JPH0470079B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for anaerobically digesting organic sludge such as sewage sludge and agricultural and fishery waste. [Conventional technology] Initial settling sludge, surplus sludge, digested sludge, etc. generated in sewage treatment plants are sometimes incinerated or disposed of after dehydration, but in recent years, they have been treated using anaerobic digestion methods. The number has increased. The anaerobic digestion method is a method in which organic matter in sludge is mainly decomposed into methane gas by growing so-called methane bacteria in an anaerobic state, thereby reducing the amount of sludge solids. Anaerobic digestion of sludge has many advantages, including the ability to obtain methane gas that can be used as fuel, low energy consumption, high kill rate of pathogenic bacteria, and easy disposal of digested sludge. However, since the growth of methane bacteria is extremely slow, there is a problem in that the treatment takes a long time. For example, when surplus sludge generated from an activated sludge tank is treated using this method, the digestibility rate (organic matter decomposition rate ), a long residence time of 15 to 20 days was required. Furthermore, even if the retention period was extended beyond that, the upper limit of the achievable digestion rate was about 45%. Similar problems occur when attempting to anaerobically digest waste from agricultural and marine product processing factories such as mills, livestock manure, livestock processing waste, fish iliac bones, etc. The penetration rate of anaerobic digestion is still extremely low. [Problems to be Solved by the Invention] As described above, the conventional anaerobic digestion method is inefficient, requires a large-sized digestion tank, and has an unsatisfactory effect on solid matter weight loss. Therefore, the present invention aims to solve the above-mentioned problems in the conventional anaerobic digestion method and to improve the anaerobic digestion method so that a higher digestibility can be achieved in a shorter time. [Means for Solving the Problems] In the anaerobic digestion method successfully provided by the present invention, organic sludge is first subjected to a treatment capable of crushing the cell walls of microorganisms in the sludge (hereinafter referred to as
This treatment is called cell wall disruption treatment. Cell wall disruption treatment does not include biological solubilization treatment. ) Next, the sludge solubilized by cell wall crushing was supplied to an anaerobic digestion tank in which methane bacteria, which exists in sewage sludge and grows vigorously at pH 7.8 to 8.5, was present as the main methane flora, and the pH was adjusted to 7.8. ~8.5, Anaerobic digestion is performed without returning the seed sludge under conditions where the retention period is 5 days or less. The anaerobic digestion method of the present invention will be explained in more detail below. In the method of the present invention, well-known methods known to be effective for disrupting the cell walls of microorganisms in sludge, such as wet milling, ultrasonic treatment, and heat treatment, are appropriately used for the cell wall disruption treatment performed as a pretreatment for anaerobic digestion. You can do it by doing this. However, the preferred method is wet milling. This process is
It is a process that micronizes solids by mainly applying shearing frictional force to solids suspended in water. A high degree of grinding is performed using a mill, colloid mill, etc. Among the wet milling methods, the medium agitation milling method is particularly preferable because it has the best processing effect. This media stirring type mill violently stirs the small balls and beads in the container by rotating a stirring disk inserted in a cylindrical container at high speed, creating a shearing friction force between the beads and grinding them. Beads of various sizes are used depending on the purpose, but the preferred beads for sludge solubilization are:
The particle size is 0.05 to 1 mm. In that case, the rotation speed of the stirring disk should be 1000 to 3000 rpm (peripheral speed 10 to 3000 rpm).
30 m/sec), and the residence time of the sludge to be treated is approximately 5 to 60 minutes when treating ordinary sludge. Anaerobic digestion of sludge after cell wall crushing is carried out in a single anaerobic digestion tank as follows. Solubilized sludge is adjusted to pH 7.8 by adding alkali.
Adjust to ~8.5 and feed into the digester. or,
Continuously inject alkali into the digestion tank to maintain the pH of the sludge in the tank within the above range. Normally, the pH suitable for the growth of methane bacteria in sewage sludge is considered to be weakly alkaline, less than 8. However, in the case of single-tank digestion in which acid-producing bacteria and methane bacteria coexist, a pH of 7 to 7.6 is appropriate. Therefore, in the digestion method of the present invention, the pH is set to a much higher value than conventional ones.
This further progresses the solubilization of the sludge components, making it easier for methane fermentation to occur. However, higher than 8.5
Although adjusting the pH is advantageous in terms of promoting solubilization,
This is not preferable because it becomes difficult to accumulate a sufficient amount of methane bacteria that actively proliferate in the tank. As mentioned above, alkaliphilic fast methane bacteria that proliferate vigorously in sludge with high pH are rare in normal sewage sludge and digested sludge from conventional anaerobic digestion tanks, but they are found in sludge in anaerobic digestion tanks. For example, by providing the following acclimatization period at the start of operation,
The methane bacteria can account for most of the methane bacteria flora in the tank. That is, digested sludge, which is used as seed sludge, is collected from a sewage treatment plant and put into a digestion tank, and the supply of organic sludge is started. Continuously add alkali to keep the pH within the above range. After continuing the operating conditions in which the residence time is approximately 10 days or more (for example, about 2 weeks), the residence time is gradually shortened, and finally the residence time is reduced to approximately 5 to 2 days. During this time, the sludge discharged from the digestion tank will not be returned as seed sludge. Thereby, it is possible to prepare an anaerobic digestion tank in which alkaliphilic fast methane bacteria are predominant and present in a high number of bacteria.
If acclimatization was carried out to the specified pH without providing an acclimatization period to gradually shorten the retention period as described above, and by returning the seed sludge, the alkaliphilic fast methane bacteria would be “concentrated”. is slow and remains at a low level. After acclimatization is completed, anaerobic digestion can be started as normal operation by continuing to supply the sludge to be treated. In this process, the number of days of residence should not exceed 5 days. A particularly preferable length of stay is
It will take 2-3 days. If the retention period is too long, the ratio of alkaliphilic fast methane bacteria to the methane bacteria in the tank will decrease, leading to a decrease in digestibility.
In addition, the supply of seed sludge (returning a portion of the digested sludge), which is commonly done in conventional anaerobic digestion methods, is not carried out because it causes a decrease in the proportion of alkaliphilic fast methane bacteria in the tank. In the digestion method of the present invention, since the methane bacteria used are alkaliphilic fast methane bacteria that proliferate vigorously, a high rate of digestion can be achieved even with a short retention period as described above and even if the operation is continued without the supply of seed sludge. A sufficient number of bacteria can be maintained in the tank. [Example] Anaerobic digestion according to the present invention and comparative experiments were conducted on surplus sludge and mixed sludge (mixture of surplus sludge and initial settling sludge) collected from a city sewage treatment plant. The composition of the sludge and the experimental conditions are as follows. The length of stay is 3 days in each case.
Seed sludge was not returned. Sludge composition: TS (%) VS (%) Surplus sludge 3.9 3.2 Mixed sludge 4.1 3.3 Experimental conditions: Cell wall crushing treatment* a (comparative example) None b (comparative example) Yes A (example) Yes PH in the digestion tank a ( Comparative example) No adjustment (6.3 to 6.8) b (Comparative example) Same as above A (Example) 8.0 (adjusted with casein soda) *Cell wall crushing treatment is performed using a media stirring type pearl mill (Ashizawa Co., Ltd., model PMISTS)
Bead diameter 0.2mm, disk rotation speed 1300rpm,
The test was carried out using a circumferential speed of 6 m/sec, and the residence time was 5 minutes. Digestion tank: Completely mixed type Acclimation at the start of the experiment was carried out for a total of 6 weeks using digested sludge collected from a sewage treatment plant as the initial seed sludge as follows. First 2 weeks: 10 days of residence Next 2 weeks: 5 days of residence Last 2 weeks: 3 days of residence For 10 days after acclimatization, digestibility, gas production, and methane concentration in the gas were measured. The average values of the measured values are shown in Tables 1 and 2.

【table】

〔Effect of the invention〕

According to the anaerobic digestion method of the present invention, as mentioned above, it is possible to achieve a high digestion efficiency equal to or higher than the highest level of conventional methods with an extremely short residence time. Not only can the processing capacity per volume be greatly improved or the size of the digester tank significantly reduced, the amount of digested sludge generated can also be reduced, making subsequent treatment easier.

Claims (1)

[Claims] 1 Organic sludge is treated to crush the cell walls of microorganisms in the sludge, and the sludge solubilized by cell wall crushing is added to the sewage sludge with a pH of 7.8 to 8.5.
The anaerobic digestion tank is supplied with methane bacteria, which actively proliferates in
8.5. A sludge anaerobic digestion method characterized by anaerobic digestion without returning seed sludge under conditions where the retention period is 5 days or less.