CA2073216A1 - Process for denitrifying water using metallic iron and installation for implementing same - Google Patents
Process for denitrifying water using metallic iron and installation for implementing sameInfo
- Publication number
- CA2073216A1 CA2073216A1 CA 2073216 CA2073216A CA2073216A1 CA 2073216 A1 CA2073216 A1 CA 2073216A1 CA 2073216 CA2073216 CA 2073216 CA 2073216 A CA2073216 A CA 2073216A CA 2073216 A1 CA2073216 A1 CA 2073216A1
- Authority
- CA
- Canada
- Prior art keywords
- water
- iron
- process according
- denitrifying
- ferrobacteria
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 title claims abstract description 93
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 title claims abstract description 58
- 229910052742 iron Inorganic materials 0.000 title claims abstract description 45
- 238000000034 method Methods 0.000 title claims abstract description 38
- 230000008569 process Effects 0.000 title claims abstract description 36
- 238000009434 installation Methods 0.000 title claims abstract description 11
- 241000894006 Bacteria Species 0.000 claims abstract description 24
- 239000002689 soil Substances 0.000 claims abstract description 7
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 12
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical group [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims description 11
- 229910052799 carbon Inorganic materials 0.000 claims description 11
- 229910052802 copper Inorganic materials 0.000 claims description 11
- 239000010949 copper Substances 0.000 claims description 11
- 230000007797 corrosion Effects 0.000 claims description 10
- 238000005260 corrosion Methods 0.000 claims description 10
- 239000003638 chemical reducing agent Substances 0.000 claims description 8
- VTYYLEPIZMXCLO-UHFFFAOYSA-L Calcium carbonate Chemical compound [Ca+2].[O-]C([O-])=O VTYYLEPIZMXCLO-UHFFFAOYSA-L 0.000 claims description 4
- 241001468175 Geobacillus thermodenitrificans Species 0.000 claims description 4
- 241000862970 Gallionella Species 0.000 claims description 3
- 229910021536 Zeolite Inorganic materials 0.000 claims description 2
- 229910000019 calcium carbonate Inorganic materials 0.000 claims description 2
- 239000004927 clay Substances 0.000 claims description 2
- HNPSIPDUKPIQMN-UHFFFAOYSA-N dioxosilane;oxo(oxoalumanyloxy)alumane Chemical compound O=[Si]=O.O=[Al]O[Al]=O HNPSIPDUKPIQMN-UHFFFAOYSA-N 0.000 claims description 2
- 230000001737 promoting effect Effects 0.000 claims description 2
- 239000008262 pumice Substances 0.000 claims description 2
- 239000004575 stone Substances 0.000 claims description 2
- 238000007514 turning Methods 0.000 claims description 2
- 239000010457 zeolite Substances 0.000 claims description 2
- LSNNMFCWUKXFEE-UHFFFAOYSA-N Sulfurous acid Chemical compound OS(O)=O LSNNMFCWUKXFEE-UHFFFAOYSA-N 0.000 claims 1
- 238000004090 dissolution Methods 0.000 claims 1
- DHCDFWKWKRSZHF-UHFFFAOYSA-N sulfurothioic S-acid Chemical compound OS(O)(=O)=S DHCDFWKWKRSZHF-UHFFFAOYSA-N 0.000 claims 1
- 150000002823 nitrates Chemical class 0.000 description 18
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 14
- 238000011282 treatment Methods 0.000 description 9
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 7
- 229910052757 nitrogen Inorganic materials 0.000 description 7
- 239000001301 oxygen Substances 0.000 description 7
- 229910052760 oxygen Inorganic materials 0.000 description 7
- QTBSBXVTEAMEQO-UHFFFAOYSA-N Acetic acid Chemical compound CC(O)=O QTBSBXVTEAMEQO-UHFFFAOYSA-N 0.000 description 6
- OKKJLVBELUTLKV-UHFFFAOYSA-N Methanol Chemical compound OC OKKJLVBELUTLKV-UHFFFAOYSA-N 0.000 description 6
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 5
- 150000002826 nitrites Chemical class 0.000 description 5
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 4
- 239000003673 groundwater Substances 0.000 description 4
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 4
- CWYNVVGOOAEACU-UHFFFAOYSA-N Fe2+ Chemical compound [Fe+2] CWYNVVGOOAEACU-UHFFFAOYSA-N 0.000 description 3
- 230000002093 peripheral effect Effects 0.000 description 3
- 230000009467 reduction Effects 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- CURLTUGMZLYLDI-UHFFFAOYSA-N Carbon dioxide Chemical compound O=C=O CURLTUGMZLYLDI-UHFFFAOYSA-N 0.000 description 2
- VTLYFUHAOXGGBS-UHFFFAOYSA-N Fe3+ Chemical compound [Fe+3] VTLYFUHAOXGGBS-UHFFFAOYSA-N 0.000 description 2
- 238000013459 approach Methods 0.000 description 2
- 230000001651 autotrophic effect Effects 0.000 description 2
- 238000006243 chemical reaction Methods 0.000 description 2
- OSVXSBDYLRYLIG-UHFFFAOYSA-N dioxidochlorine(.) Chemical compound O=Cl=O OSVXSBDYLRYLIG-UHFFFAOYSA-N 0.000 description 2
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 2
- 229910052737 gold Inorganic materials 0.000 description 2
- 239000010931 gold Substances 0.000 description 2
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 2
- JVTAAEKCZFNVCJ-UHFFFAOYSA-N lactic acid Chemical compound CC(O)C(O)=O JVTAAEKCZFNVCJ-UHFFFAOYSA-N 0.000 description 2
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 238000004172 nitrogen cycle Methods 0.000 description 2
- 229910052697 platinum Inorganic materials 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 229910052709 silver Inorganic materials 0.000 description 2
- 239000004332 silver Substances 0.000 description 2
- 239000002352 surface water Substances 0.000 description 2
- 241000193830 Bacillus <bacterium> Species 0.000 description 1
- ZAMOUSCENKQFHK-UHFFFAOYSA-N Chlorine atom Chemical compound [Cl] ZAMOUSCENKQFHK-UHFFFAOYSA-N 0.000 description 1
- 239000004155 Chlorine dioxide Substances 0.000 description 1
- 241001015568 Clonothrix Species 0.000 description 1
- 241001141454 Crenothrix Species 0.000 description 1
- 241000862991 Leptothrix <Bacteria> Species 0.000 description 1
- DIWRORZWFLOCLC-UHFFFAOYSA-N Lorazepam Chemical compound C12=CC(Cl)=CC=C2NC(=O)C(O)N=C1C1=CC=CC=C1Cl DIWRORZWFLOCLC-UHFFFAOYSA-N 0.000 description 1
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 description 1
- 229910002651 NO3 Inorganic materials 0.000 description 1
- NHNBFGGVMKEFGY-UHFFFAOYSA-N Nitrate Chemical compound [O-][N+]([O-])=O NHNBFGGVMKEFGY-UHFFFAOYSA-N 0.000 description 1
- CBENFWSGALASAD-UHFFFAOYSA-N Ozone Chemical compound [O-][O+]=O CBENFWSGALASAD-UHFFFAOYSA-N 0.000 description 1
- 241000589516 Pseudomonas Species 0.000 description 1
- 102000018210 Recoverin Human genes 0.000 description 1
- 108010076570 Recoverin Proteins 0.000 description 1
- BQCADISMDOOEFD-UHFFFAOYSA-N Silver Chemical compound [Ag] BQCADISMDOOEFD-UHFFFAOYSA-N 0.000 description 1
- 241001478894 Sphaerotilus Species 0.000 description 1
- 241000605118 Thiobacillus Species 0.000 description 1
- 241001141412 Toxothrix Species 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 230000000844 anti-bacterial effect Effects 0.000 description 1
- 230000031018 biological processes and functions Effects 0.000 description 1
- 230000015572 biosynthetic process Effects 0.000 description 1
- 239000011449 brick Substances 0.000 description 1
- 239000001569 carbon dioxide Substances 0.000 description 1
- 229910002092 carbon dioxide Inorganic materials 0.000 description 1
- 238000001311 chemical methods and process Methods 0.000 description 1
- 239000003153 chemical reaction reagent Substances 0.000 description 1
- 239000000460 chlorine Substances 0.000 description 1
- 229910052801 chlorine Inorganic materials 0.000 description 1
- 235000019398 chlorine dioxide Nutrition 0.000 description 1
- 235000013399 edible fruits Nutrition 0.000 description 1
- 229960004887 ferric hydroxide Drugs 0.000 description 1
- 238000001914 filtration Methods 0.000 description 1
- 230000006872 improvement Effects 0.000 description 1
- 238000011065 in-situ storage Methods 0.000 description 1
- 229910052500 inorganic mineral Inorganic materials 0.000 description 1
- 150000002500 ions Chemical class 0.000 description 1
- 150000002505 iron Chemical class 0.000 description 1
- IEECXTSVVFWGSE-UHFFFAOYSA-M iron(3+);oxygen(2-);hydroxide Chemical compound [OH-].[O-2].[Fe+3] IEECXTSVVFWGSE-UHFFFAOYSA-M 0.000 description 1
- FLTRNWIFKITPIO-UHFFFAOYSA-N iron;trihydrate Chemical compound O.O.O.[Fe] FLTRNWIFKITPIO-UHFFFAOYSA-N 0.000 description 1
- 239000004310 lactic acid Substances 0.000 description 1
- 235000014655 lactic acid Nutrition 0.000 description 1
- 239000011133 lead Substances 0.000 description 1
- 239000007788 liquid Substances 0.000 description 1
- 229910052748 manganese Inorganic materials 0.000 description 1
- 239000011572 manganese Substances 0.000 description 1
- 239000000463 material Substances 0.000 description 1
- 239000011707 mineral Substances 0.000 description 1
- 125000000449 nitro group Chemical group [O-][N+](*)=O 0.000 description 1
- 238000010899 nucleation Methods 0.000 description 1
- 239000011368 organic material Substances 0.000 description 1
- 230000003647 oxidation Effects 0.000 description 1
- 238000007254 oxidation reaction Methods 0.000 description 1
- 230000003449 preventive effect Effects 0.000 description 1
- 239000004576 sand Substances 0.000 description 1
- 230000006641 stabilisation Effects 0.000 description 1
- 238000011105 stabilization Methods 0.000 description 1
- 239000010902 straw Substances 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- LSNNMFCWUKXFEE-UHFFFAOYSA-L sulfite Chemical class [O-]S([O-])=O LSNNMFCWUKXFEE-UHFFFAOYSA-L 0.000 description 1
- 150000004764 thiosulfuric acid derivatives Chemical class 0.000 description 1
- 230000001988 toxicity Effects 0.000 description 1
- 231100000419 toxicity Toxicity 0.000 description 1
- 230000009466 transformation Effects 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/34—Biological treatment of water, waste water, or sewage characterised by the microorganisms used
- C02F3/346—Iron bacteria
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B09—DISPOSAL OF SOLID WASTE; RECLAMATION OF CONTAMINATED SOIL
- B09C—RECLAMATION OF CONTAMINATED SOIL
- B09C1/00—Reclamation of contaminated soil
- B09C1/002—Reclamation of contaminated soil involving in-situ ground water treatment
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F1/00—Treatment of water, waste water, or sewage
- C02F1/70—Treatment of water, waste water, or sewage by reduction
- C02F1/705—Reduction by metals
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/02—Aerobic processes
- C02F3/10—Packings; Fillings; Grids
-
- C—CHEMISTRY; METALLURGY
- C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
- C02F3/00—Biological treatment of water, waste water, or sewage
- C02F3/30—Aerobic and anaerobic processes
- C02F3/302—Nitrification and denitrification treatment
- C02F3/305—Nitrification and denitrification treatment characterised by the denitrification
- C02F3/306—Denitrification of water in soil
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
- Y02W10/00—Technologies for wastewater treatment
- Y02W10/10—Biological treatment of water, waste water, or sewage
Landscapes
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Water Supply & Treatment (AREA)
- Environmental & Geological Engineering (AREA)
- Hydrology & Water Resources (AREA)
- Microbiology (AREA)
- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Biodiversity & Conservation Biology (AREA)
- Soil Sciences (AREA)
- Purification Treatments By Anaerobic Or Anaerobic And Aerobic Bacteria Or Animals (AREA)
- Removal Of Specific Substances (AREA)
- Biological Treatment Of Waste Water (AREA)
- Treatment Of Water By Oxidation Or Reduction (AREA)
- Artificial Fish Reefs (AREA)
Abstract
Process for denitrifying water using metallic iron and installation for implementing same.
ABSTRACT OF THE DISCLOSURE
Process for denitrifying water, characterized in that it essentially consists in bringing the water to be treated into contact with a bed of metallic iron and in passing it subsequently through a filter bed suitable for the establishment of a biofilm of ferrobacteria and denitrifying bacteria, without its coming into contact with the air.
Application to water denitrification in a plant or directly in the soil.
No drawing.
ABSTRACT OF THE DISCLOSURE
Process for denitrifying water, characterized in that it essentially consists in bringing the water to be treated into contact with a bed of metallic iron and in passing it subsequently through a filter bed suitable for the establishment of a biofilm of ferrobacteria and denitrifying bacteria, without its coming into contact with the air.
Application to water denitrification in a plant or directly in the soil.
No drawing.
Description
~73~
Process for denitriYying water usin~ meta11ic iro~ and installation ~or implementin~ same.
The present invention relates to an ori~inal process permitting the denitriYication o~ water intended ~or human consumption.
There has, in ~act, been noted an increase in the nitrates content of underground water, and even o~ surYace water. In intensively Yarmed areas, this increase can range from 1 to 2, or even exceed ~ mgJl per year.
A high nitrates content ~over 50 Jng/l) renders this water unfit Yor certain uses: human consumption, the ~ood industry, etc.
Two approaches can be adopted in order to reduce the nitrates content of under~round or surface waters:
- either preventive action through modifying Yarming practices; this approach is very promising but takes a long kime to bear fruit;
- or curative action, that is to say the conventional denitrifying treatments.
These denitrifying treatments are either purely physico-chemical (ion exchan~e, for example), or biological.
Biological treatments can make use of denitrifying bacteria, either heterotrophic or autotrophic.
The heterotrophic bacteria used are natural bacteria of the nitrogen cycle, such as Bacillus prodigiusus, and especially Bacillus denitrificans and Pse~domonas.
The general principle of treatments u ing hetrotrophic bacteria is:
nitrate + carbon source ~ denitrifying bacteria nitrogen + carbon dioxide.
The carbon source can be ethanol, methanol, acetic acid, straw, methane or lactic acid (in France, only ethanol and acetic acid are approved for the purpose oY preparing water intended for human consumption).
., ~732~
The basic drawback o~' such a method resides in the Yact that the addition oY a liquid reagent necessitates very care~'ul supervision.
The autotrophic bacteria used depend on the mineral source used. Thus, in the case of the so-call~d 'sulYur' or 'sulYide' process, these are bacteria Of the sul~`ur cycles, such as, a~ong others, red sul~uraires, green sulYuraires or uncoloured sulYuraires.
These purely biological processes have not given entire satisfaction to date, and they are rarely used as they are very costly.
According to the invention, ~or the purpose o~' denitri~yin~ both surface water and the water oY the groundwater tables, the inventors have hit upan the idea of ~aking use of a combination of' biological and chemical phenomena bringing into play both the nitrogen cycle and the iron cycle.
A process of this type ~or denitrifying ground-water tables is already known and exploited under the name of NITRED0 ~. This process is described and commented on by C.
Braester and R. Martinell in Wat. Sci. Tech. l988, vol.
20(3), pages 149-163 and 165-1'12. It is relatively complex insofar as it necessitates the use o~' two concentric series oY peripheral wells around a central well, the series of wells further from the central well s~erving to reduce the nitrates to nitrites with the intermittent use of an appropriate oxygen consuming substance such as methanol, and the other series o~ wells serving to eliminate the gaseous `~ nitrogen and to oæidize the iron and manganese present in the soil, as well as to oxidize any nitrites. As explained in the second article cited above, (pa~es 165-172), this process can only function satis~actorily if different parameters are supervised simultaneousl~.
The object oY the present invention is thus to develop a new and original process for denitri~ying both surface 2~32~
water and the water of the groundwater ~ables that does no-t present the drawbacks of presently known processes.
According to the invention, this object is achieved thanks to a water denitrif~in~ process, characterized in that it consists essentiaLly in bringing the water into contact with a bed of metallic iron and causing it to pass thereafter through a filter bed suitable for the establishment of a biofilm of ferrobacteria and denitrifying bacteria, without its coming into contact with the air.
Water denitrification obtained using this process can be e~plained as follows.
The metallic iron in contact with the water dissolves to give ferrous iron Fe2' in solution. This iron consumes a part of the oxygen in the water and is oxidized to form ferric hydroxide [Fe(OH)3], causing a drop in the oxido-red~ction potential of the water which transforms part of the nitrates into nitrites.
In the absence of an oxygerl supply (no contact with the air), the ~errous iron is not oxidized immediately to form ferric iron which precipitates in the form of Fe(OH)~;it is thus observed a reduction of the nitrates. A succession of biological or chemical processes is then observsd.
The dissolved ferrous iron can be oxidized biologically by the ferrobacteria naturally present in wate~ and any ferrobacteria that may have been added. In the presence o~
very small quantities of oxygen in the water, the oxygen of the nitrates is consumed in this reaction, leadin directly to the formation of nitrogen, which is eliminated.
The ferrous iron also reacts chemically with the nitrites and the nitrates in the water and gives rise to ferric iron, which precipitates in the form o~ Fe(OH~, and to nitrogen.
The denitrifying bacteria naturally present in the water, and any denitrif~ing bacteria added, use as a carbon-containing substrate the organic materials produced b~ the ~7~216 fe~robacteria; they lead to the reduction of the nitrates to nitro~en.
To summarize, all these phenomena lead to the denitrification of the water through the -transformation o~
the nitrates into nitrogen using metallic iron.
The bed of metallic iron used according to the invenkion can be composed of metallic iron in any form. AdvantageouslY, it is composed of iron turnings, iron chips or iron wires.
As a filter bed suitable for the establishment of a biofilm, me~tion can be made, in particular of active carbon, pumice stone, a zeolite or a fire-clay in ~ranular form (crushed brick, BiolitetRl or Biodamine(R), for example).
However, any material sui-table for retaining the ferrobacteria and the denitrifying bacteria ~ithout interferin~ with the biological and chemical reactions occurring in the process can be used. Thus, in the case of denitrification of groundwater, the soil can advantageously be used as a biological support.
In any case, the Filter bed must possess characteristics that make it suitable for fixing a biofilm of ferrobacteria and denitrifying bacteria, and it must have dimensional characteristics such that the desired degree of denitrification is achieved under the conditions of use contemplated.
The ferrobacteria present in a natural state in the water to be treated and which become established in the form of a bio~ilm on the ~ilter bed are, in particular:
Leptothrix, Crenothrix, Toxothrix, Clonothrix, Sphaerotilus, Gallionella, Sideromonas, Siderocapsa, Siderobacter, Siderocystis, Siderococcus, Ferrobacillus metallogenium, Pseudomonas and/or Thiobacillus ferro~idans.
When the denitrifying installation starts up, in order to limit the time taken for natural seeding of the porous support or filter bed, which is usually from 15 to 21 days, ferrobacteria chosen from the list given above can 2~32~6 advantageously be added thereto, in particular those Qf the Gallionella type, as these bacteria, in the absence of oxygen, use the nitrates, which are reduced to nitrites and to nitrogen.
Those denitrifying bacteria present in a natural state in the water to be treated, and which are established in the form of a biofilm on the filter bed are, in particular, of the Bacillus denitrificans t~pe.
To activate initialization of the system, denitrifying bacteria such as, in particular, Bacillus denitrificans, can advanta~eously be addded to the porous support when the installation starts up.
To accelerate the transformation of nitrates into nitrogen, a reducing agent that removes oxygen ~rom water is advantageously added to the water to be treated. This reducing agent is chosen preferably from among physiologically compatible sulfites or thiosulfates. The quantity of reducing agent to be added is advantageously the stoichiometric quantity, calculated on the basis of the oxygen content of the water to be treated.
In order to promote the corrosion of the iron1 it is desirable to produce an electrochemical cell using an element having an electrode potential( Nernst scale) higher than that of iron. Those elements that meet this requirement are:
; 25 copper, nickel, lead, silver, platinum and gold. Of these elements, it is not possible, within the framewor~ of the process according to the invention, ts chooselead on account of its toxicity, silver because it is bactericidal, or platinum or gold on account of their excessive cost. As to nickel, its efficiency would be low as its potential is very close to that of iron. According to the invention, use is thus preferably made of copper, particularly in the form of chips.
According to this preferred form of embodiment, the water to be treated, to which a reducing agent may have been added, is passed over copper, particularly in the form o~
chips, before it is brought into contact with the iron.
To favour the corrosion of the iron, use can also be made of a corrosion current produced by a corrosion cell Pormed by a direct current generator promoting the dissolutio~ of iron (soluble anode).
The tests carried out have shown that the longer the time for which the water is in contact with the iron, the greater the amount of iron released. Contact times are ad~antageously from 2 to 8 hours. Contact times of less than 2 hours generally ~ield unsatisfactory results, while contact times longer than 8 hours do not bring about any significant improvement.
The time of contact in the filter bed is advantageously from 30 minutes to 2 hours, in particular 1 hour.
The source of carbon for the ferrobacteria is generally the mineral carbon of the water treated. Another physiologioally compatible carbon source can possibly be added, ~uch as calcium carbonate, which can be mixed with the iron, for example.
The process according to the invention can be implemented either in a treatment plant or directly in the soil ('in situ' denitrification).
The in-plant treatment e5sentially comprises the following successive steps consisting in:
a3 adding, or not adding, a reducing agent to the water;
b) passing, or not p~ssing, the water over divided copper or using, or not using, a corrosion current;
c3 passin~ the water through a bed of metallic iron, to which a carbon source may or may not have been added, the contact time being from 2 to 8 hours.
d) without its coming into contact with the air, passing the water through a filter bedsuitable for the establishment of a biofilm, to which ferrobacteria and/or denitrifying bacteria may or may not have been previously added, the 2~7~
contact time being from 30 minutes to 2 hours.
Treatment directly in the soil is applicabl~ to wells in which the water of the water table is rich in nitrates. It essentiall~ consists in digging, around a central well, a plurality of wells arranged in a circle having a radius of 4 to 10 m, introducing metallic iron, in particular in chip form, into these wells, possibly mixed with divided copper, and recoverin~ the denitrified water at the outlet from the central well.
The number of peripheral wells is a function of the quantity of nitrates to be eliminated. As a general rule, they number from 4 to 6. The soil serves as a biological support. The processes of oxidation and then reduction of the iron take place in the peripheral wells.
The invention also relates to an in-plant water denitrifying installation, characterized in that it essentially comprises a tank filled with metall~c iron, possibly oovered with metallic ~opper, and a tank containing a filter bed suitable ~or the establishment o~ a biofilm of ferrobacteria and denitrifying bacteria, connected to the outlet Prom the tank containing the iron.
Whatever the method of denitrification, the water obtained is then refined if its turbidity exceeds 0.5 NTU.
For this purpose, filtration can be carried out on l m of sand, at a rate of 5 to 10 m~hr.
In all cases, a disin~ection step is carried out using a conventional means, for example, chlorine, chlorine dioxide or ozone.
Example~ of i~plementation "Pilot" tests were conducted in a treatment plant usin~
a treatment installation of the trpe described above.
Example 1 The water to be treated, containing approximately ~732~
51 m~/l of nitrates, was introduced at a rate of 1 m/hr at the top of a 1.5 m high tank containing 1 meter of metallic iron in the form of chips, surmounted by a layer oY copper chips.
After passing through the layer of copper chips and of metallic iron, the water was routed to the top of a tank containing 1 meter of a filter bed composed of Biolite~R). The denitrified water was collected at the bottom of this tank.
The water iron contact time was 8 hours.
The water-filter bed contact time was 2 hours.
The results obtained are summarized in the table below.
, Input: nitrates ' Output: nitrates ; ' Img/l) ' (mg/l) ,-- -- ---------------l-____________ D + 6 , 51.5 , 43.6 D + 12 ' 51 , 39 D ~ 15 , 51 , 30.5 D ~ 18 ' 51 ' 34 D ~ 24 ' 51 ' 30 ~_______________________L.__________________________ At this rate of 1 m/h, stabilization was found to take place at a level of 30 mg/l.
2 5 Exampl e 2 Using the same installation at a rate of 0.2 m/h, the values obtained for short-duration tests (1 day) were 15 to 20 mg/l of nitrates at the output for water introduced with 50 mg/1 of nitratesO
Process for denitriYying water usin~ meta11ic iro~ and installation ~or implementin~ same.
The present invention relates to an ori~inal process permitting the denitriYication o~ water intended ~or human consumption.
There has, in ~act, been noted an increase in the nitrates content of underground water, and even o~ surYace water. In intensively Yarmed areas, this increase can range from 1 to 2, or even exceed ~ mgJl per year.
A high nitrates content ~over 50 Jng/l) renders this water unfit Yor certain uses: human consumption, the ~ood industry, etc.
Two approaches can be adopted in order to reduce the nitrates content of under~round or surface waters:
- either preventive action through modifying Yarming practices; this approach is very promising but takes a long kime to bear fruit;
- or curative action, that is to say the conventional denitrifying treatments.
These denitrifying treatments are either purely physico-chemical (ion exchan~e, for example), or biological.
Biological treatments can make use of denitrifying bacteria, either heterotrophic or autotrophic.
The heterotrophic bacteria used are natural bacteria of the nitrogen cycle, such as Bacillus prodigiusus, and especially Bacillus denitrificans and Pse~domonas.
The general principle of treatments u ing hetrotrophic bacteria is:
nitrate + carbon source ~ denitrifying bacteria nitrogen + carbon dioxide.
The carbon source can be ethanol, methanol, acetic acid, straw, methane or lactic acid (in France, only ethanol and acetic acid are approved for the purpose oY preparing water intended for human consumption).
., ~732~
The basic drawback o~' such a method resides in the Yact that the addition oY a liquid reagent necessitates very care~'ul supervision.
The autotrophic bacteria used depend on the mineral source used. Thus, in the case of the so-call~d 'sulYur' or 'sulYide' process, these are bacteria Of the sul~`ur cycles, such as, a~ong others, red sul~uraires, green sulYuraires or uncoloured sulYuraires.
These purely biological processes have not given entire satisfaction to date, and they are rarely used as they are very costly.
According to the invention, ~or the purpose o~' denitri~yin~ both surface water and the water oY the groundwater tables, the inventors have hit upan the idea of ~aking use of a combination of' biological and chemical phenomena bringing into play both the nitrogen cycle and the iron cycle.
A process of this type ~or denitrifying ground-water tables is already known and exploited under the name of NITRED0 ~. This process is described and commented on by C.
Braester and R. Martinell in Wat. Sci. Tech. l988, vol.
20(3), pages 149-163 and 165-1'12. It is relatively complex insofar as it necessitates the use o~' two concentric series oY peripheral wells around a central well, the series of wells further from the central well s~erving to reduce the nitrates to nitrites with the intermittent use of an appropriate oxygen consuming substance such as methanol, and the other series o~ wells serving to eliminate the gaseous `~ nitrogen and to oæidize the iron and manganese present in the soil, as well as to oxidize any nitrites. As explained in the second article cited above, (pa~es 165-172), this process can only function satis~actorily if different parameters are supervised simultaneousl~.
The object oY the present invention is thus to develop a new and original process for denitri~ying both surface 2~32~
water and the water of the groundwater ~ables that does no-t present the drawbacks of presently known processes.
According to the invention, this object is achieved thanks to a water denitrif~in~ process, characterized in that it consists essentiaLly in bringing the water into contact with a bed of metallic iron and causing it to pass thereafter through a filter bed suitable for the establishment of a biofilm of ferrobacteria and denitrifying bacteria, without its coming into contact with the air.
Water denitrification obtained using this process can be e~plained as follows.
The metallic iron in contact with the water dissolves to give ferrous iron Fe2' in solution. This iron consumes a part of the oxygen in the water and is oxidized to form ferric hydroxide [Fe(OH)3], causing a drop in the oxido-red~ction potential of the water which transforms part of the nitrates into nitrites.
In the absence of an oxygerl supply (no contact with the air), the ~errous iron is not oxidized immediately to form ferric iron which precipitates in the form of Fe(OH)~;it is thus observed a reduction of the nitrates. A succession of biological or chemical processes is then observsd.
The dissolved ferrous iron can be oxidized biologically by the ferrobacteria naturally present in wate~ and any ferrobacteria that may have been added. In the presence o~
very small quantities of oxygen in the water, the oxygen of the nitrates is consumed in this reaction, leadin directly to the formation of nitrogen, which is eliminated.
The ferrous iron also reacts chemically with the nitrites and the nitrates in the water and gives rise to ferric iron, which precipitates in the form o~ Fe(OH~, and to nitrogen.
The denitrifying bacteria naturally present in the water, and any denitrif~ing bacteria added, use as a carbon-containing substrate the organic materials produced b~ the ~7~216 fe~robacteria; they lead to the reduction of the nitrates to nitro~en.
To summarize, all these phenomena lead to the denitrification of the water through the -transformation o~
the nitrates into nitrogen using metallic iron.
The bed of metallic iron used according to the invenkion can be composed of metallic iron in any form. AdvantageouslY, it is composed of iron turnings, iron chips or iron wires.
As a filter bed suitable for the establishment of a biofilm, me~tion can be made, in particular of active carbon, pumice stone, a zeolite or a fire-clay in ~ranular form (crushed brick, BiolitetRl or Biodamine(R), for example).
However, any material sui-table for retaining the ferrobacteria and the denitrifying bacteria ~ithout interferin~ with the biological and chemical reactions occurring in the process can be used. Thus, in the case of denitrification of groundwater, the soil can advantageously be used as a biological support.
In any case, the Filter bed must possess characteristics that make it suitable for fixing a biofilm of ferrobacteria and denitrifying bacteria, and it must have dimensional characteristics such that the desired degree of denitrification is achieved under the conditions of use contemplated.
The ferrobacteria present in a natural state in the water to be treated and which become established in the form of a bio~ilm on the ~ilter bed are, in particular:
Leptothrix, Crenothrix, Toxothrix, Clonothrix, Sphaerotilus, Gallionella, Sideromonas, Siderocapsa, Siderobacter, Siderocystis, Siderococcus, Ferrobacillus metallogenium, Pseudomonas and/or Thiobacillus ferro~idans.
When the denitrifying installation starts up, in order to limit the time taken for natural seeding of the porous support or filter bed, which is usually from 15 to 21 days, ferrobacteria chosen from the list given above can 2~32~6 advantageously be added thereto, in particular those Qf the Gallionella type, as these bacteria, in the absence of oxygen, use the nitrates, which are reduced to nitrites and to nitrogen.
Those denitrifying bacteria present in a natural state in the water to be treated, and which are established in the form of a biofilm on the filter bed are, in particular, of the Bacillus denitrificans t~pe.
To activate initialization of the system, denitrifying bacteria such as, in particular, Bacillus denitrificans, can advanta~eously be addded to the porous support when the installation starts up.
To accelerate the transformation of nitrates into nitrogen, a reducing agent that removes oxygen ~rom water is advantageously added to the water to be treated. This reducing agent is chosen preferably from among physiologically compatible sulfites or thiosulfates. The quantity of reducing agent to be added is advantageously the stoichiometric quantity, calculated on the basis of the oxygen content of the water to be treated.
In order to promote the corrosion of the iron1 it is desirable to produce an electrochemical cell using an element having an electrode potential( Nernst scale) higher than that of iron. Those elements that meet this requirement are:
; 25 copper, nickel, lead, silver, platinum and gold. Of these elements, it is not possible, within the framewor~ of the process according to the invention, ts chooselead on account of its toxicity, silver because it is bactericidal, or platinum or gold on account of their excessive cost. As to nickel, its efficiency would be low as its potential is very close to that of iron. According to the invention, use is thus preferably made of copper, particularly in the form of chips.
According to this preferred form of embodiment, the water to be treated, to which a reducing agent may have been added, is passed over copper, particularly in the form o~
chips, before it is brought into contact with the iron.
To favour the corrosion of the iron, use can also be made of a corrosion current produced by a corrosion cell Pormed by a direct current generator promoting the dissolutio~ of iron (soluble anode).
The tests carried out have shown that the longer the time for which the water is in contact with the iron, the greater the amount of iron released. Contact times are ad~antageously from 2 to 8 hours. Contact times of less than 2 hours generally ~ield unsatisfactory results, while contact times longer than 8 hours do not bring about any significant improvement.
The time of contact in the filter bed is advantageously from 30 minutes to 2 hours, in particular 1 hour.
The source of carbon for the ferrobacteria is generally the mineral carbon of the water treated. Another physiologioally compatible carbon source can possibly be added, ~uch as calcium carbonate, which can be mixed with the iron, for example.
The process according to the invention can be implemented either in a treatment plant or directly in the soil ('in situ' denitrification).
The in-plant treatment e5sentially comprises the following successive steps consisting in:
a3 adding, or not adding, a reducing agent to the water;
b) passing, or not p~ssing, the water over divided copper or using, or not using, a corrosion current;
c3 passin~ the water through a bed of metallic iron, to which a carbon source may or may not have been added, the contact time being from 2 to 8 hours.
d) without its coming into contact with the air, passing the water through a filter bedsuitable for the establishment of a biofilm, to which ferrobacteria and/or denitrifying bacteria may or may not have been previously added, the 2~7~
contact time being from 30 minutes to 2 hours.
Treatment directly in the soil is applicabl~ to wells in which the water of the water table is rich in nitrates. It essentiall~ consists in digging, around a central well, a plurality of wells arranged in a circle having a radius of 4 to 10 m, introducing metallic iron, in particular in chip form, into these wells, possibly mixed with divided copper, and recoverin~ the denitrified water at the outlet from the central well.
The number of peripheral wells is a function of the quantity of nitrates to be eliminated. As a general rule, they number from 4 to 6. The soil serves as a biological support. The processes of oxidation and then reduction of the iron take place in the peripheral wells.
The invention also relates to an in-plant water denitrifying installation, characterized in that it essentially comprises a tank filled with metall~c iron, possibly oovered with metallic ~opper, and a tank containing a filter bed suitable ~or the establishment o~ a biofilm of ferrobacteria and denitrifying bacteria, connected to the outlet Prom the tank containing the iron.
Whatever the method of denitrification, the water obtained is then refined if its turbidity exceeds 0.5 NTU.
For this purpose, filtration can be carried out on l m of sand, at a rate of 5 to 10 m~hr.
In all cases, a disin~ection step is carried out using a conventional means, for example, chlorine, chlorine dioxide or ozone.
Example~ of i~plementation "Pilot" tests were conducted in a treatment plant usin~
a treatment installation of the trpe described above.
Example 1 The water to be treated, containing approximately ~732~
51 m~/l of nitrates, was introduced at a rate of 1 m/hr at the top of a 1.5 m high tank containing 1 meter of metallic iron in the form of chips, surmounted by a layer oY copper chips.
After passing through the layer of copper chips and of metallic iron, the water was routed to the top of a tank containing 1 meter of a filter bed composed of Biolite~R). The denitrified water was collected at the bottom of this tank.
The water iron contact time was 8 hours.
The water-filter bed contact time was 2 hours.
The results obtained are summarized in the table below.
, Input: nitrates ' Output: nitrates ; ' Img/l) ' (mg/l) ,-- -- ---------------l-____________ D + 6 , 51.5 , 43.6 D + 12 ' 51 , 39 D ~ 15 , 51 , 30.5 D ~ 18 ' 51 ' 34 D ~ 24 ' 51 ' 30 ~_______________________L.__________________________ At this rate of 1 m/h, stabilization was found to take place at a level of 30 mg/l.
2 5 Exampl e 2 Using the same installation at a rate of 0.2 m/h, the values obtained for short-duration tests (1 day) were 15 to 20 mg/l of nitrates at the output for water introduced with 50 mg/1 of nitratesO
Claims (20)
1. A water denitrifying process, which essentially consists in bringing the water to be treated into contact with a bed of metallic iron and causing it to pass thereafter through a filter bed suitable for the establishment of a biofilm of ferrobacteria and denitrifying bacteria, without its coming into contact with the air.
2. A process according to claim 1, wherein the bed of metallic iron is composed of iron turnings, iron chips or iron wires.
3. A process according to claim 1, wherein the filter bed suitable for the establishment of a biofilm is composed of active carbon, pumice stone, a zeolite or a fire-clay in granular form.
4. A process according to claim 1, wherein, when the installation starts up, ferrobacteria are added to the filter bed.
5. A process according to claim 4, wherein said ferrobacteria are of the Gallionella type.
6. A process according to claim 1, wherein, when the installation starts up, denitrifying bacteria are added to the filter bed.
7. A process according to claim 6, wherein said denitrifying bacteria are of the Bacillus denitrificans type.
8. A process according to claim 1, wherein a reducing agent is added to the water to be treated.
9. A process according to claim 8, wherein said reducing agent is a physiologically compatible sulfite or thiosulfate.
10. A process according to claim 1, wherein, in order to promote the corrosion of the iron, an electrochemical cell is produced by means of an element having a potential higher than that of iron.
11. A process according to claim 10, wherein said element is copper.
12. A process according to claim 1, wherein, in order to promote the corrosion to the iron, use is made of a corrosion current produced by a corrosion cell formed by a direct current generator promoting the dissolution of iron.
13. A process according to claim 1, wherein the contact time between the water and the iron is from 2 to 8 hours.
14. A process according to claim 1, wherein the contact time in the filter bed is 30 minutes to 2 hours.
15. A process according to claim 1, wherein there is added a physiologically compatible source of carbon.
16. A process according to claim 15, wherein said physiologically compatible source of carbon is calcium carbonate mixed with iron.
17. In-plant water denitrifying process, which essentially comprises the following successive steps consisting in:
a) adding, or not adding, a reducing agent to the water to be treated, b) passing, or not passing, the water to be treated through divided copper or using, or not using, a corrosion current;
c) passing the water to be treated through a bed of metallic iron, to which a carbon source may ox may not have been added, the contact time being from 2 to 8 hours;
d) without its coming into contact with the air, passing the water through a filter bed suitable for the establishment of a biofilm, to which ferrobacteria or denitrifying bacteria, or ferrobacteria and denitrifying bacteria, may or may not have been added, the contact time being from 30 minutes to 2 hours.
a) adding, or not adding, a reducing agent to the water to be treated, b) passing, or not passing, the water to be treated through divided copper or using, or not using, a corrosion current;
c) passing the water to be treated through a bed of metallic iron, to which a carbon source may ox may not have been added, the contact time being from 2 to 8 hours;
d) without its coming into contact with the air, passing the water through a filter bed suitable for the establishment of a biofilm, to which ferrobacteria or denitrifying bacteria, or ferrobacteria and denitrifying bacteria, may or may not have been added, the contact time being from 30 minutes to 2 hours.
18. A process for directly denitrifying water in the soil, which esentially comprises digging, around a central well, a plurality of wells arranged in a circle of 4 to 10 m in radius, introducing into these wells metallic iron, mixed or not with divided copper, and recovering the denitrified water at the outlet from the central well.
19. A process according to claim 18, wherein said metallic iron is in the form of chips.
20. In-plant water denitrifying installation, which essentially comprises a tank filled with metallic iron, covered or not with metallic copper, and a tank containing a filter bed suitable for the establishment of a biofilm of ferrobacteria and denitrifying bacteria, connected to the tank containing iron.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| FR9108526 | 1991-07-08 | ||
| FR9108526A FR2678923B1 (en) | 1991-07-08 | 1991-07-08 | METHOD FOR DENITRIFICATION OF WATER USING METAL IRON AND INSTALLATION FOR ITS IMPLEMENTATION. |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2073216A1 true CA2073216A1 (en) | 1993-01-09 |
Family
ID=9414813
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2073216 Abandoned CA2073216A1 (en) | 1991-07-08 | 1992-07-06 | Process for denitrifying water using metallic iron and installation for implementing same |
Country Status (6)
| Country | Link |
|---|---|
| EP (1) | EP0522946B1 (en) |
| AT (1) | ATE141244T1 (en) |
| CA (1) | CA2073216A1 (en) |
| DE (2) | DE69212736T2 (en) |
| ES (1) | ES2046162T3 (en) |
| FR (1) | FR2678923B1 (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109721154A (en) * | 2019-03-06 | 2019-05-07 | 苏州方舟环保科技有限公司 | A kind of device of sulphur iron coupling technique removal nitrate nitrogen |
| CN111003796A (en) * | 2019-12-30 | 2020-04-14 | 河海大学 | Iron-carbon coupling denitrification filter for sewage denitrification |
Families Citing this family (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5618427A (en) * | 1995-02-13 | 1997-04-08 | W. R. Grace & Co.-Conn. | Composition and method for degradation of nitroaromatic contaminants |
| NL1000794C2 (en) * | 1995-07-13 | 1997-01-14 | Holding Company Belgie Nv | Preparation comprising zeolite, method for its preparation and use thereof for controlling biological conditions in waters. |
| US6719902B1 (en) | 1997-04-25 | 2004-04-13 | The University Of Iowa Research Foundation | Fe(o)-based bioremediation of aquifers contaminated with mixed wastes |
| WO1998049106A1 (en) * | 1997-04-25 | 1998-11-05 | The University Of Iowa Research Foundation | Fe(o)-based bioremediation of aquifers contaminated with mixed wastes |
| DE19953249A1 (en) * | 1999-11-04 | 2001-05-31 | Fraunhofer Ges Forschung | Process for the purification of waste water |
| ITMI20052150A1 (en) | 2005-11-11 | 2007-05-12 | Enitecnologie Spa | PROCESS FOR THE TREATMENT OF CONTAMINATED WATERS BY MEANS OF A BIFUNCTIONAL SYSTEM MADE OF IRON AND ZEOLITH |
| DE102017111014A1 (en) | 2017-05-19 | 2018-11-22 | Gunter Buxbaum | Use of carbon iron as a reducing agent for removing nitrate from water |
Family Cites Families (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| AU592948B2 (en) * | 1984-04-30 | 1990-02-01 | Don E. Heskett | Method of treating fluids |
| GB8515101D0 (en) * | 1985-06-14 | 1985-07-17 | Anglian Water Authority | Ground water treatment |
| JP2835394B2 (en) * | 1988-10-26 | 1998-12-14 | カナツ技建工業株式会社 | Sewage purification method and apparatus |
-
1991
- 1991-07-08 FR FR9108526A patent/FR2678923B1/en not_active Expired - Fee Related
-
1992
- 1992-07-06 DE DE69212736T patent/DE69212736T2/en not_active Expired - Fee Related
- 1992-07-06 EP EP19920401933 patent/EP0522946B1/en not_active Expired - Lifetime
- 1992-07-06 AT AT92401933T patent/ATE141244T1/en not_active IP Right Cessation
- 1992-07-06 CA CA 2073216 patent/CA2073216A1/en not_active Abandoned
- 1992-07-06 ES ES92401933T patent/ES2046162T3/en not_active Expired - Lifetime
- 1992-07-06 DE DE92401933T patent/DE522946T1/en active Pending
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN109721154A (en) * | 2019-03-06 | 2019-05-07 | 苏州方舟环保科技有限公司 | A kind of device of sulphur iron coupling technique removal nitrate nitrogen |
| CN111003796A (en) * | 2019-12-30 | 2020-04-14 | 河海大学 | Iron-carbon coupling denitrification filter for sewage denitrification |
Also Published As
| Publication number | Publication date |
|---|---|
| EP0522946A1 (en) | 1993-01-13 |
| DE69212736D1 (en) | 1996-09-19 |
| FR2678923A1 (en) | 1993-01-15 |
| ES2046162T1 (en) | 1994-02-01 |
| DE522946T1 (en) | 1994-03-17 |
| ES2046162T3 (en) | 1997-03-16 |
| FR2678923B1 (en) | 1993-11-05 |
| DE69212736T2 (en) | 1997-04-17 |
| EP0522946B1 (en) | 1996-08-14 |
| ATE141244T1 (en) | 1996-08-15 |
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