JP3636035B2 - Digestion method of organic sludge - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for digesting organic sludge, and more particularly, to a method for biological digestion of organic sludge containing hardly biodegradable biological sludge.
[0002]
[Prior art]
Biological sludge generated by aerobic or anaerobic treatment of organic wastewater is biologically stable and hardly biodegradable because it has undergone biological treatment, and its treatment is difficult. As a method for digesting organic sludge containing such biological sludge, there is a method of performing anaerobic digestion such as methane fermentation, and the decomposition rate of sludge is usually about 40%. In order to improve the decomposition rate of sludge, pretreatment methods such as ozone treatment, ball mill treatment, ultrasonic treatment, heat treatment, etc. have been proposed, but even with these pretreatment methods, the sludge decomposition rate is 50-60. % Is the limit, and the cost is high.
As a method other than the pretreatment method, a method of concentrating the anaerobically digested sludge, returning the concentrated sludge to the anaerobic digester, and increasing the sludge concentration in the anaerobic digester has been proposed. However, even in this method, the decomposition rate of sludge is limited to about 60%.
[0003]
As another method, JP-A-9-136100 describes a method in which anaerobic digested sludge is treated with ozone, and this ozone treated sludge is digested aerobically, and 90% or more of the raw sludge weight is reduced. It is described.
However, the conventional method has a problem that the cost is high because a large amount of ozone is consumed by a large amount of hydrogen sulfide contained in the anaerobic digested sludge.
[0004]
[Problems to be solved by the invention]
An object of the present invention is to propose a method for digesting organic sludge that can digest organic sludge containing biological sludge at a low cost and at a high decomposition rate.
[0005]
[Means for Solving the Problems]
The present invention introduces organic sludge containing biological sludge into an aerobic digester equipped with a submerged membrane separation device, and aerobic digestion step for aerobically biotreating,
An ozone treatment process for ozone treatment of aerobic digested sludge biologically treated in the aerobic digestion process;
An anaerobic digestion process for anaerobically digesting the ozone-treated sludge ozone-treated in the ozone treatment process in an anaerobic digestion tank;
A solid-liquid separation process for solid-liquid separation of the anaerobic digested sludge biologically treated in the anaerobic digestion process into a concentrated sludge and a separation liquid;
This is a method for digesting organic sludge, which includes a returning step in which part of the concentrated sludge separated in the solid-liquid separation step is returned to the aerobic digester and the remainder is returned to the anaerobic digester.
[0006]
The organic sludge to be treated in the present invention is organic sludge containing biological sludge, preferably sludge containing biological sludge as a main component, and may contain other organic substances and / or inorganic substances in addition to biological sludge. Good. Examples of biological sludge include sludge generated by biological treatment of organic wastewater such as sewage, human waste, and industrial wastewater, and include activated sludge and digested sludge. Organic sludge is sludge containing such biological sludge, and may be mixed sludge with other sludge containing organic or inorganic substances such as sediment sludge such as sewage in addition to biological sludge. Such sludge includes not only slurry-like sludge generated by water treatment or the like, but also dehydrated cake obtained by dehydrating the sludge.
[0007]
The aerobic digestion process of the present invention introduces organic sludge containing biological sludge and concentrated sludge returned from the subsequent return process into the aerobic digestion tank, mixes it with the biological sludge in the tank, aerated and aerobic This is a process of biological treatment. As an aeration method, an ordinary aeration method using air, an aeration method using oxygen-enriched air, or the like can be employed.
[0008]
In the aerobic digestion process, organic matter accumulated in microbial cells, sludge mucilage, and the like are biologically digested, and sludge solubilization and weight reduction are promoted. In this reaction, the longer the residence time, the better. However, if it is too long, the amount of methane gas generated in the subsequent anaerobic digestion process is reduced and the resource recovery rate is lowered. Therefore, the residence time in the aerobic digestion process is usually 1 It is desirable that the time is not less than 6 hours, preferably 6 to 48 hours.
[0009]
Organic sludge often rots during normal transportation and storage, and therefore contains hydrogen sulfide, and anaerobic digested sludge also contains a large amount of hydrogen sulfide. In the present invention, hydrogen sulfide in the sludge is biologically oxidized to sulfuric acid by aerobically treating these sludge containing hydrogen sulfide in the aerobic digestion step. The residence time required for the oxidation of hydrogen sulfide is usually about 1 hour. In addition, most of the soluble biodegradable organic matter is decomposed by aerobic digestion.
[0010]
In the aerobic digester, a submerged membrane separation device is immersed. This submerged membrane separation device has a separation membrane, and is a device that filters water from the liquid in the tank and separates it from sludge while the membrane is immersed in the liquid in the tank. This submerged membrane separation device performs filtration while preventing the concentration of suspended substances on the membrane surface by applying a water flow caused by aeration caused by the aeration device provided in the aerobic digestion tank to the separation membrane surface. It is preferable to be configured as described above. Filtration can be performed by suction means such as a pump. The water separated by the submerged membrane separator is discharged out of the system as treated water. An aerobic digester can be provided with a partition wall such as a baffle plate.
[0011]
As the type of the membrane of the membrane separator, an ultrafiltration (UF) membrane, a microfiltration (MF) membrane or the like can be used. As a material of the membrane, a membrane of any material such as cellulose acetate (CA) membrane, polyamide (PA) membrane, aramid membrane, polysulfone membrane, hydrophilic polyethylene can be used. The membrane can be of any shape such as a flat membrane, spiral membrane, tubular membrane, hollow fiber membrane.
[0012]
The ozone treatment process is a process in which the aerobic digested sludge treated in the aerobic digestion process is contacted with ozone to perform ozone treatment. Usually, the solution in the aerobic digestion tank can be directly introduced into the ozone treatment tank and brought into contact with ozone. As a contact method, a method in which aerobic digested sludge is introduced into an ozone treatment tank and ozone is blown in, a method by mechanical stirring, a method using a packed bed, or the like can be employed. As ozone, ozone gas, ozone-containing air, ozonized air, or the like can be used. The sludge is modified to easily biodegradable organic matter by ozone treatment.
[0013]
The conditions for the ozone treatment are not particularly limited, but the treatment is performed with the amount of ozone used being 0.002 to 0.05 g-O ₃ / g-VSS, preferably 0.005 to 0.03 g-O ₃ / g-VSS. Is desirable. Moreover, it is preferable that pH is 5 or less.
[0014]
In the present invention, the aerobic digested sludge to be subjected to the ozone treatment contains almost no hydrogen sulfide and soluble organic matter. Therefore, even if such aerobic digested sludge is treated with ozone, the oxidation of hydrogen sulfide, etc. In addition, ozone is not wasted. Further, since the aerobic digested sludge is filtered by a submerged membrane separator to remove water, the amount to be treated is reduced. Therefore, the ozone treatment can be efficiently performed at a low cost with a small amount of ozone.
[0015]
The anaerobic digestion step is a step of anaerobically biotreating the ozone-treated sludge ozone-treated in the ozone treatment step in an anaerobic digestion tank. In the anaerobic digestion process, readily biodegradable organic substances and other organic substances in the ozone-treated sludge are decomposed through the steps of organic acid generation and methane generation, and converted to methane. Anaerobic digestion can be performed in a two-phase system in which organic acid generation and methane generation are performed in separate reaction tanks, or in a single-phase system in one reaction tank.
[0016]
As the anaerobic digestion, a floating method used in a normal anaerobic digestion method can be generally employed, but a high-load anaerobic biological treatment such as a fixed bed method, a fluidized bed method, and a UASB method can also be employed. The anaerobic digestion treatment conditions are not particularly limited, and normal anaerobic treatment conditions can be employed. For example, the temperature can be 30-60 ° C. A mesophilic methane-fermenting bacterium acts at 30 to 40 ° C, and a high-temperature methane-fermenting bacterium acts at 45 to 60 ° C. The residence time is 5 to 30 days, preferably 10 to 20 days, and the pH is 6 to 8.5, preferably 7 to 8.
[0017]
In the anaerobic digestion according to the present invention, water is filtered and separated by the submerged membrane separator in the aerobic digestion step, so that the volume of sludge to be subjected to the anaerobic digestion process is reduced. For this reason, an anaerobic digester can be made small, the amount of heat used for heating is reduced, and efficient processing is possible at low cost.
[0018]
In the solid-liquid separation step, anaerobic digested sludge is subjected to solid-liquid separation into concentrated sludge and separated liquid. As the solid-liquid separation means, ordinary solid-liquid separation means such as precipitation separation, pressurized flotation, centrifugation, MF membrane and UF membrane can be employed. When employing precipitation separation or pressurized flotation, etc., it is preferable to avoid contact with air as much as possible because methane-fermenting bacteria are vulnerable to oxygen. Moreover, when employ | adopting centrifugation etc., in order to reduce SS density | concentration in a separation liquid, it is preferable to use a polymer flocculant together.
[0019]
The separated liquid separated in the solid-liquid separation process is discharged out of the system as treated water, the concentrated sludge is returned to the aerobic digestion tank in the aerobic digestion process, and the remainder is anaerobic in the anaerobic digestion process. Return to sex digester. Alternatively, a part of the system may be pulled out of the system and other treatments such as dehydration may be performed.
[0020]
The ratio of returning the concentrated sludge varies depending on the sludge concentration and residence time in the anaerobic digester, but it is preferable to return the amount returned to the anaerobic digester more than the amount returned to the aerobic digester. For example, if the sludge concentration in an anaerobic digester is 1% and the residence time is 20 days, about 10-30% of the concentrated sludge is returned to the aerobic digester, and 70-90% is returned to the anaerobic digester. desirable.
[0021]
Since the aerobic digested sludge is ozone-treated in the method of the present invention, ozone is not wasted in the oxidation of hydrogen sulfide, and therefore, the amount of ozone used can be reduced and treated at a low cost. it can.
In the method of the present invention, water is extracted from the aerobic digestion tank by the submerged membrane separation apparatus, so that the sulfur compound is extracted from the aerobic digestion tank as treated water in the form of sulfate ions. On the other hand, if the treated water is not withdrawn from the aerobic digester, the sulfur compound flows into the anaerobic digester again to become S ^2- , and forms an insoluble salt with metal ions such as iron and enters the system. Accumulate. Therefore, the sludge can be digested at a high decomposition rate by extracting water from the aerobic digester using the submerged membrane separator. For this reason, the decomposition rate of sludge in an anaerobic digestion process improves to 90% or more, and it is also possible to digest sludge completely.
[0022]
【The invention's effect】
As described above, according to the present invention, organic sludge containing biological sludge is aerobically digested and then subjected to ozone treatment. The ozone-treated sludge is subjected to anaerobic digestion and then solid-liquid separation, and a part of the concentrated sludge is subjected to aerobic digestion. Since the remainder is returned to the anaerobic digestion treatment, the organic sludge can be digested at a low cost and at a high decomposition rate.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a system diagram showing an organic sludge digester according to an embodiment. In FIG. 1, reference numeral 1 denotes an aerobic digestion tank, in which a submerged membrane separation device 2 is disposed so as to be immersed in the tank. 3 is an ozone treatment tank, 4 is an anaerobic digestion tank, and 5 is a solid-liquid separation tank.
[0024]
In order to treat organic sludge containing biological sludge with the apparatus of FIG. 1, organic sludge containing biological sludge is introduced into the aerobic digestion tank 1 from the sludge introduction path 11 and concentrated returned from the return sludge path 12. The sludge is introduced into the aerobic digestion tank 1 and mixed with the biological sludge in the tank, and the air supplied from the oxygen supply path 13 is diffused from the aeration device 14 for aerobic biological treatment. In this way, the sludge is aerobically digested and the pump 15 is driven to permeate the separation membrane 16 and filter the water. The filtered water is discharged from the drainage channel 17 to the outside of the system.
[0025]
The solution in the aerobic digestion tank 1 is taken out from the communication path 21 as aerobic digestion sludge and introduced into the ozone treatment tank 3. In the ozone treatment tank 3, ozone generated by the ozone generator 22 is introduced from the ozone supply path 23, and contacted with the aerobic digested sludge introduced from the communication path 21 for ozone treatment. The ozone exhaust gas is discharged from the ozone exhaust gas passage 24. Ozone-treated sludge is introduced into the anaerobic digester 4 from the communication path 25.
[0026]
In the anaerobic digestion tank 4, the ozone treatment sludge introduced from the communication path 25 and the concentrated sludge returned from the return sludge path 26 are introduced and mixed with the biological sludge in the tank to maintain anaerobic and perform anaerobic digestion. Do. When anaerobic digestion is performed with mesophilic methane fermentation bacteria, it is heated to 30-40 ° C. by a heating means (not shown), and when it is performed with high temperature methane fermentation bacteria, it is heated to 45 ° C.-60 ° C. . The generated methane gas is discharged from the methane gas discharge path 27.
[0027]
Anaerobic digested sludge is introduced into the solid-liquid separation tank 5 from the communication path 28, and is separated into solid-liquid separation into separated sludge and separated liquid. The separated sludge is concentrated sludge, a part of which is returned from the return sludge passage 12 to the aerobic digester 1 and the remainder is returned from the return sludge passage 26 to the anaerobic digester 4. When excess sludge is generated, it is discharged from the sludge discharge passage 29. The separated liquid is discharged from the drainage channel 30.
[0028]
In the above treatment method, hydrogen sulfide contained in the organic sludge and concentrated sludge is oxidized in the aerobic digestion tank 1 and converted into sulfuric acid, so that hydrogen sulfide is not introduced into the ozone treatment tank 3. Therefore, ozone is not wasted due to the oxidation of hydrogen sulfide, and the ozone treatment can be performed at low cost by reducing the amount of ozone used. Moreover, since water is filtered by the submerged membrane separator 2 and extracted from the aerobic digester 1, the sulfur compound is extracted in the form of sulfate ions and the volume of sludge to be treated in the anaerobic digester 4 is reduced. Therefore, sludge can be digested at a high decomposition rate, the anaerobic digester 4 can be made smaller, and the amount of heat used for heating is reduced, enabling efficient treatment at low cost. It is.
[0029]
In FIG. 1, the anaerobic digestion tank 4 is performed in one reaction tank, but organic acid fermentation and methane fermentation can be performed in separate reaction tanks.
[0030]
【Example】
Comparative Example 1
Organic sludge mixed with surplus sludge from the activated sludge process treating the sewage and the first sedimentation basin sludge at 1: 1 was adjusted to a concentration of 1.5%, and this was treated sludge (hydrogen sulfide concentration: 12 mg / L). This treated sludge was continuously passed through a 1 liter aerobic digester at a flow rate of 0.5 L / day. The aeration amount was 2 L / hr. This aerobic digested sludge (hydrogen sulfide concentration: below detection limit) 0.5 liter was taken into a washing bottle with a volume of 1 liter, and ozone-containing gas with a concentration of 25 g / m ³ was aerated at a rate of 10 mL / min. The inflow and exhaust gas concentrations were measured, and the ozone treatment was terminated when the ozone consumption reached 50 g / kg-sludge.
[0031]
The ozone-treated sludge was taken out and put into an anaerobic digester. The same amount was withdrawn from the anaerobic digester and this was used as anaerobic digested sludge. In addition, a part of the anaerobic digested sludge was analyzed for sludge concentration. The anaerobic digester had an effective volume of 7.5 liters, and 75 g of sludge from the sewage anaerobic digester was added as seed sludge in terms of dry weight. This operation was continued once a day for 3 months. The ozone treatment and the ozone treatment sludge input to the anaerobic digester were performed 5 times a week.
[0032]
The anaerobic digested sludge was centrifuged at 15000 rpm for 10 minutes, and the supernatant was discarded. The precipitated sludge was divided into three equal parts, 1/3 was returned to the aerobic digester, and the remainder was returned to the anaerobic digester.
As a result, the sludge concentration in the anaerobic digester was initially 15.2 g / L, but after 3 months it became 22.2 g / L, and the sludge concentration increased about 1.5 times in 3 months. .
[0033]
Example 1
The same sludge to be treated as in Comparative Example 1 was continuously passed through a 1 liter aerobic digester at a flow rate of 0.5 L / day. A hollow fiber immersion membrane manufactured by Mitsubishi Rayon Co., Ltd. was immersed in this aerobic digestion tank, and treated water was drawn out at a flow rate of 250 mL / day. The aeration amount was 5 L / hr. The aerobic digested sludge was subjected to ozone treatment and anaerobic digestion in the same manner as in Comparative Example 1.
As a result, the sludge concentration in the anaerobic digester was 15.2 g / L at the beginning and 16.0 g / L after 3 months, and no significant increase was observed.
[Brief description of the drawings]
FIG. 1 is a system diagram showing an organic sludge digester of an embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Aerobic digestion tank 2 Submerged membrane separation apparatus 3 Ozone treatment tank 4 Anaerobic digestion tank 5 Solid-liquid separation tank 11 Sludge introduction path 12, 26 Return sludge path 13 Oxygen supply path 14 Aeration apparatus 15 Pump 16 Separation membrane 17, 30 Drainage channels 21, 25, 28 Communication channel 22 Ozone generator 23 Ozone supply channel 24 Ozone exhaust gas channel 27 Methane gas discharge channel 29 Sludge discharge channel

Claims (1)

An aerobic digestion process in which organic sludge containing biological sludge is introduced into an aerobic digester equipped with a submerged membrane separation device, and aerobically biotreated;
An ozone treatment process for ozone treatment of aerobic digested sludge biologically treated in the aerobic digestion process;
An anaerobic digestion process for anaerobically digesting the ozone-treated sludge ozone-treated in the ozone treatment process in an anaerobic digestion tank;
A solid-liquid separation process for solid-liquid separation of the anaerobic digested sludge biologically treated in the anaerobic digestion process into concentrated sludge and separated liquid;
A method for digesting organic sludge comprising a returning step of returning a part of the concentrated sludge separated in the solid-liquid separation step to an aerobic digester and returning the remainder to the anaerobic digester.

Images