Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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).
.,
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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
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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
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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
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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
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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
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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
