Method for removal of suspended solid particles

Abstract

The present invention relates to a method for removal of solid particles suspended in an aqueous medium. The method comprises adding to the aqueous medium a cationic biopolymer selected from cationic linear α-(1,3-glucan) polymers, cationic branched α-(1,3-glucan) polymers, and any of their mixtures. The solid particles suspended in the aqueous medium are allowed to interact with the cationic biopolymer to form agglomerates, and the agglomerates are separated from the aqueous medium.

Description

[0001] METHOD FOR REMOVAL OF SUSPENDED SOLID PARTICLES

[0002] The present invention relates to a method for removal of suspended solid particles from an aqueous medium according to the preambles of the enclosed independent claims.

[0003] Turbidity is a measure of the clarity of a liquid, and high turbidity value indicates that the liquid is cloudy, opaque or hazy. Turbidity is usually a visible defect in an aqueous medium, such as wastewater or surface water. Turbidity is not only unaesthetic, but the suspended solid particles, such as silt, algae, small inorganic and organic particles, can be harmful in itself and / or facilitate microbial growth in the aqueous medium. This means that the suspended solid particles, i.e. turbidity, should be reduced or removed before the aqueous medium can be safely used or returned back to the environment. Removal of suspended solids particles from the aqueous medium reduces the amount of microorganisms and contaminants in the aqueous medium and can improve effectiveness of downstream treatment processes, such as filtration and / or disinfection of the aqueous medium.

[0004] Removal of the suspended solid particles which cause turbidity by sedimentation is slow, inefficient and sometimes even impossible. Due to the small particle size, solid particles tend to remain suspended in the aqueous medium even if the medium is allowed to settle for prolonged times. A common way to remove suspended solid particles from an aqueous medium is by using chemical coagulants, such as coagulant metal salts or cationic synthetic coagulant polymers, e.g. polyepiamine. The chemical coagulants enable the suspended solid particles to come closer together, usually due to charge neutralisation, and to form larger particle agglomerates that can then be separated from the aqueous liquid medium, possibly after a following flocculation step. Typically the used chemical coagulants end up into the sludge formed in the treatment process of the aqueous medium.

[0005] Traditionally, the sludge formed in the treatment processes of aqueous media, such as wastewater, is seen as problematic waste, which disposal is expensive. Sludge can be conventionally disposed by landfill deposition or by incineration for production of energy. However, rising awareness of resource management and circular economy are changing the conventional thinking. The formed sludge can contain valuable compounds, such as nutrients, and instead of an ecological problem it could be seen as a possible resource and opportunity for sustainable recycling. For example, some sludges could be used for fertilizing purposes. However, this shift in thinking puts a new focus on chemicals used for turbidity removal. For example, if the content of inorganic metal coagulants in the formed sludge is too high, it may render the sludge unsuitable for recycling. High content of inorganic metal coagulants may even disturb the conventional incineration by increasing the ash formation.

[0006] There is also a current growing incentive towards sustainable industrial processes and towards bio-based and / or biodegradable chemicals. This desire to use biobased and / or biodegradable chemicals has induced a strong interest to find replacements for petroleum-based synthetic polymers, which have been traditionally used as coagulants in treatment of aqueous media for removal of turbidity.

[0007] WO 2021 / 247810 discloses dextran-alpha-glucan graft copolymers and derivatives thereof, as well as their use in various applications and products.

[0008] In view of above, there is a need to find new sustainable ways of effectively remove suspended solid particles from an aqueous medium.

[0009] An object of this invention is to minimise or even eliminate the disadvantages existing in the prior art.

[0010] An object is also to provide a sustainable method for removal of solid particles suspended in an aqueous medium, i.e. for removal of turbidity from an aqueous medium, e.g. wastewater. A further object of the invention is to provide a method which enables reduction of, or even complete abstention from, the use of conventional coagulants in turbidity removal.

[0011] These objects are attained with the invention having the characteristics presented below in the characterising part of the independent claim. Some preferable embodiments are disclosed in the dependent claims.

[0012] The embodiments mentioned in this text relate, where applicable, to all aspects of the invention, even if this is not always separately mentioned.

[0013] A typical method according to the present invention for removal of solid particles suspended in an aqueous medium comprises

[0014] - adding to the aqueous medium a cationic biopolymer selected from cationic linear a-(1 ,3-glucan) polymers, cationic branched a-(1 ,3-glucan) polymers, and any of their mixtures,

[0015] - allowing the solid particles suspended in the aqueous medium to interact with the cationic biopolymer by formation of agglomerates, and

[0016] - separating the agglomerates from the aqueous medium.

[0017] A typical use according to the present invention of a cationic biopolymer is as a coagulant for removal of solid particles suspended in an aqueous medium, wherein the cationic biopolymer is selected from cationic linear a-(1 ,3-glucan) polymers, cationic branched a-(1 ,3-glucan) polymers, and any of their mixtures.

[0018] Now it has been surprisingly found that a biopolymer selected from cationic linear and branched a-(1 ,3-glucan) polymers is able to effectively interact with solid particles suspended in an aqueous medium and to create agglomerates that can be easily separated from the aqueous medium. The used cationic biopolymer notably reduces the turbidity of the aqueous medium and makes it suitable for further processing or for return to environment. The cationic biopolymer reduces or even eliminates the need to use of inorganic coagulants, which facilitates the use of the separated agglomerates, i.e. sludge, in further processes or recycling. In the present context the terms “solid particles” and “suspended solid particles” are used synonymously and fully interchangeably. Both terms denote solid particles having a particle size in a range of 20 - 120 pm, typically 30 - 100 pm. The solid particles may comprise inorganic particles, organic particles or their mixtures. Cellulosic fibres are preferably excluded from solid particles. For example, the suspended solid particles may originate from clay, silt, algae, plankton, and microorganisms. Typically the solid particles remain suspended in the aqueous medium even if allowed to settle for 1 hour. The individual solid particles may be so small that they cannot be clearly seen by naked eye, but their presence can be detected as haziness, cloudiness and / or colouration of the aqueous medium.

[0019] In the present context the terms “a-(1 ,3-glucan) polymer” and “a-(1 ,3-glucan)” are used synonymously and fully interchangeably and they both denote a polymeric structure having a polysaccharide backbone which comprises D-glucose units linked together by glycosidic linkages. At least 70 %, preferably at least 80 %, more preferably at least 90 % or at least 95 %, sometimes even at least 99 % or 100 %, of the glycosidic linkages of the polymeric structure are a-1 ,3-linkages. This means that in the polysaccharide backbone the a-D-glucose units are connected to each other through carbons 1 and 3 on adjacent a-D-glucose rings. The form of glycosidic linkages can be determined by a person skilled in the art by using methods known as such, for example1HNMR.

[0020] According to one preferable embodiment, the cationic biopolymer may be linear cationic a-(1 ,3-glucan) polymer, i.e. the biopolymer may be unbranched. The cationic biopolymer may also be a mixture of one or more linear cationic a-(1 ,3- glucan) polymers.

[0021] Alternatively, the cationic biopolymer may be branched cationic a-(1 ,3-glucan) polymer or a mixture of one or more branched cationic a-(1 ,3-glucan) polymers.

[0022] The cationic biopolymer may be a mixture of one or more linear and branched cationic a-(1 ,3-glucan) polymers. When the cationic biopolymer, selected from linear a-(1 ,3-glucan) polymers and branched a-(1 ,3-glucan) polymers, is brought into a contact with the aqueous medium comprising suspended solid particles, the cationic biopolymer interacts with the suspended solid particles and transforms them into agglomerates that can be separated from the aqueous medium with any suitable manner, such as sedimentation or filtration. The cationic biopolymer may be added to the aqueous medium in form of solution or dispersion, without any preceding processing or treatment steps. Especially, the present method is free of subjecting the cationic biopolymer to a high shear treatment or the like. Preferably, the cationic biopolymer is only diluted or dissolved to the appropriate concentration and added to the aqueous medium, preferably under normal mixing to guarantee proper distribution within the aqueous medium and interaction with the suspended solid particles.

[0023] The cationic biopolymer is an a-(1 ,3-glucan) polymer, either linear or branched, preferably linear a-(1 ,3-glucan) polymer, having cationic substitution groups attached to its polymeric structure. The cationic substitution groups may be substituted ammonium groups, preferably quaternary ammonium groups, more preferably trialkyl ammonium groups. The alkyl group in the trialkyl ammonium group may be, for example a methyl group, a hydroxymethyl group, a hydroxyethyl group or a hydroxypropyl group. The substituted ammonium group may be, for example, trimethylammonium group or hydroxyethyl group.

[0024] The cationic substitution groups may be attached to the polymeric structure of a- (1 ,3-glucan) polymer, either linear or branched, via an ether, ester, carbarn ate / carbamoy I, or sulfonyl linkage. The cationic biopolymer may thus be a cationic ester- or ether-derivative of linear or branched a-(1 ,3-glucan) polymer or a carbamate derivative of a linear or branched a-(1 ,3-glucan) polymer.

[0025] According to one embodiment of the invention the cationic biopolymer may be an a- (1 ,3-glucan) polymer, either linear or branched, having a degree of cationic substitution >0.05, more preferably >0.1 . The degree of cationic substitution may be in a range of 0.05 - 1 .2, preferably 0.1 - 1 .0, more preferably 0.15 - 0.7, even more preferably 0.2 - 0.55 or 0.25 - 0.45. The degree of cationic substitution refers to the average number of hydroxyl groups substituted with cationic groups in each glucose unit in the polymeric structure. The cationic substitution, as defined, provides the a- (1 ,3-glucan) polymer with the ability to effectively interact with the suspended solid particles, e.g. neutralize at least some of their electrical charges, and facilitate the formation of agglomerates. In this manner the turbidity of the aqueous medium can be significantly reduced, even without an addition of an inorganic metal coagulant. However, the moderate substitution degree provides the cationic biopolymer with a cationic charge density which is still within appropriate limits.

[0026] The cationic biopolymer selected from linear and branched a-(1 ,3-glucan) polymers may have a charge density of 0.3 - 3.5 meq / g, preferably 0.5 - 3.2 meq / g, more preferably 0.8 - 2.6 meq / g or 0.9 - 3.0 meq / g, even more preferably 1 .0 - 2.2 meq / g or 1.5 - 2.0 meq / g. Charge densities can be measured by Mutek titration. Even if the charge density of the cationic biopolymer is relatively low, it is still able to substantially reduce the turbidity when added to the aqueous medium. It is unexpected that the cationic biopolymer with a relatively low charge density can effectively interact with the suspended solid particles. The low cationicity makes the biopolymer safe and suitable for use in applications, where the use of highly charged chemicals might be considered problematic, e.g. drinking water applications, or when the treated water is discharged to a receiving water body, for example to a natural water body with a sensitive ecosystem, such as lake, river or sea.

[0027] Preferably the cationic biopolymer selected from linear and branched a-(1 ,3-glucan) polymers is at least partly, preferably fully, water-soluble.

[0028] According to one embodiment of the invention, the cationic biopolymer may comprise or be a cationic branched a-(1 ,3-glucan) polymer. The cationic branched a-(1 ,3-glucan) polymer may be a cationic graft copolymer of dextran and a-(1 ,3- glucan). In the present context the term “dextran” denotes an a-glucan where at least 50%, preferably at least 60 %, more preferably at least 70 %, even more preferably at least 80 % or at least 90 %, of the glycosidic linkages are a-1 ,6- glycosidic linkages, wherein the balance to 100 % is typically a-1 ,3-glycosidic linkages. Dextran has substantially linear structure, which means that is has 0 - 5 % of branches before formation of graft copolymer with a-1 ,3-glucan. Possible branches in dextran itself are usually short, one to three glucose monomers in length. The cationic graft copolymer of dextran and a-(1 ,3-glucan) may comprise 5 - 75 weight-%, preferably 10 - 70 weight-%, more preferably 20 - 60 weight-%, even more preferably 30 - 50 weight-%, of dextran, calculated from the dry weight of the graft copolymer. The cationic graft copolymer of dextran and a-(1 ,3-glucan) may comprise, for example 35 - 95 weight-%, preferably 30 - 90 weight-%, more preferably 40 - 80 weight-%, even more preferably 50 - 70 weight-%, of a-(1 ,3- glucan), e.g. a-(1 ,3-glucan) side chains, calculated from the dry weight of the graft copolymer.

[0029] According to one embodiment, the cationic biopolymer may be cationic branched graft copolymer comprising a dextran backbone and a-(1 ,3-glucan) side chains. The weight average molecular weight of the cationic graft copolymer may be 500 000 g / mol or more, for example 600 000 or more. The weight-average molecular weight can be determined, for example, by using size exclusion chromatography. The a- (1 ,3-glucan) side chains are preferably linked to the dextran backbone via a-1 ,2 and / or a-1 ,3 and / or a-1 ,4 linkages. The a-(1 ,3-glucan) side chains may comprise at least 70%, preferably at least 80%, more preferably at least 90% or at least 95%, sometimes even of 99% or 100%, of a-1 ,3-glycosidic linkages.

[0030] According to one preferable embodiment the cationic biopolymer may be or comprise a cationic linear a-(1 ,3-glucan) polymer. The cationic linear a-(1 ,3-glucan) polymer preferably has a degree of polymerization in a range of 55 -10 000, preferably 55 - 5000, more preferably 100 - 3000 or 550 - 2500. The cationic linear a-(1 ,3-glucan) polymer is preferably free of structural units originating from dextrin or dextran. At least 70 %, preferably at least 80 %, more preferably at least 90 % or at least 95 %, sometimes even at least 99 % or 100 %, of the glycosidic linkages of its polymeric structure are a-1 ,3-linkages.

[0031] According to one embodiment the cationic biopolymer selected from linear and branched a-(1 ,3-glucan) polymers may be crosslinked. The crosslinked a-(1 ,3- glucan) polymer suitable for use in the present invention may be obtained by contacting the a-(1 ,3-glucan) polymer, either linear or branched, with a crosslinker and a solvent, e.g. water. The amount of used crosslinker may be 20 - 5000 ppm, preferably 100 - 5000 ppm, calculated of polymer dry weight. According to one embodiment it is possible to use a crosslinker selected from a group comprising epihalohydrins, epoxy compounds, diglycidyl ethers, polyvalent metals, glyoxal and polycarboxylic acids. For example, the crosslinker may be selected from glyoxal and diglycidyl ethers, such as poly(ethylene glycol) diglycidyl ether, polypropylene glycol) diglycidyl ether, 1 ,4-butanediol diglycidyl ether, neopentyl glycol diglycidyl ether, glycerol diglycidyl ether, poly(dimethylsiloxane) diglycidyl ether and trimethylolpropane triglycidyl ether. Crosslinking of the cationic biopolymer modifies the three-dimensional structure of the biopolymer, and may improve its interaction and removal of suspended solid particles, especially when the cationic biopolymer has a linear structure before crosslinking.

[0032] The cationic linear crosslinked a-(1 ,3-glucan) polymer may have a viscosity in water in a range of 10 - 500 000 mPas, preferably 100 - 50 000 mPas, more preferably 500 - 5000 mPas, measured for 2 weight-% polymer solution as described in the experimental section. The cationic linear crosslinked a-(1 ,3-glucan) polymer may have a salt viscosity in a range of 1 - 50 000 mPas, preferably 10 - 5000 mPas, more preferably 50 - 500 mPas, measured for 1.8 weight-% concentration of polymer in an aqueous solution comprising 9.1 weight-% of NaCI, as described in the experimental section (“Salt viscosity 2%”).

[0033] Preferably, the cationic biopolymer is cationic linear a-(1 ,3-glucan) polymer is noncrosslinked.

[0034] The cationic biopolymer selected from linear and branched a-(1 ,3-glucan) polymers may be added to the aqueous medium in an amount of 0.1 - 150 ppm, preferably 0.5 - 100 ppm, more preferably 1 - 50 ppm, given as mg dry active polymer per litre of aqueous medium. In case two or more cationic biopolymers are added, either successively or as a mixture, the total amount of cationic biopolymers may be in a range of 0.1 - 150 ppm, preferably 0.5 - 100 ppm, more preferably 1 - 50 ppm. The cationic biopolymer can be added in relatively low amounts and it is still able to provide effective removal of suspended solid particles.

[0035] According to one preferable embodiment, in addition to the cationic biopolymer selected from linear and branched a-(1 ,3-glucan) polymers, a coagulant metal salt is added to the aqueous medium. One or more cationic biopolymers may thus be used in combination with one or more coagulant metal salts. The coagulant metal salt may be selected, for example, from aluminium salts, iron salts or zirconium salts, aluminium salts being preferred. The coagulant metal salts may be chloride salts, sulphate salts, nitrate salts, formiate salts, sulphate chloride salts of coagulant metals. Examples of suitable coagulant metal salts are polyaluminium chloride, aluminium sulphate, ferric chloride, ferrous sulphate, ferric sulphate and polyferric sulphate. The coagulant metal salt may be added to the aqueous medium before or after the addition of the cationic biopolymer. Alternatively the coagulant metal salt can be added simultaneously with the cationic biopolymer, either separately or as a mixture.

[0036] The coagulant metal salt is preferably added in an amount that is lower than the amount of the cationic biopolymer added. The use of biopolymer thus makes it possible to use lower amounts of coagulant metal salts, i.e. reduce the amount of inorganic material in the formed agglomerates. This makes the separated agglomerates more suitable for recycling, such as fertilizer, and reduces the formation of ash, if the separated agglomerate material is incinerated. In general, the use of biopolymer for turbidity removal increases the proportion of biobased organic material in the separated agglomerates, which is beneficial for many possible new applications envisioned within circular economy.

[0037] The coagulant metal salt may be aluminium or iron salt, preferably aluminium salt, wherein the coagulant metal salt and the cationic biopolymer may be added in a weight ratio from 5:95 to 50:50, preferably from 7:93 to 45:55, more preferably from 10:90 to 40:60, given as metakbiopolymer. The coagulant metal salt may be added to the aqueous medium before or after the addition of the cationic biopolymer, preferably before the addition of the cationic biopolymer.

[0038] According to one preferable embodiment a cationic linear a-(1 ,3-glucan) polymer is added to the aqueous medium, either before or after the addition of a coagulant metal salt to the aqueous medium.

[0039] It is possible that the cationic biopolymer is used in combination with synthetic polymer coagulants, such as cationic polyacrylamide. The cationic biopolymer may enable reduction of the dosage of the synthetic polyacrylamide needed, and / or may facilitate a further reduction in turbidity.

[0040] According to one embodiment, the aqueous medium which is subjected to removal of suspended solid particles may be wastewater, especially municipal or industrial wastewater, or surface water. For example, the present invention is suitable for removal of suspended solid particles from drinking water or municipal wastewater. It is possible that the aqueous medium is industrial wastewater, for example originating from a food or beverage production or from food or beverage processing.

[0041] According to one preferable embodiment, the cationic biopolymer may be added to the aqueous medium after a biological treatment step in a water treatment process of wastewater, such as industrial wastewater or municipal wastewater. The cationic biopolymer can be used to effectively remove any remaining suspended solids from the aqueous medium.

[0042] The cationic biopolymer may, in addition of reduced turbidity, reduce also the total phosphorous and / or nitrogen content of the aqueous medium, and thus minimise the risk of eutrophication of the receiving waters.

[0043] The aqueous medium may have a dry solids content 0 - 10 g / l, preferably 0.1 - 10 g / l. According to one embodiment, the aqueous medium may have a dry solids content <5 g / l, more preferably <4 g / l, sometimes even <3 g / l. The dry solids content can be, for example, in a range of 0.1 - 4.9 g / l, preferably 0.5 - 4 g / l.

[0044] According to one embodiment, the aqueous medium may have a total phosphorus content in a range of 0 - 15 mg / l, preferably 1 - 10 mg / l. The present invention is especially suitable for aqueous medium comprising phosphorus, as the cationic biopolymer is able not only to reduce the turbidity of the aqueous medium but also provide a clear reduction in the total phosphorus content. Thus the cationic biopolymer can be used to reduce also the total phosphorus content of the aqueous medium.

[0045] The formed agglomerates can be separated from the aqueous medium in any suitable manner, such as sedimentation, decantation and / or filtration.

[0046] According to one embodiment of the invention, the aqueous medium has a turbidity value <30 NTU, preferably <20 NTU, more preferably <10 NTU or <5 NTU, after the separation of the agglomerates. Turbidity can be measured by using commercially available nephelometer, providing the results in nephelometric turbidity units (NTU).

[0047] EXPERIMENTAL

[0048] Some embodiments of the present invention are described in the following nonlimiting experiments.

[0049] Finnish municipal wastewater was used in the experiments.

[0050] The experiments were carried out by using Kemira Flocculator 2000 equipment as follows:

[0051] Each wastewater sample, volume 600 ml, was mixed for 10 s with mixing speed of 400 rpm, after which chemical(s) were added to the sample as described below. After addition of the chemical(s), mixing was continued as slow mixing, with speed of 40 rpm, for 5 min, for formation and build-up of flocs. After the slow mixing, the sample was allowed to settle for 5 min without mixing. Turbidity was measured from a sample taken about 3 cm below the surface after settling. The a-(1 ,3-glucan) polymers used in the experiments are given in Table 1. Charge densities for the a-(1 ,3-glucan) polymers were measured by Mutek titration.

[0052] In addition, polyaluminium chloride (PAX), medium basicity, in liquid form was used, either alone as a reference, or together with the a-(1 ,3-glucan) polymer. When used together, the a-(1 ,3-glucan) polymer and polyaluminium chloride were dosed separately, withl - 5 s delay between the dosages. For each experimental series, a zero test without any chemical addition was conducted.

[0053] Table 1 shows also values for viscosity in water for 2 weight-% polymer solution (“Viscosity 2% in water”) and values for viscosity in presence of salt for 2 weight-% polymer solution (“Salt viscosity 2%”). The viscosity values were determined by using a Brookfield DV-1 type viscometer with a small sample adapter at 25 °C, using spindle #18 or #31 , depending on viscosity level. The viscosity measurement was always done by using the maximum possible rotational speed (the highest speed being 100 rpm). Each biopolymer sample was first dissolved in deionized water as 2 weight-% solution. “Viscosity 2% in water” was measured from this solution. “Salt viscosity 2%” was determined by adding sodium chloride (NaCI) in weight ratio of 5:1 (NaCkpolymer) to 2 weight-% biopolymer solution. Salt was allowed to dissolve under mixing before the viscosity was measured. This means that the “Salt viscosity 2%” was measured at 1.8 weight-% concentration of biopolymer in an aqueous solution comprising 9.1 weight-% of NaCI.

[0054] Table 1 a-(1 ,3-glucan) polymers used in the experiments. Experiment 1

[0055] First the performance of the polyaluminium chloride PAX (PAX-18) at different dosages was measured. The performance results are shown in Table 2. Table 2 Performance results for the polyaluminium chloride (PAX).

[0056] Then the performance of the a-(1 ,3-glucan) polymers A, B and C together with polyaluminium chloride PAX (PAX-18) at dosage 30 mg / l were studied. The performance results are shown in Table 3. In Table 3, test 1-0 is a zero experiment (O-test) without any chemicals and test 1-1 is a test with 30 mg / l polyaluminium chloride but no cationic biopolymer.

[0057] Table 3 Performance results of for the a-(1 ,3-glucan) polymers A, B and C together with 30 mg / l PAX. It can be seen from Table 3 that the presence of cationic biopolymer clearly improves turbidity removal. When polyaluminium chloride PAX is used alone (see test 1-1 ), the turbidity of treated waste water was 104 NTU. When cationic biopolymer was used together with the polyaluminium chloride PAX, the turbidity could be reduced even down to 12 NTU.

[0058] Experiment 2

[0059] The performance of the polyaluminium chloride PAX (PAX-18) at different dosages was measured and used as reference. The performance of the a-(1 ,3-glucan) polymers A, B and C was measured, alone, without addition of polyaluminium chloride. The performance results are shown in Table 4. In Table 4, test 4-0 is a zero experiment (O-test) without any chemicals.

[0060] Table 4 Performance results for Experiment 2. It can be seen from Table 4 that the cationic biopolymer is able to effectively reduce turbidity, even if used alone, without polyaluminium chloride. The cationic biopolymer is able to reduce the turbidity of the wastewater to a level about 60 NTU, which corresponds to the turbidity level obtainable with PAX dosing of 50 mg / l.

[0061] Experiment 3

[0062] The performance of the polyaluminium chloride PAX (PAX-18) at different dosages was measured and used as reference. The performance of the a-(1 ,3-glucan) polymer A was measured, together with addition of 15 ml / g polyaluminium chloride (PAX). The performance results are shown in Table 5. In Table 5, test 5-0 is a zero experiment (O-test) without any chemicals.

[0063] Table 5 Performance results for Experiment 3.

[0064] *a-(1 ,3-glucan) polymer A was used together with 15 mg / l PAX.

[0065] It can be seen from Table 5 that when 15 mg / l PAX was used together with the cationic biopolymer, the turbidity of the wastewater could be reduced down to 8 NTU, without high biopolymer addition. The obtained turbidity reduction corresponds dosage of polyaluminium chloride of about 100 mg / l, when used alone. Significant decrease in use of coagulant metal salt could thus be achieved. Experiment 4

[0066] A plant trial was conducted at a production site of a Scandinavian manufacturer of products for agricultural use. For biological sludge, the production plant had one after-sedimentation basin, from which the treated water is directed to a polishing basin for further sedimentation. After the polishing station, the treated water is further directed through a filter arrangement before the treated water is discharged into an ecologically sensitive area. Due to sensitivity of the receiving waterbody, the removal of particulate matter is of primary interest and there are strict limits for COD, nitrogen and phosphorous in the treated water.

[0067] At the present, synthetic cationic polymer (polyamine) is used for turbidity removal. The synthetic polymer is dosed to the feed flow to the after-sedimentation basin for biological sludge. The polymer dosage is constant 50 ml / min, but the influent flow to after-sedimentation basin varies strongly.

[0068] A ferric chloride coagulant with ca. 40 weight-% FeCh content is dosed at constant dosage of 60 ml / min to the feed flow to the after-sedimentation basin, too.

[0069] In the plant trial experiment, the synthetic polymer was replaced with an a-(1 ,3- glucan) polymer, in ratio 1 :1. The a-(1 ,3-glucan) polymer was a linear cationic a- (1 ,3-glucan) polymer, degree of substitution 0.4, charge density +1.8 meq / g. No other change were made to the water treatment system. The feed of ferric chloride coagulant remained unchanged throughout the plant trial.

[0070] The dosage point for a-(1 ,3-glucan) polymer was the feed flow to the aftersedimentation basin for biological sludge, i.e. the same as for the synthetic polymer.

[0071] Samples of the clarified wastewater were taken from the exit flow of the polishing tank and analysed. The sample at 9:00 o’clock, Day 1 , gives the starting point reference for the plant trial. The analysis results are shown in Table 6. PO4-P value gives the concentration of phosphorus in the form of orthophosphate (PO4) in clarified wastewater, and NO3- N value gives the concentration of nitrogen in the form of nitrate (NO3) in clarified wastewater.

[0072] It can be seen from the results of Table 6 that the turbidity was significantly reduced during the plant trial from the initial value. Significant reduction in COD values, as well as in measured phosphorous values, was observed. The measured nitrogen values also showed generally a clear decrease from the starting value. It is seen that even a low a-(1 ,3-glucan) polymer dosage can produce surprisingly good results in wastewater treatment.

[0073] Table 6 Results of Experiment 4.

[0074] *given as active polymer, dosage per ton dry solids (DS).

[0075] **given as active polymer in relation to the volume flow of the feed flow.

[0076] ***reference value.

[0077] Experiment 5

[0078] A plant trial was conducted at a production site of a Scandinavian manufacturer of plant-based food additives. The wastewater treatment system comprised an afterclarifier for biological sludge. The treated water is discharged into an ecologically sensitive waterbody. Due to sensitivity of the receiving waterbody, the treated water must fulfil high standards at the discharge.

[0079] At the present, synthetic cationic polymer (polyamine) is used together with polyacrylamide to treat the wastewater, the dosage relation being about 50:50. The polymers are dosed to the inlet of the after-clarifier for biological sludge. The polyacrylamide dosage is constant 4 l / h, and the influent flow to after-clarifier is quite constant.

[0080] In the plant trial experiment, the synthetic cationic polymer was replaced with an a- (1 ,3-glucan) polymer. The same a-(1 ,3-glucan) polymer as in Experiment 4 was used. The feed of polyacrylamide remained unchanged throughout the plant trial. The feed of a-(1 ,3-glucan) polymer was started every morning. In the evening, the feed of a-(1 ,3-glucan) polymer was stopped, and synthetic cationic polymer was fed during the night.

[0081] The dosage point for a-(1 ,3-glucan) polymer was the feed flow to the aftersedimentation basin for biological sludge, i.e. the same as for the synthetic cationic polymer.

[0082] The dosages for the a-(1 ,3-glucan) polymer and for the polyacrylamide are given in Table 7. The used dosages are given both in kg per ton dry solids (kg / t DS), which gives the real amount of dosed active polymer in relation to solid particulate matter in the feed flow, as well as in ppm, which gives the dosed active polymer in relation to the volume flow of the feed flow.

[0083] Samples were taken from the exit flow of the after-clarifier and analysed. The first sample result for each day is the reference result with the synthetic cationic polymer.

[0084] The results are shown in Table 7.

[0085] It can be seen from Table 7 that the turbidity value effectively decreased from the reference value in the morning. The value for COD unfiltered remained in the same, satisfactory level. This indicates that the a-(1 ,3-glucan) polymer efficiently removed the last small particles in the after-sedimentation. Table 7 Results of Experiment 5.

[0086] ‘reference value for the day in question.

[0087] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.

Claims

CLAIMS1. Method for removal of solid particles suspended in an aqueous medium, the method comprising- adding to the aqueous medium a cationic biopolymer selected from cationic linear a-(1 ,3-glucan) polymers, cationic branched a-(1 ,3-glucan) polymers, and any of their mixtures,- allowing the solid particles suspended in the aqueous medium to interact with the cationic biopolymer to form agglomerates, and- separating the agglomerates from the aqueous medium.

2. Method according to claim 1 , characterised in that the cationic biopolymer is a cationic linear a-(1 ,3-glucan) polymer.

3. Method according to claim 1 or 2, characterised in that the cationic biopolymer has a degree of cationic substitution in a range of 0.05 - 1.2, preferably 0.1 - 1.0, more preferably 0.15 - 0.7, even more preferably 0.2 - 0.55.

4. Method according to claim 1 , 2 or 3, characterised in that the cationic biopolymer has a charge density of 0.3 - 3.5 meq / g, preferably 0.5 - 3.2 meq / g, more preferably 0.8 - 2.6 meq / g, even more preferably 1 .0 - 2.2 meq / g.

5. Method according to any of preceding claims 1 - 4, characterised in that the cationic biopolymer comprises a cationic linear a-(1 ,3-glucan) polymer, which has a degree of polymerization in a range of 55 -10 000, preferably 55 - 5000, more preferably 100 - 3000.

6. Method according to any of the preceding claims 1 - 5, characterised in that the cationic biopolymer comprises a cationic branched a-(1 ,3-glucan) polymer selected from cationic graft copolymers of dextran and a-(1 ,3-glucan).

7. Method according to claim 6, characterised in that the cationic graft copolymer comprises- 10 - 70 weight-%, preferably 20 - 60 weight-%, more preferably 30 - 50 weight- %, of dextran, calculated from the dry weight of the graft copolymer, and / or- 30 - 90 weight-%, preferably 40 - 80 weight-%, more preferably 50 - 70 weight- %, of a-(1 ,3-glucan), calculated from the dry weight of the graft copolymer.

8. Method according to claim 6 or 7, characterised in that the cationic graft copolymer comprises a dextran backbone and a-(1 ,3-glucan) side chains.

9. Method according to any of preceding claims 1 - 8, characterised in that the cationic biopolymer is crosslinked.

10. Method according to claim 9, characterised in that the crosslinked cationic biopolymer is obtained by using a crosslinker selected from a group comprising epihalohydrins, epoxy compounds, diglycidyl ethers, polyvalent metals, glyoxal and polycarboxylic acids.

11. Method according to any of preceding claims 1 - 10, characterised in that the cationic biopolymer is added to the aqueous medium in an amount of 0.1 - 150 ppm, preferably 0.5 - 100 ppm, more preferably 1 - 50 ppm.

12. Method according to any of preceding claims 1 - 11 , characterised in that in addition to the cationic biopolymer a coagulant metal salt, such as aluminium salt, iron salt or zirconium salt, is added to the aqueous medium.

13. Method according to claim 12, characterised in that the coagulant metal salt is added in an amount that is lower than the amount of the cationic biopolymer.

14. Method according to any of preceding claims 1 - 13, characterised in that the aqueous medium is selected from surface water or wastewater, such as industrial wastewater or municipal wastewater.

15. Use of a cationic biopolymer as a coagulant for removal of solid particles suspended in aqueous medium, wherein the cationic biopolymer is selected fromcationic linear a-(1 ,3-glucan) polymers, cationic branched a-(1 ,3-glucan) polymers, and any of their mixtures.

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