A method for enhancing anaerobic acid production of excess sludge by iron-based gel beads synergistically with persulfate

Abstract

The present invention discloses a method for synergistically enhancing anaerobic acid production of excess sludge by iron-based gel beads and persulfate. The method comprises the following steps: (1) preparing iron-based gel beads; (2) adding persulfate and iron-based gel beads to the excess sludge, and performing pretreatment in a constant temperature shaker to obtain pretreated sludge; (3) adding inoculated sludge to the pretreated sludge and conducting anaerobic fermentation reaction. The present invention activates persulfate by iron-based sodium alginate gel beads to generate reactive oxygen species, so as to accelerate the anaerobic digestion acid production rate of excess sludge. The operation is simple, and the gel beads are convenient for recycling, reducing the treatment cost, and having good environmental benefits and engineering application values.

Description

Technical Field

[0001] The present invention relates to a method for enhancing anaerobic acid production of excess sludge by iron-based gel beads synergistically with persulfate, belonging to the technical field of resource treatment of excess sludge. Background Art

[0002] A large amount of excess sludge is inevitably produced as the main by-product during the wastewater treatment process. If not properly treated, it will not only reduce the effective treatment capacity of the wastewater treatment facilities, but also pose a great threat to the ecological environment and cause a huge waste of resources. Therefore, there is an urgent need for efficient and sustainable alternative solutions for excess sludge treatment to achieve excess sludge reduction and energy conversion.

[0003] Anaerobic digestion is a promising sludge stabilization and safe treatment technology, which has been widely applied to the treatment of excess sludge in wastewater treatment plants because it can reduce the sludge volume and recover renewable energy sources such as methane, biohydrogen and volatile fatty acids. Among them, the production of volatile fatty acids has received increasing attention because it can be used as a biological precursor for the production of bioenergy (i.e., biofuels and bioplastics) and also as the preferred carbon source for promoting biological denitrification. In addition, compared with biogas production, the greenhouse gas emissions during the anaerobic digestion process to produce volatile fatty acids are lower. However, the implementation of anaerobic digestion of volatile fatty acids is limited by its low organic matter hydrolysis rate. Therefore, in recent years, various methods such as mechanical, thermal, biological and chemical additives have been widely developed to improve the production efficiency of volatile fatty acids in the anaerobic digestion of excess sludge.

[0004] It is particularly noteworthy that persulfate is considered an effective method for degrading organic pollutants through its induced oxidation / reduction reaction due to its higher oxidation potential (2.5 - 3.1V), longer half-life (30 - 40 μs) and wider operating pH range. And it has been confirmed that persulfate can improve the biodegradability of excess sludge and provide a good environment for hydrolysis and acidification microorganisms, thereby increasing the production of volatile fatty acids or biogas. However, the amount of free radicals generated by persulfate decomposition alone without external force is extremely limited. Therefore, in the catalytic degradation process, the reaction activity is low, resulting in unnecessary waste of resources. Therefore, external energy (light energy, heat energy, etc.) often needs to be provided or the peroxide bond in the persulfate system is broken and free radicals are generated through the catalysis of transition metals. In particular, catalysts based on transition metals are most commonly used for persulfate activation due to their good performance, low cost and environmental friendliness. However, there are still some defects in practical applications, including the aggregation of nanomaterials, the leaching of harmful ions, environmental pollution, and difficulties in recycling and utilization. Therefore, it is of great engineering application value to develop an efficient, low-cost and sustainable method for promoting anaerobic fermentation of sludge to produce volatile fatty acids. Summary of the Invention

[0005] Objective of the Invention: To solve the problems existing in the prior art, the objective of the present invention is to provide a method for enhancing anaerobic acid production of excess sludge by synergistically using iron-based gel beads and persulfate.

[0006] Technical Solution: The method for enhancing anaerobic acid production of excess sludge by synergistically using iron-based gel beads and persulfate according to the present invention comprises the following steps:

[0007] (1) Prepare iron-based gel beads;

[0008] (2) Add persulfate and iron-based gel beads to the excess sludge, and perform pretreatment in a constant-temperature shaker to obtain pretreated sludge;

[0009] (3) Add inoculated sludge to the pretreated sludge and carry out anaerobic fermentation reaction.

[0010] Among them, in step (2), the mass ratio of persulfate to iron-based gel beads is 1:(1 - 6), preferably 1:4.

[0011] Among them, in step (2), the mass ratio of persulfate to the solid content in the excess sludge is (0.01 - 0.5):1.

[0012] Among them, in step (2), during pretreatment, the rotation speed of the constant-temperature shaker is 100 - 250 rpm / min, the pretreatment temperature is 20 - 30 °C, and the pretreatment time is 1 - 6 h.

[0013] Among them, in step (3), the mass ratio of the inoculated sludge to the solid of the excess sludge is 1:(5 - 20).

[0014] Among them, in step (3), during the anaerobic fermentation reaction, the rotation speed of the constant-temperature shaker is 100 - 250 rpm / min, the anaerobic fermentation reaction temperature is 15 - 50 °C, preferably 35 °C, and the anaerobic fermentation reaction time is 1 - 20 days.

[0015] Among them, in step (1), the preparation of iron-based gel beads comprises the following steps: dropwise add the sodium alginate solution to the iron salt solution under stirring, stand still, wash, and freeze-dry.

[0016] Among them, the iron salt includes one or several of ferric chloride, ferrous chloride, ferric nitrate, ferrous nitrate, and ferric sulfate.

[0017] Among them, the concentration of the sodium alginate solution is 0.25 - 1%, and the concentration of the iron salt solution is 1 - 5%.

[0018] Among them, the volume ratio of the sodium alginate solution to the iron salt solution is 1:(10 - 3).

[0019] Among them, the standing time is 6 - 24 h.

[0020] Under the above conditions, the mass ratio of persulfate to iron-based gel beads is preferably 1:4. The selection principle is as follows: iron-based gel beads provide abundant active sites for the activation of persulfate. If the relative proportion of gel beads is too low, the activation effect is not obvious; if the relative proportion of gel beads is too high, the excess Fe 3+ will consume the generated free radicals, and the gel beads occupy too much space in the anaerobic system, which may lead to system instability. The mass ratio of persulfate to iron-based gel beads is preferably 1:4 in this method.

[0021] Mechanism of the treatment process: Iron-based gel beads can activate persulfate to generate a large number of strongly oxidizing free radicals (such as SO4 · - and · OH), which can destroy the sludge floc structure, rapidly release dissolved organic matter, and shorten the hydrolysis time. In addition, the gel beads have a three-dimensional network structure, which provides a suitable growth environment for fermenting microorganisms, promotes the enrichment of relevant functional flora and interspecies electron transfer of microorganisms, speeds up the reaction rate, and thus greatly increases the production of volatile fatty acids.

[0022] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages:

[0023] (1) The present invention proposes a simple and green method for synthesizing recyclable iron-based gel beads. During the pretreatment of excess sludge, the iron-based gel beads can efficiently activate persulfate, generate a large number of free radicals to accelerate sludge lysis, and improve the biological solubilization rate.

[0024] (2) During the anaerobic digestion process of treating excess sludge in the present invention, the excess sludge after activation treatment promotes the hydrolysis process of excess sludge. Moreover, the synthesized iron-based gel beads can serve as excellent biological carriers to promote the adhesion of anaerobic bacteria and interspecies electron transfer, strengthen the acid production process of sludge, and significantly increase the production of volatile fatty acids.

[0025] (3) The method of the present invention is simple to operate, has high fermentation efficiency, and the iron-based gel beads can be separated and recycled without additional steps, significantly reducing the operating cost. Description of the Drawings

[0026] Figure 1 is a physical diagram of the iron-based gel beads prepared in Example 1;

[0027] Figure 2 is a scanning electron microscope image of the iron-based gel beads prepared in Example 1;

[0028] Figure 3 is a graph of the production of volatile fatty acids in the sludge treated in Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3;

[0029] Figure 4 It is the recycling effect diagram of the iron-based gel beads in Example 3 activating persulfate to promote the volume increase efficiency of excess sludge. Specific implementation manners

[0030] The technical solutions of the present invention will be further described below with reference to the accompanying drawings.

[0031] Example 1 Preparation of iron-based gel beads

[0032] Dissolve 2 g of sodium alginate in 200 mL of deionized water, heat it under the water bath condition at 60 °C, and stir mechanically for 1 h. Dropwise add the sodium alginate solution into 400 mL of 3% iron nitrate solution with a syringe while stirring. Keep the formed gel beads standing in the solution for 12 h, wash them repeatedly with deionized water for 3 times, and finally freeze-dry them in a freeze dryer at -60 °C.

[0033] Take a photo of the iron-based gel beads prepared in this example for observation, and the result is as Figure 1 shown. Figure 1 It is the physical diagram of the iron-based gel beads prepared in Example 1. From Figure 1 this, it can be seen that the iron-based gel beads synthesized in Example 1 are uniform spherical beads with a diameter of about 2.5 - 3 mm.

[0034] Perform scanning electron microscope analysis on the iron-based gel beads prepared in this example, and the result is as Figure 2 shown. Figure 2 It is the scanning electron microscope image of the iron-based gel beads prepared in Example 1. Among them, (a) is the surface morphology diagram of the iron-based gel beads, and (b) is the cross-sectional diagram of the iron-based gel beads. From Figure 2 this, it can be known that the surface of the microbeads is in an irregular gully shape. Its interior is a three-dimensional network structure with a large number of open pores.

[0035] Example 2 Iron-based gel beads as an activator to synergistically strengthen anaerobic acid production of excess sludge with persulfate

[0036] Add 200 mL of excess sludge into a customized 250 mL blue-mouth bottle. The solid content (TS) of the used excess sludge is 2%. Add persulfate and iron-based gel beads. The dosage of persulfate is 0.05 g / g TS (the mass ratio of sulfate to the solid content in the excess sludge is 0.05:1), and the mass ratio of the dosage of persulfate to the iron-based gel bead activator is 1:4. Place the blue-mouth bottle in a constant-temperature shaker, set the temperature to 25 °C, the rotation speed to 150 rpm / min, and the pretreatment time to 2 h. Then add a certain amount of inoculated sludge into the anaerobic bottle. The mass ratio of the added amount of inoculated sludge to the solid content (TS) of the excess sludge is 1:10. Then assemble the customized blue-mouth bottle into an anaerobic reactor, and purge with nitrogen one by one to remove the oxygen in the system, and seal it to maintain anaerobic conditions. The reaction system is placed in a constant-temperature shaker and carried out anaerobic digestion in the dark. The reaction temperature is 35 °C, the shaker rotation speed is 150 rpm / min, and the reaction time is 7 days. At the same time, set Comparative Examples 1-3, specifically as follows:

[0037] Comparative Example 1

[0038] The treatment process is the same as that of Example 2. The difference from Example 2 is that persulfate and iron-based gel beads are not added for pretreatment, and the same amount of sludge is pretreated under the same conditions, and the same amount of inoculated sludge is anaerobically fermented under the same conditions.

[0039] Comparative Example 2

[0040] The treatment process is the same as that of Example 2. The difference from Example 2 is that only persulfate is added and no base gel beads are added for pretreatment. The same amount of sludge is pretreated under the same conditions, and the same amount of inoculated sludge is anaerobically fermented under the same conditions.

[0041] Comparative Example 3

[0042] The treatment process is the same as that of Example 2. The difference from Example 2 is that an equal amount of persulfate is added and the activator used is pure ferric nitrate, and the amount of iron ions in pure ferric nitrate is equal to the amount of iron ions in the iron-based gel beads in Example 2. The same amount of sludge is pretreated under the same conditions, and the same amount of inoculated sludge is anaerobically fermented under the same conditions. The results are as Figure 3 shown.

[0043] Figure 3 It is the production graph of volatile fatty acids in the sludge after treatment in Example 2, Comparative Example 1, Comparative Example 2 and Comparative Example 3; from Figure 3It can be seen that the highest maximum volatile fat content in Example 2 is 2013.1 ± 41.1 mg COD / L. The content of the maximum volatile fatty acid in Comparative Example 1 is 205.5 ± 4.9 mg COD / L, the content of the maximum volatile fatty acid in Comparative Example 2 is 1038.9 ± 30.2 mg COD / L, and the content of the maximum volatile fatty acid in Comparative Example 3 is 1768.8 ± 48.5 mg COD / L. The content of the maximum volatile fatty acid in Comparative Examples 1-3 is significantly less than that in Example 2. It can be seen from this that persulfate activated by iron-based gel beads generates more hydroxyl radicals and sulfate radicals, promoting the lysis of sludge and the release of organic matter. For anaerobic fermentation to produce acid, the hydrolysis of organic matter is the rate-limiting step in sludge anaerobic digestion; the pre-oxidation of persulfate activated by iron-based gel beads cracks the sludge, accelerates the hydrolysis of organic matter, and the gel beads can provide a suitable growth environment for fermentative microorganisms, thereby promoting anaerobic acid production.

[0044] Recycling of iron-based gel beads in Example 3

[0045] Add 200 mL of excess sludge to a customized 250 mL blue-mouth bottle, and the solid content of the excess sludge used is 2%. Add persulfate and iron-based gel beads to each bottle. The addition amount of persulfate is 0.05 g / g TS, and the mass ratio of the dosage of persulfate to the iron-based gel bead activator is 1:4 respectively. Place the blue-mouth bottle in a constant-temperature shaker, set the temperature to 25 °C, and set the rotation speed to 150 rpm / min. Set the pretreatment time to 2 h, and measure the content of soluble organic matter (measured as TOC) during the pretreatment process. After the pretreatment is completed, wash the iron-based gel beads and repeat the above pretreatment test steps. The results are as Figure 4 shown, Figure 4 the recycling effect diagram of the promotion of the volume increase efficiency of excess sludge by persulfate activated by iron-based gel beads in Example 3. It can be Figure 4 seen that persulfate activated by iron-based gel beads promotes the rapid release of sludge organic matter, still has high catalytic activity after being recycled 4 times, and the solubilization efficiency remains above 40%, confirming that the gel beads have good recycling value in promoting anaerobic acid production of sludge.

[0046] Example 4

[0047] Add 200 mL of excess sludge into a customized 250 mL blue-mouth bottle. The solid content (TS) of the excess sludge used is 2%. Add persulfate and iron-based gel beads, and the dosing ratio is 1:1. The addition amount of persulfate is 0.05 g / g TS (the mass ratio of sulfate to the solid content of the excess sludge is 0.05:1). Place the blue-mouth bottle in a constant-temperature shaker, set the temperature to 25 °C, the rotation speed to 150 rpm / min, and the pretreatment time to 2 h. Then add a certain amount of inoculated sludge into the anaerobic bottle. The addition amount of the inoculated sludge is in a mass ratio of 1:10 to the solid content (TS) of the excess sludge. Then assemble the customized blue-mouth bottle into an anaerobic reactor, and purge the system with nitrogen one by one to remove the oxygen in the system, and seal it to maintain anaerobic conditions. Place the reaction system in a constant-temperature shaker and carry out anaerobic digestion in the dark. The reaction temperature is 35 °C, the shaker rotation speed is 150 rpm / min, and the highest volatile fatty acid yield after 7 days of fermentation is 1238.4 ± 69.8 mg COD / L.

[0048] Example 5

[0049] Add 200 mL of excess sludge into a customized 250 mL blue-mouth bottle. The solid content (TS) of the excess sludge used is 2%. Add persulfate and iron-based gel beads, and the dosing ratio is 1:2. The addition amount of persulfate is 0.05 g / g TS (the mass ratio of sulfate to the solid content of the excess sludge is 0.05:1). Place the blue-mouth bottle in a constant-temperature shaker, set the temperature to 25 °C, the rotation speed to 150 rpm / min, and the pretreatment time to 2 h. Then add a certain amount of inoculated sludge into the anaerobic bottle. The addition amount of the inoculated sludge is in a mass ratio of 1:10 to the solid content (TS) of the excess sludge. Then assemble the customized blue-mouth bottle into an anaerobic reactor, and purge the system with nitrogen one by one to remove the oxygen in the system, and seal it to maintain anaerobic conditions. Place the reaction system in a constant-temperature shaker and carry out anaerobic digestion in the dark. The reaction temperature is 35 °C, the shaker rotation speed is 150 rpm / min, and the highest volatile fatty acid yield after 7 days of fermentation is 1529.9 ± 72.7 mg COD / L.

[0050] Example 6

[0051] Add 200 mL of excess sludge to a customized 250 mL blue-mouth bottle. The solid content (TS) of the excess sludge used is 2%. Add persulfate and iron-based gel beads, and the dosing ratio is 1:6. The addition amount of persulfate is 0.05 g / g TS (the mass ratio of sulfate to the solid content of the excess sludge is 0.05:1). Place the blue-mouth bottle in a constant temperature shaker, set the temperature to 25 °C, the rotation speed to 150 rpm / min, and the pretreatment time to 2 h. Then add a certain amount of inoculated sludge to the anaerobic bottle, and the mass ratio of the addition amount of the inoculated sludge to the solid content (TS) of the excess sludge is 1:10. Then assemble the customized blue-mouth bottle into an anaerobic reactor, and purge the system with nitrogen one by one to remove the oxygen in the system, and seal it to maintain anaerobic conditions. The reaction system is placed in a constant temperature shaker for light-shielded oscillating reaction. The reaction temperature is 35 °C, the shaker rotation speed is 150 rpm / min, and the highest volatile fatty acid yield after 7 days of fermentation is 1189.4 ± 53.5 mg COD / L.

Claims

1. A method for enhancing anaerobic acid production of excess sludge by synergistically using iron-based gel beads and persulfate, characterized in that, It includes the following steps: (1) Prepare iron-based gel beads: Dropwise add the sodium alginate solution into the iron salt solution under stirring, let it stand, wash, and freeze-dry; (2) Add persulfate and iron-based gel beads to the excess sludge, and pretreat it in a constant temperature shaker to obtain pretreated sludge. The mass ratio of persulfate to iron-based gel beads is 1:1 - 6; (3) Add inoculated sludge to the pretreated sludge and carry out anaerobic fermentation reaction.

2. The method for enhancing anaerobic acid production of excess sludge by iron-based gel beads in synergy with persulfate according to claim 1, characterized in that In step (2), the mass ratio of persulfate to the solid content in the excess sludge is 0.01 - 0.5:

1.

3. The method for enhancing anaerobic acid production of excess sludge by synergistic effect of iron-based gel beads and persulfate according to claim 1, characterized in that, In step (2), during pretreatment, the rotation speed of the constant temperature shaker is 100 - 250 rpm / min, the pretreatment temperature is 20 - 30 °C, and the pretreatment time is 1 - 6 h.

4. The method for enhancing anaerobic acid production of excess sludge by iron-based gel beads in synergy with persulfate according to claim 1, wherein In step (3), the mass ratio of the inoculated sludge to the solid of the excess sludge is 1:5 - 20.

5. The method for enhancing anaerobic acid production of excess sludge by iron-based gel beads synergistically with persulfate according to claim 1, characterized in that In step (3), during the anaerobic fermentation reaction, the rotation speed of the constant temperature shaker is 100 - 250 rpm / min, the anaerobic fermentation reaction temperature is 15 - 50 °C, and the anaerobic fermentation reaction time is 1 - 20 days.

6. The method for enhancing anaerobic acid production of excess sludge by iron-based gel beads synergistically with persulfate according to claim 1, characterized in that, The iron salt includes one or more of ferric chloride, ferrous chloride, ferric nitrate, ferrous nitrate, and ferric sulfate.

7. The method for enhancing anaerobic acid production of excess sludge by iron-based gel beads synergistically with persulfate according to claim 1, wherein, The concentration of the sodium alginate solution is 0.25 - 1%, and the concentration of the iron salt solution is 1 - 5%.

8. The method for enhancing anaerobic acid production of excess sludge by synergistic action of iron-based gel beads and persulfate according to claim 1, characterized in that, The volume ratio of the sodium alginate solution to the iron salt solution is 1:10 - 3.

9. The method for enhancing anaerobic acid production of excess sludge by iron-based gel beads synergistically with persulfate according to claim 1, wherein The standing time is 6 - 24 h.

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