JPS598440B2 - Wastewater denitrification treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a biological denitrification treatment method for wastewater.

In the biological denitrification reaction, in the first step, ammonia nitrogen (NH4+-N) in wastewater is extracted from nitrite bacteria (the representative species being Nitrosomonas).
s)}, Nitrate bacteria {Representative species include 21. Robacter (
It is oxidized to nitrite nitrogen (NO, -1N) and nitrate nitrogen (N to -1N) by so-called nitrifying bacteria such as Nitrobacter (Nitrobacter).

Next, in the second step, nitrite nitrogen or nitrate nitrogen is removed by so-called denitrifying bacteria (representative species include Pseudomonas denitrificans).
trificans)} to reduce it to nitrogen gas (N2), and then release this nitrogen gas into the atmosphere to perform denitrification treatment of wastewater.

The nitrifying bacteria in the first stage belong to so-called autotrophic bacteria, and even if there is no organic nutrient source in the culture solution, dissolved NH4+
If 1N and carbon gas (CO2) are supplied, it can grow under aerobic conditions, and its biological reaction is as follows.

(Reaction caused by Nitrosomonas) NH store +1.5 0 2 → NO summer + H20 + 2 ■ 1 +
...(1) (Reaction with Nitrobacter) No; +0.50→N03- ...
(2)(L), From formula (2), NI−{++20 →NO +H O+2H+
...(3) 4 2 3
2 In contrast, the denitrifying bacteria in the second stage belong to facultative anaerobes and can grow under both aerobic and anaerobic conditions, but denitrification is only possible under anaerobic conditions. .

Furthermore, this denitrifying bacterium is a heterotrophic bacterium, and the denitrifying reaction requires organic matter (organic carbon source) as a hydrogen (H) supplier.

The biological reaction is as follows.

(Biological reduction of nitrite nitrogen) 2NO, - + 6H- + N2↑+2H20+20H-
...(4) (Biological reduction of nitrate nitrogen) 2NO! +10H-+N2↑+4H20+20H1...
(5) As an organic carbon source in this denitrification step, methanol, acetic acid, etc. can be used. Since it is necessary to add an equivalent amount of input, denitrification has the drawback of requiring a large amount of running cost.

For example, when methanol is used as an organic carbon source, when denitrifying ammonia nitrogen (NH4-N) after nitrification, l
For K9's ammonia nitrogen, 3-4 kg of methanol is consumed. If this is applied to a sewage treatment plant that processes 100,000 tons/day of sewage containing 25-/t ammonia nitrogen, the nitrogen compounds can be reduced. It takes 1 methanol to remove
0 tons/day is also required.

Furthermore, the organic carbon source added in this way is used not only for the reduction of nitrite nitrogen and nitrate nitrogen, but also as a nutrient source for the growth of denitrifying bacteria, resulting in an increase in the amount of surplus sludge. There are also some drawbacks.

In order to reduce the cost of organic carbon sources for denitrification and reduce the amount of surplus sludge, there is a method of using denitrification liquid or methane in the digestion treatment of surplus sludge as an organic carbon source, or a method of using sludge heat treatment separated liquid. There are suggestions on how to use it.

However, the organic substances contained in the sludge digestion desorbed liquid or the sludge heat treatment separated liquid are polymeric and are not necessarily of the type that denitrifying bacteria can easily utilize, and the methane generated during sludge digestion treatment is also low. Although it is a molecule, it is difficult for denitrifying bacteria to utilize it, and denitrification treatment using these organic carbon sources has the disadvantage that the denitrification tank becomes large because the denitrification rate is low.

In view of these points, the inventor first produced organic acids by bringing organic sludge obtained by solid-liquid separation of wastewater into contact with acidic bacteria in an anaerobic atmosphere with a pH of 6.8 or less. Afterwards, we proposed a denitrification treatment method characterized by separating this into solid and liquid in a sludge thickening tank and using the organic acid in this separated liquid as an organic carbon source for denitrification.

In this method, the sludge mixture in which organic acids have been generated is passed directly to a sludge thickening tank for solid-liquid separation, and the separated liquid is used as an organic carbon source, so there is no need to administer methanol, acetic acid, etc., and running costs are reduced. When Kokichi is produced, the organic carbon source obtained is a low molecular weight material that is easily utilized by living organisms and has high denitrification performance.

However, this method has several problems.

In other words, the sludge that enters the acid production process is the sludge drawn from the thickening tank, and has a high concentration of around 20,000 ml, so even if the sludge volume decreases by about 25% during the acid production process, the The sludge concentration is high, around 15,000 pI)m, and the sludge has poor settling properties.

Therefore, in order to separate the sludge mixture after acid generation treatment into solid and liquid by gravity concentration, it is necessary to increase the size of the thickening tank, and to perform solid-liquid separation by pressurized flotation concentration or centrifugal concentration, equipment costs and operating costs are required. (Electricity costs) will increase.

Moreover, after performing these concentration processes, the TS15,000pl)
Even if II1 sludge is concentrated to twice the concentration to TS30,000ppIn, only 1/2 of the volume of the original sludge mixture can be recovered from the separated liquid, and nearly 1/2 of the organic acids produced are on the sludge side. It cannot be used effectively as an organic carbon source for denitrification.

Furthermore, when dehydrating concentrated sludge and refining it using nitric lime {Ca(OH)2), p
The standard for operation is to raise H to 10.5 or more, but the concentration of organic acids in the thickened sludge treated with the above acid generation is high, and the p
Due to the low H, a considerable amount of slaked lime is consumed for neutralization, resulting in an increase in the amount of slaked lime added.

As mentioned above, the method proposed earlier is an excellent method, but
Solid-liquid separation requires large equipment and high running costs, and the organic acids in the sludge cannot be recovered sufficiently.
There were problems such as the need for a large amount of chemicals to neutralize the solid-liquid separated sludge.

The present invention has been made in view of the above circumstances, and its purpose is to effectively separate organic acids from sludge by washing organic acids decomposed and produced by acid-producing bacteria, and to use this as an organic carbon source. The present invention aims to provide a method for denitrifying sewage that can be used to denitrify wastewater with high efficiency, reduce chemical costs, and reduce the amount of surplus sludge generated.

That is, in a wastewater denitrification treatment method in which wastewater that has undergone a BOD removal step, a nitrification step, and an organic carbon source are introduced into a denitrification tank and denitrified by the action of microorganisms, the BOD removal step,
The organic sludge obtained by solid-liquid separation of wastewater generated in the nitrification and denitrification processes is brought into contact with acid-producing bacteria in an anaerobic atmosphere with a pH of 6.8 or less to produce organic acids, which are then subjected to primary treatment. Water, secondary treated water, nitrification treated water, denitrification treated water, tap water, ground water, etc. are added at a ratio of 1 to 5 parts by weight to 1 part by weight of sludge for cleaning, and the cleaning solution is used as it is or after ammonia removal treatment. This method of denitrifying wastewater is characterized in that the organic acid contained herein is used as an organic carbon source for the denitrifying treatment.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a flowchart showing one embodiment of the method of the present invention. In this sewage treatment, firstly, water to be treated such as sewage 1 is put into a sedimentation tank 2, sludge 3 is separated by sedimentation, and sludge (or (Storage tank) 4.

In addition, the separated liquid is placed in an aeration tank 5 and brought into contact with air 6 (!l) to perform sewage treatment using aerobic bacteria.The separated liquid is then placed in a settling tank 7 for solid-liquid separation, and a portion 8 of the separated sludge is It is returned to the aeration tank 5 and the remaining 9 is put into the thickener (or storage tank) 4.

The separated sludge enters a digestion tank 10, where it is brought into contact with air 11 and subjected to a digestion process using nitrifying bacteria to produce nitric acid or nitrous acid from nitrogen compounds.

Next, the sludge is placed in a settling tank 12 for solid-liquid separation, and a portion 13 of the separated sludge is returned to the digestion tank 10, while the remaining 14
into the thickener (or storage tank) 4.

In addition, part or all of the separated liquid 15 is added to the nitrified water 1 for cleaning.
The mixture is placed in a stirring mixing tank 16, which will be described later as 5'.

On the other hand, the remainder of the separated liquid 15 is led to a denitrification reaction tank 22 together with a cleaning liquid 21 containing a highly concentrated organic acid, which will be described later, where nitric acid and nitrous acid are denitrified by facultative anaerobes and converted into nitrogen gas. .

Next, the liquid after the denitrification treatment is degassed by blowing air 24 into a deaeration tank 23, and then put into a settling tank 25 for solid-liquid separation.
The separated liquid becomes denitrified water 26.

In addition, a portion 27 of the sludge generated by this solid-liquid separation is returned to the denitrification reaction tank 22. The facultative anaerobes contained therein are effectively used as a denitrification reaction source.

The remaining portion 28 is put into the thickener (or storage tank) 4 and then subjected to acid generation treatment.

Therefore, the present invention involves charging the sludge 3, 9, 14, and 28 generated in the settling tanks 2, 7, 12, and 25 to the thickener (or storage tank) 4, and then generating organic acids from the sludge. The organic acid is washed away, and this washing water is used as the organic carbon source and charged into the denitrification reaction tank 22.

In other words, the sludge is condensed (
or storage) and then led to the reactor 17, where a pH adjuster 29 such as sulfuric acid is added to adjust the pH to 6. 8 or less, and react with acid-producing bacteria in an anaerobic atmosphere to generate organic acids.

Here, the acid-producing bacteria are, for example, Pseudomonas (Pse
udomonas), Flavobacterium (Fla
vobacterium) etc., the concentration of organic sludge,
It decomposes large amounts of low-molecular volatile organic acids, such as acetic acid and probionic acid, which are easily utilized by living organisms, with a stable and fast reaction rate regardless of temperature, pH value, etc.

In this way, acid-producing bacteria produce organic acids regardless of the pH value, but in the present invention, the pH value is reduced to 6.
．． The reason for adjusting the pH to 8 or lower is that at pH values exceeding this value, methane-fermenting bacteria become active and further decompose the generated organic acids to generate methane, resulting in a decrease in the amount of organic acids. This is to put it away.

This became clear as a result of adjusting organic sludge to various pH values, performing anaerobic digestion, and examining the relationship between the pH value and the concentration of volatile organic acids.

That is, as shown in FIG. 3, when the pH is below 6.8, the concentration of volatile organic acid is high; The concentration of over 8 is low.

Furthermore, when examining the relationship between pH value and gas generation amount, as shown in Figure 4, when the pH is below 6.8 (point a), the action of methane fermenting bacteria is suppressed, so the amount of methane gas generated is extremely small. pH 7. 1 to 7.4 (curve b), it is clear from the fact that the amount of methane gas generated is large.

Note that pf-1 adjustment of organic sludge is not limited to using acids such as sulfuric acid, but can also be done by adjusting the pH to 6.8 or less by increasing the amount of volatile organic acids generated. Good too.

In addition, the reaction temperature of the organic sludge in the reactor 17 can be treated within the active temperature range of acid-producing bacteria, that is, a high temperature range from room temperature to a high temperature of about 70°C, which is not necessary in actual operation. Accordingly, the steam generator 30 or the like is used to set the optimum temperature in consideration of the number of processing days, processing efficiency, heating cost, etc.

The thus obtained sludge 18 which has been subjected to the organic acid generation treatment is put into the stirring mixing tank 16 together with the nitrified water 15'.

Sludge 18 and nitrified water 1 stirred in the stirring mixing tank 16
5' is introduced into the separation tank 19.

In this case, the amount of nitrate treated water 15' to be introduced is 1 to 5 parts by weight per 1 part by weight of sludge, although it depends on the properties of the sludge 18.

This is because not much organic acid is separated from the sludge at the end of the weight part, and when it exceeds 5 parts by weight, the amount of organic acid separated becomes saturated and the organic acid concentration in the cleaning liquid decreases.

The sludge 20 separated in the separation tank 19 is treated as sludge.

On the other hand, the cleaning liquid 21 contains a high concentration of organic acid, and is led to the denitrification reaction tank 22 described above, where the organic acid is used as an organic carbon source for denitrification treatment.

In this case, the amount of organic acid added in the cleaning liquid 21 should be 0.6 parts by weight or more per 1 part by weight of nitrogen compounds (nitrate nitrogen and nitrite nitrogen) in order to improve denitrification efficiency. preferable.

To summarize the sewage denitrification process shown in the above example, organic sludge obtained in the sewage treatment process is concentrated or stored, then organic acids are generated, and this sludge is treated with digested water. The organic acid in this cleaning solution was used as an organic carbon source for denitrification treatment.

According to this method, since an organic carbon source is generated from organic sludge using microorganisms, there is no need to separately input methanol or acetic acid as in the conventional method.

Furthermore, the obtained organic carbon source is a low-molecular volatile organic acid such as acetic acid or propionic acid, which is easily utilized by living organisms, so that denitrification treatment can be carried out efficiently.

In addition, organic sludge, which was previously difficult to dispose of, can be rendered harmless.
Moreover, the amount of sludge can be reduced by about 25%.

In addition, instead of solid-liquid separation methods such as gravity concentration and pressure flotation concentration,
Since cleaning is required, equipment costs and operating costs will be reduced.

In addition, according to this method, the generated organic acids are water-soluble and migrate to the cleaning liquid side by washing, so it is possible to separate about 80 to 97 of the organic acids contained in the sludge and include them in the cleaning liquid. This organic acid can be effectively used as an organic carbon source.

In addition, in this method, the sludge 2 that undergoes final treatment is
0 is pH because M is washed away with water.
It has the advantage that it is close to neutral and requires only a small amount of sludge conditioning chemicals such as Ca(OH)2.

In Figure 1, nitrification treated water is used as sludge cleaning water, but cleaning water is not limited to digestion treated water, but can also be primary treated water, secondary treated water, denitrification treated water, tap water, groundwater, etc. It is also possible to use

In addition, in Fig. 1, the thickener (or storage tank) 4
After receiving the sludge, it was charged into the reactor 17, but it is also possible to remove the thickener (or storage tank) 4 and charge the sludge drawn from each settling tank directly into the reactor 17.

It is also possible to return the washed sludge 20 in FIG. 1 to the reactor 17 and repeat the treatment.

In addition, FIG. 1 shows the weight reduction of generated sludge through acid generation treatment and the denitrification treatment using the generated organic acid.

In treatment plants that are not equipped with denitrification treatment equipment, i.e., treatment plants up to secondary treatment or treatment plants up to nitrification treatment, this method can be used to perform treatment for the sole purpose of reducing the volume of generated sludge. It is possible.

That is, by guiding the cleaning liquid 21 in FIG. 1 to the secondary treatment aeration tank 5 or the digestion tank 10 and performing aeration treatment,
It is possible to reduce the amount of generated sludge.

This is because acid-producing bacteria decompose generated sludge anaerobically, and the process of propagating acid-producing bacteria consumes energy, so the amount of decrease in generated sludge is greater than the amount of acid-producing bacteria multiplying. By performing the generation treatment, the amount of mixed sludge decreases as a whole.

In addition, the generated organic acid was introduced into the aeration tank, and aerobic bacteria (2
Energy is also consumed in the process of oxidizing and decomposing the sludge (sub-treated sludge or nitrification tank sludge) and multiplying it.

Therefore, by repeating the above operations, the amount of generated sludge can be reduced.

Next, another embodiment will be described based on the flowchart shown in FIG.

The organic sludge 3, 9, 14, and 28 are introduced into the thickener (or storage tank) 4, and organic acid generation treatment is performed in the reactor 17 by involving a pH adjuster 29, a steam generator 30, etc. as necessary. .

The sludge mixing plate 18 after this treatment and water 15' such as primary treated water or secondary treated water are stirred in a mixing tank 16, and further in a separating tank 1.
9, the sludge in the sludge mixture 18 is washed with water 15', and washed sludge 20 and washing liquid 21 are obtained.

The flow up to this point is the same as that shown in FIG.

In this embodiment, the cleaning liquid 21 is introduced into a stirring tank 31, and a pH adjusting agent 32 such as slaked lime or caustic soda is added thereto to adjust the pH to a range of 10 to 13.

Next, this adjusted liquid is led to the settling tank 33, and the CaC generated is
Supernatant water 35 from which solid content 34 such as O3 has been separated and removed is led to an ammonia stripping tower 36 and brought into gas-liquid contact with air 37.

It is preferable to suck this air from the vicinity of the stirring tank 31 and the settling tank 33 from the viewpoint of preventing bad odors.

Next, the liquid 38 subjected to ammonia removal treatment in the ammonia stripping tower 36 is led to the denitrification reaction tank 22 shown in FIG. 1, and denitrification treatment is performed using an organic acid as an organic carbon source.

For ammonia 39, use this as ammonia treatment tank 40.
At the same time, the treated gas 42 is released into the atmosphere, and the liquid 43 that has absorbed ammonia is led to the nitrification tank 10 shown in FIG. 1 for nitrification treatment, followed by denitrification treatment. conduct.

In this case, water such as primary treated water, secondary treated water, nitrification treated water, denitrification treated water, aqua regia, or an acid such as sulfuric acid can be used as the absorption liquid 41.

The difference between the embodiment shown in FIG. 2 and the embodiment shown in FIG. 1 is that the ammonia nitrogen contained in the cleaning solution of the acid-generated sludge mixing plate is removed and then introduced into the denitrification tank.

According to this embodiment, ammonia nitrogen eluted from organic sludge during the acid production process is directly led to the denitrification tank.
It does not come out into the denitrification treated water.

Therefore, it is effective when the ammonia nitrogen concentration in the cleaning solution is higher than the organic acid concentration, or when the quality of the water required for denitrification treatment is severe.

In this example, the so-called ammonia stripping method was used to remove ammonia nitrogen from the cleaning solution. ) etc. can also be used.

Further, in the embodiment shown in FIG. 2, the settling tank 33 was installed between the stirring tank 31 and the ammonia stripping tower 36, but this settling tank 33 was placed after the ammonia stripping tower 36, and the ammonia removed concentrated liquid 38 It is also possible to carry out a precipitation treatment.

Next, an example of actual processing using the processing steps shown in the embodiment of FIG. 1 will be described.

Example 1 In the treatment process shown in FIG. 1, sewage was treated by a sedimentation and activated sludge method to separate wastewater and a sludge mixture (including organic sludge), and the sludge mixture was concentrated.

The components of this sludge mixture are shown in Table 1. The concentrated sludge mixture was subjected to a process for generating volatile organic acids using microorganisms (pH 6.8 or lower, temperature 35° C., 10 days under anaerobic conditions).

Table 1 shows the composition of the sludge mixture after the acid generation treatment.

Table 1 shows the composition of the cleaning solution which was washed in continuous flow with 3 parts of tap water per 1 volume part of the anaerobic acid production treated sludge as described above.

For comparison, solid-liquid separation was performed by gravity concentration without washing the sludge, and the analysis results are also listed in Table 1.

From the above table, it can be seen that treatment with microorganisms (acid-producing bacteria) reduces the sludge concentration (TS, VTS) and significantly increases volatile organic acids.

In addition, the volatile organic acid in the separated liquid according to the comparative method was 1600 ~
In contrast, in the method of the present invention, the volatile organic acids in the cleaning solution are as high as 500 to 800772li+/4.
Although the amount of organic acids recovered was low, the method of the present invention could recover 920 IO of the organic acids contained in the sludge, whereas the comparative method could only recover 74 IO.

This is because the amount of organic acid recovered is expressed as the product of [volume of recovered liquid] and [concentration of organic acid in recovered liquid], and in the method of the present invention, the concentration of organic acid is low, but the volume of liquid is large. This is because the amount of organic acid increases as a result.

Furthermore, while the sludge after washing has a pH of 6.2 to 7.1, which is close to neutral, the sludge that acts as a solid-liquid separator has a low pH of 5.9 to 6.8. According to the present invention, chemicals are used to neutralize the sludge. It was recognized that there was no need for much.

Next, the results of denitrification treatment using this cleaning solution are shown in Table 2.

For comparison, the same table shows the denitrification treatment results when methanol was added as an organic carbon source and when a separated liquid of solid-liquid separation was used.

As is clear from the above results, according to the present invention, since organic acids are separated from sludge by washing, the equipment is simple and the lining cost is low, and the organic acids can be effectively used as an organic carbon source for denitrification. It has various remarkable effects, such as increasing denitrification efficiency and reducing the cost of neutralizing sludge.

[Brief explanation of drawings]

FIG. 1 is a flow sheet diagram showing an embodiment of the method of the present invention;
Figure 2 is a flow sheet diagram showing another embodiment of the method of the present invention, Figure 3 is a characteristic diagram showing the relationship between pH value and volatile organic acid concentration in anaerobic digestion, and Figure 4 is a flow sheet diagram showing the relationship between pH value and volatile organic acid concentration in anaerobic digestion. FIG. 2 is a characteristic diagram showing the relationship between pH value and gas generation amount during digestion. 1... Water to be treated, 2... Sedimentation basin, 3.
...sludge, 4 ... Utsukuna (or storage tank), 5 ... aeration tank, 6 ... air 7
... Sedimentation basin, 8 ... Sludge, 9 ...
...Sludge, 10...Nitrification tank, 11...
Air, 12...Sedimentation tank, 13...Sludge, 14...Sludge, 15...Separated liquid, 1
6... Mixing stirring tank, 1γ... Reactor,
18...Sludge mixing plate subjected to acid generation treatment, 19
... Separation tank, 20 ... Washed sludge,
21...Cleaning liquid, 22...Denitrification reaction tank, 23...Deaeration tank, 24...Air, 2
5...Sedimentation tank, 26...Denitrification treated water,
27...Sludge, 28...Sludge, 29.
...pH adjuster, 30...steam generator,
31... Stirring tank, 32... PH adjuster,
33... Sedimentation tank, 34... Solid content, 3
5...Supernatant water, 36...Ammonia stripping tower, 37...Air 38...Liquid subjected to ammonia removal treatment, 39...Ammonia Air rich in gas, 40...Gas cleaning tower, 41...
- Absorbing liquid, 42...Gas, 43...Liquid that has absorbed ammonia.

Claims (1)

[Claims] I. A wastewater denitrification treatment method in which wastewater that has undergone a BOD removal step, a nitrification step, and an organic carbon source are introduced into a denitrification tank and denitrified by the action of microorganisms, the BOD removal step, the nitrification step and Sewage generated in the denitrification process is separated into solid and liquid, and the resulting organic sludge is brought into contact with acid-producing bacteria in an anaerobic atmosphere with a pH of 6.8 or less to produce organic acids, which are then added to 1 part by weight of sludge. A method for denitrifying wastewater, which comprises cleaning with 1 to 5 parts by weight of a cleaning liquid, and using an organic acid contained in the cleaning liquid as an organic carbon source for the denitrification treatment. 2. A method for denitrifying wastewater according to claim 1, characterized in that the cleaning liquid is directly introduced into a denitrification tank and the organic acid contained therein is used as an organic carbon source for denitrification treatment. 3 Does the washing water undergo ammonia removal treatment and is led to the denitrification tank? death,
2. A method for denitrifying wastewater according to claim 1, wherein the organic acid contained therein is used as an organic carbon source for denitrifying treatment.