JPS5825519B2 - Improved methane fermentation method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a methane fermentation method that is effective in treating organic waste and organic waste liquid, and in particular to a method for improving methane bacteria sludge in a fermenter and controlling the concentration of methane bacteria sludge.

The spread of methane fermentation is remarkable in the fields of sewage sludge treatment and night soil treatment, but little has been done to manage the sludge inside the fermentation tank, for example, to maintain the sludge concentration. The occupancy rate of bacterial groups) is still at the research stage, and there are no examples of its practical application.

Therefore, if the target of methane fermentation is significantly different in composition, BOD concentration, etc. from the sewage sludge or human waste that has been used to date, applying the conventional method will ensure that the environmental conditions for methane bacteria in the tank are optimal. Since it deviates from the target, efficiency is extremely low.

To give an example, wastewater from an bean paste factory (with an organic matter concentration of 4500
In the case of dilute wastewater such as ppm 1 BOD 3000 ppm, ), the conventional method fails to balance the overflow of methane bacteria sludge and the production of methane bacteria sludge, resulting in a decrease in the sludge concentration in the tank and a loss of fermentation ability.

For this reason, a method has been adopted in which the sludge that overflows into the treated water for methane fermentation is collected and returned to the tank.

Moreover, it has become common practice to maintain the sludge concentration in the tank at 110,000 pp or more (conventional method).

However, in this case, it is difficult to perform solid-liquid separation between the methane bacteria sludge that overflows into the treated water and the treated water due to the following points.

(1) When the sludge concentration is 110,000 pp or more, the settling velocity (LV = m/h) is 0.01 to 0.1 m/h.
This requires a sedimentation tank with a very large surface area, and the second stage of the conventional two-tank system fulfills this role.

(2) When sludge is forcibly dehydrated, an auxiliary agent is required for dewatering, and it is usually carried out in the open to the atmosphere, so even if the dehydrated sludge is returned to the tank, it is not very effective.

Therefore, the load of organic matter contained in the wastewater to be treated on the methane fermentation tank capacity is set to about 0.2 to 4 to 9/m/day, and the sludge concentration in the tank is set to 2000 to 8000 pp.
A method is being considered, in which the fermenter is operated at a speed of about 100 m and solid-liquid separation is carried out in an ordinary settling tank, or a batch system is used in which both a fermenter and a settling tank are used.

However, the growth rate △S of methane bacteria is △5=aXLa a=0.05~0.3 La: amount of decomposed organic matter This is lower than the normal activated sludge method.

Therefore, when performing solid-liquid separation in a sedimentation tank, it is necessary to capture as much sludge as possible and prevent it from overflowing into the treated water to maintain a balance between production and overflow.

However, in a methane fermentation tank, a large amount of CO□ gas is dissolved in a free state, and if the settling tank is an open type, CO2
Temporary atmospheric dissipation occurs, and organic matter in the process of decomposition gasifies in the settling tank, making the specific gravity of the sludge appear lighter, which often results in the generation of a large amount of scum. solid-liquid separation is not possible.

Furthermore, when methane fermentation treated water containing sludge is sent to a settling tank, it has foaming properties, which often impedes solid-liquid separation.

In addition, a system that combines a methane fermentation tank and a solid-liquid separation tank (see % formula %) has been proposed, but simply adopting this system would require the following: There are drawbacks.

■ Gas is generated in the tank during solid-liquid separation, and the apparent sludge settling velocity (LV) becomes extremely low, and with the high sludge concentration as in the conventional method, the solid-liquid separation time becomes extremely long, resulting in
Solid-liquid separation time of 5 to 20 hours per day is required.

(2) As a result, the fermentation reaction time is not sufficient and the acid production reaction takes precedence, leading to a decrease in the pH in the tank and a decrease in the gas generation rate, ultimately making it impossible to carry out methane fermentation.

■ In the case of a single tank type, it is impossible to capture and recover the SS carryover that flows out, and depending on the amount of carryover, the sludge concentration in the tank decreases drastically, making methane fermentation impossible.

■ In the low concentration range of sludge concentration of 2000 to 8000 ppm, single-tank solid-liquid separation has a low consolidated concentration;
When gas is generated in the tank, sludge floats to the surface, making SS carryover more likely.

Therefore, the present inventors have conducted repeated research on a method that can optimize the organic matter decomposition conditions for methane bacteria, easily perform solid-liquid separation, and eliminate the drawbacks of the conventional methane fermentation method as described above. The present invention was achieved by discovering that this object can be achieved by combining the following conditions.

That is, the conditions are as follows: (1) Adding 100 to 200% of an inorganic coagulation aid to the raw water in advance.
Add 0 ppm (1/10' to 115 x 102 by weight).

■ Control the sludge concentration in the tank to 2,000 to 8,000 ppm.

(2) The ratio of inorganic matter in sludge to organic matter in sludge that is flocculated by an inorganic flocculant is controlled within the range of 0.01 to 0.5.

That's true.

The above-mentioned inorganic flocculant must have flocculating ability in the neutral pH range of 6.0 to 8.0 and must not be toxic to methane bacteria, and must not be toxic to methane bacteria, such as ferric chloride F e C13 or A4 (S 04.
') 3rd grade is used.

The present invention will be explained in more detail with reference to FIG.

Raw water for methane fermentation is supplied from line 11 to adjustment tank 1
guided by.

This adjustment tank 1 has a sufficient stirring mechanism (mechanical or gas type), and around it there are a nutrient tank 4, a pH adjustment chemical cook 5, an inorganic coagulant dissolving tank, and lines 15, 16, and 16, respectively. It enters the adjustment tank 1 from 17 onwards.

The adjustment tank 1 contains a pH adjustment chemical solution, for example, NaOH1Ca(O
An alkaline agent such as H)2 or an acidic agent such as HCl1H2SO4 is introduced through line 16 to adjust the pH of the raw water to 6-8.

Next, N and P sources such as urea and ammonium phosphate are led from line 15 to adjustment tank 1, and the organic matter in the raw water is
, P is 1/10 to 1/20.1150 to 1, respectively.
Input within the range of /200.

Furthermore, the weight ratio to the amount of raw water is 1/10' to 115 x 102
of inorganic flocculant is added through line 17.

The raw water thus adjusted enters the methane fermentation tank 2 through the line 12 and is decomposed under anaerobic conditions.

A mixed gas of methane gas and carbon dioxide gas generated in the methane fermentation tank 2 is stored in the gas holder 7 via a line 18.

This gas can be led back to the fermenter via the line 25 and used for stirring, and the excess generated gas can be effectively used as boiler or drying fuel through the line 23.

On the other hand, the methane fermentation treated water enters the settling tank 3 from the line 13 and is separated into solid-liquid into methane bacteria sludge and clear water.

The solid-liquid separated sludge is partially returned to the methane fermentation tank 2 from line 19 through line 20, and the concentration of the methane fermentation tank is reduced to 20%.
Maintain between 00 and 8000 ppm.

The remaining sludge enters the dehydrator 8 through line 21 and is discharged through line 22 after being dehydrated.

Since the clarified liquid from the settling tank 3 contains undecomposed organic matter, it is discharged from the line 14 for secondary treatment such as activated sludge method or coagulation sedimentation method.

In the case of a batch system, it is also possible to combine the gas holder 7 and the settling tank 3 with the methane fermentation tank 2 to make it more compact.

The method of the present invention is particularly effective in a semi-batch system in which methane fermentation and solid-liquid separation are carried out in the same tank.

The method of the present invention provides the following effects.

■ Adding an inorganic flocculant increases the clarity of treated water and captures fine suspended solids (SS), improving the quality of treated water.

■ Methane bacteria sludge concentration from 2,000 to 8,000 pp
Since solid-liquid separation is controlled to m, solid-liquid separation is easy, and the synergistic effect with (2) improves the sludge settling rate and shortens the solid-liquid separation time.

■ The sludge becomes more flocculent, which promotes the effect of ■ and reduces the generation of scum in the methane fermentation tank.

■ As the composition ratio in sludge, (inorganic substances in sludge flocculated by inorganic flocculant)/(organic substances in sludge) is 0.0.
By maintaining the ratio within the range of 1 to 0.5, the activity of the methane bacteria sludge can be maintained at a high level.

In addition, the composition control of the methane bacteria sludge in section (2) can be easily controlled by selecting the sludge concentration in the tank and the addition rate of the inorganic flocculant.

Although the method of the present invention can be applied to methane fermentation equipment for municipal waste, agricultural and livestock waste, and industrial water, it is particularly useful for treating wastewater containing highly viscous polymeric substances such as pectin.

Examples of waste liquids containing highly viscous polymeric substances include waste liquids containing highly viscous polysaccharides called pectic substances, which are discharged from mandarin orange canning factories and juice factories. The sludge gradually swells due to the proliferation of bacteria, making solid-liquid separation impossible with ordinary methane fermentation methods, but by applying the method of the present invention, methane fermentation can be continued sufficiently.

Example 1 Chemical treatment waste liquid for canned mandarin oranges (B, 0.D, 7000pp
The methane fermentation test was carried out using 15,000 ppm of organic matter in the tank at a tank sludge concentration of 1,000 ppm and 2,000 ppm.

4000ppm, 8000ppm, 12000ppm
At each point, FFe is present in the raw water (-・-) and in the raw water.
The results for the sample to which 300 pp of C73 was added (-he-) are shown in Figures 2-5.

Figure 2 shows the amount of gas generated against COD, and it is clear that the addition of FeCl3 does not change the amount of gas generated against COD, indicating that there is no adverse effect of adding a flocculant. Also, from Figure 3, which shows the pH in the tank, It can be seen that the pH does not change due to the addition of the flocculant.

FIG. 4 shows that the SS carryover of treated water is reduced by adding a coagulant, but as the sludge concentration in the tank increases, the effect of adding a coagulant disappears.

Regarding the sludge settling rate, FIG. 5 shows that the addition of a coagulant increases the sludge settling rate and makes sedimentation separation advantageous, but it becomes ineffective when the sludge concentration exceeds 110,000 pp.

The reason why the clarity of the treated water increases and the quality of the water improves by being able to capture suspended solids in the present invention is thought to be that the addition of a flocculant generates hydroxide flocs, which precipitate together with fine suspended solids.

Example 2 A beaker test for methane fermentation was conducted on the same wastewater as in Example 1 under the same conditions as in Example 1.
The results shown in the figure were obtained.

The symbol in the figure is C0D (-〇-1) when flocculant is added.
, 5S (-contains-), pH (-()-), COD (-・-) without flocculant added, 5S (-&-), pH (+)
represents.

As is clear from this figure, when it comes to pH, the sludge concentration is 20
Below 0.00 ppm, the shift to the acidic side is significant, which is the optimum condition for methane fermentation.

6 (pH (out of the range of 8).

The reason for this is that the balance between methane gas-producing bacteria and facultative anaerobes is disrupted due to differences in growth rates, and acid production by facultative anaerobes precedes acid accumulation.

As for SS in the treated water, when the sludge concentration exceeds 8000 ppm, the SS removal rate rapidly decreases, SS carryover increases, and at the same time, the COD value removal rate decreases due to SS.

Regarding COD, it was found that when the sludge concentration was 2000 ppm or less, only acid production proceeded and the removal rate decreased.

On the other hand, when no coagulant was added, both COD and SS removal rates were significantly worse than in the case of the present invention.

The effect of adding a flocculant was found in the sludge concentration range of 2000 ppm to 8000 ppm.

From this result, one of the major reasons why the methane fermentation method could not be implemented in areas where the sludge concentration was extremely low is that
It is clear that there is sludge carryover.

As an example, if a sludge concentration of 4000 ppm is taken, the amount of grown sludge can be expressed by equation (1).

△S= a XL a-b XF (1
)ΔS: Amount of propagated sludge (kg/day) a: Growth rate of methane bacteria sludge (ky/ky) La: Organic matter or BOD in the stock solution (kg/day) b: Carryover concentration of sludge (kg/m') )F:
Inflow amount of stock solution (m/day') Looking at the data of Example 1, when no flocculant is added, a=0.15, La=400, b=o, s, F=80
Therefore, Δ5=-4.

In other words, the sludge concentration in the tank decreases by 4 kg/day, and eventually the sludge concentration becomes less than 11,000 pp, making methane fermentation impossible. Therefore, methane fermentation is normally almost impossible in the low concentration range. there were.

On the other hand, in the case of adding a flocculant, b=0.25 (1)
According to the formula △5-40, 40 k is required to maintain the sludge concentration.
Normal methane fermentation can be maintained by removing g/day of sludge from the tank.

Therefore, it can be seen that sludge carryover is a very important factor when performing methane fermentation at low sludge concentrations.

In addition, when we set the sludge concentration time of the control area without flocculant as 1, we compared the case with flocculant addition (adding 500 ppm of FeC11s to the stock solution), as shown in Figure 7. It was found that the effect of adding a flocculant was excellent in this area.

Example 3 Using the same sample as in Example 1, FeCl in raw water was
When methane fermentation was carried out at a sludge concentration in the tank of 4000 ppm by changing the amount of 3 added, the results shown in Fig. 8 were obtained regarding the amount of gas generated relative to COD.

As can be seen from this figure, when the amount of FeCl3 added exceeds 2000 ppm, the amount of gas produced decreases significantly.

We also analyzed the sludge when gas production decreased.
Fe2O3/organic matter was 0.58.

Furthermore, data for comparison with the case where a flocculant was added is shown in FIG. 9, with the gas generation amount in the case where no flocculant was added being 1.

As a result, up to the addition of 2000 ppm of flocculant, the amount of gas generated was the same as or slightly higher than that without the addition of flocculant.
If it exceeds 2000 ppm (weight ratio 115xlO2), it becomes lower, so it is clear that the addition of flocculant must be kept below 2000 ppm.

In addition, the sludge composition at the end of methane fermentation was analyzed, and the loss on ignition in the sludge was taken as the amount of organic matter, and the main metal oxide (Fe
When the ratio of CAa to Fe20a) is calculated and displayed, it can be seen that the gas generation ability decreases when Fe2O3/organic matter is 0.5 or more.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet showing one embodiment of the present invention;
6 are graphs comparing the effects of the method of the present invention and the conventional method, and FIGS. 7 to 9 are graphs showing the effects of the present invention corresponding to changes in the sludge concentration in the tank and the amount of flocculant.

Claims (1)

[Claims] 1 In the methane fermentation method, an inorganic flocculant is added to raw water at a weight ratio of 1710' to 115 x 102, and the sludge concentration in the methane fermentation tank is adjusted to 2000 to 8000 ppm.
A methane fermentation method characterized by operating the ratio of inorganic matter in sludge to organic matter in sludge flocculated by an inorganic flocculant in a range of 0.01 to 0.5.