JPH02211297A - Treatment of sludge - Google Patents

Abstract

PURPOSE:To reduce a large amount of sludge in a short period of time and decompose organic substances by alkalizing organic sludge for the solubilization of microbial biomass and subjecting the same to anaerobic digestion after the breaking thereof and the liquid phase to aerobic biological treatment. CONSTITUTION:Alkali such as caustic soda is added to organic sludge until its pH value reaches 9-11 for the solubilization of microbial biomass therein, followed by a wet milling for the breaking of the residual biomass. The sludge is then supplied into a single anaerobic digestor chamber for effecting the anaerobic digestion thereof. The sludge discharged from said chamber is subjected to solid-liquid separation, the resulting liquid phase undergoing aerobic biological treatment. The aforesaid treatment method brings about anaerobic digestion in a short period of time and at a high digestion rate, thereby permitting a remarkable reduction in the amount of the sludge.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] 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 urban sewage in 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 when they are disposed of, there is also the problem of dispersing pathogenic microorganisms and deteriorating the environment, so it is necessary to take steps to reduce the amount of these microorganisms as much as possible before disposal. A typical treatment method for this purpose is anaerobic digestion treatment,
Decomposes organic matter 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 2 days in the case of mesophilic digestion (VA degree 35-37℃).
It is long, from 0 to 30 days. Moreover, even if such a number of days are spent, the decomposition rate of organic matter remains at about 40 to 50%, and therefore the effluent still contains a high concentration of organic matter, which becomes a heavy burden for subsequent water treatment.

[Problem to be solved by the invention]

As mentioned above, conventional sludge treatment methods are inefficient, require large anaerobic digestion tanks, and have unsatisfactory solids reduction effects. In addition, the discharged water has a high concentration of organic matter, so it cannot be discharged directly to the bottom of the tank, and further treatment 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 to solve the problem]

In the sludge treatment method provided by the present invention, organic sludge is
Solubilization of microbial cells in the sludge is caused by adding alkali until the value reaches 9 to 11, further crushing remaining microbial cells etc. by wet milling, and subjecting the treated sludge to anaerobic digestion, The method is characterized in that the water separated from the sludge after the anaerobic digestion treatment is subjected to aerobic microbial treatment.

Hereinafter, the sludge treatment method of the present invention will be explained in more detail.

For alkaline treatment of sludge, add alkali such as sodium hydroxide or soda carbonate until the pH of the sludge reaches 9 to 11, and heat the sludge at a temperature of about 2.
This is carried out by holding at 0 to 30°C for about 0.5 to 1 hour. This destroys the cell walls of microorganisms in the sludge,
So-called solubilization occurs in which intracellular components such as proteins and sugars are eluted. To make it strongly alkaline with a pH of 11 or higher,
Although it is effective in promoting solubilization, it becomes difficult to accumulate a sufficient amount of methane bacteria that proliferate in such strongly alkaline sludge in the digester during the subsequent anaerobic digestion process.
Undesirable.

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 because it has the best processing effect.

This media stirring type mill violently stirs the 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. Although beads of various sizes are used depending on the purpose, preferred beads for solubilizing sludge have a particle size of 0.05 to 1 square centimeter. In that case, the rotational speed of the stirring disk is 1000 to 3000 rpm (peripheral speed lO to 3
0 m/5ec), and the residence time of the sludge to be treated is approximately 5 to 60 minutes when ordinary sludge is treated.

Microbial cells and fibrous organic matter that were not solubilized by alkaline treatment are also solubilized by this wet mill treatment.
Alternatively, it is crushed into a form that is more amenable 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 on 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 has a pH of 9~ due to the alkaline treatment.
No. 11 is somewhat strongly alkaline, but it is supplied as is to the anaerobic digestion tank without neutralization. Normally, the pH suitable for the growth of methane bacteria in sewage sludge is considered to be weakly alkaline, less than 8. In particular, in the case of single-tank digestion in which acid-producing bacteria and methane bacteria coexist, PAL is The 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 conventional ones. In the digestion method of the present invention, due to such a high pH value, firstly, the sludge components are further solubilized, making them more 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, By providing an acclimatization period in which the pH is maintained at the same level at the beginning, it is possible to ensure that the anaerobic bacteria in the tank occupy the majority of the bacterial flora.

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 a temperature of 30 to 40° C. is appropriate, and a retention period of 2 to 5 days is appropriate.

The sludge discharged from the anaerobic digestion tank can be appropriately disposed of as it is after separating solid and liquid, but in the treatment method of the present invention, the liquid phase portion is further subjected to aerobic microbial treatment. This treatment is for aerobic microorganisms to decompose dissolved organic matter that was solubilized by alkali treatment and wet mill treatment but not decomposed by anaerobic microorganisms, and this process reduces the amount of dissolved organic matter by about 50%.
The above water can be decomposed into water that can be discharged as is, or the burden of subsequent water treatment processes can be reduced.

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 at sewage treatment plants, as well as at manufacturing plants and agricultural and fishery processing plants. It can also be applied to the disposal of waste from plants, livestock manure, plaster treatment waste, fish iliac bones, etc.

〔Example〕

Excess sludge and mixed sludge collected from urban sewage treatment plants (
A 1:1 mixture of surplus sludge and initial settling sludge) was treated according to the present invention and a comparative experiment was 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% aqueous soda solution until the sludge pH reaches 1O, and stir at room temperature for 30 minutes.

Wet mill processing conditions: Equipment used: Media stirring type mill/bar mill (Ashizawa Co., Ltd., model PMISTS, bead diameter 0.2 mm.

Disc rotation speed: 130 rpm, turbidity rate: 6 m/sec Residence time: 5 minutes Anaerobic digestion conditions: Using a 51-volume fermentor, continuous stirring at 300 rpm Digestion temperature: 37°C Residence days: 2 days Acclimatization at the start of the experiment: Sewage Mesophilic digested sludge collected from the treatment plant was used as the initial seed sludge and carried out as follows: First two weeks: Retention days: 10 days Next two weeks: Retention days: 5 days Last two weeks: Retention days: 2 days Aerobic Microbial treatment: The supernatant obtained by centrifuging the anaerobic digested sludge is supplied to a bioreactor filled with porous ceramics.
The aeration amount was adjusted so that the dissolved oxygen concentration was approximately 1 ppm, and the residence time was adjusted to 6 hours.

For comparison, an experiment was conducted in which the same anaerobic digestion and aerobic microbial treatment 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. A similar experiment (Comparative Example 2) was conducted.

For 10 days after the acclimatization of the anaerobic digester, the digestibility (VS removal rate), amount of gas generated, methane content of gas, and amount of volatile organic acids in the anaerobic digester were measured, and further aerobic microbial treatment was performed. The BOD was measured.

The average values of the measured values are shown in Tables 1 and 2.

Table 1 Comparative example 1 of sludge and surplus sludge Comparative example 2 Example anaerobic digestion Digestion tank pH Digestibility (%) Gas generation amount (ffi/VS-tg) Methane in gas (%) Volatile organic acid (mg) /A) 8.2 8.3 9.1 BOD before aerobic microbial treatment (mg/I) BOD after 2300 treatment
(mg/I) 83 BOD removal rate (%) 96
．． 4 97.2 98.2 Table 2 Sludge: Mixed sludge Comparative Example 1 Comparative Example 2 Example Anaerobic Digestion Digestion tank pH Digestion rate (%) Gas generation amount (a/VS-Methane in garlic gas (%) Volatile organic acid (+u/A) BOD before aerobic microbial treatment (-g/I) BOD after treatment (at/1) BOD removal rate (%) [Effect of the invention] 8.1 8.4 9 .2 97.0 96.8 98.4 According to the sludge treatment method of the present invention, as mentioned 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 time. In addition to significantly reducing the amount of sludge, it also improves the quality of the effluent water.As a result, the processing capacity per unit volume of the digestion tank is significantly improved compared to conventional methods, or the This has the remarkable effect of making it possible to significantly downsize the tank, reduce the amount of digested sludge generated, and make subsequent treatment easier.Also, because it is treated with strong alkalinity, anaerobic digestion is possible. The digested sludge discharged from the tank not only does not contain pathogenic microorganisms, but also contains the remaining anaerobic microorganisms that have grown in strong alkaline conditions, and most of them will die if released into the natural world. Therefore, there is an advantage that there is less risk of polluting the environment when the digested sludge is disposed of.

Claims (1)

[Claims] Alkali is added to organic sludge until the pH reaches 9 to 11 to solubilize the microbial cells in the sludge, and residual microbial cells are crushed by wet milling, and the treated sludge is A method for treating sludge, which comprises subjecting the sludge to anaerobic digestion and subjecting water separated from the sludge after the anaerobic digestion to aerobic microbial treatment.