JPH0470078B2 - - 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. has become more common. 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 was a problem in that the treatment required 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] The anaerobic digestion method provided by the present invention involves adding alkali to organic sludge to a concentration of 0.01 to 0.1N and subjecting it to wet mill treatment, and the treated sludge is , methane bacteria that exist in sewage sludge and multiply vigorously at pH 7.8 to 8.5 (hereinafter referred to as alkaliphilic fast methane bacteria)
is supplied to an anaerobic digestion tank where methane bacteria exist as the main flora, and anaerobic digestion is performed without returning the seed sludge under conditions where the pH is 7.8 to 8.5 and the residence time is 5 days or less. shall be. The anaerobic digestion method of the present invention will be explained in more detail below. In the method of the present invention, the wet milling process performed as a pretreatment for the anaerobic digestion method is a process in which the solids suspended in water are pulverized by mainly applying shear frictional force to the solids. Specifically, a high degree of grinding is performed using a rotating cylindrical mill, a vibrating ball mill, a centrifugal ball mill, a media stirring mill, a colloid mill, or the like. Among the wet milling methods, the media agitation mill is particularly preferred since 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 beads with a particle size of 0.05 to 1 mm are preferred for sludge solubilization. In that case, the rotation speed of the stirring disk is
Approximately 1000 to 3000 rpm (circumferential speed 10 to 30 m/sec) and residence time of the sludge to be treated is approximately 5 to 60 minutes when ordinary sludge is treated. The wet mill treatment of the present invention, which is performed with the addition of an alkali, significantly improves the solubilization rate of organic components (proteins, sugars, etc.) than when no alkali is added. Anaerobic digestion of sludge after wet milling is carried out in a single anaerobic digestion tank as follows. The pH of the sludge is adjusted to 7.8 to 8.5 by adding alkali or acid as necessary before supplying it to the digestion tank. Alternatively, alkali is continuously injected into the digestion tank to maintain the pH of the sludge in the tank within the above range. Normally, the optimum pH for the growth of methane bacteria in sewage sludge is 8.
However, in the case of single-vessel digestion in which acid-producing bacteria and methane bacteria coexist, a pH of 7 to 7.6 is considered appropriate. is set to a much higher value than before. This further progresses the solubilization of the sludge components, making it easier for methane fermentation to occur. however,
Although setting the pH higher than 8.5 is advantageous in terms of promoting solubilization, it is not preferable because it makes it difficult to accumulate a sufficient amount of actively proliferating methane bacteria in the tank. As mentioned above, alkaliphilic fast methane bacteria, which proliferate vigorously in sludge with high pH, are rare in normal sewage sludge and digested sludge from conventional anaerobic digestion tanks, but in anaerobic digestion tanks. By providing the following acclimatization period for the start of operation,
The methane bacteria can account for most of the methane bacterial flora in the tank. That is, digested sludge to be used as seed sludge is collected from a sewage treatment plant and put into a digestion tank, and supply of organic sludge solubilized by wet milling and addition of alkali is started. Continuously add alkali to keep the PH in the high range mentioned above. After maintaining operating conditions for a while (for example, about 2 weeks) with a residence time of approximately 10 days or more, the residence time is gradually shortened until the residence time is 5 to 1.5 days.
Shorten it to about a day. 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 the training 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 preferred residence time is 2 to 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. Sludge composition: TS (%) VS (%) Surplus sludge 4.2 3.4 Mixed sludge 3.9 3.2 Wet mill processing conditions: Equipment used: Media stirring mill/pearl mill (Ashizawa Co., Ltd., model PMISTS, bead diameter 0.2 mm,
Disk rotation speed 1300 rpm, peripheral speed 6 m/sec Residence time: 5 minutes Added alkali: 25% caustic soda with NaOH concentration
Anaerobic digestion conditions: Hydraulic residence time: 2 days Digestion temperature: 37℃ Digestion tank: Fully mixed type No return of seed sludge Acclimatization at start of experiment: Digested sludge collected from a sewage treatment plant Use the initial seed sludge as follows:
It was conducted for a week. First 2 weeks: 10 days of residence Next 2 weeks: 5 days of residence Last 2 weeks: 2 days of residence In parallel with the above examples, the following comparative examples were carried out. Comparative Example 1 The same anaerobic digestion is performed without any pretreatment for the anaerobic digestion treatment. Comparative Example 2 Same as above except that no alkali was added in the wet milling process. Comparative Example 3 The procedure was the same as above except that the wet milling process for adding alkali was replaced with simple stirring. Digestion efficiency, amount of gas generated, methane content of gas, and amount of volatile organic acids in each case were measured for 10 days after acclimatization. The average values of the measured values are shown in Tables 1 and 2. In Comparative Example 3, in which the pretreatment was solely alkali treatment, no improvement was observed in either the digestibility or the amount of gas generated. In Comparative Example 2, which was milled without adding alkali, the digestibility was 1.4 to 1.7 times higher, and the amount of gas generated was 1.5 to 1.7 times higher. In contrast, in the examples of the present invention, the digestibility was 2.1 to 2.6 times higher.
The amount of gas generated increased approximately three times.

【table】

〔Effect of the invention〕

According to the anaerobic digestibility of the present invention, as mentioned above, it is possible to achieve a high digestibility that is 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 Add alkali to organic sludge at a concentration of 0.01~
The sludge was added to a concentration of 0.1N and subjected to wet mill treatment, and the treated sludge was subjected to anaerobic digestion in which methane bacteria, which exist in sewage sludge and proliferate vigorously at pH 7.8 to 8.5, were present as the main methane flora. Supply to the tank and adjust the pH to 7.8~
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.