Recovery of nitrogen from waste water

Abstract

A method for recovering nitrogen from wastewater, the method comprising: providing (S10) wastewater having dissolved therein ammonium ions. The ratio between the alkalinity, expressed in equivalents per litre, and the molar concentration of dissolved ammonium ions in the wastewater is at least 0.5:1. Adding (S20) struvite to the wastewater, causing struvite to precipitate. During at least a part of the struvite precipitation, degassing (S30) the wastewater. Then separating (S50) the precipitated struvite from the wastewater. Also disclosed is a struvite precipitation reactor arrangement and a circulation arrangement for recovering nitrogen from wastewater.

Description

Technical Field

[0001] This technology generally relates to the recovery of commercial substances from waste materials, and specifically to methods and arrangements for recovering nitrogen from wastewater. Background Technology

[0002] Biogas production generates a significant amount of wastewater. One reason for this large volume of wastewater is the presence of ammonium ions (NH4+) exceeding 3000 ppm. + High nitrogen levels in biogas can be highly toxic to the microorganisms that produce it. Therefore, water is added to biogas plants to dilute the nitrogen content to below toxic levels, generating large amounts of wastewater. This wastewater typically contains high concentrations of ammonium ions, making it unsuitable for release into the environment.

[0003] Meanwhile, ammonia is the required substance. Currently, most ammonia production is based on the Haber-Bosch process, which requires a large amount of natural gas. Nitrogenous effluents should be used as a nitrogen source, not released.

[0004] In the prior art, struvite precipitation has been used to primarily remove phosphorus from waste, but also nitrogen. This is achieved by adding a magnesium source and adjusting the pH to an alkaline level. The pH increase is typically performed by adding sodium hydroxide. Using cheaper calcium hydroxide results in undesirable calcium phosphate precipitation instead of struvite precipitation, and is therefore not preferred. Struvite can then be processed into other commercially more valuable materials, for example, according to published international patent application WO2020 / 256622 A1. In these processes, the magnesium source can be recovered and recycled back into the precipitation process. The magnesium source can be, for example, magnesium phosphate.

[0005] In the published European patent application EP 2 431 336 A1, magnesium phosphate is combined with sodium hydroxide in a single reactor with pH control applied.

[0006] Sodium hydroxide is a relatively expensive substance, and up to 60% of the total cost of a nitrogen recovery process can be the cost of sodium hydroxide. Furthermore, sodium hydroxide is not only consumed by struvite precipitation but is also consumed by other reactions in typical wastewater compositions. The high cost associated with existing struvite precipitation methods makes them less attractive for treating large volumes of wastewater.

[0007] Furthermore, due to the accumulation of sodium ions, recycling wastewater treated with sodium hydroxide back to the biogas reactor to dilute the nitrogen content is not feasible to any extent, and sodium ions at high levels also have a negative effect on the microorganisms that produce biogas.

[0008] Therefore, there is a need to improve the recovery of nitrogen from liquid solutions. Summary of the Invention

[0009] Therefore, the overall objective of this technology is to find an improved method for recovering nitrogen from aqueous solutions with high ammonium ion content.

[0010] The above-mentioned objective is achieved by the method and apparatus according to the independent claim. Preferred embodiments are defined in the dependent claims.

[0011] In general, in the first aspect, a method for recovering nitrogen from wastewater includes providing wastewater containing dissolved ammonium ions. The ratio between the alkalinity of the wastewater and the molar concentration of dissolved ammonium ions is at least 0.5:1, and the alkalinity is expressed in equivalents per liter. Magnesia phosphate rock is added to the wastewater, causing struvite to precipitate. During at least a portion of the struvite precipitation, the wastewater is degassed. The precipitated struvite is then separated from the wastewater.

[0012] In a second aspect, a struvite precipitation reactor arrangement for recovering nitrogen from wastewater includes a reactor vessel having an inlet for wastewater and an inlet for magnesium phosphate rock. The struvite precipitation reactor arrangement further includes a degassing device configured to degas the reactor vessel. The struvite precipitation reactor arrangement further includes a struvite separation arrangement configured to separate the struvite precipitated in the reactor vessel from the wastewater.

[0013] In a third aspect, a circulating arrangement for recovering nitrogen from wastewater includes a struvite precipitation reactor arrangement and a struvite decomposition reactor as described in the second aspect. The struvite decomposition reactor is connected to or integrated into the struvite precipitation reactor arrangement and has an input for acid. The struvite decomposition reactor further has an output for precipitated magnesium phosphate and an output for a liquid containing dissolved ammonium salts and acid anions.

[0014] One advantage of the proposed technique is that it enables efficient nitrogen recovery from wastewater using less sodium hydroxide. Other advantages will be understood when reading the detailed embodiments. Attached Figure Description

[0015] The invention and its further objects and advantages can be best understood by referring to the following description taken in conjunction with the accompanying drawings, in which:

[0016] Figure 1 This is a flowchart of the steps of an embodiment of a method for recovering nitrogen from wastewater;

[0017] Figure 2 This is a graph showing the ammonium nitrogen content during the struvite precipitation process;

[0018] Figure 3 It is a graph showing the changes in ammonium nitrogen content and pH value during the sedimentation process of struvite;

[0019] Figure 4 This is a flowchart of the steps of another embodiment of a method for recovering nitrogen from wastewater;

[0020] Figure 5 This is a graph showing the ammonium nitrogen content during the two-stage struvite precipitation process;

[0021] Figure 6A This is a schematic diagram of the various parts of an embodiment of a struvite sedimentation reactor arrangement;

[0022] Figure 6B This is a schematic diagram of the various parts of another embodiment of the struvite sedimentation reactor arrangement;

[0023] Figure 7 This is a flowchart of the steps of an embodiment of a recycling method for recovering nitrogen from wastewater;

[0024] Figure 8 This is a schematic diagram of various parts of an embodiment of a circulating arrangement for recovering nitrogen from wastewater; and

[0025] Figure 9 This is a flowchart of the steps of another embodiment of a method for recovering nitrogen from wastewater. Detailed Implementation

[0026] In all the accompanying drawings, the same reference numerals are used for similar or corresponding elements.

[0027] The assumed reaction when guanostone is precipitated from magnesium phosphate at an alkaline pH is:

[0028]

[0029] However, using sodium hydroxide to increase the pH value may also cause other side reactions, such as:

[0030]

[0031] Magnesium hydroxide may precipitate instead of struvite (which removes ammonium ions from the solution). Therefore, sodium hydroxide is consumed, but it does not contribute to nitrogen removal. To avoid this "misuse" of sodium hydroxide, alternative reaction mechanisms can be used.

[0032] Wastewater from sources such as biogas production or wastewater treatment plants typically contains significant alkalinity. Alkalinity is the concentration of neutralized hydrogen ions in water. The most well-known alkaline components are bicarbonates, carbonates, and hydroxides. The quantitative definition of alkalinity is A = [HCO3-] - ] + 2[CO32- ] + [OH - ] - [H + Alkalinity is expressed in milliequines per liter (mEq / L). The carbonate ion concentration is counted in double because this ion has the ability to absorb two hydrogen ions. The definition of alkalinity can be simplified to the buffering capacity of a solution; when applied to wastewater, this alkalinity is typically expressed in mg HCO3. - / L indicates the ratio. In the presence of alkalinity, and especially bicarbonate ions (i.e., bicarbonate ions), the following reaction can occur:

[0033]

[0034] Unfortunately, tests have shown that, according to this reaction protocol, simply adding magnesium phosphate to alkaline wastewater only causes a relatively slow and incomplete reaction.

[0035] However, it was unexpectedly found that if degassing was performed during the process, the rate and completeness increased significantly. The conclusion is that degassing helps reduce dissolved carbon dioxide gas in the water, which leads to a further driving effect on struvite precipitation. For example, it was found that if the reaction was performed in a sealed container, the reaction completely stopped after a period of time.

[0036] Figure 1 A flowchart illustrating the steps of an embodiment of a method for recovering nitrogen from wastewater is provided. In step S10, wastewater containing dissolved ammonium ions is provided. The ratio between the alkalinity of the wastewater and the molar concentration of dissolved ammonium ions is at least 0.5:1, and this alkalinity is expressed in equivalents per liter. Lower alkalinity slows down any reaction kinetics and reduces the total amount of ammonium that can be extracted. A ratio of 0.5:1 is currently considered the lower limit for achieving a method suitable for industrial purposes. Preferably, the ratio between the alkalinity of the wastewater and the dissolved ammonium ions is at least 0.7:1, more preferably at least 1:1, and ideally greater than 1.5:1. This ratio provides an indication of the maximum amount of ammonium that can be extracted from the water.

[0037] In step S20, magnesium phosphate is added to the wastewater, causing struvite to precipitate. In a preferred embodiment, the step of adding magnesium phosphate to the wastewater includes adding magnesium phosphate in an amount that gives a molar ratio between magnesium phosphate and dissolved ammonium ions of at least 0.5:1, preferably at least 0.7:1, more preferably at least 1:1, and most preferably at least 2:1.

[0038] In step S30, the wastewater is degassed. Step S30 at least partially overlaps with step S20, i.e., degassed occurs during at least a portion of the struvite settling period. Preferably, degassed occurs throughout the entire struvite settling period.

[0039] In step S50, the precipitated struvite is separated from the wastewater. If any excess magnesium phosphate is present that has not yet reacted, it will be in essentially solid form and will be separated together with the struvite.

[0040] The reaction kinetics of the magnesium phosphate-ammonium reaction depends on the initial amount of magnesium phosphate added, the initial ammonium-nitrogen concentration, and the initial alkalinity.

[0041] Figure 2 This graph shows the change in ammonium-nitrogen concentration over time in wastewater from a wastewater treatment plant after the addition of magnesium phosphate while the solution is stirred. Curve 101 corresponds to a molar ratio of 1.0 between the initial magnesium phosphate and the initial dissolved ammonium. Curve 102 corresponds to a molar ratio of 1.4 between the initial magnesium phosphate and the initial dissolved ammonium. Curve 103 corresponds to a molar ratio of 2.1 between the initial magnesium phosphate and the initial dissolved ammonium. It can be seen that for this wastewater, the efficiency of ammonium capture increases with the amount of magnesium phosphate, with a molar ratio at least as high as 2. It is further noted that diminishing returns exist when the total magnesium phosphate solids content in the reactor is increased above a certain limit.

[0042] While the ratio of magnesium phosphate to ammonium is indeed important for efficiency, the absolute concentration of magnesium phosphate in the mixture is also crucial. For example, for wastewater with low ammonium content, diminishing returns with increasing magnesium phosphate solids content appear to begin at higher molar ratios.

[0043] Similar tests were performed on aqueous solutions containing ammonium chloride dissolved in clean water without substantial alkalinity. No ammonium capture was detected in such solutions. Therefore, alkalinity is crucial for capture to occur. This indicates that the aforementioned reaction (3) between magnesium phosphate and bicarbonate ions produces the majority of the ammonium capture. Since the molar ratio between magnesium phosphate and bicarbonate ions in reaction (3) is 1:1, the theoretical limit of ammonium capture capacity is equal to the existing quantity of bicarbonate ions. Therefore, a ratio of 0.5:1 between alkalinity and dissolved ammonium ions allows for the capture of up to half of the ammonium ions. To increase the capture amount, the alkalinity should therefore preferably be at least 1:1 relative to the initial amount of ammonium ions. However, a large excess of alkalinity beyond this amount does not significantly improve or decrease the completeness of the reaction, but rather improves the reaction kinetics to some extent.

[0044] Figure 3This is a graph (104) showing the pH changes during ammonium capture using magnesium phosphate rock via struvite precipitation from wastewater obtained from a biogas production unit. Curve (105) indicates the ammonium concentration in the solution. After the initial drop in pH, the buffering capacity of alkalinity begins to take effect, and in this particular example, the entire process occurs essentially within a pH range of 6.5 to 8. Depending on the initial level of alkalinity in the wastewater, slightly higher pH values ​​may be present, sometimes up to 10.

[0045] In other words, in one embodiment, the precipitation of struvite occurs at a pH value between 6.5 and 10.

[0046] As mentioned above, the reaction rate is affected by degassing during ammonium capture. In the test experiment, 5 g of magnesium phosphate was added to two containers containing 200 mL of highly alkaline wastewater from a wastewater treatment plant. One container was sealed, and the other was open. Other conditions were the same. The initial wastewater had an initial ammonium-nitrogen content of 480 mg / L. After 18 hours, the ammonium-nitrogen content was analyzed again. As shown in Table 1, the water in the open container showed an ammonium-nitrogen content of only 10 mg / L, while the sealed container still showed an ammonium-nitrogen content of 228 mg / L. Therefore, the increase in carbon dioxide gas content in the container significantly inhibited further degassing of the water, which in turn caused the reaction to stop. Therefore, the reaction in the open container will drive the ammonium capture process faster than in the absence of the possibility of atmosphere exchange.

[0047]

[0048] Table 1. Ammonium capture via magnesium phosphate in open and sealed containers

[0049] Compared to laboratory tests, in a volume of 1 m³ 3 Scale-up of the reaction in an open container resulted in significantly slower kinetics. The reason is currently explained as slower degassing due to the smaller surface area to volume ratio during scale-up. The conclusion is that the kinetics are too slow for industrial applications. Active degassing is necessary to obtain kinetics suitable for industrial implementation.

[0050] Active degassing can be performed in various ways as known in the art. Some examples include degassing by generating eddies with a stirrer, degassing by centrifugal force, vacuum degassing, ultrasonic degassing, and pump degassing, or combinations thereof.

[0051] Several tests have been conducted using different degassing principles. Most of these did indeed serve their intended purpose, but some appeared to be more efficient than others. In one experiment, a standard mixing arrangement was used, including a reactor with baffles. A reduction was obtained from an initial NH4-N concentration of 879 mg / L to a final NH4-N concentration of 319 mg / L. When the experiment was repeated under the same conditions, but with baffles and aerators, a final NH4-N concentration of 222 mg / L was obtained. Finally, when the same input conditions were applied to a vortex mixer, the final NH4-N concentration became 136 mg / L. It is currently believed that centrifugal force is responsible for expelling dissolved CO2 from the solution and incorporating air into the vortex to exchange CO2, thereby improving the conditions for the rapid ammonium capture reaction.

[0052] In other words, in one embodiment, degassing is performed by generating eddies using a stirrer.

[0053]

[0054] Table 2. Ammonium extraction under different degassing methods.

[0055] In the published European patent application EP 2 431 336 A1, degassing of initially ammonium-containing water was employed before the addition of sodium hydroxide and magnesium phosphate. Therefore, experiments were conducted comparing degassing performed before adding magnesium phosphate to the water with degassing performed during the actual reaction. Table 2 shows some results from these tests. Samples indicated by "A" are those where degassing was allowed before vortexing, not during vortexing. Samples indicated by "B" are those where degassing was performed during vortexing. Sample numbers 1, 2, and 3 indicate different vortexing speeds. It is readily apparent from these tests that degassing during vortexing significantly reduces the residual ammonium-nitrogen in the solution.

[0056] As concluded from experiments involving the addition of magnesium phosphate rock to water containing ammonium ions and alkalinity, even with sufficient alkalinity and magnesium phosphate rock quantity, complete capture of ammonium ions was not achieved within a reasonable timeframe. To make this process suitable for an industrial context, it is preferable to interrupt the process at some stage. This interruption could, for example, occur after a predetermined reaction time. Alternatively, the reaction can be allowed to continue until a residual fraction of ammonium ions is obtained in the solution. A reasonable fraction could be approximately 5%–25%.

[0057] In other words, in one embodiment, struvite precipitation is allowed to continue for a predetermined time or until the molar content of remaining ammonium ions in the solution in the wastewater decreases by a predetermined fraction.

[0058] If the process terminates at this stage, the wastewater will still contain a certain concentration of ammonium. In one embodiment, additional steps can therefore be performed. Figure 4 This is a flowchart of the steps of an embodiment of a method for recovering nitrogen from wastewater. (In addition to already combined...) Figure 1 In addition to the steps described, there is an additional step S40 performed after step S30 terminates. In this step, most of the remaining ammonium ions in the solution are removed by adding alkali, thereby significantly increasing the pH value.

[0059] This step itself is known in the prior art, but that is used for the entire process. Instead, by initiating this step only when a small fraction of the initial ammonium ions remain in the solution, the drawbacks of the prior art method are reduced. For example, the amount of alkali required to precipitate the remaining ammonium ions is only a fraction of the amount required if alkali had already been used in the initial water. The above reaction (3) reduces the buffering capacity of the alkalinity present in the wastewater. This saves hydroxide ions from being "buffered" and can be used alternatively, for example, in reaction (1).

[0060] In other words, in one embodiment, the method for recovering nitrogen from wastewater includes an additional step of adding an alkali to the wastewater after a predetermined time or after a predetermined fraction has been reached. Preferably, the alkali is sodium hydroxide.

[0061] Figure 5 This is a graph illustrating the process of capturing ammonium ions from initial wastewater containing 1520 mg / L NH4-N and an alkalinity of 6060 meq / L. Magnesium phosphate was added under degassing, and the reaction was allowed to continue for 3 hours, as shown by curve 106. At this stage, denoted as t1, only 200 mg / L of NH4-N remained in the solution. Sodium hydroxide was then added, which allowed most of the remaining ammonium ions to precipitate as struvite according to reaction (1), as shown by curve 107. In this way, a very high fraction of ammonium ions were precipitated as struvite and could be removed from the solution by a separation step. Meanwhile, the amount of sodium hydroxide added was only a small amount compared to adding sodium hydroxide directly to the initial water.

[0062] As mentioned above, adding alkali to wastewater can also lead to side reactions, such as those given in reaction (2). These conditions can even cause magnesium from dissolved magnesia phosphate to precipitate, at least partially, as magnesium hydroxide. However, such processes are generally relatively slow, and such side reactions can be limited by subjecting the wastewater to alkali only for a short period of time. If the precipitated struvite is separated relatively quickly after the magnesia phosphate treatment step and alkali is added, the degree of magnesium hydroxide precipitation can be kept very low.

[0063] Struvite sedimentation reactors can be designed to perform the above ideas. Figure 6AThis is a schematic diagram of the various parts of a struvite precipitation reactor arrangement 10 for recovering nitrogen from wastewater. The struvite precipitation reactor arrangement 10 includes a reactor vessel 20. The reactor vessel 20 has an input 22 for wastewater 200 leading to the reactor vessel 20 and an input 24 for magnesium phosphate 201 leading to the reactor vessel 20. Inputs 22 and 24 can also be combined into a single input.

[0064] The reactor vessel 20 further includes a degassing device 30 configured to degas the reactor vessel 20. The degassing device 30 can be of various types. Non-exclusive examples are agitators that generate vortices, centrifugal degassing devices, vacuum devices, ultrasonic degassing devices and degassing pumps, or combinations thereof. In this embodiment, the degassing device 30 includes an agitator 32 that generates vortices in the solution within the reactor vessel 20. Preferably, the degassing device 30 also includes a gas conduit 34 configured to remove carbon dioxide gas 202 escaping from the generated vortices. Typically, the removed carbon dioxide gas 202 is replaced by another gas (typically air 208) through a vent 35.

[0065] The reactor further includes a struvite separation arrangement 40 configured to separate struvite deposited in reactor vessel 20 from wastewater. In this embodiment, struvite 203 removal is carried out via output 42. Depleted ammonium water 204 is removed via output 44.

[0066] In this embodiment, reactor vessel 20 further includes an input 26 for alkali 205 leading to reactor vessel 20. This input 26 is operated after a predetermined time following the introduction of magnesium phosphate into the vessel or when the molar concentration of remaining ammonium ions in the wastewater solution decreases by a predetermined fraction. This addition of alkali 205 initiates a second stage of struvite precipitation by increasing the pH of the solution.

[0067] The second-stage reaction has two main functions. The first function is to reduce the effluent nitrogen to the desired level. The second function is to minimize the loss of dissolved magnesium phosphate. The loss of dissolved magnesium phosphate results in the loss of Mg and P, which need to be compensated for and added to the system.

[0068] Based on these principles, another preferred arrangement is to operate continuously by connecting two continuously stirred reactors in series. An example of this is... Figure 6B The struvite sedimentation reactor arrangement 10 here includes a first reactor vessel 20A and a second reactor vessel 20B. The first reactor vessel 20A has an input 22A for wastewater 200A leading to the first reactor vessel 20A and an input 24 for magnesium phosphate 201 leading to the first reactor vessel 20A. Inputs 22A and 24 can also be combined into a single input.

[0069] Similar to the previous embodiments, the first reactor vessel 20A further includes a degassing device 30 configured to degas the reactor vessel 20A. In this embodiment, the degassing device 30 includes a stirrer 32 that generates eddies in the solution within the first reactor vessel 20A. Preferably, the degassing device 30 also includes a gas conduit 34 configured to remove carbon dioxide gas 202 escaping from the generated eddies. Typically, the removed carbon dioxide gas 202 is replaced by another gas (typically air 208) passing through a ventilation duct 35.

[0070] In this embodiment, the first reactor vessel 20A further includes a first struvite separation arrangement 40A configured to separate struvite precipitated in the first reactor vessel 20A from wastewater 200A. Removal of struvite 203 is carried out via a first output 42A. Partially depleted ammonium wastewater 200B is removed via a second output 44A to be supplied to the second reactor vessel 20B.

[0071] In an alternative embodiment, the first struvite separation arrangement 40A and output 42A are omitted. The precipitated struvite slurry in the partially depleted ammonium wastewater is then transferred to the second reactor vessel using the second output 44A.

[0072] In this embodiment, the second output 44A is connected to the input 22B of the second reactor vessel 20B. The second reactor vessel 20B further includes an input 26 for alkali 205 leading to the second reactor vessel 20B. The transfer of partially depleted ammonium wastewater 200B to the second reactor vessel is performed after a predetermined time following the introduction of magnesium phosphate into the vessel, or when the molar concentration of remaining ammonium ions in the solution of wastewater 200B decreases by a predetermined fraction. This addition of alkali 205 initiates a second stage of struvite precipitation by increasing the pH of the solution.

[0073] The second reactor vessel 20B further includes a second struvite separation arrangement 40B, which is configured to separate struvite precipitated in the second reactor vessel 20B from partially depleted ammonium wastewater 200B. In this embodiment, the removal of struvite 203 is carried out via a third output 42B. The almost completely depleted ammonium wastewater 204 is removed via a fourth output 44B.

[0074] In other words, magnesium phosphate is added to a first reactor vessel, which has active degassing (e.g., by generating eddies) and a predetermined residence time. The overflow from the first reactor vessel is then treated in a second reactor for the predetermined residence time. Alkali is continuously added only to the second reactor vessel.

[0075]

[0076] Table 3. Nitrogen removal in the second stage.

[0077] Experiments were conducted to investigate the effect of sodium hydroxide dosage. A first-stage reaction was used with a retention time of approximately 5.4 hours. A molar ratio of 1.8:1 between magnesium phosphate and dissolved ammonium ions was used, and eddy currents were generated at 550 RPM. This yielded an NH₄⁻ content of 143 mg / L–147 mg / L in partially depleted ammonium wastewater at pH 7.4. Subsequently, for many different samples, this partially depleted ammonium wastewater was treated in a second stage by adding NaOH at varying flow rates over a period of approximately 10 minutes. The final pH and ammonium content were then measured. Results were presented in… Figure 3 As shown in the image.

[0078] These experiments demonstrate that the concentration of effluent NH4-N can be controlled by the amount of NaOH added. With increasing dosage, the effluent NH4-N decreases and the pH increases proportionally. However, the recovered NH4-N is not entirely equivalent to the number of moles of hydroxide added. In all cases, excess hydroxide was added for the recovered NH4-N. The pH, effluent NH4-N, and NaOH all varied, with the sodium hydroxide dosage increasing from the minimum to the maximum dosage by approximately 150%, and the pH varying between 8.15 and 8.55.

[0079] The results for magnesium and phosphorus were also analyzed to understand how magnesium phosphate loss was affected. As seen in Table 3, the fixed residence time experiments showed that dissolved magnesium and phosphorus remained relatively constant in the effluent, even with variations in the influent values. The average effluent value for Mg was approximately 40 mg / L, and the average effluent value for P was approximately 58 mg / L. Therefore, under all different sodium hydroxide dosages, dissolved Mg and P decreased compared to the influent.

[0080] The next parameter to be studied is the change in residence time. Table 4 shows the decrease in dissolved magnesium phosphate as residence time decreases, as indicated by the decrease in both Mg and P. This holds true for both sodium hydroxide dosage tests conducted. Simultaneously, NH₄⁻ and pH increase as residence time decreases. The lowest dissolved Mg and P concentrations reached in these experiments were 17.6 mg / L and 31.4 mg / L, respectively. The losses can be calculated by comparing them to the values ​​in the original wastewater. This can be further reduced by decreasing the residence time. It appears that the first reaction that occurs when sodium hydroxide enters the solution is the precipitation of a 1:1 Mg:P magnesium phosphate compound.

[0081] The first six rows in Table 4 show that longer retention times mean more struvite precipitation and a higher pH for the same amount of hydroxide added. Therefore, as retention time progresses, the initially precipitated magnesium phosphate subsequently dissolves and reforms into struvite, but also increases the pH and the amount of dissolved magnesium and phosphate. The exact mechanism is not yet known; however, it is clear that to minimize magnesium phosphate loss, the retention time in the second reactor should be reduced.

[0082] In other words, in one embodiment, the step of separating the precipitated struvite from the wastewater after the step of adding alkali to the wastewater is performed for less than 30 minutes, preferably less than 5 minutes.

[0083]

[0084] Table 4. Residence time experiment.

[0085] To ensure the losses and water collection benefits of the two-stage reactors are realistic, they need to be compared with the results of adding the same amount of sodium hydroxide directly to the main reactor instead. The experiments presented in the last two rows of Table 4 (labeled with M instead of residence time) show that, compared to the two-stage reactor system, when the same amount of sodium hydroxide is used directly in the main reactor, the final NH4-N and dissolved magnesium phosphate values ​​increase, while the pH value decreases significantly. This is independent of the residence time being compared. This confirms that using two reaction steps in this system allows for optimal utilization of sodium hydroxide.

[0086] One advantage of using magnesium phosphate as a reagent for capturing ammonium is that magnesium phosphate can be regenerated from struvite. This is discussed, for example, in published international patent application WO 2020 / 256622 A1. A similar idea can be applied in this context.

[0087] Figure 7 This is a flowchart of the steps of an embodiment of a method for recovering nitrogen from wastewater. Steps S10, S20, S30, and S50, and optionally step S40, follow the preceding description. In an additional step S60, the separated precipitated struvite is exposed to an acid in a liquid solution. The acid is at least one of sulfuric acid, phosphoric acid, nitric acid, and carbonic acid. The struvite dissolves in the liquid solution by the acid. Conversely, in step S70, magnesium phosphate precipitates from the liquid solution. This can be performed, for example, according to the principles presented in published International Patent Application WO 2020 / 256622 A1. In step S80, the precipitated magnesium phosphate is separated from the liquid solution, yielding a remaining ammonium salt solution. In step S90, the precipitated magnesium phosphate is recycled to the step of adding magnesium phosphate to the wastewater.

[0088] Figure 8The illustration schematically shows portions of an embodiment of a circulation arrangement 1 for recovering nitrogen from wastewater. The circulation arrangement 1 for recovering nitrogen from wastewater includes a struvite sedimentation reactor arrangement 10 and a struvite decomposition reactor 50. The struvite sedimentation reactor arrangement 10 is any embodiment of the technology presented elsewhere in this disclosure, for example, according to... Figure 6A or Figure 6B It's configured.

[0089] In this embodiment, the struvite decomposition reactor 50 has an input 54 for struvite 203 and an input 52 for acid 206, the struvite input being directly or indirectly connected to the output 42 of the struvite precipitation reactor arrangement 10. The struvite decomposition reactor 50 further has an output 58 for precipitated magnesium phosphate 201 and an output 56 for liquid 207 containing dissolved ammonium salts and anions of acid 206. The struvite decomposition reactor 50 typically has a magnesium phosphate separation arrangement 60 for separating the precipitated magnesium phosphate 201 and the liquid 207. Preferably, a recycling arrangement 70 is present, configured to recycle at least a portion of the precipitated magnesium phosphate 201 to the input 24 of the struvite precipitation reactor arrangement 10. This recycling arrangement 70 may also include any type of storage arrangement.

[0090] Precipitation of magnesium phosphate 201 from struvite 203 dissolved in acid 206 can be performed by any method known to those skilled in the art. Some alternatives are presented, for example, in published international patent application WO 2020 / 256622 A1.

[0091] In an alternative embodiment, the struvite decomposition reactor 50 may be integrated into the struvite sedimentation reactor arrangement 10. In such an embodiment, the struvite separation arrangement 40 (or 40B, if using...) Figure 6B The embodiment is configured to separate struvite precipitated in reactor vessel 20 (or 20B) from wastewater. Depleted ammonium-rich water 204 is removed via output 44 (or 44B), leaving the struvite in reactor vessel 20 (or 20B). Input 52 for acid 206 then directs the acid to the combined reactor vessel 20 and struvite decomposition reactor 50, where the decomposition process occurs.

[0092] The current idea of ​​recovering nitrogen in the form of ammonium from wastewater is based on the assumption that the wastewater contains a considerable amount of alkalinity. This is typical in many cases. Wastewater from biogas production almost always contains high alkalinity sufficient to satisfy the idea presented herein. Therefore, in one embodiment, wastewater from biogas production is used as the input wastewater in this technology. Similarly, most wastewater from wastewater treatment plants does indeed contain alkalinity sufficient to allow the recovery of at least a substantial portion of the ammonium content.

[0093] However, if the goal is to recover ammonium from wastewater that lacks sufficient alkalinity for the desired amount of ammonium recovery, the alkalinity can be increased. Sodium carbonate and / or potassium carbonate are useful sources of alkalinity. By adding such substances to the raw wastewater, which has low alkalinity, the alkalinity can be increased to a level suitable for the process of this invention. Figure 9 The diagram illustrates a flowchart of the steps in an embodiment of a method for recovering nitrogen from wastewater. Most steps are similar to those described previously. However, in this embodiment, step S10, which provides wastewater, comprises two parts. In step S12, raw wastewater is provided. This wastewater may itself have low alkalinity. In step S14, an alkali is added to the raw wastewater to form wastewater with increased alkalinity. Preferably, the alkali is at least one of sodium carbonate, potassium carbonate, sodium hydroxide, and ammonium hydroxide. Most preferably, the alkali is at least one of sodium carbonate and potassium carbonate.

[0094] Another way to add alkalinity to raw wastewater is to mix two waste streams (one high-ammonia waste stream) with a high-alkalinity stream, usually for cost-saving reasons.

[0095] The above embodiments should be understood as illustrative examples of the present invention. Those skilled in the art will understand that various modifications, combinations, and changes can be made to the embodiments without departing from the scope of the invention. In particular, where technically possible, different partial solutions from different embodiments can be combined in other configurations. However, the scope of the invention is defined by the appended claims.

Claims

1. A method for recovering nitrogen from wastewater, comprising the following steps: - Provide (S10) wastewater (200) containing dissolved ammonium ions and at least one of carbonate ions and bicarbonate ions; Wherein, the ratio between the alkalinity of the wastewater (200) and the molar concentration of dissolved ammonium ions is at least 0.5:1, and the alkalinity is expressed in equivalents per liter; - Adding magnesium phosphate (201) (S20) to the wastewater (200) causes struvite (203) to precipitate; - During at least a portion of the sedimentation of the struvite (203), the wastewater (200) is degassed (S30); and - Separate the precipitated struvite (203) from the wastewater (S50).

2. The method according to claim 1, characterized in that, The step of adding magnesium phosphate (201) (S20) to the wastewater includes adding magnesium phosphate (201) in an amount that gives a molar ratio between magnesium phosphate (201) and dissolved ammonium ions of at least 0.5:1, preferably at least 0.7:1, more preferably at least 1:1, and most preferably at least 2:

1.

3. The method according to claim 1 or 2, characterized in that, The guanostone (203) precipitation occurs at pH values ​​between 6.5 and 10.

4. The method according to any one of claims 1 to 3, characterized in that, The degassing (S30) is selected from: Use a stirrer (32) to generate eddies; Degassing is achieved through centrifugal force; Vacuum degassing; Ultrasonic degassing; and Pump degassing.

5. The method according to claim 4, characterized in that, The degassing (S30) is achieved by generating a vortex using a stirrer (32).

6. The method according to any one of claims 1 to 5, characterized in that, The degassing (S30) occurs throughout the sedimentation time of the struvite (203).

7. The method according to any one of claims 1 to 6, characterized in that, The guano (203) is allowed to precipitate for a predetermined time or until the molar content of the remaining ammonium ions in the solution in the wastewater decreases by a predetermined fraction.

8. The method according to claim 7, characterized in that, The following additional steps: - After the predetermined time or after the predetermined fraction is reached, alkali (205) is added (S40) to the wastewater (200).

9. The method according to claim 8, characterized in that, The alkali (205) is sodium hydroxide.

10. The method according to claim 8 or 9, characterized in that, After the step of adding (S40) the alkali (26) to the wastewater (200), the step of separating (S50) the precipitated struvite (203) from the wastewater is performed for less than 30 minutes, preferably less than 5 minutes.

11. The method according to any one of claims 1 to 10, characterized in that, The following additional steps: - The separated precipitated struvite (203) is exposed (S60) to acid (52) in a liquid solution; Wherein, the acid (52) is at least one of sulfuric acid, phosphoric acid, nitric acid and carbonic acid; - Precipitate magnesium phosphate (201) from the liquid solution (S70); - Separate the precipitated magnesium phosphate (201) from the liquid solution (S80), yielding the remaining ammonium salt solution (207); and - The precipitated magnesium phosphate (201) is recycled (S90) to the step of adding magnesium phosphate (201) to the wastewater (200) (S20).

12. The method according to any one of claims 1 to 11, characterized in that, The wastewater (200) includes wastewater generated from biogas production.

13. The method according to any one of claims 1 to 12, characterized in that, The step of providing (S10) wastewater (200) includes the following steps: - Provide (S12) raw wastewater; and - Add alkalinity (S14) to the original wastewater, preferably by adding at least one of sodium carbonate, potassium carbonate, sodium hydroxide and ammonium hydroxide, most preferably by adding at least one of sodium carbonate and potassium carbonate, to form the wastewater (200) with increased alkalinity.

14. A struvite precipitation reactor arrangement (10) for recovering nitrogen from wastewater (200), comprising: - Reactor vessel (20); - Input (22) for wastewater (200) to the reactor vessel (20); - An input (24) for magnesium phosphate (201) to the reactor vessel (20) to enable it to react with the wastewater (200); as well as - A struvite separation arrangement (40) configured to separate struvite (203) deposited in the reactor vessel (20) from the wastewater. Its features are, - Degassing device (30), which is configured to remove the gas generated in the reactor vessel (20) by the reaction between magnesium phosphate (201) and the wastewater (200).

15. The arrangement according to claim 14, characterized in that, The degassing device (30) is one of the following: Agitator that generates vortices (32); Centrifugal degassing equipment; Vacuum equipment; Ultrasonic degassing equipment; and Degassing pump.

16. The arrangement according to claim 15, characterized in that, The degassing device (30) is a stirrer (32) that generates vortices.

17. The arrangement according to any one of claims 14 to 16, characterized in that... An input (26) for alkali (205) to the struvite precipitation reactor arrangement (10) is provided, which is operated after a predetermined time following the introduction of the magnesium phosphate (201) into the reactor vessel or when the molar content of the remaining ammonium ions in the solution in the wastewater decreases by a predetermined fraction.

18. A circulating arrangement (1) for recovering nitrogen from wastewater (200), comprising: - The struvite sedimentation reactor arrangement (10) according to any one of claims 14 to 17; - A struvite decomposition reactor (50), which is connected to or integrated into the struvite precipitation reactor arrangement (10) and has an input (52) for acid (206). The struvite decomposition reactor (50) further has an output (58) for precipitated magnesium phosphate (201) and an output (56) for a liquid (207) containing dissolved ammonium salt and the acid (52) anions.

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