JP4671928B2 - Wastewater treatment method - Google Patents

Description

  The present invention relates to a wastewater treatment method, and more particularly to a wastewater treatment method for treating condensate demineralizer drainage of a coal-fired power plant.

  In coal-fired power plants, wastewater is generated by various devices and processes, and is treated according to the characteristics of each wastewater. For example, drainage generated regularly includes desulfurization drainage, pure water drainage, condensate desalination drainage, unit drain drainage, etc., and wastewater generated non-steadyly includes various equipment washing drains (for example, Air preheater cleaning wastewater, electric dust collector cleaning wastewater), boiler chemical cleaning wastewater, boiler system blow wastewater, etc. Desulfurization effluent is discharged from flue gas desulfurization equipment and contains soot, fluorine, heavy metals, etc. in boiler exhaust gas, and is treated separately from other effluents to treat these. .

  In order to treat these wastewaters and discharge them outside the power plant, it is necessary to treat them so as to meet certain emission standards. For example, it is determined to discharge chemical oxygen demand (COD), suspended solids, heavy metals, oil, etc. below a predetermined reference value by law or pollution control agreement. This discharge standard is getting stricter, and various wastewater treatment technologies have been developed to satisfy this discharge standard (for example, Patent Documents 1 and 2).

  Patent Document 1 discloses a water treatment apparatus having a biological treatment unit that decomposes ammonia nitrogen contained in waste water into nitrate nitrogen, and detects the temperature of the waste water or the outside air temperature and when the temperature is equal to or lower than a predetermined temperature. A water treatment apparatus including an electrolytic treatment unit that converts ammonia nitrogen into nitrogen gas by electrolytic treatment is disclosed.

In Patent Document 2, the concentration of toxic substances, particularly persulfuric acid, contained in flue gas desulfurization wastewater is detected, and the wastewater is treated by changing the order of the activated carbon treatment step and the nitrification denitrification step according to the detection result. A wastewater treatment method is disclosed.
JP 2004-267967 A JP 2003-136092 A

  However, in the apparatus disclosed in Patent Document 1, since the electrolytic treatment is performed when the temperature is low, extra energy is required, the energy efficiency is deteriorated, and the cost for the wastewater treatment is increased.

  The method disclosed in Patent Document 2 is a method for efficiently treating flue gas desulfurization drainage according to changes in the operating conditions of the flue gas desulfurization apparatus, and is applicable to wastewater other than flue gas desulfurization drainage. Can not.

  This invention is made | formed in view of this point, The place made into the objective is to provide the waste water treatment method which processes a condensate demineralizer waste_water | drain efficiently.

  In order to achieve the above object, a wastewater treatment method of the present invention is a wastewater treatment method for treating condensate demineralizer drainage of a coal-fired power plant, wherein the drainage tank is provided with condensate demineralizer drainage and desulfurization drainage. And a holding step for holding the drainage tank for 1 day or more and 4 days or less after the charging step. By adopting such a configuration, the temperature of the condensate demineralizer drainage is raised by the high temperature desulfurization drainage, and the ammonia in the condensate demineralizer drainage is kept in the drainage tank by keeping it warm for 1 day or more and 4 days or less. The speed of decomposition is greatly improved.

  It is preferable that air preheater washing waste water is further added to the drain tank.

  In the holding step, it is preferable to keep the temperature of the waste water in the drain tank at 30 ° C. or higher and 50 ° C. or lower.

  In a preferred embodiment, the method further includes adding sodium hypochlorite to the waste water after the holding step.

  In a preferred embodiment, the amount of nitrite ions in the waste water is measured after the holding step, and sodium hypochlorite is added to the waste water when the amount of nitrite ions exceeds a predetermined value.

  Since the condensate demineralizer drainage and the desulfurization drainage are put into the drainage tank and the temperature is kept, the treatment of ammonia in the condensate demineralizer drainage is promoted.

  Before describing the embodiment, the background of how the inventor of the present application came to invent the present application will be described.

  As mentioned above, regulations on wastewater from power plants have become stricter, and various technologies for wastewater treatment have been developed over the years. How to treat all wastewater generated from power plants efficiently and Whether the cost is low is still being studied. Looking at the current wastewater treatment method from such a viewpoint, conventionally, the desulfurization wastewater discharged from the flue gas desulfurization equipment contains fluorine and heavy metals, and it is necessary to perform these treatments. The desulfurization effluent was treated alone and was never considered to be mixed with other effluents. As a result of considering the total treatment of all wastewater discharged from coal-fired power plants, the inventor of the present application conforms to the current wastewater regulations and, as a whole, performs wastewater treatment most efficiently and at low cost. It came to the idea that it would be better to mix desulfurization waste water with other waste water.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following drawings, components having substantially the same function are denoted by the same reference numerals for the sake of brevity.

(Embodiment 1)
The flow of the waste water treatment of Embodiment 1 is shown in FIG.

  In this embodiment, condensate demineralizer drainage and desulfurization wastewater that are regularly discharged are first stored in the drainage tank 11. The drainage tank 11 is provided with a heat insulating material on the tank wall, and retains and holds the stored wastewater. There are a plurality of drain tanks 11 (here, three), and when one drain tank 11 is filled with drainage, drainage is put into another drain tank 11.

  In the drainage tank 11, condensate demineralizer drainage and desulfurization drainage are placed and stored while keeping the temperature for 1 day to 4 days. During this storage, ammonia in the wastewater is decomposed into nitrous acid and further decomposed into molecular nitrogen. This decomposition will be described in detail later. This storage process is a holding process.

  The drainage after the holding step flows into the drainage storage tank 21 from the drainage tank 11 and is stored. Then, the waste water is sent from the waste water storage tank 21 to the pH adjustment tank 22, and the pH adjustment for the subsequent separation membrane treatment is performed in the pH adjustment tank 22. Then, a treatment for removing heavy metals and fluorine in the waste water is performed in the reaction tank 23. The waste water after the treatment is caused to flow into the circulation tank 24.

  Next, wastewater is sent from the circulation tank 24 to the separation membrane 31 and wastewater that could not be separated by the separation membrane 31 is returned to the circulation tank 24 again. The waste water that has been separated by the separation membrane 31 is sent to the relay tank 25.

  The waste water that has flowed into the relay tank 25 is sent to the COD adsorption tower 41 by a pump, and the COD adsorbed by the activated carbon in the COD adsorption tower 41 is removed.

  The waste water discharged from the COD adsorption tower 41 flows into the neutralization tank 26 where the pH is adjusted to 7.0 so that it can be discharged.

  Further, the wastewater flows from the neutralization tank 26 into the monitoring tank 27, where it is checked whether COD, heavy metal, etc. satisfy the emission standard, and if it meets the emission standard, it is discharged outside the power plant. If the emission standard is not met, further processing is performed until the emission standard is met.

  In the present embodiment, the condensate demineralizer drainage and the desulfurization wastewater are treated and discharged outside the power plant according to the above flow. What is important in this embodiment is that the condensate demineralizer drainage and the desulfurization wastewater are first mixed and stored in the drainage tank 11 (storage), and this storage is performed for 1 day or more and 4 days or less while keeping the temperature. It is. This ensures that ammonia is efficiently decomposed. This will be described below.

  First, the condensate demineralizer waste water and the desulfurization waste water contain a relatively large amount of ammonia. In order to discharge the waste water outside the power plant, it is necessary to decompose the ammonia. The ammonia is decomposed in the drainage tank 11 by a biological denitrification method that is performed by nitrification by nitrifying bacteria and molecular nitrogenation by denitrifying bacteria. The biological denitrification method is an excellent treatment method that can remove almost all forms of nitrogen and consumes less energy than other physicochemical treatment methods.

Nitrification is carried out by nitrifying bacteria such as the scientific names Nitrosomonas and Nitrosocystis, and the following formula NH ₄ ⁺ + 3 / 2O ₂ → NO ₂ ⁻ + 2H ⁺ + H ₂ O
NO ₂ ⁻ + 1 / 2O ₂ → NO ₃ ⁻
It is a reaction represented by. Since the specific growth rate μ [1 / d] of nitrifying bacteria is expressed by μ = 0.18 × 1.128 ^t−15 (t is a processing temperature [° C.]), when the temperature becomes 15 ° C. or less, the nitrification rate Is significantly reduced. On the other hand, when the temperature rises by 10 ° C., the nitrification rate becomes about 3.3 times.

Molecular nitrogenation (denitrification) is carried out by a denitrifying bacterium such as the scientific name Pseudomonas denitrificans, and the following formula 2NO ₃ ⁻ + 5H ₂ → N ₂ + 2OH ⁻ + 4H ₂ O
It is a reaction represented by. The specific growth rate of denitrifying bacteria is also expressed by a formula similar to that of nitrifying bacteria, and the denitrifying rate increases as the temperature rises. In addition, denitrifying bacteria denitrify under anaerobic conditions, but do not denitrify under aerobic conditions because they use dissolved oxygen.

  In this embodiment, the desulfurization effluent and the condensate demineralizer effluent are put together in the drainage tank 11. The desulfurization effluent is a high temperature effluent discharged from the flue gas desulfurization device, and is relatively high at about 40 to 55 ° C. when entering the drainage tank 11. Since the drainage tank 11 is kept warm, even after the desulfurization drainage is mixed with the condensate demineralizer drainage, the drainage in the tank is maintained at about 30 to 50 ° C. if it is 4 days or less. Therefore, as described above, the activities of nitrifying bacteria and denitrifying bacteria become active, and the decomposition of ammonia is greatly accelerated as compared with the case of normal temperature. Here, when the temperature in the drainage tank 11 is lower than 30 ° C. in the holding step, it is not preferable because the effect of promoting the decomposition of ammonia is small compared to the case of room temperature particularly in summer. In winter, even when the temperature is 20 ° C. or higher and lower than 30 ° C., the effect of promoting ammonia decomposition is recognized as compared with normal temperature. Further, if the temperature in the drainage tank 11 is higher than 50 ° C. in the holding step, the activities of nitrifying bacteria and denitrifying bacteria are suppressed, which is not preferable.

  Furthermore, in the holding process, the waste water is kept warm for 1 day or more and 4 days or less, but if it is less than 1 day, the decomposition of ammonia is insufficient, and if it is longer than 4 days, a lot of drainage tanks 11 are required. This is not preferable because the cost increases. Condensate demineralizer wastewater and desulfurization wastewater are kept together at 30 to 50 ° C. for 1 day or more and 4 days or less in the drainage tank 11, even if the amount of wastewater for heavy metal and fluorine treatment increases, Compared with the case where the desalination apparatus waste water and the desulfurization waste water are treated separately, the total treatment time and treatment cost are reduced. This is because the waste heat of the desulfurization wastewater that has been wasted so far can be effectively utilized.

(Embodiment 2)
The wastewater treatment according to the second embodiment is different from the first embodiment in that a part of the wastewater to be input to the drainage tank 11 and a sodium hypochlorite storage tank exist, and the other parts are almost the same as those in the first embodiment. Since they are the same, only the parts different from the first embodiment will be described.

  The flow of the waste water treatment of this embodiment is shown in FIG. In this embodiment, in addition to the condensate demineralizer drainage and the desulfurization drainage, the air preheater cleaning wastewater is put into the drainage tank 11. Air preheater cleaning wastewater is wastewater discharged irregularly and contains a lot of ammonia.

The three types of wastewater thrown into the drainage tank 11 are kept warm in the same manner as in the first embodiment and are held for 1 day or more and 4 days or less. While being held, ammonia is decomposed by the biological denitrification method as described in the first embodiment. However, in this embodiment, denitrification is not sufficiently performed, and a large amount of NO ₃ ⁻ remains in the waste water after the holding process is completed. This is because the denitrification reaction is the next stage of the nitrification reaction, and the denitrification bacteria perform the denitrification reaction under anaerobic conditions and when there is sufficient organic substance (for example, methanol) to donate hydrogen. It depends on being.

When the amount of NO ₃ ^{− in} the waste water is measured after the holding process is finished and exceeds a predetermined value (for example, 20 mg / l), sodium hypochlorite is discharged from the sodium hypochlorite storage tank 12 to the waste water. throw into. If a large amount of NO ₃ ⁻ remains in the wastewater, the value of COD increases, so it is necessary to reduce the NO ₃ ⁻ by performing some treatment. In this embodiment, NO ₃ ⁻ is added by adding sodium hypochlorite. ₃ ^- it is reducing. In normal waste water treatment, but the sodium hypochlorite is used as a medicine for treating the ammonia to the molecular nitrogen, the inventors have, sodium hypochlorite NO ₃ ^- can also be used for treatment It has been found that it is possible to use this embodiment. The amount of sodium hypochlorite added is, NO ₃ ^- a is an amount 2 to 4 times is appropriate for COD weight when converted to the COD.

  Since the subsequent steps are the same as those in the first embodiment, the description thereof is omitted.

  In addition to the effects of the first embodiment, the present embodiment has an effect that the COD can be reliably made to be equal to or lower than the wastewater standard, and the wastewater treatment can be performed efficiently.

(Other embodiments)
The above-described embodiments are examples of the present invention, and the present invention is not limited to these examples. The drain tank 11 may be further charged with other drainage containing a large amount of ammonia such as ammonia tank blow drainage. In the wastewater treatment flow, other treatment steps (for example, a treatment step of a compound having an NS bond) may be included in the middle. Moreover, you may add the tank required for a process further.

  In order to promote denitrification, it is preferable to add a compound that donates hydrogen, such as methanol, to the drain tank 11.

  As described above, the waste water treatment method according to the present invention can treat ammonia efficiently, and is thus useful as waste water treatment for a coal-fired power plant.

FIG. 3 is a flowchart of waste water treatment according to the first embodiment. FIG. 6 is a flowchart of wastewater treatment according to Embodiment 2.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Drain tank 12 Sodium hypochlorite storage tank 21 Drain storage tank 22 pH adjustment tank 23 Reaction tank 24 Circulation tank 25 Relay tank 26 Neutralization tank 27 Monitoring tank 31 Separation membrane 41 COD adsorption tower

Claims (4)

A wastewater treatment method for treating condensate demineralizer drainage from a coal-fired power plant,
An input process for supplying condensate demineralizer drainage and desulfurization drainage to the drainage tank;
After said adding step, seen including a holding step of holding less than one day 4 days while kept the drainage tank,
A wastewater treatment method in which in the holding step, the wastewater in the drainage tank is kept at a temperature of 30 ° C to 50 ° C.   The waste water treatment method according to claim 1, wherein an air preheater cleaning waste water is further added to the waste water tank. The wastewater treatment method according to claim 1 or 2 , further comprising a step of adding sodium hypochlorite to the wastewater after the holding step. The wastewater treatment according to claim 3 , wherein the amount of nitrite ions in the wastewater is measured after the holding step, and sodium hypochlorite is added to the wastewater when the amount of nitrite ions exceeds a predetermined value. Method.

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