Sludge anaerobic fermentation acid production strengthening method and system based on methylglycine triacetic acid pretreatment

Abstract

The invention discloses a method for producing short-chain fatty acid (SCFAs) by utilizing MGDA (methylglycine diacetic acid) to pretreat and reinforce anaerobic fermentation of sludge. According to the technical scheme, the method comprises the following steps: mixing municipal sludge with an MGDA solution with the mass fraction of 0.05-0.3%, pretreating at 25-45 DEG C for 12-48 hours, then adjusting the pH value to 8.0-10.0, conveying to an anaerobic fermentation tank, and fermenting at 35-55 DEG C for 5-15 days. The yield of the SCFAs is increased by 40-80% through triple synergistic effects of complexing heavy metals, destroying cell wall structures and promoting dissolution of organic matters by the MGDA. The industrial pain points that a traditional pretreatment method is low in efficiency, high in cost and prone to secondary pollution are solved, and an efficient and environment-friendly new way is provided for sludge recycling.

Description

Technical Field

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

[0002] With the acceleration of urbanization and the widespread use of sewage treatment facilities, global municipal sludge production has exceeded 150 million tons (in dry solids terms), with my country accounting for over 40% and continuing to grow at an average annual rate of 10%. As the end product of sewage treatment, sludge contains significant amounts of organic matter, nitrogen and phosphorus nutrients, and pollutants such as heavy metals and pathogens. Traditional disposal methods include landfilling, which consumes land resources and poses the risk of leachate contamination. Incineration, while reducing waste, is energy-intensive and releases highly toxic substances such as dioxins. In contrast, anaerobic fermentation and acidification technology can convert organic matter in sludge into short-chain fatty acids (SCFAs). These high-value-added chemicals are widely used in the synthesis of bioplastics (such as polyhydroxyalkanoates, PHAs), as carbon sources for wastewater denitrification and phosphorus removal, and offer significant resource utilization potential. However, the dense structure of microbial cell walls 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] In order to break the above bottlenecks, existing technologies focus on the development of pretreatment processes. Pyrolysis (80-180°C) destroys cell walls through high temperatures, but the energy consumption is as high as 1.2-2.5 kWh / kg DS, and difficult-to-degrade melanin-like substances are easily generated at high temperatures; although acid-base pretreatment (pH ≤ 3 or pH ≥ 12) can effectively dissolve cell walls, strong acids and alkalis cause equipment corrosion, and additional neutralizers are required, resulting in a large amount of salt-containing wastewater. Although ozone oxidation has high efficiency in cell wall breaking, its operating cost is as high as US$15-25 / ton DS, and the hydroxyl radicals (·OH) produced by ozone decomposition will indiscriminately attack organic molecules, leading to excessive oxidation of the target product SCFAs. In addition, the above methods have limited passivation effects on heavy metals, and the concentration of free heavy metals in the fermentation broth is still generally higher than 30 mg / L, which seriously inhibits the hydrogenase and coenzyme activity of 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 ethylenediaminetetraacetic acid (EDTA), promoting sludge hydrolysis and anaerobic fermentation to produce acid. However, EDTA has poor biodegradability and there is a risk of heavy metals seeping into the environment. Literature (Environ.Pollut.2024,361:124809) shows that although nitrilotriacetic acid (NTA) is biodegradable, its complexing ability is weak and it is easy to release heavy metal ions under acidic conditions. As a new generation of green chelating agent, trisodium methylglycine diacetate (MGDA) has the advantages of high water solubility, strong complexing ability and complete biodegradability, and has been widely used in detergents and industrial cleaning fields. However, its application research in sludge pretreatment is almost blank. Existing literature only involves the removal effect of MGDA on single heavy metals, but lacks systematic understanding of its multi-effect synergistic mechanism in complex sludge systems, and there is no research on its integrated optimization with anaerobic fermentation processes.

[0005] The current technical pain points are mainly reflected in the following aspects: first, the single pretreatment method makes it difficult to simultaneously achieve cell wall disruption, heavy metal passivation, and fermentation environment regulation; second, chemical agent residues can easily cause secondary pollution, such as EDTA residue, which can increase effluent COD by 20% to 30%; third, the poor matching of process parameters, and the disconnect between pretreatment conditions (such as pH and temperature) and subsequent fermentation requirements. Therefore, developing an efficient, environmentally friendly sludge pretreatment technology with multiple synergistic effects has become the key to breaking through the bottleneck of industrializing 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 developing an integrated "pretreatment-fermentation" process based on this. The study found that when MGDA acts on sludge at a mass fraction of 0.05% to 0.3%, the yield of SCFAs can be significantly improved 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, 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 the microbial cell wall through hydrogen bond cleavage and ion exchange, increasing the dissolution rate of intracellular organic matter to 58% to 72%; third, MGDA can regulate the redox potential of the fermentation system under alkaline conditions, promote the metabolic activity of butyric acid bacteria, and increase the proportion of acetic acid.

[0007] The technical solution of the present invention breaks through the single limitation of traditional pretreatment process and realizes process optimization through the three-stage combined mechanism of "chemical wall breaking-heavy metal passivation-environmental regulation". 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 of 15 to 30 rpm, intermittent operation of 5 to 15 minutes / hour) is used to ensure uniform dispersion of the agent to avoid local excessive concentration leading to inactivation of microorganisms; 2) The mixed solution is transferred to a constant temperature anaerobic fermentation tank (35 to 55 ° C) after adjusting the pH to 8.0 to 10.0. The redox state is monitored in real time by a built-in ORP sensor, and the pH is adjusted by feedback 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. The fermentation tank is integrated with a gas recovery module to reuse the generated biogas in the pretreatment heating system, reducing overall energy consumption by 30% to 40%.

[0008] Through the coordinated implementation of the above technical solutions, the present invention achieves significantly superior results to existing technologies: First, a breakthrough in acid production efficiency. The SCFAs yield reaches 350-480 mg COD / g VS, a 60%-80% increase compared to 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, product selectivity is optimized. The proportion of acetic acid is increased to 55%-65% (the traditional process is <45%), which is more conducive to the subsequent bioplastic synthesis (PHA conversion rate is increased by 20%-30%); third, deep removal of heavy metals. The removal rates of MGDA for Cu, Zn, and Cd reach 85%, 89%, and 78%, respectively, and the heavy metal leaching toxicity of the fermentation residue (TCLP test) is lower than the limit of the "Standard for Agricultural Use of Sludge" (GB 4284-2018); fourth, the entire process is green. The complete biodegradability of MGDA combined with TiO2 photocatalytic degradation technology reduces the effluent chemical oxygen demand (COD) increment to <50 mg / L, which is much lower than the EDTA treatment process (200 mg / L).

[0009] The expanded application value of this invention lies in: first, its synergistic processing capabilities. It can be co-fermented with food waste (mass ratio of 1:1 to 1:3), leveraging the high carbon-nitrogen ratio (C / N = 20-30) of food waste to compensate for the carbon source deficiency in sludge, further increasing SCFA yield by 15% to 25%. Second, it allows for high-value product utilization. The produced SCFAs can be directly used as a carbon source in sewage treatment plant denitrification and phosphorus removal systems (increasing denitrification rates by 0.15-0.25 mg N / (g VSS·h)) or purified through distillation for bio-based chemical production. Third, its process compatibility: This pretreatment system seamlessly integrates with existing anaerobic digestion facilities, requiring only the addition of an MGDA dosing module and a stirring device. The modification cost is 30% less than that of traditional methods. Through the deep integration of theoretical innovation and engineering design, this invention addresses the three long-standing technical bottlenecks in the sludge resource utilization field: difficulty in wall breaking, strong inhibition, and heavy pollution, providing a scalable solution for the efficient conversion of organic solid waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 Process flow chart;

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

[0012] Figure 3 Schematic diagram of system structure. DETAILED DESCRIPTION

[0013] Example 1: Basic MGDA pretreatment process

[0014] Step 1: Sludge pretreatment

[0015] 10 kg of dewatered sludge from a sewage treatment plant (with characteristics of 82% moisture content, pH 6.8, VS 16.2 g / L, and total heavy metals of 120 mg / kg Cu and 280 mg / kg Zn) was added to 30 L of a 0.1% MGDA solution (solid-to-liquid ratio of 1:3) and injected into a jacketed pretreatment reactor. A double-screw agitator was operated at 20 rpm, with a 5-minute pause every 10 minutes. The temperature was maintained at 35 ± 1°C via circulating water in the jacket for 24 hours.

[0016] Step 2: pH adjustment and fermentation

[0017] After pretreatment, the sludge was filtered through a plate and frame filter at a pressure of 0.6 MPa. The filtrate pH was raised from an initial pH of 6.8 to 8.5. The residue and filtrate were mixed and transferred to an anaerobic fermenter. The fermentation was inoculated with thermophilic anaerobic sludge (VS) = 21.5 g / L, with an inoculum size of 20%. The fermentation temperature was set at 50 ± 1°C, the stirring rate at 15 rpm, and the fermentation was run continuously. 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 phase was separated by centrifugation at 3000 rpm for 15 minutes. The total SCFA yield was measured to be 398 mg COD / g VS, with acetic acid accounting for 61%, propionic acid 23%, and butyric acid 16%. Heavy metal removal rates were 83% for Cu, 86% for Zn, and 75% for Cd. The residual MGDA content 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 applied simultaneously during the pretreatment stage: a 40kHz ultrasonic generator with an acoustic energy density of 0.35W / mL was used, and the probe was inserted into the pretreatment reactor. The treatment was continued for 30 minutes with a 15-minute interval. The MGDA concentration was increased to 0.2%, the pretreatment time was shortened to 18 hours, and the other conditions were the same as in Example 1. The SCFAs yield was measured to be 452mg 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 rupture rate to 68% (54% in the control group), and the heavy metal removal rate was simultaneously improved, with heavy metal removal rates of Cu 89% and Zn 92%, respectively. The ORP of the fermentation broth was stabilized at -345mV, and the proportion of acetic acid increased to 65%.

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

[0023] Sludge was mixed with food waste in a mass ratio of 1:2, yielding a TS ratio of 22%, a VS / TS ratio of 88%, and a C / N ratio of 28, for a total treatment capacity of 15 kg. Pretreatment was performed using a 0.15% MGDA solution at a solid-to-liquid ratio of 1:4 at 40°C for 30 hours. The pH was adjusted to 9.2, and a composite inoculum (containing Clostridium butyricum and Bacteroides vulgatus) was inoculated and fermented at 45°C for 10 days. This increased SCFA yield to 486 mg COD / g VS, with acetic acid accounting for 67%. The addition of readily degradable carbon sources (such as starch and oil) to the food waste accelerated the peak acid production rate by two days. The fermentation residue achieved a humification index of 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, with an equal amount of water replacing the MGDA solution, and identical pretreatment conditions (e.g., time, temperature, and agitation), the SCFA yield was measured to be only 226 mg COD / g VS, and the fermentation cycle was extended to 16 days. The heavy metal removal rate was less than 25%, with acetic acid accounting for 41%, and the fermentation broth ORP dropped to only -205 mV, indicating limited metabolic activity of the acid-producing bacteria.

[0026] Comparative Example 2: EDTA replaces MGDA

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

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

[0029] Table 1 Comparative analysis of the effects of various embodiments

[0030]

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

Claims

1. A method for enhancing acid production by anaerobic fermentation of sludge based on trisodium methylglycine diacetate, characterized in that: The following steps are involved: a) mixing the sludge with a 0.05% to 0.3% by mass solution of trisodium methylglycine diacetate (MGDA); b) pretreatment at 25 to 45° C. for 12 to 48 hours; c) adjusting the pH of the mixed solution to 8.0-10.0; d) The pretreated sludge is transported to an anaerobic fermentation tank and fermented at 35-55°C for 5-15 days.

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

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

4. The method according to claim 1, characterized in that In the 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 the MGDA to the heavy metals in the sludge is 1.2:1 to 2:

1.

6. The method according to claim 1, characterized in that During the pretreatment stage, 20-50 kHz ultrasonic treatment is simultaneously applied, and the acoustic energy density is 0.2-0.5 W / mL.

7. A system for implementing the method according to any one of claims 1 to 6, characterized in that: include: MGDA pretreatment reactor: equipped with pH online monitoring module; Double helix stirring device; Constant temperature anaerobic fermentation tank: built-in gas collection unit.

8. The system according to claim 7, characterized in that The inner wall of the pretreatment reactor is coated with a nano titanium dioxide photocatalytic coating.

9. The method according to claim 1, characterized in that The method is applicable to the coordinated fermentation of food waste.

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

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