Digested liquid treatment method

Abstract

To provide a digested liquid treatment method capable of effectively reusing both solid content and liquid content in digested liquid as fertilizers.SOLUTION: Bentonite as a cation exchanger is added to digested liquid generated from a biogas plant, and solid content of the digested liquid is adhered to the thickened bentonite. In this case, bentonite powder having an average particle diameter of 10 μm to 30 μm is used. Ammoniacal nitrogen is adsorbed onto the bentonite, and the bentonite is separated from the liquid part of the digested liquid using a centrifugal separator. The bentonite is dried to be used as a solid fertilizer with a high nitrogen concentration, while the remaining liquid part is used as a liquid fertilizer for hydroponics with a low ammoniacal nitrogen concentration.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a digestive fluid treatment method for treating digestive fluid generated in a biogas plant, and more specifically, to a digestive fluid treatment method that enables the digestive fluid to be reused as solid fertilizer or liquid fertilizer. [Background technology]

[0002] Generally, when using a biogas plant to process food waste, food residues, livestock waste, etc., the food waste, etc. are placed in a fermentation tank inside a tank, and the gas produced by fermentation is burned to generate electricity, while the digestive fluid stored in the fermentation tank is stored in a digestive fluid storage tank and used as fertilizer (Patent Document 1, Patent Document 2, etc.). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6426863 [Patent Document 2] JP 2019-34870 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, the digested liquid discharged from such a digested liquid storage tank has an overall ammonia nitrogen concentration of 0.1% to 0.2%, which is significantly lower than the nitrogen content of general chemical fertilizers, which is 10% to 20%, and therefore must be sprayed in large quantities using a spraying device such as a large tank truck in order to use it as a substitute for chemical fertilizer.On the other hand, when using the digested liquid as liquid fertilizer for hydroponics, the ammonia nitrogen concentration of the digested liquid solution is high, at 2000 ppm or more, while the ammonia nitrogen concentration of general liquid fertilizer for hydroponics is 100 ppm, and a large amount must be diluted, so that not all of the digested liquid can be used and a large amount remains.

[0005] On the other hand, Patent Document 2 proposes a method in which the digested liquid is decompressed in a vacuum distillation apparatus, the dehydrated filtrate in an evaporator is heated and boiled, and then cooled to reduce the concentration of ammoniacal nitrogen, thereby making it usable as liquid fertilizer. However, this method does not allow the liquid to be used as a fertilizer to be spread on soil.

[0006] Therefore, the present invention has been made with attention to the above-mentioned problems, and aims to provide a digestive fluid treatment method that enables the solid and liquid components in the digestive fluid to be effectively reused as fertilizer. [Means for solving the problem]

[0007] That is, in order to solve the above-mentioned problems, the present invention provides a method for treating the digestive liquid generated from a biogas plant, comprising the steps of adding a cation exchanger to the digestive liquid generated from a biogas plant and causing the cation exchanger to adsorb cations from the digestive liquid, flocculating the cation exchanger to which the cations have been adsorbed, and separating the solids from the liquid portion of the digestive liquid.

[0008] In this way, the cation exchanger can adsorb ammonia nitrogen, sodium ions, calcium ions, magnesium ions, etc. in the digestive fluid, and the solid fraction obtained by flocculating this can be used as a solid fertilizer with a high nitrogen concentration. Meanwhile, the remaining liquid fraction can be used as a liquid fertilizer for hydroponics with a low concentration of ammonia nitrogen, since ammonia nitrogen has been largely removed from the remaining liquid fraction by the cation exchanger, without the need for large-scale dilution.

[0009] In the present invention, bentonite powder is used as the cation exchange agent.

[0010] In this way, the cation exchange properties of the bentonite allow it to adsorb ammoniacal nitrogen, metal ions, etc., and the thickening and expansion of the bentonite powder allows foreign matter in the digestive fluid to adhere and be sealed in the bentonite powder. Furthermore, when such bentonite powder is spread on soil, the water retention properties of the bentonite allow the soil to maintain its water retention, and the expansion and contraction properties of the bentonite allow it to create voids in the soil, softening the soil.

[0011] Furthermore, the mixing ratio of the digestive fluid to the cation exchanger is set to 100 parts by weight of the digestive fluid to 20 to 60 parts by weight of the cation exchanger.

[0012] By doing this, the concentration of ammoniacal nitrogen can be reduced to about 1 / 2 to 1 / 4.

[0013] Furthermore, as the cation exchange agent, a powder having an average particle size of 10 μm to 30 μm is used.

[0014] In this way, a large amount of ammonia nitrogen can be adsorbed by the powder of the cation exchange agent, and when bentonite powder is used as the cation exchange agent, the thickening and expansion of the bentonite makes it possible to confine the foreign matter in the bentonite powder when separating it from the digestive fluid, which makes it easier to separate the digestive fluid from the foreign matter. Effect of the Invention

[0015] According to this invention, the cation exchanger can adsorb ammonia nitrogen, sodium ions, calcium ions, magnesium ions, etc. in the digestive fluid, and the solid fraction obtained by flocculating this can be used as a solid fertilizer with a high nitrogen concentration. Meanwhile, the remaining liquid fraction can be used as a liquid fertilizer for hydroponics with a low concentration of ammonia nitrogen, since ammonia nitrogen has been largely removed from the remaining liquid fraction by the cation exchanger, without the need for large-scale dilution. [Brief description of the drawings]

[0016] [Figure 1] A process diagram of a digestive fluid treatment method according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0018] The digestive liquid treatment method in this embodiment is to simultaneously produce solid fertilizer and liquid fertilizer using the digestive liquid discharged from the digestive liquid storage tank of the biogas plant. Bentonite powder is added as a cation exchange agent to the digestive liquid discharged from the digestive storage tank, and in a state where the bentonite is thickened, the solid content of the digestive liquid is attached to the bentonite to be flocculated, and the solid content and the liquid part of the digestive liquid are separated. By using the cation exchange agent in this way, ammonia nitrogen, metal ions, etc. are adsorbed on the bentonite so that it can be used as a solid fertilizer with a high nitrogen content, and the remaining liquid part can be used as a liquid fertilizer with a low concentration of ammonia nitrogen. The digestive liquid treatment method in this embodiment will be described in detail below using the process diagram in Figure 1.

[0019] First, when collecting digestive fluid, the digestive fluid discharged from a biogas plant is used. This biogas plant is equipped with a receiving tank for receiving livestock manure, agricultural waste, food residues, etc., a grinder for grinding the raw materials received in the receiving tank, a primary fermentation tank for performing pretreatment such as moisture adjustment and primary fermentation, a gas storage tank for desulfurizing the gas generated in the primary fermentation tank and storing it in a gas holder, and a secondary fermentation tank for performing secondary fermentation of the raw materials generated in the primary fermentation tank to extract liquid fertilizer. Among these, the gas stored in the gas holder is used to generate electricity and can be used as electrical or thermal energy. Meanwhile, the digestive fluid generated from the secondary fermentation tank is stored in a digestive fluid tank and is sequentially discharged from there for use (step S1).

[0020] The composition of the digestive fluid stored in the digestive fluid tank depends on the composition of the waste, but generally, it contains total nitrogen, ammonia nitrogen, nitrate nitrogen, phosphoric acid, potassium, total carbon, etc., in a water content of 95% by weight. The digestive fluid containing these components contains 1.0 to 2.0 kg of ammonia nitrogen per 1,000 kg of digestive fluid, 1.0 to 1.5 kg of phosphoric acid, and 3.5 to 4.5 kg of potassium. The digestive fluid containing these components is stored in a storage tank and used as liquid fertilizer and solid fertilizer.

[0021] The cation exchange agent used in separating the digestive fluid is a material such as a cation exchange resin having a carboxyl group or a sulfo group in its molecule, clay ore, or zeolite, which adsorbs cations contained in the water and exchanges them for its own cations. In this example, bentonite powder is used.

[0022] This bentonite is a weakly alkaline clay rock mainly composed of the clay mineral montmorillonite, with quartz, α-cristobalite, opal, and other silicate minerals as secondary components, and associated silicate minerals such as feldspar, mica, and zeolite, carbonate minerals such as calcite, dolomite, and gypsum, sulfate minerals, and sulfide minerals such as pyrite, and contains silicon, aluminum, iron, magnesium, calcium, sodium, potassium, oxygen, hydrogen, sulfur, carbon, etc. As this bentonite contains a large amount of layered aluminum phyllosilicate, it has very high cation exchange, water absorption, viscosity, and caking properties, and in this embodiment, these properties are used to adsorb and separate ammonia nitrogen in the digestive fluid.

[0023] When such bentonite is used to adsorb ammoniacal nitrogen in the digestive fluid, if the bentonite particles are made large, it can be used as a solid fertilizer as it is, but the surface area is small and it is not possible to adsorb a large amount of cations including ammoniacal nitrogen. On the other hand, if the bentonite particles are made small, the surface area is large and it is possible to adsorb a large amount of ammoniacal nitrogen, but it becomes too viscous. Therefore, it is preferable to crush the bentonite to a size of about 10 μm to 30 μm as the average particle size (step S2). With such a particle size, the surface area is increased and it is possible to adsorb a large amount of ammoniacal nitrogen and cations such as metal ions, and the bentonite separated from the digestive fluid can be used as a liquid fertilizer with a low concentration of ammoniacal nitrogen.

[0024] The crushed bentonite powder is then added to the digestive liquid (step S3). When adding the bentonite to the digestive liquid, the bentonite powder is added in a weight ratio of 20 to 60 parts by weight of 100 parts by weight of the digestive liquid, and mixed until uniform (step S4). When the bentonite is added in a ratio of 20 to 40 parts by weight, the concentration of ammoniacal nitrogen in the digestive liquid can be reduced to about 1 / 2, while when the bentonite is added in a ratio of 40 to 60 parts by weight, the concentration of ammoniacal nitrogen in the digestive liquid can be reduced to 1 / 4.

[0025] When such bentonite powder is added to the digestive fluid, it thickens by absorbing water, and foreign matter in the digestive fluid can be attached to the thickened bentonite, and the foreign matter can be sealed in the bentonite by expanding. Then, in order to separate the thickened and expanded bentonite from the digestive fluid, it is centrifuged (step S5). When separating bentonite using such a centrifuge, a batch-type centrifuge that can continuously input digestive fluid is used. Then, the rotating cylinder of the centrifuge is rotated at a rotation speed of 5000 rpm to 7000 rpm (3000 G to 4000 G) for about 30 minutes to separate the digestive fluid from the bentonite. Then, the separated digestive fluid is taken out (step S6), and the remaining bentonite is taken out of the centrifuge (step S7) and dried (step S8).

[0026] In this way, the digested liquid discharged from the centrifuge has a low concentration of ammoniacal nitrogen, while the bentonite powder absorbs a large amount of ammoniacal nitrogen and the nitrogen content becomes high. In other words, the bentonite can be used to separate the digested liquid into ammoniacal nitrogen concentrations suitable for liquid fertilizer and nitrogen concentrations suitable for solid fertilizer.

[0027] The digestive fluid thus separated is stored in a separate storage tank, and when it is used as liquid fertilizer, it is diluted to a concentration suitable for liquid fertilizer (step S9).On the other hand, when bentonite is used as solid fertilizer, the bentonite with adsorbed ammonia nitrogen is dried, bagged, transported to a farm field, and spread on the farm field (step S10).

[0028] When dried bentonite is spread on soil and mixed with the soil by a tractor, the bentonite absorbs water and expands 10 to 15 times its size, forming voids in the soil, while when it dries it returns to its normal volume, and this expansion and contraction creates voids in the soil. This allows air to enter the soil and promotes the growth of plant roots.

[0029] According to the above embodiment, the digestive liquid is treated by adding powdered bentonite to the digestive liquid generated from the biogas plant, adhering the solids of the digestive liquid to the thickened bentonite, agglomerating the solids with the bentonite, and separating the solids from the liquid of the digestive liquid. The cation exchange performance of the bentonite allows the ammoniacal nitrogen in the digestive liquid to be adsorbed, making it possible to use the resulting liquid as a solid fertilizer with a high nitrogen content. Meanwhile, the remaining liquid has had ammoniacal nitrogen largely removed by the bentonite, making it possible to use the resulting liquid as a liquid fertilizer for hydroponic cultivation with a low ammoniacal nitrogen concentration without the need for large-scale dilution.

[0030] The present invention is not limited to the above-described embodiment, but can be embodied in various forms.

[0031] For example, in the above embodiment, bentonite is used as the cation exchange agent, but other materials such as cation exchange resin may also be used.

[0032] In the above embodiment, the bentonite powder is added to the digestive fluid and then centrifuged, but the digestive fluid may be centrifuged once before the bentonite is added to condense the foreign matter, and then the bentonite powder may be added. In this way, when the foreign matter and bentonite powder are separated again in the centrifuge later, the foreign matter and bentonite can be separated at a low rotation speed.

[0033] Furthermore, in the above embodiment, bentonite powder with a particle size of approximately 10 μm to 30 μm is used, but if the bentonite powder is intended to be spread directly on soil as fertilizer, powder with a size of approximately 100 μm to 1000 μm may be used. EXAMPLES

[0034] When 7.5g of bentonite powder and 0.5g of coagulant were added to 30g of digested liquid and centrifuged at 3750G for 30 minutes, the ammonia nitrogen concentration of the liquid fertilizer decreased from 2003PPM to 1001PPM.

[0035] On the other hand, when 50g of bentonite powder was added to 100g of digested liquid, 50g of the liquid was taken, 0.1g of coagulant was added, and the liquid was centrifuged at 3750G for 30 minutes, the concentration of the liquid fertilizer went from 2733PPM to 565PPM. The ammonia nitrogen concentration of the liquid fertilizer was the same when the liquid was centrifuged in the same way without adding coagulant.

Claims

1. The present invention relates to a digestive liquid treatment method, comprising the steps of: introducing a cation exchanger into a digestive liquid generated from a biogas plant, and allowing the cation exchanger to adsorb cations in the digestive liquid; flocculating the cation exchanger to which the cations have been adsorbed; and separating the solid content from the liquid portion of the digestive liquid. A digestive fluid treatment method comprising the steps of:

2. 2. The method for treating digestive fluid according to claim 1, wherein the cation exchange agent is bentonite powder.

3. 2. The method for treating digestive fluid according to claim 1, wherein the mixing ratio of the digestive fluid to the cation exchanger is, in weight ratio, 20 to 60 parts of the cation exchanger to 100 parts of the digestive fluid.

4. 2. The method for treating digestive fluid according to claim 1, wherein the cation exchange agent is a powder having an average particle size of 10 μm to 30 μm.

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