Method and system for producing short-chain fatty acid based on synergistic fermentation of waste molasses and municipal sludge

Abstract

The invention discloses a method and a system for efficiently producing short-chain fatty acid by utilizing waste molasses in the sugar industry and municipal sludge through synergistic anaerobic fermentation, and belongs to the technical field of organic solid waste recycling. Waste molasses and municipal sludge are mixed according to a specific ratio, the carbon-nitrogen ratio is optimized to 20-30: 1, trace metal catalysts (Fe < 2 + >, Co < 2 + > and Ni < 2 + >) are added, and two-stage pH regulation (the initial pH is 6.0-7.0, and the later pH is 5.5-6.0) and a medium-temperature (35-37 DEG C) anaerobic fermentation process are combined, so that the yield of short-chain fatty acid is remarkably increased to 5.2-6.8 g COD / L and is increased by 200-300% compared with that of traditional sludge single-substrate fermentation. The method realizes recycling and high-value conversion of the organic solid waste, and has the characteristics of low cost, high acid production efficiency and environmental friendliness.

Description

Technical Field

[0001] The present invention belongs to the technical field of organic solid waste resource utilization, and specifically relates to a collaborative anaerobic fermentation process utilizing waste molasses from the sugar industry and sludge from municipal sewage treatment plants, which achieves efficient synthesis of short-chain fatty acids by directional regulation of microbial metabolic pathways. Background Art

[0002] With global population growth and accelerated urbanization, the annual output of industrial and municipal organic solid waste has exceeded 10 billion tons. Among them, waste molasses from the sugar industry and municipal sewage sludge have become the focus of solid waste management due to their dual properties of high pollution load and coexisting resource potential. According to statistics from the International Sugar Organization (ISO), every ton of sucrose produced globally produces 0.3 to 0.5 tons of waste molasses, with an annual total exceeding 120 million tons. This viscous, dark brown liquid contains 50 to 60% fermentable sugars (sucrose, glucose) and a variety of trace elements, with a COD (chemical oxygen demand) of up to 80 to 120 g / L. If discharged directly, one ton of waste molasses can pollute water equivalent to 30 standard swimming pools, and the methane produced by the fermentation of its sugars has a greenhouse effect 28 times that of CO2. At the same time, the United Nations Environment Programme (NEP) shows that urban sewage treatment plants around the world generate more than 300 million tons of wet sludge each year, or about 60 million tons on a dry basis. The organic matter content (VS) is 40-60%, equivalent to the calorific value of 24 million tons of standard coal. However, more than 70% of the sludge is still disposed of through landfills or open-air dumping in traditional treatment methods, which not only occupies land but also poses a lasting threat to groundwater and soil due to the leaching of heavy metals and the spread of pathogens.

[0003] Faced with the need to treat waste molasses and sludge, existing technologies have fallen into a vicious circle of "treatment equals waste". Taking waste molasses as an example, the mainstream disposal methods include: (1) Concentration and incineration: extracting residual sugars through evaporation and crystallization and then incinerating the ash. However, the energy consumption of the evaporation process is as high as 200-300 kWh / ton, and the potassium salt (K2O content 15-20%) in the ash is difficult to purify due to impurities, and ultimately becomes a low-end building material filler with an economic value of less than 800 yuan / ton; (2) Animal feed addition: using its sugar as feed binder, but heavy metals (such as As and Pb) in waste molasses are prone to exceeding the standard, and the EU has clearly limited its addition ratio to <5%; (3) Ethanol fermentation: It requires inoculation of special yeast and strict temperature control at 30-35°C, and the COD of the fermentation residual liquid is still as high as 20-30 g / L, which increases the subsequent treatment cost by 40%.

[0004] The treatment of municipal sludge is equally challenging. Although anaerobic digestion technology is widely adopted, its core goal is to produce methane, and it faces three major bottlenecks: First, the conversion efficiency is low. The complex extracellular polymer (EPS) structure of sludge leads to a slow hydrolysis rate, a VS removal rate of less than 40%, and a methane yield of only 0.2-0.3 m 3 / kg VS; secondly, the reaction cycle is lengthy. Medium-temperature digestion takes 20 to 30 days, while high-temperature processes can shorten it to 10 to 15 days, but energy consumption soars by 50%; seventhly, the product value is single. Methane calorific value (35.8MJ / m 3 ) is high, but direct combustion produces CO2, and the carbon footprint is difficult to offset.

[0005] In recent years, short-chain fatty acids (including acetic acid, propionic acid, and butyric acid) have become a star product in the high-value conversion of organic solid waste due to their widespread application in bioplastics (PHA), wastewater treatment carbon sources, and feed additives. The market price of short-chain fatty acids exceeds US$3,000 per ton, and demand is growing at an annual rate of 12%. However, traditional short-chain fatty acid production processes face two major constraints: First, raw material dependence: Chemical synthesis requires petroleum-based feedstocks (such as ethylene oxidation to produce acetic acid), and costs are significantly affected by oil price fluctuations; while biofermentation is environmentally friendly, it relies on grain-based starch (such as corn), raising ethical concerns about "competing with people for food." Second, process efficiency ceilings: Single solid waste fermentation has low acid production efficiency. For example, pure sludge fermentation, due to an imbalance in the carbon-nitrogen ratio (C / N = 8-10:1), limits the metabolism of acid-producing bacteria, resulting in short-chain fatty acid yields of only 1.5-2.5g COD / L, with acetic acid accounting for less than 40%.

[0006] To break through the limitation of a single substrate, researchers have tried to co-ferment sludge with other organic wastes, but most of the technical routes have fallen into the dilemma of "taking one thing from another to fill another". For example, co-fermentation of food waste (patent CN111876444A): Although it can effectively improve the C / N ratio, food waste contains a large amount of oil, which forms a hydrophobic film that wraps the substrate, significantly reducing the hydrolysis efficiency and requiring additional de-oiling pretreatment (cost increase of 300 yuan / ton); co-fermentation of straw (Bioresour.Technol.2021,337:125437): The lignin-cellulose structure will lead to a prolonged hydrolysis cycle, which means that more enzyme preparations may be required, thereby increasing costs; co-fermentation of plastics (patent CN113186231A): Plastic particles are difficult to control the C / N ratio, which is not conducive to the acid production reaction, and a large amount of plastic particles may cause the reactor to clog.

[0007] What is more serious is that the existing processes generally ignore the regulation of microbial communities. In traditional single-phase anaerobic reactors, acid-producing bacteria (such as Clostridium and Bacteroides) form a symbiotic network with methanogens (Methanosaeta and Methanosarcina). Short-chain fatty acids are consumed by methanogens within a few hours of production (conversion rate exceeds 80%). Although there are patents that use pH shock (such as patent CN114774487A, which raises the pH to 10 to inhibit methanogens), strong alkaline conditions may cause hydrolytic enzymes to be inactivated, and subsequent neutralization requires the addition of a large amount of hydrochloric acid (1.2 tons of HCl is consumed per ton of short-chain fatty acids), resulting in secondary pollution. Summary of the Invention

[0008] The core breakthrough of this invention lies in overturning the single substrate thinking of traditional anaerobic fermentation. Through the precise matching and process control of waste molasses and municipal sludge, a three-in-one technical system of "carbon source complementarity, microbial community synergy, and metabolic orientation" is constructed. This system is based on "waste molasses to supplement carbon, sludge to supply nitrogen, and metal catalysis to enhance efficiency". Combined with a two-stage environmental stress strategy, it transforms the two wastes that originally constrained each other into a "golden partner" of high-purity short-chain fatty acids. Experimental data show that the short-chain fatty acid yield of this process is 320% higher than that of single sludge fermentation, and the proportion of acetic acid in the product exceeds 60%, providing a new path for the industrial production of high-value-added bio-based chemicals.

[0009] The purpose of the present invention is achieved through the following technical solutions:

[0010] (a) Raw material pretreatment: waste molasses was diluted to a COD of 40-60 g / L and then centrifuged to remove suspended particles with a particle size greater than 0.5 mm. At the same time, municipal sludge was thermally hydrolyzed at 90-100° C. for 30-60 minutes.

[0011] (b) Mixing and blending: Mix waste molasses and volatile solids (VS) of sludge in a mass ratio of 1:1 to 3:1, and adjust the carbon-nitrogen ratio (C / N) of the system to 20 to 30:1;

[0012] (c) Anaerobic fermentation: Add 0.05-0.2 mmol / L of Fe to the mixture. 2+ 、Co 2+ 、Ni 2+ The composite catalyst was fermented in two stages at 35-37°C. The first stage maintained pH at 6.5-7.0 for 24 hours, and the second stage adjusted pH to 5.5-6.0 and introduced trace oxygen (DO = 0.1-0.3 mg / L) for 5-7 days.

[0013] (d) Product separation: The fermentation broth is subjected to ceramic membrane ultrafiltration and electrodialysis to extract the short-chain fatty acid concentrate, and the residue is dried and granulated to produce organic fertilizer.

[0014] Preferably, the viscosity of the waste molasses after dilution is ≤1500 cP, and the centrifugal speed is 2000-4000 rpm.

[0015] Preferably, the sludge thermal hydrolysis pressure is 0.1-0.3 MPa, and the SCOD improvement rate after thermal hydrolysis is ≥50%.

[0016] Preferably, the VS mixing ratio of waste molasses to sludge is 2:1.

[0017] Preferably, the Fe 2+ 、Co 2+ 、Ni2+ The molar concentration ratio is 5:2.5:1.

[0018] Preferably, in the first stage, pH=6.8±0.2, ORP=-250 to -300 mV, and in the second stage, microaeration is performed for 10 to 15 minutes every 6 hours, and 0.05 to 0.15 g / L of sodium 2-bromoethane sulfonate (BES) is added.

[0019] Preferably, the H2 concentration in the reactor is monitored in real time, and when the H2 volume fraction is greater than 1%, an alarm is triggered and the frequency of micro-oxygen aeration is increased.

[0020] Preferably, the pore size of the ceramic membrane ultrafiltration is 20-100 nm, the operating temperature is 35-45° C., the voltage of the electrodialysis is 10-20 V, and the pH value of the concentrated solution is 2.5-3.5.

[0021] Preferably, when the sludge C / N is less than 8:1, the VS mixing ratio of waste molasses is increased to 2.5-3:1; when the COD of waste molasses is less than 50 g / L, glucose is supplemented to COD = 50±5 g / L.

[0022] Preferably, the system for implementing the method according to any one of claims 1 to 9 is characterized in that it includes: a dilution tank (1), a waste molasses centrifuge (2), a sludge thermal hydrolysis tank (3), a sludge cooler (4); a mixing and blending tank (5), a two-phase anaerobic reactor (6) containing a pH / ORP sensor and a micro-aeration device; a ceramic membrane ultrafiltration machine (7), an electrodialysis device (8), a belt dryer (9); and a central controller (10) for jointly adjusting pH, temperature, and aeration parameters.

[0023] The principle of the present invention is:

[0024] (1) Raw material pretreatment: Although waste molasses is rich in fermentable sugars, its sticky texture (viscosity > 5000 cP) and suspended particles (bagasse residue accounts for 3-5%) seriously hinder the contact efficiency of microorganisms. To this end, the present invention innovatively adopts a dilution-centrifugation impurity removal coupling process: based on the initial COD value of waste molasses (80-120 g / L), water is injected and diluted to 40-60 g / L at a ratio of 1:1 to 1:2, which not only avoids high osmotic pressure from inhibiting bacterial activity, but also ensures that the carbon source concentration is in the optimal metabolic range for acid-producing bacteria; centrifugation is carried out at 3000 rpm for 10 minutes to remove bagasse particles with a particle size > 0.5 mm and reduce the viscosity to below 1200 cP. The pretreatment of municipal sludge focuses on the dual goals of releasing intracellular organic matter and inactivating methanogens: the sludge is heated to 90-100°C and maintained for 45 minutes. The high temperature and high pressure (0.1MPa) environment breaks the β-glycosidic bonds of the sludge EPS (extracellular polymers), increases the SCOD (soluble chemical oxygen demand) release rate by 65-70%, and completely inactivates heat-resistant methanogen spores (survival rate <0.01%); after hydrolysis, the sludge pH naturally drops to 6.0-6.5, and when mixed with waste molasses, the amount of subsequent acid adjustment reagents can be reduced by 40%.

[0025] (2) Mixing and blending: The mixing of waste molasses and sludge is not a simple physical superposition, but a precise calculation based on the metabolic needs of microorganisms. The present invention realizes the "golden ratio of nutrition" of fermentation substrates through the dynamic model of carbon-nitrogen ratio (C / N) and the synergistic matrix of trace elements: waste molasses (C / N=60:1) and sludge (C / N=8:1) are mixed at a volatile solid (VS) mass ratio of 2:1, so that the system C / N is stabilized at 25:1. Under this ratio, acid-producing bacteria can use sufficient carbon sources to synthesize acetyl-CoA (SCFAs precursor), and the nitrogen source provided by the sludge (amino acids produced by protein hydrolysis) just meets the proliferation needs of the bacteria, avoiding metabolic inhibition caused by ammonia nitrogen accumulation (concentration <150 mg / L). Adding Fe 2+ (0.1mmol / L), Co 2+ (0.05mmol / L), Ni 2+ (0.02mmol / L) sulfate complex. Among them, Fe 2+ Acts as an electron shuttle to accelerate NADH regeneration in the glycolytic pathway; Co 2+ Activate the key enzyme for propionate production - methylmalonyl-CoA mutase; Ni 2+ This promotes the activity of hydrogenases in acid-producing bacteria, converting excess H2 into reducing power to drive carbon chain elongation (butyrate synthesis increased by 15%). The relative abundance of Clostridium (butyric acid bacteria) and Bacteroides (acetic acid bacteria) in the mixed substrate increased 8-10 times compared to the single sludge, while the proportion of Methanosaeta (acetic acid-type methanogens) dropped sharply from 12% to 0.3%, successfully establishing an acid-dominated microbial community.

[0026] (3) Two-stage fermentation: In traditional single-phase fermentation, the “niche overlap” between acid-producing bacteria and methanogens leads to the rapid consumption of SCFAs. The present invention creatively introduces a two-stage regulation strategy of "promoting hydrolysis first and then locking metabolism", precisely controlling the metabolic flow of microorganisms through environmental factors: Phase I hydrolysis and acidification period (0-24 hours) pH control: maintaining pH = 6.8±0.2. This weakly acidic environment is optimal for the activity of hydrolases (such as amylase and protease). The sucrose conversion rate in waste molasses reaches more than 95% within 12 hours, and the efficiency of sludge protein hydrolysis into small molecular peptides (molecular weight <1kDa) is increased by 70%; redox potential (ORP) control: through microaerobic aeration (DO = 0.05-0.1 mg / L), the ORP is stabilized at ~250 to ~300 mV, which not only avoids the premature activation of methanogens under strictly anaerobic conditions, but also stimulates the rapid proliferation of facultative anaerobic acid-producing bacteria (such as Bacillus subtilis), with the biomass density reaching 8×108 CFU / mL within 24 hours. Phase II: Acid production enhancement period (24 hours to 7 days): Acidic environment locking: The pH gradient is lowered to 5.5±0.2. Under this condition, the coenzyme F420 activity of methanogens is inhibited by more than 90%, while the pyruvate decarboxylase activity of acid-resistant acid-producing bacteria (such as Acetobacter pasteurianus) is increased by 3 times; Metabolic blocker intervention: Sodium 2-bromoethane sulfonate (BES, 0.1g / L) is added to specifically inhibit the methyl-CoA reductase of methanogens, completely blocking the conversion pathway of SCFAs to methane; Directed carbon chain extension: Through intermittent microaerobic stimulation (aeration for 10 minutes every 6 hours, DO = 0.2-0.3mg / L), acid-producing bacteria are induced to initiate the reverse β-oxidation pathway, promoting the conversion of acetic acid to butyric acid. The proportion of butyric acid increases from 12% to 18%, and the added value of the product is further improved.

[0027] (4) Product separation: Efficient separation of fermentation broth is the last barrier to the commercialization of SCFAs. Traditional centrifugation-distillation processes consume high energy and are prone to loss of volatile acids. The present invention develops a membrane separation-electrodialysis coupling technology: using a 50nm pore size ceramic membrane for cross-flow filtration at 40°C to remove residues (SS removal rate > 99%) and macromolecular pigments (waste molasses pigment retention rate 98%), with a stable flux of 80L / (m 2h); at a voltage of 15V and a flow rate of 2L / min, SCFAs (pKa ≈ 4.8) pass through the anion exchange membrane in their molecular form at pH 3.0, while impurity ions such as SO₄⁻ and Cl⁻ are retained. The resulting SCFA concentrate has a purity of >92% and a concentration of 120-150g / L, which can be directly used for PHA production or esterification purification. The residue is converted into organic fertilizer through thermal drying and granulation, reducing the moisture content from 80% to 20%. The total nitrogen, phosphorus, and potassium content of the granules (NPK = 6-8%) meets the GB / T23349-2020 standard, achieving zero waste discharge throughout the entire process.

[0028] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0029] First, the acid production efficiency is high. The SCFA yield reaches 6.8±0.3g COD / L, with an ideal composition of 58% acetic acid, 28% propionic acid, and 14% butyric acid, meeting the raw material requirements for PHA synthesis.

[0030] Second, the cost is low. The production cost of a ton of SCFAs has been reduced to 1,800 yuan, a 60% reduction compared to the corn starch fermentation method (4,500 yuan / ton), and it can be profitable without government subsidies.

[0031] Third, it has carbon emission reduction benefits. Every ton of waste molasses-sludge mixture processed can reduce CO2 equivalent emissions by 12.6 kg, while replacing petroleum-based SCFAs reduces carbon emissions by 4.8 tons. The combined carbon reduction potential is equivalent to the annual carbon sequestration of 340 fir trees.

[0032] This technology not only builds a circular economy bridge for sugar mills and sewage treatment plants to "treat waste with waste and turn waste into treasure", but also creates a new paradigm for the resource utilization of organic solid waste in the context of global carbon neutrality. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 Process flow chart of SCFAs production by synergistic fermentation of waste molasses and sludge

[0034] Figure 2 Schematic diagram of the collaborative fermentation system DETAILED DESCRIPTION

[0035] Example 1: Synergistic fermentation of waste molasses and municipal sludge to produce SCFAs (optimal conditions)

[0036] Using waste molasses (COD = 108 g / L, VS = 55%) and municipal sludge (VS = 38%, C / N = 8.3:1) as raw materials, they were mixed at a VS mass ratio of 2:1 (system C / N = 24.8:1), and Fe 2+ (0.1mmol / L), Co 2+(0.05mmol / L), Ni 2+ A two-stage anaerobic fermentation process was performed using a 0.02 mmol / L composite catalyst. The first stage (24 hours) maintained a pH of 6.8 and an ORP of -280 mV to promote hydrolysis. The second stage (5-7 days) adjusted the pH to 5.5, added a BES inhibitor (0.1 g / L), and intermittent microaeration (DO = 0.2 mg / L) to inhibit methanogens. The final SCFA yield reached 6.7 g COD / L (58% acetic acid, 27% propionic acid, and 13% butyric acid), with a cost of 1,820 yuan per ton. The COD removal rate was 87%, the sludge reduction was 69%, and carbon emissions were reduced by 11.8 kg CO₂-eq per ton of raw material, achieving optimal overall performance.

[0037] Example 2: Adaptability adjustment of high ammonia nitrogen sludge

[0038] For high ammonia nitrogen sludge (TN = 8.5g / kg, C / N = 6.5:1), strengthen pretreatment (thermal hydrolysis at 100℃ for 60 minutes), increase the VS mixing ratio of waste molasses to sludge to 2.5:1 (system C / N = 28:1), extend the acid production period to 8 days, and reduce Fe 2+ The concentration was increased to 0.15 mmol / L to alleviate ammonia nitrogen inhibition. Results showed a SCFA yield of 6.1 g COD / L (a 9% decrease compared to Example 1), but the proportion of propionic acid increased to 33% (adapting to high-nitrogen environments). The cost per ton was 2,039 yuan, the COD removal rate was 79%, and the ammonia nitrogen concentration was stabilized below 180 mg / L, demonstrating significant process adaptability.

[0039] Example 3: Emergency Plan for Low-COD Molasses

[0040] When the COD of the waste molasses was low (42g / L), the dilution step was omitted and glucose was supplemented to a COD of 50g / L. 0.2% cellulase (enzyme activity 2000U / g) was added to promote fiber hydrolysis. The SCFA yield was 5.8g COD / L (a 13% decrease from the optimal conditions), but the proportion of butyrate increased to 18% (enzymatic hydrolysis promotes the synthesis of long-chain acids). The cost per ton was 1,966 yuan, and the COD removal rate was 68%. Although the cost increased by 8% (due to the glucose supplementation), it was still far lower than the traditional process, demonstrating the ability to flexibly adjust to raw material defects.

[0041] Comparative Example 1: (Single sludge fermentation)

[0042] Using only sewage sludge (C / N = 8.3:1), without the addition of waste molasses or metal catalysts, and in a single-stage fermentation (pH = 6.8, 7 days), the SCFA yield was only 1.8 g COD / L (42% acetic acid, 18% propionic acid), with a cost per ton of 3,200 yuan and a COD removal rate of 35%, demonstrating the inefficiency and economic disadvantages of a single substrate.

[0043] Comparative Example 2: (without metal catalyst)

[0044] Waste molasses and sludge were mixed at a VS ratio of 2:1, but no Fe was added 2 + / Co 2 + / Ni 2 + catalyst, two-stage fermentation. SCFA yield dropped to 3.2 g COD / L (50% acetic acid, 22% propionic acid), with a cost per ton of 2,500 yuan and a COD removal rate of 63%, demonstrating the key role of metal ions in promoting the metabolism of acid-producing bacteria.

[0045] Comparative Example 3: (Single-stage fermentation)

[0046] The raw material combination was the same as in Example 1, but the pH was kept at 6.8 throughout the process and no BES inhibitor was added, resulting in active methanogens. The SCFAs yield was only 2.1 g COD / L (acetic acid 44%, propionic acid 31%), the cost per ton was RMB 2,712, the COD removal rate was 25%, and the methane by-production reached 0.35 m3. 3 / ton, highlighting the necessity of two-stage regulation to block SCFAs consumption.

[0047] The above examples show that the present invention can maintain stable and efficient SCFAs output under complex scenarios such as raw material fluctuations and changes in processing scale, and has a solid technical foundation for industrial promotion.

[0048] The effects of the embodiment and the comparative example are shown in Table 1:

[0049] Table 1 Comparative analysis of the effects of various embodiments

[0050]

[0051] In the comparison of the examples and comparative examples in Table 1, the SCFAs yield of Example 1 of the present invention reached 6.7 g COD / L, significantly higher than that of single sludge fermentation (Comparative Example 1, 1.8 g COD / L) and co-fermentation without metal catalysts (Comparative Example 2, 3.2 g COD / L). The acetic acid content was increased to 58%, and the product composition was more in line with the needs of high-value applications. Through the precise combination of waste molasses and sludge and a two-stage control strategy, the production cost per ton was reduced to 1,820 yuan, a 60% reduction compared to the traditional corn-based fermentation method (4,500 yuan / ton). The COD removal rate was as high as 87%, and the sludge reduction rate reached 69%. Examples 2 and 3 show that even under complex conditions with high ammonia nitrogen in the raw material (C / N = 6.5:1) or low COD in waste molasses (42 g / L), SCFAs yields of 5.8 to 6.1 g COD / L can be maintained, and the proportions of propionic acid and butyric acid are optimized and improved, demonstrating the strong adaptability of the process. In contrast, single-stage fermentation (Comparative Example 3) yielded only 2.1 g COD / L of SCFAs due to the lack of methanogen inhibition, and the production of methane byproducts increased carbon emissions. In summary, this invention, through substrate synergy, metabolic targeting, and process optimization, achieves multi-dimensional breakthroughs in efficient short-chain fatty acid synthesis, cost control, and environmental benefits, demonstrating significant industrial application value.

[0052] The above embodiments are only some examples of the present invention, but the implementation methods of the present invention are not limited to the embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for synergistically producing short-chain fatty acids using waste molasses and municipal sludge, characterized in that The following steps are involved: (a) Raw material pretreatment: waste molasses was diluted to a COD of 40-60 g / L and then centrifuged to remove suspended particles with a particle size greater than 0.5 mm. At the same time, municipal sludge was thermally hydrolyzed at 90-100° C. for 30-60 minutes. (b) Mixing and blending: Mix waste molasses and volatile solids (VS) of sludge in a mass ratio of 1:1 to 3:1, and adjust the carbon-nitrogen ratio (C / N) of the system to 20 to 30:1; (c) Anaerobic fermentation: Add 0.05-0.2 mmol / L of Fe to the mixture. 2+ 、Co 2+ 、Ni 2+ The composite catalyst was fermented in two stages at 35-37°C. The first stage maintained pH at 6.5-7.0 for 24 hours, and the second stage adjusted pH to 5.5-6.0 and introduced trace oxygen (DO = 0.1-0.3 mg / L) for 5-7 days. (d) Product separation: The fermentation broth is subjected to ceramic membrane ultrafiltration and electrodialysis to extract the short-chain fatty acid concentrate, and the residue is dried and granulated to produce organic fertilizer.

2. The method according to claim 1, characterized in that The viscosity of the waste molasses after dilution in step (a) is ≤1500 cP, and the centrifugal speed is 2000-4000 rpm.

3. The method according to claim 1, characterized in that The sludge thermal hydrolysis pressure in step (a) is 0.1-0.3 MPa, and the SCOD improvement rate after thermal hydrolysis is ≥50%.

4. The method according to claim 1, characterized in that The VS mixing ratio of waste molasses to sludge in step (b) is 2:

1.

5. The method according to claim 1, characterized in that Fe in step (c) 2+ 、Co 2+ 、Ni 2+ The molar concentration ratio is 5:2.5:

1.

6. The method according to claim 1, characterized in that In step (c), the pH in the first stage is 6.8±0.2, and the ORP is -250 to -300 mV. In the second stage, microaeration is performed for 10 to 15 minutes every 6 hours, and 0.05 to 0.15 g / L of sodium 2-bromoethane sulfonate (BES) is added.

7. The method according to claim 1, characterized in that In step (c), the H2 concentration in the reactor is monitored in real time. When the H2 volume fraction is greater than 1%, an alarm is triggered and the frequency of micro-oxygen aeration is increased.

8. The method according to claim 1, characterized in that The pore size of the ceramic membrane ultrafiltration in step (d) is 20-100 nm, the operating temperature is 35-45° C., the voltage of the electrodialysis is 10-20 V, and the pH value of the concentrated solution is 2.5-3.

5.

9. The method according to claim 1, wherein: When the sludge C / N is less than 8:1, increase the VS mixing ratio of waste molasses to 2.5-3:1; when the COD of waste molasses is less than 50g / L, add glucose to COD = 50±5g / L.

10. A system for implementing the method according to any one of claims 1 to 9, characterized in that include: Pretreatment unit: dilution tank (1), waste molasses centrifuge (2), sludge thermal hydrolysis tank (3), sludge cooler (4); fermentation unit: mixing tank (5), two-phase anaerobic reactor (6) including pH / ORP sensor and micro-aeration device; separation unit: ceramic membrane ultrafiltration machine (7), electrodialysis device (8), belt dryer (9); control unit: central controller (10) to adjust pH, temperature and aeration parameters in a coordinated manner.

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