Method for promoting anaerobic fermentation of excess sludge to produce acid

Abstract

The invention discloses a method for promoting anaerobic fermentation of excess sludge to produce acid, and belongs to the technical field of sludge treatment and recycling. The method comprises the following steps: preparing excess sludge into a sludge suspension; adding a percarbonate adhesive and carrying out ultrasonic synergistic treatment; and mixing with inoculated sludge, and fermenting to produce acid under an anaerobic condition. The ultrasonic cavitation effect is utilized to promote decomposition of percarbonate to generate hydroxyl free radicals and free ammonia, sludge flocs and cell structures are synergistically destroyed, the dissolution and hydrolysis efficiency of organic matters is greatly improved, and abundant substrates are provided for acid-producing bacteria, so that the yield of volatile fatty acid is remarkably increased. The method has the advantages of strong synergistic effect, high treatment efficiency, less agent dosage, low cost, mild operation conditions, environmental friendliness and the like, and is suitable for efficient resourceful treatment of the excess sludge.

Description

Technical Field

[0001] This invention relates to the field of sludge treatment and resource utilization technology, specifically to a method for promoting anaerobic fermentation and acid production from excess sludge. Background Technology

[0002] Wastewater sludge is an unavoidable solid waste generated during wastewater treatment. It has a complex composition, rich in incompletely degraded organic matter such as proteins, polysaccharides, and lipids, and may also carry pathogenic microorganisms and various trace pollutants. Improper disposal can easily cause secondary pollution, threatening the ecological environment and public health. Therefore, developing efficient, safe, and economical technologies for the resource utilization and harmless treatment of wastewater sludge has become an important research direction in the current water treatment field.

[0003] Anaerobic fermentation for the production of volatile fatty acids is a highly promising pathway for the resource utilization of sludge. In this process, the organic components in the sludge are converted into volatile fatty acids such as acetic acid, propionic acid, and butyric acid through microbial action. These volatile fatty acids are not only important chemical raw materials, but can also serve as high-quality carbon sources for bioenergy (such as for the synthesis of polyhydroxyalkanoates) or denitrification in wastewater treatment, exhibiting significant environmental and economic value.

[0004] However, the efficient acidification of residual sludge still faces key bottlenecks. Its dense floc structure and microenvironment rich in extracellular polymers, together with the robust microbial cell walls, hinder the release and hydrolysis of organic matter, resulting in low hydrolysis efficiency, which in turn limits subsequent acidification reactions and ultimately restricts the yield and generation rate of volatile fatty acids.

[0005] To address this challenge, existing research primarily employs pretreatment methods to enhance sludge hydrolysis. While physical methods (such as ultrasonic treatment) or chemical methods (such as oxidation with potassium permanganate and sodium hypochlorite) can disrupt sludge structure and promote organic matter dissolution to some extent, they generally suffer from limited treatment efficiency, high energy or reagent costs, and potential environmental risks from byproducts. More importantly, these methods struggle to achieve a good balance between treatment effectiveness and operating costs, limiting their widespread application in large-scale engineering projects.

[0006] Therefore, there is an urgent need to develop a new pretreatment or synergistic regulation strategy that can effectively overcome sludge hydrolysis barriers, significantly improve the yield of volatile fatty acids, and is both economical and environmentally friendly. Summary of the Invention

[0007] Waste sludge has a complex flocculent structure and cell walls, making it difficult to hydrolyze. This hydrolysis becomes the rate-limiting step in anaerobic fermentation for acid production, thus limiting the production of volatile fatty acids. This invention provides a method to promote anaerobic fermentation and acid production from waste sludge. By using ultrasound-enhanced percarbamide synergistic treatment of waste sludge, the hydrolysis process is accelerated, the degree of hydrolysis is increased, more substrate is provided for microorganisms, and anaerobic fermentation for acid production is promoted.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution: This invention provides a method for promoting anaerobic fermentation and acid production from waste sludge, comprising the following steps: (1) Raw material pretreatment: Prepare the remaining sludge into a sludge suspension; (2) Ultrasonic-percarbamide synergistic treatment: Percarbamide is added to the sludge suspension and ultrasonic treatment is performed simultaneously; (3) Anaerobic fermentation: The ultrasonically treated sludge suspension is mixed with the inoculated sludge and anaerobic fermentation is carried out to produce volatile fatty acids.

[0009] The core principle of the above scheme lies in utilizing the synergistic effect of percarbonamide and ultrasound to achieve efficient sludge breakdown and subsequent anaerobic fermentation for acid production: In the pretreatment stage, percarbonamide hydrolyzes to generate H2O2 and ammonia nitrogen. Under ultrasound-promoted conditions, H2O2 decomposes to produce hydroxyl radicals (·OH) and superoxide anion radicals (O2··OH). - The ultrasonic cavitation process generates active free radicals, and the two work synergistically to significantly increase the concentration of free radicals, indiscriminately oxidizing and degrading extracellular polymeric substances (EPS). At the same time, the hydrolysis of percarbonamide raises the pH of the system, causing some ammonia nitrogen to be converted into highly destructive free ammonia (NH3), further disintegrating the EPS structure and cell membrane. Meanwhile, the mechanical shearing and homogenizing effects of ultrasound enhance mass transfer efficiency and promote the breakage of sludge particles, jointly strengthening the hydrolysis effect. After entering the anaerobic fermentation stage, the alkaline substances remaining after pretreatment maintain the sludge in a weakly alkaline environment, effectively buffering the pH drop caused by rapid acidification in the early stage of fermentation, and avoiding the inhibition of acid-producing bacteria activity. Combined with the inoculation of active bacteria and optimization of fermentation conditions, the efficient conversion of organic matter into volatile fatty acids is achieved.

[0010] As a preferred technical solution of the present invention, in step (1), the total suspended solids mass fraction of the sludge suspension in the raw material pretreatment is 3-8%.

[0011] More preferably, in step (1), the total suspended solids mass fraction of the sludge suspension in the raw material pretreatment is 5%.

[0012] The above scheme limits the total suspended solids mass fraction of the sludge suspension to 3-8%, preferably 5%. Within this concentration range, the sludge has moderate fluidity, which is conducive to the transmission of ultrasonic cavitation effect and facilitates subsequent mixing and fermentation; the 5% concentration showed the best hydrolysis and acid production balance point in the experiment, taking into account both treatment efficiency and operating economy.

[0013] As a preferred technical solution of the present invention, in step (1), the residual sludge is the residual sludge generated during the treatment of domestic sewage.

[0014] As a preferred technical solution of the present invention, in step (2), the amount of percarbonamide added is 5 to 40 mg / g of the total suspended solids of the sludge suspension.

[0015] More preferably, in step (2), the amount of percarbonamide added is 10 mg / g of the total suspended solids in the sludge suspension.

[0016] The above scheme limits the dosage of percarbonate gum to 5–40 mg / g TSS, with 10 mg / g TSS being preferred. A significant synergistic effect can be achieved with low dosage, greatly reducing reagent costs; 10 mg / g TSS showed the best acid-producing synergistic coefficient (1.67 times) in the experiment, achieving the optimal balance between economy and effectiveness.

[0017] As a preferred technical solution of the present invention, in step (2), the intensity of the ultrasonic treatment is 200-300W, the frequency is 30-50KHz, the treatment temperature is 20-30℃, and the treatment time is 0.5-2h.

[0018] The above-mentioned scheme specifies the ultrasonic treatment parameters as follows: intensity 200–300 W, frequency 30–50 kHz, time 0.5–2 h, and temperature 20–30 °C. This parameter range can effectively induce cavitation and promote the generation of free radicals; the treatment time is short and energy consumption is controllable, making it suitable for continuous or intermittent engineering operations.

[0019] As a preferred technical solution of the present invention, in step (3), the inoculated sludge is a sludge suspension prepared in step (1), and the volume of the inoculated sludge is 5 to 15% of the volume of the ultrasonically treated sludge suspension.

[0020] The above scheme specifies that the inoculated sludge is a suspension of sludge from the same source, and the inoculation amount is 5-15%. Using sludge from the same source as the inoculation source avoids the problem of adaptation of foreign microorganisms, resulting in rapid start-up and high stability; the inoculation amount is moderate, which can provide sufficient active microorganisms without over-diluting the reaction system.

[0021] As a preferred technical solution of the present invention, in step (3), the fermentation conditions include: a fermentation temperature of 30-40°C, a fermentation time of 5-10 days, and a pH value of 6.5-7.5 in the fermentation system.

[0022] The above scheme specifies the fermentation conditions as follows: temperature 30–40℃, time 5–10 days, pH 6.5–7.5. Mesophilic fermentation is conducive to the metabolic activity of acid-producing bacteria and improves the efficiency of volatile acid accumulation; the near-neutral pH environment avoids excessive acidity that inhibits bacterial activity and ensures stable fermentation.

[0023] As a preferred technical solution of the present invention, in step (3), the anaerobic fermentation is carried out in an anaerobic environment, which is achieved by introducing an inert gas into the fermentation container and sealing it.

[0024] The above method limits the creation of an anaerobic environment to the introduction of an inert gas and sealing. It is simple and reliable to operate, effectively removes oxygen, and prevents interference from aerobic bacteria; it is suitable for common fermentation reactors and is easy to implement in engineering.

[0025] The specific beneficial effects of this invention are summarized as follows: (1) Multiple mechanisms work together to significantly improve yield: After ultrasonic-enhanced percarbamide pretreatment, free radicals, free ammonia and ultrasound jointly promote the hydrolysis of residual sludge, change the microbial community structure, and provide good conditions for acid-producing bacteria. After the combined treatment, the peak concentration of dissolved organic matter reached 2622 mg COD / L, which is 229% higher than the blank group (795 mg COD / L) (as in Example 2); the peak concentration of volatile fatty acids during anaerobic fermentation reached 545 mg COD / L, which is 329% higher than the blank group (127 mg COD / L) (as in Example 2), which greatly improves the efficiency of sludge resource utilization.

[0026] (2) Outstanding synergistic effect: There is a strong synergistic effect between ultrasound and percarbonamide pretreatment, and the effect is far greater than the sum of the effects of the two individual processes. The concentration of soluble organic matter in the ultrasound-enhanced percarbonamide treatment group was increased by 58% and 140% compared with the ultrasound-treated group and the percarbonamide-treated group alone, respectively, which is 1.35 times the sum of the soluble organic matter after hydrolysis of the two groups (as in Example 2); the acid production in the ultrasound-enhanced percarbonamide treatment group was increased by 71% and 301% compared with the ultrasound-treated group and the percarbonamide-treated group alone, respectively, which is 1.67 times the sum of the acid production of the two groups (as in Example 2), showing a significant synergistic effect compared with the sum of the effects of the two individual processes.

[0027] (3) Reduce costs: The low-dose percarbonamide and ultrasonic synergistic treatment mode (the percarbonamide dosage in this method is 10mg / g TSS) is adopted, which greatly reduces the amount of reagents consumed and significantly reduces operating costs; at the same time, it reduces the environmental risks and safety hazards caused by the introduction of chemical reagents, and achieves a balance between treatment effect, cost control and environmental safety. Attached Figure Description

[0028] Figure 1 The graph shows the changes in the concentration of soluble organic matter during the fermentation process of Example 1 and Comparative Examples 1 to 3 of the present invention.

[0029] Figure 2 This is a graph showing the changes in the total concentration of volatile fatty acids during the fermentation process of Example 1 and Comparative Examples 1 to 3 of the present invention.

[0030] Figure 3 The graph shows the changes in the concentration of soluble organic matter during the fermentation process of Examples 2, 1, 2 and 4 of the present invention.

[0031] Figure 4 This is a graph showing the changes in the total concentration of volatile fatty acids during the fermentation process of Examples 2, 1, 2 and 4 of the present invention.

[0032] Figure 5 The graph shows the changes in the concentration of soluble organic matter during the fermentation process of Examples 3, 1, 2 and 5 of the present invention.

[0033] Figure 6 This is a graph showing the changes in the total concentration of volatile fatty acids during the fermentation process of Examples 3, 1, 2 and 5 of the present invention.

[0034] Figure 7 The graph shows the changes in the concentration of soluble organic matter during the fermentation process of Examples 4, 1, 2 and 6 of the present invention.

[0035] Figure 8 This is a graph showing the changes in the total concentration of volatile fatty acids during the fermentation process of Examples 4, 1, 2 and 6 of the present invention. Detailed Implementation

[0036] To enable those skilled in the art to better understand the technical solutions of the present invention, the preferred embodiments of the present invention are described below in conjunction with specific examples. However, these should not be construed as limiting the present invention and are merely examples.

[0037] Unless otherwise specified, the test methods or experimental methods described in the following examples are conventional methods; unless otherwise specified, the reagents and materials are obtained from conventional commercial sources or prepared by conventional methods.

[0038] Unless otherwise specified, all raw materials used in the embodiments of this invention are commercially available chemical raw materials that are known in the market.

[0039] Example 1

[0040] This embodiment provides a method for promoting anaerobic fermentation and acid production from excess sludge, including the following steps: (1) Raw material pretreatment The sludge was taken from the dewatering unit of a domestic wastewater treatment plant in Wuhan that uses the AAO process. The concentration of the sludge after dewatering was 13.6±1 g / L. The sludge was then adjusted to prepare a sludge suspension with a total suspended solids (TSS) mass fraction of 5% for later use.

[0041] (2) Ultrasonic-percarbamide synergistic treatment Take 300ml of sludge suspension and put it into a fermentation container (serum bottle). Add percarbonamide at a concentration of 5mg / g TSS to the sludge suspension and stir evenly. At the same time, use ultrasound treatment with an ultrasonic intensity of 240W, a frequency of 40KHz, a treatment time of 1h, and a treatment temperature of 25±1℃. During the process, keep the sludge suspension continuously stirred to ensure that the ultrasound and percarbonamide can fully interact.

[0042] (3) Anaerobic fermentation process Add 30 ml of inoculum sludge to the ultrasonically treated sludge suspension. The inoculum sludge is a sludge suspension with a total suspended solids (TSS) mass fraction of 5% prepared in step (1) and a pH value of 6.9-7.3. Introduce nitrogen into the fermentation container for 3 min to completely remove oxygen from the bottle. Then seal the serum bottle to ensure an anaerobic fermentation environment. Control the fermentation temperature at 35±1℃ (this temperature is the optimal range for mesophilic fermentation, which can balance the fermentation cycle and the accumulation efficiency of volatile fatty acids). The total fermentation time is 7 days. Stir the fermentation system regularly during the period to ensure that the microorganisms and the substrate are in full contact.

[0043] Example 2

[0044] The difference between this embodiment and embodiment 1 is that the amount of percarbonamide added in step (2) is 10 mg / g TSS, and the rest is basically the same as in embodiment 1.

[0045] Example 3

[0046] The difference between this embodiment and embodiment 1 is that the amount of percarbonamide added in step (2) is 20 mg / g TSS, and the rest is basically the same as in embodiment 1.

[0047] Example 4

[0048] The difference between this embodiment and embodiment 1 is that the amount of percarbonamide added in step (2) is 40 mg / g TSS, and the rest is basically the same as in embodiment 1.

[0049] Comparative Example 1 The difference between this embodiment (i.e., the blank group) and embodiment 1 is that: in step (2), ultrasonic treatment is not performed and percarbonamide is not added, while the rest is basically the same as embodiment 1.

[0050] Comparative Example 2 The difference between this embodiment (i.e., the ultrasound group alone) and embodiment 1 is that: in step (2), only ultrasound treatment with the same parameters as in embodiment 1 is performed, and percarbonamide is not added, while the rest is basically the same as in embodiment 1.

[0051] Comparative Example 3 The difference between this embodiment (i.e., the percarbamide group alone) and Example 1 is that: in step (2), only 5 mg / g TSS of percarbamide is added and no ultrasonic treatment is performed; otherwise, it is basically the same as Example 1.

[0052] Comparative Example 4 The difference between this embodiment (i.e., the percarbamide group alone) and embodiment 2 is that: in step (2), only 10 mg / g percarbamide TSS is added and no ultrasonic treatment is performed; otherwise, it is basically the same as embodiment 2.

[0053] Comparative Example 5 The difference between this embodiment (i.e., the percarbamide group alone) and Example 3 is that: in step (2), only 20 mg / g percarbamide TSS is added, and ultrasonic treatment is not performed. The rest is basically the same as Example 3.

[0054] Comparative Example 6 The difference between this embodiment (i.e., the percarbamide group alone) and embodiment 4 is that: in step (2), only 40 mg / g percarbamide TSS is added, and ultrasonic treatment is not performed; otherwise, it is basically the same as embodiment 4.

[0055] The index detection methods for each embodiment and comparative example are as follows: Throughout the fermentation cycle, the concentration changes of dissolved organic matter (characterized by dissolved chemical oxygen demand, SCOD) in the fermentation system were periodically sampled and recorded, and its peak value was recorded. At the same time, the total concentration changes of volatile fatty acids (including acetic acid, propionic acid, butyric acid, etc.) were detected and their peak values ​​were recorded. The enhancement effects of different treatment groups were compared and analyzed.

[0056] For a dosage of 5 mg / g TSS percarbamide: (e.g.) Figure 1As shown, the concentration of dissolved organic matter in the ultrasonically enhanced percarbamide treatment group (2402 mg COD / L) was increased by 44% and 155% compared to the ultrasonically treated group (1658 mg COD / L) and the percarbamide treated group (940 mg COD / L) alone, respectively. This was 1.33 times the sum of the dissolved organic matter after hydrolysis of both groups (1658 + 940 - 795, minus one blank group; the calculation method is the same below). Figure 2 As shown, the acid production of the ultrasonically enhanced percarbonamide treatment group (457 mg COD / L) was increased by 44% and 257% compared with the ultrasonic treatment group alone (318 mg COD / L) and the percarbonamide treatment group alone (128 mg COD / L), respectively. It was 1.43 times the sum of the acid production of the two groups (318 + 128 - 127, minus one blank group, the same calculation method below), showing a significant synergistic effect compared with the superposition of the effects of the two individual processes.

[0057] For a dosage of 10 mg / g TSS percarbamide: (e.g.) Figure 3 As shown, ultrasound and percarbonamide pretreatment have a strong synergistic effect, far exceeding the combined effect of either process. The concentration of dissolved organic matter in the ultrasound-enhanced percarbonamide treatment group (2622 mg COD / L) was increased by 58% and 140% compared to the ultrasound-only treatment group (1658 mg COD / L) and the percarbonamide-only treatment group (1089 mg COD / L), respectively, which is 1.35 times the sum of the dissolved organic matter after hydrolysis of the two groups. Figure 4 As shown, the acid production of the ultrasonically enhanced percarbonamide treatment group (545 mg COD / L) was increased by 71% and 301% compared with the ultrasonic treatment group (318 mg COD / L) and the percarbonamide treatment group (136 mg COD / L) alone, respectively. It was 1.67 times the sum of the acid production of the two groups, showing a significant synergistic effect compared with the combined effect of the two individual treatments.

[0058] For a dosage of 20 mg / g TSS percarbamide: (e.g.) Figure 5 As shown, ultrasound and percarbonamide pretreatment have a strong synergistic effect, far exceeding the combined effect of either process. The concentration of dissolved organic matter in the ultrasound-enhanced percarbonamide treatment group (3068 mg COD / L) was increased by 84% and 123% compared to the ultrasound-only treatment group (1658 mg COD / L) and the percarbonamide-only treatment group (1368 mg COD / L), respectively, which is 1.37 times the sum of the dissolved organic matter after hydrolysis of the two groups. Figure 6As shown, the acid production of the ultrasonically enhanced percarbonamide treatment group (603 mg COD / L) was increased by 89% and 138% compared with the ultrasonic treatment group (318 mg COD / L) and the percarbonamide treatment group (253 mg COD / L) alone, respectively. It was 1.35 times the sum of the acid production of the two groups, showing a significant synergistic effect compared with the combined effect of the two individual treatments.

[0059] For a dosage of 40 mg / g TSS percarbamide: (e.g.) Figure 7 As shown, the concentration of dissolved organic matter in the ultrasonically enhanced percarbamide treatment group (4143 mg COD / L) was increased by 149% and 63% compared to the ultrasonically treated group (1658 mg COD / L) and the percarbamide treated group (2540 mg COD / L) alone, respectively, which was 1.21 times the sum of the dissolved organic matter after hydrolysis of the two groups; Figure 8 As shown, the acid production of the ultrasonically enhanced percarbonamide treatment group (813 mg COD / L) was increased by 155% and 96% compared with the ultrasonic treatment group (318 mg COD / L) and the percarbonamide treatment group (414 mg COD / L) alone, respectively. It was 1.34 times the sum of the acid production of the two groups, showing a significant synergistic effect compared with the combined effect of the two individual treatments.

[0060] In summary, under four different dosage conditions (5 mg / g TSS, 10 mg / g TSS, 20 mg / g TSS, and 40 mg / g TSS), the combined treatment of ultrasound and percarbonamide showed a significant and strong synergistic effect, with treatment results far exceeding the combined effect of the two individual processes. Regarding the increase in dissolved organic matter concentration, the effect of ultrasound-enhanced percarbonamide treatment first increased and then decreased with increasing percarbonamide dosage, reaching its highest synergistic effect at a dosage of 20 mg / g TSS (1.37 times the sum of the two). Regarding the increase in acid production, the effect of ultrasound-enhanced percarbonamide treatment first increased and then decreased with increasing percarbonamide dosage, reaching its highest synergistic effect at a dosage of 10 mg / g TSS (1.67 times the sum of the two). This fully demonstrates the synergistic advantage of combining the two processes.

[0061] The above are merely preferred embodiments of the present invention. It should be noted that the above preferred embodiments should not be considered as limitations on the present invention, and the scope of protection of the present invention should be determined by the scope defined in the claims. For those skilled in the art, several improvements and modifications can be made without departing from the spirit and scope of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for promoting anaerobic fermentation and acid production from excess sludge, characterized in that, Includes the following steps: (1) Raw material pretreatment: Prepare the remaining sludge into a sludge suspension; (2) Ultrasonic-percarbamide synergistic treatment: Percarbamide is added to the sludge suspension and ultrasonic treatment is performed simultaneously; (3) Anaerobic fermentation: The ultrasonically treated sludge suspension is mixed with the inoculated sludge for anaerobic fermentation to produce volatile fatty acids.

2. The method according to claim 1, characterized in that, In step (1), the total suspended solids mass fraction of the sludge suspension in the raw material pretreatment is 3-8%.

3. The method according to claim 2, characterized in that, In step (1), the total suspended solids mass fraction of the sludge suspension in the raw material pretreatment is 5%.

4. The method according to claim 1, characterized in that, In step (1), the residual sludge is the residual sludge generated during the treatment of domestic sewage.

5. The method according to claim 1, characterized in that, In step (2), the amount of percarbonamide added is 5 to 40 mg / g of the total suspended solids in the sludge suspension.

6. The method according to claim 5, characterized in that, In step (2), the amount of percarbonamide added is 10 mg / g of the total suspended solids in the sludge suspension.

7. The method according to claim 1 or 5, characterized in that, In step (2), the intensity of the ultrasonic treatment is 200-300W, the frequency is 30-50KHz, the treatment temperature is 20-30℃, and the treatment time is 0.5-2h.

8. The method according to claim 1, characterized in that, In step (3), the inoculated sludge is a sludge suspension prepared in step (1), and the volume of the inoculated sludge is 5 to 15% of the volume of the ultrasonically treated sludge suspension.

9. The method according to claim 1 or 8, characterized in that, In step (3), the fermentation conditions include: a fermentation temperature of 30-40°C, a fermentation time of 5-10 days, and a pH value of 6.5-7.5 in the fermentation system.

10. The method according to claim 9, characterized in that, In step (3), the anaerobic fermentation is carried out in an anaerobic environment, which is achieved by introducing an inert gas into the fermentation container and sealing it.

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