Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This invention relates to a process and appara-tus
particularly for use in treating a s-tream of waste wa-ter
of the type resulting from pulp and paper manufacturing
processes -to reduce the oxygen demand of the waste water
and to improve the colouration of this water so that
the treated wa-ter will be more environmentally acceptable.
Waste water from pulp and paper processes
includes many organic substances which are too numerous
and too complex to treat individually. Quantitative
tests have been devised for comparing one such waste
water with another waste water, and also for comparing
waste water before treatment with the resulting water
after treatment. One such test produces a value known
as the Chemical Oxygen Demand (C.O.D.) and is a measure
of the quantity of oxidizable components present in the
waste water. Because the carbon and hydrogen in organic
matter are oxidized by chemical oxidants, the oxygen
concerned is a measure only of the chemically oxidizable
compounds.
?.0 Another test produces a value known as the
Biochemical Oxygen Demand (B.O.D.). This value is an
expression of the quantity,of dissolved oxygen required
during stabilization of the decomposable organic matter
by aerobic biochemical action. Because C.O.D. does not
diEferentiate stable from uns-table organic matter, C.O.D~
will not correlate with B.O.D.
Other tests include colour -tests and ~oxicity
tests. A typical colour test involves measuring the
degree of dilution which must be used to cause waste
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water to have a similar light transmission property to
that of a s-tandard liquid.
A commonly used toxicity test is done using fish
which are placed in a given volume of waste water to
determine how many fish survive after a given period of
time in the waste water.
Tests such as those mentioned are described in
aetail in publications such as ~'Standard Methods for the
Examination of Water and Waste Water" published jointly
by The American Public Health ~ssociation, The American
Water Working Association and The Wa~er Pollution Control
Federation.
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A typical waste water having a high organic content
is that from pulp and paper manufacturing processes. Such
~ waste water includes many polluting materials which are
; derived either from the wood used as a raw material, or
from materials introduced during the pulp and paper manu-
facturing process.
The wood based mate~ials include tree bark, wood
debris~ lignins and their decomposition products, and the
other materials include starches, resins, gums and proteins.
The complexity o~ the problem o~ improving the quality
o~ waste water associated with pulp and paper processes is
illustrated by reviewing some o-E the ma-terials which have
been identified in such waste water. For instance, toxicity
is believed to be due to resin acids, monochloro and
dichloro dihydro abiet;c acids, trichloro and tetrachloro
guaiacol, 9, 10-dichloro, and 9, 10-epoxy-stearic acid.
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In addition, unsaturated fatty acids such as palmitoleie,
oleic, linoleic and linolenic sometimes contribute to
toxicity in waste water~ Other toxic subs-tances sometimes
present are diterpene alcohols, principally pimarol and
isopimerol and corresponding aldehydes as well as the
resin acids, abietic, dehydro abietic, isopimaric, palustric
and pimaric.
The colour of the waste water from pulp and paper
manufacturing processes is usually due mainly to lignin
alone or to lignin in combination wi-th the decomposition
products of lignin. Some of the chromophorie substances
such as the dihydroxystilbenes, the quinonemethides and
the stilbenequinones also eontribute to colouration. All
of these substances may be oxidized to form colourless
degradation products. Although the various standards of
the water regulating organizations are less stringent in
relation to eolour than they are in relation to toxieity,
a clean-looking waste water is obviously desirable.
Various processes for treating waste water from
~ pulp and paper manufacturing processes have been used,
but biochemieal and physico-chemical processes are most
comrnon. O~ these; the biochemical methods are used rnost
frequen~ly.
Although biochemical methods provide satisfactory
decreases in B.O.D. levels, -they provide uncertain
detoxification results, no reduction in colour and only
small decreases in C.O.D. values. Moreover, biochemical
systems are less effective in winter temperatures, require
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large lagoon areas, and require at least several days
to effect any significant improvement in the quality of
the waste water. Because of this, a number of physico-
chemical treatments have been investigated such as lime
treatment, alum coagulation, activated carbon absorption,
absorption with X.A.D. resins, ion-exc;hange, ozonation,
reverse osmosis and flocculation.
Generally, the physico-chemical processes provide
a satisfactory reduction of colour, C.O.D. levels and to a
limited extent toxicity, but a satisfactory decrease in
B.O.D. levels is not generall~ achleved. Also, these methods
tend to be too expensive to be cor~mercially viable.
Attempts have also been made to use electro-
chemical methods involving consumable electrodes of a
;~ material which goes into solution and induces flocculation
in the waste water.
The present invention is directed to the problem
of providing a process and an apparatus which will be
useful in treating waste water of a type such as that
emanating pulp and paper manufacturing processes to make
the B.O.D., C.O.Dc, toxicity, and colour values more
acceptable and to thereby,prod~ce a resulting water which
is more environmentally acceptable.
In one of its aspects the invention provides an
eleckrochemical process for treatin~ a stream of was-te
watex as it emanates from pulp and paper manufacturin~
plants. rrhe waste water has unacceptable biochqmical and
chemical oxygen dernands, an unacceptable -toxicity level,
and an unacceptable colouration. The process comprises
the steps of flowing the waste water through an electro-
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chemical reactor which has a fibrous anode and a cathode. The
fibrous anode has a high surface area to volume ratio and
a relatively large oxygen overpotential. Electrolytic
continuity is provided between the anode and the cathode
and the anode is at a generally constant
electrical potential different from that of the cathode.
As a result at least some of the compounds present in the
stream are electrochemically oxidized at the anode and
gases are produced at the anode. These gases produce
turbulence and movement in the hydrodynamic boundary
layers so that these layers are broken down to further
increase the mass transfer rates at the anode. The quality
of the waste water is thereby improved for the purpose
of discharging the waste water stream into natural water
sources.
; The invention will be better understood with
reference to the following description and drawing (Fig. 1)
which is a diagrammatic sectional side view of a preferred
embodiment of apparatus according to the invention.
As seen in Fig. 1, a reactor 10 is provided
having a housing 12, inlet 14 and outlet 1~. The reactor
includes an anode 18 of carbon fibres retained in a bed
by a diaphragm 20 made from a nylon mesh, a plate-like
cathode %2 is provided adjacent the nylon mesh, and the
anode and cathode are coupled to an electrical supply
23 to provide an electrical potential di~erence between
the ele~trodes.
The reactor housing 12 includes a trough 24
adjacent the inlet 14 to absorb the flow energy of the
waste water so that there is a gentle flow over the work-
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ing electrode. Similarly, a trough 26 is provided
adjacent the outlet 16 to collect the resulting treated
water thereby ensuring a more uniform flow over the anode
prior to discharge through outlet 16. f
The flow of water through the reactor will beessentially over and through -the anode 18. Some of the
water flowing above the anode will contact the cathode 22
and will be affected by the process less than water which
passes continuously over and through the anode. It is
therefore desirabIe in this design of reactor,to arrange
for a maximum flow ,to take place over and through the anode '
'rather than over the cathode.
The diaphragm 20 should have a mesh which is
as large as possible without permitting the carbon fibres
'of the anode to touch the cathode.
Because of the large overpotential of oxygen
at the carbon fibre anode, it is possible to electro-
oxidize and thus break up a large proportion of the
organic compounds before the production of oxygen
becomes predominant. Also if the waste water con~ains
sufficient sodium chloride, chlorine and hypochlorite
will be formed at the anode, a,nd these substances further
oxidize and break up the polluting substances in the
waste water~ This effect can be enhanced where necessary
by the addition of sodium chloride, calcium chloride,
po-tassium chloride or brine, to the waste water~ Also,
oxygen is produced at the anode and this further oxidizes
the pollu~ing substances.
It has been found that the reactor can be used
to treat waste watér of the type emanating from pulp
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and paper manufacturing processes using a very short
residence time in the order of minutes. This is because
of the high mass transfer rates which are achieved to some
extent because of the production of gases (for example
chlorine and oxygen at the anode~. These gases create
turbulence or movement in the hydrodynamic boundary layers
resulting in breakdown of the layers.
The carbon fibre anode is preferably of relatively
long carbon fibres in a bed, although shorter lengths can
be used. Such short lengths will require a diaphragm
having a close mesh to prevent these fibres from passing
through the diaphragm. Typical carbon fibres are sold
under the Trade Mark GRAFIL by Courtaulds Limited of
England. A carbon fibre anode was used in the following
examples.
EXAMPLE I
Waste water from a pulp and paper mill was col-
lected and found to have 9,000 colour units and a C.O.D.
of 1,800 mg/litre.
The waste water was passed through a reactor
consisting of a carbon fi~re.anode, a nylon diaphragr
with 10 micron mesh size, and a steel cathode. About 99
percent of the solution passed through and over the anode
and only about 1 percent passed over the cathode.
A reactor voltage of 3.5V was used with a current
of 1 amp, which leads to an electrochemical poténtial o~
1.7V being set up between the carbon fibre anode and a
saturated calomel electrode (which was used as a reference
electradé to measure the electrode/solution potential).
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After a single pass through the reactor the
colour units dropped by 60 percent to become 4,000 colour
units and the C.O.D. level dropped to 1,000 mg/litre.
EXAMPLE I I
Example I was repeated except that the waste
water was passed through the reactor five times. It
was found that the colour was lowered to 1,500 colour
units.
In some instances it may be preferable to change
the form of the reactor while continuing to cause a major
portion of the flow to pass through and over the anocle.
Such changes are within the scope of the invention.
A modiflcation to the process is to pass the
waste water through a reactor such as that described and
then to pass the resulting water through a second reactor
after first mixing some sodium chloride (or any other
suitable chloride) into the resulting water. The treated
water leaving the second reactor has an enhanced quality
not found by using a single pass through a reactor after
adding a chloride. , , '
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EXAMPLE III
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The same waste water as that used in Example I
was used. Howe~er, some sodium chloride was added to
the solution such that the solution had 2 percent sodium
chloride. A reactor voltage of 3.5V could now support
a current of 2.5 amps owing to the decrease in solution
resistance. Here a single pass leads to a colour reduction
from 9,000 units to 1,000 units. This is a 90 percent
reduction, while C.O.D. dropped from 1,800 mg/litre to
500 mg/litre, a drop of 72 percent.
EXAMPLE IV
A different pulp and paper mill waste water was
taken which had initial colour of 12,500 units, a C.O.D.
level of 2,125 mg/litre and a B.O.D. of 690 mg/litre.
The waste water toxicity was also determined. At 60
percent concentration, the initial toxicity resulted in
100 percent mortality of fish in 22 hours.
A first pass of the waste water was made at 1 amp
and 3.5V and the colour decreased to 6,000 units. Then
some sodium chloride was added to produce a 2 percent solu-
tion and multiple passes were made at 3.5V and 2.5 amps.
The colour was found to have decreased from 12,500 units
to 1,000 units, a reduction of 92 percent. The C.O.D.
dropped from 2,125 mg/litre to 1,336 mg/litre, a decreas~
of 48 percent. The B~O.D. dropped from 690 mg/litre to
365 mg/litre, a decrease of 48 percent and toxicity on a
60 pexcent concentration decreased from 100 percent mortal-
ity in 22 hours to 0 percent mortality after 96 hours.
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S [~PPLEMENTARY D I SCLOSURE
In some instances it may be preferable to
change the form of the reactor while con-tinuing to cause
a major portion of the flow to pass through and over the
anode. Such changes are within the scope of the invention
and an example of such a modification is shown in Fig. 2
which is a diagrammatic sectional side view. In this
embodiment an anode 28 is contained in a housin~
30 and separated from a chamber 32 containing a
' metal cathode 34 by a diaphragm 36. In this instance the
diaphragm is of NAFION which is a proprietary product of
Dupont of Canada Limited. The electrodes are connected
to an electrical power supply 38 and respective inlets
40, 42 and outlets 44, 46 are providea for the anode and
cathode.
The Fig. 2 arrangement ensures complete
separation of waste water from the cathode 34 so that all
of the waste water passing between inlet 40 and outlet 44
must pass through housing 30 filled with anode 28. Electro-
lytic continuity is provided by circulating water ;~which is
preferably alkaline) through chamber 32. As a result, while
the waste water is treated at the anode 28, hydrogen is
formed as a by-product and is collected downstream of
outlet ~6.
Although specific diaphragms have been
described with reference to the illustrated embodiments
it is within the scope o~ the invention to change these
diaphragms for other equivalent diaphragms. Many diaphragms
are available and it is expected skillin the art to select
a suitable alternative where desired.
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