CN115362135A

CN115362135A - Method for treating water to be treated and method for treating wastewater comprising the same

Info

Publication number: CN115362135A
Application number: CN202180025040.XA
Authority: CN
Inventors: 村木务
Original assignee: Sumitomo Metal Mining Engineering Co Ltd
Current assignee: Sumitomo Metal Mining Engineering Co Ltd
Priority date: 2020-03-27
Filing date: 2021-03-05
Publication date: 2022-11-18
Anticipated expiration: 2041-03-05
Also published as: CN115362135B; KR102646503B1; WO2021192246A1; JP7010564B1; KR20220146712A; WO2021192922A1; JPWO2021192922A1

Abstract

The invention provides a method for effectively and stably obtaining a clarified liquid to which ammonia stripping can be applied when wastewater treatment is performed on a liquid to be treated containing high-concentration ammonia nitrogen, COD components, SS and other solids such as anaerobic digestion liquid. The present invention is a method for treating a liquid to be treated containing ammoniacal nitrogen at a concentration of 1000mg/L or more, COD at a concentration of 2000mg/L or more, and solids, the method comprising: adding an inorganic flocculant and an acid to a liquid to be treated to adjust the pH to 3 or less, and dissolving all or a part of the inorganic flocculant to separate the inorganic flocculant from the liquid; neutralizing the separated liquid after removing the solids, and separating the precipitate thus formed.

Description

Method for treating water to be treated and method for treating wastewater comprising the same

Technical Field

The present invention relates to a method for treating a liquid to be treated containing high concentrations of ammoniacal nitrogen and COD components and solids. In particular to a treatment method for obtaining a clarified liquid to which ammonia stripping can be effectively applied in the wastewater treatment of the liquid to be treated, and a wastewater treatment method comprising the treatment method.

Background

Anaerobic digestion solution is waste water produced by biologically treating organic matter such as human excrement and livestock waste in an anaerobic state, recovering methane gas such as methane as an energy source, and discharging the resultant. Since the anaerobic digestion solution contains suspended matter (SS fraction) at a high concentration of several thousand to several tens of thousands mg/L or more and ammonia nitrogen at a high concentration of several thousand mg/L derived from proteins and the like, it is necessary to perform wastewater treatment. Specifically, as a method for treating such wastewater, a biological treatment method is generally performed.

In a common biological treatment method, BOD: n =100: about 5 is preferable. However, in anaerobic digestate, ammoniacal nitrogen is present in high concentrations, BOD: n = N: about 1 is often the case, and the ratio of nitrogen is higher than the normal appropriate ratio, so that it is difficult to apply the biological treatment method.

Therefore, as a wastewater treatment for anaerobic digestion liquid, it is considered to improve the BOD/N ratio by selectively reducing the nitrogen concentration by performing ammonia stripping. For example, patent document 1 proposes a method including the following steps: ammonia dispersing step, controlling the pH value of the ammonia-containing wastewater containing solid and organic matters within the range of 7-12 to disperse ammonia; an ammonia dissolving step of allowing the absorption water to absorb the ammonia gas dispersed in the gas; and a biological nitrogen removal step of decomposing and removing ammonia by nitritation and anammox treatment of the ammonia water having absorbed ammonia gas.

However, in the method as in patent document 1, ammonia stripping is performed in a state of containing solids, and therefore, a general ammonia stripping column (dissipation column) cannot be used. Because if solids are present, a malfunction such as a blockage may occur in the apparatus. Therefore, patent document 1 discloses a method of heating the liquid to be treated to a temperature in the range of 30 to 100 ℃ by a heating method, preferably a method of promoting the gas diffusion by gas stirring, mechanical stirring, steam injection, heating, and the like, whereby the gas diffusion of ammonia gas from the liquid to be treated can be efficiently performed in the range of pH7 to 10, and the amount of the alkaline agent to be used can be reduced. However, there is a risk of lowering the stripping efficiency as compared with a dissipation column in which the subject liquid is brought into countercurrent contact with steam.

In addition, the dissipated ammonia gas is generally not dissolved in the solution, but is burned directly in the form of gas using a catalyst, which can be disposed of at low cost. However, in the technique of patent document 1, the solution is made to absorb the dissipated gas (ammonia dissolving step), and biological nitrogen removal treatment (biological nitrogen removal step) is applied to the solution. It is presumed that this is because the manner of stripping is different from that in the ordinary dissipation tower premised on the continuous dissipation treatment, and therefore it is difficult to apply the ammonia combustion system.

As described above, in order to efficiently perform ammonia treatment, it is preferable to use a dissipation tower and burn the dissipated ammonia gas. For this purpose, it is necessary to separate the solids in a previous stage to obtain a clear separation liquid. However, anaerobic digestate has a high SS concentration and a negatively charged surface, and is not only inferior in cohesiveness but also in dehydration properties, and thus is difficult to separate solid from liquid.

Patent document 2 proposes a method of performing activated sludge treatment on a liquid obtained by diluting an anaerobic treatment liquid (digestion liquid) of human excrement 15 times with water, and performing coagulation sedimentation treatment on the treated water. In this method, although a predetermined treatment effect is obtained, since the anaerobic digestion solution is diluted 15 times and subjected to activated sludge treatment and then to coagulation sedimentation, the SS concentration is reduced to about 40mg/L at the inlet of the coagulation sedimentation. Diluting the anaerobic digestion solution to such a degree that the SS concentration is lowered requires not only a large amount of dilution water but also a large-scale apparatus in the subsequent treatment. Therefore, it is preferable to perform solid-liquid separation without dilution as much as possible.

In addition, patent document 3 proposes a method of aggregating SS portions and separating solid and liquid by treating with a polymer flocculant alone, as a treatment of wastewater having a high organic nitrogen content in SS and a high organic nitrogen concentration in wastewater. The organic wastewater containing the digestion liquid is preferably coagulated and subjected to solid-liquid separation by a treatment with a polymer coagulant alone.

However, according to the example described in patent document 3, the polymer flocculant is added at a ratio of about 2% to the total evaporation residue of the wastewater. In the case of the anaerobic digestion solution, the concentration of the total evaporation residue is usually 2 to 3 mass% although the case may be different, and therefore, when the concentration of the total evaporation residue is assumed to be 2 mass%, and the polymer flocculant is added in an amount of 2% relative to the total evaporation residue, the concentration of the polymer flocculant after the addition becomes 400mg/L. The amount of the polymer flocculant added was 400mg/L, which is much larger than the amount of the polymer flocculant added in the ordinary wastewater treatment. Since the polymer flocculant has a higher unit price than an inorganic flocculant, the addition of a high concentration of the polymer flocculant increases the cost. In the above examples, the SS recovery rate was 82 to 99% with respect to the SS concentration in the raw water, and therefore the separated liquid contained at least several hundred mg/L of SS. That is, no clear liquid was obtained.

As described above, the method proposed in patent document 3 is not a method for efficiently recovering a solid substance, unlike a method for obtaining a clear liquid. Although the technical advantage of solid-liquid separation by treatment with a polymer flocculant alone is great, it is difficult to say that the method is effective in view of cost and clarity of the separated liquid.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 2010-000444

Patent document 2: japanese examined patent publication No. 53-041463

Patent document 3: international publication No. 2018/199330

Disclosure of Invention

Problems to be solved by the invention

The present invention has been made in view of such circumstances, and an object thereof is to provide a method for efficiently and stably obtaining a clarified liquid to which ammonia stripping is applicable when a liquid to be treated containing high-concentration ammonia nitrogen, COD components, and solids such as SS, for example, an anaerobic digestion liquid, is subjected to wastewater treatment.

Means for solving the problems

The present inventors have intensively studied to solve the above problems. As a result, the inventors have found that a clear liquid to which ammonia stripping can be applied can be efficiently and stably obtained by adding an inorganic flocculant and an acid to a liquid to be treated to adjust the pH to a predetermined pH or lower, dissolving all or a part of the inorganic flocculant to separate the inorganic flocculant into a solid and liquid, and then subjecting the separated liquid to neutralization treatment, and have completed the present invention.

(1) A first aspect of the present invention is a method for treating a liquid to be treated containing ammoniacal nitrogen at a concentration of 1000mg/L or more, COD at a concentration of 2000mg/L or more, and solids, the method comprising: adding an inorganic coagulant and an acid to the liquid to be treated to adjust the pH to 3 or less, and dissolving all or a part of the inorganic coagulant to perform solid-liquid separation; and a step of neutralizing the separated liquid from which the solid matter has been separated and removed, and separating the formed precipitate.

(2) The second invention of the present invention is a treatment method using an aluminum salt as the inorganic coagulant in the first invention.

(3) The third invention of the present invention is a method of treating the liquid to be treated as described in the first or second invention as an anaerobic digestion liquid.

(4) A fourth aspect of the present invention is the treatment method according to any one of the first to third aspects, wherein when the liquid to be treated contains a sulfide, the sulfide precipitate contained in the liquid to be treated is removed under a condition that the pH is in a neutral region before the acid is added to the liquid to be treated.

(5) The fifth aspect of the present invention is a method for treating wastewater containing a liquid to be treated containing ammoniacal nitrogen at a concentration of 1000mg/L or more, COD at a concentration of 2000mg/L or more, and solids, the method comprising: adding an inorganic coagulant and an acid to the liquid to be treated to adjust the pH to 3 or less, and dissolving all or a part of the inorganic coagulant to perform solid-liquid separation; neutralizing the separated liquid from which the solid is removed, and separating the generated precipitate; and a step of subjecting the separated liquid from which the precipitate has been separated and removed to ammonia stripping treatment.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a clarified liquid to which ammonia stripping can be applied can be efficiently and stably obtained. In addition, since the clear liquid can be obtained by efficiently separating the solid matter, the COD concentration can be effectively reduced.

Detailed Description

Hereinafter, a specific embodiment of the present invention (hereinafter, referred to as "the present embodiment") will be described in detail. The present invention is not limited to the following embodiments, and can be modified as appropriate within a range not changing the gist of the present invention. In addition, in this specification, unless otherwise specified, a symbol of "x to y" (x and y are arbitrary numerical values) means "x or more and y or less".

Method for treating Water to be treated (method for pretreating Ammonia stripping)

The treatment method of the present embodiment is a treatment method for a liquid to be treated containing ammoniacal nitrogen, and is a treatment method preferable as a pretreatment step in a wastewater treatment method including an ammonia stripping treatment step.

Specifically, the liquid to be treated contains ammoniacal nitrogen at a concentration of 1000mg/L or more, COD at a concentration of 2000mg/L or more, and solids. Examples of the liquid to be treated include anaerobic digestion liquids. As described above, the anaerobic digestion solution is waste water discharged from a treatment of biologically treating organic matter such as human excrement and livestock waste in an anaerobic state and recovering methane gas such as methane as an energy source.

In the treatment of wastewater for a liquid to be treated such as an anaerobic digestion liquid, a biological treatment method is generally used. However, the liquid to be treated contains ammonia nitrogen in a high concentration. Therefore, prior to the biological treatment, it is necessary to selectively reduce the nitrogen concentration by subjecting the liquid to be treated to ammonia stripping treatment.

Ammonia stripping treatment generally uses a dissipator tower to dissipate ammonia gas, but if solids such as SS are contained, it may not be possible to effectively treat the ammonia gas from the viewpoint of occurrence of troubles such as clogging in the apparatus.

The method of treating a liquid to be treated according to the present embodiment is a pretreatment method before wastewater treatment in which ammonia stripping treatment is performed in a post-step, and is a method of effectively separating and removing solids in a liquid to be treated to effectively obtain a clarified liquid to which ammonia stripping can be applied. Specifically, the processing method comprises: adding an inorganic coagulant and an acid to a liquid to be treated to adjust the pH to 3 or less, and dissolving all or a part of the inorganic coagulant to separate the solid from the liquid; neutralizing the separated liquid after removing the solids, and separating the precipitate thus formed.

In this treatment method, when the liquid to be treated contains sulfide, the sulfide precipitate contained in the liquid to be treated may be removed under a condition where the pH is in a neutral region before the acid is added to the liquid to be treated.

According to this method, a clarified liquid to which ammonia stripping is applied can be efficiently and stably obtained. In addition, since the clear liquid can be obtained by efficiently separating the solid matter, the COD concentration can be effectively reduced. Hereinafter, each step will be described in detail by taking an example in which an anaerobic digestion solution is used as a liquid to be treated.

[ first step ]

As a first step, sulfide precipitates contained in an anaerobic digestion solution, which is a liquid to be treated, are removed. In the treatment method of the present embodiment, although not an essential step, when the anaerobic digestion solution contains sulfides, in a second step described later, it is preferable to perform a treatment of removing sulfide precipitates under a condition of setting the pH to a neutral region before adding an acid to the anaerobic digestion solution.

In general, in anaerobic treatment for obtaining an anaerobic digestion solution, sulfuric acid or the like is reduced to generate hydrogen sulfide, but if it is discharged as a gas, the grade of biogas (methane gas) is lowered. Therefore, in the anaerobic treatment, an iron salt is added for the purpose of suppressing the generation of hydrogen sulfide gas. Accordingly, in the anaerobic digestion liquid obtained by the anaerobic treatment, iron derived from the added iron salt may precipitate as iron sulfide.

In this method of treating an anaerobic digestion solution, since the pH of the anaerobic treatment solution is lowered to acidity by adding an acid such as sulfuric acid in a second step described later, a hydrogen sulfide gas is generated in acidity without separating sulfide precipitates in advance. Therefore, in the first step, in order to prevent the generation of hydrogen sulfide gas, solid-liquid separation is performed under a neutral region condition in which iron sulfide solubility is low, and sulfide is removed.

In the separation and removal of the sulfide, a flocculant such as a cationic polymer flocculant may be added. In order to set the neutral range, a pH adjuster may be added as appropriate. The method of solid-liquid separation is not particularly limited, and may be performed by centrifugal separation or the like.

As described above, the first step is not essential, and may be omitted when the precipitation of sulfides in the anaerobic digestion solution is not significant.

[ second Process ]

In the second step, an inorganic flocculant and an acid are added to an anaerobic digestion solution as a liquid to be treated to adjust the pH to 3 or less, and after the addition, the inorganic flocculant is dissolved in its entirety or in part to perform solid-liquid separation. The second step treatment is mainly intended to remove COD components having low solubility or high flocculation property under acidic conditions.

Most of the COD components contained in the anaerobic digestion solution are present as solids. The solid surface is negatively charged and is difficult to agglomerate, and solid-liquid separation is difficult. Therefore, the pH of the anaerobic digestion solution is lowered by adding an acid to positively shift the surface potential of the solid matter, and the surface potential is neutralized by an inorganic coagulant added together.

The acid to be added is not particularly limited, and hydrochloric acid, sulfuric acid, or the like can be used.

In the second step, an acid is added in this manner to lower the pH, and carbon dioxide is generated in the process to perform decarboxylation. Therefore, by removing the pH buffering action of the carbonate, the amount of the alkaline agent consumed in the ammonia stripping treatment performed later can be reduced.

The inorganic coagulant is preferably an aluminum salt as long as it has a positive surface charge under the conditions of use. Further, a cationic polymer flocculant may be used together.

In the second step, the whole or part of the added inorganic coagulant is dissolved. Thus, the added inorganic coagulant is dissolved and present in the solution as a positive multivalent ion, thus having an effect of neutralizing the negative surface charge of the solid. In addition, by performing solid-liquid separation at this stage, COD components having low solubility under acidic conditions or high flocculation property can be effectively separated.

The amount of the inorganic coagulant to be added is not particularly limited, and the concentration of the inorganic coagulant to be added may be about 1000mg/L to 3500mg/L (in terms of the concentration of the metal component). In this way, in the present embodiment, the treatment can be performed by adjusting the pH to 3 or less by adding an acid, adding an inorganic flocculant, and dissolving all or a part of the inorganic flocculant to neutralize the surface charge of the solid matter, thereby controlling the amount of the inorganic flocculant to be added to a small range.

The method of solid-liquid separation is not particularly limited, and may be performed by centrifugal separation or the like.

[ third Process ]

In the third step, the separated liquid from which the solid matter has been removed in the second step is neutralized, and the formed precipitate is separated.

As described above, since the COD component having low solubility under acidic conditions or high flocculation property is already removed by the solid-liquid separation in the second step, the COD component which is soluble or non-condensable under acidic conditions mainly remains in the separated liquid after the solid-liquid separation. In addition, most of the inorganic coagulant is present in a dissolved state. Therefore, in the third step, the separated liquid is neutralized to precipitate the inorganic flocculant, and the COD component remaining in the liquid is flocculated. This enables further removal of the COD component and clarification of the separated liquid. Further, by separating the organic solid matter in this way, the COD concentration is also reduced.

In the neutralization, for example, a pH adjuster such as an aqueous sodium hydroxide solution is added to adjust the pH of the separated liquid to a range of about 5 to 6.5.

The separated liquid (treated liquid) obtained as described above is a clear solution from which solids are effectively separated and removed. Therefore, as the wastewater treatment of the anaerobic digestion solution, the ammonia stripping treatment can be appropriately performed on the treated solution. As described above, the method for treating a liquid to be treated according to the present embodiment is effective as a pretreatment method for performing an ammonia stripping step, and can stably obtain a clear liquid while suppressing the amount of a flocculant containing an inorganic flocculant to be added. In addition, the COD concentration can be effectively reduced by separating the generated solid.

Method for treating waste water of liquid to be treated

The method of treating wastewater with a liquid to be treated according to the present embodiment is a method of treating wastewater with a liquid to be treated containing ammoniacal nitrogen at a concentration of 1000mg/L or more, COD at a concentration of 2000mg/L or more, and solids. Examples of the liquid to be treated include anaerobic digestion liquids.

The wastewater treatment method comprises the following steps: a step (pretreatment step) of clarifying the liquid to be treated as a pretreatment step of ammonia stripping treatment; and a step (ammonia stripping step) of subjecting the post-treatment liquid obtained by the clarification treatment to ammonia stripping.

[ pretreatment Process ]

The pretreatment step is a treatment step of separating and removing solids from a liquid to be treated containing ammoniacal nitrogen at a concentration of 1000mg/L or more, COD at a concentration of 2000mg/L or more, and solids, and clarifying the separated solids, and reducing the concentration of COD components.

Specifically, the pretreatment step comprises: adding an inorganic flocculant and an acid to a liquid to be treated to adjust the pH to 3 or less and dissolving all or a part of the inorganic flocculant to separate the inorganic flocculant from the liquid; neutralizing the separated liquid after removing the solids, and separating the precipitate thus formed.

The respective steps are the same as those of the above-described method for treating water to be treated, and therefore, the description thereof is omitted here. By the treatment in the pretreatment step, not only the addition amount of the flocculant including the inorganic flocculant can be suppressed, but also a clear liquid can be stably obtained, and the COD concentration in the treated liquid can be effectively reduced by separating the produced solids.

[ Ammonia stripping Process ]

The ammonia stripping step is a treatment step of performing ammonia stripping using the treated liquid obtained in the pretreatment step.

Generally, ammonia stripping uses an ammonia stripper (dissipation tower) to dissipate, as ammonia gas, ammonia nitrogen at a high concentration contained in a liquid to be treated. This makes it possible to effectively reduce the nitrogen concentration derived from ammoniacal nitrogen. In this case, in the wastewater treatment method of the present embodiment, since the pretreatment step is performed before ammonia stripping, solids contained in the liquid to be treated are effectively removed and clarified. Therefore, even if the ammonia stripping column is installed, the occurrence of a trouble of blocking the column in the column can be suppressed, and the wastewater treatment can be efficiently performed.

Further, with respect to the ammonia stripping treatment, there is no particular limitation, and a well-known method may be performed.

Examples

Hereinafter, specific examples of the present invention will be described in detail. The scope of the present invention is not limited to any of the examples described below.

[ example 1]

To 100ml of the anaerobic digestion solution having a water quality shown in Table 1 below, 20mg/L of a cationic molecular flocculant was added at the concentration after the addition to coagulate the solution, and the resultant solution was centrifuged (first step).

TABLE 1

Then, 2ml (about 1300mg/L in terms of aluminum concentration) of polyaluminum chloride (PAC: 10% solution in terms of alumina) was added to the obtained separated liquid, and hydrochloric acid was added thereto to adjust the pH to 3. Since carbon dioxide is generated in this process, the mixture is once left to stand, and after the generation of gas is completed, centrifugal separation is performed (second step).

Then, a liquid alkali was added to the obtained separation liquid to adjust the pH to 6 for neutralization, and a precipitate was produced and centrifuged. The solids are separated by centrifugation to obtain a clear separated liquid (third step).

The clarified separated liquid obtained in this way is subjected to ammonia stripping to obtain a treated liquid. The measurement results of the obtained treated liquid are shown in table 2 below.

Further, since it is difficult to secure the liquid amount of the treatment water, evaluation of solid-liquid separation was performed by the degree of transillumination. The transmittance is measured by a method according to JIS K0102 using a transmissometer which is a glass cylinder with a lower opening and graduated every 10mm and which has a marking plate with a double cross marked on the bottom. Specifically, the refractometer is filled with a measurement sample, the bottom is seen through from the top, the scale of the water surface is read when the double cross from the lower port to the marking plate is clearly recognized, and the average value is measured when this operation is repeated a plurality of times.

[ example 2]

In the second step, the experiment was performed in the same manner as in example 1, except that the pH of the separated liquid was adjusted to 2. The measurement results of the obtained separation liquid are shown in table 2 below.

Comparative example 1

In comparative example 1, 20mg/L of cationic polymer flocculant was added to 100ml of anaerobic digestion solution to coagulate the solution, and the solution was centrifuged.

Then, 2ml (about 1300mg/L in terms of aluminum concentration) of polyaluminum chloride (10% in terms of alumina) was added to the obtained separation liquid, and centrifugation was performed without adjusting pH (maintaining neutrality).

As a result of this treatment, no clear separated liquid was obtained. Therefore, ammonia stripping was not performed. The measurement results of the separated liquid are shown in table 2 below.

Comparative example 2

In comparative example 2, a test was conducted in the same manner as in comparative example 1 except that 6ml (3800 mg/L in terms of aluminum concentration) of polyaluminum chloride (10% in terms of alumina) was added to the separated liquid.

As a result, a clearer separated liquid was obtained as compared with comparative example 1, but no clarified separated liquid was obtained as in examples 1 and 2. Therefore, ammonia stripping was not performed. The measurement results of the separated liquid are shown in table 2 below.

Comparative example 3

In comparative example 3, the first step was performed under the same conditions as in example 1, and 2ml (about 1300mg/L in terms of aluminum concentration) of polyaluminum chloride (PAC: 10% solution in terms of alumina) was added to the obtained separated liquid, and hydrochloric acid was added to adjust the pH to 3. During this process, carbon dioxide was generated, and therefore, the mixture was left to stand for a while, and after the generation of gas was completed, liquid alkali was added to adjust the pH to 6 for neutralization, thereby producing a precipitate, and centrifugal separation was performed. That is, in comparative example 3, unlike example 1, the solid-liquid separation treatment by centrifugal separation was not performed in the second step, but the neutralization was performed as it is to form a precipitate.

As a result, a clear separated liquid as in example 1 was not obtained. Therefore, ammonia stripping was not performed. The measurement results of the separated liquid are shown in table 2 below.

TABLE 2

From the above, it was confirmed that a clear separated liquid was obtained by the method described in examples with a small amount of the inorganic coagulant added. Further, it was confirmed that the nitrogen concentration can be effectively reduced by performing ammonia stripping treatment after the clarification treatment.

Claims

1. A method for treating a liquid to be treated containing ammoniacal nitrogen at a concentration of 1000mg/L or more, COD at a concentration of 2000mg/L or more, and solids, the method comprising:

adding an inorganic coagulant and an acid to the liquid to be treated to adjust the pH to 3 or less, and dissolving all or a part of the inorganic coagulant to perform solid-liquid separation; and

neutralizing the separated liquid after removing the solids, and separating the formed precipitate.

2. The processing method according to claim 1,

an aluminum salt is used as the inorganic coagulant.

3. The processing method according to claim 1 or 2,

the treated liquid is anaerobic digestion liquid.

4. The processing method according to any one of claims 1 to 3,

in the case where the liquid to be treated contains a sulfide, the sulfide precipitate contained in the liquid to be treated is removed under a condition where the pH is in a neutral region before the acid is added to the liquid to be treated.

5. A method for treating wastewater containing ammoniacal nitrogen at a concentration of 1000mg/L or more, COD at a concentration of 2000mg/L or more, and solids, the method comprising:

adding an inorganic coagulant and an acid to the liquid to be treated to adjust the pH to 3 or less, and dissolving all or a part of the inorganic coagulant to perform solid-liquid separation;

neutralizing the separated liquid from which the solid is removed, and separating the generated precipitate; and

and a step of subjecting the separated liquid from which the precipitate has been removed to ammonia stripping treatment.