations of processing parameters, and after these perturbations the production is stopped, long periods of time (tens of hours) being necessary to come back to the desired concentration, these stops determining big energetic consumption on the product unit. The deuterium depleted water obtained by applying these processes in the installations according to patent RO 115148 B1 has a deuterium concentration of 1 - 80 ppm D/(D+H) with the following characteristics: pH=6,5...7,5 ; SiO2 < 0,1 mg/l ; CP < 0,1 mg/l ; K+ < 0,1 mg/l ; SO42" < 0,1 mg/l ; PO43" < 0,1 mg/l; Ca2+ < 0,1 mg/l ; Mg2+ < 0,1 mg/l ; Fe2+ < 0,1 mg/l ; Cu2+ < 0,1 ; Mn2+ < 0,1 mg/l ; Consumption KMnO4 < 1 ,5 mg/l ; Na+ < 0,1 mg/l ; Conductivity < 1 , 5 μS/cm ; Oxidability < 0,5 mg/l ; Turbidity < 3,0 ppm SiO2; Bacteriologic exam: pathogen free. The disadvantages of these deuterium depleted waters consist in the fact that they cannot be considered potable waters because they are demineralized waters, they do not contain the mineral substances imposed by the standard, and the manufacture of a deuterium depleted water with a constant deuterium concentration, is perturbed by the permanent tendency of contamination with heavy water, a very toxic product for living organisms. DISCLOSURE OF INVENTION The technical problem solved by the inventions belonging to the group of inventions consists in obtaining continuously water for food use, with a constant deuterium content of 20-30 ppm D/(D+H) and a controlled content of mineral salts, without toxicity for living organisms, without the presence of pathogens or ozone concentrations at harmful levels and there is no waste which negatively influences the natural equilibrium of the environment. Surprisingly, we noticed that maintaining a 1 : 5 ratio between the raw material flow with a temperature of 450C, and the vapor flow with a temperature of 6O0C, when the pressure at the top of the isotopic distillation column is 100 mm col. Hg, and the temperature 51.50C respectively, the column having a height : diameter ratio of 45 : 1 , leads to a product flow consisting of deuterium depleted water representing 4.5% of the vapor flow with a concentration of 20-30 ppm D/(D+H). Also in this context the discarding of the waste is done without negative effects on the natural equilibrium of the environment, since the water with under 200 ppm deuterium content is not harmful and in contact with the humidity in the atmosphere it equilibrates isotopically in a short time of about 72 hours to the concentration and quality of natural water having a 145 ppm D/(D+H) content. The process according to the invention does not have the disadvantages shown before because after purification, water is fed in a continuous isotopic distillation column which has a height : diameter ratio of 45 : 1 in the presence of a packing of minimum 100 theoretical plates with an efficiency of 8 theoretical plates per meter, but not less than 4.2 theoretical plates per meter with a liquid load of 2000 I/ (h x m2) and preferably of 2555 I/ (h x m2), a vacuum of 100 mm Hg being achieved and maintained at the top of the column, the temperature of the vapors at the top being 51.50C, and the ratio of the raw material flow and of the vapor flow with a 6O0C temperature is 1 : 5, the reflux flow being fed on the last plate of the superior stripping zone which in minimal conditions is the 90th plate - the equivalent of a concentration of 30 ppm D/(D+H), the ratio between the number of theoretical plates in the stripping zone and of those from a lower enrichment zone is 10 : 1 and preferably 9 : 1 , resulting in this latter case 10.5% of the reflux flow as a waste which flows through the enrichment zone with a concentration of under 200 ppm D/(D+H), obtaining 4.5% of the reflux flow deuterium depleted water with a concentration of 20-30 ppm D/(D+H), which is mineralized by contacting it, in the presence of static mixing promoters, with mineralizing additive at ambient temperature. The cooling water for the deuterium depleted vapors, collected at the top of the column with a temperature of 450C, comes from the purified water, dechlorinated and deionized respectively, which, after cooling the vapor flow, is introduced in the feeding water of the isotopical distillation column. In the superior stripping zone in the packing from bottom to top a constant efficiency is distributed, preferably maximum 8 theoretical plates per meter and it can decrease with 50% theoretical plates per meter over the height of this zone, when a deuterium depleted water with a maximum concentration of 20 ppm D/(D+H) is obtained. The mineralization of water with a 20-30 ppm D/(D+H) content is achieved with a mineralizing additive obtained by dissolving in 100 I water with a 20-30 ppm D/(D+H) content, in the indicated order, of salts from naturally mineralized waters of pharmaceutical quality consisting of minimum. 200 g calcium chloride, 100 g magnesium sulfate and 100 g sodium hydrogenocarbonate, resulting in a deuterium depleted potable water, which is not toxic for living organisms. The installation according to the invention, in which the process is applied, does not have the disadvantages shown before because a steam boiler is connected through a pipe with the lower part of a lower enrichment zone of a packing, which also contains an upper stripping zone, at the intersection of these zones being connected to a thermally insulated isotopic distillation column in which the packing is placed, a feeding pipe, between the steam boiler and the column being connected a level regulation loop consisting of a level meter and a regulating tap, placed under the zone in which in the column are injected vapors provided by the steam boiler, between the vacuum ramp and the top zone and respectively a secondary condenser being connected a vacuum regulation loop, a collecting pipe being connected to an intermediate storage tank to which a short pipe is connected, provided with a dosing device, which communicates with a mixture pipe fitted with static mixing promoters, at which another short pipe is connected, having another automatic dosing device, connected to a storage tank for mineralizing additive, the mixing pipe being connected, through a pipe and through an ozonization subsystem with a final storage tank, to which is connected a final evacuation pipe connected to a UV sterilizing subsystem. At the deionizing and purification subsystem the anterior and posterior pipes are connected. BRIEF DESCRIPTION OF DRAWINGS In the following an embodiment of the process and of the installation according to the group of inventions is given in connection with the Fig.1 which represents the principle scheme of an installation for obtaining mineralized deuterium depleted water. BEST MODE FOR CARYING OUT The installation according to the invention comprises a purification subsystem 1 , water dechlorination and deionization respectively, which communicates, by a dosing device 2 fitted with a tap 3 on a pipe 4, with a source 5 providing water with physico-chemical characteristics which make it potabie water. Subsystem 1 is known per se and comprises, preferably, an automatized purification mini-installation, of water dechlorination and deionization respectively, a situation not presented in the scheme. Water from subsystem 1 is gravitationally vehiculated through a feeding pipe 6 fitted with a flow meter 7 with automatic regulation of the flow in a thermally insulated isotopic distillation column 8, having a ratio height : diameter of 45:1. The distillation column 8 is up to 45 m high and preferably 36 m high, when the diameter is 0.8 m. Column 8 is fitted with a packing 9, for example an ordered packing, phosphor-bronze wire mesh, having the following technical features: - density 360 kg/m3; - specific free volume 0.952 m3/m3; - specific free surface 670 m2/m3 The height on which the packing 9 is disposed is equivalent to a minimal theoretical height of 100 theoretical plates which is 22.5 m, possibly attaining for example 12 m, when an efficiency of 8 plates per meter is wanted. Along the height in the packing 9 two zones a and b are delimitated respectively the lower enrichment zone and the upper stripping zone containing a different number of theoretical plates, so that zone a can have a maximum of 90% of the equivalent height of the theoretical plates. Pipe 6 is connected to column 8 in the upper part of zone a which at its turn is adjacent to a zone c at the bottom of column 8. The total height of the packing 9 is 22.5 m, distributed as 2.25 m on each of the 10 parts. The height of zone a is 3 m, that of zone b is 27 m, and that of zone c is 5 m. The feeding water accumulates in the bottom zone c and when its level in column 8 reaches a value representing 25% of the filling volume delimited by zone c the liquid excess is transferred by a level regulating loop A of the type level - steam flow, which comprises a level meter 10 and a regulating tap 11 , in a steam boiler 12 which generates vapors at 6O0C transported by a pipe 13 in column 8 in which they are injected near by at the bottom of zone a. The resulting condensate is removed by a pipe 14 fitted with a closing tap 15 and it is sent to the condensate recovery system. The vapor flow at 6O0C is ascendent in column 8 through zones a and b of the packing 9 and enters a zone d at the top, exits in the upper part through a evacuation pipe 16 connected to a primary condenser 17, which is serially connected to the secondary condenser 18 through a pipe 19. The condenser 18 is connected to an anterior pipe 20 through which the vapor flow subjected to condensation passes in counter flow with the cooling water, which has at the inlet preferably a temperature of 150C, this water being circulated through an intermediate pipe 21 to the primary condenser 17, from which it is evacuated with a temperature of 450C through posterior pipe 22 and enters subsystem 1. The column 8 at the upper zone d level is connected to a pipe 23 provided with a manometer 24 and a tap 25 respectively in a vacuum regulating loop B, pipe 23 being connected through a pipe 26 to a ramp 27 which achieves a vacuum of preferably 100 mm Hg at the top zone d. At the same time vacuum in column 8 is measured with another manometer 28 placed at the lower part of enrichment zone a and preferably it equals 150 mm Hg, value determined by the pressure drop in the ordered packing 9. The liquid flow resulted from the condensation of vapors in the primary condenser 17 which represents 75% from that of the vapor flow is taken through a pipe 29 to a reflux tank 30 from which it is suctioned through lower pipe 31 by a centrifugal pump 32 and pushed as reflux, with a flow of 1250 l/h with a pressure of 2 bar through a pipe 33 fitted with a regulating flow meter 34, in column 8 at the lower part of zone d, in contact with the first plate of zone b. After charging the installation with water and stabilization of the hydrodynamic regimen, the supplemental accumulation of liquid in column 8 is discharged by opening lower regulating tap 35 placed on lower evacuation pipe 36 connected to column 8 at the bottom part of zone c. On pipe 36 is fitted, after tap 35 a regulating flow meter 37, and pipe 36 is connected to a collector 38 of deuterium enriched water with a maximum content of 200 ppm D/(D+H). When deuterium concentration of the distillate from reflux tank 30 decreases from 144 ppm under 30 ppm D/(D+H), due to a signal provided by analyzer 39, tap 40 is opened, the tap being fitted on collecting pipe 41 connected at the lower medium part of tank 30. After tap 40, pipe 41 is fitted with a flow meter 42 which measures and regulates the deuterium depleted water flow which is stored in an intermediate storage tank 43. In this context, pipe 41 is connected to tank 43 at its upper part. The deuterium depleted water from tank 43 circulates through a short pipe 44 fitted with an automatic dosing device 45 to a mixing pipe 46, provided with statical mixing promoters, and to it another short pipe 47 is connected which is fitted with another automatic dosing device 48. Pipe 47 is connected to a storage tank 49 with natural mineralizing additive. Pipe 46 is in connexion through another pipe 50, with an ozonizing subsystem 51 ' which communicates through a pipe 52 with a final storage tank 53, fitted at the lower part with pipe 54 of final evacuation fitted with a tap 55 and a regulating flow meter 56, as well as with a UV sterilizing subsystem 57. This later one has the following technical characteristics: working capacity 1000 l/h, preferably 500 l/h and a lamp power of 35 W. The subsystem 57 communicates through pipe 58 with a closed circuit bottling subsystem 59, known perse. The secondary condenser 18 is connected through pipe 60 with ramp 27, and through another end pipe 61 is connected to pipe 41. The whole installation is made of stainless steel for food industry. Through pipe 62, pipe 36 communicates up-flow from tap 35 with the lower part of boiler 12 in order to recycle through this boiler a liquid flow representing 92% from the total flow evacuated through pipe 36. The subsystem 1 is preferably connected though pipe 63 fitted with a tap 64, a pump 65 which pumps water cooled with cryostat 67 at a temperature of 150C through pipe 20 connected in turn to condenser 18. Pipe 22 fitted with tap 66 is connected to subsystem 1. The process according to the invention applied in the installation consists in treating used water as raw material having the physico-chemical characteristics of potable tap water, so that the end product has the following characteristics: - pH : 6,5 -7,5 - oxidability : maximum 5 mg KMnO4/! - conductivity: maximum 10 μS/cm - free chlorine: none - ammonium: none - total hardness: none This water is subjected to a treatment comprising dechlorination and purification leading to a conductivity of less than 10 μS/cm, in subsystem 1 fed with liquid at a constant flow equivalent with 15% of the reflux flow, regulated in an automatic manner by flow meter 7, when the vacuum at the feeding point in column 8 is 145 mm Hg. Distillation column 8 is fitted with packing 9, which has preferably a height equivalent to 100 theoretical plates and a maximum height equivalent with 180 theoretical plates. The water feeding point is placed towards the bottom zone c at a height representing 10% of packing height 9. Feeding water accumulates in bottom zone c and when the level equals 50% of the indicating panel of flow meter 10 feeding with steam at 2000C is automatically started from boiler 12 provided with level regulating pipe A for zone c of the bottom, through regulating tap 11 and level meter 10. Vapors fed in column 8, having a temperature of 6O0C circulate from bottom to top passing through zone b and are evacuated from zone d, 90% of them being condensed in primary condenser 17 and the rest in secondary condenser 18, which are cooled in counter-flow with water. The 100 mm Hg vacuum in zone d at the top of column 8 is achieved with vacuum ramp 27 and is regulated by vacuum regulating system B through manometer 24 and a regulating tap 25, and at the bottom zone c the vacuum is measured with manometer 28. The liquid resulted from the condensation of vapors is gravitationally transferred in reflux tank 30 from which it is taken by pump 32 and pumped with a 2 bar pressure as reflux in zone d at the top end of column 8. Reflux flow is measured and automatically regulated with flow meter 34 and preferably it is maintained at 1250 l/h. Functioning is regulated so as to maintain a liquid charge in the top d zone of preferably 2555 I/ (h x m2), and when the liquid amount from column 8 is secured, tap 35 is opened and evacuation of the waste is performed at 6O0C and at atmospheric pressure by a barometric leg at a flow measured and regulated with flow meter 37 equal with that fed to column 8 through flow meter 7. In these conditions the installation is run to attain a stationary regimen, signifying maintaining constant flow rates, temperatures and pressures at the measuring and regulating points presented. At the contact point of the liquid with water vapors at the surface of packing 9 by mass transfer is achieved a deuterium depletion of vapors and a deuterium enrichment of the liquid. At the same time, the feeding point with liquid of column 8 separates two zones a and b on the height of packing 9, i.e. upper stripping zone b having minimum 90 theoretical plates and bottom enrichment zone a with a minimum of 10 theoretical plates. In order to have the previously mentioned liquid charge in zone d, high performance packing 9 has an efficiency of 8 plates per meter but it can get to 4.2 plates per meter. When the concentration of the distillate collected in condenser 18 drops to 20-30 ppm D/(D+H), being determined by analyzer 39, tap 40 is opened and, at the coldest point, an extraction is performed of the product consisting in deuterium depleted water with a temperature of 250C, representing preferably 4.5% of the reflux flow automatically measured and regulated with flow meter 42 and stored in tank 43, and the waste flow rate is reduced at a value representing preferably 10.5% of reflux flow rate, respectively the difference between the feeding and the extraction flow rate. Deuterium depleted water is mineralized by mixing it with mineralizing additive from tank 49 in volume ratio 99 : 1 ensured by two automatic dosing devices 45 and 48, then the mixture is subjected to antibacterial treatment by ozonization in ozonization subsystem 51 by forced bubbling at 25°C and atmospheric pressure of ozone enriched air ensuring a remanent ozone concentration in water of up to 0.15 mg/l. Then the mixture is stored in tank 53 with a minimum stay time of 2 hours for completion of the antibacterial treatment. The mineralizing additive is prepared by dissolving in 100 liters of deuterium depleted water containing 20-30 ppm D/(D+H) of 200 g calcium chloride, 100 g magnesium sulfate and 100 g sodium hydrogenocarbonate, pharmaceutical grade salts from naturally mineralized waters and is stored, at 250C with a pH of 6.5-7.5, in tank 49. Before bottling, mineralized deuterium depleted water is passed with a flow rate of 500 l/h through the UV sterilizing subsystem 57, which besides the antibacterial treatment of water, destroys the remanent ozone traces. Deuterium depleted water has a constant deuterium concentration of 20 - 30 ppm D/(D+H) with the following quality characteristics: - pH : 6,5-8,5 ; - oxidability : maximum 2 mg KMnO4/! ; - calcium : maximum 100 mg/ 1; - magnesium: maximum 50 mg/ 1; - total hardness : minimum 90 mg / 1 CaCO3 ; - ammonium: none; - free chlorine: none; - chlorides: maximum 250 mg /I ; - sulfates: maximum 250 mg / 1; - nitrates: maximum 50 mg / 1; - nitrites: none; - bacteriological activity: sterile. Because of these physico-chemical characteristics, this is a potable water for human consumption, which abides the standard regulation concerning the drinking water. The bottling is performed in vessels manufactured, preferably, from a polymeric material, for example polyethylene for food industry, at a temperature of 250C. Usually water has a expiration time of minimum 12 months.