JPH0470080B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a method for treating organic sludge such as sewage sludge and agricultural and fishery waste. [Prior Art] In standard microbial treatment of municipal sewage at a sewage treatment plant, sewage is first subjected to aerobic microbial treatment in an aeration tank, and then sent to a settling tank. The supernatant discharged from the settling tank is sent to a secondary treatment process, while the large amount of settled sludge, or surplus sludge, is sometimes incinerated or disposed of after thickening, but it is difficult to dewater and incinerate it. Not only do they have dangerous properties, but disposal also poses the problem of dispersing pathogenic microorganisms and degrading the environment, so it is necessary to take steps to reduce the amount as much as possible before disposal. A typical treatment method for this purpose is anaerobic digestion, in which organic matter is decomposed into organic acids, methane gas, carbon dioxide gas, etc. using the metabolism of anaerobic microorganisms. However, excess sludge is like a mass of microorganisms and is extremely difficult to decompose, so the number of days required for anaerobic digestion is as long as 20 to 30 days in the case of mesophilic digestion (temperature of 35 to 37°C). Moreover, even after spending that many days, the decomposition rate of organic matter remains at about 40 to 50%, so the effluent still contains a high concentration of organic matter, which poses a heavy burden for subsequent water treatment. [Problems to be Solved by the Invention] As mentioned above, the conventional sludge treatment method is inefficient, requires a large anaerobic digestion tank, and has an unsatisfactory solids reduction effect.
In addition, the discharged water has a high concentration of organic matter, so it cannot be discharged as is, and even if it were to be further treated, it would be a heavy burden. Therefore, the present invention aims to solve the above-mentioned problems in conventional sludge treatment methods and to provide a sludge treatment method that can achieve a higher rate of weight loss and decomposition of organic matter in a shorter period of time. [Means for Solving the Problems] The sludge treatment method provided by the present invention includes adding alkali to organic sludge until its pH becomes 9 to 11 to cause solubilization of microbial cells in the sludge, Furthermore, residual microbial cells etc. are crushed by wet milling, and the treated sludge is subjected to anaerobic digestion treatment at pH 9 to 11 for a retention period of 2 to 5 days. It is characterized by subjecting water to aerobic digestion treatment using alkaliphilic microalgae. Hereinafter, the sludge treatment method of the present invention will be explained in more detail. Alkali treatment of sludge is carried out by adding an alkali such as caustic soda or soda carbonate until the pH of the sludge reaches 9 to 11, and maintaining the mixture at a temperature of about 20 to 30°C for about 0.5 to 1 hour. This destroys the cell walls of microorganisms in the sludge and causes so-called solubilization, in which intracellular components such as proteins and sugars are eluted. Although making the sludge strongly alkaline to PH11 or above is effective in promoting solubilization, it may cause a sufficient amount of methane bacteria to accumulate in the digester, which grows in such strongly alkaline sludge during the subsequent anaerobic digestion process. This is not desirable because it becomes difficult. The alkali-treated sludge is further subjected to wet mill treatment. This wet milling process is a process in which solids are pulverized by mainly applying shearing frictional force to the solids suspended in water. A high degree of grinding is performed using a type ball mill, a media stirring type mill, a colloid mill, etc. 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 beads in the container by rotating a stirring disk inserted into 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 solubilizing sludge.
In that case, the rotation speed of the stirring disk should be 1000~
Approximately 3000 rpm (peripheral speed 10 to 30 m/sec), the residence time of the sludge to be treated is 5 to 60 when treating normal sludge.
About a minute is appropriate. Microbial cells and fibrous organic matter that were not solubilized by the alkali treatment are also solubilized by this wet mill treatment or crushed into a form that is susceptible to anaerobic digestion. Although solubilization and crushing can be achieved by wet milling alone, the effects of wet milling are more pronounced and appear in a shorter time for sludge constituents that have swelled and been partially solubilized by alkali treatment. Anaerobic digestion of sludge after wet milling is carried out in a single anaerobic digestion tank as follows. The sludge is strongly alkaline due to the alkali treatment, but if necessary, alkali is added to the sludge and the sludge is supplied to the anaerobic digestion tank at a pH of 9 to 11. Alternatively, alkali is continuously injected into the digestion tank to keep the inside of the tank at the above pH. Normally, the optimum pH for the growth of methane bacteria in sewage sludge is 8.
It is said to be weakly alkaline, with a PH of less than
Conventionally, pH is around 7 both when adjusted and when not adjusted, so in the digestion method of the present invention, the pH is set to a much higher value than conventionally. In the digestion method of the present invention, due to such a high pH value, firstly, the solubilization of the sludge components further progresses, making them susceptible to digestion by anaerobic microorganisms. Second, pathogenic microorganisms are killed off to the point where they are no longer detectable. As mentioned above, alkaliphilic methane bacteria, which proliferate vigorously in sludge with a high pH, are rare in normal sewage sludge and digested sludge from conventional anaerobic digestion tanks, but when operating an anaerobic digestion tank, Tank pH at start
By providing an acclimatization period in which the anaerobic bacterial flora is maintained at 9 to 11, 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 2 to 40℃.
It will be 5 days. If the retention period is too long, the proportion of alkaliphilic methane bacteria among the methane bacteria in the tank will decrease, leading to a decrease in digestibility. The sludge discharged from the anaerobic digestion tank can be appropriately disposed of as it is after separating the solid and liquid, but in the treatment method of the present invention, the liquid phase portion is further subjected to aerobic digestion treatment using alkaliphilic microalgae. I do. The treatment requires a residence time of 4 to 10 days. This treatment reduces the content of organic matter in wastewater by using the organic matter that has been solubilized through alkali treatment and wet milling that has not been decomposed by anaerobic microorganisms as a nutrient source for the algae. Suitable algae to be used are macroscopic microalgae that can be easily separated later, such as Spirulina and Anabaena. The reason why alkaliphilic algae is particularly used in this treatment is that it can treat wastewater from an anaerobic digestion tank that has been subjected to strong alkalinity without neutralizing it, there is little fear of bacterial growth, and culture management is easy. . The present invention has been described above for the case of treating surplus sludge, but the treatment method of the present invention is of course suitable for treating other sludge generated in sewage treatment plants, as well as in agricultural and fishery processing plants such as mills. It can also be applied to the disposal of waste from animals, livestock manure, livestock processing waste, fish iliac bones, etc. [Example] Surplus sludge and mixed sludge collected from a city sewage treatment plant (1:1 mixture of surplus sludge and initial settling sludge)
For this purpose, treatment according to the present invention and comparative experiments were conducted. The composition of the sludge and the experimental conditions are as follows. Sludge composition: TS (%) VS (%) Surplus sludge 4.0 3.0 Mixed sludge 3.8 3.1 Alkaline treatment conditions: Add 25% caustic soda solution until the sludge pH reaches 10 and stir for 30 minutes at room temperature. Wet mill processing conditions: Equipment used: Media stirring type pearl mill (Ashizawa Co., Ltd., model PM1STS, bead diameter 0.2
mm, disc rotation speed 1300 rpm, peripheral speed 6 m/
sec Residence time: 5 minutes Anaerobic digestion conditions: Use a 5 volume fermenter, continuously stir at 300 rpm, and maintain PH at 10 with a PH controller. Digestion temperature: 37℃ Residence days: 2 days Acclimatization at the start of the experiment: Mesophilic digested sludge collected from a sewage treatment plant was used as the initial seed sludge and carried out as follows First 2 weeks: Residence days 10 days Next 2 weeks : Retention days: 5 days Last 2 weeks: Residence days: 2 days Treatment with alkaliphilic algae: Algae used: Spirulina (Spirulina sp.) Residence days: 4 days Culture tank: Capacity 25, liquid depth 5 cm, PH10 500W flat lamp to lower the liquid level For comparison, an experiment was conducted in which anaerobic digestion and alkaliphilic algae digestion were performed without any alkali treatment or wet mill treatment (Comparative Example 1)
and an experiment similar to the above except that only wet mill treatment was performed and no alkali treatment was performed (Comparative Example 2)
I did this. For 10 days after the acclimatization of the anaerobic digester, we measured the digestibility (VS removal rate), gas generation amount, methane content of gas, and volatile organic acid content in the anaerobic digester, and also measured the VS removal rate in the anaerobic digester. Digestibility, volatile organic acid content and nitrogen content (centrifugation supernatant) were measured. Table 1 shows the average values of the measured values.
and shown in Table 2.

【table】

〔Effect of the invention〕

According to the sludge treatment method of the present invention, as described above, a high digestibility equal to or higher than the highest conventional level can be achieved in the anaerobic digestion process in an extremely short period of time, and the amount of sludge can be significantly reduced. At the same time, the quality of the waste water can also be improved.
Therefore, compared to conventional systems, it is possible to significantly improve the processing capacity per unit volume of the digester, or make the digester significantly smaller, and the amount of digested sludge generated is reduced, making subsequent treatment easier. , a remarkable effect is achieved. In addition, because the sludge is treated with strong alkalinity, the digested sludge discharged from the anaerobic digestion tank not only does not contain pathogenic microorganisms, but also contains the remaining anaerobic microorganisms that have grown in the strong alkalinity. Since most of the digested sludge will die if it is released into the natural world, it has the advantage that there is little risk of contaminating the environment when it is disposed of.

Claims (1)

[Claims] 1. Add alkali to organic sludge until its pH reaches 9 to 11 to solubilize the microbial cells in the sludge, and then use a wet mill to crush remaining microbial cells, and add the remaining microbial cells to the treated sludge. A sludge characterized by subjecting the sludge to an anaerobic digestion treatment at a pH of 9 to 11 for a retention period of 2 to 5 days, and subjecting alkaline water separated from the sludge after the anaerobic digestion treatment to an aerobic digestion treatment using alkaliphilic microalgae. processing method.