Recovery of nitrogen from reject water

Abstract

A method for recovering nitrogen from waste-water comprises providing (S10) of waste-water having dissolved ammonium ions A ratio between alkalinity, expressed in equivalents per liter, and molar concentration of dissolved ammonium ions in the waste-water is at least 0.5:1. Newberyite is added (S20) to the waste-water, which causes precipitation of struvite. The waste-water is degassed (S30) during at least a part of the precipitation of struvite. The precipitated struvite is then separated (S50) from the waste-water. A struvite precipitation reactor arrangement and a cyclic arrangement for recovering nitrogen from waste-water are also disclosed.

Description

[0001] RECOVERY OF NITROGEN FROM REJECT WATER

[0002] TECHNICAL FIELD

[0003] The present technology refers in general to recovery of commercial substances from reject substances, and in particular to methods and arrangements for recovery of nitrogen from reject water.

[0004] BACKGROUND

[0005] In the production of biogas, substantial amounts of reject water are produced. One reason for the large amounts of reject waters is that a high nitrogen content of above 3 000 ppm ammonium ions (NH4+) causes a severe toxicity for the biogas producing microorganisms. Therefore, water is added to biogas plants to dilute the nitrogen content to below the toxicity level which results in large volumes of reject water. This reject water typically comprises high concentrations of ammonium ions, which makes it unsuitable to be released into the environment.

[0006] At the same time, ammonia is a requested substance. Today, most ammonia production is based on the Haber-Bosch process, which requires large consumption of natural gas. Nitrogen-containing effluents should instead of being released be used as a nitrogen source.

[0007] In prior art, precipitation of struvite has been used for removal of mainly phosphorus but also nitrogen from waste material. This is obtained by adding a magnesium source and adjusting the pH to an alkaline pH. The increase in pH is usually performed by adding sodium hydroxide. Use of cheaper calcium hydroxide results in unwanted precipitation of calcium phosphates in preference to struvite and is thus not preferred. The struvite may then be processed into other more commercially valuable substances, e.g. according to the published international patent application WO 2020 / 256622 Al. In these processes, magnesium sources may be regained and may be recycled into the precipitation process again. The magnesium source may e.g. be newberyite.

[0008] In the published European Patent Application EP 2 431 336 Al, newberyite is used in combination with sodium hydroxide in a single reactor applying pH control.

[0009] Sodium hydroxide is a relatively expensive substance and as much as 60% of the total costs for the nitrogen recovery process may be costs for sodium hydroxide. Furthermore, the sodium hydroxide does not only assist in the precipitation of struvite but is also consumed by other reactions in typical reject water compositions. The high costs associated with the prior art struvite precipitation approach makes it less attractive for treatment of large amounts of reject waters.

[0010] Furthermore, the recycling of reject water treated by sodium hydroxide back to the biogas reactor for diluting the nitrogen content is also not feasible to any larger extent due to accumulation of sodium ions, which at high levels also have negative effect on the biogas producing microorganisms.

[0011] There is thus a need for improvements in recovery of nitrogen from liquid solutions.

[0012] SUMMARY

[0013] A general object of the present technology is thus to find improved processes for recovering of nitrogen from water solutions having high ammonium ion contents.

[0014] The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims. In general words, in a first aspect, a method for recovering nitrogen from waste-water comprises providing of waste-water having dissolved ammonium ions. A ratio between alkalinity, expressed in equivalents per liter, and molar concentration of dissolved ammonium ions in the waste-water is at least 0.5: 1. Newberyite is added to the waste-water, which causes precipitation of struvite. The waste-water is degassed during at least a part of the precipitation of struvite. The precipitated struvite is then separated from the waste-water.

[0015] In a second aspect, a struvite precipitation reactor arrangement for recovering nitrogen from waste-water comprises a reactor vessel, having an input for waste-water to the reactor vessel and an input for newberyite to the reactor vessel. The struvite precipitation reactor arrangement further comprises a degassing equipment, configured to degas the reactor vessel. The struvite precipitation reactor arrangement further comprises a struvite separation arrangement. The struvite separation arrangement is configured to separate struvite precipitated in the reactor vessel from the waste-water.

[0016] In a third aspect, a cyclic arrangement for recovering nitrogen from wastewater comprises a struvite precipitation reactor arrangement according to the second aspect and a struvite decomposition reactor. The struvite decomposition reactor is connected to or integrated in the struvite precipitation reactor arrangement and has an input for an acid. The struvite decomposition reactor further has an output for precipitated newberyite and an output for a liquid comprising a dissolved salt of ammonium and an anion of the acid.

[0017] One advantage with the proposed technology is that an efficient recovery of nitrogen from waste-water can be obtained with less utilization of sodium hydroxide. Other advantages will be appreciated when reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

[0019] FIG. 1 is a flow diagram of steps of an embodiment of a method for recovering nitrogen from waste-water;

[0020] FIG. 2 is a diagram illustrating ammonium nitrogen contents during a struvite precipitation process;

[0021] FIG. 3 is a diagram illustrating ammonium nitrogen content and pH changes during a struvite precipitation process;

[0022] FIG. 4 is a flow diagram of steps of another embodiment of a method for recovering nitrogen from waste-water;

[0023] FIG. 5 is a diagram illustrating ammonium nitrogen contents during a two-stage struvite precipitation process;

[0024] FIG. 6A is a schematic illustration of parts of an embodiment of a struvite precipitation reactor arrangement;

[0025] FIG. 6B is a schematic illustration of parts of another embodiment of a struvite precipitation reactor arrangement;

[0026] FIG. 7 is a flow diagram of steps of an embodiment of a recirculation method for recovering nitrogen from waste-water;

[0027] FIG. 8 is a schematic illustration of parts of an embodiment of a cyclic arrangement for recovering nitrogen from waste-water; and

[0028] FIG. 9 is a flow diagram of steps of yet another embodiment of a method for recovering nitrogen from waste-water.

[0029] DETAILED DESCRIPTION

[0030] Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

[0031] When struvite is precipitated from newberyite in alkaline pH, the presumed reaction is: MgHPO4-3H20(s) + 2H2O(l)+NH4+(aq) + OH-(aq) NH4MgPO4-6H20(s). (1)

[0032] However, using sodium hydroxide for increasing the pH may also give rise to other side reactions, for instance:

[0033] MgHPO4-3H20(s) + 2NaOH(aq)

[0034] Mg(OH)2(s) + Na3PO4(aq) + 3H2O(1). (2)

[0035] Instead of precipitation of struvite, which removes ammonium ions from the solution, magnesium hydroxide may be precipitated. Sodium hydroxide is thereby consumed without contributing to the removal of nitrogen. In order to avoid this “misuse” of sodium hydroxide, other reaction mechanisms can be used.

[0036] Waste-water from e.g. biogas production or wastewater treatment plants does normally comprise significant alkalinity. The concentration of ions in the water that neutralize the hydrogen ion is known as alkalinity. The most well-known alkalinity components are bicarbonate, carbonate, and hydroxide, respectively. The quantitative definition of alkalinity is A=[HCO3_] + 2[CO32] + [OH ] - [H+], i.e. expressed in equivalents per liter. The alkalinity is typically defined in milliequivalents per liter. The carbonate ion concentration is counted double, since the ion has the ability to take up two hydrogen ions. The definition of alkalinity may be simplified as being the buffer capacity of a solution, which when applied to wastewater typically is expressed in mg HCO3 / L. In the presence of alkalinity and in particular bicarbonate ions (i.e. hydrogen carbonate ions), the following reaction may take place:

[0037] MgHPO4-3H20(s) + 2H2O(l)+NH4+(aq) + HCO3(aq)

[0038] NH4MgPO4-6H20(s) + CO2(g) (3) Unfortunately, tests have shown that the mere addition of newberyite to waste-water with alkalinity only give rise to relatively slow and uncomplete reactions according to this reaction scheme.

[0039] However, it was surprisingly found that if degassing was performed during the process, speed and completeness increased considerably. It was concluded that the degassing contributed to a decrease of dissolved carbon dioxide gas in the water, which resulted in driving the reaction further in the direction of struvite precipitation. It was for instance found that the reaction stopped completely after some time if the reaction was performed in a sealed vessel.

[0040] Figure 1 illustrates a flow diagram of steps of an embodiment of a method for recovering nitrogen from waste-water. In step S10 waste-water having dissolved ammonium ions was provided. A ratio between alkalinity, expressed in equivalents per liter, and molar concentration of dissolved ammonium ions in the waste-water is at least 0.5: 1. A lower alkalinity slows down any reaction kinetics and reduces the total amount of ammonium possible to extract. The ratio of 0.5: 1 is presently considered as a lower limit for achieving a process suitable for industrial purposes. Preferably, the ratio between alkalinity and dissolved ammonium ions in the waste-water is at least 0.7: 1, and more preferably at least 1: 1, and ideally larger than 1.5: 1. The ratio gives an indication of the maximum amount of ammonium that can be extracted from the water.

[0041] In step S20, newberyite is added to the waste-water, causing precipitation of struvite. In a preferred embodiment, the step of adding newberyite to the waste-water comprises adding newberyite in an amount giving a molar ratio between newberyite 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.

[0042] In step S30, the waste-water is degassed. Step S30 overlaps at least to a part with step S20, i.e. the degassing takes place during at least a part of the precipitation of struvite. Preferably, the degassing takes place during the entire time of the precipitation of struvite.

[0043] In step S50, the precipitated struvite is separated from the waste-water. If there is any excess newberyite that has not reacted, this newberyite will essentially be in solid form and will be separated together with the struvite.

[0044] The reaction kinetics of the newbeiyite-ammonium reaction are dependent on the initial dosing of newberyite, the initial ammonium-nitrogen concentration and the initial alkalinity.

[0045] Figure 2 is a diagram illustrating the ammonium-nitrogen concentration in reject water from a wastewater treatment plant over time after addition of newberyite, while stirring the solution. Curve 101 corresponds to a molar ratio between initial newberyite and initial dissolved ammonium of 1.0. Curve 102 corresponds to a molar ratio between initial newberyite and initial dissolved ammonium of 1.4. Curve 103 corresponds to a molar ratio between initial newberyite and initial dissolved ammonium of 2. 1. It is thus seen for this reject water that the efficiency of the ammonium capture increases with an increased amount of newberyite, at least up to a molar ratio of 2. It was further noticed that there is diminishing returns on increasing the total newberyite solid content in the reactor above a certain limit.

[0046] The ratio of newberyite to ammonium is indeed of importance for the efficiency, but so is also the absolute concentration of newberyite in the mixture. For instance, for reject waters having a low ammonium content, the diminishing returns upon increased newberyite solid content seems to start at higher molar ratios.

[0047] Similar tests were also performed on water solutions comprising ammonium chloride dissolved in clean water without substantial alkalinity content. In such solution, no ammonium capture was detected. The alkalinity content is therefore essential for the capture to take place. This indicates that the above- mentioned reaction (3) between newberyite and bicarbonate ions is responsible for the main part of the ammonium capture. Since the molar ratio between newberyite and bicarbonate ions in the reaction (3) is 1: 1, a theoretical limit of the ammonium capture ability is equal to the existing amount of bicarbonate ions. A ratio between alkalinity and dissolved ammonium ions of 0.5: 1 thus allows at the most half the ammonium ions to be captured. In order to increase the captured amount, the alkalinity should therefore preferably be at least 1: 1 compared to the initial amount of ammonium ions. However, a large excess of alkalinity over this amount does not considerably improve the reaction completeness, nor decreasing it, but improves the kinetics of the reaction somewhat.

[0048] Figure 3 is a diagram illustrating the changed pH 104 during ammonium capture with newberyite by struvite precipitation from a waste-water obtained from a biogas production unit. Curve 105 indicated the Ammonium concentration in solution. After an initial drop of pH, the buffering capacity of the alkalinity comes into operation and the entire process in this particular example takes place essentially in the pH range of 6.5 to 8. Depending on the initial level of alkalinity in the waste-water, somewhat higher pH values may be present, sometimes up to a pH of 10.

[0049] In other words, in one embodiment, the precipitation of struvite takes place at a pH between 6.5 and 10.

[0050] As mentioned above, the reaction speed is influenced by degassing during the ammonium capture process. In a test experiment, 5 g of newberyite was added to two vessels containing 200 mL wastewater treatment plant reject water having a high alkalinity. One of the vessels was sealed and the other one was left open. Otherwise the conditions were the same. The initial reject water had an initial content of ammonium-nitrogen of 480 mg / L. After 18 hours, the ammonium-nitrogen content was again analyzed. As shown in Table 1, the water in the open vessel showed a content of ammonium-nitrogen of only 10 mg / L, while the sealed vessel still showed a content of ammonium-nitrogen of 228 mg / L. The increased content of carbon dioxide gas within the vessel thus significantly prohibited further degassing of the water, which in turn stopped the reaction to continue. Reaction in an open vessel will thus drive the ammonium capture process faster than without possibilities for exchanging the atmosphere.

[0051] Table 1. Ammonium capture by newberyite in open and sealed vessels

[0052] Scaling up the reaction in an open vessel having a volume of 1 m3resulted in significantly slower kinetics compared to the lab test. The reason is currently interpreted to be a slower degassing due to a smaller surface area to volume ratio upon upscaling. The kinetics were concluded too slow for an industrial application. Active degassing is necessary for obtaining kinetics sufficient for industrial implementation.

[0053] Active degassing may be performed in different ways, as such known in prior art. Some examples are creating a vortex with an agitator, degassing by centrifugal forces, vacuum degassing, ultrasonic degassing and pump degassing, or a combination thereof.

[0054] Some tests have been performed by different degassing principles. Most of them are indeed operable for the intended purpose, but some seem to be more efficient than others. In one experiment, a standard mixing arrangement comprising a reactor with baffles was used. A reduction of an initial NH4-N content of 879 mg / L into a final NH4-N content of 319 mg / L was obtained. When the experiment was repeated with the same conditions, but with baffles with aerators, it resulted in a final NH4-N content of 222 mg / L. Finally, when the same input conditions were applied to a vortex mixer, the final NH4-N content became 136 mg / L. It is presently considered that the centrifugal forces are responsible for pushing the dissolved CO2 out from the solution, as well as incorporating air into the vortex exchanging the CO2, and thereby improve the conditions for a fast ammonium-capture reaction.

[0055] In other words, in one embodiment, the degassing is performed by creating a vortex with an agitator.

[0056] Table 2. Ammonium extraction under different degassing approaches. In the published European Patent Application EP 2 431 336 Al, degassing of the initial ammonium-ion-containing water before addition of sodium hydroxide and newberyite was employed. Experiments were therefore also made comparing performing of degassing before the newberyite was added to the water with performing degassing during the actual reaction. Table 2 shows some result from these tests. Samples denoted by “A” refer to samples that were allowed to be degassed before vortexing, but not during vortexing. Samples denoted by “B” refer to samples where degassing was performed during the vortexing. The sample numbers 1, 2 and 3 denoted different vortexing speed. From these tests, it is easily seen that degassing during the vortexing significantly decreases the remaining ammonium-nitrogen in solution.

[0057] As concluded from the experiments made on addition of newberyite to water having ammonium ions and alkalinity, no complete capture of ammonium ions was achieved within a reasonable time, for being industrially acceptable, even if the amounts of alkalinity and newberyite were sufficient. In order to make the process applicable in industrial contexts, it is therefore advisable to interrupt the process at some stage. Such an interruption may e.g. be made after a predetermined reaction time. Alternatively, the reaction could be allowed to continue until less than a certain fraction of remaining ammonium ions in solution was obtained. Reasonable fractions could be in the order of 5- 25%.

[0058] In other words, in one embodiment, the precipitation of struvite is allowed to continue for a predetermined time or until an ammonium ion molar content remaining in solution in the waste-water is reduced by a predetermined fraction.

[0059] If the process is terminated in this stage, the waste-water will still comprise some concentration of ammonium. In one embodiment, an additional step may therefore be performed. Figure 4 is a flow diagram of steps of an embodiment of a method for recovering nitrogen from waste-water. Besides the steps already described in connection with Figure 1, there is an additional step S40 that is performed after the step S30 is terminated. In this step, most of the last remains for the ammonium ions in the solution is removed by adding a base and thereby increasing the pH considerably. This step is, as such, known in prior art, but then for the entire process. By instead initiating this step only when a small fraction of the initial ammonium ions is remaining in solution, the disadvantages of prior art approaches are reduced. For instance, the amount of base needed for precipitating the remaining ammonium ions is only a fraction of what would have been needed if the base would have been used on the initial water. The reaction (3) above reduces the buffer capacity of the alkalinity present in the wastewater. This saves the hydroxide ions from being “buffered” and can instead be used in e.g. reaction (1).

[0060] In other words, in one embodiment, the method for recovering nitrogen from waste-water comprises the further step of adding a base, after the predetermined time or after the predetermined fraction is reached, to the waste-water. Preferably, the base is sodium hydroxide.

[0061] Figure 5 is a diagram illustrating a process of ammonium ion capture from an initial reject water comprising 1520 mg / L NH4-N and an alkalinity of 6060 meq / L. Newberyite was added under degassing and the reaction was allowed to continue for 3 hours, illustrated by curve portion 106. At this stage, denoted as tl, only 200 mg / L of NH4-N remained in solution. Sodium hydroxide was then added, which caused most of the remaining ammonium ions to precipitate as struvite, probably according to the reaction ( 1), as indicated by curve portion 107. In this way, a very high fraction of the ammonium ions is precipitated as struvite and can be removed from the solution by the separating step. At the same time, the amount of sodium hydroxide that is added is just a small amount compared to if sodium hydroxide would have been added directly to the initial water.

[0062] As mentioned above, the addition of a base to the waste-water may also cause side reactions to happen, e.g. as given in reaction (2). The conditions may even be so that magnesium from dissolved newberyite at least to a part instead may precipitate as magnesium hydroxide. However, such processes are typically relatively slow and by letting the base act on the waste-water only during a shorter period of time, such side reactions may be limited. If the precipitated struvite is separated relatively soon after the newberyite treatment step ends and the base is added, the degree of magnesium hydroxide precipitation can be kept very low.

[0063] A struvite precipitation reactor can be designed to be suitable for performing the above-described ideas. Figure 6A is a schematic drawing of parts of a struvite precipitation reactor arrangement 10 for recovering nitrogen from waste-water. The struvite precipitation reactor arrangement 10 comprises a reactor vessel 20. The reactor vessel 20 has an input 22 for waste-water 200 to the reactor vessel 20 and an input 24 for newberyite 201 to the reactor vessel 20. The inputs 22, 24 may also be combined into a single input.

[0064] The reactor vessel 20 further comprises a degassing equipment 30, configured to degas the reactor vessel 20. The degassing equipment 30 can be of different kinds. Non-exclusive examples are an agitator creating a vortex, a centrifugal degassing equipment, a vacuum equipment, an ultrasonic degassing equipment and degassing pumps, or a combination of these. In the present embodiment, the degassing equipment 30 comprise an agitator 32 creating a vortex in a solution in the reactor vessel 20. Preferably, the degassing equipment 30 also comprises a gas duct 34 configured for removing carbon dioxide gas 202 escaping from the created vortex. Typically, the removed carbon dioxide gas 202 is replaced by another gas, typically air 208, through a venting duct 35.

[0065] The reactor further comprises a struvite separation arrangement 40, configured to separate struvite precipitated in the reactor vessel 20 from the waste-water. Removal of the struvite 203 is in this embodiment made through an output 42. Clear water 204, depleted from ammonium, is removed through an output 44. In the present embodiment, the reactor vessel 20 further comprises an input 26 for a base 205 to the reactor vessel 20. This input 26 is operational after a predetermined time after input of the newberyite into the vessel or when an ammonium ion molar content remaining in solution in the waste-water is reduced by a predetermined fraction. Such an addition of a base 205 initiates a second stage of the struvite precipitation by means of increasing pH in the solution.

[0066] The second stage reaction has two main functions. A first function is to lower the outgoing nitrogen to a desired level. The second function is to to minimize the losses of dissolved newberyite. Loss of dissolved newberyite gives losses of Mg and P that need to be compensated and added to the system.

[0067] According to these principles another preferred setup is to operate continuously by connecting two continuously stirred reactors in series. An embodiment of this is illustrated in Figure 6B. The struvite precipitation reactor arrangement 10 here comprises a first reactor vessel 20A and a second reactor vessel 20B. The first reactor vessel 20A has an input 22A for wastewater 200A to the first reactor vessel 20A and an input 24 for newberyite 201 to the first reactor vessel 20A. The inputs 22A, 24 may also be combined into a single input.

[0068] The first reactor vessel 20A further comprises a degassing equipment 30, configured to degas the reactor vessel 20A in analogy with earlier embodiment. In the present embodiment, the degassing equipment 30 comprise an agitator 32 creating a vortex in a solution in the first reactor vessel 20A. Preferably, the degassing equipment 30 also comprises a gas duct 34 configured for removing carbon dioxide gas 202 escaping from the created vortex. Typically, the removed carbon dioxide gas 202 is replaced by another gas, typically air 208, through a venting duct 35.

[0069] In this very embodiment, the first reactor vessel 20A further comprises a first struvite separation arrangement 40A, configured to separate struvite precipitated in the first reactor vessel 20A from the waste-water 200A. The removal of the struvite 203 is made through a first output 42A. Waste-water 200B, partially depleted from ammonium, is removed through a second output 44A to be provided to a second reactor vessel 20B.

[0070] In an alternative embodiment, the first struvite separation arrangement 40A is omitted as well as the output 42A. The second output 44A is then used for transferring a slurry of precipitated struvite in waste-water, partially depleted from ammonium, to the second reactor vessel.

[0071] In the present embodiment, the second output 44A is connected to an input 22B of a second reactor vessel 20B. The second reactor vessel 20B further comprises an input 26 for a base 205 to the second reactor vessel 20B. The transfer of the waste-water 200B, partially depleted from ammonium, to the second reactor vessel is operational after a predetermined time after input of the newberyite into the vessel or when an ammonium ion molar content remaining in solution in the waste-water 200B is reduced by a predetermined fraction. Such an addition of a base 205 initiates the second stage of the struvite precipitation by means of increasing pH in the solution.

[0072] The second reactor vessel 20B further comprises a second struvite separation arrangement 40B, configured to separate struvite precipitated in the second reactor vessel 20B from the waste-water 200B partially depleted from ammonium. Removal of the struvite 203 is in this embodiment made through a third output 42B. Clear water 204, almost entirely depleted from ammonium, is removed through a fourth output 44B.

[0073] The newberyite is in other words added to the first reactor vessel which has active degassing e.g. by creating a vortex, and for a pre-determined retention time. The overflow from the first reactor vessel is thereafter treated in the second reactor for a pre-determined retention time. Base is continuously added only to the second reactor vessel.

[0074] Table 3. Second stage nitrogen removal.

[0075] An experiment was performed, where the effect of the sodium hydroxide dosing was investigated. A first stage reaction was used with a retention time of approximately 5.4 hours. A molar ratio between newberyite and dissolved ammonium ions of 1.8: 1 was used and with a vortex generated at 550 RPM. This resulted in a NH4-N content of 143-147 mg / L in a partially ammonium depleted wastewater, having a pH of 7.4. This partially ammonium depleted wastewater was thereafter treated in a second stage by adding NaOH during about 10 minutes with different flow rates for a number of different samples. The final pH and ammonium content was then measured. The result is shown in Table 3.

[0076] These experiments show that the outgoing NH4-N concentration can be controlled by the NaOH dosing. As dosing increases, the outgoing NH4-N decreases and pH increases proportionally. However, the recovered NH4-N is not exactly equivalent to the moles of hydroxide dosed. In all cases, an excess of hydroxide is dosed for the recovered NH4-N. The pH, outgoing NH4-N and NaOH are all varying, where the sodium hydroxide dosing increased by - 150% from the lowest to the highest dosing ratio and the pH varied between 8.15 and 8.55.

[0077] The results were also analysed for magnesium and phosphorous to see how the newberyite losses were impacted. The fixed retention time experiments as seen in Table 3 show that the dissolved magnesium and phosphorous remained relatively constant in the outgoing stream even with a variation in the incoming values. The average outgoing value for Mg was ~40 mg / L and for P was ~58 mg / L. Thus, with all different sodium hydroxide dosing values, dissolved Mg and P was reduced when compared to the incoming stream.

[0078] The next parameter to investigate was variations in retention time. Table 4 shows that dissolved newberyite decreases with decreasing retention time, as indicated by the decrease of both Mg and P with decreased retention time. This is true for both sodium hydroxide dosing rates tested. Simultaneously NF -N and pH increase with decreasing retention time. The lowest dissolved Mg and P reached in these experiments were 17.6 and 31.4 mg / L respectively. The losses can be calculated by comparing with values of the original waste-water. This could possibly be pushed lower by lowering the retention time even further. It appears that the first reaction to take place when sodium hydroxide enters solution is the precipitation of a 1: 1, Mg:P magnesium phosphate species.

[0079] The first six rows in table 4 shows that longer retention times mean more precipitation of struvite, and a higher pH for the same amount of hydroxide added. So as retention time progresses, the initial magnesium phosphate species that precipitates, subsequently dissolves and reforms as struvite but also increases pH and dissolved magnesium and phosphate. The exact mechanism is presently not known in detail, however, it is clear that in order to minimize newberyite losses, the retention time in the second reactor should be lowered.

[0080] In other words, in one embodiment, the step of separating the precipitated struvite from the waste-water is performed less than 30 minutes, preferably less than 5 minutes, after the step of adding the base to the waste-water.

[0081] Table 4. Retention time experiments.

[0082] To ensure that the losses and catchment benefits of a two-stage reactor are real, they need to be compared to results when the same amount of sodium hydroxide is instead dosed directly into the main reactor. Experiments presented at the two last rows of Table 4, marked with an M instead of a retention time, shows that when the same amount of sodium hydroxide is used directly in a main reactor, as opposed to the two-stage reactor system, the final NH4-N and dissolved newberyite values are all increased, with a significant pH decrease. This is regardless of which retention time it is compared with. This confirms that two reaction steps are used to make optimal use of sodium hydroxide in this system.

[0083] One of the advantages of using newberyite as agent for capturing ammonium is that it is possible to regenerate newberyite again from struvite. This is e.g. discussed in the published international patent application WO 2020 / 256622 Al. Similar ideas can be applied also in the present context.

[0084] Figure 7 is a flow diagram of steps of an embodiment of a method for recovering nitrogen from waste-water. Steps S10, S20, S30 and S50 and optionally step S40 follow the previous description. In the additional step S60, the separated precipitated struvite is exposing 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. Instead, in step S70, newberyite is precipitated from the liquid solution. This can be performed e.g. according to the principles presented in the published international patent application WO 2020 / 256622 Al. In step S80, precipitated newberyite is separating from the liquid solution, giving a remaining ammonium salt solution. In step S90, the precipitated newberyite is recirculated to the step of adding newberyite to the waste-water.

[0085] Figure 8 illustrates schematically parts of an embodiment of a cyclic arrangement 1 for recovering nitrogen from waste-water. The cyclic arrangement 1 for recovering nitrogen from waste-water comprises a struvite precipitation reactor arrangement 10 and a struvite decomposition reactor 50. The struvite precipitation reactor arrangement 10 is configured according to any embodiment of the technology presented elsewhere in the present disclosure, e.g. according to Fig. 6A or 6B.

[0086] The struvite decomposition reactor 50 has in this embodiment an input 54 for struvite 203 directly or indirectly connected to the output 42 of the struvite precipitation reactor arrangement 10 and an input 52 for an acid 206. The struvite decomposition reactor 50 further having an output 58 for precipitated newberyite 201 and an output 56 for a liquid 207 comprising a dissolved salt of ammonium and an anion of the acid 206. The struvite decomposition reactor 50 typically has a newberyite separation arrangement 60 for separating precipitated newberyite 201 and the liquid 207. Preferably, there is a recirculation arrangement 70 configured to recirculate at least a part of the precipitated newberyite 201 to the input 24 of the struvite precipitation reactor arrangement 10. Such recirculation arrangement 70 may also comprise any kind of storage arrangements.

[0087] The precipitation of newberyite 201 from struvite 203 dissolved by the acid 206 can be performed according to any process known by a person skilled in the art. Some alternatives are e.g. presented in the published international patent application WO 2020 / 256622 Al.

[0088] In an alternative embodiment, the struvite decomposition reactor 50 can be integrated in the struvite precipitation reactor arrangement 10. In such an embodiment, the struvite separation arrangement 40 (or 40B if the embodiment of Fig. 6B is used) is configured to separate struvite precipitated in the reactor vessel 20 (or 20B) from the waste-water. Clear water 204, depleted from ammonium, is removed through an output 44 (or 44B), leaving the struvite behind in the reactor vessel 20 (or 20B). The input 52 for an acid 206 then leads the acid to the combined reactor vessel 20 and struvite decomposition reactor 50 and the decomposition process takes place.

[0089] The present ideas of recovering nitrogen in the form of ammonium from wastewater are based on the assumption that there is a considerable alkalinity in the waste-water. This is also typically the situation in many cases. Reject water from biogas production does almost always comprise high alkalinity, sufficient for the here presented ideas. Thus, in one embodiment, reject water from biogas production is used as the input waste-water in the present technology. Likewise, most waste-water from waste-water treatment plants do also present an alkalinity that is sufficient to allow recovery of at least a considerable part of the ammonium content.

[0090] However, if ammonium is to be recovered from waste-water that do not comprise enough alkalinity for recovering requested amounts of the ammonium, the alkalinity may be improved. Sodium carbonate and / or potassium carbonate are useful sources of alkalinity. By adding such substances to an original waste-water having a low alkalinity, the alkalinity can be increased to a level suitable for the present processes. In Figure 9, a flow diagram of steps of an embodiment of a method for recovering nitrogen from waste-water is illustrated. Most steps are similar to the previously described ones. However, step S10 of providing waste-water comprises in this embodiment two part steps. In step S12, an original waste-water is provided. This waste-water may in itself have a low alkalinity. In step S14, a base is added to the original waste-water to form the waste-water with an 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.

[0091] Another way to add alkalinity to the original waste-water can be to mix two waste streams, one that is high in ammonium with a stream that is high in alkalinity, typically for cost saving reasons.

[0092] The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims

CLAIMS1. A method for recovering nitrogen from waste-water, comprising the steps of:- providing (S10) waste-water (200) having dissolved ammonium ions and at least one of carbonate ions and hydrocarbonate ions; wherein a ratio between alkalinity, expressed in equivalents per liter, and molar concentration of dissolved ammonium ions in said waste-water (200) is at least 0.5: 1;- adding (S20) newberyite (201) to said waste-water (200), causing precipitation of struvite (203);- degassing (S30) said waste-water (200) during at least a part of said precipitation of struvite (203); and- separating (S50) said precipitated struvite (203) from said wastewater.

2. The method according to claim 1, characterized in that said step of adding (S20) newberyite (201) to said waste-water comprises adding newberyite (201) in an amount giving a molar ratio between newberyite (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 said precipitation of struvite (203) takes place at a pH between 6.5 and 10.

4. The method according to any one of the claims 1 to 3, characterized in that said degassing (S30) is selected from: creating a vortex with an agitator (32); degassing by centrifugal forces; vacuum degassing; ultrasonic degassing; and pump degassing.

5. The method according to claim 4, characterized in that said degassing (S30) is creating a vortex with an agitator (32).

6. The method according to any one of the claims 1 to 5, characterized in that said degassing (S30) takes place during the entire time of said precipitation of struvite (203).

7. The method according to any one of the claims 1 to 6, characterized in that said precipitation of struvite (203) is allowed to continue for a predetermined time or until an ammonium ion molar content remaining in solution in said waste-water is reduced by a predetermined fraction.

8. The method according to claim 7, characterized by the further steps of:- adding (S40) a base (205), after said predetermined time or after said predetermined fraction is reached, to said waste-water (200).

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

10. The method according to claim 8 or 9, characterized in that said step of separating (S50) said precipitated struvite (203) from said waste-water is performed less than 30 minutes, preferably less than 5 minutes, after said step of adding (S40) said base (26) to said waste-water (200).

11. The method according to any one of the claims 1 to 10, characterized by the further steps of:- exposing (S60) said separated precipitated struvite (203) to an acid (52) in a liquid solution; wherein said acid (52) being at least one of sulfuric acid, phosphoric acid, nitric acid and carbonic acid;- precipitating (S70) newberyite (201) from said liquid solution;- separating (S80) precipitated newberyite (201) from said liquid solution, giving a remaining ammonium salt solution (207); and- recirculating (S90) said precipitated newberyite (201) to said step of adding (S20) newberyite (201) to said waste-water (200).

12. The method according to any one of the claims 1 to 11, characterized in that said waste-water (200) comprises reject water from biogas production.

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

14. A struvite precipitation reactor arrangement (10) for recovering nitrogen from waste-water (200), comprising:- a reactor vessel (20);- an input (22) for waste-water (200) to said reactor vessel (20);- an input (24) for newberyite (201) to said reactor vessel (20) for enabling a reaction with said waste-water (200); and- a struvite separation arrangement (40), configured to separate struvite (203) precipitated in said reactor vessel (20) from said waste-water, characterized by- a degassing equipment (30), configured to degas said reactor vessel (20) of gases resulting from said reaction between newberyite (201) and said waste-water (200).

15. The arrangement according to claim 14, characterized in that said degassing equipment (30) is one of: an agitator (32) creating a vortex;a centrifugal degassing equipment; a vacuum equipment; an ultrasonic degassing equipment; and degassing pumps.

16. The arrangement according to claim 15, characterized in that said degassing equipment (30) is an agitator (32) creating a vortex.

17. The arrangement according to any of the claims 14 to 16, characterized by an input (26) for a base (205) to said struvite precipitation reactor arrangement (10), operational after a predetermined time after input of said newberyite (201) into said reactor vessel or when an ammonium ion molar content remaining in solution in said waste-water is reduced by a predetermined fraction.

18. A cyclic arrangement (1) for recovering nitrogen from waste-water (200), comprising:- a struvite precipitation reactor arrangement (10) according to any of the claims 14 to 17;- a struvite decomposition reactor (50) being connected to or integrated in said struvite precipitation reactor arrangement (10) and having an input (52) for an acid (206); said struvite decomposition reactor (50) further having an output (58) for precipitated newberyite (201) and an output (56) for a liquid (207) comprising a dissolved salt of ammonium and an anion of said acid (52).

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