Wastewater treatment method and plant

Abstract

The invention relates to a method for treating water comprising nitrogen in the form of ammonium and phosphorus in the form of phosphate, the method comprising a step of co-precipitation of the nitrogen and phosphorus in the form of struvite, the co-precipitation step being carried out by adding magnesium and phosphorus in the form of phosphate, the water to be treated being subjected to alternating stirring phases and rest phases during the co-precipitation step. The invention also relates to a wastewater treatment plant for implementing the method according to the invention, the plant comprising a precipitation tank with a water inlet and an outlet, a phosphorus addition device configured to introduce phosphorus in the form of phosphate into the precipitation tank, a magnesium addition device configured to introduce magnesium into the precipitation tank, and a stirring device.

Description

Wastewater treatment process and installation Technical field

[0001] The present invention relates to the field of wastewater treatment, in particular containing phosphates and ammoniums, and more particularly to the recovery of useful phosphate and ammonium compounds during wastewater treatment. State of the art

[0002] Phosphorus is a common constituent of agricultural fertilizers, manure, and organic waste in wastewater and industrial effluents. It is essential for plant life, but when there is too much of it in water, it can cause plant and algae growth and deplete oxygen in the water at a rate greater than ecosystems can sustain, potentially causing serious ecological effects, including toxic algae blooms, the death of native aquatic species, and biodiversity loss. This is called aquatic eutrophication.

[0003] Similarly, when the phosphorus content is too high in the soil, this causes an ecological imbalance in the development of terrestrial plants, for example by favoring the growth of species with strong growth and strong branches at the expense of other species deprived, in fact, of light. This is called soil eutrophication.

[0004] Nitrogen, particularly in the form of ammonium, also contributes to the eutrophication of soils and aquatic environments. Furthermore, once in the soil, ammonium - through a process of nitrification - will form nitric acid and thus also contribute to soil acidification.

[0005] Phosphorus and nitrogen come mainly from human and animal excrement, and are found in wastewater to be treated by treatment plants, or in water extracted, for example, from manure, slurry, etc. in biomethanization plants. Due to their harmful effect on the environment, their discharge into the natural environment is now prohibited and it is necessary to develop processes to extract these elements from wastewater, especially since the recovered nitrogen and phosphorus can be used as fertilizers.

[0006] Wastewater treated in a wastewater treatment plant produces waste called sewage sludge. The vast majority of this sewage sludge is incinerated. This is an expensive process, and elements such as phosphorus and nitrogen are lost.

[0007] To minimize these volumes of sewage sludge, treatment plant operators are forced to dewater it. This dewatering is most often done using presses or centrifuges. During this stage, ions such as phosphate, ammonium, and others are partially transferred from the sludge back into the press water, which must then be reprocessed to eliminate these elements.

[0008] Press water treatment processes have been developed in which press water containing nitrogen and phosphorus is returned to the inlet of the treatment process to repeat the entire process again. Such processes make it possible to limit nitrogen and phosphorus discharges but have the disadvantage of being expensive, as a relatively small volume of water with a high concentration of nitrogen and phosphorus must again pass through all the stages of the treatment plant.

[0009] In recent years, new processes have been developed to reduce the phosphorus and nitrogen content of wastewater while allowing the recycling of these elements. For example, some processes are based on the discovery that adding magnesium to the water to be treated allows nitrogen and phosphorus to be precipitated in the form of struvite, a slow-release fertilizer that can be used in agriculture. Such processes are known, for example, from WO 2012 / 060768 A1 and EP 4 023 611 A1.

[0010] However, such processes allow the recovery, in the form of struvite, of a small part of the nitrogen present in the water to be treated. In order to recover, i.e. eliminate the rest of the nitrogen present in the water, numerous subsequent treatment steps are necessary. These steps are costly in terms of time and energy and involve, for example, the transformation of nitrogen in the form of ammonium into ammonia followed by the reverse transformation to give back ammonium, which is, for example, precipitated in the form of ammonium chloride. The transition from ammonium to ammonia and back to ammonium represents a waste of time, money and resources that must be limited or even delete in a world where recycling and saving resources is increasingly important. Subject of the invention

[0011] An object of the present invention is to provide an improved wastewater treatment method and wastewater treatment plant for extracting phosphorus and nitrogen in a recoverable form, and which are free from the drawbacks mentioned above. General description of the invention

[0012] The present invention provides, in a first aspect, a method for treating water comprising nitrogen in the form of ammonium NH , and phosphorus in the form of phosphate PO 4 , the method comprising a step of co-precipitation in the form of struvite of nitrogen and phosphorus. According to the invention, the co-precipitation step is carried out by adding magnesium and phosphorus in the form of phosphate, and the water to be treated is subjected to alternating phases of agitation and rest during the co-precipitation step.

[0013] Thus, during the precipitation step, phosphorus is added (i.e. introduced) into the water to be treated, i.e. the amount of phosphorus present in the water is intentionally increased by addition (i.e. introduction). In other words, the water to be treated comprises, during the precipitation step, the phosphorus that it initially contained (i.e. the phosphorus contained in the water before it was subjected to the process according to the invention) as well as additional phosphorus (or supplementary phosphorus) added during the precipitation step. The precipitation step is therefore carried out by adding additional (i.e. additional) phosphorus to the water to be treated.

[0014] Furthermore, it is not excluded from the scope of the present invention that the water to be treated also initially comprises magnesium, so that during the precipitation step, the magnesium concentration is increased by adding (i.e. introducing) additional (or supplementary) magnesium. In other words, the water to be treated comprises, during the precipitation step, the magnesium that it possibly initially comprised (i.e. the magnesium contained in the water before it was subjected to the process according to the invention) as well as additional magnesium (or supplementary magnesium) added during the precipitation step. The precipitation step is therefore carried out by adding additional magnesium (i.e. additional) to the water to be treated.

[0015] The invention is based on the identification that the addition of phosphorus to the water to be treated, during the precipitation step, makes it possible to improve the efficiency of the precipitation step and in particular to increase the rate of precipitation of nitrogen. Indeed, the inventors have surprisingly discovered that adding phosphorus to the water to be treated so as to increase the phosphorus concentration of this water - and although phosphorus constitutes one of the elements to be eliminated from wastewater - makes it possible to promote the precipitation of nitrogen and thus its separation and therefore its elimination.

[0016] In other words, the present invention is based on the counter-intuitive discovery by the inventors that the addition of phosphorus to water to be treated makes it possible to significantly increase the recovery rate of nitrogen and phosphorus present in said water to be treated. The addition of phosphorus thus surprisingly makes it possible to improve the extraction in solid form, i.e. the precipitation, of the nitrogen and phosphorus present in the water. The increase in the precipitation rate of nitrogen, in particular of nitrogen present in the form of ammonium, also advantageously makes it possible to eliminate the need for subsequent treatment steps involving, for example, an acid as in EP 4 023 611 A1. The water treatment process is thereby facilitated and accelerated, and the quantity of waste is greatly reduced (in particular by the absence of generation of acidic water to be reprocessed).

[0017] Another merit of the method according to the invention is that it can be implemented easily and at low cost in a conventional wastewater treatment installation, in particular in a conventional treatment plant, requiring only the implementation of a device configured to introduce (i.e. add) phosphorus to the water to be treated, as well as, where appropriate, the implementation of a device configured to introduce (i.e. add) magnesium to the water to be treated.

[0018] Furthermore, struvite is formed by the following reaction between magnesium, phosphate and ammonium: Mg 2+ + NHt + POl~ -> MgNH4P04

[0019] One of the advantages of producing struvite is that this mineral precipitates in the form of well-defined crystals, easily separable from the water to be treated. In addition, the precipitated struvite constitutes a valuable reaction by-product in the form of slow-release fertilizer, and allows the recycling of phosphorus and nitrogen removed from the water to be treated.

[0020] Yet another merit of the method according to the invention is to have identified that subjecting the water to be treated to alternating phases of agitation and rest during the precipitation step, the duration of an agitation phase preferably being substantially equal to the duration of a rest phase, can advantageously make it possible to locally homogenize the concentrations of nitrogen, phosphorus and magnesium so as to promote the co-precipitation, in the same solid, of these three elements in equimolar form.

[0021] Alternating agitation phases and rest phases during the co-precipitation of phosphorus, magnesium and nitrogen in the form of struvite (i.e. during the formation of struvite) advantageously improves the reduction of the nitrogen content in the form of ammonium in the water to be treated (i.e. increases the rate of nitrogen precipitation) and promotes the formation of struvite MgNH4PO4 compared to other solid compounds (struvite-K i.e. MgKPCU, magnesium nitride, vivianite, etc.).

[0022] According to certain embodiments, phosphorus is added to the water to be treated in the form of hydrogen phosphate, or phosphoric acid. The affinity of phosphorus with nitrogen, in particular in the form of ammonium, is maximum when the phosphorus is in the form of phosphate, which facilitates the co-precipitation, i.e. the simultaneous precipitation in a solid, of these two elements (nitrogen and phosphorus).

[0023] Preferably, the phosphorus is added to the water to be treated in an amount such that the number of moles of phosphorus in the water after addition corresponds to the number of moles of nitrogen. In other words, after addition of phosphorus, the water to be treated comprises as much phosphorus as nitrogen, that is to say that the phosphorus and nitrogen are present in stoichiometric quantities, or that the nitrogen and phosphorus are equimolar in the water to be treated. Advantageously, this makes it possible to increase the nitrogen precipitation rate.

[0024] According to other equally preferred embodiments, the phosphorus is added in an amount such that the phosphorus is present in a substoichiometric amount relative to the nitrogen, i.e. the water to be treated comprises less phosphorus than nitrogen. In particular, the number of moles of phosphorus - after addition - may be less than the number of moles of nitrogen by 1%, 5%, 10% or 15%.

[0025] Thus, according to certain embodiments, the water treatment method further comprises a step of dosing (i.e. quantifying) the nitrogen and phosphorus present in the water to be treated. The dosing of each of these elements can be done according to methods well known to those skilled in the art. According to certain embodiments, the dosing (i.e. quantifying) can be done within the installation in which the present water treatment method is carried out. Alternatively, the dosing can be carried out upstream and the nitrogen and phosphorus content of the water to be treated are known when the water enters the installation in which the present water treatment method is carried out. Advantageously, the quantity of magnesium present in the water to be treated (i.e. the magnesium content of the water) can also be determined.

[0026] According to the same or other embodiments, magnesium is added to the water to be treated in the form of a magnesium salt, for example in the form of magnesium chloride (MgCl2), basic magnesium chloride (MgChNaOH) or magnesium hydroxide (Mg(OH)2). Preferably, the magnesium is added in the form of magnesium hydroxide. Thus, the pH suitable for precipitation can be obtained solely by introducing magnesium, without requiring the addition, to the water to be treated, of another compound in order to modify the pH, which facilitates the implementation of the method and reduces costs, in particular the costs related to the adjuvants necessary for precipitation.

[0027] According to preliminary studies, it is particularly advantageous to introduce phosphorus in the form of phosphoric acid and magnesium in the form of magnesium hydroxide. Indeed, magnesium hydroxide dissolves poorly at high pH but well at low pH. The addition of phosphoric acid lowers the pH of the water to be treated and promotes the dissolution of magnesium hydroxide, which raises the pH.

[0028] Preferably, magnesium is added to the water to be treated in a super-stoichiometric quantity relative to the nitrogen contained in the water, in order to promote the precipitation of the latter. The number of moles of magnesium may be greater than the number of moles of nitrogen by 1%, 5%, 10%, 15%, 20% or 25%. Indeed, during the precipitation of nitrogen and phosphorus, magnesium competes with calcium potentially present in the water to be treated, and it must advantageously be introduced in a super-stoichiometric quantity in order to promote the precipitation of magnesium at detriment of calcium precipitation. This particularly favors the precipitation of struvite compared to the precipitation of other solid compounds such as calcium phosphate Cas(PO4)2 or hydroxyapatite Ca5(PO4)3(OH).

[0029] According to certain embodiments, magnesium and phosphorus in phosphate form are added to the water to be treated so as to obtain an equimolarity of phosphorus (in particular in phosphate form) and nitrogen (in particular in ammonium form) and an overmolarity of magnesium in the water to be treated, that is to say that the number of moles of nitrogen is substantially identical to the number of moles of phosphorus and less than the number of moles of magnesium in the water to be treated during the precipitation step.

[0030] According to other embodiments, the phosphorus is introduced in a substoichiometric quantity relative to the nitrogen present in the water to be treated, so as to minimize the quantity of residual phosphorus after precipitation of the solid. The number of moles of phosphorus - after addition - in the water to be treated may be less than the number of moles of nitrogen by 1%, 5%, 10% or 15%.

[0031] According to still other embodiments, the phosphorus in the form of phosphate is introduced in a super-stoichiometric quantity relative to the nitrogen present in the water to be treated, so as to promote the precipitation of nitrogen (in particular in the form of ammonium) and therefore the reduction of nitrogen in the water to be treated. The number of moles of phosphorus - after addition - in the water to be treated may be greater than the number of moles of nitrogen by 1%, 5%, 10% or 15%.

[0032] According to certain preferred embodiments, phosphorus, in particular in the form of phosphate, is added to the water to be treated before the magnesium. In other words, the method comprises, in this order during the precipitation step, a sub-step of adding phosphorus and then a sub-step of adding magnesium, that is to say that the magnesium is added only once the water to be treated contains the desired quantity of phosphorus. According to particularly preferred embodiments, the phosphorus is added to the water to be treated in an amount such that the water comprises the quantity of phosphorus necessary for the precipitation of all the nitrogen, in particular in the form of struvite, before the addition of the magnesium.

[0033] However, it is not excluded from the scope of the present invention that the phosphorus and magnesium are added simultaneously to the water to be treated, or that the magnesium is added before the phosphorus.

[0034] The pH of the water to be treated during the precipitation stage depends on the composition of the water and in particular its origin (treatment plant, biomethanization plant, etc.) and any pre-treatment stages. Advantageously, the pH of the water during the precipitation stage is between 6.5 and 10.0, preferably between 7.5 and 9.0, which advantageously makes it possible to have basic water promoting the precipitation of struvite and preventing its dissolution, which mainly takes place in an acidic environment.

[0035] According to certain embodiments, the pH of the water to be treated is not constant during the precipitation step but varies. For example, and without this being limiting, the pH of the water to be treated may vary globally during the addition of magnesium and / or phosphorus, or the pH may vary locally during agitation of the water during the precipitation step. Thus, the ranges of pH values ​​mentioned above refer to maximum and minimum values ​​of the pH of the water during the precipitation step, without assuming that this pH is constant either in time (i.e. duration of the precipitation step) or in space (i.e. the volume of water to be treated).

[0036] According to the same or other embodiments, the water to be treated is subjected to gas extraction (also called stripping or stripping), preferably gas extraction by injection of compressed gas, in particular air (air stripping or air stripping), during the precipitation step. Advantageously, such gas extraction makes it possible to extract CO2 from the water to be treated, which makes it possible to increase its pH and therefore promote the precipitation of nitrogen and phosphorus in solid form, for example in the form of struvite.

[0037] The precipitate formed during the precipitation step is preferably separated from the treated water during a step of separating the precipitated solid, carried out after the precipitation step. According to certain particularly preferred embodiments, the step of separating the precipitated solid is carried out using a lamella separator, or lamella decanter. The precipitated solid, preferably the precipitated struvite, can thus be recovered and recovered, for example sold, separately from the treated water.

[0038] Advantageously, the step of separating the precipitated solid may be followed by a filtration step, in particular reverse osmosis filtration. Reverse osmosis may be described as a liquid-entrained membrane process, with reverse osmosis membranes being capable of allowing water to pass through while rejecting solutes, such as ions, salts or low molecular weight organic matter. The reverse osmosis step makes it possible to retain residual nitrogen and phosphorus in the water resulting from the precipitation step, i.e. the nitrogen and phosphorus not precipitated during the precipitation step. The reverse osmosis step thus advantageously makes it possible to increase the rate of reduction of the nitrogen and phosphorus content in the water to be treated.

[0039] The step of separating the precipitated solid also advantageously serves as protection for the membrane used during the reverse osmosis step, the solid separated during the separation step being abrasive for said membrane.

[0040] This water treatment process applies to any type of water containing nitrogen and phosphorus, such as domestic, industrial, agricultural or rainwater wastewater, and is particularly advantageously applied to press water from treatment plants or methanization installations.

[0041] In addition to nitrogen and phosphorus, the water to be treated may also contain other pollutants, for example solid pollutants such as traces of excrement, grease, or metals. Advantageously, the water treatment process includes a sedimentation step, before the precipitation step, in order to separate the solid pollutants by sedimentation.

[0042] According to this embodiment, the method may also comprise a membrane filtration step between the sedimentation step and the precipitation step, in order to remove (i.e. extract) from the water to be treated solid compounds that are too small to precipitate, such as for example metals, greases, or flocculation agents added during the sedimentation step to promote the sedimentation of solid pollutants of large size, or added during a pretreatment step, for example during a wastewater press step.

[0043] In a second aspect, the present invention also relates to a wastewater treatment installation for implementing a method according to the invention, the installation comprising: - a precipitation tank with a water inlet and an outlet, - a phosphorus addition device configured to introduce phosphorus in phosphate form into the precipitation tank, and - a magnesium addition device configured to introduce magnesium into the precipitation tank.

[0044] The advantages and beneficial effects of the method according to the invention apply mutatis mutandis to the installation according to the invention. The present installation allows the addition (i.e. the addition) of phosphorus to wastewater to be treated during a precipitation step. The addition of phosphorus, in addition to the addition of magnesium, advantageously makes it possible to increase the precipitation rate of certain elements present in the water, in particular phosphorus and nitrogen, and makes it possible to increase the rate of reduction of the nitrogen and phosphorus content of said wastewater to be treated compared to an installation allowing only the addition (i.e. the addition of magnesium).

[0045] The devices for adding phosphorus and magnesium can be made in any suitable form known to those skilled in the art. According to a preferred embodiment, the magnesium addition device is a worm screw, preferably a worm screw with a storage tank, capable of operating at slow speed in order to avoid any foam formation during the introduction of the magnesium. According to the same or other embodiments, the phosphorus addition device is a metering pump, preferably a diaphragm pump, capable of operating at low speed in order to avoid any foam formation during the introduction of the phosphorus. Indeed, the foam traps, i.e. captures / comprises, phosphorus and magnesium during their introduction, which are not available to precipitate with nitrogen.

[0046] In this text, "low speed" refers to an introduction rate of less than 500 mL / min, preferably less than 250 mL / min, more preferably between 25 and 200 mL / min, particularly preferably between 50 and 150 mL / min.

[0047] In the present text, "slow speed" refers to an introduction speed of less than 500 g / min, preferably less than 350 g / min, more preferably between 50 and 300 g / min, particularly preferably between 100 and 300 g / min.

[0048] According to some embodiments, the precipitation tank may further comprise at least one of: - an agitation device for agitating the water present in the precipitation tank; - a pH measuring probe; - a water level measuring probe in the tank; - an air compressor configured to inject a flow of compressed air into the tank.

[0049] According to particularly preferred embodiments, the precipitation tank comprises a stirring device configured to stir the water present in the precipitation tank according to an alternation of stirring and rest phases. In other words, the precipitation tank may comprise a stirring device configured to subject the water present in the precipitation tank to alternating stirring and rest phases, the duration of a stirring phase preferably being substantially equal to the duration of a rest phase.

[0050] According to some embodiments, the wastewater treatment facility may further comprise at least one of: - a unit configured to measure (i.e. quantify) nitrogen, in particular in the form of ammonium, present in water to be treated using said installation; - a unit configured to measure (i.e. quantify) phosphorus, in particular in the form of phosphate, present in water to be treated using said installation.

[0051] According to the same or other embodiments, the wastewater treatment facility may further comprise at least one of: - a sedimentation basin arranged upstream of the precipitation tank, the sedimentation basin comprising an outlet in fluid communication with the water inlet of the precipitation tank; - a filtration device, preferably a membrane filtration device, comprising a filtration inlet in fluid communication with the outlet of the sedimentation tank and a filtered water outlet in fluid communication with the water inlet of the precipitation tank; - a device for separating the precipitated solid, preferably a lamella separator, comprising an inlet in fluid communication with the outlet of the precipitation tank; - a reverse osmosis installation arranged in fluid communication with an outlet of the precipitated solid separation device. Brief description of the drawings

[0052] Other features and characteristics of the invention will emerge from the detailed description of some advantageous embodiments presented below, by way of illustration, with reference to the appended drawings. These show: [Fig. 1] the different stages of a water treatment process according to a preferred embodiment; [Fig. 2] a schematic representation of a water treatment plant according to a preferred embodiment for implementing the method of Fig. 1; [Fig. 3] curves showing the evolution of the pH during the co-precipitation step for a process according to the invention (A) and a comparative process (B). Description of favorite executions

[0053] The operating principle of the water treatment method according to the invention will be explained with reference to Fig. 1 and 2. As mentioned previously, the present invention surprisingly proposes a method in which phosphorus is added to water to be treated, the phosphorus and nitrogen content of which is to be reduced.

[0054] With reference to Fig. 1, a preferred but non-limiting embodiment of a method 100 according to the invention is described. With reference to Fig. 2, a preferred but non-limiting embodiment of a water treatment installation 10 allowing the carrying out of a method 100 according to the invention as presented with reference to Fig. 1.

[0055] A sedimentation tank 14 is filled with water to be treated 12 comprising nitrogen and phosphorus, for example press water from a wastewater treatment plant or a biomethanization facility (for example, and without limitation, slurry). According to certain embodiments, the sedimentation tank 14 is made of stainless steel and is made up of different compartments separated by baffles. In order to promote the sedimentation 102 of solid pollutants (for example, and without limitation, traces of excrement, grease, or metals), a flocculation agent (not shown) may be added to the water in the sedimentation tank.

[0056] At the end of the sedimentation step 102, the supernatant water 18 is supplied to a first filtration device 20 while the sedimented solid pollutants 16 are removed. For example, the supernatant liquid portion of the sedimentation tank 14 may be pumped to the filtration device 20.

[0057] The filtration device 20 is for example a membrane filtration device and makes it possible to filter the supernatant water during the filtration step 104. The pollutants too small to sediment during the step 102, such as for example flocculation agents, are then retained (i.e. separated) from the filtered water 24. According to certain embodiments, the membrane may be a ceramic membrane whose retention threshold (or Molecular Weight Cut-Off MWCO) is located between filtration and ultrafiltration. An example of a membrane is an ultrafiltration membrane, e.g. of the UF50A type, but those skilled in the art will know how to adapt the membrane to be used to the nature of the pollutants to be eliminated. The part of the supernatant water 18 retained by the membrane of the filtration device 20 is called retentate 22 and can advantageously be returned to the start of the process and added to the water to be treated 12.

[0058] According to certain embodiments, a sample of filtered water 24 is taken and brought to a nitrogen and phosphorus dosing installation 26 (or quantification installation) configured to determine the nitrogen content, in particular in the form of ammonium, and phosphorus content, in particular in the form of phosphate, of the filtered water 24. The methods for dosing nitrogen and phosphorus are well known to those skilled in the art and will not be described in further detail.

[0059] The filtered water 24, also called permeate, is introduced, for example using a pump, into the precipitation tank 28, in which the simultaneous precipitation step 106 of nitrogen and phosphorus takes place.

[0060] According to certain embodiments, the precipitation tank, or reactor, 28 may comprise an agitator controlled by a frequency converter, hydraulic baffles, a gas extraction unit 34 (or stripping unit or stripping unit) powered by compressed air, a phosphorus addition device 30, a magnesium addition device 32, a pH measuring probe and a level measuring probe.

[0061] The level measuring probe measures the filling level of the precipitation tank 28 and can be configured to communicate with a filling device, for example a pump, and stop filling the precipitation tank (or reactor) once a certain level is reached.

[0062] The phosphorus addition device 30 may be of any type known to those skilled in the art. Preferably, it is a diaphragm metering pump configured to operate at low speed to limit or even prevent foam formation. The metering pump 30 introduces phosphoric acid into the precipitation tank 28 during the phosphorus addition step 110.

[0063] The magnesium addition device 32 may be of any type known to those skilled in the art. Preferably, it is a worm screw provided with a storage tank configured to operate at low speed to limit or even prevent the formation of foam. The worm screw 32 introduces magnesium hydroxide into the precipitation tank 28 during the magnesium addition step 112.

[0064] The quantities of phosphorus (in the form of phosphoric acid) and magnesium (in the form of magnesium hydroxide) to be added are determined using the results of the assays (i.e. quantifications) carried out in step 108 and are defined as follows (n designating a number of moles): n P (added = n N (present in water) — n P present in the water)

[0065] According to a preferred embodiment, the amounts of phosphorus to be added in the form of phosphoric acid and of magnesium to be added in the form of magnesium hydroxide are defined as follows:

[0066] The quantities added and to be added are advantageously controlled over time, so that the elements are found in the water to be treated in the quantity necessary and sufficient for the precipitation of the solid including nitrogen and phosphorus.

[0067] Advantageously, the agitator is started as soon as the precipitation tank 28 is filled, so as to agitate the water contained in the precipitation tank during the steps 110 of adding the phosphorus and 112 of adding the magnesium.

[0068] According to a preferred but non-limiting embodiment, the agitator operates until the pH of the water added with phosphorus and magnesium in the precipitation tank stabilizes or begins to rise. The pH is determined using the pH measuring probe.

[0069] The agitator is then stopped for a predefined time, preferably between 170 and 190 seconds, for example for 180 seconds, then successive agitation-rest cycles are carried out in step 114. The number of cycles depends on the nature of the water to be treated and is advantageously between three and ten, for example it can be equal to six. During the cycles, the duration of the agitation phase is preferably equal to the duration of the rest phase. Preferably the duration of the agitation phase is between 170 and 190 seconds, for example it is 180 seconds and the duration of the rest phase is also 180 seconds.

[0070] In a second step, step 114 consists of several successive cycles each consisting of a first phase during which the agitator and the gas extraction unit 34 powered by compressed air are started, and a second phase during which the agitator and the unit 34 are turned off. The number of cycles is advantageously between three and ten, for example it can be equal to six. Preferably, the two phases have the same duration, in particular each phase has a duration between 170 and 190 seconds, preferably each phase lasts 180 seconds.

[0071] Step 114 is followed by a rest phase of a predetermined duration allowing the crystals formed during the simultaneous precipitation step, in particular the struvite crystals formed, to settle at the bottom of the precipitation tank 28. The duration of this rest phase depends on the quantity of crystals formed, and may for example be equal to 170, 180 or 190 seconds.

[0072] In order to separate the crystals formed and deposited at the bottom of the tank from the water contained therein, the contents of the precipitation tank (or reactor) 28 are brought to a device for separating the precipitated solid 36. The device 36 may take any form known to those skilled in the art. According to a preferred embodiment, it is a lamella separator 36 configured to separate the precipitated solid 38, in particular the precipitated struvite 38, from the water 40, during a solid / liquid separation step 116.

[0073] The precipitated solid 38 is collected and stored in a suitable container for subsequent recovery. The water 40 separated from the solid is collected in a tank of a second filtration installation 42. The second filtration installation 42 is, according to a preferred embodiment, a reverse osmosis filtration installation comprising, in addition to the tank, a pump and a reverse osmosis membrane, the pump being configured to suck the water 40 from the tank and pass it through the reverse osmosis membrane. The water passing through the reverse osmosis membrane is filtered during step 118, in order to remove residual ammonium and phosphate ions, but also other ions that may be present (for example calcium, magnesium, iron, etc.). The water passing through the membrane is treated water 44 whose nitrogen and phosphate content is substantially zero, and in any case much lower than the threshold values ​​authorized for discharged water.The portion of the water retained in the tank by the reverse osmosis membrane is called concentrate 46 (or osmosis retentate). According to certain embodiments, it can be used for cleaning and / or rinsing upstream installations, such as for example the precipitation tank 28, and / or where appropriate the sedimentation basin 14, the filtration device 20, or the device 36 for separating the precipitated solid. However, according to a preferred embodiment, the concentrate 46 is advantageously returned to the start of the process and added to the water to be treated 12.

[0074] Water containing phosphorus in the form of phosphate and nitrogen in the form of ammonium, treated using the process described above, or using the installation described above, has a reduction in its ammonium content of more than 92% and a reduction in its phosphate content of more than 98%. Experimental results

[0075] I) Materials and methods

[0076] Press water from a treatment plant was collected and analyzed to determine its content (i.e. concentration) of nitrogen in the form of ammonium, nitrite, nitrate, phosphorus in the form of ortho-phosphate, sodium, potassium and calcium.

[0077] Wastewater treatment tests were carried out with a process A according to the invention and a comparative process B different from the process according to the invention only in that the agitation is continuous during the co-precipitation step (i.e. absence of rest phases).

[0078] The methods implemented are as follows. In the absence of further details, the steps are identical for methods A - according to the invention - and B - comparative.

[0079] A volume of 200 mL of press water is taken, to which 0.37 mL of phosphoric acid and 669 mg of brucite (magnesium hydroxide Mg(OH)2) are added.

[0080] A stirrer is started and operated (i.e. stirred) until the pH of the water containing phosphoric acid and brucite begins to rise (step a). The stirrer is then stopped for 180 seconds (step b).

[0081] Then, the co-precipitation step takes place during which: - for method A according to the invention, the agitator is started intermittently in order to alternate between stirring phases (lasting 180 seconds) and rest phases (also lasting 180 seconds) (step c). - for comparative process B, the agitator is started continuously (step c').

[0082] The co-precipitation step has a total duration of 1 h 06 for both process A and process B, and is followed by a rest period to allow the crystals obtained to settle at the bottom of the precipitation tank, and are separated i.e. collected using a centrifuge. A sample of supernatant liquid is taken and analyzed in the same way as the initial press waters in order to determine the residual nitrogen content in the form of ammonium, nitrite, nitrate, phosphorus in the form of ortho-phosphate, sodium, potassium and calcium after the treatment process.

[0083] Finally, the solid obtained is analyzed in order to determine its composition, in particular the nature of the crystals obtained and their relative preponderance.

[0084] II) Characterization methods

[0085] The nitrogen contents in the form of ammonium, nitrite, nitrate, phosphorus in the form of orthophosphate, sodium, potassium and calcium are respectively determined using the following standards: - Nitrogen in the form of ammonium NH4: ISO 11732 - Nitrogen in the form of nitrite NO2: ISO 13395 - Nitrogen in the form of nitrate NO3: ISO 13395 - Phosphorus in the form of ortho-phosphates PO4: ISO 15681-2 - Sodium: ISO 9964-3 - Potassium: ISO 9964-3 - Calcium: SOP 114

[0086] The pH is determined using a pH measuring probe, for example a Digital pH-meter, model 903 manufactured by Nahita.

[0087] The obtained solid is characterized by X-ray diffraction and energy dispersive X-ray spectroscopy (EDS) after dispersion on a carbon tape substrate.

[0088] The device used to record the X-ray diffractogram is a Malvern-PANalytical X'Pert Pro MPD with an X-ray anode corresponding to the Ka line of copper and a PIXcel 3D sensor (2 ème generation).

[0089] EDS characterizations are performed using an FEI Quanta 200 F scanning electron microscope with an AMETEK EDAX 32 energy dispersive X-ray spectrometer.

[0090] In particular, a quantitative analysis of the different elements is carried out from the X-ray diffractograms using an analysis according to the Rietveld method which consists of simulating a diffractogram from a crystallographic model of the sample, then adjusting the parameters of this model so that the simulated diffractogram is as close as possible to the measured diffractogram.

[0091] III) Results

[0092] Table 1 below shows the ammonium, nitrite, nitrate, ortho-phosphate, sodium, potassium and calcium content for press water before and after treatment using process A according to the invention or comparative process B. [Table 1]

[0093] As can be seen in Table 1, the amount of ammonium present in the press water is reduced by approximately 40% using comparative method B while it is reduced by approximately 55% using method A according to the invention. The alternation between agitation phases and rest phases makes it possible to significantly increase the reduction of the ammonium content in the water.

[0094] Table 2 below shows the composition of the solid obtained during the precipitation step. [Table 2] In Table 2, the contents are expressed in % by weight relative to the total weight of the solid.

[0095] The results in Table 2 highlight that: whether for the solid obtained using the process according to the invention or according to the comparative process, approximately one third (by weight, relative to the total weight) of the solid is brucite (Mg(OH)2) - corresponding to brucite introduced before the co-precipitation step and not dissolved; for water treated using process A according to the invention, the remainder of the solid obtained is struvite, i.e. approximately 67% by weight, relative to the total weight of the solid obtained is struvite; for water treated using comparative process B, i.e. without rest phases, only 29% by weight, relative to the total weight of the solid obtained is struvite. struvite, the remainder being struvite-K (in which ammonium NH4 is replaced by potassium K - 20% by weight, relative to the total weight of the solid obtained) and magnesium nitride (21% by weight, relative to the total weight of the solid obtained).

[0096] The alternation between stirring phases and rest phases during the co-precipitation step significantly promotes the precipitation of struvite compared to other crystals (struvite-K, magnesium nitride).

[0097] Without wishing to be bound by any theory, the inventors are of the opinion that the difference in composition between the solids obtained using process A according to the invention and comparative process B may be a consequence of the difference in pH evolution during the precipitation step. As visible in Fig. 3, the initial and final pH values ​​of the water treated according to the two processes are substantially identical. However, while the pH increases continuously after starting agitation for process B (Fig. 3B step c), for process A according to the invention, the pH undergoes a succession of increases and decreases during the agitation and rest phases (Fig. 3A step c). Intermittent decreases in pH may favor the formation of struvite in the face of other compounds.

Claims

Claims 1. Method for treating water (100) comprising nitrogen in the form of ammonium and phosphorus in the form of phosphate, the method comprising a step of co-precipitation (106) in the form of struvite of nitrogen and phosphorus, characterized in that the co-precipitation step (106) is carried out by adding magnesium (112) and phosphorus in the form of phosphate (110), and in that the water to be treated is subjected to alternating phases of agitation and rest (114) during the co-precipitation step.

2. Method (100) according to claim 1, in which the phosphorus is added to the water to be treated in the form of hydrogen phosphate.

3. Method (100) according to claim 1 or 2, wherein the phosphorus is added (110) to the water to be treated in an amount such that the number of moles of phosphorus in the water after addition corresponds to the number of moles of nitrogen in the water to be treated or wherein the phosphorus is added (110) to the water to be treated in an amount such that the number of moles of phosphorus in the water after addition is less than the number of moles of nitrogen in the water to be treated.

4. Method (100) according to any one of the preceding claims, in which the magnesium is added (112) to the water to be treated in the form of magnesium hydroxide and / or in which the magnesium is added (112) to the water to be treated in a superstoichiometric quantity relative to the nitrogen contained in the water.

5. Method (100) according to any one of the preceding claims, wherein the pH of the water during the precipitation step is between 6.5 and 10.0, preferably between 7.5 and 9.

0.

6. Method (100) according to any one of the preceding claims, in which the duration of an agitation phase is substantially equal to the duration of a rest phase.

7. Method (100) according to any one of the preceding claims, wherein the water to be treated is subjected to gas extraction, preferably gas extraction by injection of compressed gas, during the coprecipitation step (106) and / or 8. Method (100) according to any one of the preceding claims, wherein the method further comprises a sedimentation step (102) before the precipitation step (106), preferably wherein the method further comprises a filtration step (104), in particular membrane filtration, after the sedimentation step (102) and before the precipitation step (106).

9. Method (100) according to any one of the preceding claims, wherein the method further comprises a step of separating (116) the precipitated solid, in particular a step of separating the precipitated solid using a lamella separator, after the precipitation step (106), preferably wherein the method further comprises a filtration step (118), in particular a reverse osmosis filtration step, after the step of separating (116) the precipitated solid.

10. Method (100) according to any one of the preceding claims, wherein the water is waste water, in particular press water.

11. Wastewater treatment plant (10) for implementing a method (100) according to any one of the preceding claims, the plant comprising: a precipitation tank (28) with a water inlet and an outlet, a phosphorus addition device (30) configured to introduce phosphorus in the form of phosphate into the precipitation tank (28), a magnesium addition device (32) configured to introduce magnesium into the precipitation tank (28), and a stirring device.

12. Installation (10) according to claim 11, in which the phosphorus addition device (30) is a metering pump, preferably a membrane pump, and / or wherein the magnesium adding device (32) is a screw conveyor, preferably a screw conveyor with a storage tank.

13. Installation (10) according to any one of claims 11 to 12, in which the precipitation tank (28) comprises a pH measuring probe and / or a level measuring probe and / or an air compressor.

14. Installation (10) according to any one of claims 11 to 13, further comprising a sedimentation basin (14) arranged upstream of the precipitation tank (28), the sedimentation basin comprising an outlet in fluid communication with the water inlet of the precipitation tank, preferably further comprising a filtration device (20), more preferably a membrane filtration device, comprising a filtration inlet in fluid communication with the outlet of the sedimentation basin (14) and a filtered water outlet in fluid communication with the water inlet of the precipitation tank (28).

15. Installation (10) according to any one of claims 11 to 14, further comprising a device (36) for separating the precipitated solid (38), preferably a lamella separator, comprising an inlet in fluid communication with the outlet of the precipitation tank (28), preferably further comprising a second filtration installation (42), more preferably a reverse osmosis installation, arranged in fluid communication with an outlet of the device (36) for separating the precipitated solid (38).

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