JP5321475B2 - Nitrogen-containing wastewater treatment method - Google Patents

Description

  The present invention relates to a nitrogen-containing wastewater treatment method for removing nitrogen contained in wastewater by biological denitrification, a method for suitably selecting a hydrogen donor used in the treatment, and the use of the hydrogen donor. The present invention relates to a method for treating nitrogen wastewater.

  Nitrogen components contained in wastewater from food factories and sewage treatment facilities are controlled to be less than the amount regulated by local ordinances by biological denitrification to worsen the surrounding environment. .

  Conventionally, when biologically denitrifying nitrogen-containing wastewater, depending on the nature of the wastewater, organic carbon sources such as methanol (hydrogen It is common practice to add donors. However, because methanol is expensive, alternative hydrogen donors have been studied.

  Patent Document 1 relates to a biological denitrification method for organic wastewater. The separation liquid after methane fermentation, such as raw garbage and livestock waste, needs to be denitrogenated. Describes the use of waste milk as a denitrification hydrogen donor (organic carbon source) to perform nitrogen removal at low cost and high efficiency because of the high efficiency and the low ratio of organic substances that serve as denitrification hydrogen donors. Has been.

  Patent Document 2 relates to a wastewater treatment method and wastewater treatment equipment for nitrifying and denitrifying wastewater containing ammonia with activated sludge, and adding a whiskey initial distillate that is a waste liquid generated in the whiskey manufacturing process. It is described that denitrification efficiency substantially equal to that obtained when methanol is added is obtained, and the treatment cost of ammonia-containing wastewater is reduced.

JP 2005-74263 A JP 2007-275747 A

"Sewage test method 1984 version" Japan Sewerage Association June 25, 1985 Second edition issued

  By the way, in the case of steelworks, nitrogen-containing wastewater containing nitric acid as a nitrogen component is generated from the acid cleaning process of cold rolling mills, etc., and biological desorption is performed by reducing nitrate ions to nitrogen in the presence of a hydrogen donor. Treated with nitrogen treatment. In the case of wastewater derived from such steelworks, organic carbon sources for denitrification tend to be extremely short, and it is necessary to add organic carbon sources such as methanol from outside the system. .

In this reaction, denitrifying bacteria act, and when methanol is used as a hydrogen donor, it is represented by the formula (1).
NO ₃ ⁻ + 5 / 6CH ₃ OH → 5/6 CO ₂ + 1 / 2N ₂ + 7 / 6H ₂ O + OH ⁻ (1)

  However, the existence of production facilities for discharging waste milk and whiskey first-run products used in Patent Documents 1 and 2 is limited, and when these production facilities are not in the vicinity of the steel works, hydrogen donors are used. It is difficult to secure a sufficient amount of the material itself, and even if it can be ensured, transportation and transportation from a remote place are costly.

  In addition, since the operation status of the steelworks and the operation status of the factories that supply these hydrogen donors are basically unrelated, the operation status of the steelworks is good and the amount of nitrogen-containing wastewater discharged is large. When a hydrogen donor is required, there may be a situation where a sufficient amount of hydrogen donor cannot be supplied unless the operating condition of the hydrogen donor supplier is good.

  In such a case, methanol must be temporarily used, but the denitrifying bacteria to be treated cannot adapt to changes in the hydrogen donor in a short period of time, and the denitrification ability is inevitably lowered for a certain period. Therefore, there is a concern about an increase in the concentration of treated water nitrogen.

Therefore, it is difficult to use these hydrogen donors as a substitute for methanol, and it is necessary to use a hydrogen donor appropriately selected from waste organic substances derived from steelworks.

  Therefore, the present invention uses a method for selecting a hydrogen donor that can be used as an alternative to methanol in biological denitrification treatment of nitrogen-containing wastewater derived from steelworks, from waste organic matter discharged from the steelworks, and the same. It aims at providing the processing method of nitrogen-containing wastewater.

In order to solve the above-mentioned problems, the present inventors have intensively studied the suitability as a hydrogen donor for waste organic substances discharged from steelworks, and found that those satisfying certain selection criteria are effective. The object of the present invention can be achieved by the following means.
1. As a hydrogen donor used in the biological denitrification treatment of nitric acid-containing waste liquor generated in the pickling process of stainless steel in steelworks, which is nitrogen-containing wastewater , BOD / CODcr is used as waste hydrogen or oil- containing wastewater discharged from steelworks. value selected ones of 0.4 or more, and further selects the nitrogen removal rate obtained in advance performed denitrification continuous experiment from its what is more nitrogen removal rate that the specification on the target, waste oil or oil-containing wastewater Is used as a hydrogen donor . A method for treating nitrogen-containing wastewater.
2 . A method for treating nitrogen-containing wastewater, comprising using methanol and waste oil or oil- containing wastewater discharged from an iron mill selected by the method described in 1 in addition to methanol as a hydrogen donor.
3 . 3. The method for treating nitrogen-containing wastewater according to 2 , wherein the ratio of the amount of addition of methanol and other than methanol in the hydrogen donor is determined by at least the total nitrogen concentration in the treated water and COD _Mn .

  According to the present invention, in biological denitrification treatment of nitrogen-containing wastewater derived from steelworks, waste organic substances discharged from the steelworks are selected as those that can be used as hydrogen donors. The operation cost can be reduced while obtaining the same treated water quality, which is extremely useful industrially.

The schematic diagram explaining the structure of the nitrogen-containing wastewater treatment apparatus which performs the biological denitrification process suitable for implementation of this invention. The schematic diagram explaining the structure of an experimental apparatus. The figure explaining the operating conditions of the experimental apparatus shown in FIG. The figure which shows the denitrification continuous experiment result.

The present invention relates to a hydrogen donor used for biological denitrification treatment of nitric acid-containing waste liquid, which is nitrogen-containing wastewater derived from steelworks, as follows: 1. The hydrogen donor's own BOD / CODcr value and The selection is based on the value of the nitrogen removal rate obtained in a denitrification continuous experiment performed in advance. Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the description, BOD is a biochemical oxygen demand (according to Non-Patent Document 1 p123) and is an indicator of the amount of organic matter eaten by organisms, and CODcr is the amount of oxygen consumed by potassium dichromate (JIS K 0102 15). It is an indicator of quantity. In the water treatment field in Japan, oxygen consumption by potassium permanganate COD _Mn (JIS K 0102 13) is common as COD, but the oxidizing power of potassium permanganate is higher than that of potassium dichromate. Since there are many organic substances that cannot be oxidized due to their weakness and are inappropriate as an indicator of total organic substances, CODcr was used as an indicator of total organic substances in the present invention.

  FIG. 1 is a schematic diagram illustrating the configuration of a nitrogen-containing wastewater treatment apparatus that performs biological denitrification suitable for the practice of the present invention. In the figure, 1 is a raw water tank, 2 is a denitrification tank, 3 is an aeration tank, 4 Is a settling tank, 5 is a stirrer, 6 is an aeration device, 7 is a blower, 8 is a methanol addition device which is a hydrogen donor addition device (1), and 9 is a waste organic substance addition device which is a hydrogen donor addition device (2). Reference numeral 10 denotes a return sludge pipeline, 11 denotes a raw water pumping motor, and 12 denotes a raw water feed pipeline.

  The nitrogen-containing wastewater is stored in the raw water tank 1 after being subjected to coagulation sedimentation and filtration. At this point, the BOD of the nitrogen-containing wastewater has a very low concentration, and has a drainage property in which almost no organic substance serving as a hydrogen donor exists. The nitric acid-containing waste liquid generated in the pickling process such as stainless steel is stored in the raw water tank 1 after being subjected to coagulation precipitation and filtration. At this point, the total nitrogen concentration is about 50 to 150 mg / L, most of which is nitrate nitrogen, while BOD is very low in concentration, and has a drainage property with almost no organic substance serving as a hydrogen donor.

  The raw water is pumped from the raw water tank 1 by the pump 11 for pumping the raw water, and sent to the denitrification tank 2 through the raw water supply pipe 12, where it is biologically denitrified by activated sludge (biological denitrification process). Is done. A hydrogen donor is added from the methanol addition device 8 and the waste organic matter addition device 9 in accordance with the nitrogen concentration of the raw water.

The waste water biologically denitrified in the denitrification tank 2 is then guided to the aeration tank 3. Here, the organic matter (COD component) remaining without being used in the denitrification step is biologically oxidatively decomposed with activated sludge and converted into CO ₂ and water. Thereafter, solid-liquid separation is performed in the sedimentation tank 4, and the supernatant is treated water. A part of the settled sludge is circulated through the return conduit 10, and a part is extracted as surplus sludge. By such treatment, waste water treatment capable of removing nitrogen and COD components with good methanol cost can be realized.

  In the present invention, the hydrogen donor added from the hydrogen donor addition device 9 is 1. 1. The hydrogen donor's own BOD / CODcr value and Select based on the value of nitrogen removal rate in continuous denitrification experiments using biological denitrification.

  Fig. 4 shows the frequency distribution and cumulative ratio of the BOD / CODcr value of raw water collected under operating conditions that meet the standard value of treated water set in the specifications in the sewage treatment plant and industrial wastewater treatment facility that are actually in operation. Is shown. In FIG. 4, the horizontal axis represents BOD / CODcr values in order from the left, less than 0.3, 0.3 or more and less than 0.4, 0.4 or more and less than 0.5, and 0.5 or more and 0.6. Less than, 0.6 or more. Since the BOD / CODcr value at which the cumulative ratio is 0.5 or more is 0.4 or more, it is appropriate to select a hydrogen donor that satisfies this condition as a candidate capable of performing stable nitrogen removal. Thought. The hydrogen donor thus selected is subsequently subjected to continuous denitrification experiments.

  FIG. 2 shows the apparatus configuration of the denitrification continuous experiment, and FIG. 3 shows the experimental procedure. A hydrogen donor from the hydrogen donor storage tank 14 (corresponding to the methanol addition apparatus 8 in FIG. 1), methanol from the methanol storage tank 15 (corresponding to the waste organic substance addition apparatus 9 in FIG. 1), and nitrogen from the raw water storage tank 16 The waste water is fed to the aeration tank 21 by the pump 20 and the mixed solution 23 is only stirred by the stirring device 22 to realize an anaerobic state without oxygen to cause a biological denitrification reaction, and then the air pump 24 The aerobic state is realized by aeration with air from, and the organic matter remaining during the denitrification reaction is aerobically decomposed. The acid / alkaline is fed from the acid / alkali storage tank 18 by the pump 20 so that the pH of the mixed solution 23 becomes an appropriate value by the pH controller 17. The liquid mixture 23 is fed to the treated water storage tank 19 by a pump 20 after the experiment is completed.

  An example of this denitrification continuous experiment is shown below. Oil-containing wastewater (waste organic matter A in Table 1) and other (waste organic matter B and C in Table 1) discharged from the steelworks as waste organic matter derived from the steelworks as hydrogen donors, and the raw water is the acid of the stainless steel of the steelworks A nitric acid-containing waste liquid generated in the washing step was used. In this experimental example, an experiment was performed for each waste organic matter A, B, and C, and the content of each waste organic matter in the used hydrogen donor was 100%.

  Table 1 shows the BOD / CODcr value of each organic waste and the results of denitrification continuous experiments. Waste organic substances A and B had BOD / CODcr values of 0.7 and 0.4, respectively, and high nitrogen removal rates of 98% and 97% were obtained by continuous denitrification experiments, respectively. On the other hand, the waste organic matter C had a BOD / CODcr value of 0.4, but the nitrogen removal rate in the continuous denitrification experiment was only 70%, and the nitrogen removal rate targeted in the specifications of the apparatus (in this experimental example) 88%). This means that selection based on the BOD / CODcr value alone is not sufficient, and after selection based on the BOD / CODcr value, the nitrogen removal rate (88% in this example) is higher than the target nitrogen removal rate by continuous denitrification experiments. Indicates that you need to check. On the other hand, in the selection method in which all waste organic substances are confirmed by continuous denitrification experiments from the beginning, continuous experiments must be performed on all waste organic substances. This is not preferable because it takes time. Therefore, the selection can be made efficiently and quickly by first selecting the BOD / CODcr value of each waste organic substance and then further selecting by denitrification continuous experiment.

  In the case of this experimental example, an actual device that biologically denitrifies the nitric acid-containing waste liquid generated in the steel pickling process at a steel works is assumed, and the target nitrogen removal rate is the actual device specifications. Although described as 88% or more, this numerical value varies depending on the assumed device, and may be changed as appropriate depending on the quality of the influent water, the type of treatment, the desired effluent quality, and the like.

  According to the present invention, in the case of biological denitrification treatment of nitric acid-containing waste liquid generated in the steel pickling process of steelworks, in addition to methanol as a hydrogen donor, oil-containing wastewater discharged from steelworks (the waste of Table 1) When the organic substance A) is added, the following effects are obtained.

Nitrogen-containing wastewater derived from steelworks has almost no BOD source, so the amount of methanol used is enormous, and even if only a part of it can be replaced, sufficient cost merit can be expected. In this case, the waste organic matter addition ratio can be determined within a range that does not exceed the predetermined water quality by gradually increasing the addition ratio while monitoring at least the total nitrogen concentration and COD _Mn in the treated water. it can.

  In addition, when waste oil derived from steelworks or oil-containing wastewater is used as waste organic matter, a certain degree of correlation can be expected between the amount of nitrogen-containing wastewater generated and the amount of waste organic matter that can be secured.

  Both nitrogen-containing wastewater and waste organic matter are generated with the production of steel materials at steelworks, and the amount of nitrogen-containing wastewater and waste organic matter generated is considered to be proportional to the production amount of steel materials.

  Therefore, the amount of waste organic matter required increases or decreases depending on the increase or decrease in the amount of nitrogen-containing wastewater generated, but both are correlated to some extent, so when there is a large amount of nitrogen-containing wastewater generated and you want to secure a large amount of waste organic matter, A situation such as an extremely shortage of waste organic matter is extremely unlikely to occur. Therefore, it is possible to add waste organic matter at an almost constant addition rate.

  A denitrification treatment experiment was carried out in a laboratory-level experimental apparatus simulating the batch activated sludge method shown in FIG. This apparatus operates according to the time chart shown in FIG. 3 and removes nitrogen and COD components.

  Table 2 shows the processing conditions. The nitrogen-containing wastewater to be treated was subjected to experiments by adding sodium nitrate to the wastewater discharged from the stainless steel pickling process at the steelworks and adjusting the nitric acid concentration to 150 mg / L in terms of nitrogen. At this time, the total nitrogen concentration in the waste water was all nitrate nitrogen.

  In the experiment, oil-containing wastewater (waste organic matter A in Table 1) discharged from steelworks was used as waste organic matter. Table 3 shows the analysis values.

  Part of methanol was replaced with oil-containing wastewater by changing the ratio. The amount of methanol and oil-containing wastewater added is theoretically such that the total amount of BOD required to denitrify nitrate nitrogen is the total amount of BOD added / total amount of nitrogen to be treated = 2.86. You may adjust to. This is due to the following reason.

The reaction in which nitric acid is denitrified by a hydrogen donor is represented by formula (2).
2NO ₃ ⁻ + 5H ₂ → N ₂ + 4H ₂ O + 2OH ⁻ (2)
Further, H ₂ (= organic substance as a hydrogen donor) consumed in the denitrification reaction is converted into BOD, and 1 mol of H ₂ corresponds to 1/2 mol of O ₂ according to the formula (3).
H ₂ + 1 / 2O ₂ → H ₂ O (3)
Therefore, when NO ₃ ^- is denitrified in terms of nitrogen by the reaction of formula (2),
[5 × (1/2) × 2 × O atomic weight] / (2 × N atomic weight) = 2.86 (4)
This leads to the consumption of 2.86 mg of BOD.

  However, not all of the added BOD is used for the denitrification reaction, and BOD is consumed for energy acquisition by respiration and synthesis of bacterial cells. In practice, the BOD consumed in addition to these denitrification reactions is about 30%, so the total amount of methanol to be added and the BOD of the oil-containing wastewater is about 4 times the total amount of nitrogen to be treated (required total) From BOD × (1-0.3) = 2.86, 2.86 / 0.7≈4) was added.

Table 4 shows the mixing ratio of methanol and oil-containing wastewater in Experiment Nos. 1 to 4. Also sets the acclimatization period of about one month at any conditions, nitrate body nitrogen and COD _Mn in the treated water was measured periodically, and determines that the change has reached a steady state was no longer observed. The analysis of nitrate nitrogen in the treated water was carried out using nitric acid (zinc reduction-naphthylethylenediamine colorimetric method JISK0102 43.2.3 modified by Pactest Kyoritsu Riken), nitrous acid (naphtholethylenediamine absorption by Paktest Kyoritsu Riken). The photometric method JIS K 0102 (43.1.1) was analyzed, and the analysis values of both were added. COD _Mn was analyzed by the method described above (JIS K 0102 13).

After the steady state, the treatment was continued for about one week, and nitrate nitrogen and COD _Mn in the treated water were measured every day, and then an average value was obtained and used as a representative value of treated water quality. Further, the nitrogen removal rate was determined as nitrogen removal rate (%) = nitric acid nitrogen concentration in treated water / 150 mg / L × 100. In addition, 150 mg / L of the denominator is the nitrate nitrogen concentration in terms of nitrogen contained in the nitrogen-containing wastewater before treatment.

Table 4 also shows the experimental results. In addition, the target value on the specification set in the present embodiment is such that the total nitrogen concentration in the treated water is 18 mg / L or less and the COD _Mn is 40 mg / L or less. It shows together with Table 4.

  In Experiment Nos. 1 and 2, 5% and 25% of the hydrogen donor can be replaced with oil-containing wastewater while maintaining good treated water quality well below the target specification value, and methanol costs are reduced to 5% and 25%, respectively. It was possible to reduce.

In both experiment number 3 (methanol: oil-containing wastewater = 50: 50) and experiment number 4 (methanol: oil-containing wastewater = 25: 75), the total nitrogen concentration in the treated water is the same as in experiment numbers 1 and 2, and good nitrogen treatment is achieved. However, COD _Mn increased in proportion to the oil-containing wastewater addition ratio, and the quality of treated water deteriorated. This is because there is a biodegradable COD _Mn component present in the oil-containing wastewater, and some of it is removed from the treated water by adsorption to sludge, but it cannot be removed if the amount added is increased. This is probably because the biodegradable COD _Mn component flows out into the treated water. That is, in Experiment No. 3, it is considered that careful operation management is required, but the specification target value is satisfied, and the methanol cost can be reduced by 50%. On the other hand, in experiment number 4, the specification target value is not satisfied.

  Therefore, in this embodiment, the methanol replacement ratio of oil-containing wastewater is appropriately about 5 to 50%, and methanol is between 5 and 50% depending on the amount of oil-containing wastewater that can be secured and the actual processing conditions. The replacement rate should be determined.

  As a matter of course, this example is merely an example, and the quality of treated water as a target value in the specification is the presence or absence of subsequent treatment, and in some cases, the type (coagulation sedimentation, sand filtration, activated carbon treatment, etc.), It should be determined according to performance and the like, and the target numerical values are not limited by this embodiment.

DESCRIPTION OF SYMBOLS 1 Raw water tank 2 Denitrification tank 3, 21 Aeration tank 4 Precipitation tank 5 Stirrer 6 Aeration device 7 Blower 8 Methanol addition device 9 Waste organic substance addition device 10 Return sludge pipeline 11 Raw water pumping motor 12 Raw water supply pipeline 14 Hydrogen supply Body storage tank 15 Methanol storage tank 16 Raw water storage tank 17 pH controller 18 Acid / alkali storage tank 19 Treated water storage tank 20 Pump 22 Stirrer 23 Mixture 24 Air pump

Claims (3)

As a hydrogen donor used in the biological denitrification treatment of nitric acid-containing waste liquor generated in the pickling process of stainless steel in steelworks, which is nitrogen-containing wastewater , BOD / CODcr is used as waste hydrogen or oil- containing wastewater discharged from steelworks. value selected ones of 0.4 or more, further nitrogen removal rate obtained in advance performed denitrification continuous experiments among them is selected as at least a nitrogen removal rate of the specification on the target, the waste oil or oil-containing wastewater Is used as a hydrogen donor . A method for treating nitrogen-containing wastewater. A method for treating nitrogen-containing wastewater, characterized by using methanol and waste oil or oil- containing wastewater discharged from a steel mill selected by the method according to claim 1 in addition to methanol as a hydrogen donor. The method for treating nitrogen-containing wastewater according to claim 2 , wherein the ratio of the amount of methanol added to the hydrogen donor other than methanol is determined by at least the total nitrogen concentration in the treated water and COD _Mn .

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