A slow release fertilizer

Abstract

The invention pertains to a method for converting an aqueous stream comprising soluble nitrogen and / or potassium into a solid slow-release fertilizer product, comprising the steps of concentrating the aqueous stream comprising soluble nitrogen and / or potassium, such that at >80% but <95% of the water is removed, to obtain a recovered water fraction and a concentrated aqueous fraction comprising the soluble nitrogen and / or potassium; and absorbing said concentrated aqueous fraction in an organic absorbent material to obtain an organic absorbent loaded with the nitrogen and / or potassium from the concentrated aqueous fraction, thereby obtaining a water fraction and a solid slow-release fertilizer comprising said organic absorbent loaded with the nitrogen and / or potassium from the concentrated aqueous fraction. Also provided is the slow-release fertilizer and a method for soil improvement.

Description

[0001] A SLOW RELSEASE FERTILIZER

[0002] Field of the Invention

[0003] The present invention relates to the production of slow-release fertilizers derived from aqueous streams containing soluble nitrogen and / or potassium compounds. Specifically, the invention encompasses a method for converting such streams, for example farm liquid animal manure into clean water and a nutrient-rich solid fertilizer product that allows for the slow release of essential nutrients over an extended period. It also pertains to the slow-release fertilizer product and methods of use thereof.

[0004] Background of the Invention

[0005] Manure from animal farms is an increasingly pressing environmental issue due to its significant pollution and regrowth of recipients as a result of nutrient runoff, but also due to its contribution to greenhouse gas emissions. Recent studies have highlighted that the scale and intensity of modem cattle farming produces far more manure than local ecosystems can absorb, leading to overloaded soils and contaminated waterways.

[0006] The raising of farm animals gives rise to feces and urine, most notably so for cattle and hogs, but applicable also for other animals. Sometimes the feces and urine are mixed in the handling system at the farm and sometimes they are kept apart as solid and liquid manure. Manure in general contains a lot of nutrients valuable as fertilizers. However, whereas the solid manure has a high content of nutrients economically allowing it to be transported long distances for its use as a fertilizer, the opposite goes for the liquid manure. Liquid manure contains a lot of water, meaning it is not economically sustainable to transport the liquid manure any long distances. The obvious solution - spreading the liquid manure on the nearby fields - could work in small scale where the number of animals is low in relation to the surrounding land. However, on industrial animal farms, where large numbers of animals are stocked at one place, the surrounding land is often simply insufficient to absorb the manure.

[0007] Liquid manure, once applied on a field, easily runs off the field and into trenches and streams and into recipients. This excess can run off into nearby water bodies, resulting in eutrophication - a process where harmful algal blooms form, deplete oxygen, and devastate aquatic ecosystems. Concentrating liquid manure through evaporation in ponds would reduce the volume. However, large evaporation ponds pose severe risks in relation to occupational health and safety, as workers exposed to these areas may face life-threatening hazards if they accidentally fall into the liquid. Moreover, these ponds can emit corrosive and unpleasant odors as well as greenhouse gases, such as ammonia and methane, contributing to air pollution and climate change. The process is also highly weatherdependent, functioning effectively only in warm and dry climates, which limits its applicability in cooler or wetter regions. Also, if the concentrated liquid manure is applied to agricultural land, the highly soluble nature of the concentrated liquid manure can lead to rapid nutrient run-off. This means that instead of being absorbed by crops, a significant portion of the nutrients can be washed away into nearby water bodies, such as rivers, lakes, or groundwater systems. Thus, while the concentrated form addresses the logistical problem of economically sustainable transportation, its environmental impact during use remains a critical issue.

[0008] Beyond animal agriculture, various industrial processes also generate aqueous waste streams rich in highly soluble nitrogen and / or potassium compounds. For example, mining operations can produce water contaminated with nitrogen from the use of ammonium nitrate-based explosives. Similarly, chemical manufacturing, such as the production of tartaric acid from wine residues or potassium chlorate for matches, can result in effluents with significant concentrations of potassium. These streams, like liquid manure, pose environmental risks if discharged directly and represent a lost resource.

[0009] Thus, there is a need for new methods and strategies for handling aqueous streams containing soluble nitrogen and / or potassium, such as liquid manure from large animal farms, in a sustainable manner both ecologically, socially and economically. Waste management is complex and a contentious issue for livestock farming operations.

[0010] Summary of the Invention

[0011] Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above mentioned problems by providing a method for converting an aqueous stream containing soluble nitrogen and / or potassium into a solid nitrogen-rich slow-release fertilizer product, comprising the steps of concentrating said aqueous stream, such that at >80 % but <95 % of the water is removed, to obtain a recovered water fraction and a concentrated aqueous fraction comprising the soluble nitrogen and / or potassium; and absorbing said concentrated aqueous fraction in an organic absorbent material to obtain an organic absorbent loaded with said concentrated aqueous fraction, thereby obtaining a water fraction and a solid slow-release fertilizer comprising said organic absorbent loaded with the nitrogen and / or potassium from the concentrated aqueous fraction.

[0012] In one aspect, the aqueous stream is farm animal liquid manure, or a digestate effluent thereof.

[0013] In one further aspect, the farm animal liquid manure is liquid manure from cattle, swine, sheep, goat, bird, horse, and / or buffalo or any mixture thereof.

[0014] In one aspect, the farm animal liquid manure is from cattle or from swine.

[0015] In one aspect, the aqueous stream is an industrial effluent, such as from mining operations or chemical production.

[0016] In one aspect, the aqueous stream has an initial nitrogen and / or potassium concentration of 0.1-5 wt.-%, and more typically 0.3-3 wt.-%.

[0017] In one aspect, during the concentrating step, >85 % but <90 % of the water is removed.

[0018] In one aspect, the method further comprises a pretreatment step of removing any suspended solids from the aqueous stream.

[0019] In one further aspect, the pretreatment step comprises fdtration, sedimentation, or electrocoagulation (EC).

[0020] In one aspect, the aqueous stream is concentrated using an evaporation or an osmosis technique, such as reverse osmosis (RO) or forward osmosis (FO), or a combination thereof.

[0021] In one aspect, the recovered water fraction is used for cleaning purposes and / or for the irrigation of crops.

[0022] In one aspect, the absorption step is >1 minute, such as >20 minutes, such as >1 hour, but <24 hours, preferably between 1 minute to 2 hours.

[0023] In one aspect, during the absorption step, the concentrated aqueous fraction is added to the organic absorbent material until the organic absorbent material is saturated.

[0024] In one aspect, the concentrated aqueous fraction is absorbed by the organic absorbent material at a weight ratio of 0.35: 1 to 1 :0.35, preferably from 1 : 1 to 1 :0.5 of concentrated aqueous fraction to organic absorbent material.

[0025] In one aspect, the resulting slow-release fertilizer has a nitrogen content of 0.5% to 20% by weight. In one aspect, the absorption step further comprises mechanically stirring or mixing the concentrated aqueous fraction with the organic absorbent material.

[0026] In one aspect, a soluble potassium salt or nitrogen salt is dissolved in the concentrated aqueous fraction before said fraction is absorbed in the organic absorbent material.

[0027] In one aspect, the organic absorbent material is selected from lignin-based absorbents, organic fiber-based absorbents, or cellulose-based absorbents.

[0028] In one aspect, the organic absorbent material is selected from: kraft lignin; lignosulfonate; chaff (such as chopped straw, rice husks, or pellets thereof); humus; peat; sawdust; compressed wood pellets; paper pulp (such as fluff pulp, chemical pulps, mechanical pulps, or pulp rejects); bagasse pulp; gels made of pulp (e.g. microcrystalline cellulose); organic litter; hydrogels; biochar; or composted organic matter.

[0029] In one aspect, the absorbent material is kraft lignin or lignosulfonate.

[0030] In one aspect, the slow-release fertilizer comprises both biochar and nutrients.

[0031] In one aspect, the resulting slow-release fertilizer is for use as a soil fertilizer, including application to agricultural soil, or for incorporation into formulations comprising other agricultural products.

[0032] Further is provided a slow-release fertilizer, obtainable by the method.

[0033] In one aspect, the slow-release fertilizer having further been dried, pelletized, coated, sterilized, pH adjusted or nutrient enriched.

[0034] In one aspect, the slow-release fertilizer further being mixed with other fertilizers or lime.

[0035] In one aspect, the slow-release fertilizer further being mixed with gypsum.

[0036] Also provided is a method for soil improvement, wherein the slow-release fertilizer is added to top soil.

[0037] Also provided is a method for simultaneously providing slow-release nutrients to soil and enhancing the water retention capacity of said soil, comprising the step of applying the slow-release fertilizer to said soil.

[0038] Brief Description of the Drawings

[0039] These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which: Fig. l is a figure showing an absorption series of bentonite clay, chopped dried grass (hemp) and paper pulp after overnight absorption of a biologic dye;

[0040] Fig. 2 is a figure showing the absorption series of viscous biologic dye absorbed in bentonite clay, chopped dried grass (hemp) and paper pulp after a first extraction;

[0041] Fig. 3 is a figure showing a control sample of diluted (1 / 10) viscous biological staining agent and the first filtrate from the absorption series of viscous biologic dye absorbed in bentonite clay, chopped dried grass (hemp) and paper pulp, respectively;

[0042] Fig. 4 is a figure showing the result from a similar experiment as shown in figure 1 to 3, but for a non-viscous biologic dye, showing a control sample of diluted (1 / 10) non-viscous biologic dye and the first filtrate from the absorption series of non- viscous biologic dye absorbed in bentonite clay, chopped dried grass (hemp) and paper pulp, respectively;

[0043] Fig. 5 is a figure showing the result from a slow-release experiment for a viscous biologic staining agent, from left to right: filtrates from bentonite, hemp, and paper pulp. From the top of the image downward: first, second, and third filtrates after the addition of 1 dl of water at a time;

[0044] Fig. 6 is a figure showing the result from a lignin absorbance experiment showing (from top left to lower right) pure lignin powder, lignin powder mixed with water, lignin powder mixed with raw urine, lignin powder mixed with urine concentrated to 80%, and lignin powder mixed with urine concentrated to 90%;

[0045] Fig. 7 is a figure showing the result from a lignin absorbance experiment of figure 6 after two days of drying;

[0046] Fig. 8 shows a schematic representation of a method for the manufacture of a solid slow release fertilizer product from an aqueous stream comprising soluble nitrogen and / or potassium according to one embodiment; and

[0047] Fig. 9 shows a schematic representation of a method for the manufacture of a solid nitrogen rich slow release fertilizer product from farm animal liquid manure using a ligning absorbentaccording to one embodiment.

[0048] Description of embodiments

[0049] The following description focuses on an embodiment of the present invention applicable to a method for the manufacture of a solid nitrogen and / or potassium rich slow-release fertilizer product, as well as of purified water, from an aqueous stream containing soluble nitrogen and / or potassium. Tn a preferred embodiment, said aqueous stream is farm animal liquid manure.

[0050] A significant problem with using aqueous streams rich in soluble nutrients, such as liquid manure, such as cattle or hog urine, as fertilizer is that a large portion of its nutrients is often lost through irrigation or washed out of the soil during rainfall. This runoff can carry excessive amounts of nitrogen and other nutrients into nearby water bodies, leading to eutrophication. The nutrient overload in aquatic systems causes harmful algal blooms, which deplete oxygen levels and severely damage aquatic life. As a result, while such streams, such as liquid manure, is rich in nutrients beneficial for crops, improper management and timing of their application can contribute to serious environmental damage.

[0051] Since many producers of such streams, for example animal farms, are not users of fertilizer, transport of the nutrient-rich aqueous stream from the source to crop fields or processing facilities is often required.

[0052] An aqueous stream like liquid manure can be concentrated before use as a fertilizer, since this would result in less volume for transportation. Urine consists mostly of water, but otherwise also of urea and other organic nitrogen compounds and salts of various kinds. Depending on how the farm animals are kept, it is also conceivable that drug residues are found in the urine or suspended material that comes from the handling of the urine rather than from the pigs as such. One should also expect microbes to be present in the urine. A summary of the most common urine constituents is given in Table 1.

[0053] Table 1.

[0054] However, since urine is high in urea (which degrades into ammonia), concentrated amounts could lead to ammonia toxicity, which can bum plant roots and foliage, reducing plant growth and even killing the plants.

[0055] It is a well-known fact that a dilute aqueous stream like manure from economic points of view cannot be transported far away. Since application on the nearby lands will cause run-off detrimental for the recipient, one needs to come up with another strategy. Dilution of the aqueous stream presents itself as one option, however, calling for large amounts of water - which are not always there, and which regardless comes at a cost. Moreover, the amount of liquid is not affected by the dilution scheme risking to lead just to a build-up of the aqueous stream at the source over time, thus not solving the problem of too much volume.

[0056] The other option presenting itself is instead to dry the aqueous stream, such has liquid manure, e.g. by letting it stay in the open air to make the sun dry the water away over time - at least when the location is in a hot area. Here the obstacle is that such a regime calls for big ponds, ponds that also present a hazard to workers and animals to accidentally fall into if they should be open to air. Moreover, the end product being crystalline nutrients (such as crystalline liquid manure), on the one hand now may be economic to transport long distance, but still is as prone as ever rapidly to dissolve during rain and then still run off into the recipient. The solution to the problem of how to get rid of big volumes of liquid manure in a sustainable way thus must be another one. One could consider diluting the concentrated stream before use as a fertilizer, however, since both diluted and concentrated stream can lead to nutrient runoff, particularly of nitrogen and phosphorus, this would not solve the problem.

[0057] It was realized that it would be more beneficial if the contained nutrients (such as liquid manure) could be applied as fertilizer in smaller, more frequent doses rather than a single large application. This method would allow crops to absorb nutrients gradually, minimizing nutrient runoff and leaching, and thus reducing the risk of eutrophication through nutrient leakage, and prevent concentrations that are toxic. However, such an approach would require increased labour, specialized equipment, and more time, increased fuel consumption and emissions, making it less sustainable from both an economic and ecological perspective. It might also be impractical to distribute the fertilizer for certain crops while they are growing.

[0058] In the invention, it was realized that many of these problems could be solved if the liquid stream (such as animal liquid manure) could be treated in such a way as to produce a concentrated slow-release fertilizer product. This way, the fertilizer could slowly be released into the soil, minimizing risks of nutrient run-off.

[0059] One possibility that was considered was to transform the nutrients in the aqueous stream (such as liquid manure) into struvite crystals. Struvite is a well-known slow-release fertilizer component by being a salt with low solubility in water. It includes both nitrogen in the form of ammonium and phosphorus in the form of phosphate. However, in order to transfer or make a full conversion of the nitrogen content in the aqueous stream (such as liquid manure) into struvite, all the nitrogen must be present in ammonium form - not in urea as is the case for the lion’s share of the nitrogen in the manure. Conversion is possible, e.g. by employing an anaerobic digestion unit, which, then calls for big investments - and, not least importantly, a significant space for the unit itself. The latter could be troublesome in densely populated areas. Moreover, the magnesium does not have any positive effect as such, but is only there in order for the ammonium to precipitate (as struvite). There is a fixed ratio between nitrogen and phosphorus in struvite, which is not necessarily the same ratio between these two important fertilizer components as would be desired on a specific field. Finally, precipitating ammonium as struvite calls for purchase of magnesium and phosphate salts, handling of these and conducting the precipitation - and the separation, rapidly turning the liquid manure into an advanced chemical processing plant.

[0060] Therefore, although struvite crystals indeed do possess a desirable slow-release functionality (due to low solubility in water), the precipitation of struvite does not present an economical solution for handling large volumes of aqueous stream (such as liquid manure) in a smooth fashion, since it both requires large (possibly non-local) infrastructure investments and use of make-up chemicals.

[0061] In the invention, it was thus found a method for the manufacture of a solid nitrogen and / or potassium rich slow-release fertilizer product from an aqueous stream comprising soluble nitrogen and / or potassium (such as animal liquid manure). First, any suspended solids from the aqueous stream may be removed during a pre-treatment. This is followed by concentrating the aqueous stream such that a large portion (at least >80 %) of the water is removed - but equally important - neither such that all of the water is removed (<95 %). Doing so, a recovered water fraction and a concentrated aqueous fraction comprising the soluble nitrogen and / or potassium are obtained. It was found that the concentrated aqueous fraction could be absorbed in an organic absorbent material to obtain an organic absorbent loaded with the concentrated aqueous fraction. This procedure thus provides two useful products: a water fraction and a solid nitrogen and / or potassium rich slow release fertilizer product made up of an organic absorbent loaded with the nitrogen and / or potassium from the concentrated aqueous fraction.

[0062] It was found that by absorbing the concentrated aqueous fraction (such as liquid manure) in an organic absorbent, one does not only reap the benefits of low volume and easier transportation. Further, the production of clean water from the aqueous stream - instead of just letting it evaporate - is particularly relevant for hot areas suffering from permanent or temporary water shortages. It was also found to solve several of the problems of using a concentrated nutrient stream as a fertilizer, such as nutrient runoff.

[0063] In a preferred embodiment, it was found that the method can be used for different kinds of farm animal liquid manure, such as liquid manure from cattle, swine, sheep, goat, bird, horse, and / or buffalo or a mixture thereof. The liquid manure may be from cattle or from swine, such as swine urine.

[0064] Each type of manure has its unique properties, and the choice of liquid manure for specific applications depends on the desired nutrient profile, ease of handling, and separation requirements. For instance, swine is relatively easy to separate into liquid and solid fractions, due to high water and low fiber content, and simple mechanical separation methods, such as screens, can be used to separate the urine fraction (liquid) from solids. For manure that has higher fiber content, such as horse or bovine manure, it may require further steps, such as pressing, to separate out a liquid fraction.

[0065] The method also works for a digestate effluent of the liquid manure. When animal manure (including urine) is treated in an anaerobic digestion plant, three main phases typically result: gas phase, solid phase, and liquid phase. The liquid phase, often known as "liquid digestate" or "liquid phase of digestate", is a nutrient-rich liquid containing dissolved nutrients like nitrogen (often as ammonium), potassium, and dissolved organic compounds.

[0066] Thus, farm animal liquid manure, or a digestate effluent thereof, may be used in the method. Further, the digestate effluent may be the liquid phase from farm animal liquid manure treated in an anaerobic digestion plant.

[0067] More broadly, the method is applicable to any aqueous stream containing soluble nitrogen and / or potassium compounds at a concentration that makes concentration and recovery economically viable. It is estimated that an initial nitrogen concentration in the range of 0.1-5 wt.-%, and more typically 0.3-3 wt.-%, is suitable.

[0068] Examples of such streams, in addition to animal manure, include industrial effluents such as mine water, or waste streams from chemical production involving tartaric acid or potassium chlorate. For industrial streams contaminated with transition metal ions, the pre-treatment step using electrocoagulation (EC) is particularly advantageous for removing such contaminants prior to the concentration step.

[0069] Depending on the origin exact type, origin or state of the aqueous stream containing soluble nitrogen and / or potassium compounds, it may be beneficial to have a pre-treatment to remove any suspended solids from the aqueous stream.

[0070] Such a pre-treatment may for instance use filtering, sedimentation, and / or electrocoagulation (EC) techniques.

[0071] Different techniques have different advantages, where filtration and sedimentation are simple and cheap. If the aqueous stream (such as animal liquid manure) has a high content of transition metal ions (for instance from the animal feed), it may be suitable to use electrocoagulation (EC) techniques, since this will remove suspended solids, phosphorus and transition metals. A large part of any microbes will also be removed. However, if no major transition metal content exists in aqueous stream, it may not be economically viable to do such a pre-treatment. However, since large amounts of pathogens are removed, an EC pre-treatment also results in a cleaner end product (fertilizer), which may be preferred and economically viable.

[0072] In the method, the aqueous stream (such as animal liquid manure) is concentrated, resulting in a recovered water fraction and a concentrated concentrated aqueous fraction comprising the soluble nitrogen and / or potassium (such as a concentrated liquid manure fraction). Two cost-effective methods for this are evaporation and osmosis techniques.

[0073] Examples of evaporation techniques are rotary evaporation techniques, low pressure evaporation, thermal evaporation, membrane distillation, ultrafiltration or multi-effect distillation. For osmosis techniques, examples are reverse osmosis (RO), forward osmosis (FO) or a combination of these. Reverse osmosis often has an energy consumption of about 3-10 kWh / m3, and results in a clean aqueous phase and a concentrate. If the aqueous stream is relatively clean (for instance after pre-treatment), RO is likely a preferred method.

[0074] Evaporation, usually exhibits an energy consumption of >10 kWh / m3and results in a clean water fraction and a concentrate. The evaporation may be advantageous, if the aqueous stream is dirty still, such as if there are still a large amount of solids.

[0075] It was found that preferred to remove >80 % of the water. However, if economically viable, the removal of water may be >85 %, such as >90 %, but <95 % in order for the fertilizer product to be as concentrated in nutrients as possible while still remaining a liquid fraction. However, eventually the viscosity will increase, and salts may also start to precipitate, why a water removal of between >80 % to <95% seems to be preferred.

[0076] Another advantage is that the recovered water does not require transportation, since it can be used on site, for instance for cleaning purposes and / or for irrigation of crops.

[0077] In the invention, the concentrated aqueous fraction comprising the soluble nitrogen and / or potassium is then absorbed into an absorbent rendering a solid slow- release fertilizer product.

[0078] In the absorption step, the concentrated aqueous fraction comprising the soluble nitrogen and / or potassium is added to the absorbent. The absorption may be made more even and / or speeded up by mechanical stirring or mixing of the absorbent and concentrated aqueous fraction mixture.

[0079] Depending on the absorbent material used and the degree of concentration of the concentrated aqueous fraction, the ratio of concentrated aqueous fraction (such as concentrated animal liquid manure) to absorbent may vary. The ratio will likely be between 10: 1 to 1 : 10, such as between 5: 1 to 1 :5.

[0080] One may use small test batches to find the exact ratio for optimal saturation. Testing with incremental additions of concentrate is usually the best way to determine the exact liquid-to-absorbent ratio for your specific materials.

[0081] The concentrated aqueous fraction comprising the soluble nitrogen and / or potassium (such as concentrated liquid manure fraction) may be added to the absorbent material until it is saturated. One simple way to accomplish this is by adding batches of concentrated aqueous fraction to the absorbent until it becomes clear that it is no longer being absorbed.

[0082] The absorption is generally a relatively quick process. Static absorption without stirring generally takes from several minutes to hours, depending on the absorbent’s porosity and the concentrate's viscosity. With stirring, the absorption is generally even quicker. If one would use pressure or vacuum absorption, the process is usually just a matter of minutes. However, one can also let the absorption take place for a longer time, such as overnight.

[0083] Thus, the absorption step may be > 1 minute, such as at least 20 minutes, such as at least 1 hour, but <24 hours, preferably between 1 minute to 2 hours.

[0084] Conventional techniques to recover the concentrated water stream (such as concentrated liquid manure), could be used to create a fertilizer directly, or processing of the aqueous fraction (such as liquid manure) in a biogas plant. However, none of these methods would solve the problems of how to get rid of the nitrogen rich fraction in a responsible manner.

[0085] Instead, in the method, the concentrated aqueous fraction (such as concentrated animal liquid manure) is absorbed in an absorbent to obtain an absorbent loaded with the nitrogen and / or potassium from the concentrated aqueous fraction. This is a very simple and robust method that requires very little investment. Further, it results in a product that was found to be suitable for use as a slow-release fertilizer product.

[0086] It was found that the absorbent material can be several different kinds of absorbent materials, as long as these are in themselves not toxic to the soil. It is important that the absorbent material is organic since it will be included in the slow-release fertilizer (once soaked with the concentrated aqueous fraction) then in itself also contributing to biochar to the soil where it is spread rather than building up the mineral content. Given that the costs both of the organic absorbent as such and for its transportation - for instance to a farm as a raw material and from the farm in the form of the final fertilizer product (=the organic absorbent loaded with concentrated liquid manure and potentially dried thereafter) - needs to be low, the organic absorbent should be sought for based on a number of criteria as follows:

[0087] Having a low cost, meaning alternative uses are few or not giving a high economic return, calling for some sort of by-product.

[0088] Having a low weight and at the same time a high weightweight absorption capacity, calling for an organic product rather than a mineral one. Being compact making its transportation cheap, for instance to farm or it may be a residual from the farm’s own produce or locally sourced.

[0089] This all means that what is the most suitable organic absorbent could differ from different areas around the globe.

[0090] One particularly interesting organic absorbent presents itself in the form of kraft lignin or lignosulfonate. This is for several reasons: The cost is low. The production is large scale and could potentially be carried out on all inhabited continents. The product is rich in sulfur, itself a desirable fertilizer. The product is also compact, being a powder thus not calling for milling or other mechanical operations to be ready for use. Furthermore, kraft lignin demonstrates excellent water retention capabilities, as seen in figures 6 and 7, contributing to prolonged soil moisture and reduced irrigation needs, particularly in dry or arid regions. Its particulate nature also facilitates even distribution when applied as a fertilizer.

[0091] Other materials of general interest none-the-less include chaff (such as chopped straw, rice husks, etc., or pellets thereof) humus, peat, sawdust or compressed wood pellets, paper pulp of some kind (e.g. fluff pulp, chemical pulps, mechanical pulps, pulp rejects, bagasse pulp or gels made of pulp e.g. microcrystalline cellulose), cat litter (in the form of organic litter), hydrogels, biochar or composted organic matter.

[0092] To try the absorbent properties of different materials, three different classes of commercially available absorbents were tried with two different types of organic dyes. The different absorbents were:

[0093] Organic fiber-based absorbents are made from plant materials and have a high carbohydrate or lignin content, which allows them to absorb moisture. These fibers can trap and retain liquids within their cellular structure. Hemp is particularly absorbent due to its fibrous composition and ability to absorb water, making it useful in agricultural applications, animal bedding, and even insulation materials. Other examples of organic fiber-based absorbents are coconut coir, straw, wood chips.

[0094] One cellulose-Based Absorbent (paper pulp). Cellulose-based absorbents are derived from processed plant fibers, primarily cellulose, which is highly hydrophilic and can absorb large amounts of water. Paper pulp, which consists of processed wood, pulping rejects or recycled paper, is a good example, as it absorbs water and swells, making it ideal for applications like hygiene products, cleaning materials, and packaging. These absorbents are typically soft, lightweight, and used in less industrial applications compared to mineral absorbents. Other examples of cellulose-based absorbents are cotton, sawdust, recycled paper products. For comparison, one mineral absorbent (Bentonite clay) was also tested. Mineral absorbents are inorganic, often crystalline materials with high surface areas and layered structures that enable them to absorb liquids. The mineral adsorbents have Bentonite clay, specifically, is a highly absorbent material due to its ability to expand and trap water molecules within its structure. These absorbents are commonly used for heavy-duty applications, such as in oil spills, cat litter, or industrial waste management. Other examples of mineral absorbents are silica gel, zeolites, perlite. However, using an inorganic absorbent mitigates several of the advantages of using an organic absorbent.

[0095] For dyes, one viscous biologic dye and one less-viscous biological staining agent were used. The viscous biological staining agent was a food product (mushroom soy sauce), which contains pigments such as melanoidins formed through the Maillard reaction during fermentation. The staining agent was selected since it tended to be thicker and more viscous, resembling the concentrated urine. It was also preferred, since it is also quite salty and had a dark colour facilitating the visual analysis of the test results. The non-viscous biological staining agent was a food berry concentrate from black currant, which contains anthocyanins, naturally occurring pigments. The staining agent was more watery, and the pigments smaller than in the viscous staining agent but still also possessed a dark colour.

[0096] In Figure 1 is shown an absorption series of bentonite clay, chopped dried grass (hemp) and paper pulp after overnight absorption of a viscous biological staining agent. It was found that all three classes of material could absorb a large amount of the viscous biological staining agent. Further, while dry it does not release the staining agent. This means that it will not be toxic to the plants in the soil, compared to if urine concentrated urine was dispersed directly into the soil.

[0097] In Figure 2 is shown the absorption series of viscous biological staining agent absorbed in bentonite clay, chopped dried grass (hemp) and paper pulp after a first filtration. As seen in the figure, the absorbent starts to release the staining agent once wet. However, it does not release all the staining agent directly.

[0098] This is seen in Figure 3, where is shown a control sample of diluted (1 / 10) viscous biological staining agent and the first filtrate from the absorption series of viscous biological staining agent absorbed in bentonite clay, chopped dried grass (hemp) and paper pulp, respectively. As can be seen in the figure, the released staining agent is more diluted than the 1 / 10 diluted staining agent. This shows that the solid fertilizer will have slow release of the nutrients. This can thus be compared to the desire to apply the fertilizer in smaller, more frequent doses rather than a single large application. Also, it minimizes nutrient runoff and leaching, and thus reducing the risk of eutrophication through nutrient leakage, and since the concentrated urine will be diluted, it helps prevent concentrations that are toxic.

[0099] Finally, Figure 4 shows the result from a similar experiment as shown in Figures 1 to 3, but for a non-viscous biological staining agent, showing a control sample of diluted (1 / 10) non-viscous biological staining agent and the first filtrate from the absorption series of non-viscous biological staining agent absorbed in bentonite clay, chopped dried grass (hemp) and paper pulp, respectively.

[0100] Also, here was shown that the colouring agent is released in a slow release behaviour, where the concentration after a first filtration is lower than the 1 / 10 diluted control sample. The slow release is further shown in figure 5, where filtrates are taken by addition of water at three different timepoints from each same soaked absorbent. The figure clearly illustrates the slow-release function of the sorbed concentrated liquid manure, as both the second filtrate and the third filtrate still show a distinct soy color. Interestingly, the bentonite appears to release the color more quickly than the organic absorbents. The chopped hemp seems to exhibit the best slow-release functionality in this case. Given that it is also the lightest material, the result is particularly intriguing. Seeing the advantages of using organic absorbents, a sample series was also performed using lignin powder. In figure 6 is shown an absorption series, showing (from top left to lower right) pure kraft lignin powder, kraft lignin powder mixed with water, kraft lignin powder mixed with urine, kraft lignin powder mixed with urine concentrated to 80%, and kraft lignin powder mixed with urine concentrated to 90%.

[0101] It was found that kraft lignin effectively serves as an organic absorbent material. In the trials, it was found that a weight ratio of about between 0.5: 1 to 1 :0.5, such as between 0:75: 1 to 1 : 1 , between kraft lignin and liquid resulted in a good consistency (however, for other organic absorbents, the absorption may differ somewhat, such that an organic absorbent to liquid ratio of 0.35: 1 to 1 :0.35 may be acceptable). In figure 6, 0.8 grams of kraft lignin, occupying a volume of approximately 2.5 ml, absorbed around 1 gram of liquid, consistently yielding a manageable, lumpy consistency, for various liquid compositions, from water to concentrated urine, indicating the material's robustness in handling diverse concentrations of liquid manure fractions. This can be seen in figures 6 and 7.

[0102] Thus, the concentrated aqueous fraction (such as concentrated liquid manure) fraction may be absorbed by the organic absorbent material at a weight ratio of 0.35: 1 to 1 :0.35, such as from 0.5: 1 to 1 :0.5, preferably from 1 : 1 to 1 :0.75 of concentrated liquid manure fraction to organic absorbent material.

[0103] Trying larger volumes, it was found that the absorption step may benefit from mechanically stirring or mixing the concentrated aqueous fraction with the organic absorbent material.

[0104] Based on experimental data, the final slow-release fertilizer product, after absorption but before any final drying step, may comprise a nitrogen and / or potassium content in the range of 0.5-20% by weight, more typically 1 - 10% by weight, and even more typically 2-6% by weight.

[0105] Thus, the slow-release fertilizer may have a nitrogen and / or potassium content of 0.5% to 20% by weight, such as 1-10% by weight, such as 2-6% by weight.

[0106] It was observed that when using highly concentrated liquids, such as urine evaporated to 1 / 1 Oth of its original volume, the resulting mixture with lignin became notably stickier, indicating a practical trade-off between nutrient concentration and the physical handling properties of the final product, whereby it was found favorable to remove >80 % but <95 % of the water, such as >85% to 90%.

[0107] An additional effect of the use of the slow-release fertilizer as a soil improvement is the water retention capacity of the absorbent over time. Initially the absorbent is loaded with the concentrated liquid manure when put into the soil. When water is spread on the land either in the form of rain or in the form of active water supply for irrigation the absorbent will swell and then begin slowly to release the nitrogen and / or potassium from the concentrated aqueous fraction (such as concentrated liquid manure) by means of both dissolution and diffusion. However, the absorbent will keep the soil moist for longer as the water will not merely penetrate through the soil but actually being kept in the absorbent. This aspect of the slow-release fertilizer and water retention absorbent is of particular importance in dry areas or areas with irregular rain. It was found that relatively small amounts of the slow release fertilizer, particularly when it is based on a powder-formed lignin absorbent, were required per square meter of soil.

[0108] This dual-functionality, providing both slow-release nutrients and long-term water retention, represents a significant advantage of the invention. The organic absorbent acts as both a carrier for the nutrients and a sponge for water. When applied to soil, the absorbent matrix physically hinders the rapid dissolution of the highly soluble nutrients from the concentrated aqueous fraction, ensuring they are released gradually. Simultaneously, this same matrix absorbs and holds ambient moisture from rain or irrigation. This retained water not only supports plant growth during dry spells but also facilitates the gradual diffusion and release of the entrapped nutrients from the absorbent into the surrounding soil, creating a synergistic effect where water retention and nutrient release are intrinsically linked.

[0109] Thus, it was found that the bulk volume of the slow release fertilizer could be enlarged, while at the same time further improving the water retention, by mixing the slow release fertilizer with gypsum. This not only simplified distribution of the fertilizer - It also improved the water binding capacity providing further water retention as well as providing sulfur as a fertilizer and calcium to improve soil structure. The mix might also help prevent clogging during distribution.

[0110] It was also found that potassium salt could be dissolved in the concentrated aqueous fraction, such as concentrated liquid manure, before the drying or absorption step. Similarly, in case of a potassium rich concentrated aqueous fraction, a nitrogen salt can be dissolved in in the concentrated aqueous fraction before the drying or absorption step. This would turn the final slow release fertilizer product into an NK fertilizer instead of merely an N or K fertilizer.

[0111] Thus, the organic absorbent material may be selected from lignin-based absorbents, organic fiber-based absorbents, or cellulose-based absorbents.

[0112] By using an organic absorbent material, biochar is added to the soil together with the delivery of the nutrients.

[0113] Preferably, the organic absorbent material is renewable, whereby carbon is sustainably added to the soil, contributing directly to enhanced soil health and crucial carbon sequestration efforts. The advantages of this sustainable carbon addition are manifold and include improved soil structure through enhanced soil aggregation leading to better aeration and reduced compaction; enhanced water retention due to the improved soil structure and inherent porosity of organic materials; increased nutrient retention by binding nutrients and preventing leaching; boosted microbial activity by providing a vital energy source and habitat for beneficial soil microorganisms; and climate change mitigation through carbon sequestration in the soil. Thus, the invention provides a multi-functional product that not only fertilizes but also acts as a long-term soil improver and a tool for environmental sustainability.

[0114] It was found that some different absorbents have different advantages, such as chaff (husks or dry, scaly protective casings of seeds, typically removed during the threshing or winnowing of grains). While chaff is a sustainable biomaterial which improves soil structure and adds biochar to the soil, chaff does not contribute to high levels of nutrients like compost or manure. Thus, it works very well when combined with the nutrient-rich concentrated nutrients from the absorbed aqueous fraction (like liquid manure).

[0115] Thus, the method results in a slow release fertilizer and a usable water fraction. An overview of the method can be seen in Figure 8, and a more specific application using fam animal liquid manure and a lignin absorbent can be seen in Figure 9.

[0116] The slow release fertilizer may be used as a soil fertilizer, such as for crops, or in a combination with other products.

[0117] Thus, is provided a slow release fertilizer product, being slow-release fertilizer obtained in the method of the invention.

[0118] The slow-release fertilizer may further be dried, pelletized, coated, sterilized, pH adjusted or nutrient enriched. This may be to ensure it meets specific crop needs.

[0119] For instance, the slow-release fertilizer may be mixed with other fertilizer components and / or lime.

[0120] The slow release fertilizer may be added to the top soil. During limited tests, it was also found that it not only worked as a slow release fertilizer, it also helped retain moisture in the top soil, if the top soil was fast drying.

[0121] Also, should since the fertilizer (concentrated aqueous fraction, such as liquid manure) is absorbed in the absorbent material, the risk of so-called fertilizer bum during drought is reduced, since the soil surface concentration of fertilizer will be lower, due to the slow-release.

[0122] Although the present invention has been described above with reference to (a) specific embodiment(s), it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims, e.g. different than those described above.

[0123] In the claims, the term "comprises / comprising" does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or process. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and / or advantageous. In addition, singular references do not exclude a plurality. The terms “a”, “an”, “first”, “second” etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

CLAIMS1. A method for converting an aqueous stream comprising soluble nitrogen and / or potassium into a solid slow-release fertilizer product, comprising the steps of concentrating said aqueous stream, such that at >80 % but <95 % of the water is removed, to obtain a recovered water fraction and a concentrated aqueous fraction comprising the soluble nitrogen and / or potassium; and absorbing said concentrated aqueous fraction in an organic absorbent material to obtain an organic absorbent loaded with the nitrogen and / or potassium from the concentrated aqueous fraction, thereby obtaining a water fraction and a solid slow-release fertilizer comprising said organic absorbent loaded with the nitrogen and / or potassium from the concentrated aqueous fraction.

2. The method according to claim 1, wherein the aqueous stream is farm animal liquid manure, or a digestate effluent thereof.

3. The method according to claim 2, wherein the farm animal liquid manure is liquid manure from cattle, swine, sheep, goat, bird, horse, and / or buffalo or any mixture thereof.

4. The method according to claim 3, wherein the farm animal liquid manure is from cattle or from swine.

5. The method according to claim 1, wherein the aqueous stream is an industrial effluent, such as from mining operations or chemical production.

6. The method according to any one of claims 1 to 5, wherein the aqueous stream has an initial nitrogen and / or potassium concentration of 0.1-5 wt.-%, and more typically 0.3-3 wt.-%.

7. The method according to any one of claims 1 to 6, wherein during the concentrating step, >85 % but <90 % of the water is removed.

8. The method according to any one of claims 1 to 7, further comprising a pretreatment step of removing any suspended solids from the aqueous stream.

9. The method according to claim 8, wherein the pretreatment step comprises filtration, sedimentation, or electrocoagulation (EC).

10. The method according to any one of claims 1 to 9, wherein the aqueous stream is concentrated using an evaporation or an osmosis technique, such as reverse osmosis (RO) or forward osmosis (FO), or a combination thereof.

11. The method according to any one of claims 1 to 10, wherein the recovered water fraction is used for cleaning purposes and / or for the irrigation of crops.

12. The method according to any one of claims 1 to 11, wherein the absorption step is >1 minute, such as >20 minutes, such as >1 hour, but <24 hours, preferably between 1 minute to 2 hours.

13. The method according to any one of claims 1 to 12, wherein during the absorption step, the concentrated aqueous fraction is added to the organic absorbent material until the organic absorbent material is saturated.

14. The method according to any one of claims 1 to 13, wherein the concentrated aqueous fraction is absorbed by the organic absorbent material at a weight ratio of 0.35:1 to 1:0.35, preferably from 1:1 to 1:0.5 of concentrated aqueous fraction to organic absorbent material.

15. The method according to any one of claims 1 to 14, wherein the resulting slow- release fertilizer has a nitrogen content of 0.5% to 20% by weight.

16. The method according to any one of claims 1 to 15, wherein the absorption step further comprises mechanically stirring or mixing the concentrated aqueous fraction with the organic absorbent material.

17. The method according to any one of claims 1 to 16, wherein a soluble potassium salt or nitrogen salt is dissolved in the concentrated aqueous fraction before said fraction is absorbed in the organic absorbent material.

18. The method according to any one of claims 1 to 17, wherein the organic absorbent material is selected from lignin-based absorbents, organic fiber-based absorbents, or cellulose-based absorbents.

19. The method according to any one of claims 1 to 18, wherein the organic absorbent material is selected from: kraft lignin; lignosulfonate; chaff (such as chopped straw, rice husks, or pellets thereof); humus; peat; sawdust; compressed wood pellets; paper pulp (such as fluff pulp, chemical pulps, mechanical pulps, or pulp rejects); bagasse pulp; gels made of pulp (e.g. microcrystalline cellulose); organic litter; hydrogels; biochar; or composted organic matter.

20. The method according to any one of claims 1 to 19, wherein the absorbent material is kraft lignin or lignosulfonate.

21. The method according to any one of claims 1 to 20, wherein the slow-release fertilizer comprises both biochar and nutrients.

22. The method according to any one of claims 1 to 21, wherein the resulting slow release fertilizer is for use as a soil fertilizer, including application to agricultural soil, or for incorporation into formulations comprising other agricultural products.

23. A slow-release fertilizer, obtainable by the method of any one of claims 1 to 22.

24. The slow-release fertilizer according to claim 23, having further been dried, pelletized, coated, sterilized, pH adjusted or nutrient enriched.

25. The slow-release fertilizer according to any one of claims 23 to 24, further being mixed with other fertilizers or lime.

26. The slow-release fertilizer according to any one of claims 23 to 25, further being mixed with gypsum.

27. A method for soil improvement, wherein the slow-release fertilizer according to any one of claims 23 to 26 is added to top soil.

28. A method for simultaneously providing slow-release nutrients to soil and enhancing the water retention capacity of said soil, comprising the step of applying the slow-release fertilizer according to any one of claims 23 to 26 to said soil.

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