JP3447037B2 - Aerobic digestion of activated sludge - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for significantly reducing excess activated sludge discharged from an activated sludge treatment facility for organic wastewater. 2. Description of the Related Art A large amount of excess activated sludge is generated from an activated sludge treatment facility for organic sewage such as a sewage activated sludge treatment facility, and the treatment and disposal of such excess activated sludge is currently the biggest problem. Has become. Conventionally, excess activated sludge is difficult to dewater, so a large amount of dehydration aid (polymer, etc.) is added, dewatered by a dehydrator, and then incinerated.
Dehydration aid costs are high, and dehydrators and incinerators are large-scale, and equipment costs and maintenance costs are high. In addition,
Conventionally, there are anaerobic digestion and aerobic digestion as sludge reduction methods for biologically reducing the excess activated sludge.
It is hardly used recently because the weight reduction rate is inferior to the one requiring a long residence time of 0 day or more.
Also, as shown in FIG. 2, a method of directly adding ozone to activated sludge to solubilize the sludge and then aerobically digesting it (Japanese Patent Publication No. 577-1)
9719), but applying this method to the reduction of excess activated sludge from biological treatment facilities for organic sewage requires a large amount of added ozone and increases the cost of ozone. poor. [0003] The above-mentioned technique of directly adding ozone to surplus activated sludge to solubilize sludge also has the following problems. In other words, when ozone is directly added to the excess activated sludge to partially solubilize the sludge and then sent to the aerobic digestion tank to perform aerobic digestion, there is considerable sludge that is not solubilized in the supplied activated sludge and is not solubilized. Aerobic digestion of sludge is extremely difficult. A large amount of new sludge is produced from the supplied biological sludge containing BOD produced by partially solubilizing. Since ozone is not added to this newly generated sludge, aerobic digestion of these sludges does not easily proceed. SUMMARY OF THE INVENTION The present invention is directed to a process for aerobic digestion of activated sludge by directly adding ozone to the activated sludge, and the surplus generated when aerobic digestion of activated sludge is performed at low cost. The challenge is to provide a new technology that can completely reduce the amount of activated sludge. It is an object of the present invention to eliminate the need for a sludge dehydrator and an incinerator and to streamline the sludge treatment process. SUMMARY OF THE INVENTION The present invention combines aerobic digestion and solubilization of sludge by ozone oxidation in a novel manner,
Ozone oxidation while aerobic digestion of activated sludge realizes effective sludge reduction. That is, in the aerobic digestion method of the present invention, the excess activated sludge discharged from the activated sludge treatment facility of the organic wastewater is subjected to aerobic digestion and then to solid-liquid separation, and a large amount of the separated sludge separated by the solid-liquid separation is obtained. Returning the part to the aerobic digestion step, treating the sludge by adding ozone to a part of the separated sludge, and supplying the separated sludge to the aerobic digestion step together with most of the separated sludge. It is an aerobic digestion method. Hereinafter, the sludge generated from the activated sludge treatment facility of the organic wastewater is referred to as activated sludge, and the sludge obtained by aerobically digesting the activated sludge is referred to as “biologically treated sludge”. As described above, when ozone is directly added to activated sludge to chemically solubilize it by ozone oxidation, a large amount of ozone is required. In the embodiment of the present invention, activated sludge is preferably used. Ozone is added to the biologically treated sludge which is easily digested by the aerobic microorganisms by digestion into a biologically treated sludge, and ozone is oxidized and solubilized to reduce the amount thereof. In addition, when ozone is added to a part of the biologically treated sludge and solubilized, the amount of ozone remaining in the solubilized sludge is small. Is less affected. In the aerobic digestion process, maintaining the sludge concentration in a suitable and constant range makes the digestion treatment efficient, but in order to maintain the predetermined concentration, a part of the digested sludge is preferably used. It is desirable to withdraw from the temper digestion process. In the present invention, ozone is added to a part of the sludge extracted from the system from the aerobic digestion step to solubilize the sludge by chemical oxidation, and the excess sludge is returned by returning the solubilized sludge to the aerobic digestion step. Along with the reduction, the sludge concentration in the aerobic digestion process can also be maintained at an appropriate range of sludge concentration. [0008] The appropriate addition amount of ozone for effectively solubilizing the biotreated sludge is approximately 10 to 20% based on the biotreated sludge solids. In the present invention, ozone is added to a part of the biologically treated sludge extracted from the aerobic digestion process to the outside of the system, and the ozone is oxidized, so that the biological sludge is directly chemically ozone-oxidized and solubilized. Since the amount of ozone used can be significantly reduced, the cost of ozone can also be significantly reduced. The sludge solubilized by chemical oxidation by adding ozone to the biologically treated sludge (solubilized sludge) is then supplied to an aerobic digestion process in which the solubilized sludge is further aerated in the presence of aerobic microorganisms, and solubilized. Decompose sludge into carbon dioxide and water by aerobic microorganisms. In another embodiment of the present invention, the aerobic digestion step of adding ozone to the biologically treated sludge, oxidizing it with ozone, and further aerobic digesting the solubilized solubilized sludge includes the aerobic digestion step of the preceding step. Instead of directly refluxing the wastewater, a biological treatment step in which an aerobic digestion step such as an activated sludge method for wastewater such as another sewage is performed may be used or may be interposed therebetween. In the present invention, as means for solid-liquid separation in the aerobic digestion step, (i) a method of storing in a sedimentation basin, and sedimentation of the settling sludge and solid-liquid separation by (i)
i) A method of performing solid-liquid separation using a filter equipped with a separation membrane, and the like. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One example of the method for reducing organic sludge of the present invention will be described below with reference to FIG. Surplus activated sludge 1 discharged from activated sludge treatment facility of organic wastewater such as sewage
Is supplied to the aerobic digestion tank 2 and oxygen-containing gas (air, etc.)
Aerobically digested with aerosol. Activated sludge self-digests by aerobic digestion, and a part (about 30%) of sludge is decomposed into carbon dioxide and water. However, most of the sludge (about 70%) is hardly self-digested even if it is aerated for a long time, and weight loss does not progress. The biologically treated sludge is sent from the aerobic digestion tank 2 to the solid-liquid separation tank 3. In the present invention, the precipitated separated sludge 4 is withdrawn from a solid-liquid separation tank 3 (for example, a settling tank) subsequent to the aerobic digestion tank 2, and most of the separated sludge is returned to the aerobic digestion tank 2 as returned sludge 5, and a part of the sludge is returned Is transferred to the ozone oxidation tank 7 as the transfer sludge 6, and the ozone 8 is added to the sludge in the ozone oxidation tank 7 to solubilize the sludge by an oxidation reaction.
The solubilized sludge 9 is returned to the aerobic digestion tank 2 together with the return sludge 5. In the method of the present invention, a part of the biologically treated sludge obtained by digestion by aerobic digestion is branched, and the branched sludge is solubilized by adding ozone, and the solubilized sludge is subjected to aerobic digestion. As a result of returning to the tank 2, it was recognized that the self-digestion of the sludge proceeded extremely effectively. It is considered that the reason for this is that the oxidizing action of ozone easily breaks down polysaccharides and strong microbial cell walls that hinder the progress of autolysis of sludge, and facilitates autolysis. According to the present invention, activated sludge is reduced almost completely, and almost no sludge is generated outside the system from the process of FIG. Therefore, a sludge dewatering machine, a sludge incinerator and the like are not required, or the size thereof can be significantly reduced. The hydraulic retention time of the aerobic digestion tank in the present invention may be about 7 days, which is reduced to about の of the number of days required for the conventional aerobic digestion method. In the present invention, an appropriate amount of ozone to be added is as small as 3 to 5% per kg of solid matter of the branched biological treatment sludge. Since the present invention employs a method of adding a small amount of ozone while aerobic digestion to reduce the factors inhibiting digestion, the above-mentioned known technique shown in FIG. (Aerobic digestion after solubilization)) requires less ozone. In the method for reducing the amount of activated sludge according to the present invention, an interesting effect was observed when the biologically treated sludge digested in the aerobic digestion tank was subjected to solid-liquid separation by a membrane separation means. That is, it was recognized that membrane contamination was reduced in membrane separation. For this reason, in the conventionally known excess sludge treatment step, it is necessary to perform solid-liquid separation by a precipitation method, whereas in the present invention, even if solid-liquid separation is performed by more efficient membrane separation means, there is almost no need to replace the separation membrane. The advantage of sufficient use was recognized. The experimental conditions based on the process of FIG. 1 are shown in Table 1. Excess activated sludge from sewage (solid concentration: 5.0 g / liter) was treated according to the treatment flow shown in FIG.
The sludge throughput is 10 liters / day (50 g / day).
The volume of the aerobic digestion tank 2 used in the aerobic digestion process is 70
Liter, the digestion tank 2 is aerated with air, the amount of dissolved oxygen in the liquid to be treated in the tank is maintained at 5 to 6 mg / liter, the temperature in the digestion tank 1 is 25 ° C., and the sludge concentration is 5 to 6 g. / Liter. The biologically treated sludge generated by the aerobic digestion is subjected to solid-liquid separation by a precipitation method, and the separated water separated there is discharged out of the system. Most of the separated sludge 4 is returned to the aerobic digestion tank 2 as returned sludge 5. I will send it back. A part of the separated sludge is used as the transfer sludge 6 in the ozone oxidation tank 7 having the specifications shown in Table 1 below.
Then, ozone 8 is added to the sludge, and the sludge is solubilized by an oxidation reaction. The solubilized sludge 9 is returned to the aerobic digestion tank 2 together with the return sludge 5. [Table 1] As a result of continuing the test under these conditions for 9 months, 96% of the total weight of the supplied excess activated sludge (9 kg) was decomposed, and the amount of sludge discharged outside the system was very small. Advantages of the Invention The effect of reducing excess activated sludge is remarkably large, and the sludge treatment and disposal process is rationalized. The amount of sludge discharged out of the system can be made very small. 2. Since the amount of ozone used for activated sludge is significantly reduced, the cost of adding ozone is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a process flow of sludge reduction according to the present invention in which activated sludge is subjected to aerobic digestion and then a part of the separated sludge is ozone-oxidized. FIG. 2 shows a process flow of conventional sludge reduction by directly oxidizing activated sludge with ozone. [Description of Signs] 1 Activated activated sludge 2 Aerobic digestion tank 3 Solid-liquid separation tank 4 Separated sludge 5 Returned sludge 6 Transfer sludge 7 Ozone oxidation tank 8 Ozone 9 Solubilized sludge 10 Supply biological sludge 11 Ozone oxidation tank 12 Aerobic digestion Tank 13 Settling tank 14 Returned sludge

Claims (1)

(57) [Claims] [Claim 1] Excess activated sludge discharged from an activated sludge treatment facility of organic wastewater is subjected to aerobic digestion and then to solid-liquid separation.
Most of the separated sludge separated by the solid-liquid separation is returned to the aerobic digestion step, and ozone is added to a part of the separated sludge for treatment, and the aerobic digestion is performed together with most of the separated sludge. An aerobic digestion method for activated sludge, which is supplied to a process.