Flocculant composition, its use and method for liquid-solid separation

Abstract

The invention relates to a flocculant composition for liquid-solid separation, preferably for increasing a solid content of sludge. The flocculant composition comprises a particulate mixture of a cationic polyacrylamide and cationized starch in a ratio from 30:70 to 80:20. The invention relates also the use of the flocculant composition and to a method for flocculating a process medium in a liquid-solid separation process, where the flocculant composition is added to the process medium comprising an aqueous phase and suspended solid material, whereafter the process medium is flocculated and dewatered.

Description

[0001] FLOCCULANT COMPOSITION, ITS USE AND METHOD FOR LIQUID-SOLID

[0002] SEPARATION

[0003] The present invention relates to a flocculant composition, its use and to a method for liquid-solid separation according to the preambles of the enclosed claims. The present invention relates especially to dewatering or thickening of a process medium, preferably sludge, for increasing the solid content of the sludge after dewatering or thickening and / or for producing a separated liquid phase with a low concentration of suspended solid particles.

[0004] Many industrial processes include a liquid-solid separation step, where solid particles are separated from a liquid aqueous phase. For example, liquid-solid separation steps are employed in water treatment processes as well as treatment of various sludges, such as sludge thickening or sludge dewatering. Sludges to be treated usually comprise solid material particles and / or microorganisms suspended in an aqueous phase. In a liquid-solid separation step the water content of the sludge is reduced so that the solid material fraction of the sludge can be processed further, for example deposited, used as a fertilizer, or incinerated for energy production.

[0005] Liquid-solid separation often includes flocculation of the solid material particles, as it improves their separation from the liquid phase. Use of flocculants aims generally to produce sludge with as high solids content as possible and a treated liquid phase having as low content of suspended solids as possible. In addition, flocculants and / or coagulating chemicals are used to improve the formation and / or the quality, such as strength, of the formed flocs. Cationic polymers are conventionally used in liquid-solid separation, and they are highly effective as flocculants.

[0006] Conventional cationic polymers which are used in liquid-solid separation are petroleum-based synthetic polymers. The current environmental issues and need to reduce carbon emissions has created a need to decrease the use of petroleumbased synthetic polymers. Decrease in the amount of petroleum-based synthetic polymers is also advantageous, as it makes the separated solids (dried sludge) more suitable for use in composting, landfilling or as fertilizers. Consequently, there is a need for more sustainable alternatives in liquid-solid separation.

[0007] Flocculants are usually dosed in the liquid-solid separation process as liquids. Some applications employing liquid-solid separation require relatively large amount of flocculant for reaching the desired dry solids content for the sludge and / or the desired low suspended solids content for the treated liquid phase. The volume of added flocculant cannot be indefinitely increased, though, as there are physical restrictions, in form of tank and pump volumes, etc. Furthermore, adding a large volume of dilute liquid flocculant increases the amount of water that must be separated from the solids in the later process steps, e.g. by centrifuging. This naturally reduces the efficiency of the separation process. In such case, or when the added flocculant volume cannot be increased from the start, the alternative is to increase the concentration of the liquid flocculant in order to provide more of the effective chemical while maintaining the addition volume on the low / same level. However, cationic synthetic polymers often have high viscosity at high concentrations, making their handling, e.g. transfer in pipelines, inconvenient or impossible. Furthermore, flocculants with high solution viscosity might show reduced ability to mix homogenously with the process medium to be treated, such as sludge or waste water.

[0008] An object of this invention is to minimise or even eliminate the disadvantages existing in the prior art.

[0009] An object is also to provide an effective and more sustainable flocculant composition for a liquid-solid separation process, especially for sludge treatment, such as sludge dewatering and / or sludge thickening.

[0010] A further object of the invention is to provide a method which provides an effective flocculation of a process medium and a high solids content for the separated solids phase after a liquid-solid separation step. 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, both the flocculant composition, its use as well as the method for liquid-solid separation, even if this is not always separately mentioned.

[0012] A typical flocculant composition according to the present invention for liquid-solid separation, preferably for dewatering of sludge and / or for increasing a solid content of sludge, comprises a particulate mixture of a cationic polyacrylamide and cationized starch in a ratio from 30:70 to 80:20.

[0013] A typical use according to the invention of a flocculant composition according to the invention is for flocculating a process medium comprising an aqueous phase and suspended solid material, such as sludge, in a liquid-solid separation process, preferably for increasing a solid content of sludge.

[0014] A typical method according to the present invention for flocculating a process medium in a liquid-solid separation process, preferably for dewatering of sludge and / or increasing a solid content of sludge, comprises

[0015] - dissolving a flocculant composition according to the present invention in water,

[0016] - optionally diluting the dissolved flocculant composition to a dosage concentration,

[0017] - adding the dissolved and optionally diluted flocculant composition according to the present invention to a process medium comprising an aqueous phase and suspended solid material, such as sludge,

[0018] - flocculating the process medium, and

[0019] - dewatering the process medium.

[0020] Now it has been surprisingly found that a novel flocculant composition in particulate form can be obtained by co-formulating polyacrylamide flocculants with a derivatized polysaccharide component which is cationized starch. Unexpectedly, a flocculant composition comprising cationic polyacrylamide and a significant amount of highly cationized starch is able to provide similar or even better results in liquid-solid separation process as the state-of-the-art fully synthetic polymer flocculants, at about the same dosage. In practice this means that the amount of used synthetic polymer can be reduced to 50% or more, without compromising the efficiency of the liquid-solid separation step. This is highly surprising, as cationized starch alone cannot produce the same flocculation or separation effect, at least at the same dosage levels. The flocculant composition according to the present invention is able to provide equal drainage and dewatering results as conventional high-performance synthetic flocculant polymers. However, the flocculant composition of the present invention is able to significantly reduce the CO2 footprint of the flocculation step in liquid-solid separation process.

[0021] Furthermore, the present flocculant composition is easy to apply as it behaves in a similar manner as the conventional petroleum-based flocculation agents and it can be applied in a same way and at the same process locations, using the same flocculant solution preparation and dosing equipment. It was surprisingly found that the viscosity of the flocculant composition according to the invention remains low or at least on an appropriate level when the flocculant composition comprises cationized starch, even when the concentration of the cationized polyacrylamide present in the flocculant composition is increased.

[0022] According to the present invention the flocculant composition for liquid-solid separation comprises a cationic polyacrylamide and cationized starch in a ratio from 30:70 to 80:20 (Polyacrylamide:Cationized starch). The ratio is given as dry weight ratio. According to one preferable embodiment the flocculant composition may comprises cationic polyacrylamide and cationized starch in the ratio from 40:60 to 70:30, preferably from 43:57 to 65:35, more preferably 45:55 to 60:40. For example, the dry weight ratio of cationic polyacrylamide and cationized starch in the flocculant composition may be from 53:47 to 63:37, or from 55:45 to 65:35. The invention thus provides a possibility to significantly reduce the amount of petroleum-based polymers used in liquid-solid separation. According to the present invention the flocculant composition is in form of dry particulate material comprising solid particles. The flocculant composition thus comprises or consists of particles of cationic polyacrylamide and particles of cationized starch. The flocculant composition may be obtained by mixing dry particulate cationic polyacrylamide and dry particulate cationized starch in suitable ratios as defined above. The flocculant composition may have a moisture content <10 weight-%, typically 1 - 10 weight-%. The dry solids content of the flocculant composition may typically be 80 - 99.9 weight-%, preferably 85 - 99 weight-%, more preferably 90 - 97 weight-%, calculated from the total weight of the flocculant composition. The particulate dry form of the flocculant composition makes it easy to transport and store, as well as easy to use. Furthermore, the particulate dry form of the flocculant composition minimises the risk for microorganism attacks against the cationized starch component of the composition.

[0023] The flocculant composition may further comprise, in addition to cationic polyacrylamide and cationized starch, one or more auxiliary agents for improving its performance, stability or use. For example, the flocculant composition may comprise a buffering agent, such as citric acid, adipic acid or their mixture, filler salt(s), anticaking agent(s), and / or urea. Suitable filler salt may be selected from inorganic chloride salts, such as sodium chloride or ammonium chloride; or inorganic sulphate salts, such as sodium sulphate, ammonium sulphate or aluminium sulphate; or any of their combinations. Anticaking agent may be selected from dry silica compounds. According to one preferable embodiment the flocculant composition comprises a buffering agent, advantageously selected from citric acid, adipic acid and their mixtures. The amount of auxiliary agent(s), if present, is usually 10 weight-% or less, calculated from total dry weight of the composition. The flocculant composition may comprise 0.5 - 10 weight-%, preferably 1 - 7 weight-%, preferably 1.5 - 5 weight- %, of auxiliary agent(s). The presence of auxiliary agent(s) is fully optional, as the flocculant composition provides good performance and stability even without any auxiliary agents. Preferably the auxiliary agents are in dry particulate form, which makes them suitable for incorporation into dry flocculant composition, preferably having a moisture content <10 weight-%, as described above. According to one advantageous embodiment the flocculant composition in particulate form consists of particulate cationic polyacrylamide and particulate cationized starch, the remaining composition weight being absorbed atmospheric moisture.

[0024] The flocculant composition in particulate form comprises particles of which preferably at least 90 %, e.g. 90 - 99 %, have a particle size <2.5 mm, preferably <2 mm, for example 0.1 - 2.5 mm or 0.1 - 2 mm, preferably 0.125 - 1 .5 mm, more preferably 0.2 - 1 .25 mm. The particle size of the cationic polyacrylamide may be such that preferably at least 90 %, e.g. 90 - 99 %, of the particles of cationic polyacrylamide are, for example, in a range of 0.1 - 2.5 mm or 0.1 - 2.0 mm, preferably 0.125 - 1 .5 mm, more preferably 0.2 - 1 .25 mm, and / or the particle size of the cationized starch may be such that preferably at least 90 %, e.g. 90 - 99 %, of the particles of cationized starch are for example, in a range of 0.1 - 2.5 mm or 0.1 - 2.0 mm, preferably 0.125 - 1.5 mm, more preferably 0.2 - 1.25 mm. The particle size may be determined by using a sieve analysis. According to one preferable embodiment of the present invention, the flocculant composition comprises or consists of particulate cationic polyacrylamide and particulate cationized starch, wherein the difference between the particle size Dv50 of the particulate cationic polyacrylamide and the particle size Dv50 of the particulate cationized starch is at maximum of 30 %, preferably at maximum of 20 %, more preferably at maximum of 10 %, even more preferably at maximum of 7 % or at maximum of 5 %. The difference between the particle size Dv50 of the particulate cationic polyacrylamide and the particle size Dv50 of the particulate cationized starch may be between 0 - 30 %, preferably between 0 - 20 %, more preferably between 0.01 - 10 %, even more preferably between 0.1 - 7 % or between 0.1 - 5 %. In practice this means that the average particle size of the cationic polyacrylamide and the average particle size of the cationized starch are very close to each other. The particle size Dv50 is measured by using Malvern Mastersizer particle size analyzer (Malvern Pananalytical Ltd). When the particle size Dv50 values of the cationic polyacrylamide and the cationized starch are close to each other, the risk for component separation in the flocculant composition is low, even during transport and / or storage. In the present context, particle size Dv50 means, as conventional in the art, volume median particle size or volume average particle size, indicating that 50% of the particles are smaller than the given D50 value, and 50% of the particles are bigger than the given D50 value. The particle size Dv50 and average particle size are considered synonyms in the present text.

[0025] The cationized starch suitable for use in the present invention may be obtained by any available cationization method, which is able to provide cationized starch with a suitable particle size distribution and with an appropriate degree of cationization. Preferably starch is cationized by using cationization reagent selected from 2,3- epoxypropyltrimethylammonium chloride or 3-chloro-2-hydroxypropyltrimethyl- ammonium chloride, 2,3-epoxypropyltrimethylammonium chloride being preferred. It is also possible to provide cationized starch by using other applicable cationization methods, e.g. methods providing quaternary ammonium esters of starch.

[0026] According to one preferable embodiment of the present invention the flocculant composition may comprise cationized starch which has a degree of substitution (DS) in a range of 0.45 - 2.0 or 0.45 - 1 .5, preferably 0.5 - 1 .2, more preferably 0.6 - 1 .0. The degree of substitution refers, as conventional in the art, to the average number of the hydroxyl groups substituted by a cationizing reagent per anhydrous glucose unit (AGU). The used cationized starch thus has a high degree of substitution, which means that it is highly cationic.

[0027] The cationized starch may have charge density in a range of 2.0 - 4.0 or 2.0 - 3.5 meq / g, preferably 2.1 - 3.3 meq / g, more preferably 2.4 - 3.1 meq / g, measured at pH 4. The charge density values here are given per gram cationized cellulose material as dry and determined at pH 4 by using AFG Analytics’ CAS-II touch! Charge Analyzing System. For the determination, a cationized starch sample is dissolved as 0.025 - 0.05 weight-% solution in deionized water, pH is adjusted to 4.0 with 0.1 M acetic acid and titrated using 0.001 N sodium polyethylenesulfonate (PES-Na) solution as the titrant. During titration pH may increase 0 1 - 0.2 pH units. The charge density, given as meq / g, dry sample is calculated from the titration result. It is speculated that the high degree of substitution and high charge density of the cationized starch may have an advantageous effect on floc formation in liquid-solid separation, providing strong flocs which are able withstand breaking forces during separation treatments, such as filtration, centrifuging, etc.

[0028] The cationized starch may originate from any natural starch, such as potato starch, tapioca starch, corn starch, waxy corn starch, waxy potato starch, wheat starch, barley starch, rice starch, waxy rice starch, pea starch, legume starch, sweet potato starch, or any of their mixtures. According to one embodiment the cationized starch is obtained by cationization of non-degraded starch. In the present context the term “non-degraded starch” denotes cationized starch which is essentially untreated by oxidative, thermal, enzymatical and / or acid treatment in a manner that would cause hydrolysis of glycosidic bonds or degradation of starch molecules or units prior to cationization.

[0029] The cationized starch suitable for use in the present invention is water-soluble. Preferably the cationized starch may be at least soluble in water having a temperature of 40 °C. This means that the cationized starch is fully miscible with water, preferably having a temperature of 40 °C, providing transparent or nearly transparent solution. When mixed with excess of water, the cationized starch is dissolved and the obtained solution of cationized starch is preferably essentially free from visible discrete solid particles, granules or coagulated clots of cationized starch. Excess of water means that the obtained solution is not a saturated solution. No cooking of starch is required for dissolving, i.e. the preparation of flocculant composition is free of steps involving cooking of starch.

[0030] The cationized starch may have a turbidity value less than 1000 NTU, preferably less than 500 NTU, more preferably less than 250 NTU, when dissolved in water, preferably at 40 °C. The turbidity may be, for example, 0 - 1000 NTU, 1 - 500 NTU or 1 - 250 NTU. The turbidity values are measured from a water solution of the cationized starch, at 1 weight-% concentration, by using HACH, 2100 AN IS Laboratory Turbidimeter. The flocculant composition according to the present invention comprises also cationic polyacrylamide. Cationic polyacrylamide is here understood as copolymer of (meth)acrylamide and at least cationic monomer. The possible cationic monomers may be selected from a group comprising 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl] trimethylammonium chloride (ADAM-CI), 2-(dimethylamino)- ethyl acrylate benzylchloride, 2-(dimethylamino)ethyl acrylate dimethylsulphate, 2- dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl] trimethylammonium chloride (MADAM-CI), 2-dimethylaminoethyl methacrylate dimethylsulphate, [3-(acryloylamino)propyl] trimethylammonium chloride (APTAC), [3-(methacryloylamino)propyl] trimethylammonium chloride (MAPTAC), and diallyldimethylammonium chloride (DADMAC).

[0031] The cationic polyacrylamide used in the present invention may have a charge density in a range of 0.5 - 5.0 meq / g, preferably 1 .2 - 4.1 meq / g, more preferably 2.1 - 3.3 meq / g, measured at pH 4. The charge density may be measured by using AFG Analytics’ CAS-II touch! Charge Analyzing System, and in a similar manner as described above for the starch.

[0032] The cationic polyacrylamide suitable for the present invention may be obtained by free radical polymerisation of (meth)acrylamide and 4 - 90 mol-%, preferably 10 - 60 mol-%, more preferably 20 - 40 mol-%, of cationic monomers, calculated from the total amount of monomers present in the polymerisation.

[0033] The cationic polyacrylamide used in the present invention may be obtained, for example, by solution polymerisation, suspension polymerisation, inverse emulsion polymerisation, gel polymerisation or by dispersion polymerisation. According to one preferable embodiment the cationic polyacrylamide is obtained by gel polymerisation which produces a cationic polyacrylamide, which is in form of a gel or a highly viscous liquid. After the gel polymerisation, the obtained cationic polyacrylamide in gel form may be comminuted, such as shredded or chopped, as well as dried and optionally milled, whereby a dry particulate polyacrylamide is obtained. According to one embodiment of the invention, the flocculant composition may comprise cationic polyacrylamide, which has a standard viscosity (SV) in a range of 2 - 6 mPas, preferably 2.5 - 5.5 mPas, more preferably 3 - 5 mPas. Standard viscosity (SV) is measured at 0.1 weight-% product sample content in an aqueous 1 M NaCI solution, using Brookfield LV viscometer equipped with UL adapter and UL Adapter Spindle, at 25 °C using rotational speed 60 rpm. A general relationship between the standard viscosity of the cationic polyacrylamide and its average molecular weight is given in Table 1 .

[0034] Table 1 Relationship between the standard viscosity and the average molecular weight.

[0035] The relationship shown in Table 1 is based on standard viscosity and intrinsic viscosity measurements and using Mark-Houwink-Sakurada constants K=2.57-10’4dl / g and a=0.67.

[0036] The flocculant composition is fully water-soluble over a wide range of temperatures, for example from 5 °C to 50 °C. The flocculant composition is usually dissolved in water and / or optionally diluted to a dosage concentration with water before its use in liquid-solid separation process. Typically the flocculant composition is dissolved, and optionally diluted, with available process water. Advantageously the water used for dissolving and optional dilution is free of organic solvents. When the flocculant composition is dissolved water, it was surprisingly found that the viscosity of the solution of the flocculant composition remains at an appropriate level, even at higher concentrations, when the flocculant composition comprises cationized starch in addition to cationic polyacrylamide. This reduces the amount of water needed for dilution, as the flocculant composition can be dosed at higher concentrations. The reduction of dilution water may also be important factor in environments where there are limitations in water usage. Possibility to use higher concentrations reduces also the amount of water to be added to the liquidseparation process. Furthermore, the flocculant composition according to present invention can be used as high concentration solution, at concentrations where the viscosity of the cationic polyacrylamide alone could be too high for practical applications.

[0037] The flocculant composition may be dissolved in water and diluted to a suitable dosage concentration before its addition to the process medium, for example, sludge to be thickened or dewatered. A suitable dosage concentration for the flocculation composition may be, for example, in a range of 0.025 - 5 weight-%, preferably 0.05 - 3 weight-%, more preferably 0.1 - 1 or 0.1 - 0.5 weight-%.

[0038] The flocculant composition according to the present invention may be used for flocculating solid particles in any liquid-solid separation process where solid particles are separated from an aqueous liquid phase. The flocculant composition according to the present invention is advantageously intended for flocculating, dewatering and / or thickening a process medium, such as wastewater and / or sludge, preferably sludge formed in a wastewater treatment. For example, the flocculant composition may be used in a liquid-solid separation process which is a treatment process of wastewater, preferably municipal, industrial or agricultural wastewater, or it may be used for conditioning drinking water. The industrial wastewater may originate from food or beverage processing or manufacturing.

[0039] The flocculant composition according to the present invention is especially suitable for increasing a solid content of a process medium, preferably sludge. Sludge is here understood as an aqueous suspension, which comprise a continuous aqueous liquid phase and organic and / or inorganic solid material and / or particles suspended in the aqueous liquid phase. The sludge may be rich in material of bacterial origin. The aqueous liquid phase of the sludge may contain also dissolved organic substances, such as polysaccharides, humic substances and fatty acids. The sludge may be municipal wastewater sludge or agricultural sludge, or it may originate from a biological treatment process of wastewater and / or sewage. Alternatively, the aqueous suspension may originate from an industrial process, especially from wastewater treatment of an industrial process, or from food or beverage production or from food or beverage processing.

[0040] The liquid-solid separation process is preferably a dewatering step, pre-thickening step or thickening step, wherein the process medium is sludge. The flocculant composition may be advantageously used for dewatering, pre-thickening and / or thickening of sludge. For example, the flocculant composition may be used for sludge dewatering as a final step in a wastewater treatment process or for sludge pre-thickening step of a wastewater treatment process before the final dewatering. In the present context the terms “sludge thickening” and “sludge pre-thickening” are used interchangeably, and they both denote a treatment step where the dry solid content of the sludge is increased, e.g. prior to digester, sludge hydrolysis step, etc.

[0041] According to one embodiment, the flocculant composition may be used for dewatering, pre-thickening and / or thickening of digested sludge, mixed nondigested sludge, primary sludge, secondary sludge, tertiary sludge, anaerobically digested sludge or biological sludge.

[0042] The flocculant composition may also be used for increasing dry solids content of the process medium, such as sludge, originating from commercial biogas production, such as from municipal or agricultural biogas facilities or from co-digestion biogas facility or for increasing dry solids content of a process medium, such as sludge, to be used in these facilities. For example, sludge is treated with the flocculant composition in wastewater treatment process for increasing its solids content. The treated sludge, with increased solids content may be transported in a biogas facility, where it is advantageously used for biogas production. The increased solids content of the treated sludge reduce the transport costs. Furthermore, when the sludge is rediluted for the biogas production, it is advantageous that the flocculant composition has increased degree of biodegradability.

[0043] The flocculant composition according to the present invention is added to the process medium comprising an aqueous phase and suspended solid material, preferably sludge. The dissolved, and optionally diluted, flocculant composition may be added at any suitable position or location of the liquid-solid separation process. It may be added, for example, at least before or at one or more dewatering units of the liquid-solid separation process, such as decanter centrifuges, filter presses, screw presses, belt filter presses, belt filters, gravity settlers and / or thickening drums.

[0044] According to one embodiment of the invention the flocculant composition may be added in amount of 3 - 20 kg / ton dry process medium (e.g. sludge), preferably 4.5 - 15 kg / ton dry process medium or 4.5 - 14 kg / ton dry process medium, more preferably 5 - 12 kg / ton dry process medium, even more preferably 6.5 - 10 kg / ton dry process medium. The dosages are given as total dry active agent. These values are especially applicable, when the flocculant composition is used for final dewatering of the sludge in a wastewater treatment.

[0045] According to another embodiment the flocculant composition may be added in amount of 2 - 10 kg / ton dry process medium( e.g. sludge), preferably 4 - 5.5 kg / ton dry process medium, more preferably 4.5 - 5 kg / ton dry process medium. These values are especially applicable, when the process medium is sludge and the flocculant composition is used for pre-thickening of the sludge. The dosages are given as total dry active agent.

[0046] The flocculant composition is preferably used for increasing the solid content of a process medium, such as sludge. The present invention provides effective increase in process medium dryness and therefore a significant decrease in process medium volume after the liquid-solid separation. This reduces the handling costs of the treated process medium, such as sludge. At the same time the present invention provides a possibility to use sustainable composition and minimise the overall impact on the environment. After the liquid-solid separation process, comprising flocculation with the flocculant composition of the invention, the solid content of the process medium, such as sludge, may be 5 - 40 weight-%, typically 10 - 35 weight- %, more typically 15 - 35 weight-%.

[0047] The flocculant composition also provides relatively low suspended solids value for the liquid phase which is separated in the liquid-solid separation process. Typically the turbidity of the separated liquid phase is <1000 NTU, more typically <800 NTU. For example, the turbidity may be 100 - 600 NTU. Turbidity of the liquid phase may be measured by using Hach 2100Qis Portable Turbidimeter or a corresponding equipment.

[0048] When the process medium is sludge, the treated sludge, obtained from the liquidsolid separation process involving treatment of the sludge with the flocculant composition of the present invention, is suitable for use on agricultural lands or as fertilizer for agriculture. The treated sludge may also be suitable for use in composting or landscaping, due to its low content of non-biodegradable material.

[0049] According to one embodiment of the invention, an inorganic coagulant comprising a metal salt may be added to the process medium, such as sludge, before the addition of the flocculant composition. The inorganic coagulant may provide for effective phosphorous removal and / or sulphide control. The inorganic coagulant may be an aluminium or iron based coagulant, preferably an iron based coagulant, such as ferric chloride, ferrous chloride, ferric sulphate, ferrous sulphate, ferric chloro sulphate, or any of their mixtures. It has been observed that process medium, such as sludge, first treated with inorganic coagulant, preferably with iron based coagulant, provide flocs with good structure and strength when the flocculant composition according to the invention is added.

[0050] According to one embodiment of the invention, an organic coagulant may be added to the sludge before the addition of the flocculant composition. The organic coagulant may be selected from polyamine, poly-DADMAC, polyethyleneimine, polydicyandiamide and polyvinylamine.

[0051] EXPERIMENTAL

[0052] Some embodiments of the present invention are described in the following nonlimiting examples.

[0053] EXAMPLE 1

[0054] Example 1 demonstrates the beneficial effect of cationized starch to the viscosity of flocculant composition.

[0055] Six different cationic polyacrylamides CPAM1 - CPAM6 were used in Example 1. The properties of the cationic polyacrylamides CPAM1 - CPAM6 are given in Table 2. Standard viscosity (SV) was measured at 0.1 weight-% polymer content in an aqueous 1 M NaCI solution, using Brookfield LV viscometer equipped with UL adapter and UL Adapter Spindle, at 25 °C using rotational speed 60 rpm.

[0056] Table 2 Characteristic properties of the polyacrylamides CPAM1 - CPAM6 used in Example 1 .

[0057] * mol-% expresses the share of the cationic monomer in the acrylamide copolymer

[0058] Solution viscosities of cationic polyacrylamides CPAM1 - CPAM6 were measured at different polymer concentrations. The measured solution viscosity values are given in Table 3, as reference. The same cationic polyacrylamides (CPAM1 - CPAM6) were then mixed with cationized starch in a weight ratio 1 :1 to provide corresponding flocculant compositions FC1 - FC6. The solution viscosities of the obtained flocculant compositions FC1 - FC6 were measured at different polymer concentrations, and the measured solution viscosities are given in Table 3.

[0059] The same cationized starch was used for each flocculant composition. The charge density of the cationized starch was 2.9 meq / g, determined at pH 4 using AFG Analytics’ CAS-II touch! Charge Analyzing System using sodium polyethylenesulfonate (PES-Na) solution as the titrant. The charge density as meq / g dry sample was calculated from the titration result. Further, the cationized starch had a 3% salt viscosity value 24 mPas. 3% Salt Viscosity value for the cationized starch solution in water, in the presence of salt, was measured by using a Brookfield LV viscometer with a small sample adapter at 25 °C, using spindle #18. The 3% salt viscosity measurement was performed by using maximum possible rotational speed. The cationized starch was first dissolved in deionized water to provide a 3 weight-% starch solution. Then sodium chloride (NaCI) in weight ratio of 5:1 (NaCkcationized starch) was added to the starch solution and allowed to dissolve under mixing before the 3% salt viscosity was measured. This means that the viscosity of the cationized starch is measured at 2.6 weight-% concentration of cationized starch in an aqueous solution comprising 13.0 weight-% of NaCI.

[0060] The solution viscosity for cationized starch alone, at different starch concentrations, was also measured and the results are given in Table 3.

[0061] Cationic polyacrylamides (CPAM1 - CPAM6) and flocculant compositions FC1 - FC6 were dissolved / diluted with water. The cationized starch was dissolved in water. The solution viscosities were measured by using Brookfield DV-1 viscometer. A small sample adapter and spindle #18 were used. Temperature was 25 °C. The rotational speed used was always the maximum possible one from 12, 30, 60 and 100 rpm.

[0062] It can be clearly seen from the results of Table 2 that it is possible to almost double the concentration of the active polymer content (cationized starch and cationic polyacrylamide) when using the flocculant composition comprising cationic polyacrylamide and cationized starch while maintaining the viscosity of the solution on a level that is manageable in practical applications in industry.

[0063] Table 3 Solution viscosities of cationic polyacrylamides CPAM1 - CPAM6 and flocculant compositions FC1 - FC6 comprising corresponding cationic polyacrylamides CPAM 1 - CPAM6 and cationized starch, in ratio 1 :1 .

[0064] APPLICATION EXAMPLE 1

[0065] A flocculant composition of cationic polyacrylamide (PAM) and cationized starch (VHCS) was made by mixing powders of PAM and VHCS. The flocculant composition comprised PAM, VHCS and citric acid in mass ratio 48:48:4. PAM was a commercial PAM with Standard viscosity (SV) of 4.2 mPas and cationicity of 20 mol-%. Cationized starch had charge density of 2.9 meq / g dry starch, corresponding to degree of substitution 0.8.

[0066] The flocculant composition was used for dewatering of municipal anaerobically digested sludge in a Swedish wastewater treatment plant (WWTP). The dewatering was carried out with an industrial decanter centrifuge. The flocculant composition was dissolved in water and dosed as 0.14-0.22 weight-% aqueous solution. The sludge feed flow rate to the centrifuge was 17 m3 / h and dry solids in the feed flow was at 1 .8-2.0 weight-%. The dosage of the flocculant composition was 10 kg / ton dry sludge. With this dosage, the reject had turbidity of ca. 400-500 NTU and dry solids of the sludge cake was 26 weight-%.

[0067] The observed performance was compared to the performance of a commercial PAM with the same properties as the PAM used in the flocculant composition. In the reference run with the commercial PAM and the same operation conditions of the centrifuge, the polymer dosage was 10 kg / ton dry sludge, yielding the reject turbidity of ca. 400-500 NTU and dry solids of the sludge cake 27 weight-%. With these results, the WWTP was able to reach their target turbidity and sludge cake dry solids by using the flocculant composition of the invention.

[0068] APPLICATION EXAMPLE 2

[0069] A flocculant composition of cationic polyacrylamide (PAM) and cationized starch (VHCS) was made by mixing powders of PAM and VHCS. The flocculant composition comprised PAM, VHCS and citric acid in mass ratio of 48:48:4. PAM was a commercial PAM with Standard viscosity (SV) of 4.2 mPas and cationicity of 20 mol-%. Cationized starch had charge density of 2.9 meq / g dry starch, corresponding to degree of substitution 0.8.

[0070] The flocculant composition was used for dewatering of municipal mixed sludge in a Finnish wastewater treatment plant (WWTP). The dewatering was carried out with an industrial decanter centrifuge. The flocculant composition was dissolved in water and dosed as 0.25-0.28 weight-% aqueous solution. The sludge feed flow rate to the centrifuge was 20 m3 / h and dry solids in the feed flow was at 2.0-2.1 weight-%. The dosage of the flocculant composition was 7.5 kg / ton dry sludge. With this dosage, the reject had turbidity of ca. 400 NTU and dry solids of the sludge cake was 22 weight-%.

[0071] The observed performance was compared to the performance of a commercial PAM with the same properties as the PAM used in the flocculant composition. In the reference run with the commercial PAM and the same operation conditions of the centrifuge, the polymer dosage was 6.7 kg / ton dry sludge, yielding the reject turbidity of ca. 400 NTU and dry solids of the sludge cake 22 weight-%. With these results, the WWTP was able to reach their target turbidity and sludge cake dry solids by using the flocculant composition according to the invention.

[0072] APPLICATION EXAMPLE 3

[0073] A flocculant composition of cationic polyacrylamide (PAM) and cationized starch (VHCS) was made by mixing powders of PAM and VHCS. The flocculant composition comprised PAM, VHCS and citric acid in mass ratio 48:48:4. PAM was a commercial PAM with Standard viscosity (SV) of 4.8 mPas and cationicity of 30 mol-%. Cationized starch had charge density of 2.8 meq / g dry polymer, corresponding to degree of substitution 0.8.

[0074] The flocculant composition was used for dewatering of municipal anaerobically digested sludge in an Estonian wastewater treatment plant (WWTP). The dewatering was carried out with an industrial decanter centrifuge. The flocculant composition was dissolved in water and dosed as 0.4 weight-% aqueous solution. The sludge feed flow rate to the centrifuge was 31 m3 / h and dry solids in the feed flow was at 2.8-2.9 weight-%. The dosage of the flocculant composition was 8.8 kg / ton dry sludge. With this dosage, the reject had suspended solids of ca. 200 mg / L and dry solids of the sludge cake was 21 weight-%.

[0075] The observed performance was compared to the performance of a commercial PAM with Standard viscosity (SV) of 4.2 mPas and cationicity of 33 mol-%. In the reference run with the commercial PAM and the same operation conditions of the centrifuge, the polymer dosage was 8.8 kg / ton dry sludge, yielding the reject suspended solids of ca. 150 mg / L and dry solids of the sludge cake 22 weight-%. With these results, the WWTP was able to reach their target turbidity and sludge cake dry solids by using the flocculant composition according to the invention.

[0076] APPLICATION EXAMPLE 4

[0077] A flocculant composition of cationic polyacrylamide (PAM) and cationized starch (VHCS) was made by mixing powders of PAM and VHCS. The flocculant composition comprised PAM, VHCS and citric acid in mass ratio of 48:48:4. PAM was a commercial PAM with Standard viscosity (SV) of 4.2 mPas and cationicity of 33 mol-%. Cationized starch had charge density of 2.8 meq / g dry starch, corresponding to degree of substitution 0.8.

[0078] The flocculant composition was used for dewatering of municipal anaerobically digested sludge in a German wastewater treatment plant (WWTP). The dewatering was carried out with an industrial decanter centrifuge. The flocculant composition was dissolved in water and was dosed as 0.4 weight-% aqueous solution. The sludge feed flow rate to the centrifuge was 26-27 m3 / h and dry solids in the feed flow was at 2.1 weight-%. The dosage of flocculant composition was 13 kg / ton dry sludge. With this dosage, the reject had turbidity of 530 NTU and dry solids of the sludge cake 29.0 weight-%.

[0079] The observed performance was compared to the performance of a commercial PAM with the same properties as the PAM used in the flocculant composition. In the reference run with the commercial PAM and the same operation conditions of the centrifuge, the polymer dosage was 11.5 kg / ton dry sludge, yielding the reject turbidity of 460 NTU and dry solids of the sludge cake 29.7 weight-%. With these results, the WWTP was able to reach their target turbidity and sludge cake dry solids by using the flocculant composition according to the invention.

[0080] APPLICATION EXAMPLE 5

[0081] A flocculant composition of cationic polyacrylamide (PAM) and cationized starch (VHCS) was made by mixing powders of PAM and VHCS. The flocculant composition comprised PAM, VHCS and citric acid in mass ratio of 53:43:4. PAM was a commercial PAM with Standard viscosity (SV) of 3.4 mPas and cationicity of 20 mol-%. Cationized starch had charge density of 2.9 meq / g dry starch, corresponding to degree of substitution 0.8.

[0082] The flocculant composition was used for pre-thickening of municipal primary and secondary mixed sludge in a WWTP located in UK. The pre-thickening was carried out with an industrial pre-thickening drum. The flocculant composition was dissolved in water and dosed as 0.55 weight-% aqueous solution. The sludge feed flow rate to the centrifuge was 15 m3 / h and dry solids in the feed flow was at 3.8-4.6 weight- %. The dosage of the flocculant composition was 4.5-5.5 kg / ton dry sludge. With this dosage, the reject had suspended solids of ca. 6500-7000 mg / L and dry solids of the sludge cake 7-10 weight-%.

[0083] The observed performance was compared to the performance of a commercial PAM with the same properties as the PAM used in the flocculant composition. In the reference run with the commercial PAM and the same operation conditions of the centrifuge, the polymer dosage was 4.5-5.0 kg / ton dry sludge, yielding the reject suspended solids of ca. 6000-9000 mg / L and dry solids of the sludge cake of 7.5- 8.5 weight-%. With these results, the WWTP was able to reach their target sludge cake dry solids by using the flocculant composition according to the invention.

[0084] APPLICATION EXAMPLE 6

[0085] A flocculant composition (FC) of cationic polyacrylamide (PAM) and cationized starch (VHCS) was made by mixing powders of PAM and VHCS. The flocculant composition comprised PAM, VHCS and citric acid in mass ratio 48:48:4. PAM was a commercial PAM with Standard viscosity (SV) of 4.2 mPas and cationicity of 20 mol-%. Cationized starch had charge density of 2.9 meq / g dry starch, corresponding to degree of substitution 0.8.

[0086] The flocculant composition was used for dewatering of municipal anaerobically digested sludge in a Swedish wastewater treatment plant (WWTP). The dewatering was carried out with an industrial decanter centrifuge. The flocculant composition was dissolved in water and dosed as 0.2 weight-% or 0.3 weight-% aqueous solution. The flow rate of the sludge feed to the centrifuge was from 15.5 m3 / h to 17 m3 / h, and the dry solids content in the sludge feed flow varied from 1 .9 weight-% to 2.3 weight-%. The dosage of the flocculant composition was from 9.1 to 11 .5 kg / ton dry sludge. The dosage of the flocculant composition was dynamically adapted to the varying feed flow of the sludge.

[0087] The results are shown in Table 4. It can be seen that with these dosages of the flocculant composition, the liquid phase (reject) from the decanter centrifuge had a suspended solids content of varying from. 525 to 1114 mg / l and dry solids of the sludge cake after centrifuge was 27.4 to 31 .5 weight-%.

[0088] The performance of the flocculant composition comprising cationic polyacrylamide and cationized starch was compared to the performance of a commercial cationic polyacrylamide (CPAM) with the same properties as the cationic polyacrylamide used for the flocculant composition. In the reference runs with the commercial CPAM, the same operation conditions of the decanter centrifuge were employed and the dosage of the commercial cationic polyacrylamide was from 8.5 to 9.2 kg / ton dry sludge. The commercial cationic polyacrylamide was dosed in concentration of 0.2 weight-%. The results of reference runs are also shown in Table 4. It can be seen that the wastewater treatment plant was able to improve their suspended solids in the reject and sludge cake dry solids by using the flocculant composition of the invention.

[0089] Table 4 A Scandinavian WWTP sludge, dewatered on decanter centrifuge

[0090] APPLICATION EXAMPLE 7

[0091] A flocculant composition (FC) of cationic polyacrylamide (PAM) and cationized starch (VHCS) was made by mixing powders of CPAM5 and VHCS. The flocculant composition comprised CPAM5, VHCS and citric acid in weight ratio 48:48:4. CPAM5 was a commercial PAM with standard viscosity (SV) of 4.8 mPas and cationicity of 30 mol-%. Cationized starch had charge density of 2.9 meq / g dry starch, corresponding to degree of substitution 0.8. For the cationized starch, 90 % of the particles were within the range 0.26 - 2.1 mm and for the cationic polyacrylamide 90 % of the particles were within the range 0.15 - 1 .7 mm.

[0092] The flocculant composition was used for dewatering of municipal anaerobically digested sludge from a Finnish wastewater treatment plant (WWTP) in a laboratory scale. The dewatering was carried out with a Lemitec laboratory scale decanter centrifuge. The flocculant composition was dissolved in water and dosed as 0.4-0.6 weight-% aqueous solutions. The sludge feed flow rate to the centrifuge was 5 l / h and dry solids in the feed flow was at 2.5 weight-%. The dosage of the flocculant composition was 11 to 18 kg / ton dry sludge.

[0093] The observed performance of the flocculant composition was compared to the performance of CPAM5. CPAM5 was dissolved in water and dosed as 0.4 weight- % aqueous solution. The polymer dosage was 8-10 kg / ton dry sludge.

[0094] The results are given in Table 5.

[0095] It turned out that the 0.4 weight-% solution of CPAM5 was difficult to use. The solution of CPAM5 was probably too viscous, which impaired the performance of the polymer. After dewatering, the cake dry solids remained low and turbidity of the filtrate remained high indicating high suspended solids in the filtrate. Increase of the dosage of CPAM5 did not improve performance, and there was not any clear response, as can be seen from the results of Table 5.

[0096] Flocculant composition according to the invention was used at 0.4 weight-% concentration without difficulty for sludge dewatering. The performance of the flocculant composition was good, as can be seen from the results of Table 5: the obtained cake dry solids was significantly higher than with CPAM5 alone and turbidity of the filtrate was drastically reduced, indicating effective removal of suspended solids. The flocculant composition was tested even at 0.6 weight-% concentration without any difficulty in handling. The increased concentration allowed higher dosing, which was seen as even more improved performance.

[0097] The results of Table 5 confirm that the use of flocculant composition according to the invention is advantageous: it provides more flexibility in dosing, enables the increase the dosage, and improves the handling of the flocculant composition and its mixing even at higher flocculant concentrations. All this is especially important in cases where the available pumping and / or dissolution capacity of the treatment facility is the limiting factor for the increased dosage. Higher concentration of used the flocculant solution also saves dilution water needed and reduces the amount of water to be pumped into the system.

[0098] Table 5 Result of application example 7. 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 . Flocculant composition for liquid-solid separation, preferably for increasing a solid content of sludge, the composition comprising a particulate mixture of a cationic polyacrylamide and cationized starch in a ratio from 30:70 to 80:20.

2. Flocculant composition according to claim 1 , characterised in that the cationized starch has a degree of substitution in a range of 0.45 - 1 .5, preferably 0.5 - 1 .2, more preferably 0.6 - 1 .0.

3. Flocculant composition according to claim 1 or 2, characterised in that the cationic polyacrylamide has a charge density in a range of 0.5 - 5.0 meq / g, preferably 1 .2 - 4.1 meq / g, more preferably 2.1 - 3.3 meq / g.

4. Flocculant composition according to claim 1 , 2 or 3, characterised in that the cationic polyacrylamide has a standard viscosity (SV) in a range of 2 - 6 mPas, preferably 2.5 - 5.5 mPas, more preferably 3 - 5 mPas, when dissolved in 1 M NaCI solution and measured as defined in the specification.

5. Flocculant composition according to any of preceding claims 1 - 4, characterised in that the flocculant composition comprises cationic polyacrylamide and cationized starch in the ratio from 40:60 to 70:30, preferably from 43:57 to 65:35, more preferably 45:55 to 60:40.

6. Flocculant composition according to any of preceding claims 1 - 5, characterised the flocculant composition has a moisture content <10 weight-%.

7. Flocculant composition according to any of preceding claims 1 - 6, characterised in that the flocculant composition comprises particles of which at least 90 % have a particle size <2.5 mm, preferably <2 mm, more preferably 0.1 - 2 mm, even more preferably 0.125 - 1 .5 mm.

8. Flocculant composition according to any of preceding claims 1 - 7, characterised in that the cationized starch comprises particles of which at least 90 % are in a range of 0.1 - 2.5 mm, preferably 0.1 - 2.0 mm, more preferably 0.125 - 1.5 mm, even more preferably 0.2 - 1 .25 mm.

9. Flocculant composition according to any of preceding claims 1 - 8, characterised in that the difference between the particle size Dv50 of the particulate cationic polyacrylamide and the particle size Dv50 of the particulate cationized starch is at maximum of 30 %, preferably at maximum of 20 %, more preferably at maximum of 10 %, even more preferably at maximum of 7 % or at maximum of 5 %.

10. Flocculant composition according to any of preceding claims 1 - 9, characterised in that the flocculant composition comprises a buffering agent.11 . Use of a flocculant composition according to any of claims 1 - 10 for flocculating a process medium comprising an aqueous phase and suspended solid material, such as sludge, in a liquid-solid separation process, preferably for increasing a solid content.

12. Use according to claim 11 , characterised in that the liquid-solid separation process is a treatment process of wastewater, such as municipal, industrial or agricultural wastewater.

13. Use according to claim 11 or 12, characterised in that the liquid-solid separation process is a dewatering step, pre-thickening step or thickening step, wherein the process medium is sludge.

14. Use according to claim 13, characterised in that the sludge is digested sludge, mixed non-digested sludge, primary sludge, secondary sludge, tertiary sludge, anaerobically digested sludge or biological sludge.

15. Use according to any of preceding claims 11 - 14, characterised in that the flocculant composition is used for increasing the solid content of the processmedium originating from commercial biogas production, such as from municipal or agricultural biogas facilities or from co-digestion biogas facility.

16. Method for flocculating a process medium in a liquid-solid separation process, preferably for increasing a solid content of sludge, the method comprising- dissolving a flocculant composition according to any claims 1 - 10 in water,- optionally diluting the dissolved flocculant composition to a dosage concentration,- adding the dissolved and optionally diluted flocculant composition to a process medium comprising an aqueous phase and suspended solid material, such as sludge,- flocculating the process medium, and- dewatering the process medium.

17. Method according to claim 16, characterised in that the flocculant composition is added in amount of 3 - 20 kg / ton dry process medium, preferably 4.5 - 14 kg / ton dry process medium, more preferably 5 - 12 kg / ton dry process medium, even more preferably 6.5 - 10 kg / ton dry process medium.

18. Method according to claim 17, characterised in that the flocculant composition is added in amount of 2 - 10 kg / ton dry process medium, preferably 4 - 5.5 kg / ton dry process medium, more preferably 4.5 - 5 kg / ton dry process medium.

19. Method according to claim 16, 17 or 18, characterised in that an inorganic coagulant comprising metal salt, such as iron or aluminium salt, is added to the process medium before the addition of the dissolved flocculant composition.

20. Method according to any of preceding claims 16 - 19, characterised in that the dissolved flocculant composition is added at one or more dewatering units of the liquid-solid separation process, such as decanter centrifuges, filter presses, screw presses, belt filter presses, belt filters, gravity settlers and / or thickening drums.

Images