JP2006205097A - Biological treatment method of wastewater - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biological treatment method of wastewater which can efficiently reduce and remove nitrogen and selenium in wastewater by improving the selenium reducing capacity of activated sludge. <P>SOLUTION: The biological treatment method comprises a first anaerobic treatment process (6) for bringing the wastewater (1) into contact with activated sludge adhering to a carrier in presence of first organic matter (5) under an anaerobic condition, and a second anaerobic treatment process (8) for bringing the treated water of the first anaerobic treatment process (6) into contact with activated sludge adhering to a carrier in presence of second organic matter (7) under an anaerobic condition. This method may additionally contain a solid-liquid separation process (10) for performing the solid-liquid separation of the treated water of the second anaerobic treatment process (8), a first aerobic treatment process (2) for bringing the wastewater into contact with activated sludge adhering to a carrier under an aerobic condition before introducing the wastewater to the first anaerobic treatment process (6), and a second aerobic treatment process (13) for bringing the treated water (14a) of the second anaerobic treatment process or the separated water (11) of the solid-liquid separation process into contact with activated sludge adhering to a carrier under an aerobic condition. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a biological treatment method of wastewater, and in particular, nitrogen and soluble selenium (hexavalent and selenium) such as wastewater discharged from a wet flue gas desulfurization device that removes sulfur oxides in combustion exhaust gas such as coal. The present invention relates to a biological treatment method of wastewater containing tetravalent).

  Wastewater discharged from wet flue gas desulfurization equipment that removes sulfur oxides from combustion exhaust gas using coal, etc. as fuel, includes trace metals and nitrogen compounds (ammonia nitrogen, nitric acid) in exhaust gas (including ash) The water quality that satisfies the discharge regulation value is rendered harmless and discharged.

  Metals such as selenium (Se) have become regulated substances as part of the tightening of effluent quality regulations today. Incidentally, the concentration of selenium in wastewater discharged from wet flue gas desulfurization equipment that removes sulfur oxides in combustion exhaust gas using coal as fuel is about 5 mg / l (mainly hexavalent selenium occupies most). On the other hand, since the selenium release regulation value is 0.1 mg / l or less, the selenium treatment is indispensable.

Selenium is contained in exhaust gas and discharged when coal is burned, and is dissolved in wastewater as hexavalent or tetravalent selenate (SeO ₄ ²⁻ , SeO ₃ ²⁻ ). Among these selenates, tetravalent selenium (mainly present in the form of SeO ₃ ²⁻ ) forms a hardly soluble metal salt with iron or the like, and therefore can be removed by a coagulation precipitation treatment. However, hexavalent selenium (present in the form of SeO ₄ ²⁻ ) does not form a hardly soluble metal salt with iron or the like, and cannot be removed by ion exchange resin or activated carbon adsorption treatment. For this reason, there is a method in which hexavalent selenium is once reduced to tetravalent selenium with a divalent iron salt or metallic iron, and then a hardly soluble metal salt is formed with the iron salt. However, there is a disadvantage that the amount of sludge generation is very large.

  In the conventional method described above, it is difficult to treat selenium below the regulation value, but if hexavalent selenium in the waste water can be reduced to tetravalent selenium, it can be removed by a physicochemical treatment method. . As the method, the following biological treatment methods have been proposed.

  In Patent Document 1, selenium-containing water and nitrate ions are supplied to an anaerobic reaction tank, and an excess of organic matter is supplied in excess of the denitrification reaction equivalent of nitrate ions to make contact with denitrifying bacteria in the tank in an anaerobic state. And a method of solid-liquid separation after reducing hexavalent and tetravalent selenium to simple selenium by biological reaction is described.

  In Patent Document 2, in a method in which a first anaerobic reaction tank and a second anaerobic reaction tank are provided to perform anaerobic treatment of wastewater in multiple stages, a first solid-liquid separation tank and a second The biological sludge of one anaerobic treatment tank is not moved to the other anaerobic treatment tank by using the solid-liquid separation tank of It is described that an organic substance (methanol) and nitric acid (nitrous acid) salt are added to both the first anaerobic reaction tank and the second anaerobic reaction tank.

  Patent Document 3 describes a method for reducing selenic acid in which a novel microorganism belonging to the genus Bacillus and having a selenate reducing ability is brought into contact with selenate or its inclusion.

  Patent Document 4 describes a method for treating waste water containing selenium and a nitrogen compound shown in FIG. In addition, when the nitrogen compound contained in the waste water 1 is ammonia, it is brought into contact with activated sludge in advance under aerobic conditions by a nitrification tank (not shown) provided in the previous stage to convert ammonia nitrogen to nitrate nitrogen. Let me. This waste water 1 is introduced into the anaerobic treatment tank 20, an organic substance 21 is added under anaerobic conditions, and nitrate nitrogen is decomposed by being brought into contact with floating denitrifying bacteria. At the same time, hexavalent selenium contained in the waste water is reduced to tetravalent selenium by denitrifying bacteria, and further reduced to single selenium to be solidified. Note that methanol is used as the organic substance 21, and nitric acid (or nitrous acid) salt 22 is added to the anaerobic treatment tank 20 when the nitrogen concentration in the wastewater 1 is low.

Next, the treatment liquid of the anaerobic treatment tank 20 is introduced into the aerobic treatment tank 24, and the air 4 is supplied from the aeration device 25 to decompose the remaining organic matter (methanol) by the action of the aerobic microorganisms. Further, the treatment liquid in the aerobic treatment tank 24 is introduced into the solid-liquid separation tank 26 to precipitate single selenium that has become a solid. A part of the sludge is returned to the anaerobic treatment tank 20 as a return sludge 27 in order to keep the sludge concentration in the anaerobic treatment tank 20 constant, and the remaining part is taken out as a surplus sludge 28 and disposed of. On the other hand, the supernatant liquid from the solid-liquid separation tank 26 is discharged out of the system as treated water 14b.
Japanese Patent Application Laid-Open No. 08-299986 Japanese Patent Laid-Open No. 09-122688 JP 09-248595 A Japanese Patent Laid-Open No. 11-188387

  The treatment method (Patent Document 4) shown in FIG. 2 has an advantage that nitrogen compound and selenium can be treated simultaneously, but floating activated sludge is used in the anaerobic treatment tank 20 and the aerobic treatment tank 24. Selenium is known to be toxic to microorganisms, and as the amount of selenium accumulated in the activated sludge increases, it deteriorates the selenium treatment performance and the cohesiveness of the activated sludge. It tends to cause a decline.

  Further, in order to maintain the aerobic state in the aerobic treatment tank 24 subsequent to the anaerobic treatment tank 20, the tetravalent selenium and the single selenium generated in the anaerobic treatment tank 20 are oxidized again into difficult-to-treat hexavalent selenium by activated sludge. Re-eluted, reducing the ability to remove selenium. In order to prevent this, ORP (oxidation reduction potential) control is required in the aerobic treatment tank 24.

  The treatment methods described in Patent Document 1 and Patent Document 2 both obtain nitrate ions (nitrite ions) for denitrification (as an oxygen source) by nitrification of ammonia in wastewater, or forcibly nitrate ions. Is essential. Among them, the treatment method described in Patent Document 2 is a treatment using different biota by two-stage anaerobic treatment, but since both use an activated sludge in the anaerobic treatment tank, Similar to Patent Document 4, there is a problem with floating activated sludge.

  In the treatment method using a novel microorganism described in Patent Document 3, complicated procedures such as culture, pure separation, inoculation to contaminated wastewater and culture, and long-term preparation are required. Further, there is no disclosure about the treatment in the case where a nitrogen compound coexists other than selenium.

  Therefore, in view of the above problems, the present invention provides a biological treatment method of wastewater that can efficiently reduce and remove nitrogen and selenium in wastewater by increasing the selenium reduction ability of activated sludge. For the purpose.

  As a result of studying selenium reduction treatment using various activated sludges in industrial wastewater treatment facilities and sewage treatment plants, the present inventors, unlike the biological treatment method described above, in any activated sludge, It was found that the coexistence of selenium reduction treatment ability of activated sludge is rather lowered and becomes unstable, and it has a great influence on the required duration of acclimatization of activated sludge.

  That is, the biological treatment method of wastewater according to the present invention includes a first anaerobic treatment step in which wastewater is brought into contact with activated sludge attached to a carrier in the anaerobic state in the presence of the first organic substance, And a second anaerobic treatment step in which the treated water of the anaerobic treatment step is brought into contact with the activated sludge attached to the carrier in the anaerobic state in the presence of the second organic substance.

  The waste water is preferably waste water containing nitrogen and selenium. For example, in addition to nitrate nitrogen mainly discharged from wet flue gas desulfurization equipment that removes sulfur oxides in combustion exhaust gas such as coal, ammonia Wastewater containing nitrogen containing nitrous acid, nitrite nitrogen and selenium mainly comprising hexavalent selenium is more preferable. The first organic substance is preferably methanol.

  The second organic substance is acetic acid and acetate, ethanol, glycerol, lactic acid and salt thereof, pyruvic acid and salt thereof, fumaric acid and salt thereof, malic acid and salt thereof, succinic acid and salt thereof, glutamic acid and salt thereof, It is preferably at least one selected from citric acid and its salts, glucose, fructose and sucrose.

  The biological treatment method of waste water according to the present invention can further include a solid-liquid separation step of solid-liquid separation of the treated water in the second anaerobic treatment step. In this solid-liquid separation step, it is preferable to add a ferric salt or an alkali agent.

  The biological treatment method of waste water according to the present invention is a first aerobic treatment step in which the waste water is brought into contact with activated sludge adhering to a carrier in an aerobic state before being introduced into the first anaerobic treatment step. Can further be included.

  In the biological treatment method of waste water according to the present invention, the treated water in the second anaerobic treatment step or the separated treated water in the solid-liquid separation step is brought into contact with activated sludge attached to a carrier in an aerobic state. A second aerobic treatment step can be further included.

  The biological treatment method of wastewater according to the present invention is the wastewater discharged from the first anaerobic treatment step and the second anaerobic treatment step, and the precipitate separated in the solid-liquid separation step, It can further include a step of discharging out of the system without being mixed in the first aerobic treatment step and the second aerobic treatment step.

  The biological treatment method of waste water according to the present invention is a step of discharging the precipitate separated in the solid-liquid separation step out of the system without being mixed in the first aerobic treatment step and the second aerobic treatment step. Can further be included.

  In the present invention, the fixed bed system is used as a carrier for attaching microorganisms to all devices applied to the denitrification process and the selenium reduction process, preferably further the nitrification process and the biooxidation process. Thus, there is no modulation such as agglomeration deterioration accompanying an increase in the amount of accumulated selenium. Therefore, according to the present invention, when nitrogen and selenium in waste water are treated, they can be effectively removed without impairing the treatment performance of activated sludge.

  In addition, by adopting the fixed bed method in the present invention, the sludge concentration can be maintained higher than that of the floating activated sludge, and the nitrate nitrogen concentration in the treated water is reduced by the concentration gradient of nitrate nitrogen in the fixed bed. It can be stably achieved below the influence limit concentration (threshold value) of nitrate nitrogen for reducing bacteria. Therefore, it is resistant to fluctuations in the load of nitrogen and selenium in waste water and inhibitors (persulfates, etc.).

  In floating activated sludge, insoluble selenium (single selenium) must be discharged artificially together with surplus sludge. Depending on the nitrogen and selenium load of the wastewater, high concentration accumulation of selenium in the activated sludge may be unavoidable. is there. On the other hand, in the fixed bed of the present invention, the sludge and the insoluble selenium that are naturally separated from the filter bed can be efficiently discharged by washing the filter bed periodically, and as a result, the selenium in the filter bed can be discharged. It is possible to avoid a decrease in processability associated with an increase in the accumulation amount.

Conventionally, when the nitrogen concentration in the wastewater is low (NO ₃ -N is about 50 mg / l or less), the floating activated sludge has a nitrogen concentration of about 100 mg / l or more (NO ₃ -N in order to maintain the sludge concentration and treatment performance. Nitrogen compound (ammonia or nitrate nitrogen) needs to be added. In the present invention, since the fixed bed system is adopted, the sludge concentration is naturally controlled according to the nitrogen concentration of the waste water, and the nitrogen and selenium treatment properties are maintained, so that it is not necessary to add a nitrogen compound artificially. The present invention is particularly effective when the concentration of nitrogen in the wastewater is low.

  Furthermore, in the present invention, the solid matter is immediately separated in the solid-liquid separation step subsequent to the second anaerobic treatment step, so that tetravalent selenium and simple selenium produced by reduction are again hexavalent selenium in the second aerobic treatment step. Selenium treatment performance is maintained well without being oxidized.

  In the present invention, since tetravalent selenium and simple selenium are separated in the solid-liquid separation step in the previous stage and are not contained in the supernatant water, it is possible to prevent re-elution of hexavalent selenium in the second aerobic treatment step. Control of ORP (oxidation-reduction potential) or the like is unnecessary, and the equipment can be simplified.

  With reference to the attached drawings, an embodiment of the biological treatment method of waste water according to the present invention will be described. FIG. 1 is a schematic view showing an example of a biological treatment system for wastewater suitable for carrying out the biological treatment method for wastewater according to the present invention.

  As shown in FIG. 1, this system mainly includes a first anaerobic treatment tank 6 that denitrifies wastewater 1 and a second anaerobic treatment tank 8 that treats treated water in the first anaerobic treatment tank 6 with selenium. It is configured. A first aerobic treatment tank 2 for nitrifying the waste water 1 is provided in the front stage of the first anaerobic treatment tank 6. Further, a second aerobic treatment tank 13 for biologically oxidizing the treated water is provided at the subsequent stage of the second anaerobic treatment tank 8. Furthermore, a solid-liquid separation tank 10 for solid-liquid separation of the treated water in the second anaerobic treatment tank 8 is provided between the second anaerobic treatment tank 8 and the second aerobic treatment tank 13.

  The first anaerobic treatment tank 6 and the second anaerobic treatment tank 8 are each provided with a pipe for supplying backwash water 15 for backwashing the fixed bed in the tank at the bottom of the tank. Pipes for discharging the washing waste water are respectively provided in the tank ceiling.

  According to such a configuration, first, waste water 1 containing nitrogen compound such as flue gas desulfurization waste water and selenium is introduced into the first anaerobic treatment tank 2. Here, examples of the nitrogen compound include ammonia nitrogen and nitrate nitrogen. In the figure, waste water containing ammonia nitrogen is 1a, and waste water containing nitrate nitrogen alone is 1. The wastewater 1a containing ammonia nitrogen is introduced into the first aerobic treatment tank 2. Moreover, the waste water 1 containing only nitrate nitrogen is directly introduced into the first anaerobic treatment tank 6 described later.

The first aerobic treatment tank 2 is provided with a fixed bed as a microorganism adhesion carrier, adjusted to pH 5-9, preferably pH 6.5-7.5 with an alkaline agent 17, and dispersed in the tank. Aeration is performed by supplying air 4 to the air device. Thus, ammoniacal nitrogen in the wastewater 1a in an aerobic state reacts by the action of nitrifying bacteria fixed on the carrier as shown by the following formula and is oxidized to nitrate nitrogen.
^{_{NH 4 + + O 2 → NO}} 2 - + 4H +
^{_{2NO 2 - + O 2 → 2NO}} 3 -

  As the microorganism-adhering carrier for the fixed bed, sand, activated carbon, anthracite (anthracite), plastic flat plate, or the like can be used.

Next, the treatment liquid 3 of the first aerobic treatment tank 2 or the waste water 1 containing nitrate nitrogen is introduced into the first anaerobic treatment tank 6. The first anaerobic treatment tank 6 is provided with a fixed bed of a downward flow (or may be an upward flow) as a microorganism adhesion carrier, and when methanol 5 is added to the first anaerobic treatment tank 6 as a first organic substance. Under anaerobic conditions, nitrate nitrogen in the wastewater reacts and is decomposed by the action of denitrifying bacteria as follows:
^{_{6NO 3 - + 5CH 3 OH →}} 5CO 2 + 3N 2 + 7H 2 O + 6OH -

  As the microorganism-adhering carrier for the fixed bed, in addition to sand, activated carbon, anthracite (anthracite), plastic particles, those fixed in polyvinyl alcohol or polyethylene glycol may be used.

In the fixed bed of the first anaerobic treatment tank 6, the adhering sludge is kept at a high concentration of 10,000 to 20000 mg / l. Therefore, unlike the activated sludge of the floating system, nitric acid is added to the water flow direction in the fixed bed. A concentration gradient of basic nitrogen is generated, and denitrification action according to the concentration of nitrate nitrogen in the wastewater occurs stably. That is, the nitrate nitrogen load supplied to the first anaerobic treatment tank 6 is 0.5 to 5 kg / day per 1 m ^{3 of the} filler volume, and the methanol addition amount is 2 to 3 (weight ratio) with respect to the nitrate nitrogen amount in the waste water. ), The concentration of nitrate nitrogen and nitrite nitrogen in the treated water of the first anaerobic treatment tank 6 is 20 mg / l or less.

Next, acetic acid or acetate 7 is added as a second organic substance to the treatment liquid in the first anaerobic treatment tank 6, and this is introduced into the second anaerobic treatment tank 8. The second anaerobic treatment tank 8 is provided with a fixed bed having a downward flow (or may be an upward flow) as a microorganism-adhering carrier as described above, and nitrate nitrogen and nitrite nitrogen under anaerobic conditions. The concentration is further denitrified, and selenium is reduced by the action of the selenium reducing bacteria, so that hexavalent selenium becomes tetravalent selenium or simple selenium and accumulates in the attached sludge of the second anaerobic treatment tank 8. The reaction is estimated as follows:
SeO ₄ ^2- → SeO ₃ ^2- → Se ⁰

  The second organic substance added to the second anaerobic treatment tank 8 includes, in addition to the above-mentioned acetic acid or acetate 7, ethanol, glycerol, lactic acid and its salt, pyruvic acid and its salt, fumaric acid and its salt, apple One or more selected from acids and salts thereof, succinic acid and salts thereof, glutamic acid and salts thereof, citric acid and salts thereof, glucose, fructose and sucrose can be used.

  If the selenium concentration supplied to the second anaerobic treatment tank 8 is about 5 mg / l or less (as hexavalent selenium), the amount of the second organic substance added to the second anaerobic treatment tank 8 is the amount of selenium in the waste water. The selenium concentration in the treated water 14a of the second anaerobic treatment tank 8 is 0 by setting the ratio to 0.3 to 50 (weight ratio as each organic substance), preferably 2 to 40 (weight ratio as each organic substance). Can be less than 1 mg / l.

  In the first anaerobic treatment tank 6 and the second anaerobic treatment tank 8, since the filtration differential pressure of the fixed bed increases with the growth of the activated sludge adhering to the carrier, the backwash waters 15a and 15b are periodically treated with the first anaerobic treatment. The fixed bed is supplied from the bottom of the fixed bed of the tank 6 and the second anaerobic treatment tank 8 to wash each fixed bed. The frequency of backwashing is once every 1 to 30 days. The discharged backwash effluents 16a and 16b contain insolubilized single selenium as well as exfoliated sludge from the sludge adhering to the carrier. These backwash effluents 16 are the first aerobic treatment tank 2 and will be described later. The mixture is led out of the system so as not to be mixed into the second aerobic treatment tank, separated into solid and liquid, dehydrated as a solid, and disposed of (not shown). As the solid-liquid separation method, a membrane separation method or the like can be used in addition to the coagulation sedimentation treatment.

  The treated water 14 a of the second anaerobic treatment tank 8 is directly introduced into the first aerobic treatment tank 13 through the solid-liquid separation tank 10 or without passing through the solid-liquid separation tank 10. When tetravalent selenium or simple selenium remains in the treated water 14a of the second anaerobic treatment tank, it is preferable to go through the solid-liquid separation tank 10, and when no tetravalent selenium and simple selenium remain, go through the solid-liquid separation tank 10. Without being introduced directly into the second aerobic treatment tank 13.

In the solid-liquid separation tank 10, the flocculant 9 is added to the treated water 14 a, and the pH is adjusted to 5 to 9, preferably 6 to 8 with the alkali agent 17. As the solid-liquid separation tank 10, MF membrane separation can be used in addition to coagulation sedimentation treatment. As the aggregating agent 9, a ferric salt such as ferric chloride or ferric sulfate can be used, and as the alkali agent 17, sodium hydroxide, potassium hydroxide, calcium hydroxide or the like can be used. The tetravalent selenium contained in the treated water 14a of the second anaerobic treatment tank is insolubilized by the ferric salt as shown in the following reaction formula, and the simple selenium is co-precipitated by the ferric hydroxide aggregation reaction.
3SeO ₃ ^2- + 2Fe ³⁺ → Fe ₂ (SeO ₃ ) ₃ ↓

  The insolubilized tetravalent selenium is separated into solid and liquid to form a precipitate 12, which is then dehydrated outside the system (not shown), and selenium is mixed into the first aerobic treatment tank 2 and the second aerobic treatment tank 13. Do not.

  The second aerobic treatment tank 13 is preferably a fixed bed filled with a carrier to which activated sludge is adhered. As the adhesion carrier, sand, activated carbon, anthracite (anthracite), a plastic flat plate, and the like can be used as in the first aerobic treatment tank 2. In order to maintain the aerobic state in the second aerobic treatment tank 13, air 4 is supplied to a diffuser provided in the tank, and organic matter (methanol) remaining in the treated water 14 a of the second anaerobic treatment tank. , Acetic acid or acetate, etc.) is decomposed into carbon dioxide and water by aerobic microorganisms.

  In addition, when tetravalent selenium contained in the treated water 14a of the second anaerobic treatment tank is supplied to the first aerobic treatment tank 13 without being removed by the solid-liquid separation tank 10, a part of the tetravalent selenium is an action of microorganisms. Is re-oxidized to difficult-to-process hexavalent selenium to increase the selenium concentration in the treated water 14 and deteriorate the selenium removal efficiency.

  Using the apparatus shown in FIG. 1 as an example of the present invention and the apparatus shown in FIG. 2 as a comparative example, a test for treating wastewater was performed according to the following procedure.

As waste water containing nitrogen compounds and selenium, flue gas desulfurization waste water at a coal-fired thermal power plant was used. The properties of this drainage are as follows.
Ammonia nitrogen: 0.7 mg / l
Nitrate nitrogen: 40 mg / l
Hexavalent selenium: 0.3 mg / l

  Using the waste water, each was treated under the treatment conditions of Example 1 and Comparative Example below. In addition, since nitrogen in waste water was mostly nitrate nitrogen in both Example 1 and Comparative Example, the first aerobic treatment (nitrification treatment) was not carried out, and the treatment was performed directly in the following treatment steps.

(Processing conditions of Example 1)
Wastewater supply amount: 32 l / day Contact time of the first anaerobic treatment process: 0.7 hours Methanol addition amount: 100 mg / l
Contact time of the second anaerobic treatment step: 0.4 hours Acetic acid addition amount: 5 mg / l
Residence time of the solid-liquid separation process: 1 day Contact time of the second aerobic treatment process: 0.5 days Frequency of backwashing of the second anaerobic treatment process: once every 5 days

(Processing conditions of the comparative example)
Wastewater supply amount: 2 l / day Residence time of anaerobic treatment process: 1 day MLSS: about 4000 mg / l
Methanol addition amount: 100 mg / l
Nitrate addition amount: 60 mg / l (as N)
Residence time for aerobic treatment process: 0.8 days Residence time for solid-liquid separation process: 1.5 days

  In Example 1, about 1000 ppm of sewage sludge was inoculated for each step, and habituation was started under the above treatment conditions. The period (acclimation period) until the selenium treatment ability was exhibited in the second anaerobic treatment step was within about one week, and the selenium concentration in the treated water 14 at that time was 0.06 mg / l.

  In the comparative example, as a result of inoculating 2000 to 3000 ppm of sewage sludge in an anaerobic treatment tank and starting acclimatization under the treatment conditions, it took about one month for the selenium treatment ability to be exhibited, and the treated water 14b at that time The selenium concentration varied from 0.07 to 0.15 mg / l.

  The properties of the treated water obtained about one week after completion of the adaptation in Example 1 and Comparative Example were as follows.

  From the above results, it was verified that by adopting the present invention, the acclimation period required for exhibiting the selenium processing capability can be greatly shortened, and labor can be saved. In the present invention, it was verified that the quality of the treated water after completion of the acclimatization was stable.

  In place of acetic acid added in the second anaerobic treatment step of Example 1, ethanol, glycerol, lactic acid, pyruvic acid, fumaric acid, malic acid, succinic acid, glutamic acid, citric acid, glucose, fructose, sucrose were added A test was conducted under the same drainage properties and treatment conditions as in Example 1, and the selenium concentration of the treated water was measured. The results are shown in Table 2.

  As shown in Table 2, it was verified that all the organic substances can achieve the total selenium concentration of the treated water of 0.1 mg / l or less, and are effective in the selenium reduction treatment.

It is a flowchart which shows one Embodiment of the biological treatment method of the waste_water | drain based on this invention. It is a flowchart which shows the biological treatment method of the conventional waste_water | drain.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 1a Waste water 2 1st aerobic treatment tank 3 1st aerobic treatment tank treated water 4 Air 5 Methanol 6 1st anaerobic treatment tank 7 Acetic acid or acetate 8 2nd anaerobic treatment tank 9 Flocculant 10 Solid-liquid separation tank 11 Separated treated water 12 Precipitate 13 Second aerobic treatment tank 14, 14a, 14b Treated water 15 Backwash water 16 Backwash drainage 17 Alkaline agent 20 Anaerobic treatment tank 21 Organic matter (methanol)
22 Nitric acid (nitrous acid) salt 23 Stirrer 24 Aerobic treatment tank 25 Air diffuser 26 Solid-liquid separation tank 27 Return sludge 28 Excess sludge

Claims (7)

  A first anaerobic treatment step in which wastewater is brought into contact with activated sludge adhering to the carrier in the anaerobic state in the presence of the first organic matter, and treated water in the first anaerobic treatment step in the presence of the second organic matter. A biological treatment method for wastewater, comprising a second anaerobic treatment step in which the activated sludge adhered to the carrier in an anaerobic state is brought into contact.   The second organic substance is acetic acid and acetate, ethanol, glycerol, lactic acid and salt thereof, pyruvic acid and salt thereof, fumaric acid and salt thereof, malic acid and salt thereof, succinic acid and salt thereof, glutamic acid and salt thereof, The biological treatment method of waste water according to claim 1, wherein the biological treatment method is one or more selected from citric acid and salts thereof, glucose, fructose and sucrose.   The wastewater biological treatment method according to claim 1 or 2, further comprising a solid-liquid separation step of solid-liquid separation of the treated water in the second anaerobic treatment step.   The first aerobic treatment step of bringing the waste water into contact with the activated sludge adhering to the carrier in an aerobic state before introducing the waste water into the first anaerobic treatment step. A biological treatment method of waste water according to item 1.   A second aerobic treatment step in which the treated water in the second anaerobic treatment step or the separated treated water in the solid-liquid separation step is brought into contact with activated sludge attached to the carrier in an aerobic state. Item 5. The biological treatment method for wastewater according to any one of Items 1 to 4.   The washing waste water discharged from the first anaerobic treatment step and the second anaerobic treatment step, and the precipitate separated in the solid-liquid separation step, the first aerobic treatment step and the second aerobic treatment step. The biological treatment method of waste water according to any one of claims 1 to 5, further comprising a step of discharging it outside the system without being mixed into the system.   7. The method according to claim 3, further comprising a step of discharging the precipitate separated in the solid-liquid separation step out of the system without being mixed in the first aerobic treatment step and the second aerobic treatment step. A biological treatment method of waste water according to item 1.

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