JP2000218294A - Anaerobic treatment of organic sludge - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an anaerobic treatment method of org. sludge capable of recovering soluble org. matters (lower fatty acids). SOLUTION: In the anaerobic treatment method using membrane separation in which a separation membrane 3 is installed in a biological reaction tank 2 for subjecting the org. sludge to the anaerobic treatment and released liq. is separated by the separation membrane 3, the recovery of membrane flux and decreasing of the molecular weights of hardly decomposable org. matters are attained by treating the surface of membrane with ozone. Ozone treatment is executed preferably after stopping the circulation of fermentation liq. The recovered soluble org. matters can be effectively used as a hydrogen donor for denitrifying bacteria.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anaerobic treatment method for organic sludge such as sewage sludge, human waste sludge, and sludge from village settlements.

[0002]

2. Description of the Related Art An anaerobic treatment method of subjecting such organic sludge to anaerobic treatment in a biological reaction tank and subjecting the sludge to methane fermentation to obtain treated water having a low organic content is well known. At this time, if anaerobic bacterial cells flow out together with the treated water, the bacterial cell concentration in the biological reaction tank will decrease. Therefore, a membrane separator is used to separate the desorbed liquid using a separation membrane to prevent the bacterial cell concentration from decreasing. Anaerobic digestion has been proposed. However, this method has a serious problem that organic substances are easily accumulated on the membrane surface of the separation membrane and the membrane flux is easily reduced, which has hindered the practical application. Conventionally, organic sludge is decomposed into water and methane by methane fermentation, and the technical concept of recovering and effectively utilizing soluble organic matter (lower fatty acids) generated in acid fermentation in the previous stage of methane fermentation. There was no.

[0003]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned conventional problems and recovers a desorbed liquid containing a soluble organic substance (lower fatty acid) while preventing a decrease in membrane flux of a separation membrane. It has been made to provide a method for anaerobic treatment of organic sludge that can be used.

[0004]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides a biological reaction tank for anaerobically treating organic sludge, in which a separation membrane is attached, and the separation liquid is separated by the separation membrane. In the method for anaerobic treatment of organic sludge, the membrane surface of the separation membrane is treated with ozone.

[0005] As described above, the present invention combines ozone treatment, which is considered to be incompatible with anaerobic treatment, and which is considered to inhibit the activity of organic acid-producing bacteria, with anaerobic digestion with membrane separation, thereby providing a membrane for a separation membrane. The organic matter accumulated on the surface can be decomposed to prevent a decrease in film flux. In addition, by performing the ozone treatment, acid fermentation of the organic sludge under anaerobic conditions can be promoted, and the desorbed liquid containing lower fatty acids can be separated and collected. This lower fatty acid can be used as a hydrogen donor for denitrifying bacteria in a denitrification treatment step of sewage or the like, and it is not necessary to use an expensive alcohol as a hydrogen donor as in the related art.

[0006]

Embodiments of the present invention will be described below.
FIG. 1 is a schematic view of an apparatus used in the present invention, wherein 1 is a sludge storage tank, 2 is a biological reaction tank, 3 is a separation membrane, 4 is an ozone generator, and 5 is a desorbed liquid storage tank. The coagulated raw sludge stored in the sludge storage tank 1 was sent to the biological reaction tank 2 and fermented while being circulated between the separation membrane 3 and the circulation pump 6. The temperature inside the biological reaction tank 2 was maintained at about 35 °, and the anaerobic state was maintained while controlling by air aeration so that the ORP (oxidation-reduction potential) did not become −350 mV or less. A stirrer 7 was installed in the biological reaction tank 2.

As the separation membrane 3, a ceramic membrane that can withstand ozone treatment is preferable, and in particular, a monolithic ceramic membrane as shown in FIG. 2 is preferable. In this embodiment, a microfiltration membrane made of alumina and having a pore diameter of 1 μm was used. The fermentation liquor sent from the biological reaction tank 2 is supplied from one end face as shown, and the filtered effluent is sucked from the outer peripheral surface by the suction pump 8 in FIG. .
Although the separation membrane 3 is provided in the circulation line of the biological reaction tank 2 in this embodiment, the separation membrane 3 may be provided inside the biological reaction tank 2.

In this embodiment, ozone treatment is performed on the membrane surface by supplying the ozone-containing air generated by the ozone generator 4 to the sludge-side membrane surface of the separation membrane 3. As shown in FIG. 2, by supplying the ozone-containing air in a cross flow or in a parallel flow, it is possible to scrape off the deposits from the film surface. The ozone treatment may be performed continuously, but it is preferable to stop the circulation of the fermentation solution to the separation membrane 3 at predetermined time intervals and perform the ozone treatment intermittently. Alternatively, a method of intermittently backwashing the separation membrane 3 with ozone water from the permeate side may be employed.

By such an ozone treatment, proteins, polysaccharides, amino acids, polyhydroxy aromatics, fats, and other deposits accumulated on the membrane surface of the separation membrane 3 are physically peeled off, and these deposits are removed. It can be chemically oxidatively decomposed to recover the decrease in membrane flux as shown in the examples described later. This effect is more remarkable in the case of intermittent ozone treatment in which ozone is supplied without filtration pressure.

[0010] Further, acid fermentation is promoted by such ozone treatment, and organic substances which have been hardly decomposed in the conventional method can be reduced to soluble organic substances (lower fatty acids). Moreover, despite the use of ozone having a strong bactericidal activity, the activity of the organic acid-producing bacteria is not unexpectedly impaired, and the desorbed liquid containing lower fatty acids can be separated and recovered. This lower fatty acid can be effectively used as a hydrogen donor for denitrifying bacteria in a denitrification treatment step of sewage or the like. Hereinafter, the operation and effect of the present invention will be specifically shown by the data of the examples.

[0011]

EXAMPLE Using the apparatus shown in FIG. 1, the degree of flux recovery on the membrane surface of the separation membrane 3 was measured while changing the amount of ozone injected into the membrane surface. The result is shown in FIG. The black circles show the case where the circulation of the fermentation liquid to the separation membrane 3 was intermittently stopped to perform the ozone treatment, and the white circles show the case where the ozone treatment was performed while circulating the fermentation liquid. The vertical axis is the flux, and the horizontal axis is the ozone injection amount per volume occupied by the fermentation liquor inside the membrane. As is clear from the graph of FIG. 3, the flux on the film surface is 0.7 m / d or less when the injection amount of ozone is 0, but the flux is 1 to 1 by injection of 15 to 50 g / L of ozone.
It recovered to 1.3 m / d. Also, it can be seen that the intermittent ozone treatment is more excellent in the flux recovery effect.

FIG. 4 shows the flux recovery rate of the membrane surface after ozone treatment after filtering the fermentation liquor. The vertical axis indicates the pure water flux as 100%. When the fermentation liquor is circulated, the flux at the start of filtration can be recovered by injecting 43 g / L or more of ozone, and when the circulation of the fermentation liquor is stopped, filtration can be started by injecting 27 g / L or more of ozone. The flux of time could be restored. It is considered that such a difference is caused when the efficiency of contact between ozone and the film is reduced when circulation and ozone injection are performed simultaneously.

FIG. 5 shows the membrane permeation resistance (permeation resistance due to the adhered layer captured on the membrane surface + permeation resistance formed by the interaction between the components in the liquid and the membrane) on the vertical axis, and the relationship with the ozone injection amount. FIG. As is clear from this graph, by stopping the circulation of the fermentation liquid and performing the ozone treatment, the membrane permeation resistance can be reduced with a small amount of ozone.

Next, the ozone injection amount was set to a minimum (65 g / L) constant value, and the ozone concentration and the contact time were changed as shown in Table 1, and the flux change on the film surface was examined.

[Table 1]

FIG. 6 shows the result. RUN3 and RUN
6. Since a high flux was obtained in No. 6, it was found that, when the generated ozone concentration was equal to or higher than a certain value, increasing the contact time between the separation membrane 3 and ozone was effective in recovering the flux. Was. However, since RUN4 shows the lowest flux despite the long contact time and the high feed rate, it was found that the concentration of generated ozone needs to be higher than a certain level in order to recover the flux. Further, from the results of RUN3 and RUN5, it was found that, for the same generated ozone concentration, it was more effective to recover the flux by making the contact time longer than the amount of the supplied gas.

Next, the result of the continuous operation is shown in FIG. No ozone injection was performed during the first 30 days, during which the flux on the membrane surface was 0.7-0.8 m / d. Thereafter, the separation membrane 3 was replaced with a new one, and after continuous operation for 2 hours, the circulation of the fermentation liquid was stopped, and ozone treatment for 20 minutes was performed for 2 hours 2
This was repeated in a 0 minute cycle. As a result, the flux on the membrane surface was 1 m /
d could be stabilized at a high level exceeding d.

FIG. 8 is a graph showing the change in the absorbance (E ₂₆₀ ) of the fermented liquid during the continuous operation. E
₂₆₀ is an index of the amount of the hardly decomposable organic matter, and it was confirmed that the molecular weight of the hardly decomposable organic matter was reduced by the continuous operation with the ozone treatment of the film surface.

FIG. 9 is a graph showing a change in the TOC (total organic carbon) concentration in the desorbed liquid during the continuous operation period. The ozone treatment showed a high TOC concentration, an increase in soluble organic matter due to solubilization of solids, and recovery of carbon equivalent to 31 to 51% of the amount of inflowing organic carbon as a desorbed liquid. As described above, the present invention is also effective in recovering organic substances, and the recovered soluble organic substances can be used as a hydrogen donor of denitrifying bacteria in a denitrification treatment step of sewage or the like.

As shown in FIG. 10, it has been confirmed that the number of organic acid-producing bacteria in the fermentation broth temporarily decreases when ozone injection is started, but thereafter recovers to a level at which ozone injection is not performed. It can be seen that ozone injection into the anaerobic digestion process according to the present invention is possible.

[0020]

As described above, according to the present invention, by combining the anaerobic digestion method with membrane separation and the ozone treatment on the membrane surface, it is possible to prevent the membrane flux of the separation membrane from being lowered.
An eluate containing a soluble organic substance (lower fatty acid) can be recovered, and there is an advantage that the recovered soluble organic substance can be effectively used as a hydrogen donor.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of an apparatus used in the present invention.

FIG. 2 is a sectional view of a monolithic ceramic film.

FIG. 3 is a graph showing a relationship between an ozone injection amount and a film surface flux.

FIG. 4 is a graph showing a relationship between an ozone injection amount and a recovery rate of a film surface flux.

FIG. 5 is a graph showing a relationship between an ozone injection amount and a membrane permeation resistance.

FIG. 6 shows RUN1 and RUN2 in which the ozone concentration and the contact time are changed.
It is a graph which shows the film surface flux in RUN6.

FIG. 7 is a graph showing the change over time of the film surface flux in the case of continuous operation.

FIG. 8 is a graph showing the daily change of the absorbance of the fermentation liquid in the case of continuous operation.

FIG. 9 is a graph showing the daily change of TOC in the case of continuous operation.

FIG. 10 is a graph showing the daily change of organic acid-producing bacteria in the case of continuous operation.

[Explanation of symbols]

1 sludge storage tank, 2 biological reaction tank, 3 separation membrane, 4 ozone generator, 5 desorbed liquid storage tank, 6 circulation pump, 7
Stirrer, 8 suction pump

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C02F 1/44 C02F 1/44 F C12M 1/00 C12M 1/00 HF Term (Reference) 4B029 AA02 DA03 DB01 DB11 DF01 DF04 DF06 DF09 DF10 DG10 4D006 GA07 JA01A JA01C JA31A JA51A JA56A JA70C KA02 KA43 KA63 KA67 KB23 KC03 KC14 KC16 KD21 KE02Q KE02R MC03 PB08 PC62 4D059 AA01 DA11 BA11 BA11

Claims (6)

[Claims] In a method for anaerobic treatment of organic sludge, a separation membrane is attached to a biological reaction tank for anaerobically treating organic sludge, and the separation surface is separated by the separation membrane. An anaerobic treatment method for organic sludge. 2. The anaerobic treatment method for organic sludge according to claim 1, wherein the separation membrane is provided in the biological reaction tank or in a circulation line of the biological reaction tank. 3. The method for anaerobic treatment of organic sludge according to claim 1, wherein a monolithic ceramic membrane is used as the separation membrane. 4. The method for anaerobic treatment of organic sludge according to claim 1, wherein the ozone treatment of the separation membrane is performed intermittently. 5. The anaerobic treatment method for organic sludge according to claim 4, wherein the ozone treatment is performed on the membrane surface by intermittently supplying air containing ozone to the sludge-side membrane surface of the separation membrane. 6. The ozone treatment of the membrane surface by intermittently backwashing the separation membrane with ozone water from the permeated water side.
The method for anaerobic treatment of organic sludge according to the above item.