WO2009007514A2

WO2009007514A2 - Reducing the phosphorus content of liquid manure

Info

Publication number: WO2009007514A2
Application number: PCT/FI2008/050426
Authority: WO
Inventors: Liisa Pietola; Ulla Kulokoski
Original assignee: Yara Suomi Oy
Priority date: 2007-07-12
Filing date: 2008-07-11
Publication date: 2009-01-15
Also published as: WO2009007514A3; FI20085608L; FI20085608A0; FI20070545A0

Abstract

The amount of water-soluble phosphorus of liquid manure is easily and effectively reduced by adding thereto a precipitation product containing calcium and magnesium compounds. After the treatment, the liquid portion of the liquid manure that is treated by the method can be pumped back to the fields of the farm as fertilizers. The precipitation product according to the invention is easy to use in the farm environment and it is also suited to be used to considerably decrease the phosphorus content of the filtered liquors of dry manure.

Description

REDUCING THE PHOSPHORUS CONTENT OF LIQUID MANURE

The present invention relates to a method of treating the animal manure of farms, such as liquid manure or the run-off water of dry manure, so that the phosphorus content of its liquid portion can be reduced. The invention also relates to a precipitation product used in the method and the use of the product in question.

Prior art

When farmyard manure is to be further utilized by recycling it back to the fields from slurry tanks, a situation arises, wherein the phosphorus content of the ma- nure that is spread in the fields becomes too high, causing well-known ill effects. Regarding the need of cultivated plants, liquid manure contains too much phosphorus in relation to nitrogen. To keep the amount of phosphorus within permissible limits, attempts have been made to limit the size of livestock production units, or the farmers must acquire more land to spread the liquid manure with high phos- phorus content in a larger field area. It has been assessed that the need for extra land be in the order of 4 to 10-fold because of the tightening environmental regulations. One alternative is to carry the liquid manure out of the farm, which, in turn, causes transportation costs because of the large volume of slurry. The phosphorus in the liquid manure is partly in an organic form and partly as an inorganic phosphate, neither of which evaporates nor turns into a gaseous state through microbial activity.

Attempts have been made to solve the problem of liquid manure by reducing the phosphorus content by various means. For example, the liquid manure can be mechanically separated, whereby the majority of phosphorus remains in the pre- cipitate, but a considerable amount also remains in the liquid portion. The use of polymers in the separation also helps to reduce the amount of phosphorus that remains in the liquid portion; however, it is not capable of sufficiently and effectively removing it. Flocculation with polymers, which is based on the use of acryl amides, is also questionable on a large scale: the possible accumulation of acryl amides in soil is hardly desirable. On the other hand, the preparation of polymer solutions is demanding, and they do not keep for long. Thus, alongside with polymer treatments, there should be an alternative user-friendly treatment. Furthermore, cationic polymers loose their efficiency in the alkaline conditions that enable the precipitation reactions of phosphorus. Precipitation methods have also been used for the removal of phosphorus, comprising the precipitation of phosphorus as phosphates by means of aluminium or iron, such as aluminium sulphate or iron sulphate, for example, resulting in aluminium and iron phosphates, the phosphorus of which is useless to plants in the pH of cultivated land. Furthermore, soluble phosphorus has been bound to microbial masses by means of microbes.

One solution in removing phosphorus from liquid manure is to precipitate it chemically and to crystallize it in controlled conditions as large enough particles of stru- vite, MgNH₄Pθ46H₂θ. In this way, a decrease in the phosphorus content of liquid manure to an acceptable level is provided, and after the removal of struvite pre- cipitate, the liquid manure can be spread in the fields. The method works on a laboratory scale, but there are hardly any applications on a farm scale. Furthermore, the present liquid manure treatment methods have mostly been developed for pig slurry. In calcareous, alkaline fields, the phosphorus of struvite is not as usable as in acidic land with a pH of less than 7, which probably has, for its part, lim- ited the development of the method to be implementable in practice.

Some patent specifications refer to the precipitation of struvite in animal liquid manure under field conditions. However, in these specifications, the liquid manure has been pre-treated by mechanical separation or the nitrification of ammonium nitrogen in order for the dosages of the precipitation product not to increase to un- reasonable levels, as in patent specifications WO 03/027022 and US 2004/0265266, or the dosages have been extremely large, perhaps limiting the application in practice, as in patent application WO 2004/050584.

Patent specification WO 03/027022 discloses the treatment of waste water in a multi-stage process, wherein before precipitating phosphorus, ammonium nitrogen is removed by a nitrification pre-treatment. Before precipitating the phosphorus, the level of carbonates should also be low, as both ammonium and carbonates buffer the increase in pH and thus complicate the precipitation of phosphorus. When the ammonium and carbonate buffers are removed, a smaller dose of precipitant of phosphorus is needed for the dephosphorization, but the effectiveness of the dephosphorization with the amounts of precipitation chemicals used remains low. In addition, the pH requirement is at least 9. The precipitating agent of phosphorus comprises an alkali earth metal compound, such as Ca or Mg hydroxides or oxides or a mixture thereof. The specification provides no information on the dry matter content of waste water, which has an essential effect on the phosphorus content of the slurry, the buffer capacity of pH, and the mass amounts of additives needed for that. In the method of patent specification US 2004/0265266, Mg(OH)₂ and/or calcium carbonate (or, more generally, a calcium compound) with or without a carrier are added to the slurry to decrease the evaporation of nitrogen and to precipitate the phosphorus. The addition of the compounds causes the precipitation of phospho- rus and nitrogen as struvite, magnesium ammonium phosphate. In the method, pH is adjusted to high enough so that hydrogen sulphide would not evaporate but the struvite would precipitate; however, it is kept low enough to prevent the evaporation of nitrogen. The chemical is added to the waste slurry either as powder or wet slurry. The specification does not mention the amounts of dry matter. To prevent the evaporation of nitrogen, the specification suggests a dosage of 50 - 300lbs/ton (US), i.e. 25 - 150kg per ton for different types of manure.

Patent specification WO 2004/050584 describes a method, wherein a by-product of electrolysis and foundry is added as a precipitation product to a manure pit so that the ratio of Ca to Mg is 0.94. In addition, the by-product contains 12% of Na and, on the basis of Fig. 5, the Na content in the soil profile increases as a result of using the surplus of manure that is treated with BPEF. As is well known, this is disadvantageous for the physical structure of the soil and the water conductivity, as the aggregates of soil are dispersed. This promotes erosion and the transportation of phosphorus that is bound to the soil to the water system. Neither does the specification disclose information on the pH value of the slurry, which is an essential factor for a successful precipitation of phosphorus.

Patent specification US7005072 discloses a method and equipment for removing phosphorus from a pig slurry lagoon as crystalline struvite. The equipment contains a continuous crystallizer and fluidized bed with struvite seed crystals. In the method, ammonium is lead into the crystallizer for adjusting the pH of the liquid manure to a high enough level. At the same time, magnesium, such as magnesium carbonate or magnesium oxide in the form of a solution, is fed into the crystallizer to bring the required magnesium cation into the reaction system. The slurry treated in the crystallizer is lead into the fluidized bed reactor, wherein a struvite product with large enough particles was grown from the struvite seed crystals, settling on the bottom of the reactor. The struvite is removed from the bottom of the fluidized bed and the treated liquid manure is removed from the upper part of the fluidized bed. Crystallizing the phosphorus in a controlled manner as struvite on the farm level requires strictly controlled conditions. Furthermore, this equipment seems complex for the needs of a single farm, requiring equipment investments and a place of location for the equipment, and cannot thus be implemented by the facilities that are normally found on farms. In addition, chemicals in liquid form are needed for using the method, as well as knowledge about the functioning of the equipment.

Publication Heinonen-Tanski, H. Hirvonen, A. & Tanni, K., Phosphorous loads caused by slurry fertilization to waters may be reduced by slurry aeration and waste gypsum powder, Pro Terra 4, 106 - 107, describes, on a laboratory scale, the use of waste gypsum and calcium chloride to bind phosphorus in treating pig slurry. Especially in connection with the calcium treatment, it was observed that the aeration of liquid manure had a considerable effect on the reduction of the amount of water-soluble phosphorus, i.e., phosphate, in the remaining liquid portion of the liquid manure. The publication does not examine the effect of pH on the precipitation of phosphorus. In addition, the aeration used causes mixing of the liquid manure, which furthers the evaporation of nitrogen.

Publication Cao, X., Harris, W., Josan, M. & Nair, V., Inhibition of Calcium Phos- phate Precipitation under conditions Typical of Manure-amended soil Solutions, ASA-CSSA-SSSA Abstracts, International Meetings, Indianapolis, November 12 to 16, 2006, examines the possible inhibitors of the precipitation of calcium phosphate in manure-amended soil. The tests showed that Mg ions disturb the precipitation of calcium phosphates in soil. On the basis of this publication, it would thus be advisable to avoid the use of magnesium-bearing compounds, when wanting to precipitate phosphorus as calcium phosphate.

In water purification, various methods and chemicals have been used to remove phosphorus. However, as a target of purification, the liquid manure originating in cattle farms essentially deviates from water, waste waters, in particular, because of its high dry matter content and its different chemical composition. It is a well- known fact that waste water contains larger amounts of heavy metals; correspondingly, liquid manure contains large amounts of plant nutrients, such as phosphorus and nitrogen, which make the water system eutrophic. It is a completely different matter to treat dilute waste water than concentrated liquid manure that may con- tain as much as 5 to 6% of suspended matter. Contrary to purified waste water, liquid manure is not permitted to be discharged at the water system. It is ecologically, economically, and also logically wise to use liquid manure as fertilizer, preferably in the farm's own fields, which is the case on several farms. The treatment of liquid manure should take place on the farm without having to considerably transfer or move the slurry to avoid harmful effects of odour and a partial evaporation of nitrogen. Water purification processes, again, are based on a continual flow of water and they are continuous by nature. Treatment of liquid manure on farms is almost invariably carried out on a one-off basis or in batches.

By means of the solutions described above, phosphorus can be removed from liquid manure so that the remaining phosphorus content is quite low. However, in farm-specific dephosphorizing, it is essential to lower the phosphorus content enough but, however, in a simple and economic manner so that no substances harmful to plants are added to the liquid manure. For this, methods are needed, which do not require purchasing extra equipment and user training. In addition, the method required should work reliably enough in various environmental conditions, such as outdoors in varying weather conditions, and it should not be difficult to control. Furthermore, it should be possible to implement the treatment in the farm environment, as the purpose is to reuse the major part of liquid manure in the fields of the farm as fertilizer, without an unnecessary recycling of the nutrients needed through several different process stages. The purpose of the invention

The purpose of the present invention is to provide a simple, effective and economic method of lowering the phosphorus content of manure, such as liquid manure on farms, and of preventing environmental damages.

Another purpose of the invention is to solve how to decrease the phosphorus con- tent to a suitable level without making special investments in the equipment on the farm, by using the existing basic equipment on the farm.

The purpose is to be able to use as fertilizer at least half of the volume of the liquid manure so that it can be spread directly in the field, and that the phosphorus content of the portion that is spread is not too high.

Another purpose is to decrease the soluble phosphorus that is washed out with the filtered liquor from the manure heap of dry manure.

Short description of the invention

When carrying out tests in the slurry tank of the farm to chemically reduce the phosphorus content of the liquid portion by precipitating, it was surprisingly ob- served that when adding to the tank gypsum and magnesium oxide, in particular, a quick and effective precipitation and settling of phosphorus to the bottom of the slurry tank was provided. Furthermore, when adding the mixture of gypsum and magnesium oxide to dry manure, it was observed that the content of water-soluble phosphorus in its filtered liquor decreased.

One advantage in the method and the chemicals used in the treatment of liquid manure was that it was possible to control their use under various conditions, such as temperatures, different sizes or shapes of slurry tank, or variations in the composition of the liquid manure; it was possible to eliminate any disturbing factors by means of the composition, dosage and treating time of the precipitation product. The method is particularly attractive to farmers because in the method, the basic equipment of the farm can be utilized, such as the slurry tank with its agitator, flexi- ble intermediate bulk container cranes, front loading units, and the spreading equipment of manure, and no new investments in machinery are needed. The floating slurry of the clarified liquid manure, the low in phosphorus liquid portion, which is obtained as a result of the method, can thus be pumped into the ordinary spreading equipment of the farm or even into sprinkler equipment to be spread di- rectly in the fields. The high in phosphorus portion with a higher content of solid matter, the sediment, which settles to the bottom of the slurry tank, can be further used as phosphorus fertilizer in applications that need it, or as raw material for bio- gas production. If it is appropriate to transport the sediment out of the farm, its volume is now at least half compared with that of the original liquid manure, which considerably lowers the transportation costs. Furthermore, the precipitation product needed is user-friendly; its dust formation is minor and it can be added directly to the slurry tank from a flexible intermediate bulk container. The precipitation product used contains plant nutrients, such as calcium, sulphur, and magnesium, thus improving the plant nutritional quality of the manure. The precipitation product precipitates the phosphorus to a form usable to plants at the pH (5 to 7) of the field, contrary to the phosphorus of slurry that is treated with iron and aluminium preparations, for example. Furthermore, the calcium and the magnesium of the precipitation product improve the structure of soil and reduce the compacting effect of slurry haulage. The precipitation product can also be used in binding phosphorus and nitrogen in dry manure and its filtered liquor, whereby no phosphorus leaks to the base in the storage or further use of dry manure, and the evaporation of nitrogen is decreased. In that case, the precipitation product is mechanically mixed with dry manure, for example, in a shallow manure pit or when spread in the field, whereby the leaching of soluble phosphorus in the manure yard or the field is decreased to as low as one tenth. The present invention relates to the method according to Claim 1 for reducing the water-soluble phosphorus and the total phosphorus content of the manure-based liquid portion, such as the liquid-bearing floating slurry of liquid manure or the filtered liquor of dry manure. The invention also discloses a precipitation product ac- cording to Claim 21 , which can be utilized in the method according to Claim 1 and which can be used for treating dry manure according to Claim 29.

Description of the figures

Fig. 1 shows a change in the water-soluble phosphorus, when treating pig slurry with the precipitation product according to the invention. Fig. 2 shows a change in the pH of pig slurry, when treated with the precipitation product according to the invention.

Fig. 3 compares the use of kiserite, MgSO₄H^O, and MgO as a source of magnesium in the precipitation product according to the invention.

Fig. 4 shows the improvement of the water-soluble phosphorus content of samples that were taken at a depth of 0 to 5cm of the surface of cattle slurry that was treated with the precipitation product according to the invention, in the course of the treatments.

Fig. 5 shows the water-soluble phosphorus contents of samples that were taken at a depth of 30cm of cattle slurry that was treated with the precipitation product ac- cording to the invention.

Fig. 6 shows the measuring results of the water-soluble phosphorus of the slurry- storage tank.

Fig. 7 shows the behaviour of pH in the slurry-storage tank.

Fig. 8 shows the phosphorus content of the runoff water of poultry manure treated with the precipitation product according to the invention.

Fig. 9 shows the phosphorus measurement results of the slurry-storage tank treated with the precipitation product according to the invention.

Detailed description of the invention

The manure of a farm is typically pig, cattle or poultry manure but it may also con- tain manure of the same type from the other animals of the farm. According to its dry matter content, the manure either comprises dry manure or liquid manure, whereby the dry manure is typically poultry manure and the liquid manure is pig or cattle slurry. Dry manure can be handled and stored as such, but liquid manure is generally collected in a slurry tank for a precipitation treatment. Generally, the liq- uid manure contains less than 7% by weight, preferably 1.5 - 6% by weight, more preferably 2.5 - 6% by weight of solid matter, suspended in an aqueous phase. The solid portion sediments by itself within a sufficient time, whereby the more solid sediment remains in the bottom part of the slurry tank, and the floating slurry that contains more liquid remains on the surface. However, this slow separation process can be accelerated by a chemical precipitation. In addition, the water- soluble phosphorus remains in the liquid portion in the spontaneous sedimentation.

In the method according to the invention, the water-soluble phosphorus content of the manure, such as the more liquid-bearing surface part of the liquid manure, the floating slurry, and of the corresponding liquid manure formed from the filtered liquor of dry manure, is reduced by adding, at a first stage, to the slurry tank a solid precipitation product containing at least gypsum as the source of calcium and at least magnesium oxide as the source of magnesium, which, when added to the liquid manure, provides binding of the soluble phosphorus that is in the floating slurry and its precipitation to the sediment both aerobically and anaerobically, into a form of compound that can be utilized by plants, so that the pH of the slurry mixture thus obtained is 7.5 or more. After this, at second stage B, the slurry mixture thus obtained is mixed, and at stage C, the slurry mixture is allowed to clarify, whereafter at stage D, the floating slurry containing a larger amount of liquid, which is separated in the upper part of the slurry mixture, is removed from the slurry mixture to be reused as fertilizer.

In order to precipitate the phosphorus, the pH of the phosphorus-bearing slurry mixture should be over 7.5. The speed of precipitation depends on the pH of the slurry and also on the quality, the solids content, and the phosphorus content of the slurry, as well as other factors, such as the temperature and the oxygen content of the slurry. The pH of the slurry is preferably within 7.6 and 8.8; if the pH rises too high, a nitrogen loss develops in the liquid manure, the nitrogen evaporating as ammonia. The pH of the slurry is most preferably within 7.8 and 8.6, whereby the speed of precipitation is as high as possible and the precipitation as effective as possible, but the nitrogen loss is not yet considerable. The pH can be measured by any conventional pH measuring instrument suitable for the purpose, such as EUTECH Instruments (IP67), by immersing an electrode in the slurry. Easy-to-carry measuring instruments that work in outdoor locations are commercially available. The measuring instruments are easy to use and they give the pH reading directly. The pH is preferably measured at each separate stage A, B and/or C to maintain, correct and/or adjust the desired level. The dry matter content (the solid matter) of the slurry can be measured by any known means with a hydrometer, which can be customized and connected to the slurry treatment method according to the invention.

According to a preferred embodiment of the invention, the solid precipitation prod- uct that contains at least gypsum as the source of calcium and at least magnesium oxide as the source of magnesium is added to the slurry tank, causing, when added to the liquid manure, binding of the soluble phosphorus that is in the more liquid-bearing, floating slurry, and the precipitation thereof both aerobically and anaerobically to the more solid sediment, into a form of compound that can be util- ized by plants, and which has a pH-increasing effect. Now, the mixture of Mg or Ca compounds that is used forms compounds that increase the alkalinity, when dissolving in water. The Mg compound used contains at least MgO. In addition, it can contain Mg(OH)₂, MgCI₂ MgCO₃, such as magnesite, or MgSO₄, such as kis- erite, or a mixture thereof or ammonia water, resulting in a pH-increasing effect. The Mg compound more preferably contains essentially MgO. The Mg compound most preferably comprises MgO. The Ca compound used contains at least gypsum, such as natural gypsum, CaSO₄2H₂O, synthetic phosphogypsum, CaSO₄ ¹^H₂O, CaSO₄, or a mixture thereof. In addition, it can also contain Ca(OH)₂, CaCO₃, CaO, chalk lime, liming material or a mixture thereof. The term "synthetic phosphogypsum" in connection with this invention refers to gypsum CaSO₄2H₂O, which is generated as a by-product in the phosphoric acid industry. The term "liming material" refers to soil amendments, which have pH-increasing effects, such as calcite-based and dolomite-based limestone, slag from the metal industry, filter dust from the lime industry, biotite, siliceous lime, and compressed lime. The Ca compound is preferably essentially gypsum. The Ca compound more preferably contains synthetic phosphogypsum, which quickly dissolves in liquid manure. The Ca compound is most preferably phosphogypsum. For example, the solubility of CaCO₃ in the pH of liquid manure is about a hundred times weaker than that of gypsum. Although it has been observed that Mg ions are harmful to the precipitation of calcium phosphate, in the method according to the invention it was observed, however, that the Mg ions that were added to the slurry tank also furthered the formation of struvite, which is essential for the total dephosphorizing. Presumably, the precipitation of phosphorus takes places simultaneously both aerobically, whereby calcium phosphate is formed, for example, and anaerobically, whereby struvite is formed, for example; however, without making a commitment to this theory. Simultaneous precipitation into several compounds, the mechanism depending on the place and the conditions inside the slurry tank, provides an effective precipitation of soluble phosphorus and clarification of the floating slurry.

In the method according to the invention, a mechanical mixture of MgO and gypsum is preferably used as precipitation product.

The precipitation product according to the invention can be added to the liquid manure in one or more batches before removing the floating slurry that contains a larger amount of liquid. The need for addition can be followed by means of a pH measurement: If the pH is not over 7.5 after two days from the treatment, more precipitation product should be added, and then most preferably a product, wherein the pH-increasing component is prevailing.

Furthermore, the components of the precipitation product can be added to the liquid manure together or separately. The Ca and Mg compounds are preferably added together to the liquid manure; the Ca and Mg compounds are more preferably added in one compact or granulated form, whereby one granule can contain one or more compounds. The Ca and Mg compounds are most preferably added in one compact or granulated form, so that one granule contains both compounds in an essentially dustless form. Dustless in the present invention refers to a product, the dust formation of which is less than 2000mg/kg, preferably less than 1000mg/kg, measured as a loss. The "loss" herein means that the said amount of product forms dust outside the slurry tank. According to a preferred embodiment, the precipitation product is added to the liquid manure in an amount of 500 to 1000kg/100m³. The amount to be added depends on the quality, the phosphorus content, and the dry matter content of the slurry. The precipitation product is preferably added to pig slurry in an amount of 500 to 900kg/100m³. The aim is that (Ca+Mg):P in the treated slurry is within 1 :1 - 10:1 , depending on the dry matter content of the manure. As the dry matter content is typically about 4 - 5%, the amount added to pig manure is more preferably 700 to 800kg/100m³, most preferably about 750kg/100m³. The amount of precipitation product that is added to cattle slurry is preferably within 700 to 1000kg /100m³, more preferably about 900kg/100m³, as cattle manure often contains more dry matter, such as 5 to 6%, whereby the dry matter weakens the precipitation. Cattle slurry needs proportionally more adjustment of pH than pig slurry, the pH of which is naturally higher and the dry matter content lower. Generally, the more dry matter in the slurry, the larger amounts of precipitation product are needed, which

_, those skilled in the art can assess by visually examining the manure and/or by means of hydrometer and/or pH measurements.

When needed, the slurry can be separated mechanically before precipitation. In that case, the dry matter content of the slurry is typically 3% by weight or less. To treat mechanically separated slurry, typically, less precipitation product is needed, about 30 - 50% compared with non-separated manure. The precipitation product is added to the separated slurry in an amount of 150 - 800kg/100m³, preferably 150 - 500kg/100m³, more preferably 150 - 300kg/100m³, most preferably 200 - 300kg/100m³, such as 240 - 260kg/100m³.

A situation similar to the separation can result, if a lot of rain or washing water enters the slurry well, among others, whereby the amounts mentioned above can be used.

The precipitation can be furthered by polymers. According to an embodiment of the invention, in addition to the precipitation product, polymer is added to the liquid manure, if the liquid manure contains a considerably large amount of dry matter. The polymer can be any polymer that is well-known in the field for being intended for or used in treating liquid manure, preferably cationic polymer, more preferably polyacrylamide or diallyl dimethyl ammonium chloride, or a mixture thereof. The amount of polymer added is typically less than 40ppm, preferably 10 to 30ppm, so that the polymer that is dissolved in liquid does not increase the liquid volume of the slurry, most preferably about 10 to 15ppm. The polymer can be added at any stage A, B or C before removing the more liquid-bearing floating slurry.

Immediately after adding the precipitation product, the slurry mixture in the slurry tank is agitated by a screw or another slurry agitator, which is typically used on the farm in connection with emptying the slurry tank. The mixing time for liquid pig or cattle manure containing less than 7% by weight, preferably 1.5 - 6% by weight, more preferably 2.5 - 6% by weight of dry matter is at least half an hour, more preferably 1 hour, most preferably 3 hours in order to evenly mix the precipitation product with the liquid manure. The mixing time depends on the amount of manure, its dry matter content, the volume and the shape of the tank, and the effectiveness of the screw or the agitator. The sedimentation time of liquid manure, to which the precipitation product has been added and which has been mixed, depends on the amount and the quality of the manure. Generally, after a few days or weeks, floating slurry that is purified of water-soluble phosphorus has developed on the surface of the slurry tank, its amount being about 50%, more preferably 70%, and most preferably 80% of the volume, and on the bottom, a sediment containing phosphorus, its amount being about 50%, more preferably 30%, and most preferably 20% of the volume, wherein the phosphorus is in a form usable to plants. In the case of pig slurry, the sedimentation time is preferably 2 to 7 days, more preferably 1 to 2 days. In the case of cattle slurry, the sedimentation time is preferably 3 days to 3 weeks, more preferably 3 to 5 days. The length of the reaction is influenced by the temperature: at over 15°C, the precipitation reaction only takes a few days, but at 5 - 10 degrees, over a week. The sedimentation can be observed visually, and the boundary layer of the precipitate by measuring with a wooden stick, for example: When the resistance of the wooden stick to push suddenly increases substantially, the floating slurry and the sediment have been separated. If a temperature sensor is connected to the measuring rod of the push resistance, the temperature of the slurry can be measured at the same time, which in the precipitation layer of phosphorus has been observed to be higher than in the slurry above: The precipitation of phosphorus typically releases heat. The content of water-soluble phosphorus in the floating slurry can most preferably be analyzed in a commercial laboratory or, possibly, tested on the farm by commercial phosphate strips, for example.

After the sedimentation, when the soluble phosphorus has disappeared from the floating slurry that contains a larger amount of liquid, or the phosphorus content has decreased to a low enough level with respect to the farmer's need for fertilizer, the farmer pumps the floating slurry portion that is low in or free of phosphorus and contains more liquid, which is separated in the upper part of the slurry mixture, into spreading equipment. It can be pumped directly to the spreading equipment and spread in the fields of the farm, or before spreading, the floating slurry can be fur- ther processed with acid to lower the pH mainly to prevent the evaporation of nitrogen and odours. Nitrogen acid or sulphuric acid is preferably added, so that the nitrogen in the liquid does not evaporate and that the liquid does not smell, when being spread.

After the treatment, a phosphorus-bearing skin with a thickness of 1 to 2mm may sometimes appear on the surface of the slurry. According to a preferred embodiment, when pumping the floating slurry that is low in phosphorus, the end of the hose should be placed in the middle of the liquid phase, i.e., at a depth of about VA from the surface of the slurry. Accordingly, the liquid portion is preferably removed by pumping from the middle of the floating slurry.

The stages of the method can be repeated, until the water-soluble phosphorus content of the floating slurry that contains more liquid is on a desired level, preferably less than 50mg/kg, more preferably less than 20mg/kg, and most preferably less than 10mg/kg. In addition, the precipitation product can be added in a required amount and as many times as needed, until the desired end result is reached. Typically, the phosphorus content of cattle slurry is more difficult to lower than that of pig slurry. The ratio of the phosphorus content of the treated floating slurry, which contains more liquid, to the phosphorus content of the liquid portion of untreated liquid manure is preferably less than 1 :4, more preferably 2:10, and most preferably less than 1 :10, whereby the cultivated area needed for direct spreading also decreases by 1 :2, 2:5, or 1 :5, respectively, taking into account the entire amount of manure, or the permissible amount of slurry per surface area can be larger in proportion, decreasing the transportation of slurry. The phosphorus content can be monitored by measuring it from the floating slurry by any means well-known in the art, preferably by a manure analysis of the surface portion or using phosphorus strips, which is a quick commercial way of measuring. In an embodiment according to the invention, the method further includes the recovery and further use of the phosphorus-rich sediment. The sediment can be used as phosphorus fertilizer, but it can also be used as complete fertilizer, as the sediment contains the plant nutrients contained in the precipitation product, such as calcium, sulphur, and magnesium. Furthermore, as its dry matter content is close to the goals of biogas facilities, for example, the sediment can be used for energy applications, such as a source of bio-energy, and, when needed, the dry matter can further be increased for the needs of energy utilities, for example, by mixing with the slurry sediment the drying agents of cattle farms, such as sawdust, peat, and/or straw. Precipitation of the soluble phosphorus of the slurry into com- pounds usable to plants is an ecologically more sustainable solution than the precipitation of phosphorus out of the nutrient cycle as iron or aluminium compounds, for example.

Typically, the method according to the invention is carried out in the slurry tank of the farm. Alternatively, the method can also be applied to other large tanks, where the slurry is allowed to sediment undisturbed for several days, preferably 1 to 7 days, after the treatment without moving. Accordingly, the treatment is carried out in batches or on a one-off basis, and not continuously, as in water purification plants, for example.

One advantage of the method according to the invention is that the dephosphorizing of the liquid fraction of the liquid manure can be carried out on cattle farms without significant investments in machinery, in the prevailing environmental conditions and varying weather conditions. The slurry can be treated in the slurry tanks of the farm, and the treatment requires no separate space, equipment or plant. Being solid and its handling corresponding to emptying a fertilizer bag, which the farms generally always have the facilities and the experience for, the precipitation product is simple to use and transport.

Another advantage of the method according to the invention is that the total amount of slurry does not have to be moved outside the farm to be treated or spread, and so the transportation costs of the manure are reduced by at least a half. Being based on the basic equipment of the farm, the method requires no in- vestments in new machinery. One significant advantage over the other methods is the enrichment of slurry with sulphur, when using gypsum in the precipitation product; the fertilizer requirement of sulphur having been stressed along with the decrease in the sulphur deposition from the atmosphere during the past years. The slightly alkaline slurry of the end product according to the invention is well- suited to increasing the fertility of acidic cultivated land, as the end product has a liming, pH-increasing effect, which influences, in addition to the exploitation of nutrients, also the soil structure, the aeration, and the supply. The present invention stresses the increase in pH to a sufficient level, whereby the precipitation of phosphate is possible. For example, as the adjustment of pH also enables the anaero- bic crystallization of struvite, no separate aeration is then needed.

When gypsum is used in the precipitation product, the slurry is enriched with sulphur. This is also advantageous for the evaporation of ammonium nitrogen, as the strong sulphate anion supposedly prevents the ammonium ions from gasifying into volatile ammonia. This effect has been proven by means of dry cattle manure. The use of the precipitation product according to the invention is described in Claims 27 and 30.

The invention discloses the use of the solid precipitation product according to the method described above for decreasing the content of water-soluble phosphorus in the more liquid-bearing floating slurry of liquid manure and/or the corresponding liquid slurry formed from the filtered liquor of dry manure.

In addition, the precipitation product according to the invention is suitable for use in the treatment of dry manure for decreasing its soluble phosphorus content, when getting into contact with water, whereby the soluble phosphorus is allowed to release.

The solid precipitation product according to the invention is added to dry manure, preferably poultry manure, whereby the pH of the developing mixture is 7.5 or more. The mixture thus obtained is stirred, preferably for at least half an hour like liquid slurry, more preferably for at least 10min. The precipitation product now begins to react with the manure. The precipitation product is preferably allowed to react for 1 - 5 days without stirring. The mixture of dry manure and the precipitation product can be preserved and stored for long periods of time. However, as this mixture at some point is brought into contact with water, the precipitation product is able to bind the water-soluble phosphorus contained in the manure.

The precipitation product is preferably mixed in a manure yard, manure tank, manure transfer car or animal locations with dry manure, which after the treatment retains the phosphorus and the phosphorus is not washed out, when the treated manure gets into contact with water and forms filtered liquor. Typically, the contact with water takes place when flushing the facilities with water or, when being spread in the fields in the rain or during irrigation. When the precipitation product according to the invention is used, the evaporation of nitrogen is also prevented or at least decreased. In this way, the nitrogen and phosphorus of the manure remain in the manure, and the recycling of nutrients back to the field as plant nutrients is more restrained.

The dry matter content of the dry manure is 20 - 60% by weight and its density preferably 0.25 - 0.50kg/dm³. The amount of precipitation product added to the dry manure is from 200 to 4000kg/100m³, preferably 500 - 3000kg/100m³, more preferably 700 - 1500kg/100m³, most preferably 800 - 1000kg/100m³, depending of the phosphorus content, the density and the dry matter content of the manure.

The invention also discloses the solid precipitation product according to Claim 21.

According to the present invention, the solid precipitation product contains at least gypsum as the source of calcium and at least magnesium oxide as the source of magnesium, and when added to phosphorus-bearing liquid that contains water- is soluble phosphorus or to solid matter that is brought into contact with water, the product causes binding and precipitation of the phosphorus both aerobically and anaerobically into a form of compound usable to plants. Typically, the water- soluble phosphorus precipitates under the effect of calcium, by means of an aero- bic reaction, mainly into amorphous calcium phosphate and, under the effect of magnesium, into crystalline struvite by an anaerobic reaction. The precipitation product that contains both cations enables the precipitation of phosphorus as phosphates both in the aerobic surface layers and in the anaerobic conditions of the lower layers of the slurry tank, so that, in future, the plants will be able to fur- ther utilize the nutrients contained in the manure. As both precipitation mechanisms of phosphate are utilized in the treatment, the processing conditions do not have to be as controlled as when precipitating struvite alone, for example. Furthermore, the formation of amorphous calcium phosphate can also be utilized in the method. Accordingly, the precipitation product provides an effective and quick precipitation of soluble phosphorus from the liquid phase and the binding thereof into a solid form that is advantageous to plants, for further use.

According to a preferred embodiment, when added to liquid manure, the precipitation product increases the pH of the liquid manure.

According to a preferred embodiment, the precipitation product is compact or granulated. The granulation decreases the dust formation of the product and thus renders it more user-friendly, convenient to handle, and reduces the health hazards resulting from use. When measured as a loss, the dust formation of the precipitation product is preferably less than 2000mg/kg, more preferably less than 1000mg/kg. Analogously to fertilizer bulk bags, the precipitation product can be packed in flexible intermediate bulk containers or bulk bags, which are ready for implementation. Typically, farms have the equipment for handling fertilizer bulk bags and they can be used for handling the precipitation product bulk bags and, for example, discharging the contents of the bag in a controlled manner in an application, such as the slurry tank. The precipitation product according to the inven- tion is added directly to the object of treatment, such as the slurry tank of the farm, from its sales package, i.e., the fertilizer bulk bag, or in the case of bulk products, the platform of a truck, from where a heap can be made, which can be transferred to the use by means of a front end loader.

The Mg source of the precipitation product according to the invention preferably contains at least MgO. In addition, it can contain Mg(OH)2, MgCI₂, MgCO₃, such as magnesite, or MgSO₄, such as kiserite, or a mixture thereof or ammonia water. The Mg compound more preferably essentially contains MgO. The Mg compound most preferably comprises MgO. One advantage of MgO is its ability to increase the pH in an aqueous solution. In addition, it was observed to remove the surface precipitate, which rises to the surface of the slurry as a result of the gas formation of microbial activity during the clarification process. One problem of using MgO, which is often presented, is its poor solubility, whereby regarding the expected value, the reactions are slow and some insoluble oxide is left over. However, this is not the case with the method according to the present invention. Using MgO provided a sufficiently quick precipitation and it dissolved effectively in the slurry, and no insoluble Mgo was found in the slurry precipitate in the XRF measurements that were conducted, for example, although the amount of MgO added was 0.3% by weight, for example. Magnesium chloride dissolves well and is easy to handle, but it provides a slightly acidic aqueous solution. Neither was kiserite found to increase the pH. The Ca source used preferably contains at least gypsum, such as natural gypsum, CaSO₄2H₂O, or synthetic phosphogypsum, CaSO₄ ¹^H₂O, CaSO₄, or a mixture thereof. In addition, it can contain Ca(OH)₂, CaCO₃, CaO, chalk lime, liming agent, or a mixture thereof. The Ca source more preferably essentially comprises gypsum, more preferably essentially phosphogypsum, the gypsum dissolving and working better than calcite or the other liming agents, for example, and most preferably synthetic phosphogypsum, which is obtained as a by-product from a phosphoric acid process and which is thus an extremely economic raw material.

The precipitation product is preferably a mixture of MgO and gypsum, or more preferably a mechanical mixture. In the treatment of dry manure, it is also possible to use pure gypsum. Dry manure is often more alkaline than liquid manure; therefore, MgO that increases the pH is not necessary. However, an MgO addition obviously enhances the effect of gypsum.

As in addition to the property of precipitating phosphorus, the purpose of the precipitation product is also to preferably increase the pH of the liquid manure, all Ca and Mg compounds as such are not feasible. For example, gypsum slightly decreases the pH of the slurry, whereby the other component of the precipitation product needed should also increase the pH, in this respect. In that case, it may be necessary to also use other additives to increase the pH, as described above. Typically, for example, MgO and dolomite increase the pH and, at the same time, work as sources of Mg. However, it is a well-known fact that a carbonate ion disturbs the precipitation of phosphorus. One advantage of the use of gypsum, again, is its ability to whiten the colour of the slurry solution, whereby it is possible to measure the phosphorus content by means of phosphorus strips. Although in the literature, kiserite, MgSO₄H₂O, has been found to be the best source of Mg in precipitating struvite, MgO works better, apparently because of its ability to increase the pH.

The most advantageous precipitation product for cattle and pig slurry is the mechanical mixture of gypsum and MgO, wherein the portion of gypsum is 20 - 70%, whereby the Ca:Mg ratio is less than 0.9, preferably within the range of 0.09 - 0.88, more preferably 0.3 - 0.7, most preferably 0.4 - 0.7. When treating dry manure, the corresponding mixture of MgO and gypsum, or pure gypsum can be used.

Typically, cattle slurry is more acidic and thus needs a larger amount of pH- increasing component.

However, cattle slurry is not always more acidic than pig slurry, the pH of which can also be 6, depending on feeding. The pH of cattle manure can also be 7, similarly to that of pig slurry. The Ca/Mg ratio of the precipitation product primarily depends on the pH of the slurry. The ratio is the bigger, the higher the pH of the slurry at the beginning.

Table 1 shows the Ca/Mg ratios according to the invention in a case, where the precipitation product is gypsum-MgO.

The present invention also discloses the use of the precipitation product in reducing the content of water-soluble phosphorus in the liquid manure.

The water-soluble phosphorus content of farmyard slurry in the more liquid- bearing floating slurry of farmyard slurry can be reduced by adding the solid precipitation product, which contains the Ca and Mg compounds and which, when added to the phosphorus-bearing liquid manure, provides binding and precipitation of the phosphorus both aerobically and anaerobically into a form of compound exploitable to plants. The precipitation product that is added to the liquid manure preferably acts increasing the pH of the liquid manure.

The precipitation product can be added to the liquid manure before the other known processing stages, or between or after them. Furthermore, the precipitation product can be added to the slurry tank on the farm directly from its sales pack- age, such as the flexible intermediate bulk container.

In the following, the invention is illustrated by means of examples; however, without being limited to them.

Examples

Example 1 To isolate the phosphorus contained in liquid pig and cattle manure, the following tests were conducted on the liquid phase of the manure:

- Reference 1 pig manure: no additives, self-precipitation on the basis of time and gravity.

- Sample 1 pig manure: gypsum, CaSO₄2H₂O, 0.35% by weight (Kemira Growhow Oyj, phosphogypsum) and MgO 0.3% (technical grade, Kemira Growhow Oyj). - Sample 2 pig manure: as Sample 1 , but in connection with adding the precipitation product, 15ppm of polymer are added (6 ml as a 0.25% aqueous solution, K3459 / Fennopol, Kemira Oyj).

- Reference 2 cattle manure: no additives, self-precipitation on the basis of time and gravity.

- Sample 3 cattle manure: gypsum, CaSO₄2H₂O, 0.35% by weight (Kemira Grow- how Oyj, phosphogypsum ) and MgO (technical grade).

- Sample 4 cattle manure: as Sample 3, but in connection with adding the precipitation product, 15ppm of polymer are added (6 ml as a 0.25% aqueous solution, K3459 / Fennopol, Kemira Oyj).

A precipitation product according to each sample is added to a sample of liquid manure (1dl); it is mixed for one minute and allowed to sediment with the bottle cap ajar. The phosphorus contents of the floating slurry are measured in a phosphate form after 7 days. Table 1 shows the remaining phosphorus content in rela- tion to Reference 1 (=100%).

Table 1

Example 2 To isolate the phosphorus contained in pig manure, the following tests were conducted on the liquid phase of the manure: To 1dl of liquid manure sample (dry matter content of 3%), the following were added:

- Reference 1 : nothing (self-precipitation on the basis of time and gravity), - Reference 2: gypsum, CaSO₄2H₂O, 0.35% by weight (KemphosOy, phosphogyp- sum from the Siilinjarvi Plants, where phosphoric acid is manufactured from apatite, and the gypsum in question is generated as a by-product),

- Reference 3: 15ppm of polymer (K3459, Fennopol, Kemira Oyj), as an aqueous solution (0.25% g/g) 6ml/1dl of slurry, - Reference 4: gypsum 0.35% by weight and polymer, as in Reference 3.

- Reference 5: dolomite 0.75% by weight (Kemphos Oy),

- Sample 1 : 0.35% by weight of gypsum and 15ppm of polymer, and 0.3% by weight of MgO (Kemphos Oy, technical grade).

The samples are mixed for 1 min; they are allowed to rest with the bottle cap ajar, and their phosphorus contents are measured in a phosphate form for the first time after 7 days.

Table 2 shows the remaining phosphorus content in relation to Reference 1 (=100%).

Table 2

On the basis of the tests conducted, it was observed that the isolation of pig slurry phosphate from the liquid phase works well even by means of gypsum alone (Reference 2) at room temperature, whereby the pH easily rises to over 7.5. The amount of phosphate decreased from a reference of 170mg/kg to a level of 20mg/kg even without the polymer.

The use of MgO (Sample 1 ) continues to reduce the amount of phosphorus. In addition, it was observed that the use of MgO accelerates the precipitation of phosphorus. Although the gypsum alone works well as the precipitator of phosphate, the addition of MgO considerably improves the settling of the surface deposit on the bottom, in particular.

The polymer test (Reference 3) proves that it alone is not enough to remove the phosphorus, and not even together with gypsum (reference 4) does it yield as good a result as when MgO is added. Neither does the use of dolomite (Reference 5) replace the MgO treatment. On the contrary, Benelux dolomite that contains a small amount of phosphorus even increased the phosphorus content of the liquid fraction.

Example 3

A farm test was conducted to precipitate the phosphorus of pig slurry. Into the slurry tank of about 60m³ of the farm, almost dust-free mixture was added from a flexible intermediate bulk container, which had been placed in a tractor-mounted FIBC crane, containing 300kg of gypsum (4.5kg/ton as dry, with a moisture of 10%) and 180kg of MgO (3.0kg/ton). Furthermore, 1801 of polymer that had been dissolved in water was simultaneously added to the manure tank before mixing. The phosphorus content in the mixed liquid manure that was measured before the additions was 690 - 720mg/kg in a water-soluble form (1050mg/kg as acid-soluble total phosphorus), decreasing to a level of about 50mg/kg the next day, at a measuring depth of 0 to 10cm, and being within 46 and 56mg/kg during the following days. After five days, a sample was taken at a depth of 30cm (the depth of the slurry tank was about 90cm), and the reading was 47mg/kg (the total phosphorus was 87mg/kg). Fig. 1 shows a change in the water-soluble phosphorus during the treatment of pig slurry. On May 16, samples are taken at a depth of 0 to 10cm; on May 21 and 30, also at a depth of 30cm, however, their contents proved to be the same as on the surface. After a week from the treatment, the content of water-soluble phosphorus has decreased to 13mg/kg, and this level remains during the next week. The measurements taken on May 30 show 11 to 13mg/kg on the surface of the slurry, and 13 to 15mg/kg at the depth of 30cm.

Example 4 Fig. 2 shows the change in pH caused by the treatment of pig slurry in a test according to Example 3 in the pig slurry tank of the farm: The water-soluble phosphorus in the floating slurry of the pig slurry decreased after the treatment that was carried out on May 16. The total phosphorus decreased from a level of 1050mg/kg (before the treatment on May 16) to a level of 87mg/kg (on May 21 ). The same mixed samples as the phosphorus (Fig. 1 ) were measured for pH in a laboratory by means of a pH electrode (Merck). The samples were taken at a depth of 0 to 10cm; on May 21 and 30, also at a depth of 30cm; however, their pH is the same as on the surface, about 8.6. In the test conducted, the pH rises immediately after the treatment with the precipitation product, within about 24h, from 7.45 to 8.6, where it stays during the progress of the test. The quick increase in pH to over 8 ensures an effective dephosphorization.

Example 5

It proved to be more challenging to treat cattle slurry than pig slurry. A test was conducted on a laboratory scale, showing that the phosphorus content did not de- crease by a gypsum treatment alone, as in the case of pig slurry in Example 2, but the decrease required an MgO treatment. Table 3 shows the corresponding treatments as Example 2, and the water-soluble phosphorus contents obtained from them as percents from the content of Reference 1 (=100%).

Reference 1 : untreated liquid manure Reference 2: gypsum 0.35% by weight and polymer 15ppm

Sample 1 : gypsum 0.35% by weight and polymer 15ppm, and 0.3% by weight of MgO Table 3

On the basis of the tests conducted, it was observed that isolating the phosphate of cattle slurry from the liquid phase did not work with gypsum alone (Reference 2), as along with the addition of gypsum, the pH 7.3 of cattle slurry decreased to a level of 7.0, and cattle slurry naturally contains less magnesium in relation to calcium than pig slurry. The use of MgO (Sample 1 ) increased the pH to 8.7, which provided correct conditions for the precipitation reactions and dephosphorization. The escape of surface deposition along with the use of MgO could also be ob- served.

Example 6

On a laboratory scale, kiserite, MgSO₄H₂O, was also tested as a source of Mg in a corresponding manner as using MgO in the test according to Example 5. As the pH in the kiserite treatment remains below 7.5, on an average, the phosphate con- tents remain high. Thus, kiserite does not increase the pH sufficiently, like MgO. Fig. 3 compares the use of kiserite and MgO as sources of magnesium. The water-soluble phosphorus content of cattle slurry (1dl) in untreated floating slurry (0), and both in gypsum-MgO treatments (1 , 3, 5, 7) and gypsum-kiserite treatments (2, 4, 6, 8). In treatments 1 to 4, 15ppm of polymer was used, in treatments 5 to 8, 30ppm (K3311 , Kemira Oyj). MgO raises the pH close to 9, resulting in precipitation and a decrease in the phosphorus content in each odd test, contrary to the even kiserite tests.

Example 7

On a cattle farm, a test was conducted, observing the increase in phosphorus con- tent in 1000-liter tanks on 900-liter batches of slurry by the following treatments:

- Reference, no additions - Gypsum and polymer (30ppm, K3311 , Kemira Oyj) were added for the first time on March 13, and gypsum for the second time on April 2, and MgO was added thereto on April 17.

- Gypsum and polymer were added for the first time on March 13, and gypsum on April 2, and Mg sulphate as kiserite was added thereto on April 17.

At the beginning, gypsum was dosed in an amount of 0.35% by weight (3.15kg / 900I), but after a couple of weeks, on April 2, an additional dose of 3.5kg/ton was supplied, as the decrease in phosphorus that was observed during the first two weeks did not continue and the pH remained near 7. On April 17, an MgO treat- ment (0.3% by weight) was carried out and kiserite (0.3% by weight) was used as a reference, respectively. Fig. 4 shows the development of the phosphorus content of the surface (0 to 5cm) during the treatments. The water-soluble phosphorus content of cattle slurry (900I) at a depth of 0 to 5cm in untreated floating slurry (D, not mixed after March 13), and in gypsum-MgO treatments (■), and in gypsum- kiserite treatments (Δ). The Mg compounds were added on April 17, half of the gypsum and the polymer on March 13. At the beginning of April, 2 weeks from the beginning of the test, the air cooled down and natural mixing took place under the effect of temperature differences. At this stage (on April 2), gypsum was added to the tanks, the addition being visible as a peak on April 8. It was not until the addi- tion of Mgo that the phosphorus content decreased to below 50mg/kg, when the pH rose to a level of 7.9. The pH in the reference was 7.2.

Correspondingly, Fig. 5 shows the phosphorus contents of the samples that were taken at a depth of 30cm. The water-soluble phosphorus content of cattle slurry (900I) at the depth of 30cm in untreated floating slurry (G, not mixed after March 13) and in the gypsum-MgO treatments (■) and the gypsum-kiserite treatments (Δ). The Mg compounds were added on April 17, half of the gypsum and the polymer on March 13. The gypsum-polymer-MgO treatments proved to be the best in reducing the phosphorus content. Although the gypsum alone worked at the beginning of the test, the phosphorus content did not clearly decrease until after the MgO treatment. The temperature was only 10 to 12 degrees, which for its part influenced the slowness of the reactions compared to laboratory conditions.

In the reference test, the phosphorus content decreased because of a natural sedimentation, but not as quickly as in the gypsum treatment. The water-soluble phosphorus of the reference was first (on March 13) 240mg/kg, at the end of the test (on May 23) 135mg/kg, correspondingly, the total phosphorus content of the reference at the beginning was 345mg/kg and at the end 235mg/kg. The phosphorus content increased because of admixing additional gypsum (on April 2) and additional Mg (on April 17). MgO lowered the phosphorus content again, 57mg/kg as total phosphorus, and 18mg/kg (on May 23) as water-soluble phosphorus. Example 8

A further examination was conducted in the slurry-storage tank of the cattle farm on April 16, wherein 1501 of polymer and 540kg of gypsum (5kg/ton as dry, moisture 30%) were admixed in a slurry tank of 75 cubic metres. The slurry-storage tank was 2m deep; its length was 13m and width 5m. At first, the cattle slurry seemed thick, and solid matter accumulated on the surface because of the gas formation of microbial activity. After a week (on April 24), MgO (220kg or 3kg/ton) were admixed in the tank. Now the solid matter exits the surface of the slurry in the tank and the slurry looks clear.

Fig. 6 shows the results of the phosphorus content measurements of the slurry- storage tank, and Fig. 7 shows the behaviour of pH. The water-soluble phosphorus content of the floating slurry of the cattle slurry (75m³) at a depth of 30cm (■) and a depth of 80cm (A). It is not possible to increase the pH until another MgO batch (3kg/ton) is admixed in the tank on May 23.

Example 9 When treating liquid pig or cattle manure by the mixture of gypsum and MgO, it was also observed that the colour of the liquid manure became lighter. The lightening effect was slightly weaker for the cattle slurry than for the pig slurry. When carried out on light slurry, the phosphorus determination by a quick analysis strip yields a more reliable result, as the measurement is based on a colour reaction, whereby the dark measuring strip impedes the reading and, thus, distorts the result.

Example 10

The gypsum-MgO mixture was compacted to lower the dust formation. Compacting was carried out by a large pressing machine, wherein the solid matter was pressed into a plate, crushed, and screened into suitable-size granules. The moisture of the solid matter was adjusted by means of the moisture of the gypsum. The moisture and the particle size were adjusted so that the dust formation was below 2000mg/kg (the standard requirement for the dust formation of compacted fertilizer granule is less than 1000mg/kg, the minimum requirement of granulated or prilled fertilizer granule is 500mg/kg). The amount of dust was measured by fluidizing the sample by means of air in a special column and by collecting the loosened dust on a filter. The amount of dust was calculated on the basis of weighing. Example 11

It proved to be easier to treat cattle slurry, when the slurry was mechanically pre- separated so that the dry matter content decreased form 6% to less than 3%. In 1000-litre containers, a test was conducted on separated cattle slurry having a water content of 96.6%, showing that the content of soluble phosphorus decreased to a fourth, when the precipitation product was added in an amount of 2.2kg per ton of slurry so that the product contained 1.2kg of gypsum and 1.0kg of MgO, the slurry was stirred for 2min and allowed to sediment for 2 weeks. The pH value increased from 7.5 to a level of 8.0.

Reference: untreated liquid manure. Sample: gypsum 0.24% by weight and 0.11 % by weight of MgO.

As a result, the phosphorus content P (%) according to Table 4 was obtained:

Table 4

Example 12 2, 6 or 10% by weight of gypsum was mixed with dry manure, such as poultry manure, as well as gypsum and magnesium so that the amount of gypsum was 1 , 3 and 5g per a batch of manure and 30% of MgO₁ i.e., 0.43; 1.29 and 2,14g. The dry matter content of manure was about 40% and the total weight 5Og. After about 4 days, 100ml of water was run through the manure that was in cans with a hole in the bottom, and the phosphate phosphorus of the runoff water was measured. In untreated manure (no gypsum, no MgO), 25mg of phosphorus came into the water. From gypsum-treated manure, about 50% less phosphorus, 10 - 13mg were washed to the water. The amount of phosphorus washed from the manure that had been treated with gypsum and MgO was only about a fourth, 3 - 6mg. Fig. 8 shows the phosphorus content of the water running through, which was obtained in treating dry manure with the reference/gypsum /gypsum/MgO mixtures.

Example 13

400kg of gypsum (340kg as dry matter) and 270kg of MgO were mixed with cattle slurry in a farm tank (90m³, depth 190cm), its dry matter being 5.5% and pH 7.0, by means of a slurry mixer. The stirring took 3 hours. After 5 and 12 days, slurry samples were taken from the slurry tank at three different depths. The total phosphorus contents and the contents of soluble phosphorus in the slurry decreased to 1/7 and 1/10 up to a depth of 60cm. pH in the treatment increased over 8.7. At the middle of the tank, there was a limit of upper flowing part and sediment fraction, as the phosphorus contents had not decreased as in the upper layers (Fig. 9A and 9B).

Claims

CLAIMS:

1. A method of reducing the water-soluble phosphorus content of the more liquid-bearing floating slurry of manure, such as liquid manure and/or corre- sponding liquid manure that is formed from the filtered liquor of dry manure, characterized in that at stage

A), a solid precipitation product that contains at least gypsum as the source of calcium and at least magnesium oxide as the source of magnesium is added to the liquid manure, increasing the pH and, when added to the manure, providing bind- ing and precipitation of the soluble phosphorus, which is in the more liquid-bearing floating slurry, both aerobically and anaerobically, to a more solid sediment and into a form of compound more exploitable to plants, so that the pH of the thus provided slurry mixture is 7.5 or more, and

B), the mixture obtained from stage A is mixed, and C), the mixture is allowed to settle, and

D), the more liquid-bearing floating slurry, which is separated in the upper part of the mixture, is removed from the slurry mixture to be reused as a fertilizer.

2. A method according to Claim 1 , characterized in that the said precipitation product is added to the liquid manure in an amount of 500 to 1000kg/100m³.

3. A method according to Claim 2, characterized in that the said precipitation product is added to the liquid manure in an amount of 500 to 900kg/100m³, if the manure is pig slurry.

4. A method according to Claim 2, characterized in that the said precipitation product is added to the liquid manure in an amount of 700 to 1000kg/100m³, if the manure is cattle slurry.

5. A method according to Claim 1 , characterized in that the said precipitation product is added to the liquid manure in an amount of 150 - 800kg/100m³, if the manure is separated liquid manure.

6. A method according to any of Claims 1 to 5, characterized in that the said precipitation product is added to the liquid manure in one or more batches.

7. A method according to any of Claims 1 to 6, characterized in that, in addition to the said precipitation product, polymer is added to the liquid manure.

8. A method according to Claim 7, characterized in that the amount of said polymer that is added is less than 40ppm.

9. A method according to Claim 7 or 8, characterized in that the said polymer is polyacrylamide, diallyl dimethyl ammonium chloride or a mixture thereof.

10. A method according to any of Claims 1 to 9, characterized in that, at stage B, the mixture is mixed for at least half an hour, the solid matter content be- ing less than 7% by weight, preferably 1.5 - 6% by weight, more preferably 2.5 - 6% by weight.

11. A method according to any of Claims 1 to 10, characterized in that, at stage C, the said slurry mixture is allowed to settle for 2 to 7 days, if it is pig slurry, or 3 days to 3 weeks, if it is cattle slurry.

12. A method according to any of Claims 1 to 11 , characterized in that the pH is measured at least at one of the stages A, B or C.

13. A method according to any of Claims 1 to 12, characterized in that, after stage A, B or C, the pH of the mixture is within 7.6 and 8.8.

14. A method according to any of Claims 1 to 13, characterized in that the stages A, B and/or C are repeated to bring the water-soluble phosphorus content of the said liquid portion to a desired range.

15. A method according to any of Claims 1 to 14, characterized in that the method is implemented in the slurry tank of a farm.

16. A method according to any of Claims 1 to 15, characterized in that, at stage D, the more liquid-bearing floating slurry, which is separated in the upper part of the slurry mixture, is removed by pumping it into spreading equipment.

17. A method according to Claim 16, characterized in that the said pumping is carried out from the middle of the floating slurry.

18. A method according to any of Claims 1 to 17, characterized in that the method further includes stage E, wherein the sediment is removed to be used as a fertilizer or in energy applications.

19. A method according to any of Claims 1 to 18, characterized in that the ratio of the phosphorus content of the said more liquid-bearing floating slurry, which is treated with the precipitation product, to the phosphorus content of the floating slurry of untreated liquid manure is less than 1 :4.

20. A method according to any of Claims 1 to 19, characterized in that the Ca and Mg compounds of the said precipitation product are added to the liquid manure as separate compounds at the same time.

21. A solid precipitation product containing at least gypsum as the source of calcium and at least magnesium oxide as the source of magnesium, which, when added to a liquid or solid matter that contains water-soluble phosphorus and is brought into contact with water, provides binding and precipitation of the phos- phorus both aerobically and anaerobically into a form of compound that is exploitable to plants.

22. A precipitation product according to Claim 21 , characterized in that adding it to an aqueous solution increases the pH of the solution.

23. A precipitation product according to Claims 21 and 22, characterized in being compacted and granulated and that its dust formation is less than

2000mg/kg.

24. A precipitation product according to any of Claims 21 to 23, characterized in that the said gypsum is natural gypsum, CaSO₄2H₂O, synthetic phosphogypsum, CaSO₄I /2H₂O, CaSO₄, or a mixture thereof.

25. A precipitation product according to Claim 24, characterized in that the said gypsum is synthetic phosphogypsum.

26. A precipitation product according to any of Claims 21 to 25, characterized in that the ratio Ca:Mg of calcium and magnesium is less than 0.9.

27. The use of the solid precipitation product according to Claim 21 for decreasing the content of water-soluble phosphorus in the more liquid-bearing floating slurry of liquid manure.

28. The use according to Claim 27, characterized in that the said precipitation product is added directly to the slurry tank of a farm from its sales package.

29. The use of the solid precipitation product according to Claim 21 for decreasing the content of water-soluble phosphorus of dry manure, when it gets in contact with water.

30. The use according to Claim 29, characterized in that the solid precipitation product according to Claim 21 is added to dry manure, preferably poultry manure, whereby the pH of the thus developing mixture is 7.5 or more, and the developing mixture is stirred and it is brought into contact with water.