JP7629351B2 - Organic wastewater treatment system - Google Patents

Description

This disclosure relates to an organic wastewater treatment system.

Food-derived organic wastewater, including sugars, discharged from food factories, waste food processing facilities, etc., has a high biochemical oxygen demand (BOD value). From the perspective of preventing water pollution, it is necessary to treat the wastewater to remove the organic matter before it is discharged into the environment.
In particular, organic wastewater containing food-derived sugars is generated in large quantities and has a low content of organic components, so that, compared to general organic wastewater, there is a problem that the introduction of, for example, methane fermentation related to anaerobic treatment is limited. For example, the concentration of sugars contained in organic wastewater discharged from a noodle factory, which is a food factory that produces noodles, is about 5 mass %.

In recent years, in the treatment of organic wastewater, there has been a demand for reducing energy consumption, the amount of residual waste, and costs in order to improve the efficiency of the treatment.
Methods for treating organic wastewater include aeration, in which part of the organic matter is separated as solid matter by pressure flotation and removed as sludge, and the remaining wastewater is aerobically treated to convert it to _CO2 and discharged into the sewer. Aeration has the problem that a lot of energy is consumed for aeration during the treatment process, and furthermore, solid by-products such as scum and sludge are generated, which must be treated separately as waste.
Another method for treating organic wastewater is to carry out anaerobic treatment using methanogens in an upflow anaerobic sludge blanket reactor (UASB reactor) or the like.
Anaerobic treatment consumes less energy than aerobic treatment, but from the perspective of recovering valuable materials to reduce treatment costs, there is still room for improvement in terms of cost because methane gas recovered as a valuable material is inexpensive.

From the viewpoint of effective utilization of organic wastewater, methods have been attempted to extract valuable materials from organic matter in biomass to reduce wastewater treatment costs. A technology has been proposed in which furfural is extracted from hemicellulose in lignocellulosic biomass by treating it with hot compressed water in the presence of an aqueous phosphoric acid solution (see Patent Document 1).
A method has been proposed in which a prehydrolyzed liquid generated in a pulp production process is subjected to ethanol fermentation to extract ethanol, the remaining treated liquid is subjected to an acid treatment for a dehydration reaction to produce furfural, and the remaining treated liquid is subjected to methane fermentation to obtain methane and a methane fermentation effluent (see Patent Document 2).
As a technique focusing on raw sugar, a method for producing 5-hydroxymethyl-2-furfural suitable for industrial-scale production has been proposed, in which carbohydrates containing hexose as a constituent sugar or derivatives thereof are dehydrated using activated carbon as a catalyst. It has been disclosed that the use of activated carbon as a catalyst also makes it easy to recover the 5-hydroxymethyl-2-furfural obtained (see Patent Document 3).

JP 2014-214086 A JP 2014-166172 A JP 2018-2701 A

Both of the technologies described in Patent Documents 1 and 2 have an issue of recovering valuable materials from waste liquid, but they are technologies related to waste liquid treatment aimed at effective utilization of cellulosic biomass, and there is no focus on application to the treatment of low-concentration organic wastewater that is generated in large quantities, such as organic wastewater derived from food.
Patent Document 3 relates to a technology for industrially producing 5-hydroxymethyl-2-furfural with high efficiency from limited raw sugar, and is not suitable for application to the treatment of organic wastewater containing various organic substances.

There is a need to develop a wastewater treatment system that reduces wastewater treatment costs and the environmental burden by preventing the generation of undesirable by-products, efficiently recovering valuable materials using low energy, and reducing the organic components in the wastewater, even for organic wastewater derived from food that is generated in large quantities and contains low concentrations of organic components.

The objective of one embodiment of the present invention is to provide a wastewater treatment system that consumes little energy and produces little undesirable by-products, and is suitable for treating organic wastewater that contains sugars derived from food.

Means for solving the above problems include the following embodiments.
<1> A wastewater treatment system comprising: an organic wastewater concentrating section that concentrates sugars in wastewater containing sugars derived from food; a hydrothermal treatment section that hydrothermally treats the sugars contained in the concentrated wastewater to convert them into at least one selected from furfural and 5-hydroxymethylfurfural (hereinafter also referred to as 5-HMF); a recovery section that recovers the at least one selected from furfural and 5-hydroxymethylfurfural converted from the wastewater that has been subjected to the hydrothermal treatment; and a methane fermentation section that decomposes organic matter remaining in the wastewater that has been subjected to the hydrothermal treatment by methane fermentation to convert it into biogas containing methane gas, and recovers the biogas.

According to the first embodiment of the present disclosure, first, sugars are concentrated in the organic wastewater concentrating section, and the water content in the wastewater is reduced, thereby reducing the energy consumption in the subsequent hydrothermal treatment section, suppressing the production of by-products, and efficiently obtaining valuable materials such as furfural and biogas.
Furthermore, since the remaining waste liquid from which organic matter such as furfural has already been recovered in the recovery section has a reduced organic matter content, the volume load of the fermentation tank in the methane fermentation section downstream of the recovery section is improved, and the fermentation tank can be made smaller.
In addition, the residual wastewater treated by methane fermentation and the wastewater separated in the organic wastewater concentrating section have a low organic matter content, which can reduce the load on wastewater treatment equipment, such as an aerobic treatment section, that is optionally provided downstream of the methane fermentation section.

<2> The wastewater treatment system described in <1>, in which the organic wastewater concentrating section is equipped with a reverse osmosis membrane (hereinafter also referred to as an RO membrane).

According to the second embodiment of the present disclosure, in the first embodiment of the present disclosure, by applying membrane technology to separate organic matter and water in the organic wastewater concentrating section, it is possible to efficiently concentrate organic matter with less energy consumption. In addition, since the wastewater separated by the RO membrane contains almost no organic components, it can be discharged as it is as treated water.

<3> The wastewater treatment system described in <1> or <2> further includes an aerobic treatment section that aerobically treats at least one of the wastewater separated in the organic wastewater concentration section and the wastewater separated in the methane fermentation section.

According to the third embodiment of the present disclosure, by further providing an aerobic treatment section in the first or second embodiment of the present disclosure, at least one of the wastewater separated in the organic wastewater concentration section and the wastewater separated in the methane fermentation section can be further treated aerobically, thereby enabling discharge into the sewer with less environmental impact.
In the third embodiment, the wastewater supplied to the aerobic treatment unit has had its organic component content reduced in advance in an organic wastewater concentration unit or a methane fermentation unit, so that the energy consumption in the aerobic treatment unit is reduced compared to commonly performed aerobic treatment.

According to one embodiment of the present invention, it is possible to provide a wastewater treatment system that consumes little energy and produces little undesirable by-products, and is suitable for treating organic wastewater that contains sugars derived from food.

1 is a system configuration diagram showing a wastewater treatment system according to an embodiment of the present disclosure. FIG. 2 is a system configuration diagram showing a wastewater treatment system according to another embodiment of the present disclosure. FIG. 1 is a system configuration diagram showing an embodiment of a conventional wastewater treatment system to which aerobic treatment is applied.

The wastewater treatment system of the present disclosure will be described in detail below.
The following description of the configuration elements may be based on representative embodiments of the present disclosure, but the present disclosure is not limited to such embodiments.
In the present disclosure, the use of "to" indicating a range of values means that the values before and after the range are included as the lower limit and upper limit.
In the numerical ranges described in the present disclosure in stages, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. In addition, in the numerical ranges described in the present disclosure, the upper or lower limit value of the numerical range may be replaced with a value shown in the examples.
In the present disclosure, combinations of two or more preferred aspects are more preferred aspects.
In the present disclosure, 5-hydroxymethyl-2-furfural and 5-hydroxymethylfurfural may be referred to as "5-HMF." In addition, at least one selected from the group consisting of furfural and 5-HMF may be collectively referred to as "furfurals."
In this disclosure, "room temperature" refers to 25° C. unless otherwise specified.
In the present disclosure, the term "step" refers not only to an independent step, but also to a step that cannot be clearly distinguished from other steps, as long as the intended purpose of the step is achieved.
In the various drawings, components designated by the same reference numerals are meant to be the same components.
The dimensional ratios in the drawings do not necessarily represent the actual dimensional ratios.

Below, we will explain a wastewater treatment system according to one embodiment of the present disclosure, with reference to the drawings.

(Wastewater Treatment System: First Embodiment)
FIG. 1 shows a system configuration diagram of a wastewater treatment system 10 according to a first embodiment of the present disclosure.

The wastewater treatment system 10 is composed of an organic wastewater concentrating section 12 that concentrates the organic components of wastewater containing organic components derived from food, a hydrothermal treatment section 14 that performs hydrothermal treatment on the concentrated wastewater, a recovery section 16 that recovers furfurals converted from the organic components in the hydrothermal treatment section 14, a methane fermentation section 18 that performs methane fermentation treatment on the wastewater from which the furfurals have been recovered, and transport equipment 30A-30G that includes pumps and the like (not shown).

The transport equipment 30A to 30G is configured with piping equipped with a pump (not shown) that is useful for transporting wastewater as needed.

As shown in FIG. 1, wastewater containing organic components including sugars derived from food (shown as "organic wastewater" in FIG. 1) is transported to organic wastewater concentrating section 12, where it is separated into a concentrate in which the organic components are concentrated and separated wastewater containing a large amount of water.
The concentrate concentrated in the organic wastewater concentrating section 12 is transported to the hydrothermal treatment section 14 through a pipe 30A which is a transport facility.
The separated wastewater containing a large amount of water separated by the concentration is discharged into the sewer through the pipe 30B, or is transported to an aerobic treatment section (not shown) as necessary.

-Organic wastewater concentration section-
The organic wastewater concentrating section 12 in the wastewater treatment system 10 of the present disclosure concentrates the organic wastewater containing sugars derived from food.
Organic wastewater containing sugar is discharged in large quantities from food factories and the like due to the food manufacturing process, food washing process, and the like. Compared to wastewater derived from food waste that contains solids, for example, the concentration of organic components in this wastewater is lower and the content of liquid components is higher.
Organic wastewater containing sugars derived from food contains organic components other than sugars, such as proteins and lipids, and the types of sugars and the types and amounts of organic components other than sugars in organic wastewater discharged from food factories, etc. vary.
When food-derived sugar-containing organic wastewater contains protein in addition to sugar, the protein is decomposed in the hydrothermal treatment section described below and converted to methane in the methane fermentation section, so there is no need for separation and the wastewater can be treated with the wastewater treatment system of the present disclosure. On the other hand, lipids are difficult to decompose in the hydrothermal treatment section, so for organic wastewater containing a large amount of lipids, it is preferable to consider separating and removing lipids before applying the wastewater treatment system of the present disclosure.

To convert sugars contained in organic wastewater from food sources into furfurals, the organic wastewater must be heated, but a large amount of energy is required to heat the wastewater, which contains a large amount of solvent.
In the wastewater treatment system of the present disclosure, an organic wastewater concentrating section is provided to concentrate the organic wastewater containing sugars, thereby obtaining a concentrated liquid from the organic wastewater that has a higher concentration of organic components, particularly sugars. In addition, the separated wastewater that is separated from the concentrated liquid and contains a large amount of solvent, mainly water, is separated wastewater with very little remaining organic components such as sugars.
By increasing the concentration of sugars in the organic wastewater concentrating section and obtaining a concentrated liquid with a reduced amount of solvent such as water, the energy consumption required for heating can be reduced in the hydrothermal treatment to obtain valuable components such as furfurals from the concentrated liquid.

There is no particular restriction on the concentration rate of the organic wastewater in the organic wastewater concentrating section, and the effect is obtained as long as the amount of the solvent is reduced compared to the organic wastewater introduced into the organic wastewater concentrating section.
In particular, from the viewpoint of improving the efficiency of the treatment in the hydrothermal treatment section, it is preferable to concentrate the obtained concentrated liquid so that the sugar content is in the range of 1% by mass to 20% by mass, and it is more preferable to concentrate the concentrated liquid so that the sugar content is in the range of 3% by mass to 10% by mass.

The method for concentrating the organic wastewater in the concentrating section is not particularly limited, and any known method can be applied.
Examples of the concentration method include a method of concentrating the organic wastewater by filtering it using a membrane, for example, a reverse osmosis membrane (RO membrane), a boiling concentration method in which the organic wastewater is heated at a high temperature to evaporate the water and concentrate it, a vacuum concentration method in which the organic wastewater is depressurized at a low temperature to evaporate the water, freeze concentration in which the water contained in the organic wastewater is frozen and the ice crystals are separated and extracted, and a concentration method that combines these methods in any combination.

Among these, the concentration method using an RO membrane is preferred from the viewpoints of low energy consumption, efficient concentration, and clean separated wastewater. An RO membrane generally has pores of 2 nm or less, and in order to continuously obtain a concentrated solution containing a large amount of sugars, it is necessary to apply a pressure exceeding the osmotic pressure of the sugars.
In the present disclosure, the term "RO membrane" is used to include not only a general reverse osmosis membrane (RO membrane) but also a nanofiltration membrane (NF membrane) having a relatively larger opening size than an RO membrane, for example, an NF membrane having an opening diameter of 1 nm to 2 nm and having a separation efficiency of ions and the like of 70% or less, which is lower than that of an RO membrane.
Separation of the concentrated liquid and the separated wastewater by the RO membrane can be performed by a known method. Since the amount of water permeated through the RO membrane depends on temperature, the pressurization condition of the osmotic pressure or higher required for separation may be appropriately adjusted in consideration of the temperature of the organic wastewater, the type of sugars contained in the organic wastewater, etc.

- Hydrothermal Treatment Section -
The concentrated liquid obtained in the organic wastewater concentrating section 12 is transported to the hydrothermal treatment section 14 through a pipe 30A which is a transport facility.
In the hydrothermal treatment section, the concentrated liquid is treated at high temperature and high pressure in a closed system, whereby sugars in the organic wastewater can be converted into furfurals.
According to the hydrothermal treatment, the system is heated to a high temperature and pressure, so that water remains in a subcritical liquid state and is believed to have a high hydrolysis ability.
The concentrated liquid to be treated in the hydrothermal treatment section is not particularly limited as long as it contains sugars and water as a solvent.
In the hydrothermal treatment process, an autoclave, a tubular reactor, a flow reactor, or the like can be used.
The concentrated liquid is introduced into a reaction apparatus of the hydrothermal treatment section, and is subjected to hydrothermal treatment at the temperature and pressure shown below, whereby the reaction proceeds and the sugars in the concentrated liquid are converted into furfurals.

The temperature of the hydrothermal treatment section can be 100°C to 400°C, preferably 140°C to 260°C, and more preferably 140°C to 230°C. By heating to the above temperature range, furfurals can be efficiently obtained from the concentrated liquid.

In the hydrothermal treatment section, furfurals are produced by treating the concentrated liquid within the above temperature range, so no particular catalyst is required.
From the viewpoint of improving the production efficiency of furfurals, the hydrothermal treatment may be performed in the presence of a catalyst. When a catalyst is used, the catalyst may be appropriately selected from known catalysts. Examples of catalysts that can be used in the hydrothermal treatment section include modified or unmodified activated carbon, organic acids such as citric acid, inorganic acids such as sulfuric acid, solid acid catalysts such as ion exchange resins, and zeolite catalysts with controlled catalytic activity sites.
The pressure in the hydrothermal treatment step may be normal pressure, but from the viewpoint of reaction efficiency, pressurization is also a preferred embodiment.
When the hydrothermal treatment is performed, the pressure conditions are preferably 0.15 MPa to 10 MPa, and more preferably 1.0 MPa to 3.5 MPa.
The reaction time is preferably 0.2 to 3.0 hours, and more preferably 0.5 to 1.5 hours.
After the reaction is completed, it is preferable to cool the temperature inside the apparatus in the hydrothermal treatment section to room temperature (25° C.).

In general, various attempts have been made to efficiently produce furfurals from sugars, cellulose, etc. However, the wastewater treatment system disclosed herein has the problem of consuming less energy and producing less undesirable by-products in the treatment of organic wastewater containing sugars. According to the wastewater treatment system disclosed herein, furfurals are obtained as a valuable resource in the hydrothermal treatment section, thereby reducing the cost of the entire wastewater treatment system. Furthermore, biogas can be recovered as a valuable resource in the subsequent methane fermentation section. For this reason, extremely high efficiency in the production of furfurals in the hydrothermal treatment section is not important in the wastewater treatment system disclosed herein.

The concentrated liquid hydrothermally treated in the hydrothermal treatment section 14 contains at least furfurals produced by the hydrothermal treatment, organic acids and other organic components derived from the organic wastewater that were not converted to furfurals, and water as a solvent. The concentrated liquid hydrothermally treated in the hydrothermal treatment section 14 is cooled to room temperature and then transported to the recovery section 16 through the piping 30C, which is a transport facility.

-Collection Department-
In the recovery section 16, furfurals produced in the concentrated liquid are recovered.
The furfurals in the concentrated liquid in the recovery section can be recovered by a known method.
Examples of recovery methods include a method in which the concentrated liquid is introduced into a container that is impermeable to the concentrated liquid and the furfurals are adsorbed onto an adsorbent such as activated carbon or an ion exchange resin, and a method in which the furfurals are extracted with an organic solvent such as a hexane/acetone mixed solvent.
It is also preferable to first subject the concentrated liquid to solid-liquid separation to remove fine solid components before contacting the concentrated liquid with the adsorbent, and then contact only the separated liquid component with the adsorbent. By removing the solid components from the concentrated liquid in advance, it is possible to prevent the solid components from adhering to the pores of the adsorbent such as activated carbon, thereby preventing a decrease in adsorption efficiency.
In the case of the method of adsorbing furfurals onto an adsorbent, the adsorbent is washed with an organic solvent, whereby the furfurals are dissolved in the organic solvent, and high-purity furfurals are recovered. The recovered furfurals are recovered and stored as valuable materials through the pipe 30D, which is a transport facility.

Furfurals are attracting attention as biomass-derived chemical raw materials that can replace fossil resources.
Furfural is useful as a solvent, a raw material for synthetic rubber, a raw material for producing furan resin, and a raw material for producing adipic acid, which is a raw material for polyamide. 5-HMF is useful as a raw material for biofuel and synthetic resin, and is also used as a biologically active substance.
Therefore, the furfural and furfural-related compounds such as 5-HMF recovered in the recovery section 16 can be sold as valuable materials, which is expected to reduce the operating costs of the wastewater treatment system.
The wastewater treatment system of the present disclosure has the advantage of being able to continuously recover furfurals at a constant yield.

The remaining wastewater from which furfurals have been recovered in the recovery section 16 is transported through the piping 30E, which is a transport facility, to the methane fermentation section 18, which serves as a purification treatment section. The remaining wastewater still contains organic components other than furfurals, such as organic acids, and it is difficult to discharge it as treated water as is. For this reason, further organic matter is recovered in the methane fermentation section 18, which serves as a purification treatment section.

-Methane fermentation department-
In the methane fermentation section 18, methane fermentation is carried out using the wastewater remaining after the furfurals have been removed in the recovery section 16 as a raw material, and biogas containing methane gas is produced and recovered.

The waste liquid after the furfurals are extracted contains organic matter such as fine organic matter and organic acids derived from food.
The organic components in the wastewater include soluble components such as organic acids dissolved in the wastewater, and insoluble organic components contained in the wastewater.

In the methane fermentation section, anaerobic treatment, that is, under anaerobic conditions, is carried out using anaerobic microorganisms such as acid-producing bacteria and methanogenic bacteria to decompose the organic components of the remaining wastewater and obtain biogas such as methane gas and carbon dioxide.
A known fermenter can be used for the methane fermentation section. As the fermenter, for example, a UASB reactor, a fluidized bed reactor, a fixed bed reactor, etc. are preferably used.
As the anaerobic bacteria used in the methane fermentation section, known anaerobic bacteria capable of decomposing organic components under anaerobic conditions can be used.

Since methanogens have a low proliferation rate, there is a concern that they will be washed away with the treated water, resulting in a decrease in treatment capacity. Therefore, in order to maintain treatment capacity, it is preferable to allow the anaerobic bacteria to self-granulate or to immobilize them on a carrier and place them in the fermenter. For example, in a method using a UASB reactor, good reactivity is achieved by forming granular sludge with excellent settling properties of about 0.5 mm to 2 mm, which is formed by self-granulation of methanogens.
There are no particular limitations on the fermentation conditions, and they are appropriately selected taking into consideration the remaining amount of organic components, the amount of wastewater to be treated, and the like.

Anaerobic microorganisms such as methanogens are placed in the fermenter. Organic components in the wastewater containing organic acids and the like, which has passed through the recovery section 16 and is supplied via pipe 30E, are decomposed by methane fermentation in the methane fermentation section.
The anaerobic treatment conditions in the methane fermentation section include a mesophilic method in which the reaction temperature is about 37° C., and a high temperature method in which the reaction temperature is about 55° C., and either method can be used.
From the viewpoint of fermentation efficiency, the high-temperature method has a better treatment efficiency, but requires energy for heating. In the case of the mesophilic method, heating is not required. In the wastewater treatment system of the present disclosure, wastewater from which valuable materials as organic components have been removed in advance is treated, so that stable treatment efficiency can be expected even in the mesophilic method. If desired, two fermentation tanks for the mesophilic method can be provided to perform two-stage fermentation.
Since the optimum condition for anaerobic microorganisms is near neutral, the pH in the fermenter is preferably 6.0 to 8.5, more preferably 7.0 to 8.0. If desired, the pH of the wastewater introduced into the methane fermentation section may be adjusted.
In order to maintain the treatment conditions of the anaerobic microorganisms within a suitable range, the anaerobic state of the fermenter can be controlled, for example, by measuring the oxidation-reduction potential or the like.

In the methane fermentation section, there is no particular limit to the contact time between the wastewater to be treated and the anaerobic microorganisms in the fermenter, i.e., the residence time of the wastewater in the fermenter. For example, the residence time of the wastewater in the fermenter can be 5 hours to 30 days, preferably 6 hours to 20 days, and more preferably 6 hours to 3 days.

The fermentation tank has an anaerobic atmosphere, and when the mesophilic process is used, no heating is required, and aeration energy as in aerobic treatment is also not required.
The biogas produced in the methane fermentation section is composed mainly of methane gas and carbon dioxide, and is a valuable resource that can be used as fuel, etc. Depending on the conditions, hydrogen sulfide may be produced as a by-product. In such cases, a process is carried out to remove the hydrogen sulfide from the biogas.
The biogas may be effectively utilized by being supplied to the outside as fuel, for example, it may be used as fuel for heating in a hydrothermal treatment section.
According to the wastewater treatment system of the present disclosure, not only furfurals but also biogas is produced, which can be provided as a valuable resource or used as an energy source for heating the wastewater treatment system, thereby achieving the goal of reducing the cost of the entire wastewater treatment system.

The biogas produced in the methane fermentation section 18 is recovered outside the system via pipe 30G. If the amount of remaining organic components in the separated wastewater after biogas recovery is small, it is directly discharged into the sewer via pipe 30F. Note that even after treatment in the methane fermentation section 18, insoluble organic components such as components that have not been decomposed by fermentation may remain in the separated wastewater. In such cases, it is preferable to remove the insoluble organic components by solid-liquid separation before discharging into the sewer. The insoluble organic components separated by solid-liquid separation may be separately treated as waste.
The solid-liquid separation method is not particularly limited, and can be performed by a known method such as filtration, centrifugation, or leaving the mixture to settle out of the solids.

In the wastewater treatment system of the present disclosure, the separated wastewater supplied to the methane fermentation section has organic matter such as furfural previously collected in the collection section, and the organic matter content in the wastewater is reduced, which allows the fermentation tank in the methane fermentation section downstream of the collection section to be made smaller.
In addition, the residual wastewater treated by methane fermentation and the wastewater separated in the organic wastewater concentrating section have a low organic matter content. Therefore, when the organic component content is below a predetermined amount, the wastewater can be directly discharged into the sewer. In addition, the processing load in the wastewater treatment equipment, such as an aerobic treatment section, which is installed as desired downstream of the methane fermentation section, can be reduced.

(Wastewater treatment system: second embodiment)
FIG. 2 shows a system configuration diagram of a wastewater treatment system 20 according to a second embodiment of the present disclosure.

The wastewater treatment system 20 is composed of an organic wastewater concentrating section 12 that concentrates organic components of wastewater containing organic components derived from food, a hydrothermal treatment section 14 that performs hydrothermal treatment on the concentrated wastewater, a recovery section 16 that recovers furfurals converted from the organic components in the hydrothermal treatment section 14, a methane fermentation section 18 that performs methane fermentation treatment on the wastewater from which the furfurals have been recovered, an aerobic treatment section 22 that aerobicly treats the separated wastewater separated from the methane fermentation section 18, and transport equipment 30A-30H that includes pumps and the like not shown.

The transport facilities 30A to 30H are configured to have piping equipped with a pump (not shown) useful for transporting wastewater as necessary.
The wastewater treatment system 20 has the same configuration as the wastewater treatment system 10 according to the first embodiment, except that it includes an aerobic treatment section 22 downstream of the methane fermentation section 18. Therefore, the organic wastewater concentrating section 12, the hydrothermal treatment section 14, the recovery section 16, and the methane fermentation section 18 will not be described.

- Aerobic treatment section -
The aerobic treatment section 22 shown in FIG. 2 is provided downstream of the methane fermentation section 18, and is a treatment section that further treats the separated wastewater from which biogas has been recovered in the methane fermentation section 18 under aerobic conditions, thereby performing a purification process suitable for discharge into the sewerage system.
The aerobic treatment section 22 is a treatment section that applies an activated sludge method or a carrier fluidized bed method in which an aerobic microorganism group decomposes organic components in the wastewater.
The aerobic treatment section 22 generally includes an aeration tank and a settling tank.

The aeration tank contains water with suspended aerobic flocs and an exhaust pipe that pumps air into the water to activate the aerobic microbial population. The exhaust pipe is usually located at or near the bottom of the aeration tank.
The aeration tank is filled with water containing suspended flocs containing a large amount of aerobic microorganisms. Air is blown into the tank as fine bubbles from an air diffuser located near the bottom of the tank, activating the aerobic microorganisms and decomposing the organic components in the wastewater.
The settling tank, which is connected to the aeration tank, has the function of separating the treated water from which organic components have been decomposed and removed, and the activated sludge flocs, by settling.
The supernatant liquid in the settling tank can be discharged as treated water into the sewage system. The flocs as activated sludge settled in the settling tank can be collected from the bottom of the settling tank and fed back to the aeration tank. In order to increase the volumetric load, carriers for retaining the bacteria can be added to the aeration tank.
The aerobic treatment section 22 is supplied with the separated wastewater separated from the concentrated liquid in the organic wastewater concentrating section 12 and the separated wastewater from which biogas has been recovered in the methane fermentation section 18, and undergoes aerobically treated.
The supernatant liquid in the settling tank, in which the organic component content has been reduced through treatment in the aerobic treatment unit 22, is discharged into the sewer through pipe 30H.

In the wastewater treatment system 20 according to the second embodiment, the separated wastewater from the organic wastewater concentrating section 12 and the methane fermentation section 18 is further subjected to aerobic treatment, thereby further improving the quality of the wastewater discharged into the sewer system.
The separated wastewater from the organic wastewater concentrating section 12 is preferably separated by an RO membrane and is water containing almost no organic components, and the separated wastewater from the methane fermentation section 18 is also water from which organic components have been removed in advance in the hydrothermal treatment section 14 and the methane fermentation section 18. Therefore, according to the wastewater treatment system of the present disclosure, it is possible to improve the quality of the wastewater with less energy than general aerobic treatment.

A system configuration diagram showing an example of a conventional wastewater treatment system is shown in Fig. 3. The conventional wastewater treatment system 40 as an example is a system that utilizes aerobic treatment, and has an aerobic treatment section 22 and a pressurized flotation treatment section 24 for pressurized flotation that is performed prior to the aerobic treatment.
In the pressurized flotation treatment unit 24, the sludge contained in the organic wastewater is floated and separated by a pressurized flotation method. Specifically, air is compressed and dissolved in the organic wastewater in the pressurized flotation treatment unit 24 by a pressure pump (not shown), and the sludge in the wastewater is caused to adhere to the fine air bubbles and float up. This method is also called aeration treatment. The scum separated by the aeration treatment is sucked by a suction pump (not shown) and collected through a pipe 30I, and is treated as waste.
The wastewater remaining in the pressure flotation treatment unit 24 is sent to the aerobic treatment unit 22 via pipe 30J. The aerobic treatment unit 22 holds carriers with attached microorganisms, and the organic components in the wastewater are biologically treated. The solid content of the treated organic wastewater is collected via pipe 30K equipped with a discharge valve (not shown) and treated as sludge.
The configuration of the aerobic treatment section 22 is similar to that in the wastewater treatment system 20 according to the second embodiment of the present disclosure.
The separated wastewater, from which the organic components have been reduced in the aerobic treatment section 22, is discharged into the sewer via a pipe 30L equipped with a discharge valve (not shown).
The conventional wastewater treatment system 40 requires energy to compress and dissolve the air in the aeration process, and further has a problem in that waste materials such as scum and sludge are generated.

The wastewater treatment system of the present disclosure may have other components as necessary in addition to the above-mentioned treatment sections, that is, the organic wastewater concentrating section, the hydrothermal treatment section, the recovery section, and the methane fermentation section.
Other configurations include a foreign matter removal device that removes solid foreign matter other than organic components from organic wastewater containing sugar derived from food, an oil-water separation device or a pressurized flotation device for removing oils such as lipids from organic wastewater, and the like.
As another configuration, there may be mentioned a pressure flotation treatment section which separates the sludge generated in the aerobic treatment section by floating it by a pressure flotation method.

Example 1
The wastewater treatment system used in the treatment was a wastewater treatment system 20 equipped with an aerobic treatment section 22 shown in FIG.

(Preparation of model wastewater and measurement of TOC)
A 1% by mass starch aqueous solution was prepared as a model for food-derived wastewater.
The processing volume per day is 50 t.
In the following, unless otherwise specified, the amount of wastewater, the amount of power consumption, the amount of heat energy, etc. are all indicated per day.
The amount of organic carbon contained in 50 t of 1% by mass starch aqueous solution is about 222 kg, and the amount of total organic carbon (TOC) present in the water is about 4 g/L (liter).
The TOC in wastewater can be measured, for example, by the combustion oxidation method described below. In this example, since model wastewater containing no inorganic carbon is used, the inorganic carbon removal step that is usually performed is omitted.
(Combustion Oxidation Method)
Model water and air are supplied to a combustion tube filled with an oxidation catalyst such as cobalt oxide, platinum, or palladium oxide and heated to 900° C. to 950° C., where organic matter is oxidized to carbon dioxide.
The amount of carbon dioxide generated is measured by an infrared analyzer, specifically, a non-dispersive infrared gas detector (NDIR), to determine the total carbon amount.

(Organic wastewater concentrating section: hereinafter abbreviated as "wastewater concentrating section")
The model wastewater is introduced into the wastewater concentrating section 12, where the wastewater is concentrated. The concentration is performed using a reverse osmosis (RO) membrane type concentrator (RO98pHt, manufactured by Alfa Laval) equipped with an ultrafiltration membrane (UF membrane: MWCO 1,000).
Through the treatment in the wastewater concentrating section 12, the wastewater is separated into 5.0 t, i.e., a ten-fold concentrated concentrate containing starch (organic carbon amount: 211 kg) and 45 t of separated wastewater (residual carbon amount: 11 kg).
In the wastewater concentrating section 12, the energy consumption used for concentration, such as pressurization required for the reverse osmosis membrane method, is 600 kWh.
The concentrated liquid is transported through a pipe 30A to the hydrothermal treatment section 14. The separated wastewater is transported to the aerobic treatment section 22 through a pipe 30B.

(Hydrothermal Treatment Section)
In the hydrothermal treatment section 14, the concentrated liquid transferred from the wastewater concentrating section 12 is heated to obtain furfurals. The temperature in the hydrothermal treatment section 14 is 230° C., and the pressure is 3.0 MPa.
The thermal energy required for heating to treat 5.0 t of the concentrated liquid was 3780 MJ, and the power consumption in the hydrothermal treatment unit 14 was 200 kWh.
Analysis showed that 89 kg of furfural was produced in the concentrated solution by the treatment in the hydrothermal treatment unit 14. The concentrated solution containing furfural after the hydrothermal treatment is transported to a recovery unit via a pipe 30C.

(Collection Department)
The concentrated liquid containing furfural is recovered in the recovery section 16 by adsorbing the furfural contained in the concentrated liquid onto activated carbon. The furfural etc. adsorbed onto the activated carbon is recovered using an organic solvent, and the organic solvent is evaporated to increase the purity of the furfural etc.
The thermal energy required for heating the organic solvent was 500 MJ, and the power consumption in the recovery section 16 was 100 kWh.
The total amount of the remaining concentrated liquid from which furfural has been recovered in the recovery section 16 remains almost unchanged after the recovery of furfural, at approximately 5.0 t. The amount of organic carbon converted to furfural is 40.6 kg, and the amount of organic carbon in the remaining concentrated liquid is 144 kg.
The remaining concentrated liquid is transported to the methane fermentation section 18 via pipe 30E.

The furfural adsorbed onto the activated carbon in the recovery section 16 is recovered by washing the activated carbon with acetone, an organic solvent, and removing the organic solvent. The amount of furfural produced is 71.2 kg. The price of furfural converted to the current market price in March 2021 is estimated to be 50,000 yen.

(Methane fermentation department)
A UASB reactor is used as the fermentation tank in the methane fermentation section 18. A carrier carrying methanogens is provided in the fermentation tank.
The separated wastewater separated in the recovery section 16 is carried into the fermentation tank. The fermentation tank is equipped with an agitator, and the separated wastewater comes into contact with the methanogens while being stirred. The temperature in the fermentation tank is maintained at about 37°C, and the pH is maintained at about 7.5. The separated wastewater is retained in the fermentation tank for about 24 hours, and biogas is generated by decomposing the organic components in the wastewater due to the action of the methanogens. The generated biogas is collected from the top of the fermentation tank through a pipe 30G.
The separated wastewater from which the biogas has been recovered is transported to the aerobic treatment section 22 via pipe 30F.
In the methane fermentation section 18, the power consumption used for maintaining the temperature and stirring the fermentation tank is 164 kWh.
Of the collected biogas, 102 kg of methane gas was produced. The calorific value that can be generated from this 102 kg of methane gas is estimated to be 5,125 MJ.

(Aerobic treatment section)
The separated wastewater carried into the aerobic treatment unit 22 is introduced into the aeration tank in the aerobic treatment unit 22. The aeration tank is equipped with water in which clumps (flocs) of aerobic microorganisms are suspended, and an exhaust pipe that sends air into the water to activate the aerobic microorganisms, and aeration is performed by spraying air as fine bubbles from an aeration pipe located near the bottom of the aeration tank. The aeration activates the aerobic microorganisms and further decomposes the organic components in the wastewater.
The wastewater that has been aerated in the aeration tank is transported to a settling tank that is connected to the aeration tank. In the settling tank, treated water from which organic components have been decomposed and removed in the aeration tank is separated by settling from flocs as activated sludge.
The supernatant liquid in the settling tank is discharged into the sewer through pipe 30H as treated water with a reduced content of organic components.
The flocs that settle in the settling tank as activated sludge are collected from the bottom of the tank and fed back to the aeration tank, where the deactivated sludge is separated by settling in the settling tank and then treated as sludge.
The amount of power consumed for aeration treatment in the aerobic treatment section 22 is 471 kWh.

In the wastewater treatment system 20 of Example 1, the total amount of power consumption is 1535 kWh per day, and the total amount of heat consumed by the hydrothermal treatment unit 14 and the like is 4413 MJ per day.
As described above, the calorific value of the biogas obtained in the methane fermentation section 18 was 5,125 MJ per day. Therefore, in the wastewater treatment system 20 of Example 1, it is estimated that the calorific value obtained by treating the wastewater can supplement the calorific value for heating the system.
The amount of sludge discharged is 86 kg per day. The discharged sludge is further separated into solid and liquid, and the separated water has a low organic component content like the supernatant liquid, so it can be discharged into the sewer. When solid-liquid separation is performed, the amount of water that can be discharged into the sewer is estimated to be 30 kg per day, and the amount of sludge generated can be reduced to 56 kg.
Therefore, the wastewater treatment system of Example 1 can continuously recover furfural, which is a valuable resource. Furthermore, the system has the advantages of being able to keep power consumption low and being able to cover the energy required to heat the system with the heat energy contained in the valuable biogas.
The wastewater treatment system of Example 1 has the effect of reducing energy consumption and generating less undesirable by-products when treating organic wastewater containing sugar derived from food.

[Control Example 1]
In Comparative Example 1, aerobic treatment is applied as the wastewater treatment system, and power consumption etc. are estimated when treatment is performed using a wastewater treatment system that aerates wastewater and separates it into sludge and treated water.
In Control Example 1, similarly to Example 1, a 1% by mass starch aqueous solution is used as a model for wastewater derived from food, and treatment is carried out with a daily treatment amount of 50 t.
As in Example 1, the amount of organic carbon contained in 50 t of the 1% by mass starch aqueous solution was about 222 kg, and the total organic carbon (TOC) present in the water was about 4 g/L.

The wastewater treatment system of Comparative Example 1 uses an aerobic treatment unit equipped with an aeration tank and a sedimentation tank, similar to the device applied in Example 1. A 1% by mass starch aqueous solution is charged into the aeration tank provided in the aerobic treatment unit.
If the treatment volume is 50 tons per day, the total amount of power consumed for heating the wastewater and aeration in the aeration tank is estimated to be 2,919 kWh per day.
The wastewater that has been aerated in the aeration tank is transported to a settling tank that is connected to the aeration tank. In the settling tank, treated water from which organic components have been decomposed and removed in the aeration tank is separated by settling from flocs as activated sludge.
In the settling tank, 921 kg of sludge is collected. The supernatant liquid in the settling tank is discharged into the sewer.
The sludge collected in the settling tank may be further separated into solid and liquid. The water separated by solid-liquid separation has a low organic content and can be discharged into the sewer. When solid-liquid separation is performed, the amount of water that can be discharged into the sewer is estimated to be 30 kg per day, and the amount of sludge generated can be reduced to 186 kg.

Valuable materials are not recovered in the wastewater treatment system of Comparative Example 1. Moreover, in the wastewater treatment system of Comparative Example 1, heating as in the hydrothermal treatment section and recovery section of Example 1 is not necessary, and no heat is consumed.
On the other hand, in the wastewater treatment system of Comparative Example 1, the power consumption required for aeration treatment was 2919 kWh, which was 1.9 times the power consumption in the wastewater treatment system of Example 1. The amount of sludge generated was 10.7 times as much by mass.
In the wastewater treatment system of Control Example 1, no valuables are recovered, and the power consumption and amount of sludge generated are greater than those of the wastewater treatment system of Example 1, so the superiority of the wastewater treatment system of Example 1 over the wastewater treatment system of Control Example 1 is clear.

10, 20, 40 Wastewater treatment system 12 Organic wastewater concentration section 14 Hydrothermal treatment section 18 Methane fermentation section 22 Aerobic treatment section 24 Pressurized flotation treatment section 30A, 30B, 30C, 30D, 30E, 30F, 30G, 30H, 30I, 30J, 30K, 30L Piping (transportation equipment)

Claims (3)

an organic wastewater concentrating section for concentrating sugars in wastewater containing sugars derived from food;
a hydrothermal treatment section for hydrothermally treating the sugars contained in the concentrated wastewater to convert them into at least one kind selected from furfural and 5-hydroxymethylfurfural;
a recovery section for recovering at least one selected from furfural and 5-hydroxymethylfurfural obtained through the hydrothermal treatment;
a methane fermentation section for decomposing organic matter remaining in the wastewater from which at least one selected from furfural and 5-hydroxymethylfurfural has been recovered in the recovery section, converting the organic matter into biogas by methane fermentation, and recovering the biogas;
A wastewater treatment system comprising: The wastewater treatment system according to claim 1, wherein the organic wastewater concentrating section is equipped with a reverse osmosis membrane. 3. The wastewater treatment system according to claim 1, further comprising an aerobic treatment section for aerobic treatment of at least one of the wastewater separated in the organic wastewater concentrating section and the wastewater separated in the methane fermentation section.