Method for treating wastewater containing cod component and selenium

Abstract

To purify waste water containing a COD component and selenium in high concentration, especially coal gasification waste water, at a lower cost than a conventional method.SOLUTION: In the method for treating wastewater containing a COD component and selenium, at least a part of the COD component contained in the wastewater is removed by biologically treating the wastewater, and then at least a part of the selenium contained in the biologically treated wastewater and the residual COD component are removed by Fenton treatment.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for treating wastewater containing high concentrations of COD components and selenium, such as coal gasification wastewater, eco-cement wastewater, and metal refining wastewater. [Background technology]

[0002] In conventional thermal power plants, fossil fuels such as coal, oil, and natural gas are burned in boilers to produce high-temperature, high-pressure steam, which then drives a steam turbine to turn a generator and generate electricity. In recent years, however, priority has been given to the construction of coal-fired power plants, which use coal as fuel, as reserves are more abundant than those of oil and natural gas, which are feared to be depleted in the future. In conventional coal-fired power plants, finely powdered coal (pulverized coal) is burned by injecting it into the boiler's combustion chamber together with air using a pulverized coal burner.

[0003] Meanwhile, in recent years, there have been promising power generation methods, such as integrated coal gasification combined cycle (IGCC), in which coal (pulverized coal) is heated in a furnace with a small amount of oxygen (usually air) to generate coal gas, which is primarily composed of carbon monoxide and hydrogen. This coal gas is then used as fuel to drive a gas turbine, and the residual heat from the combustion exhaust gas is used in a heat recovery boiler to generate steam, which drives a steam turbine, and both the gas turbine and the steam turbine turn a generator to generate electricity. Furthermore, integrated coal gasification fuel cell combined cycle (IGFC), which converts coal gas into hydrogen-rich gas (the remainder is mainly carbon dioxide) through a reaction (shift reaction) between the carbon monoxide in the coal gas and steam, and then combines this with fuel cell power generation to use the hydrogen extracted from this hydrogen-rich gas, is expected to be a promising power generation method in the future due to its high power generation efficiency.

[0004] In conventional coal-fired power plants, flue gas from coal combustion is purified using a flue gas desulfurization system before being released into the atmosphere. However, in integrated coal gasification combined cycle (IGCC) and integrated coal gasification fuel cell (IGFC) power plants, impurities are removed from the coal gas produced by coal gasification, and the purified gas is then supplied to a gas turbine as fuel or used to produce hydrogen-enriched gas for fuel cells. This coal gas purification process is typically carried out using a wet gas scrubbing process, and the resulting scrubbing wastewater is called coal gasification wastewater. This coal gasification wastewater often contains soot, cyanides, thiocyanides, polythionic acids, various organic acids, ammonia, boron, fluorine, and various metals. Of these, thiocyanides, polythionic acids, and various organic acids are components that contribute to COD measurements (COD components).

[0005] As methods for treating such coal gasification wastewater, the following have been proposed: a method in which the wastewater is subjected to thermal hydrolysis under alkaline conditions to decompose cyanide, and metals separated and precipitated from the cyano complexes are removed, followed by wet catalytic oxidation (Patent Document 1); a method in which the wastewater is subjected to coagulation and sedimentation to remove fluorine, then wet oxidation or thermal hydrolysis to decompose cyanide, and further reduction of selenate ions with a metal reductant is performed on the resulting product, followed by wet catalytic oxidation or biological treatment to remove COD components and ammonia (Patent Document 2); a method in which the wastewater is aerated under acidic conditions to remove free cyanide, then biological treatment of the wastewater after free cyanide removal treatment, and then accelerated oxidation of the wastewater after biological treatment using a strong oxidizing agent such as persulfate (Patent Document 3); and a method in which the wastewater is subjected to high-temperature alkaline chlorination to decompose cyanide compounds, ammoniacal nitrogen, and COD components, followed by coagulation and sedimentation or adsorption to remove fluorine compounds and reduction and removal of selenium compounds (Patent Document 4). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-289841 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-221151 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-76058 [Patent Document 4] International Publication No. 2014 / 083903 Summary of the Invention [Problem to be solved by the invention]

[0007] The method described in Patent Document 1 involves wet catalytic oxidation of oxidizable substances (particularly COD components) other than cyanides and metals under high temperature and pressure, resulting in high catalyst costs due to catalyst degradation and scaling. The method described in Patent Document 2 involves the high cost of the reducing agent used in the reduction treatment of selenate ions, and the additional cost of the catalyst when COD components are oxidized using wet catalytic oxidation. Furthermore, when COD components are biologically treated, COD components that cannot be removed remain in the wastewater. The method described in Patent Document 3 involves the biological treatment being affected by cyano complexes that are not completely removed in the free cyanide aeration step, resulting in unstable COD treatment performance. Furthermore, the use of expensive persulfuric acid for accelerated oxidation treatment increases costs. Furthermore, the use of sodium hypochlorite in the cyano complex decomposition method results in a low cyano complex removal rate. The method described in Patent Document 4 involves the decomposition and removal of all ammonia nitrogen and COD components using high-temperature alkaline chlorination, resulting in the high cost of sodium hypochlorite used to treat coal gasification wastewater containing high concentrations of COD components. That is, all of the methods described in Patent Documents 1 to 4 have problems in terms of economic efficiency, such as high costs for catalysts, oxidizing agents, reducing agents, etc., when treating wastewater containing high concentrations of COD components and selenium, such as coal gasification wastewater. [Means for solving the problem]

[0008] The present invention provides a method for treating wastewater containing COD components and selenium, which comprises biologically treating the wastewater to remove at least a portion of the COD components contained in the wastewater, and then simultaneously removing at least a portion of the selenium contained in the biologically treated wastewater and the remaining COD components by Fenton treatment, in which the treatment pH and chemical addition conditions are optimized. This method solves the above-mentioned problems. [Effects of the Invention]

[0009] Wastewater containing high concentrations of COD components and selenium, particularly coal gasification wastewater, can be purified at a lower cost than conventional methods. [Brief explanation of the drawings]

[0010] [Figure 1] An example of an embodiment of the method of the present invention will be described below. DETAILED DESCRIPTION OF THE INVENTION

[0011] The basic concept of the method of the present invention is to decompose and remove most of the COD components contained in wastewater that can be removed by biological treatment (e.g., formic acid) through biological treatment, and then decompose and remove only the COD components that remain in the wastewater that are not removed by biological treatment through Fenton treatment under specified conditions. This significantly reduces the COD load required for the subsequent Fenton treatment, thereby reducing the cost of chemicals used in the Fenton treatment (hydrogen peroxide and ferrous salts). In addition, by performing the Fenton treatment under specific conditions (pH 3-5, ORP 400-700 mV), the method of the present invention not only decomposes and removes COD components, but also reduces the cost of the chemicals used in the Fenton treatment (hydrogen peroxide and ferrous salts) by reducing selenate ions with ferrous ions, which are then separated and removed together with iron as sludge. This eliminates or reduces the cost of the subsequent selenium removal treatment.

[0012] Figure 1 shows an example of an embodiment of the method of the present invention. In the method of the present invention, biological treatment 1 and Fenton (oxidation) treatment 2 are sequentially performed on the water to be treated (raw water). However, if cyanide is contained in the raw water, it is preferable to perform cyanide treatment 3 prior to biological treatment because cyanide has an adverse effect on the aquatic microorganisms responsible for the biological treatment. Furthermore, if necessary, selenium treatment 4 or activated carbon adsorption treatment 5 may be performed following the Fenton treatment to further remove selenium and COD components remaining after the Fenton treatment.

[0013] (Biological treatment) In biological treatment 1, COD components (mainly organic matter) that can be decomposed by microorganisms in the water are decomposed at room temperature. Biological treatment includes aerobic biological treatment, which decomposes organic matter under conditions of abundant dissolved oxygen, and anaerobic biological treatment, which decomposes organic matter under conditions of a lack of dissolved oxygen. In the present invention, aerobic biological treatment is performed. There are two types of aerobic biological treatment: one in which the microorganisms in the water are suspended in the water to be treated, and one in which the microorganisms in the water are attached to a carrier. A typical example of the former is the activated sludge method, and a typical example of the latter is the carrier method, such as the trickling filter method or the rotating disk method. Either method may be used in the present invention. Generally, the activated sludge process is used to treat large volumes of wastewater because it provides efficient contact between microorganisms and the organic matter and dissolved oxygen in the wastewater. However, the activated sludge process requires advanced technology for operation and management when there are large fluctuations in the quality and volume of the water being treated. Therefore, when treating wastewater from small-scale plants where there are large fluctuations in water quality and volume, or from plants that are started and stopped frequently, carrier methods such as trickling filter beds and rotating disk processes that use carriers may be more suitable.

[0014] In the activated sludge process, aeration with air or oxygen is performed to replenish dissolved oxygen in the water and suspend microbial flora in the water. Microorganisms grow their cells using energy obtained by decomposing organic matter dissolved in the water. If the nitrogen and phosphorus required for this growth are insufficient, ammonium salts or phosphate salts are added to replenish them. Furthermore, as the amount of microbial flora (activated sludge) in the water gradually increases with microbial growth, portions of the activated sludge are periodically withdrawn and incinerated to maintain a constant amount. In contrast, trickling filters and rotating disk processes are designed so that the microbial flora held on the carriers are constantly exposed to air (i.e., "aerated"), thereby supplying dissolved oxygen to the treated water. Because biological treatment utilizes microorganisms that live in nature, it is carried out at room temperature and under nearly neutral conditions.

[0015] (Fenton treatment) In the Fenton process 2, hydrogen peroxide and a ferrous salt (usually ferrous chloride or ferrous sulfate) are added to the water to be treated. Hydroxy radicals (·OH) generated from the hydrogen peroxide oxidize and decompose COD components (reducible substances, including organic matter) at room temperature. The pH of the solution is preferably adjusted to 3–5. Hydrogen peroxide (HO) is preferably added at a ratio of 1–10 moles per mole of COD(O), and ferrous salt (Fe) is preferably added at a ratio of 1–10 moles per mole of COD(O) (COD (mg-O / L) converted to moles of oxygen atoms) and 300–3000 moles per mole of selenium (Se) (COD (mg-O / L) converted to moles of selenium atoms). Under these conditions, a 30–90-minute Fenton process not only oxidizes and decomposes COD components, but also reduces and precipitates selenium, which is then incorporated into the sludge along with the iron and separated for removal.

[0016] (Preliminary processing) In the method of the present invention, COD components are removed by biological treatment carried out prior to Fenton treatment, but it is desirable to remove substances that hinder biological treatment, such as cyanides and heavy metals, in advance.

[0017] (Final processing) In the method of the present invention, COD components are removed in two stages: a first stage, biological treatment, and a second stage, Fenton treatment. In the first stage, most of the COD components (mainly organic matter) that can be decomposed by aerobic microorganisms are decomposed and removed. In the second stage, only the COD components that were not decomposed by the biological treatment (residual organic matter and other reducible substances) are decomposed and removed. This two-stage treatment typically reduces the COD value to a level that satisfies wastewater standards. However, if further COD value reduction is desired, additional treatments such as activated carbon adsorption treatment can be added. Furthermore, in the method of the present invention, selenium is also reduced and separated and removed in the Fenton treatment. However, if further reduction of residual selenium is desired, a conventional selenium removal treatment using metallic iron as a reducing agent can be added. Even in such cases, because the majority of COD components and selenium have already been removed by the biological treatment and Fenton treatment, the costs of activated carbon and reducing agents (metallic iron) are reduced compared to conventional methods.

[0018] (Example) Raw water (simulant solution prepared by adding reagents to ion-exchanged water) with the composition shown below was prepared. <Raw water properties> Formic acid 7,000mg / L Thiosulfate 1,100mg / L Thiocyanate 650mg / L COD 1,970mg / L Selenium 5.0 ppm

[0019] The raw water with the above properties was subjected to biological treatment under the following conditions. <Biological treatment conditions> BOD volumetric load 1.5kgBOD / m 3 / d Liquid temperature 30℃ Treatment pH 6.0 BOD:N:P 100:5:1 The properties of the biologically treated water obtained were as follows: <Biologically treated aqueous solution> COD 350mg / L Selenium 5.0 ppm

[0020] Furthermore, the treated water after the biological treatment was subjected to Fenton treatment under the following conditions. <Fenton treatment conditions> pH 3.0 Temperature 23℃ Hydrogen peroxide concentration: 880mg-H2O2 / L Fe addition concentration 730mg-Fe / L Reaction time: 60 minutes The properties of the obtained Fenton-treated water were as follows: <Fenton treated aqueous solution> COD 20mg / L Selenium 3.9 ppm

[0021] As described above, by carrying out biological treatment and Fenton treatment under appropriate conditions in the present invention, it was possible to reduce COD and selenium concentrations.

[0022] (Comparative Example) The same raw water as used in the examples was subjected to biological treatment under the same conditions as in the examples, and the resulting treated water after biological treatment was subjected to COD treatment using persulfate under the following conditions. <Persulfate treatment conditions> pH 10.0 Temperature 80℃ Persulfuric acid addition concentration 70000mg-S2O8 / L Reaction time: 60 minutes The properties of the obtained persulfate-treated water were as follows: <Persulfate treated aqueous solution> COD 35mg / L Selenium 5.0 ppm

[0023] As described above, biological treatment and persulfate treatment were able to reduce COD, but were unable to reduce selenium concentrations. [Industrial Applicability]

[0024] It can be used to treat wastewater containing high concentrations of COD and selenium, especially coal gasification wastewater, at low cost. [Explanation of symbols]

[0025] 1. Biological treatment process 2. Fenton treatment process 3 Cyanide treatment process (if necessary) 4. Selenium treatment process (if necessary) 5. Activated carbon adsorption treatment process (if necessary)

Claims

1. A method for treating wastewater containing COD components and selenium, comprising biologically treating the wastewater to remove at least a portion of the COD components contained in the wastewater, and then removing at least a portion of the selenium contained in the biologically treated wastewater and the remaining COD components by Fenton treatment.

2. The method of claim 1, wherein the Fenton treatment is carried out under the following treatment conditions: pH: 3 to 5 H ２ O ２ Amount added: 1 to 10 mol H ２ O ２ / Mol COD Amount of iron added: 1 to 10 moles Fe / mole COD and 300 to 3000 mol Fe / mol Se ORP: 400-700mV

3. The method of claim 1 , wherein the biological treatment is an aerobic biological treatment.

4. The method according to claim 1, wherein the selenium remaining in the wastewater is removed by a retention reduction method in the subsequent stage of the Fenton treatment.

Images