Method for treating selenium-containing wastewater

Abstract

The invention provides a selenium-containing wastewater treatment method, and belongs to the technical field of wastewater treatment. The treatment method comprises the following steps: adjusting the pH value of the selenium-containing wastewater to 7-10, adding an organic-inorganic composite flocculant, carrying out stirring reaction, and carrying out solid-liquid separation to obtain primarily treated wastewater; introducing the primarily treated wastewater into a tower oxidation reactor for ozone oxidation reaction to oxidize tetravalent selenium into hexavalent selenium; the wastewater subjected to ozone oxidation is adjusted to be weakly acidic and then flows through a manganese sand filter, and selenium in the wastewater is adsorbed by manganese sand; wherein in the tower-type oxidation reactor, the wastewater and ozone gas are subjected to gas-liquid countercurrent contact reaction and then subjected to standing reaction. According to the method, three-stage processes are set, including flocculation and sedimentation of an organic and inorganic composite flocculant, step-by-step oxidation of ozone, filtration and adsorption of manganese sand under weak acidity, and efficient oxidation of selenite (tetravalent selenium) into selenate (tetravalent selenium), so that the selenium removal efficiency is improved, and the removal rate of selenium in selenium-containing wastewater reaches up to 99% or above.

Description

Technical Field

[0001] The invention belongs to the technical field of wastewater treatment, and particularly relates to a method for treating selenium-containing wastewater. Background Art

[0002] Selenate (SeO4 2- ) and selenite (SeO3 2- ) are all water-soluble and toxic to organisms at high concentrations. Elemental selenium is generally considered to be non-bioavailable because it is insoluble in water. Hydrogen selenide (H2Se) is a highly toxic gas that can be rapidly oxidized into non-toxic elemental selenium in the air. Under strong reducing conditions, it will form an insoluble metal selenium compound precipitate. In most cases, selenium in selenium-containing wastewater coexists in the form of selenate ions and selenite ions. The treatment of selenium-containing wastewater generally adopts methods such as ion exchange, biological method and flocculation precipitation.

[0003] Ion exchange method for removing selenium is to use anion resin to fully contact with selenium-containing raw water, and the exchange resin selectively absorbs selenate ions in the water, thereby achieving the purpose of removing selenium from the water. This method is not only effective in removing selenium, but also simple to operate, and the residual selenium concentration in the water is low, which can meet the specified content of water quality standards. However, the ion exchange method is costly and has high requirements for the solid particles and other impurities in the raw water, making it difficult to apply industrially.

[0004] In the biological method, anaerobic biological treatment is generally used to reduce soluble selenium to elemental selenium. Both selenate ions and selenite ions can be removed by reducing them to elemental selenium. For selenium-containing wastewater with low concentration, although the selenium concentration can be reduced to below the limit through anaerobic biological reaction alone, wastewater with a relatively high selenium concentration or wastewater with varying selenium concentration will make the biological treatment unstable due to load changes, and the treatment process will be difficult to maintain stability. It is difficult to always keep the selenium concentration below the allowable value.

[0005] In the flocculation sedimentation method, metal salts are usually added to the biologically treated wastewater to react with soluble selenium to form insoluble selenium compounds, and the residual soluble selenium is removed in the form of insoluble selenium compounds. However, when the pollutant is in the form of selenate (SeO4 2- ) form, the removal rate of selenium is very limited. For example, selenium-containing wastewater is treated by reducing selenium with sulfate and sulfite and then adsorbing the iron salt product in two stages; or ferric chloride and potassium permanganate are added to selenium-containing wastewater to produce manganese dioxide precipitation to remove selenium. Studies have found that the above is only effective for the removal of selenite, and the removal efficiency of selenate is limited, resulting in a decrease in the removal effect of selenium. For example, selenate can be efficiently removed by adsorption through manganese sand filter material, but the removal effect of selenite is low.

[0006] In summary, due to the presence of hexavalent selenium (SeO4 2- ) and tetravalent selenium (SeO3 2- ) There are two forms of selenium substances, and it is difficult to remove selenate and selenite at the same time with the same treatment process. In order to solve this problem, the prior art converts tetravalent selenium and hexavalent selenium into the same selenium substance by oxidation or reduction reaction before removing selenium. For example, selenite (quadrivalent selenium) is oxidized to selenate (quadrivalent selenium) by oxidation reactions such as ozone and sulfite, and then selenate is adsorbed by adsorption to achieve selenium removal. However, studies have found that in conventional oxidation reactions in industrial production, the conversion efficiency of tetravalent selenium to hexavalent selenium is not high, so that some tetravalent selenium is still contained in the wastewater, resulting in low selenium removal efficiency. Summary of the invention

[0007] In view of this, the object of the present invention is to provide a method for treating selenium-containing wastewater, aiming to solve at least one technical problem in the background technology.

[0008] The present invention is achieved in that:

[0009] A method for treating selenium-containing wastewater comprises the following steps:

[0010] The pH value of the selenium-containing wastewater is adjusted to 7-10, an organic-inorganic composite flocculant is added, and solid-liquid separation is performed after stirring the reaction to obtain the preliminarily treated wastewater;

[0011] The wastewater after preliminary treatment is introduced into a tower oxidation reactor for ozone oxidation reaction to oxidize tetravalent selenium into hexavalent selenium;

[0012] The wastewater after ozone oxidation is adjusted to weak acidity and then flows through a manganese sand filter, where the manganese sand absorbs selenium in the wastewater;

[0013] Among them, in the tower oxidation reactor, the wastewater and ozone gas are first contacted by gas-liquid countercurrent reaction and then left to react statically.

[0014] Furthermore, in the ozone oxidation reaction, the amount of ozone used exceeds the theoretical amount, and the gas-liquid countercurrent reaction followed by the static reaction specifically includes:

[0015] Ozone gas enters from the lower part of the tower oxidation reactor, and the wastewater after preliminary treatment is sprayed or centrifuged from the top of the tower oxidation reactor. The ozone gas flows upward in countercurrent and contacts and reacts with the wastewater from top to bottom.

[0016] After the ozone gas ventilation and wastewater spraying are completed, the two are allowed to react in the tower oxidation reactor for 5 minutes to 10 minutes.

[0017] Furthermore, in the ozone oxidation reaction, ozone gas enters the lower part of the tower oxidation reactor through an aeration plate.

[0018] Furthermore, the organic-inorganic composite flocculant includes an organic flocculant and an inorganic flocculant, the organic flocculant is grafted polyacrylamide, and the inorganic flocculant is at least one of polymerized iron salt or iron salt.

[0019] Furthermore, the dosage of the organic flocculant is 1 mg / L to 6 mg / L; the dosage of the inorganic flocculant is 6 mg / L to 36 mg / L.

[0020] Furthermore, the reagent for adjusting the pH value of the selenium-containing wastewater is sodium hydroxide, potassium hydroxide or lime milk, preferably lime milk.

[0021] Furthermore, the solid-liquid separation adopts one of the methods of centrifugal separation, filter press or membrane filtration.

[0022] Furthermore, the filtration rate in the manganese sand filter is 5m / h to 10m / h.

[0023] Furthermore, the wastewater after ozone oxidation is adjusted to be weakly acidic, with a pH value of 4 to 6.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention sets up a three-stage process, including flocculation and precipitation of organic-inorganic composite flocculants, step-by-step oxidation with ozone, and filtration and adsorption of manganese sand under weak acidity, so as to efficiently oxidize selenite (quadrivalent selenium) into selenate (quadrivalent selenium), thereby improving the selenium removal efficiency and making the selenium removal rate in selenium-containing wastewater as high as 99% or more.

[0026] 2. In the ozone oxidation reaction of the present invention, the wastewater and ozone gas are first contacted by gas-liquid countercurrent reaction, and the gas and liquid are initially contacted in the air, rather than by immersion; then the reaction is allowed to stand to further ensure that the oxidation reaction is complete.

[0027] 3. In the present invention, ozone gas enters the lower middle part of the tower oxidation reactor through an aeration disk. The aeration air intake method can increase the flow rate and diffusivity of the gas. On the other hand, air intake from the lower middle part can maximize the initial air-liquid contact in the reactor rather than immersion contact, thereby increasing the gas-liquid contact surface and reaction efficiency.

[0028] 4. The present invention performs manganese sand filtration and adsorption on wastewater in a weakly acidic system, which is helpful for the protonation of the surface of filter materials such as manganese sand. The enhancement of protonation can increase the positive charge on the surface of the adsorption material, thereby improving the adsorption of SeO4 2- The electrostatic attraction can achieve higher adsorption capacity and adsorption rate. DETAILED DESCRIPTION

[0029] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific implementation cases described herein are only used to explain the present invention and are not used to limit the present invention.

[0030] A method for treating selenium-containing wastewater comprises the following steps:

[0031] S1. Adjust the pH value of the selenium-containing wastewater to 8-10, add an organic-inorganic composite flocculant, stir the reaction, and separate the solid and liquid to obtain the preliminarily treated wastewater; the solid-liquid separation is carried out by one of centrifugal separation, filter press or membrane filtration;

[0032] The organic-inorganic composite flocculant is a composite of an organic flocculant and an inorganic flocculant, wherein the organic flocculant is a grafted polyacrylamide, and the inorganic flocculant is at least one of an iron salt or a polymerized iron salt. In some preferred embodiments, the inorganic flocculant is preferably an inorganic polymerized iron salt because (1) iron salts are generally highly corrosive (such as ferric chloride), and the treated water is prone to showing the color of iron, and the chromaticity of the effluent water exceeds the standard; (2) polymerized iron salts are inorganic substances that use trivalent iron ions to polymerize large molecules through hydroxyl groups, and have high polymerization properties. The generated hydrolysis products also include products with high flocculation effects such as multinuclear complexes, and their flocculation effects are better than those of iron salts.

[0033] The polyferric salt can be polyaluminium iron, polyferric chloride, polyferric sulfate or polyferric silicate, preferably polyferric sulfate, because of its low cost, ease of use and no secondary pollution; the iron salt can be ferric sulfate, ferric chloride, etc.;

[0034] The organic flocculant uses grafted polyacrylamide, and any grafted polyacrylamide permitted in the art can be used, such as starch grafted polyacrylamide or cellulose grafted polyacrylamide, etc. It is prepared by using starch, carboxymethyl cellulose and other materials as the base material / skeleton, and adopting a grafting modification method under the action of an initiator, with acrylamide as the modifier. Compared with conventional polyacrylamide, grafted polyacrylamide has both polyhydroxyl and amide groups and presents a branched structure, which disperses the functional groups of flocculation, promotes the "bridging" effect in the flocculation process, and thus improves the flocculation effect;

[0035] In terms of dosage, the dosage of organic flocculants is 1mg / L to 6mg / L; the dosage of inorganic flocculants is 6mg / L to 36mg / L; the specific dosage is adjusted according to the concentration of pollutants in the selenium-containing wastewater, and is not specifically limited here;

[0036] The functions of organic-inorganic composite flocculants include: (1) removing pollutants such as suspended solids, organic matter, and phosphorus from wastewater to avoid affecting the efficiency of subsequent oxidation reactions, or to avoid the growth of biofilm on the filter material of subsequent manganese sand filters after long-term use, thereby reducing the filtration adsorption efficiency and service life; (2) when iron salt flocculants are added to water, hydrolysis reactions will occur, and the iron salt hydrolysis products can initially remove selenium from wastewater;

[0037] The method of the present invention adjusts the selenium-containing wastewater to alkaline before flocculation. Sodium hydroxide, potassium hydroxide and lime milk can be used to adjust the pH value of the selenium-containing wastewater. Preferably, saturated lime milk is added for adjustment, because saturated lime milk can not only make the ozone oxidation reaction proceed at the optimal pH value, but also make the calcium salt and magnesium salt in the water react with the lime milk to undergo chemical precipitation reaction, thereby removing a part of the calcium salt and magnesium salt in the wastewater.

[0038] S2, introducing the preliminarily treated wastewater into a tower oxidation reactor for ozone oxidation reaction to oxidize tetravalent selenium into hexavalent selenium;

[0039] Selenite ion SeO3 2- Hexavalent selenium exists in the form of selenate ion SeO4 2- In the form of ozone, selenite ions SeO3 2- Oxidized to selenate ion SeO4 2- ; Selenite and selenate both exist in selenium-containing wastewater; however, the adsorption amount of manganese sand on the two is quite different, and the adsorption amount of hexavalent selenium by manganese sand is 5 to 10 times that of tetravalent selenium; this step oxidizes tetravalent selenium into hexavalent selenium, thereby improving the selenium removal rate;

[0040] In the tower oxidation reactor, gas-liquid countercurrent reaction is first adopted followed by static reaction, which specifically includes:

[0041] (1) Ozone gas enters from the lower part of the tower oxidation reactor, and the wastewater after preliminary treatment is sprayed or centrifuged from the top of the tower oxidation reactor. The ozone gas flows upward in countercurrent and contacts and reacts with the wastewater from top to bottom; and the ozone gas enters the lower part of the tower oxidation reactor through the aeration plate.

[0042] (2) After the ozone gas ventilation and wastewater spraying are completed, the two are allowed to react in the tower oxidation reactor for 5 minutes to 10 minutes.

[0043] In the ozone oxidation reaction, the reaction mechanism is: O3+3SeO3 2- =3SeO4 2-The theoretical dosage of ozone is: the molar ratio of ozone to selenite ion is 1:3. In some preferred embodiments, the dosage of ozone can be appropriately exceeded by the theoretical dosage, generally 1.05 to 1.2 times of the theoretical dosage, which can reduce the waste of ozone and increase the cost. In the specific implementation, it can also be adjusted according to actual needs. The half-life of ozone is short, and the excess ozone can be self-decomposed in a short time without secondary pollution. In addition, the present invention uses ozone as an oxidant, which avoids the high cost of adding a large amount of oxidants (such as sulfites) to water, and the problems of secondary pollution and increased salt content.

[0044] In the present invention, ozone gas enters the lower middle part of the tower oxidation reactor through an aeration plate. The aeration air intake method can improve the flow rate and diffusivity of the gas. On the other hand, from the lower middle part (generally, the air intake area is set to 1 / 3 to 1 / 2 of the height of the tower oxidation reactor), it can maximize the initial contact of gas and liquid in the reactor in the air rather than immersion contact, thereby increasing the gas-liquid contact surface and reaction efficiency. Then, a static reaction is adopted to further ensure the complete oxidation reaction.

[0045] After the oxidation reaction, the pH will drop slightly, and the pH in the wastewater is between 6-8. Studies have found that in conventional gas-liquid contact reactions, the ozone oxidation reaction is incomplete, resulting in the presence of tetravalent selenium in the wastewater, which affects the subsequent adsorption and selenium removal. The present invention adopts a gas-liquid countercurrent reaction followed by a static reaction, which can promote the ozone oxidation reaction to be complete and completely oxidize the tetravalent selenium into hexavalent selenium.

[0046] S3, adjusting the wastewater after ozone oxidation to weak acidity (pH 4-6), and then passing it through a manganese sand filter, where the manganese sand absorbs selenium in the wastewater;

[0047] In this step, manganese sand adsorbs selenate, and then deeply removes selenium from the wastewater. The manganese sand filter adopts a manganese sand filter bed or a manganese sand filter column, and a multi-stage filtering device can be arranged in series; in a specific implementation, the filtration rate in the manganese sand filter is 5m / h~10m / h, and the selenium content in the effluent is tested after the manganese sand filter adsorbs and filters; the filtration and adsorption time of the manganese sand filter is about 1h~2h; the height of the manganese sand filler in the manganese sand filter is set to 1m~3m; the diameter or length and width are set to 20cm~40cm, and the specific parameters of the filler can also be adjusted according to actual needs, and are not specifically limited here;

[0048] Before entering the manganese sand filter, the wastewater is adjusted to weak acidity. The reason is that when the wastewater to be treated is acidic, the removal rate of selenium is better than that of neutral or alkaline wastewater. 2- When the adsorption system is acidic, it is helpful to protonate the surface of filter materials such as manganese sand. The enhancement of protonation can increase the positive charge on the surface of the adsorption material, thereby improving the adsorption of SeO4 2- Electrostatic attraction, thereby obtaining a higher adsorption amount and adsorption rate;

[0049] The study found that as the pH value decreased, the manganese sand filter material had a significant effect on SeO4 2- The adsorption amount and adsorption rate show a trend of first increasing and then decreasing. The reason is that when the reaction system changes from neutral to weakly acidic, the adsorption effect can be increased due to the increase in electrostatic attraction. However, when the reaction system increases to strong acidity, the main component of the manganese sand filter material (manganese dioxide) will be partially dissolved, resulting in a weakened adsorption effect. For this reason, the initial pH of the adsorption system is set to 4-6 in the present invention.

[0050] Example 1

[0051] This embodiment includes a method for treating selenium-containing wastewater, comprising the following steps:

[0052] S1. Use lime milk to adjust the pH value of selenium-containing wastewater to 8, then add 1 mg / L grafted polyacrylamide and 6 mg / L polyferric sulfate as flocculants, stir and react for about 15 minutes, and obtain the preliminarily treated wastewater after membrane filtration;

[0053] The polyferric salt may be polyaluminium iron, polyferric chloride, polyferric sulfate or polyferric silicate; the iron salt may be ferric sulfate, ferric chloride, etc.;

[0054] S2, ozone gas enters from the middle of the tower oxidation reactor through the aeration plate, and the wastewater after preliminary treatment is sprayed into the tower oxidation reactor from the top, and the molar ratio of ozone to selenite ion is 1.05:3; the ozone gas flows upward in countercurrent and contacts and reacts with the wastewater from top to bottom; after the ozone gas ventilation and wastewater spraying are completed, the two are allowed to react in the tower oxidation reactor for 10 minutes;

[0055] S3. Adjust the pH of the wastewater after ozone oxidation to 4-6, and then flow it through a manganese sand filter column (manganese sand filler 1.5m, diameter 30cm), the filtration rate is 5m / h, the adsorption time is set to 2h, and the manganese sand adsorbs selenium in the wastewater.

[0056] The selenium content (SeO3 2- and SeO4 2- ) were measured, and the results are shown in Table 1.

[0057] Example 2

[0058] This embodiment includes a method for treating selenium-containing wastewater, comprising the following steps:

[0059] S1. Use sodium hydroxide or the like to adjust the pH value of the selenium-containing wastewater to 9, then add 3 mg / L grafted polyacrylamide and 20 mg / L polyferric sulfate as flocculants, stir and react for about 10 minutes, and obtain the preliminarily treated wastewater after membrane filtration;

[0060] S2, ozone gas enters from the middle of the tower oxidation reactor through the aeration plate, and the wastewater after preliminary treatment enters the tower oxidation reactor through cyclone centrifugation at the top, and the molar ratio of ozone to selenite ion is 1.1:3; the ozone gas flows upward in countercurrent and contacts and reacts with the wastewater from top to bottom; after the ozone gas ventilation and wastewater spraying are completed, the two are allowed to react in the tower oxidation reactor for 7 minutes;

[0061] S3. Adjust the pH of the wastewater after ozone oxidation to 5, and then flow it through a manganese sand filter column (manganese sand filler 1.5m, diameter 30cm), the filtration rate is 8m / h, the adsorption time is set to 1.5h, and the manganese sand adsorbs selenium in the wastewater.

[0062] The selenium content (SeO3 2- and SeO4 2- ) were measured, and the results are shown in Table 1.

[0063] Example 3

[0064] This embodiment includes a method for treating selenium-containing wastewater, comprising the following steps:

[0065] S1. Use potassium hydroxide to adjust the pH value of the selenium-containing wastewater to 10, then add 6 mg / L grafted polyacrylamide and 36 mg / L polyferric sulfate as flocculants, stir the reaction for about 5 minutes, and centrifuge to obtain the preliminarily treated wastewater;

[0066] S2, ozone gas enters from the middle of the tower oxidation reactor through the aeration plate, and the wastewater after preliminary treatment is sprayed into the tower oxidation reactor from the top, and the molar ratio of ozone to selenite ion is 1.2:3; the ozone gas flows upward in countercurrent and contacts and reacts with the wastewater from top to bottom; after the ozone gas ventilation and wastewater spraying are completed, the two are allowed to react in the tower oxidation reactor for 5 minutes;

[0067] S3. Adjust the pH of the wastewater after ozone oxidation to 6, and then flow it through a manganese sand filter column (manganese sand filler 1.5m, diameter 30cm), the filtration rate is 10m / h, the adsorption time is set to 1h, and the manganese sand adsorbs selenium in the wastewater.

[0068] The selenium content (SeO3 2- and SeO4 2- ) were measured, and the results are shown in Table 1.

[0069] Example 4

[0070] This embodiment includes a method for treating selenium-containing wastewater, comprising the following steps:

[0071] S1. Use lime milk to adjust the pH value of selenium-containing wastewater to 7-10, then add 1 mg / L grafted polyacrylamide and 6 mg / L ferric chloride as flocculants, stir and react for about 15 minutes, and obtain the preliminarily treated wastewater after membrane filtration;

[0072] The polyferric salt may be polyaluminium iron, polyferric chloride, polyferric sulfate or polyferric silicate; the iron salt may be ferric sulfate, ferric chloride, etc.;

[0073] S2, ozone gas enters from the middle of the tower oxidation reactor through the aeration plate, and the wastewater after preliminary treatment is sprayed into the tower oxidation reactor from the top, and the molar ratio of ozone to selenite ion is 1.05:3; the ozone gas flows upward in countercurrent and contacts and reacts with the wastewater from top to bottom; after the ozone gas ventilation and wastewater spraying are completed, the two are allowed to react in the tower oxidation reactor for 10 minutes;

[0074] S3. Adjust the pH of the wastewater after ozone oxidation to 5, and then flow it through a manganese sand filter column (manganese sand filler 1.5m, diameter 30cm), the filtration rate is 6m / h, the adsorption time is set to 2h, and the manganese sand adsorbs selenium in the wastewater.

[0075] The selenium content (SeO3 2- and SeO4 2- ) were measured, and the results are shown in Table 1.

[0076] Comparative Example 1

[0077] This comparative example includes a method for treating selenium-containing wastewater, which differs from Example 1 in that the S2 ozone oxidation reaction step is removed, and other reaction parameters and step conditions are the same as those in Example 1.

[0078] The selenium content (SeO3 2- and SeO4 2- ) were measured, and the results are shown in Table 1.

[0079] Comparative Example 2

[0080] This comparative example includes a method for treating selenium-containing wastewater, which differs from Example 1 in that S2 is an ozone oxidation reaction step, and other reaction parameters and step conditions are the same as those of Example 1. The specific details of this comparative example S2 are:

[0081] The wastewater treated in step S1 is transported to an oxidation reactor, and ozone is then blown into the oxidation reactor for an oxidation reaction for at least 20 minutes.

[0082] The selenium content (SeO3 2- and SeO4 2- ) were measured, and the results are shown in Table 1.

[0083] Comparative Example 3

[0084] This comparative example includes a method for treating selenium-containing wastewater, which differs from Example 1 in that S2 is an ozone oxidation reaction step, and other reaction parameters and step conditions are the same as those of Example 1. The specific details of this comparative example S2 are:

[0085] Ozone gas enters from the middle of the tower oxidation reactor through an aeration plate, and the wastewater treated in step S1 is sprayed into the tower oxidation reactor from the top. The ozone gas flows upward in a countercurrent manner to react with the wastewater from top to bottom.

[0086] The selenium content (SeO3 2- and SeO4 2- ) were measured, and the results are shown in Table 1.

[0087] Comparative Example 4

[0088] This comparative example includes a method for treating selenium-containing wastewater, which differs from Example 1 in the S3 manganese sand filtration step. In this comparative example, the pH is not adjusted during the manganese sand filtration, and the wastewater after the S2 ozone oxidation reaction is neutral or weakly alkaline. The other reaction parameters and step conditions are the same as those in Example 1.

[0089] The selenium content (SeO3 2- and SeO4 2- ) were measured, and the results are shown in Table 1.

[0090] Table 1

[0091]

[0092]

[0093] It can be seen from the data in Table 1 that after the ozone oxidation of the present invention, more than 96% of the tetravalent selenium is oxidized to hexavalent selenium; and then filtered by manganese sand to achieve deep selenium removal.

[0094] It can be seen from the data of Comparative Example 1 that if the wastewater after flocculation treatment is directly filtered with manganese sand, most of the hexavalent selenium can be removed, but nearly half of the tetravalent selenium is still not removed;

[0095] It can be seen from Comparative Examples 2 and 3 that the oxidation reaction is not sufficient using conventional ozone oxidation;

[0096] It can be seen from Comparative Example 4 that the effect of manganese sand filtration in a weakly alkaline environment is inferior to that in a weakly acidic environment.

[0097] The data in Table 1 were converted into the total selenium content of the inlet and outlet water, and the selenium removal rate was calculated. The results are shown in Table 2 below.

[0098] Table 2

[0099]

[0100] Among them, total selenium content = 78.96*(C 四价硒 +C 六价硒 ), C 四价硒 , C 六价硒 SeO3 2- and SeO4 2- Molar amount; Selenium removal rate = (total selenium content before treatment - total selenium content of effluent after treatment in each embodiment) / total selenium content before treatment * 100%.

[0101] It can be seen from the data in Table 2 that after the selenium-containing wastewater is treated in sequence by the three steps of the present invention, the final effluent selenium removal rate is above 99%, and the data of Comparative Example 1 shows that the selenium removal rate is greatly reduced compared with Example 1 by direct filtration and adsorption with manganese sand without ozone oxidation; it can be seen from Comparative Examples 2 and 3 that the conventional ozone oxidation is used, and the selenium removal rate is slightly reduced compared with Example 1, which is about 97%; it can be seen from the data of Comparative Example 4 that under a non-acidic system, the manganese sand filtration and adsorption effect is significantly reduced, and its selenium removal rate is greatly reduced compared with Example 1.

[0102] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for treating selenium-containing wastewater, characterized in that: The processing method comprises the following steps: The pH value of the selenium-containing wastewater is adjusted to 7-10, an organic-inorganic composite flocculant is added, and solid-liquid separation is performed after stirring the reaction to obtain the preliminarily treated wastewater; The wastewater after preliminary treatment is introduced into a tower oxidation reactor for ozone oxidation reaction to oxidize tetravalent selenium into hexavalent selenium; The wastewater after ozone oxidation is adjusted to weak acidity and then flows through a manganese sand filter, where the manganese sand absorbs selenium in the wastewater; Among them, in the tower oxidation reactor, the wastewater and ozone gas are first contacted by gas-liquid countercurrent reaction and then left to react statically.

2. The method for treating selenium-containing wastewater according to claim 1, characterized in that: In the ozone oxidation reaction, the amount of ozone used exceeds the theoretical amount, and the gas-liquid countercurrent reaction followed by the static reaction specifically includes: Ozone gas enters from the lower part of the tower oxidation reactor, and the wastewater after preliminary treatment is sprayed or centrifuged from the top of the tower oxidation reactor. The ozone gas flows upward in countercurrent and contacts and reacts with the wastewater from top to bottom. After the ozone gas ventilation and wastewater spraying are completed, the two are allowed to react in the tower oxidation reactor for 5 minutes to 10 minutes.

3. A method for treating selenium-containing wastewater according to claim 2, characterized in that: In the ozone oxidation reaction, ozone gas enters the lower part of the tower oxidation reactor through an aeration plate.

4. The method for treating selenium-containing wastewater according to claim 1, characterized in that: The organic-inorganic composite flocculant comprises an organic flocculant and an inorganic flocculant. The organic flocculant is grafted polyacrylamide, and the inorganic flocculant is at least one of polymerized iron salt or iron salt.

5. A method for treating selenium-containing wastewater according to claim 4, characterized in that: The dosage of the organic flocculant is 1 mg / L to 6 mg / L; the dosage of the inorganic flocculant is 6 mg / L to 36 mg / L.

6. The method for treating selenium-containing wastewater according to claim 1, characterized in that: The reagent for adjusting the pH value of selenium-containing wastewater is sodium hydroxide, potassium hydroxide or lime milk.

7. The method for treating selenium-containing wastewater according to claim 1, characterized in that: The solid-liquid separation adopts one of centrifugal separation, filter press or membrane filtration methods.

8. The method for treating selenium-containing wastewater according to claim 1, characterized in that: The filtration rate in the manganese sand filter is 5m / h to 10m / h.

9. The method for treating selenium-containing wastewater according to claim 1, characterized in that: Adjust the wastewater after ozone oxidation to weak acidity and its pH value to 4-6.

Images