JPH06178995A - Anaerobic digestion treatment of organic waste water - Google Patents

Abstract

PURPOSE:To provide the anaerobic digestion treatment method for org. waste water capable of lowering the ammonia concn. and efficiently effecting methane fermentation without using chemicals. CONSTITUTION:The org. waste water 1 is treated in an anaerobic digestion treatment stage 2, by which the org. matter in the waste water is decomposed to gaseous methane 3 in the anaerobic digestion treatment method for the org. waste water contg. ammonia. The ammonia is then removed from the anaerobic digestion treated liquid and a part 6 of the ammonia treated water 5 obtd. in such a manner is circulated to the anaerobic digestion treatment stage 2 where the ammonia concn. in the liquid to be treated is diluted. An ammonia removal method or biological nitrification and denitrification treatment method, etc., are usable for the removal of the ammonia.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for anaerobic digestion of organic wastewater, and more particularly to a method for treating anaerobic digestion of organic wastewater containing a high concentration of ammonia without using a chemical.

[0002]

2. Description of the Related Art A method of anaerobic digestion of ammonia-containing organic wastewater such as human waste and the methanation of organic matter in the wastewater by a methane bacterium has been conventionally known, and has been widely adopted as a practical method. It was In addition, at the sewage treatment plant, a method of anaerobically treating the sludge in the first settling tank and the surplus activated sludge to reduce the volume is widely used.

When the methane bacterium, which plays an important role in anaerobic digestion, has a high ammonia concentration, the toxicity of the methane bacterium reduces the activity of the methane bacterium. Therefore, the ammonia nitrogen concentration in the methane fermentation tank is 1500 mg / l or less, preferably 1000
It is better to have a concentration of mg / l or less, or a lower concentration. For example,
In human urine, which requires 30 days for anaerobic digestion, the ammonia nitrogen concentration is as high as about 3000 to 4000 mg / l, which is a concentration that inhibits smooth methane fermentation.

As shown in the formula (1), ammonia in water exists in equilibrium with ammonia ions (NH ₄ ⁺ ) and free ammonia (NH ₃ ). If it is released and reduced, the inhibition by ammonia will be alleviated. NH ₄ ⁺ + OH ^- ⇔ NH ₃ + H ₂ O (1)

However, in wastewater containing a high concentration of an acidic substance such as an organic acid together with ammonia, such as human waste, the pH balance is destroyed due to the emission of ammonia, which is an alkaline substance, and the pH of the liquid is lowered to release ammonia. It will not (gasify). Therefore, even if ammonia in human waste is heated and dissipated using water vapor, the removal rate of ammonia is limited to about 50% at the maximum. In order to increase the removal rate of ammonia by emission to about 100%, a method of moving ammonia to a free state by adding an alkaline agent such as sodium hydroxide (NaOH) is known, but since the amount of chemicals used increases, Practical application was economically difficult.

This is because, in addition to sodium hydroxide for increasing the pH, when the organic acid is methanated, as shown in the formula (2), hydroxide ions are released and the pH of the treated water increases. This is because an acid for neutralizing the pH of the treated water is also required. _{CH 3 COONa + H 2 O →} CH 4 + CO 2 + Na + + OH - (2) Also in the anaerobic digestion process of sewage sludge, in ammonia eluted with solubilization of the sludge, the ammonium nitrogen is a high concentration in the fermentor It accumulates and inhibits the rapid reaction of methane fermentation.

[0007]

DISCLOSURE OF THE INVENTION In view of the above-mentioned problems of the prior art, the present invention reduces the concentration of ammonia without using chemicals and enables organic methane fermentation to be performed efficiently. An object is to provide an anaerobic digestion treatment method.

[0008]

In order to solve the above problems, the present invention provides a method for anaerobic digestion of organic wastewater containing ammonia, wherein the organic wastewater is treated in an anaerobic digestion treatment step. The organic matter in the wastewater is decomposed into methane gas, ammonia is removed from the obtained anaerobic digestion treatment liquid, and a part of this ammonia-treated water is circulated to the anaerobic digestion treatment step to change the concentration of ammonia in the liquid to be treated. It was decided to dilute.

Next, the present invention will be described in detail with reference to FIG. In FIG. 1, the raw water 1 flows into the anaerobic digestion treatment step 2 together with the circulating ammonia-treated water 6 introduced from the ammonia removal step 4, and the organic matter in the raw water is converted to methane 3
After being reduced and decomposed into ammonia, it is introduced into ammonia removal step 4 to remove ammonia. A part 6 of the ammonia-treated water 5 is circulated to the anaerobic treatment step 2 to dilute and reduce the ammonia concentration of the raw water 1 flowing into the step 2. The rest of the ammonia-treated water 5 is discharged or subjected to higher-level treatment.

The anaerobic treatment step 2 is carried out by the UASB even in the floating methane fermentation system in which the conventional gas stirring and the like are performed.
Any anaerobic treatment method such as (upflow anaerobic sludge filter bed) method can be used. Ammonia removal process 4
Ammonia stripping method or biological nitrification / denitrification treatment method is recommended. Further, known ammonia removal techniques such as ion exchange with zeolite capable of adsorbing ammonia and decomposition of ammonia such as chlorine can be used, but these techniques are expensive at present and are not practical.

When the biological nitrification denitrification treatment method is applied to the ammonia removal step 4, it is desirable that the concentrations of ammonia and nitrogen oxides (nitric acid, nitrous acid) in the ammonia-treated water 5 are as low as possible. . This is because the toxicity of ammonia to methane bacteria is as described above, but nitrogen oxide also inhibits methane fermentation. Oxygen in nitrogen oxides interferes with the maintenance of absolute anaerobic conditions in methane fermentation, hinders smooth methane fermentation, and reduces the amount of methane produced by oxidizing and eliminating organic matter in raw water.
The amount of nitrogen oxides that can flow into the anaerobic digestion treatment process 2 differs depending on the amount of organic matter in the raw water that flows into the anaerobic digestion treatment process 2. The biological nitrification and denitrification treatment method should be selected as the ammonia removal step 4.

[0012]

In the treatment method of the present invention as described above, the concentration of ammonia nitrogen in the anaerobic digestion treatment step 2 is
It is determined by the circulation rate of the ammonia-treated water 5 to the raw water 1 (circulation water amount / human waste treatment amount). For example, the ammonia nitrogen concentration is 30 for raw water and ammonia-treated water.
If the circulation rate is set to 4 times, the concentration of ammonia nitrogen in the anaerobic treatment step 2 is 600 mg / liter {= 3000 mg / liter × 1 /
(1 + 4)}.

It is desirable that the concentration of ammonia nitrogen in the anaerobic digestion treatment step 2 is lower, but increasing the circulation rate more than necessary increases the water amount load of the anaerobic digestion step 2 and the ammonia removal step 4, and also circulates it. This is not preferable because it consumes a large amount of power. By confirming the optimum value for the circulation rate by preliminary experiments, it is possible to set the ammonia concentration at which methane fermentation can be efficiently performed without affecting the anaerobic digestion treatment.

[0014]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. Example 1 FIG. 2 is a process diagram when a biological nitrification denitrification treatment method is applied to the ammonia removal process. In FIG. 2, raw water 1
Part of the water is transferred to the denitrification tank as the dispensed raw water 7 and 8, and the rest flows into the anaerobic digestion treatment step 2 together with the circulating ammonia-treated water 6 flowing out from the nitrification denitrification step 9, and the organic matter in the raw water is methane. After being reduced and decomposed into 3, returned sludge 1
0, and the circulating nitrification solution 11 is introduced into the first denitrification tank 12 under anaerobic conditions.

The nitrate nitrogen in the circulating nitrification solution 11 is biologically denitrified with the residual BOD component in the effluent of the anaerobic digestion treatment step 2 and the BOD component in the dispensed raw water 7 as a reducing substrate. First, the activated sludge mixed solution is introduced into the nitrification tank 13 under aerobic conditions, ammonia is nitrified into nitric acid, and a part of the nitrification solution is circulated to the first denitrification tank 12. The remaining nitrification liquid flows into the second denitrification tank 14 under anaerobic conditions, and the nitrate nitrogen in the liquid is biologically denitrified after the biological denitrification using the BOD component in the dispensed raw water 8 as a reducing substrate. After flowing into the re-aeration tank 15 under aerobic conditions to remove residual BOD,
It flows into the settling tank 16 and is separated into the ammonia-treated water 5 and the activated sludge. The activated sludge is returned to the first denitrification tank 12, and a part 6 of the ammonia-treated water 5 is circulated to the anaerobic digestion treatment step 2.

Next, an example of processing performed according to the process diagram of FIG. 2 will be shown. As the water to be treated, the concentrated artificial raw water shown in Table 1 was used and treated under the following conditions. (a) Equipment volume, anaerobic digestion tank 2 (UASB method): 900 liters (raw water retention days 9 days), first denitrification tank 12: 300 liters, nitrification tank 13: 300 liters, second denitrification tank 14: 100 liters Re-aeration tank 15: 50 liters, Settling tank 16: 200 liters,

(B) Operating conditions, raw water amount 1: 100 liters / day, circulating ammonia water amount 6: 200 liters / day, raw water content injection amount 7: 7 liters / day, raw water content injection amount 8: 3 liters / day, Return sludge amount 10: 290 liters / day, circulating nitrification liquid amount 11: 4000 liters / day, nitrification tank MLSS concentration: 9500 mg / l, and the execution results are shown in Table 1. From Table 1, in the anaerobic digestion treatment process, the concentrated artificial wastewater was treated with BOD after treatment for 9 days.
It can be seen that the concentration has decreased by 99% or more.

[0018]

[Table 1]

Example 2 FIG. 3 is a process diagram in the case of applying ammonia emission by vapor to the ammonia removal process to perform heat exchange. In FIG. 3, the raw water 1 flows into the anaerobic digestion treatment step 2 together with the circulating ammonia-treated water 6 that has flowed out from the ammonia diffusion step 18, and the organic matter in the raw water is reduced and decomposed into methane 3 and then the heat exchanger 17 The ammonia 20 is countercurrently contacted with the vapor 19 in the ammonia diffusion step 18 in which the contact material is filled with the ammonia 20 and is removed by diffusion.

A part of the ammonia-treated water 5 is circulated as the circulating ammonia-treated water 6 via the heat exchanger 17 to the anaerobic digestion treatment step 2, and the rest is discharged or further treated. When the water temperature in the anaerobic digestion treatment step 2 is around 60 ° C. or higher, the anaerobic bacteria are inactivated,
It is desirable that the water temperature of the inflow water (mixed liquid of the raw water 1 and the circulating ammonia-treated water 6) in the anaerobic digestion treatment step 2 is 60 ° C. or lower.

The water temperature in the anaerobic digestion process 2 is the circulation ratio of the ammonia-treated water 5 to the raw water 1 (circulation water amount / circulation water amount /
Raw water treatment amount). For example, if the water temperatures of the raw water 1 and the ammonia-treated water 5 are respectively 20 and 100 ° C., the heat exchange rate of the heat exchanger 17 is 100%, and the heat loss of the entire treatment process is zero, the water temperature of the anaerobic treatment step 2 is set. The circulation rate to bring the temperature to 50 ° C is calculated to be 1.6 times. Actually, the amount of heat loss varies depending on the material of the process, the thickness of the heat insulating material, etc., and the heat exchange rate varies depending on the model of the heat exchanger and the operating conditions. I have to decide.

The liquid introduced into the ammonia stripping tower 18 via the heat exchanger 17 is stripped of ammonia by the steam 19. The stripping tower 18 sprays the liquid from the top of the tower by a scrubber type or multistage contact type. It is advisable to contact the water vapor 19 with an alternating current. Although not shown, if a storage tank is provided between the anaerobic digestion treatment step 2 and the heat exchanger 17, it is possible to flexibly cope with the fluctuation of the inflow amount of the raw water 1.

Ammonia in the liquid is dissociated into ammonia ions and free ammonia as shown in the equation (1). Although free ammonia is released from the liquid as a gas, the heating facilitates the release of ammonia, and the use of water vapor is desirable for improving the removal efficiency of ammonia. The stripped ammonia 3 can be treated by a known ammonia treatment technique such as oxidative decomposition by a catalyst and absorption by an acid. Methane 3 can be used as fuel for the steam 19 generator and the like.

Next, an example of processing performed according to the process diagram of FIG. 3 will be shown. As the liquid to be treated, 10 kl / day of filtered urine shown in Table 2 was used and treated under the following conditions. (a) Equipment volume and operating conditions Anaerobic digestion tank 2 (UASB method): 30 m ³ , Ammonia stripping tower 18 (multistage contact stripping tower): 1 m ³
(0.45m × 6.3m), the amount of water vapor 19: 250kg / kl raw sewage, circulation ammonia water 6: 20m ³ / day,

The results of the tests are shown in Table 2. From Table 2, in the anaerobic digestion treatment step, BOD concentration was reduced by about 95% or more by the treatment of retaining urine for 3 days, and 99% or more of ammonia was removed by the ammonia emission treatment.
It can be seen that ammonia-treated water is obtained.

[0026]

[Table 2]

[0027]

According to the present invention, the following effects which cannot be obtained by the prior art can be obtained. (1) By circulating the ammonia-treated water, it is possible to economically treat the BOD component (organic matter), which is a pollutant component of wastewater, without using a chemical for ammonia treatment. (2) Since the concentration of ammonia, which is a harmful substance in anaerobic digestion treatment, can be easily reduced, the efficiency of anaerobic digestion treatment (methane fermentation) can be easily increased.

[Brief description of drawings]

FIG. 1 is a basic process diagram showing a processing method of the present invention.

FIG. 2 is a process diagram in which a biological nitrification denitrification process is used in the ammonia removal process.

FIG. 3 is a process diagram in which a diffusion process using steam is used in an ammonia removal process.

[Explanation of symbols]

1: Raw water, 2: Anaerobic digestion process, 3: Methane, 4:
Ammonia removal step, 5: Ammonia treated water, 6: Circulated ammonia treated water, 7, 8: Dispensing raw water, 9: Nitrification denitrification step, 10: Return sludge, 11: Circulating nitrification solution, 12: First denitrification tank, 13: Nitrification tank, 14: Second denitrification tank, 15: Re-aeration tank, 16: Precipitation tank, 17: Heat exchanger, 18: Ammonia emission step, 19: Water vapor, 20: Ammonia

Claims (1)

[Claims] 1. A method for anaerobic digestion treatment of an organic wastewater containing ammonia, wherein the organic wastewater is treated in an anaerobic digestion treatment step to decompose organic matter in the wastewater into methane gas, and the resulting anaerobic digestion treatment is obtained. Anaerobic digestion treatment method of organic wastewater, characterized in that ammonia is removed from the liquid, and a part of this ammonia-treated water is circulated to the anaerobic digestion treatment step to dilute the ammonia concentration in the liquid to be treated. .