JP4712297B2 - Method for reducing excess sludge - Google Patents

Description

  The present invention relates to a surplus sludge reduction system, more specifically, a method for reducing surplus sludge generated by microbial treatment of organic wastewater by electrolysis (represented by “electrolysis” by simplifying “electrolysis”), The present invention relates to a method for reducing excess sludge, which can prevent the precipitation of non-conductors on an electrode during electrolysis and efficiently reduce excess sludge.

  In microbial treatment of organic sewage, the microorganisms digest the soluble organic matter present in the sewage while causing microbial growth. The sludge containing the grown microorganisms needs to be discharged as so-called surplus sludge. This surplus sludge is either landfilled or incinerated after being dehydrated, but the surplus sludge is difficult to dehydrate, so it usually has 70 to 80% by mass of water even after dehydration. It is. Therefore, even if this is landfilled as it is, it will be transported and landfilled with a solid content of about 20 to 30% by mass, and the mass and volume of the majority will be spent as moisture transportation and landfill costs. This is the current situation. In addition, even if it is incinerated, most of the energy is spent on drying and latent heat of evaporation due to the large amount of moisture in the excess sludge, and the cost is high. It has become.

  The amount of excess sludge has been increasing year by year, and the disposal cost is high for both landfill disposal and incineration. The problem has become obvious in both environmental and environmental aspects.

Under such a background, effective utilization of microbial treatment for the treatment of surplus sludge has been studied. It is well known that surplus sludge is digested with activated sludge in the same way as digestion of organic matter once the microorganisms contained therein are killed to form soluble organic matter. After surplus sludge grown by digestion of soluble organic matter is killed and converted to soluble organic matter, the microorganisms digest the soluble organic matter by returning it to the activated sludge tank, reducing the excess sludge (or reducing the volume). . That is, by digesting the soluble organic matter in the excess sludge with microorganisms, the microorganisms in the excess sludge are decomposed into CO ₂ and H ₂ O via the soluble organic matter, and the amount is reduced.

Various methods have been proposed for reducing the amount of excess sludge. For example, by adding chloride to the excess sludge and electrolyzing it, the microorganisms in the excess sludge are killed and activated. A method of reducing excess sludge by returning it to the sludge tank has been proposed (see, for example, Patent Document 1).
JP 2002-126782 A

  According to this method, hypochlorous acid (HClO) is generated at the electrode during electrolytic treatment, and microorganisms in excess sludge are killed using the oxidizing power of hypochlorous acid generated at the electrode, and the killed microorganisms Is returned to the activated sludge tank to digest the microorganisms as soluble organic matter.

  However, when the excess sludge is directly electrolytically treated by the above-mentioned electrolytic treatment, the mineral component-containing material present in the excess sludge is deposited on the anode (negative electrode) as a compound such as an oxide or hydroxide, and the electrode is not a conductor. There has been a problem that the conductivity is lowered by reducing the conductivity, or the electrode is insulated to make it difficult for current to flow during electrolysis.

  As a method for solving this problem, there can be considered a method of removing a non-conductive precipitate by mechanical maintenance or a method of removing a non-conductive precipitate by reversing the polarity of an electrode. The latter method of removing non-conductive precipitates by reversing the polarity of the electrodes is generally used in equipment for producing alkaline components such as water purifiers, and in this application, polarity reversal takes approximately 1 second every 60 seconds. In general, the inversion is periodically performed, for example, in order to produce alkali, acid, or the like at each electrode and flow from each pipe.

  However, when the electrode polarity reversal method is used for surplus sludge electrolysis, the surplus sludge contains much more minerals than tap water, so periodic polarity reversal makes the electrode nonconductive. The precipitate could not be removed sufficiently.

  The present invention solves the problems during the electrolytic treatment in reducing the conventional excess sludge as described above, prevents the decontamination from being deposited on the electrode during the electrolytic treatment, It is an object of the present invention to provide a method capable of efficiently killing microorganisms and efficiently reducing excess sludge.

In the present invention, in reducing surplus sludge generated by microbial treatment of organic sludge, the activated sludge-containing liquid from the activated sludge tank is separated into concentrated sludge and supernatant liquid, and a part of the concentrated sludge is converted into the activated sludge tank. A first step of returning the second sludge, and applying a direct current to the surplus sludge in an electrolytic cell ; the second material is an electrolytic treatment using an electrode whose cathode and anode are both graphite and whose carbon purity is 95% or more . process and, and a third step of returning again activated sludge tank the excess sludge treatment time ratio of polarity reversal of the electrodes during the electrolysis process carried out in the second step is 50% and so as to periodically 10 minutes The polarity is reversed every time, a stop time of 0.1 second to 1 minute is provided when the polarity of the electrode is reversed, and the current when the polarity of the electrode is reversed is increased from 0% to 100% or from 100% to 0%. The time it takes to decrease By reversing the polarity of the electrode at a constant cycle from 0.01 second to 1 minute, the above problem was solved by reducing the excess sludge while preventing the deposition of the non-conductive material on the electrode. Is.

That is, in the present invention, during the electrolytic treatment of as the second factory, for instance, a switching element provided in the power supply, by reversing the electrode using a method such as reversing the applied voltage is switched alternately to the electrode , Preventing mineral components such as calcium and magnesium contained in the excess sludge from depositing on the electrode as a deconductor, thereby reducing the conductivity of the electrode due to the non-conductive deposit on the electrode and electrolysis based on it. A decrease in efficiency can be prevented. Therefore, according to the present invention, it is possible to efficiently reduce excess sludge without being hindered by the non-conducting precipitate on the electrode generated during electrolysis.

  According to the present invention, it is possible to efficiently reduce excess sludge generated by microbial treatment of organic wastewater.

That is, in the present invention, as described above, the activated sludge-containing liquid from the activated sludge tank is separated into concentrated sludge and supernatant liquid, and a part of the concentrated sludge is returned to the activated sludge tank; The surplus sludge is energized with a direct current in an electrolytic cell , the material is a cathode and an anode, both of which are graphite, and the carbon purity is 95% or more. in reduction system excess sludge by electrolysis with a third step for returning sludge again in the activated sludge tank, wherein the second period so that the time ratio of the polarity reversal of the electrodes during the electrolysis process carried out in step is 50% 10 The polarity is reversed every minute, and when the polarity of the electrode is reversed, a stop time of 0.1 second to 1 minute is provided, and the current when the polarity of the electrode is reversed is increased from 0% to 100% or from 100% to 0%. Decrease to By reversing the polarity of the electrode so that the time required for the treatment is from 0.01 second to 1 minute, it is possible to prevent the deposition of the non-conductive material on the electrode, and the excess sludge is generated by the non-conductive material on the electrode. Thus, it is possible to efficiently reduce the excess sludge so that the electrolytic treatment can be performed efficiently without being hindered.

In the present invention, the electrode material used for the electrolytic treatment in the second step may be graphite for both the cathode and the anode. When other materials are used, problems such as electrode dissolution or non-conductivity occur due to periodic inversion. The carbon purity of the graphite is preferably 95% or more. When the carbon purity is low, problems such as unstable electric potential and severe exhaustion occur.

Time ratio of the polarity reversal of the electrodes (i.e., the time ratio of the time you are an anode while the one electrode and the cathode) is Taken together exhaustion of precipitation removal and electrodes nonconductor product to the electrode, 50% Is preferred. More specifically, since the anode is oxidized during electrolysis, if carbon such as graphite is used as an electrode material, the reaction of carbon into carbon dioxide occurs and the anode is consumed. Therefore, in order to uniformly consume the anode and cathode present in pairs the time ratio of the electrode polarity reversal, preferably 50%. By bringing the time ratio of polarity reversal closer to 50%, it is possible to prevent the deposition of non-conductors on the electrode and to make the electrode wear even. Further, it is preferable to provide an electrolysis rest period of 0.01 second to 1 minute when switching the polarity inversion of the electrode. That is, stress is applied to the electrode transiently during polarity reversal and wear is accelerated, but by providing a pause, stress generated during polarity reversal can be mitigated and wear can be suppressed.

  Further, the time required for the current during polarity reversal to increase from 0% to 100% or decrease from 100% to 0% is preferably from 0.1 second to 1 minute. By doing so, the stress transiently given to an electrode at the time of polarity reversal can be relieved.

  Next, an embodiment of a method for reducing excess sludge generated by microbial treatment of organic wastewater according to the present invention will be described with reference to FIGS.

  FIG. 1 is a diagram showing an outline of a treatment process for carrying out the method for reducing excess sludge generated by microbial treatment of organic wastewater of the present invention, and FIG. 2 is a diagram showing an essential part thereof and an electrolyzer used for electrolytic treatment. It is.

  In the above treatment process, the organic sewage 1 is first introduced into an activated sludge tank (aeration tank) 2 and digested by microorganisms in the activated sludge tank 2. And the activated sludge containing liquid 3 discharged | emitted from the activated sludge tank 2 is introduce | transduced into the sedimentation tank 4, and activated sludge is settled by the gravity sedimentation method in the sedimentation tank 4, and it isolate | separates into a concentrated sludge and a supernatant liquid. The The supernatant liquid in the settling tank 4 is discharged from the settling tank 4 as treated water 5, and the treated water 5 is sterilized and discharged out of the treatment process system.

  Then, the activated sludge precipitated at the bottom of the sedimentation tank 4 and having a high concentration is taken out from the sedimentation tank 4 as concentrated sludge 6, and a part of the sludge is returned to the activated sludge tank 2 as return sludge 7. The process so far is the same as in the case of conventional microbial treatment of organic wastewater, but in the present invention, a part of the concentrated sludge in this conventional process is extracted as excess sludge and specified as the excess sludge as described below. In this embodiment, electrolysis is performed to kill the microorganisms in the excess sludge and change to soluble organic substances to facilitate digestion with microorganisms.

  The remaining concentrated sludge 6 is sent to the chemical mixing tank 9 as surplus sludge 8, and usually chloride is added to the surplus sludge 8 in the chemical mixing tank 9 and an acid is added to adjust the pH to 2. The solid content concentration is adjusted to 0.2 to 3% by mass.

  As said chloride, sodium chloride, potassium chloride, magnesium chloride, calcium chloride etc. are preferable, for example. This is because those chlorides tend to generate hypochlorous acid having a high bactericidal action in the subsequent electrolytic treatment process. And the addition amount of the chloride to this excess sludge 8 is preferably 0.1 to 3.5% by mass.

  In the chemical mixing tank 9, the acid added to the excess sludge 8 is added to enhance the bactericidal action (microbe killing action). As this acid, for example, a strong acid such as hydrochloric acid or sulfuric acid is preferable. In particular, hydrochloric acid is preferable because it also serves as a source of hypochlorous acid.

  In the present invention, by adding acid to excess sludge as described above and adjusting to pH 2-6, hypochlorous acid can be efficiently generated from the chloride during electrolysis, and the bactericidal action can be improved. However, this pH is particularly preferably pH 4-6. When the pH is lower than 2, harmful chlorine gas is generated, and when the pH is higher than 6, the bactericidal action may be lowered. In addition, when alkaline, not only the bactericidal action is reduced, but also the cell wall of the microorganism is easily damaged, and it becomes impossible to dissolve the cell wall from inside the microorganism by protease, and the insoluble organic substance can be efficiently converted into the soluble organic substance. There is a risk that it will not be possible. However, pH adjustment by addition of chloride and oxidation as described above, solid content concentration of excess sludge, and the like are not necessarily limited to the above examples.

In the second step of the present invention, surplus sludge mixed with the chemical (that is, chloride and acid) in the chemical mixing tank 9 is introduced into the electrolytic tank 11 as the chemical mixed sludge 10 and electrolyzed in the electrolytic tank 11. Thus, sterilization (determination of microorganisms) by hypochlorous acid generated on the electrode surface is performed. The larger the current density during the electrolysis, the shorter the electrolysis time, but the voltage between the electrodes rises and water is decomposed to reduce the sterilization efficiency. Therefore, the current density is preferably 1 to 100 mA / cm ² , 2-50 mA / cm < ² > is especially preferable.

  Here, details of the electrolytic treatment and polarity inversion of the electrodes in the electrolytic treatment will be described with reference to FIG. First, reference numerals and objects in FIG. 2 will be described. 21 is an electrolytic cell, which corresponds to the electrolytic cell 11 in FIG. Reference numeral 22 denotes an excess sludge to which sodium chloride and hydrochloric acid are added, which corresponds to the chemical liquid mixed sludge 10 in FIG. 1, and corresponds to an electrolytic solution in an electrolytic process. Reference numerals 23 and 24 denote an electrode A and an electrode B, respectively, and the surfaces of these electrodes A and B are both composed of graphite having a carbon purity of 95% or more, although only one sheet is shown in FIG. Actually, a plurality of each is connected in parallel. Reference numeral 25 denotes a power source for electrolysis, and this power source 25 has a characteristic capable of turning on / off the output, gradually increasing from 1 second to 1 minute, gradually rising and falling by a command from the timing generation circuit. . Reference numeral 27 denotes a switch drive circuit, and the four switches (switching elements) 28 to 31 are represented by SW1, SW2, SW3, and SW4 in FIG. The operation for the electrolytic process proceeds at a timing generated by the timing generation circuit 26. For example, when the description starts from the downtime, the output of the power supply 25 is initially off, and all the switches 28 to 31 (SW1 to SW4) are open. After this period lasts from 1 second to 1 minute, in the next period, 28 switches (SW1) and 31 switch (SW4) are closed, and the output of the power supply 25 is changed from 0% to 100% in 1 second to 1 minute. In the next period, a 100% output is continuously applied to the electrodes A and B, and the electrolytic treatment is performed. After this period lasts from 1 second to 10 hours, the power supply 25 falls from 100% to 0% in 1 second to 1 minute.

  After the power supply 25 is turned off, the switches 28 to 31 (SW1 to SW4) are all open, and this state lasts from 0.01 second to 1 minute, and then 30 switches (SW3) with 29 switches (SW2). Is closed, and the anodes and cathodes of the electrodes A and B are exchanged. Next, the output of the power source 25 rises from 0% to 100% in 1 second to 1 minute. In the next period, 100% output is continuously applied to the electrodes, and reverse electrolysis is performed.

  EXAMPLES Next, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

Example 1
Based on the treatment process shown in FIG. 1, the excess sludge was reduced by electrolytic treatment under the following conditions (i), (ii), and (iii). In the following, “%” indicating concentration is “% by mass” unless otherwise specified.
(I) As surplus sludge 8, 200 ml of surplus sludge having a solid content concentration of 0.95% was prepared and put into a 200 ml beaker as the chemical mixing tank 9. In addition, as shown in FIG. 2, this excess sludge introduce | transduces the organic sludge 1 into the activated sludge tank 2, is digested with the microorganisms in the activated sludge tank 2, and the activated sludge containing liquid 3 from the activated sludge tank 2 is shown. The activated sludge-containing liquid 3 is introduced into the sedimentation tank 4, the activated sludge is precipitated in the sedimentation tank 4, and the supernatant is discharged from the sedimentation tank 4 as treated water 5 (this treated water 5 Is discharged to the outside of the treatment system), and the sludge having a high concentration after precipitation is taken out from the sedimentation tank 4 as the concentrated sludge 6 and a part of the concentrated sludge 6 is returned as the sludge 7 to the activated sludge tank 2. In this case, surplus sludge 8 is obtained from the remaining portion.

(Ii) 1.0 g of sodium chloride and hydrochloric acid for pH adjustment were used as the chemicals, which were added to the excess sludge 8 to adjust the pH to about 5 to obtain a chemical mixed sludge 10. The concentration of sodium chloride in the drug mixed sludge 10 was 0.5%.

(Iii) The beaker containing the drug-mixed sludge 10 is used as an electrolytic cell 11, and an amorphous carbon electrode (mass when dried: 8.0 g) having an effective electrode area of 25 cm ² is used as a positive electrode and a negative electrode for electrolysis. At 15 mm, it is inserted into the chemical mixed sludge, and while stirring, the polarity is reversed every 10 minutes with a direct current of 100 mA using the electrolysis apparatus shown in FIG. 2, and the stop (rest) time is 1 second. The current change time from 0 mA to 100 mA and 100 mA to 0 mA was 0.5 seconds, respectively, and the current was applied for 1 hour. The voltage at this time was 4.2 V, and the temperature of the sludge was 25 ° C.

  As a result of the above treatment, a large amount of microorganisms survived at the time of mixing the chemicals, but were sterilized by the subsequent electrolytic treatment (the sterilization rate was 95% as determined from the decrease rate of dissolved oxygen), and the electrode after electrolysis. No precipitate was observed on the surface.

The sterilization rate (microbe kill rate) was determined as follows. That is, the excess sludge after electrolytic treatment was bubbled with air for 10 minutes, and then the reduction rate of dissolved oxygen was measured. Prior to the measurement of the rate of decrease in dissolved oxygen, the sterilization rate obtained from the rate of decrease in dissolved oxygen in the stock solution (excess sludge with a solid content of 0.95%) was set to 0%, and the rate of decrease in dissolved oxygen from distilled water was The obtained sterilization rate is set to 100%, and the sterilization rate of 20% of excess sludge (activated sludge) diluted with distilled water is set to 20% and 50% of excess sludge diluted with distilled water (active) The sterilization rate of sludge) is 50%, 70% is diluted with distilled water, the sterilization rate of surplus sludge (activated sludge) is 70%, a calibration curve is drawn, and the measurement result of the decrease rate of dissolved oxygen using this calibration curve Was determined as the sterilization rate (microbe kill rate). As a result, as described above, the sterilization rate (microbe kill rate) was 95%. Further, the (i) ~ potential during electrolysis Engineering as an after repeating 10 times of (iii) is 4.2 V, showed that passivated electrodes does not occur. Moreover, when the dry mass of the electrode was measured, the dry mass of the electrode was 7.7 g and 7.6 g. Of course, there was no increase in the mass of the electrode due to the non-conductive precipitate, and the mass decrease in the electrode was almost uniform. And there was little consumption of the electrode.

Comparative Example 1
The treatment was the same as in Example 1 except that the current was supplied for 1 hour without polarity reversal and stop (pause) at a DC current of 100 mA using a normal DC power supply, and deposition on the electrode after electrolysis in those cases The adhesion state and sterilization rate of the object were investigated. As a result, the sterilization rate was 64%, and the entire surface of the cathode after electrolysis was covered with white precipitates. As a result of analysis of the white precipitate, as shown in Table 1, it was a precipitate containing, as a main component, a mineral such as calcium contained in excess sludge. Further, that has arisen Engineering more (however, the polarity reversal is not performed) electrolytic treatment when the potential 5.2V next after repeated 10 times, passivated electrode of (i) ~ (iii) Was showing. Further, when the dry weight of the electrode was measured, the anode was 7.1 g while the cathode was 8.1 g, the anode caused a decrease in mass, and the cathode increased in mass due to the deposition of a non-conductor. I was waking up.

  As is clear from the comparison between Example 1 and Comparative Example 1, the deposition of non-conductors on the surface of the electrode during electrolytic treatment is prevented, efficient electrolytic treatment is performed, and consumption of the electrode is prevented. For this purpose, it is understood that it is necessary to reverse the polarity of the electrode during the electrolytic treatment.

FIG. 1 is a diagram showing an outline of a treatment process for carrying out a method for reducing excess sludge generated by microbial treatment of organic wastewater according to the present invention. It is a figure which shows the outline of the electrolysis apparatus used for the principal part and electrolytic treatment of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Organic sludge 2 Activated sludge tank 3 Activated sludge containing liquid 4 Precipitation tank 5 Treated water 6 Concentrated sludge 7 Return sludge 8 Surplus sludge 9 Chemical mixing tank 10 Chemical mixing sludge 11 Electrolytic tank 21 Electrolytic tank 22 Surplus sludge (chemical mixed sludge )
23 Electrode A
24 Electrode B
25 Power supply 26 Timing generator circuit 27 Switch drive circuit 28 Switch (SW1)
29 switch (SW2)
30 switch (SW3)
31 switch (SW4)

Claims (1)

A method of reducing excess sludge generated by microbial treatment of organic sludge, separating the activated sludge-containing liquid from the activated sludge tank into concentrated sludge and supernatant liquid, and a part of the concentrated sludge into the activated sludge tank A first step of returning the second sludge, and applying a direct current to the surplus sludge in an electrolytic cell ; the second material is an electrolytic treatment using an electrode whose cathode and anode are both graphite and whose carbon purity is 95% or more . process and, and a third step of returning again activated sludge tank the excess sludge treatment time ratio of polarity reversal of the electrodes during the electrolysis process carried out in the second step is 50% and so as to periodically 10 minutes The polarity is reversed every time, a stop time of 0.1 second to 1 minute is provided when the polarity of the electrode is reversed, and the current when the polarity of the electrode is reversed is increased from 0% to 100% or from 100% to 0%. 0 time to decrease A method for reducing excess sludge, wherein the polarity of the electrode is reversed so as to be from 1 second to 1 minute, thereby preventing excess sludge from being reduced while preventing deposition of a non-conductive material on the electrode.

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