TECHNICAL FIELD

The present disclosure relates to a method and system for the treatment of biosludge and to a product obtainable by the method.

BACKGROUND

Pulp and paper industry waste waters typical ly contain wood, either in its original form or in an altered form (e.g. as lignin, starch, alcohols) . Waste waters also contain a variety of process substances and chemicals, also either in their original form or in a form altered in some way.

The content of compounds harmful for the en vironment in effluent or waste water coming from plants such as pulping processes or from municipal sewage is currently quite strictly controlled for en vironmental reasons. Plants employ various types of solutions that are used for waste water treatment in order to ensure that the plant fulfils environmental regulations .

Most effluents of pulp and paper industry are treated in biological waste water (sewage) treatment plants (active sludge plants) . Some of them treat also effluents from the surrounding community. Biological waste water treatment systems can be used especially for decreasing the amount of small-molecule organic substances. Biological purification uses the ability of the microorganisms to live in effluents. Microor ganisms dissipate dissolved and colloidal wastes, us ing them as nutrition. Wastes are removed in part me chanically and in part by the microorganisms trans forming them to carbon dioxide and water.

Waste water treatment processes typically in volve a number of chemical and mechanical phases. At an early phase, fibrous primary sludge is removed from the waste water. Then the pH may be adjusted, as the most suitable pH for the biological sludge may be 7 - 7.5 and a suitable range may be 6.8 - 8. The pH may be controlled by dosing alkaline (for example, lime) or acid to the waste water before aeration depending on the pH of the incoming effluent. The nutrient concen tration of the remaining waste water may also be ad justed, as the subsequent aerobic treatment using bac terial processes requires an amount of nutrients. Bio logical purification employs natural microbes, which use organic materials of effluents as their nutrition. Biological purification methods may involve, for exam ple, active sludge processes, anaerobic treatment, bi ological filtration or an aerated pond. Typically the bacteria present in the biological processes oxida tively degrade organic compounds present in the waste water, thus lowering its chemical oxygen demand (COD) . As oxygen is required in the process, it may be added by aeration. After the bacterial processes have oper ated for a sufficient time, for instance approx, three weeks, biosludge is separated from the waste water.

Sludges may be processed after thickening e.g. in a screw press or a filter belt press, either alone or in mixture with other materials, and burned e.g. in a bark boiler or in a soda recovery unit. Sludges can also be treated biologically by anaerobic digesting to provide valuable biogas or by composting, so that sludges can be utilized as a fertilizer or in landscaping. Disposal of biosludge currently poses various challenges, however. It has a foul smell and is potentially harmful to the environment, as it con tains bacteria and other possible microorganisms. It may also contain phosphorus, other nutrients, potassi um, chloride, residues of chemicals, various non process elements (NPEs) and/or heavy metals. Biosludge may retain a large amount of water, so disposal of the biosludge by burning often requires a supplementary fuel, even if some of the water is removed prior to burning. On the other hand, certain components of the biosludge tend to interfere in processes in which bi osludge could be used or recycled, for example when conveyed to a soda recovery unit in admixture with black liquor.

SUMMARY

A method for the treatment of biosludge is disclosed, wherein the biosludge comprises cellular structures. The method may comprise mixing the biosludge with an acid to obtain a mixture; breaking the cellular structures of the biosludge in the mixture at least partially; and separating a solid phase from the mixture, thereby obtaining a product comprising the solid phase.

A product comprising the solid phase obtainable by the method is disclosed.

A system for the treatment of biosludge is also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a further understanding of the embodiments and constitute a part of this specification, illustrate various embodiments. In the drawings:

Fig. 1 illustrates a method and a system for the treatment of biosludge;

Fig. 2A illustrates a cross-sectional side view of an embodiment of a high shear mixer; and

Fig. 2B shows the same embodiment of the high shear mixer as a cross-sectional top view.

DETAILED DESCRIPTION Disposal of biosludge may be quite challenging, because it tends to retain a large amount of water. Biosludge may in some cases also contain various components that may need to be released and possibly removed prior to disposal, for example heavy metals, such as cadmium (Cd) .

It has now been found, however, that it is possible to treat biosludge such that the solids content of the biosludge may be increased. The product obtainable by the method may thus have a significantly increased energy content and may be burnt with an improved energy efficiency. The amount, e.g. volume and total weight, of the biosludge, which may be considered a waste, can be decreased. The use of water may also become more effective. Furthermore, its heavy metal content, for example Cd content, can be reduced. The product obtainable by the method may be suitable for use e.g. as a raw material for fertilizer products and/or for soil improvement. The use of supplementary fuels or other materials during the processing or disposal of biosludge may also be reduced or even eliminated. The method may be considered to be a chemi-mechanical treatment method, as it employs both a chemical agent (an acid) and a mechanical treatment.

A method for the treatment of biosludge is disclosed. The biosludge comprises cellular structures. The method may comprise mixing the biosludge with an acid to obtain a mixture; breaking the cellular structures of the biosludge in the mixture at least partially; and separating a solid phase from the mixture, thereby obtaining a product comprising the solid phase.

The method may be suitable for removing heavy metals and/or for releasing and optionally removing water from the biosludge, i.e. increasing the dry solids (DS) content of the biosludge. The acid may assist in the method e.g. to extract or chemically separate certain components, such as heavy metals. In combination with the breaking of the cellular structures, the acid can significantly improve the removal of water and/or potentially harmful components from the biosludge.

The cellular structures of the biosludge may be derived from plant material and/or from the microbes employed in the biological waste water treatment process from which the biosludge is obtained. When the cellular structures of the biosludge are broken at least partially, i.e. at least a part of the cellular structures of the biosludge are broken, intracellular water can be released. Water within (i.e. intracellular water) or otherwise bound to the cellular structures can thus be released and removed from the biosludge at least partially with the present method. However, the breaking of the cellular structures may need to be done such that the biosludge or the mixture does not form a gel-like slurry. From such a gel-like slurry, it may be challenging to separate the solid phase.

The term "biosludge" may be understood as re ferring to sludge obtainable from biological waste wa ter treatment, i.e. residual, semi-solid material left from a biological waste water treatment process. Bi osludge typically contains various organic, inorganic and microbiological contaminants. Biosludge may con tain cellular structures derived from the microbes used in the biological waste water treatment.

The biosludge may be obtainable from the treatment process of waste water obtainable from a pulping process, such as a chemical pulping process. Such biosludge may contain cellular structures derived from the wood material used in the pulping process. The biosludge may also be obtainable from municipal sewage or from any other source of biosludge. The bi osludge may therefore contain cellular structures de- rived from various sources, for example cellular structures derived from plant-based materials (other than wood material) .

The acid may, in principle, be any acid that can be mixed with the biosludge, including any inorganic and organic acids and any mixtures or combinations thereof. Examples of possible inorganic acids may include hydrogen halides, such as hydrochloric acid (HC1) , and their solutions; sulphuric acid (H ₂ SO ₄ ) ; nitric acid (HN0 ₃ ) ; phosphoric acid (H ₃ PO ₄₎ ; or any mixtures or combinations thereof. Examples of possible organic acids include carboxylic acids, such as acetic acid (CH ₃ COOH) , citric acid, formic acid (HCOOH) , gluconic acid, lactic acid, oxalic acid, tartaric acid, or any mixtures or combinations thereof. In an embodiment, the acid is an inorganic acid, such as sulphuric acid, hydrochloric acid, or nitric acid; an organic acid, such as a carboxylic acid; or any mixture or combination thereof. In an embodiment, the acid is sulphuric acid, hydrochloric acid, or nitric acid, or any mixture thereof. The acid may be, in an embodiment, understood as referring to an aqueous solution of an acid, including any of the acids described above. The concentration of the aqueous solution of the acid may be selected such that a desired pH and/or volume of the mixture of the acid, i.e. the aqueous solution of the acid, and the biosludge is achieved. A concentrated acid solution, for example a concentrated sulphuric acid, hydrochloric acid or nitric acid may be used, for example so as to avoid introducing large volumes of water into the mixture.

The pH of the mixture may be adjusted to a pH of 4 or lower. It may also be adjusted to a pH of 2.5 or lower, or to a pH of 2 or lower. In general, a lower pH may improve the results. For example, a lower pH may be associated with improved removal of heavy metals, such as Cd. The pH may however also be optimized such that the consumption of the acid remains economical.

A flocculant may be added to the mixture, to the biosludge or to the acid. The flocculant may in some situations improve the results.

The biosludge and the acid may be premixed, i.e. admixed to form a mixture prior to breaking the cellular structures of the biosludge at least partially. This may be done e.g. in a suitable mixing apparatus, for example in a mixing tank. During the premixing, the properties of the mixture, for example the volume, viscosity, and/or consistency of the mixture, may be adjusted. Alternatively or additionally, the biosludge and the acid may be fed separately to the apparatus for breaking the cellular structures of the biosludge in the mixture at least partially, in which they are simultaneously intimately mixed. For example, the acid and the biosludge may be fed separately to a mixer in which the cellular structures of the biosludge are subsequently broken at least partially.

Various means can be employed for breaking the cellular structures of the biosludge in the mixture at least partially. For example, the biosludge and acid and/or the mixture thereof may be fed to a mixer and subjected simultaneously to shear forces, thereby breaking the cellular structures of the biosludge at least partially. Suitable mixers for this purpose may be e.g. a blade mixer comprising a dispersion blade, a high shear impeller, or a rotor- stator mixer. Such a mixer may also impart impact forces to the mixture.

The breaking of the cellular structures may be particularly efficient, if a high shear mixer is used. In an embodiment, the biosludge and the acid or the mixture thereof are fed to a zone of high shear forces within a high shear mixer and subjected simultaneously to the zone of high shear forces, thereby breaking the cellular structures of the biosludge at least partially. Since the biosludge and the acid are fed to and subjected to the zone of the high shear forces, these forces may be exerted to substantially the entire volume of the biosludge and acid. For example, blade mixers may produce high shear forces at the rim of the blade, but they do not form a zone of high shear forces, through which all or essentially all of the biosludge and acid would be forced. On the other hand, high shear mixers such as impact mixers (e.g. Atrex-type mixers) may produce high shear forces and all or essentially all of the biosludge and acid, due to the geometry of the mixer, is forced through the zone of high shear forces formed by the rotors. For example, at least about 90 w-%, or at least about 95 w-%, or at least about 99 w-% of the biosludge and acid or the mixture thereof may pass through the zone of high shear forces.

The mixing of the biosludge and the acid is therefore efficient and may be faster than e.g. using a conventional mixer, i.e. the residence time through the high shear mixer may be reduced. Energy consumption may also be reduced. Furthermore, the high shear forces may efficiently break the cellular structures .

The energy intensity of the high shear forces to which the biosludge and the acid, or the mixture thereof, are subjected in the high shear mixer mixture may be selected depending on various factors. The energy intensity may be such that the cellular structures in the biosludge are broken to such an extent that heavy metals, water and/or other components are released and removed at least partially. The energy intensity may therefore be, for example, at least 200 kWh/m ³ or at least 300 kWh/m ³ . The energy intensity may be, for example, up to 650 kWh/m ³ , or in the range of about 200 to about 650 kWh/m ³ .

However, high energy intensities may cause the biosluddge to form a gel-like slurry. Separating the solid phase from such a gel-like slurry may be complicated. The energy intensity may therefore be such that the biosludge does not form a gel-like slurry. The energy intensity may therefore be, for example, at most 650 kWh/m ³ , or at most 500 kWh/m ³ .

In an embodiment, the biosludge and the acid, or the mixture thereof, are subjected to an energy intensity of 300 - 500 kWh/m ³ in the high shear mixer. Such an energy intensity may be well suited for breaking the cellular structures and may avoid significant formation of the gel-like slurry.

Any energy intensities described in this specification may be calculated on the basis of the volume occupied by the biosludge and the acid or the mixture thereof fed into the high shear zone.

The residence time of the biosludge and the acid in the zone of high shear forces may be about

0.01 to 60 seconds, or longer, if desired.

The high shear mixer may be capable of operating continuously.

In an embodiment, the zone of high shear forces is formed by a mixing zone of a high shear mixer having at least one rotating rotor element.

In an embodiment, the zone of high shear forces is formed by a mixing zone of a high shear mixer having at least one static stator element and at least one rotating rotor element.

In an embodiment, the zone of high shear forces is formed by a mixing zone of a high shear mixer having at least two counter-rotating rotors.

An example of a high shear mixer may be an impact mixer, for example an impact mixer sold under the trade name Atrex® (Megatrex Oy) . Such a high shear mixer may comprise a first rotor provided with blades and a second rotor provided with blades, wherein the first and second rotor are arranged concentrically with each other and configured to rotate to opposite directions in relation to each other, and the biosludge and the acid or the mixture thereof are supplied through the rotors such that they are repeatedly subjected to shear forces by the effect of the blades, the effect of the blades thereby forming the zone of high shear forces and breaking the cellular structures of the biosludge in the mixture at least partially. The rotor elements may be capable of rotating at a speed of about 500 to 5000 rpm. Examples of such a high shear mixer are described e.g. in WO 2013/072559 (page 7, line 1 - page 11, line 17 and Figures 1-4); or in FI 105112 B (e.g. the apparatus described in Figs. 1 - 5 and associated paragraphs in the text, e.g. p. 5, 1. 30 to p. 8, 1. 31), which are herein incorporated by reference in their entirety.

Another type of high-shear mixer is the mixer sold under the trade name Cavitron® (Hagen & Funke GmbH) . Such a Cavitron-type high-shear mixer may comprise a dispersing unit or shock-wave reactor. In the dispersing unit or shock-wave reactor, a zone of high shear forces is induced by a rotor/stator system having passage gaps at the rotor and stator. The Cavitron-type high-shear mixer may be configured to fill the gaps arranged in a row with the biosludge and the acid or the mixture thereof, such that they are/it is centrifugally accelerated by the rotor to gaps in an adjacent row of gaps, thereby generating alternating pressure fields. Examples of possible high shear mixers may be described e.g. in US3165299A and US3589363.

After breaking the cellular structures of the biosludge in the mixture at least partially, the solid phase may be separated from the mixture, thereby obtaining a product comprising the solid phase. Thus the solid phase and a liquid phase may be obtained.

The solid phase may be separated using suitable means, for example a filtering apparatus or a centrifugal apparatus. In an embodiment, the solid phase is separated using at least one of a decanter centrifuge or a pressure filter. The pressure filter may be a vertical pressure filter. A vertical pressure filter may have a relatively good performance for separating the solid phase. However, the pressure filter may, additionally or alternatively, be a horizontal pressure filter. The separated solid phase may then be recovered. The liquid phase, for example a filtrate obtainable by separating the solid phase using a filter, may be recovered and/or discarded. At least a part of the heavy metals, such as Cd, derived from the biosludge can thus be leached into the liquid phase .

The treatment may be repeated. In other words, the solid phase obtained may be mixed with an acid to obtain a second mixture; the cellular structures of the solid phase in the second mixture may be broken at least partially; and a second solid phase may be separated from the second mixture, thereby obtaining a product comprising the second solid phase. The acid may be the same as the acid mixed with the biosludge, or it may be selected independently. Repeating the treatment may improve the result .

The liquid phase may be conveyed to effluent. However, the liquid phase may also be treated further. For example, it may be treated to remove metals e.g. by an electrochemical treatment or ion exchange. Such treated liquid phase could be reused as a process water . The product, i.e. the solid phase, may be in the form of a cake after it has been separated from the liquid phase. The method may therefore further comprise increasing the surface area of the product. This can be done mechanically, e.g. by crushing. For example, a crusher may be used, or any other apparatus suitable for increasing the surface area of the prod uct .

Heating of the biosludge and/or the mixture may improve the results, such as the dry solids content of the product obtainable and/or the removal of heavy metals, such as Cd. The mixture may be heated to a temperature of at least 30°C, or at least 40°C, or at least 50°C before separating a solid phase from the mixture. However, it is also possible to heat the biosludge and/or the mixture to a higher temperature, for example to a temperature of at least 60°C, or at least 70°C, or up to 70°C. The heating may be done e.g. using steam. The biosludge and/or the mixture may be heated prior to, during or after breaking the cellular structures of the biosludge in the mixture. It may also be possible to heat the mixture after breaking the cellular structures and prior to separating the solid phase.

The method may further comprise increasing the dry solids content of the product. This can be done e.g. by thermal heating and/or drying the product .

A product comprising the solid phase obtainable by the method according to one or more embodiments described in this specification is also disclosed .

The product may comprise at least 30 w-%, or at least 35 w-%, or at least 40 w-% of dry solids derived from the biosludge by total weight of the product . The cadmium (Cd) content of the product may be less than or equal to 3 mg/kg, or less than or equal to 2.5 mg/kg, or less than or equal to 2 mg/kg, or less than or equal to 1.5 mg/kg, based on the total dry weight of the product.

The product, i.e. the solid phase obtainable after dewatering, may be used for various purposes, such as for fertilizing or soil conditioning. The product may be a fertilizer product or a soil conditioner. The product may also be incinerated. Due to the relatively high dry solids content of the product, it may be incinerated without a supplementary fuel. Some embodiments of the product may have a lower heating value greater than 6 MJ/kg, or even greater than 10 MJ/kg.

Use of the product comprising the solid phase obtainable by the method according to one or more embodiments described in this specification for fertilizing or soil conditioning is also disclosed.

A system for the treatment of biosludge, wherein the biosludge comprises cellular structures, is also disclosed. The system may comprise

an apparatus for breaking the cellular structures of the biosludge in a mixture of the biosludge and an acid at least partially; and

a separation apparatus for separating a solid phase from the mixture to obtain a product comprising the solid phase.

The apparatus for breaking the cellular structures of the biosludge in the mixture of the biosludge and the acid at least partially may comprise a high shear mixer.

In an embodiment, the high shear mixer comprises a first rotor provided with blades and a second rotor provided with blades, wherein the first and second rotor are arranged concentrically with each other and configured to rotate to opposite directions in relation to each other, so that the biosludge and the acid or the mixture thereof are repeatedly subjected to shear forces by the effect of the blades, the effect of the blades thereby breaking the cellular structures of the biosludge in the mixture of the biosludge and the acid at least partially. Such a high shear mixer is thus configured to form the zone of high shear forces by the effect of the blades.

In an embodiment, the high shear mixer is a Cavitron-type mixer.

The separation apparatus may be, for example, a centrifugal apparatus or a filter. In an embodiment, the separation apparatus is a pressure filter, such as a vertical pressure filter or a horizontal pressure filter .

The apparatus for breaking the cellular structures of the biosludge in a mixture of the biosludge and an acid at least partially may be configured to intimately mix the biosludge and acid fed or injected therein separately. The mixing may be done simultaneously as the breaking of the cellular structures. However, alternatively or additionally, the system may comprise a mixing apparatus for mixing the biosludge with an acid to obtain the mixture prior to breaking the cellular structures. The mixing apparatus may be suitable for premixing the biosludge with the acid to obtain the mixture prior to conveying the mixture to the apparatus for breaking the cellular structures of the biosludge in the mixture of the biosludge and the acid at least partially.

The system may further comprise a storage vessel, such as a buffer tank, for storing the mixture after breaking the cellular structures at least partially .

The system may further comprise a crusher for increasing the surface area of the product. The crusher may be e.g. a mechanical crusher. The system may further comprise a heater and/or a dryer for increasing the dry solids content of the product. For example, the heater and/or dryer may be a thermal dryer.

The system may, in an embodiment, further comprise at least one of:

a mixing apparatus for mixing the biosludge with an acid to obtain the mixture;

a crusher for increasing the surface area of the product; or

a heater and/or a dryer for increasing the dry solids content of the product, such as a thermal dryer .

EXAMPLES

Reference will now be made in detail to various embodiments, an example of which is illustrated in the accompanying drawing.

The description below discloses some embodiments in such a detail that a person skilled in the art is able to utilize the embodiments based on the disclosure. Not all steps or features of the embodiments are discussed in detail, as many of the steps or features will be obvious for the person skilled in the art based on this specification.

For reasons of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components.

Fig. 1 illustrates a method and a system 9 for the treatment of biosludge 1. The biosludge may be obtained from biological waste water treatment of various waste waters from a chemical pulping process and therefore may comprise cellular structures derived from wood material used in the chemical pulping process. The system 9 may comprise a mixing apparatus 10, for example a mixing tank, for mixing the biosludge 1 with an acid 2 to obtain the mixture. Such a mixing apparatus or tank may comprise e.g. a simple blade mixer. The biosludge 1 and the acid 2 may be premixed in the mixing apparatus 10. The mixture 3 thus obtained may be fed into an apparatus 7 for breaking the cellular structures of the biosludge in a mixture 3 of the biosludge and the acid 2 at least partially. Alternatively or additionally, the biosludge 1 and the acid 2 may be fed directly to the apparatus 7 for breaking the cellular structures of the biosludge in the mixture least partially. The apparatus 7 may simultaneously mix the biosludge 1 and the acid 2 and break the cellular structures at least partially .

For heating the biosludge and/or the mixture, heat 13 may be supplied to them. Steam, for example steam at a pressure of about 3.5 bar and at a temperature of about 137°C, may be readily available e.g. at a chemical pulping plant and may be utilized for this purpose as a source of heat. The biosludge and/or the mixture may be heated using steam and a suitable heat exchanger.

The apparatus 7 for breaking the cellular structures of the biosludge in the mixture of the biosludge and the acid at least partially is or comprises, in this exemplary embodiment, a high shear mixer. The high shear mixer may be configured to form a zone 6 of high shear forces within the high shear mixer and to subject the biosludge and the acid, or the mixture thereof, simultaneously to the zone 6 of high shear forces, thereby breaking the cellular structures of the biosludge at least partially. An embodiment of such a high shear mixer is described in detail in Figs. 2A and 2B. However, any mixer, in particular any high shear mixer (for example, a Cavitron-type mixer) , described in this specification could be contemplated instead. All or essentially all of the mixture may pass through the zone 6 of high shear forces, such that all or essentially all of the mixture is subjected to the high shear forces.

Subsequently, the mixture may be conveyed to and stored in a suitable storage vessel, such as a buffer tank 14. The buffer tank 14 could also, in some embodiments, be omitted.

The mixture may be heated, additionally or alternatively, at a later stage, for example after breaking the cellular structures. In this exemplary embodiment, heat may be supplied to the mixture prior to separating the solid phase by heating the mixture in the buffer tank 14.

The mixture 3 may subsequently be conveyed to a separation apparatus 8 for separating a solid phase 4 and a liquid phase 15 from the mixture. Thus a product 5 comprising the solid phase 4 can be obtained. The separation apparatus 8 may be or comprise, for example, a centrifugal apparatus, such as a decanter centrifuge. The separation apparatus 8 may be or comprise, for example, a filtering apparatus, such as a pressure filter. The pressure filter may be a vertical pressure filter or a horizontal pressure filter. A vertical pressure filter may, at least in some cases, perform better than e.g. a horizontal pressure filter. Filter cloth and e.g. the running parameters of the pressure filter may be selected such that the separation operates reliably and efficiently. Ash may be mixed with the biosludge, for example in an amount of 5 to 10 w-%, to facilitate the removal of the cake from the pressure filter.

The solid phase 4 may be recovered as a cake. The cake can be used as such, or the product 5 comprising the solid phase can then be processed further .

The system may further comprise a crusher 11 for increasing the surface area of the product. The system may further comprise a heater and/or a dryer 12 for increasing the dry solids content of the product, such as a thermal dryer.

The system could further comprise e.g. a granulator or extruder (not shown) for forming granules of the product.

The system may further comprise a biological waste water treatment plant (not shown) .

The system may further comprise a chemical pulping plant and a waste water treatment plant for treating waste water obtainable from the chemical pulping plant.

The system may further comprise conduits or flow connections, for example pipings, between the parts of the system for conveying the mixture or the product. The system may further comprise a conduit for conveying the liquid phase to waste water treatment or to discharge.

Additives or other substances may be added to the product, if desired. Examples of such additives may include e.g. additives for assisting with forming granules of the product, coatings, or components for improving the nutritional content of the product. For example, nitrogen-containing compounds such as nitrogen salts or urea may be added to increase the nitrogen content of the product; ash; one or more of other nutrients, such as one or more of phosphorus, potassium, calcium, magnesium, sulphur, boron, chlorine, manganese, iron, zinc, copper, cobalt, molybdenum, nickel, silicon, selenium or sodium, or other components of a fertilizer or soil conditioner. The product may thus comprise other components, for example any of the components described above.

Fig. 2A illustrates a cross-sectional side view of an exemplary high shear mixer 7 which can be used for breaking the cell walls at least partially. Fig. 2B shows the same exemplary high shear mixer 7 as a cross-sectional top view. This embodiment of the high shear mixer 7 is merely an example, and it is clear to a skilled person that various other high shear mixers or other mixers may be utilized for the same purpose and that their structures and operations may differ from the one described herein. The high shear mixer 7 described in these Figs, is similar e.g to Atrex CD 500 G45 which has been used in the experimental examples below.

The high shear mixer 7 comprises a first rotor 16 and a second rotor 17 arranged concentrically within each other such that they are configured to rotate around a common rotation axis 18. The first rotor 16 and the second rotor 17 are configured to rotate to opposite directions in relation to each other. The first and second rotor 16, 17 thus form a pair of counter-rotating rotors. However, the high shear mixer 7 may comprise two, three or more rotors. In other embodiments, one of the first or second rotors 16, 17 could be replaced by a stator. However, solutions comprising at least two counter-rotating rotors may be more efficient at causing high shear forces .

The rotors 16, 17 are provided with blades or ribs 29. The blades 29 are arranged in at least two concentric rims 19, 20, 21, 22, 23, 24. The blades 29, connected to the rotors 16, 17, are thus also configured to rotate around the common rotation axis 18. The at least two concentric rims are configured to rotate to opposite directions in relation to each other. The exemplary embodiment of the high shear mixer 7 shown in Figs. 2A and 2B comprises blades arranged in a plurality of rims 19, 20, 21, 22, 23, 24 - specifically six in this embodiment. The blades 29 of three of the rims 19, 21, 23 are configured to rotate in the same direction. The blades of the other three rims 20, 22, 24 are configured to rotate in the opposite direction. The rims 19, 20, 21, 22, 23, 24 are arranged pairwise such that one rim is always followed and/or preceded in the radial direction by a rim of counter-rotating blades. The rotors may be capable of rotating at a speed of about 500 to 5000 rpm, but speeds lower than about 500 or greater than about 5000 may also be contemplated.

The blades 29 may be elongated pieces, the height of which may be greater than their width (i.e. their dimension in the radial direction of the rotors) . Figs. 2A and 2B show measurements of certain dimensions of the exemplary high shear mixer 7 in millimetres, but various geometries and dimensions for the rotors and the blades may be contemplated. For example, in Fig. 2B the blades 29 are parallel to the radial direction, but at least some of the blades 29, for example those arranged in the outermost rim 24 or in the two outermost rims 23, 24, may be arranged at an angle to the radial direction. Further, the dimensions of the high shear mixer 7 may be selected e.g. on the basis of the amount of the biosludge to be treated in a given time period.

The high shear mixer 7 comprises a housing 25 within which the rotors 16, 17 may be arranged. The high shear mixer 7 further comprises an inlet 26 for feeding in the biosludge and the acid and/or the mixture 3 thereof. The inlet 26 opens to the innermost rim 19. The high shear mixer 7 further comprises an outlet 27 for removing the mixture. The outlet 27 opens to the outermost rim 24 and extends through the housing 25. As the mixture 3 is fed into the high shear mixer 7 via the inlet 26, it is subsequently fed through the zone 6 of high shearing forces towards the outlet 27. The rotors 16, 17 can be considered to be flow-through rotors, such that the mixture may pass through the rotors via gaps between the blades 29 extending in the direction of the rotation axis 18 and therefore also through the zone 6 of the high shear forces. The mixture may pass through the zone 6 of high shear forces in a given residence time.

The mixture 3 may thus be supplied outwards in the radial direction with respect to the rotation axis 18 of the rotors 16, 17 such that the mixture is repeatedly subjected to shearing and impacting forces by the effect of the blades 29 of the counter-rotating rotors. The blades 29 provide collision surfaces for impacting. The zone 6 of high shearing forces is thus generated in the space along which the blades move upon the rotation of the rotors. The extent of the zone 6 of high shearing forces is shown in Fig. 2A in the direction of the rotation axis 18 and in Fig. 2B in the radial direction. The biosludge and the acid or the mixture 3 thereof may thus be supplied through the rotors 16, 17 such that they are repeatedly subjected to shear forces by the effect of the blades 29, the effect of the blades 29 thereby forming the zone 6 of high shear forces and breaking the cellular structures of the biosludge in the mixture at least partially.

Further, the rotary movement of the blades 29 produce narrow gaps 28 between the blades 29, in which the mixture is subjected to shear forces. As each pair of counter-rotating rims, i.e. 19 and 20; 21 and 22; and 23 and 24, of blades generates a number of the narrow gaps 28 and correspondingly reversals of impact directions during each complete rotation of a pair of rims, the direction of the impacts caused by the blades 29 changes at a high frequency.

The mixture is therefore repeatedly impacted by and subjected to the shearing forces caused by the blades 29. These impacting forces and shearing forces therefore cause the breaking of cellular structures of the biosludge at least to some extent. The mixture 3 fed into the zone 6 of high shear forces thus can occupy a volume within the rims, i.e. the space along which the blades 29 move upon the rotation of the rotots and which is not occupied by the blades or other parts of the rotors.

Various parameters such as the number of the rotors and rims, the number and/or density of the blades in each rim and/or rotor, the geometry (e.g. angles) of the blades and/or the rotation speeds of the rotors may be used to affect the shear forces and the energy intensity to which the mixture is subj ected .

The mixture may be subjected to an energy intensity in the zone 6 of high shear forces that is sufficient to cause the breaking of the cellular structures at least to some extent. However, the energy intensity may be such that there is no significant gelling of the mixture or formation of a gel-like slurry.

The energy intensity in the zone 6 of high shear forces may be calculated or estimated by dividing the input power of the high shear mixer by the volume of the mixture 3 fed into the zone 6 of high shear forces. An example of the calculation of the energy intensity is described in Example 4.

The high shear mixer may be operated at an energy intensity of at least 200 kWh/m ³ , or at least 300 kWh/m ³ , or at most 500 kWh/m ³ , or at most 650 kWh/m ³ or 300 - 500 kWh/m ³ for breaking the cellular structures. The high shear mixer 7 may be operated at a radial velocity of at least 13 m/s, or at most 18 m/s, or about 13 to 18 m/s of the outermost rim for breaking the cellular structures.

The residence time of the biosludge and the acid in the zone 6 of high shear forces may be about 0.01 to 60 seconds, or longer, if desired. The type of high shear mixer described in these Figs, may however be quite efficient, and residence times of a few seconds may in many cases suffice. Example 1

Biosludge from the waste water treatment of a chemical pulping mill containing approximately 3-4 mg/kg DS cadmium was obtained. The initial pH of the biosludge was 7.0. Test samples of the biosludge were treated by acidifying them with H ₂ SO ₄ or HN0 ₃ . The pH values in acid treated test samples of the biosludge were approx. 1.6-1.9. The flocculant used in some of the test points was a charged polymer flocculant.

The the biosludge was warmed to a temperature of approx. 28 to 38°C.

The acidified biosludge samples were treated using an Atrex CD 500 G45 high shear mixer having a 45 kW drive motor capable of a maximum rotation speed of 1500 rpm. The rotors had a total diameter of 500 mm and a total of 6 rims. After the Atrex treatment, the solid phase was separated from the mixture using a de- canter centrifuge GEA UCD 205 having a maximum rota tion speed of 5600 rpm, comparative capacity of 5000 1/h, a bowl diameter of 200 mm and a L/D-ratio of 4.0.

The trial record and cadmium results are pre sented in Table 1. The amount of biosludge in the sam ples was 1028 kg for samples 7A & 7B, 955 kg for sam ple 8, 1030 kg for sample 9 and 1020 kg for samples

10A & 10B .

Neither the Atrex treatment nor the nitric acid treatment alone reduced the content of the cadmi um in cake, but together these treatments decreased the cadmium content .

Example 2

In a second trial, sulphuric acid and nitric acid were compared. Two pH levels were also tested. A decanter centrifuge and a horizontal Larox pressure filter were tested for the separation of the solid phase in some of the trial points.

K ₂ HPO ₄ was included in the trial for testing whether the addition of salts would improve the cadmi um removal efficiency in acid solution. The trial plan is presented in Table 2.

Table 2. Trial plan.

The process parameters of the decanter centrifuge were the same in all trial points. The rotation speed of the centrifuge was 5600, the rotation speed difference 10 rpm and the frequency of the feed pump was 10 Hz. The trial point 1 was a reference point, trial points 2-13 were actual trial points, and trial point 14 was repetition of the trial point 2.

The results of the trial are presented in Table 3. The initial cadmium content of the biosludge was 3.6 mg/kg DS and the dry matter 4.6 %.

Table 3. Results and certain process parameters of the trial.

Some test point were also pressed with the horizontal Larox pressure filter.

The results from the pressure filtration are presented in table 4. The comparison of the test points is not complete, because they are only a part of the trial.

In certain trial points, the cadmium content was below the targeted limit value of the Finnish leg islation concerning fertilizers.

Example 3

The aim of this trial was to optimize the pa rameters relating to the decanter centrifuge based on the results of the previous trial and to test horizon tal and vertical Larox pressure filters.

The trial plan of the fourth trial is pre sented in table 5. Trial point one was the reference point, the biosludge was pressed without any treat ment. The initial pH was 7.21 and the dry matter was 5.2 w-%. The biosludge was taken from the biosludge thickener of the waste water treatment plant of a chemical pulping plant. All of the test points were performed in both Larox pressure filters (horizontal and vertical) .

Some additional trial points were performed. Atrex treatments 600 rpm and 800 rpm were compared. In this trial, pH 2 was tested because it could decrease acid consumption and therefore be more economical. A charged polymer flocculant was also tested. Reference point (trial point 1), shorter pressing times (trial points 14B and 14C) , increased Atrex speed, 1100 rpm (trial point 15) and different flocculant dosing (tri al point 16) were performed outside the matrix.

Two different feed speeds of the biosludge were tested as part of the trial plan. The used pres sure rises are presented in the table 6.

All the produced cakes were processed so that the composition of the sample would be as constant as possible. The cadmium contents and dry matters of the cakes and filtrates of the vertical and horizontal Larox pressures filters are presented in table 7.

The initial cadmium content of the biosludge was 4.26 mg/kg ds and the dry matter was 4.8 %. In table 8, the percentage amounts of cadmium remaining in the cake after vertical Larox pressure filter are pre- sented.

The warming of the biosludge improved the cadmium removal. The lower the pH was, the lower the cadmium content of the cake. The use of the decanter centrifuge was further tested. The used and measured process parameters are presented in table 9. Concentrated sulphuric acid was used to adjust the pH, mixing time was one day and the temperature of the biosludge was increased up to 45.5 °C . A container mixer was used. The used dam plate in centrifuge was 120. The trial procedure was similar to those in the previous Examples.

Table 9. Plan for the decanter centrifuge points of the trial and measured values of pH and tem- perature.

Table 10. Decanter centrifuge results

To summarize, the cadmium content of the bi- osludge could be decreased by the use of a decanter centrifuge .

Example 4 A further trial was run by treating biosludge in an Atrex mixer at temperatures of approx. 20 °C and approx. 50°C with a trial design similar to the previous Examples.

After pressure filtration, a higher dry solids content could be obtained with mixtures that had been Atrex treated.

Heating the mixture to a higher temperature appeared to increase the dry solids content after pressure filtration.

The biosludge and the acid were treated using an Atrex CD 500 G45 high shear mixer having a 45 kW drive motor capable of a maximum rotation speed of 1500 rpm. The rotors had a total diameter of 500 mm and a total of 6 rims (two counter rotating rotors, both rotors comprising three rims of blades) ; the structure of the high shear mixer and the rotors was similar to the mixer described in connection with

Figs. 2A and 2B.

The energy intensity to which the mixture was subjected was estimated on the basis of the total calculated volume of the rims, which corresponds to the volume occupied by the mixture fed into the zone of high shear forces, and of the input power at each rotation speed as shown in Tables 11 and 12. The energy intensity was calculated by dividing the input power by the total calculated volume of the rims.

Table 11. Calculation of the total volume of the rims of the Atrex mixer.

Energy intensity was calculated by dividing the input power at each RPM by the total volume of the rims calculated in Table 11.

Table 12. Calculation of energy intensities, rim radial velocities at outermost rim and frequencies of the Atrex mixer at different rpm values.

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways. The embodiments are thus not limited to the examples described above; instead they may vary within the scope of the claims.

The embodiments described hereinbefore may be used in any combination with each other. Several of the embodiments may be combined together to form a further embodiment. A product, a system, a method, or a use, disclosed herein, may comprise at least one of the embodiments described hereinbefore. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be understood that reference to 'an' item refers to one or more of those items. The term "comprising" is used in this specification to mean including the feature (s) or act(s) followed thereafter, without excluding the presence of one or more additional features or acts.