JPH0438480B2 - - 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 at 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 decomposes organic matter in sludge mainly into methane gas by growing anaerobic microorganisms such as methane bacteria in an anaerobic state, thereby reducing the amount of sludge solids. Anaerobic digestion of sludge has many advantages, including the production of methane gas that can be used as fuel, low energy consumption, and a high rate of killing pathogenic bacteria. However, the conventional anaerobic digestion, which has a pH of 5 to 7, a temperature of 30 to 40 degrees Celsius, and a retention period of 20 to 30 days, requires a long time, but the digestibility is low at about 40 to 50%, and the sludge The weight loss effect is not sufficient. In addition, anaerobic pathogenic bacteria such as Clostridium tetani and Clostridium botulinum often survive in the sludge, and if the digested sludge is disposed of as is, there is a risk of contaminating the environment with pathogenic bacteria. [Problems to be Solved by the Invention] Therefore, an object of the present invention is to provide an anaerobic digestion method that has a higher organic matter digestibility and a higher pathogen killing rate than conventional anaerobic digestion methods. [Means for Solving the Problems] The anaerobic digestion method of the present invention, which has succeeded in achieving the above object, involves adding alkali to organic sludge.
It is characterized by adjusting the pH to 9 to 11 and performing anaerobic digestion using a group of alkaliphilic microorganisms that can grow in an anaerobic state under this pH condition. The anaerobic digestion method of the present invention will be explained in more detail below. Examples of organic sludge that can be treated by the method of the present invention include surplus sludge generated in sewage treatment plants, initial settling sludge, and mixtures of these sludges, as well as those generated during the treatment of agricultural and fishery waste and food factory wastewater. This includes, but is not limited to, sludge. Before the sludge to be treated is subjected to anaerobic digestion according to the present invention, microbial cell walls may be crushed by wet milling, ultrasonic treatment, heat treatment, etc., and cell components may be solubilized. The sludge charged to the anaerobic digestion tank is made of caustic soda,
Add an alkali such as soda carbonate to adjust the pH to 9-11 before feeding it to the digestion tank. Alternatively, by continuously injecting alkali into the digester tank, the pH of the sludge in the tank can be adjusted.
be maintained 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, the PH has traditionally been adjusted. Since the pH value is around 7 in both cases and cases without adjustment, the pH in the digestion method of the present invention is set to a much higher value than in the past. In the digestion method of the present invention, due to such a high pH value, the sludge components first become solubilized and become susceptible to digestion by microorganisms. Second, pathogenic microorganisms are killed off to the point where they are no longer detectable. However, it is preferable to set the pH higher than 11 because, although it is advantageous in terms of promoting solubilization, it becomes difficult to accumulate a sufficient amount of methane bacteria that grow in such strongly alkaline sludge in the tank. do not have. As mentioned above, alkaliphilic 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 they are Same PH to start
By providing an acclimatization period during which the anaerobic bacteria are maintained at a constant temperature, the anaerobic bacteria can occupy the majority of the anaerobic bacterial flora in the tank. Once the alkaliphilic anaerobic bacteria group is prepared by acclimatization, the sludge to be treated is supplied to the digestion tank and anaerobic digestion is started as normal operation. In this treatment, the temperature inside the tank is not particularly limited, but 30 to 40℃ is appropriate, and the residence time is 10 to 40℃.
20 days is appropriate. [Example] Surplus sludge and mixed sludge collected from a city sewage treatment plant (1:1 mixture of surplus sludge and initial settling sludge)
We conducted anaerobic digestion according to the present invention and comparative experiments. The composition of the sludge and the experimental conditions are as follows. Sludge composition: TS(%) VS(%) Surplus sludge 4.0
3.2 Mixed sludge 3.8
3.0 Experimental conditions: Temperature: 35℃ Retention days: 20 days PH: Adjusted to 9.0 or 10 with caustic soda (no adjustment in comparative examples) Acclimation: Digested sludge collected from a sewage treatment plant was used as seed sludge and carried out for 8 weeks Acclimation For 10 days after completion, the digestibility, amount of gas generated, methane content of gas, and amount of volatile organic acids were measured. Tables 1 and 2 show the average values of the measured values and the results of examining the presence or absence of pathogenic microorganisms in the digested sludge.

[Table] Pathogenic microorganisms Detected Not detected Not detected

[Table] Pathogenic microorganisms Detection Not detected Not detected [Effects of the invention] As described above, the anaerobic digestion method of the present invention can achieve a high digestibility that exceeds the conventional level. In addition, the digested sludge discharged when carrying out the anaerobic digestion of the present invention not only does not contain pathogenic microorganisms, but also contains the remaining anaerobic microorganisms that have grown in strong alkaline conditions and are naturally occurring. Since most of the digested sludge will die if released into the environment, it has the advantage of being less likely to pollute the environment when disposed of.

Claims (1)

[Claims] 1 Add alkali to organic sludge to raise the pH to 9-11
An anaerobic digestion method for sludge, which is characterized by adjusting the pH to 100% and performing anaerobic digestion using a group of alkaliphilic microorganisms that can grow in an anaerobic state under these pH conditions.