Method for flocculating solid particles and use of high shear treatment

Abstract

The invention relates to a method for flocculating solid particles in a liquid-solid separation process. The method comprises obtaining a flocculant solution comprising a cationic biopolymer having a charge density in a range of 0.5 – 2.6 meq / g and selected from cationic crosslinked α-1,3-glucan polymers, cationic graft copolymers of dextran and α-1,3-glucan, or any mixture thereof. The flocculant solution is subjected to a high shear treatment. The flocculant solution is brought after the high shear treatment into a contact with a suspension, where solid particles are dispersed in a continuous aqueous phase, and the solid particles are flocculated. The flocculated solid particles are then separated from the continuous aqueous phase.

Description

[0001] METHOD FOR FLOCCULATING SOLID PARTICLES AND USE OF HIGH SHEAR

[0002] TREATMENT

[0003] The present invention relates to a method for flocculating solid particles and to the use of a high shear treatment according to the preambles of the enclosed independent claims.

[0004] Many industrial processes comprise a step, where solid particles are separated from a liquid phase to provide two separate fractions, namely a solid fraction and a liquid fraction. This kind of liquid-solid separation steps are used, for example, in water treatment processes, in manufacture of pulp, paper, board or the like, as well as in mining industry. One typical example of a liquid-solid separation step is sludge dewatering in water treatment. The sludge usually comprises various solid particles and / or microorganisms suspended in an aqueous phase. The success of the liquidsolid separation step is comparable to the clarity of the liquid fraction and to the dryness of the solids fraction after separation. The solid fraction of the sludge can be processed further, for example deposited, used as a fertilizer, or incinerated for energy production.

[0005] Often liquid-solid separation step includes flocculation of the solid particles present in a liquid phase. Flocculating and / or coagulating chemicals are used to improve the formation and / or the quality of the formed flocs. For example, in a water treatment process the sludge may be conditioned before the dewatering step by addition of flocculating agents, such as inorganic compounds of iron and lime, or synthetic organic polymers. These flocculating agents are added to the sludge in order to improve the sludge handling and to increase the dewatering effect in the liquid-solid separation.

[0006] Petroleum-based synthetic cationic polymers are conventionally used as flocculants in liquid-solid separation. Due to the non-degradable nature of the synthetic polymers, it may be impossible to use the separated solid fraction from waste water treatment for landfills, composting, or for soil improvement, when synthetic cationic polymers have been employed in the liquid-solid separation step. Furthermore, there is a current growing incentive towards more 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 cationic polymers. Also the increasing price of petroleum has reduced its attractivity as a raw material. Consequently, there is on-going search for more sustainable alternatives that would still provide appropriate process behaviour and separation results.

[0007] An object of this invention is to minimise or even eliminate the disadvantages existing in the prior art.

[0008] An object is also to provide more a sustainable method for flocculation in a liquidsolid separation process, especially for sludge dewatering in a water treatment process.

[0009] A further object of the invention is to provide a method which provides an effective flocculation and a high solids content for the separated solids after the liquid-solid separation step.

[0010] 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.

[0011] The embodiments mentioned in this text relate, where applicable, to all aspects of the invention, even if this is not always separately mentioned.

[0012] A typical method for flocculating solid particles in a liquid-solid separation process comprises

[0013] - obtaining a suspension, where solid particles are dispersed in a continuous aqueous phase;

[0014] - obtaining a flocculant solution comprising a cationic biopolymer having a charge density in a range of 0.5 - 2.6 meq / g and selected from cationic crosslinked a-1 ,3- glucan polymers, cationic graft copolymers of dextran and a-1 ,3-glucan, or any mixture thereof;

[0015] - subjecting the flocculant solution to a high shear treatment;

[0016] - bringing the flocculant solution after the high shear treatment into a contact with the suspension, and flocculating the solid particles; and

[0017] - separating the flocculated solid particles from the continuous aqueous phase.

[0018] Typical use according to the present invention of a high shear treatment is for increasing cationicity of a flocculant solution comprising a cationic biopolymer having a charge density in a range of 0.5 - 2.6 meq / g and selected from cationic crosslinked a-1 ,3-glucan polymers, cationic graft copolymers of dextran and a-1 ,3- glucan, or any mixture thereof.

[0019] Now it has been surprisingly found that subjecting a flocculant solution comprising a specific cationic biopolymer to a high shear treatment significantly improves the flocculating ability of the flocculant solution. The high shear treatment significantly improves the dewatering efficiency of these cationic biopolymers, and makes them suitable for use even in industrial scale. The specific cationic biopolymer for the present invention has a charge density in a range of 0.5 - 2.6 meq / g and is selected from cationic crosslinked a-1 ,3-glucan polymers as well as cationic graft copolymers of dextran and a-1 ,3-glucan and any mixtures thereof. The effect obtained with the high shear treatment for these biopolymers is completely unexpected, as similar effect cannot be obtained, for example, for cationic linear a-1 ,3-glucan polymers. The present invention thus provides a flocculant solution, which is biodegradable and obtainable from renewable sources.

[0020] The flocculant solution comprising the cationic biopolymer, selected from cationic crosslinked a-1 ,3-glucan polymers, cationic graft copolymers of dextran and a-1 ,3- glucan, or any mixtures thereof, is used for flocculating an aqueous suspension, where solid particles are dispersed or suspended in a continuous aqueous phase. The cationic biopolymer is assumed to interact with the solid particles, whereafter flocs are formed. In the present context the terms “a-1 ,3-glucan polymer” and “a-1 ,3-glucan” denote a polymeric structures 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 are a-1 ,3-linkages.

[0021] 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.

[0022] According to one embodiment the flocculant solution may comprise or consists of a cationic biopolymer, which is a cationic crosslinked a-1 ,3-glucan polymer. The cationic crosslinked a-1 ,3-glucan polymer suitable for use in the present invention comprises cationic substitution groups attached to its 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. It is assumed that the cationic substitution groups of the a-1 ,3-glucan polymer are able to interact with the solids particles present in the aqueous suspension and provide the floc formation. It is further assumed, without wishing to be bound by a theory that the high shear treatment somehow releases or makes the cationic groups more available for interaction.

[0023] 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 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 glyoxal and diglycidyl ethers, such as polyethylene 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.

[0024] Alternatively, the flocculant solution used in the present invention may comprise or consists of a cationic biopolymer, which is a cationic biopolymer selected from cationic ester derivatives or ether derivatives of a graft copolymer of dextran and a- 1 ,3-glucan. Suitable graft copolymer derivatives, methods for their preparation and determination of their glycosidic linkage profile are described, for example, in WO 2021 / 247810. The graft copolymer has a branched structure, comprising dextran and a-1 ,3-glucan chains linked together. The degree of polymerization of the a-1 ,3- glucan in the graft copolymer may be in a range of 20 - 3000, preferably 500 - 2000. For example, the degree of polymerisation may be in a range of 20 - 2000 or 55 - 1000. Degree of polymerization refers here to the number of glucose units comprised within an individual a-1 ,3-glucan chain.

[0025] According to one embodiment of the invention, the flocculant solution may comprise or consist of a cationic graft copolymer comprising 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 before ester derivatization or ether derivatization. The cationic graft copolymer 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 before ester derivatization or ether derivatization.

[0026] The ester derivative or ether derivative of a graft copolymer of dextran and a-1 ,3- glucan may comprise one or more cationic groups linked to the graft copolymer via an ester or ether linkage. The cationic group may comprise a substituted ammonium group, such as primary, secondary, tertiary or quaternary ammonium group, preferably a quaternary ammonium group, more preferably a trialkyl ammonium group. An ammonium group may be substituted with alkyl and / or aryl group(s), for example with C1 - C4 alkyl or C6 - C24 alkyl groups. One of the groups of the substituted ammonium group comprises one carbon or a carbon chain in ether or ester linkage to the graft copolymer.

[0027] According to one embodiment, the cationic biopolymer may be cationic graft copolymer comprising a dextran backbone and a-1 ,3-glucan side chains, where preferably the said side chains are 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.

[0028] According to one embodiment of the invention the flocculant solution comprises a cationic biopolymer, as defined above, which may have a degree of cationic substitution in the range of 0.05 - 1.2, preferably 0.1 - 1.0, more preferably 0.1 - 0.7, even more preferably 0.15 - 0.6, determined after the high shear treatment. The degree of substitution refers to the average number of hydroxyl groups substituted with cationic groups via ether or ester linkage or via other linkage in each glucose unit in the graft copolymer or in the crosslinked copolymer. The high cationicity improves the water-solubility of the biopolymer. Excess cationicity is, however, preferably avoided as it may cause ecotoxicity for aquatic organisms. The high shear treatment according to the present invention makes it possible to increase the availability of the cationic groups of the biopolymer for the interaction with solid particles of the aqueous suspension. This means that effective dewatering results may be achieved when biopolymers even with lower cationicity is used. Biopolymers with lower cationicity might also contain less impurities and make them more suitable e.g. for drinking water applications.

[0029] According to one embodiment the cationic biopolymer is water-soluble. The cationic biopolymer may be partially water-soluble or fully water-soluble. Water-solubility in the present context is defined as the amount of insolubles in the biopolymer solution, measured by using the procedure described in the experimental section, Example 12. For example, the cationic biopolymer may preferably have <25 %, more preferably <20 %, even more preferably <15 % of insolubles, measured before the high-shear treatment. After the high shear treatment, the amount of insolubles in the flocculant solution comprising the cationic biopolymer may be <10 %, preferably <5 %, more preferably <3 %, measured as described in the experimental section.

[0030] The cationic biopolymer has the charge density in a range of 0.5 - 2.6 meq / g. It has been observed that if the charge density is too high, no increase in charge density can be observed after the high shear treatment. The flocculant solution may comprise a cationic biopolymer which has a charge density of 0.6 - 2.6 meq / g, preferably 1 .0 - 2.5 meq / g, more preferably 1 .5 - 2.5 meq / g, even more preferably 1 .9 - 2.4 meq / g, determined before the high shear treatment. It is possible that the charge density is, for example, in a range of 1 .0 - 2.4 meq / g, preferably 1 .1 - 2.4 meq / g or 1.8 - 2.3 meq / g, determined before the high shear treatment. Charge density is measured by Mutek titration, as described in the experimental part.

[0031] According to one embodiment of the invention the flocculant solution may comprise a crosslinked cationic graft copolymer of dextran and a-1 ,3-glucan. The crosslinking can be performed by using the same crosslinkers as defined above. Crosslinking of the branched structure of the graft copolymer further modifies the three- dimensionality of the cationic biopolymer.

[0032] The cationic biopolymer may have a salt viscosity of 50 - 150 000 mPas or 100 - 100 000 mPas, measured at 2 weight-% biopolymer concentration, before the high shear treatment. According to one embodiment the cationic biopolymer may have the salt viscosity in a range of 50 - 10 000 mPas, preferably 50 - 5000 mPas, sometimes 200 - 4000 mPas, measured at 2 weight-% biopolymer concentration, before the high shear treatment. The measurement method has been described in detail in the experimental section. The salt viscosity can be used to measure or estimate the molecular size of the biopolymer. It has been observed that the cationic biopolymer is able to provide especially effective dewatering effect, when the viscosity of the cationic biopolymer is within the given ranges. The flocculant solution comprising the selected cationic biopolymer is subjected to a high shear treatment where a shear power of at least 5 W / kg, preferably at least 20 W / kg, more preferably at least 35 W / kg, is applied on the flocculant solution comprising or consisting of the cationic biopolymer. According to one preferable embodiment the flocculant solution may be subjected in the high treatment to the shear power which is in a range of 5 - 1000 W / kg, preferably 20 - 500 W / kg, more preferably 35 - 300 W / kg, even more preferably 35 - 209 W / kg. Sometimes the shear power may be up to 500 W / kg. The duration of the high shear treatment may preferably be at most 600 s, preferably 1 - 600 s, more preferably of 5 - 400 s, even more preferably 10 - 300 s. The duration of the high shear treatment may be, for example, 10 - 180 s or 30 - 150 s. In general, the higher the power used in the high shear treatment, the shorter the duration of the high shear treatment may be and vice versa.

[0033] During the high shear treatment the flocculant solution may comprise 0.01 - 5 weight-%, preferably 0.05 - 4 weight-% of the cationic biopolymer, calculated from the weight of the flocculant solution.

[0034] The flocculant solution comprising, or consisting of, the cationic biopolymer may be subjected to the high shear treatment in any suitable high shear apparatus or high shear device, which is able to create appropriate high shear power in aqueous systems. For example, the cationic biopolymer may be subjected to the high shear treatment in a homogenizer, a high speed mixer, a disperser, a rotor-stator mixer, a mixer with two counterrotating rotors, centrifugal pumping device providing a straight flow or back rotation flow, high pressure devices, shear pumps or the like. In some cases also ultrasonic treatment is applicable as high shear treatment. Suitable high shear mixing apparatuses and homogenizers are well-known for a person skilled in the art and commercially available, for example, under tradenames Ultra Turrax®, Polytron®, Atrex®, Silverson®, Ystral®, Cavitron™ and Waukesha shear pumps™. For example, the cationic biopolymer may be dissolved in water, subjected to the high shear treatment by high shear homogenisation and then brought into contact with the aqueous suspension comprising solid particles to be flocculated. The high shear homogenisation may be achieved, for example, by using rotational speed of at least 2000 rpm. The appropriate shear power may be achieved alternatively in a high shear treatment where the selected cationic biopolymer is subjected to the high shear power by centrifugal pumps or the like, e.g. during pumping of the cationic biopolymer after it has been dissolved or dispersed in water.

[0035] Before the high shear treatment, the flocculant solution comprising the selected cationic biopolymer may be subjected to a heat treatment in a temperature of 60 - 115 °C, preferably 85 - 100 °C.

[0036] After the high shear treatment the flocculant solution is brought into a contact with the aqueous suspension, whereby the cationic biopolymer interacts with the solid particles and flocculates them. The present invention is suitable for flocculating of aqueous suspensions, where solid particles are dispersed in a continuous aqueous phase, in a liquid-solid separation process, where the liquid phase and the solid particles are to be separated from each other. For example, the present invention is suitable for conditioning drinking water. According to one preferable embodiment the liquid-solid separation process may be a treatment process of wastewater, such as municipal wastewater or industrial wastewater, or a sludge dewatering step in a treatment process of such wastewater. The aqueous suspension subjected to the liquid-solid separation process may be municipal wastewater sludge or agricultural sludge, or it may originate from a biological treatment process of wastewater and / or sewage. It is possible that the liquid-solid separation process may be a treatment process of sludge, such as primary sludge, secondary sludge, tertiary sludge or digested sludge, originating from a treatment of wastewater. The liquid-solid separation may be a treatment process of a suspension originating from a food or beverage production or from food or beverage processing.

[0037] According to one embodiment, the aqueous suspension, such as wastewater or sludge, comprises a continuous aqueous liquid phase and organic and / or inorganic solid material and / or particles suspended in the aqueous liquid phase. The suspension may be rich in material of bacterial origin, especially if it is sludge originating from a water treatment process or from an agricultural process. The aqueous liquid phase of the suspension may contain also dissolved organic substances, such as polysaccharides, humic substances and fatty acids. The suspension, such as wastewater or sludge, may have a biological oxygen demand (BOD) >50 mg / l, chemical oxygen demand (COD) in a range of 15 - 45 g / l, preferably 20 - 40 g / l, and / or a dry solids content in a range of 5 - 80 g / l, preferably 10 - 60 g / l, more preferably 20 - 55 g / l. pH of the suspension may be in a range from pH 6 to pH 9, preferably from pH 7 to pH 8. The conductivity of the suspension may be in a range of 5 - 14 mS / cm, preferably 5 - 10 mS / cm, and / or the charge density may be in a range from -5.5 to -1 .5 peq / g, preferably from -5.0 to -1 .8 peq / g. Total phosphorous value for the suspension may be in a range of 400 - 1400 mg / l, preferably 450 - 1200 mg / l and / or the total nitrogen value in a range of 1 .2 - 3.5 g / l, preferably 1 .5 - 3.0 g / l.

[0038] According to one preferable embodiment, the suspension comprises solid organic particles that are dispersed in a continuous aqueous phase. This means that the aqueous suspension, such as wastewater or sludge, comprises a continuous aqueous liquid phase and at least organic solid material and / or particles suspended in the aqueous liquid phase. The flocculant solution comprising the cationic biopolymer is after the high shear treatment especially suitable for flocculating organic material and / or particles, which otherwise can be demanding to flocculate and / or dewater.

[0039] The flocculant solution comprising the cationic biopolymer interacts directly with solid material and / or particles suspended in the aqueous liquid phase of the suspension. The present method is preferably free of pre-treatment steps, where the suspension is brought into contact with anionic flocculants, anionic additives or anionic modifiers before the suspension is brought into contact with the flocculant solution.

[0040] The flocculant solution may be brought into a contact with the aqueous suspension in amount that provides 1 - 30 kg of cationic biopolymer / ton dry suspension, preferably 2 - 20 kg of cationic biopolymer / ton dry suspension, more preferably 3 - 15 kg of cationic biopolymer / ton dry suspension. The flocculated solid particles are separated from the continuous aqueous phase in any suitable manner. For example, the separation of aqueous phase, i.e. dewatering of flocculated solid particles, be performed by using mechanical dewatering means, such as centrifuge(s), belt press or chamber press, preferably centrifuge(s).

[0041] EXPERIMENTAL

[0042] Some embodiments of the present invention are described in the following nonlimiting examples.

[0043] Characterisation of Cationic Biopolymers

[0044] Cationic biopolymers used in the examples were characterised by measuring their charge density at pH 4 and by measuring their salt viscosity. Charge density was measured also after high shear treatment.

[0045] Charge Density

[0046] Charge density at pH 4 was determined by using BTG’s Mutek PCD-04 particle charge titrator. Each cationic biopolymer to be tested was first dissolved in deionized water to form 0.20 weight-% flocculant solution. The flocculant solution was diluted to 0.01 - 0.05 weight-% concentration for the measurement, depending on the charge density of the biopolymer. pH was adjusted to 4.0 with 0.1 M acetic acid. Charge density was determined by using 0.001 N sodium polyethylenesulfonate (PES-Na) solution as a titrant. During titration, pH normally increased 0.1 - 0.3 pH units. Charge density is expressed as meq / g of dry substance.

[0047] Salt Viscosity

[0048] Salt viscosity of cationic biopolymer solution in presence of a salt was determined by using a Brookfield DV-1 viscometer with a small sample adapter at 25 °C, using spindle #18 or #31 , depending on the viscosity level (called hereafter “salt viscosity”). The pH of the biopolymer solutions was not adjusted, as it did not have any impact on measured viscosity results. The pH of the solutions was generally in a range from pH 6 to pH 11.4. The salt viscosity measurement was performed by using maximum possible rotational speed. The cationic biopolymer was first dissolved in deionized water as 2 weight-% solution. Then sodium chloride (NaCI) in a weight ratio of 5:1 (NaCkcationic biopolymer) was added and allowed to dissolve under mixing before the salt viscosity was measured. This means that the salt viscosity of the cationic biopolymer was measured at 1.8 weight-% concentration of cationic biopolymer in an aqueous solution comprising of 9.1 weight-% of NaCI.

[0049] Conductivity

[0050] The conductivity of 0.5 weight-% cationic biopolymer in deionized water was measured using a Knick Portavo 902 COND conductivity meter equipped with a Knick SE 204 sensor.

[0051] High Shear Treatment of Cationic Biopolymer

[0052] High shear treatment of cationic biopolymers was carried by mixing 150 ml of the freshly made 2 % or 200 ml of the 0.2 % cationic biopolymer solutions by using an IKA T25 digital Ultra Turrax® homogenizer with a S 25 N - 25 F blade at 16000 rpm for 3 - 5 minutes. In the examples, the samples subjected to the high shear treatment are marked with “After UT”.

[0053] Sludge Dewatering Test

[0054] Digested sludges, collected from a Finnish wastewater treatment plant, were used in the examples. The pH and solids content of each sludge is given in the associated example, as well as the CST time for the sludge without any chemical addition, indicated as “zero test” in parenthesis.

[0055] Sludge dewatering tests were carried out by using capillary suction time (CST) test, by using Triton type 319 Multi-purpose CST (Triton Electronics Ltd, UK). Mixing was done with either Heidolph RZR 2021 or IKA RW 20 digital mixer using a 4-blade stirrer with blade total width of 30 mm and blade height of 15 mm.

[0056] In the CST test the mixing speed was 1000 rpm. The used cylinder had a diameter of 18 mm. The flocculant solution comprising the cationic biopolymer was added to 100 g of the digested sludge in a 250 ml beaker and mixed 10 s after addition. After 10 s of mixing a 4.5 ml sample was taken to the cylinder and the CST value was measured. A low CST value indicates good dewatering.

[0057] The flocculant solutions for CST testing were prepared as follows: selected cationic biopolymers were first dissolved as 0.5 % solutions overnight and then diluted to 0.2 % solutions for testing. The solutions were used either as such or after high shear treatment of the solution.

[0058] Example 1

[0059] The used sludge had a dry solids content 2.6 %, pH 7.2 and a CST time of 350 s (zero test).

[0060] The biopolymers, used as flocculant solutions, were cationic graft copolymers of dextran and a-1 ,3-glucan, denoted as aG1 and aG2. Characteristics of biopolymers aG1 and aG2, before and after the high shear treatment, are shown in Table 1. It can be seen from Table 1 that the high shear treatment increased both the charge density and the viscosity of the biopolymers.

[0061] Table 1 Characteristics of the biopolymers used in Example 1.

[0062] It was also investigated if a temperature treatment could influence the dewatering performance. Therefore a sample of biopolymer aG2 was cooked at 95°C for 1 h, before it was used as flocculant solution, either with or without high shear treatment.

[0063] The efficiency of the biopolymers as flocculant solutions, with or without the high shear treatment, were tested by using the sludge dewatering test, describe above. The results are shown in Table 2.

[0064] It can be seen from the results of Table 2 that the high shear treatment clearly improved the dewatering performance of the biopolymer, which was seen as significantly lower CST values at corresponding dosing levels. It can also be seen that the temperature treatment alone provided only a moderate improvement in dewatering efficiency, but a significant improvement could be obtained with the high shear treatment. It can be also concluded that the temperature treatment did not disturb the effect obtainable with the high shear treatment.

[0065] Table 2 Sludge dewatering test results of Example 1 .

[0066] *kg active biopolymer per ton dry sludge

[0067] Example 2

[0068] The used sludge had a dry solids content 2.4 %, pH 7.6 and a CST time of 296 s (zero test).

[0069] The biopolymers, used as flocculant solutions, were cationic graft copolymers of dextran and a-1 ,3-glucan, denoted as aG3, aG4, and aG5. Their characteristics, before and after high shear treatment, are shown in Table 3. It can be seen from Table 3 that when the charge density before the high shear treatment was 1 .5 meq / g or lower, a clear increase in charge density could be observed after the high shear treatment. An increase in the viscosity of the flocculant solution could be observed for all the samples.

[0070] Table 3 Characteristics of the biopolymers used in Example 2. The efficiency of the biopolymers as flocculant solutions, with or without the high shear treatment, were tested by using the sludge dewatering test, describe above.

[0071] The results are shown in Table 4.

[0072] Table 4 Sludge dewatering test results of Example 2.

[0073] *kg active biopolymer per ton dry sludge

[0074] It can be seen from the results of Table 4 that the high shear treatment clearly improved the dewatering performance of the biopolymer, when it was used as a flocculant solution. The increased efficiency is seen as significantly lower CST values at corresponding dosing levels.

[0075] Example 3

[0076] The used sludge had a dry solids content 2.4 %, pH 7.2, and a CST time of 342 s (zero test).

[0077] The biopolymers, used as flocculant solutions, were a cationic graft copolymer of dextran and a-1 ,3-glucan, denoted as aG7, and a crosslinked a-1 ,3-glucan polymer, denoted as aG8. Their characteristics, before and after the high shear treatment, are shown in Table 5. It can be seen from Table 5 that the high shear treatment provided a clear increase in charge density for both biopolymers. The viscosity value decreased for aG8 after the high shear treatment. On the other hand, the high shear treatment produced a significant increase in viscosity of aG7. Table 5 Characteristics of the biopolymers used in Example 3.

[0078] *too viscotic, could not be measured

[0079] The efficiency of the biopolymers as flocculant solutions, with or without the high shear treatment, were tested by using the sludge dewatering test, describe above.

[0080] The results are shown in Table 6.

[0081] Table 6 Sludge dewatering test results of Example 3.

[0082] *kg active biopolymer per ton dry sludge

[0083] It can be seen from Table 6 that the high shear treatment improved the dewatering performance of the biopolymers, which is seen as clearly lower CST values at corresponding dosing levels. The high shear treatment improved the CST test performance of aG8 even if the obtained increase in charge density was moderate.

[0084] Example 4

[0085] The used sludge had a dry solids content 2.3 %, pH 7.2, and a CST time of 244 s (zero test).

[0086] The biopolymer, used as the flocculant solution, was a crosslinked a-1 ,3-glucan polymer, denoted as aG9. Characteristics of aG9, before and after the high shear treatment, are shown in Table 7. It can be seen from Table 7 that the high shear treatment provided a clear increase in charge density but decrease in the salt viscosity value. Table 7 Characteristics of the biopolymer used in Example 4.

[0087] It was also investigated if temperature treatment could influence the dewatering performance of biopolymer aG9. Therefore a part of aG9 was cooked at 95°C for 1 h, before it was used as flocculant solution, either with or without high shear treatment.

[0088] The efficiency of the biopolymer aG9 as flocculant, with or without the high shear treatment, was tested by using the sludge dewatering test, describe above. The results are shown in Table 8.

[0089] Table 8 Sludge dewatering test results of Example 4.

[0090] *kg active biopolymer per ton dry sludge

[0091] It is seen that the high shear treatment improved the dewatering performance of the biopolymer aG9. The temperature treatment alone could only provide a moderate improvement in dewatering efficiency, but a significant improvement could be obtained when the temperature treated biopolymer was subjected to the high shear treatment.

[0092] Example 5

[0093] The used sludge had a dry solids content 3.2 %, pH 7.3 and a CST time of 285 s (zero test).

[0094] The biopolymer, used as the flocculant solution, was a cationic graft copolymer of dextran and a-1 ,3-glucan, denoted as aG10. Characteristics of aG10, before and after the high shear treatment, are shown in Table 9. It can be seen that the high shear treatment provided a moderate increase in the charge density but lowered the salt viscosity of the biopolymer.

[0095] Table 9 Characteristics of the biopolymers used in Example 5.

[0096] The efficiency of the biopolymer aG10 as flocculant, with or without the high shear treatment, was tested by using the sludge dewatering test, describe above. The results are shown in Table 10. It is seen that the high shear treatment improved the dewatering performance of the biopolymer aG10.

[0097] Table 10 Sludge dewatering test results of Example 5.

[0098] *kg active biopolymer per ton dry sludge

[0099] Example 6

[0100] The used sludge had a dry solids content of 3.2 %, pH 7.3 and a CST time of 285 s (zero test).

[0101] The biopolymers, used as flocculant solutions, were cationic graft copolymers of dextran and a-1 ,3-glucan, denoted as aG11 , aG12, and aG13. Their characteristics, before and after the high shear treatment, are shown in Table 11. It can be seen from Table 3 that a clear increase in charge density could be observed after the high shear treatment. An increase in the viscosity of the flocculant solution could be observed for all the samples. The efficiency of the biopolymers as flocculant solutions, with or without the high shear treatment, was tested by using the sludge dewatering test, describe above. The results are shown in Table 12. It is seen from Table 12 that the high shear treatment significantly improved the dewatering performance of biopolymers aG11 , aG12 and aG13.

[0102] Table 11 Characteristics of the biopolymers used in Example 6.

[0103] Table 12 Sludge dewatering test results of Example 6.

[0104] *kg active biopolymer per ton dry sludge

[0105] Comparative Example 7

[0106] The sludge had a dry solids content of 3.2 %, pH 7.3 and a CST time of 337 s, (zero test).

[0107] The biopolymers, used as flocculant solutions, were cationic linear a-1 ,3-glucan polymers, denoted as aG16, aG17 and aG19. Their characteristics, before and after the high shear treatment, are shown in Table 13. It is seen that the high shear treatment did not provide any significant change in charge density, even if an increase in salt viscosity could be observed. Table 13 Characteristics of the biopolymers used in Comparative Example 7.

[0108] The efficiency of the biopolymers as flocculant solutions, with or without the high shear treatment, were tested by using the sludge dewatering test, describe above. The results are shown in Table 14. The high shear treatment did not provide any improvement to sludge dewatering efficiency of the tested linear biopolymers.

[0109] Table 14 Sludge dewatering test results of Comparative Example 7. Comparative Example 8

[0110] The sludge had a dry solids content of 2.5 %, pH 7.3 and a CST time of 392 s, (zero test).

[0111] The biopolymers, used as flocculant solutions, were cationic linear a-1 ,3-glucan polymers, denoted as aG20 and aG21. Their characteristics, before and after the high shear treatment, are shown in Table 15. It is seen that the high shear treatment did not provide any significant change in charge density.

[0112] Table 15 Characteristics of the biopolymers used in Comparative Example 8. The efficiency of the biopolymers as flocculant solutions, with or without the high shear treatment, were tested by using the sludge dewatering test, describe above. The results are shown in Table 16. The high shear treatment did not provide any improvement to sludge dewatering efficiency of the tested linear biopolymers.

[0113] Table 16 Sludge dewatering test results of Comparative Example 8.

[0114] Comparative Example 9

[0115] The sludge had a dry solids content of 3.0 %, pH 7.3 and a CST time of 238 s, (zero test).

[0116] The biopolymer, used as the flocculant solution, were cationic linear a-1 ,3-glucan polymers, denoted as aG22. Its characteristics, before and after the high shear treatment, are shown in Table 17. It is seen that the high shear treatment did not provide any significant change in charge density.

[0117] Table 17 Characteristics of the biopolymers used in Comparative Example 9.

[0118] The efficiency of the biopolymer as the flocculant solution, with or without the high shear treatment, was tested by using the sludge dewatering test, describe above. The results are shown in Table 18. The high shear treatment did not provide any improvement to sludge dewatering efficiency of the tested linear biopolymer.

[0119] Table 18 Sludge dewatering test results of Comparative Example 9.

[0120] Comparative Example 10

[0121] The sludge had a dry solids content of 3.0 %, pH 7.3 and a CST time of 344 s, (zero test).

[0122] The biopolymers, used as the flocculant solutions, were cationic graft copolymer of dextran and a-1 ,3-glucan, denoted as aG23, aG24 and aG25. Their characteristics, before and after the high shear treatment, are shown in Table 19. It is seen that the high shear treatment did not provide any increase in charge density or salt viscosity.

[0123] Table 19 Characteristics of the biopolymers used in Comparative Example 10.

[0124] The efficiency of the biopolymers as flocculant solutions, with or without the high shear treatment, were tested by using the sludge dewatering test, describe above. The results are shown in Table 20. The high shear treatment did not provide any improvement to sludge dewatering efficiency of the tested linear biopolymers. Table 20 Sludge dewatering test results of Comparative Example 10.

[0125] Comparative Example 11

[0126] The used sludge had a dry solids content of 3.2 %, pH 7.3 and a CST time of 285 s (zero test).

[0127] The biopolymers, used as flocculant solutions, were cationic linear a-1 ,3-glucan polymers, denoted as aG14 and aG15. Their characteristics before and after high shear treatment are shown in Table 21 . It is seen that the high shear treatment did not provide any significant change in charge density. An increase in the viscosity of the flocculant solution could be observed.

[0128] Table 21 Characteristics of the biopolymers used in Comparative Example 11 .

[0129] The efficiency of the biopolymers as flocculant solutions, with or without the high shear treatment, was tested by using the sludge dewatering test, describe above. The results are shown in Table 22. It is seen from Table 22 that the high shear treatment could not provide improvement for the linear a-1 ,3-glucan polymers aG14 and aG15. Table 22 Sludge dewatering test results of Comparative Example 11 .

[0130] *kg active biopolymer per ton dry sludge

[0131] Example 12

[0132] The amount of insolubles in the biopolymer solutions comprising biopolymers aG7, aG11 and aG13 were determined before and after the high shear treatment.

[0133] The insolubles were determined as follows:

[0134] 1 g of biopolymer was mixed into 900 ml of tap water (25°C) at 450 rpm for 60 minutes, after which the solution was filtered through a 300 pm sieve. The beaker was washed thoroughly with cold tap water, pouring the washings through the sieve. The sieve was washed under a cold running tap until the effluent was free from polymer (approx. 5 - 10 mins). The sieve was allowed to drain. If any insolubles remained on the sieve, a cooled preconditioned tray was weighed on an analytical balance (= W1 ), the insolubles was scraped from the sieve onto the tray and dried for a minimum of 240 minutes at 110 - 120 °C. The tray with the dried insolubles was weighed on the analytical balance (= W2).

[0135] The % insolubles was calculated as ((W2 - W1 ) / 1 g) x 100%.

[0136] If the amount of insolubles was too small to be collected and weighed, but still visible, the insolubles (gel particles) were counted. In this case the result was close to 0 %.

[0137] The results are shown in Table 23. Table 23 Insolubles in biopolymers before and after the high shear treatment.

[0138] It is seen that the high shear treatment improved the solubility of all tested biopolymers.

[0139] For biopolymer aG2, the amount of insolubles was determined also without cooking and after cooking. The amount of insolubles was 9.6% without cooking, and 0.2% after cooking. This shows that the improved dewatering effect of biopolymer aG2 was not due to the decrease in the amount of insolubles.

[0140] 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 flocculating solid particles in a liquid-solid separation process, the method comprising- obtaining a suspension, where solid particles are dispersed in a continuous aqueous phase;- obtaining a flocculant solution comprising a cationic biopolymer having a charge density in a range of 0.5 - 2.6 meq / g and selected from cationic crosslinked a-1 ,3- glucan polymers, cationic graft copolymers of dextran and a-1 ,3-glucan, or any mixture thereof;- subjecting the flocculant solution to a high shear treatment;- bringing the flocculant solution after the high shear treatment into a contact with the suspension, and flocculating the solid particles; and- separating the flocculated solid particles from the continuous aqueous phase.

2. Method according to claim 1 , 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 0.1 - 0.7, even more preferably 0.15 - 0.6, determined after the high shear treatment.

3. Method according to claim 1 or 2, characterised in that the cationic biopolymer has the charge density in the range of 0.6 - 2.6 meq / g, preferably 1 .0 - 2.5 meq / g, more preferably 1 .5 - 2.5 meq / g, even more preferably 2.0 - 2.4 meq / g, determined before the high shear treatment.

4. Method according to claim 1 , 2 or 3, characterised in that in the high shear treatment the flocculant solution is subjected to a shear power which is in a range of 5 - 1000 W / kg, preferably 20 - 500 W / kg, more preferably 35 - 209 W / kg.

5. Method according to claim 4, characterised in that the high shear treatment has a duration of 1 - 600 s, preferably of 5 - 400 s, more preferably of 10 - 300 s.

6. Method according to any of preceding claims 1 - 5, characterised in that the cationic graft copolymer comprises 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.

7. Method according to any of preceding claims 1 - 6, characterised in that the cationic graft copolymer comprises 35 - 95 weight-%, preferably 30 - 90 weight-%, more preferably 40 - 80 weight-%, even more preferably 50 - 70 weight-%, of a- 1 ,3-glucan, calculated from the dry weight of the graft copolymer.

8. Method according to any of preceding claims 1 - 7, characterised in that the cationic graft copolymer is selected from ester derivatives or ether derivatives of graft copolymers of dextran and a-1 ,3-glucan.

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

10. Method according to any of preceding claims 1 - 9, characterised in that the cationic crosslinked biopolymer is obtained by using a crosslinker selected from a group comprising glyoxal and diglycidyl ethers, such as polyethylene 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.11 . Method according to any of preceding claims 1 - 10, characterised in that the flocculant solution comprising the cationic biopolymer is subjected to a heat treatment in a temperature of 60 - 115 °C, preferably 85 - 100 °C, before the high shear treatment.

12. Method according to any of preceding claims 1 - 11 , characterised in that the flocculant solution comprises 0.01 - 5 weight-%, preferably 0.05 - 4 weight-% of the cationic biopolymer, calculated from the weight of the flocculant solution.

13. Method according to any of preceding claims 1 - 12, characterised in that the liquid-solid separation process is a treatment process of wastewater, such as municipal wastewater or industrial wastewater, or a treatment process of sludge, such as primary sludge, secondary sludge, tertiary sludge or digested sludge, originating from a treatment of wastewater.

14. Method according to claim 13, characterised in that the wastewater has a biological oxygen demand (BOD) >50 mg / l, and / or a chemical oxygen demand (COD) in a range of 15 - 45 g / l, preferably 20 - 40 g / l, and / or a dry solids content in the range of 5 - 80 g / l, preferably 10 - 60 g / l, more preferably 20 - 55 g / l.

15. Method according to any of preceding claims 1 - 14, characterised in that the flocculant solution is brought into the contact with the suspension in amount that provides of 1 - 30 kg of the cationic biopolymer / ton dry suspension, preferably 2 - 20 kg of the cationic biopolymer / ton dry suspension, more preferably 3 - 15 kg of the cationic biopolymer / ton dry suspension.

16. Use of a high shear treatment for increasing cationicity of a flocculant solution comprising a cationic biopolymer having a charge density in a range of 0.5 - 2.6 meq / g and selected from cationic crosslinked a-1 ,3-glucan polymers, cationic graft copolymers of dextran and a-1 ,3-glucan, or any mixture thereof.

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