WO2020209719A1

WO2020209719A1 - Method, device and wastewater treatment system for phosphorus, such as phosphate, removal from a feed solution

Info

Publication number: WO2020209719A1
Application number: PCT/NL2020/050242
Authority: WO
Inventors: Yang Lei; Machiel Saakes; Renata Dorothea Van Der Weijden; Cees Jan Nico Buisman
Original assignee: Stichting Wetsus, European Centre Of Excellence For Sustainable Water Technology
Priority date: 2019-04-10
Filing date: 2020-04-09
Publication date: 2020-10-15
Also published as: NL2022919B1

Abstract

The present invention relates to a method, device and wastewater treatment system for phosphorus, such as phosphate, removal from a seed solution, the method according to the invention comprises the steps of: - providing a reactor and packing at least part of the reactor with granules; - arranging at least one cathode and/or at least one anode in or at the reactor; - providing a waste stream to the reactor; - providing a current over the cathode and anode; and - forming phosphate salt at and/or on the cathode and/or the granules.

Description

METHOD, DEVICE AND WASTEWATER TREATMENT SYSTEM FOR PHOSPHORUS, SUCH AS PHOSPHATE, REMOVAL FROM A FEED SOLUTION

The present invention relates to a method, device and wastewater treatment system for phosphorus, such as phosphate, removal from a feed solution.

Methods, devices and wastewater treatment systems for phosphate or other phosphorus removal from a feed solution are known from practice. These methods, devices and wastewater treatment systems may use membranes for removal of phosphate from a feed solution. Membranes usually have high maintenance costs due to clogging and degradation, and thus require frequent cleaning and/or replacement. Furthermore, the use of one or more membranes results in an increase in the pressure drop over the system. This results in additional costs for the phosphate removal.

Other methods for the removal of phosphate from a seed solution are based on chemical precipitation, adsorption by (natural) materials and biological treatments. It is known that adsorption is one of the techniques to remove phosphorus, and is comparatively more useful and more efficient than the chemical precipitation and biological treatments. Furthermore, mesoporous materials, such as lanthanum-doped mesoporous silica oxide, have been used for the removal of phosphates from wastewater. This requires relatively complex and costly treatments. In addition, in practice, neither of these methods, devices and wastewater treatment systems can be applied easily at large scale.

Removal of phosphorus is desired as this element is associated with eutrophication.

An objective of the present invention is to provide a method, device and wastewater treatment system for the enhancement of the phosphate removal from a feed solution that obviates or at least reduces the aforementioned problems and/or is more efficient as compared to conventional methods, devices and wastewater treatment systems for the phosphate removal from a feed solution.

The objective is achieved with the method for phosphorus, such as phosphate, removal from a feed solution according to the invention, the method comprising the steps of:

providing a reactor and packing at least part of the reactor with granules; arranging at least one cathode and/or at least one anode in or at the reactor; providing a waste stream to the reactor;

providing a current over the cathode and anode; and

forming phosphate salt at and/or on the cathode and/or the granules.

A feed solution may involve one or more of waste streams, industrial waste stream, domestic waste solution, agricultural waste solution or general waste solution, or other feed solution comprising valuable resources, such as phosphorus and in particularly phosphate. Instead of or in addition to the feed solution, solid waste materials such as slags and amorphous solids can be used. The method according to the invention aims at removal of a substantial amount of the phosphorus, such as phosphate, from the feed flow.

The feed solution may be a solution comprising dissolved ions comprising phosphorus, or a suspension. It will be understood that a suspension comprising particles without phosphorus may be filtered before use. Therefore, in a preferred embodiment the method comprises the step of filtering the feed solution, wherein the filtering step is performed before the step of providing a waste stream to the reactor.

According to the invention there is provided a reactor that is at least partly packed with granules to enable the reactor to trap phosphate salts. The granules will effectively and efficiently provide a purified solution with a reduced phosphate concentration. It will be understood the granules can form a sort of electrochemical precipitation reactor, wherein precipitation of for example phosphate under electrochemical conditions can occur.

In the rest of the description according to the invention the advantages and effects discussed refers to phosphate. It will be understood that this also applies to phosphorus.

To form phosphate salt at and/or on the cathode and/or the granules and/or in a liquid medium a feed solution, such as a waste stream, is provided to the reactor. This will result in the ability of removing phosphates from the feed solution and effectively and efficiently remove a substantial amount of the phosphate from the feed solution.

The method according to the invention makes use of electrodes that are able to conduct electricity. In order to form phosphate salt at and/or on the cathode and/or the granules a current is required. The electrodes are arranged such that at least one cathode and/or at least one anode is provided in or at the reactor. A current source is provided such that a current is provided at the electrode(s). Providing a current over the cathode and anode will result in electrochemical induced phosphate salt precipitation.

An advantage of the electrodes being exposed to the feed solution is that less maintenance is required. This will then result in lower operation costs and fewer downtime of the device the method is applied to.

For example, when the feed solution, such as a waste stream, comprises water it can be reduced to hydroxide ions (OH ) at the cathode and oxidised to protons (referred to as H₃0⁺ or H⁺) at the anode by electrolysis. The reactions at the cathode and anode can be defined as:

Cathode:

Anode: 2

The protons will at least partly react with an electrochemical precipitation compound and/or granules comprising one or more electrochemical precipitation compound, and dissolve the electrochemical precipitation compound, yielding (alkaline earth) metal cations and anion species. Therefore, an advantage of the method according to the invention is that neutralisation of hydroxide and protons formed at the anode is minimised. Furthermore, the granules prevent severe mixing of the feed solution, and thus severe mixing of the protons and hydroxide. A further effect is that the phosphorus, such as phosphate, removal can be performed in a single cell/single compartment. A single compartment is an undivided compartment. This obviates the need for a membrane or membrane system. The effectiveness of hydroxide utilization is increased by reducing the recombination with protons. As a result of the dissociation, the hydroxide increases the local pH near the cathode and increases the degree of supersaturation for (alkaline earth) metal phosphate. Therefore, (alkaline earth) metal ions will react with phosphate ions, which are dissolved in the waste stream, and precipitate on the cathode and/or granules. Thus, the method according to the invention will result in electrochemical induced (alkaline earth) metal phosphate precipitation.

The current provides the required energy to induce electrochemical phosphate salt precipitation. The, for example, water which is part of the feed solution, will split into an anion and cation. The dissociated water, especially the protons of the dissociated water, are capable to react with the electrochemical precipitation compound, for example the granules. The water may act as an electrolyte.

Furthermore, it is noted that, for the purpose of the invention, the current can be provided by (rechargeable) batteries, a power grid, windmills, solar panels, or any other type of electrical energy source. In such configuration, the positive pole is connected to the cathode or anode and the negative pole is connected to the other electrode. For example, when the positive pole of the electrical energy source is connected to the cathode, the negative pole of the electrical energy source is connected to the anode. It will be understood this can also be in the opposite way. For example in case of polarity reversal for cleaning the reactor.

Another advantage of the method according to the invention is that a feed solution with a pH of at least approximately 4 can be treated to remove the phosphate. As a result the solution after the treatment comprises a pH of around 7 (neutral pH). The treated solution could be disposed to the surface water. Costs will therefore be reduced as special treatment of the feed solution is not required.

Experiments showed that a feed solution with a pH of approximately 4 could be used to remove phosphate/phosphorus from a feed solution. After the treatment the pH is approximately 7 (neutral) and the water can be disposed to the surface water or the like. Providing a feed solution comprising a pH of at least 4 before treatment results in the ability to use a wide variety of sources of the feed solution. For example, the feed solution may originate from domestic waste, industrial waste, farmlands, hospitals, and the like.

In a presently preferred embodiment the granules are provided with pores, also referred to as openings, having a pore size. Preferably, the granules are provided with pores to achieve a (relatively) high specific surface area. This increases the effective granule surface. Such configuration improves the trapping of phosphate salt.

A further advantage of the method according to the invention is that phosphate salt precipitates, for example phosphate salt is an alkaline earth metal phosphate salt. As a result the phosphate salt will precipitate on the cathode and/or on the granules and/or forms a suspension with the waste stream. The formation of solid phosphate salt is based on the solubility of the salt in the medium. Therefore, a low solubility of the phosphate salt is preferred, and the phosphate salt precipitates easily on the cathode and/or on the granules and/or forms a suspension with the waste stream. A suspension can be formed in batch as well as in a continuous process.

As mentioned before, when a suitable current is passing through the cathode, water molecules will be reduced at the cathode, producing H₂ and hydroxide ions. The formation of hydroxide ions increases the pH close to the cathode, so-called the high local pH (wherein the high local pH is compared to the pH of the feed solution). With the increase of pH in the vicinity of the cathode, the solubility of ions comprising phosphate decreased and the driving force for compounds comprising phosphate precipitation increases. Close to the cathode the concentration of the compound comprising phosphate becomes saturated and precipitates on and/or close to the cathode in the form of the compound comprising phosphate solids. Preferably, use is made of electrochemical precipitation compounds that are preferably soluble in (mildly) acidic conditions and are able to precipitate phosphate by forming insoluble or less soluble (compared to the starting material) salts.

In a preferred embodiment according to the invention, the reactor is provided with an undivided reactor volume comprising the at least one cathode and/or at least one anode, and the granules. The reactor volume is the space for the formation of the phosphate salt. Therefore, the method does not require the use of a membrane. This reduces the complexity and/or costs of the method and system. In addition, it reduces the risk of fouling and the amount of maintenance that is required.

The granules prevent severe mixing of the feed solution, and thus severe mixing of the protons and hydroxide. A further effect is that the phosphorus, such as phosphate, removal can be performed in a single cell/single compartment. As a result, the method does not require the use of a membrane and is therefore more efficient due to less maintenance. Furthermore, such method is cheaper as the cost of a membrane is higher compared to the granules.

In a preferred embodiment according to the invention, the method comprises the step of recovering the phosphate, wherein the phosphate salt is recovered from the reactor.

In such preferred embodiment of the invention the method anticipates the formation of phosphate salts, wherein the phosphate is recovered from the feed solution. Recovery of phosphate from the feed solution includes filtering and/or removal. Filtering of the phosphate salt removes phosphate salts from the feed solution. For example, phosphate salts can be bound by and/or precipitate on the granules. Saturated granules can be replaced and the used granule bed disposed. Filtering has the advantage that the influence of eutrophication will be reduced.

Preferably, the phosphate salts are recovered from the feed solution and/or from the granules and/or from at least the cathode. Recovering has the advantage that valuable phosphorus is harvested from the feed solution as the worldwide supply of phosphorus is limited. Recovery opens up the possibility to re-use the phosphorus.

Furthermore, this method provides removal and/or filtering and/or recovery from (organic) phosphorus by a single reactor. The valuable phosphorus can be used as fertilizer and/or nutrient.

In addition, phosphorus can be collected effectively by the recovering step. A further advantage of the method according to the invention is that phosphorus is recovered and less phosphorus needs to be mined. This contributes to a reduction of the mining process of phosphorus on the environment. This enables a more sustainable operation.

In a further preferred embodiment according to the invention granules are provided that comprise calcium carbonate and/or calcium sulphate, for forming of phosphate salt.

In stead of, or in addition to, calcium carbonate and/or calcium sulphate, the granules comprise magnesium carbonate and/or magnesium sulphate.

Calcium carbonate, magnesium carbonate, calcium sulphate and magnesium sulphate are also referred to as electrochemical precipitation compounds. It will be understood that also granules comprising other electrochemical precipitation compounds, such as for example calcium nitrate tetrahydrate (Ca(N0 )₂-4H₂0), iron(II) carbonate, barium carbonate, lead carbonate and calcium magnesium carbonate, can be used. The compounds suitable for the method according to the invention require to be soluble in (mildly) acidic conditions and are able to precipitate phosphate by forming insoluble or less soluble (compared to the starting material) salts. In the present invention, salts can also be referred to as minerals. These electrochemical precipitation compounds (iron(II) carbonate, barium carbonate, lead carbonate, calcium magnesium carbonate) provide the same or similar advantages and effects as calcium carbonate, calcium sulphate, magnesium carbonate and magnesium sulphate. However, presently, the application of calcium carbonate, calcium sulphate, magnesium carbonate and/or magnesium sulphate is preferred as these components are available and enable a cost effective and sustainable removal of phosphate from a feed solution.

In a presently preferred embodiment according to the invention, the waste stream is an ammonium poor feed solution.

The ammonium poor feed solution is defined by the molar ratio ammonium (NH₄ ⁺) to phosphate (P0 ³ ). Preferably, the NH₄ ⁺/P0 ³ ratio is below 1. An advantage of the NH₄ ⁺/P0 ³ ratio below 1 in the ammonium poor feed solution is that the formation of struvite (MgNH₄P0 · 6H₂0) is reduced. Struvite comprises equal moles masses of NH₄ ⁺, P0 ³ , and Mg²⁺. In a preferred embodiment, the NH₄ ⁺/P0 ³ ratio is below 0.5, more preferably the NH₄ ⁺/P0 ³ ratio is 0.25, even more preferably the NH₄ ⁺/P0 ³ ratio is 0.16.

An advantage of an ammonium poor feed solution is that more efficient and effective recovery of phosphate is achieved. Furthermore, an ammonium poor feed solution requires less treatment steps after phosphate removal.

It will be understood a substantially ammonium free solution as waste stream can also be used. This has the same advantages and effects as an ammonium poor solution.

In a presently preferred embodiment of the invention, in order to achieve

precipitation/formation of a phosphate salt it is preferred to provide granules comprising electrochemical precipitation compounds of which the metal component is able to precipitate phosphate. The solubility of the formed phosphate salt is preferably lower than the solubility of the electrochemical precipitation compound.

Preferably, the electrochemical precipitation compound comprises a carbonate. The advantage of a carbonate group is the equilibrium with the atmosphere as carbon dioxide (gas) can escape from the reactor without a build up of the acidity.

Preferably, the electrochemical precipitation compound is calcium carbonate and/or magnesium carbonate as the price to buy is lower compared that the other before mentioned electrochemical precipitation compounds.

An advantage of the method according to the invention is that calcium and magnesium are able to form (solid) phosphate salts. Therefore, the phosphate salt in such embodiment is calcium phosphate and/or magnesium phosphate. The formation of calcium phosphate and/or magnesium phosphate reduces the amount of solubilised phosphate and will remove the phosphate from the waste stream. This helps to remove and/or filter and/or recover the phosphate and provides a sustainable method to minimise the disposal of valuable phosphorus.

A further advantage of the method according to the invention is that calcium phosphate and/or magnesium phosphate have a relatively low solubility in water. This will result in precipitation of the phosphate salt and therefore relatively easy removal and/or filtering and/or recovery of the phosphate salt. Furthermore, high recovery of the valuable phosphate can be achieved. In addition, calcium phosphate and/or magnesium phosphate also have a relatively low solubility in the majority of (other) solvents. The low solubility will result in precipitation of the phosphate salt and therefore easy recovery and/or filtering and/or removal can be achieved.

Furthermore, high recovery of the valuable phosphate can be achieved.

A further advantage of the method according to the invention is that such phosphate salts can be removed easily and less labour is required to remove and/or filter and/or recover the phosphate. In the presence of calcium carbonate particles, the calcium carbonate solids present at the anode dissolve to some extent due to the acidity at the anode. The reaction may be defined as:

CaC0₃ + 2H⁺ Ca²⁺ + H₂C0₃

Along with the reaction of H⁺ with CaC0 , the cathodically produced OH ions will be accumulated in the bulk solution, increasing the pH of the bulk solution. Meanwhile, Ca²⁺ ions are released from CaC0 particles. In the case of a feed solution comprising phosphate or ions comprising phosphor, the phosphate or ions comprising phosphorus will be removed by calcium phosphate precipitation, either on the cathode surface, which has a local high pH or on the surface of CaC0₃ particles.

In a further preferred embodiment according to the invention, the granules comprising a carrier material configured for the forming of phosphate salt, such as but not limited to, clay, activated charcoal, silicon particles, and the like. Preferably, the carrier material is clay or activated charcoal.

An advantage of such particles is that the particles provide a larger surface. This will result in a higher tendency for phosphate precipitation. Furthermore, this will reduce the downtime of the reactor as more phosphate can be precipitated. This also influences the costs, which are reduced, as the granules need to be changed less frequently.

A further advantage of using such particles in an embodiment of the invention is that these particles are inert. The formed phosphate salt can precipitate on these particles which can then be used for the removal and/or filtering and/or recovery of the valuable phosphorus containing precipitate. For example, the inert granules can be provided to the bottom of the reactor to form a bed and the granules comprising the electrochemical precipitation compound can be provided on top of the inert granules.

For example, the precipitated phosphate salt is recovered by dissolving it in a suitable solvent. Preferably, the granules with the precipitated salt are combined with a solvent suitable for dissolving the phosphate salt.

Therefore, the carrier material may promote the precipitation of phosphate salt.

In a further preferred embodiment according to the invention, the granules have a size distribution with an average particle size between 0.5 mm and 3 mm, preferably between 1 and 2 mm.

It will be understood that the relative surface of the granules will increase with smaller diameter when weight of the granules is the substantially the same. Furthermore, the surface of the granules will increase when these are porous. This is in particularly effective when a continuous process is operated. Furthermore, the packing density of the granules leaves preferably enough space between the granules in order for migration of metal ions between the granules. Migration of the metal ions may be reduced by the build-up of phosphate salts between the granules. Preferably the average particle size distribution is 90% (with 90% of the particles in the range), more preferably the average particle size distribution is 95%, most preferably the average particle size distribution is 99%.

A further advantage of the method according to an embodiment of the invention is that phosphate salts can precipitate well on the granules. Furthermore, this size distribution of the granules results in convenient saturation of the reactor with phosphate salt. If the reactor reaches the saturation point with phosphate salt too quickly, the granules needs to be replaced too frequently. This is due to the fact that phosphate salt precipitation rate will drop when approaching the saturation point.

In a further preferred embodiment according to the invention, the method comprises the step of sensing and/or measuring the phosphate concentration.

An advantage of sensing and/or measuring the phosphate concentration is that the precipitation rate of the phosphate salt and/or phosphate concentration in the waste stream can be analysed. As a result the substantially maximum saturation of the reactor can be anticipated and/or the quality of the out flow can be determined before the out flow is discharged, for example. Furthermore, sensing/measuring enables to control the reactor conditions, e.g. temperature, pH, flow rate (if applicable), purity, and the like.

A further advantage is that the recovery of the phosphate can be optimised, and thus an efficient and effective recovery of valuable phosphorus can be achieved.

In a further preferred embodiment according to the invention, the method comprises the step of providing an additional electrolyte, wherein the additional electrolyte is chosen from the group of sodium sulphate, sodium perchlorate, sodium chloride, or a mixture thereof.

Providing an additional electrolyte to the reactor will increase conductivity and the formation of phosphate salt. This will result in a more efficient and effective removal and/or filtering and/or recovery of phosphate from a waste stream. Furthermore, the turn over of the reactor will increase due to the fact that a larger amount of valuable phosphorus per reactor can be recovered.

A further advantage of providing an electrolyte to the reactor is that this enables the use of a suspension as feed solution.

In a further preferred embodiment according to the invention, the method comprises the step of oxidation of phosphorus and/or phosphate, wherein the oxidation step is preferably performed in-situ, wherein the phosphorus and/or phosphate is organic phosphorus and/or organic phosphate.

An advantage of the oxidation step is that the molecular structure of compounds can be broken down. As a result, organic as well as inorganic compounds can be removed and/or filtered and/or recovered. This will increase the suitability of the system for domestic waste streams as well as industrial waste streams.

A further advantage of the oxidation step is that organic phosphorus and/or organic phosphate can be converted into inorganic phosphate. The inorganic phosphate can than form a phosphate salt and be removed and/or filtered and/or recovered. This will increase the ability to recover a higher amount of valuable phosphorus.

It will be understood that the in-situ oxidation may be performed within the feed solution which is present inside the reactor.

Preferably, before the oxidation step a pre-treatment step is performed. The pre-treatment of the waste stream will result in a feed solution that is able to be oxidised more effectively. The pre-treatment step comprises the oxidation of organic phosphorus and/or organic phosphate to inorganic phosphate and/or the mixing of the feed solution with an oxidizing agent and/or treating the feed solution comprising the oxidizing agent with UV-light, heat, and/or other suitable techniques.

In a preferred embodiment, the pre-treatment step of the waste stream is performed in a single step operation with the providing a waste stream to the reactor step. This will result in an efficient and effective phosphorus removal.

The invention further also relates to a device for phosphate removal, with the device comprising:

a reactor with a reactor housing;

at least one cathode and at least one anode which are operatively connected with each other;

an effluent outlet and effluent inlet, both operatively connected with the reactor housing; and

a bed of granules, at least partly filling the reactor,

wherein the at least one cathode and/or anode and/or the bed of granules are configured for formation of phosphate salt.

The device provides similar advantages and effects as described for the method.

By providing at least one cathode and at least one anode which are operatively connected with each other and provided to the housing the forming of phosphate salts can be initiated. This enables the device to electrochemically treat a feed solution, such as a waste stream.

In a preferred embodiment according to the invention, the reactor comprises an undivided reactor volume comprising the at least one cathode and/or at least one anode, and the granules. Therefore, the device does preferably not comprise a membrane.

A device without a membrane is cheaper to produce and to run. Furthermore, less maintenance needs to be performed. In a preferred embodiment according to the invention, the reactor housing of the device is the cathode or anode.

By providing the housing as the cathode or anode a large surface for at least one of the electrodes is achieved. Preferably the housing is the cathode, to increase the surface where the phosphate salt can precipitate. Furthermore, this will minimise the device from clogging. In addition, this minimises the number of parts.

In a further preferred embodiment according to the invention, the cathode or anode comprises a tube which is assembled within the reactor housing. Preferably, the tube comprises at least one side wall with a number of openings, wherein the tube defines an inner space for receiving and/or holding phosphorus and/or phosphate derivatives from the inlet.

By providing the cathode or anode as a tube which is configured within the reactor housing, oxidation can occur in the tube, while precipitation of the phosphate salt occurs outside the tube. An advantage of the device according to the invention is that organic phosphorus is often solubilised in non aqueous solvents/non water miscible solvents.

Preferably, the cathode forms the housing of the reactor and the anode comprises a tube which is assembled within the reactor. It is also possible that the anode forms the housing of the reactor and the cathode comprises a tube which is assembled within the reactor. Choosing the cathode to be the housing of the reactor limits the anode to be at least one conventional electrode and/or tube assembled within the reactor housing and the like.

In a further preferred embodiment according to the invention, the anode and/or cathode are made of and/or comprise platinum coated titanium, platinum-iridium coated titanium, ruthenium- iridium coated titanium, or a mixture thereof, and the reactor further preferably comprises a sensor for sensing and/or measuring the degree of saturation of the reactor with phosphorus and/or phosphate and/or concentration of phosphorus and/or phosphate.

By providing a platinum coated titanium cathode and/or anode an efficient and effective conductivity is achieved. Furthermore, by providing a sensor the phosphate precipitation rate and/or phosphate/phosphorus concentration and/or in the waste stream can be analysed.

Furthermore, the sensor can also measure the oxidation reaction of the organic phosphorus.

In a preferred embodiment according to the invention, the device for phosphate removal is an electrochemical device comprising at least one cathode and one anode. In use, when a current is passing through the cathode, water molecules will be reduced at the cathode, forming hydrogen (H₂) and hydroxide ions. The formation of hydroxide ions increases the pH close (up to 2 centimetre, preferably up to 4 centimetre) to the cathode. With the increase of the pH close to the cathode, the solubility of the phosphate species, for example calcium phosphate, decreases and the driving force of phosphate salt, for example calcium phosphate, precipitation is increased. The invention further also relates to a wastewater treatment system comprising a device for phosphate removal by the formation of phosphate salt at the cathode according to the invention and configured for performing the method according to the invention.

The wastewater treatment system provides similar advantages and effects as described for the method and/or the device.

The wastewater treatment system provides an efficient and effective way to remove and/or filter and/or recover phosphate from a waste stream and regain valuable recourses.

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:

Figure 1 shows a schematic overview of the method according to the present invention;

Figure 2 shows an embodiment of the device comprising a housing, electrodes, inlet, outlet and granules capable of performing the method described in the description; Figure 3 shows an alternative embodiment of the device wherein the housing constitutes one of the electrodes;

Figure 4 shows a further alternative embodiment of the device comprising a tube; Figure 5 schematically shows an embodiment of the wastewater treatment system; Figure 6 shows removal efficiency relative to time (day) with the method and device according to the invention;

Figure 7 shows removal of organic phosphorus by electrochemical precipitation with the method and device according to the invention;

Figure 8A shows removal of organic phosphorus for different anode materials with the method and device according to the invention;

Figure 8B shows removal of organic phosphorus for different electrolytes with the method and device according to the invention;

Figure 9A shows phosphorus removal (mg/L) relative to time (h) with the method and device according to the invention;

Figure 9B shows relative to with the method and device according to the invention; Figure 9C shows relative to with the method and device according to the invention; Figure 9D shows relative to with the method and device according to the invention; Figure 10 shows the phosphorus removal efficiency with different currents;

Figure 11 shows the phosphorus removal efficiency with domestic waste water as feed solution at 5 mA and 10 mA; and

Figure 12 shows the phosphorus removal efficiency with and without calcium carbonate particles. Method 10 (Figure 1) for phosphate removal from a feed solution follows a sequence of different steps. In the illustrated embodiment this starts with preparation A involving preparing and packing 12 a reactor with granules. At least one cathode and at least one anode are arranged in or at the reactor in arranging step 14. Optionally, (additional) electrolyte is provided to the reactor in optimization step 16.

Preparation A is followed by operation B. This involves starting the supply of feed solution 2, such as a waste stream, to the reactor in operation step 18. In operation, a current is provided over the cathode and anode to dissociate the electrolyte, which is for example water which can be present in the waste stream, into components 4. In forming step 20 phosphate salts 6 are formed on the cathode(s) and/or granules. In a presently preferred embodiment forming step 20 is followed by recovering step 22 to recover phosphate salts 6 formed at the cathode and/or granules and providing recovered phosphate salts 8. These recovered phosphate salts 8 are optionally treated further in posttreatment step 24 to produce end-product 9, such as fertilizer. It will be understood that the removal and/or recovery of phosphate salt can monitored/analysed by sensing and/or measuring to enable control of operation B.

In an illustrated embodiment of the invention, device 30 (Figure 2) is configured for phosphate removal by the formation of phosphate salt at the cathode, and comprises reactor 32 and optionally sensor 31, wherein sensor 31 comprises sensor body 31 A and sensor analyser 3 IB. Reactor 32 comprises reactor housing 34, effluent inlet 36 which operatively coupled with reactor housing 34 and effluent outlet 38 which is operatively coupled with reactor housing 34. Reactor 32 is filled with granules 40 and 42. Granules 40 and 42 can be the same type of granules or different type of granules. Two electrodes, cathode 44 and anode 46, are assembled within reactor 32.

Cathode 44 and anode 46 are operatively coupled with current/voltage supply 48 via

circuit/coupling element 50 and circuit/coupling element 52. Waste stream 54 flows from effluent inlet 36 via reactor 32 to effluent outlet 38 as purified stream 55.

In an illustrated alternative embodiment of the invention, device 56 (Figure 3) is configured for phosphate removal by the formation of phosphate salt at the cathode, and comprises reactor 58 and optionally sensor 57, wherein sensor 57 comprises sensor body 57A and sensor analyser 57B. Reactor 58 comprises reactor housing 60, effluent inlet 62 which operatively coupled with reactor housing 60 and effluent outlet 64 which is operatively coupled with reactor housing 60. Reactor 58 is filled with granules 66 and 68. Granules 66 and 68 can be the same type of granules or different type of granules. Two electrodes, cathode 70 and anode 72, are assembled within reactor 58. Cathode 70 forms reactor housing 60. Cathode 70 and anode 72 are operatively coupled with current/voltage supply 74 via circuit/coupling element 76 and circuit/coupling element 78. Waste stream 80 flows from effluent inlet 62 via reactor 58 to effluent outlet 64 as purified flow 81. In an illustrated embodiment of the invention, device 82 (Figure 4) is configured for phosphate removal by the formation of phosphate salt at the cathode, and comprises reactor 84 and optionally sensor 83, sensor 83 comprises sensor body 83 A and sensor analyser 83B. Reactor 84 comprises reactor housing 86, effluent inlet 88 and effluent outlet 90. Reactor 84 is filled with granules 92 and 94. Granules 92 and 94 can be the same type of granules or different type of granules. Two electrodes, cathode 96 and anode 98, are assembled within reactor 84. Cathode 96 forms reactor housing 86. Anode 98 is configured as hollow tube 100 comprising holes 102 and stop 104. Stop 104 stops the effluent flowing through anode 98. Cathode 96 and anode 98 are operatively coupled with current/voltage supply 106 via circuit/coupling element 108 and circuit/coupling element 110. Waste stream 112 flows from effluent inlet 88 via holes 102 and reactor 84 to effluent outlet 90.

In a preferred embodiment of device 82, oxidation of organic phosphorus and/or organic phosphate can take place within anode 98. It will be understood that other configurations for the conversion by the oxidation of organic phosphorus and/or organic phosphate can also be envisaged in accordance with the present invention, including performing a pre-treatment on the feed solution before entering the reactor.

It will also be understood that individual parts of the different embodiments can be applied to other embodiments and/or can be foreseen in alternative embodiments. For example, circuit/coupling element 50, 76, 108 and/or circuit/coupling element 52, 78, 110 are for example a wires of conductive material such as copper, salt bridge, graphite, direct coupling and the like. It will be understood that the reactor housing can also be formed by the anode. Furthermore, the cathode can also be a hollow tube.

In an illustrated embodiment of the invention, wastewater treatment system 114 (Figure 5) comprises a device for phosphate removal according to the invention which is configured for performing the method according to the invention. Wastewater treatment system 114 comprises removal device 116 which is operatively coupled with effluent inlet 118 to provided effluent to wastewater treatment system 114. Removal device 116 is also operatively connected via connection element 120 with denitrification device 122. Furthermore, wastewater treatment system 114 comprises nitrification device 126 which is provided with effluent from denitrification device 122 via coupling element 124. Nitrification device 126 is aerated by aeration 128 and can be configured with internal sludge recycle 130, wherein internal sludge recycle 130 is operatively coupled with nitrification device 126. Internal sludge recycle 130 provides denitrification device 122 with sludge from nitrification device 126. Nitrification device 126 further comprises waste sludge outlet 132 to remove waste sludge from nitrification device 126 and wastewater treatment system 114. Wastewater treatment system 114 further comprises settler device 134 which is operatively coupled with nitrification device 126 via coupling element 136. Settler device 134 further comprises effluent outlet 138 to remove effluent from wastewater treatment system 114 and sludge recycle 140 to recycle sludge from settle device 134 to removal device 116. Connection elements 120, 124, 136 can be a pipe, tube, shackle, weld, and the like. It is also possible to form a virtual connection element by a drop down or via communicating vessels.

In an experiment of the invention, continuous removal and recovery of phosphate in the device according to the invention by the method according to the invention (Figure 6) was performed. Removal fraction (y-axis) is plotted against time in days (x-axis). The conditions applied are 10 mA (current), 4.2 h (HRT), 0.4 L/d (flow rate, L/d is litre per day), 82.35 g (packed calcite weight, < 0.5 mm (diameter particle size), 10 mM Na₂S0₄ (electrolyte). Figure 6 shows that the removal efficiency drops below 80% after about 35 days.

In an experiment of the invention, removal and recovery of organic phosphorus by electrochemical of phosphate in the device according to the invention by the method according to the invention (Figure 7) was performed. Removal fraction (y-axis) is plotted against electrolysis time in hours (x-axis). The condition applied are 1.0 mM Ca²⁺ (40 mg/L), 0.3 mM organic phosphorus (9.3 mg/L), 50 mM Na₂S0₄, current is 100 mA at 23 °C. The anode was a platinum coated titanium electrode and the cathode was a titanium electrode.

In a further experiment the removal efficiency of organic phosphorus such as

nitrilotris(methylene)triphosphonic acid) (NTMP) was tested at 23 °C in the device according to the invention by the method according to the invention (Table 1).

Table 1 : Removal efficiency of organic phosphorus.

In a further experiment removal efficiency of organic phosphorus such as NTMP for different anode materials (Figure 8A) and different electrolytes (Figure 8B) was tested at 23 °C. Removal fraction (y-axis) is plotted against electrolysis time in hours (x-axis). The condition applied are 1.0 mM Ca²⁺ (40 mg/L), 0.3 mM organic phosphorus (9.3 mg/L), current is 100 mA at 23 °C. The anode was a platinum coated titanium electrode and the cathode was a titanium electrode.

In a further experiment the phosphorus removal efficiency as a function of current, particle size and flow rate was shown (Table 2) wherein standard values were flow rate 2.4 L/d, particles size 1-2 mm, current 5 mA. The total particles mass of about 85 g remained constant during the experiments.

Table 2: Phosphorus removal efficiency as a function of current.

Surprisingly it becomes clear that electrical current is crucial to achieve high removal efficiencies.

In a further experiment, different electrochemical precipitation compounds were tested (Figures 9A - 9D). For Figure 9A - 9C the concentration of respectively phosphorus, calcium and magnesium in mg/L (y-axis) is plotted against time in hour (x-axis). For Figure 9D the pH value (y-axis) is plotted against time in hours (x-axis). Applied current was 100 mA and cell voltage was 2.6 V, the electrode distance was about 3 cm. Electrodes were a Ti cathode, a Pt anode having electrode surfaces of about 36 cm².

This experiment showed that a feed solution with a pH of approximately 4 could be used to remove phosphate/phosphorus from a feed solution. After the treatment the pH is approximately 7 (neutral) and the water can be disposed to the surface water or the like.

The anode and/or cathode electrode used in the various experiments as described above, had a rod shape with a length of approximately 15 cm and a diameter of approximately 3 mm. The reactor used in the various experiments had a volume of approximately 70 mL and was packed with approximately 1 g/mL of granules. The applied current density was between 10 mA/m³ and 500 A/m³.

In a further experiment, a feed solution obtained from cheese manufacturing was treated by the method according to the invention in the device according to the invention. The feed solution comprised a NH₄ ⁺/P0₄ ³ ratio of 0.16. The feed solution further comprises 827 mg/L phosphorus comprising ions, 2173 mg/L calcium ions, and 77 mg/L ammonium. The pH of the feed solution was 4.4.

The device according to the invention comprised a platinum mesh anode and a platinum coated titanium cathode, which were provided with a current of 200 mA. The removal of phosphorus comprising ions is provided in Table 3. It was found that a removal efficiency of 92.3% was achieved after 96 hours.

Table 3: Phosphorus removal efficiency from a feed solution comprising a NH₄ ⁺/P0₄ ³ ratio of 0.16.

In a further experiment, the influence of current on the removal of phosphorus comprising ions in a calcium carbonate packed electrochemical precipitation column was tested. The phosphorus comprising ion removal efficiency in the calcium carbonate packed electrochemical precipitation column as a function of the current is shown in Figure 10 and values are provided in Table 4.

The phosphorus comprising ion removal efficiency was achieved by using calcium carbonate particles with a size in the range of 1 to 2 mm, the calcium carbonate packed electrochemical precipitation column was fed at a rate of 2.4 L/d and hydraulic retention time (HRT) of 0.7 hour, wherein the feed solution comprised 10 mM Na₂S0 , and 0.6 mM P0 -P with an initial pH of 7.5. It was found that a removal efficiency of up to 40.7% was achieved using a current of 10 mA.

Table 4: Phosphorus removal efficiency with different current.

In a further experiment, domestic waste water was used as feed solution and the efficiency of the calcium carbonate packed electrochemical precipitation at 5 mA and 10 mA was analysed. The calcium carbonate particle size was in the range of 1 to 2 mm, and flow rates of 2.4 L/d (HRT = 0.7 hour) and 0.4L/d (HRT = 4.2 hour) were used (Figure 11 and Table 5). The initial ions comprising phosphorus concentration was 3 mg/L. It was found that a removal efficiency of up to 43.2% was achieved using a current of 10mA and flow rate of 0.4 L/d.

Table 5: Phosphorus removal efficiency from domestic waste as feed solution.

In a further experiment, the removal efficiency with and without calcium carbonate particles was analysed (Figure 12 and Table 6). The calcium particle size was in the range of 1 to 2 mm, a current of 5mA and flow rate of 2.4 L/d (HRT = 0.7 hour) was used. The initial phosphorus concentration was 0.6 mM which relates to about 18.6 mg/L. It was found that a higher removal efficiency could be achieved in a device comprising calcium carbonate particles, compared to a device which does not comprise calcium carbonate particles (Figure 12 and Table 6).

Table 6: Phosphorus removal efficiency with and without calcium carbonate particles.

Experiments show the feasibility of the method, device and wastewater treatment system according to the invention.

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

1. Method for phosphorus, such as phosphate, removal from a feed solution, comprising the steps of:

providing a reactor and packing at least part of the reactor with granules; arranging at least one cathode and/or at least one anode in or at the reactor;

providing a waste stream to the reactor;

providing a current over the cathode and anode; and

forming phosphate salt at and/or on the cathode and/or the granules.

2. Method according to claim 1 , wherein the reactor is provided with an undivided reactor volume comprising the at least one cathode and/or at least one anode, and the granules.

3. Method according to claim 1 or 2, further comprising a recovering step, wherein the phosphate salt is recovered from the reactor.

4. Method according to any one of the preceding claims, wherein the granules comprise calcium carbonate and/or calcium sulphate, for forming phosphate salt.

5. Method according to any one of the preceding claims, wherein the waste stream is an ammonium poor feed solution.

6. Method according to any one of the preceding claims, wherein the forming of phosphate salt comprises forming of calcium phosphate and/or magnesium phosphate.

7. Method according to any one of the preceding claims, wherein feed solution comprises a pH of at least 4 before treatment.

8. Method according to any one of the preceding claims, wherein the granules comprising a carrier material configured for the forming of phosphate salt, such as clay, activated charcoal, silicon particles.

9. Method according to claim 8, wherein the carrier material is clay or activated charcoal.

10. Method according to any one of the preceding claims, wherein the granules have a size distribution with an average particle size between 0.5 mm and 3 mm, preferably between 1 and 2 mm.

11. Method according to any one of the preceding claims, further comprising the step of sensing and/or measuring the phosphate concentration.

12. Method according to any one of the preceding claims, further comprises the step of providing an additional electrolyte.

13. Method according to claim 12, wherein the additional electrolyte is chosen from the group of sodium sulphate, sodium perchlorate, sodium chloride, or a mixture thereof.

14. Method according to any one of the preceding claims, further comprising the step of oxidising phosphorus and/or phosphate in the feed solution.

15. Method according to claim 14, wherein the oxidising is performed in-situ.

16. Method according to any one of the preceding claims, wherein the phosphorus and/or phosphate is organic phosphorus and/or organic phosphate.

17. Device for phosphate removal from a feed solution, comprising:

a reactor with a reactor housing;

a bed of granules, at least partly filling the reactor,

18. Device according to claim 17, wherein the reactor comprises an undivided reactor volume comprising the at least one cathode and/or at least one anode, and the granules.

19. Device according to claim 17 or 18, wherein the reactor housing is the cathode or anode.

20. Device according to claim 17, 18 or 19, wherein the cathode or anode comprises a tube which is assembled within the reactor housing.

21. Device according to claim 20, wherein the tube comprises at least one side wall with a number of openings, wherein the tube defines an inner space for receiving and/or holding phosphorus and/or phosphate derivatives from the inlet.

22. Device according to any one of the claims 17 to 21, wherein the anode and/or cathode comprise platinum coated titanium, platinum-iridium coated titanium, ruthenium-iridium coated titanium or a mixture thereof.

23. Device according to any one of the claims 17 to 22, further comprising a sensor for sensing and/or measuring the degree of saturation of the reactor with phosphorus and/or phosphate and/or concentration of phosphorus and/or phosphate.

24. Wastewater treatment system comprising a device for phosphate removal according to any one of the claims 17 to 23 configured for performing the method according to any one of the claims 1 to 16.