EP3687942A1

EP3687942A1 - Recovery of high purity calcium phosphate

Info

Publication number: EP3687942A1
Application number: EP18778466.5A
Authority: EP
Inventors: Haiyan QU; Liubov VASENKO
Original assignee: Syddansk Universitet
Current assignee: Syddansk Universitet
Priority date: 2017-09-25
Filing date: 2018-09-25
Publication date: 2020-08-05
Also published as: WO2019057984A1

Abstract

The present invention relates to a method for recovery of high purity calcium phosphate from waste waters. In particular, the present invention relates to precipitation of dibasic calcium phosphate dihydrate (DCPD) or hydroxyapatite (HAp). The DCPD and HAp obtained by the method complies with the European Commission Regulation No 231/2012 laying down specifications for food additives. Thus, the high purity calcium phosphate obtained by the method of the invention may be suitable for use in the food industry.

Description

Recovery of high purity calcium phosphate

Technical field of the invention

The present invention relates to the technical field of crystallization of calcium phosphate. In particular, the present invention relates to a method for recovery of high purity calcium phosphate from waste waters comprising organic matter. More particular, the present invention relates to crystallization of dibasic calcium phosphate dihydrate (DCPD) or hydroxyapatite (HAp). Background of the invention

Crystallization of phosphorus compounds from digester supernatant is one of the common ways to recover phosphorus at wastewater treatment plants. Digester supernatant is a liquid phase, which is produced during sludge digestion. The phosphorus concentration in the digester supernatant is the highest compared to all the other streams in the process. The digester supernatant can contain up to 1800-1900 mg phosphate (P04^3~) per liter and all of the phosphate is in the dissolved state. The presence of a high phosphate concentration in a digester supernatant, makes phosphate-recycling from the digester supernatant one of the preferable ways for achieving phosphorus sustainability. Technologies based on the crystallization of struvite (MAp) or calcium phosphate (CaP), with fluidized bed or continuous flow stirred-tank reactors, have been developed in the past decades and are successfully implemented at wastewater treatment plants. However, in these known technologies, the final product can only be used in agriculture as a fertilizer due to low purity of the MAp or CaP. The development of a more sophisticated technology can significantly improve the economy of the process, if pure CaP can be produced for the further industrial use.

The crystallization of pure CaP from digester supernatant is however challenging due to the high number of different reactions, which can take place during crystallization, and the presence of different impurities. The presence of heavy metals significantly decreases the quality of obtained product. US 6,569,342 Bl discloses a single stage method of precipitating HAp from waste waters using alkaline earth metal peroxides. However, the purity of the HAp is low due to mineralization of organic matter and co-precipitation with HAp.

Hence, an improved method that solves the above mentioned problems of the prior art would be advantageous. In particular a method resulting in CaP of high purity from digester supernatant is needed.

Summary of the invention

An object of the present invention is to provide a two-stage method for recovery of high purity calcium phosphate from waste waters comprising phosphate but also organic matter, such as humic substances, biopolymers, low molecular weight organic compounds etc. In particular, it is an object of the present invention to provide a method that solves the above mentioned problems of the prior art with low purity of the calcium phosphate. In addition, it is known that organic matters, such as humic substances, have an inhibitory effect on crystallization of calcium phosphate. With the present invention, the inventors have found a method where organic matter (especially humic substances) efficiently can be oxidized such that the calcium phosphate more efficiently can be recovered, both with regard to amount but also the quality of the calcium phosphate.

Thus, one aspect of the invention relates to a two-stage process for recovery and/or purification of calcium phosphate from an aqueous solution compring organic matter and phosphate, comprising the steps of;

a) contacting an aqueous solution comprising phosphate and organic matter with an oxidant for a time period sufficient to allow oxidation

b) adding a calcium salt to the oxidized solution of step a) and adjusting pH to be in the range of 5.5 to 8.5 to form a precipitate comprising calcium phosphate c) separate said precipitate from said solution

Brief description of the figures

Figure 1 shows Raman spectra of the different products obtained together with a reference sample of DCPD and HAp. Figure 2 shows PXRD patterns of different products obtained.

Figure 3 shows SEM images from the samples obtained in Example 1 with pH 3.9 in the oxidation, a) Sample obtained from experiment No. 5; b) Sample obtained from experiment No. 6; c) Sample obtained from experiment No. 7; d) Sample obtained from experiment No. 8.

Figure 4 shows SEM images from the samples obtained in Example 1 with pH 7.9 in the oxidation, a) Sample obtained from experiment No. 1; b) Sample obtained from experiment No. 2; c) Sample obtained from experiment No. 3; d) Sample obtained from experiment No. 4.

Figure 5 shows the influence of the pH value at the oxidation stage on phosphorus recovery (P-recovery) during crystallization when oxidation with hydrogen peroxide and ultrasound treatment lasted for 24 hours.

Figure 6 shows the influence of the pH value at the oxidation stage on the color of obtained products when oxidation with hydrogen peroxide and ultrasound treatment lasted for 24 hours.

Figure 7 shows the influence of ozonation conditions such as dose of ozone and ozonation pH on the reduction of Color436, chemical oxygen demant (COD) and total organic carbon (TOC) concentrations.

Figure 8 shows the influence of ozonation conditions such as dose of ozone and ozonation pH on phosphorus recovery (P-recovery) rate durind calcium phosphate precipitation from ozonated digester supernatant Figure 9A shows the effect of pH of ozonation on total alkalinity (TAC) of digester supernatant.

Figure 9B shows the effect of pH of ozonation on the color of solid products. Figure 10 Shows the effect of crystallization conditions such as crystallization pH and seeding addition on phosphorus recovery rate.

The present invention will now be described in more detail in the following.

Detailed description of the invention

Thus, one aspect of the invention relates to a two-stage process for recovery and/or purification of calcium phosphate from an aqueous solution comprising organic matter and phosphate comprising the steps of;

b) adding a calcium salt to the oxidized solution of step a) and adjusting pH to be in the range of 5.5 to 8.5 to form a precipitate comprising calcium phosphate c) separate said precipitate from said solution.

In the context of the present invention, the following abbreviations have been used :

ACP Amorphous Calcium Phosphate

CaP Calcium phosphate

COD Chemical Oxygen Demand

DCPA Dicalcium Phosphate Anhydrous

DCPD Dicalcium Phosphate Dihydrate

HAp Hydroxyapatite

MAp Magnesium Ammonium Phosphate

NOM Natural Organic Matter

OCP Octacalcium Phosphate

P Phosphorus

P-Recovery Phosphorus Recovery Rate

PXRD Powder X-Ray Diffraction

SEM Scanning Electron Microscopy

TAC Total alkalinity

TOC Total Organic Carbon

US Ultrasonication WWTP Wastewater Treatment Plant

In the context of the present invention, the term "two-stage process" refers to separately conducting a first stage of oxidation followed by a second stage of precipitation.

In the context of the present invention, the term "recovery" refers to the percentage of a particular substance that is isolated or removed from the aqueous solution.

In the context of the present invention, the term "solution" has the usual meaning in the art being a homogeneous mixture composed of two or more substances. In such a mixture, a solute is a substance dissolved in another substance, known as a solvent. Hence, an aqueous solution refers to a solution wherein the majority of solvent is water. In the context of the present invention, the term "majority" refers to at least 95 % volume by volume.

In the context of the present invention, the term "purification" has the usual meaning in the art and refers to removal of one or more unwanted contaminant(s) to obtain a product of higher purity. Purity may be determined by a range of methods known by the skilled person chosen from but not limited to, HPLC methods such as ion chromatography, LC-MS, ICP-MS, FT-Raman or NMR.

In the context of the present invention, the term "calcium phosphate" refers to a family of materials and minerals comprising calcium ions (Ca²⁺) together with inorganic phosphate anions. Hence, this group consists of materials and minerals comprising Ca²⁺ ions together with orthophosphate (P0₄ ^3"), monohydrogen phosphate (HP0₄ ^2") or dihydrogen phosphate (H₂P0₄ ^"), such as but not limited to monocalcium phosphate (Ca(H₂P0₄)2 or Ca(H₂P0₄)₂ H₂0), dicalcium phosphate (dibasic calcium phosphate, CaHP0₄ or CaHP0₄ 2H₂0), tricalcium phosphate (Ca₃(P0₄)₂), octacalcium phosphate (Ca₈H₂(P0₄)₅ 5H₂0), hydroxyapatite

(Ca₅(P0₄)₃(OH)) or tetracalcium phosphate( Ca₄(P0₄)₂0).

In the context of the present invention, the term "phosphorus" refers to any molecule comprising the chemical element phosphorus (P). Hence phosphorus may be present in any known oxidation state such as 5, 4, 3, 2, 1, - 1, -2, -3 in the aqueous solution . Typically the phosphorus is present as phosphate (P0₄ ^3"), HP0₄ ^2", H₂P0₄ ^" or hbPC depending of the pH of the aqueous solution . The phosphorus concentration of the aqueous solution in step a) may vary depending on the origin . In an embodiment of the invention, the phosphorus concentration may be up to 600 mg elementary phosphorus (P) present as phosphate. Thus, the phosphate concentration in the aqueous solution may be as high as 1840 mg phosphate (P0₄ ^3~) per liter. In the process for recovery and/or purification of calcium phosphate according to the present invention from an aqueous solution comprising phosphate and organic matter, the oxidation step is crusial, since the oxidation step provides oxidation and degradation of the organic matter in order to prevent the inhibitory effect of organic matter in crystallization of calcium phosphate .

By oxidant is meant any substance capable of oxid ising a substrate in a reaction, the oxidant itself in turn being red uced . Oxidation has the usual meaning to the skilled person in the art being the complete, net removal of one or more electrons from a molecular entity and an increase in the oxidation state of any atom within the substrate.

In an embod iment of the invention, the oxidant is selected from the group consisting of peroxides or alkali metal salts thereof, ozone, oxygen, hypochlorites, perchlorates, peroxy acids or alkali metal salts thereof, ammonium cerium(IV) nitrate, sulphur based oxidants, or mixtures thereof.

In the context of the present invention, the term "peroxide" has the usual meaning in the art as a compound containing an oxygen-oxygen single bond . Hence, a peroxide may be an unbranched or branched monoalkyl peroxide (RO- OH) such as tert-butyl peroxide, an unbranched or branched dialkyi peroxide (RO- OR) such as di-tert- butyl peroxide, hydrogen peroxide (HO-OH) or a peroxide anion (Ό-0^") . Thus, alkali metal salts of peroxides in the present context should be understood as a peroxide that forms a salt with an alkali metal such as but not limited to (RO-ONa) or (HO-ONa) . In an embodiment of the invention, sulphur based oxidants may be chosen from but are not limited to sulphur trioxide (SO3), persulfates such as ammonium persulfate (NH₄)2S208 and monopersulfate compound (KHSOs ^■ 0.5KHSO₄ ^■ 0.5K₂SO₄) known as Oxone.

In an embod iment of the invention the oxidant is selected from the g roup of from peroxides or alkali metal salts thereof, ozone, oxygen or mixtures thereof. The oxidant may for example be oxygen . In a embodiment of the invention, the oxidant is selected from peroxides or alkali metal salts thereof, ozone or mixtures thereof. In a preferred embodiment of the invention, the oxidant is peroxides or alkali metal salts thereof, and may be but are not limited to hydrogen peroxide (H₂0₂) or sod ium hydrogen peroxide (NaOOH) or a combination thereof. In an even further preferred embodiment, the peroxide is H₂0₂. The oxidant may be added in different concentrations depending on the desired time period used in the oxidation in step a) . Thus, if a short oxidation time period in step a) is desired the oxidant may be added in a hig her concentration and vice versa . The oxidant may be added to obtain a final concentration of 0.01 to 2.5 mol/L, such as 0.05 to 2.0 mol/L, preferably 0.1 to 1.8 mol/L, preferably 0.5 to 1.6 mol/L. By final concentration is meant the concentration of the oxidant after addition i .e. the initial concentration of the oxidant in the aq ueous solution in step a) .

If ozone is used as oxidant, the concentration may be from 0.75 to 3 mg 03/mg Total Organic Carbon (TOC), preferably, the concentraton of ozone is 1.0 to 2.5 mg 03/mg TOC, such as 1.0 to 2.0 mg 03/mg TOC, most preferably 1.25 mg

In an embodiment of the invention, the calcium (Ca²⁺) concentration in the aq ueous solution is no higher than 300 mg/L . A too high Ca²⁺ concentration may initiate precipitation of calcium phosphate during the oxidation stage and result in lower purity of the calcium phosphate.

In an embod iment of the invention, the pH in step a) is in the range of 4 to 9 such as 5.0 to 8.8, such as 6.0 to 8.6, preferably 7.0 to 8.4, more preferably 7.5 to 8.2, most preferably 7.7 to 8.0. Surprisingly, the inventors found that phosphorus recovery in form of calcium phosphate was higher when the pH during oxidation was from 4 to 9 as compared to when a lower or higher pH was used . In particular, the phosphorus recovery was found to be highest when the pH in step 5 a) was around 7.7 to 8.0. Furthermore, the product appeared more white when the pH in step a) was around pH 7.7 to 8.0.

In a further embodiment of the invention, the oxidation in step a) is performed for a time period of at least 15 min.

10

The oxidation time varies dependent on the oxidant used, For example, if ozone is used as oxidant, the oxidation time could be lower than when for example hydrogen peroxide is used as oxidant. When ozone is used as oxidant, the time periode for oxidation may for example be from 15 minutes to 90 minutes, such as 15 from 20 minutes to 60 minutes, preferably form 30 minutes to 50 minutes. When for example hydrogen peroxide is used as oxidant, the time periode for oxidation is at least 20 minutes, such as at least 30 min, such as at least 1 h, preferably at least 2 h, such as at least 3 h, more preferably at least 4h, such as at least 5 h, even more preferably at least 10 h, such as at least 20 h.

20

In a further embodiment of the invention, the oxidation in step a) is performed for a time period of at least 25 h, such as at least 30 h, such as at least 35 h, preferably at least 40 h, such as at least 45 h, more preferably at least 50 h, such as at least 24 h, even more preferably at least 48 h.

25

In an embodiment of the invention the time period of the oxidation in step a) is from 15 min to 70 h, such as from 30 min to 65 h, such as from 2 to 60 h, such as from 4 to 55 h, such as from 6 to 50 h, such as from 8 to 45 h, such as from 10 to 40 h, such as from 12 to 35 h, such as from 14 to 30 h, such as from 16 to 30 25 h. In a preferred embodiment, the time period for oxidation when hydrogen peroxide is used as oxidant is from 20 to 28 h, in an even more preferred embodiment the time period is from 22 to 26 h.

The inventors of the present invention has found out that the oxidation in step a) with hydrogen peroxide in the laboratory should be performed for about 20 to 28 hours with the amount of oxidant used in the examples of the present application. In industrial scale, it might be relevant to increase the time of oxidation and this will still provide a process with high calcium phosphate recovery and high purity. In an embodiment of the invention, the oxidant is added to the aqueous solution comprising phosphorus in step a) under ultrasound treatment. The ultrasound treatment aids in the dissolution of the oxidant if added as a solid to the aqueous solution in step a). The ultrasound treatment is used to decompose H2O2 to OH- radicals. Ultrasound treatment can also decrease contact time of the oxydation step. Furthermore, without being bound by any theory, the ultrasound treatment aids in the formation of the highly reactive hydroxyl radical (ΌΗ) from the H2O2. HO^* is formed due to the energy involved during cavitation bubble collapse when water is treated with ultrasound. The ultrasound treatment is performed for a period of at least 5 minutes such as at least 10 min., such as at least 20 min., such as at least 30 min., such as at least 40 min., such as at least 50 min., such as at least 60 min. In an embodiment the ultrasound treatment is performed for 5 min. to 48 hour, such as 10 min. to 40 hour, such as 15 min. to 32 hour, such as 20 min. to 24 hour, such as 25 min. to 16 hour, such as 30 min. to 8 hour. In yet another embodiment of the invention, the ultra sound treatment is performed throughout the oxidation.

In the context of the present invention, "ultrasound" may also be called

"sonication" or "ultrasonication". The terms may be used interchangeably and refers to the same, namely applying sound energy to the solution at ultrasonic frequencies (>20 kHz).

In a further embodiment of the invention, the process comprises no addition of FeCl2. FeCl2 will have a negative effect on the calcium phosphate recovery products, since the iron can be accumulated in the solid products and high amounts of iron are not suitable for food products and will not be allowed by the EU regulations on food and feed additives.

In an embodiment of the invention, a pH adjusting agent is added to the oxidized solution after step a) and before step b) to adjust the pH to be 5 or below, such as pH 1 to 5, preferably pH 1.5 to 2.5, even more preferably pH 2. The pH is adjusted to obtain an undersaturated state before the addition of the calcium salt in step b). By undersaturated state is meant any state wherein the phosphate is soluble in presence of calcium (Ca²⁺) such that crystallization of calcium

phosphate does not occur. Immediately after obtaining the undersaturated state the calcium salt in step b) is added in an amount to obtain a Ca/P molar ratio of 2. The calcium salt may be added as a solid or dissolved in an aqueous solution. In a preferred embodiment, the calcium salt is added in an aqueous solution.

In an embodiment of the invention, the calcium salt is added to obtain a Ca/P ratio in the range of from 1 to 5, preferably from 1 to 2, even more preferably from 1 to 1.7 depending on which product is aimed to be crystallized.

By Ca/P is meant the molar ratio between calcium and phosphorus, 1 mol phosphate comprises 1 mol phosphorus. After addition of the calcium salt, the pH is adjusted to be in the range of 5.5 to 8.5. The pH is increased to be in the range of 5.5 to 8.5 by addition of a base selected from the list consisting of alkali metal hydroxides or alkali metal carbonates. In a preferred embodiment, the base is an alkali metal hydroxide such as sodium hydroxide (NaOH). The base may be added as a solid or predissolved in an aqueous solution. The pH in step b) is adjusted to be in the range of 5.5 to 8.5 such that the nucleation point is reached . Nucleation has the typical meaning in the art and is to be understood as the first step in the formation of a new thermodynamic phase or a new structure via self-assembly or self-organization. Nucleation is typically defined to be the process that determines how long an observer has to wait before the new phase or self-organized structure appears, in this case a precipitate. Nucleation is reached when the change in turbidity can be detected.

In a preferred embodiment of the invention, the pH during precipitation in step b) is adjusted to pH 6.0 to 8.2, such as 6.2 to 8.2, preferably 6.5 to 8.0, even more preferably 7.0 to 8.0. In a preferred embodiment of the invention, the pH during precipitation in step b) is adjusted to pH 6.5 to 8.2, such as 7.2 to 8.2, preferably 7.7 to 8.0 for precipitation of HAp. If DCPD is to be crystallized, pH should be adjusted to 5.5 - 7.0, for example 5.5 to 6.5, such as 5.7 - 6.3, preferably 5.8 - 6.0. In an embodiment of the invention, the pH of the nucleation is selected as the operation pH for the crystallization and is kept constant during the crystallization by addition of a base such as NaOH . In another embodiment a slight increase of the pH value after nucleation was observed . Then pH was then kept stable at the value to which it increased .

At the end of the stage b) pH can be adjusted to a slightly higher value than the nucleation pH in order to increase phosphorus recovery in the form of calcium phosphate, but no higher than 8.5. pH values at the end of the stage b) above 8.5 is not desired, as the supersaturation will be too hig h, causing a to fast

spontaneous nucleation resulting in precipitation of amourphous calcium

phosphate thereby reducing the purity of the prod uct. In addition, the pH at stage b) should be at least pH 5.5, since at a lower pH the system will be in the undersaturated state and no crystallization will happen, or the system will be very slightly supersaturated and the supersaturation will not be enough to start crystallization, i .e no precipitation .

In an embod iment of the invention, step a) and/or, b) is performed under stirring . Stirring should be understood in the broadest sense as any means of causing a movement in a liquid resulting in mixing . Stirring may be performed by any means known to the person skilled in the art such as but not limited to mag netic stirring, mechanical stirring, using a static mixer, using a shaker, using a fluidized bed reactor or using a bubble column reactor. In a preferred embodiment stirring is performed using a fluidized bed reactor.

In an embod iment of the invention, separation in step c) is performed using filtration, centrifugation and/or decantation or any other means for separation of a solid from a liquid known to the skilled person . The filtration may be performed with or without vacuum . In a preferred embodiment of the invention, separation is performed using vacuum filtration .

In yet an embodiment of the invention, step b) furthermore comprises add ition of dibasic calcium phosphate dihydrate (DCPD) crystals, hydroxyapatite (HAp) crystals or octacalcium phosphate (Ca₈H2(P04)6-5H₂0; OCP) crystals for seeding . Without being bound to theory, crystallization is improved by an initiation step which can be spontaneous or can be done by adding a small amount of a seed crystal such as DCDP, HAp or OCP to a supersaturated solution. In a preferred embodiment, DCDP is added to the solution in step b). The seeding crystal is added after the addition and dissolution (if added as a solid) of the calcium salt in step b) and after the pH adjustment to the range of 5.5 to 8.5. In a preferred embodiment the seed crystals is added right before the nucleation point is reached. In an embodiment of the invention, the precipitate formed in step b) comprises dibasic calcium phosphate dihydrate (DCPD), hydroxyapatite (HAp), octacalcium phosphate (OCP), amorphous calcium phosphate (ACP), dibasic calcium

phosphate anhydrous (DCPA) or mixtures thereof. Several factors may influence the composition of the precipitate such as but not limited to the pH in the oxidation in step a), the time period for which oxidation is performed in step a), the oxidant used in step a), whether or not seeding in step b) is performed, the seed crystal used such as DCPD, OCP or HAp, the calcium salt added in step b) and the pH used at precipitation in step b). Precipitation should be understood in the broadest sense as the formation of a solid from a solution the solid being either crystalline or amorphous. Thus, precipitation may also encompass crystallization. In one embodiment the precipitate is a crystalline solid. In another embodiment the precipitate is an amorphous solid. In a yet another embodiment the precipitate is a mixture of crystalline and amorphous solid . In a preferred embodiment the precipitate is a crystalline solid. Crystalline and amorphous solid has the usual meaning in the art. A crystalline solid thus means any solid material whose constituents are arranged in a highly ordered microscopic structure forming a crystal lattice, i.e. it is the presence of three-dimensional order on the level of atomic dimensions. Crystalline solid may either be single crystals or polycrystals composed of many microscopic crystals also known as crystallites. Several techniques known to the skilled person can be used for the detection of crystallinity such as but not limited to Powder X- Ray Diffraction (PXRD) and Single-crystal X-Ray Diffraction. In an embodiment of the invention, the calcium salt added in step b) is selected from the group consisting of CaCI₂, CaC03, Ca(OH)₂ or mixtures thereof. In a preferred embodiment, the calcium salt is CaCI₂. In an embodiment of the invention, the process is conducted at a temperature of 5 to 40 C, such as 7 to 38 C, such as 9 to 36 C, preferably 11 to 34 C, such as 13 to 32 C, even more preferably 15 to 30 C, such as 17 to 28 ^°, such as 19 to 26 C or at room temperature. The temperature may be the same or different in step a), b) and c). Heating and/or cooling may be performed with any conventional methods known to the person skilled in the art depending on scale on which the process is performed. In a preferred embodiment, the process is conducted at room temperature such that no heating and cooling is required. This allows for a more energy sufficient process. In an embodiment of the invention, the pH is maintained constant during step b) after the pH adjustment. As calcium phosphate precipitates during step b), the pH of the solution decreases. In order to achieve maximum recovery of calcium phosphate, the pH is kept constant by addition of a base. In an embodiment the base added is selected from the list consisting of alkali metal hydroxides. In a preferred embodiment the base is NaOH. The base may be added as a solid or predissolved in an aqueous solution. In a preferred embodiment the base is predissolved in water.

In an embodiment of the invention, the precipitation in step b) is for a period of 1 to 24 h, such as 2 to 22 h, such as 3 to 20 h, 4 to 18 h, preferably 5 to 16 h, such as 6 to 14 h, more preferably 8 to 12 h, preferably 1 to 3 h, even more preferably 2 h.

In an aspect of the invention, the aqueous solution comprises organic matter. In the context of the present invention, the term "organic matter" is to be

understood as a group of carbon based compounds originating from plants or animals and can be found in soils and aquatic environments. The organic matter may for example include one or more humic substances, biopolymers, and low molecular organic compounds. In the context of the present invention, the organic matter is present in he aqueous solution in small amounts and therefore the term "organic matter" does not include "organic solvents". In a preferred embodiment of the invention, organic matter comprises humic substances. Humic substances are the major organic constituents of soil (humus) and humus refers to a fraction of soil organic matter. Humic substances encompass a complex diverse class of molecules with humic acids being the principal component of humic substances. Humic substances can be found in water such as waste water from treatment plants. Humic substances complex ions such as calcium and can therefore make precipitation of DCPD, HAp, OCP and/or DCPA less effective. Thus, in the present invention, the oxidation is performed to degrade organic matter, such as humic substances. Hence, the quality of the calcium phosphate increases and the color of the precipitate isolated in step c) is becoming more white.

In an embodiment of the invention, the aqueous solution is waste water from a waste water treatment plant, waste water from industry or household waste water. In principle, any aqueous solution comprising sufficient phosphorus in the form of phosphate to be precipitated may be used . By sufficient phosphorus is meant phosphorus in a minimum concentration 5 mg of phosphorus (P) per liter. In a preferred embodiment, the aqueous solution is a digester supernatant from a waste water treatment plant. When sludge is allowed to settle in a digester, a supernatant develops. Anaerobic digester supernatant is commonly returned to the head of wastewater treatment plants and mixed with the influent. Although the supernatant is relatively small in volume, it contains dissolved and suspended organic and inorganic materials such as phosphorus in the form of phosphate . These materials add suspended solids, nutrients (nitrogen and phosphorus), and organic compounds to the influent.

The aqueous solution according to the present invention comprises phosphate (P0₄), but the aqueous solution does not comprise phosphorous acid (H PO3) . The process of the invention does not only recover valuable high purity calcium phosphate products but also prevents eutrophication if the waste water is returned into the eco system . Eutrophication is to be understood as the

enrichment of a water body with nutrients, usually with an excess amount of nutrients. This process induces growth of plants and algae and due to the biomass load, may result in oxygen depletion of the water body. Eutrophication is almost always induced by the discharge of phosphate-containing detergents, fertilizers, or sewage, into an aquatic system.

In an embodiment of the invention, the aqueous solution is filtered prior to 5 oxidation to remove suspended solids. Filtration should be understood in the

broadest sense as any mechanical or physical operation that separate solids from fluid by adding a medium through which only the fluid can pass. Filtration may be performed with any method known by the skilled person such as but not limited to pressure driven filtrations. Filtration may be performed with a filter with mean

10 pore size of 20 - 25 Mm. Other pore sizes may be applicable depending on the size of the suspended solids in the aqueous solution to be treated. In another embodiment of the invention, no filtration is needed prior to oxidation when the aqueous solution contains low amount of suspended solids. In yet an embodiment of the invention, methods for removing suspended solids may include but are not

15 limited to centrifugation and/or decantation.

In an embodiment of the invention, the pH is adjusted with a pH adjusting agent selected from the group of acids or bases such as NaOH, and/or HCI. Suitable acids include but are not limited to HCI, HBR, HNO3, H3BO3, CH3COOH, H2CO3 and 20 H3PO4. Suitable bases include but are not limited to LiOH, KOH, NaOH, NH3,

NH4OH, Na₂C03, Ca(OH)₂. In a preferred embodiment the acid is HCI and the base is NaOH. The acid or base may be added as a solid, in an aqueous solution or as a gas depending on the acid or base used.

25 The precipitate obtained by the process of the present invention is of food grade quality. In the context of the present invention, food grade quality is to be understood as a product complying with the European Commission Regulation No 231/2012 laying down specifications for food additives in terms of heavy metals content listed in Annexes II and III to Regulation No 1333/2008 of the European

30 Parliament and of the Council.

In an embodiment of the invention, the precipitate comprises at least 65% by weight of DCPD, HAp, OCP and/or DCPA or their mixture, such as at least 70%, such as at least 75%, such as at least 80%, such as at least 85%, such as at least 35 90%, such as at least 95%. By using the process of the present invention, a high recovery of phosphorus in the form of calcium phosphate is obtained . In add ition, the phosphorus recovery will include a hig h amount of the calcium phosphates DCPD and HAp. At different conditions of the procees, DCPA or OCP can also be obtained in the precipitate obtained by the present process. However, the precipitate obtained by the process of the present invention will comprise no or neglible amounts of struvite.

The phosphorus recovery from the aqueous solution of step a), when using the two-stage process of the present invention, is at least 70% . In particular, the phosphorus recovery of the aqueous solution of step a) when using the process of the present invention is at least 75%, such as at least 80%, in particular at least 85% . It is possible to further improve the process by amending the process parameters, for example by cooling/heating, improve filtration, amend pressure etc, and thus obtain an even higher phosphorus recovery, i .e. a phosphorus recovery of at least 90%, such as at least 92%, for example at least 97% . By phosphorus recovery is meant the percentage of phosphorus recovered from the aq ueous solution in step a) . The phosphorus recovery is calculated as defined below and is refered to as P-recovery later on in the text

P-recovery :

100 %, where P_rec is the % of phosphorus (P) recovered from the aq ueous solution ; P_in is the initial phosphorus (P) concentration in the aqueous solution; P_fin is the phosphorus (P) concentration in the aqueous solution after step c) .

The calcium phosphate recovery obtained in the process according to the present invention will be measured by measuring phosphorus recovery, since all phosphorus present in the digester supernatant is in the form of phosphate. In an embodiment of the invention the process is conducted under ambient atmosphere and/or ambient pressure. By ambient atmosphere and/or pressure is meant the atmosphere and pressure of the immediate surroundings. In another embodiment of the invention the process is conducted under and artificial atmosphere and/or pressure. The artificial atmosphere may be chosen from but is not limited to oxygen and/or ozone. The pressure may be in the range of 0.5 to 20 bar, such as 0.6 to 17 bar, such as 0.7 to 14 bar, such as 0.8 to 11 bar, such as 0.9 to 8 bar, such as 1.0 to 5 bar, such as 1.1 to 2 bar. In a preferred embodiment, the process is conducted at ambient atmosphere and/or pressure.

It should be noted that embodiments and features described in the context of one of the aspects of the present invention also apply to the other aspects of the invention.

The invention will now be described in further details in the following non-limiting examples.

Materials and methods (used in example 1, 2, 3)

Materials

Aaby Wastewater treatment plant, located in Aarhus, Denmark, provided a digester supernatant with initial total phosphate (P0₄ ^3") concentration of 0.01 mol/L and calcium concentration 0.0002 mol/L.

Munktell filter paper with the mean pore size of 20 - 25 Mm was used for the filtration of the digester supernatant before all experiments in order to remove suspended solids. Hydrogen peroxide solution (30 % (w/w) in H2O) was used as oxidant for the oxidation of the digester supernatant. pH adjustment was done using 2M standard NaOH, 23% w/w NaOH, or 37% w/w HCI solutions. 1M CaCI₂ solution for dosing Ca during the crystallization stage was prepared from reagent grade CaCI₂.

Reagent grade DCPD and HAp were used as the control samples for the FT-Raman analysis. Reagent grade DCPD was used for the seeding experiment. pH electrodes were calibrated using pH 4 and pH 7 IUPAC buffer solutions before each experiment and were stored in a 3M KCI solution between measurements.

Advantec Glass Fiber filter with the pore size of 0.45 Mm were used for vacuum filtration of solid and liquid phases after crystallization.

All chemicals were purchased from Sigma Aldrich.

Methods

Analysis of liquid phase with ion chromatography

A 761 SD Compact IC with an 813 Compact Autosampler by Metrohm is used for determination of calcium and phosphate ions in the solution. Phosphate's concentration is analyzed with Metrosep A Supp 5 - 150 (6.1006.520) using 3.2 mM Na₂CO3/1.0 mM NaHC03 + 10 % acetone as an eluent. Metrosep Cation 4 (6.1010.000) column and a 1.7 nM nitric acid/0.70 mM dipicolinic acid as an eluent is used for calcium determination.

All phosphorus (P) was present in the form of phosphate (P0₄ ^3") in the digester supernatant provided by Aaby Wastewater treatment plant, located in Aarhus, Denmark. Hence, P-recovery could be calculated using the phosphate (P0₄ ^3") concentration.

Thus for the examples, P-recovery was calculated as:

where P_rec is the % of phosphorus (P) recovered from the aqueous solution; is the initial phosphate-(P0₄ ^3") concentration in the aqueous solution;

[PO^^"] _in is the phosphate-(P0₄ ^3") concentration in the aqueous solution after step c). Analysis of solid phase

FT-Raman

A Bruker MultiRAM FT-Raman Spectrometer was used with 1064 nm laser at 100 - 400 mW laser power depending on the color of the samples. Baseline correction of spectra was done using CrystalSleuth software.

Powder X-Ray Diffraction

The Powder X-ray Diffraction (PXRD) analysis was performed using A Rigaku Miniflex 600 X-ray diffractometer Cu source of X-rays and a Ni filter operating at voltage - 40 kV, current - 15 mA, speed -10°/min. The patterns were recorded at 2Θ range of 2 - 70° and a step size 0.02 °. A COD-Inorg database with the demo version of Match! Ver.3. were used to interpret the PXRD patterns.

Scanning Electron Microscope imaging

EI Quanta 200E Scanning Electron Microscope was used for imaging of the obtained crystalline products. It was operated at the low vacuum mode (at the pressure of 30 Torr) in order to prevent charging.

Determination of the metal contents

The selected solid samples were analyzed using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to determine the metal contents. Oxidation of the digester supernatant

The initial pH of the digested supernatant was 7.9, therefore for the oxidation at the pH 7.9 no adjustment was needed. For the oxidation at the pH 3.9 and pH 2, the solution was adjusted by addition of 37 % HCI. During addition of HCI the formation of gas was observed. For the oxidation at pH 10, pH was adjusted by adding NaOH. When the pH value increased to above pH 9, a brown sludge started to coagulate. This sludge was further destroyed during oxidation. The change of the solution's volume during pH adjustments was less than 1%.

Oxidation was performed in a 1 L glass reactor with magnetic agitator. During the ultrasonication, mixing was set up to 50 rpm. Tip horn was dipped into the solution for approximately 2 cm. Ultrasonificator Hielscher UP200St was operated at power of 50W, amplitude of 94 %, C=70 %. 20 ml of hydrogen peroxide were added to the reactor at the beginning of the ultrasound treatment. The ultrasonication lasted 10 min. After ultrasonication, the solution was mixing at 250 rpm for 24h or 48h for the prolonged oxidation. After 24h or 48h, 10 ml of sample was taken out for the analysis and the crystallization stage was proceeded on the remaining.

Crystallization of calcium phosphates

Crystallization was performed in the 1 L glass reactor with overhead magnetic agitator set up to 100 rpm. The combination pH electrode and the Crystal

Eyes turbidity probe were dipped into the solution. They were connected to the Crystal Eyes software for the continuous pH and turbidity measurement. When the oxidation was done at pH 2.0 and pH 3.9 no pH adjustment before adding CaCI₂ was needed. When the oxidation was performed at pH 7.9 and pH 10, the pH of the digester supernatant was first lowered to 2.0 using 37 % HCI to ensure an undersaturated state of the solution in order to avoid spontaneous nucleation. Immediately after obtaining the undersaturated state a 1M CaCI₂ solution was added with a pipette to obtain a Ca/P molar ratio of 2. Afterwards, the pH was increased using 23 % NaOH solution until the change of turbidity can be detected, which attested the nucleation.

For the seeding experiments, right before the nucleation point was reached, 0.2 g of DCPD seeds was added to the reactor.

The nucleation point was pre-determined for the different solutions in order to add the seeding crystals right before the nucleation point was reached. Table 1 shows the nucleation point of the different solutions. DCPD was preferably added right before nucleation, because if added too early, the seeds can dissolve and if added after nucleation, the seeding will not be effective.

Table 1 : Nucleation pH at different conditions of the oxidation stage

pH at the oxidation state Time of oxidation (hours) Nucleation pH

2.0 (±0.1) 24 6.10 (±0.05)

3.9 (±0.1) 24 6.10 (±0.05)

48 6.10 (±0.05)

7.9 (±0.1) 24 6.00 (±0.07)

48 6.04 (±0.1)

10.0 (±0.1) 24 5.8 (±0.1) All mass measurements were done using Mettler Toledo Analytical Balances (±0.001 g). pH of the nucleation was selected as the operation pH for

crystallization and during the crystallization, it was kept stable using 2M NaOH solution. But in some cases, the slight increase of the pH value after nucleation was observed . Then pH was kept stable at the value to which it increased .

The crystallization stage lasted for 2 hours and after this time crystalline products were separated from solution using vacuum filtrations. Solid samples were weighted and analyzed with FT-Raman, PXRD and SEM as described above. Liquid samples were analyzed for total phosphate content as described above.

Examples Example 1. Effect of DCPD seeding, pH and oxidation time on P-recovery and the nature of product obtained.

The oxidation was performed at pH 3.9 and pH 7.9 for either 24 h or 48 h with or without DCPD seeding. A digester supernatant from Aaby Wastewater treatment plant was used and the conditions of oxidation and crystaiisation are as mentioned under "Methods" above. After the oxidant was added to the system the formation cavitation (bubbles formation) was observed. A significant improvement of color was noticed after the solution was left for oxidation for 24 h or 48 h. The color of the solutions changed from dark brown to light yellow. The color improvement of the liquid was more significant for oxidation at the higher pH values. Moreover, after the oxidation at the pH 7.9 (±0.1) the solutions were clear while the solutions, which were oxidized at the pH 3.9 (±0.1), were slightly turbid. The possible reason for that is that during the oxidation some organic products were formed that are not soluble at low pH values. When the pH of these solutions was increased during crystallization by addition of NaOH they became clear around pH 5.5 - 5.7 and then turbid again when nucleation pH was achieved.

The product obtained and P-recovery are presented in Table 2. All of the experiments no. 1 to 8 were run in duplicates. All experiments gave identical products except no. 3 where 1^st run gave a mixture of DCPD and HAp and 2^nd run gave HAp as the only product. Table 2. Experimental parameters and results. Products in bold are the major product of the mixture.

As can be seen from Table 2, DCPD seeding promotes crystallization of DCPD at all experimental conditions. Whenever DCPD crystals were added to the solution the only obtained product was DCPD. Mixtures of DCPD and HAp were obtained for the experiments without seeding (experiments No. 2, 4, 6, 8). For the experiments No. l, No.5 and No.7 in both runs DCPD were crystallized as a major product, but some minor amounts of HAp present in the sample. The major and minor products were determined by analyzing the SEM imaged of the samples.

For the oxidation at pH 7.9 (±0.1), the effectiveness of the P-recovery was significantly higher than for oxidation at pH 3.9 (±0.1). The highest achieved P- recovery was 88.8 % for sample No. l when the oxidation was done at the pH 7.9 (±0.1). For the oxidation at pH 3.9, the highest P-recovery was 65 %.

No strict correlation between application of seeding and P-recovery efficiency was observed. In some cases experiments without seeding showed slightly higher P- recovery than seeding experiments (No. 1 compared to No.2, No.7 compared to No.8). At oxidation pH 3.9 the P-recovery was higher when the prolonged oxidation lasted for 24 hours instead of 48. At oxidation pH 7.9 when no seeding was applied the P-recovery was higher for 24 hours of oxidation, but for the seeded experiment the P-recovery was higher for 48 hours of oxidation.

Example 2. Effect of pH of the oxidation on P-recovery and the nature of product obtained.

In the Example 1, the highest P-recovery was obtained for the oxidation pH 7.9 with 24 hours of prolonged oxidation and crystallization without seeding.

Therefore, 24 hours of prolonged oxidation with hydrogen peroxide and

ultrasound treatment and a non-seeded crystallization were chosen as the conditions for testing the effect of the pH value at the oxidation stage on the P- recovery. It was studied by performing the oxidation at four different pH values: 2.0; 3.9; 7.8 and 10. Table 3 shows the products and the P-recovery obtained. The P-recovery obtained is shown in figure 5.

Table 3.

* denotes that the product obtained was characterized as HAp, but during its crystallization, formation of some sludge flocks was observed prior to the nucleation detection.

Figure 5 and table 3 shows that the P-recovery is highest when a pH of 7.9 is used during the oxidation step. In figure 6, pictures of the 4 samples is shown in order to show the influence of the pH value during oxidation step on the color of the obtained products. The oxidation was performed with hydrogen peroxide and ultrasound treatment for 24 hours. From figure 6 it is shown that calcium phosphate obtained when having a pH during oxidation at 2.0, 3.9 and 10.0 is more colored than when pH 7.9 was used. When pH 7.9 was used, a lower content of impurities is present.

Example 3. ICP-MS analysis of metal contents and calcium phosphate purity

Two samples from Example 1 (No. l and No.3) were chosen for ICP-MS analysis. In Table 4 the results of the metal content determinations are presented compared with the requirements for DCPD and HAp as food grade additives according to the commission regulation No 231/2012.

Table 4. Selected elements content in the samples No. 1 and No. 3 compared to the commission regulation No 231/2012.

Element/Ion Sample Sample Specification Specification

No. 1 No. 3 for food for food

DCPD+HAp HAp+DCPD additives additives

89% P-rec. 86% P-rec. (Dicalcium (Hydroxyapati phosphate)¹ te)¹

Analyzed Analyzed

with ICP- with ICP- MS MS

Total 16.7 % w/w 16.1 % w/w

Phosphorus (P)

Phosphorus (P) 16.1 % w/w 16.0 % w/w

present as

phosphate

Phosphate 49.4 % w/w 49.4 % w/w

Calcium, Ca 28.8 % w/w 29.6 % w/w - -

Arsenic, As 0.46 mg/kg 0.29 mg/kg < 1 mg/kg < 1 mg/kg

Lead, Pb 0.89 mg/kg 0.58 mg/kg < 1 mg/kg < 1 mg/kg

Cadmium, Cd 0.028 0.034 < 1 mg/kg < 1 mg/kg

mg/kg mg/kg

Mercury, Hg 0.023 0.014 < 1 mg/kg < 1 mg/kg

mg/kg mg/kg

Aluminum, Al 80 mg/kg 105 mg/kg < 80 mg/kg for < 150 mg/kg for the dihydrated the dihydrated form (food for form (food for infants and infants and young young children) children)

< 200 mg/kg

< 200 mg/kg (for all uses (for all uses except food for except food for infants and infants and young children) young

children)

Fluoride Was not Was not Not more than Not more than measured measured 50 mg/kg 50 mg/kg

Comparison of the heavy metals content in samples No. 1 and No. 3 to the requirements for the metals content in the food additives confirms that the obtained samples are of a high purity. The amount of phosphorus (P) present as phosphate in the samples (16.1% and 16.0% respectively - shown in table 4) is slightly lower than the phosphorus (P) content in DCPD (17.9 %) and HAp (19.5 %) according to their chemical formula. This small difference of phosphorus (P) content of 1.8 and 3.5 % respectively can be explained by the presence of other impurities in the samples.

The amount of DCPD and HAp in the samples that were analyzed by ICP-MS were calculated.

Table 5. DCPD and HAp content in % weight/weight

Sample 1 mg phosphorus g phosphorus g DCPD/kg % of DCPD (calculated (P) per kg (P) per kg in the as pure sample sample sample DCPD) (w/w)

161063 161.06 894.97 89.5

Sample 2 mg phosphorus g phosphorus g HAp/kg % HAp in (calculated (P) per kg (P) per kg the sample as pure sample sample (w/w) HAp)

160014 160.01 865.10 86.5 This calculation is based on the assumption that all the present ortho-phosphate in the samples belongs to DCPD in the sample 1 and to HAp in the sample 2. This calculation is an approximation because some minor amount of HAp was present in the sample 1 and some minor amount of DCPD was present in sample 2.

Example 4 - Effect of oxidation with ozone

A digester supernatant from Lille Marquettee WWTP (France) was used which was produced during mesophilic digestion of sewage sludge. The supernatant and sludge was separated by centrifugation at the WWTP. Concentrations of the selected components in the supernatant is given in table 6.

Table 6 : Selected compounds of digester supernatant from Lille Marquette WWTP

Suspended solids were removed from the digester supernatant using vacuum filtration with NY20 Millipore nylon net filter (20 Mm). 5 M NaOH and 37% HCI solution were used for the pH adjustments. Powdered calcium chloride dehydrate (>99%) ware used for the preparation of the 1 M solution for addition of calcium. Carmin Indigo, 85% Ortho-phosphoric acid and powdered sodium dihydrogen phosphate were used for the preparation of the stock and working solution for the determination of the dissolved ozone. Reagent grade calcium hydrogen phosphate dehydrate and hydroxyapatite were used for the seeding experiments. The effect of ozone for oxidation of humic substances in the digester supernatant was analysed by using two different doses of ozone, 1.25 and 2.5 mg ozone per mg total organic carbon (TOC). The dose of ozone was regulated by adjusting the flow and concentration of the ozone/oxygen mixture. Ozonation was performed at different initial pH values: 8.1, 6.2 and 2.8, in order to test the efficiency of degradation of organic matter by different mechanisms of ozonation. At the neutral and slightly alkaline pH, oxidation is performed at highy reactive OH radicals, while at acidic pH, direct oxidation by O3 takes place. Conditions for the ozonation experiments are shown below in table 7.

Table 7 : Experimental conditions for ozonation experiment

A glass column was filled with 2.5 L of a filtrated digester supernatant from WWTP and the pH was adjusted to the value mentioned in table 7. Recirculation of the supernatant in the column was done using the pump. Change of pH during ozonation was monitored using the pH probe inserted into the recirculation cirle. O2 was supplied to the ozone generator. In the beginning of the experiment, the flow of O2 and concentration og ozone in the produced Ο2/Ο3 mixtures were set to the values mentioned in table 7.

Concentration of ozone in the produced Ο2/Ο3 mxture was analysed using the intel ozone analyser. During the setting up period Ο2/Ο3 mixture was discharged to the fume hood.

When the concentration anmd the flow rate of Ο2/Ο3 mixture was set, Ο2/Ο3 mixture was directed to the column by changing the positions of valves. The beginning of ozonation was attested by appearance of first bubbles at the bottom of the column. The gas that passed through the column was leaving the column from the top, passing the outlet ozone analyser and was discharged to the fume hood . Every 10 minutes, samples of digester supernatant was taken from the sample valve and stripped with N₂ in order to stop the reaction. After 40 minutes supply of 0₂/03 mixture to the column, the reaction was stopped by closing the valve. The column was stripped with N₂ until the concentration of O3 in the outlet gas was lower than 1 ppm. Afterwards, the column was drained and the digester supernatant was collected in the tank for collection of effluent.

Analysis of the liquid phase during ozonation

The collected samples was analysed for Total Organic Carbon (TOC), Color436, Chemical Oxygen Demand (COD) and dissolved ozone.

Concentration of dissolved ozone was determined by the Indigo method.

TOC was determined using the total organic carbon analyzer TOC-VCPN equipped with ASI-V auto-sampler from Shimadzu using Non-Purgeable Organic Carbon (NPOC) method.

Spectrophotometer HACH DR 5000 with 10 mm quartz cuvettes were used for determination of Color436.

COD of the collected samples was measured using COD cuvettes test LCK 514 from HACH.

Crystallization procedure

Crystallization was performed in a 1 L beaker with an overhead stirrer driven by motor at 100 rpm. pH of supernatant was adjusted to the required pH of crystallization using NaOH/HCI standard solutions. 1 M CaCI₂ solution was dosed into the reactor with a peristaltic pump at the rate of 10 ml/min. pH of solution was monitored and kept stable during crystallization. Crystallization lasted for 2 hours. Thereafter, solid and liquid phases were separated using vacuum filtration. Collected solid product was dried at the room temperature and analyzed .

Analysis of the solid phase

A Bruker MultiRAM FT-Raman Spectrometer equipped with 1064 nm laser was used for collection of FT-Raman spectra of the solid samples. Bruker Alpha FTIR Spectrometer was used for collection of FTIR Spectra. Rigaku Miniflex 600 X-ray diffractometer with Cu source of X-rays and a Ni filter was used for Powder X-ray Diffraction (PXRD) analysis. Analysis of the liquid phase

Digester supernatant was analyzed for phosphate (P0₄) and calcium content before ozonation, after ozonation/before crystallization and after crystallization. HACH cuvettes test LCK 350 was used for determination of phosphate

concentration. Calcium concentration was determined by using Water Hardness cuvette test LCK 327.

Example 4A - Effect of ozonation conditions

To show the effect of ozonation, different parameters was measured of the samples shown in table 7; reduction of Color₄36, COD and TOC during ozonation was calculated and plotted versus the time of ozonation. The result is presented in Figures 7A to Figure 7C. From figures 7A-C, it is shown that for the initial pH of ozonation 6.2 and 8.1 no difference of Color436 and COD reduction was observed. However, increasing the dose from 1.25 to 2.5 mg 03/mg TOC resulted in a slight increase of decoloring of supernatant and COD reduction. TOC reduction is slightly higher when ozonation was performed at the initial pH 6.2 as compared to pH 8.1

Different mechanisms of ozonation are involved at different pH. At the pH 8.1 oxidation is performed by highly reactive OH- radicals that are formed after O3 decomposition. At acidic pH (2.8) direct oxidation by O3 takes place. O3 is more reactive towards the double bonds that are creating dark brown the color of digester supernatant. Therefore the reduction of Color₄36 was the highest when the ozonation was performed at the initial pH 2.8, even though the dose of ozone was lower (1.25 mg 03/mg TOC). At the same time, COD and TOC reduction was the lowest for initial pH of ozonation 2.8. It means that after ozonation at initial pH 2.8 the highest amount of organic matter is still present in the supernatant even though the color of supernatant was improved the most.

For all ozonation conditions, the reduction of Color₄36 was the highest comparing to COD and TOC reduction. Only 9.5 - 14 % of organic matter in digester supernatant were completely mineralized (TOC reduction). This means that some of the organic matter were degraded to lower molecular weight substances and still remained in the digester supernatant. The difference of Color₄36, COD and TOC reduction during ozonation at different conditions was similar and with small dfferences. Therefore, it is preferential to perform ozonation at the initial pH of digester supernatant (8.1) and the lower dosis of ozone (1.25 mg 03/mg TOC) as it results in a decrease of the costs associated with chemicals and energy consumption. Example 4B- Effect of ozonation conditions on the efficiency of

crystallization

The effect of ozonation was analysed by measuring the P-recovery of the samples shown in table 7. The crystallization was performed with molar ratio Ca/P=2 at pH 8 without addition of a seed crystal.

In figure 8 is the effect of ozonation conditions on P-recovery shown.

The P-recovery rate slightly increases when crystallization is performed after ozonation of organic matter comparing to crystallization from a non-ozonated supernatant. Figure 8 shows that the lower the pH of ozonation was, the higher was the P-recovery rate. This can be attributed to pH dependence of carbonate speciation in supernatant. At acidic pH (2.8) all carbonate is present as CO2, which can be stripped from the supernatant during ozonation. Therefore, during crystallization, less calcium is consumed on the reaction with carbonate and more of it is available for the reaction with phosphate. This hypothesis can be supported by the values of alkalinity of supernatant before and after ozonation at different pH (Figure 9).

Example 4C - Effect of ozonation conditions on the total alkalinity (TAC) of the digester supernatant.

The effect of ozonation on total alkalinity (TAC) of the samples from table 7 at different pH values and in the dosis of 1.25 and 2.5 mg 03/mg TOC was also analysed. The samples were compared to a sample of the initial digester supernatant. The result is shown in figure 9A. Ozonation at pH 2.8 resulted in TAC being zero. Therefore, there is no column shown in figure 9A for ozonation at pH 2.8.

A photo of the solid samples are shown in figure 9B. It illustrates the effect of pH values during ozonation from table 7 with the dose of 1.25 03/mg TOC on the color of the crystallized samples. It was compared with a sample crystallized from digester supernatant without ozonation. In figure 9B is sample (I) showing the solid product obtained without ozonation, sample (II) is with ozonation at pH 8.1, sample (III) is with ozonation at pH 6.2, while sample (IV) is with ozonation at pH 2.8.

Figure 9B shows that the color of the sample with no ozonation is much darker than the other samples. Furthermore, the samples with ozonation at pH 8.1 and pH 6.2 appears much more white as compared to the sample with ozonation at pH 2.8 which is colored.

Hence, even though decoloring of digester supernatant was the highest for the initial pH of ozonation 2.8, the product crystallized had the worst characteristic in terms of its color. The sample crystallized after ozonation at initial pH 8.1 had the best color.

Example 4D - Effect of crystallization conditions on the efficiency of the process

To test the effect of crystallization pH, ozonation was performed at the initial pH 8.1 and 6.2 at the dose of ozone 2.5 mg 03/mg TOC. Tested crystallization conditions are listed in Table 8 below.

Table 8. Conditions for studying the effect of crystallization pH.

Ozonation conditions Crystallization conditions

Dose of Initial pH of Ca/P pH of Seeding ozone mg/ ozonation crystallization conditions mg TOC

No ozonation 2 8 No seeding

No ozonation 2 7 No seeding 2.5 8.1 (±0.05) 2 8 No seeding

2.5 8.1 (±0.05) 2 7 No seeding

2.5 6.2 (±0.05) 2 8 No seeding

2.5 6.2 (±0.05) 2 7 No seeding

In figure 10A, the effect of crystallization pH on P-recovery rate is shown.

Figure 10A shows that for all tested conditions (no ozonation, ozonation at pH 8.1 and at pH 6.2), the P-recovery rate decreased with the decrease of crystallization pH from 8 to 7, due to lower supersaturation rate. No differences in the products nature or color were observed with the change of pH of crystallization.

Experimental conditions for testing the seeding conditions are listed in Table 9 and its effect of P-recovery rate is presented in Figure 10B.

Table 9. Conditions for studying the effect of seeding

From figure 10B, it is shown that for product crystallized with HAp seeds addition a strong peak on FT-Raman and FTIR spectra is shown attesting that seeds addition promoted crystallization of HAp.

In order to recover DCPD, crystallization pH is supposed to be slightly acidic.

Nevertheless, when the concentration of phosphate in digester supernatant is low, low pH of crystallization is not enough to create supersaturation. Therefore, recovery of DCPD from digester supernatant with low phosphate concentration might not be possible even with addition of DCPD seeds. This can be confirmed by a very low P-recovery rate achieved by crystallization at pH 6.5 with addition of 0.1 g of DCPD seeds.

Claims

1. A two-stage process for recovery and/or purification of calcium phosphate from an aqueous solution comprising phosphate and organic matter comprising the steps of;

a) contacting an aqueous solution comprising phosphate and organic matter with an oxidant at a pH in the range of from 6.5 to 8.5 for a time period sufficient to allow oxidation of organic matter

2. The process according to claim 1, wherein the oxidant is selected from the group consisting of peroxides or alkali metal salts thereof, ozone, oxygen and hypochlorites, perchlorates, peroxy acids or alkali metal salts thereof, ammonium cerium (IV) nitrate or sulphur based oxidants.

3. The process according to claim 2, wherein the peroxide or the alkali metal salt of peroxide is hydrogen peroxide, sodium hydrogen peroxide or a combination thereof.

4. The process according to any of the claims 1 to 3, wherein the pH in step a) is in the range of 4 to 9.

5. The process according to any of the claims 1 to 4, wherein the oxidation in step a) is performed for at time period of at least 15 min.

6. The process according to any of the claims 1 to 5, wherein the oxidant is added to the aqueous solution comprising phosphate in step a) under ultrasound treatment.

7. The process according to claim 6, wherein ultrasound treatment is for a period of at least 5 minutes.

8. The process according to any of the claims 1 to 7, wherein a pH adjusting agent is added to the oxidized solution after step a) and before step b) to adjust the pH to be 5 or below.

9. The process according to any of the claims 1 to 8, wherein step b) furthermore comprises addition of dibasic calcium phosphate dihydrate (DCPD) crystals, hydroxyapatite (HAp) crystals or octacalcium phosphate (Ca₈H2(P04)6- 5H₂0; OCP) crystals.

10. The process according to any of the claims 1 to 9, wherein the calcium salt added in step b) is selected from the group consisting of CaCI₂, CaC03, Ca(OH)₂ or mixtures thereof.

11. The process according to any of the claims 1 to 10, wherein the process is conducted at a temperature of 5-40 °C.

12. The process according to any of the claims 1 to 11, wherein the precipitation in step b) is for a period of 1 to 24 h.

13. The process according to any of the claims 1 to 12, wherein the aqueous solution is waste water from a treatment plant, waste water from industry or household waste water.

14. The process according to any of the claims 1 to 13, wherein the aqueous solution is a digester supernatant.

15. The process according to claim 13, wherein the waste water is filtered prior to oxidation.