JP7813184B2 - Method for producing nitrogen-containing concentrate from digested liquid - Google Patents

Description

The present invention relates to a method for producing a nitrogen-containing concentrate from digested liquid.

The residual liquid after methane fermentation, the digested liquor, has a solids concentration of approximately 3-5%. Conventional wastewater treatment requires dehydration using chemicals such as flocculants and separation into solids and separated liquid. This dehydration process requires high costs for chemicals such as flocculants, resulting in poor economic efficiency. Furthermore, the separated liquid after dehydration contains many sources of BOD (biochemical oxygen demand), such as ammonia. This process does not meet the standards for discharge into public rivers or sewers, necessitating advanced treatments such as nitrification, denitrification, flocculation, and activated carbon. The initial and running costs, complex flow, and operational management required for digested liquor treatment have hindered the widespread adoption of methane fermentation facilities. Furthermore, approximately 70% of the nitrogen contained in the digested liquor is ammoniacal nitrogen, which exists in aqueous solution. Therefore, the nitrogen component migrates to the separated liquid after dehydration, limiting the nitrogen content of the solids. As a result, the resulting solids have little value as fertilizer.
In response to this, for example, Patent Document 1 describes removing solids from a methane fermentation residual liquid using a screen with wedge wires, and then adjusting the pH of the resulting separated liquid to an acidic level. This fixes the ammonia component in the separated liquid, and the pH-adjusted separated liquid is then evaporated and concentrated to produce a liquid fertilizer with a stable nitrogen component. Patent Document 1 describes that sulfuric acid is preferable as an acid for adjusting the pH, and the addition of sulfuric acid converts the ammoniacal nitrogen component in the separated liquid into ammonium sulfate, which is fixed in the liquid fertilizer.
On the other hand, the digested liquid, which is the residual liquid from methane fermentation, is saturated with carbon dioxide produced during methane fermentation, so the addition of sulfuric acid or other substances causes rapid and large amounts of foaming in the digested liquid.The large amounts of foaming in the digested liquid hinder the progress of the ammonium sulfate production reaction caused by the addition of sulfuric acid, and the time required for the foaming to disappear is the rate-limiting factor in the digested liquid treatment system.

JP 2019-34870 A

In light of this situation, the present invention aims to provide a method for producing a nitrogen-containing concentrated liquid from digested liquid that can efficiently carry out the reaction to produce ammonium sulfate from the ammoniacal nitrogen in the digested liquid without using a flocculant in the digested liquid dehydration process.

To solve the above problems, the inventors came up with the idea of using an antifoaming agent to prevent foaming when sulfuric acid is added to produce ammonium sulfate. After extensive research, they discovered that mixing an antifoaming agent into the digestive fluid before adding sulfuric acid to fix the ammonia nitrogen in the digestive fluid as ammonium sulfate can significantly reduce foaming when sulfuric acid is added. Furthermore, the inventors discovered that, of the various antifoaming agents available, alcohol-based antifoaming agents exhibit significantly superior defoaming effects. The present invention was completed based on this finding and includes the following aspects.

One aspect of the present invention is
[1] The present invention relates to a method for producing a nitrogen-containing concentrated liquid from a digestive liquor, the method comprising: (a) a step of mixing an alcohol-based antifoaming agent with the digestive liquor; (b) a step of adding sulfuric acid to the digestive liquor obtained in the step (a) to adjust the pH; and (c) a step of concentrating the digestive liquor obtained in the step (b), wherein the nitrogen-containing concentrated liquid and condensed water are separated and obtained.
In one embodiment, the manufacturing method of the present invention is [2] the manufacturing method described in [1] above,
The digested liquid is characterized in that it is obtained by methane fermentation.
In one embodiment, the manufacturing method of the present invention is [3] the manufacturing method described in [1] or [2] above,
The digestive fluid is characterized in that it is obtained by solid-liquid separation without using a flocculant.
In one embodiment, the manufacturing method of the present invention is [4] the manufacturing method described in any one of [1] to [3] above,
The method is characterized in that the alcohol-based defoaming agent in the step (a) is added to the digestive liquid so as to have a concentration in the range of 20 ppm to 1000 ppm.
In one embodiment, the manufacturing method of the present invention is [5] the manufacturing method described in any one of [1] to [4] above,
The pH of the digestive fluid adjusted in the step (b) is within the range of pH 4.0 to pH 6.0.
In one embodiment, the manufacturing method of the present invention is [6] the manufacturing method described in any one of [1] to [5] above,
The amount of sulfuric acid to be added in the step (b) is determined based on the evaporation residue, ammonia nitrogen, or M alkalinity of the digested liquid.
In one embodiment, the manufacturing method of the present invention is [7] the manufacturing method described in any one of [1] to [6] above,
The amount of sulfuric acid added in the step (b) is an amount such that the amount of 20% sulfuric acid dropped per 1 g of M alkali amount based on the M alkalinity of the digested liquid is 5.8 to 7.8 mL .
In one embodiment, the manufacturing method of the present invention is [8] the manufacturing method described in any one of [1] to [7] above,
the step (a) is a step of continuously flowing the digestion liquid and the alcohol-based defoaming agent into a mixing tank, and continuously flowing the digestion liquid sufficiently mixed with the alcohol-based defoaming agent into a pH adjusting tank,
The step (b) is a step of adjusting the pH of the digested liquid continuously flowing in from the mixing tank by adding sulfuric acid to the pH adjustment tank, and continuously discharging the pH-adjusted digested liquid into a concentration raw water tank.
Manufacturing method.
In one embodiment, the manufacturing method of the present invention is [9] the manufacturing method described in any one of [1] to [8] above,
(d) A step of drying the nitrogen-containing concentrate obtained in the step (c) is further included.

Another aspect of the present invention is
[10] An apparatus for producing a nitrogen-containing concentrated liquid from a digestive liquid, comprising:
The apparatus comprises a mixing tank for mixing the digestive liquid with an antifoaming agent, a pH adjusting tank for adjusting the pH of the digestive liquid by adding sulfuric acid to the digestive liquid mixed with the antifoaming agent, and a concentrating means for concentrating the pH-adjusted digestive liquid.
Here, in one embodiment of the device of the present invention, [11] the device described in [10] above,
the mixing tank is provided with a digestion fluid supply means for supplying a desired amount of digestion fluid and a defoaming agent supply means for supplying a desired amount of defoaming agent;
The pH adjusting tank is characterized by being equipped with a sulfuric acid supply means.
In one embodiment of the device of the present invention, [12] the device according to [10] or [11] above, further comprising a solid-liquid separation means for separating the digestive fluid into solid and liquid,
The solid-liquid separation means and the digested liquid supply means are connected, and the separated liquid obtained by the solid-liquid separation means is sent to the mixing tank via the digested liquid supply means.
In one embodiment of the device of the present invention, [13] the device according to any one of [10] to [12] above,
The mixing tank and the pH adjustment tank are directly connected to each other at their lower parts, and the digested liquid introduced into the mixing tank flows directly into the pH adjustment tank after a certain residence time in the mixing tank.
In one embodiment of the device of the present invention, [14] the device according to any one of [10] to [13] above,
The system further includes a digestion liquid storage tank, a defoaming agent storage tank, and a sulfuric acid storage tank.
In one embodiment of the device of the present invention, [15] the device according to any one of [10] to [14] above,
The method further comprises a sulfuric acid dilution tank for adjusting the sulfuric acid to be added to the pH adjustment tank.
In one embodiment of the device of the present invention, [16] the device according to any one of [10] to [15] above,
The method further includes a drying means for drying the nitrogen-containing concentrated liquid obtained by the concentrating means, wherein the drying means and the concentrating means are connected, and the concentrated liquid is sent from the concentrating means to the drying means.

According to the method for producing a nitrogen-containing concentrated liquid from a digested liquid of the present invention, foaming that occurs when ammoniacal nitrogen in the digested liquid is fixed as ammonium sulfate can be defoamed in a short time with a smaller amount of defoaming agent.
Furthermore, the method for producing a nitrogen-containing concentrate according to the present invention can provide a concentrate containing a high concentration of nitrogen. Furthermore, the condensed water separated during the production of the concentrate does not require wastewater treatment such as nitrification or denitrification. Therefore, the cost and treatment of the flocculant used in the production process of the nitrogen-containing concentrate, as well as the cost and treatment of the condensed water can be reduced.

FIG. 1 shows a flow chart of one embodiment of the method for producing a nitrogen-containing concentrate from digested liquid according to the present invention. FIG. 2 is a schematic diagram showing an embodiment of an apparatus for producing a nitrogen-containing concentrated liquid from a digested liquid according to the present invention. FIG. 3 is a schematic diagram showing one embodiment of an apparatus for producing a nitrogen-containing concentrated liquid from a digested liquid according to the present invention, which is equipped with a two-tank tank in which a mixing tank and a pH adjustment tank are connected. FIG. 4 is a schematic diagram showing one embodiment of an apparatus for producing a nitrogen-containing concentrated liquid from a digested liquid according to the present invention, which further includes a two-tank tank in which a mixing tank and a pH adjustment tank are connected, and a sulfuric acid dilution tank. 5 is a graph showing the results of the post-foaming addition test (Comparative Example 1) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the defoaming time of the foam generated in the digestive liquid, and the pH change of the digestive liquid. FIG. 6 is a photograph showing the state of the digestive fluid at the start (FIG. 4A) and when 3 ml of sulfuric acid was added dropwise (FIG. 4B) in the post-foaming addition test (Comparative Example 1) described in the Examples below. 7 is a graph showing the results of a pre-addition test (Example 1) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the time it takes for bubbles generated in the digestive liquid to disappear, and the pH change of the digestive liquid. FIG. 8 is a photograph showing the state of the digestive fluid when 6 ml of sulfuric acid was added dropwise in the prior addition (Example 1) described in the following Examples. 9 is a graph showing the results of a pre-addition test (Example 2) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the time it takes for bubbles generated in the digestive fluid to disappear, and the pH change of the digestive fluid. FIG. 10 is a photograph showing the state of the digestive fluid when 5 ml (FIG. 10A) and 6 ml (FIG. 10B) of sulfuric acid were added dropwise in the prior addition test (Example 2) described in the following Example. 11 is a graph showing the results of a pre-addition test (Example 3) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the time it takes for bubbles generated in the digestive fluid to disappear, and the pH change of the digestive fluid. FIG. 12 is a photograph showing the state of the digestive fluid when 3 ml (FIG. 12A) and 5 ml (FIG. 12B) of sulfuric acid were added dropwise in the prior addition test (Example 3) described in the following Example. 13 is a graph showing the results of a pre-addition test (Comparative Example 2) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the time it takes for bubbles generated in the digestive liquid to disappear, and the pH change of the digestive liquid. FIG. 14 is a photograph showing the state of the digestive fluid when 6 ml of sulfuric acid was added dropwise in the prior addition test (Comparative Example 2) described in the following Example. 15 is a graph showing the results of a pre-addition test (Comparative Example 3) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the time it takes for bubbles generated in the digestive liquid to disappear, and the pH change of the digestive liquid. FIG. 16 is a photograph showing the state of the digestive fluid when 6 ml of sulfuric acid was added dropwise in the prior addition test (Comparative Example 3) described in the following Example. 17 is a graph showing the results of a pre-addition test (Comparative Example 4) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the time it takes for bubbles generated in the digestive liquid to disappear, and the pH change of the digestive liquid. FIG. 18 is a photograph showing the state of the digestive fluid when 6 ml of sulfuric acid was added dropwise in the prior addition test (Comparative Example 4) described in the Examples below. 19 is a graph showing the results of a pre-addition test (Comparative Example 5) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the time it takes for bubbles generated in the digestive liquid to disappear, and the pH change of the digestive liquid. FIG. 20 is a photograph showing the state of the digestive fluid when 6 ml of sulfuric acid was added dropwise in the prior addition test (Comparative Example 5) described in the following Example. 21 is a graph showing the results of a pre-addition test (Comparative Example 6) described in the following Examples. The graph shows the relationship between the amount of sulfuric acid added, the time it takes for bubbles generated in the digestive liquid to disappear, and the pH change of the digestive liquid. FIG. 22 is a photograph showing the state of the digestive fluid when 6 ml of sulfuric acid was added dropwise in the prior addition test (Comparative Example 6) described in the following Example. FIG. 23 is a photograph showing the state of condensed water (FIG. 23A) and nitrogen-containing concentrated liquid (FIG. 23B) that are the subject of analysis in Example "II. Analysis of properties after digestive fluid treatment" below. FIG. 24 is a graph showing the average titration results for each digestive fluid in the following Example "III. Relationship between the amount of sulfuric acid injected and the properties of the digestive fluid." FIG. 25 is a graph showing the relationship between the amount of sulfuric acid dripped and the evaporation residue measured in Example "III. Relationship between the amount of sulfuric acid injected and the properties of the digested liquid" below. FIG. 26 is a graph showing the relationship between the amount of sulfuric acid dripped and NH ₄ —N measured in the following Example "III. Relationship between amount of sulfuric acid injected and properties of digested liquid." FIG. 27 is a graph showing the relationship between the amount of sulfuric acid dripped and M alkalinity measured in the following Example "III. Relationship between sulfuric acid injection amount and properties of digested liquid." FIG. 28 is a graph showing the change in the amount of sulfuric acid dropped and the pH when each alcohol-based defoaming agent was used in Example "IV. Examination of alcohol-based defoaming agents" below. FIG. 29 is a graph showing the defoaming time at each pH when each alcohol-based defoaming agent was used in Example "IV. Examination of Alcohol-Based Defoaming Agents" below.

One aspect of the present invention relates to a method for producing a nitrogen-containing concentrate from a digested liquid. The method for producing a nitrogen-containing concentrate of the present invention comprises the following steps (a) to (c), as shown in FIG.
(a) A step of mixing an alcohol-based defoaming agent with the digestive liquid (defoaming agent mixing step)
(b) A step of adding sulfuric acid to the digested liquid obtained in step (a) to produce ammonium sulfate (pH adjustment step).
(c) A step of concentrating the digested liquid obtained in step (b), which separates the nitrogen-containing concentrate from the condensed water (concentration step).

As described above, the method for producing a nitrogen-containing concentrate of the present invention includes the step (a) of mixing an alcohol-based defoaming agent with a digested liquid.
In this specification, the term "digestion liquid" refers to a methane fermentation residual liquid obtained after methane fermentation of organic waste. The digestion liquid that can be used in the present invention is a methane fermentation residual liquid containing ammonia and ammonium salts, and the origin of the organic waste and the methane fermentation method are not limited.
The digested liquid can be the methane fermentation residual liquid obtained after methane fermentation. In this case, the digested liquid can be supplied, for example, from a storage tank for the methane fermentation residual liquid to a mixing tank for mixing with an antifoaming agent. The digested liquid is supplied to the mixing tank in a quantitative manner so as to obtain a desired amount.

The digestive fluid may also be a separated fluid obtained by solid-liquid separation. Therefore, one embodiment of the method for producing a nitrogen-containing concentrated fluid from a digestive fluid of the present invention includes a step of subjecting the digestive fluid to solid-liquid separation prior to step (a). Solid-liquid separation may be performed to separate solids with large particle sizes that may cause blockages in the concentration means or pump. Examples of solids to be removed in solid-liquid separation include, but are not limited to, substances with particle sizes or diameters of 20 mm or more. The solid-liquid separation means is not limited as long as it can separate the solids in the digestive fluid, and examples thereof include a juicer and a centrifugal dehydrator.
The method for producing a nitrogen-containing concentrate of the present invention does not require the use of a flocculant in the solid-liquid separation process, and therefore the digestive liquor does not contain a flocculant. In this specification, a flocculant refers to a substance that changes the properties of particles in the digestive liquor to aggregate or coarsen them so that water and solids can be separated during the dehydration process of the digestive liquor. Known flocculants include, but are not limited to, inorganic flocculants such as iron salts, aluminum salts, and calcium salts, and organic flocculants such as cationic polymer flocculants and anionic polymer flocculants.
In a preferred embodiment, the digestive liquid is a digestive liquid after solid-liquid separation and does not contain a flocculant.

An "alcohol-based defoaming agent" refers to a defoaming agent used to eliminate foam that forms in a liquid, and is prepared with alcohol as the primary component. Examples of the primary alcohol include, but are not limited to, higher alcohols having 12 to 30 carbon atoms, or straight-chain alcohols having 18 to 22 carbon atoms. Examples of higher alcohols include, but are not limited to, one or more selected from the group consisting of natural alcohols having 12 to 30 carbon atoms and synthetic alcohols having 12 to 30 carbon atoms. Examples of natural alcohols having 12 to 30 carbon atoms include saturated alcohols such as lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, eicosanol, docosanol, tetracosanol, hexaconol, octacosanol, and myricyl alcohol, as well as unsaturated alcohols such as oleyl alcohol. Examples of synthetic alcohols having 12 to 30 carbon atoms include linear primary alcohols or branched primary alcohols synthesized by the Ziegler method, mixtures of these alcohols with different carbon numbers, and linear secondary alcohols produced by air oxidation of paraffin. These may be used alone as defoamers, or two or more may be used in combination. The defoamer may also contain alcohols with fewer than 12 carbon atoms or alcohols with more than 30 carbon atoms. While not limited to the following, higher alcohols may be present in an amount of, for example, 5 to 90 wt.% of the total weight of the defoamer. In addition to alcohol, alcohol-based defoamers may also contain inorganic salts, surfactants (e.g., anionic surfactants, nonionic surfactants), fatty acid esters, natural oils and fats, mineral oils, hydrocarbons, water, and other known components of alcohol-based defoamers. The alcohol-based defoamers that can be used in the present invention are not limited, and any known alcohol-based defoamers can be used as long as they are added to the digestive liquor before the addition of sulfuric acid to inhibit foaming and/or provide a defoaming effect upon the addition of sulfuric acid. Examples of such alcohol-based defoaming agents include higher alcohol-based defoaming agents disclosed in JP-A-2011-215235, JP-A-2012-143700, JP-A-2014-079699, JP-A-2018-51513, JP-A-2021-098156, and JP-A-2015-054259.
The form of the defoaming agent is not particularly limited, and any form of defoaming agent can be used, such as oil type, oil compound type, solution type, emulsion type, self-emulsifying type, etc., but emulsion type is preferred. The defoaming agent may contain additives (such as separation inhibitors, preservatives, emulsifiers) that are preferred for each form.
Commercially available alcohol-based defoaming agents that can be used in the present invention include, but are not limited to, Kuriles 653 (manufactured by Kurita Water Industries Ltd.), Dappo H-312 (San Nopco Ltd.), Bismer FS series (Nissin Chemical Research Institute Co., Ltd.), SN-650 (Taki Chemical Industry Co., Ltd.), FALC-108 (Kubota Kasui Co., Ltd.), AE-3100 (Mitsubishi Chemical Aqua Solutions Co., Ltd.), Pronal EM-38N (Toho Chemical Industry Co., Ltd.), and EL-101 (E-Life Co., Ltd.).

The antifoaming agent is preferably added to the digestive fluid so as to have a concentration in the range of 20 ppm to 1000 ppm, more preferably in the range of 50 ppm to 500 ppm. In a more preferred embodiment, the antifoaming agent is added to the digestive fluid so as to have a concentration of 100 ppm. If the amount of antifoaming agent added is less than 20 ppm, a desirable antifoaming effect cannot be obtained, and if it exceeds 1000 ppm, the cost increases, which is not preferable.
Furthermore, after adding the antifoaming agent to the digestive liquid, it is preferable to thoroughly mix the mixture using a stirring means. The stirring process is preferably carried out until the antifoaming agent is uniformly dispersed in the digestive liquid. The digestive liquid is stirred, for example, for 5 minutes or more (e.g., about 5 to 10 minutes). By thoroughly stirring the digestive liquid, foaming during the production of ammonium sulfate can be suppressed. The stirring means is not limited as long as it can mix the antifoaming agent and the digestive liquid, and a known stirrer can be used.

The method for producing a nitrogen-containing concentrate of the present invention includes, after the above step (a), "(b) a step of adding sulfuric acid to the digested liquid obtained in step (a) to adjust the pH."
The digested liquid that has been thoroughly mixed with the antifoaming agent in the mixing tank is sent to the pH adjustment tank, where ammonium sulfate is produced from the ammoniacal nitrogen in the digested liquid.
It is preferable to use sulfuric acid to produce ammonium sulfate, as the use of sulfuric acid can reduce costs and also lower the possibility of equipment corrosion compared to the use of hydrochloric acid or the like.
When sulfuric acid is used, it can be supplied to the pH adjustment tank at, for example, 10 to 40 wt % sulfuric acid. If the concentration of sulfuric acid supplied is high, the foaming force will be intense, while if the concentration of sulfuric acid is low, it will take time for the ammonium sulfate production reaction to proceed. Those skilled in the art can appropriately set a preferred concentration by taking into consideration the balance between the foaming state when sulfuric acid is added and the time required for the sulfuric acid production reaction. In a preferred embodiment, the concentration of sulfuric acid supplied to the pH adjustment tank is 15 to 25 wt %.
Sulfuric acid may be supplied until the pH of the digested liquid reaches, for example, a range of pH 4.0 to pH 6.0. In a preferred embodiment, sulfuric acid is supplied until the pH of the digested liquid reaches a range of 4.5 to 5.5, and in a more preferred embodiment, sulfuric acid is supplied until the pH of the digested liquid reaches 5.0. Because the volatilization rate of ammonia (the rate of ammonia mixing into the condensed water) is the same at pH 4.0 and pH 5.0, a pH of 5 is preferable because it reduces the amount of sulfuric acid and antifoaming agent used. However, adjusting the pH to less than 4.0 is undesirable because the pH of the nitrogen-containing concentrate will be low, which may cause problems when used as fertilizer, and adjusting the pH to more than 6.0 is undesirable because ammonia nitrogen will not be sufficiently fixed and will volatilize.

In one embodiment of the method for producing a nitrogen-containing concentrate of the present invention, the amount of sulfuric acid to be added to the digested liquid can be determined from the evaporation residue, ammoniacal nitrogen, or M alkalinity of the digested liquid. By measuring the evaporation residue, ammoniacal nitrogen, or M alkalinity of the digested liquid in advance, the amount of sulfuric acid required to adjust the pH can be calculated based on the description of Example III and the results of Figures 25 to 27 of the present application. For example, when adjusting the pH of the digested liquid to 4.8, the amount of 20% sulfuric acid to be added can be set to 5.8 to 7.8 mL per 1 g of M alkalinity based on the M alkalinity of the digested liquid .
The sulfuric acid is preferably supplied from a sulfuric acid supply port connected to the bottom or lower portion of the pH adjustment tank containing the digestion liquid. Supplying sulfuric acid from the lower portion of the digestion liquid can efficiently promote the reaction.
After adding sulfuric acid to the digestive liquid, it is preferable to thoroughly mix the mixture using a stirring means. The digestive liquid is preferably stirred for 5 minutes or more (e.g., 5 to 10 minutes). This makes it possible to fix nitrogen (especially ammoniacal nitrogen) contained in the digestive liquid without volatilizing it.
After adjusting the pH, the digested liquid can be sent to the concentrated raw water tank by overflow.

The method for producing a nitrogen-containing concentrate of the present invention includes, after step (b), "(c) a step of concentrating the digested liquid obtained in step (b), which involves separating the nitrogen-containing concentrate from condensed water." In step (c), the digested liquid from which ammonium sulfate has been produced is sent from a pH adjustment tank or a raw concentrate tank to a concentrating means. The digested liquid is concentrated by the concentrating means and separated into a nitrogen-containing concentrate and condensed water. At this time, the ammoniacal nitrogen contained in the digested liquid is transferred to the concentrate. In a preferred embodiment, the nitrogen-containing concentrate obtained in step (c) can contain ammoniacal nitrogen that is 1.5 times or more, 2 times or more, and more preferably 2.5 times or more concentrated compared to the ammoniacal nitrogen in the digested liquid before concentration. Furthermore, for example, in a preferred embodiment, the nitrogen-containing concentrate obtained by the production method of the present invention can contain 5,000 mg/kg or more of nitrogen. A known reduced-pressure concentrating device can be used as the concentrating means for step (c). For example, hot water at about 90°C can be used as a heat source for the reduced-pressure concentrating device. Furthermore, by using a heat pump to reuse the latent heat of the condensed water and thereby increasing thermal efficiency, it is possible to configure the system so that condensation can be performed using only the surplus heat source of the generator.
The condensed water obtained from the concentration process has low solids (SS) and nitrogen content, meets the general wastewater standards set by the Ministry of the Environment, and can be discharged into rivers and sewers.
In one embodiment, the condensate obtained by the method of the present invention has an SS (solids) of less than 100 mg/L, preferably less than 50 mg/L, and more preferably less than 30 mg/L. Also in one embodiment, the condensate obtained by the method of the present invention has a t-n (total nitrogen) of less than 100 mg/L, preferably less than 50 mg/L, and more preferably less than 30 mg/L. Also in one embodiment, the condensate obtained by the method of the present invention has a BOD (biochemical oxygen demand) of less than 100 mg/L, preferably less than 50 mg/L, and more preferably less than 30 mg/L. Also in one embodiment, the condensate obtained by the method of the present invention has a COD _Mn (chemical oxygen demand) of less than 100 mg/L, preferably less than 50 mg/L, and more preferably less than 30 mg/L.

The nitrogen-containing concentrate obtained through the above step (c) can be used as a liquid fertilizer as it is, or can be dried to form a dry fertilizer.
Therefore, in one embodiment, the method for producing a nitrogen-containing concentrate from a digested liquid of the present invention further comprises a step (d) of drying the nitrogen-containing concentrate obtained in step (c). This makes it possible to provide a nitrogen-containing solid as a dried fertilizer. A known drying device can be used as a drying means in step (d).

In another aspect, the present invention provides an apparatus (digestive fluid treatment apparatus) for producing a nitrogen-containing concentrated liquid from digestive fluid. The apparatus includes a mixing tank for mixing the digestive fluid with an antifoaming agent, a pH adjustment tank for adding sulfuric acid to the digestive fluid mixed with the antifoaming agent to adjust the pH, and a concentration means for concentrating the pH-adjusted digestive fluid.

In one embodiment of the apparatus for producing a nitrogen-containing concentrated liquid from a digested liquid of the present invention, the mixing tank is equipped with a digested liquid supply means for supplying a desired amount of digested liquid, an antifoaming agent supply means for supplying a desired amount of antifoaming agent, and an agitation means, and the pH adjustment tank is equipped with a sulfuric acid supply means and an agitation means. The digested liquid supply means may be a configuration such as a combination of a digested liquid storage tank, a pump, and a flow meter, as exemplified in FIG. 2. The antifoaming agent supply means may be a configuration such as a combination of an antifoaming agent storage tank, a pump, and a flow meter, as exemplified in FIG. 2. The sulfuric acid supply means may be a configuration such as a combination of a sulfuric acid storage tank, a pump controlled by a pH control panel, and a pH meter.
With this configuration, it is possible to supply the antifoaming agent in a preferred range to a desired amount of the digestive liquid, and to thoroughly mix the digestive liquid and the antifoaming agent in the mixing tank before adding sulfuric acid.

In one embodiment, the apparatus for producing a nitrogen-containing concentrated liquid from a digested liquid of the present invention further includes a solid-liquid separation means for separating the digested liquid into solid and liquid. The solid-liquid separation means is connected to the digested liquid supply means, and the separated liquid obtained by the solid-liquid separation means is sent to the mixing tank via the digested liquid supply means. As described above, the solid-liquid separation means is not limited as long as it can separate the solids in the digested liquid, and known juice extractors, centrifugal dehydrators, etc. can be used.
This configuration allows large-sized solids contained in the digestion liquid to be removed in advance, thereby improving the efficiency of the subsequent defoaming agent mixing step and pH adjustment step.

In one embodiment, the apparatus for producing a nitrogen-containing concentrate from a digested liquid of the present invention further includes a drying means for drying the nitrogen-containing concentrate obtained by the concentrating means. The drying means is connected to the concentrating means, and the nitrogen-containing concentrate obtained by the concentrating means can be sent to the drying means. The drying means is not limited as long as it can dry the nitrogen-containing concentrate into a solid, and a known dryer can be used.
This configuration allows the nitrogen-containing concentrate to be provided as a dry or solid product.

One embodiment of the apparatus of the present invention will now be described with reference to Figure 2. Note that the referenced drawings are used to explain the technical features that can be adopted by the present invention, and the apparatus of the present invention is not limited to the configuration shown in the drawings. The digestive fluid treatment apparatus 1 shown in Figure 2 comprises a digestive fluid storage tank 2, an antifoaming agent storage tank 3, a mixing tank 4, a sulfuric acid storage tank 5, a pH adjustment tank 6, a concentrated raw water tank 7, and a vacuum concentration means 8.

The digested liquid, which is the residual liquid from methane fermentation, is stored in the digested liquid storage tank 2. The digested liquid storage tank 2 is connected to the mixing tank 4 by piping. The digested liquid in the digested liquid storage tank 2 is supplied quantitatively to the mixing tank 4 by the pump 21 and flow meter 31. The defoaming agent is adjusted to the desired concentration in the defoaming agent storage tank 3 and stored therein. The defoaming agent storage tank 3 is equipped with an agitation means 51 and is connected to the mixing tank 4 by piping. The defoaming agent in the defoaming agent storage tank 3 is supplied quantitatively to the mixing tank 4 by the pump 22 and flow meter 32. The mixing tank 4 is equipped with an agitation means 52 and mixes the digested liquid and the defoaming agent. The mixing tank 4 is connected to the pH adjustment tank 6 by piping, and the digested liquid mixed with the defoaming agent is sent to the pH adjustment tank 6. In the sulfuric acid storage tank 5, sulfuric acid is adjusted to the desired concentration and stored. The sulfuric acid storage tank 5 is connected to the pH adjustment tank 6 by piping, and sulfuric acid in the sulfuric acid storage tank 5 is sent to the pH adjustment tank 6 by a pump 23. The pH adjustment tank 6 is equipped with an agitator 53 and a pH meter 41. The pH meter 41 and pump 23 are controlled by a pH control panel, and sulfuric acid is sent to the pH adjustment tank 6 to achieve the desired pH. The pH adjustment tank 6 is connected to the concentrated raw water tank 7 by piping, and the digested liquid overflowing from the pH adjustment tank 6 is sent to the concentrated raw water tank 7. The concentrated raw water tank 7 is equipped with an agitator 54 and is connected to the reduced pressure concentration means 8 by piping. The digested liquid in the concentrated raw water tank 7 is sent to the reduced pressure concentration means 8 by control of a flow regulator and a control valve. The reduced pressure concentration means 8 separates the digested liquid into a nitrogen-containing concentrate and condensed water.

Another embodiment of the apparatus of the present invention is shown in Figure 3. The digestive fluid treatment apparatus 1 shown in Figure 3 comprises a digestive fluid storage tank 2, an antifoaming agent storage tank 3, a two-tank tank consisting of a mixing tank 4 and a pH adjustment tank 6 connected together, a sulfuric acid storage tank 5, a concentrated raw water tank 7, and a vacuum concentration means 8. The antifoaming agent and digestive fluid are introduced into the top of the two-layer mixing tank 4. The mixing tank 4 can be designed to have a sufficient residence time for the digestive fluid to mix with the antifoaming agent within the tank. As a result, the digestive fluid that has been sufficiently mixed with the antifoaming agent is sent from the bottom of the mixing tank 4 to the pH adjustment tank 6, where the pH is adjusted with sulfuric acid. The pH adjustment tank 6 can be designed to have a sufficient residence time for the digestive fluid to be adjusted to the desired pH. A constant amount of digested liquid and antifoaming agent is continuously added to the mixing tank 4, and the digested liquid adjusted to the desired pH in the pH adjustment tank 6 overflows and is sent to the concentrated raw water tank 7. As in the digested liquid treatment device shown in Figure 3, the mixing tank 4 and pH adjustment tank 6 may be connected to form a two-tank system. Using a two-tank system like this is preferable as it reduces costs compared to separate tanks.

Another embodiment of the apparatus of the present invention is shown in Figure 4. The digestive fluid treatment apparatus 1 shown in Figure 4 comprises a digestive fluid storage tank 2, an antifoaming agent storage tank 3, a two-tank tank consisting of a mixing tank 4 and a pH adjustment tank 6 connected together, a sulfuric acid storage tank 5, a sulfuric acid dilution tank 9, a concentrated raw water tank 7, and a vacuum concentration means 8. The sulfuric acid dilution tank 9 can be used to dilute the sulfuric acid to the desired concentration. As with the digestive fluid treatment apparatus shown in Figure 4, an additional sulfuric acid dilution tank may be provided. Two or more sulfuric acid dilution tanks may also be provided. The provision of the sulfuric acid dilution tank 9 allows the use of concentrated sulfuric acid (98%), thereby reducing the amount of sulfuric acid to be transported, which is preferable as it saves space for the apparatus.

The present invention will be described in detail below using examples. However, the present invention is not limited to the following embodiments.

(I. Antifoam Test)
In this example, sulfuric acid was added dropwise to the digestion liquid, and after foaming was confirmed from the digestion liquid, an alcohol-based defoaming agent was added after foaming (Comparative Example 1), and the amount of defoaming agent added and the defoaming time were confirmed.
In addition, the amount of antifoaming agent added and the defoaming time when sulfuric acid was added dropwise after an alcohol-based antifoaming agent was added to the digestion liquid in advance were confirmed (pre-addition test group: Examples 1 to 4), and compared with the post-foaming addition group.
Furthermore, in order to verify the defoaming effect of the silicone-based defoaming agent, tests were conducted in the same manner as in the pre-addition test group using a silicone-based defoaming agent (Comparative Examples 2 to 6).

1-1. The digested fluid used as the reagent was the digested fluid obtained after methane fermentation of livestock waste. The analysis results of the digested fluid are shown in Table 1 below. Before use, the digested fluid was sieved to remove floating matter so as not to affect the pH measurement or stirring operation.

The sulfuric acid used in the titration was 96% concentrated sulfuric acid diluted to 20%.
The defoaming agents used were "Kuriles (registered trademark) 653" (manufactured by Kurita Water Industries Ltd.) as an alcohol-based defoaming agent, and "Kuriles (registered trademark) S-117" (manufactured by Kurita Water Industries Ltd.) and "KM-73" (manufactured by Shin-Etsu Silicones Co., Ltd.) as silicone-based defoaming agents.

1-2. Test Method The test for the post-foaming addition group was carried out as follows. 150 ml of the digestion liquid was transferred to a 500 mL beaker, and while stirring, sulfuric acid was added dropwise in 1 mL increments until the digestion liquid reached a pH of 4. If foaming occurred during the addition of sulfuric acid, the addition was stopped and antifoaming agent was added in small amounts, and the time required for foaming to disappear was recorded. After foaming had disappeared, sulfuric acid was again added dropwise in 1 mL increments until the digestion liquid reached a pH of 4, and the addition of antifoaming agent after foaming and the time required for foaming to disappear were recorded. This procedure was repeated until the digestion liquid reached a pH of around 4.
The test for the pre-addition test group was carried out as follows: 150 ml of digestion liquid was transferred to a 500 ml beaker, and a certain amount of antifoaming agent was added to the digestion liquid in advance. After the antifoaming agent was added, the digestion liquid was stirred and thoroughly mixed. Next, sulfuric acid was added dropwise in 1 ml increments while stirring until the digestion liquid reached a pH of 4.
If foaming occurred during the addition of sulfuric acid, the addition of sulfuric acid was stopped and stirring was continued until the foaming disappeared, and the time from foaming to disappearance was recorded. After the foaming disappeared, sulfuric acid was again added dropwise in 1 mL increments until the digestion liquid reached a pH of 4. If foaming occurred, the addition of sulfuric acid was stopped and stirring was continued until the foaming disappeared, and the time from foaming to disappearance was recorded. This procedure was repeated until the digestion liquid reached a pH of approximately 4.
The defoaming time was the time required from the formation of a foam layer until the liquid surface became visible.

1-3. Test Conditions In the pre-addition test section, the antifoaming agent was added in advance to the concentration shown in the table below. When the digestive fluid foamed, resistance was generated in the fluid, causing the agitation to stop, so the agitation speed was increased as appropriate. Table 2 below shows the initial agitation speed and the increased agitation speed for each test section.

2. result
2-1. Comparative Example 1
Figure 5 shows a graph illustrating the relationship between pH change and defoaming time in the post-foaming addition (Comparative Example 1). As shown in Figure 5, foaming was observed when the pH dropped below 7. When 3 ml of sulfuric acid was added dropwise, the pH reached 6.7, and a 10 mm-thick layer of foam was formed. Flow of the digestion liquid was observed during stirring, but once the foam layer formed, the liquid stopped flowing. The stirrer rotation speed was increased to 1,200 rpm. 333 ppm of antifoaming agent was added. Since defoaming occurred 30 seconds after the addition of the antifoaming agent, sulfuric acid was continued to be added dropwise. Although it took some time after the addition of the antifoaming agent, defoaming occurred simply by stirring. Figure 6 shows the state at the start and after 3 ml of sulfuric acid was added dropwise. As shown in Figure 6, in the test area where 333 ppm of antifoaming agent was added after foaming, it took a total of approximately 12.5 minutes for the defoaming treatment to reach pH 4.

2-2. Example 1
A graph showing the relationship between pH fluctuation and defoaming time for Example 1 (pre-addition amount 1000 ppm) is shown in Figure 7. As shown in Figure 7, when 6 ml of sulfuric acid was added dropwise, a layer of foam (approximately 5 mm) formed. The pH at this time was 6.3. A fine layer of foam about 1 mm thick formed, but the fine layer of foam disappeared within 30 seconds. When 7 ml to 8 ml of sulfuric acid was added dropwise, a small amount of foam formed, but the foam disappeared within 30 seconds. After 8 ml of sulfuric acid was added dropwise, the rotation speed was increased to 1,000 rpm. The state after 6 ml of sulfuric acid was added is shown in Figure 8.
By mixing the antifoaming agent with the digestion liquid before adding sulfuric acid, the defoaming time could be significantly shortened compared to Comparative Example 1 in which the antifoaming agent was added after foaming.

2-3. Example 2
FIG. 9 is a graph showing the relationship between pH change and defoaming time in Example 2 (pre-addition amount 100 ppm). As shown in FIG. 9, a layer of foam formed when 2 ml of sulfuric acid was added dropwise. When 2 ml of sulfuric acid was added dropwise, stirring stopped, so the rotation speed was increased to 1,000 rpm. When 5 ml of sulfuric acid was added dropwise, a layer of foam approximately 3 mm thick formed. The pH at that time was 6.3. After foaming, the foam disappeared within 1 minute. When 6 ml of sulfuric acid was added dropwise, a layer of foam approximately 5 mm thick formed. The pH was 6.0. After foaming, the foam disappeared within 1 minute. As shown in FIG. 9, in this example, it took a total of approximately 5.5 minutes for the foam to disappear. FIG. 10 shows the state when 5 ml and 6 ml of sulfuric acid were added dropwise.
Even though the amount of antifoaming agent used (100 ppm) was one-third or less of that used in Comparative Example 1 (333 ppm), by mixing the antifoaming agent with the digestion liquid in advance before adding sulfuric acid, the defoaming time could be significantly shortened compared to Comparative Example 1 in which the antifoaming agent was added after foaming.

2-4. Example 3
FIG. 11 is a graph showing the relationship between pH change and defoaming time for Example 3 (pre-addition amount 20 ppm). As shown in FIG. 11, a layer of foam formed when 2 ml of sulfuric acid was added dropwise. When 2 ml of sulfuric acid was added dropwise, stirring stopped, so the rotation speed was increased to 1,000 rpm. When 3 ml of sulfuric acid was added dropwise, a layer of foam about 3 mm in size formed. The pH was 6.4. After foaming, the foam defoamed in one minute. When 5 ml of sulfuric acid was added dropwise, a layer of foam about 5 mm in size formed. The pH was 6.0. After foaming, the foam defoamed in one minute. FIG. 12 shows the state when 3 ml and 5 ml of sulfuric acid were added dropwise.
Even though the amount of antifoaming agent used (20 ppm) was 1/15 or less of that used in Comparative Example 1 (333 ppm), by mixing the antifoaming agent with the digestion liquid in advance before adding sulfuric acid, the defoaming time could be significantly shortened compared to Comparative Example 1 in which the antifoaming agent was added after foaming.

2-5. Comparative Example 2
FIG. 13 is a graph showing the relationship between pH change and defoaming time in Comparative Example 2 (pre-addition amount 10 ppm). As shown in FIG. 13, a foam layer was formed when 3 ml of sulfuric acid was added dropwise. A foam layer of about 2 mm was also formed when 3 ml of sulfuric acid was added dropwise. The pH was 6.5. After foaming, the foam defoamed in 2 minutes. When 6 ml of sulfuric acid was added dropwise, a foam layer of 5 mm was also formed. The pH at this time was 5.9. After foaming, the foam defoamed in 4 minutes. The state after 6 ml of sulfuric acid was added dropwise is shown in FIG. 14.

2-6. Comparative Example 3
FIG. 15 is a graph showing the relationship between pH change and defoaming time for Comparative Example 3 (using silicone-based defoamer "CURIRES S-117"; pre-added amount 1000 ppm). As shown in FIG. 15, when 2 ml of sulfuric acid was added dropwise, a layer of foam (5 mm) formed. The pH was 6.8. Because stirring could not be confirmed from the side of the beaker, the rotation speed was increased to 750 rpm, and the foam defoamed in 3 minutes. After 3 ml of sulfuric acid was added dropwise, stirring could no longer be seen from the side of the beaker, so the rotation speed was increased to 1000 ppm. When 6 ml of sulfuric acid was added dropwise, a layer of foam about 5 mm thick also formed. The pH was 6.0. After foaming, the foam defoamed in 2 minutes. FIG. 16 shows the state when 2 ml and 6 ml of sulfuric acid were added dropwise.

2-7. Comparative Example 4
FIG. 17 is a graph showing the relationship between pH change and defoaming time in Comparative Example 4 (using silicone-based defoaming agent "KM-73"; pre-added amount 1000 ppm). As shown in FIG. 17, when 3 ml of sulfuric acid was added dropwise, a layer of foam (3 mm) was formed. The pH was 6.6. When 6 ml of sulfuric acid was added dropwise, a layer of foam about 5 mm was also formed. The pH was 6.0. After foaming, the foam defoamed in 5 minutes. FIG. 18 shows the state when 3 ml and 5 ml of sulfuric acid were added dropwise.

2-8. Comparative Example 5
FIG. 19 is a graph showing the relationship between pH change and defoaming time for Comparative Example 5 (using silicone-based defoaming agent "CURIRES S-117"; pre-added amount 100 ppm). As shown in FIG. 19, when 3 ml of sulfuric acid was added dropwise, a layer of foam (5 mm) was formed. The pH was 6.4. The foam defoamed 5 minutes after foaming. When 6 ml of sulfuric acid was added dropwise, a layer of foam about 5 mm thick was also formed. The pH was 5.5. The foam defoamed 5 minutes after foaming. The state when 5 ml and 6 ml of sulfuric acid were added dropwise is shown in FIG. 20.

2-9. Comparative Example 6
FIG. 21 is a graph showing the relationship between pH change and defoaming time in Comparative Example 5 (using silicone-based defoamer "KM-73"; pre-added amount 100 ppm). As shown in FIG. 21, when 3 ml of sulfuric acid was added dropwise, a foam layer (5 mm) was formed. The pH was 6.4. Foaming deactivated 2 minutes after foaming. When 4 ml of sulfuric acid was added dropwise, a foam layer (5 mm) was formed. The pH was 6.2. Foaming deactivated 5 minutes after foaming. When 5 ml of sulfuric acid was added dropwise, a foam layer (3 mm) was formed. The pH was 6.0. Foaming deactivated 4 minutes after foaming. As shown in FIG. 22, in this Comparative Example, it took a total of approximately 16 minutes for the foam that had formed to defoam. FIG. 22 shows the state when 3 ml and 4 ml of sulfuric acid were added dropwise.
The results of Example 2 (alcohol-based defoaming agent; pre-added amount 100 ppm) and Comparative Example 6 (silicone-based defoaming agent; pre-added amount 100 ppm) show that the use of an alcohol-based defoaming agent can shorten the total time required for defoaming by about one-third of the time required for defoaming using a silicone-based defoaming agent.

(II. Analysis of properties after digestive fluid treatment)
In this example, a nitrogen-containing concentrate and condensed water were obtained from the digested liquid using the method for producing a nitrogen-containing concentrate from the digested liquid according to the present invention, and the properties of the nitrogen-containing concentrate and condensed water were analyzed.
1. Digestive fluid vacuum concentration test
An antifoaming agent was added to the digestion liquid to a concentration of 100 ppm, and the mixture was thoroughly mixed. Then, while stirring the digestion liquid, 20 wt % sulfuric acid was added to adjust the pH to 4. The digestion liquid, in which ammonium sulfate had been generated by pH adjustment, was subjected to vacuum evaporation and concentration using a rotary evaporator to obtain a nitrogen-containing concentrate and condensed water. The rotary evaporator was operated under the following conditions: operating pressure 20 kPa.A, constant temperature bath temperature 82°C, and rotation speed 70 rpm. The above-mentioned vacuum concentration test was performed twice, and two samples for analysis were obtained.

2. Results The properties of the nitrogen-containing concentrate and condensed water obtained as a result of the above test were analyzed. The digested fluid vacuum concentration test was conducted twice, and the properties of the nitrogen-containing concentrate and condensed water obtained in each test were analyzed. The results are shown in the table below (in the table, - indicates that no data measurement was performed). The state of the nitrogen-containing concentrate and condensed water is also shown in Figure 23.

As shown in Table 3, the ammonia concentration of the raw digested liquor was 2,300 mg/kg. On the other hand, the ammonia concentration in the condensed water after concentration was almost zero (less than 5 mg/L). In this test, the raw digested liquor was concentrated 2.5 times. Therefore, it is calculated that approximately 5,700 mg/kg of ammoniacal nitrogen was fixed in the nitrogen-containing concentrate.
According to the method for producing a nitrogen-containing concentrate from a digested liquid of the present invention, the resulting nitrogen-containing concentrate contains a high concentration of ammonia nitrogen. Furthermore, the condensed water obtained together with the nitrogen-containing concentrate contains almost no ammonia, and can be directly discharged without requiring nitrification or denitrification treatment.

(III. Relationship between sulfuric acid injection amount and digestion liquid properties)
In this example, the amount of sulfuric acid injected for concentrating the digested fluid was examined. The relationship between the amount of sulfuric acid injected for concentrating the digested fluid and the values of ammonia nitrogen and M alkalinity was also examined.

1-1. The reagent digestive fluids used were digestive fluids obtained after methane fermentation of livestock waste from Inagawa Farm (hereinafter referred to as digestive fluid I), Kobayashi Farm (hereinafter referred to as digestive fluid K), and Minna no Bokujo Farm (hereinafter referred to as digestive fluid M). Digestive fluids I and K were separated using a mesh with 1 mm openings, and the passed digestive fluid was used. Digestive fluid M was the digestive fluid after separation and was used as is.
The analysis results of the initial values for the three types of digested fluids are shown in Table 4 below. Evaporation residue was measured in accordance with the measurement method specified in JIS K 0102 14.2. NH ₄ -N was measured in accordance with the measurement method specified in JIS K 0102 42.1 42.3. M alkalinity was measured in accordance with the measurement method specified in 5.1.13 of the Sewerage Testing Methods (Japan Sewage Works Association). As shown in Table 4, digested fluid I had the highest evaporation residue, NH ₄ -N (ammonia nitrogen), and t-VFA (total volatile fatty acids), followed by digested fluid K, and digested fluid M had the lowest. Digested fluid I had the highest alkalinity, followed by digested fluid M, and digested fluid K had the lowest.
In addition, 96% sulfuric acid was diluted with water to prepare 20% sulfuric acid, which was then used.

1-2. Test Method: 150 ml of each digestion liquid was transferred to a 500 mL beaker, and 100 ppm of CRYLES 653 was added to each digestion liquid. While stirring at 1,000 rpm, 20% sulfuric acid was added dropwise until the pH reached 4.8. After the sulfuric acid addition, the digestion liquid samples were analyzed. The test was performed twice for each digestion liquid, and the average value was used as the analysis result.

2. Results Table 5 shows the titration results for the three digestion fluids up to pH 4.8. Figure 24 shows the average titration results for each digestion fluid. The sulfuric acid titration volume for digestion fluid I was 10.4 mL, the sulfuric acid titration volume for digestion fluid K was 9 mL, and the sulfuric acid titration volume for digestion fluid M was 9.7 mL.

Figure 25 shows the relationship between the amount of sulfuric acid added and evaporation residue, Figure 26 shows the relationship between the amount of sulfuric acid added and the amount of ammonia nitrogen (NH ₄ -N), and Figure 27 shows the relationship between the amount of sulfuric acid added and M alkalinity. As shown in Figures 25 to 27, there is a proportional correlation between the amount of sulfuric acid added and each water quality item, and a calibration curve could be drawn. The sulfuric acid titration rate was 6-6.8% for a digestion liquid volume of 150 mL. Furthermore, the amount of sulfuric acid added per 1 g of M alkalinity was 1.2-1.6 mg-H ₂ SO ₄ /g alkalinity.
The average values of the digestive fluid analysis results before and after the test are shown in Table 6.

(IV. Consideration of Alcohol-Based Antifoaming Agents)
1-1. Reagents
The digestate used was the digestate obtained after methane fermentation of livestock waste from Minna no Bokujo (digested liquid M). This digested liquid was used as is, since it was a digested liquid after solid-liquid separation.
The antifoaming agents used were Kuriles 653, SN Deformer 170, FK Antifoamer FALC-108, or Dappo H312.

1-2. Test method: 150 ml of each digestion liquid was transferred to a 500 mL beaker, and 100 ppm of each defoamer was added to each digestion liquid. While stirring at 1,000 rpm, 20% sulfuric acid was added dropwise until the pH reached 5.0. The time until foaming disappeared when sulfuric acid was added dropwise and the thickness of the foam layer were measured, and the effectiveness of alcohol-based defoamers from each manufacturer was compared.
The condition for foaming to start was when bubbles could be seen on the liquid surface, and the condition for foaming to stop was when bubbles could no longer be seen on the liquid surface.

2. Results: 100 ppm of each antifoaming agent was added to three digestive fluid samples (Digestive Fluid M: Minna no Bokujo). The relationship between the amount of 20% sulfuric acid added dropwise until the pH reached 5.0 and the pH is shown in Table 7 and Figure 28. As shown in Table 7 and Figure 28, when each alcohol-based antifoaming agent was added to the digestive fluid, there was no difference in the amount of sulfuric acid added dropwise until the pH reached 5.0, regardless of the antifoaming agent used.

Figure 29 also shows the defoaming time for each defoaming agent when sulfuric acid was added dropwise. As shown in Figure 29, when any of the alcohol-based defoaming agents was used, the defoaming time was less than one minute at each pH point. In this way, alcohol-based defoaming agents other than CRYLES 653 also showed favorable defoaming time results.

(V. Example of Production of Nitrogen-Containing Concentrate)
A nitrogen-containing concentrated liquid was produced from the digested liquid using the digested liquid treatment apparatus shown in FIG.
Specifically, in a mixing tank, an antifoaming agent was added to 38 L/h of digested liquor (after solid-liquid separation) after methane fermentation to a concentration of 100 ppm, and the mixture was stirred for 5 minutes. The digested liquor with sufficient antifoaming agent mixed therein was transferred to a pH adjustment tank, where 20% sulfuric acid was added to adjust the pH to 5, and the mixture was stirred for 5 minutes. After the pH adjustment, the digested liquor was transferred to a raw water concentration tank and separated into a nitrogen-containing concentrate and condensed water using a vacuum concentration device. The results of component analysis of the digested liquor used, as well as the resulting nitrogen-containing concentrate and condensed water, are shown in the table below.
As shown in Table 8, the nitrogen-containing concentrate produced by the method of the present invention contained a high concentration of nitrogen. This concentrate containing a high concentration of nitrogen can be used as liquid fertilizer or dried to produce solid fertilizer. Meanwhile, the condensed water separated by the concentration process of the digested liquid had low values for SS, TN, BOD, and COD _Mn , meeting the general wastewater standards.

REFERENCE SIGNS LIST 1 Digestive liquid treatment device 2 Digestive liquid storage tank 3 Antifoaming agent storage tank 4 Mixing tank 5 Sulfuric acid storage tank 6 pH adjustment 7 Concentrated raw water tank 8 Reduced pressure concentration means 9 Sulfuric acid dilution tank 21, 22, 23, 24 Pumps 31, 32 Flow meter 41 pH meters 51, 52, 53, 54, 55 Stirring means

Claims (16)

A method for producing a nitrogen-containing concentrated liquid from a digestive liquid, comprising: (a) a step of mixing an alcohol-based antifoaming agent with the digestive liquid; (b) a step of adding sulfuric acid to the digestive liquid obtained in the step (a) to adjust the pH; and (c) a step of concentrating the digestive liquid obtained in the step (b), wherein the nitrogen-containing concentrated liquid and condensed water are separated. The method of claim 1,
The production method, wherein the digested liquid is obtained by methane fermentation. The method of claim 1,
The production method, wherein the digestive fluid is obtained by solid-liquid separation without using a flocculant. The method of claim 1,
The method for producing the digestive liquid, wherein the alcohol-based defoaming agent in the step (a) is added to the digestive liquid so as to have a concentration in the range of 20 ppm to 1000 ppm. The method of claim 1,
The production method, wherein the pH of the digestive fluid adjusted in the step (b) is within the range of pH 4.0 to pH 6.0. The method of claim 1,
The production method, wherein the amount of sulfuric acid to be added in the step (b) is determined based on the evaporation residue, ammonia nitrogen, or M alkalinity of the digested liquid. The method of claim 1,
The amount of sulfuric acid added in the step (b) is an amount such that the amount of 20% sulfuric acid dropped per 1 g of M alkali amount based on the M alkalinity of the digested liquid is 5.8 to 7.8 mL . The method of claim 1,
the step (a) is a step of continuously flowing the digestion liquid and the alcohol-based defoaming agent into a mixing tank, and continuously flowing the digestion liquid sufficiently mixed with the alcohol-based defoaming agent into a pH adjustment tank,
The step (b) is a step of adjusting the pH of the digested liquid continuously flowing in from the mixing tank by adding sulfuric acid to the pH adjustment tank, and continuously discharging the pH-adjusted digested liquid into a concentration raw water tank.
Manufacturing method. The method of claim 1,
(d) A method for producing a nitrogen-containing concentrate, further comprising the step of drying the nitrogen-containing concentrate obtained in the step (c). 1. An apparatus for producing a nitrogen-containing concentrate from a digested liquid, comprising:
An apparatus comprising: a mixing tank for mixing the digestive liquid with an antifoaming agent; a pH adjustment tank for adjusting the pH of the digestive liquid mixed with the antifoaming agent by adding sulfuric acid; and a concentrating means for concentrating the pH-adjusted digestive liquid. 11. The apparatus of claim 10,
the mixing tank is provided with a digestion fluid supply means for supplying a desired amount of digestion fluid and a defoaming agent supply means for supplying a desired amount of defoaming agent;
The pH adjusting tank is equipped with a sulfuric acid supply means.
Device. 11. The apparatus of claim 10,
Further comprising a solid-liquid separation means for separating the digestive liquid into solid and liquid,
The solid-liquid separation means and the digestion liquid supply means are connected, and the separated liquid obtained by the solid-liquid separation means is sent to the mixing tank via the digestion liquid supply means. 11. The apparatus of claim 10,
The mixing tank and the pH adjustment tank are directly connected to each other at their lower parts, and the digested liquid introduced into the mixing tank flows directly into the pH adjustment tank after a certain residence time in the mixing tank. 11. The apparatus of claim 10,
The apparatus further includes a digester fluid reservoir, a defoamer reservoir, and a sulfuric acid reservoir. 15. The apparatus of claim 14,
The apparatus further comprises a sulfuric acid dilution tank for adjusting the sulfuric acid to be added to the pH adjustment tank. 16. The device according to any one of claims 10 to 15,
The apparatus further comprises a drying means for drying the nitrogen-containing concentrate obtained by the concentrating means, said drying means and said concentrating means being connected, and said concentrate being sent from said concentrating means to said drying means.