Nitrogen recovery from backflow water

Abstract

A method for recovering nitrogen from wastewater includes the step (S10) of providing wastewater having dissolved ammonium ions. The ratio of alkalinity, expressed as an equivalent per liter in the wastewater, to the molar concentration of dissolved ammonium ions is at least 0.5:1. Newberite is added to the wastewater (S20), which causes the precipitation of struvite. The wastewater is degassed during at least a portion of the precipitation of struvite (S30). Subsequently, the precipitated struvite is separated from the wastewater (S50). A struvite precipitation reactor device and a circulation device for recovering nitrogen from wastewater are also disclosed.

Description

Technology Field

[0001] The present technology generally relates to the recovery of commercial substances from reject substances, and in particular to a method and apparatus for recovering nitrogen from reject water. Background Technology

[0002] A significant amount of backwash water is generated during biogas production. One of the reasons for the large volume of backwash water is the presence of ammonium ions (NH4) exceeding 3,000 ppm. + This is because the high nitrogen content of ) causes severe toxicity to biogas-producing microorganisms. Therefore, water is added to biogas plants to dilute the nitrogen content below toxic levels, which results in a large amount of return water. Since this return water typically contains high concentrations of ammonium ions, it is unsuitable for discharge into the environment.

[0003] At the same time, ammonia is a substance in demand. Today, most ammonia production is based on the Haber-Bosch process, which requires the consumption of large amounts of natural gas. Nitrogen-containing effluents must be used as a nitrogen source instead of being discharged.

[0004] In the prior art, the precipitation of struvite has been used primarily to remove phosphorus, but also nitrogen, from waste material. This is achieved by adding a magnesium source and adjusting the pH to an alkaline pH. The increase in pH is usually carried out by adding sodium hydroxide. Using the cheaper calcium hydroxide is undesirable because calcium phosphate precipitates unintentionally preferentially over struvite. The struvite can then be processed into other commercially more valuable materials, for example, according to published international patent application WO 2020 / 256622 A1. In this process, the magnesium source can be recovered and recycled back to the precipitation process. The magnesium source may be, for example, newberyite.

[0005] In the disclosed European patent application EP 2 431 336 A1, Newberite is used in combination with sodium hydroxide in a single reactor to apply pH control.

[0006] Sodium hydroxide is a relatively expensive substance, and the cost of sodium hydroxide can account for 60% of the total cost of the nitrogen recovery process. In addition, sodium hydroxide not only assists in the precipitation of struvite but is also consumed by other reactions in typical backflow water compositions. The high costs associated with conventional struvite precipitation approaches reduce the attractiveness of large-scale backflow water treatment.

[0007] In addition, recirculating backwater treated with sodium hydroxide to dilute nitrogen content back into the biogas reactor cannot be carried out on a larger scale due to the accumulation of sodium ions, and sodium ions also have a negative effect on biogas-producing microorganisms at high levels.

[0008] Therefore, improvements are needed in the recovery of nitrogen from liquid solutions. The problem to be solved

[0009] Therefore, the general objective of the present technology is to find an improved process for recovering nitrogen from an aqueous solution having a high ammonium ion content. means of solving the problem

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

[0011] Generally speaking, in the first embodiment, a method for recovering nitrogen from wastewater comprises the step of providing wastewater having dissolved ammonium ions. The ratio of alkalinity, expressed as an equivalent per liter in the wastewater, to the molar concentration of dissolved ammonium ions is at least 0.5:1. Newberite is added to the wastewater, which causes the precipitation of struvite. The wastewater is degassed during at least a portion of the precipitation of struvite. Subsequently, the precipitated struvite is separated from the wastewater.

[0012] In a second embodiment, a struvite precipitation reactor device for recovering nitrogen from wastewater comprises a reactor vessel, said reactor vessel having an inlet for wastewater into said reactor vessel and an inlet for Newberite into said reactor vessel. The struvite precipitation reactor device further comprises a degassing device configured to degasify the reactor vessel. The struvite precipitation reactor device further comprises a struvite separation device. The struvite separation device is configured to separate the struvite precipitated within the reactor vessel from the wastewater.

[0013] In a third embodiment, a circulation device for recovering nitrogen from wastewater comprises a struvite precipitation reactor device and a struvite decomposition reactor according to the second embodiment. The struvite decomposition reactor is connected to or integrated with the struvite precipitation reactor device and has an inlet for acid. The struvite decomposition reactor further has an outlet for precipitated Newberite and an outlet for a liquid containing anions of dissolved ammonium salt and acid. Effects of the invention

[0014] One advantage of the proposed technology is that it can efficiently recover nitrogen from wastewater while using less sodium hydroxide. Other advantages will be understood by reading the detailed description. Brief explanation of the drawing

[0015] The present invention is best understood by referring to the following description together with the accompanying drawings, along with its additional purposes and advantages: FIG. 1 is a flowchart of the steps of an embodiment of a method for recovering nitrogen from wastewater; FIG. 2 is a diagram showing the ammonium nitrogen content during the struvite precipitation process; Figure 3 is a diagram showing the changes in ammonium nitrogen content and pH during the struvite precipitation process; FIG. 4 is a flowchart of the steps of another embodiment of a method for recovering nitrogen from wastewater; FIG. 5 is a diagram showing the ammonium nitrogen content during the two-stage struvite precipitation process; FIG. 6a is a schematic diagram of parts of one embodiment of a struvite precipitation reactor apparatus; FIG. 6b is a schematic diagram of parts of another embodiment of a struvite precipitation reactor apparatus; FIG. 7 is a flowchart of the steps of an embodiment of a recirculation method for recovering nitrogen from wastewater; FIG. 8 is a schematic diagram of parts of an embodiment of a circulation device for recovering nitrogen from wastewater; and FIG. 9 is a flowchart of the steps of another embodiment of a method for recovering nitrogen from wastewater. Specific details for implementing the invention

[0016] Throughout the drawing, the same reference number is used for similar or corresponding elements.

[0017] When struvite precipitates from Newberite at alkaline pH, the presumed reaction is as follows:

[0018] MgHPO4·3H2O(s) + 2H2O(l) + NH4 + (aq) + OH - (aq) →

[0019] → NH4MgPO4·6H2O(s). (1)

[0020] However, using sodium hydroxide to increase pH can also cause other side reactions, such as the following:

[0021] MgHPO4·3H2O(s) + 2NaOH(aq) →

[0022] → Mg(OH)2(s) + Na3PO4(aq) + 3H2O(l). (2)

[0023] Instead of the precipitation of struvite to remove ammonium ions from the solution, magnesium hydroxide may be precipitated. As a result, sodium hydroxide is consumed without contributing to nitrogen removal. To avoid this “misuse” of sodium hydroxide, a different reaction mechanism may be used.

[0024] For example, wastewater from biogas production or wastewater treatment plants typically contains significant alkalinity. The concentration of ions in water that neutralize hydrogen ions is known as alkalinity. The most well-known alkaline components are bicarbonates, carbonates, and hydroxides, respectively. The quantitative definition of alkalinity is A=[HCO3 - ] + 2[CO3 2- ] + [OH - ] - [H +...and is expressed as an equivalent per liter. Alkalinity is typically defined as a milliequivalent per liter. Carbonate ion concentration is calculated twice because the ion has the ability to accept two hydrogen ions. The definition of alkalinity can be simplified to the buffering capacity of a solution, which is typically mg HCO3 when applied to wastewater. - It is expressed as / L. Under alkalinity, particularly in the presence of bicarbonate ions (i.e., hydrogen ions), the following reaction may occur:

[0025] MgHPO4·3H2O(s) + 2H2O(l) + NH4 + (aq) + HCO3 - (aq) →

[0026] → NH4MgPO4·6H2O(s) + CO2(g) (3)

[0027] Unfortunately, tests have revealed that simply adding Newberite to alkaline wastewater results in only a relatively slow and incomplete reaction according to this equation.

[0028] However, surprisingly, it was found that the rate and completeness increase significantly when degassing is performed during the process. It was concluded that degassing contributes to the reduction of dissolved carbon dioxide gas in the water, which consequently drives the reaction further toward struvite precipitation. For example, it was found that when the reaction is carried out in a sealed container, the reaction stops completely after a certain period of time.

[0029] FIG. 1 shows a flowchart of the steps of an embodiment of a method for recovering nitrogen from wastewater. In step S10, wastewater containing dissolved ammonium ions is provided. The ratio of alkalinity, expressed as an equivalent per liter in the wastewater, to the molar concentration of dissolved ammonium ions is at least 0.5:1. If the alkalinity is lower, the reaction kinetics slow down and the total amount of extractable ammonium decreases. The ratio of 0.5:1 is currently considered a lower limit for achieving a process suitable for industrial purposes. Preferably, the ratio of alkalinity to dissolved ammonium ions in the wastewater is at least 0.7:1, more preferably at least 1:1, and ideally greater than 1.5:1. This ratio provides an indicator of the maximum amount of ammonium that can be extracted from water.

[0030] In step S20, Newberite is added to the wastewater to cause a precipitation of struvite. In a preferred embodiment, the step of adding Newberite to the wastewater comprises adding Newberite in an amount such that the molar ratio of Newberite to dissolved ammonium ions is at least 0.5:1, preferably at least 0.7:1, more preferably at least 1:1, and most preferably at least 2:1.

[0031] In step S30, the wastewater is degassed. Step S30 overlaps at least partially with step S20, that is, degassing occurs during at least part of the struvite sedimentation. Preferably, degassing occurs for the entire time that the struvite is sedimented.

[0032] In step S50, the precipitated struvite is separated from the wastewater. If there is any unreacted Newberite remaining, this Newberite will be in an essentially solid form and will be separated along with the struvite.

[0033] The reaction rate of the Newberite-ammonium reaction depends on the initial dose of Newberite, the initial ammonium nitrogen concentration, and the initial alkalinity.

[0034] Figure 2 is a diagram showing the ammonium nitrogen concentration in the return water from a wastewater treatment plant over time after adding Newberite while stirring the solution. Curve 101 corresponds to a molar ratio of 1.0 for initial Newberite to initial dissolved ammonium. Curve 102 corresponds to a molar ratio of 1.4 for initial Newberite to initial dissolved ammonium. Curve 103 corresponds to a molar ratio of 2.1 for initial Newberite to initial dissolved ammonium. Therefore, in the case of this return water, it can be seen that the efficiency of ammonium capture increases as the amount of Newberite increases, at least up to a molar ratio of 2. Furthermore, it was confirmed that when the total content of Newberite solids in the reactor increases beyond a certain limit, there is a phenomenon of diminishing returns.

[0035] The ratio of Newberite to ammonium is indeed important for efficiency, but so is the absolute concentration of Newberite in the mixture. For example, in the case of recirculating water with low ammonium content, the decrease in the above effect with increasing Newberite solid content appears to begin at a higher molar ratio.

[0036] A similar test was performed on an aqueous solution in which ammonium chloride was dissolved in clean water with no substantial alkalinity. In such a solution, no ammonium capture was detected. Therefore, alkalinity is essential for capture to occur. This indicates that the above-described reaction (3) between Newberite and bicarbonate ions is responsible for the main part of ammonium capture. Since the molar ratio of Newberite to bicarbonate ions in reaction (3) is 1:1, the theoretical limit of the ammonium capture capacity is equal to the amount of bicarbonate ions present. Therefore, if the ratio of alkalinity to dissolved ammonium ions is 0.5:1, at most half of the ammonium ions can be captured. To increase the amount of capture, the alkalinity must preferably be at least 1:1 relative to the initial amount of ammonium ions. However, an excess alkalinity that significantly exceeds this amount neither significantly improves nor decreases the completeness of the reaction, but does improve the reaction rate somewhat.

[0037] FIG. 3 is a diagram showing the pH (104) changed during ammonium capture using Newberite by struvite precipitation from wastewater obtained from a biogas production unit. Curve 105 represents the ammonium concentration in the solution. After an initial drop in pH, the buffering capacity of alkalinity begins to work, and in this particular example, the entire process essentially takes place in a pH range of 6.5 to 8. Depending on the initial level of alkalinity in the wastewater, somewhat higher pH values ​​may appear, sometimes up to pH 10.

[0038] In other words, in one embodiment, the precipitation of struvite occurs at a pH between 6.5 and 10.

[0039] As described above, the reaction rate is affected by degassing during the ammonium capture process. In the test experiment, 5 g of Newberite was added to two containers holding 200 mL of return water from a wastewater treatment plant with high alkalinity. One container was sealed, and the other was left open. Other conditions were identical. The initial return water 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. In other words, the increased carbon dioxide gas content within the container significantly hindered further degassing of the water, which eventually stopped the reaction from continuing. Therefore, the reaction in the open container would accelerate the ammonium capture process compared to cases where there is no possibility of atmosphere exchange.

[0040] Table 1. Ammonium capture by Newberite in open and closed containers Open container chest Newberite [g] 5 5 Banryusu [ml] 200 200 Duration [h] 18 18 Initial NH 4 -N [mg / L] 480 480 Final NH 4 -N [mg / L] 10 228

[0041] 1 m 3 When the reaction was scaled up in an open vessel with a volume, a significantly slower reaction rate was observed compared to laboratory tests. This is currently interpreted as being due to slower degassing resulting from the smaller surface area-to-volume ratio associated with the scale-up. It was concluded that this reaction rate is too slow for industrial applications. Active degassing is required to achieve a reaction rate sufficient for industrial implementation.

[0042] Active degassing can be performed in different ways, as is known in the prior art. Some examples include forming a vortex with a stirrer, degassing by centrifugal force, vacuum degassing, ultrasonic degassing and pump degassing, or combinations thereof.

[0043] Several tests were conducted using different degassing principles. While most of these are actually capable of operating for their intended purposes, some appear to be more efficient than others. In one experiment, a standard mixing apparatus including a reactor with baffles was used. An initial NH4-N content of 879 mg / L was reduced to a final NH4-N content of 319 mg / L. When the experiment was repeated under the same conditions but using baffles with aerators, a final NH4-N content of 222 mg / L was obtained. Finally, when the same input conditions were applied to a vortex mixer, the final NH4-N content became 136 mg / L. Currently, centrifugal force is believed to be responsible for pushing dissolved CO2 out of the solution and incorporating air into the vortex that exchanges CO2, thereby improving conditions for a rapid ammonium capture reaction.

[0044] In other words, in one embodiment, degassing is performed by forming a vortex with a stirrer.

[0045] Table 2. Ammonium extraction under different degassing methods Sample A1 A2 A3 B1 B2 B3 Vortex formation speed [rpm] 500 1100 1650 500 1100 1650 Initial NH 4 -N [mg / L] 922 911 843 922 902 905 initial pH 7.99 7.99 8.00 7.99 7.94 7.94 NH after pre-degassing 4 -N [mg / L] 886 916 866 - - - pH after pre-degassing 8.64 8.88 8.93 - - - NH after vortex formation without degassing 4 -N [mg / L] 362 320 314 - - - pH after vortex formation without degassing 7.16 7.15 7.18 - - - NH after vortex formation along with degassing 4 -N [mg / L] - - - 311 170 161 pH after vortex formation along with degassing - - - 7.52 7.59 7.83

[0046] In the published European patent application EP 2 431 336 A1, degassing of water containing initial ammonium ions was applied prior to the addition of sodium hydroxide and Newberite. Accordingly, experiments were also conducted to compare performing degassing before the addition of Newberite to water with performing degassing during the actual reaction. Table 2 shows some of the results from these tests. Samples labeled “A” refer to samples that were allowed to be degassed before vortex formation but were not degassed during vortex formation. Samples labeled “B” refer to samples that underwent degassing during vortex formation. Sample numbers 1, 2, and 3 exhibited different vortex formation rates. From these tests, it is easy to see that degassing during vortex formation significantly reduces the ammonium nitrogen remaining in the solution.

[0047] As concluded from experiments on the addition of Newberite to water containing ammonium ions and alkalinity, complete capture of ammonium ions was not achieved within a reasonable time that is industrially acceptable, even when the alkalinity and the amount of Newberite were sufficient. Therefore, to make the process applicable in an industrial context, it is desirable to stop the process at some point. Such a stoppage may occur, for example, after a predetermined reaction time. Alternatively, the reaction may be allowed to continue until the remaining ammonium ions in the solution fall below a certain fraction. A reasonable fraction may be approximately 5 to 25%.

[0048] In other words, in one embodiment, the precipitation of struvite is allowed to continue for a preset time or until the molar content of residual ammonium ions in the wastewater in the solution is reduced by a preset fraction.

[0049] If the process ends at this stage, the wastewater will still contain a certain concentration of ammonium. Therefore, in one embodiment, additional steps may be performed. FIG. 4 is a flowchart of the steps of one embodiment of a method for recovering nitrogen from wastewater. In addition to the steps already described in relation to FIG. 1, there is an additional step S40 performed after step S30 is completed. In this step, most of the remaining ammonium ions in the solution are removed by adding a base to significantly increase the pH.

[0050] This step is known in itself in the prior art, but in the prior art, it pertains to the entire process. Instead, by initiating this step only when a small fraction of the initial ammonium ions remain in the solution, the disadvantages of the prior art approach are reduced. For example, the amount of base required to precipitate the residual ammonium ions is only a fraction of the amount that would have been required if the base had been used in the initial water. The above reaction (3) reduces the buffering capacity of the alkalinity present in the wastewater. This prevents the hydroxide ions from being “buffered,” and the hydroxide ions can instead be used, for example, in reaction (1).

[0051] In other words, in one embodiment, a method for recovering nitrogen from wastewater includes an additional step of adding a base to the wastewater after a preset time has elapsed or after reaching a preset fraction. Preferably, the base is sodium hydroxide.

[0052] FIG. 5 is a diagram illustrating the process of capturing ammonium ions from an initial backflow water containing 1520 mg / L of NH4-N and 6060 meq / L of alkalinity. Newberite was added under degassing and the reaction was allowed to continue for 3 hours, which is indicated by curve portion 106. At this point, indicated as t1, only 200 mg / L of NH4-N remained in the solution. Subsequently, sodium hydroxide was added, which precipitated most of the remaining ammonium ions as struvite, possibly according to reaction (1), as indicated by curve portion 107. In this way, a very high fraction of ammonium ions is precipitated as struvite and can be removed from the solution by a separation step. At the same time, the amount of sodium hydroxide added is only a small amount compared to when sodium hydroxide is added directly to the initial water.

[0053] As described above, adding a base to the wastewater may cause side reactions to occur, for example, as given in reaction (2). Depending on the conditions, magnesium from the dissolved Newberite may even precipitate at least partially as magnesium hydroxide instead. However, such a process is typically relatively slow, and such side reactions can be limited by allowing the base to act on the wastewater for only a shorter period. If the precipitated struvite is separated relatively quickly after the Newberite treatment step is finished and the base is added, the degree of magnesium hydroxide precipitation can be kept very low.

[0054] A struvite precipitation reactor can be designed to be suitable for carrying out the above-described idea. FIG. 6a is a schematic diagram of parts of a struvite precipitation reactor device (10) for recovering nitrogen from wastewater. The struvite precipitation reactor device (10) includes a reactor vessel (20). The reactor vessel (20) has an inlet (22) for wastewater (200) into the reactor vessel (20) and an inlet (24) for Newberite (201) into the reactor vessel (20). The inlets (22, 24) may be combined into a single inlet.

[0055] The reactor vessel (20) further comprises a degassing device (30) configured to degas the reactor vessel (20). The degassing device (30) may be of different types. Non-exclusive examples include a vortex-generating stirrer, centrifugal degassing device, vacuum device, ultrasonic degassing device and degassing pump, or a combination thereof. In this embodiment, the degassing device (30) comprises a stirrer (32) that generates a vortex in the solution within the reactor vessel (20). Preferably, the degassing device (30) also comprises a gas duct (34) configured to remove carbon dioxide gas (202) escaping from the generated vortex. Typically, the removed carbon dioxide gas (202) is replaced by another gas, typically air (208), through a ventilation duct (35).

[0056] The reactor further includes a struvite separation device (40) configured to separate the struvite precipitated within the reactor vessel (20) from the wastewater. In this embodiment, the removal of the struvite (203) is carried out through the outlet (42). The clear water (204) from which the ammonium has been consumed is removed through the outlet (44).

[0057] In this embodiment, the reactor vessel (20) further includes an inlet (26) for a base (205) into the reactor vessel (20). This inlet (26) is activated after a preset time has elapsed after Newberite is introduced into the vessel, or when the molar content of residual ammonium ions in the wastewater in the solution decreases by a preset fraction. The addition of this base (205) initiates a second stage of struvite precipitation by increasing the pH in the solution.

[0058] The second stage reaction has two main functions. The first function is to reduce the effluent nitrogen to a desired level. The second function is to minimize the loss of dissolved Newberite. The loss of dissolved Newberite results in the loss of Mg and P, which must be replenished and added to the system.

[0059] Based on this principle, another desirable configuration is to connect two continuous stirred reactors in series and operate them continuously. One embodiment of this is shown in FIG. 6b. Here, the struvite precipitation reactor device (10) comprises a first reactor vessel (20A) and a second reactor vessel (20B). The first reactor vessel (20A) has an inlet (22A) for wastewater (200A) into the first reactor vessel (20A) and an inlet (24) for Newberite (201) into the first reactor vessel (20A). The inlets (22A, 24) may be combined into a single inlet.

[0060] The first reactor vessel (20A) further comprises a degassing device (30) configured to degassing the reactor vessel (20A) in a manner similar to the preceding embodiment. In this embodiment, the degassing device (30) comprises a stirrer (32) that generates a vortex in the solution within the first reactor vessel (20A). Preferably, the degassing device (30) also comprises a gas duct (34) configured to remove carbon dioxide gas (202) escaping from the generated vortex. Typically, the removed carbon dioxide gas (202) is replaced by another gas, typically air (208), through a ventilation duct (35).

[0061] In this very embodiment, the first reactor vessel (20A) further comprises a first struvite separation device (40A) configured to separate the struvite precipitated within the first reactor vessel (20A) from the wastewater (200A). The removal of the struvite (203) is carried out through the first outlet (42A). The wastewater (200B) from which the ammonium has been partially consumed is removed through the second outlet (44A) and supplied to the second reactor vessel (20B).

[0062] In an alternative embodiment, the first struvite separation device (40A) and the outlet (42A) are omitted. In this case, the second outlet (44A) is used to transfer the slurry of precipitated struvite in the wastewater in which the ammonium has been partially consumed to the second reactor vessel.

[0063] In this embodiment, the second outlet (44A) is connected to the inlet (22B) of the second reactor vessel (20B). The second reactor vessel (20B) further includes an inlet (26) for the base (205) to the second reactor vessel (20B). The transfer of the ammonium-depleted wastewater (200B) to the second reactor vessel is activated when a preset time has elapsed after the Newberite is introduced into the vessel, or when the molar content of the remaining ammonium ions in the wastewater (200B) in the solution decreases by a preset fraction. The addition of this base (205) initiates a second stage of struvite precipitation by increasing the pH in the solution.

[0064] The second reactor vessel (20B) further includes a second struvite separation device (40B) configured to separate the struvite precipitated within the second reactor vessel (20B) from the ammonium-partially consumed wastewater (200B). In this embodiment, the removal of the struvite (203) is carried out through the third outlet (42B). The clear water (204) from which the ammonium has been almost completely consumed is removed through the fourth outlet (44B).

[0065] In other words, Newberite is added for a preset residence time to a first reactor vessel, for example, where active degassing by vortex generation takes place. Then, the overflow from the first reactor vessel is processed in a second reactor for a preset residence time. The base is continuously added only to the second reactor vessel.

[0066] Table 3. Stage 2 Nitrogen Removal NaOH flow rate [mmol / min] pH Inflow NH 4 -N [mg / L] Leak NH 4 -N [mg / L] NaOH [% of initial N] 0.16 8.15 146 64.0 10.2 0.18 8.25 146 53.4 11.5 0.20 8.33 144 47.3 12.7 0.22 8.45 143 40.6 14.0 0.24 8.55 147 34.3 15.3

[0067] Experiments were conducted to investigate the effects of sodium hydroxide administration. A first-stage reaction with a retention time of approximately 5.4 hours was used. A molar ratio of Newberite to dissolved ammonium ions of 1.8:1 was used, and a vortex was generated at 550 RPM. As a result, an NH4-N content of 143 to 147 mg / L was obtained in the ammonium-partially consumed wastewater with a pH of 7.4. Subsequently, this ammonium-partially consumed wastewater was treated in a second stage by adding NaOH for approximately 10 minutes at different flow rates for multiple different samples. Afterward, the final pH and ammonium content were measured. The results are shown in Table 3.

[0068] These experiments demonstrate that the effluent NH4-N concentration can be controlled by the administration of NaOH. As the dosage increases, the effluent NH4-N decreases and the pH increases proportionally. However, the recovered NH4-N is not exactly equivalent to the moles of the administered hydroxide. In all cases, an excess of hydroxide is administered relative to the recovered NH4-N. pH, effluent NH4-N, and NaOH all vary, and the sodium hydroxide dosage increased by approximately 150% from the lowest dosage rate to the highest dosage rate, while the pH varied between 8.15 and 8.55.

[0069] To determine how Newberite loss is affected, the results were also analyzed for magnesium and phosphorus. As shown in Table 3, the fixed retention time experiments demonstrate that dissolved magnesium and phosphorus remained relatively constant in the effluent stream despite variations in the inflow values. The average effluent values ​​were approximately 40 mg / L for Mg and 58 mg / L for P. Therefore, at all different sodium hydroxide dosage values, dissolved Mg and P were reduced compared to the inflow stream.

[0070] The next parameter to investigate was the variation in residence time. Table 4 shows that dissolved Newberite decreases with decreasing residence time, as indicated by the fact that both Mg and P decrease with decreasing residence time. This is true for both sodium hydroxide dosing rates tested. At the same time, NH4-N and pH increase with decreasing residence time. The lowest dissolved Mg and P reached in these experiments were 17.6 and 31.4 mg / L, respectively. The loss can be calculated by comparing it to the values ​​of the original wastewater. This could likely be made even lower by further reducing the residence time. The first reaction that occurs when sodium hydroxide enters the solution appears to be the precipitation of a 1:1 Mg:P magnesium phosphate species.

[0071] The first six rows of Table 4 show that as the residence time increases, the precipitation of struvite increases, and the pH becomes higher for the same amount of added hydroxide. That is, as the residence time progresses, the magnesium phosphate species that initially precipitate are subsequently dissolved and reformed into struvite, but the pH and dissolved magnesium and phosphate also increase. Although the exact mechanism is not currently known in detail, it is clear that the residence time in the second reactor must be reduced to minimize Newberite loss.

[0072] In other words, in one embodiment, the step of separating the precipitated struvite from the wastewater is performed within a time of less than 30 minutes, preferably within a time of less than 5 minutes, after the step of adding the base to the wastewater.

[0073] Table 4. Residence time experiment Time of stay [min] NaOH rate [μL / min] NH 4 -N [mg / L] Mg [mg / L] P [mg / L] Na [mg / L] pH 10.0 240 36.7 47.7 66.9 69.8 8.4 8.0 240 41.0 33.7 50.0 52.5 8.8 5.4 240 49.4 22.5 35.6 37.2 9.0 10.0 300 18.3 42.9 61.9 64.9 9.1 8.0 300 26.1 31.0 47.0 49.2 9.3 5.4 300 34.6 17.6 31.4 33.0 9.5 M 240 95.1 36.6 59.8 62.0 7.6 M 300 78.9 61.2 93.6 96.9 7.6

[0074] To confirm that the loss and catchment advantages of the two-stage reactor are real, it is necessary to compare the results with those obtained when the same amount of sodium hydroxide is administered directly into the main reactor instead. The experiments presented in the last two rows of Table 4, labeled M instead of residence time, show that unlike the two-stage reactor system, using the same amount of sodium hydroxide directly into the main reactor results in an increase in both final NH4-N and dissolved Newberite values, and a significant decrease in pH. This holds true regardless of the residence time. This confirms that two reaction stages are used in this system to optimize the use of sodium hydroxide.

[0075] One of the advantages of using Newberite as an ammonium capture agent is that it is possible to regenerate Newberite from struvite. This is discussed, for example, in the published international patent application WO 2020 / 256622 A1. A similar idea may be applied in this context as well.

[0076] FIG. 7 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 and 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 is dissolved into the liquid solution by the acid. Alternatively, in step S70, Newberite is precipitated from the liquid solution. This can be carried out, for example, according to the principles set forth in the published international patent application WO 2020 / 256622 A1. In step S80, the precipitated Newberite is separated from the liquid solution to provide a residual ammonium salt solution. In step S90, the precipitated Newberite is recycled to the step of adding Newberite to the wastewater.

[0077] FIG. 8 schematically illustrates parts of an embodiment of a circulation device (1) for recovering nitrogen from wastewater. The circulation device (1) for recovering nitrogen from wastewater includes a struvite precipitation reactor device (10) and a struvite decomposition reactor (50). The struvite precipitation reactor device (10) is configured according to any embodiment of the technology presented elsewhere in this disclosure, for example, according to FIG. 6a or FIG. 6b.

[0078] In this embodiment, the struvite decomposition reactor (50) has an inlet (54) for struvite (203) and an inlet (52) for acid (206) connected directly or indirectly to the outlet (42) of the struvite precipitation reactor device (10). The struvite decomposition reactor (50) further has an outlet (58) for precipitated Newberite (201) and an outlet (56) for a liquid (207) containing anions of dissolved ammonium salt and acid (206). The struvite decomposition reactor (50) typically has a Newberite separation device (60) for separating the precipitated Newberite (201) and the liquid (207). Preferably, there is a recirculation device (70) configured to recirculate at least a portion of the precipitated Newberite (201) to the inlet (24) of the struvite precipitation reactor device (10). Such a recirculation device (70) may also include any type of storage device.

[0079] The precipitation of Newberite (201) from struvite (203) dissolved by acid (206) can be carried out according to any process known to a person skilled in the art. Some alternatives are presented, for example, in the published international patent application WO 2020 / 256622 A1.

[0080] In an alternative embodiment, the struvite decomposition reactor (50) may be integrated into the struvite precipitation reactor device (10). In such an embodiment, the struvite separation device (40) (or 40B if the embodiment of FIG. 6b is used) is configured to separate the struvite precipitated within the reactor vessel (20) (or 20B) from the wastewater. The clear water (204) from which the ammonium has been consumed is removed through the outlet (44) (or 44B), and the struvite is left within the reactor vessel (20) (or 20B). Then, the inlet (52) for the acid (206) introduces the acid into the combined reactor vessel (20) and the struvite decomposition reactor (50), and the decomposition process takes place.

[0081] The present idea of ​​recovering nitrogen in the form of ammonium from wastewater is based on the assumption that the wastewater has a significant alkalinity. This is also a common situation in many cases. Return water from biogas production almost always contains a high alkalinity sufficient for the idea presented herein. Therefore, in one embodiment, return water from biogas production is used as the input wastewater in this technology. Likewise, most wastewater from wastewater treatment plants also exhibits sufficient alkalinity to allow for the recovery of at least a significant portion of the ammonium content.

[0082] However, if the intention is to recover ammonium from wastewater that does not contain sufficient alkalinity to recover the required amount of ammonium, the alkalinity may be improved. Sodium carbonate and / or potassium carbonate are useful sources of alkalinity. By adding such substances to raw wastewater having low alkalinity, the alkalinity can be increased to a level suitable for the process. FIG. 9 shows a flowchart of the steps of an embodiment of a method for recovering nitrogen from wastewater. Most of the steps are similar to those described above. However, in this embodiment, step S10 of providing wastewater comprises two sub-steps. In step S12, raw wastewater is provided.

[0083] This wastewater itself may have low alkalinity. In step S14, a base is added to the raw wastewater to form wastewater having increased alkalinity. Preferably, the base is at least one of sodium carbonate, potassium carbonate, sodium hydroxide, and ammonium hydroxide. Most preferably, the base is at least one of sodium carbonate and potassium carbonate.

[0084] Another method of adding alkalinity to raw wastewater, typically for cost-saving reasons, may be to mix two waste streams: a stream with a high ammonium content and a stream with high alkalinity.

[0085] The embodiments described above should be understood as some exemplary examples of the present invention. A person skilled in the art will understand that various modifications, combinations, and changes may be made to the embodiments without departing from the scope of the invention. In particular, different partial solutions in different embodiments may be combined into different configurations where technically feasible. However, the scope of the invention is defined by the appended claims.

Claims

Claim 1 A method for recovering nitrogen from wastewater, comprising the steps of: providing wastewater (200) containing dissolved ammonium ions and at least one of carbonate ions and hydrocarbonate ions—wherein the ratio of alkalinity expressed in equivalents per liter in the wastewater (200) to the molar concentration of dissolved ammonium ions is at least 0.5:1—(S10); adding newberyite (201) to the wastewater (200) to cause a precipitation of struvite (203) (S20); degassing the wastewater (200) during at least a portion of the precipitation of the struvite (203) (S30); and separating the precipitated struvite (203) from the wastewater (S50). Claim 2 A method according to claim 1, wherein the step (S20) of adding the Newberite (201) to the wastewater (200) comprises adding the Newberite (201) in an amount such that the molar ratio of the Newberite (201) to the dissolved ammonium ions is at least 0.5:1, preferably at least 0.7:1, more preferably at least 1:1, and most preferably at least 2:

1. Claim 3 A method according to claim 1 or 2, characterized in that the precipitation of the struvite (203) takes place at a pH between 6.5 and 10. Claim 4 A method according to any one of claims 1 to 3, wherein the degassing (S30) is selected from forming a vortex with a stirrer (32); degassing by centrifugal force; vacuum degassing; ultrasonic degassing; and pump degassing. Claim 5 A method according to claim 4, characterized in that the degassing (S30) forms a vortex with a stirrer (32). Claim 6 A method according to any one of claims 1 to 5, wherein the degassing (S30) is performed during the entire time that the struvite (203) is precipitated. Claim 7 A method according to any one of claims 1 to 6, characterized in that the precipitation of the struvite (203) is allowed to continue for a preset time or until the molar content of residual ammonium ions in the wastewater in the solution is reduced by a preset fraction. Claim 8 A method characterized by further including the step (S40) of adding a base (205) to the wastewater (200) after the above-set time has elapsed or after the above-set fraction has been reached. Claim 9 A method according to claim 8, characterized in that the base (205) is sodium hydroxide. Claim 10 A method according to claim 8 or 9, wherein the step (S50) of separating the precipitated struvite (203) from the wastewater is performed within a time of less than 30 minutes, preferably within a time of less than 5 minutes, after the step (S40) of adding the base (26) to the wastewater (200). Claim 11 A method according to any one of claims 1 to 10, further comprising: a step (S60) of exposing the separated and precipitated struvite (203) to an acid (52) in a liquid solution—the acid (52) being at least one of sulfuric acid, phosphoric acid, nitric acid, and carbonic acid; a step (S70) of precipitating Newberite (201) from the liquid solution; a step (S80) of separating the precipitated Newberite (201) from the liquid solution to provide a residual ammonium salt solution (207); and a step (S90) of recirculating the precipitated Newberite (201) to the step (S20) of adding the Newberite (201) to the wastewater (200). Claim 12 A method characterized in that, in any one of claims 1 to 11, the wastewater (200) comprises reject water from biogas production. Claim 13 A method characterized in that, in any one of claims 1 to 12, the step (S10) of providing the wastewater (200) comprises: the step (S12) of providing original wastewater; and the step (S14) of adding alkalinity to the original wastewater, preferably by at least one of sodium carbonate, potassium carbonate, sodium hydroxide, and ammonium hydroxide, most preferably by at least one of sodium carbonate and potassium carbonate, to form the wastewater (200) having increased alkalinity. Claim 14 A struvite precipitation reactor device (10) for recovering nitrogen from wastewater (200), comprising: a reactor vessel (20); an inlet (22) for wastewater (200) into the reactor vessel (20); an inlet (24) for Newberite (201) into the reactor vessel (20) to enable a reaction with the wastewater (200); and a struvite separation device (40) configured to separate the struvite (203) precipitated from the wastewater within the reactor vessel (20); and a degassing device (30) configured to degassing the gas generated from the reaction between the Newberite (201) and the wastewater (200) in the reactor vessel (20). Claim 15 In claim 14, the above degassing equipment (30) is characterized as being one of a stirrer (32) that generates a vortex; a centrifugal degassing equipment; a vacuum equipment; an ultrasonic degassing equipment; and a degassing pump. Claim 16 In item 15, the device is characterized in that the above-mentioned degassing equipment (30) is a stirrer (32) that generates a vortex. Claim 17 A device characterized by an inlet (26) for a base (205) to a struvite precipitation reactor device (10), which is activated after a preset time has elapsed following the introduction of the Newberite (201) into the reactor vessel in any one of claims 14 to 16, or when the molar content of the residual ammonium ions in the wastewater in the solution decreases by a preset fraction. Claim 18 A circulation device (1) for recovering nitrogen from wastewater (200), comprising: a struvite precipitation reactor device (10) according to any one of claims 14 to 17; a struvite decomposition reactor (50) connected to or integrated with the struvite precipitation reactor device (10) and having an inlet (52) for an acid (206); wherein the struvite decomposition reactor (50) further comprises an outlet (58) for precipitated Newberite (201); and an outlet (56) for a liquid (207) containing anions of dissolved ammonium salt and the acid (52).

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