A method and system for enhancing acid production through anaerobic fermentation of sludge based on trisodium methylglycine diacetate pretreatment

Abstract

This invention discloses a method for enhancing the production of short-chain fatty acids (SCFAs) from anaerobic fermentation of sludge using trisodium methylglycine diacetate (MGDA) pretreatment. The technical solution includes: mixing municipal sludge with a 0.05%–0.3% (w / w) MGDA solution, pretreating at 25–45°C for 12–48 hours, then adjusting the pH to 8.0–10.0 and transferring the sludge to an anaerobic fermenter for fermentation at 35–55°C for 5–15 days. MGDA, through a triple synergistic effect of complexing heavy metals, disrupting cell wall structure, and promoting the dissolution of organic matter, increases the SCFA yield by 40%–80%. This invention solves the industry pain points of low efficiency, high cost, and easy secondary pollution associated with traditional pretreatment methods, providing a new, efficient, and environmentally friendly approach for sludge resource utilization.

Description

Technical Field

[0001] This invention relates to the field of organic solid waste resource utilization technology, specifically to a novel process for enhancing anaerobic fermentation and acid production of sludge through chemical pretreatment, and particularly to the application method and supporting system of trisodium methylglycine diacetate (MGDA) in sludge pretreatment. Technical Background

[0002] With the acceleration of urbanization and the widespread adoption of wastewater treatment facilities, the global annual production of municipal sludge has exceeded 150 million tons (by dry solids), with my country accounting for over 40% and continuing to grow at an average annual rate of 10%. As the final product of wastewater treatment, sludge contains large amounts of organic matter, nitrogen and phosphorus nutrients, as well as pollutants such as heavy metals and pathogens. Traditional disposal methods include landfilling, which consumes land resources and poses a risk of leachate pollution; and incineration, while reducing sludge volume, is energy-intensive and releases highly toxic substances such as dioxins. In contrast, anaerobic fermentation acidification technology can convert the organic matter in sludge into short-chain fatty acids (SCFAs), which are widely used as high-value-added chemicals in the synthesis of bioplastics (such as polyhydroxyalkanoates, PHA), wastewater denitrification and phosphorus removal, and carbon source replenishment, demonstrating significant resource utilization potential. However, the dense structure of the cell walls of microorganisms in sludge results in an organic matter dissolution rate of less than 25%, and the inhibitory effect of heavy metals on acid-producing bacteria severely restricts the yield and industrial application of SCFAs.

[0003] To overcome these bottlenecks, existing technologies focus on the development of pretreatment processes. Pyrolysis (80–180℃) destroys cell walls through high temperatures, but energy consumption is as high as 1.2–2.5 kWh / kg DS, and it easily generates recalcitrant melanoidins at high temperatures. Acid-base pretreatment (pH ≤ 3 or pH ≥ 12) can effectively dissolve cell walls, but strong acids and alkalis cause equipment corrosion and require additional neutralizing agents, generating large amounts of saline wastewater. While ozone oxidation has highly efficient cell wall disruption capabilities, its operating cost is as high as $15–25 / ton DS, and the hydroxyl radicals (·OH) generated by ozone decomposition indiscriminately attack organic molecules, leading to over-oxidation of the target product SCFAs. Furthermore, the above methods have limited passivation effects on heavy metals; the concentration of free heavy metals in the fermentation broth is still generally higher than 30 mg / L, severely inhibiting the activity of hydrogenases and coenzymes in acid-producing bacteria.

[0004] In recent years, aminopolycarboxylate chelating agents have attracted attention due to their heavy metal complexing ability. For example, patent (CN102994578A) removes polyvalent metals by adding disodium ethylenediaminetetraacetate (EDTA), promoting sludge hydrolysis and anaerobic fermentation to produce acid. However, EDTA has poor biodegradability and poses a risk of heavy metal leaching into the environment. Literature (Environ. Pollut. 2024, 361:124809) shows that while NTA is biodegradable, its complexing ability is weak, and it easily releases heavy metal ions under acidic conditions. Methylglycine diacetate trisodium (MGDA), as a new generation of green chelating agent, has advantages such as high water solubility, strong complexing ability, and complete biodegradability, and has been widely used in detergents and industrial cleaning. However, research on its application in sludge pretreatment is almost nonexistent. Existing literature only covers the removal effect of MGDA on single heavy metals, but lacks a systematic understanding of its multi-effect synergistic mechanism in complex sludge systems, and there is no research on its integration and optimization with anaerobic fermentation processes.

[0005] Current technological challenges are mainly manifested in three aspects: First, the pretreatment methods are limited, making it difficult to simultaneously achieve cell wall disruption, heavy metal passivation, and fermentation environment control; second, chemical residues can easily cause secondary pollution, such as EDTA residues leading to a 20%–30% increase in effluent COD; and third, the process parameters are poorly matched, with pretreatment conditions (such as pH and temperature) being out of sync with the requirements of subsequent fermentation. Therefore, developing a highly efficient, environmentally friendly sludge pretreatment technology with multiple synergistic effects has become crucial to overcoming the industrialization bottleneck of anaerobic fermentation acid production technology. Summary of the Invention

[0006] The core innovation of this invention lies in revealing the multi-dimensional synergistic mechanism of MGDA in sludge pretreatment, and based on this, developing an integrated "pretreatment-fermentation" process. Research has found that when MGDA is applied to sludge at a mass fraction of 0.05%–0.3%, it can significantly improve the yield of SCFAs through a triple effect: First, MGDA's strong chelating ability can effectively passivate heavy metals, reducing the concentration of free heavy metals in the fermentation broth to below 5 mg / L, thus relieving their inhibitory effect on the hydrogenase of acid-producing bacteria; Second, the carboxylic acid groups in the MGDA molecule specifically attack the β-1,4 glycosidic bonds of the peptidoglycan layer of microbial cell walls through hydrogen bond breaking and ion exchange, increasing the intracellular organic matter dissolution rate to 58%–72%; Third, under alkaline conditions, MGDA can regulate the redox potential of the fermentation system, promote the metabolic activity of butyric acid bacteria, and increase the proportion of acetic acid.

[0007] The technical solution of this invention breaks through the limitations of the single nature of traditional pretreatment processes, and achieves process optimization through a three-stage combined mechanism of "chemical cell disruption - heavy metal passivation - environmental control". Specifically: 1) Sludge with a moisture content of 80% to 85% is mixed with MGDA solution at a ratio of 1:3 to 1:5 (w / v) and pretreated at 25 to 45°C for 12 to 48 hours. During this period, a double-helix stirring device (speed 15 to 30 rpm, intermittent operation for 5 to 15 minutes / hour) is used to ensure uniform dispersion of the agent and avoid excessive local concentration that could lead to microbial inactivation; 2) After adjusting the pH of the mixture to 8.0 to 10.0, it is transferred to a constant temperature anaerobic fermenter (35 to 55°C). The oxidation-reduction state is monitored in real time by a built-in ORP sensor, and the pH is adjusted accordingly to increase the abundance of acid-producing bacteria by 3 to 5 times. 3) The inner wall of the pretreatment reactor is coated with a nano-titanium dioxide (TiO2) photocatalytic coating (thickness 50-100nm), which can degrade residual MGDA under visible light irradiation, completely eliminating the risk of secondary pollution from chemical agents. In addition, the fermenter integrates a gas recovery module to reuse the generated biogas in the pretreatment heating system, reducing the overall energy consumption by 30%-40%.

[0008] Through the synergistic implementation of the above technical solutions, this invention achieves significantly better results than existing technologies: First, it achieves a breakthrough in acid production efficiency. The yield of SCFAs reaches 350–480 mg COD / g VS, which is 60%–80% higher than that of the traditional pyrolysis method (220–280 mg COD / g VS), and the fermentation cycle is shortened to 5–15 days (the traditional process requires 15–25 days); Second, it optimizes product selectivity. The proportion of acetic acid is increased to 55%–65% (the traditional process is <45%), which is more conducive to subsequent bioplastic synthesis (PHA conversion rate is increased by 20%–30%); Third, it achieves deep removal of heavy metals. The removal rates of Cu, Zn, and Cd by MGDA reach 85%, 89%, and 78%, respectively, and the heavy metal leaching toxicity of fermentation residue (TCLP test) is lower than the limit of the "Standard for Agricultural Use of Sludge" (GB 4284-2018); Fourth, it achieves greening of the entire process. The complete biodegradability of MGDA, combined with TiO2 photocatalytic degradation technology, results in an increase in chemical oxygen demand (COD) of the effluent of less than 50 mg / L, which is far lower than that of the EDTA treatment process (200 mg / L).

[0009] The extended application value of this invention lies in three aspects: First, its co-processing capability. It can be co-fermented with kitchen waste (mass ratio 1:1 to 1:3), utilizing the high carbon-to-nitrogen ratio (C / N = 20 to 30) of kitchen waste to compensate for the insufficient carbon source in sludge, further increasing the yield of SCFAs by 15% to 25%. Second, its high-value utilization of products. The generated SCFAs can be directly used as a carbon source in wastewater treatment plant denitrification and phosphorus removal systems [increasing the denitrification rate by 0.15 to 0.25 mg N / (g VSS·h)], or purified by distillation for the production of bio-based chemicals. Third, its process compatibility: This pretreatment system can be seamlessly integrated with existing anaerobic digestion facilities, requiring only the addition of an MGDA dosing module and a stirring device, with modification costs lower than 30% of traditional methods. Through the deep integration of theoretical innovation and engineering design, this invention solves the three major technical bottlenecks of "difficult cell wall breaking, strong inhibition, and heavy pollution" that have long existed in the field of sludge resource utilization, providing a scalable solution for the efficient conversion of organic solid waste. Attached Figure Description

[0010] Figure 1 Process flow diagram;

[0011] Figure 2 Schematic diagram of the mechanism of action of MGDA;

[0012] Figure 3 System structure diagram. Detailed Implementation

[0013] Example 1: Basic MGDA Pretreatment Process

[0014] Step 1: Sludge Pretreatment

[0015] 10 kg of dewatered sludge from a wastewater treatment plant was taken. The sludge characteristics were: moisture content 82%, pH = 6.8, VS = 16.2 g / L, total heavy metals Cu = 120 mg / kg, Zn = 280 mg / kg. 30 L of 0.1% MGDA solution was added, with a solid-liquid ratio of 1:3, and the mixture was injected into a jacketed pretreatment reactor. A double-helix agitator was started at 20 rpm, with a 5-minute pause every 10 minutes of operation. The temperature was maintained at 35 ± 1℃ by circulating water through the jacket, and the pretreatment lasted for 24 hours.

[0016] Step 2: pH Adjustment and Fermentation

[0017] After pretreatment, the sludge was filtered through a plate and frame filter press at a pressure of 0.6 MPa. The pH of the filtrate increased from an initial 6.8 to 8.5. The filter residue and filtrate were then mixed and transferred to an anaerobic digester. High-temperature anaerobic sludge was inoculated at a concentration of 20% (VS = 21.5 g / L). The fermentation temperature was set at 50 ± 1℃, and the stirring rate was 15 rpm, with continuous operation. SCFA content was monitored starting on day 5 of fermentation, and the yield stabilized by day 12.

[0018] Step 3: Product Collection and Analysis

[0019] After fermentation, the liquid products were separated by centrifugation at 3000 rpm for 15 minutes. The total yield of SCFAs was measured to be 398 mg COD / gVS, with acetic acid accounting for 61%, propionic acid 23%, and butyric acid 16%. The heavy metal removal rates were Cu 83%, Zn 86%, and Cd 75%. The residual MGDA in the pretreatment stage was 12 mg / L, which was reduced to 0.8 mg / L after photocatalytic degradation.

[0020] Example 2: MGDA-ultrasound synergistic pretreatment

[0021] Based on Example 1, ultrasonic enhancement was simultaneously applied during the pretreatment stage: a 40kHz ultrasonic generator with a sound energy density of 0.35W / mL was used, and the probe was inserted into the pretreatment reactor, with a 15-minute interval between every 30 minutes of treatment. The MGDA concentration was increased to 0.2%, and the pretreatment time was shortened to 18 hours, with other conditions remaining the same as in Example 1. The SCFAs yield was measured to be 452 mg COD / g VS, an increase of 13.6%, and the fermentation cycle was shortened to 9 days. The cavitation effect of ultrasound increased the cell wall disruption rate to 68% (compared to 54% in the control group), and the heavy metal removal rate was simultaneously improved, with Cu removal rates of 89% and Zn removal rates of 92%. The fermentation broth ORP stabilized at -345mV, and the acetic acid content increased to 65%.

[0022] Example 3: Co-fermentation of kitchen waste

[0023] Sludge and food waste were mixed at a mass ratio of 1:2, with TS = 22%, VS / TS = 88%, C / N = 28, and a total processing capacity of 15 kg. A 0.15% MGDA solution was used, with a solid-liquid ratio of 1:4, and pretreatment was performed at 40℃ for 30 hours. The pH was adjusted to 9.2, and a compound microbial agent (containing Clostridium butyricum and Bacteroides vulgatus) was inoculated, followed by fermentation at 45℃ for 10 days. The SCFAs yield increased to 486 mg COD / g VS, with acetic acid accounting for 67%. The addition of readily degradable carbon sources (such as starch and oil) from the food waste advanced the peak acid production rate by 2 days, and the humification index of the fermentation residue reached 0.82, making it suitable for direct use as a soil conditioner.

[0024] Comparative Example 1: No MGDA pretreatment

[0025] Using the same sludge as in Example 1, but replacing the MGDA solution with an equal amount of clean water, and with identical pretreatment conditions (such as time, temperature, and stirring), the SCFAs yield was only 226 mg COD / g VS, and the fermentation cycle was extended to 16 days. The heavy metal removal rate was less than 25%, the acetic acid content was 41%, and the ORP of the fermentation broth only decreased to -205 mV, indicating limited metabolic activity of the acid-producing bacteria.

[0026] Comparative Example 2: EDTA replacing MGDA

[0027] MGDA was replaced with an equimolar concentration of EDTA (0.1%), with all other conditions remaining the same as in Example 1. During the pretreatment stage, EDTA achieved removal rates of 78% and 81% for Cu and Zn, respectively. However, significant inhibition occurred on day 3 of fermentation, with SCFA yield stagnating at 185 mg COD / g VS. EDTA residue of 38 mg / L led to an increase in effluent COD to 620 mg / L (compared to 380 mg / L in the control group). Furthermore, EDTA degradation intermediates, such as iminodiacetic acid at a concentration of 9.2 mg / kg, were detected in the fermentation residue, posing an ecological risk.

[0028] The effects of the examples and comparative examples are shown in Table 1.

[0029] Table 1 Comparison and analysis of the effects of each embodiment

[0030]

[0031] The data from the embodiments of this invention show that the process system based on MGDA pretreatment is significantly superior to traditional methods in terms of sludge acidification efficiency and environmental friendliness. Example 1, using basic MGDA pretreatment, increased the SCFAs yield by 76% compared to untreated sludge (Comparative Example 1), shortened the fermentation cycle by 25%, and achieved heavy metal removal rates of 83% and acetic acid content of 61%, respectively, verifying the core role of MGDA in cell wall disruption, heavy metal passivation, and optimized product distribution. Example 2, by introducing ultrasonic synergistic treatment, further increased the yield to 452 mg COD / g VS and shortened the fermentation cycle to 9 days, demonstrating that physical-chemical coupling can enhance cell wall disruption efficiency (increasing by 14.8%) and accelerate the reaction process. Example 3, combined with co-fermentation of food waste, achieved an SCFAs yield of 486 mg COD / g VS and increased the acetic acid content to 67%, highlighting the synergistic potential of multi-source organic solid waste co-treatment. Compared to EDTA pretreatment (Comparative Example 2), the MGDA process exhibits significant environmental advantages in terms of reagent residue and effluent COD increase, while avoiding secondary pollution. In summary, through differentiated technology combinations, the various embodiments systematically demonstrate the multi-dimensional breakthroughs of the present invention in terms of acid production performance, process economy, and ecological safety, providing reliable data support for industrial applications.

Claims

1. A method for enhancing acid production through anaerobic fermentation of sludge based on trisodium methylglycine diacetate, characterized in that, Includes the following steps: a) Mix the sludge with a 0.05%~0.3% (w / w) solution of trisodium methylglycine diacetate (MGDA); b) Pretreatment at 25~45℃ for 12~48 hours; c) Adjust the pH of the mixture to 8.0~10.0; d) The pretreated sludge is transported to an anaerobic digester and fermented at 35-55℃ for 5-15 days.

2. The method according to claim 1, characterized in that, In step a), the solid-liquid ratio of sludge to MGDA solution is 1:3 to 1:5 (w / v).

3. The method according to claim 1, characterized in that, In step b), during the pretreatment stage, intermittent mechanical stirring is applied at a frequency of 5-15 minutes per hour.

4. The method according to claim 1, characterized in that, In step d), the volatile solids (VS) content of the inoculated sludge during the anaerobic fermentation stage is 15% to 25%.

5. The method according to claim 1, characterized in that, The molar ratio of MGDA to heavy metals in the sludge is 1.2:1 to 2:

1.

6. The method according to claim 1, characterized in that, The pretreatment stage involves simultaneous application of 20-50 kHz ultrasonic waves with an acoustic energy density of 0.2-0.5 W / mL.

7. The method according to claim 1, characterized in that, The method described is applicable to the co-fermentation of kitchen waste.

8. The method according to claim 1, characterized in that, The SCFAs produced by the method can be used as a carbon source in biological nitrogen and phosphorus removal systems.

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