JPS6120356B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for treating organic wastewater using an activated sludge method. As is well known, the activated sludge method involves an aeration process in which organic waste liquid and activated sludge returned from a settling tank are mixed, the mixture is aerated using air or oxygen, and organic matter is adsorbed and oxidized in the activated sludge, followed by an aeration process. The process consists of a sedimentation process in which treated wastewater and activated sludge are separated by sedimentation, and a sludge return process in which the sedimented and separated sludge is returned to the aeration process. Among these, activated sludge, which plays the most important role in treating organic wastewater, is composed of a wide variety of bacterial groups. The composition of this bacterial group varies depending on the quality of the wastewater, the season, and other environmental factors, but in general, it consists of the bacteria that most easily assimilate the various organic substances contained in the wastewater, or the intermediate metabolites produced by these bacteria. Bacteria that assimilate sludge propagate and maintain a certain balance of environmental conditions within the activated sludge treatment system, such as water temperature, pH, and dissolved oxygen concentration. Conventionally, in activated sludge treatment, the method of artificially changing the composition of the bacterial group that forms this activated sludge to make the treatment more effective has been to add a separate pure culture of special bacterial cells. There were almost no other bacteria, and the composition was left to the composition of the bacterial groups that naturally formed during activated sludge treatment. For this reason, depending on the organic components contained in wastewater, bacteria that assimilate them may not easily proliferate and remain in the treated water, or bacteria that are undesirable during treatment, such as sedimentation and separation of sludge, may be difficult. However, filamentous bacteria that cause bulking may proliferate, which remains a serious problem in activated sludge treatment. In order to solve the above-mentioned problems in conventional activated sludge treatment, the present invention allows the composition of the bacterial group that forms activated sludge to be changed as necessary, thereby making the operation more efficient and easier. It is intended for this purpose. Next, to explain the structure of the present invention, when organic waste liquid is processed by sequentially passing through at least two aeration sections and the subsequent settling section, the activated sludge mixture of the second aeration section is added to the first aeration section. send back some of the
It is characterized by operation by adjusting the BOD-MLSS load in the front aeration section to an arbitrary value. The present invention will be explained with reference to FIG. 1, which is a schematic diagram, in which an organic waste liquid 11 to be treated is sequentially passed through an aeration section 2, 3 consisting of at least two stages, followed by a settling section 4. In the treatment method, the first stage is a bacteria selection section 2, and a part of the activated sludge mixture from the aeration section 3 at the next stage is returned to this section via a return passage 6 using a return sludge pump 5, and at the same time a blower is used. 9 to supply air or oxygen for mixed aeration. Next, the mixed liquid of the activated sludge in the bacteria selection section 2 and the organic waste liquid that has been partially treated in this section is led to the subsequent aeration section 3, and at the same time, the activated sludge that has been sedimented and separated in the settling section 4 is returned to the sludge. The organic waste liquid is returned to the subsequent aeration section 3 via the return path 8 using the pump 7, and air or oxygen is sent using the blower 9 for mixed aeration to completely treat the organic waste liquid. Next, a part of the activated sludge mixture in the aeration section 3 at the rear stage is returned to the aeration section 2 at the front stage.
Most of the remaining water is sent to the sedimentation section 4, where it is separated into treated water and activated sludge by gravity sedimentation, and the supernatant water is discharged as treated water. The activated sludge that has been sedimented and separated is returned to the aeration section 3 in the latter stage, but a portion of it may be returned to the aeration section in the former stage. Furthermore, an alkaline agent or an acidic agent may be added to the return path 6 returning from the rear aeration section 3 to the front aeration section 2 or directly to the front aeration section 2 to change the pH of the activated sludge mixture. Alternatively, the activated sludge in the return path 6 or the activated sludge mixed liquid in the front aeration section 2 may be heated or cooled to maintain the temperature of the mixed liquid in the front aeration section 2 at an arbitrary value. The above is the configuration of the present invention, but in the present invention, the BOD-MLSS load of the bacteria selection section 2 at the front stage can be set to an arbitrary value or slightly changed by adjusting the amount of sludge returned from the aeration section 3 at the rear stage. The purpose is to maintain the bacterial parts that form activated sludge in a favorable state by operating the system with a certain width. To explain this in detail, bacterial growth is generally characterized in that its specific growth rate changes depending on the concentration of organic matter, unless other limiting factors become rate-limiting. (The specific growth rate is usually expressed as μ (1/hr), and indicates the amount of bacteria that grows per unit weight per unit time. The larger this value is, the faster the growth rate of the bacteria is. It can be said that it is a thing.)
The relationship between this specific growth rate and organic matter concentration is generally expressed by the formula
It is said to follow the Monod equation shown in (1). μ(1/hr)=μmax・S/Ks+S……(1) where μ: Specific growth rate (1/hr) μmax: Maximum value of specific growth rate (1/hr) Ks: Specific growth rate of bacteria is the maximum value
Organic matter concentration (saturation constant) when it shows 1/2 S: Organic matter concentration In this case, μmax and Ks differ depending on the individual bacterial species and organic matter components, so μ is different for each bacterial species.
The relationship between and S is different. Similar relationships apply to the individual bacterial species that make up activated sludge. (In this case, because the organic nutrient sources contained in the wastewater are diverse and the relationships between bacteria are complicated, Monod's equation does not necessarily match.) Figure 2 shows a hypothetical example of activated sludge. The relationship between the specific growth rate μ of the three types of bacterial cells (A, B, and C) and the organic matter concentration (hereinafter this will be represented by the BOD value) is illustrated. As is clear from this figure, the relative magnitude of the specific growth rate of the three types of bacterial cells (A, B, C) changes as follows depending on the BOD concentration. BOD concentration Relative ranking according to the magnitude of specific growth rate S ₁ A>B>C S ₂ B>A>C S ₃ B>C>A S ₄ C>B>A Therefore, the BOD concentration can be set to any value. By culturing the activated sludge while keeping the activated sludge at a constant temperature, the ratio of bacterial cells constituting the activated sludge becomes the same value as the rank according to the magnitude of the specific growth rate. Therefore, by changing the BOD concentration, it is possible to change the bacterial composition forming activated sludge. Applying this characteristic, for example, if C bacteria is a bacteria that prefers to assimilate difficult-to-decompose components contained in wastewater, under conditions where the BOD concentration is S ₁ , its proportion in activated sludge is low. However, by setting the BOD concentration condition to S ₃ or S ₄ , the proportion of C bacteria will increase,
This also makes it possible to completely remove difficult-to-decompose components. As mentioned above, the areas where various bacterial groups proliferate
By changing the BOD concentration, it is possible to change the composition ratio of the bacterial group and, in turn, increase the proliferation of desirable bacterial species. However, incorporating this method directly into the activated sludge method will result in residual bacteria remaining in the treated water.
There is a possibility that there is a lot of BOD. Therefore, in the present invention, the aeration section is divided into two or more stages,
The front aeration section is designed to adjust the BOD concentration in the tank to allow more desirable bacterial species to proliferate, and the BOD remaining in the front aeration section is completely disposed of in the rear aeration section. The BOD concentration in the tank of the front aeration section can be kept constant by adjusting the amount of sludge returned from the rear aeration section. In other words, the residual BOD concentration in the aeration section is
There is a positive correlation with the MLSS load, and by adjusting the amount of sludge returned to the front stage, the BOD-MLSS load of the front stage aeration section can be set, and thus the desired BOD concentration can be obtained. At this time, the activated sludge from the rear aeration section was returned to adjust the BOD-MLSS load at the front aeration section for the following reason. (1) Since the sludge concentration of the activated sludge mixture in the latter stage has a relatively stable value, the sludge concentration returned to the former stage aeration section is constant, making it easy to adjust the BOD-MLSS load in that section. (2) In the aeration section of the first stage, the organic matter in the wastewater is taken up by microorganisms, so the oxygen demand in this section increases; however, since the activated sludge mixture in the second stage contains sufficient dissolved oxygen, some oxygen is supplied. Can play a supporting role. (3) The activated sludge present in the latter aeration section is well adapted to the wastewater and is maintained under aerobic conditions.
High activity in organic matter removal ability. Therefore, by returning this sludge to the former aeration section, sufficient bacterial growth can be achieved. As described above, in the present invention, when treating organic waste liquid, the activated sludge mixture from the latter aeration section is returned to the first aeration section, and the BOD-MLSS load in the first aeration section is adjusted, so that any tank can be used. Obtain the inner BOD concentration,
The aim is to maintain the bacterial population of activated sludge in a favorable state and to treat it completely stably in the subsequent aeration section. Furthermore, in Figure 2, the specific growth rate of bacteria and
Since the relationship with BOD concentration can be changed by pH, water temperature, etc., it is recommended to add alkaline or acidic chemicals to the return path that returns activated sludge to the aeration section in the first stage or directly to the aeration section in the first stage. By changing the pH of the activated sludge mixture in the first stage or by changing the water temperature in the first stage aeration section, the specific growth rate of each bacterial species changes, and the ranking relationship of the relative growth rates of bacterial species can be determined. By changing this, the bacterial composition that forms activated sludge can be changed. Next, one of the applications of the present invention is to prevent bulking. Conventional activated sludge treatment has been able to treat organic wastewater extremely efficiently, but activated sludge often has poor settling and thickening properties, making it difficult to separate the treated water and activated sludge in the sedimentation tank, resulting in bulking. There was a problem. This is mainly due to the abnormal growth of filamentous bacteria in the activated sludge. Therefore, if the growth of filamentous bacteria can be suppressed to a low level, it will be sufficiently effective in preventing bulking. Filamentous bacteria that generally cause bulking are
It is said that BOD-MLSS proliferates significantly in the range of 0.5 to 2.0 Kg/Kg/day. However, BOD―
When the MLSS load is 2.0 kg/kg/day or more, the growth of filamentous bacteria actually decreases, and the growth of significantly dispersed bacterial groups such as stem bacteria and cocci is observed. This thing is BOD
- When the MLSS load is increased, the BOD concentration in the aeration area increases, but under these conditions, bacterial groups with forms such as stem bacteria and cocci have a higher specific growth than filamentous bacteria. It is understood to have speed. That is, if we explain with reference to FIG. 2 above, the relationship between the specific growth rate of filamentous bacteria and the BOD concentration is thought to correspond to bacteria A, and the dispersed stem bacteria and cocci correspond to bacteria B or C. Therefore, in the present invention, the BOD of the front aeration section is
By setting the MLSS load to 2.0 kg/kg/day or more, dispersed stem bacteria, cocci, etc. can grow more than filamentous bacteria, and as a result, filamentous bacteria can be eliminated. In addition, by keeping the dispersed bacterial stem cells in a low-nutrient state of organic matter in the subsequent aeration section for a long period of time, they can become cohesive and form flocs with good sedimentation properties. Based on the above, in the present invention, filamentous bacteria can be removed and bulking can be prevented by adjusting the amount of sludge returned from the rear aeration section so that the BOD-MLSS load in the front aeration section is 2 kg/kg/day or more. can do. Next, examples will be shown. Example 1 Wastewater from a certain tannery has a high BOD concentration and also contains some protein and fat from skin tissue, so it was necessary to give it a long residence time in an aeration tank when performing activated sludge treatment. A bench-scale continuous treatment experiment was conducted using the aeration tank of the present invention, which consists of a front aeration section 4 and a rear aeration section 6, for this wastewater. At this time, as a comparison, a conventional method with 10 aeration tank volumes was also operated in parallel. The wastewater used in the experiment had been subjected to pH adjustment and coagulation and sedimentation treatment. Table 1 shows the experimental conditions at this time. Table 2 shows the average values of raw water quality and treated water quality for both methods.

【table】

[Table] Figure 3 shows the results of adding wastewater to both types of activated sludge and examining the BOD removal rate in batches. The results of continuous treatment experiments clearly showed that the quality of the water treated by the present invention was lower. Also, in Figure 3
The present invention is also superior in BOD removal, especially in the latter half of Sa・t (MLSS concentration x time) (this part is thought to mainly indicate the removal of difficult-to-decompose components contained in wastewater). I understand. From the above, it has been proven that the present invention is extremely effective compared to conventional methods. Example 2 A standard activated sludge treatment equipment with an aeration tank total volume of 4000 m ³ was delivered to a certain pulp manufacturing factory, but the SVI value of the activated sludge was high and filamentous bacteria were growing. According to the present invention, using this activated sludge as a seed sludge, the activated sludge mixture in the rear aeration section is returned to the front aeration section using a bench scale device with a volume of 3.2 in the front aeration section, a volume of 12.8 in the rear aeration section, and a settling tank volume of 10. A pulp wastewater treatment experiment was conducted. At this time, activated sludge treatment using the conventional method using an aeration tank with a capacity of 16 was also operated in parallel. The average BOD ₅ value of wastewater is approximately 200 mg/, and both methods have a BOD value of
The MLSS load is 0.23Kg/Kg・day, but the BOD-MLSS load of the front aeration section in the present invention is 3Kg/Kg/day.
The MLSS concentration in the front aeration section was adjusted so that it was kg/day (MLSS 800-1100 mg/day). Figure 4 shows the daily change in SVI at this time. It can be seen from the figure that in the treatment method of the present invention, sludge with extremely good settling properties was obtained with an SVI of 150 ml/g after 30 days when the SVI gradually decreased. During this period, filamentous bacteria also clearly decreased. On the other hand, with the conventional method, even in long-term continuous experiments, the settling properties of sludge did not improve, but rather the SVI value gradually increased.
From the above, it was proven that the present invention is sufficiently effective in preventing bulking.

[Brief explanation of the drawing]

Figure 1 schematically shows the embodiment of the present invention, Figure 2 shows organic matter concentration (BOD) and specific growth rate of bacterial cells (μ), and Figure 3 shows MLSS concentration x time and residual BOD. Concentration (mg/), Figure 4 shows SVI (mg/
) shows the change over time. 1... Organic waste liquid, 2... Bacteria selection section, 3...
Rear stage aeration section, 4... Sedimentation section, 5... Return sludge pump, 6... Return path, 7... Return sludge pump, 8...
...Return route, 9...Blower.

Claims (1)

[Claims] 1. When treating organic waste liquid, at least 2
A method for treating organic waste liquid comprising a stage aeration section, characterized in that a part of the activated sludge mixture in the latter stage aeration section is returned to the previous stage aeration section. 2 BOD-MLSS load of the front aeration section is 2.0Kg/
2. A method for treating organic waste liquid according to claim 1, which comprises operating at a pressure of at least Kg/day.