Note: Descriptions are shown in the official language in which they were submitted.
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1 Title: ELECTROCHEMICAL TREATMENT OF WATER CONTAMINATED WITH
z NITROGENOUS COMPOUNDS
3
4 This invention relates to the treatment ofi polluted or contaminated
water by
a electrochemical treatment (which includes both galvanic and
electrolytic treatment).
~ The invention is intended to be applied to waters contaminated
by dissolved
nitrogenous compounds.
s
9 in conventional cases where electrolysis has been used for
treating contaminated
1 o water, the reactions that have been utilised have been those,
for example, which take
1 ~ metals out of solution by cathode deposition.
12
13 Work has been done on the use of electrolysis for treating
effiluent from manufacturing
14 establishments, where the water is polluted with a specific
pollutant. But those
1 s systems were special in that the effiluent water was already
contained, and the
~ s research effort was directed to cleaning up or recovering specific
contaminants, at
17 known concentrations, and in a system that was designed to
cater for the pollutant
1 a when the factory was built.
19
2o Ammonium (NH4+) in sewage water can be broken down by bacterial
action. Much
21 consideration and effort has been applied over many years to
engineering the
zz ammonium breakdown reaction on a large scale, in the context
of municipal sewage
z3 water treatment. Generally, bacteria promote oxidation of the
ammonium into nitrate;
z4 other bacteria then promote denitrification or reduction of
the nitrate to nitrogen gas.
25 The reaction has been engineered by providing an environment
in which bacteria can
z6 effect the oxidation and reduction.
27
is Some shortcomings of this conventional procedure will now be
discussed. The
29 biological oxidation of ammonium to nitrate and the subsequent
reduction of the
so nitrate to nitrogen gas, has undesirable side-effects. Other
compounds of nitrogen
31 are fiormed, such as nitric oxide (NO), nitrous oxide (N20),
and nitrogen dioxide (N02),
3z which are considered to be either directly toxic to humans
and other animals, or
33 harmful to the ozone layer or other aspects of the environment.
34
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2
1 One problem is that the conversion of ammonium to nitrogen, via nitrate,
done
z microbiologically, as in the conventional systems, has been far from
complete.
3 Significant quantities of the nitrogen oxides can be released, as gases,
during the
reactions, in addition to the nitrogen gas. When, as in the conventional
systems, the
s oxidation of ammonium /ammonia occurred biologically (nitrification), the
reaction
s pathway may be described as:
7 NH4+ / NH3 ___~ N02 ___~ NO (N20) ___> N03
a whereby the harmful intermediate gaseous compounds may escape into the
s atmosphere.
11 Aiso, the nitrate produced by the ammonium breakdown cannot be left in the
water.
12 And nitrate itself, for example from agricultural run-off, may be present
as a
13 nitrogenous contaminant per se, in groundwater.
14
1 5 When the reduction of nitrate occurred biologically (denitrification), the
reaction
1 6 pathway may be described as:
17 N03 ___~ NO2_ ___~ NOX ___~ NZO ___~ N2
1 a Again, the intermediate gaseous compounds may escape.
19
zo Another point that should be noted is that the efficiency of the
conventional
21 biochemical nitrification and denitrification reactions is affected by cold
weather,
zz whereby it is found that the gases released to the atmosphere in the winter
contain an
z3 even larger proportion of N20 and NOX gases. This is a marked disadvantage
of the
z4 conventional systems. In some cases, also, the temperature can be so low
that
z~ biological reactions substantially do not take place at a11, and breakdown
of the
zs contaminants has to await warmer weather.
27
za It is recognised that the electrochemical reactions as described herein
have the
zs potential to proceed at lower temperatures than the conventional biological
processes.
3 ~ They may therefore be suitable for cold-climate applications, where
biological
31 remediation is ineffective for most of the year. '
3z
33
3Q GENERAL FEATURES OF THE INVENTION
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3
1 It is recognised in the invention that, in order to remove nitrogenous
contamination,
z the nitrogenous contaminants can be transformed, by engineered
electrochemical
s processes, directly into nitrogen gas. (Nitrogen gas of course already
comprises 4/5
° 4 of the atmosphere, and its release is not harmful.)
s
' s It is also recognised that inorganic electrochemical reactions can be used
to drive
both kinds of breakdown reaction, i.e the oxidation of e.g ammonium, and the
a reduction of e.g nitrate, both to gaseous nitrogen directly, whereby the
production and
9 release of the harmful intermediate gaseous compounds is eliminated or
reduced.
1 1 Electro-chemical treatment of water containing nitrogenous contaminants
has been
1 2 proposed (Lin & Wu, 1995 Jrnl. Env. Sci. & Health, A30, i 445-1456). What
is not
13 present in the prior art is the recognition that nitrogenous contaminants
can be
14 transformed, by commercially practicable electrochemical processes,
directly into
1 s nitrogen gas, and thereby eliminated from the system.
16
In assessing whether a particular reaction will proceed electrotytically, the
reaction
1 s may be compared to the Nernst equilibrium equation:
~ s E = Eo - (RT/nF)loge(Q)
zo in which E is the cell voltage, E° is the cell voltage of a standard
cell (calculated, or
z1 derived from tables), Q is the concentration quotient, being the ratio of
the
za mathematical product of the concentrations of all the reaction products,
divided by the
zs mathematical product of the concentrations of all the reactants. For
example, at 25°C,
z4 the term (RT/nF)fog°(Q) can be evaluated as (0.0592/n)loglo(Q).
zs One aspect of the present invention lies in determining, from an assessment
of the
z7 Nernst equation, whether electrolysis will be effective in a particular
situation, to cause
is contaminants to break down, and if so to what, and under what treatment
conditions.
29
sa The Nernsi relationships for many different transformations and conditions
may be
° 31 plotted on a predominance field diagram, or phase diagram, or
Pourbaix diagram, i.e
3z a nitrogen-oxygen-hydrogen Eh-pH solution phase diagram. Such a diagram is
33 published and available on a chemistry text book basis, as it relates to
aqueous
s4 nitrogenous compounds. Sets of the Nernst relationships appropriate to the
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4
1 nitrogenous compounds are also available in table form, again on a text-book
basis,
z and that may also be used.
3
In carrying out a preferred form of the invention, the designer of the system
may use
s the Nernst relationships in a phase-diagram form, for example. The designer
enters
s on the phase diagram the Eh and pH that are measured in the contaminated
system. '
7 He then notes, from the diagram, the voltage V-Ngas at which, at the
measured pH,
s the predominating form of nitrogen is nitrogen gas. A computation is made as
to the
s voltage difference between V-Ngas and the voltage V-Eh as actually measured.
11 The designer then provides an electrochemical cell, having two electrodes
(i.e an
1 z anode and a cathode). If the nitrogenous contaminant is one (e.g dissolved
nitrate)
1 s which requires the addition of electrons in order to be transformed into
gaseous
14 nitrogen, then it can pick up the electrons it needs from the cathode, and
the designer
1 s should arrange the cell so that the water flows by the cathode. The cell
should be so
1 s arranged as to its size, and the velocity of movement of water
therethrough, that the
17 residence time of the water near the cathode is long enough for the
transformation
1 s reaction to be substantially completed.
19
zo Similarly, if the nitrogenous contaminant is one (e.g dissolved ammonium)
which
z 1 requires the subtraction of electrons in order to be transformed into
gaseous nitrogen,
22 then it can shed the excess electrons to the anode, and the designer should
arrange
z3 the cell so that the water flows by the anode. Again, the cell should be so
arranged
z4 a5 to its size, and the velocity of movement of water therethrough, that
the residence
25 time of the water near the anode is long enough for the transformation
reaction to be
z6 substantially completed.
27
zs In either case, the engineer should set the voltage V-cell which will be
applied
z9 between the electrodes to a value at which the Eh voltage as measured in
the vicinity
so of one of the electrodes, after the electro-chemical reaction has been
initiated, lies at
s 1 a voltage level, V-Ngas, at which nitrogen gas predominates. That is to
say, the ~
3z engineer adjusts the electrical energy available at the electrodes to the
effect that the
33 Eh voltage near one of the electrodes changes from V-Eh as measured
initially to a
s4 voltage that lies within the range of Eh voltages at which nitrogen gas
predominates.
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1
2 In either case, it may be noted, the other electrode basically
serves no fiunction in the
s transformation reaction. (On the other hand, as will be explained
later, sometimes
4 both the anode and the cathode of the same cell can be instrumental
in releasing
s gaseous nitrogen.)
a 6
It should be noted that in cases where very reductive conditions
prevail, the Eh
a voltage, V-Eh, as measured, may be negative.
9
1 o As an example, in a particular case, a body of water contaminated
with dissolved
11 ammonium may be identifiied as having an Eh voltage of, say,
-0.53 volts. The pH of
12 the body may be measured at, say, 4.5. Given that the pH is
not to be changed, the
1 s designer traces up the diagram, at a constant pH, until he comes
to an Eh voltage at
14 which the predominant phase of the nitrogen is nitrogen gas.
It is found that nitrogen
1 s gas predominates over a range of Eh voltages at that pH, i.e
from about 0.0 volts to
1 s about +0.3 volts. Given that range, the designer preferably
should aim for an Eh
17 voltage of +0.15 volts.
18
a s (It is assumed that the pH of the water is a given, and that
nothing is to be done to
zo change the pH. Sometimes, however, it is possible economically
to change the pH,
21 and in that case such changes can be factored into the computations
-- but it is a
22 benefit of the system as described herein that usually the pH
may be left at whatever
2s value obtains naturally, and only the Eh voltage need be
manipulated.)
24
25 In the exemplary case of water contaminated by ammonium at a
pH of 4.5, the
zs designer should aim to set up the cell so that the Eh voltage
of the water in the region
27 of the anode of the cell is about +0.15 volts. The engineer
then adjusts the electro-
2s chemical parameters (including the voltage applied between the
electrodes) in order
z9 to produce the required Eh voltage V-Ngas near the anode --
being a voltage between
so the limits V-Ngas-upper (0.3 volts) and V-Ngas-lower (0.0 volts).
Preferably, the cell
31 is engineered to provide an Eh voltage in the vicinity of the
anode of +0.15 volts.
s2
33 To achieve this, the Eh voltage near the cathode might go even more
negative, i.e
a4 further away firom promoting the gaseous phase of the nitrogen. But that
does not
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s
i matter: when the water passes near the anode, nitrogen gas will bubble off,
and when
2 the water passes the cathode, nitrogen gas will not bubble off. Only one of
the
s electrodes -- the anode in this case -- is effective.
4
Preferably, it should be arranged that the water moves through the cell (or
cells) in
s such a manner that all the contaminated water is caused to be close to the
anode for '
7 an adequate residence time.
s
s It will be understood that when the contaminant is nitrate, the cathode now
becomes
i o the electrode which is effective to transform the nitrogen into nitrogen
gas. In an
1 ~ exemplary case, the Eh might be measured at +1.1 volts, and the pH at 9Ø
Now,
12 having carried out the Nernst calculations (or having inspected the phase
diagram) the
i s engineer knows to arrange the electrochemical characteristics of the cell
so that the
Eh voltage as measured in the vicinity of the cathode of the cell lies at
about -0.1
volts, since that is a voltage at the middle of the range which, at a pH of
9.0, nitrogen
~ s gas is the predominant phase.
The cathode and anode of the cell might be physically the same. In fact, the
polarity
~ s of the cell may be reversed periodically or cyclically. This can be useful
for ensuring
2o that all the water gets treated, and also, switching the electrodes might
serve to
21 prevent a build up of a coating on the anode, which sometimes is a problem.
22 '
23 In some cases, the Nernst equation indicates that the transformation to the
nitrogen
z4 gas phase can take place galvanically, i.e without the input of electrical
energy from
z5 an outside source, if the cell is engineered appropriately. In that case,
the electrodes
2s would be of different materials, the anode being the material that is the
more active in
27 the electro-chemical series.
as
29 It should be noted that the change of phase between the nitrogen gas phase
and the
3o contaminant phase is gradual, not sudden. Thus, generally, the release of
nitrogen,
although at a maximum at one particular Eh (for a given pH), still occurs, in
most
sz cases, over quite a wide range of Eh. Of course, the expert knows that it
is not
33 possible to measure Eh voltages of contaminated waters to a high degree of
s4 consistency and accuracy, nor is it possible to set the cell voltage all
that accurately.
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7
1 It is recognised herein, however, that the margins of Eh voltage between
which an
z effective release of nitrogen gas will take place are wide enough that the
inaccuracies
3 of measurement and adjustment, which are inevitable in practice, can be
accommodated.
s The designer must see to it that sufficient electrical energy
is available in the cell to
keep the Eh voltage in the contaminated water at the desired
level, i.e between the
a calculated /indicated limits. The resistance of the water should
not be expected to
9 remain constant as treatment takes place, and in fact generally
the resistance of the
1 o cell may be expected to increase as the contaminant is eliminated.
Thus, in order to
11 maintain the required Eh voltage in the water, more energy must
be passed through
1 z the cell. There may come a point, of course, at which it is
no longer practical to
13 maintain voltage between the electrodes: but it is recognised
herein that, in general,
waters that are contaminated with nitrogenous contaminants are
of such a conductivity
1 s that the required voltages at the electrodes can be maintained
while supplying
1 s electrical energy in an economical manner.
17
1 a In many cases, it is economical to provide automatic control of the
voltages, which
1 s can be useful in a case where the Eh, pH, concentration of the
contaminants, etc
zo might be subject to variation over a period of time.
z1
zz It may be noted that carbonates, sulphates, chlorides, etc, and the like,
may all be
23 present in addition to the nitrogenous contaminants in the water, and all
these
z4 substances can have an effect on the Eh voltage. However, it is recognised
that the
zs single measurement of Eh takes care of all the substances. By contrast, it
would be
zs difficult to determine what the Eh voltage should be, by calculation,
taking account of
z7 the various substances in their various concentrations. But, in most cases,
a single
zs measure of the actual value of the Eh of the water is all that is needed:
the voltage
29 that must be obtained to transform the nitrogen to nitrogen gas can also be
easily
so determined, e.g by inspection from the nitrogen phase diagram. This makes
the
s ~ system easy to automate: even though the composition of the contaminants
may
3z change, as the water passes by, and as a result of the treatment, all that
need be
ss done to maintain efficiency is to periodically check the Eh and the pH, and
adjust the
cell voltage accordingly.
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s
1
2 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
3
4 By way of further explanation of the invention, exemplary embodiments of the
s invention will now be described with reference to the accompanying drawings,
in
s which: .
7
s Fig 1 is an Eh-pH diagram, which indicates the phases of nitrogen under the
various
9 conditions;
1 o Fig 2 is a schematic diagram of a municipal sewage treatment installation,
which
11 embodies the invention;
12 Fig 3 is a cross-section of an area of contaminated ground around a well,
undergoing
1 s electrolytic treatment;
14 Fig 4 is a section corresponding to Fig 3, showing galvanic treatment;
1 5 Fig 5 is a cross-section of an area of contaminated ground, undergoing
electrolytic
~ s treatment;
17 Fig 6 is a plan view of an area of ground, undergoing treatment of the kind
as shown
18 in Fig 5.
19
2o The systems shown in the accompanying drawings and described below are
examples
21 which embody the invention. It should be noted that the scope of the
invention is
zz defined by the accompanying claims, and not necessarily by specific
features of
zs exemplary embodiments.
24
25 Fig 1 is an Eh-pH diagram for aqueous nitrogen species under the conditions
stated.
2s Whether the predominant form of the nitrogen in the water is ammonium,
ammonia,
27 nitrate, nitrite, or nitrogen gas, may be derived, according to the
diagram, by entering
za the actual conditions of the variables, and reading off the redox voltages.
Since it is
z9 desired that nitrogen gas be the predominant form, the engineer may read
off the
so redox voltages, for a given pH, between which N2 gas will predominate.
37
sz For example, at a pH of 7 the predominance field of nitrogen gas occurs
between
33 -250 mV (moderately reducing) and +150 mV (moderately oxidising) at a
nitrogen
s4 activity of 0.8 atm, oxygen gas activity at 0.2 atm, and the activity of
aqueous nitrogen
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9
1 species fixed at 10-3 moles/litre.
2
s (For the purposes of this specification, the term "activity" is used to
indicate the ideal
4 thermodynamic concentration of a chemical species present in the system
under
s consideration. If the species is present as a gas, the activity may be
measured as a
s partial pressure; if the species is present as a non-volatile component, it
may be
7 measured in terms of a concentration value, for example moles/litre.)
s
s In applying the invention to the treatment of sewage water, an electrolytic/
galvanic
1 o cell is set up, in which the sewage water is the electrolyte. The purpose
of the cell is
11 to enhance the electrochemical oxidation of ammonium in the sewage water,
and the
12 reduction of any residual or generated nitrate in the sewage water, both
directly to
13 nitrogen gas, thereby minimising the escape of the toxic nitrogenous gases,
and
14 allowing the preferential escape of non-toxic nitrogen gas.
~ s Preferably, a bank of electrochemical cells is provided, each cell having
anodic and
1 ~ cathodic compartments connected by a salt bridge or an ion-selective
membrane.
1a
1 s Fig 2 is an example of a water treatment plant. Raw sewage enters the
primary
zo clarifier 20, where initial settling of solid material takes place. The
water then passes
21 to a cell 23. The cell is divided by an ion-selective membrane 25, the two
zz compartments of the cell being given anode 27 and cathode 29 status by the
action of
23 a voltage source 30.
24
2s Having passed through the anode 27, in which the ammonium in the sewage
water is
z6 oxidised and removed, the water passes to the aerobic tank 32, where BOD
27 (Biological Oxygen Demand) is reduced. (The tank 32 is of conventional
design,
2a being, for example, a trickling tank, or an activated sludge tank.) Sludge
is separated
2s and removed from the water in a settling tank 34. Biomass recycling takes
place due
3o to the interaction of the tanks 32 and 34.
31
32 Water from the settling tank 34, which contains nitrate, passes to the
cathode
33 compartment 29 of the cell 23, where the nitrate is broken down and
removed. As
s4 shown, the water from the settling tank 34 may first be passed through one
or more
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powered cells 36, which serve to ensure that the breakdown and removal of all
the
2 ammonium and nitrate is as complete as possible.
3
4 The clean effluent water can be further treated with chlorine, as at 38, if
desired.
5
s The electrons released in the anodic compartment 27 by the oxidation of the
ammonium are collected by the anode, and pass via external wiring to the
cathode
a 29, there to promote the reduction of residual nitrate to N2 gas. (If
nitrate is absent,
9 under anaerobic conditions hydrogen (1-I+) wilt probably serve as the
electron acceptor,
~ o only minor oxygen being present under the existing redox conditions.)
11
~ 2 Each cell operates independently of the others and can be isolated when
maintenance
13 is required.
14
1 s So long as the redox voltage is maintained within the indicated limits
(Fig i ) only a
1 s single-stage treatment system is required. This treatment cell is arranged
to control
17 the redox potential of the solution within the predominance field of
nitrogen gas.
1s
~ s Two reactions of interest are:
z1 2 NHa+ - N2(gas) + 8 H+ + 6 e-
22
23 2 N03 + 12 H' + 10 e- - N2(gas) + 6 H20
24
zs Nitrogen gas is the most stable phase under typical earth surface
conditions (pH 3-10,
2s redox potential -0.5 to +1.2 volts), so the conversion to gas is nearly
always
27 thermodynamically favoured. It is recognised herein that, with the
nitrogenous
2$ contaminants, the conditions needed to transform the contaminant to
nitrogen gas are
2s within comparatively easy reach, from which it is recognised that the size
of the facility
3o needed to treat the water, residence times, etc, can be engineered on an
economical
3 ~ scale.
32
s3 As to their physical structure, the electrodes of the cell 23 may be of
porous graphite,
34 iron, magnetite, etc. The set-up generally requires separate anodic and
catholic
CA 02247135 1998-08-19
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~i
1 compartments, connected via an ion-selective membrane.
z
s The anodic reactions may be described as follows. More than 90% of the
nitrogen in
° 4 sewage water is present as ammonium (NHa+), and it is the aim to
oxidise the
s ammonium directly to nitrogen gas. The reaction to be encouraged is:
' s 2NH4+ - N2 + 8H+ + 6e-
7 The standard cell voltage, E° in the Nernst equation, typically would
be -0.28 V.
a
9 The electrolytic cell is used to provide the electrical energy required to
initiate this
otherwise unfavourable chemical reaction.
11
12 The Nernst equation is utilized to evaluate the optimal extent of the
electrolytic
13 conversion. The Nernst relationship shows how redox potential of the half-
cell
14 changes with concentrations of reactants and products:
1 s E = -0.28V + 0.0788 pH - 0.0098 !og PN2 + 0.0197 log NH4+
1 s from which it can be seen that E (voltage) becomes more positive by (a)
maximising
1 ~ the pH, (b) maximising the NH4+, and (c) minimising the p(N2) by
withdrawing the
1 a . nitrogen gas as it is produced.
19
ao However, the small coefficients on the various terms in the Nernst equation
indicate
z1 that the potential should be affected only very slightly by changes in the
pH and
az ammonium content of the influent solution, and by the pressure of nitrogen
(in
23 practice, fixed to a value near the atmospheric level of 0.78 atm).
24
is The nitrate is broken down in the cathode compartment 29. The cathodic
reactions
is may be described as follows. The intended cathodic reaction is:
a~ 2 N03 + 12 H' + 10 e- - N2(gas) + 6 H20
zs The standard cell voltage, E°, would be 1.24 V, whereby this is a
spontaneous
as reaction. The reactions that result in the formation of Nz0 and NO are also
3o spontaneous; however, by controlling the voltage within the field of
predominance of
31 N2 gas, as described, the formation of the nitrous and nitric oxide species
may be
3a inhibited.
33
34 The Nernst relationship for the above equation is:
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12
1 E = 1.24 V - 0.0709 pH - 0.0059 log PN2 + 0.0118 fog N03_
z
a It is an aim of the system as described that ammonia /ammonium in sewage
water
4 may be broken down, by electrolysis, directly into nitrogen gas. This is
contrasted '
s with the conventional biochemical reaction, which has many intermediate
stages that
s can lead to the release of toxic gases. '
a The invention may be applied in applications other than municipal-scale
sewage
s treatment, as will now be described.
The invention may be used to treat groundwater contaminated with nitrate. The
1 z cathodic reaction is as described previously; but the anodic reaction will
probably
13 involve the oxidation of water, as follows:
2 H20 = 02 + 4H+ + 4e- Eo = -1.23 V
i s Generally, treatment of groundwater contaminated with nitrate has been
considered
17 very expensive. It is recognised that major cost benefits arise from
carrying out
~ a electrolysis on groundwater in-situ, as compared with electrolysis
treatment systems
1 s which involve passing the water to be treated through an engineered
treatment facility.
zo Such systems involve the expense of the provision of a means for routing
the water to
z1 be treated out of the ground, and into and through the facility, and
because the facility
za itself has to be physically large to contain the volumes of water that need
to be
z3 treated.
24
In-situ electrolytic treatment of groundwater incurs reduced expense,
especially as to
zs capital cost, because the treatment is carried out with the water remaining
in the
z7 ground, and therefore the cost of engineering a treatment facility, and of
engineering a
zs means for moving the water into and out of the facility, is avoided or
reduced.
29
so Fig 3 shows a drinking-water well 40, having a metal (steel) casing 43.
Nitrate, e.g
31 from agricultural run-off, is contaminating the local groundwater, to the
extent of
sz polluting the water drawn up from the well. Carbon rods 45 are inserted
into the
33 ground in such a way as to make contact with the nitrate-contaminated
water. A
s4 voltage source 47 is connected as shown, so as to turn the well-casing 43
into a
CA 02247135 1998-08-19
WO 99!30943 PCTlCA97100l22
13
1 cathode and the rods 45 into an anode.
2
3 The influence of the cathode at the well extends several meters into the
surrounding
ground, and of course is strongest close to the well. As described, the
cathodic
s reaction serves to reduce or eliminate the nitrate from the water.
' 6
7 Sometimes, even a galvanic cell (i.e a cell having no source of electrical
energy) can
a create a sufficiently vigorous reduction of the nitrate that the nitrate is
treated
s effectively. Fig 4 shows the anode 49 in such a case. The anode is of an
electro-
1 o active metal, such as magnesium.
11
1 z Fig 5 shows a case where nitrate-laden water is to be treated, not at a
well as in Figs
13 3 and 4, but while passing through the ground. Two carbon rods 50,52 and a
voltage
14 source 54 are so placed that the cathode 50 lies within the area of
contaminated
1 s groundwater, and the anode 52 lies in the groundwater outside the
contaminated
1 s area. (The demarcations between contaminated and uncontaminated of course
are
1 ~ characterised by gradual, not sharp, changes in concentration of the
contaminant.)
18
1 s As shown in Fig 5, sensors 56 are provided for monitoring the pH and Eh of
the
ao groundwater. Signals from these sensors are processed by a computer 58, and
the
z1 result fed to the voltage source 54. Thus, if the Eh (or indeed the pH) of
the water
za should change, the voltage is adjusted. Similarly, a check can be kept that
the right
i3 amount of electrical energy is being supplied to keep the voltage at the
desired level.
24 The "before" and "after" measurements can be compared, for checking the
efficacy of
is the treatment system.
2s
a7 Sometimes, only one cell is needed. If more than one is needed, the extra
cells are
as added in the same manner, ie with the cathode inside the contaminated area
and the
as anode outside. Fig 6 shows a plan of a typical area of ground in which the
3o groundwater is contaminated with nitrate. Two cells are shown, and other
cells may
31 be added later, as the contaminated zone moves, or to give more efficacy to
the
32 treatment.
33
34 In Fig 6, two cells are provided. The required voltage is supplied between
the
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14
1 cathode 60 and the anode 62 of the first cell 63. The cathode 60 creates an
area of
z influence 64, in which the nitrate is transformed into nitrogen gas. The
anode has
s substantially no effect on the contaminant. The second cell 67 is similar.
4
s In Fig 6, as shown, the reason treatment is being undertaken is because
drinking-
s water wells 68 are located in the path of the oncoming plume of nitrate.
These wells
7 may be further protected from any nitrate that escaped the cells 63,67. The
protection
a is the same as shown in Fig 3.
s
1 o When more cells are added, these can be added as electrically-separate
units, or they
11 may be connected in series, or in parallel, with the existing cells. The
engineer
1 z should make sure the voltage at the electrodes are maintained, and the
manner of
1 s connection is secondary to that.
14
15 The invention is intended for use generally in setting up large scale
electrolysis cells,
1 s preferably in-situ, with or without added electrical energy. Electrodes
may be
17 engineered to be suitable for the needs of the individual case.
18
1 s The engineer should take account of the effects of electrochemical
activity at the
ao anode (in both the electrolytic and the galvanic cases) in that
electrolysis of the water
a1 at the anode will lead to the generation of hydrogen ions, and an increase
in acidity.
22
zs 1f the water is naturally alkaline, a little extra acidity would not
matter. However, if the
a4 water is naturally acidic, further acidity might be a problem. If so, the
engineer might
25 decide to provide the anode in a sacrifical material, whereby acidity
generated at the
is anode would be reduced.
z7
as Since the material of the sacrifical anode passes into the water, the
material should
29 be selected on the basis of being environmentally friendly in water.
Magnesium, for
so example, has very low toxicity in water, and is a good choice for the
sacrifical anode.
s1 Aluminum, on the other hand, can be toxic in water, and is contra-
indicated. In some
3a cases, for example if the water contains such potentially toxic materials
as dissolved
ss aluminum and other metals, the preferred sacrificial anodic material would
be iron,
s4 because iron can promote co-precipitation of dissolved metals.
CA 02247135 1998-08-19
WO 9713094f PCTlCA97/OOI22
1 Other areas of applicability for the technology are:
z - Treatment of well water. Many wells are contaminated with nitrate. The
electrolytic
s celi in the well converts nitrate to nitrogen gas.
4 - Treatment of an aquifer contaminated with nitrate, at depth (where there
is little
5 biological activity to reduce the nitrate). The electrolytic cell arises by
placing
° s electrodes in the aquifer. Electrolysis reduces the nitrate to
nitrogen gas.
7 - Treatment of animal waste collection tanks. Here, the waste material is,
for
s example, in a tank under a pig barn. in addition to addressing the ammonium,
as
s described, electrolysis converts hydrogen sulphide to H* and S04 and
converts
1 o methane to H* and C02.
- Treatment of mine waste water. Ammonium nitrate + diesel fuel, as used for
1 z explosives, leaves ammonia (NH3) dissolved in the water. Electrolysis
converts the
1 s ammonia to nitrogen gas.
- Treatment of waste water from the food processing industry (e.g pickling
plants).
1 s Other points to be considered in relation to the treatment of contaminated
water by
1 ~ electrolysis as described herein are:
1 a - The efficacy of the electrolysis reaction is generally much less
dependent on
19 temperature than biochemical reactions. Biological denitrification at
10°C may be
zo , expected to be an order of magnitude less than at 25°C. The
electrolysis reaction can
z 1 be expected to take place efficiently even in prolonged freezing weather.
22 - The conventional biological treatment produces sludge. The electrolysis
reaction
z3 can be expected to produce a comparatively smaller amount of sludge.
z~ - Electrolysis circumvents the need to add a carbon source, which is needed
in
z5 conventional biological denitrification for maintaining reaction rate and
completion.
zs This may also reduce the volume of sludge produced.
z7 - Biochemical reactions typically proceed slowly, whereby long residence
times are
za required for treatment to be completed. The electrolysis reactions can be
expected to
zs be completed in shorter periods, thus avoiding the tong residence times.
so - If an electrolysis system is found to be inadequate, often the inadequacy
can be
s1 remedied simply by adding a further electrolytic treatment facility, in
series with the
3z already-present facility. The expense of doing that is hardly more than the
expense of
ss providing the larger system originally would have been. By contrast, if a
biological
34 system is found to be inadequate, generally it cannot be remedied in that
way, i.e
CA 02247135 1998-08-19
WO 97/30941 PCT/CA97/00122
16
1 simply by adding another small system. Rather, what is required is that the
a inadequate biochemical system must be removed, and a whole larger system
installed
3 in its place. Therefore, the prudent designer ofi a conventional biochemical
system
had to take care to make a contingency provision, at the time the system was
installed, often at considerable expense, for future increased demands on the
system.
6 The designer of the electrolytic system, on the other hand, can engineer the
system '
7 just for today's needs, knowing that the system can easily be upgraded later
if the
s need should arise.
9
1 o The term Eh as used herein is defined as follows. The Eh voltage of a
solution is the
11 redox potential generated in the solution by comparison with a standard
hydrogen
~ a electrode. A standard hydrogen electrode comprises a platinum wire with
hydrogen
1 3 bubbling around it, contained within a solution of hydrogen ions in
solution of 10°
14 moles per litre (the zero pH condition).
