JPH09239397A - Controlling method for methane fermentation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To take an appropriate measure by perceiving the sign of abnormal fermentation (system failure) in a methane fermentation tank utilizing waste in its early stages. SOLUTION: A pH meter 12, a device 13 for separately measuring the components of a volatile org. acid and a substrate supply control means 15 are set in a methane fermentation tank 11, the pH value of the liq. phase is measured, and a nonionizable volatile org. acid except acetic acid is measured by the device 13. Subsequently, the concn. of the nonionizable volatile org. acid is obtained by a calculating means 14, and the supply of substrate to the tank 11 is controlled with the calculated concn. as an index so that the concn. of the nonionizable volatile org. acid does not exceed 15-20mg/l. Further, the pH of the methane feremntation tank is controlled by a pH control means based on the index.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling methane fermentation of organic wastewater and wastes, and particularly to measuring the pH of the liquid phase in a methane fermentation tank and the concentration of volatile organic acids for each component other than acetic acid. The present invention relates to a methane fermentation control method in which the concentration of a non-ionizing volatile organic acid is calculated, and the amount of substrate input to a methane fermentation tank or a pH value is controlled so that the concentration becomes equal to or lower than a limiting concentration using this as an index.

[0002]

2. Description of the Related Art In a method of methane fermentation of organic wastewater and wastes by anaerobic treatment, abnormalities are caused due to overload of organic matter, excessive shortening of hydraulic retention time (HRT) and inflow of toxic substances. Fermentation (system failure) may occur, and in such a case, the concentration of volatile organic acids (hereinafter abbreviated as VFA) increases and the pH often decreases as the amount of methane gas generated decreases.
If the above abnormal fermentation occurs, the situation will only get worse if you do not deal with it immediately, and it often takes a long time to recover, so detect the signs of abnormal fermentation early. It is essential to take appropriate measures.

With respect to the above, the literature "Dynamic Behavior and Stability of Anaerobic Processes-On the Effect of Hydrogen on Propionic Acid Accumulation" (Water and Wastewater, 30, 11, p33-3).
(1988, 1988, Shinji Ide), a control method using the hydrogen concentration in the gas phase as an index is proposed by utilizing the phenomenon that the hydrogen concentration in the gas phase rises as a precursor of the system failure of methane fermentation. There is.

In addition, when a system failure of methane fermentation occurs, an abnormal pH is often observed and the measurement is easy, so that the pH is generally measured. However, the pH abnormality occurs as a result of the system failure, and it is often too late at the stage of detecting the pH abnormality, and the control using only the pH as an index is effective as a methane fermentation control method. I can't say.

On the other hand, an inhibitory effect of non-ionized VFA on methanogenic bacteria has been reported as a major cause of system failure of methane fermentation. (Eg Kroecker.e
tal: Water.Pollu.Control.Fed., 51, p718-727,1979, or Duarte and Anderson: Water.Sci.Technol., 14, p749
~ 763.1982) The above report is summarized as follows. Kroecker.e
According to tal, the inhibition of non-ionizing VFA is 30-60 (m
g / l), and Duarte and Anderson
Said that at a non-ionizing VFA concentration of 10 (mg / l) methane production was inhibited by 50%. Besides these reports, propionic acid is more toxic than acetic acid,
There is also a report that even if the total propionic acid concentration of both the non-ionized type and the ionized type is 1000 (mg / l), it inhibits methane production.

On the other hand, however, there is a report that the propionic acid concentration is not inhibited even at a concentration of 10,000 (mg / l), so that no clear consensus is obtained on the toxicity of non-ionized VFA. Therefore, the reality is that the method for controlling methane fermentation using the non-ionized VFA concentration as an index has not been realized.

As another method for controlling methane fermentation, there is also a method in which the number of methane bacteria, F ₄₂₀ , ATP, etc. are measured and these are used as indicators, but the measuring technology for these factors is incomplete, and the measurement accuracy is assumed. However, even if the measurement time is shortened and the measurement time is shortened, there remains the problem of how to distinguish normal and abnormal methane fermentation from the measured values. Furthermore, the above-mentioned measurement items, the number of methane bacteria, F ₄₂₀ , and ATP are related to the microorganism concentration, and it is presumed that it is difficult to correlate them with the system failure even if a minute change in the microorganism concentration is detected. .

To briefly explain methane fermentation, the methanogenic bacterium directly involved in methane production uses acetic acid or hydrogen and carbon dioxide as substrates, and the other substrates undergo hydrolysis and organic production to produce acetic acid. After producing hydrogen, carbon dioxide, etc., it is methanated. Certain methanogens are known to directly utilize methanol to produce methane.

[0009] The applicant of the present invention previously filed Japanese Patent Application No. 6-324159 to treat organic waste and wastewater anaerobically.
As an index of the amount of raw water input to the reaction tank, a control factor called organic matter / acetic acid assimilating methane bacteria load was determined, and the flow rate of the anaerobic treatment was adjusted so that the control factor was within an appropriate range. A control method was proposed. Furthermore, Japanese Patent Application No. 7-
According to No. 266555, sludge collected from the sludge digestion tank and digested sludge as seed sludge were added to the basic medium, ethanol was added as a substrate, and sodium bicarbonate was further added to adjust the pH. A methane fermentation method using ethanol as a substrate below was proposed. The above pH conditions were adjusted to 5.8 to 6.5.

To briefly explain this, in general, the control of the substrate input amount in various anaerobic treatment systems is carried out by HRT (hydraulic retention time) and organic matter volume loading. However, when the solid content is low, a volume load such as TOC (total amount of organic carbon), COD (chemical oxygen demand), BOD (biochemical oxygen demand) is used instead of the organic matter volume load.

According to the above-mentioned Japanese Patent Application No. 6-324159, based on the value measured by the acetic acid-utilizing methane bacteria concentration measuring means attached to the reaction tank, an organic substance / acetic acid-utilizing methane bacterium is calculated using a formula. The load is calculated and from this result optimal control of the raw water flow rate to the reactor is implemented. In particular, when the organic matter / acetic acid assimilating methane bacterial load does not fall within an appropriate range, it is possible to calculate the flow rate of raw water that falls within the appropriate range by calculation, and to control the flow rate of the raw water pump. Become.

According to Japanese Patent Application No. 7-266555,
By adding ethanol as a substrate and further adding sodium bicarbonate NaHCO ₃ to adjust the pH, normal methane fermentation can be progressed even in a low pH condition of about pH 5.8 in methane fermentation using ethanol as a substrate. It becomes possible, and the addition of sodium bicarbonate promotes the production of methane from hydrogen and carbon dioxide, and the action of promoting the production of VFA from ethanol. Furthermore, the culture medium conditioned at pH 5.0 with methanol as the substrate is used as seed sludge, and the ethanol is used as the substrate with pH.
By carrying out methane fermentation under a relatively low pH condition of 5.8 and 6.5, acetic acid-assimilating methanogens having high resistance to non-ionized VFA can be accumulated and cultured.

[0013]

As described above, as a control method for detecting an abnormality in methane fermentation at an early stage to prevent system failure, at present, non-ionized VFA is measured and this non-ionized VFA concentration is used as an index. As a method, it is considered promising to control the pH and the input amount of the substrate. This non-ionized VFA concentration can be calculated by measuring the liquid phase pH and VFA, and both have the advantage that they can be automatically measured.

Currently, the VFA is measured by a method of measuring the total VFA by an acid / base titration method. Furthermore, it is possible to measure VFA for each component by ion chromatography, and an automated measuring device has been developed.

In addition to non-ionized VFA, long-chain fatty acids are known to have an inhibitory effect, and long-chain fatty acids may be produced as an intermediate product in the process of anaerobic decomposition, such as hydrocarbons. In methane fermentation using complex substrates such as fats and proteins, long-chain fatty acids inhibit methanogens. Therefore, it is desirable to study the inhibitory property of non-ionized VFA in a methane fermentation system using a substrate that does not produce long-chain fatty acids.

According to the conventional theory, it is said that non-ionized VFA has an inhibitory effect on methanogens of 10 to 20 (mg / l), and non-ionized propionic acid has a stronger inhibitory effect than non-ionized acetic acid. . This propionic acid is hardly decomposable, and methanogens cannot directly use propionic acid as a substrate.

It is naturally expected that the components of VFA have different inhibitory strengths. However, according to the conventional research reports on this point, non-ionized propionic acid has a stronger inhibitory effect than other VFAs. And the view to deny it coexist. It seems that there is no established theory about the inhibitory properties of various VFAs other than propionic acid.

From the above viewpoint, in controlling the methane fermentation system using the non-ionized VFA concentration as an index, the non-ionized VF is used.
There is a need to clarify the inhibitory effect of A.

Therefore, the present invention has been made in view of the above, and calculates the non-ionizing volatile organic acid concentration other than acetic acid from the pH of the liquid phase in the methane fermentation tank and the volatile organic acid concentration of each component. By controlling the amount of substrate input to the methane fermentation tank or the pH value using as an index, it is possible to detect signs of abnormal fermentation (system failure) at an early stage and take appropriate measures. It is intended to provide.

[0020]

In order to achieve the above object, the present invention firstly provides a methane fermentation tank with a pH meter, a volatile organic acid component-by-ingredient measuring device, and a substrate input control means. It is installed and the non-ionizing volatile organic acid other than acetic acid is measured by measuring the liquid phase pH value and the component-specific measuring instrument, and then the non-ionizing volatile organic acid concentration is calculated by calculation means. Ionizing volatile organic acid concentration is 15
Provided is a methane fermentation control method in which the amount of a base material charged into a methane fermentation tank is controlled so as not to exceed -20 (mg / l).

Further, according to the present invention, a methane fermentation tank is provided with a pH meter, a component-by-component measuring device for volatile organic acids and a pH control means, and a liquid phase pH value is measured and a component-by-component measuring device is used to measure non-acetic acid After the ionized volatile organic acid is measured, the non-ionized volatile organic acid concentration is calculated by a calculating means, and the non-ionized volatile organic acid concentration is 15 to 20 as an index.
PH of methane fermentation tank should not exceed (mg / l)
There is provided a method for controlling methane fermentation adapted to control

According to such a method for controlling methane fermentation, the system fouling caused by the overload of organic substances, the shortening of HRT, the inflow of toxic substances, etc. when the waste is subjected to methane fermentation by anaerobic treatment is non-ionizable. It becomes possible to detect early from the change of the organic acid concentration and take measures such as control of the amount of input substrate and control of the liquid phase pH value.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Specific examples of the method for controlling methane fermentation according to the present invention will be described below. In this example, first, for the purpose of clarifying the inhibitory effect of non-VFA in methane fermentation, a pH dependency test during methane fermentation was carried out using various VFAs or lower alcohols as substrates.

FIG. 3 is a schematic diagram of the experimental apparatus used for culturing the methanogenic bacterium according to the present embodiment. Reference numeral 1 denotes a water tank made of plastic or the like. It has been deployed. The volume of the water tank 1 is 20
It was liter.

Two support rods 4, 4 are installed over both ends of the water tank 1. The other ends of the strings 5, 5 each having one end connected to the support rods 4, 4 have a narrow mouth of 1 liter in volume. A total of six narrow-mouthed bottles 3, 3 connected to the bottles 3, 3 are immersed in the water tank 1. Reference numeral 7 denotes an air-washing bottle, and reference numeral 8 denotes a gas holder. The gas holder 8 is provided with a scale 9 for measuring the vertical movement of gas.

According to such an apparatus, the narrow-mouthed bottles 3, 3 float on the water in the water tank 1, and the throw-in heater 2 keeps the inside of the water tank 1 at a temperature condition which will be described later.
The sludge and the medium are put into the container, and the mixture is stirred by the impeller, so that each of the narrow-mouthed bottles 3 and 3 swings to generate gas.

The gas thus generated passes through gas conduits 6 and 6 from the gas outlets at the upper portions of the narrow-mouthed bottles 3 and 3, and passes through a gas-washing bottle 7 filled with 1N NaOH to remove CO ₂ . The gas passes through the gas conduit 10 and passes through the gas holder 8.
Is stored in Then, the vertical movement of the gas holder 8 is read by the scale 9 to calculate the amount of methane gas generated.

As the medium, a basic medium having the composition shown in Table 1 was used.

[0029]

[Table 1]

In practice, a 10-fold concentration medium was prepared and used as a stock solution diluted 10-fold each time it was used.

First of all, the VFA as the substrate was formic acid, acetic acid, propionic acid, and isobutyric acid, and the lower alcohols were methanol and ethanol. Then, the sludge that has undergone anaerobic digestion is collected from the sludge digestion tank, and the digested sludge stored at room temperature is used as seed sludge, and 250 ml of this seed sludge and the basic medium 500
ml into the narrow mouth bottles 3 and 3 and 1N, HCl or 1
The pH was adjusted to 7.3, 6.5, 5.8, and 5.5 using N and NaOH.
The culture was adjusted to 0.

During the experimental period, the temperature in the water tank 1 was set to 30 ° C., and the methane generation amount and pH were measured, pH adjustment, and methane generation rate were measured once a day or two. In addition, a small amount of the culture solution was sampled at appropriate times and filtered using filter paper, and the filtrate was sampled to measure the VFA (volatile organic acid) concentration by ion chromatography.

As for the substrate input, in principle, the same substrate is used for pH.
The same was applied regardless of the above, and the input amount per one time was increased with the passage of time. Then, calculate the maximum methane production rate for each substrate and set this as 100%
The methane production rate in H was expressed as a percentage. When VFA was used as the substrate, the VFA decomposition rate was calculated in addition to the methane production rate, and the ratio to the maximum VFA decomposition rate was expressed as a percentage. Further, it was carried out when formic acid or acetic acid was added to the system in which propionic acid remained.

In the non-ionized VFA, the existence ratio of acetate ion and non-ionized acetic acid is a function of pH expressed by the following equation (1), and the existence ratio is first calculated from pH,
It was calculated by multiplying this by the VFA concentration. However, since propionic acid, butyric acid, and valeric acid have similar pK values to acetic acid, it can be calculated by this method, but since formic acid is a strong acid, there is no non-ionizing type in the pH range of 5 to 7. It is considered to be a thing.

FIG. 4 shows the relationship between the abundance ratio of free acetic acid (non-ionized) and acetate ion (ionized) and pH dependency calculated using the above formula (1).

The results of these methane fermentation pH dependence tests will be described below. Figure 5 shows the relationship between pH and maximum methane production rate in methane fermentation of various VFAs (COD equivalent, gC
FIG. 6 is a graph showing OD / l · day, NaHCO ₃ addition amount 5 g / l), and FIG. 6 shows pH in methane fermentation of various VFAs.
And the maximum VFA decomposition rate (COD conversion, gCOD /
1 is a graph showing the amount of NaHCO ₃ added per day (5 g / l). 5 and 6 that acetic acid has a higher methane generation rate and VFA decomposition rate than VFA other than acetic acid, and that both rates decrease gradually even when pH is lowered to 5.

VFAs other than propionic acid have a pH of 6.
When it is increased from 5 to 7.3, the decomposition rate tends to decrease, and when the pH of VFA other than acetic acid is decreased from 5.8 to 5.0, the decomposition rate is significantly decreased as compared with acetic acid. . The decomposition rate of formic acid also decreased remarkably at pH 5.0, but considering that non-ionized formic acid is almost negligibly low, it is considered to be due to the inhibitory property due to the increase of hydrogen ion concentration at pH 5.0.

Further, comparing the cases of pH 6.5 and 5.8, it can be seen that pH 5.8 has higher methane generation rate and VFA decomposition rate (acetic acid and formic acid).
It can be considered that, in the range of pH 6.5 to pH 5.8, the methanogenesis-inhibiting effect due to the increase of the hydrogen ion concentration is small.

On the other hand, FIG. 7 is a graph showing the relationship between the non-ionizing VFA and the VFA decomposition rate in a methane fermentation pH dependence test using various VFAs as substrates. When acetic acid was used as the substrate, the non-ionizing VFA concentration was 150 (mg / L) even VFA
Since the decomposition rate is 84% of the maximum value of the VFA decomposition rate when the conditions other than pH are the same, it is considered that acetic acid has a lower non-ionization type inhibitory property than propionic acid or isobutyric acid.

PH of decomposition rate of acetic acid and VFA other than acetic acid
The difference in dependence and the non-ionizing VFA inhibitory difference are due to the fact that the number of hydrogen-producing acetogenic bacteria that have the action of producing hydrogen from propionic acid and the like to produce acetic acid is small, and that they are easily inhibited by non-ionizing VFA. Conceivable.

Next, in order to clarify the methane fermentation inhibitory effect of the non-ionized VFA concentration in the methane fermentation pH dependence test of VFA and lower alcohol, the pH at which the maximum methane generation rate or the maximum VFA decomposition rate was obtained for each substrate was determined. A period was selected and the ratio of methane generation rate to VFA decomposition rate under other pH conditions during that period was calculated. The results are shown in Table 2.

[0042]

[Table 2]

The data in Table 2 are only for pH 6.5 and pH 5.8. In Table 2, methane generation rate and VFA
It is considered that the lower the ratio with the decomposition rate, the stronger the inhibitory effect on methane production. Also, the highest methane production rate and the highest VFA
Degradation rates were recorded at pH 6.5 except for propionic acid.
Non-ionized VFA, non-ionized propionic acid, non-ionized VFA other than acetic acid are averaged at the beginning and end of the target period, whereas non-ionized VFA and non-ionized VFA other than acetic acid are converted to acetic acid equivalent concentrations. Non-ionized propionic acid is not converted to acetic acid.

Using the data in Table 2, non-ionized VFA, non-ionized propionic acid, non-ionized VFA other than acetic acid, etc. and [methane production rate] / [maximum methane production rate] and [VFA decomposition rate] / [maximum VFA decomposition] Speed] correlation analysis,
The correlation coefficient and regression equation were obtained. The results are shown in Tables 3, 4 and 5. Further, among the correlations obtained by the above analysis results, particularly noteworthy relations are selected and shown in FIGS.

[0045]

[Table 3]

[0046]

[Table 4]

[0047]

[Table 5]

Correlation between [non-ionized VFAs other than formic acid and non-ionized VFAs other than acetic acid and [methane production rate] / [maximum methane production rate] and [VFA decomposition rate] / [maximum VFA decomposition rate] is respectively -0.930, -0.759
And non-ionized propionic acid (-0.834, -0.69).
2) and non-ionized VFA (-0.063, -0.116) have a higher negative correlation.

Even when VFA and lower alcohol are used as substrates, the negative correlation between [methane production rate] / [maximum methane production rate] is relatively high.
It was FA (correlation coefficient -0.731). On the other hand, the correlation coefficient of non-ionized VFA was low both when the substrate was VFA alone and when both VFA and lower alcohol were combined. Therefore, when assessing the inhibitory strength of non-ionized VFAs, VF other than acetic acid was used more than total VFAs and propionic acid alone.
It is considered better to target A.

When non-ionized VFA other than acetic acid is used as an index of inhibition, the methane production rate is 80% and 50% at the maximum.
The concentration decreases to 15% when the concentration shown in FIGS.
˜20, 70 (mg / l). When actually performing methane fermentation, the methane production rate is 80% of the maximum
Since it is necessary to control so as not to decrease below, the concentration of non-ionized VFA other than acetic acid may be suppressed to 15 to 20 (mg / l) or less.

Therefore, in this example, the methane fermentation method shown in the following two specific examples was realized based on the experimental facts described above.

[0052]

【Example】

[Embodiment 1] FIG. 1 shows a system configuration for controlling the embodiment 1. In the figure, 11 is a methane fermentation tank, 12 is a pH meter, 13 is a VFA component measuring instrument, and 14 is a non-ionizing VF.
A calculation means, 15 is a substrate input amount control means.

As shown in the figure, in Example 1, a pH meter 12 and a VFA component-specific measuring device 13 were installed in a methane fermentation tank 11, and a pH value of the liquid phase was measured by the pH meter 12 and a VFA component-specific measuring device 13 was used. Non-ionized VFA other than acetic acid is measured, and then the non-ionized VFA is calculated by the non-ionized VFA calculation means.
The substrate input amount control means 15 controls the input substrate mass to the methane fermentation tank 11 so that the concentration does not exceed 15 to 20 (mg / l).

Specifically, when the concentration of non-ionized VFA other than acetic acid exceeds 15 to 20 (mg / l), the above-mentioned concentration of 15 to 20 is stopped by stopping the input of the substrate or reducing the input amount. Wait until (mg / l) or less.

[Second Embodiment] FIG. 2 shows a system configuration for controlling the second embodiment. Since the basic configuration is the same as that of FIG. 1, the same reference numerals are given and displayed. In the second embodiment, the substrate input amount control means 1 in the first embodiment is used.
Instead of 5, pH control means 16 is provided.

Then, the pH meter 12 measures the pH of the liquid phase in the methane fermentation tank 11, and the VFA component-specific measuring device 13 measures the non-ionized VFA other than acetic acid.
The non-ionizing VFA is calculated by the non-ionizing VFA calculating means, and the non-ionizing VFA concentration is 15 to 20 (mg /
The pH is controlled by the pH control means 16 so as not to exceed l). As a pH control method, there is a method in which an alkali is added or the gas is washed with water to remove CO ₂ in the gas phase and volatilize into the liquid phase. When pH increases, [non-ionizing VFA]
/ [Total VFA] ratio decreases, and non-ionized V other than acetic acid
FA concentration also decreases.

[0057]

As described in detail above, the methane fermentation control method according to the present invention comprises a methane fermentation tank equipped with a pH meter, a volatile organic acid component-by-ingredient measuring device, and a substrate input amount control means or a pH control means. The concentration of the non-ionizing volatile organic acid other than acetic acid is measured by measuring the liquid phase pH value and the component-specific measuring instrument, and then the non-ionizing volatile organic acid concentration is calculated by a calculation means. The system failure (abnormal fermentation) can be accelerated by controlling the mass of the base material input to the methane fermentation tank or by controlling the pH so that the volatile organic acid concentration does not exceed 15 to 20 (mg / l). Therefore, it is possible to take appropriate measures.

In particular, there has been no unified view on the methane fermentation inhibition property of non-ionizing volatile organic acids, but according to this Example, the methane fermentation pH of volatile organic acids and lower alcohols
From the results of the dependence test, it was found that non-ionized acetic acid had weak inhibitory properties and strong inhibitory effects on non-ionized VFAs other than acetic acid. By calculating VFA and controlling the substrate input amount or pH using this as an index, the above-mentioned problems can be efficiently solved.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a methane fermentation control system according to a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a methane fermentation control system according to a second embodiment of the present invention.

FIG. 3 is a schematic diagram of an experimental apparatus used for culturing a methanogenic bacterium according to this example.

FIG. 4 is a graph showing the relationship between the abundance ratio of free acetic acid (non-ionized) and acetate ion (ionized) and pH dependence.

FIG. 5 is a graph showing the relationship between pH and the maximum methane production rate in methane fermentation of VFA.

FIG. 6 pH and maximum V in methane fermentation of various VFAs
The graph which shows the relationship of FA decomposition rate.

FIG. 7 is a graph showing the relationship between non-ionized VFA and VFA decomposition rate in a pH dependence test of methane fermentation using various VFAs as substrates.

FIG. 8: Nonionized VFA other than acetic acid and [Methane production rate]
The graph which shows the correlation of / [maximum methane production rate].

FIG. 9: Nonionized VFA other than acetic acid and [Methane production rate]
The graph which shows the correlation of / [maximum methane production rate].

[Explanation of symbols]

 1 ... Water tank 2 ... Throwing heater 3 ... Narrow mouth bottle 4 ... Support rod 5 ... String 6, 10 ... Gas conduit 7 ... Rinsing bottle 8 ... Gas holder 9 ... Scale 11 ... Methane fermentation tank 12 ... pH meter 13 ... VFA component measurement Container 14 ... Non-ionizing VFA calculation means 15 ... Substrate input amount control means 16 ... pH control means

Claims (2)

[Claims] 1. A methane fermentation tank is provided with a pH meter, a volatile organic acid component-specific measuring device, and a substrate input amount control means,
Non-ionizing volatile organic acids other than acetic acid are measured by liquid phase pH value measurement and component-by-component measuring instrument, and then the non-ionizing volatile organic acid concentration is calculated by calculation means. A method for controlling methane fermentation, which comprises controlling a mass of a base material charged into a methane fermentation tank so that an acid concentration does not exceed 15 to 20 (mg / l). 2. A methane fermentation tank is provided with a pH meter, a volatile organic acid component-by-ingredient measuring device and a pH control means, and a liquid phase p
After measuring non-ionizing volatile organic acids other than acetic acid by measuring H value and component-specific measuring instrument, the non-ionizing volatile organic acid concentration is calculated by calculation means, and the non-ionizing volatile organic acid concentration is used as an index. The method for controlling methane fermentation, which comprises controlling the pH of the methane fermentation tank so that the value does not exceed 15 to 20 (mg / l).