JPH09155368A - Treatment process for flue gas desulfurization drain - Google Patents

Abstract

PROBLEM TO BE SOLVED: To treat efficiently flue gas desulfurization drain containing hard-to-reduce toxic oxygen acid such as peroxy sulfuric acid, iodic acid, selenic acid and the like by adding hydrochloric acid water solution containing ferrous iron ions into flue gas desulfurization drain and carrying out the reduction reaction treatment, and then carrying out the flocculation treatment and solid-liquid separation. SOLUTION: Hydrochloric acid and iron fine particles are added into treated water in an iron elution tank 1, and ferrous iron ions are eluted into hydrochloric acid water solution. The hydrochloric acid water solution containing iron ions is fed into a reaction tank 3 and mixed with the flue gas desulfurization drain therein, and pH is set almost 5 or less, preferably approximately 2-3, and an oxygen acid in the flue gas desulfurization drain is reduced. Then, water to be treated is fed into a flocculation reaction tank 4, and sodium hydroxide is injected therein to adjust pH to approximately 8 or more, preferably approximately 9-10, and iron ions are flocculated as ion hydroxide. Then the water to be treated having caused the flocculation reaction is fed into a solid-liquid separation tank 5 and if necessary, the treated water is transferred into a process such as the COD removing process or the nitrogen removing process and purified therein.

Description

Detailed Description of the Invention

[0001]

[0001] The present invention relates to a method for treating flue gas desulfurization wastewater. More specifically, the present invention provides a flue gas desulfurization effluent for efficiently treating effluent containing oxygen acids such as peroxosulfuric acid, iodic acid and selenate discharged from a flue gas desulfurization device such as a thermal power plant. Regarding processing method.

[0002]

2. Description of the Related Art Flue gas desulfurization wastewater discharged from a desulfurization device for exhaust gas generated when coal or oil is burned in a thermal power plant or the like contains various harmful substances such as heavy metals and non-metals. It is necessary to remove these harmful substances from the flue gas desulfurization wastewater. The water quality of flue gas desulfurization effluent changes due to more efficient combustion of fuel in thermal power plants and improvement of flue gas desulfurization methods, and there is a need for a method of treating flue gas desulfurization wastewater that corresponds to changes in water quality. The present inventor previously added an inorganic coagulant, an organic coagulant or a pH adjuster to perform coagulation treatment as a method for removing COD components, suspended substances and metal salts contained in flue gas desulfurization wastewater, A method has been proposed in which solid-liquid separation is performed to remove suspended substances and metal salts, and a COD component is adsorbed and removed with a synthetic adsorption resin having a basic active group (Japanese Patent Publication No. 55-12312). According to this method, COD components and metal salts in the flue gas desulfurization wastewater can be removed, but if oxidative substances such as peroxosulfuric acid and iodic acid and selenium are contained in the flue gas desulfurization wastewater, these Is difficult to remove. Further, as a method for treating flue gas desulfurization wastewater containing peroxosulfuric acid and COD components, the present inventors have adjusted the flue gas desulfurization wastewater to pH 5 or less, decomposed and removed peroxosulfuric acid by contacting with activated carbon, and then adsorbed COD. A method of contacting with a resin has been proposed (JP-A-7-75778).
No.). According to this method, the COD adsorption efficiency is enhanced, the COD adsorption resin is prevented from deterioration over time, and high-quality treated water can be obtained. However, the consumption of activated carbon varies depending on the wastewater quality, and the treated water quality is Sufficient management of wastewater treatment is required to stabilize it. Further, when the oxidizing substance has an extremely high concentration, the quality of the treated water is deteriorated, and when selenium is contained in the flue gas desulfurization effluent, there is a problem that selenium cannot be expected to be removed. The present inventor has also adjusted the pH of flue gas desulfurization wastewater to 5 or less as a method for treating flue gas desulfurization wastewater containing iodate ions and COD components,
A reducing agent is added to decompose iodic acid, and then COD
A method of contacting with an adsorption resin has been proposed (JP-A-7-12).
4575). According to this method, iodate ions that cannot be treated by the COD adsorption resin are removed, and
It is possible to stabilize the performance of the COD adsorption resin for a long period of time and obtain treated water of high water quality, but it is necessary to perform two-stage treatment, that is, treatment by adding a reducing agent and treatment by the COD adsorption resin. Regarding the treatment of selenium-containing water, a number of improved coagulation-sedimentation methods have been proposed, for example, adding a neutralizing agent after adding ferrous sulfate or ferrous chloride as an iron salt to wastewater,
A method of removing selenium by coprecipitating it with iron hydroxide floc has been proposed (Japanese Patent Laid-Open No. 6-79286). However, in this method, the stability of the aqueous solution in which ferrous sulfate or ferrous chloride is dissolved is poor and there is a problem in storage, and the selenium concentration in the final treated water is usually 0.2 to 0.4 mg / liter. However, it was necessary to inject an extremely large amount of chemical liquid to achieve the drainage standard of 0.1 mg / liter, and a large amount of sludge was generated, which made practical application difficult.

[0003]

DISCLOSURE OF THE INVENTION The present invention provides a flue gas desulfurization effluent containing a non-reducing harmful oxyacid such as peroxosulfuric acid, iodic acid and selenate with a simplified and simplified treatment system. The object of the present invention is to provide a method for treating flue gas desulfurization wastewater that can be efficiently treated.

[0004]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the inventors of the present invention have contacted an aqueous hydrochloric acid solution with iron and used leached ferrous ions as a reducing agent for flue gas desulfurization. It was found that the treatment of the non-reducing oxyacid can be easily carried out by performing the coagulation treatment and the solid-liquid separation after the reduction treatment reaction in addition to the wastewater, and the present invention was completed based on this finding. Came to do. That is, according to the present invention, (1) an aqueous hydrochloric acid solution containing ferrous ions eluted by contacting with iron is added to flue gas desulfurization effluent to perform a reduction reaction treatment, and then a coagulation treatment and solid-liquid separation are performed. A method for treating flue gas desulfurization wastewater, which is characterized by the above. Further, as a preferred embodiment of the present invention,
(2) The method for treating flue gas desulfurization wastewater according to item (1), wherein the concentration of the hydrochloric acid aqueous solution is 0.05 to 1 mol / liter.
(3) The method for treating flue gas desulfurization wastewater according to item (1) or (2), wherein the iron is electrolytic iron, low carbon steel, mild steel, pig iron or cast iron,
(4) The shape of iron is powder, granular, spherical, block,
A method for treating flue gas desulfurization wastewater according to the item (1), item (2) or item (3), which is linear or crushed, and (5) an aqueous hydrochloric acid solution,
Item (1), item (2), item (2), which is brought into contact with iron in the stirring tank
(3) or (4) flue gas desulfurization wastewater treatment method, (6)
The hydrochloric acid aqueous solution is passed through a packed bed filled with iron to bring it into contact with iron. (1), (2), (3) or
(4) Treatment method of flue gas desulfurization wastewater, (7) The concentration of ferrous ion in hydrochloric acid aqueous solution is 1,000 to 15,000 mg /
Liters (1), (2), (3), (4)
Item, the method for treating flue gas desulfurization wastewater according to Item (5) or (6), (8) The concentration of iron ions in the water to be treated is 100 to 5
Add a hydrochloric acid aqueous solution containing ferrous ions to the flue gas desulfurization effluent so that the concentration becomes 00 mg / liter. (1), (2)
Clause, (3), (4), (5), (6) or
The method for treating flue gas desulfurization wastewater according to (7), (9) the coagulation treatment is performed by adding an alkali agent, (1), (2),
(3), (4), (5), (6), (7) or (8) described in the flue gas desulfurization wastewater treatment method, and (1
0) The method for treating flue gas desulfurization wastewater according to the item (9), wherein the alkali agent is sodium hydroxide.

[0005]

BEST MODE FOR CARRYING OUT THE INVENTION The method of the present invention comprises:
It can be applied to flue gas desulfurization wastewater containing oxyacids that are difficult to reduce such as iodic acid and selenate. According to the method of the present invention, when heavy metals such as copper, lead, and zinc are contained in addition to these hard-to-reduce oxygen acids, these heavy metals can be simultaneously removed. In the method of the present invention,
The ferrous ion is eluted into the aqueous hydrochloric acid solution by contacting the aqueous hydrochloric acid solution with iron. The concentration of the aqueous hydrochloric acid solution used is preferably 0.05 to 1 mol / liter, and more preferably 0.1 to 0.5 mol / liter. When the concentration of the hydrochloric acid aqueous solution is less than 0.05 mol / liter, it takes a long time to elute iron when it is brought into contact with iron, and iron may not be sufficiently eluted. If the concentration of the hydrochloric acid aqueous solution exceeds 1 mol / liter, the equipment requires a corrosion resistant material. In the method of the present invention, instead of hydrochloric acid,
It is possible to use other mineral acids such as sulfuric acid, but since calcium ions and sulfate ions are usually dissolved in the flue gas desulfurization wastewater in a saturated state, when sulfuric acid is used, gypsum is generated as SS, causing troubles. Because it may cause
Hydrochloric acid can be used particularly preferably. In the method of the present invention, the type of iron to be contacted with the aqueous hydrochloric acid solution is not particularly limited, and pure iron, crude steel, alloy steel, other iron alloys, and the like can be mentioned. Among these, electrolytic iron, low carbon Steel, mild steel, pig iron or cast iron can be preferably used. In the method of the present invention, iron is powdery, granular, spherical,
A shape having a large surface area such as a block shape, a linear shape, or a crushed shape is preferable, and the maximum diameter is preferably 3 mm or less, more preferably 1 mm or less. In the method of the present invention, the method of preparing an aqueous hydrochloric acid solution and bringing it into contact with iron is not particularly limited. For example, in a stirring tank, hydrochloric acid is added to water to prepare an aqueous hydrochloric acid solution, and then powdery, granular, linear The aqueous hydrochloric acid solution can be brought into contact with iron by adding iron and the like and stirring. Also, in the stirring tank,
By adding hydrochloric acid and iron to the water at the same time, or
It is possible to bring the aqueous hydrochloric acid solution into contact with iron by adding iron to water and then adding hydrochloric acid. Further, hydrochloric acid is added to water to prepare an aqueous hydrochloric acid solution, and the aqueous hydrochloric acid solution is passed through a packed bed filled with iron in the form of powder, granules, spheres, blocks, lines, etc. Can be contacted.

In the method of the present invention, industrial water can be used to prepare an aqueous hydrochloric acid solution to be brought into contact with iron. However, hydrochloric acid is added to treated water of flue gas desulfurization wastewater to prepare an aqueous hydrochloric acid solution,
Alternatively, hydrochloric acid may be added to a part of the flue gas desulfurization wastewater to form a hydrochloric acid aqueous solution. By using the treated water or the flue gas desulfurization wastewater for the preparation of the hydrochloric acid aqueous solution, it is possible to reduce the water cost, which is economically and practically preferable. In the method of the present invention, the concentration of ferrous ion in the hydrochloric acid aqueous solution contacted with iron can reduce the amount of ferric ion-containing hydrochloric acid aqueous solution added to the flue gas desulfurization wastewater if the concentration is high. However, since a hydrochloric acid aqueous solution of 0.05 to 1 mol / liter is actually used from the viewpoint of safety and practical use, the ferrous ion concentration in the hydrochloric acid aqueous solution is 1,000 to 1
It is often 5,000 mg / liter. In the method of the present invention, an aqueous hydrochloric acid solution containing ferrous ions is added to the flue gas desulfurization wastewater. The amount of ferric ion-containing hydrochloric acid aqueous solution to be added to the flue gas desulfurization wastewater can be appropriately selected according to the amount of oxygen acid contained in the flue gas desulfurization wastewater, etc. Ferrous ion concentration is 100 ~
It is preferable to add it so as to be 500 mg / liter, and it is more preferable to add it so that the ferrous ion concentration is 200 to 300 mg / liter. When the ferrous ion concentration is 100 mg / liter or more, the reduction reaction of oxygen acid easily proceeds. Normally, the concentration of ferrous iron is 500
mg / liter is sufficient, and if the concentration of ferrous ion is higher than necessary, the amount of sludge generated in the coagulation treatment and solid-liquid separation will unnecessarily increase. In the method of the present invention, the pH of the flue gas desulfurization effluent added with the aqueous hydrochloric acid solution containing ferrous ion is preferably 5 or less, and the pH is 2 to 2.
It is more preferably 3. If the pH exceeds 5, the reduction reaction rate of oxygen acid may slow down. In the method of the present invention, the control of ferrous ion concentration in the water to be treated is
It can be performed by measuring the redox potential. By measuring the redox potential, it is possible to reliably remove selenium and the like. For example, ferrous iron is added to a reduction reaction tank such as selenium so that the concentration becomes 100 mg / liter, and the value of the oxidation-reduction potential in the agglomeration reaction tank is set to −300 mV or less. It is possible to ensure the removal of selenium by adjusting the supply amount of the aqueous hydrochloric acid solution containing ions.

In the method of the present invention, ferrous ion is considered to react with peroxosulfuric acid, iodic acid and selenic acid as shown in the following formula. Therefore, the concentration of these oxygen acids in the flue gas desulfurization effluent is determined and theoretically necessary. You can know the amount of ferrous iron. _{^{S 2 O 8 2+ 2Fe 2+ →}} 2SO 4 2+ 2Fe 3+ (1) 2IO 3+ 10Fe 2+ + 12H + → I 2 + 10Fe 3+ + 6H 2 O (2) SeO 4 2+ 6Fe 2+ + 8H + → Se ⁰ + 6Fe ³⁺ + 4H ₂ O (3) In the method of the present invention, the oxygen acid in the water is reduced by the reaction with the ferrous ion, and then the treated water is subjected to a coagulation treatment. The method of coagulation treatment is not particularly limited, but by adding an alkaline agent, ferrous ions and ferric ions in water are made into water-insoluble ferrous hydroxide and ferric hydroxide, and iron flocs It is preferable to form and aggregate. The pH of the water to be treated is preferably adjusted to 8 or higher, more preferably 9 to 10 by adding the alkaline agent. If the pH of the water to be treated is less than 8, the aggregation of iron flocs may be insufficient. By setting the pH of the water to be treated to 8 or more, the ferrous ion in the water becomes water-insoluble ferrous hydroxide and the ferric ion becomes water-insoluble ferric hydroxide as shown in the following formula. Become. 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. In the method of the present invention, by adjusting the pH of the water to be treated to 8 or more, other metal ions whose hydroxides are insoluble in water also become hydroxides to form flocs. Further, at this time, suspended matter contained in the flue gas desulfurization wastewater is also adsorbed by the iron flocs and simultaneously aggregates. Further, when ultrafine iron particles are suspended in water, the ultrafine iron particles are also adsorbed by the iron flocs and aggregated. Also, when the reaction system is open to the air, ferrous ions undergo air oxidation and some become ferric oxide fine particles, and the ferric oxide fine particles are adsorbed by iron flocs and aggregate. To do.

In the method of the present invention, there is no particular limitation on the alkaline agent for adjusting the pH of the water to be treated to 8 or more. For example, sodium hydroxide, slaked lime, potassium hydroxide, sodium carbonate, potassium carbonate, carbide slag, etc. Can be used, but sodium hydroxide can be particularly preferably used. 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 aggregating agents, the anionic polymer aggregating agent has an excellent aggregating effect, and thus can be used particularly preferably. In the method of the present invention, the floc generated by the coagulation treatment is removed by performing the solid-liquid separation after the coagulation treatment, and the treated water 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.

FIG. 1 is a process flow chart of one embodiment of the method of the present invention. In the iron elution tank 1, hydrochloric acid and iron fine particles are added to industrial water or treated water to elute ferrous ions in an aqueous hydrochloric acid solution. If necessary, a reservoir 2 for the aqueous hydrochloric acid solution containing ferrous ions can be provided. A hydrochloric acid aqueous solution containing ferrous ions is sent to the reaction tank 3 and mixed with flue gas desulfurization wastewater to adjust the pH to 5 or less, preferably 2 to 3, and oxygen oxygen in the flue gas desulfurization wastewater is reduced. The water to be treated is then sent to the flocculation reaction tank 4, sodium hydroxide is injected, the pH is adjusted to 8 or higher, preferably 9 to 10, and iron ions are flocculated as iron hydroxide. The water to be treated which has completed the agglutination reaction is sent to the solid-liquid separation tank 5, and the treated water is separated by solid-liquid separation. The treated water is further sent to steps such as COD removal and nitrogen removal as necessary. In the method of the present invention, the aqueous hydrochloric acid solution is brought into contact with iron in advance to elute the ferrous ions, so that a hydrochloric acid aqueous solution containing highly reducible ferrous iron ions in a high concentration can be prepared with a small apparatus. , It becomes easy to manage the iron supply. According to the method of the present invention, harmful oxygen acids can be removed in the first stage of the flue gas desulfurization wastewater treatment system, and the wastewater treatment system can be simplified and simplified.
According to the method of the present invention, flue gas desulfurization wastewater is treated to reduce peroxosulfuric acid in treated water to 0.5 mg / liter or less and iodic acid to 0.1 mg / liter or less. Selenium can also be stably reduced to 0.1 mg / liter or less. The method of the present invention has a simple and simplified reaction method and is excellent in practical operation management.

[0010]

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. Example 1 pH is 6.8, peroxosulfate ion 4.2 mg / l, iodate ion 2.0 mg / l, selenium 0.5.
Flue gas desulfurization waste water containing mg / liter, lead 0.2 mg / liter, zinc 0.5 mg / liter and copper 0.5 mg / liter was treated using the apparatus shown in FIG. The iron elution tank was charged with industrial water, and the hydrochloric acid concentration was 0.2 mol / liter.
It was added to 0 g / liter and stirred at 300 rpm for 30 minutes. The concentration of ferrous ion in this hydrochloric acid aqueous solution was 4,000 mg / liter. In the reaction tank, 50 ml of an aqueous hydrochloric acid solution containing ferrous ion was added to 950 ml of flue gas desulfurization wastewater, and the mixture was stirred at 300 rpm for 15 minutes to carry out a reduction reaction treatment. Then, the water to be treated is sent to the flocculation reaction tank, and an aqueous sodium hydroxide solution is injected to adjust the pH.
Was set to 9, and the mixture was agitated at 300 rpm for 15 minutes to perform the aggregation treatment. The coagulated water to be treated was sent to a solid-liquid separation tank for solid-liquid separation. The quality of the treated water is pH 8.0, peroxosulfate ion 0.1 mg / liter or less, and iodate ion 0.1.
1 mg / liter or less, selenium 0.1 mg / liter or less,
Lead was less than 0.1 mg / liter, zinc was less than 0.1 mg / liter and copper was less than 0.1 mg / liter. Comparative Example 1 Instead of adding an aqueous hydrochloric acid solution containing ferrous ions,
The same operation as in Example 1 was repeated except that the ferrous sulfate aqueous solution was injected so that the concentration of ferrous ions was 200 mg / liter. The quality of treated water is pH 8.1, peroxosulfate ion 0.8 mg / liter, iodate ion 0.9 mg.
/ Liter, selenium 0.45 mg / liter, lead 0.1 mg /
Liter or less, zinc 0.1 mg / liter or less and copper 0.1
It was below mg / liter. The results of Example 1 and Comparative Example 1 are collectively shown in Table 1.

[0011]

[Table 1]

In Example 1 according to the method of the present invention, the treated water contains selenium and harmful oxygenate ions at a concentration of 0.1 mg /
It has been removed to less than 1 liter, and the heavy metals lead, zinc and copper have also been removed to less than 0.1 mg / liter. On the other hand, Comparative Example 1 using an aqueous ferrous sulfate solution
, The heavy metals are removed, but the removal of selenium and harmful oxygenate ions is insufficient. Example 2 An aqueous hydrochloric acid solution containing ferrous ions was prepared by using flue gas desulfurization drainage instead of industrial water. The flue gas desulfurization wastewater used had a pH of 6.7, and had peroxosulfate ion 7.2 mg / liter, iodate ion 4.0 mg / liter, selenium 0.6 mg / liter, lead 0.3 mg / liter,
Zinc is 0.2 mg / liter and copper is 0.2 mg / liter. The above-mentioned flue gas desulfurization waste water was charged into the iron elution tank of the apparatus shown in FIG. 1 so that the concentration of hydrochloric acid became 0.6 mol / liter, and the amount of iron of 100 mesh iron particles became 10 g / liter. Add and stir at 300 rpm for 30 minutes.
The concentration of ferrous ion in this hydrochloric acid aqueous solution was 1400
It was 0 mg / liter. In the reaction tank, 20 ml of the aqueous hydrochloric acid solution containing ferrous ion was added to 980 ml of flue gas desulfurization wastewater, and the mixture was stirred at 300 rpm for 30 minutes to carry out a reduction reaction treatment. Then, the water to be treated is sent to the coagulation reaction tank, and a sodium hydroxide aqueous solution is injected to adjust the pH to 3
Agitation was performed by stirring at 00 rpm for 30 minutes. The coagulated water to be treated was sent to a solid-liquid separation tank for solid-liquid separation. The water quality of the treated water is pH 7.5, peroxosulfate ion is 0.1 mg / liter or less, iodate ion is 0.1 mg / liter or less, selenium is 0.1 mg / liter, lead is 0.1 mg / liter or less, and zinc is 0.1 mg / liter. Less than liter and 0.1 mg / copper
It was below liter. Comparative Example 2 Instead of adding an aqueous hydrochloric acid solution containing ferrous ions,
The same operation as in Example 2 was repeated, except that the ferrous sulfate aqueous solution was injected so that the ferrous ion concentration was 300 mg / liter. The quality of treated water is pH 7.5, peroxosulfate ion 0.1 mg / liter or less, and iodate ion 0.1.
1 mg / liter, selenium 0.4 mg / liter, lead 0.1 mg
/ Liter or less, zinc 0.1 mg / liter or less, and copper 0.1.
It was 1 mg / liter or less. The results of Example 2 and Comparative Example 2 are shown in Table 2 together.

[0013]

[Table 2]

In Example 2 according to the method of the present invention, selenium was removed from the treated water to a concentration of 0.1 mg / liter, harmful oxygenate ions were removed to a concentration of 0.1 mg / liter or less, and lead, which is a heavy metal, was removed. , Zinc and copper have also been removed to below 0.1 mg / liter. On the other hand, Comparative Example 2 using the aqueous ferrous sulfate solution has a concentration of ferrous ion 1.5 times that of Comparative Example 1 and a reaction time of 2 times that of Comparative Example 1, and thus is more harmful than Comparative Example 1. The removal rate of substances is high, but the removal of selenium is still insufficient.

[0015]

According to the method for treating flue gas desulfurization effluent of the present invention, harmful oxygen acids such as peroxosulfuric acid and selenate and heavy metals can be effectively removed, and the conventional wastewater treatment system can be simplified. Can be realized. Compared with the conventional method in which ferrous sulfate or the like is used as the reducing agent, the amount of iron added can be reduced, operation management becomes easier, and the quality of treated water is excellent.

[Brief description of the drawings]

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

[Explanation of symbols]

 1 Iron elution tank 2 Reservoir 3 Reaction tank 4 Aggregation reaction tank 5 Solid-liquid separation tank

Claims (1)

[Claims] 1. A method of adding an aqueous hydrochloric acid solution containing ferrous ions eluted by contacting with iron to flue gas desulfurization effluent for reduction reaction treatment, and then performing coagulation treatment and solid-liquid separation. Flue gas desulfurization wastewater treatment method.