AU2021103930A4 - Method for Regulating the Degradation of Straw Silage to Produce Organic Acid and Methane by Using Black Liquor - Google Patents

Abstract

OF THE DISCLOSURE A method for regulating the degradation of straw silage to produce an organic acid and methane by using black liquor, including: (1) preparation of raw materials: preparing the straw silage, using a papermaking black liquor as the black liquor; (2) microbial domestication; (3) conducting an acid production fermentation in the presence of a high concentration of solid, maintaining the pH of the leachate to be 8 and the fermentation temperature at 35 °C, and continuously percolating and degrading the organic components in the straw in the presence of microorganisms for 5 days to obtain a final leachate rich in organic acids; and (4) conversion of the methanogenic phase in the final leachate into methane; gradually increasing the organic load of the system in a gradient loading method, conducting for 10 days using each organic load, and continuously fermenting the final leachate in an up-flow anaerobic sludge bed as a methane production reactor to generate methane and carbon dioxide.

Description

METHOD FOR REGULATING THE DEGRADATION OF STRAW SILAGE TO PRODUCE ORGANIC ACID AND METHANE BY USING BLACK LIQUOR TECHNICAL FIELD

[01] The present disclosure relates to the technical field of organic waste resource utilization, in particular to a method for regulating the degradation of straw silage to

produce an organic acid and methane by using black liquor.

BACKGROUNDART

[02] Straw, as conventional by-product in agricultural production, has attracted much

attention in terms of its comprehensive utilization. Since anaerobic digestion technology

makes it possible to realize the energy recovery of organics in straw, it is widely used as

a clean energy utilization technology of organic solid waste in the actual application. At

present, the technology used in projects is mainly single-phase fermentation; however,

due to the influence of fermentation concentration, slagging and floating crust and large

amount of biogas liquid produced, the development of large-scale biogas projects is

severely restricted. In view of this, two-phase process has gradually been widely

researched and promoted. However, in this process, the organic acid yield of the

acid-producing phase directly affects the volumetric gas production rate of the entire

process, and it is urgent to optimize the process to improve the production and

metabolism of the target product of the acid-producing phase. Black liquor is a polluting

wastewater with a high concentration produced in the papermaking process. It contains

a large number of suspended solids, organic pollutants and toxic substances, and its

direct discharge into water will cause serious pollution. The black liquor mainly has the

following hazards: wastewater containing a lot of pigments and inorganic salts will

make water black and stink; black liquor with a high concentration of organic pollutants

has a chemical oxygen demand (COD) of as high as 10000-100000 mg/L, and will

consume a large amount of oxygen dissolved in water, which affects water quality. If

not handled properly, it will pose a serious threat to the environment.

SUMMARY

[03] An object of the present disclosure is to develop a process that could achieve high-efficiency degradation of straw silage and effective treatment of organic pollutants

in black liquor by combining black liquor with anaerobic fermentation and acidification

of straw silage, thus providing an important reference method for actual production.

[04] In order to achieve the above object, the present disclosure provides the following technical solutions:

[05] A process that could achieve high-efficiency degradation of straw silage and

effective treatment of organic pollutants in black liquor, comprising the following steps:

(1) preparation of raw materials: preparing the straw silage, using a papermaking

black liquor as the black liquor;

(2) microbial domestication: adding the straw silage, the black liquor, NH 4 C1 as a

nitrogen source, KH 2 PO 4 as a phosphorus source and an anaerobic sludge into a serum

bottle with an effective working capacity of 1 L to obtain a mixture, wherein the mixture

has a concentration of the straw silage of 10 g/L, a COD concentration of the black

liquor of 0.2 g/L, and a weight ratio of COD:N:P of 100:5:1; cultivating the mixture at a

medium temperature of 35 °C for 30 days as one fermentation cycle, during which

observing the gas production to ensure a normal process; repeating above fermentation

cycle for 4 times to obtain a domesticated sludge which is used as an inoculum for the

acid-producing phase and the methanogenic phase of the straw silage;

[06] (3) acid production and fermentation in the presence of a high concentration of

solid;

[07] heating the domesticated sludge at 90 °C for 30 minutes to kill methanogens; using a 1 L leaching bed reactor (LBR) as an acid-producing device, mixing 100 g of

the straw silage, 50 g of the heated domesticated sludge and 200 mL of water to be

uniform to obtain a mixture as an initial leachate, and placing the mixture into the

leaching bed reactor, wherein a weight ratio of the heated domesticated sludge to

volatile solid (VS) in the straw silage is 1:4, and the initial leachate has a pH of 4.5

due to the generation of organic acid such as lactic acid in the preparation of the straw silage; concentrating the black liquor to half of the original volume by boiling to obtain a concentrated black liquor, adjusting the pH of the initial leachate to 8 with the concentrated black liquor, and then driving the initial leachate from the top to the bottom of the leaching bed reactor with a peristaltic pump to start a percolation acid production process, conducting the percolation acid production process for 5 days to obtain a final leachate rich in organic acids, during which the leachate collected at the bottom of the leaching bed reactor is circularly driven from the top to the bottom of the leaching bed reactor, the pH thereof is maintained at 8 by continuously adding the concentrated black liquor through an automatic pH control dosing machine (CPH-1A, Changsha Croma Pharmaceutical Technology Co., Ltd., China) to avoid the decrease of the pH causing by the continuous production of organic acids, and the temperature of the percolation acid production process is maintained at 35 °C by a water bath jacket, the organic components in the straw silage are continuously percolated and degraded in the presence of microorganisms, and at the same time, parts of the refractory components in the black liquor are bio-transformed;

[08] (4) conversion of the methanogenic phase in the final leachate into methane;

[09] diluting the final leachate, to obtain diluted leachates with different organic loads (1 g COD/L, 2 g COD/L, 3 g COD/L, 4 g COD/L, 6 g COD/L, 8 g COD/L, 9 g COD/L, and 10 g COD /L), adjusting the pH of the diluted leachates to 7, driving the diluted leachates from low organic loading rate (1 g COD/L-d) to high organic loading rate (10 g COD/L-d) in sequence to a 2 L up-flow anaerobic sludge bed (UASB) which is used as a methane-producing reactor and with the domesticated sludge by using a peristaltic pump, and conducting a fermentation, to form methane and carbon dioxide, wherein the fermentation is conducted at 35 °C using a water bath jacket, and the speed of the peristaltic pump is controlled such that a hydraulic retention time is maintained for 24 hours, the fermentation using each diluted leachate is conducted for 10 days, and the effluent from UASB could be used for supplying water and adjusting organic load in the entire fermentation process.

[10] The specific features of the present disclosure also comprise that the present disclosure mainly focuses on anaerobic digestion of straw silage for producing acid and methane conversion. The raw materials mainly involve straw silage and black liquor, wherein the straw silage is provided by Shandong Laoling Shengli New Energy Co.,

Ltd., China, and prepared by crushing fresh corn straws to 1-2 cm, stacking and sealing

for storage.

[11] The papermaking black liquor is provided by the State Key Laboratory of

Bio-based Materials and Green Papermaking of Qilu University of Technology. It is an

organic wastewater with a high concentration obtained by subjecting corn straws to a

sulfite pulping process, and is rich in lignin derivatives.

[12] The anaerobic sludge is taken from the sugar wastewater anaerobic fermentation

plant of Shandong Xingguang Sugar Co., Ltd., China.

[13] The present disclosure has the following beneficial effects:

[14] Using the alkalinity of the black liquor to adjust the pH in the anaerobic acid production process of the straw silage improves the degradation degree of cellulose and

hemicellulose, thereby obtaining a higher yield of organic acids, and in this process, the

biotransformation of decomposition products such as lignin in black liquor by using

domesticated microorganisms improves the resource utilization value of black liquor,

thus successfully achieving the purpose of methane conversation.

[15] 1. In the acid-producing phase of two-phase process, the straw silage plays a

positive role in improving the degradation of substrate and the accumulation of organic

acids by adjusting the pH value of leachate; compared with dry straws, the production

rate of organic acids increases by 218%, indicating that alkaline pH could improve the

acid production process of straw silage; compared with straw silage without pH control,

the production rate of organic acids increases by 191.7%. This is due to the following

specific aspects: (1) pH in the alkaline range may destroy the bonds such as ester bonds

and ether bonds between lignin and hemicellulose and those between hemicellulose and

cellulose, which is conducive to the contact of microorganisms with the substrate; (2)

pH in the alkaline range may reduce the concentration of free acids, thereby reducing

the inhibition of acid-producing bacteria; (3) pH is closely related to the distribution of

functional microorganisms for degrading cellulose. The co-acid-producing metabolites

of straw silage and black liquor and the unused organic components in the black liquor may be efficiently converted within a certain organic load range in the methanogenic reactor, and the production rate of methane could reach 301 mL CH4/g COD; compared with sodium hydroxide, the cumulative volume of methane increases by 5%. When black liquor is used, it is advisable to control the pH of the leachate to 8. If the pH is too high, the increase in the amount of the black liquor will increase the content of refractory components in the acid-producing liquor, which will reduce the organic loading rate and decrease the gas production rate in the subsequent methane production process.

[16] 2. Since the production of organic acids in acid production fermentation will

significantly reduce the pH of the leachate, the consumption of alkali liquor is large.

According to the present disclosure, the alkaline characteristics of the wastewater (i.e.

black liquor) produced in the papermaking process is fully used, and the wastewater is

used to adjust the pH of the system appropriately after concentration, which reduces the

addition of external alkali sources, and broadens the use of black liquor, thereby

reducing the cost for applying pH control technology to biogas projects and contributing

to improvement of the economics of the project. The necessary concentration of black

liquor could significantly reduce the amount of the black liquor for the adjustment of

pH and avoid the decrease of the concentration of acidified product due to the addition

of excessive water.

[17] 3. In the entire acid production process, in addition to being used as an alkaline regulator, black liquor contains a large number of lignin degradation products, most of

them are aromatic compounds, and they are highly toxic to microorganisms, and the

microorganisms currently present in anaerobic sludge have poor tolerance and

utilization to these substances. Therefore, in the process of acid production of straw

silage, the domesticated microorganisms are used to obtain the enrichment of functional

flora, and the organics in the black liquor are synchronously transformed to further

improve the acid production rate (13% higher than that of NaOH as regulator) and the

value of resource utilization.

[18] 4. Due to the excessive high concentration of black liquor, it is very difficult for

direct biological treatment. Therefore, it needs to be diluted during treatment. In the present disclosure, black liquor is used as an alkaline regulator and is diluted to a considerable extent when co-acid-producing with straw silage, which avoids the toxic effects on microorganisms resulting from the direct use of black liquor and improves the possibility of degradation of the components in black liquor.

[19] 5. Compared with the fully mixed fermentation process, the percolation fermentation avoids the high-concentration organic acid and black liquor contacting with the microorganisms for a long time, reducing the product feedback inhibition of the organic acid on the microorganisms and the toxic effect of the black liquor on the microorganisms; the two-phase process could increase the fermentation concentration, shorten the fermentation cycle to 6 days (5 days for acid production and 1 day for methane production), recycle water in the whole process, and avoid the problems such as slagging and floating crust, large amount of biogas liquid and long cycle in the single-phase process. The final methane content in the biogas is up to 80% or more, which increases the calorific value of the biogas, and provides higher-quality raw gas for subsequent purification, and reduces energy consumption.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[20] Example 1 This example relates to an investigation on the acid production and methane production of straw silage fermentation when the pH of the leachate is controlled by black liquor to be 8.A process that could achieve high-efficiency degradation of straw silage and effective treatment of organic pollutants in black liquor was conducted as follows:(1) Preparation of raw materials:

[23] The present disclosure is mainly based on anaerobic digestion of straw silage for acid production and methane conversion. The raw materials mainly involve straw silage and black liquor. The straw silage was provided by Shandong Laoling Shengli New Energy Co., Ltd., China, and prepared by crushing fresh corn straws to 1-2 cm, stacking, and sealing for storage. The black liquor was a papermaking black liquor from the State Key Laboratory of Bio-based Materials and Green Papermaking of Qilu University of Technology, which was an organic wastewater with a high concentration, obtained by subjecting corn straws to a sulfite pulping process, and was rich in lignin derivatives. (2)

Microbial domesticationThe straw silage, the black liquor, NH 4 Cl as a nitrogen source,

KH2PO4 as a phosphorus source and an anaerobic sludge (which is taken from the sugar

wastewater anaerobic fermentation plant of Shandong Xingguang Sugar Co., Ltd, China)

were added into a 1 L serum bottle and mixed to obtain a mixture, wherein the mixture

has a concentration of the straw silage of 1% by weight, a COD concentration of the

black liquor of 0.2 g/L, and a weight ratio of COD:N:P of 100:5:1; the mixture was

cultivated at a medium temperature of 35 °C for 30 days as one fermentation cycle,

during which the gas production was observed to ensure a normal process; the above

fermentation cycle was repeated for 4 times to obtain a domesticated sludge, which is

used as an inoculum for the acid-producing phase and the methanogenic phase of straw

silage.(3) Acid production and fermentation in the presence of a high concentration of

solid.The domesticated sludge was heated at 90 °C for 30 minutes to kill methanogens.

100 g of the straw silage, 50 g of the heated domesticated sludge and 200 mL of water

were mixed uniformly to obtain a mixture as an initial leachate, and the initial leachate

was placed in a leaching bed reactor (LBR) with an effective working capacity of 1 L,

The initial leachate has a pH of 4.5 due to the generation of organic acids such as lactic

acid in the preparation of straw silage. The black liquor was concentrated to half of the

original volume by boiling, obtaining a concentrated black liquor. The initial leachate

was adjusted to a pH of 8 by the concentrated black liquor, and driven from the top to

the bottom of the leaching bed reactor by a peristaltic pump, to start a percolating acid

production process; the percolating acid production process was performed for 5 days,

obtaining a final leachate rich in organic acids, during which the leachate collected at

the bottom of the leaching bed reactor was circularly driven from the top to the bottom

of the leaching bed reactor, the pH thereof was maintained at 8 by continuously adding

the concentrated black liquor through an automatic pH control dosing machine

(CPH-1A, Changsha Croma Pharmaceutical Technology Co., Ltd., China) to avoid the

decrease of the pH causing by the continuous production of organic acids, and the

temperature of the percolation acid production process was maintained at 35 °C by a

water bath jacket, the organic components in the straw silage were continuously

percolated and degraded in the presence of microorganisms, and at the same time, parts of the refractory components in the black liquor were bio-transformed. After calculation, the organic acid production rate is 0.35 g volatile fatty acids (VFAs)/g VS.(4) Conversion of leachate into methane in methanogenic phase. In order to investigate the methane conversion and utilization of the final leachate, a methanogenic reactor was coupled with the leaching bed reactor to evaluate the methane conversion effect of the product. The final leachate was diluted with water to 1 g COD/L, and adjusted to a pH of 7, obtaining a diluted leachate as a methanogenic phase. The diluted leachate was driven into an up-flow anaerobic sludge bed (UASB) by a peristaltic pump and subjected to a continuous fermentation, wherein the up-flow anaerobic sludge bed was used as the methanogenic reactor, and has an effective working capacity of 2 L, the fermentation had a hydraulic retention time of 24 hours which is controlled by the speed of the peristaltic pump, and was maintained at 35 °C by a water bath jacket. Methane and carbon dioxide were generated by fermentation in the presence of the domesticated sludge in the UASB. After performing for 10 days, the organic load of the diluted leachate was increased to 2 g COD/L, 3 g COD/L, 4 g COD/L, 6 g COD/L, 8 g COD/L, 9 g COD/L, and 10 g COD/L in sequence. The fermentation using each organic load was conducted for 10 days, and the gas production status was observed. The results show that within the test loading rate range, when the organic load of the diluted leachate is not higher than 9 g COD/L, the gas production is normal; when the organic load of the diluted leachate is equal to 9 g COD/L, the methane production rate is 301 mL CH 4 /g COD, and the methane content reaches 80.9%; when the organic load of the diluted leachate increases to 10 g COD/L, it begins to exceed the load bearing capacity of the system, resulting in a significant decrease in the gas production rate, and the methane production rate is only 195 mL CH 4 /g COD.

[30] Example 2

[31] This example is conducted as described in Example 1, except that in step (3), the pH of the leachate was changed from 8 to 10. As a result, the organic acid production rate is 0.37 g VFAs/g VS in the process of acid production. Compared with Example 1, the increase of the pH results in an increase in the organic acid production rate. However, when the final leachate was subjected to a methane conversion in step (4), the organic loading rate decreases, and the methane production rate decreases significantly at 6 g COD/L, which is 219 mL CH4/g COD. The results show that adding too much black liquor when adjusting to a higher pH will result in more refractory components in the acid producing liquor, which will make the components in black liquor exhibit toxic effects on microorganisms.

[32] Example 3 This example is conducted as described in Example 1, except that in step (3), the pH of the leachate was changed from 8 to 7. As a result, the organic acid production rate is 0.18 g VFAs/g VS in process of acid production, which is significantly lower than that in Example 1. Since the acid production rate directly affects the methane production rate of the two-phase process, it is not appropriate to control the pH of the leachate to 7.Example 4 This example is conducted as described in Example 1, except that in step (3), the pH of the leachate was changed from 8 to 6. As a result, the organic acid production rate is 0.11 g VFAs/g VS in the process of acid production, which is significantly lower than that in Example 1 and Example 2, indicating that it is not appropriate to control the pH of the leachate within an acidic range.It can be seen from Examples 1-4 that when the pH value of the leachate from straw silage is adjusted by black liquor, it should not be too high or too low, otherwise the organic loading rate or acid production rate in the methanogenic phase will be low, making the anaerobic fermentation system unable to operate normally.Example 5 This example relates to an investigation on the acid production and fermentation of straw silage without adjusting pH. This example is conducted as described in Example 1, except that in step (3), the pH of the leachate was not adjusted. As a result, the final organic acid production rate is 0.12 g VFAs/g VS. Compared with Example 1, it can be seen that the pH in the acid production metabolism of the straw silage has a significant effect on the degradation of substrate and accumulation of organic acids.Example 6This example is conducted as described in Example 1, except that in step (3), the pH of the leachate was adjusted to 10 by NaOH. As a result, the organic acid production rate is 0.38 g VFAs/g VS in the acid production process, which is not significantly different from that in Example 2, indicating that adjusting the pH to 10 is beneficial to the acid production metabolism of the substrate. However, when the organic loading rate of the diluted leachate reaches 10 g COD/L, the content of methane in the biogas is 81.1%, and the methane production rate is 305 mL CH4/g COD, which is significantly higher than that in Example 2, indicating that there is a significant difference between black liquor and NaOH, and the addition of excessive black liquor has a certain negative effect on methane conversion, which will seriously affect the conversion of organic acids in the methanogenic phase.Example 7 This example relates to an investigation on the acid production and fermentation of straw silage using NaOH as the alkaline regulator. Steps (1) - (3) are the same as those in Example (1), except that in step (3), the pH of the leachate was adjusted to 8 by a NaOH solution with a concentration of 6 mol/L. In this example, the degradation of organics in the black liquor does not contribute to the organic acid in the final product, and thefinal organic acid production rate is 0.33 g VFAs/g VS which is lower than the acid production rate in Example

.Step (4): Methane production, which is the same as that in Example 1. When the

organic load of the diluted leachate is 8 g COD/L, the methane production rate is 307

mL CH4/g COD, and the cumulative volume of methane produced is reduced by 5%

compared with Example 1, indicating that the components in the black liquor and their

metabolites in the acid-producing phase in Example 1 have a certain contribution to the

methane production process in Example 1.Example 8 Direct anaerobic fermentation

was carried out using the black liquor. A 500 mL serum bottle was used as a

fermentation device, and 350 mL of papermaking black liquor as a mother liquor was

added thereto, and the domesticated sludge in Example 1 was inoculated thereto in a

ratio of the domesticated sludge to the VS in the substrate of 1:1, and the pH was

adjusted from 14 to 7 using a hydrochloric acid solution with a concentration of 6 mol/L.

The resulting mixture was placed in a water bath with a temperature of 35 °C. The gas

production was observed, and it is found that no methane is produced from the first day

to the 20th day, and no organic acid is detected in the system, indicating that the

biochemical utilization performance of the black liquor mother liquor is poor, and it is

difficult to be effectively transformed and utilized by microorganisms at high

concentrations.Example 9This example relates to an investigation on the acid production and fermentation of dry straw when the pH was controlled by black liquor.

[50] The example was conducted as described in Example 1, except that the straw silage was replaced with dry a straw, and in step (3), 30 g of the dry straw and 50 g of

the domesticated sludge were mixed uniformly and placed in the reactor, 270 mL of

water was added thereto, and the pH was adjusted to 8 with the concentrated black

liquor, and the percolation was started. The final organic acid production rate is 0.11 g

VFAs/g VS, showing that when the pH is 8, the dry straw does not exhibit the same acid

production characteristics as the straw silage, this is because the microorganisms and

the acid produced in the preparation process of the straw silage has a certain

pretreatment effect on the straw, the anti-degradation barrier produced by the complex

structure of straw makes it difficult to achieve a better acid production effect for dry

straw without any treatment other than pH control.Example 10 The example was

conducted as described in Example 1, except that in step (4), the continuous

fermentation was changed to batch fermentation, and 1000 mL serum bottles were used

as the fermentation devices, and the domesticated sludge was inoculated into the

fermentation devices in a ratio of 20 g/L, the effective working capacity was 750 mL,

the final leachate was diluted to diluted leachates with organic loads of 1 g COD/L, 2 g

COD/L, 3 g COD/L, and 4 g COD/L, respectively, and the diluted leachates were added

to the fermentation devices. An acetic acid fermentation experiment using the same

organic loads was used as a control. As a result, when the organic load is 4 g COD/L,

the methane conversion of the diluted leachate, being 201 mL CH 4 / g COD, is

significantly reduced, while the acetic acid fermentation group as a control group

produces gas normally, further confirming that the black liquor contains substances that

inhibit anaerobic fermentation microorganisms. A gradient increase loading method for

continuous fermentation, with the organic load of diluted leachate for the UASB

increased gradually from 1 g COD/L to 10 g COD/L, makes it possible for normal

accumulation of methane. When the organic load of the diluted leachate is 9 g COD/L,

the methane production rate is 301 mL CH4/g COD; the organic load is continued to

increasing, the gas production is suppressed. However, compared with conventional

batch fermentation, the organic loading rate of the fermentation process in the continuous fermentation is significantly increased.Example 11 The example was conducted as described in Example 1, except that the leaching bed reactor (LBR) was replaced by a continuously stirred tank reactor (CSTR) as the acid producing device.

Compared with Example 1, the acid production rate in the acid-producing phase is

significantly reduced, which is 0.15 g VFAs/g VS. This is because when the substrate,

inoculum and fermentation broth are mixed for fermentation, the feedback inhibition of

products on the microbial activity by organic acids in acid-producing phases will

seriously affect the degradation of the substrate. Therefore, the leaching bed reactor as

an acid producing device could reduce the effect of the product on the microorganisms

and achieve higher organic acid production rate in the fermentation of high solids

concentration of straw.

Claims (4)

WHAT IS CLAIMED IS:

1. A method for regulating the degradation of straw silage to produce an organic acid and methane by using black liquor, comprising: (1) preparation of raw materials: preparing the straw silage, using a papermaking black liquor as the black liquor; (2) microbial domestication: adding the straw silage, the black liquor, NH 4 Cl as a nitrogen source, KH2PO4 as a phosphorus source and an anaerobic sludge into a 1 L serum bottle and mixing to obtain a mixture, wherein the mixture has a concentration of the straw silage of 1% by weight, a COD concentration of the black liquor of 0.2 g/L, and a weight ratio of COD:N:P of 100:5:1; cultivating the mixture at 35 °C for 30 days as one fermentation cycle, during which observing the gas production to ensure a normal process; repeating the above fermentation cycle for 4 times to obtain a domesticated sludge, which is used as an inoculum for an acid-producing phase and a methanogenic phase of the straw silage; (3) acid production and fermentation in the presence of a high concentration of solid; heating the domesticated sludge at 90 °C for 30 minutes to kill methanogens; using a 1 L leaching bed reactor (LBR) as an acid-producing device, mixing 100 g of the straw silage, 50 g of the heated domesticated sludge and 200 mL of water to be uniform to obtain a mixture as an initial leachate, and placing the mixture into the leaching bed reactor, wherein a weight ratio of the heated domesticated sludge to volatile solids in the straw silage (VS) is 1:4, and the initial leachate has a pH of 4.5 due to the generation of organic acid such as lactic acid in the preparation of the straw silage; concentrating the black liquor to half of the original volume by boiling, to obtain a concentrated black liquor, adjusting the pH of the initial leachate to 8 with the concentrated black liquor, and driving the initial leachate from the top to the bottom of the leaching bed reactor by a peristaltic pump to start a percolation acid production process, conducting the percolation acid production process for 5 days to obtain a final leachate rich in organic acids, during which the leachate collected at the bottom of the leaching bed reactor is circularly driven from the top to the bottom of the leaching bed reactor, the pH thereof is maintained at 8 by continuously adding the concentrated black liquor through an automatic pH control dosing machine to avoid the decrease of the pH causing by the continuous production of organic acids, and the temperature of the percolation acid production process is maintained at 35 °C, the organic components in the straw silage are continuously percolated and degraded in the presence of microorganisms, and at the same time, parts of the refractory components in the black liquor are bio-transformed;

(4) conversion of leachate into methane in methanogenic phase;

diluting the final leachate, to obtain diluted leachates with different organic load: 1

g COD/L, 2 g COD/L, 3 g COD/L, 4 g COD/L, 6 g COD/L, 8 g COD/L, 9 g COD/L,

and 10 g COD /L, adjusting a pH of the diluted leachates to 7, driving the diluted

leachates from low organic loading rate to high organic loading rate in sequence to an

up-flow anaerobic sludge bed which is used as a methane-producing reactor and filled

with the domesticated sludge by using a peristaltic pump, and conducting a fermentation,

to form methane and carbon dioxide, wherein the fermentation is conducted at 35 °C,

and the speed of the peristaltic pump is controlled such that a hydraulic retention time is

maintained for 24 hours, the fermentation using each diluted leachate is conducted for

days, and the effluent from the up-flow anaerobic sludge bed is used for supplying

water and adjusting organic load in the entire fermentation process.

2. The method of claim 1, wherein the anaerobic sludge is taken from a sugar

wastewater anaerobic fermentation plant of Shandong Xingguang Sugar Co., Ltd,

China.

3. The method of claim 1, wherein preparing the straw silage comprises crushing

fresh corn straws to 1-2 cm, stacking and sealing for storage.

4. The method of claim 1, wherein the papermaking black liquor is an organic

wastewater with a high concentration and rich in lignin derivatives, which is obtained

by subjecting corn straws to a sulfite pulping process.