JPH0910779A - Denitrification treatment of organic nitrogen-containing waste solution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently denitrify a waste soln. containing at least EDTA as a nitrogen-containing org. compd. without performing biological treatment. SOLUTION: EDTA is sedimented from a waste soln. under a strong acidic pH condition to be removed and an org. substance is oxidized and decomposed by an oxidizing agent to generate ammonia and, after the pH of the waste soln. is made alkaline, ammonia gas is stripped in a stripping column. The ammonia gas is subjected to oxidation treatment and, if necessary, residual org. matter and inorg. salts are removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for denitrifying an organic nitrogen-containing waste liquid containing at least EDTA as an organic substance, and more specifically, a step of precipitating and removing EDTA in the waste liquid, organic nitrogen. The present invention relates to a method for denitrifying an organic nitrogen-containing waste liquid, which mainly includes a step of oxidatively decomposing an organic substance such as oxidizer with an oxidant and a step of oxidatively decomposing produced ammonia gas into nitrogen gas.

In a boiler of a thermal power plant, some of impurities contained in the boiler water adhere to the inside of an evaporation pipe or the like to form a scale. Conventionally E to remove scale
A chemical cleaning liquid containing DTA as a main component and several kinds of organic acids, a corrosion inhibitor and the like, an alkaline cleaning liquid containing a surfactant, hydrazine and the like are used. The amount of cleaning liquid or alkaline cleaning liquid used varies considerably depending on the type of scale and the amount of attached organic substances.

In a nuclear power plant, an oxidizing agent, a reducing agent (hydrazine, etc.), a chelating agent (EDTA, etc.) are used in order to decontaminate radioactive corrosion products (clads) generated in the primary cooling system of a nuclear reactor. , Corrosion inhibitors, surfactants, etc. are used alone or in combination. The waste liquid after washing or decontamination contains the above-mentioned organic substances and inorganic substances, and the COD of the waste liquid is very high.

In order to reduce the COD of the waste liquid, conventionally, when the waste liquid contains heavy metals, (1) the pH of the waste liquid is adjusted to alkaline (12 to 12.5) to precipitate the heavy metals. Then, (2) the pH of the waste liquid is adjusted to acidic (1.5 to 3) and EDTA is precipitated as crystals to separate, and finally (3) the organic substance in the waste liquid is chemically oxidized. A treatment method such as oxidative decomposition is used.

[0005]

However, in the above treatment method, only a part of the nitrogen component in the waste liquid is removed,
Although it was discharged as treated water containing nitrogen components, eutrophication of rivers, bays, and inland seas became a social problem, and now nitrogen and phosphorus in the treated water are strictly regulated and denitrification of waste liquid is performed. Is needed.

As a method for removing organic nitrogen in waste water, a biological treatment method using microorganisms is known.
The waste liquid contains a large amount of components such as EDTA and surfactants that cannot be removed by biological treatment, and a denitrification treatment method that does not rely on biological treatment is required. Therefore,
An object of the present invention is to provide an efficient denitrification method of a waste liquid containing at least EDTA as a nitrogen-containing organic compound, which does not depend on biological treatment.

[0007]

The above object is achieved by the present invention described below. That is, the present invention provides at least sodium ethylenediamine tetraacetate (E
In the denitrification treatment of the waste liquid containing DTA), the waste liquid is
(1) a step of removing EDTA by precipitation under strongly acidic pH, (2) a step of oxidatively decomposing an organic substance with an oxidizing agent to generate ammonia, (3) adjusting the pH to alkaline, and stripping the ammonia with a stripping column A step of stripping the gas, a step (4) of oxidizing the stripped ammonia gas, and a step (5) of removing residual organic substances and inorganic salts, if necessary, from step (1). This is a denitrification treatment method for waste liquid containing organic nitrogen.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below with reference to preferred embodiments. The waste liquid treatment method of the present invention utilizes the pH and the heated state of the waste liquid of the previous step in the next step to efficiently perform the denitrification treatment in each step. That is, the pH of the waste liquid is made highly acidic to precipitate EDTA for separation, and then the acidic waste liquid from which EDTA has been removed is oxidatively decomposed under heating with or without the use of an oxidation catalyst to convert organic nitrogen into ammonia and nitrogen. Decomposes into gas and nitrogen oxides. At this time, the oxidation conditions are adjusted to minimize the production of nitrogen oxides. Next, by stripping the ammonia in the heat treatment waste liquid and oxidizing and stripping the stripped ammonia gas, the organic nitrogen in the waste liquid can be efficiently denitrified.

The waste liquid used in the present invention is at least E as organic nitrogen generated in a thermal power plant or a nuclear power plant.
It is a waste liquid such as a cleaning liquid and a decontaminating agent containing DTA. Each step will be described below step by step.

(1) EDTA removal step EDTA in the waste liquid precipitates as insoluble crystals in the acidic region. Therefore, the pH of the waste liquid is adjusted to 1 to 3 with sulfuric acid or the like, and the waste liquid is sent to the EDTA separation layer to generate EDTA.
The crystals are precipitated, and the supernatant is transferred to the next oxidation treatment step. Mechanical separation means such as centrifugation can be used for the separation of EDTA. EDTA is 95-
99% recovered and reused.

(2) Oxidative decomposition treatment step In this step, C in the waste liquid of acidic pH which has undergone the step (1) is
Oxidizing OD and organic substances to oxidize and decompose carbon dioxide gas and water, and organic nitrogen such as residual EDTA and hydrazine to nitrogen gas and ammonia. In this oxidative decomposition, organic nitrogen is converted into ammonia, nitrogen gas, and nitrogen oxides, but it is desirable to minimize the production of nitrogen oxides by adjusting the oxidizing conditions. Most of the ammonia gasifies, and some remains in the processing liquid. Ammonia gas can be collected as ammonium sulfate in the sulfuric acid absorption liquid with an ammonia scrubber, for example.

The preferred oxidation treatment method in this step is to use the waste liquid p
Since H is acidic, a chemical oxidation method in which an oxidation treatment is performed using a metal or metal ion as a catalyst and an oxidant is preferable. A preferable oxidation treatment condition is pH 0.7 to 5.0,
The temperature is in the range of 40 to 75 ° C.

As the oxidation catalyst, any metal or metal ion used in the conventionally known chemical oxidation method such as ferrous ion can be used, and it is not particularly limited, but the waste liquid contains metal or metal ion. In some cases, these can be used as a catalyst. Since the use of the catalyst causes an increase in solid waste, the catalyst may not be used depending on the type and amount of organic substances in the waste liquid. As the oxidizing agent, any oxidizing agent such as hydrogen peroxide used in the conventionally known chemical oxidation method can be used and is not particularly limited.

Oxidative decomposition conditions (addition amount of oxidizing agent, reaction p
Depending on H, reaction temperature, etc.), EDTA can be decomposed almost completely, but if the oxidation reaction is promoted too much, organic nitrogen such as EDTA is oxidized to nitrate nitrogen via ammonia nitrogen. The reaction can be suppressed by ammonia nitrogen by controlling the oxidative decomposition conditions. In such a case, since EDTA will remain, a post-oxidation treatment step can be provided after the above-mentioned oxidative decomposition treatment in order to further increase the EDTA removal rate. The conditions for the oxidation treatment are the same as above. The high temperature waste liquid subjected to the oxidation treatment in this step is transferred to the next stripping column and subjected to the stripping treatment.

(3) Ammonia Stripping Treatment Step In this step, the high temperature waste liquid containing residual ammonia and the like that has undergone the preceding oxidation treatment step is sent to a stripping column in order to strip ammonia from the waste liquid.
At this time, in order to enhance the stripping efficiency of ammonia, the pH of the waste liquid is adjusted to be alkaline with caustic soda or the like, preferably 10 to 12. P for alkaline
The H-adjusted waste liquid is further heated by steam or the like in the stripping column to strip ammonia gas. The stripping temperature varies depending on the concentration of ammonia in the waste liquid, but is preferably 60 to 1
00 ° C.

The preferred stripping column used in this step is a plate column or a packed column, but it is not particularly limited as long as it is a column capable of efficiently stripping ammonia gas. The operating conditions of the column are not particularly limited. The stripped ammonia gas is sent to the next ammonia oxidation treatment step (4), and the treatment waste liquid discharged from the stripping column is sent to the step (5) of removing residual organic substances and inorganic salts, if necessary. It is processed.

(4) Ammonia Oxidation Treatment Step The ammonia gas extracted from the stripping column is oxidatively decomposed into nitrogen gas and water in the presence of an oxidation catalyst in this step. As the oxidizing agent and the oxidizing catalyst, all the conventionally known oxidizing agents and oxidizing catalysts used for the oxidation of ammonia can be used. For example, as the oxidizing agent, oxygen gas in the air, etc., and as the oxidizing catalyst, platinum, rhodium, etc. Is mentioned.
A preferable oxidation temperature is 250 to 350 ° C.

(5) Detreatment process of residual organic substances and inorganic salts Discharge from the stripping column in the preceding stage (1)-
The treated waste liquid that has undergone each treatment step of (4) is, if necessary, subjected to a removal (removal) treatment of residual organic substances and inorganic salts in the waste liquid in this step so that it can be recycled and reused as treated water. For removing the residual organic matter, a known treatment method such as the above-mentioned chemical oxidation method or a method of oxidatively decomposing the waste liquid in which hydrogen peroxide and ozone are dissolved by irradiating with ultraviolet rays can be used, and the treatment method itself is particularly preferable. Not limited.

The demineralization treatment is not particularly limited in the demineralization method itself, and known demineralization treatment methods such as an ion exchange method and a membrane separation method using a reverse osmosis membrane can be used. The concentrated salt-containing liquid subjected to the desalting treatment can reduce the amount of the processing waste liquid by removing water by evaporation and precipitating an inorganic salt to form a solid waste. By going through the treatment steps (1) to (5) above, the waste liquid containing organic nitrogen such as EDTA is efficiently denitrified and desalted, and the treated water is reused as highly treated water. You can do it.

[0020]

Next, the present invention will be described specifically with reference to examples. Example 1 (1) Removal of EDTA A simulated chemical cleaning waste liquid (simulated raw water: pH 7.48) containing 10% by weight of EDTA and 1.0% by weight of hydrazine was used to remove EDTA. The pH of the simulated raw water was adjusted to 1.5 with concentrated sulfuric acid, and this raw water was reacted at room temperature for 2 hours with stirring. The precipitated EDTA crystals were sufficiently precipitated, and the TOC and total nitrogen content (TN) of the supernatant (treated water) were measured and shown in Table 1 together with these values of raw water. The removal rate of EDTA was 98.8%.

[0021]

[Table 1] Results of EDTA removal treatment

(2) -1 Chemical oxidation treatment of hydrazine etc. Based on the result of (1), the treated water after removing EDTA was simulated to contain 1.0 wt% of EDTA and 1.0 wt% of hydrazine. The simulated waste water was oxidatively decomposed under the two conditions shown in Table 2, and the treatment results shown in Table 3 were obtained. In the oxidative decomposition under the condition (I), the hydrazine removal rate was 55.3%,
The oxidative decomposition under the condition (II) was 99.9%. E
It was confirmed that DTA was also removed to some extent by oxidative decomposition, and the results in Table 3 show that the removal rate of EDTA is further improved by performing the two-stage oxidation treatment of condition (I) and condition (II). There is.

[0023]

[Table 2]

[0024]

[Table 3] Results of chemical oxidation treatment

(2) -2 Oxidation Conditions and Decomposition State of Organic Nitrogen Oxidation conditions and decomposition state of organic nitrogen are as follows: EDTA 10% by weight, hydrazine 1% by weight, surfactant (polyethylene glycol alkylphenyl ether) 0.01 weight%,
Simulated wastewater including others [TOC 40036mg / l,
T-N 9260 mg / l (nitrogen N derived from hydrazine is not included because it cannot be detected by a nitrogen meter)]. (I) Amount of oxidizing agent added Under the oxidizing conditions of pH = 2, reaction temperature = 65 ° C., reaction time = 5 hours, the amount of the oxidizing agent (hydrogen peroxide) used was changed to perform oxidative decomposition. Table 4 shows the amount of oxidant used and the result of oxidative decomposition.
Shown in

[0026]

[Table 4]

(Ii) Reaction pH Oxidizing agent (using hydrogen peroxide) Dose = 112000 mg / l
Oxidative decomposition was performed by changing the pH of the treatment system under the oxidizing conditions of as O (as oxygen), reaction temperature = 75 ° C., and reaction time = 5 hours. Table 5 shows the results of pH and oxidative decomposition during oxidation.
Shown in

[0028]

[Table 5]

(3) Ammonia stripping Based on the result of (2), various ammonia contents (45
Simulated waste water (0-1600 mg / l) was applied to the stripping column to strip ammonia gas.
The pH of each simulated wastewater was adjusted to 12.5 with caustic soda in advance. Stripping column is 10 cm (diameter) x 10
0 cm (length) was used, the internal temperature of the column was maintained at 60 to 70 ° C., and simulated waste water was continuously supplied. 55 kinds of simulated waste water were stripped, but in any case, ammonia was not detected in the simulated waste water after the stripping treatment, and the removal rate of ammonia was substantially 100%. The results are shown in FIG.

(4) Treatment of residual organic matter In the stripped simulated wastewater, 100% of ammonia was removed by stripping the simulated wastewater subjected to oxidative decomposition under the condition (II) in (2) in the same manner as in (3). The residual hydrazine and EDTA were removed (pH before treatment was 6.5, T-N was 8.0 mg / l, and TOC was 2).
3 mg / l). Add 20mg / l hydrogen peroxide to the simulated waste water, and bubbling ozone at room temperature for 2
Irradiated for hours. A high pressure ultraviolet lamp was used as the ultraviolet source. As a result of the treatment, pH was 6.5 and T-N was 7.5 mg /
1 and TOC were 5 mg / l.

[0031]

According to the present invention as described above, the denitrification treatment of the waste liquid containing organic nitrogen such as EDTA can be carried out very efficiently, and the denitrification-treated waste liquid is reprocessed as highly treated water. It can be recycled.

[Brief description of the drawings]

FIG. 1 is a diagram showing an effect of an ammonia stripping treatment of Example 1.

Claims (4)

[Claims] 1. In a denitrification treatment of a waste liquid containing at least ethylenediaminetetratetraacetic acid sodium salt (EDTA) as organic nitrogen, the waste liquid is (1) a step of removing EDTA by precipitation under strongly acidic pH, (2) A step of oxidatively decomposing an organic substance with an oxidizing agent to generate ammonia, (3) a step of adjusting pH to alkaline and stripping ammonia gas with a stripping column,
(4) A step of oxidizing the ammonia gas, and (5) a step of removing the residual organic substances and inorganic salts, if necessary,
A method for denitrifying an organic nitrogen-containing waste liquid, which is performed sequentially from (1). 2. The oxidation treatment in step (2) is performed at room temperature to 100 ° C.
The denitrification treatment method of the organic nitrogen-containing waste liquid according to claim 1, which is carried out in 1. 3. The method for denitrifying an organic nitrogen-containing waste liquid according to claim 1, wherein the oxidation treatment in the step (2) is performed at least once. 4. The method for denitrifying an organic nitrogen-containing waste liquid according to claim 1, wherein the waste liquid containing the residual organic matter is irradiated with ultraviolet rays in the presence of hydrogen peroxide and ozone.