JPH0938694A - Treatment of waste water of flue gas desulfurization - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to efficiently remove the selenium in waste water of flue gas desulfurization and to efficiently remove the ammonia nitrogen in the waste water of a condensate desalting device by combining the waste water of flue gas desulfurization and the waste water of the condensate desalting device and subjecting the waste water to biological digestion and anaerobic treatments. SOLUTION: The waste water of the flue gas desulfurization is subjected to a pretreatment, such as settling and/or flocculating and settling, etc., and is mixed with the waste water of the condensate desalting device in a mixing stage 2. The waste water mixture is subjected to nitrification of the ammonia nitrogen to nitric (nitrous) acid nitrogen in a nitrifying stage 3. The waste water mixture is subjected to denitrification and reduction insolubilization of selenium in an anaerobic treatment stage 4. The anaerobically treated liquid is subjected to a sepn. of solid from the liquid in a stage 5 for separating the solid from the liquid. The sludge on which the selenium compd. is adsorbed is discharged and the separated liquid is post treated in a post treating stage 6, by which the treated water is obtd. The waste water of the condensate desalting device is otherwise nitrified in the nitrifying stage 3 and is then mixed with the waste water of the flue gas desulfurization pretreated in the pretreating stage 1 in the mixing stage 2; thereafter, the waste water is subjected to the anaerobic treatment in the anaerobic treatment stage 4, the solid-liquid sepn. in the stage 5 for separating the solid from the liquid and the post treatment in the post treating stage 6.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for mixing selenium-containing flue gas desulfurization wastewater with condensate desalination equipment wastewater containing ammoniacal nitrogen.

[0002]

2. Description of the Related Art When flue gas generated by combustion of fuel such as coal or oil in a thermal power plant is desulfurized by a desulfurizer, the flue gas desulfurized waste water discharged from the desulfurizer contains selenium. There is. Since such flue gas desulfurization wastewater contains a COD component in addition to selenium, it is adsorbed by activated carbon or a COD adsorption resin. Since such an adsorption process is a peculiar treatment to this wastewater, the flue gas desulfurization wastewater was treated independently of other condensate desalination equipment wastewater and dust collection wastewater generated in a thermal power plant.

On the other hand, the condensate demineralizer is an ion exchange device for demineralizing the condensate generated in the condenser so as to use it as boiler feed water. When sulfuric acid is regenerated with sulfuric acid, wastewater containing ammonium sulfate is produced. Conventionally, such wastewater containing ammonia nitrogen is subjected to neutralization and other chemical treatments together with other wastewater generated in a desalination apparatus.

By the way, as a treatment method for detoxifying waste water containing selenium compounds such as Se ⁶⁺ and Se ⁴⁺ , there is a biological treatment method for reducing selenium compounds by contacting selenium-containing water with biological sludge in an anaerobic state. . For example, as a biological reaction of selenium compounds, the Annual Meeting of the Japan Society on Water Environment, 1995, p.
176, it is reported that Se ⁶⁺ and Se ⁴⁺ are reduced in the presence of lactose by a (sub) selenate-reducing bacterium. This method separates (sub) selenate-reducing bacteria from a place contaminated with selenium compounds and utilizes them for the reduction of selenium compounds, but it is difficult to process because special bacteria are used.

[0005]

An object of the present invention is to efficiently remove selenium from flue gas desulfurization wastewater by treating flue gas desulfurization wastewater and condensate desalination equipment wastewater in combination. It is an object of the present invention to propose a method for treating flue gas desulfurization effluent that can efficiently remove ammoniacal nitrogen in the effluent of desalination equipment.

[0006]

According to the present invention, there is provided a pretreatment step for removing biological inhibitors from selenium-containing flue gas desulfurization wastewater, and ammoniacal nitrogen contained in condensate desalination equipment wastewater, Before or after mixing with desulfurization effluent, nitrification step of biologically nitrifying by contacting biological sludge containing nitrifying bacteria in an aerobic state, flue gas desulfurization effluent after pretreatment, and before or after nitrification A mixing process of mixing the condensate demineralizer wastewater with the mixed wastewater that has been mixed and nitrified is contacted with biological sludge containing denitrifying bacteria in an anaerobic state to perform denitrification,
A method for treating flue gas desulfurization wastewater, which includes an anaerobic treatment step of reducing a selenium compound and a solid-liquid separation step of solid-liquid separation of sludge having adsorbed a selenium compound insolubilized by the reduction.

In the present invention, "(selen) selenic acid" means "selenic acid and / or selenious acid". Also, “Se ⁶⁺ ”, “Se ⁴⁺ ”, “Se ⁰ ”, or “S
“E ²⁻ ” means selenium having the respective oxidation number + VI, + IV, zero or −II. These may be simply described as Se. Further, in the present invention, "(nitrous) nitric acid"
Means "nitric acid and / or nitrous acid".

The flue gas desulfurization wastewater to be treated in the present invention is wastewater discharged from a device for wet desulfurization of flue gas generated by combustion of fuel such as coal and petroleum.
Wastewater containing a selenium compound such as (sub) selenic acid, and usually contains sulfate ions and COD components in addition to the selenium compound.

Condensate demineralizer wastewater to be treated in the present invention is wastewater containing a regenerated waste liquid for regenerating the ion exchange resin of the desalinizer for desalting the condensate generated in the condenser.
Of the wastewater discharged from the condensate demineralizer, the ammoniacal nitrogen-containing wastewater including the regenerated wastewater desorbing ammonium ions from the ammonia-type cation exchange resin and its cleaning wastewater is suitable for the treatment of the present invention, It may be mixed drainage with other drainage.

In the present invention, first, in the pretreatment step, a pretreatment for removing biological inhibitors from the flue gas desulfurization wastewater is performed. Since the flue gas desulfurization wastewater contains inhibitors such as heavy metal ions and fluoride ions, pretreatment such as precipitation and / or coagulation precipitation is carried out to remove them. Precipitation is to separate the already deposited solid matter by gravity by standing. In the coagulation precipitation, an inhibitor, an aggregating agent or the like is added to precipitate an inhibitory substance, and the inhibitory substance is separated by precipitation. It is also possible to combine other pretreatments.

On the other hand, the condensate desalination equipment wastewater is nitrified as a nitrification step before or after mixing with the flue gas desulfurization wastewater to oxidize ammoniacal nitrogen to (nitro) nitrite nitrogen.
Nitrification is biologically nitrified by excessive aeration in the presence of biological sludge containing nitrifying bacteria. The operation in the nitrification step in the biological nitrification denitrification method which is widely used in water treatment can be applied to this step as it is.

As the biological sludge containing nitrifying bacteria, it is possible to collect the nitrifying sludge from a system for carrying out a biological nitrification denitrification treatment and use it as it is, but the biological sludge is returned from the anaerobic treatment step described later, It can also be generated by mixing condensate demineralizer wastewater containing ammoniacal nitrogen or mixed wastewater and aerating excessively. When excess aeration is performed in the presence of such biological sludge, ammoniacal nitrogen is oxidized to (nitrite) nitrogen.

In the mixing step, the wastewater of the condensate desalination equipment that has undergone nitrification is mixed with the flue gas desulfurization wastewater that has undergone pretreatment. When mixing before nitrification, nitrification is performed after mixing. Either may be applied first, but in the mixed state, all of the ammoniacal nitrogen should be oxidized to (nitrite) nitrogen. The flue gas desulfurization wastewater is continuously discharged in approximately fixed amounts, but the condensate desalination equipment wastewater is discharged once every 10 to 20 days, so the condensate desalination equipment wastewater is stored in the storage tank. However, it is preferable to discharge so as to equalize the flow rate and mix with the flue gas desulfurization wastewater.

In the anaerobic treatment step, the mixed effluent that has been mixed and nitrified in this manner is brought into contact with biological sludge containing denitrifying bacteria in an anaerobic state to denitrify and reduce the selenium compound.

The biological sludge used in the anaerobic treatment step is a sludge containing denitrifying bacteria, is a biological sludge produced by maintaining the mixed wastewater in an anaerobic state, and is an aerobic treatment of wastewater as in the activated sludge treatment method. It can also be produced spontaneously by collecting biological sludge (activated sludge) in the method, adding it to mixed wastewater and maintaining it in an anaerobic state.
Denitrifying bacteria predominate in such biological sludge, and the denitrifying bacteria reduce (sub) selenic acid in the mixed wastewater.

The denitrifying bacteria that can be used in the anaerobic treatment step are bacteria that separate organic substances by utilizing oxygen of (nitrite) ions by respiration of nitric acid, and are found in facultative anaerobic bacteria such as Pseudomonas. Such denitrifying bacteria are used in the denitrification process in the denitrification method by nitrification denitrification utilizing the biological reaction of wastewater containing ammoniacal nitrogen. As the above-mentioned denitrifying bacterium, the denitrifying bacterium in such a biological denitrifying method can be used as it is, and aerobic sludge (activated sludge) in the aerobic treatment of wastewater such as the activated sludge treatment method is collected and used as an organic matter and It can also be generated spontaneously by maintaining it in an anaerobic state in the presence of (sub) nitrite.

The biological sludge containing such denitrifying bacteria is usually a floc-like biological sludge, and in the present invention, the floc-like biological sludge can be used as it is in a suspended state, but it is granular, fibrous, etc. It can also be used by supporting it on a carrier having a high porosity. The carrier is not limited as long as it can support biological sludge, and examples thereof include sand, activated carbon, alumina gel, and foamed plastic. The biological sludge can be carried on the carrier by acclimation or treatment in the presence of the carrier. In addition, a selenate-reducing bacterium may be fixed in a gel such as polyvinyl alcohol or polyethylene glycol.

In the anaerobic treatment step, the mixed effluent is brought into contact with the above-mentioned biological sludge in an anaerobic state, whereby the (nitro) nitrite nitrogen in the mixed effluent is reduced to nitrogen gas and denitrified. Selenate ie Se ⁶⁺ and / or Se
⁴⁺ is reduced and becomes insoluble. At this time, it is estimated that Se ⁶⁺ is reduced to Se ⁰ and / or Se ²⁻ via Se ⁴⁺ . The anaerobic state in the anaerobic treatment step means a state in which oxygen is blocked, but a slight amount of oxygen that does not inhibit the reduction of the selenium compound can be mixed.

The above-mentioned anaerobic treatment process requires an oxygen source and a nutrient source for respiration of biological sludge. Since the oxygen source is anaerobic, not molecular oxygen but (nitrous) nitric acid produced by nitrification is used. As the nutrient source, organic substances contained in the mixed effluent or organic substances contained in the biological sludge can be used as a substrate, but when they are not contained, methanol or the like is added separately. As a result, the biological sludge is maintained at a high activity, and (s) selenic acid is reduced along with the decomposition thereof.

In the case of the mixed waste water, the selenium compound is reduced after the (nitro) nitrite nitrogen is reduced, so that it is necessary to add the organic substance in an amount necessary for these reductions. Since (nitrite) ions are present in such mixed effluent, denitrifying bacteria appear in the biological sludge and the selenium reduction activity is maintained high, which results in efficient reduction of (selenite) selenate. It

In the above anaerobic treatment step, raw water is placed in the anaerobic treatment tank,
It is carried out by introducing organic substances. An anaerobic treatment tank is used for contact between the mixed wastewater and the biological sludge, and any method such as a floating method and a biofilm method can be adopted. The floating method is a method in which floc-like biological sludge containing denitrifying bacteria is stirred and brought into contact with each other in a floating state, and is performed in the same manner as the denitrifying step in the biological denitrifying method. In the biofilm method, a biological sludge is supported on a carrier to form a biofilm,
This is a method in which this is brought into contact with mixed wastewater, and the same method as that used in the denitrification process such as a fixed bed type, a fluidized bed type, or an upward flow type or a downward flow type can be used.

The residence time in the anaerobic treatment tank is the time required for the reduction of the (selenite) selenite ion, which can also be regarded as the time required for the decomposition of the organic substances present in the system. When performing denitrification inside, it can be 1.1 times or more of the time required for denitrification. The anaerobic treatment is performed at a predetermined sludge concentration (500
~ 50,000 mg / l, preferably 2000-2000
The reaction is carried out maintaining 0 mg / l).

When the selenium compound is reduced in the above anaerobic treatment step, an insoluble selenium compound is produced and adsorbed on the biological sludge. The selenium compound adsorbed on the biological sludge is removed from the system by solid-liquid separating and discharging the biological sludge in the solid-liquid separation step.

Since the anaerobic treatment liquid taken out from the above anaerobic treatment step contains excessively added organic substances and other oxidizable substances, the anaerobic treatment is carried out in order to oxidize and decompose them. An aerobic treatment step can be provided as a post-treatment after the step. Examples of such an aerobic treatment step include an activated sludge treatment step for decomposing excess organic matter used as a substrate in the anaerobic treatment step. In addition, an adsorption step using a COD adsorption resin or the like can be provided to remove COD that is hardly biodegradable.

When the flue gas desulfurization wastewater is treated alone, it is necessary to add (nitro) nitrite nitrogen as an oxygen source, but when it is treated in combination with the condensate desalination equipment wastewater, it is included ( ) It is possible to maintain high selenium removal activity by utilizing nitrate nitrogen. Since all of these wastewaters are generated in thermal power plants, combination treatment is easy.

When the Se content adsorbed in the biological sludge exceeds 20% by weight with respect to the MLSS, it exhibits inhibitory effect on the biological sludge, but the flue gas generated in ordinary thermal power plants When the desulfurization wastewater and the condensate desalination equipment wastewater are combined and treated, the Se content in the biological sludge is 5 to 15% by weight, and the biological inhibition is not exhibited.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. 1A and 1B are system diagrams showing different embodiments. In FIG. 1 (A), the flue gas desulfurization wastewater is subjected to pretreatment such as precipitation and / or coagulation sedimentation in the pretreatment step 1, and the condensate desalination equipment wastewater is mixed in the mixing step 2. In the nitrification step 3, the mixed wastewater nitrifies ammoniacal nitrogen into (nitro) nitrite nitrogen, and in the anaerobic treatment step 4, denitrification and reduction insolubilization of selenium are performed. The anaerobic treatment liquid is subjected to solid-liquid separation in the solid-liquid separation step 5, discharging sludge having adsorbed a selenium compound, and
The separated liquid is post-treated in post-treatment step 6 to obtain treated water.

In FIG. 1B, after condensate demineralizer wastewater is nitrified in nitrification step 3, it is mixed with flue gas desulfurization wastewater pretreated in pretreatment step 1 in mixing step 2 and then anaerobic treatment step 4 The anaerobic treatment, the solid-liquid separation in the solid-liquid separation step 5, and the post-treatment in the post-treatment step 6 are performed.

In any of the above cases, when an aerobic treatment for removing organic substances is performed as a post-treatment, solid-liquid separation may be performed in this aerobic treatment step, and the preceding solid-liquid separation step 5 may be omitted. . The sludge separated in these solid-liquid separation process can be returned to any process of nitrification process, anaerobic treatment process and aerobic treatment process, but in order to avoid dissolution of selenium, solid-liquid separation process It is also possible to provide.

[0030]

【Example】

Example 1 Flue gas desulfurization wastewater discharged from a coal-fired thermal power plant (pH 6.9, total Se 2.76 mg / l, S ₂ O ₆
109 mg / l, Fe 212 mg / l, CODcr
^{_{203mg / l, SO 4 2- 5800mg}} / l, F 5
5 mg / l) was subjected to a precipitation treatment as a pretreatment step, and to the supernatant water was added Na ₂ CO ₃ mg / l for softening treatment, and then 250 mg / l of a sulfuric acid band was added to perform coagulation precipitation treatment at pH 7.0. did. As a result, Fe was 0.9 mg / l,
F decreased to 8.4 mg / l, but CODcr and Se were hardly removed.

Next, condensate demineralizer wastewater (NH ₄ -N 8
00 mg / l) was added in an amount of 1/30, and the mixture was aerated in a 2 liter aeration tank (with a 3 liter sedimentation tank) for a retention time of 6 hours for nitrification. In this step, the pH is adjusted to 7.
Adjusted to zero. This treated water is treated with a 2 liter anaerobic tank (3 lit.
er's settling basin) and add methanol to NO ₃ -N 4
A double amount was added and anaerobic treatment was carried out with a residence time of 12 hours to denitrify and reduce selenium. The re-aeration tank as a post-treatment was retained for 30 minutes. Anaerobic tank MLSS is 3500-4000
It was maintained at mg / l. As a result, the treated water Se is 0.1 m
It became g / l or less.

[0032]

According to the present invention, the flue gas desulfurization wastewater and the condensate desalination equipment wastewater are combined for biological nitrification and anaerobic treatment. Nitrogen can be used to maintain a high selenium removal activity in biological sludge, which makes it possible to efficiently remove selenium in flue gas desulfurization wastewater as well as ammonia nitrogen in condensate desalination equipment wastewater. It is possible to treat both wastewater efficiently at low cost.

[Brief description of drawings]

FIG. 1A and FIG. 1B are system diagrams showing different embodiments.

[Explanation of symbols]

 1 Pretreatment step 2 Mixing step 3 Nitrification step 4 Anaerobic treatment step 5 Solid-liquid separation step 6 Posttreatment step

[Procedure amendment]

[Submission date] August 1, 1995

[Procedure amendment 1]

[Document name to be amended] Drawing

[Correction target item name] All figures

[Correction method] Change

[Correction contents]

FIG.

Claims (1)

[Claims] 1. A pretreatment step of removing biological inhibitors from flue gas desulfurization wastewater containing selenium, and ammonia nitrogen contained in the wastewater of condensate desalination equipment before or before mixing with the flue gas desulfurization wastewater. Later, the nitrification step of biologically nitrifying by contacting biological sludge containing nitrifying bacteria in an aerobic state, flue gas desulfurization wastewater after pretreatment, and condensate desalination equipment wastewater before or after nitrification The mixing process of mixing and the mixed wastewater after mixing and nitrification are contacted with biological sludge containing denitrifying bacteria in an anaerobic state for denitrification, and an anaerobic treatment process for reducing selenium compounds and selenium insolubilized by reduction. A method for treating flue gas desulfurization wastewater, which comprises a solid-liquid separation step of solid-liquid separation of sludge that has adsorbed compounds.