JPH09290297A - Treatment of waste water from thermal electric power plant - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively remove the harmful matter, especially selenium, contained in the waste water from a thermal electric power plant by bringing the waste water with the pH specified into contact with iron, conducting flocculation and solid-liq. separation and then biologically denitrifying the waste water. SOLUTION: The waste water from a thermal electric power plant contg. such oxidative matter as iodic acid and selenic acid is introduced into a pH regulating tank 1, controlled to <=pH5 with hydrochloric acid and then sent to a fine iron grain-packed column 3 by a pump 2 to elute the iron as ferrous ion and to reduce the oxidative matter and selenium compd. in the water. Sodium hydroxide is injected into the water flowing out from the column 3 in a reaction tank 4 to flocculate the iron ion as iron hydroxide, the water contg. the iron floc is sent to a settling tank 5, and solid is separated from liq. The supernatant water in the settling tank 5 is aerobically nitrated in a nitration tank 6, anaerobically denitrified in a denitrification tank 7 and further subjected to solid-liq. separation in a settling tank 8 to obtain treated water.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating wastewater from a thermal power plant. More specifically, the present invention relates to a method of treating wastewater from a thermal power plant, which can efficiently perform biological denitrification on wastewater from a thermal power plant containing oxidizing substances such as peroxosulfuric acid, iodic acid and selenate.

[0002]

In a thermal power plant, flue gas desulfurization wastewater,
Various kinds of wastewater such as condensate desalination wastewater and general wastewater are generated. Flue gas desulfurization wastewater contains nitrate nitrogen and nitrite nitrogen, and condensate desalination wastewater contains ammonia nitrogen. Therefore, it is necessary to remove nitrogen from these wastewaters by denitrification.
Various wastewaters generated in a thermal power plant are treated separately depending on the situation or mixed and treated collectively. If the flue gas desulfurization method is a suit mixed type, the wastewater contains heavy metals, fluorine, nitrogen, etc. as harmful substances,
Further, so-called oxygen acid such as peroxosulfuric acid, iodic acid and selenate is often contained as an oxidizing substance. Flue gas desulfurization wastewater is generally sent from a wastewater storage tank to a biological denitrification process via coagulation sedimentation treatment and filtration treatment, and further to coagulation sedimentation treatment and filtration treatment and then to a treated water tank. However, according to this treatment flow, the nitrogen removal function in the biological denitrification treatment step is often lowered, and it is difficult to stably treat the wastewater from the thermal power plant. In addition, it is necessary to remove harmful substances such as peroxosulfuric acid, iodic acid, and selenate contained in wastewater from thermal power plants, but the conventional treatment method reduces the selenium concentration to 0.1 mg / liter or less, which is the wastewater standard. It was difficult to do.

[0003]

DISCLOSURE OF THE INVENTION The present invention is a thermal power plant capable of exhibiting a stable nitrogen removal function in a biological denitrification process and efficiently removing harmful substances, especially selenium, contained in wastewater. It was made for the purpose of providing a method for treating wastewater.

[0004]

As a result of intensive studies to solve the above problems, the inventors of the present invention have found that the functional deterioration in the biological denitrification process is caused by the oxidizing substances contained in the wastewater of thermal power plants. Furthermore, by adjusting the pH of the waste water to 5 or less and bringing it into contact with iron, and then performing coagulation treatment and solid-liquid separation, it was possible to efficiently remove oxidizing substances. As a result, the present invention has been completed based on these findings. That is, the present invention
(1) A method for treating thermal power plant wastewater, which comprises subjecting the thermal power plant wastewater to pH 5 or less, bringing it into contact with iron, performing coagulation treatment and solid-liquid separation, and then performing biological denitrification treatment, Is provided. Furthermore, as a preferred embodiment of the present invention, (2) the use of hydrochloric acid or sulfuric acid for pH adjustment
(1) Thermal power plant wastewater treatment method described in (1), (3) iron,
A method for treating wastewater from a thermal power plant according to item (1) or (2), which is fine particles having a maximum diameter of 3 mm or less, wire or granular iron, or an iron alloy having an iron content of 85% by weight or more, (4) The contact with iron is carried out until the pH of the water to be treated reaches 5 to 7 or until the oxidation-reduction potential of the water to be treated reaches −100 mV or less, (1), (2) or The method for treating wastewater from a thermal power plant according to (3), (5) performing coagulation treatment by adding an alkaline agent to the water to be treated and adjusting the pH to 7 or higher (1)
Item, (2), (3) or (4) the thermal power plant wastewater treatment method described in (4), and (6) biological denitrification treatment, floating organism method, biofilm method or these Combined first (1)
The method of treating wastewater from a thermal power plant according to item (2), item (3), item (4) or item (5) can be mentioned.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The method of the present invention can be applied to the treatment of wastewater from thermal power plants. The method of the present invention, even in the case of treating flue gas desulfurization wastewater alone in a thermal power plant,
In addition, flue gas desulfurization wastewater, other wastewater generated at thermal power plants,
For example, it can be applied to the case of mixing and treating general wastewater and condensate desalination wastewater. Wastewater from a thermal power plant usually contains oxidizing substances such as peroxosulfuric acid, iodic acid, and selenic acid. Since these oxidizing substances have an inhibitory effect on nitrifying bacteria and denitrifying bacteria, in the method of the present invention, First removes oxidizing substances from the wastewater of thermal power plants and then performs biological denitrification. In the method of the present invention,
A pH adjusting agent is added to the wastewater of the thermal power plant to adjust the pH to 5 or less, preferably to 2 to 3. The pH adjuster used is not particularly limited, but hydrochloric acid and sulfuric acid can be preferably used. When the pH of the water to be treated exceeds 5, it may take a long time to elute iron into the water to be treated when it is brought into contact with iron, or iron may not be sufficiently eluted into the water to be treated. If the pH of wastewater from a thermal power plant is 5 or less, it is not always necessary to adjust the pH. It is preferable for the pH of the water to be treated to be 2 to 3 because iron is quickly eluted to contribute to the reaction. When the pH of the water to be treated is 1 or less, the elution of iron is too fast and excess iron may elute. In the method of the present invention, pH adjustment can be carried out at any place, for example, it is possible to preliminarily adjust the pH by providing a pH adjusting tank, a pH adjusting agent can be added in the reaction tank, or This can be done by directly supplying the pH adjusting agent to the water pipe. In the method of the present invention, examples of the iron with which the water to be treated whose pH is adjusted to 5 or less are brought into contact include pure iron, crude steel, alloy steel, and other iron alloys. Iron is
It is preferable that the particles have a large surface area such as iron fine particles, iron wire, and granular iron, and the maximum diameter is preferably 3 mm or less, more preferably 0.1 to 1 mm. Also,
When iron is an iron alloy, the iron content is preferably 85% by weight or more.

In the method of the present invention, there is no particular limitation on the method of contacting iron with the water to be treated whose pH is adjusted to 5 or less,
For example, it can be brought into contact by passing water through a column packed with iron fine particles, iron wire, granular iron, or the like, or iron fine particles, iron wire, granular iron, etc. are added to the water to be treated in the reaction tank. They can be brought into contact with each other.
The contact time between the water to be treated and iron is usually preferably 2 to 30 minutes, but it can be controlled by measuring the pH value or redox potential of the water to be treated. The pH increases because the acid is consumed by the dissolution of iron, and the pH of 5 to 7 can be used as a criterion for determining an appropriate contact time.
Since the redox potential decreases due to the reduction of the oxidizing substance, it is possible to use the fact that the redox potential reaches −100 mV or less as a criterion for determining an appropriate contact time. In the method of the present invention, when the water to be treated whose pH is adjusted to 5 or less is brought into contact with iron, iron becomes divalent iron ions and is eluted in water. Divalent iron ions react with oxidizing substances such as peroxosulfate, iodic acid, and selenate in water. In the method of the present invention, peroxosulfate is considered to be decomposed according to the following formula. Fe → Fe ²⁺ + 2e S ₂ O ₈ ²⁻ + 2Fe ²⁺ → 2SO ₄ ² + 2Fe ³⁺ It is considered that iodic acid is reduced according to the following formula. _{^{2IO 3+ 10Fe 2+ + 12H + →}} I 2 + 10Fe 3+ +
6H ₂ O Further, it is considered that selenate is reduced according to the following formula. SeO ₄ ^2- + 6Fe ²⁺ + 8H ⁺ → Se ⁰ + 6Fe ³⁺ +4
H ₂ O

In the method of the present invention, the oxidizing substance in the water is reduced by contacting with iron, and then the water to be treated is coagulated. There is no particular limitation on the method of aggregation treatment,
By adding an alkaline agent, it is preferable that divalent iron ions and trivalent iron ions in water are converted to water-insoluble ferrous hydroxide and ferric hydroxide to form iron flocs and aggregate. The pH of the water to be treated is adjusted to 7 by adding an alkaline agent.
The above is preferable, and the pH is more preferably 9 to 10. If the pH of the water to be treated is less than 7, aggregation of iron flocs and the like may be insufficient. By adjusting the pH of the water to be treated to 7 or more, divalent iron ions in water become water-insoluble ferrous hydroxide as shown in the following formula, and trivalent iron ions become water-insoluble ferric hydroxide. Becomes iron. Fe2 ⁺⁺ 2NaOH → Fe (OH) ₂ + 2Na ^{+ Fe3} ++ 3NaOH → Fe (OH) ₃ + 3Na ⁺ At this time, the reduced selenium is adsorbed on the flocs of the iron hydroxide to be produced and separated by aggregation. Further, part of the fluorine is also adsorbed on the iron flocs and aggregated and separated. In the method of the present invention, by adjusting the pH of the water to be treated to 7 or more, other metal ions whose hydroxides are insoluble in water also become hydroxides to form flocs. At this time, suspended matter, reduced selenium, fluoride components, etc. contained in the wastewater of the thermal power plant are adsorbed by the iron flocs and coagulate at the same time. Further, when ultrafine iron particles are suspended in water, the ultrafine iron particles are also adsorbed by the iron flocs and aggregated. Further, when the reaction system is open to the air, the divalent iron ions undergo air oxidation and part of them become ferric oxide fine particles, and the ferric oxide fine particles are adsorbed on the iron flocs. Aggregate.

In the method of the present invention, the alkaline agent for adjusting the pH of the water to be treated to 7 or more is not particularly limited, and examples thereof include sodium hydroxide, slaked lime, potassium hydroxide, sodium carbonate, potassium carbonate, and carbide slag. Although it can be used, sodium hydroxide and slaked lime can be used particularly preferably. In the method of the present invention, a polymer flocculant can be further added during the flocculation treatment by adding the alkali agent. The addition of the polymeric flocculant coarsens the flocs and facilitates their separation from water.
The polymer flocculant used is not particularly limited, and examples thereof include polyacrylamide, polyethylene oxide, nonionic polymer flocculants such as urea-formalin resin, polyaminoalkylmethacrylate, polyethyleneimine, halogenated polydiallylammonium, and chitosan. Use cationic polymer flocculant, sodium polyacrylate, polyacrylamide partial hydrolyzate, partially sulfomethylated polyacrylamide, anionic polymer flocculant such as poly (2-acrylamide) -2-methylpropane sulfate be able to. Among these polymer flocculants,
Since the anionic polymer flocculant has an excellent flocculating effect,
It can be particularly preferably used. In the method of the present invention, the floc generated by the coagulation treatment is removed by performing solid-liquid separation after the coagulation treatment, and the water to be treated is separated. The solid-liquid separation method is not particularly limited, and any solid-liquid separation method such as precipitation, filtration, centrifugation or membrane separation can be used. Among these solid-liquid separation methods, the membrane separation can remove even fine SS, and the apparatus can be downsized, so that it can be particularly preferably used.

In the method of the present invention, the water to be treated obtained by solid-liquid separation is subsequently subjected to biological denitrification treatment. The biological denitrification treatment is performed by a combination of a nitrification process and a denitrification process. In the nitrification process, ammonia is oxidized by nitrifying bacteria into nitrous acid and nitric acid. Nitrifying bacteria are autotrophic bacteria that exist widely in nature, and Nitrosomonas
Oxidizes ammonia to nitrous acid, and Nitrobacte
r oxidizes nitrous acid to nitric acid, and the energy released in each case is used for reductive assimilation of carbon dioxide. In the denitrification step, nitrous acid and nitric acid are reduced to nitrogen gas by denitrifying bacteria. As denitrifying bacteria, Pseudomon
As, Micrococcus, Spirillum and the like are present, and in the reduction process of nitric acid and nitrous acid, they replace oxygen and become a final oxidant for biological oxidation. As the nitrogen content in the wastewater, there is little ammonia nitrogen, nitrate nitrogen, when mainly nitrite nitrogen can be omitted and only treatment with denitrifying bacteria can be omitted, but usually,
The nitrification process and the denitrification process are used. Nitrification proceeds by bringing water to be treated into contact with activated sludge under aerobic conditions where aeration is performed to supply oxygen. The denitrification proceeds by bringing the water to be treated into contact with activated sludge while supplying a carbon source under anaerobic conditions. In the method of the present invention, the biological denitrification method can be carried out by any treatment method such as a suspended biological method such as an activated sludge method or a suspended particle biofilm method, and a biofilm method such as a fixed biofilm method. A combination of methods can be used. Further, the biological denitrification treatment consisting of nitrification and denitrification can adopt a generally known method, for example, a nitrification tank, a two-tank type in which a denitrification tank is provided, or nitrification and denitrification in one tank. It can be performed by the method described in 1. Further, either a continuous treatment method or a batch treatment method may be used, and further, a circulating denitrification method of circulating nitrification-treated water in a denitrification tank, a step denitrification method of dispensing raw water, or the like can be used. In the method of the present invention, the water to be treated after contact with iron and coagulation treatment can be subjected to biological denitrification treatment by adjusting the pH as necessary. In the nitrification step in the biological denitrification treatment, the pH of the water to be treated decreases, so it is preferable to maintain the pH of the water in the nitrification tank at weak alkaline. According to the method of the present invention, the water to be treated after the coagulation treatment is usually alkaline, and it is often possible to introduce the water as it is into the nitrification tank.
Even if the pH is adjusted, a small amount of the pH adjuster is required.

In the method of the present invention, when the flue gas desulfurization effluent is mixed with another effluent to be treated, the effluent desulfurization effluent which has been mixed in advance can be treated according to the method of the present invention. After the treatment in the iron contacting step and the coagulating step, it can be mixed with other waste water to perform biological denitrification treatment. The method of treating only the flue gas desulfurization wastewater in the iron contacting step and the aggregating step is preferable because it requires a smaller processing apparatus and uses less chemicals. In the method of the present invention,
Before the step of contacting with iron, a pretreatment step such as coagulation treatment or filtration can be provided. By providing the pretreatment step, it is possible to remove a part of easily-removed suspensions and contaminants in the wastewater and reduce the load of the subsequent steps. Especially,
When using the column for iron contact, it is preferable to remove it beforehand so that the suspension does not clog the iron packed bed. Further, after the biological denitrification treatment step, a post-treatment step such as coagulation or filtration can be provided depending on the desired treated water quality. Further, the COD adsorption step can be provided at any position. FIG. 1 is a process flow chart of an embodiment of the method of the present invention. Wastewater from thermal power plant pH adjustment tank 1
And adjust the pH to 5 or less by adding hydrochloric acid. The water to be treated whose pH has been adjusted is sent to the iron fine particle packed column 3 by the pump 2 to elute iron as divalent iron ions to reduce the oxidizing substances and selenium compounds in the water to be treated. The water to be treated flowing out from the column packed with iron fine particles is sent to the reaction tank 4, sodium hydroxide is injected, and the pH is adjusted to 7 or higher to aggregate iron ions as iron hydroxide. The water to be treated in which iron flocs are formed in the reaction tank is sent to the precipitation separation tank 5 and is subjected to solid-liquid separation. The supernatant water of the precipitation separation tank is subjected to aerobic nitrification in the nitrification tank 6, then anaerobic denitrification in the denitrification tank 7, and further subjected to solid-liquid separation in the precipitation separation tank 8 to obtain treated water.

[0011]

EXAMPLES The present invention will be described in more detail with reference to the following Examples, which should not be construed as limiting the present invention. Reference Example 1 (Inhibition of nitrification by peroxosulfuric acid) Coagulated sediment filtered water containing 100 mg / l of peroxosulfuric acid ion (S ₂ O ₈ ²⁻ ) was used as it was and 2 times, 10 times,
Dilute 20 times to give a peroxosulfate ion concentration of 5 to 1
Four kinds of water of 00 mg / liter were prepared. Ammonia concentration of 50 in pure water and water containing peroxosulfate ion
It was added to be mg / l. A fixed amount of a carrier to which nitrifying bacteria were attached was added to these five types of test water, and the mixture was shaken in a rotary shaker at 25 ° C, the ammonia concentration was measured at regular intervals, and the nitrification rate was calculated from the decreasing rate of ammonia concentration. I asked. The ratio of the nitrification rate to the sample water containing no peroxosulfate ion was defined as the nitrification activity ratio. When the peroxosulfate ion concentration was 5, 10, 50 and 100 mg / liter, the nitrification activity ratios were 82, 77, 50 and 3, respectively.
4%. The results are shown in FIG. It was found that the nitrification activity decreased with the increase of the peroxosulfate ion concentration. Reference Example 2 (Inhibition of nitrification by iodate ion) Pure water having an iodate ion concentration of 1, 5, 10 and 20 mg
Iodic acid is added to adjust the concentration to be 1 / liter, and ammonia is added to pure water and water containing iodate ions at a concentration of 50
It was added to be mg / l. A fixed amount of a carrier to which nitrifying bacteria were attached was added to these five types of test water, and the mixture was shaken in a rotary shaker at 25 ° C, the ammonia concentration was measured at regular intervals, and the nitrification rate was calculated from the decreasing rate of ammonia concentration. I asked. The ratio of nitrification rate to that of sample water containing no iodate ion was defined as the nitrification activity ratio. The nitrification activity ratios at the iodate ion concentrations of 1, 5, 10 and 20 mg / liter were 100, 100, 68 and 38%, respectively. The results are shown in FIG. When the iodate ion concentration is 5 mg / liter or less, the nitrification activity ratio is 100% and no inhibition of nitrification is observed, but when the iodate ion concentration exceeds 5 mg / liter, the nitrification activity ratio sharply decreases and the iodate ion concentration It was found to be halved at about 15 mg / liter.
From the results of Reference Example 1 and Reference Example 2, peroxosulfate ion inhibits nitrification even at a concentration of 5 mg / liter or less, and the nitrification activity is halved at a concentration of 50 mg / liter. on the other hand,
Iodiate ion does not inhibit nitrification at a concentration of 5 mg / liter or less, but the nitrification activity is halved at a concentration of 15 mg / liter. Thus, the peroxosulfate ion and the iodate ion have strong inhibitory effects on nitrifying bacteria, although the states of inhibiting nitrification differ in details. Example 1 (Treatment of wastewater from thermal power plant) pH is 6.5, peroxosulfate ion 10 mg / liter, iodate ion 5 mg / liter, copper 1 mg / liter, lead 0.5 mg / l, zinc 0.5 mg / liter Liter, selenium 0.5 mg / liter, fluorine 30 mg / liter, and flue gas desulfurization wastewater containing nitrate nitrogen 20 mg / liter and general wastewater containing ammoniacal nitrogen 100 mg / liter 1: 1
Were mixed and treated using the apparatus shown in FIG. In the pH adjusting tank, 300 mg / liter of hydrochloric acid was injected to adjust the pH to 2.5, and this water was pumped up through a column packed with iron particles having a particle diameter of 0.6 mm in an upward flow rate SV20 hr ^-1.
The water flowing out was led to the reaction tank. In the reaction tank, 350 mg / liter of sodium hydroxide was injected to adjust the pH to 9, and the aggregation reaction was performed. After that, the solid-liquid separation was conducted by introducing it into a precipitation separation tank. The quality of the supernatant water after solid-liquid separation is
pH is 7.0, peroxosulfate ion is 0.1 mg / liter or less, iodate ion is 0.1 mg / liter or less,
The concentrations of copper, lead, zinc and selenium were all 0.1 mg / liter or less. The concentration of fluorine was 10 mg / liter. This water was then passed through a nitrification tank and a denitrification tank to remove nitrogen, and a suspension was removed in a precipitation tank. The concentration of total nitrogen in the treated water was 8 mg / liter. Comparative Example 1 A 1: 1 mixed water of flue gas desulfurization wastewater and general wastewater used in Example 1 was subjected to coagulation sedimentation treatment and then biological denitrification treatment. In the coagulation treatment step, 200 mg / liter of sodium hydroxide and 0.5 mg / liter of anionic polymer coagulant were added to perform coagulation treatment and solid-liquid separation, and then the mixture was passed to a nitrification tank and a denitrification tank as in Example 1. After water treatment, the suspension was removed in a settling tank. The concentration of ammonia nitrogen in the water to be treated at the outlet of the nitrification tank is 10 mg / liter,
It was confirmed that the water flow gradually increased as the water flow continued. The pH of the treated water was 7.5, and 2.5 mg / liter of iodate ion, 0.3 mg / liter of selenium, and 20 mg / liter of total nitrogen remained. The results of Example 1 and Comparative Example 1 are
It is shown in Table 1.

[0012]

[Table 1]

[0013]

EFFECTS OF THE INVENTION According to the method of the present invention, wastewater from a thermal power plant containing an oxidizing substance, which has been difficult to treat in the past, is brought into contact with iron in advance for reduction reaction treatment, coagulation treatment and solid-liquid separation. It is possible to improve the function of biological denitrification treatment and perform nitrification and denitrification stably. Further, it is possible to reduce selenium in wastewater, which was conventionally difficult to remove to a low concentration, to a regulated value of 0.1 mg / liter or less.

[Brief description of drawings]

FIG. 1 is a process flow chart of an embodiment of the method of the present invention.

FIG. 2 is a graph showing the relationship between peroxosulfate ion concentration and nitrification activity ratio.

FIG. 3 is a graph showing the relationship between iodate ion concentration and nitrification activity ratio.

[Explanation of symbols]

 1 pH adjusting tank 2 Pump 3 Iron fine particle packed column 4 Reaction tank 5 Precipitation separation tank 6 Nitrification tank 7 Denitrification tank 8 Precipitation separation tank

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kenichi Muto 6-15-1, Ginza, Chuo-ku, Tokyo Power source development Co., Ltd. (72) Inventor Tsutomu Ogoshi 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo No. Kurita Industry Co., Ltd. (72) Inventor Haruka Tsurumaru 3-4-7 Nishishinjuku, Shinjuku-ku, Tokyo Kurita Industry Co., Ltd.

Claims (1)

[Claims] 1. Treatment of wastewater from a thermal power plant, which comprises adjusting the pH of the wastewater from a thermal power plant to 5 or less, bringing it into contact with iron, performing coagulation treatment and solid-liquid separation, and then performing biological denitrification treatment. Method.