Method for phosphate and / or arsenate recovery from an acidic stream, system therefore, and use of a precipitate obtainable by said method

Abstract

The present invention relates to a method for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, a system for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, use of a precipitate obtainable by the method according to the invention, and precipitate obtainable by the method according to the invention.

Description

[0001] METHOD FOR PHOSPHATE AND / OR ARSENATE RECOVERY FROM AN ACIDIC

[0002] STREAM, SYSTEM THEREFORE, AND USE OF A PRECIPITATE OBTAINABLE BY SAID

[0003] METHOD

[0004] The present invention relates to a method for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, a system for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, use of a precipitate obtainable by the method according to the invention, and precipitate obtainable by the method according to the invention.

[0005] Phosphorus is an essential element for life. It is responsible for various functions in all forms of life. For example, it is often a limiting nutrient for crops and, thus, a crucial part of fertilizers. The source of phosphorus is primarily from mining phosphate rock. Mined phosphorus is mainly used for agricultural fertilizers (80%) but also for animal feed additives (5%), detergents (12%), metal treatments, and other industrial applications (3%). The demand for phosphorus is rising, but its use is not sustainable, and its availability is uncertain because phosphate rock reservoirs are diminishing, and quality issues arise due to cadmium content / contamination.

[0006] In addition, regional phosphorus imbalances result in phosphate accumulation or excess phosphorus discharge to the ecosystem causing pollution.

[0007] Phosphorus recovery from wastewater can take place from the liquid side streams, sludge, or ash which have elevated phosphate concentrations. Phosphorus chemical precipitation is the most well-established phosphorus recovery method from phosphorus-rich streams using metal salts such as magnesium, calcium, or iron. For example, Mg-struvite recovery is the most widely implemented technology of commercialized phosphorus recovery techniques due to its simplicity, and struvite crystals forms at pH ~ 9. Calcium phosphate recovery occurs when calcium is added in the form of Ca(OH)2, and then calcium precipitates with phosphates at pH > 9. To enhance the settleability of struvite and calcium phosphate crystals, crystallizers are used. Vivianite (Fe2+3(PC>4)2- 8H2O) is also recovered at a pH range of 6 to 8 when Fe : P ratio = 1.5.

[0008] Furthermore, recovery of arsenate from a waste water stream with a neutral or alkaline pH is conventionally performed using sulphides or via adsorption. A disadvantage of using sulphides and / or an absorbent is that said techniques produces a hazardous and difficult to dispose of sludge and / or absorbent.

[0009] All these conventional techniques to recover phosphorus require neutral or alkaline pH conditions, as these minerals (such as struvite, calcium phosphates, and vivianite) are soluble at low pH conditions. Therefore, there is a need for phosphorus recovery from low-pH phosphorus- rich streams, when said phosphorus is available in acidic aqueous streams. These aforementioned problems prevent an efficient and effective phosphorus / phosphate recovery. This problem is even bigger for large scale recovery of phosphorus and / or phosphate.

[0010] The present invention aims at obviating or at least reducing one or more of the aforementioned problems and to enable efficient and effective method for phosphate and / or arsenate recovery from an acidic stream.

[0011] This objective is achieved with the method for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, the method comprising the steps of:

[0012] - providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate;

[0013] - dosing an iron salt to and / or controlling an iron salt in the aqueous stream;

[0014] - dosing an oxidant to and / or controlling an oxidant in the aqueous stream;

[0015] - forming a precipitate in the aqueous stream, wherein the precipitates include salts comprising iron, and phosphate and / or arsenate; and

[0016] - recovering phosphate and / or arsenate from the precipitate.

[0017] Streams may comprise phosphate (PO42) that is the main molecule in the relevant streams that comprises the element phosphorus (P), and / or streams may comprise arsenate (AsCH3) that is the main molecule in the relevant streams that comprises the element arsenic (As). These streams relate to (wet) acidic (aqueous) streams, such as waste flows, sewage streams, manure streams, or sludge streams, wherein said stream may be an acidic wastewater stream.

[0018] It is noted that controlling may include one or more of the following measuring, providing input for dosing, regulating, and the like.

[0019] An advantage of the step of controlling an oxidant in the aqueous stream enables to perform the step of adjusting the dosing rate in response to and / or to adjust the settleability.

[0020] The method according to the invention may start with the step of providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate. Said step may be followed by the step of dosing an iron salt to and / or controlling an iron salt in the aqueous stream, and the step of dosing an oxidant to and / or controlling an oxidant in the aqueous stream. The step of dosing may be followed by the step of forming a precipitate in the aqueous stream, wherein the precipitate includes salts comprising iron, and phosphate and / or arsenate. Furthermore, the step of forming may be followed by the step of recovering phosphate and / or arsenate from the precipitate.

[0021] The method according to the invention enables that precipitates are formed in the aqueous stream, wherein the precipitates include salts comprising iron, and phosphate and / or arsenate, wherein the precipitate comprises more than 60% of the initial amount of phosphate and / or arsenate in the aqueous stream. Alternatively, the step of dosing an iron salt to and / or controlling an iron salt in the aqueous stream is performed before the step of providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate.

[0022] An advantage of the method according to the invention is that the method enables to provide a precipitate which comprises more than 60% of the initial amount of phosphate and / or arsenate in the aqueous stream.

[0023] Furthermore, the step of dosing an oxidant to and / or controlling an oxidant in the aqueous stream enables to achieve an optimal settleability of the obtained precipitate.

[0024] A further advantage is that the oxidation of iron(II) to iron(III) is accelerated by increasing the dosing of the oxidant and / or selecting an oxidant which is stronger / more reactive. In addition, or alternatively, the oxidation rate may be increased by increasing the partial pressure of oxygen by pressurizing the system and using pure oxygen. The iron(II) oxidation under acidic conditions, may, without being bound to theory, be explained by the equilibrium chemistry of iron(II) in aqueous streams. At a low pH, such as a pH below 4, the oxidation rate of iron(II) becomes very low. Said slow oxidation rate may, without being bound to theory, be explained by: the equilibrium chemistry of iron(II) in aqueous streams is the most dominant and ferrous hydroxide (FeOH+and Fe(OH)2) species are almost negligible. Ferrous hydroxide species are the most significant for oxidation, as shown below; thermodynamically, all oxidation steps are endergonic, but AG° (Gibbs free energy) decreases going from Fe2+to FeOH+and Fe(OH)2, which explains the faster reaction at higher pH.

[0025] It was surprisingly found that through oxidation of Fe(II) to Fe(III) a better settling precipitate was achieved despite the fact that the particle size is not very different compared to direct dosing of Fe(III).

[0026] In addition, oxidising Fe(II) to Fe(III) enables to control the Fe(III) concentration in the aqueous stream and, as a result, the supersaturation for strengite. Without being bound by theory, controlling the oxidation of the Fe(II) to Fe(III) can reduce supersaturation. In other words, the level of supersaturation can be controlled, and thus a lower supersaturation may be achieved. Therefore, the amount of crystallisation nuclei and via that the amount of particles are formed is controlled. However, the solubility of strengite is very low in an aqueous environment so it is difficult to control this supersaturation. The method according to the invention surprisingly enables that a more efficient and effective separation through better settling of the particles at certain oxidation rates is achieved.

[0027] In a presently preferred embodiment, the aqueous stream with a pH below 7 comprising an initial amount of phosphate.

[0028] An advantage of the method according to the invention is that said method enables both high recovery and quick precipitate settleability by controlling the supply of iron(III) via a controlled iron(II) oxidation. Said high recovery and quick precipitate settleability may be achieved without the need to add carriers or further crystallizers. This improves the practical implementation of the method according to the invention.

[0029] In addition, the method according to the invention enables that the pH of an acidic (aqueous) waste stream does not need to be adjusted. In other words, an acidic waste stream may be used in the method for recovery of phosphorus and / or arsenate. As a result, chemical dosing to adjust the pH such that the (aqueous) waste stream has a neutral or alkaline pH is prevented. Therefore, the method according to the invention enables to form minerals, such as strengite and metastrengite, which precipitate (are solid) in an acidic environment.

[0030] Thus, a further advantage of the method according to the invention is that solids of the desired compounds / minerals comprising phosphate are formed under acidic conditions.

[0031] Yet another advantage of the method according to the invention is that the dosing of the iron salt is lower compared to phosphate and / or arsenate recovery with conventional methods. For example, the iron to phosphorus ratio (Fe : P ratio) in an acidic stream is about 1 , wherein the ratio of dosing vivianite to phosphate (Fe : P ratio) to a neutral or an alkaline stream is 1.5. This improves the cost effectiveness of the present invention as compared to conventional processes.

[0032] Yet another advantage of the method according to the invention is that reducing conditions are not required to form solids comprising phosphorus and / or arsenic. As a result, an efficient and effective method for phosphate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream is achieved.

[0033] Yet another advantage of the method according to the invention is that the use of membranes may be avoided. Therefore, the method according to the invention avoids high capital-investments towards membranes, and additionally maintenance costs during operation.

[0034] It was found that the formed solids / precipitates have a higher purity compared to conventional method for phosphate recovery, because contaminants such as heavy metals are soluble or stay in solution at low pH. This further improves the possible further uses of the solids / precipitates.

[0035] The (acidic) aqueous stream may originate from for example cheese production, production of semiconductors or LCD manufacturing, phosphoric acid production, pulp and paper industries, leaching processes of steelmaking slag, extracellular polymeric substances (EPS) extraction process from aerobic granular sludge, sewage sludge, and ash acidic leaching streams (wherein the latter three originate from municipal waste sources). It will be understood that the stream may also originate from other processes. In addition, EPS extraction process may result in high recovery yields as aerobic granular sludge wastewater plants are becoming more common these days.

[0036] It was found that said waste streams may comprise high loads of phosphate, and are often acidic.

[0037] In a preferred embodiment, the method according to the invention does not comprise the use of crystallizers and / or seed material.

[0038] An advantage of not using crystallizers and / or seed material is that the costs are reduced compared to method including a crystallizer and / or seed material, as materials such as the crystallizer and / or the seed material does not need to be purchased. In addition, the crystallizer does not need to be maintained (recovered and / or regenerated), saving additional costs.

[0039] Another advantage is that the method for phosphate and / or arsenate recovery from an acidic stream is less complex compared to conventional methods, as less (starting) material is used.

[0040] Yet another advantage of the method according to the invention is that less troubleshooting needs to be performed. As a result, the method according to the invention may be performed for a longer period of time.

[0041] In a preferred embodiment according to the invention, the precipitates includes iron(III) salts, preferably the iron salts in the precipitate comprises 90% iron(III), more preferably 95% iron(III), most preferably 99% iron(III). Preferably, the iron of the iron salts in the precipitates consists of iron(III).

[0042] An advantage of the method according to the invention is that the used amount of iron for phosphate and / or arsenate recovery is reduced to a minimum. As a result, lower amounts of iron are necessary for phosphate and / or arsenate recovery compared to conventional phosphate and / or arsenate recovery.

[0043] For example, recovery of phosphate and / or arsenate using iron(II) species requires 33% more iron than phosphate and / or arsenate recovery using iron(III) species.

[0044] In a presently preferred embodiment according to the invention, the step of dosing an oxidant to and / or controlling an oxidant in the aqueous stream and the step of forming a precipitate in the aqueous stream provides a volume index in the range of 3 ml gP1to 100 ml gP ', preferably in the range of 3 ml gP1to 85 ml gP ', within 30 minutes. It was found that the method according to the invention enables to substantially fully recover the phosphorus from the aqueous stream.

[0045] In a further presently preferred embodiment according to the invention, the precipitates comprise strengite-like structures.

[0046] It is noted that strengite-like structures includes pure strengite, phosphosiderite, and / or metastrengite, and also structures including some impurities like magnesium or calcium.

[0047] It was found that precipitates comprising strengite-like structures, such as FePO4-2H2O, may be recovered from acidic aqueous streams in an efficient and effective manner. In other words, the stream provided to the method according to the invention may have a pH below 7.

[0048] The recovery of said precipitates may be performed in a simple, and highly efficient, manner. Said recovery of the precipitate avoids the addition of pH adjustments agents and / or the extensive / excessive usage of an iron salt.

[0049] In a further presently preferred embodiment according to the invention, the precipitates may be FePC>4-2H2O (strengite) and / or FeAsCH-2H2O (scorodite). Preferably, the precipitate is FePO4-2H2O.

[0050] It was found that the method according to the invention enables the precipitation of FePCH- 2H2O and / or FeAsCH- 2H2O.

[0051] An advantage of FePCH- 2H2O and / or FeAsCH- 2H2O is that said compounds are not soluble in an acidic environment. Therefore, a clean and pure precipitate may be achieved. In particular, a clean and pure precipitate may be achieved from an aqueous steam which may contain pollutants, such as heavy metals, organic materials, and the like. The acidic environment reduces or prevent the incorporation of undesired elements, such as said heavy metals and / or organic materials, into the precipitate. Furthermore, FePCH- 2H2O and / or FeAsCH-2H2O may efficiently and effectively be formed, such that the precipitate comprises more than 60% of the initial amount of phosphate and / or arsenate in the aqueous stream.

[0052] In a further presently preferred embodiment according to the invention, the aqueous stream has a pH in the range of 0 to 6.5, preferably a pH in the range of 0 to 6, more preferably a pH in the range of 0 to 5, and most preferably a pH in the range of 1 to 4.

[0053] In a preferred embodiment according to the invention, the aqueous stream has a pH in the range of 1 to 6.5, preferably a pH in the range of 1 to 6, more preferably a pH in the range of 1 to 5, and most preferably a pH in the range of 1 to 4.

[0054] It was found that the method according to the invention enables an efficient and effective formation of the precipitate the precipitate comprising more than 60% of the initial amount of phosphate and / or arsenate in the aqueous stream. An advantage of the method according to the invention is that the acidic aqueous stream does not need to be adjusted and (highly) acidic aqueous streams may be provided to the method according to the invention.

[0055] In a further presently preferred embodiment according to the invention, the iron salt may be an iron(II) salt. Preferably, the iron salt may be one or more selected from the group of iron(II) chloride, iron(II) sulphate, Fe2+3(PC>4)2- 8H2O, Fe2+3(AsC>4)2- 8H2O.

[0056] In a preferred embodiment, the iron salt may be an iron(II) salt. Preferably, the iron salt may be one or more selected from the group of iron(II) chloride, iron(II) sulphate, Fe2+3(PC>4)2- 8H2O, more preferably the iron salt may be Fe2+3(PC>4)2- 8H2O (also referred to as vivianite).

[0057] In a further presently preferred embodiment according to the invention, the iron salt is insoluble in an aqueous environment having a pH of equal to or greater than 6, preferably a pH of equal to or greater than 5, and is soluble at a pH of equal to or less than 4. Preferably, the iron salt may be Fe2+3(PC>4)2- 8H2O (also referred to as vivianite), Fe2+3(AsC>4)2- 8H2O (also referred to as symplesite).

[0058] It is noted that vivianite is different from iron(II) sulphate and iron(II)chloride because it is insoluble at normal pH values whereas the latter can be bought and dosed as solutions. Also, it contains phosphate. Nevertheless, vivianite can still be used to make strengite and recover phosphorus because it has a surplus of iron to phosphorus compared to strengite. Moreover, using vivianite enables a method to remove impurities via recrystallisation into strengite and recover or make use of the surplus of the iron in vivianite compared to strengite.

[0059] In a further presently preferred embodiment according to the invention, the oxidant may be one or more selected from the group of a peroxide, fluorine, hydroxyl radical, ozone, oxygen, hydrogen peroxide, potassium permanganate, chlorine oxide, chlorine. Preferably, the peroxide is hydrogen peroxide.

[0060] It was found that both high recovery and quick precipitate settleability may be achieved by controlling the supply of iron(III), preferably via a controlled iron(II) oxidation. In fact, said high recovery and quick precipitate settleability may be achieved without the need to add carriers or further crystallizers. The high recovery and quick precipitate settleability was in particular achieved using one or more oxidants selected from the group of peroxide, fluorine, hydroxyl radical, ozone, oxygen, hydrogen peroxide, potassium permanganate, chlorine oxide, chlorine.

[0061] In a further presently preferred embodiment according to the invention, the step of dosing an oxidant to and / or controlling an oxidant in the aqueous stream may be performed for a period in the range of 5 minutes to 8 hours, preferably for a period in the range of 30 minutes to 6 hours, more preferably for a period in the range of 1 hours to 4 hours. It was found that dosing an oxidant to and / or controlling an oxidant in the aqueous stream for an above mentioned period provides an efficient and effective recovery of phosphate and / or arsenate from an acidic stream.

[0062] In a further presently preferred embodiment according to the invention, the oxidation rate is in the range of 0.5 x 104mol L1min1to 100 x 104mol L1min1, preferably in the range of 1.0 x 104mol L1min1to 75 x 104mol L1min more preferably in the range of 1.0 x 104mol L1min1to 50 x 104mol L1min most preferably in the range of 1.0 x 104mol L1min1to 40 x 104mol L1min A

[0063] It was found that there is a clear connection between the optimum for oxidation rate and the settleability. This also becomes clear from the experiments, for example this becomes clear from figure 5.

[0064] An advantage of dosing an oxidant to and / or controlling an oxidant in the aqueous stream for a period in the range of 5 minutes to 8 hours is that at least 90% of the initial phosphate and / or arsenate in the acidic aqueous stream is precipitated. Said level of precipitation is achieved in a presently preferred embodiment of the invention as the oxidation of iron(II) to iron(III) is controlled, and thus the desired amount of iron(III) is dosed.

[0065] Another advantage of the dosing and / or oxidation rate is that at least 90% of the initial phosphate and / or arsenate present in the aqueous stream is precipitated, and that gravity-based separation techniques are sufficient. Thus, the method according to the invention enables a good settleability.

[0066] In a further presently preferred embodiment according to the invention, the method further comprises the step of measuring the phosphate and / or arsenate concentration in the aqueous stream.

[0067] An advantage of the step of measuring the phosphate and / or arsenate concentration in the aqueous stream is that the phosphate concentration may be measured in the aqueous stream before, during, and / or after the step of dosing of an iron salt to and / or controlling an iron salt in the aqueous stream, and / or the step of dosing an oxidant to and / or controlling an oxidant in the aqueous stream, such that precipitates are formed in the aqueous stream, wherein the precipitates include salts comprising iron, and phosphate and / or arsenate, wherein the precipitate comprises more than 60% of the initial amount of phosphate and / or arsenate in the aqueous stream. As a result, the phosphate and / or arsenate concentration may be monitored and the dosing of iron salt may be adjusted.

[0068] In a further presently preferred embodiment according to the invention, the method further comprises the steps of controlling the dosing of an iron salt in response to a measurement of the initial amount of phosphate and / or arsenate in the aqueous stream. In a further presently preferred embodiment according to the invention, the method further comprises the step of controlling the dosing of an oxidant in response to a measurement of the oxidation rate of iron(II) to iron(III).

[0069] It is noted that the oxidation rate may be measured via the redox potential using ASTM D1498-14(2022)E01.

[0070] The controlling of the dosing of an iron salt, such as iron(II) chloride, iron(II) sulphate, and / or in one of the presently preferred embodiments Fe2+3(PC>4)2- 8H2O and / or Fe2+3(AsC>4)2- 8H2O, in response to a measurement of the initial amount of phosphate and / or arsenate in the aqueous stream, and / or controlling the dosing of an oxidant, such as peroxide, fluorine, hydroxyl radical, ozone, oxygen, hydrogen peroxide, potassium permanganate, chlorine oxide, and / or chlorine, in response to a measurement of the oxidation rate of iron(II) to iron(III) enables to control the rate of precipitation. Therefore, the desired precipitate, preferably being strengite-like structures, is formed.

[0071] In a further presently preferred embodiment according to the invention, the method further comprises the step of separating the precipitates from the aqueous stream. Preferably, the step of separating the precipitates from the aqueous stream comprises gravity-based separation.

[0072] Separating the precipitates from the aqueous stream enables to recover the phosphate and / or arsenate. As a result, the valuable phosphate and / or arsenate is recovered from the aqueous stream.

[0073] An advantage of the method according to the invention is that the (undesired) elements, such as heavy metals, remain in solution. As a result, a high throughput may be achieved as only the desired elements precipitate. In addition, said separation enables to further separate the dissolved metals from the precipitate. Therefore, the method according to the invention enables to further recover (desired) valuable compounds / metals.

[0074] In addition, using an optimal oxidation rate enables an efficient and effective separation of the precipitate from undesired compounds / materials.

[0075] In addition or alternatively, the step of separating may be performed using magnetic separation of the precipitate. The precipitates, such as strengite-like precipitates, are magnetic both from iron(II) and iron(III).

[0076] An advantage of magnetic separation is that said technique may be useful for toxic and / or high organic matter waste streams where co-settling can be problematic. As a result, the method according to the invention is enabled to handle a wide variety of aqueous streams.

[0077] In a further presently preferred embodiment according to the invention, the step of separating is performed using one or more of a sedimentation tank, a filtration unit, a centrifuge.

[0078] An advantage of using one or more of a sedimentation tank, a filtration unit, a centrifuge is that said separation means are cost efficient. In a preferred embodiment according to the invention, the method further comprises the step of neutralising the aqueous stream to a pH in the range of 6.5 to 7.5, wherein said step is performed after the step of separating the precipitates from the aqueous stream.

[0079] An advantage of the step of neutralising is that the aqueous stream, comprising a reduced phosphate and / or arsenate content, may be released to the environment, such as surface water.

[0080] The method according to the invention comprises the step of recovering phosphate and / or arsenate from the precipitate. Preferably, the step of recovering phosphate and / or arsenate from the precipitate further comprises the step of recovering the iron from the precipitate.

[0081] It is noted that strengite (FePO4-2H2O) is in particular relevant for phosphate recovery, and scorodite (FeAsCH- 2H2O) is in particular relevant for arsenate recovery.

[0082] An advantage of the step of recovering phosphate and / or arsenate from the precipitate is that the phosphate and / or arsenate is achieved. Said phosphate and / or arsenate may be used efficiently and effectively in further processes and / or a variety of different products. For example, this may include fertilizer, flame retardant, in a lithium-ion battery, such as a precursor for a lithium-ion battery, adsorbent, such as an adsorbent for heavy metals. Said heavy metals are for example Pb, Cu, Cd, Zn.

[0083] A further advantage is that recovered scorodite and / or scorodite-like structures is chemically inert. Therefore, a safe way to immobilize the arsenic is achieved. Said immobilized arsenic may be used in the production of elemental As, which has been used in the semiconductor industry.

[0084] In a preferred embodiment according to the invention, the step of recovering comprises recovering of more than 60%, preferably more than 70%, more preferably more than 80%, even more preferably more than 90%, most preferably more than 95%, of the initial amount of phosphate and / or arsenate in the aqueous stream.

[0085] In a further presently preferred embodiment according to the invention, the step of recovering the phosphates and / or arsenate from the precipitate comprises treating the precipitate to produce iron oxide.

[0086] In a further presently preferred embodiment according to the invention, treating the precipitate comprises performing an alkaline treatment to produce an alkaline phosphate solution and / or an alkaline arsenate solution. Preferably, the alkaline treatment is performed using potassium hydroxide and / or sodium hydroxide.

[0087] By forming iron oxide precipitates from the precipitate, such as the strengite-like structures, the phosphates and / or arsenate can be recovered effectively and separated from the iron. Preferably, treating the strengite-like structures involves performing an alkaline treatment. This alkaline treatment may involve the use of sodium hydroxide (caustic soda, potassium hydroxide (caustic potash). In a further presently preferred embodiment according to the invention, the method further comprises the step of treating the iron oxide with hydrochloric acid to produce iron(II) chloride. Preferably, the method further comprises the step of recycling the iron(II) chloride in the step of dosing iron salt.

[0088] An advantage of treating the iron oxide with hydrochloric acid to produce iron(II) chloride is that said iron(II) chloride may be used to trap phosphate and / or arsenate again. Therefore, a more circular and environmental friendly use of the iron salt is achieved.

[0089] Yet another advantage of the method according to the invention is that the precipitate comprises more than 70%, preferably more than 80%, even more preferably more than 90%, most preferably more than 95%, of the initial amount of phosphate and / or arsenate in the aqueous stream.

[0090] It is noted that, without being bound to theory, the precipitate comprises at most 100% of the initial amount of phosphate in the aqueous stream.

[0091] It was found that the method according to the invention enables an efficient and effective recovery of phosphate and / or arsenate.

[0092] In a further presently preferred embodiment according to the invention, the step of dosing iron salt to the aqueous stream comprises adding a total amount of iron with a molar ratio iron to phosphorus of the phosphate and / or arsenic of the arsenate of at least 1, preferably at least 1.25, and more preferably at least 1.5.

[0093] It is noted that the total amount relates to the amount of iron added to the aqueous stream before the precipitate is separated from the aqueous stream.

[0094] The method according to the invention comprising the step of adding a total amount of iron with a molar ratio iron to phosphorus of the phosphate and / or arsenic of the arsenate of at least 1 , preferably at least 1.25, and more preferably at least 1.5 enables a more efficient method for phosphate and / or arsenate recovery compared to conventional methods. In particular, said efficiency difference is achieved when the precipitates comprise strengite-like structures. Said strengite-like structures have a lower mol mass of iron compared to structures like vivianite.

[0095] Therefore, the method according to the invention is even more cost efficient compared to the conventional methods.

[0096] In a further presently preferred embodiment according to the invention, the step of dosing an oxidant to the aqueous stream comprises adding a total amount of oxidant with a molar ratio oxidant to phosphorous of the phosphate and / or arsenic of the arsenate of at least 1, preferably at least 1.25, and more preferably at least 1.5.

[0097] It is noted that the total amount relates to the amount of oxidant added to the aqueous stream before the precipitate is separated from the aqueous stream. In a presently preferred embodiment, the step of dosing an oxidant to the aqueous stream comprises adding a total amount of oxidant with a molar ratio oxidant to phosphorous of the phosphate and / or arsenic of the arsenate of at least 1 , when said oxidant is preferably hydrogen peroxide.

[0098] An advantage of a molar ratio to phosphorous of the phosphate and / or arsenic of the arsenate of at least 1, preferably at least 1.25, and more preferably at least 1.5, is that an efficient and effective phosphate and / or arsenic recovery is achieved.

[0099] In a further presently preferred embodiment according to the invention, the method further comprises the step of filtering the aqueous stream, wherein the step of filtering is performed before the step of providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate.

[0100] An advantage of filtering the aqueous stream is that solid contamination, such as sand and organic matter, is removed from the aqueous stream. As a result, the precipitate is cleaner and does not include said contamination.

[0101] In a further presently preferred embodiment according to the invention, further comprising the step of dosing an acid to and / or controlling a pH in the aqueous stream.

[0102] The step of dosing an acid to and / or controlling a pH in the aqueous stream may be performed before and / or simultaneous to the step of providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate.

[0103] The step of dosing an acid to and / or controlling a pH in the aqueous stream enables to modify the pH of the aqueous stream, such that the desired pH is achieved.

[0104] An advantage of modifying the pH such that said pH is below 7 is that the iron salts may dissolve.

[0105] The step of dosing an acid to and / or controlling a pH in the aqueous stream is preferably combined with the step of controlling an iron salt in the aqueous stream.

[0106] Said controlling step enables to determine the concentration of iron salt in the aqueous stream.

[0107] An advantage of controlling the iron salt is that the concentration of iron salts may be determined / measured and the amount of oxidant dosed to the aqueous stream may be calculated on said measurement.

[0108] In a preferred embodiment, the step of dosing an acid to and / or controlling a pH in the aqueous stream includes the dosing of one or more acid selected from the group of hydrochloric acid, sulphuric acid, nitric acid, organic acid, such as oxalate acid and / or citric acid.

[0109] In a preferred embodiment, the aqueous stream comprises an iron salt before said aqueous stream is acidified, wherein said iron salt is substantially insoluble in a neutral aqueous environment. Preferably, the iron salt is an iron(II) P salt and / or iron(II) As salt. For example, the iron salt is vivianite.

[0110] An advantage of providing an aqueous stream comprising an iron salt before said aqueous stream is acidified is that other appearances of iron, such as iron(II), may be used for phosphate and / or arsenate recovery.

[0111] An advantage of the method is that the used amount of iron for phosphate and / or arsenate recovery is reduced to a minimum. As a result, lower amounts of iron are necessary for phosphate and / or arsenate recovery compared to conventional phosphate and / or arsenate recovery.

[0112] In addition, lowering the pH of the aqueous stream by adding an acid, the iron salt is dissolved and the iron and P and / or As can recrystallise to compounds such as strengite and / or scorodite. As a result, purification of the precipitate (impurities will stay in solution) and reduction of the Fe / P ratio and / or Fe / As ratio is achieved. It is noted that the iron salts that are used in the method according to the invention would have to be soluble under acidic conditions but insoluble at higher pH (such as vivianite (for P) or symplesite (for As).

[0113] For example, recovery of phosphate and / or arsenate using iron(II) species requires 33% more iron than phosphate and / or arsenate recovery using iron(III) species.

[0114] To determine the (dissolved) iron salt / iron species the step of controlling an iron salt in the aqueous stream is preferably performed.

[0115] In a preferred embodiment, the method for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, comprises the steps of: providing a stream comprising iron salt, wherein the iron salt is an iron(II) P salt and / or iron(II) As salt, preferably the stream is an aqueous stream; optionally dosing an acid to and / or controlling an acid in the stream to reach a pH below 7 ; and dosing an oxidant to and / or controlling an oxidant in the stream, such that precipitates are formed in the stream, wherein the precipitates include salts comprising iron, and phosphate and / or arsenate, wherein the precipitate comprises more than 60% of the initial amount of phosphate and / or arsenate in the aqueous stream.

[0116] It is noted that an iron(II) P salt refers to an iron salt that comprises iron(II) and phosphorus, for example said iron(II) P salt may be one or more selected from the group of vivianite-like structures from the so called vivianite group. Examples of said vivianite group are Fe2+3(PO4)2- 8H2O, (Mg,Fe)3(PO4)2- 8H2O, Fe2+Fe3+2(PO4)2(OH)2- 6H2O.

[0117] It is also noted that vivianite-like structures include pure vivianite and also structures including some impurities like magnesium or calcium. It is also noted that iron(II) As salt refers to an iron salt that comprises iron(II) and arsenic, for example said iron(II) As salt may be one or more selected from the group of symplesite or parasymplesite (both with formula Fe2+3(AsC>4)2- 8H2O).

[0118] In a preferred embodiment, the precipitates are iron(III) salts, preferably iron(III) P salts and / or iron(III) As salts.

[0119] For example, said iron(III) P salts are strengite-like structures (FePO^FfcO).

[0120] It is noted that strengite may also be referred to as phosphosiderite or metastrengite.

[0121] In addition, iron(III) As salts may be scorodite or scorodite-like structures comprising the formula FeAsCH- 2H2O.

[0122] The invention also relates to a system for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, the system comprising: a reactor comprising: an inlet for an incoming aqueous stream comprising an initial amount of phosphate and / or arsenate; an iron salt dosing device configured for dosing iron salt in the reactor; an oxidant dosing device configured for dosing an oxidant in the reactor such that precipitates are formed in the aqueous stream, wherein the precipitate comprises more than 60% of the initial amount of phosphate and / or arsenate in the incoming aqueous stream; and an outlet.

[0123] The system for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream provides the same or similar effects and advantages as those described for the method for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream according to the invention. In particular, the system is capable of performing the method according to one of the embodiments of the invention.

[0124] In a preferred embodiment according to the invention, there is provided a system for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, the system comprising: a reactor comprising: an inlet for an incoming aqueous stream comprising an initial amount of phosphate and / or arsenate; an iron salt dosing device configured for dosing iron salt in the reactor; an oxidant dosing device configured for dosing an oxidant in the reactor such that precipitates are formed in the aqueous stream, wherein the precipitate comprises more than 60% of the initial amount of phosphate and / or arsenate in the incoming aqueous stream; and an outlet.

[0125] The system for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, enables that both high recovery and quick precipitate settleability may be achieved.

[0126] In a presently preferred embodiment according to the invention, the system further comprises a separator for separating the precipitate from the stream.

[0127] Said separator enables to recover the phosphate and / or arsenate efficiently and effectively from the aqueous stream.

[0128] In a further presently preferred embodiment according to the invention, the system further comprises a treatment system for treating the separated precipitate to produce iron oxide precipitates.

[0129] In a further presently preferred embodiment according to the invention, the system further comprises a dosing controller and a phosphate and / or arsenate measurement system that are configured to control dosing of iron salt and / or oxidant in response to a measurement of the initial amount of phosphate and / or arsenate in the incoming and / or outgoing aqueous stream.

[0130] The invention also relates to a use of a precipitate obtainable by the method according to the invention.

[0131] The use of a precipitate obtainable by the method according to the invention provides the same or similar effects and advantages as those described for the method and / or system for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream according to the invention.

[0132] In a presently preferred embodiment according to the invention, the precipitate is used as fertilizer, flame retardant in a lithium-ion battery, such as a precursor for a lithium-ion battery, adsorbent, such as an adsorbent for heavy metals. Said heavy metals are for example Pb, Cu, Cd, Zn. For example, the precipitate may be used as an adsorbent for lead pollutants.

[0133] A further advantage is that recovered scorodite is chemically inert. Therefore, a safe way to immobilize the arsenic is achieved. Said immobilized arsenic may be used in the production of elemental As, which has been used in the semiconductor industry.

[0134] The invention also relates to a precipitate obtainable by the method according to the invention, wherein the purity of the precipitate is at least 95%. Preferably, the purity of the precipitate is at least 98%.

[0135] The precipitate obtainable by the method according to the invention provides the same or similar effects and advantages as those described for the method and / or system for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream according to the invention, and the use of the precipitate obtainable by the method according to the invention. Further advantages, features and details of the invention are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which:

[0136] - Figure 1 shows a schematic overview of a method according to the invention;

[0137] - Figure 2 shows a schematic overview of the system according to the invention comprising phosphate and / or arsenate recovery;

[0138] - Figure 3 shows a schematic overview of a method according to the invention;

[0139] - Figure 4 shows a schematic overview of the system according to the invention comprising iron(II) recycling; and

[0140] - Figure 5 shows the oxidation rate on settling and recovery fitted to a sigmoidal function.

[0141] Method 10 (Figure 1) for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, follows a sequence of different steps.

[0142] In an illustrated embodiment method 10 may start with step 12 of providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate. Alternatively, method 10 may start with step 11 of filtering the aqueous stream, wherein the step of filtering, followed by step 12 of providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate.

[0143] In a preferred embodiment, step 11 of filtering the aqueous stream may be followed by step 13 of dosing an acid to and / or controlling a pH in the aqueous stream. Step 13 may be performed before step 12 and / or step 14.

[0144] Step 12 of providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate may be followed by step 14 of dosing an iron salt to and / or controlling an iron salt in the aqueous stream, and step 16 of dosing an oxidant to and / or controlling an oxidant in the aqueous stream, such that precipitates are formed in the aqueous stream.

[0145] In a preferred embodiment, step 14 comprises step 18 of controlling the dosing of an iron salt in response to a measurement of the initial amount of phosphate and / or arsenate in the aqueous stream, and / or step 19 of measuring the initial amount / the concentration of phosphate and / or arsenate in the aqueous stream. Preferably, step 19 is part of step 18. Furthermore, step 16 may comprise step 20 of controlling the dosing of an oxidant in response to a measurement of the oxidation rate of iron(II) to iron(III), and / or step 21 of measuring the oxidation rate of iron(II) to iron(III). Preferably, step 21 is part of step 20.

[0146] Furthermore, method 10 may comprise step 15 of measuring the phosphate and / or arsenate concentration in the aqueous stream. Step 15 may be performed before, during, and / or after any one of the steps 11, 12, 14, and 16.

[0147] Step 16 of dosing an oxidant to and / or controlling an oxidant in the aqueous stream, such that precipitates are formed in the aqueous stream may be followed by step 22 of separating the precipitates from the aqueous stream. Preferably, step 22 of separating the precipitates from the aqueous stream comprises gravity-based separation 24.

[0148] Furthermore, step 22 of separating the precipitates from the aqueous stream may be followed by step 26 of recovering phosphate and / or arsenate from the precipitate. Preferably, step 26 of recovering phosphate and / or arsenate from the precipitate comprises step 28 of treating the precipitate to produce iron oxide, and preferably step 30 of performing an alkaline treatment to produce an alkaline phosphate solution and / or an alkaline arsenate solution.

[0149] Step 26 may be followed by step 32 of recycling the iron(II) chloride in the step of dosing iron salt. Step 26 may also be followed by step 33 of further processing the recovered phosphate and / or arsenate from the precipitate.

[0150] In addition, step 22 may be followed by step 34 of neutralising the aqueous stream to a pH in the range of 6.5 to 7.5, wherein said step is performed after the step of separating the precipitates from the aqueous stream.

[0151] In an illustrated embodiment system 40 (Figure 2) comprises reactor 42, wherein reactor 42 comprises inlet 44 configured for an incoming aqueous stream comprising an initial amount of phosphate and / or arsenate and operatively coupled with aqueous stream supply 46. Furthermore, reactor 42 comprises iron salt dosing device 48 configured for dosing iron salt in the reactor, oxidant dosing device 50 configured for dosing an oxidant in the reactor such that precipitates are formed in the aqueous stream, wherein the precipitate comprises more than 60% of the initial amount of phosphate and / or arsenate in the incoming aqueous stream, and outlet 52.

[0152] In a preferred embodiment, reactor 42 may comprise mixing device 54, configured for mixing the aqueous stream, the iron salt, and the oxidant.

[0153] Furthermore, system 40 comprises separator 56, wherein outlet 52 is operatively connected with separator 56. In addition, separator 56 comprises outlet 58 which is configured for transporting the precipitate to collection tank 62. In addition, separator 56 further comprises outlet 60 which is configure for removing the aqueous stream which has no or low concentrations of phosphate and / or arsenate from separator 56.

[0154] Optionally, system 40 comprises treatment system 64. Treatment system 64 comprises mixing unit 66, alkaline inlet 68, phosphate moiety outlet 70, and iron oxide outlet 72. Alkaline inlet 68 is operatively coupled with mixing unit 66 and is configured to provide an alkaline to mixing unit 66. Furthermore, phosphate moiety outlet 70 is configured to provide phosphate moieties to collection unit 74, and iron oxide outlet 72 is configured to provide iron oxide to collection unit 76.

[0155] System 40 may further comprise dosing controller 78, configured for at least controlling the dosing of one or more selected from the group of iron salt, oxidant, acid, aqueous stream. Furthermore, system 40 may further comprise measurement system 80 that in the illustrated embodiment is configured to control at least one or more selected from the group of dosing of iron salt and / or oxidant with dosing devices 48, 50 in response to a measurement of the initial amount of phosphate and / or arsenate in the incoming and / or outgoing aqueous stream.

[0156] Method 100 (Figure 3) for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, follows a sequence of different steps.

[0157] In an illustrated embodiment method 100 may start with step 112 of providing a stream comprising iron salt, wherein the iron salt is an iron(II) P salt and / or iron(II) As salt, preferably the stream is an aqueous stream.

[0158] Step 112 providing a stream comprising iron salt, wherein the iron salt is an iron(II) P salt and / or iron(II) As salt may optionally be followed by step 114 of dosing an acid to and / or controlling an acid in the (aqueous) stream to reach a pH below 7, and step 116 of dosing an oxidant to and / or controlling an oxidant in the (aqueous) stream, such that precipitates are formed in the (aqueous) stream, wherein the precipitates include salts comprising iron, and phosphate and / or arsenate, wherein the precipitate comprises more than 60% of the initial amount of phosphate and / or arsenate in the (aqueous) stream.

[0159] In a preferred embodiment, step 114 comprises step 118 of controlling the dosing of an acid in response to a measurement of the initial amount of phosphate and / or arsenate in the (aqueous) stream. Furthermore, step 116 may comprise step 120 of controlling the dosing of an oxidant in response to a measurement of the oxidation rate of iron(II) to iron(III).

[0160] Furthermore, method 100 may comprise step 119 of measuring the phosphate and / or arsenate concentration in the aqueous stream. Step 119 may be performed before, during, and / or after any one of the steps 112, 114, and 116.

[0161] Step 116 of dosing an oxidant to and / or controlling an oxidant in the (aqueous) stream, such that precipitates are formed in the (aqueous) stream may be followed by step 122 of separating the precipitates from the (aqueous) stream. Preferably, step 122 of separating the precipitates from the (aqueous) stream comprises gravity-based separation 124.

[0162] Furthermore, step 122 of separating the precipitates from the (aqueous) stream may be followed by step 126 of recovering phosphate and / or arsenate from the precipitate. Preferably, step 126 of recovering phosphate and / or arsenate from the precipitate comprises step 128 of treating the precipitate to produce iron oxide, and / or step 130 of performing an alkaline treatment to produce an alkaline phosphate solution and / or an alkaline arsenate solution.

[0163] Step 126 may be followed by step 132 of recycling the iron(II) chloride in the step of dosing iron salt.

[0164] In addition, step 122 may be followed by step 134 of neutralising the aqueous stream to a pH in the range of 6.5 to 7.5, wherein said step is performed after the step of separating the precipitates from the aqueous stream, and / or step 136 of recovering iron, for example iron(III) chloride (FeCls) from the (aqueous) stream.

[0165] In a preferred embodiment, step 138 may be followed by step 138 of recycling iron(III) back to an (aqueous) stream comprising phosphate and / or arsenate for phosphate and / or arsenate recovery.

[0166] In an illustrated embodiment system 90 (Figure 4) comprises reactor 92, iron salt dosing device 94 configured for dosing iron salt in reactor 92, and acid dosing device 96 configured for dosing acid in reactor 92. It is noted that iron dosing device 94 is enabled to dose an iron(II)P moiety, such as vivianite-like structures, to reactor 92. Preferably, iron dosing device 94 is enabled to dose an aqueous stream comprising the iron salt to reactor 92.

[0167] Acid dosing device 96 may acidify the iron salt, preferably an aqueous stream comprising the iron salt, provided by iron salt dosing device 94. The combination of providing the iron salt and acid enables reactor 92 to solubilise the iron salt, such as vivianite-like structures.

[0168] Furthermore, system 90 comprises mixing unit 100 which is operatively connected via connector 98 with reactor 92, and fed with the (aqueous) stream from reactor 92. Mixing unit 100 is further supplied with an oxidant via oxidant dosing device 102 which is configured for dosing an oxidant to mixing device 100 such that precipitates, such as strengite-like structures, are formed in the (aqueous) stream.

[0169] Mixing device 100 is further operatively connected to separator 104 via connector 106. Separator 104 is configured to separate the precipitate, such as strengite-like structures, via outlet 108 and the (aqueous) stream via outlet 110, wherein said (aqueous) stream comprises (dissolved) iron salt, for example iron(III) chloride (FeCh). Said (aqueous) stream comprising (dissolved) iron salt, for example iron(III) chloride (FeCh) may be used for further phosphate recovery in a wastewater treatment plant.

[0170] System 90 further comprises precipitate, such as strengite-like structures, collection tank 112, wherein precipitate collection tank 112 is operatively connected with outlet 108.

[0171] Furthermore, in the illustrated embodiment, system 90 may further comprise dosing controller 114 and phosphate and / or arsenate measurement system 116 that are configured to control dosing of acid and / or oxidant with the use of dosing devices 94, 96 in response to a measurement of the initial amount of phosphate and / or arsenate in the incoming and / or outgoing (aqueous) stream.

[0172] Various experiments were performed. For example, the optimum initial pH (a pH between 1.5 and 5) for recovery was tested. Furthermore, the temperature (25 °C to 80 °C) was tested as a way that higher temperatures could induce partial crystallization and better settleability of the precipitate. In addition, controlled iron(III) supply was tested as a way to prevent high supersaturation where nucleation dominates over crystal growth and improve settleability. Thus, different iron(III) dosing rates, iron(II) dosage, and different oxidation oxidations (pCT = 0.2 bar, pCT = 1 bar, and pOz = 1.5 bar, and different H2O2 dosing rates) were tested.

[0173] FeNH^SCH - I2H2O, Fe(NH4)2(SC>4)2- 6H2O, and NH4H2PO4 salts (Sigma- Aldrich) were used for precipitation experiments adopted from Lundager Madsen & Koch, 2018 protocol. 1 M NaOH (VWR chemicals) was prepared with -99% pure NaOH pellets for different initial pH experiments. 0.01 M H2O2 (VWR chemicals) was used for chemical oxidation experiments. Milli- Q water was used as a solvent to prepare these solutions.

[0174] Furthermore, 0.085 M equimolar solution of FeNH4(SC>4)2 was prepared as a source of iron(III) and NH4H2PO4 as a phosphorus (P) source was prepared. Iron(III) solutions were first adjusted by adding IM NaOH in pH experiments. The temperature was controlled by shaking incubators in different temperature experiments. Iron(III) dosing to a solution including moieties comprising phosphorus was performed in two ways: one-time quick addition and drop-wise (for 3 hours) using a glass lab decanter funnel while stirring. The solid precipitation was separated from the liquid fraction by centrifugation at 4000 rpm for 10 mins. The precipitate was washed three times by adding MilliQ water and separated by centrifugation.

[0175] Furthermore, 0.085 M equimolar solution of Fe(NH4)2(SC>4)2- 6H2O as a source of iron(II) and NH4H2PO4 as a phosphorus (P) source was prepared. Open-air oxidation (pO2 = 0.2 bar) experiments were left on the lab bench overnight and for 10 days at room temperature. Nonepressurized (pO2 = 1 bar) and pressurized (pO2 = 1.5 bar) pure oxygen experiments were performed in serum-stoppered bottles and left overnight. In H2O2 oxidation experiments, H2O2 was dosed to iron(II)P (Fe(II)P) equimolar solution: one-time quick and drop-wise (for 4 mins, 10 mins, 3 hours) using a glass lab decanter funnel. Said iron(II)P equimolar solution was prepared using two salts, Fe(II) salt (Fe(NH4)2(SO4)2- 6H2O) and P salt (NH4H2PO4). The dosage of H2O2 was calculated according to Fenton's reaction equation, having a molar ratio Fe : H2O2 of 2:1.

[0176] Acidic liquid samples from EPS extraction installation in Epe wastewater treatment plant were collected as an example of complex wastewater (containing organic and inorganic impurities) to confirm the potential of phosphate recovery. At Epe a demo-scale EPS extraction process is operated to valorise carbon in excess granular sludge. The acidic phosphate-rich liquid stream is produced as a by-product. To perform the experiments, the phosphate concentration of the acidic stream from EPS extraction was measured, and iron(III) or iron(II) and H2O2 dosage was determined according to the theoretical molar ratio of Fe : P is 1 and Fe : H2O2 is 2. The elemental composition of the (acidic) aqueous stream is shown in Table 1.

[0177] Furthermore, Table 1 shows the results when strengite recovery was performed on a real wastewater sample (acidic stream produced by the EPS extraction process from aerobic granular sludge). The second column (EPS-acidic stream) shows the acidic stream composition (initially) that was used for in the experiment, and the third column (precipitate) show the composition of the FePO4-2H2O precipitate expressed as % of the TS.

[0178] Furthermore, precipitate settleability measurements were performed by pouring IL samples into Imhoff sedimentation cones and allowed to settle. The settled solids volume was recorded at 5, 10, and 30 minutes, and results were recorded as ml L1. Volumetric indexes were calculated as ml gP1recovered.

[0179] The samples obtained by various experiments were analysed. Fore example, the solid samples may be destroyed by microwave digestion to convert them to liquid analyses. Samples were digested in an Ethos Easy from Milestone with an SK-15 High-Pressure Rotor. Around 50 mg of solids were put in a Teflon vessel in which 10 mL of ultrapure HNO3 (64.5% to 70.5% from VWR chemicals) was poured. The digester is set to reach 200 °C in 15 minutes, run at this temperature for 15 minutes, and cool down for 1 hour.

[0180] The elemental inorganic composition may be determined via Inductively Coupled Plasma (Perkin Elmer, type Optima 5300 DV) with an Optical Emission Spectroscopy as a detector (ICP- OES). The device was equipped with an Autosampler, Perkin Elmer, type ESI-SC-4 DX fast, and the data were processed with the software Perkin Elmer WinLab32. The rinse and standard internal solutions were 2% HNO3 and 10 mg L1of Yttrium.

[0181] Ion chromatography samples were analysed using liquid samples. Said liquid samples were pre-treated first by filtering the samples through 0.45 pm followed by 0.22 pm membrane filters before analysis. Anions and cations (free dissolved ions) were measured by Metrohm Compact ion chromatograph Flex 930.

[0182] The dried precipitate was observed under scanned electron microscopy (SEM) (Jeol JSM- 6480LV) to determine the shape and size of the precipitate. The samples were spotted with a 10 nm layer of gold at 15 Pa and 25 mA to make the surface electrically conductive. The following settings were used: Accelerating Voltage: 6 kV, Working distance: 10 mm. The software used was JEOL SEM Control User Interface.

[0183] Room-temperature dried samples were used for X-ray diffraction analysis (XRD analysis). The sample was filled in a 0.7 mm glass capillary and tamped so the solid settled. The capillaries were sealed with a burner and mounted in a sample holder. The device used was a Bruker D8 Advance diffractometer with Cu Ka radiation (coupled 0 - 20 scan 10° to 110°, step size 0.030° 20, counting time per step 2s). The data evaluation was performed using Burker software DiffracSuite.EVA vs. 6.

[0184] Raman Spectroscopy was used to compare the iron-phosphate precipitates composition prepared from synthetic solution experiments with strengite-type minerals from the literature. The processed data of samples was obtained from Raman (LabRam Olympus MPlan N 100x / 0.9 Lens). The following settings for Raman Analysis were used: Exposition: 100 seconds, Spectro: Auto, Accumulation: 1x8, Binning: 1, Slit: 100 pm, Hole: 100 pm, Laser: 532.13 nm, Grating: 600, Objective: xlOO, Detector: Synapse CCD, Detector size: 1024. The spectrum was processed using Origin-pro by performing baseline correction and smoothing.

[0185] Field-dependent magnetization (M-H) curves at a temperature of 300 K were measured in a superconducting quantum interference devices (SQUID) MPMS-XL magnetometer equipped with a reciprocating sample option (RSO). The SQUID MPMS provides exceptional sensitivity, as high as 105Am2. The typical sample mass used in this work is about 2 to 3 mg. The temperature range is between 1.7 and 400 K, and the applied magnetic field is up to 5 T.

[0186] Liquid samples were analysed under a fight microscope. The sample was placed on the glass slide, and then a cover slide was placed. The sample was observed under 4x, lOx, 20x, and lOOx magnifications bright field using the Leica (DMI 6000B) stereo microscope and Olympus (Model BX43F) light microscope. The pictures were captured using the Leica Application Suite (LAS V4.6) and cellSens Standard. The solid precipitate from the real acidic samples experiments was analysed using an Elemental analyser (Mettler Toledo, America) to check the estimate of the organics coprecipitated.

[0187] The gas composition was analysed using gas chromatography to ensure that the serum bottles were completely purged by pure oxygen in iron(II) experiments. A glass syringe (1 mL) was connected to the serum bottles, and a sample was collected and analysed using Micro GC (CP- 4900) using argon as carrier gas. The module was connected to a Thermal Conductivity Detector (TCD) for data acquisition, and the Galaxy Chromatography Data System controls the instrument.

[0188] In this application, iron(II) and iron(III) were measured using iron Hach Lange kits which depend on the strong iron(II) -binding ligand phenanthroline. Three molecules of phenanthroline chelate a single iron(II) molecule to form an orange -red complex. Any iron(III) ions in the sample are reduced to iron(III) in a secondary step by ascorbic acid before the complex is formed again. Phenanthroline is better than ferrozine (another standard iron(II) colorimetric reagent) at low pH samples. Ferrozine can induce reduction and interfere with the reliable measurement of iron(II).

[0189] In an experiment, iron (III) (0.085 M of FeNH^SO^) has been dosed in amounts of 5% of the total iron salt amount with intervals of 9 minutes to an aqueous stream comprising 0.085 M of NH4H2PO4. The pH of the aqueous stream was initially about 1.5. The phosphate of NH4H2PO4 reacted with the iron (III) which led to the (instantaneous) formation of FePO4- 2H2O white precipitate.

[0190] In a further experiment, the effect of pH was studied at room temperature at pH values 1.5 (no pH adjustment), 3, 4, and 5 after iron(III) dosage. Total phosphorus (TP) removal from the liquid fraction and TP precipitated are reported (Table 2). Initial pH 1.5 had the lowest recovery, around 60 %, while at initial pH 3 or higher the phosphorus recovery reached ~98%.

[0191] Table 2: Phosphate recovery % in iron(III) tests (different pH and temperature) and iron(II) tests (different oxidation conditions). TP% (liquid) is phosphate removal from the liquid fraction measured by ion chromatography (IC), and TP% (precipitate) is phosphates recovered as a solid fraction measured by microwave digestion+ inductively coupled plasma, and the molar ratio of Fe : P of the precipitate is calculated. All experiments were performed in triplets, and averages and standard deviations were calculated.

[0192] In an experiment, iron(II) (0.085 M of Fe(NH4)2(SO4)2- 6H2O) has been dosed in amounts of 5% of the total iron salt amount with intervals of 15 minutes to an aqueous stream comprising 0.085 M of NH4H2PO. In 5 addition, hydrogen peroxide or oxygen (0.01 M) has been dosed in amounts of 5% of the total hydrogen peroxide amount with intervals of 15 minutes to the aqueous stream. Said hydrogen peroxide or oxygen acts as oxidant, and enables the oxidation of iron(II) to iron(III). The pH of the aqueous stream was initially about 3.5. The phosphate of NH4H2PO reacted with the iron(III) which then led to the formation of FePCH- 2H2O precipitate.

[0193] Phosphate recovery is very low in open-air oxidation systems (pO2= 0.2 bar), as about 7% of total phosphorus was recovered overnight (Table 2). When the experiment was extended to 10 days, the recovery went up to about 45%, as more precipitation formed over time. It was found that the oxidation of iron(II) to iron(III) under atmospheric conditions is very slow and is the ratelimiting step.

[0194] Hydrogen peroxide (H2O2) is a stronger oxidant than oxygen that can be used to increase the oxidation rate of iron(II) to iron(III). The addition of a peroxide, such as H2O2, increased the phosphate recovery. Indeed, said phosphate recovery was around 99%, which is higher compared to phosphate recovery using oxygen.

[0195] In a further experiment, the settling behaviour of the precipitate was determined and compared between iron(II) and iron(III) experiments. The precipitate was instantly formed with the (quick) addition of iron(III) (addition of full amount). The settling was poor under all pH and temperature conditions tested with a volumetric index (VI) equal to 263 ml gP '. The controlled addition of iron(III) over time (5% every 9 minutes) enhanced the settleability (VI = 40 ml gP (Table 3). Iron(II) addition instead of iron(III) with O2 and / or H2O2 oxidation further enhanced the settleability of the precipitate. Oxygen's slow oxidation results in the most settleable precipitate, while HzC s rapid oxidation can influence settling behaviour. Comparing H2O2 dosage over 3 hours, 10 mins, 4 mins, or one-time addition reveals this impact. These different oxidation conditions reflect that controlling the iron(II) oxidation rate is a way to control the settleability of the precipitate without the need for crystallizers or seed addition. Iron(II) oxidation rates from different experiments were calculated based on iron(II) consumption over time (Table 3). The effect of oxidation rate on settling and recovery was fitted to a sigmoidal function (Figure 5). Combining the phosphate recovery curve (time-dependent) with the settling curve shows a preferred method / system for phosphate and / or arsenate recovery that combines both a high recovery percentage and low volumetric index (ml gP ') at iron(II) oxidation rate of 4.7 x 104mol L1min1. It was also found that iron(III) and iron(II) / H2O2 identical dosing rates over three hours showed that iron(II) / H2O2 achieved better settleability.

[0196] Table 3: Volumetric index of the precipitate formed in different iron(II) and iron(III) experiments, calculated by measuring the volume of settled solids after 30 min as ml g phosphorus.

[0197] The precipitates formed from iron(III) and iron(II) oxidation systems were examined by SEM and a light microscope to relate the difference in settleability to particles shape / size. It was found that strengite-like structures were made performing a dosing regime of the iron salt and oxidant.

[0198] Said strengite-like structures has an equimolar ratio of iron and phosphorus based on its formula FePO 2H2O. The strengite-like structures precipitates were collected, solubilized, and analysed by inductively coupled plasma to determine the Fe : P molar ratio. The molar ratio of Fe : P was calculated (Table 1). In all experiments, the Fe : P molar ratio in the precipitate was around 1.

[0199] In a further experiment, magnetic separation of the precipitate being strengite-like structures was performed. Mass susceptibility measurements were 6.7 x 105, 5.4 x 105, and 5.4 x 105emu Oe1g1for iron(III) quick addition, wherein the aqueous stream had a pH of 3, and hydrogen peroxide and iron(II) chloride were dosed with 5% over 3 hours. It was found that these measurements are significantly higher (xlOO) than the values reported for vivianite 8 x 107emu Oe1g1(https: / / doi.org / 10.5194 / cp-9-433-2013). Since strengite-like structures has higher magnetic susceptibility, this allows for higher recovery / separation compared to vivianite with the same magnetic strength. This advanced separation technique can be useful for toxic or high organic matter waste streams where co-settling can be problematic. In a further experiment, (acidic) aqueous samples from an EPS extraction process from aerobic granular sludge (AGS) wastewater treatment plants was performed to confirm the potential of integrating this technology to recover phosphorus from EPS installations. Experiments showed that the theoretical iron dosage to keep Fe : P molar ratio at 1 and the theoretical H2O2 dosage to keep Fe : H2O2 at 2 was sufficient to achieve a phosphate recovery of over 95%. Therefore, it was surprisingly found that no higher iron and / or H2O2 dosage was required due to organics presence in these samples (COD = 11600 mg L ').

[0200] Following from Figure 5, it is known that the settleability using the method according to the invention was tested using the conditions as described in the previous experiments. It was found that up to 100% of the phosphorus in the initial aqueous stream could be recovered within 30 minutes.

[0201] Table 4: correlation oxidation rate and phosphorus recovery within 30 minutes.

[0202] The present invention is by no means limited to the above described preferred embodiments and / or experiments thereof. The rights sought are defined by the following claims within the scope of which many modifications can be envisaged.

Claims

CLAIMS1. Method for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, the method comprising the steps of: providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate; dosing an iron salt to and / or controlling an iron salt in the aqueous stream; dosing an oxidant to and / or controlling an oxidant in the aqueous stream; forming a precipitate in the aqueous stream, wherein the precipitates include salts comprising iron, and phosphate and / or arsenate; and recovering phosphate and / or arsenate from the precipitate.

2. Method according to claim 1, wherein the step of the step of dosing an oxidant to and / or controlling an oxidant in the aqueous stream and the step of forming a precipitate in the aqueous stream provides a volume index in the range of 3 ml gP1to 100 ml gP ', preferably in the range of 3 ml gP1to 85 ml gP ', within 30 minutes.

3. Method according to any one of the preceding claims, wherein the precipitates comprise strengite-like structures.

4. Method according to any one of the preceding claims, wherein the precipitates are FePCU- 2H2O and / or FeAsCU- 2H2O.

5. Method according to any one of the preceding claims, wherein the aqueous stream has a pH in the range of 0 to 6.5, preferably a pH in the range of 0 to 6, more preferably a pH in the range of 0 to 5, and most preferably a pH in the range of 1 to 4.

6. Method according to any one of the preceding claims, wherein the iron salt is an iron(II) salt.

7. Method according to any one of the preceding claims, wherein the iron salt is one or more selected from the group of iron(II) chloride, iron(II) sulphate, Fe2+3(PO4)2- 8H2O, Fe2+3(AsO4)2- 8H2O.

8. Method according to any one of the preceding claims, wherein the iron salt is insoluble in an aqueous environment having a pH of equal to or greater than 6, preferably a pH of equal to or greater than 5, and is soluble at a pH of equal to or less than 4.

9. Method according to any one of the preceding claims, wherein the iron salt is Fe2+3(PC>4)2- 8H2O and / or Fe2+3(AsO4)2- 8H2O.

10. Method according to any one of the preceding claims, wherein the oxidant is one or more selected from the group of a peroxide, fluorine, hydroxyl radical, ozone, oxygen, hydrogen peroxide, potassium permanganate, chlorine oxide, chlorine.

11. Method according to claim 10, wherein the peroxide is hydrogen peroxide.

12. Method according to any one of the preceding claims, wherein the step of dosing an oxidant to and / or controlling an oxidant in the aqueous stream is performed for a period in the range of 5 minutes to 8 hours, preferably for a period in the range of 30 minutes to 6 hours, more preferably for a period in the range of 1 hours to 4 hours.

13. Method according to any one of the preceding claims, wherein the oxidation rate is in the range of 0.5 x 104mol L1min1to 100 x 104mol L1min1, preferably in the range of 1.0 x 104mol L1min1to 75 x 104mol L1min ', more preferably in the range of 1.0 x 104mol L1min1to 50 x 104mol L1min ', most preferably in the range of 1.0 x 104mol L1min1to 40 x 104mol L1min '.

14. Method according to any one of the preceding claims, further comprising the step of measuring the phosphate and / or arsenate concentration in the aqueous stream.

15. Method according to any one of the preceding claims, further comprising the step of controlling the dosing of an iron salt in response to a measurement of the initial amount of phosphate and / or arsenate in the aqueous stream.

16. Method according to any of the preceding claims, further comprising the step of controlling the dosing of an oxidant in response to a measurement of the oxidation rate of iron(II) to iron(III).

17. Method according to any one of the preceding claims, further comprising the step of separating the precipitates from the aqueous stream.

18. Method according to the foregoing claim, wherein the step of separating the precipitates from the aqueous stream comprises gravity-based separation.

19. Method according to claim 17 or 18, wherein the step of separating is performed using one or more of a sedimentation tank, a filtration unit, a centrifuge.

20. Method according to any one of the foregoing claims, wherein the step of recovering the phosphates and / or arsenate from the precipitate comprises treating the precipitate to produce iron oxide.

21. Method according to the foregoing claim, wherein treating the precipitate comprises performing an alkaline treatment to produce an alkaline phosphate solution and / or an alkaline arsenate solution.

22. Method according to the foregoing claim, wherein the alkaline treatment is performed using potassium hydroxide and / or sodium hydroxide.

23. Method according to any one of the claims 20 to 22, further comprising the step of treating the iron oxide with hydrochloric acid to produce iron(II) chloride.

24. Method according to the foregoing claim, further comprising the step of recycling the iron(II) chloride in the step of dosing iron salt.

25. Method according to any one of the preceding claims, wherein the step of dosing iron salt to the aqueous stream comprises adding a total amount of iron with a molar ratio iron to phosphorus of the phosphate and / or arsenic of the arsenate of at least 1, preferably at least 1.25, and more preferably at least 1.5.

26. Method according to any one of the preceding claims, wherein the step of dosing an oxidant to the aqueous stream comprises adding a total amount of oxidant with a molar ratio oxidant to phosphorous of the phosphate and / or arsenic of the arsenate of at least 1, preferably at least 1.25, and more preferably at least 1.5.

27. Method according to any one of the preceding claims, further comprising the step of filtering the aqueous stream, wherein the step of filtering is performed before the step of providing an aqueous stream with a pH below 7 comprising an initial amount of phosphate and / or arsenate.

28. Method according to any one of the preceding claims, further comprising the step of dosing an acid to and / or controlling a pH in the aqueous stream.

29. System for phosphate and / or arsenate recovery from an acidic stream, such as a waste flow, sewage stream, or sludge stream, the system comprising: a reactor comprising: an inlet for an incoming aqueous stream comprising an initial amount of phosphate and / or arsenate; an iron salt dosing device configured for dosing iron salt in the reactor; an oxidant dosing device configured for dosing an oxidant in the reactor such that precipitates are formed in the aqueous stream; and an outlet.

30. System according to the foregoing claim, further comprising a separator for separating the precipitate from the stream.

31. System according to claim 29 or 30, further comprising a treatment system for treating the separated precipitate to produce iron oxide precipitates.

32. System according to any one of the claims 29 to 31, further comprising a dosing controller and a phosphate and / or arsenate measurement system that are configured to control dosing of iron salt and / or oxidant in response to a measurement of the initial amount of phosphate and / or arsenate in the incoming and / or outgoing aqueous stream.

33. Use of a precipitate obtainable by the method according to any one of the claims 1 to 28, wherein the precipitate is used as fertilizer, flame retardant, in a lithium-ion battery, such as a precursor for a lithium-ion battery, adsorbent, such as an adsorbent for heavy metals.

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