JP2014128753A - Method for processing wastewater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for processing wastewater, which effectively inhibits the odor of hydrogen sulfide and methyl mercaptan generated from the sludge formed in processing of wastewater.SOLUTION: The method includes the steps of: separating preliminary sedimented sludge from wastewater by sedimentation in a preliminary sedimentation basin 12; separating the surplus sludge from the wastewater by biological processing after separation of the preliminary sedimented sludge; concentrating the preliminary sedimented sludge and the surplus sludge; mixing the preliminary sedimented sludge with the surplus sludge; and dewatering the mixed preliminary sedimented sludge and the surplus sludge. In a stage before the step of separating preliminary sedimented sludge from wastewater by sedimentation in the preliminary sedimentation basin 12, a metal salt is added to the wastewater. Also, in a stage after separation of the preliminary sedimented sludge and the surplus sludge from the wastewater, a nitrite salt is added to the preliminary sedimented sludge and/or the surplus sludge.

Description

  The present invention relates to a method for treating sewage, and in particular, effectively treats odors of hydrogen sulfide and methyl mercaptan, which are malodorous substances generated from sludge (including dehydrated cake) discharged in the process of sewage treatment at a sewage treatment plant or the like. The present invention relates to a sludge odor control method that can be easily controlled.

  The sewage treatment in a sewage treatment plant or the like includes a water treatment process for separating and removing substances that become dirty from the sewage to finally obtain clean water, and a sludge treatment process for treating sludge generated from the remaining residue.

  In the sludge treatment process, sludge is spoiled and so on, and thus malodorous substances such as hydrogen sulfide and sulfur compounds such as methyl mercaptan, ammonia, nitrogen compounds such as trimethylamine, lower fatty acids and the like are generated. Among these malodorous substances, hydrogen sulfide and methyl mercaptan, which are particularly generated in large amounts, are the cause of deterioration of the working environment around the sludge treatment system, complaints of nearby residents, and corrosion of equipment. Therefore, various odor control measures have been taken conventionally.

  That is, as a conventional countermeasure against odor, a method for suppressing odor using nitrite which is an oxidizing agent and a method for suppressing odor using metal salts such as zinc, copper, iron and the like are known. In particular, a method for suppressing odor using nitrite is typical, and is widely used because of its good handling (see, for example, JP-A-2001-340895).

JP 2001-340895 A

  However, in the case of the method of suppressing odor using nitrite which is an oxidizing agent, nitrite is acidic and destabilized, so the effect is stabilized when the pH of the target sludge is low, but the pH of the sludge is 6 or more. As a result, the duration of efficacy is significantly shortened. In addition, nitrite is decomposed by microorganisms under anaerobic conditions. Therefore, when used in sludge containing a large amount of microorganisms, such as spoiled sludge and excess concentrated sludge generated after microbial treatment of sewage, it is effective due to the decomposition action of microorganisms. Is significantly reduced.

In general, in order to avoid the above problem, a method has been employed in which the amount of nitrite added is increased or an antibacterial agent is used in combination to maintain the nitrite concentration and stabilize the effect.
However, if only the antibacterial agent is used to stop the growth of bacteria, a large amount of the antibacterial agent needs to be introduced, resulting in an increase in cost. In addition, when a large amount of nitrite is added, the microorganisms become acclimatized, so it is necessary to increase the addition amount over time.
For the above reasons, it has been difficult to suppress odors using nitrite in a sludge treatment system containing a large amount of sludge with high pH and microorganisms.

  In addition, in the case of a method for suppressing sludge odor using a metal salt such as zinc, copper, iron, etc., these metals react specifically with hydrogen sulfide and thus have a high odor control effect. Almost no deodorizing effect was obtained with respect to the substance methyl mercaptan.

  The present invention has been devised in view of the above-mentioned conventional problems, and the object of the present invention is the odor of hydrogen sulfide and methyl mercaptan, which are malodorous substances generated from sludge generated during the treatment of sewage. It is in realizing the sludge odor control method that can effectively suppress the odor.

In order to achieve the above object, a method for treating sewage according to claim 1 of the present invention comprises:
In the first settling basin, the first settling sludge is separated from the sewage, the first sewage is separated from the sludge by biological treatment, and the first settling sludge is concentrated and the excess sludge is separated. A method for treating sewage, comprising a step of concentrating, a step of mixing the first settling sludge and excess sludge after concentration, and a step of dehydrating the mixed first settling sludge and excess sludge,
In the stage prior to the step of precipitating and separating the first settling sludge from the sewage in the first settling pond, a metal salt is added to the sewage,
Nitrite is added to the primary sedimentation sludge and / or surplus sludge in a stage after separating the primary sedimentation sludge and surplus sludge from the sewage.

The wastewater treatment method according to claim 2 of the present invention is the wastewater treatment method according to claim 1,
The metal salt is any one of iron, zinc, calcium, and magnesium.

The method for treating sewage according to claim 3 of the present invention is the method for treating sewage according to claim 2,
The metal salt is an iron salt, and the iron salt added to the sewage is 0.2 to 150 mg / l in terms of iron ion weight in the iron salt.

The method for treating sewage according to claim 4 of the present invention is the method for treating sewage according to claim 2 or 3,
The metal salt is polyferric sulfate.

In the method for treating sewage of the present invention, the metal salt is added to the sewage before the step of precipitating and separating the first settling sludge from the sewage in the first settling basin, and after the first settling sludge and excess sludge are separated from the sewage. In this stage, by adding nitrite to the initial settling sludge and / or surplus sludge, the following effects can be obtained.
(1) Since the metal salt has the effect of promoting the sedimentation of sludge, the metal salt is added to the sewage before the step of precipitating and separating the primary sludge from the sewage, so that the primary sedimentation sludge in the first sedimentation basin. It is possible to reduce the time for sedimentation of the sewage, and it is possible to suppress the decay of the initial sewage sludge because the residence time of the sludge in the first sedimentation basin is short. As a result, the nitrite is prevented from being decomposed and consumed by the microorganisms in the spoiled primary sedimentation sludge in which the microorganisms are mass-produced, so that the deodorizing effect of the nitrite is sufficiently exerted, and the hydrogen sulfide by the metal salt The odor of hydrogen sulfide and methyl mercaptan generated from sludge can be effectively suppressed in combination with the effect of suppressing odor.
(2) The concentrated primary sludge has a pH of about 5 and the concentrated excess sludge has a pH of about 7, but a metal salt is added to the sewage before the step of precipitating and separating the primary sediment from the sewage. Thus, the sedimentation of the primary sedimentation sludge in the primary sedimentation basin is promoted, and the primary sedimentation sludge has a larger ratio when the primary sedimentation sludge after the concentration and the excess sludge are mixed. For this reason, since the pH rise of the mixed sludge of the primary sedimentation sludge and excess sludge after concentration is suppressed, it can prevent that the deodorizing effect of nitrite is inhibited. In addition, since the ratio of the primary sludge after concentration and excess sludge is mixed is larger in the primary sludge, the amount of microorganisms in the mixed sludge can be reduced, and the metal salt increases the activity of microorganisms. As a result, the nitrite is prevented from being decomposed and consumed by microorganisms. As a result, the odor of hydrogen sulfide and methyl mercaptan generated from sludge is effective in combination with the odor suppression effect of hydrogen sulfide by metal salts. Can be suppressed.

Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a process diagram showing an embodiment of a sewage treatment method according to the present invention in a sewage treatment plant.
This sewage treatment plant consists of a sedimentation basin 10, a first sedimentation basin 12, a biological tank 14 for performing a water treatment process, a final sedimentation site 15 and an advanced treatment tank 16, a first sedimentation sludge concentration tank 18 for performing a sludge treatment process, and an excess sludge concentration section. 20, a concentrated sludge storage tank 22, a dehydrator 24, and a hopper 26.

The treatment of sewage at the sewage treatment plant is performed in the following steps.
Sewage (sewage) that flows into the sewage treatment plant through the pump station / sewage pipe 28 is first introduced into the sand basin 10 and filtered through a screen (not shown) to remove large dust and sand. The
The sewage from which large debris, sand, etc. have been removed is then introduced into the first sedimentation basin 12 and settled at the bottom of the first sedimentation basin 12 as small debris, mud, etc. sink during the storage process for several hours. Sludge is precipitated and separated.

  The sewage from which the first settling sludge has been separated in the first settling basin 12 is introduced into the biological tank 14 and a water treatment process is performed. That is, in the biological tank 14, sewage is sent to the final sedimentation site 15 after biological treatment such as decomposition of organic matter and inorganic matter in sewage by microorganisms is performed in an aerated atmosphere. The sewage introduced into the final sedimentation site 15 is solid-liquid separated in the process of storing for several hours, and the solid components are precipitated and separated as excess sludge at the bottom of the final sedimentation site 15. As a result of biological treatment with microorganisms in the biological tank 14, the excess sludge contains a large amount of microorganisms.

  The sewage from which excess sludge has been separated in the final sedimentation site 15 is sent to an advanced treatment tank 16, where eutrophication substances such as nitrogen and phosphorus are removed and purified in the advanced treatment tank 16, and then the river, etc. They are released into public waters.

On the other hand, a sludge treatment process is performed in the following procedures.
First, the first settling sludge that has settled at the bottom of the settling basin 12 is drawn out and introduced into the first settling sludge concentration tank 18 to perform gravity concentration.
Further, surplus sludge precipitated at the bottom of the final sedimentation site 15 is drawn out and introduced into the surplus sludge concentrating unit 20, and mechanical sludge using a centrifugal separator, gravity concentration, or the like is performed to concentrate the surplus sludge.

Next, the primary sedimentation sludge concentrated by gravity in the primary sedimentation sludge concentration tank 18 and the excess concentrated sludge concentrated in the excess sludge concentration unit 20 are mixed and sent to the concentrated sludge storage tank 22 for temporary storage. .
Thereafter, the mixed primary sludge and excess concentrated sludge are sent to the dehydrator 24 and dehydrated to form a dehydrated cake that is solid sludge. The dehydrated cake is transferred to the hopper 26 via a belt conveyor (not shown) and stored, and then transported to the outside to be incinerated or landfilled.

  In the method for treating sewage according to the present invention, a metal salt is added to sewage before the step of settling and separating the first settling sludge from the sewage in the first settling basin 12; 15, nitrite is added to the primary sedimentation sludge and / or surplus sludge at a stage after separating the primary sedimentation sludge and surplus sludge from the sewage.

  Accordingly, the metal salt is added to, for example, sewage initially introduced into the settling basin 12, sewage introduced into the settling basin 10, and sewage flowing into the pump station / sewage pipe. Moreover, you may add in piping (not shown) through which sewage flows.

  Examples of the metal salt include iron salts such as polyferric sulfate, iron (II) chloride, iron sulfate and iron nitrate, zinc salts such as zinc chloride, zinc sulfate and zinc acetate, calcium such as calcium chloride and calcium nitrate. Magnesium salts such as salts, magnesium hydroxide, magnesium chloride, magnesium oxide, and magnesium hydroxide are applicable, and iron salts are preferable, and among these, ferric polysulfate is most preferable.

  In other words, while polyvalent metal ions generally have the ability to aggregate sludge, there is a problem of environmental pollution due to toxicity, but iron ions are relatively toxic and are present in large quantities in the environment, so the environmental burden is small. It is preferable to use an iron salt. Among these, ferric sulfate is optimal because it has the lowest corrosiveness compared to other iron salts such as iron chloride and iron sulfate, and hardly causes corrosion of sewage treatment facilities.

  In addition, in the case of calcium salt and magnesium salt, it is easy to cause damage to the sewage treatment equipment by generating piping scales, but since the environmental load is relatively small, it should be used suitably if appropriate measures are taken to prevent the generation of piping scales. Can do.

When the metal salt is an iron salt, the iron ions in the iron salt combine with water-soluble sulfur ions and precipitate as insoluble iron sulfide ions, but the iron salt added to the sewage is an iron salt. It is preferable to set the weight of iron ions in the range of 0.2 to 150 mg / l.
That is, since the dissolved sulfide ion concentration in the inflowing sewage is generally in the range of 1 to 7.5 mg / l, the equimolar reaction between the iron ions in the iron salt and the sulfur ions in the dissolved sulfide. In this case, the required iron ion is 2 to 15 mg / l.
However, in the case of a combined sewage pipe dredger, rainwater may join and the amount of water may be about 10 times the normal amount. In this case, the concentration of dissolved sulfide ions in the sewage is 0.1 / 10 of the normal 1/10. -0.75 mg / l. On the other hand, a large amount of sulfide ions may flow into the sewer pipe due to storm surges and the like, and the concentration of sulfide ions may be 10 to 75 mg / l, which is 10 times the normal concentration. Therefore, in consideration of such environmental changes, as described above, the iron salt added to the sewage is preferably set in the range of 0.2 to 150 mg / l in terms of the iron ion weight in the iron salt.

When the metal salt is a zinc salt, the zinc salt added to the sewage is preferably set in the range of 0.2 to 180 mg / l in terms of the weight of zinc ions in the zinc salt. That is, in the case of general sewage with a dissolved sulfide ion concentration of 1 to 7.5 mg / l, the zinc ion required for the equimolar reaction is 2 to 18 mg / l. This is because it becomes 0.2 to 180 mg / l.
Furthermore, when the metal salt is a calcium salt, the calcium salt added to the sewage is preferably set in the range of 0.1 to 100 mg / l in terms of the weight of calcium ions in the calcium salt. That is, in the case of general wastewater having a dissolved sulfide ion concentration of 1 to 7.5 mg / l, the calcium ion required for the equimolar reaction is 1 to 10 mg / l. This is because it becomes 0.1 to 100 mg / l.
Furthermore, when the metal salt is a magnesium salt, the magnesium salt added to the sewage is preferably set in the range of 0.05 to 60 mg / l in terms of the weight of magnesium ions in the magnesium salt. That is, in the case of general wastewater having a dissolved sulfide ion concentration of 1 to 7.5 mg / l, the magnesium ion required for the equimolar reaction is 0.5 to 6 mg / l. This is because 0.05 to 60 mg / l is added.

On the other hand, nitrite is added at a stage after the initial sludge and surplus sludge are separated in the first sedimentation basin 12 and the final sedimentation site 15, and therefore, for example, in FIG. Is added to the initial settling sludge introduced into the excess sludge, the excess sludge introduced into the excess sludge concentration unit 20, the initial settling sludge introduced into the concentrated sludge storage tank 22, and the excess concentrated sludge.
In addition, depending on the properties of the sludge, the sludge may not rot for a while after the concentration due to the effect of adding the metal salt. In such a case, the dehydrated cake is a solid sludge after the dehydration process. The nitrite solution may be added by spraying. Thus, when adding nitrite to a dewatered cake that is solid sludge after dehydration, the unreacted portion of nitrite ions is washed away, unlike when added to sludge containing a large amount of water. And nitrite can be used without waste.

  Sodium nitrite is preferred as the nitrite added to the sludge. That is, sodium nitrite is not very strong in oxidizing power, so the reaction is slow, it is difficult to emit harmful gases such as chlorine gas, and it does not react explosively like hydrogen peroxide. This is because the feeling is good.

  If measures are taken to prevent generation of harmful gases and control the reaction rate, hydrogen peroxide, nitrate, hypochlorite, chlorite and chlorinated isocyanuric acid can be substituted for nitrite. .

In the method for treating sewage according to the present invention, metal salt is added to the sewage before the step of settling and separating the first settling sludge from the sewage in the first settling basin 12, and in the first settling basin 12 and the final settling site 15 described above. In the stage after separating the primary sedimentation sludge and the excess sludge from the sewage, the following effects can be achieved by adding nitrite to the primary sedimentation sludge and / or the excess sludge.
(1) Since the metal salt has an action of promoting the sedimentation of sludge, the metal salt is added to the sewage before the step of precipitating and separating the primary sludge from the sewage. The time for sedimentation and separation of the sludge can be shortened, and the sludge residence time in the first sedimentation basin 12 can be shortened, so that the decay of the initial sedimentation sludge can be suppressed. As a result, the nitrite is prevented from being decomposed and consumed by the microorganisms in the spoiled primary sedimentation sludge in which the microorganisms are mass-produced, so that the deodorizing effect of the nitrite is sufficiently exerted, and the hydrogen sulfide by the metal salt The odor of hydrogen sulfide and methyl mercaptan generated from sludge can be effectively suppressed in combination with the effect of suppressing odor.
(2) Although the pH of the primary sedimentation sludge is about 5 and the pH of the excess concentrated sludge is about 7, by adding a metal salt to the sewage before the step of separating the primary sedimentation sludge from the sewage Since the sedimentation of the primary sedimentation sludge in the primary sedimentation basin 12 is promoted, the primary sedimentation sludge has a higher ratio when the primary sedimentation sludge and the excess concentrated sludge are mixed. For this reason, since the pH rise of the sludge mixed with the primary sedimentation sludge and the excess concentrated sludge is suppressed, it can be prevented that the deodorizing effect of nitrite is inhibited. In addition, since the ratio of primary concentrated sludge and excess concentrated sludge is larger in primary concentrated sludge, the amount of microorganisms in the mixed sludge can be reduced, and the metal salt increases the activity of microorganisms. As a result, the nitrite is prevented from being decomposed and consumed by microorganisms. As a result, the odor of hydrogen sulfide and methyl mercaptan generated from sludge is effective in combination with the odor suppression effect of hydrogen sulfide by metal salts. Can be suppressed.

In addition, when adding nitrite to primary sedimentation sludge and / or surplus sludge, you may add an antibacterial agent simultaneously.
As this antibacterial agent, sodium pyridine thiol-1-oxide (hereinafter referred to as NaPT), zinc bis (2-pyridylthio-1 oxide), 1,2-benzisothiazolin-3-one, 2-methyl-4-isothiazoline-3- ON, methyl-2-benzimidazole carbamate, 2- (4-thiazolyl) -benzimidazole, 2-bromo-2-nitro-1,3-propanediol, and the like, but NaPT has good dispersibility and handling. Particularly preferred.

  Thus, by using an antibacterial agent together with the addition of nitrite to the initial settling sludge and / or surplus sludge, it is possible to suppress the proliferation and activity of microorganisms and to prevent the consumption of nitrite due to the decomposition action of microorganisms. Therefore, the odor suppression effect by nitrite can be maintained.

  The present invention will be further described below with reference to examples, but the present invention is not limited thereto.

[Example 1]
(Test method)
After adding 10 mg / l of the ferric sulfate solution in terms of the iron ion weight in the ferric sulfate solution to the sewage flowing into the sewage treatment plant, the solution is first introduced into the settling basin 12, And the sludge treatment process. Sludge slurry was sampled in a 2 L beaker from the concentrated sludge storage tank 22 in which the primary sedimentation sludge and excess concentrated sludge were mixed, and a deodorizer containing nitrite was added to the collected sludge slurry and stirred.
The concentration of hydrogen sulfide and methyl mercaptan in the sludge slurry before dehydration after stirring for 30 minutes was measured using a detector tube manufactured by Gastec.
Thereafter, the sludge slurry is dehydrated using a small belt press dehydrator for testing, and 100 g of each dehydrated cake obtained by dehydration is sealed in a polyethylene container and retained in a thermostatic bath at 30 ° C. The concentration of hydrogen sulfide and methyl mercaptan in the dehydrated cake after dehydration 24 hours and after dehydration 48 hours was measured.
As a comparative example, the concentration of hydrogen sulfide and methyl mercaptan was measured for the inflowing sewage to which no polyferric sulfate solution was added by the same method as described above.

Table 1 shows the test results for measuring the concentration of hydrogen sulfide.

Table 2 shows the test results for measuring the concentration of methyl mercaptan.

As shown in Tables 1 and 2, the following test results were obtained.
(1) In the test section where the metal ferric sulfate solution, which is a metal salt, was not added to the influent sewage, both hydrogen sulfide and methyl mercaptan odors were generated, whereas the polyferric sulfate solution was In the added test section, the amount of hydrogen sulfide generated was remarkably small. Therefore, it can be seen that the effect of suppressing the odor of hydrogen sulfide is obtained by adding the polyferric sulfate solution, which is a metal salt.
(2) The ferric sulfate solution, which is a metal salt, is added to the influent sewage, but the metal salt has almost no deodorizing effect on methyl mercaptan in the test section where no nitrite deodorant is added. The amount of methyl mercaptan generated is large, and the effect decreases when the residence time after making a dehydrated cake becomes long.
(3) In the test section where the polyferric sulfate solution, which is a metal salt, is not added to the inflowing sewage, but only a nitrite deodorant is added, the deodorization effect before dehydration is high, but the effect is lost after making a dehydrated cake. It was. This is thought to be because microorganisms propagated over time, and nitrite was consumed by the decomposition action of microorganisms.
(4) On the other hand, in the test section in which the ferric sulfate solution, which is a metal salt, was added to the inflowing sewage and the nitrite deodorant was added to the sludge slurry, hydrogen sulfide and methyl were used for a long time. Odor generation of both mercaptans could be suppressed.

[Example 2]
(Test method)
The present Example 2 is a test in the case of using a nitrite deodorant sprayed on a dehydrated cake which is sludge after dehydration.
After adding 10 mg / l of the ferric sulfate solution in terms of the iron ion weight in the ferric sulfate solution to the sewage flowing into the sewage treatment plant, the solution is first introduced into the settling basin 12, And the sludge treatment process. The sludge slurry is collected in a 2L beaker from the concentrated sludge storage tank 22 in which the primary sedimentation sludge and surplus concentrated sludge are mixed, and the concentration of hydrogen sulfide and methyl mercaptan generated from the collected sludge slurry is measured using a gastec detector tube. Measured.
Thereafter, the sludge slurry is dehydrated using a small belt press dehydrator for testing, and after spraying a nitrite-based deodorant on the dehydrated cake obtained by dehydration, 100 g of each test section is sealed in a polyethylene container. The dehydrated cake was dehydrated 24 hours later and dehydrated 48 hours later, and the concentrations of hydrogen sulfide and methyl mercaptan were measured.
As a comparative example, the concentration of hydrogen sulfide and methyl mercaptan was measured for the inflowing sewage to which no polyferric sulfate solution was added by the same method as described above.

Table 3 shows the test results of the hydrogen sulfide concentration measurement.

Table 4 shows the test results for measuring the concentration of methyl mercaptan.

As shown in Tables 3 and 4, both hydrogen sulfide and methyl mercaptantan were generated in the test section where the nitrite-based deodorant was not added, regardless of whether or not the polyferric sulfate solution was added to the inflowing sewage. Further, in the test section where only the nitrite deodorant was added without adding the polyferric sulfate solution, the generation amounts of hydrogen sulfide and methyl mercaptantan increased with the passage of time.
In contrast, in the test section where the ferric sulfate solution, which is a metal salt, was added to the influent sewage, which was the method of the present invention, and the nitrite deodorant was sprayed on the dehydrated cake, both hydrogen sulfide and methyl mercaptan were used. Odor generation could be suppressed for a long time.

It is process drawing which shows one Example of the processing method of the sewage which concerns on this invention.

10 Sand basin
12 First sedimentation basin
14 Biological tank
15 Final sedimentation basin
16 Advanced treatment tank
18 First settling sludge concentration tank
20 Excess sludge concentration section
22 Concentrated sludge storage tank
24 Dehydrator
26 Hopper
28 Pumping stations and sewer pipes

Claims (4)

In the first settling basin, the first settling sludge is separated from the sewage, the first sewage is separated from the sludge by biological treatment, and the first settling sludge is concentrated and the excess sludge is separated. A method for treating sewage, comprising a step of concentrating, a step of mixing the first settling sludge and excess sludge after concentration, and a step of dehydrating the mixed first settling sludge and excess sludge,
In the stage prior to the step of precipitating and separating the first settling sludge from the sewage in the first settling pond, a metal salt is added to the sewage,
A method for treating sewage characterized by adding nitrite to the primary sedimentation sludge and / or surplus sludge in a stage after separating the primary sedimentation sludge and surplus sludge from the sewage.   The method for treating sewage according to claim 1, wherein the metal salt is a metal salt of iron, zinc, calcium, or magnesium.   The method for treating sewage according to claim 2, wherein the metal salt is an iron salt and the iron salt added to the sewage is 0.2 to 150 mg / l in terms of the weight of iron ions in the iron salt. . The method for treating sewage according to claim 2 or 3, wherein the metal salt is ferric sulfate.

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