JP2002355698A - Method of supercritical water oxidative decomposition of organic sludge and apparatus therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of supercritical water oxidative decomposition of organic sludge capable of suppressing ammonia, nitric acid, nitrous acid and total nitrogen in treated water. SOLUTION: In a method for concentrating organic sludge obtained by biologically treating organic wastewater to separate the same into concentrated sludge and separated water to oxidize and decompose the concentrated sludge in the presence of supercritical water under a condition not lower than the critical temperature of water and not less than the critical pressure thereof, separated water is added to a supercritical water oxidizing reaction system.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for oxidizing organic sludge generated from a biological treatment apparatus for organic wastewater represented by sewage sludge.

[0002]

2. Description of the Related Art Heretofore, as a method of oxidizing sewage sludge and the like, a method of mainly incineration has been generally used. However, since the sewage sludge has a high water content of 97 to 98%, in order to incinerate the sewage sludge, the sewage sludge is previously dehydrated by a dehydration means such as a screw press type dehydrator.
It was necessary to reduce the water content to the extent that it could be incinerated.

On the other hand, in recent years, a method of directly oxidizing sewage sludge using water in a supercritical state has been proposed. Since sewage sludge obtained by treating sewage with a biological treatment apparatus has a low calorific value, it is general that the sewage sludge is concentrated to 5 to 15% by weight and subjected to a supercritical water oxidation treatment.

With reference to FIG. 4, a conventional supercritical water oxidation apparatus for supercritical water oxidation of concentrated sludge will be described. FIG. 4 is a flow sheet showing the configuration of a conventional supercritical water oxidation apparatus. As shown in FIG. 4, the conventional supercritical water oxidation apparatus 30 includes a tubular long pressure-resistant closed reactor 31 as a reactor for performing a supercritical water oxidation reaction, and upstream of the reactor 31, A concentrated sludge pump 39 for supplying the concentrated sludge, an air compressor 40 for supplying the oxygen, a double-tube heat exchanger 32 for preheating the reactants, and a heat exchanger for cooling the reaction products downstream of the reactor 31. A device 33 and a cooler are provided. The concentrated sludge supplied to the reactor 31 is obtained by converting sewage sludge treated by a biological treatment device (not shown) into a concentrator 42 such as a centrifugal concentrator.
It is concentrated in. The separated liquid obtained by concentrating the sewage sludge is discharged out of the system.

When the reaction product is cooled by the heat exchanger 33, the temperature of the heat medium becomes high, and the heat medium is used by the double-tube heat exchanger 32 as a heat source for preheating the mixed fluid to be processed. Further, the supercritical water oxidation apparatus 30 includes a pressure control valve 35 for controlling the pressure in the reactor 31 downstream of the cooler 34 and a gas-liquid separator 36 for gas-liquid separation of the reaction product into gas and slurry. A solid-liquid separator 37 is provided downstream of the pressure control valve 35 and separates a solid reaction product from the reaction product by solid-liquid separation of a slurry-like reaction product. The inorganic solids separated by the solid-liquid separator 37 are mainly contained in the reactants and did not contribute to the reaction, and may also contain salts generated by the supercritical water oxidation reaction. is there.

[0006] When the concentration of sludge is low in the conventional supercritical water oxidation cracking apparatus, it is necessary to supply an auxiliary fuel to perform supercritical water oxidation.

[0007] In the supercritical water oxidation treatment of sewage sludge, in order to stably and efficiently treat the sludge, the supply of auxiliary fuel is reduced while ensuring the fluidity of the sludge, and the processing fluid after the supercritical water oxidation decomposition is processed. It is important that no special post-processing is required to process

However, since the calorific value per unit amount of organic matter in the sludge is low, it is necessary to efficiently recover heat and increase the sludge concentration in order to reduce the amount of auxiliary fuel. However, when the concentration of the sludge increases, the fluidity of the sludge extremely decreases, and it becomes difficult to supply the sludge to the supercritical hydroxylation decomposition apparatus.

In recent years, due to the problem of eutrophication in closed water bodies, the regulation of nitrogen and phosphorus emissions has tended to be strengthened. Therefore, nitrogen and phosphorus do not remain as much as possible in treated water after supercritical hydroxylation decomposition. It is required to do so.

[0010] As means for satisfying these requirements, JP-A-11-90494 discloses that the sludge concentration is adjusted to about 5 to 15% in accordance with the amount of heat exchange, and the reaction temperature is set to 600 ° C or higher. A method is described in which auxiliary fluid is eliminated while maintaining the fluidity of sludge, and supercritical water oxidation of sewage sludge is performed without leaving nitrogen in the treated water.

Heretofore, in supercritical water oxidation of sewage sludge, all nitrogen components in sewage sludge are once converted to ammonia by hydrolysis, ammonia is oxidized to nitric acid and nitrous acid, and treated by the reaction of ammonia with nitric acid or nitrous acid. It has been considered that nitrogen components disappear from water (see the following reaction formulas (1) to (3)).

[0012] Nitrogen component in sewage sludge + H ₂ O → NH ₄ ... (1) NH ₄ + O ₂ → NO ₂ or NO ₃ ... (2) NH ₄ + NO ₂ or NO ₃ → N ₂ or N ₂ O (3) In this series of reactions, the rate of oxidation of ammonia shown in reaction formula (2) is rate-determining, and the production of ammonia by hydrolysis in reaction formula (1) and the conversion of ammonia in reaction formula (3) Since the reaction with nitric acid or nitrous acid occurs rapidly, it was considered that nitric acid or nitrous acid did not remain in the treated water, and that all nitrogen components could be removed from the treated water by completely oxidizing ammonia.

However, when the supercritical hydroxylation of sewage sludge was examined in detail, organic nitrogen other than ammonia in the sludge was converted to ammonia by hydrolysis at a low temperature of about 450 ° C. as shown in reaction formula (4). Formation is the main reaction, and when the temperature rises to 500 ° C. or higher, the reaction formula (5)
As shown in the above, it was confirmed that the production of nitric acid or nitrous acid by oxidation was the main reaction.

Organic nitrogen + H ₂ O → NH ₄ (4) Organic nitrogen + O ₂ → NO ₂ or NO ₃ (5) When the concentration of sewage sludge was concentrated to about 5 to 15%, the concentration was increased. Because the ratio of organic nitrogen in the sludge increases, if the reaction temperature is supercritically hydroxylated at a high temperature of about 550 ° C,
Nitric acid and nitrous acid remain in the treated water. On the other hand, when the concentrated sludge is subjected to supercritical water oxidation at a low reaction temperature of about 400 ° C., even in the case of organic nitrogen, ammonia production becomes the main reaction, and the ammonia contained in the sludge also remains. A large amount of ammonia remains.

Therefore, when sewage sludge is subjected to supercritical water oxidation, ammonia remains at low temperatures and nitric acid remains at high temperatures, making it difficult to reduce total nitrogen in the treated water.

[0016]

As described above, in supercritical water oxidation of organic sludge such as sewage sludge, ammonia remains at a low reaction temperature, and nitric acid and nitrous acid remain at a high reaction temperature. It is necessary to control the reaction temperature strictly. For example, solid matter concentration 10% (% by weight)
In the case of concentrated sludge that has been concentrated to
By fixing the front and rear sides, the amount of ammonia in the treated water becomes extremely small, and the residual amount of nitric acid or nitrous acid can be suppressed to some extent, but about 100 mg / L of total nitrogen often remains.

In this case, in order to suppress the production of nitric acid and nitrous acid, the reaction temperature may be lowered, but if the reaction temperature is lowered too much, ammonia remains on the contrary,
In some cases, the TOC component may also remain. TO
If the C component remains, the reaction temperature must be 500 ° C. or higher because the drainage standard in the case where there is no regulation of nitrogen and phosphorus emission is not satisfied. It becomes difficult.

An object of the present invention is to solve the drawbacks of the conventional oxidation treatment of organic sludge such as sewage sludge using supercritical water, and to remove ammonia, nitric acid and nitrous acid and total nitrogen in treated water. It is an object of the present invention to provide a method and a device for supercritically hydrolyzing organic sludge, which can suppress the occurrence of organic sludge.

[0019]

Means for Solving the Problems The inventors of the present invention have repeated experiments with supercritical water oxidation of sewage sludge, and as a result, separated water obtained when concentrating sewage sludge has been discharged outside the conventional system. But,
By adding this separated water to the supercritical water oxidation reaction system, it was found that the above-mentioned problem was solved, and the present invention was completed.

That is, a first solution for solving the above-mentioned problem.
The invention of the present invention, the organic sludge obtained by biological treatment of the organic wastewater is concentrated and separated into concentrated sludge and separated water, at a temperature above the critical temperature of water and above the critical pressure, in the presence of supercritical water, The present invention relates to a method for oxidatively decomposing concentrated sludge, wherein the separated water is added to a supercritical water oxidation reaction system, the method comprising supercritical water oxidation and decomposition of organic sludge.

According to a second aspect of the present invention, there is provided an apparatus for reacting an organic sludge and an oxidizing agent in supercritical water to supercritically decompose the organic sludge.
A concentration separation means for separating the organic sludge into concentrated sludge and separated water, a pressurized supply means for pressurizing the concentrated sludge and the oxidizing agent to a pressure higher than the critical pressure of water, and a pressurized supply from the pressurized supply means A reactor for performing a supercritical water oxidation reaction of a mixed fluid of a substance to be treated composed of a concentrated sludge and an oxidizing agent, separation water supply means for supplying the separation water to the reactor, and post-reaction treatment flowing out of the reactor The present invention relates to a supercritical hydroxylation decomposition apparatus comprising a cooling means for cooling a fluid, and a take-out means for taking out a cooled processing fluid flowing out of the cooling means.

A third aspect of the present invention for solving the above problems is:
In a device that reacts organic sludge and an oxidizing agent in supercritical water and supercritically hydrolyzes organic sludge, a concentration separation means for separating organic sludge into concentrated sludge and separated water, and a part of separated water Separation water supply means for supplying the concentrated sludge, pressurized supply means for supplying concentrated sludge, separated water and oxidizing agent at a pressure higher than the critical pressure of water, and concentrated sludge pressurized and supplied from the pressurized supply means A reactor for performing a supercritical water oxidation reaction of the mixed fluid of the object to be treated comprising separated water and an oxidizing agent, and a cooling means for cooling the treated fluid after the reaction flowing out of the reactor,
The present invention relates to a supercritical hydroxylation decomposition apparatus comprising a take-out means for taking out a cooled processing fluid flowing out of the cooling means.

[0023] By adding the separating solution, the ratio of ammonia is increased appropriately, so that nitric acid and nitrous acid can be reduced even under conditions where nitric acid and nitrous acid are generated without adding the separating solution.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The organic sludge in the present invention is
It is obtained by biological treatment of organic wastewater such as domestic wastewater and industrial wastewater.

In the method for supercritically hydrolyzing organic sludge according to the present invention, the organic sludge obtained by biological treatment of organic wastewater is concentrated and separated into concentrated sludge and separated water, and the concentrated sludge is separated into supercritical water. When oxidizing below, all or a part of the separated water is added to the supercritical water oxidation reaction system. The position at which the separated water is added is not particularly limited.For example, the separated water may be added to the separated concentrated sludge and supplied to the reactor under pressure together with the concentrated sludge, or the pressure may be supplied to the subsequent stage of the reactor or the outlet of the reactor May be supplied.

In the supercritical water oxidation reaction, the concentrated sludge is oxidatively decomposed in the presence of supercritical water and an oxidizing agent.
The supercritical hydroxylation reaction performed in the reactor is not particularly limited as long as the temperature and pressure conditions bring water to a supercritical state. For example, the temperature is 374 ° C or higher, preferably 500 to 650 ° C, and The pressure may be 22 MPa or more, preferably 22 to 25 MPa. Examples of the oxidizing agent include air, pure oxygen, hydrogen peroxide, and liquid oxygen. These oxidizing agents may be used in a stoichiometrically required amount or more. The reactor for performing the supercritical hydroxylation decomposition is preferably a pipe (tubular) type.

Since water is a good solvent for organic substances and gaseous substances in the supercritical state, supercritical water, organic substances and oxidizing agent form a homogeneous phase in the reactor, and the supercritical water oxidation reaction The organic matter is oxidatively decomposed in a very short time.

Examples of the means for concentration and separation of the organic sludge include means such as centrifugal concentration, gravity concentration, and pressure flotation concentration.

By the concentration of the organic sludge, the concentrated sludge concentrated to a predetermined concentration and the separated liquid separated from the solid matter are discharged. At this time, Table 1 shows typical compositions of the sewage sludge, the concentrated sludge, and the separated liquid as raw materials.

[0030]

[Table 1]

The concentration of the concentrated sludge may be appropriately determined within the range of 5 to 15% by weight, preferably 8 to 12% by weight. When the concentration of the concentrated sludge is less than 5% by weight, a large amount of auxiliary fuel is required because the calorific value is low, and the amount of treatment increases. When the concentration exceeds 15% by weight, the fluidity of the sludge deteriorates. It is not preferable in that the supply under pressure becomes difficult.

In the present invention, when the concentrated sludge is subjected to supercritical water oxidation, the reaction temperature must be set to 500 ° C. or higher, preferably 500 to 550 ° C., in order to prevent organic substances from remaining in the treatment fluid.

The amount of the separated liquid to be added may be controlled by measuring the concentrations of ammonia, nitric acid and nitrous acid in the processing fluid and minimizing these substances. However, analytical instruments for measuring ammonia, nitric acid, and nitrous acid in the processing fluid are expensive.
Is measured. If the pH of the processing fluid decreases, it is considered that nitric acid or nitrous acid has been generated, so the amount of the separation liquid added is increased. Conversely, when the pH increases, it is conceivable that ammonia has been generated, so the amount of the separation liquid to be added is reduced.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(First Embodiment) Referring to the flowchart of FIG.
A first embodiment of the present invention will be described.

As shown in FIG. 1, the supercritical hydroxylation / decomposition apparatus of the present invention includes a tubular (tubular) long pressure-resistant closed type reactor 10 as a reactor for performing a supercritical hydroxylation reaction. Upstream of the vessel 10, a concentration separation means 2 for concentration and separation of organic sludge such as sewage sludge supplied from a sewage biological treatment facility, and a concentration sludge tank 6 for storing the concentration sludge concentrated and separated by the concentration separation means 2 And a pressure supply pump 26 as a pressure supply means for supplying a fluid to be treated such as concentrated sludge to a pressure higher than the critical pressure of water, and a pressure supply means as a pressure supply means for pressurizing and supplying an oxidant. A pressure supply pump 28 (a pressurized supply compressor when the oxidizing agent is a gas such as oxygen) and a preheater 12 of a double tube heat exchanger as preheating means for preheating the mixed fluid to be processed.

A part of the separation liquid obtained from the concentration separation means 2 is supplied to the reactor 10 under pressure by the separation liquid supply means 4.

Downstream of the reactor 10, a double tube heat exchanger 14 and a cooler 16 for cooling the processed fluid after the reaction are provided. The flow passages of the supercritical water splitting and cracking apparatus including the preheater, the reactor, the heat exchanger, and the cooler have substantially the same diameter.

Further, the supercritical water splitting / decomposing apparatus comprises a reactor 1
A pressure control valve 18 for controlling the pressure within 0 is provided downstream of the cooler 16. As means for taking out the cooled processing fluid, a gas-liquid separator 20 for gas-liquid separation into gas and slurry is used.
Is provided downstream of the pressure control valve 18 and a solid-liquid separator 22 that separates the inorganic solid from the reaction product by solid-liquid separation of the slurry-like reaction product. The inorganic solid separated by the solid-liquid separator 22 is mainly contained in the reaction product,
It does not contribute to the reaction and may additionally contain a salt formed by the supercritical hydroxylation reaction.

The ammonia, nitric acid, and nitrous acid of the processing fluid flowing out of the reactor are measured, and the amount of the added separation liquid is controlled by controlling the separation liquid supply means 4 so that these nitrogen compounds are minimized. I do. That is, when the concentration of nitric acid or nitrous acid in the fluid to be treated increases, the amount of the separation liquid added may be increased, and when the concentration of ammonia increases, the amount of the separation liquid may be decreased.

(Second Embodiment) Referring to the flowchart of FIG.
A second embodiment of the present invention will be described.

The same components as those in FIG. 1 are denoted by the same reference numerals, and detailed description will be omitted. The flow chart of FIG. 2 is different from that of FIG. 1 in that a pH measuring device 8 for measuring the pH of the processing fluid after supercritical hydroxylation decomposition is provided, and the amount of the separated solution added is feedback-controlled according to the measurement result. It is the point which was made.

That is, when the pH of the processing fluid decreases, it is considered that the concentration of nitric acid or nitrous acid in the processing fluid has increased. In that case, the separation liquid supply means 4 may be controlled to increase the supply amount of the separation liquid.

When the pH of the processing fluid rises,
It is considered that the concentration of ammonia in the processing fluid has increased. In that case, the separation liquid supply means 4 is controlled to
What is necessary is just to reduce the supply amount of the separation liquid. (Third Embodiment) Although not shown, the following embodiment may be adopted.

In the first and second embodiments, an example has been shown in which the separated liquid is supplied to the latter half of the reactor. However, the separated liquid is supplied to the pipe between the concentrated sludge tank and the pressurized supply pump. The pressure may be supplied to the reactor together with the concentrated sludge. By adopting such an embodiment, it is possible to use an ordinary pump that is not a pressurized type as a means for supplying the separated liquid.

[0046]

EXAMPLES Example 1 Reaction temperature 520 ° C., reaction pressure 24 MPa, reaction time 2
The concentrated sludge having a solid concentration of 10.0% shown in Table 1 was treated under the conditions of an oxygen ratio of 1.5 for each minute. Separation liquid having a solid concentration of 1.4% shown in Table 1 was supplied under pressure to the latter stage of the reactor while controlling the supply amount. One hour after the start of operation,
Since the reaction temperature reached 530 ° C. and the pH of the treated water dropped, the supply amount of the separation liquid was increased.

As a result, two hours after the start of operation, NO ₃ −
N decreased.

After 4 hours, the reaction temperature dropped to 500 ° C. and the pH of the treated water rose, so that the supply amount of the separation liquid was reduced.

As a result, NH ₄ —N was reduced after 5 hours. The results are shown in Table 2.

[0050]

[Table 2]

FIG. 3 is a graph plotting the results of Table 2.

As is evident from the results in Table 2, it was possible to reduce the amount of ammonia and nitric acid in the treated water by controlling the supply amount of the separated solution even when the reaction temperature fluctuated. Although not shown in Table 2, nitrous acid was also generated, and its concentration was about 1/5 of that of nitric acid.

[0053]

According to the first aspect of the present invention, the production of ammonia, nitric acid, and nitrous acid is suppressed as much as possible by adding a conventionally used separation liquid to the supercritical water oxidation reaction system. Supercritical water hydroxide can decompose sludge.

According to the second aspect of the present invention, in addition to the effects described above, the amount of auxiliary fuel used can be suppressed, and stable operation can be performed on an appropriate scale.

According to the third aspect of the present invention, in addition to the above effects, organic substances contained in sewage sludge can be further oxidatively decomposed.

According to the present invention, in addition to the above effects, the supercritical hydroxylation reaction can be controlled at low cost.

According to the fifth aspect of the present invention, by adding a separation liquid that has not been used conventionally to a supercritical water oxidation reaction system, the generation of ammonia, nitric acid and nitrous acid is suppressed as much as possible to reduce sewage sludge. Can be supercritical hydroxylated decomposition,
The supercritical hydroxylation decomposition apparatus can be operated stably for a long period of time.

According to the present invention described in claim 6, according to claim 5,
And the number of high-pressure pumps used can be reduced.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a first embodiment of the present invention.

FIG. 2 is a flowchart showing a second embodiment of the present invention.

FIG. 3 is a graph showing the concentration of each component in the processing fluid and the elapsed time in Example 1.

FIG. 4 is a flowchart showing a conventional supercritical water oxidation apparatus.

[Explanation of symbols]

 2 Concentration / separation means 4 Separation liquid supply means 6 Concentrated sludge tank 8 pH meter 10 Reactor 12 Preheater 14 Heat exchanger 16 Cooler 18 Pressure control valve 20 Gas-liquid separator 22 Solid-liquid separator 24 Heat medium pipe 26 Pressurized supply pump 28 Pressurized supply pump

Claims (7)

[Claims] An organic sludge obtained by biologically treating an organic wastewater is concentrated and separated into concentrated sludge and separated water, and is subjected to supercritical water at a temperature not lower than the critical temperature of the water and not lower than the critical pressure. A method for oxidatively decomposing the concentrated sludge, wherein the separated water is added to a supercritical water oxidation reaction system. 2. The concentrated sludge has a solid matter concentration of 5 to 15% by weight.
The supercritical hydroxylation decomposition method according to claim 1, wherein 3. The method of claim 1, wherein the oxidative decomposition temperature is 500 ° C. or higher. 4. The pH of a processing fluid subjected to supercritical hydroxylation decomposition
4. The method of claim 1, wherein the amount of the separated water is controlled. 5. An apparatus for reacting an organic sludge and an oxidizing agent in supercritical water to supercritically hydrolyze and decompose the organic sludge, comprising: a concentration separation means for separating the organic sludge into a concentrated sludge and a separation water; Pressurized supply means for supplying pressurized sludge and an oxidant to a pressure higher than the critical pressure of water, and a supercritical water oxidation reaction of a mixture fluid to be treated comprising concentrated sludge and oxidant supplied from the pressurized supply means , A separated water supply means for supplying the separated water to the reactor, a cooling means for cooling the reaction fluid flowing out of the reactor after the reaction, and a cooled treatment flowing out of the cooling means A supercritical hydroxylation decomposition apparatus comprising a take-out means for taking out a fluid. 6. An apparatus for reacting an organic sludge and an oxidizing agent in supercritical water to supercritically hydrolyze and decompose the organic sludge, comprising: a concentration separation means for separating the organic sludge into concentrated sludge and separated water; Separated water supply means for supplying a part of water to the concentrated sludge, pressurized supply means for supplying concentrated sludge, separated water and an oxidizing agent at a pressure equal to or higher than the critical pressure of water, and pressurization from the pressurized supply means A reactor for performing a supercritical water oxidation reaction of the mixture fluid to be treated comprising the supplied concentrated sludge, separated water and an oxidizing agent, a cooling means for cooling the treated fluid flowing out of the reactor after the reaction, A supercritical hydroxylation decomposition apparatus comprising a take-out means for taking out a cooled processing fluid flowing out of the means. 7. The apparatus according to claim 1, further comprising means for measuring the pH of the processing fluid flowing out of the reactor, wherein the supply amount of the separation water supplied from the separation water supply means is controlled by the pH of the processing fluid. Item 7. A supercritical hydroxylation decomposition apparatus according to item 5 or 6.