JP4768881B2 - Dehydrated methane fermentation - Google Patents

Description

  In the methane fermentation of the present invention, by extracting the moisture of the digested sludge contained in the fermenter, the digested sludge concentration (bacterial cell concentration) in the fermenter is increased, and the volume of the digested sludge decreases and stays by the amount of water reduced. The time will be longer. As a result, the present invention relates to a methane fermentation method aiming to increase the digestibility to a theoretical value (90% or more) and to achieve downsizing of the fermenter.

From the time when global warming is called out, it is very important as a CO ₂ reduction measure to collect energy from carbonaceous biomass waste. Methane fermentation seems to be optimal for energy recovery from watery biomass such as sewage sludge, drought, livestock waste, agricultural waste, food processing waste, papermaking waste, etc. A large-capacity fermenter is required to ferment 5% or less of biomass over about 4 weeks, and only a low digestion rate (conversion rate of biomass carbon to digestion gas) of 40-60% can be obtained. It was.

  Methane fermentation has long been carried out as part of sludge reduction measures at sewage treatment plants, but as sewage into which all waste flows is regarded as a treasure trove of resources, we looked at a low-carbon society and a resource recycling society. The future image is changing to a sewage treatment plant. The basic technology that supports the future is methane fermentation with high digestibility. The main resources contained in sewage are energy, ammonia, phosphorus, and rare metals. Energy, ammonia, and phosphorus are contained in the organic biomass portion, and rare metals are contained in the inorganic portion. High digestibility methane fermentation technology is indispensable for high efficiency recovery of energy and ammonia / phosphorus. When treated with the high digestibility methane fermentation technology, ammonia and phosphorus are separated from the organic matter and contained in the desorbed liquid, and the rare metal is contained in the inorganic portion of the residue from which the organic matter has been removed.

Also, when incinerating waste digested sludge, N ₂ O with a warming coefficient 310 times that of CO ₂ is generated, so high digestibility methane fermentation reduces the amount of sludge incinerated from the viewpoint of reducing greenhouse gases. Technology is essential. In other words, high digestibility methane fermentation technology is a fundamental technology that will support future sewage treatment plants.

Through research on dehydrated sludge (high-concentration biomass) methane fermentation with the aim of technological innovation in methane fermentation, the present inventors have shortened the residence time, reduced the size of the fermenter, and obtained digestibility of nearly 90%. If the concentration was too high, NH ₃ concentration disturbance occurred, so the solution was a problem. They also discovered a phenomenon in which solubilization of solid organic matter was promoted as the sludge concentration increased. (See Non-Patent Document 1 and Fig. 9)

If this phenomenon includes that the rate-limiting step of methane fermentation is a solubilization process (generally well known), the relationship between sludge concentration, methane fermentation rate, digestibility, and residence time becomes clear from the following diagram.
[Sludge Concentration] ∝ [Sludge Concentration in Tank] 菌 [Bacterial Cell Concentration in Tank] ∝ [Solubilized Enzyme Concentration in Tank] ∝ [Solubilization Rate of Solid Organic Matter] ∝ [Methane Fermentation Rate]
As is clear from the above diagram, dewatering the raw sludge and extracting the moisture from the digested sludge in the fermentor to increase the digested sludge concentration are the same effect in increasing the solubilized enzyme concentration in the fermenter From this, it can be seen that high digestibility methane fermentation technology can be developed by extracting water from digested sludge in the fermenter instead of dewatered sludge.
(See [0039])

Patent No. 2138131 Patent No. 2997833

Yoshio Kobayashi "Methane fermentation of dehydrated sludge" PPM 25 34-41 (1994)

The miniaturization of the fermenter and the achievement of a digestibility of 90% were conspicuous in the methane fermentation of dehydrated sludge (see [0034] [Calculation of digestibility]), but (1) the water content of the input sludge was reduced. If it is too low, methane fermentation will not proceed due to high concentration of NH ₃ in the fermenter. (2) It is less expensive to extract the moisture from the digested sludge whose organic matter has been reduced by methane fermentation than to dehydrate the raw material sludge (biomass). (3) In the methane fermentation facility at the sewage treatment plant, downsizing the fermenter and improving the digestibility from about 45% to about 90% are important themes.
(1) For the reasons of (2) and (3), in the present invention, a sewage treatment system based on a small fermenter and high digestibility methane fermentation technology based on the method of removing water from the fermentor described in [0003] was constructed.

  The problem of realizing this moisture-removal methane fermentation technology was conspicuous by the reaction engineering analysis of the methane fermentation process. The present inventors have studied that the solubilization of solid substrate sludge (biomass), which is the rate-limiting step of methane fermentation, is promoted when the substrate biomass concentration supplied to the fermenter increases while studying the mechanism of dehydrated sludge methane fermentation method. discovered. When the substrate sludge (biomass) concentration in the fermenter is increased, the concentration of cells that propagate in the sludge (biomass) is also increased, and the concentration of the solubilizing enzyme secreted by the solubilizing bacteria is also increased. On the other hand, since the solubilization reaction rate is proportional to the solubilizing enzyme concentration, the solubilization reaction is promoted when the solubilizing enzyme concentration increases.

  Despite the efforts of many researchers, the digestibility of methane fermentation technology that is currently in practical use is difficult to exceed 40-60%, and it is not clear where the upper limit of digestibility is. The present inventor derived the relationship between the solubilized enzyme concentration and the digestibility in terms of reaction engineering, and clarified the problems of the digestibility described above, for the realization of a moisture-removal methane fermentation technique. That is, in order to obtain a high digestibility, it was found that the digested sludge (bacteria) concentration in the fermenter should be increased, or the residence time should be increased by increasing the fermenter volume (details [ Experimental support] [See 0033]-[0038]).

Regarding the digestibility of methane fermentation (conversion rate of carbon content of substrate biomass to digestion gas), the rate of conversion to digestion gas is the ultimate when 5-10% of carbon content of substrate biomass is fixed as cells That is about 90%. In other words, the upper limit of digestibility is considered to be about 90%. This conclusion is consistent with the experimental fact (see [0035] to [0038] [Calculation of digestibility]) that a digestibility of about 90% was obtained by methane fermentation of dewatered sludge that we developed.

  The upper limit of the digestibility of water-removal methane fermentation is based on the fact that 5-10% of the carbon content of the substrate biomass is fixed as microbial cells that are difficult to undergo methane fermentation. The cells of the microbial cells are protected by cell walls that do not accept solubilizing enzymes, so it is difficult to process in methane fermentation. However, the heat treatment can destroy the cell walls and enable processing in methane fermentation. well known. This phenomenon also applies to methane fermentation treatment of sewage surplus sludge consisting of aerobic microorganisms. For example, in Fig. 7, waste digested sludge containing a large amount of bacterial cells is heat-treated and mixed with `` input substrate biomass '' in a high temperature state, so that the heating operation of the substrate biomass is omitted and the digestibility is increased to 90% or more. I can do it. [0035] ~ [0038] [Calculation basis of digestion rate of about 90%] is based on continuous experimental data of sewage primary sludge, digestion rate in methane fermentation of 40% excess sludge and 60% primary sludge We got about 80%. The difference between about 80% and the digestibility of primary sludge is about 90% is due to the cell wall of excess sludge. A digestibility of 90% can be secured by heat-treating this excess sludge together with the waste digested sludge from methane fermentation.

  The conventional methane fermentation system requires a large-capacity fermentor to ferment biomass with a solid content of 5% or less over about 4 weeks, and has a low digestibility of 40-50% (most recently 40-60) % Progress). If the residence time was set to 2 months in this methane fermentation system, the digestibility might have increased to nearly 80%, but in order to secure a residence time of 2 months, in addition to the current fermentor, 1 to 2 times the capacity However, such a giant fermenter is unrealistic, and it seems that the upper limit of the actual digestibility is 40-60% as a natural process. .

  In the normal method methane fermentation system, the cells grown during fermentation are discharged out of the fermenter as digested sludge together with undigested biomass. In other words, in the conventional methane fermentation system, the solids concentration in the fermenter decreases with the progress of fermentation, but it is directly discharged out of the fermenter as digested sludge, but here the digested sludge is removed using a gravity sedimentation tank or sludge concentrator. If moisture is discharged outside the fermenter, the concentration of digested sludge (including bacterial cells) in the fermenter can be increased, and the residence time can be increased by the amount of reduced water.

  In the system method of the present invention, by extracting water from the digested sludge in the fermenter in this way, the concentration of digested sludge (bacteria) in the fermenter is increased, and the residence time is increased by the amount of reduced water. As a result, the fermentation rate was increased, enabling 90% digestibility and downsizing of the fermenter.

Specific means for extracting water from the fermenter of the system of the present invention are the digested sludge concentrator described above, a method using a gravity sedimentation tank, a method of returning a part of the digested sludge dewatered cake for disposal to the fermenter, A method of providing a filter on the inner surface of the fermenter (see [0028] and [0029]) can be considered. In short, digested sludge (bacteria) concentration and solubilizing enzyme concentration in the fermenter should be increased. It is also effective to add a flocculant to the fermenter as an aid for the operation of extracting moisture from the digested sludge.

  If the moisture in the fermenter is extracted, the solids concentration in the fermenter will increase and the fluidity will be lost, and there is a concern that the substrate biomass will not be inoculated smoothly.・ If the solubilizing enzyme concentration increases, the fermentation rate increases and the substrate biomass disappears through solubilization and gasification, so the viscosity (solid content concentration) in the fermenter does not rise abnormally. Rather, the operation management determines the supply sludge amount and the discharged digested sludge amount while keeping the viscosity (solid content concentration) in the fermenter constant.

  The above explanation is in the steady state, but in the process up to the steady state, the undigested substrate biomass is converted into digestion gas and the digestibility continuously changes from 45% to 90%. To do. During the long-term continuous operation, the amount of undigested substrate biomass in the digested sludge decreases, and the decrease is replaced by the increased amount of bacterial cells, so the solid content concentration (viscosity) in the fermenter increases abnormally and the substrate Biomass inoculation is not inhibited.

[Application example 1 of the method of the present invention] (High digestibility and greenhouse gas reduction effect)]
The existing methane fermentation facility at the sewage treatment plant is equipped with a facility for extracting moisture from the fermenter, and the digestibility of the methane fermenter is increased from about 45% to about 90% without any major modifications, and the sludge treatment capacity. Can also be enhanced. As a result, the amount of methane gas recovered doubles, the amount of auxiliary fuel associated with incineration of waste digested sludge decreases, and the amount of N ₂ O with a global warming potential of 310 decreases.

既設のメタン発酵設備に僅かな設備投資をするだけで消化率90%の水分抜き取り式メタン発酵が実現する。表１は水分抜き取り式メタン発酵では通常法メタン発酵の約2倍量のCH₄ガス回収が得られる事と、消化汚泥の焼却処分量が少ないために温室効果ガスの排出量が通常法メタン発酵よりも圧倒的に少ないことを示している。The table below shows a comparison of greenhouse gas emissions when 1t (DS) of sewage sludge is treated by the conventional method (digestion rate 45%) methane fermentation technology and the water removal type (digestion rate 90%) methane fermentation technology.

Moisture extraction methane fermentation with a digestibility of 90% can be achieved with a small investment in existing methane fermentation facilities. Table 1 shows that the CH ₄ gas recovery of about twice the amount of conventional methane fermentation can be obtained with water extraction methane fermentation, and that the amount of greenhouse gas emissions is less than conventional method methane fermentation due to the incineration disposal of digested sludge. It is overwhelmingly less than that.

[Application example of the method of the present invention No. 2. (Resource recovery measures)]
90% of phosphorus, which is said to contain about 2% of dry sewage sludge when the digestibility reaches 90%, dissolves in the digestive fluid, but contains solids extracted from the fermenter through a filter This liquid is referred to as desorbed water, but when Mg ²⁺ (MgCl ₂ , MgSO _4, etc.) is added to the desorbed water, depletion is called out and the phosphorus resources that are currently rising in market price are hardly soluble NH ₄ MgPO ₄ -Easily recovered as 6H ₂ O crystals.

When the digestibility reaches 90%, 90% of the nitrogen contained in the sewage sludge is also dissolved in the digestive fluid as NH ₃ . The NH ₃ elution amount corresponds to about 70 kg per sludge organic component 1 (DS) t. In the usual NH ₃ treatment method, NH ₃ is converted into NO, NO ₂ and N ₂ O in the aeration tank, and NO and NO ₂ are reduced to N ₂ by the BOD component in the sewage in the following anaerobic tank, but chemically. Stable N ₂ O is directly released into the atmosphere. With this method, the amount of N ₂ O generated during aeration, the consumption of aeration power, and the required amount of BOD components are approximately double that of 45% at a digestibility of 90%. Therefore, in the present invention, an ion exchange resin (membrane) is used. Collect NH ₃ as a resource and use it as fertilizer.

  In the future, it will be possible to recover rare metals. In this way, moisture-removing methane fermentation that provides a digestibility of 90% is a fundamental technology that supports sewage treatment plants in a low-carbon society and a resource recycling society. By elevating the digestibility to 90% at low cost, the position of methane fermentation in the sewage treatment plant will advance from the supporting role of sludge reduction measures to the leading technology that supports the sewage treatment plant.

[Example of application of the method of the present invention 3. (Measures against NH ₃ concentration disorder)
When livestock waste with a high nitrogen content and low moisture content is treated with methane fermentation, the NH ₃ concentration in the fermenter becomes too high and the fermentation does not proceed smoothly due to NH ₃ concentration failure. Diluting waste with water to avoid NH ₃ concentration obstacles and adopting the method of the present invention that removes moisture through a filter provided on the inner surface of the fermenter will not increase the fermenter volume without incurring significant costs. NH ₃ is possible to avoid concentration disorder can, however NH ₃ concentration disorders by adding water to low moisture biomass to avoid, then method to remove the NH ₃ that extracts moisture from the fermentor NH ₃ in It is not a wise method because it recovers NH ₃ after diluting high-concentration desorption water with water.
NH ₃ recovered first NH ₃ from eliminated water which methanation is the TOC concentration rises not proceed due to the high concentration disorders, then led to the fermentor (eg UASB tank) using granulated anaerobic microorganisms One solution is to recover methane.

Although NH ₃ concentration is preferably 1000ppm or less in the digestive juice in the methane fermentation, NH ₃ concentration in the digestion solution for NH ₃ content is high in livestock waste that is more than 1000ppm is often also a sewage sludge treatment When the solid content concentration of the sludge put into the fermenter increases, the NH ₃ concentration in the digestive fluid exceeds 1000 ppm, causing an NH ₃ concentration disturbance, and the CH ₃ production rate decreases as the NH ₃ concentration increases. The TOC (total organic carbon) concentration in the liquid increases. Therefore, in the present invention, the TOC component is converted to CH ₄ by introducing the desorbed water into the fermentor using the anaerobic microorganism group granulated after NH ₃ is recovered from the desorbed water extracted from the fermenter.

  In this way, the system of the present invention is applied to existing and new facilities for methane fermentation of various biomass, and brings about a large economic effect by downsizing the fermenter and increasing the digestibility by about 90%. Contributes greatly to reduction.

Fig. 1 shows the material flow in the fermenter for conventional methane fermentation, and Figs. 2, 3, and 4 show the material flow for methane fermentation with water removal. In the usual method, the substrate biomass (sludge, livestock waste, agricultural waste, etc.) input is discharged outside the system as digested sludge as it grows within the residence time (see Fig. 1). In the method (moisture-removal type methane fermentation), the residence time in the fermenter is increased because the volume of the digested sludge is reduced by extracting the water from the digested sludge in the fermenter. The proliferation amount of the bacterial cells increases according to the longer residence time, and the bacterial cell concentration in the fermenter increases. When the bacterial cell concentration is increased, solubilization of the solid organic matter is promoted, and the methane fermentation rate is increased. As a result, the digestion rate has been improved from 45% to 90% and the fermenter can be downsized. (Quantitative explanation shows the mass balance of the analysis section of FIG. 6, FIG. 7, FIG. 8 [0047] [004
8])

[How to remove moisture]
Mechanical concentration (Fig. 2) and gravity sedimentation (Fig. 3) are well known as means for draining water in the fermenter, but they are installed outside the fermenter and fermenter as a much lower cost system. We propose here a new invention in which a filter is provided on the inner surface of a pipe and the like, and water is extracted through the filter using the water pressure applied to the filter (FIGS. 4 and 5). A filter is a device that allows only liquid to pass through without passing through a solid, and according to FIGS. 4 and 5, the main part of the filter is a hole formed in the wall of a fermenter or conduit (attached to the fermenter). It consists of a filter cloth affixed to the inner wall of a fermenter or conduit. In FIGS. 4 and 5, the desorbed water extracted through the filter passes through the space provided between the outer wall of the fermenter or conduit and the jacket of the fermenter or conduit, and is discharged at the bottom of the fermenter or conduit. It is discharged from the exit.

[Method by filter provided on the inner surface of fermenter or conduit]
This method uses the water pressure applied to the filter provided on the inner surface of the fermenter, conduit, etc. to extract water from the fermenter to make slurry sludge with a moisture content of about 95% (solid content concentration about 5%). The function is completely different from that of the filter-press dehydrator, which is intended to produce a solid sludge with a moisture content of about 80% (solid content concentration of about 20%). Less is enough. Among the water extraction devices incorporated in the system of the present invention, the filter system provided on the inner surface of a fermenter, a conduit or the like is the most excellent in terms of both equipment cost and energy consumption.

  The growth of digested sludge deposits on the filter is an obstacle to draining water, but when the sludge concentration increases in the growth of the deposits, solubilization is promoted and the growth of the deposits is suppressed. . In addition, when the digestibility is 90%, the ratio of inorganic components (sediment components) in digested sludge increases, but it is a well-known fact that moisture can be easily extracted when the proportion of (sediment components) increases. The solubilization effect / sediment effect and the stirring effect in the tank, and the synergistic effect of the flocculant effect and the backwashing effect do not impede the removal of moisture. This method does not require a large-scale facility other than the filter-repairing equipment such as backwashing, and the power necessary for draining water is obtained by applying the water pressure applied to the filter provided on the inner surface of the fermenter or conduit. Although there is no special power consumption to use, the connection with the fermenter of the external conduit-type moisture extractor is connected to the lower part of the fermenter where large water pressure is applied at both the inlet and outlet of the conduit-type moisture extractor. Is particularly effective. By adding a flocculant to the fermenter, water removal becomes easier.

This method is not only newly established, but also has a significant effect on CO ₂ reduction and economic efficiency by making small modifications to existing facilities. If it is difficult to modify the fermenter with the existing methane fermenter, replace the function of removing water by the filter provided on the inner surface of the fermenter with a method of connecting an external conduit-type moisture extractor to the lower part of the fermenter. I can do it. When sludge slurry is taken in from the lower part of the fermenter and returned to the lower part of the fermenter via a pump, this conduit is always under the same water pressure as the lower part of the fermenter. It has the advantage that it can be handled with a filtration area and a compact device, and is easy to maintain.

[Experimental support of the present invention]
[Upper limit of methane fermentation digestibility]
Until now, it was uncertain where the upper limit of the digestibility of methane fermentation was, but continuous experimental data for dehydrated sludge methane fermentation with a solid content of 15% (Fig. 9: Patent No. 2997833, [Non- Patent Document 1] (See PPM 25 34-41 (1994)), calculating the digestibility from the operating data of the carbon balance, energy balance and sewage treatment plant yielded about 90% ([0035] To [0038] [Calculation of digestibility]. Assuming that 5 to 10% of the carbon content is converted into cells, a digestion rate close to the theoretical value was obtained in the methane fermentation experiment of dehydrated sludge.

[Arrangement of data for calculating digestibility from basic experimental data in FIG. 9]
If we read the cumulative amount for 28 days from Fig. 9,
Input organic sludge amount: 2.733 kg
Digestion gas generation amount: 2.00m ³ N
Digestion gas generation rate per input organic sludge: 0.732m ³ N / kg

[Calculation of digestibility (1) Method based on material balance]
Analytical value of sewage sludge (organic) (wt%): H 6.33%, C
42.11%, N 6.55%
Carbon content of input substrate sludge: 421.1g / kg (sludge) = 35.09mol / kg (sludge)
Carbon content of digestion gas (CH ₄ + CO ₂ ): 0.732 m ³ N / kg (sludge) = 30.97 mol / kg (sludge)
Digestibility: 30.97mol / kg (sludge) ÷ 35.09mol / kg (sludge) = 0.88

[Calculation of digestibility (part 2) Method based on heat balance]
Digestion gas composition: CH ₄ 60%, CO ₂
40%
Calorific value of CH ₄ : 9500kcal / m ³ N
Calorific value of sewage sludge: 4500kcal / kg
Calorific value of digestion gas: 9500kcal / m ³ N × 0.60 = 5700kcal / m ³ N
Digestibility: 5700kcal / m ³ N × 0.732
m ³ N / kg ÷ 4500kcal / kg ＝
0.93

[Calculation of digestibility (part 3) Method based on operation data of sewage treatment plant]
Operating data of sewage treatment plant Organic sludge digested from Fig. 6 = 11.00t / d-6.05t / d = 4.95t / d
Generated digestion gas = 4322.3m ³ N / d
Digestion gas generated from 1kg of sludge (theoretical value): 4322m ³ N / d ÷ 4950kg / d = 0.87m ³ N / kg
Digestion gas amount corresponding to the charged organic sludge ^{amount: 2.733kg × 0.87m 3 N / kg} = 2.377m 3 N ^{digestibility: 2.0m 3 N ÷ 2.37m 3 N} = 0.84

[Summary of digestibility calculation]
Summarizing the calculation results of digestibility of dehydrated sludge methane fermentation (Figure 9)
[0035] Mass balance standard: 88%,
[0036] Heat balance standard: 93%,
[0037] Sewage treatment plant operating data criteria: 84%
Average digestibility: 88%
In other words, the analysis of continuous experimental data of 15 (DS)% concentration dehydrated sludge methane fermentation revealed that a digestibility of about 90% was obtained.

[Digested sludge concentration of dehydrated sludge methane fermentation with 15 (DS)% concentration]
When dehydrated sludge with a concentration of 15 (DS)% is subjected to methane fermentation at a digestibility of 90%, the digested sludge concentration in the fermenter becomes 4 (DS)%, which is generally indicated by [0047] [Fig. 8]. It is the same as the digested sludge concentration of the extracted water methane fermentation. In other words, the method of dehydrating the input sewage sludge in advance and the method of extracting the water from the digested sludge in the fermenter are exactly the same in terms of fermentation rate and digestibility except for the NH ₃ inhibition factor, and the dehydrated sludge with a concentration of 15 (DS)%. The digestibility of 90% obtained in the above is also applied to the water extraction type methane fermentation of [0047] [Fig. 8].

[Why is 90% digestibility achieved?]
In conventional methane fermentation, the digestibility is thought to be 45-60%, so 90% is a great improvement. The cause of this is that the concentration of the solubilizing enzyme secreted outside the body by the solubilized bacteria increases as the concentration of the substrate sludge increases and the concentration of the bacterial cells also increases. As a result, the solubilization reaction of the substrate sludge was promoted, and the relationship between the sludge concentration and the total amount of TOC (= water-soluble TOC amount + digested gas amount converted to TOC) was measured.

[Measurement of sludge concentration vs TOC total amount]
Liquid raw sludge (substrate sludge) provided by the sewage treatment plant and liquid digested sludge are mixed and precisely weighed in a centrifuge tube (500 cm ³ ). After centrifugation, the supernatant is removed and distilled water is added. Was added to adjust the concentration to 2 to 15 (DS)%. Next, a stopper was attached to this centrifuge tube, and methane fermentation was performed in a constant temperature bath at 55 ° C., and the TOC in the centrifuge tube and the amount of digestion gas generated were measured every day. FIG. 10 is a plot of the measured values of the elapsed days of 4, 7, 10, and 13. Fig. 10 shows that the sludge concentration effect on the total amount of TOC is almost constant regardless of the elapsed time.
The vertical axis (total amount of TOC) in Fig. 9 (= water-soluble TOC amount + digestion gas amount converted to TOC) is the amount of carbon eluted from sludge evenly weighed in a centrifuge tube (500 cm ³ ) (mg) Expressed in

[Reaction rate analysis of methane fermentation solubilization process]
It was demonstrated in Fig. 9 that the solubilization reaction of substrate sludge is promoted by increasing the substrate sludge concentration. Based on this, the reaction rate analysis of the solubilization step is as follows. In other words, when this phenomenon is organized, the following proportional relationship emerges.
[Sludge concentration] ∝ [Bacteria concentration] ∝ [Solubilized enzyme concentration] ∝ [Solubilization reaction rate]
This relationship can be expressed by chemical reaction equation and reaction rate equation.
Chemical reaction formula: Substrate sludge A + H ₂ O → Solubilized organic matter (organic acid, etc.) a
Solubilized enzyme C
Reaction rate formula: da / dt = k · C
Integral form of reaction rate equation: a = k · C · t
A: Solid substrate sludge
a: Solubilized organic matter amount (= methane gas production amount),
t: reaction time (residence time),
C: enzyme concentration,
k: Reaction rate constant (different values for each sludge composition)
The conclusion of the reaction engineering analysis is that the amount of solubilized organic matter a is proportional to the solubilized enzyme concentration C and the reaction time (residence time) t, but is not directly related to the substrate sludge concentration. In other words, whether the substrate sludge concentration is low or high (4% or 15%), 90% digestibility can be obtained as long as the solubilized enzyme concentration and residence time are secured. That is feasible.

According to this reaction rate equation, the digestibility of 90% obtained by dehydrated sludge methane fermentation has increased the concentration of cells that propagate in the sludge as a result of the solid content concentration of sludge being 15%, solubilizing enzymes secreted by the cells This is thought to be due to the increase in concentration. In other words, if the sludge concentration 15% is about 3 times the normal concentration 4.5% and the solubilizing enzyme concentration is also about 3 times the normal concentration, the normal method with a digestibility of 45% is 90% (value close to the theoretical value) It is reasonable to become.

According to this reaction rate equation, regardless of the substrate sludge concentration, if the bacterial cell concentration is increased, the digestibility close to the theoretical value can be obtained and the fermenter volume can be reduced. Based on this principle, the present inventor extracts moisture in the fermenter to keep the digested sludge concentration high and to reduce the volume of digested sludge to be handled. As a result, if the fermenter volume is the same, the residence time becomes longer. Based on this principle, a method of performing high-performance methane fermentation was invented.

To clarify the relationship between residence time, bacterial cell concentration, methane fermentation rate, digestibility, and fermenter volume through quantitative analysis, Fig. 1 shows the material balance of the sewage treatment plant that is actually operated. 6
In Fig. 2 of mechanical concentration, enter the mass balance assuming a digestion rate of 90% (see [0036] [Calculation of digestibility]) and a sludge concentration in the fermenter of 3 (DS)%. Figure 4 shows the water removal method using a filter provided on the inner surface of the tank. Figure 8 shows the material balance assuming a digestibility of 90% and a sludge concentration in the fermenter of 4 (DS)%.
It was.

The three figures, the mass balance diagram of the normal method (Fig. 6) and the mass balance diagram of the water extraction method (Figs. 7 and 8), have the same fermenter size, sludge concentration and amount of sludge. This shows that the concentration of bacterial cells in the fermenter and the residence time of the organic substances in the fermenter differ greatly depending on the amount of water extracted from the fermenter, and as a result, the digestibility has evolved from 45% to 90%. This is a diagram clearly illustrating the principle of the present invention.

FIG. 7 and FIG. 8 show the difference in the method of extracting moisture from the fermenter. Since the mechanical concentration in FIG. 7 is costly, the amount of water extracted is reduced and the digested sludge concentration in the fermenter is set to 3%. In FIG. 8, a filter is provided on the inner surface of the fermenter. In consideration of the fact that the water extraction cost is cheap because the water pressure is applied to the filter, the digestion sludge concentration in the fermenter is increased by increasing the water extraction amount. 4% is set.

From the mass balance charts of FIGS. 6, 7, and 8, the cell concentration increase rate and residence time increase rate in the fermenter for normal methane fermentation were calculated according to the following calculation method.
Since it is difficult to measure the amount of cells in the fermenter, the rate of increase in the concentration of cells produced by removing moisture from the fermenter using solid inorganic substances that do not change in mass as the methane fermentation progresses as an indicator of the amount of cells. (= Increase rate of solid inorganic substance concentration) was calculated by the following equation.

[Increase rate of bacterial cell concentration in the tank] for normal methane fermentation =
(Concentration of solid inorganic substance in the tank of the water removal type) ÷ (Concentration of solid inorganic substance in the tank of the normal method)
In the case of Fig. 7: [Increase rate of bacterial cell concentration in the tank] = 1.94% ÷ 0.47% = 4.12.
In the case of Figure 8: [Increase rate of bacterial cell concentration in the tank] = 2.57% ÷ 0.47% = 5.46
Similarly, [Increase rate of residence time in tank] for conventional methane fermentation =
(Exhaust digested sludge volume of normal method) ÷ (Exhaust digested sludge volume of water removal type)
In the case of Fig. 7: [Increase rate of residence time in the tank] = 421.05 t / d ÷ 103.33 t / d = 4.07
In the case of Fig. 8: [Increase rate of residence time in the tank] = 421.05 t / d ÷ 77.5 t / d = 5.43

The above calculation results are shown in Table 2 below as a comparison table between the water extraction type methane fermentation and the conventional methane fermentation.

Table 2 shows a comparison between the methane fermentation method in which water is extracted from the fermenter and the normal method methane fermentation. Even if water is extracted from the fermenter, water is removed at the input sludge (biomass) stage. Even if it is removed, the relationship between residence time / bacterial cell concentration and digestibility has the same tendency, indicating that the dehydrated sludge methane fermentation and the water extraction type methane fermentation in the fermenter are the same in principle. ing. (See [0039])

[0042] [Reaction rate analysis of methane fermentation solubilization process]
[Sludge concentration] ∝ [Bacteria concentration] ∝ [Solubilized enzyme concentration] ∝ [Solubilization reaction rate]
And [methane gas production amount] = k [solubilized enzyme concentration] x [residence time] relationship, as can be seen from Table 2, the increase in residence time and cell concentration increase rate are the same, so methane gas production It was found that the effect of removing water on the amount was effective to the square of the amount of water removed, and as a result, even a small amount of removed water greatly affected the digestibility.

This also suggests the possibility of downsizing the fermenter.
In the example of Fig. 7, [Solubilized enzyme concentration increase rate] x [Residence time increase rate] ≒ 25, and 5 is assigned as the potential to increase this digestion rate from 45% to 90%, and the remaining 5 is fermented. If it is allocated to downsizing of the tank, the fermenter volume will fit in 1/5 of normal methane fermentation. In other words, according to the newly developed calculation method, simply by setting the digested sludge (DS) concentration in the fermenter to 4%, the digestibility can be reduced from 45% to 90%, and the fermenter volume can be reduced to 1/5 of the conventional methane fermentation. The possibility has been seen.
Based on Table 2, preparations for a 1/1000 scale demonstration test at a sewage treatment plant are underway to verify dehydrated methane fermentation at the practical application stage.

Conceptual diagram of material flow of conventional methane fermentation Conceptual diagram of material flow in methane fermentation with water removal by mechanical concentration Conceptual diagram of material flow in methane fermentation with water removal by gravity concentration Conceptual diagram of methane fermentation with water removal by filter on inner surface of fermenter Conceptual diagram of a conduit-type moisture removal device attached to a fermenter Mass balance diagram of ordinary methane fermentation Material balance diagram of methane fermentation with water removal by mechanical concentration Mass balance diagram of methane fermentation with water removal by filter Bench scale continuous experimental data for dehydrated sludge methane fermentation Figure showing the relationship between sludge concentration (DS%) and total TOC (mg) (= water-soluble TOC amount + generated gas amount converted to TOC)

Claims (8)

In methane fermentation that digests substrate biomass using digested sludge that is circulated and used as cells, the digested sludge is stored in the fermenter and the digested sludge is filtered using the water pressure of the digested sludge in the fermenter. A methane fermentation method characterized by extracting moisture from digested sludge. The methane fermentation method according to claim 1, wherein sludge comprising at least one of anaerobic microorganism group and aerobic microorganism group is heat-treated to destroy microorganism cells and added to the input substrate biomass. The methane fermentation method according to claim 1, wherein a flocculant is added to the fermenter to make the digested sludge floc. The methane fermentation method according to claim 1, wherein ammonia resources are recovered from the desorbed water extracted from the fermenter by filtration using the water pressure of the digested sludge in the fermenter. 2. The methane fermentation according to claim 1, wherein phosphorus resources are recovered by adding magnesium ions (Mg ²⁺ ) to desorbed water extracted from the fermenter by filtration using the water pressure of digested sludge in the fermenter. Method. The methane fermentation method according to claim 4, wherein the methane is recovered by introducing it into a fermentor using an anaerobic microorganism group obtained by granulating the desorbed water containing the TOC component from which the ammonia resources have been recovered. In methane fermentation that digests substrate biomass using digested sludge that is circulated as fungus body, a filter that is provided in the fermenter and the fermenter and filters the digested sludge using the water pressure of the digested sludge in the fermenter And a mantle containing the fermenter, wherein the filter has a hole formed in the tank wall of the fermenter and a filter cloth affixed to the inner wall surface of the fermenter so as to cover the hole And the mantle has a discharge port for discharging the desorbed water filtered by the filter. In methane fermentation that digests substrate biomass using digested sludge that is circulated and used as bacterial cells, a fermenter, an external conduit provided in communication with the fermenter, and a conduit provided in the fermenter A filter for filtering the digested sludge using the hydraulic pressure of the digested sludge; and a mantle with the conduit inserted therein, the filter covering a hole formed in a tube wall of the conduit and the hole And a filter cloth affixed to the inner wall surface of the conduit, and the mantle has a discharge port for discharging the desorbed water filtered by the filter.

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