Production of a fibrous web containing cellulose fibres

Abstract

Provided herein is a method for treating an aqueous cellulose fibre suspension comprising at least one microorganism in a storage tank or a storage tower of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue), the method comprising using a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas to treat the aqueous cellulose fibre suspension in the storage tank or storage tower, wherein the storage tank or the storage tower comprises at least one inlet conduit for the aqueous fibre suspension to pass into the storage tank or storage tower, and wherein the method comprises administering the nanobubbles to the system such that the concentration of oxygen in the aqueous cellulose fibre suspension in the at least one inlet conduit is between 1 mg / l and 30 mg / l. Also provided are related methods of monitoring and controlling growth of cellulolytic microorganisms or cellulolytic activity, and of manufacturing a fibrous web product containing cellulose fibres, as well as related uses and kits suitable for use in the methods of the invention.

Description

[0001] PRODUCTION OF A FIBROUS WEB CONTAINING CEEEUEOSE FIBRES

[0002] Field of the Invention

[0003] The present disclosure generally relates to the field of manufacturing a fibrous web product containing cellulose fibres, such as paper, board or tissue. The disclosure relates particularly, though not exclusively, to a method for treating an aqueous cellulose fibre suspension in a storage tank or a storage tower of a system manufacturing a fibrous web containing cellulose fibres.

[0004] Background of the Invention

[0005] High levels of microbial growth are known to cause problems in paper or board mills, leading to poor process conditions and defects in the final product. Accordingly, it is common practice to control the growth of microorganisms using bio-control procedures, in particular the dosing of biocides to various points in the mills such that suitable levels of the biocide are present in the locations where growth of microorganisms is known to be particularly problematic.

[0006] In the context of these manufacturing processes, some microorganisms are known to be more problematic than others. It has recently been shown in WO 2021 / 214385 A that high levels of microorganisms exhibiting cellulolytic activity can exist in the aqueous fibre suspensions being processed in the paper and board mills, and this correlates with a loss of strength of the cellulose fibres in the final paper and board products. Based on this finding, WO 2021 / 214385 A teaches the use of a method of monitoring and controlling cellulolytic activity in an aqueous cellulose fibre suspension or process water for a production method of e.g. paper or board, the method comprising determining the cellulolytic activity in the aqueous suspension and controlling the activity using biocides.

[0007] However, the extensive use of biocides can be expensive, and therefore there remains a need to improve the methods of producing paper and board, in order to ensure that the cellulose fibres are protected as far as possible during the methods of production without the need for the use of large amounts of biocide. Summary of the Invention

[0008] Accordingly, in a first aspect, the present invention provides a method for treating an aqueous cellulose fibre suspension comprising at least one microorganism in a storage tank or a storage tower of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue), the method comprising using a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas to treat the aqueous cellulose fibre suspension in the storage tank or storage tower, wherein the storage tank or the storage tower comprises at least one inlet conduit for the aqueous fibre suspension to pass into the storage tank or storage tower, and wherein the method comprises administering the nanobubbles to the system such that the concentration of oxygen in the aqueous cellulose fibre suspension in the at least one inlet conduit is between 1 mg / 1 and 30 mg / 1.

[0009] In a further aspect, the present invention provides a method of monitoring and controlling growth of cellulolytic microorganisms or cellulolytic activity in an aqueous cellulose fibre suspension in a storage tank or a storage tower of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue) wherein the method comprises:

[0010] (a) determining the number of cellulolytic microorganisms or the cellulolytic activity in the aqueous cellulose fibre suspension in a part of the system; and

[0011] (b) controlling the growth of cellulolytic microorganisms or the cellulolytic activity by treating the aqueous cellulose fibre suspension with a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas in the storage tank or storage tower.

[0012] In addition, the present invention provides a method of manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue) from an aqueous cellulose fibre suspension in a system comprising a storage tower or a storage tank, the method comprising treating the aqueous cellulose fibre suspension with a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas in the storage tower or the storage tank, prior to using the aqueous cellulose fibre suspension to manufacture the fibrous web, wherein said treating controls the growth of cellulolytic microorganisms or cellulolytic activity in the aqueous cellulose fibre suspension while the aqueous cellulose fibre suspensions is stored in the storage tower or storage tank. Further, the present invention provides use of a biocide in a method for treating an aqueous cellulose fibre suspension, the method comprising using a combination of (i) the biocide and (ii) nanobubbles comprising an oxidising gas to treat an aqueous cellulose fibre suspension to control growth of cellulolytic microorganisms or cellulolytic activity, wherein the aqueous cellulose fibre suspension is in a storage tower or a storage tank of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue).

[0013] Still further, the present invention provides a kit of parts for controlling growth of cellulolytic microorganisms or cellulolytic activity in an aqueous cellulose fibre suspension in a storage tower or a storage tank of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue) which kit comprises:

[0014] (a) a nanobubble generator suitable for supplying nanobubbles to water in the system for forming the aqueous cellulose fibre suspension; and

[0015] (b) a device for measuring oxygen consumption rate of the aqueous cellulose fibre suspension, optionally wherein the kit further comprises one or both of a biocide dosing device for dosing biocide to the system based on the oxygen consumption rate measured by the device, and a biocide.

[0016] Also provided is use of the kit of parts for performing the method of the invention as described herein.

[0017] Moreover, the present invention also provides a method for treating or preventing growth of cellulolytic microorganisms in an aqueous cellulose fibre suspension in a storage tower or a storage tank of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue), the method comprising using a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas to treat or prevent said growth in the aqueous cellulose fibre suspension in the storage tower or storage tank, wherein the storage tank or the storage tower comprises at least one inlet conduit for the aqueous fibre suspension to pass into the storage tank or storage tower, and wherein the method comprises administering the nanobubbles to the system such that the concentration of oxygen in the aqueous cellulose fibre suspension in the at least one inlet conduit is between 1 mg / 1 and 30 mg / 1. Preferred features of all aspects of the present invention are defined in the dependent claims, which also apply to the method of the above paragraph.

[0018] In all aspects of the invention the aqueous cellulose fibre suspension may be defined as an aqueous cellulose fibre suspension comprising at least one microorganism.

[0019] The methods, uses, and kits defined herein are useful for controlling the growth of microorganisms (particularly cellulolytic microorganisms) and / or cellulolytic activity, and therefore maintaining the pulp fibres in as good a form as possible during pulp storage steps in the production methods. In particular, the present inventors have surprisingly found that the use of nanobubbles with the biocide significantly reduces the growth of microorganisms in aqueous pulp suspensions and in particular reduces the proportion of cellulolytic microorganisms present, thus protecting the pulp fibres as far as possible during the production process. This result is unexpected since cellulose degradation in nature is primarily an aerobic process and therefore it was not evident prior to the present invention that the addition of nanobubbles would decrease cellulose degradation. Moreover, the presence of the nanobubbles is shown to enhance the effect of the biocide, and therefore may be used to reduce the amount of biocide that needs to be used.

[0020] Brief Description of Figures

[0021] To assist understanding of the present disclosure and to show how embodiments may be put into effect, reference is made, by way of example only, to the accompanying Figures in which:

[0022] Figure 1 is a graph showing the oxygen levels (in mg / 1) over time (in minutes) in water samples that have been collected from paper mill process water and treated with four different treatments: nanobubbles alone; nanobubbles with 5 ppm monochloramine (MCA); nanobubbles with 10 ppm MCA; and nanobubbles with 15 ppm MCA.

[0023] Figure 2 is a graph showing the oxygen levels (in mg / 1) over time (in hours) in two of the treated water samples from Figure 1 : water treated with nanobubbles and 10 ppm MCA; and water treated with nanobubbles and 15 ppm MCA. Figure 3 is a graph showing the pH over time (in hours) in water collected from the dissolved air flotation (DAF) system outlet of a paper mill. One sample remained untreated, while three samples were treated with 15 ppm MCA alone, with nanobubbles alone, or with nanobubble and 15 ppm MCA.

[0024] Detailed Description of the Invention

[0025] As highlighted above, the present inventors have found that the combined use of nanobubbles with one or more biocides can advantageously be used to prevent or decrease the growth of cellulolytic microorganisms in aqueous cellulose fibre suspensions, and as a result protect the cellulose fibres of the suspension. The strength of these fibres is important to ensure the quality of the final product of paper and board mills.

[0026] Accordingly, in a first aspect, the present invention provides a method for treating an aqueous cellulose fibre suspension in a storage tank or a storage tower of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue), the method comprising using a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas to treat the aqueous cellulose fibre suspension in the storage tank or storage tower, wherein the storage tank or the storage tower comprises at least one inlet conduit for the aqueous fibre suspension to pass into the storage tank or storage tower, and wherein the method comprises administering the nanobubbles to the system such that the concentration of oxygen in the aqueous cellulose fibre suspension in the at least one inlet conduit is between 1 mg / 1 and 30 mg / 1.

[0027] Fibre materials used for paper or board production comprise cellulose, a substrate of cellulase enzymes. The aqueous cellulose fibre suspension used in the method of the invention is formed from or comprises cellulose or lignocellulose fibres. The fibres may be virgin fibres obtained by any known pulping process, they may be recycled fibres, and / or they may originate from broke. For example, the aqueous cellulose fibre suspension may comprise cellulose fibres obtained by mechanical pulping, chemical pulping, chemithermomechanical pulping or by repulping recycled or recovered fibres. The cellulose fibres can be refined or unrefined, bleached or unbleached. The cellulose fibres may be recycled unbleached or bleached kraft pulp fibres, hardwood semichemical pulp fibres, grass pulp fibres or any mixtures thereof. Preferably the aqueous cellulose fibre suspension comprises recycled cellulose fibres (RCF) or broke. In particular, recycled pulp fibres are often associated with the presence of higher levels of cellulolytic microorganisms.

[0028] The aqueous cellulose fibre suspension may comprise at least one microorganism, and in particular may comprise at least one cellulolytic microorganism.

[0029] Cellulolytic microorganisms (which may also be referred to as cellulose-degrading microorganisms) are those microorganisms that are capable of producing cellulolytic activity in at least one phase of their lifecycle. Cellulolytic or cellulase activity refers to a capability or potential capability of degrading of hydrolyzing cellulose by enzymes. Cellulases (including hemicellulose enzymes) are polypeptides comprising a cellulase activity, i.e., they are hydrolytic enzymes capable of catalyzing the decomposition of cellulose polymers into monosaccharides such as betaglucose, or shorter polysaccharides and oligosaccharides. Cellulolytic activity can be the result of a synergic process between different kind of cellulases, e.g. at least an endoglucanase and / or exoglucanase. Several different kinds of cellulases are known, which differ structurally and mechanistically.

[0030] Cellulolytic microorganisms or cellulase activity can be determined with nucleic acid-based detection methods (as previously described in WO 2021 / 214385). Determination of all potential cellulase genes or cellulase gene transcripts may be captured through nucleic acid or protein assays. Cellulolytic microorganisms can be measured by measuring known cellulolytic microbial taxa. Suitable RNA and / or DNA based methods include but are not limited to hybridization methods (e.g. southern or northern blotting, slot / dot blot, colony blot, fluorescence in situ hybridization, micro- array), PCR methods (e.g. qPCR, RT-PCR, qRT-PCR, multiplex-PCR, digital PCR, colony PCR), and sequencing methods (e.g. basic cloning and Sanger sequencing methods, next generation sequencing, high-throughput sequencing). Relative or total amounts of specific cellulolytic microorganisms, or families etc., thereof may be determined and / or monitored.

[0031] Non-limiting examples of cellulolytic microorganism include but are not limited to bacterial phyla Actinobacteria, Bacteroidetes, Firmicutes, and / or orders Corynebacteriales, Micrococcales, Bacteroidales, Bacillales, Lactobacillales, Clostridiales, Thermoanaerobacterales, Betaproteobacteriales, Xanthomonadales, and / or any family or genus belonging to said phyla or orders, optionally according to taxonomy of Bergey’s Manual of Systematic Bacteriology, 2ndEd., and / or Silva v. 132 Taxonomy.

[0032] The number of cellulolytic microorganisms as a proportion of the total microorganism population in the storage tank or the storage tower referred to herein may be less than 25%, preferably less than 15 %, more preferably less than 10%, and most preferably less than 6 %.

[0033] The system referred to herein is a system for manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue), e.g. a paper or board mill, in which paper and board are produced from aqueous cellulose fibre suspensions. Such systems generally have storage vessels, i.e. storage tanks and / or storage towers, where the suspensions can be stored during the manufacturing process until they are required. The present invention relates to treatment of the aqueous cellulose fibre suspensions while they are in the storage tanks / towers, i.e. during storage of the aqueous cellulose fibre suspensions, to control the growth of microorganisms, and specifically cellulolytic microorganisms, and / or to control the cellulolytic activity. Typically, control relates to reducing or preventing growth of the microorganisms and / or to preventing or slowing an increase in cellulolytic activity, and / or to decreasing cellulolytic activity. The treatment uses a combination of a biocide and nanobubbles, these having been administered to the aqueous cellulose fibre suspensions prior to the suspension passing into the storage tank / storage tower.

[0034] As described further below, the storage tank or the storage tower of the system comprises at least one inlet conduit and at least one outlet conduit, through which the aqueous cellulose fibre suspension passes into and out of the tank or tower. In one example of the invention, the proportion of cellulolytic microorganisms in the total microorganism population increase less than 25%, preferably less than 15%, more preferably less than 10%, and most preferably less than 6%, between the at least one inlet conduit and the at least one outlet conduit.

[0035] The present invention is intended to relate to a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue). The system may be a paper or board mill. The system comprises at least one storage tower or a storage tank for the aqueous cellulose fibre suspension. However, the system may comprise more than one storage tank and / or storage towers, and the methods and uses described herein may be performed in one or more of these. The storage tanks or towers may be a recycled pulp storage tower, a recycled pulp storage tank, broke storage tower and / or broke storage tank.

[0036] The storage tank(s) or storage tower(s) comprises at least one inlet conduit allowing the aqueous cellulose fibre suspension to pass into the tank or tower. In particular, the conduit allows the suspension to pass into the tank or tower from the preceding stage of the system. The tank / tower may comprise a recirculation loop that allows aqueous fibre suspension to move out of the tower, passing through the loop and back to the tank / tower. The tank / tower also comprises at least one outlet conduit, allowing the aqueous cellulose fibre suspension to pass out of the tank / tower to the subsequent stage in the system.

[0037] The storage tanks or towers are where the pulp described above is stored prior to or during use in the system to manufacture paper or board. The aqueous cellulose fibre suspension may be stored in these one or more locations for 1 to 48 hours, 1 to 24 hours, 1 to 12 hours, typically 1 to 8 hours. The present invention is particularly useful for methods involving longer storage times as the levels of cellulolytic microorganisms may otherwise increase rapidly over time.

[0038] The consistency of the aqueous cellulose fibre suspension may be at least 2 g / 1, and is preferably in the range of 10 - 150 g / 1.

[0039] The biocide used in the method of the present invention may comprise a non-oxidising biocide or an oxidizing biocide.

[0040] The non-oxidising biocide may be selected from the group consisting of: 2,2-Dibromo-3- nitrilopropionamide (DBNPA); 2-B romo-2 -nitropropane- 1 ,3-diol (Bronopol); 2-Bromo-2-nitro- propan-l-ol (BNP);2.2-Dibromo-2-cyano-N-(3-hydroxypropyl)acetamide; 2,2-

[0041] Dibromomalonamide; ; 1 ,2-Dibromo-2,4-dicyanobutane (DCB); Bis(trichloromethyl)sulfone; 2- Bromo-2-ni-trostyrene (BNS); Didecyl-dimethylammonium chlorine (DDAC); N-Alkyl-N- benzyl- N,N-dimethylammonium chloride (ADBAC) and other quaternary ammonium com pounds; 3-lodopropynyl-N-butylcarbamate (IPBC); Methyl and Dimethyl-thiocarbamates and their salts; 5-Chloro-2-methyl-4-isothiazolin-3-one (CMIT); 2-Methyl-4- isothiazolin-3-one (MIT) and their mixture; 2-n-Octyl-4-isothiazolin-3-one (OIT); 4,5- Dichloro-2-(n-octyl)-3(2H)- isothiazolone (DCOIT); 4, 5 -Dichloro- 1 ,2-dithiol-3-one; l,2-Benzisothiazolin-3-one (BIT); 2- (Thiocyanomethylthio)benzthiazole (TCMBT); 2-Methyl-l ,2-benzisothiazolin-3(2H)-one (MBIT); Tetrakis hydroxymethyl phosphonium sulfate (THPS); Tetrahydro-3, 5-dimethyl-2H-l ,3,5-thiadiazine-2-thione (Dazomet); Methylene bisthiocyanate (MBT); Ortho-phenylphenol (OPP) and its salts; Glutaraldehyde; Ortho-phthaldehyde (OP A); Guanidines and biguanidines; N- do-decylamine or n-dodecylguanidine; dodecylamine salt or dodecylguanidine salt, such as dodecylguanidine hydrochloride; Bis-(3-aminopropyl)dodecylamine; Pyrithiones, such as Zinc pyrithione; Triazines such as Hexahydro-1 ,3,5-trimethyl-l ,3,5- triazine; 3-[(4- Methylpheny l)sulfony 1] -2-propenenitrile; 3 -Pheny lsulphonyl-2-propenenitrile; 3 - [(4- trifluormethylphenyl)sulphonyl] -2 -propenenitrile; 3-[(2,4,6-trimethylphenyl)sulphonyl]-2- propenenitrile; 3 -(4-methoxyphenyl)sulphonyl-2-propenenitrile; 3 - [(4- methylphenyl)sulphonyl]prop-2-enamide; and any of their isomers; and any combination thereof.

[0042] The oxidising biocide may be selected from the group consisting of: chlorine; alkali and alkaline earth hypochlorite salts; hypochlorous acid; bromine; alkali and alkaline earth hypobromite salts; hypobromous acid; chlorine dioxide; ozone; hydrogen peroxide; peroxy compounds, such as performic acid, peracetic acid, percarbonate or persulfate; halogenated hydantoins, such as monohalodime- thylhydantoins; dihalodimethylhydantoins; perhalogenated hydantoins; monochloramine (MCA); monobromamine; dihaloamines; trihaloamines; urea reacted with an oxidant, the oxidant being e.g. alkali and alkaline earth hypochlorite salts or alkali and alkaline earth hypobromite salts; ammonium salts, e.g. ammonium bromide, ammonium sulfate or ammonium carbamate, reacted with an oxidant, the oxidant being preferably alkali and alkaline earth hypochlorite salts or alkali and alkaline earth hypobromite salts; and any combination thereof.

[0043] Preferably the biocide comprising an oxidising biocide. The oxidising biocide may preferably be selected from monochloramine (MCA), chlorine dioxide, a percarboxylic acid, a halogenated hydantoin, an alkali or alkaline earth hypochlorite salt, or chlorine gas. Most preferably the biocide comprises or consists of MCA.

[0044] The above biocides and suitable concentrations / doses for use in paper or board manufacturing systems are well-known in the art. As mentioned above, the use of nanobubbles described herein may allow lower concentrations / doses of biocides to be used than previously.

[0045] The nanobubbles used in the method of the invention can be obtained using a nanobubble generator. Nanobubbles are extremely small bubbles of less than 1 um in diameter, usually 500 nm or less in diameter. Preferable the nanobubbles for use in the present invention are 50 to 500 nm in diameter, more preferably between 50 and 150 nm in diameter. Devices for generating nanobubbles are known in the art, e.g. as described in ESI 263409, and are commercially available, e.g. from Moleaer Industries. The nanobubbles comprise an oxidising gas, e.g. O2 or O3. Preferably the nanobubbles are air bubbles or oxygen enriched air bubbles.

[0046] The biocide and the nanobubbles are generally administered to water upstream of the storage tank or storage tower. In particular, the water comprising the nanobubbles is generally used to form the aqueous cellulose fibre suspension, and the biocide is added either to the water comprising the nanobubbles or to the aqueous fibre suspension, which then passes into the storage tanks / storage towers through the at least one inlet conduit. The aqueous cellulose fibre suspension is normally formed prior to entering the tank or tower in order to ensure that the biocide and the nanobubbles are mixed in with the fibres when the aqueous fibre suspension enters the tank or tower. However, further water comprising nanobubbles or biocide can be added in the recirculation loop mentioned above. Preferably the method comprises administering the nanobubbles to water in the dissolved air flotation (DAF) system, wherein the water from the DAF system is then used to form the aqueous cellulose fibre suspension.

[0047] As noted above, the nanobubbles may be added such that concentration of oxygen in the aqueous cellulose fibre suspension at the at least one inlet conduit, i.e. in the aqueous cellulose fibre suspension entering the tank / tower, is between 1 mg / 1 ad 30 mg / 1, preferably between 5 mg / 1 and 30 mg / 1. Preferably, the combination of the biocide and nanobubbles described herein means that the concentration of oxygen in the aqueous cellulose fibre suspension at the at least one outlet conduit is more than 1 mg / 1.

[0048] The concentration of oxygen in the aqueous cellulose fibre suspension can be measured with standard probes that are known and commonly used in the paper and board industry for measuring dissolved oxygen concentration. The present inventors have noted that such probes appear to measure both dissolved oxygen concentration and oxygen that is present in the suspension in the form the nanobubbles, since the measurements that they have obtained (e.g. as reported in the examples) exceed the expected oxygen solubility levels. Without wishing to be bound by theory, the inventors consider that the nanobubbles present may be attaching to the probe, such that the probe measurement includes the oxygen present in the aqueous cellulose fibre suspension in the nanobubbles.

[0049] As is known in the art, the biocide may be added to the system at suitable concentrations / in suitable amounts for controlling the growth of microorganisms. In particular, it is already known in the art to control the total microorganism level in a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue) using biocide, and this may include assessing the various points within the system where significant microorganism growth is occurring or has occurred and adjusting the location and amount of biocide applied accordingly.

[0050] As described above, the biocide of the combination of nanobubbles and biocide used in the present invention, is present in the aqueous cellulose fibre suspension when it enters the storage tank / storage tower. The biocide may be added at the system and / or to the at least one inlet conduit in an amount of 1 to 100 ppm, preferably 1 to 20 ppm, more preferably 10 to 20 ppm. The amount of biocide and where it is added to the system may be adjusted based on the type of biocide being used, the level of microorganisms in the system, and the location within the system where the microorganisms are found.

[0051] The amount and location of the biocide addition may be adjusted based on the oxygen consumption rate of the aqueous cellulose fibre suspension entering the storage tank / storage tower. In particular, oxygen consumption rate may be assessed in a sample taken from the aqueous cellulose fibre suspension in the at least one inlet conduit. The oxygen concentration of the sample can be determined at at least two time points (e.g. at 0 minutes and at 1 minute) and the oxygen consumption rate determined accordingly.

[0052] The oxygen consumption rate of the sample is related to the number of microorganisms present. The addition of biocide can therefore be adjusted based on the oxygen consumption rate. In particular, in one example of the invention the method comprises administering biocide to the system such that the aqueous cellulose fibre suspension in the at least one inlet conduit has an oxygen consumption rate to less than 0.2 mg / min, preferably less than 0.1 mg / min.

[0053] In preferred embodiments of the present invention, the combination of the biocide and the nanobubbles additionally stabilizes the process conditions inside the storage tank / storage tower, maintaining one or more of the pH, oxidation-reduction potential (ORP), temperature adjusted oxidation reduction potential (rH), and conductivity of the aqueous cellulose fibre suspension. Preferably these parameters are maintained though-out the storage of the aqueous cellulose fibre suspension.

[0054] Preferably the pH of the aqueous cellulose fibre suspension at the outlet of the storage tank or storage tower is maintained in the range of pH 6.5 to pH 7.3, preferably in the range of pH 6.7 to 7.3. The pH of the suspension may be determined using a standard pH probe or pH meter.

[0055] Preferably the ORP of the aqueous cellulose fibre suspension at the outlet of the storage tank or tower is above -100 mV, preferably above -50 mV. The ORP may be measured using standard sensors or electrodes.

[0056] As would be understood by the skilled person, rH corresponds to a pH- and temperature- adjusted oxidation-reduction potential (ORP). In one example, rH may be calculated using the following equation: rH = 2 pH +2 Eh F / (2.3026 R T) where:

[0057] Eh = oxidation reduction potential measured using a standard hydrogen electrode (V) F = Faraday constant (96485 C mol’1)

[0058] R = Gas constant 8.314 J-K^ mol’1

[0059] T = temperature (K)

[0060] The temperature of the water may be determined using a standard thermometer.

[0061] Preferably the rH of the aqueous cellulose fibre suspension at the outlet of the storage tank or tower is above rH 5, preferably above rH 10.

[0062] Preferably the increase in conductivity of the of the aqueous cellulose fibre suspension between the at least one inlet and the at least one outlet is 10% or less. The conductivity may be measured using standard sensors or electrodes.

[0063] In one example of the invention, the proportion of cellulolytic microorganisms in the total microorganism population increase less than 25%, preferably less than 15%, more preferably less than 10%, and most preferably less than 6%, between the at least one inlet and the at least one outlet. Cellulolytic microorganisms may be as defined above.

[0064] Further aspects of the invention are described below. The description of the invention provided above applies also to these aspects.

[0065] In one further aspect the present invention provides a method of monitoring and controlling growth of cellulolytic microorganisms or cellulolytic activity in an aqueous cellulose fibre suspension in a storage tank or a storage tower of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue), wherein the method comprises:

[0066] (a) determining the number of cellulolytic microorganisms or the cellulolytic activity in the aqueous cellulose fibre suspension in a part of the system; and

[0067] (b) controlling the growth of cellulolytic microorganisms or the cellulolytic activity by treating the aqueous cellulose fibre suspension with a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas in the storage tank or storage tower. In one example in which the storage tank or storage tower comprises at least one outlet conduit for the aqueous fibre suspension to pass out of the storage tank or storage tower towards a subsequent part of the system, the part of the system in (a) is at least one outlet conduit. Determining in (a) may be performed on a sample taken from the part of the system, e.g. a sample taken from the at least one outlet conduit.

[0068] In particular, the cellulolytic microorganisms or cellulolytic activity and the determining in step (a) may be as defined above in relation to the first aspect of the invention. Preferably the treating in (b) comprises a method of treating as described above for the first aspect of the invention.

[0069] More generally, the present invention provides in another aspect a method of manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue) from an aqueous cellulose fibre suspension in a system comprising a storage tower or a storage tank, the method comprising treating the aqueous cellulose fibre suspension with a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas in the storage tower or the storage tank, prior to using the aqueous cellulose fibre suspension to manufacture the fibrous web, wherein said treating controls the growth of cellulolytic microorganisms or cellulolytic activity in the aqueous cellulose fibre suspension while the aqueous cellulose fibre suspensions is stored in the storage tower or storage tank.

[0070] Still further the present invention provides the use of a biocide in a method for treating an aqueous cellulose fibre suspension, the method comprising using a combination of (i) the biocide and (ii) nanobubbles comprising an oxidising gas to treat an aqueous cellulose fibre suspension to prevent or reduce growth of cellulolytic microorganisms or cellulolytic activity, wherein the aqueous cellulose fibre suspension is in a storage tower or a storage tank of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue).

[0071] In preferred embodiments of this method of manufacturing and use of a biocide, the treating can be performed as described above in relation to the method of the first aspect of the invention. Also provided is a kit of parts for controlling growth of cellulolytic microorganisms or cellulolytic activity in an aqueous cellulose fibre suspension in a storage tower or a storage tank of a system manufacturing a fibrous web containing cellulose fibres (such as paper, board, or tissue), which kit comprises:

[0072] (a) a nanobubble generator suitable for supplying nanobubbles to water in the system for forming the aqueous cellulose fibre suspension; and

[0073] (b) a device for measuring oxygen consumption rate of the aqueous cellulose fibre suspension.

[0074] The device for measuring oxygen consumption rate may be an oxygen concentration probe that is configured to determine the oxygen consumption rate in the aqueous cellulose fibre solution or a sample thereof, in a part of the system, e.g. an inlet conduit for the storage tank or storage tower, through which the aqueous fibre suspension passes into the storage tank or storage tower. The device may be configured to take a sample of the suspension from the system and determine the oxygen consumption rate in the sample.

[0075] The device may determine the oxygen consumption rate by taking at least two measurements of oxygen concentration over a time period and calculating the oxygen consumption rate based on these measurements. The device is preferably configured to be fitted into the system at the inlet conduit for the storage tower or storage tank. The device may determine the oxygen concentration in aqueous cellulose fibre suspension within the inlet conduit itself or in a sample of the suspension taken from the inlet conduit.

[0076] The kit may further comprise a biocide dosing device for dosing biocide to the system based on the measured oxygen consumption rate. In particular, the biocide dosing device may be configured to receive the oxygen consumption rate measured by the device for measuring oxygen consumption rate, and to adjust the dose of the biocide accordingly, e.g. in order to maintain the oxygen consumption rate in the inlet conduit at less than 0.2 mg / min., preferably less than 0.1 mg / min.

[0077] The kit may further comprise a biocide as described above. The nanobubble generator may be configured for supplying nanobubbles to a DAF system supplying water to form the aqueous cellulose fibre suspension.

[0078] The kit can be used for performing any one of the methods described herein, and accordingly, the description of these methods provided above also applies to the kit and use thereof.

[0079] The following are intended as examples only and do not limit the present disclosure.

[0080] EXAMPLES

[0081] Example 1 - Biocide Decrease Oxygen Consumption Rate and Maintains Higher pH

[0082] Water was taken from the dissolved air flotation (DAF) system outlet of a paper mill. A filter was used to further remove cellulose fibres and other particles, and then the water was placed in a feed tank for a nanobubble generator. Water with nanobubbles generated by the nanobubble generator was collected and divided into four jars. Different amounts of the biocide monochloramine (MCA) in ppm were added (0 ppm; 5 ppm; 10 ppm; and 15 ppm) and oxygen consumption was monitored for 60 minutes. Results are shown in Figure 1. A clear dose response is shown - the higher the MCA addition, the lower the oxygen consumption.

[0083] The oxygen levels in two of the treated samples (nanobubbles with 10 ppm; and nanobubbles with 15 ppm) were monitored over 24 hours. Results are provided in Figure 2 and show that when biocide and nanobubbles are used together, oxygen consumption remains linear for hours and only after 24 hours is all the oxygen consumed. As shown in Figure 1, without biocide all the oxygen is consumed after 45 minutes. It was found that pH and redox values remained at a higher level over the 24 hour period in the samples treated with both nanobubbles and MCA as compared to the sample treated with nanobubbles alone (data not shown). Example 2 - Biocide Decrease Oxygen Consumption Rate and Maintains Higher pH

[0084] Water samples were collected from the dissolved air flotation (DAF) system outlet of a paper mill. One portion of the water was split to form two samples without nanobubbles, while the remaining portion of the water was fed to the nanobubble generator to generate two water samples with nanobubbles. MCA was added into two of the samples at 15 ppm. Accordingly, four samples were monitored: water without nanobubbles; water with nanobubbles; water without nanobubbles but with 15 ppm MCA; and water with nanobubbles and with 15 ppm. The pH value of the samples was followed for 5 hours.

[0085] The reults are shown in Figure 3. Nanobubbles alone had only a small impact on pH at the beginning. MCA without nanobubbles kept pH stable for some time, but the best result was obtained when MCA was used together with nanobubbles.

[0086] Example 3 - Maintaining Broke Conditions with Combination of Biocide and Nanobubbles

[0087] Water samples from the DAF outlet with and without nanobubbles (as per Example 2) was mixed with broke (20% broke comprising approx. 4% cellulose fibres) from a mill using 100% recycled fiber raw material. MCA was added in both cases and the conditions followed for 4.5 hours. The results are shown in Table 1 below. Oxygen was present only with combination of biocide and nanobubbles. pH was also highest with this combination.

[0088] Table 1. Oxygen and pH in broke with nanobubbles and MCA Example 4 - Nanobubbles with Biocide Maintains Good Pulp Conditions for at least 15

[0089] Hours

[0090] Pulp from the top ply mixing chest in a paper mill was mixed with DAF outlet water with and without nanobubbles (produced as described above) and MCA. The pulp was 20% of the total volume. Levels of MCA used were 5 ppm, 10 ppm or 15 ppm. The conditions in the samples were measured at the beginning (0 hours) and after 15 hours. The parameters in Table 2 were measured using commercially-available probes. ATP was measured using commercially available reagents from 3M.

[0091] The results are in Table 2, where it is shown that conditions in the pulp are better with the increasing MCA concentration. This is visible in all parameters. ATP (total bacterial biomass was different and the highest ATP values were with nanobubbles and low amount of MCA.

[0092] Table 2. Process conditions in pulp with with nanobubbles and MCA

[0093] Example 5 - Nanobubbles with Biocide Maintains Good Pulp Conditions for at least 15

[0094] Hours

[0095] DAF outlet water with and without nanobubbles was mixed with poor quality RCF pulp that had been separated from the water at the DAF unit in a paper mill. The total amount of fibres was 10% of the total volume. Different amounts of MCA were added (5 ppm, 10 ppm, and 15 ppm) and pH and the oxidation / reduction potential (ORP) values were followed. The results in Table 3 show that nanobubbles increase rH but it decreases again quite quickly. With increasing amount of MCA rH remains at good level longer, until the end of the test at 16.5 hours.

[0096] Table 3. rH values from the experiments with nanobubbles and MCA

[0097] Example 6 - Bacterial Communities After Treatment with Nanobubbles and Biocide

[0098] Samples were collecected from the same test as Example 5, 16.5 hours after the test had started. Bacterial communities in the samples were determined based on the DNA in the samples: DNA was extracted, purified and quantified from the samples. Bacterial 16S rRNA genes were multiplied using a PCR method. Bacterial community composition was studied using a next generation sequencing (NGS) method. Sequence raw data was processed by removing poor quality prokaryotic 16S rRNA sequences. The remaining sequences were classified for phylum, class, order, family, and genus level identification.

[0099] The results are provided in Table 4. The data shows that the proportions of the most harmul bacteria, those from the order Bacteroidales and ananerobic firmicutes, were lower when the combination of nanobubbles and MCA (10 or 15 ppm) were used. Also, some other harmful bacteria, like acid producing Tolumonas bacteria, were less common in these samples.

[0100] Table 4. Bacterial communities in test where RCF pulp was treated with nanobubbles and MCA.

[0101] Example 7 - Proportions of Harmful Potential Cellulose Degraders Controlled with Combined Use of Nanobubbles and Biocide

[0102] Cellulose degrading bacteria are considered to have a negative impact on fiber, and therefore also on the final board or paper strength.

[0103] The samples produced in Example 2 above (after 5 hours of treatment) were assessed to determine the proportion of cellulose degrading bacteria. As shown in Table 5, with nanobubbles, and especially when used together with MCA, the proportion of cellulose degrading bacteria was lower than without nanobubbles. It is noted that the results in this example were obtained after 5 hours of treatment. It is expected that the difference would be bigger after a longer treatment period (e.g. after one or more days).

[0104] Table 5. Proportions of potential cellulose degraders in the samples.

[0105] Further, the samples produced in Example 4 above (after 15 hours of treatment) were assessed to determine the proportion of cellulose degrading bacteria. As shown in Table 6, the proportion of cellulose degrading bacteria was lower with the combined treatement with nanobubbles and MCA than without.

[0106] Table 6. Proportions of potential cellulose degraders in the samples.

[0107] The results provided herein show that the combined treatment of pulp suspensions with nanobubbles and biocide during pulp storage can surprisingly reduce the proportion of cellulose degrading bacteria, helping to maintain pulp fibre strength during the manufacture of paper and board.

[0108] Other variants or use cases of the disclosed techniques may become apparent to the person skilled in the art once given the disclosure herein. The disclosure is not limited by the described embodiments but only by the accompanying claims.

Claims

CLAIMS1. A method for treating an aqueous cellulose fibre suspension comprising at least one microorganism in a storage tank or a storage tower of a system manufacturing a fibrous web containing cellulose fibres, the method comprising using a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas to treat the aqueous cellulose fibre suspension in the storage tank or storage tower, wherein the storage tank or the storage tower comprises at least one inlet conduit for the aqueous fibre suspension to pass into the storage tank or storage tower, and wherein the method comprises administering the nanobubbles to the system such that the concentration of oxygen in the aqueous cellulose fibre suspension in the at least one inlet conduit is between 1 mg / 1 and 30 mg / 1.

2. The method of claim 1, wherein the cellulose fibres of the aqueous cellulose fibre suspension comprise recycled cellulose fibres (RCF).

3. The method of claim 1 or claim 2, wherein the storage tower or storage tank is a broke storage tower or a recycled fibre storage tower.

4. The method of any one of claims 1 to 3, wherein the method comprises administering the biocide to the aqueous pulp suspension in the at least one inlet conduit in an amount of 1 to 100 ppm.

5. The method of any one of claims 1 to 4, wherein the method comprises administering the biocide to the system such that the concentration of the biocide in the aqueous cellulose fibre suspension in the at least one inlet conduit is between 1 to 100 ppm.

6. The method of any one of claims 1 to 5, wherein the method comprises administering the biocide to the system such that the aqueous cellulose fibre suspension in the at least one inlet conduit has an oxygen consumption rate of less than 0.2 mg / min., preferably less than 0.1 mg / min.

7. The method of any one of claims 1 to 6, wherein the method comprises administering the nanobubbles to water in a dissolved air flotation (DAF) system and using the water from the DAF system to form the aqueous cellulose fibre suspension.

8. The method of any one of claims 1 to 7, wherein the concentration of oxygen in the aqueous cellulose fibre suspension in the at least one inlet conduit is between 5 mg / 1 and 30 mg / 1.

9. The method of any one of claims 1 to 8, wherein the storage tank or storage tower comprises at least one outlet conduit for the aqueous fibre suspension to pass out of the storage tank or storage tower towards a subsequent part of the system, and wherein method comprises using the combination to maintain one or more of:- the pH of the aqueous cellulose fibre suspension in the at least one outlet conduit between pH 6.5 and pH 7.3, and preferably between pH 6.7 and pH 7.3;- the oxidation-reduction potential (ORP) of the aqueous cellulose fibre suspension in the at least one outlet conduit above -100 mV, and preferably above - 50 mV;- a pH and temperature adjusted oxidation reduction potential (rH) of the aqueous cellulose fibre suspension in the at least one outlet conduit above rH 5.0; and- the increase in conductivity of the aqueous cellulose fibre suspension between the at least one inlet conduit and the at least one outlet conduit at 10% or less.

10. The method of any one of claims 1 to 9, wherein the at least one microorganism is at least one cellulolytic microorganism.

11. The method of any one of claims 1 to 10, wherein the storage tank or storage tower comprises at least one outlet conduit for the aqueous fibre suspension to pass out of the storage tank or storage tower towards a subsequent part of the system, and wherein a proportion of cellulolytic microorganisms of a total microorganism number in a sample of the aqueous cellulosefibre suspension taken from the at least one outlet conduit shows a less than 25 % increase as compared to the proportion in a sample of the aqueous cellulose fibre suspension taken from the at least one inlet conduit.

12. The method of claim 10 or claim 11 , wherein the at least one cellulolytic microorganism of claim 10 or the cellulolytic microorganisms of claim 11 are selected from the group consisting of bacterial phyla Actinobacteria, Bacteroidetes, Firmicutes, and / or orders Corynebacteriales, Micrococcales, Bacteroidales, Bacillales, Lactobacillales, Clostridiales, Thermoanaerobacterales, Betapropteobacterioales, Xanthomonadales, and / or any family or genus belonging to said phyla or orders.

13. The method of any one of claims 1 to 12, wherein the biocide comprises an oxidising biocide and / or a non-oxidising biocide, and preferably wherein the biocide comprises an oxidising biocide and is selected from monochloramine (MCA), chlorine dioxide, percarboxylic acids, halogenated hydantoins, alkali and alkaline earth hypochlorite salts, and chlorine gas, and preferably wherein the oxidizing biocide comprises MCA.

14. A method of monitoring and controlling growth of cellulolytic microorganisms or cellulolytic activity in an aqueous cellulose fibre suspension in a storage tank or a storage tower of a system manufacturing a fibrous web containing cellulose fibres, wherein the method comprises:(a) determining the number of cellulolytic microorganisms or the cellulolytic activity in the aqueous cellulose fibre suspension in a part of the system; and(b) controlling the growth of cellulolytic microorganisms or the cellulolytic activity by treating the aqueous cellulose fibre suspension with a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas in the storage tank or storage tower, preferably wherein the treating comprises a method of treating according to any one of claims 1 to 13.

15. The method according to claim 14, wherein the storage tank or the storage tower comprises at least one outlet conduit for the aqueous fibre suspension to pass out of the storage tank or storagetower towards a subsequent part of the system, and wherein the part of the system in (a) is the at least one outlet conduit.

16. A method of manufacturing a fibrous web containing cellulose fibres from an aqueous cellulose fibre suspension in a system comprising a storage tower or a storage tank, the method comprising treating the aqueous cellulose fibre suspension with a combination of (i) a biocide and (ii) nanobubbles comprising an oxidising gas in the storage tower or the storage tank, prior to using the aqueous cellulose fibre suspension to manufacture the fibrous web, wherein said treating controls the growth of cellulolytic microorganisms or cellulolytic activity in the aqueous cellulose fibre suspension while the aqueous cellulose fibre suspensions is stored in the storage tower or storage tank, and preferably wherein the treating comprises a method of treating according to any one of claims 1 to 13.

17. Use of a biocide in a method for treating an aqueous cellulose fibre suspension, the method comprising using a combination of (i) the biocide and (ii) nanobubbles comprising an oxidising gas to treat an aqueous cellulose fibre suspension to control growth of cellulolytic microorganisms or cellulolytic activity, wherein the aqueous cellulose fibre suspension is in a storage tower or a storage tank of a system manufacturing a fibrous web containing cellulose fibres, and preferably wherein the treating comprises a method of treating according to any one of claims 1 to 13.

18. A kit of parts for controlling growth of cellulolytic microorganisms or cellulolytic activity in an aqueous cellulose fibre suspension in a storage tower or a storage tank of a system manufacturing a fibrous web containing cellulose fibres, which kit comprises:(a) a nanobubble generator suitable for supplying nanobubbles to water in the system for forming the aqueous cellulose fibre suspension; and(b) a device for measuring oxygen consumption rate of the aqueous cellulose fibre suspension, optionally wherein the kit further comprises one or both of a biocide dosing device for dosing biocide to the system based on the oxygen consumption rate measured by the device, and a biocide.

19. The kit of parts according to claim 18, wherein the nanobubble generator is configured for supplying nanobubbles to a DAF system supplying water to form the aqueous cellulose fibre suspension.

20. Use of a kit according to claim 18 or claim 19 for performing the method of any of claims 1 to 16.

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