Note: Descriptions are shown in the official language in which they were submitted.
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A process for controlling microbial growth
The present invention relates to a process for controlling microbial growth in
a production
line for cellulosic products with the aid of gases. The invention also relates
to the use of
gases such as carbon dioxide, nitrogen, a noble gas and/or non-naturally
occurring
mixtures thereof for controlling microbial growth.
In the production of cellulosic products such as cellulose, paper and board
(hereinafter
referred to as paper), cellulosic fibers are treated in aqueous suspensions
under varying
conditions. The amount of water in the production line is huge and the water
is
continuously recirculated in smaller or larger loops. The conditions in the
production line
are often susceptible to microbial growth. This is especially so during
storage of the
aqueous material.
For growth all microbes require that the system should include sufficient
nutrients and that
the pH, temperature, moisture, oxygen level etc. is adequate.
For breeding the microbes need time to grow and propagate in the system. The
retention
time must be long enough; otherwise, the cells will be washed out of the
system. In a
papermaking system the microbes can easily find locations with a sufficient
retention time,
e.g. water storage tanks, stock chests, treatment of broke and long pipelines.
Normal papermaking conditions are suitable for the growth of many kinds of
microbes.
The recirculating white water in the short and long circulation contains
enough carbo-
hydrates and other essential ingredients such as inorganics and trace
elements. Also
chemicals used in the papermaking themselves represent an ideal nutrient
source, e.g.
starch, or contain as impurities quite a lot of nutrient material, e.g.
kaolin. The incoming
raw water can also contain considerable amounts of nutrients.
The microbes found in a papermaking system can be divided into three main
groups,
bacteria, fungi and algae. The bacteria are either spore forming (anaerobic)
or non spore
forming (aerobic); the fungi comprise moulds and yeasts; and the prevailing
algae are
blue-green or green algae.
Many different kinds of problems can be caused by microbial growth, such as
slime
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problems, runnability problems, corrosion problems, additive problems, and
product
problems .
Unless microbial control agents are added to the papermaking system, the
general growth
requirements for microbes are usually well satisfied in papermaking systems.
Modern
microbial control agents can roughly be divided into groups, which act in the
following
ways: oxidative decomposition of microbes (02, C102, 03, peroxides); biocides,
which
inhibit or kill the microbes (organic, synthetic chemicals); and enzymes.
Oxygen is primarily used to prevent anaerobic conditions, while chlorine
dioxide, ozone
and peroxide work as biocides and disinfectants. They are quite effective but
considered to
be costly. Conventional biocides can be used alone or in combination with
oxidative
biocides. They are effective, but toxic. They can also be environmentally
dangerous and
hazardous for the working environment. A relatively new method of slime
control is the
use of enzyme. Enzymes are active at pH 3.5-10, which is an advantage compared
with
conventional biocides. However, enzymes have limited effect on bacteria.
Oxygen and oxygen-rich gases have primarily been used for preventing the
formation of
hydrogen sulfide and other volatile gases by anaerobic bacteria in waste
waters.
Robichaud, W. T., Tappi Journal, Feb. 1991, pp 149-153, has reported on the
use of
aeration for controlling anaerobic bacteria to improve product quality and
mill safety in
papermaking systems.
Use of carbon dioxide in paper making has been suggested in the prior art for
various
reasons mainly connected with specific needs for adjusting the pH or
influencing the
carbonate or bicarbonate chemistry. Examples of patents related to the use of
carbon
dioxide in papermaking systems are US 5,378,322 (Canadian Liquid Air); US
5,262,006
(Mo och Domsjo Ab); EP 0 296 198 (AGA Aktiebolag); EP 0 281 273 (The BOC
Group); GB 2 008 562 (J.M. Voith GmbH); WO 99/24661 (AGA Aktiebolag); and WO
99/35333 (AGA Aktiebolag). None of these publications relate to the problem of
microbial growth.
Mixtures of gases have been used in the packaging of foodstuffs in order to
retain the
foodstuff's original taste, texture and appearance. The gas mixtures usually
consist of
carbon dioxide, nitrogen and oxygen, but also other gases such as nitrous
oxide, argon and
hydrogen have been used. Carbon dioxide inhibits microbial activity on the
foodstuff by
reducing the pH and by penetrating biological membranes, causing changes in
permeabili-
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ty and function. Nitrogen is primarily used to replace oxygen in packaging.
Oxygen helps
to preserve the oxygenated form of myoglobin, which gives meat its red colour.
Oxygen is
also required for fruit and vegetable respiration.
Amanatidou, A. ; Smid, E. J. ; Gorris, L. G. ; J Appl Microbiol, March 1999,
pp 429-38,
have reported on the effect of elevated oxygen and carbon dioxide on the
surface growth
of vegetable-associated micro-organisms. Consistently strong inhibition was
observed only
when the two gases were used in combination.
As mentioned above, problems with microbial growth are conventional in
papermaking
systems, especially in storage tanks and in long pipe lines. Moreover,
protection of the
environment by the closing of the white water systems and increased
recirculation of
process waters as well as the increased use of waste paper has caused a marked
increase in
the microbial growth in the papermaking systems.
The papermaking industry consequently has an increasing need for means for
reducing the
microbial growth in a technically feasible, inexpensive and environmentally
friendly way.
It has now been found that a gaseous inhibitor in the form of carbon dioxide,
nitrogen, a
noble gas or a gas mixture containing one or more of said gases may be used
for control-
ling microbial growth in a papermaking system.
Consequently, the present invention provides a process for controlling
microbial growth in
a production line for cellulosic products, which comprises providing an
aqueous material
containing water as well as suspended pulp fibers and/or additives therefor,
maintaining
said aqueous material under conditions susceptible to microbial growth,
providing a
gaseous inhibitor comprising a gas selected from carbon dioxide, nitrogen,
noble gases
and non-natural gas mixtures containing the same, and introducing said gaseous
inhibitor
to said aqueous material in an amount sufficient to significantly retard or
inhibit the
growth of microorganisms therein.
The gaseous inhibitor is preferably added immediately prior to and/or during
storage of
said aqueous material, since storage provides a sufficient time for the
microbes to
propagate.
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The gaseous inhibitor should preferably consist of or contain a significant
amount of
carbon dioxide, nitrogen and/or a noble gas such as helium, neon, argon,
krypton, xenon
and/or radon. Carbon dioxide is the preferred gas according to the present
invention.
Among the noble gases argon is preferred.
If a gas mixture is used, said mixture should be a non-natural one. The gases
of the
mixture may be mixed before introduction into the aqueous material, or they
may be
added separately, simultaneously or sequentially. Although the gas mixture may
contain
oxygen, it should not have the composition of air, since air is known to
inhibit only the
growth of anaerobic microbes.
The gaseous inhibitor of the present invention may be used in combination with
oxygen
either by combining oxygen in the mixture of carbon dioxide, nitrogen and/or
argon, or
by adding oxygen separately from the other gases. When oxygen is added to the
gas
mixture, the amount of oxygen may vary from 10 to 90% of the total gas volume:
However, according to another embodiment of the invention,, an oxygen-rich gas
is
introduced into the aqueous material separately from the gaseous inhibitor of
the present
invention. Such an oxygen-rich gas may be added either before or after the
addition of the
inhibiting gas mixture.
The present invention will now be described in greater detail with reference
to paper-
making systems. It is, however, clear that the gaseous inhibiting system of
the present
invention may be used also in the production of cellulose, board, etc. As used
in the
context of the invention, a production line for cellulosic products comprises
a line for the
production of pulp, paper, board or the like. The production line will
typically include at
least a portion of reprocessed recovered paper and/or broke and will include
loops of re-
circulating waters. The production line has a more or less closed water
system, it being
understood by those skilled in the art that the problems with microbial growth
are prone to
escalate in closed systems with an increasing and accumulating mass of
microbes circu-
lating in the system.
The temperature in the papermaking systems usually varies from 30 to 60
°C. Because of
recycling, the temperature in the white water often exceeds 50 °C.
Fungi and yeasts
generally do not tolerate temperatures above 40 °C. Contrary to this,
many bacteria thrive
well in the high temperature range. pH usually varies from 3 to 10. Acidic
conditions, pH
3-6, are very convenient for fungi and yeasts. Bacteria dominate under neutral
and
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alkaline conditions, viz. at pH levels from 7 to 10. Anaerobic conditions can
be found in
many places throughout the production line, such as storage after dithionite
bleaching,
pulp chests and white water tanks.
Slime helps microbes to adhere onto surfaces and provides a food reserve.
Microbial
growth causes operating problems by plugging filters and screens, by reducing
wire and
felt life and by causing a decline of productivity due to breaks, wash-ups,
etc. Corrosion
induced by microbes is a consequence of vigorous microbial activity on
surfaces.
The most important microbial species in this area include sulphate-reducing
bacteria,
which are anaerobic in character. There are, however, also a number of aerobic
species
which are harmful. Additives, such as starch, can deteriorate due to microbial
activity, not
to mention that a contaminated starch slurry can constitute a heavy
inoculation of the
white water system. When masses of microbes get loose from the actual growth
place, the
result may be seen as spots, holes or dirt specks in the final paper product.
Spore-forming
bacteria tolerate much heat and usually survive the drying stage. Thus they
remain alive in
the product and can be harmful later on.
In the working of the present invention note should be taken of the special
circumstances
of a papermaking system with its huge volumes of fluids, all the time on the
move and
having no definite surface where the microbes are prone to exist. This is in
sharp contrast
to, for instance, the packaging of food in protective atmospheres. The food
moves
nowhere within the package, its surface is solid and the gaseous atmosphere
surrounds the
product. In a papermaking system the aqueous material has a surface which
changes
continuously and it cannot be surrounded by the gas.
The term microbe or microorganism as used in the context of the present
invention is
intended to mean bacteria, fungi and/or algae such as described above. It
should be under-
stood that all of the microorganisms present in a papermaking system will not
be
influenced by the gaseous inhibitor of the present invention and that the
gaseous inhibitor
of the invention may therefore be used in combination with other inhibitors,
such as
oxygen-rich gases and biocides of various forms, as long as these do not
interfere with the
working of the invention itself.
According to the present invention microbial growth in the papermaking system
is reduced
by a gaseous inhibitor. The gaseous inhibitor of the present invention is a
gas or a gas
mixture capable of inhibiting, partly or totally, the growth of microorganisms
present in
the papermaking system. The preferred single gas is carbon dioxide. The
gaseous inhibitor
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may also comprise nitrogen and/or argon. Said gases may also be used alone,
but they are
more preferably used in combination with carbon dioxide. A suitable gaseous
inhibitor
consists of a mixture of carbon dioxide, nitrogen and argon. The mixture
preferably
contains at least 10 % carbon dioxide.
The gaseous inhibitor may additionally contain oxygen. The oxygen should not
be used in
a combination resembling air, since such a mixture is effective only against
anaerobic
microorganisms. In a mixture of carbon dioxide and oxygen, the proportion of
carbon
dioxide should be between 90 % and 10 % and the proportion of oxygen should be
between
% and 90 % . It should be noted that normal air contains about 21 % oxygen and
about
0.03 % carbon dioxide.
A preferred embodiment of the invention comprises the use of a gaseous
inhibitor
consisting essentially of carbon dioxide, nitrogen, argon or mixtures thereof,
which is
introduced into a liquid flow of the aqueous material entering a storage tank
for said
aqueous material or into said storage tank itself. An oxygen-free gaseous
inhibitor is
preferably added in an amount sufficient to purge said aqueous material of
oxygen and
thereby inhibiting the growth of aerobic bacteria contained therein. Most
aerobic bacteria
are sensitive to the lack of oxygen and will eventually be killed off by such
a procedure
while the carbon dioxide will adversely affect many of the anaerobic species.
In an improvement of this embodiment, oxygen is used separately from the
gaseous
inhibitor of the present invention. Thus, after a suitable retention time the
introduction of
the gaseous inhibitor is followed by and/or preceded by an introduction of an
oxygen
containing gas into said storage tank in an amount sufficient to kill
anaerobic bacteria in
the aqueous material.
The introduction of the gaseous inhibitor and the introduction of oxygen may
be repeated
in an alternating manner during storage of said aqueous material.
The inhibitor is preferably added in a position where the risk for microbial
growth is
largest, i.e. in storage tanks for aqueous pulp suspensions or aqueous
additives susceptible
to microbial attack. However, the gaseous inhibitor may also be added to
liquid flows of
the aqueous material, to recirculating waters and to fresh water prior to its
entering the
system. The main principle of the present invention is to reduce the microbial
growth at
any position where it would otherwise rise to harmful proportions. It is not
necessary to
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kill all the microbes but it is essential to reduce the microbial growth to
such proportions
that the harmful accumulations are minimized in the production line and in the
final
product.
In a preferred embodiment of the present invention the gaseous inhibitor is
introduced into
a liquid flow of the aqueous material or into a liquid flow of a diluent or
additive for said
material just prior to storage thereof. The gaseous inhibitor may also be
introduced into
any storage tank containing said aqueous material by bubbling the gas into the
aqueous
material and/or by filling the void space above the fluid.
In a typical process according to the present invention the aqueous material
comprises a
pulp suspension in a papermaking system and the suspension is treated with the
gaseous
inhibitor before it enters and/or as it is retained in a pulp storage tower, a
stock chest, a
broke tower or the like storage tank. The pulp suspension may also be stock in
the stock
preparation of a paper making system.
In a preferred embodiment the aqueous material to be treated comprises white
water in a
papermachine, preferably white water stored in the long circulation.
In another embodiment the aqueous material comprises a slurry of an additive
chemical
such as starch, coating, pigment, filler, or the like. Such additives are
usually retained in
aqueous suspension in readiness for use in the papermaking process and many of
the
additives contain nutrients making them susceptible to microbial attack.
Typically this is
true of starch, which in itself is a nutrient for many microorganisms. Many
other
additives, although inert in themselves, contain sufficient amounts of
impurities to make
them, with time, susceptible to microbial attacks. Treating such additive
tanks with inter-
mittent introductions of the gaseous inhibitor will effectively reduce the
amount of
microbes entering the system that way.
In a preferred embodiment of the present invention, the gaseous inhibitor is
added at a late
point, preferably just prior to the point where microbial attack is expected
to be most
severe, such as in a storage tower. Additional gaseous inhibitor (carbon
dioxide/nitro-
gen/argon) should, if necessary, be added to the head space of the tower.
If oxygen is used in combination with the gaseous inhibitor, the oxygen should
preferably
be added directly after a pump feeding the aqueous material to the storage
tower to make
use of any turbulence to achieve a high rate of mixing. Additional oxygen
should, if
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necessary be added to any of the tower's recirculation pipes to avoid creating
any
anaerobic areas in the storage tower.
It should be noted that although gases such as carbon dioxide and oxygen have
previously
been used in papermaking, the gaseous inhibitor of the present invention,
comprising
carbon dioxide, nitrogen or argon alone or in a non-natural gas mixture has
not previously
been used in papermaking systems for controlling microbial growth.
The aqueous materials of the cellulosic production line are processed to
cellulosic products
such as paper, board, dried pulp or the like material in a manner which is
conventional in
all other ways except for the biocidal treatment of the present invention.
The present invention will now be illustrated with the following examples.
Example 1
A set of representative bacterial strains was isolated from a white water
sample from a
Swedish recycled pulp mill. The sample originally yielded 70 bacterial strains
which each
represented a group of different bacteria, which grew under similar
conditions. The strains
were tested on different media and their oxygen demands were studied. The
results gave
an indication of eight main groups. Further investigations showed that three
out of the
eight main groups seemed to be almost similar. Those three groups were put
together to
one large group.
From this dominating group, one strain was chosen for the carbon dioxide
experiments.
Two other strains from another recycled pulp mill were included in the
experiments.
Glass bottles were tempered to 45 °C and C02 was added to create a
carbon dioxide
atmosphere. Bacteria from the three isolated strains were added to separate
bottles together
with a nutrient broth. A first sample representing "0 min" was taken out
immediately from
each bottle and then at a number of occasions during the next three hours.
Each sample
was diluted in dilution series with dilution 1/10 in six steps (101-106) and
grown on agar
plates. The plates were incubated at 45 °C for two days, after which
the number of
colonies were counted and related to the dilution.
The percentage of surviving cells as a function of growth time under the
influence of a
COZ atmosphere is indicated in Table 1.
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Table 1
Time Strain Strain Strain Average
1 2 3
(min) (%) (%) (%) (%)
0 100 100 100 100
75 141 92 103
34 76 77 62
73 137 58 89
50 48 47 48
61 94 36 64
59 46 67 57
80 39 88 44 57
100 16 55 42 38
120 18 43 16 47
140 5 35 7 16
160 4 41 4 16
180 9 24 2 12
The results clearly showed that after approximately 1 hour about 60 % of the
bacteria had
survived, after 2 hours about 50 % and after 3 hours slightly more than 10 %
(the values
> 100% result from small mistakes in sampling or dilution).
Example 2
A mill produced wood-free paper from chemical pulp. At the time of the trials,
the mill
stored pulp for long periods of time in storage towers prior to the stock
preparation. The
storage caused problems with bad smell and black spots in the pulp, which was
believed to
be caused by high microbial activity during storage.
A full-scale trial of treatment with a gaseous inhibitor was performed. Thus,
carbon
dioxide gas was introduced in an amount of 1-2 kg C02/ton pulp just prior to
the storage
tower.
Both the pulp's bad smell and the number of black spots in the pulp were
reduced. No
negative effects of the C02 introduction were observed.
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Example 3
A pulp storage tower having problems with an excess of microbial growth during
a pro-
duction stop is fed by a pump until the storage tower has been filled to about
80 % of its
capacity.
A gaseous inhibitor comprising a mixture of carbon dioxide/nitrogen/argon in
the ratio
70/25/5 is added to the feed line just before the pulp enters the tower. The
gas is fed into
the feed line at a rate of 1.5 kg gas per ton of pulp. The gas feeding is
continued about 5
min after the feeding pump has stopped feeding pulp to the tower, in order to
fill the head
space of the tower with gaseous inhibitor.
2 hours after the feeding of the gaseous inhibitor mixture, an oxygen-rich gas
(air) is
added into the pulp in the tower through a gas distribution tube. Addition of
oxygen-rich
gas is continued until a significant amount of oxygen is found to be present
in the vent
from the tower.
The following day the treatment is repeated by first feeding gaseous inhibitor
of the
present invention into the gas distribution tube, and after a residence time
of about 2
hours, oxygen-rich gas is fed through the tube. The microbiocidal treatment is
repeated
every day of the production stop.
The microbial growth in the storage tower is reduced to an acceptable level
and at start-up
no smell problems are encountered. A paper web is formed from the stored pulp
in the
normal way and only a minimal amount of dirt specks are seen in the paper.
