JPS586560B2 - Biological concentration method for sludge - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for concentrating sludge, and in particular to a biological concentration method, in which the entire sludge is floated by CO2 gas generated in the initial stage of anaerobic decomposition of organic sludge. On the other hand, the separated liquid discharged here is related to a new sludge concentration method that makes extremely dilute water containing almost no sludge pieces or sludge particles.

The amount of sludge generated in biological treatment of organic wastewater is generally said to be in the range of 1 to 3% of the inflow water, although it depends on the properties of the wastewater.

Various methods have been known for treating and disposing of sludge in such wastewater treatment, but in all cases, thickening of the sludge as a pre-treatment is essential. Quality is extremely important as it directly affects the efficiency of sludge treatment and disposal.

Thickening of sludge has conventionally been generally carried out in a sludge thickening tank, but the reduction rate of sludge volume here is about 50%.

The concentration of excess sludge depends on the properties of the sludge, but is approximately 0.
．． Since the concentration of sludge is in the range of 5% to 1.5%, the concentration of sludge passing through the sludge thickening tank is considered to be 1 to 3%, and in most cases, the annual average is around 2%.

However, many problems have been pointed out regarding sludge thickening tanks, such as the following.

In other words, in conventional thickening tanks, the residence time of sludge is long, and as a result, a lot of scum is generated, especially in the summer, and the thickening efficiency is extremely poor.In addition, sludge thickening tanks tend to have an anaerobic atmosphere, which generates bad odors. do.

Gas generation from anaerobic decomposition promotes scum formation,
This will greatly deteriorate the properties of the separated liquid.

When the separated liquid was returned to the aeration tank, the BOD and suspended solids concentration, ie, SS concentration, of the separated liquid were high, putting a heavy burden on the aeration tank.

For these reasons, the inventor has been conducting research to solve this problem for some time, and as a result, the inventor has proactively grasped the anaerobic atmosphere that inevitably occurs in conventional concentration tanks.
Gas generated from the sludge itself due to anaerobic decomposition,
Mainly C, which occurs especially in the very early stages of anaerobic decomposition.
The O2 gas completely floats and separates the sludge (naturally speaking, it is the same as scum).

A completely new biological sludge thickening method was invented.

That is, when concentrating sludge, the present invention maintains the sludge in a thickening tank in an anaerobic atmosphere without stirring, and converts sludge pieces into sludge fine particles using the gas generated in the initial stage of the gas generation process due to anaerobic decomposition. This is a biological concentration method for sludge, which is characterized by raising the sludge to the surface and then discharging the separated liquid.

The details of this invention will be explained below. Generally, the residence time of sludge in a thickening tank is 6 to 24 hours, but the inside of this tank tends to become an anaerobic atmosphere, and scum is actively generated especially in the summer.

The biochemical reaction formula for this is as follows.

And the CO2 gas and CH4 gas generated here are (2
), some of it dissolves in the solution, but most of it rises as bubbles and generates scum.

Therefore, the inventor focused on this generated gas, and used it to float all of the sludge in the tank, thereby attempting to thicken the sludge.

The biological concentration of sludge according to the present invention will be explained below.

That is, the inventor carefully observed the gas generation situation during anaerobic decomposition of sludge, and the results were as shown in FIG.

In the figure, the horizontal axis indicates the number of days of sludge retention when the sludge is retained in the thickening tank, and the vertical axis indicates the amount of sludge input 100c.
Shows the amount of gas generated per c.

As is clear from this figure, in the concentration tank, the residence time is 1
The maximum gas generation was observed around the 5th to 20th day, and in this case, the gas generation reached 8 times the amount of the input sludge.

However, when anaerobic decomposition progresses too much during sludge concentration, organic acids are generated, which further deteriorates the properties of the separated liquid.

Therefore, in this invention, the initial stage of anaerobic decomposition of sludge is captured, and the minimum retention period is defined as the number of days required to float all the sludge in the tank.

The biochemical reaction equations for anaerobic decomposition of sludge can generally be expressed as shown in equations (1) and (2) above.

The CO2 gas or CH4 gas generated in equation (1) is partially dissolved in the solution as shown in equation (2), but most of it is generated as a gas.

A sludge digester is an application of this decomposition of organic matter by anaerobic bacteria, that is, anaerobic decomposition.

The above-mentioned FIG. 1 also illustrates the stages of anaerobic decomposition divided into regions 1 to 2.

Note that the conditions for this experiment are as follows.

Experimental materials Sewage sludge (TS concentration 2-3%) Experimental temperature
35°C (typical temperature for mesophilic digestion) Stirring Intermittent stirring (3 times/day) In Figure 1, region I corresponds to the initial stage of anaerobic decomposition.

This I-region is still in the preparatory stage just before full-scale anaerobic decomposition proceeds, and bacteriologically speaking, it is in the late stage (la
g-phase).

In the present invention, this is the initial stage, and the sludge is floated and concentrated using the gas generated here without stirring.

The characteristics of this initial stage are as follows. ■ Organic acid production is still not active.

■ The gas generated is mainly CO2 gas.

■ Sufficient amount of gas is generated to float and concentrate the sludge.

It is considered that the ■-region corresponds to the logarithmic growth phase, the ■-region corresponds to the attenuated growth phase or stationary phase, and the ■-region corresponds to the intracellular respiration phase or cell decomposition phase.

From FIG. 1, it can be seen that gas is generated in an amount of 1/4 to 1/6 of the amount of sludge input at the very early stage of anaerobic decomposition of sludge, for example, during the retention period of 2 days.

It has also been found that the generation of this amount of gas is sufficient to float all the sludge.

That is, it is generally considered that most aerobic bacteria or anaerobic bacteria are adsorbed or attached to sludge.

The gas generated by anaerobic decomposition is generated in large numbers from the surface of the sludge pieces in the form of millet-like microscopic particles.

A large number of millet-like gases generated here rise due to buoyancy, but at this time, the millet-like gas does not separate from the sludge pieces, but remains adsorbed to the outer periphery of the sludge pieces, rising together with the sludge pieces, resulting in sludge pieces to the surface.

This also applies to sludge particles.

These sludge pieces and sludge particles rise while adsorbing fine sludge particles along the way, and are separated by flotation to form a sludge layer.

Furthermore, the sludge layer is consolidated by the levitation force from the lower sludge to form a thickened sludge layer.

On the other hand, a layer of dilute separated liquid (same as sewage) forms at the bottom of the concentration tank.

Surplus gas rises to the surface as digestion gas.

Further, the thickening tank can be operated more efficiently by heating the tank at 20 to 37° C. using surplus gas from a sludge digestion tank that is usually placed in parallel.

The present inventor designated the concentration tank used for such concentration as a biological concentration tank.

As already mentioned, the biological thickening tank uses gas generated during the initial stage of anaerobic decomposition of sludge to float and separate the sludge, and further thickens the sludge. .

■ A layer of highly concentrated thickened sludge is formed by floating.

■ Sludge should not settle and accumulate at the bottom of the separated liquid.

In other words, there is a two-layer separation of a thickened sludge layer and a separated liquid layer.

■ The separated liquid has a low concentration of suspended solids (SS) and BOD, and is so-called clean.To achieve this, the retention period must be significantly reduced because it is not directly affected by the liquefaction phenomenon of anaerobic decomposition.

■ Do not stir at all.

This invention method can be directly applied to conventional sewage treatment facilities.

FIG. 2 shows the flowchart.

In the figure, 1 is the inflowing sewage, which is first sent to the sedimentation tank 2.
After removing easily precipitated solids from the suspended matter, aeration is carried out in an aeration tank 3 according to a conventional method.

Thereafter, the sludge is sent to the final sludge pond 4, whereupon the supernatant liquid is disinfected and discharged, and the returned sludge 5 is returned to the front of the aeration tank 3.

The surplus sludge 6 generated here usually has a concentration of 0.5 to 1.0%.
Then, it is sent to the thickening tank 7 along with one raw sludge.

It is sent from the concentration tank 7 to the biological concentration tank 8.

The sludge concentrated here is sent to a dewatering tank 9 or to a high rate digestion tank 10.

The separated liquids separated in the concentration tank 7 and the biological concentration tank 8 are returned to the front stage of the aeration tank.

The gas generated in the high rate digester 10 is used to heat the high rate digester itself or the biological enrichment tank 8, and the separated liquid here is returned to the front of the aeration tank.

Furthermore, the surplus gas will greatly contribute to energy conservation issues such as power generation.

As a precaution, a conventional sewage treatment flowchart is shown in Figure 3.

In the figure, 11 is inflow sewage, 21 is the initial settling tank, 31 is the aeration tank, 41 is the final settling tank, 5 is the return sludge, 61 is the thickening tank, 81 is the digestion tank, and 101 is the dewatering process. .

Note that the separated liquid separated in the concentration tank 81 is returned to the front stage of the aeration tank.

The present invention introduces a sludge biological concentration method as a pretreatment to the conventional sludge treatment process, and is illustrated in FIG. 4 in comparison with the conventional method.

In addition, 71 in Figure 4 is a biological concentration tank, and 91
is a high rate digester.

As explained above, in this invention, sludge is held in a thickening tank in an anaerobic atmosphere, and the gas generated in the initial stage of the gas generation process due to anaerobic decomposition causes sludge pieces to float together with fine sludge particles, and then the separated liquid Since this is a biological concentration method that is characterized by the discharge of water, it can be seen that this concentration method is completely different from the conventional concentration method expected from natural sedimentation.

According to this method, all of the sludge pieces, sludge particles, and sludge fine particles can be completely floated, and a highly transparent and dilute separated liquid can be obtained below them.

The separated liquid obtained by this invention contains almost no sludge pieces or sludge particles when compared to the separated liquid from a conventional thickening tank, and has a water quality comparable to that of sewage or slightly putrid sewage.
The result is a very thin and clean separated liquid.

On the other hand, the sludge after discharging the separated liquid has a concentration of 5 to 5.
In the range of 10%, especially about 6 to 8% can be stably obtained.

Moreover, since these concentrations are carried out at the very early stages of anaerobic decomposition, there is almost no generation of organic acids or bad odors, and it can be said to be an extremely stable and reliable sludge concentration method.

Example: When 1,000 cc of sewage sludge with a concentration of 22,500 ppm was placed in a transparent glass container and kept in an anaerobic atmosphere at a temperature of 36°C for 24 hours, a large amount of fine gas was generated from the sludge, and the sludge and sludge particles floated to the surface. A thickened sludge layer was formed.

Next, the separated liquid was discharged from the lower end of this container, and concentrated sludge was collected.

Thickened sludge concentrations averaged 6 to 8% (dry solids), often as high as 10%.

The features of the present invention can be summarized as follows.

(1) The biological concentration method of sludge can stably supply high-concentration sludge with a sludge concentration of 6 to 8% (dry solids). processing is now possible.

(2) Introducing a high-rate digestion tank into the sludge treatment process can significantly reduce the volume compared to conventional digestion tanks that require a huge volume.

For example, the number of retention days for input sludge is 2-3% for conventional type, 30 days, and 6-8% for high-rate digestion tank, 15 days, and at least the volume can be expected to be 1/6 or less, taking the sludge concentration into consideration.

(3) The amount of digestive gas generated is as follows.

Conventional type: 4 to 6 times (of the amount of sludge input) High-rate digestion tank: 12 to 20 times (of the amount of sludge input) However, the amount of gas generated per organic matter is approximately the same.

Therefore, the surplus gas can be utilized to a large extent.

(4) When high-concentration sludge is directly dehydrated using the sludge biological concentration method, the dewatering process can be made smaller by the higher concentration of the sludge compared to conventional methods.

(5) To summarize the features of (1) to (4), if the sludge biological concentration method is introduced as a pretreatment to the conventional sludge treatment process, the sludge treatment process can be significantly downsized.

In addition, it enables a high-rate sludge digestion tank, and the amount of gas generated is 12 to 20 times the amount of sludge input, making it a place that is attracting a lot of attention as an energy-saving facility.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between the number of residence days and the amount of gas generated during anaerobic decomposition of sludge, along with classification of the lag period and other areas, Figure 2 is a flow sheet for sewage treatment using the method of this invention, and Figure 3 The figure shows a flowchart of conventional sewage treatment, and FIG. 4 shows a flowchart of sewage treatment according to the method of the present invention in comparison with the flowchart of conventional sewage treatment shown in FIG. 2... First settling tank, 3... Aeration tank, 4... Final settling tank,
8... Biological concentration tank, 10... High rate digestion tank.

Claims (1)

[Claims] 1. When concentrating sludge, the sludge is held in a thickening tank in an anaerobic atmosphere without stirring, and the sludge pieces are floated together with fine sludge particles by the gas generated in the initial stage of the gas generation process due to anaerobic decomposition. A method for biologically concentrating sludge, which is characterized in that a separated liquid is then discharged.