Electrolysis pretreatment method for improving the biodegradability of blue-green algae

Abstract

The application discloses an electrolysis pretreatment method for improving the biodegradability of blue algae, and belongs to the field of wastewater treatment and organic waste resource disposal. The method for electrolysis pretreatment of blue algae comprises the following steps: (1) dissolving blue algae powder in water, fully shaking and centrifuging, removing supernatant, diluting with water, and obtaining blue algae liquid; then adding Na2SO4 electrolyte into the blue algae liquid, stirring uniformly, and obtaining electrolyte; (2) using a stainless steel plate as an anode and a cathode, adopting a direct current power supply to supply power, continuously stirring the electrolyte, and performing electrolysis pretreatment to obtain blue algae solution after pretreatment. The blue algae solution after pretreatment obtained by the method has high soluble organic matter content, and when used for anaerobic fermentation, not only can the yield of organic acid be improved, but also the time for anaerobic fermentation to reach the maximum yield of organic acid can be shortened.

Description

Technical Field

[0001] This invention relates to an electrolytic pretreatment method for improving the biodegradability of cyanobacteria, belonging to the field of wastewater treatment and resource utilization of organic waste. Background Technology

[0002] In recent years, Taihu Lake has experienced continuous cyanobacterial blooms, with massive quantities of algae produced that are difficult to control effectively in the short term. Consequently, the hundreds of thousands of tons of algae harvested annually occupy vast amounts of land for stockpiling, and the leachate produced by these stockpiles causes secondary pollution through infiltration and rainwater runoff. Therefore, anaerobic fermentation of algae to produce acid is a rapid and efficient method for resource utilization, possessing significant economic and social implications.

[0003] Anaerobic fermentation for the resource utilization of cyanobacteria has broad prospects and practical significance. However, the unique rigid structure of cyanobacterial cells leads to low efficiency in anaerobic fermentation. To enhance the biodegradability of cyanobacterial wastewater, certain pretreatment of the cyanobacteria is necessary. On the one hand, this pretreatment promotes cell wall rupture and the dissolution of intracellular contents, accelerating the hydrolysis process of cyanobacteria and shortening the fermentation time. On the other hand, it transforms recalcitrant substances in cyanobacteria into readily degradable substances and insoluble substances into readily soluble substances, increasing the concentration of substrates available to acid-producing bacteria and thus improving acid production.

[0004] Anaerobic fermentation includes hydrolysis, acid production, and methanogenesis. Because the cell walls of cyanobacteria inhibit the hydrolysis rate, they are not easily broken under fermentation conditions, making the hydrolysis stage the primary rate-limiting stage. To enhance the hydrolysis process and improve the efficiency of anaerobic fermentation, pretreatment is necessary. The purpose of pretreatment is to fully dissolve and release the organic matter in the algal residue. These released soluble substances from the algal cells are more readily utilized by microorganisms.

[0005] Currently, publicly available anaerobic fermentation pretreatment methods for cyanobacteria include acid method, hot alkaline method, and microwave method. However, these methods still have problems such as the need to invest a large amount of chemical reagents, significant loss of organic matter, and high cost.

[0006] Therefore, there is an urgent need to develop efficient and environmentally friendly anaerobic fermentation pretreatment methods for cyanobacteria. Summary of the Invention

[0007] [Technical Issues]

[0008] Currently, the anaerobic fermentation pretreatment of cyanobacteria has problems such as low biochemical properties of soluble organic matter in cyanobacteria and the need to invest a large amount of chemical reagents, resulting in high costs.

[0009] [Technical Solution]

[0010] To address the aforementioned problems, this invention processes cyanobacteria sludge to obtain cyanobacteria powder; then dissolves and centrifuges it to obtain an algal solution; the algal solution is then mixed with Na2SO4 electrolyte to obtain an electrolyte solution; electrolytic pretreatment is then performed in an electrochemical system using stainless steel plates as both cathode and anode to obtain a pretreated cyanobacteria solution; finally, the pretreated cyanobacteria is readily used for anaerobic fermentation to produce acid. The pretreated cyanobacteria solution obtained by this invention has a high content of soluble organic matter, resulting in a high acid production rate during anaerobic fermentation.

[0011] The first objective of this invention is to provide a method for electrolytic pretreatment of cyanobacteria, comprising the following steps:

[0012] (1) Electrolyte preparation:

[0013] Dissolve cyanobacteria powder in water, shake well, centrifuge, remove the supernatant, dilute with water to obtain cyanobacteria solution; then add Na2SO4 electrolyte to the cyanobacteria solution, stir well to obtain electrolyte solution;

[0014] (2) Electrolytic pretreatment:

[0015] Stainless steel plates were used as the cathode and anode, and a DC power supply was used. The electrolyte was continuously stirred, and electrolytic pretreatment was carried out to obtain a pretreated cyanobacteria solution.

[0016] In one embodiment of the present invention, the preparation method of the cyanobacteria powder in step (1) is as follows:

[0017] The cyanobacteria mud obtained from the algae-water separation station is drained in a 500-mesh filter cloth, then dried at 60°C. After drying, it is ground and sieved (80 mesh) to obtain cyanobacteria powder. The cyanobacteria mud is taken from the cyanobacteria harvesting site at Lujiangkou in Binhu District, Wuxi.

[0018] In one embodiment of the present invention, the mass ratio of cyanobacteria powder to water in step (1) is 1:5-20.

[0019] In one embodiment of the present invention, the concentration of cyanobacterial cells in the cyanobacterial solution in step (1) is (4.5-5.5)×10⁻⁶. 6 cells / mL.

[0020] In one embodiment of the present invention, the centrifugation and removal of supernatant of the cyanobacteria water in step (1) can be repeated several times; wherein the centrifugation is centrifugation at a speed of 5000 rpm for 10 minutes.

[0021] In one embodiment of the present invention, the concentration of Na2SO4 electrolyte in step (1) is 5-15 mM / mL.

[0022] In one embodiment of the present invention, the stainless steel plate mentioned in step (2) comprises 316 stainless steel and has a size of 50mm × 80mm.

[0023] In one embodiment of the present invention, in step (2), the DC power supply provides a voltage and current of 0-25V and 0-8A.

[0024] In one embodiment of the present invention, the current density in the electrolytic pretreatment in step (2) is 10-100 mA / cm². 2 The electrolysis time is 10-60 min.

[0025] In one embodiment of the present invention, stirring is maintained during the electrolytic pretreatment in step (2), and the stirring speed is 100-300 rpm.

[0026] The second objective of this invention is to prepare a pretreated cyanobacteria solution using the method described herein.

[0027] The third objective of this invention is to provide a method for anaerobic fermentation to produce acid, which uses the pretreated cyanobacteria solution described in this invention.

[0028] In one embodiment of the present invention, the method for producing acid through anaerobic fermentation involves inoculating a pretreated cyanobacteria solution into domesticated sludge for anaerobic fermentation.

[0029] In one embodiment of the present invention, the anaerobic fermentation acid-producing acclimatized sludge is anaerobic granular sludge, taken from an IC reactor in operation at a citric acid plant in Wuxi City. The granular sludge undergoes heat pretreatment and activation treatment before inoculation. The heat pretreatment involves heat-treating the granular sludge (121°C, 15 min) to kill methanogenic bacteria and leave spores of organic acid-producing bacteria. The activation involves culturing the heat-treated sludge in a glucose solution with a COD of 5000 mg / L for one week to allow the spores of organic acid-producing bacteria to germinate, thereby restoring and enhancing the activity of the sludge to achieve a better fermentation effect.

[0030] In one embodiment of the present invention, the volume ratio of the pretreated cyanobacteria solution to the domesticated sludge in the anaerobic fermentation acid production process is 4-6:1.

[0031] In one embodiment of the present invention, the conditions for anaerobic fermentation to produce acid are as follows: after mixing the pretreated cyanobacteria solution and the acclimated sludge, the mixture is placed in an opaque container and anaerobic fermentation is carried out at 35°C and 300 rpm.

[0032] [Beneficial Effects]

[0033] (1) This invention uses waste biomass algal mud as raw material and achieves efficient and rapid resource utilization of waste through its anaerobic fermentation to produce acid. It also discloses the effect of electrolytic pretreatment on the structure of cyanobacteria cells. After electrochemical pretreatment, the soluble organic matter increased significantly and the dissolved chemical oxygen demand (SCOD) reached 2000 mg / L.

[0034] (2) The electrolytic pretreatment method of this invention not only increases the acid production of anaerobic fermentation of cyanobacteria, but also improves the quality of acid. The organic acid production of the electrolytic pretreatment group is 4664 mg / L, which is higher than 85% of that of the blank group.

[0035] (3) In this invention, under the action of charge neutralization and electroadsorption mechanism, algal cells with negative surface charge rapidly gather towards the anode and attach to the electrode surface. Under the continuous current stimulation and the action of the anode, the algal cell membrane is gradually damaged, causing the algal organic matter to leak.

[0036] (4) The electrolytic pretreatment used in this invention can promote the release of organic matter within cyanobacterial cells, increase the opportunity for anaerobic fermentation microorganisms to come into contact with organic matter, and help to quickly start anaerobic fermentation of cyanobacteria. Therefore, it can be considered that electrolytic pretreatment can not only increase the yield of organic acids, but also shorten the time for anaerobic fermentation to reach the maximum yield of organic acids. Attached Figure Description

[0037] Figure 1 The effects of different pretreatment methods on the dissolution of organic matter in cyanobacteria. Detailed Implementation

[0038] The preferred embodiments of the present invention are described below. It should be understood that the embodiments are for better explanation of the present invention and are not intended to limit the present invention.

[0039] Test method:

[0040] 1. Determination of dissolved chemical oxygen demand (SCOD):

[0041] After centrifuging the sample at 5000g for 10min, the SCOD of the supernatant was measured, and the value obtained was the dissolved chemical oxygen demand.

[0042] 2. Determination of organic acids:

[0043] The liquid chromatograph (Agilent 1100, USA) was used, and the column was a ZORBAX SB-Aq column.

[0044] Raw materials used in the examples:

[0045] 1. Blue-green algae powder:

[0046] Blue-green algae mud was obtained from the algae-water separation station at the Lujiangkou Blue-green Algae Harvesting Site in Binhu District, Wuxi. The water was drained through a 500-mesh filter cloth, and then dried at 60°C. After drying, it was thoroughly ground and sieved (80 mesh) to obtain blue-green algae powder.

[0047] 2. Sludge acclimatization:

[0048] Specifically, the anaerobic granular sludge was taken from an IC reactor that was in operation at a citric acid plant in Wuxi City.

[0049] Before inoculation, the anaerobic granular sludge undergoes heat pretreatment and activation treatment. The heat pretreatment involves heat-treating the anaerobic granular sludge (121℃, 15min) to kill the methanogenic bacteria and leave the spores of organic acid-producing bacteria. Activation involves culturing the heat-treated sludge in a glucose solution with a COD of 5000mg / L for one week to allow the spores of organic acid-producing bacteria to germinate, thereby restoring and enhancing the activity of the sludge and achieving a better fermentation effect.

[0050] The main properties of cyanobacterial sludge and anaerobic granular sludge are shown in Table 1:

[0051] Table 1 Properties of Cyanobacterial Sludge and Acclimated Sludge

[0052] raw material TS (%) VS(%) <![CDATA[TN(mg·g -1 )]]> <![CDATA[TP(mg·g -1 )]]> Blue algae mud 3.95 3.46 2.62 0.76 Anaerobic granular sludge 13.58 10.62 1.81 0.48

[0053] Example 1

[0054] A method for electrolytic pretreatment of cyanobacteria includes the following steps:

[0055] (1) Preparation of electrolyte:

[0056] Dissolve 5g of cyanobacteria powder in 50g of distilled water, centrifuge at 5000rpm for 10min, collect the supernatant, and then dilute with sterile deionized water to obtain cyanobacteria solution. The concentration of cyanobacteria cells in the solution is 5.0×10⁻⁶. 6 cells / mL; then Na2SO4 was added to the cyanobacteria solution as an electrolyte, and the sodium sulfate concentration was increased to 10mM. The solution was stirred evenly to form an electrolyte.

[0057] (2) Electrolytic pretreatment:

[0058] The experiment was conducted in a 250 mL beaker. A 50 mm × 80 mm stainless steel 316 anode plate and a 50 mm × 80 mm stainless steel 316 cathode plate were placed parallel to each other completely in the 250 mL electrolyte. The experiment was powered by a DC power supply (providing 0–25 V and 0–8 A voltage and current), with stirring maintained at 150 rpm throughout the process. The current density was 10 mA / cm². 2 Electrolysis was performed for 15 minutes under the specified conditions to obtain a pretreated cyanobacteria solution.

[0059] Example 2 Parameter Optimization

[0060] Parameter 1: Current density

[0061] Adjust the current density in step (2) of Example 1 to 10 mA / cm². 2 50mA / cm 2 100mA / cm 2 Everything else remained the same as in Example 1, resulting in a pretreated cyanobacteria solution.

[0062] The pretreated cyanobacteria solution was tested, and the results are shown in Table 2.

[0063] Table 2 shows that the SCOD of the pretreated cyanobacteria solution increases with increasing current density. Considering economic costs, increasing the current density from 10 to 50 (a 5-fold increase) only improved the effect by 33%, so a current density of 10 mA / cm² was chosen. 2 .

[0064] Table 2 Effect of Current Density

[0065] <![CDATA[Current density (mA / cm 2 )]]> SCOD (mg / L) 10 (Example 1) 2108 50 2806 100 3220 Untreated (cyanobacteria solution) 537

[0066] Parameter 2: Electrolysis time

[0067] The electrolysis time in step (2) of Example 1 was adjusted to 15 min, 30 min, and 60 min; other steps remained the same as in Example 1, and a pretreated cyanobacteria solution was obtained.

[0068] The pretreated cyanobacteria solution was tested, and the results are shown in Table 3.

[0069] Table 3 shows that the SCOD of the pretreated cyanobacteria solution increases with increasing electrolysis time. Considering economic costs, the SCOD concentration increased by 293% compared to the blank control group after 15 min of reaction; it increased by 367% after 30 min of reaction; doubling the reaction time only improved the effect by 16%; therefore, the final electrolysis time was selected as 15 min.

[0070] Table 3 Effect of Electrolysis Time

[0071] Electrolysis time (min) SCOD (mg / L) 15 (Example 1) 2108 30 2508 60 2984 Untreated (cyanobacteria solution) 537

[0072] Parameter 3: Electrolyte concentration

[0073] The electrolyte concentrations in Example 1 were adjusted to 5mM, 10mM, and 15mM, respectively; all other parameters remained the same as in Example 1, resulting in a pretreated cyanobacteria solution.

[0074] The pretreated cyanobacteria solution was tested, and the results are shown in Table 4.

[0075] Table 4 Effect of Electrolyte Concentration

[0076] Electrolyte concentration (mM) SCOD (mg / L) 5 1023 10 2108 15 2654

[0077] Table 4 shows that the SCOD of the pretreated cyanobacteria solution increases with increasing electrolyte concentration. Considering economic costs, increasing the electrolyte concentration from 5 to 10 doubles the concentration, but the SCOD only increases by 25.9%; therefore, a final electrolyte concentration of 20 mM was selected.

[0078] Example 3

[0079] A method for anaerobic fermentation to produce acid, wherein the method employs a batch fermentation process, as detailed below:

[0080] 250 mL of the pretreated cyanobacteria solution obtained in Example 1 was inoculated into 60 mL of acclimatized sludge (anaerobic granular sludge), placed in a 600 mL opaque Erlenmeyer flask, and purged with N2 for 2 min to remove the air above the flask. The Erlenmeyer flask was then sealed with a rubber stopper and placed in a constant temperature water bath shaker for anaerobic fermentation to produce acid at 35 °C and 300 rpm. The concentration of organic acid in the Erlenmeyer flask was measured after 14 days.

[0081] Comparative Example 1: Blank Group

[0082] The cyanobacterial solution after pretreatment in Example 3 was adjusted to be an untreated cyanobacterial solution, while all other aspects remained the same as in Example 3. Anaerobic fermentation was then carried out as a blank group.

[0083] Comparative Example 2: Effect of Hot Alkali Pretreatment on Anaerobic Fermentation

[0084] (1) Hot alkali pretreatment: Dissolve 5g of cyanobacteria powder in 50g of water, and add NaOH to make the pH values ​​9, 10, 11 and 12 respectively. Stir gently to mix evenly. After reacting at room temperature for 24h, adjust the pH with hydrochloric acid to make it between 4.9 and 5.1. Finally, place it in an 80℃ sterilizer and heat for 30min to obtain the alkali-treated cyanobacteria solution.

[0085] (2) Anaerobic fermentation: The alkaline-treated cyanobacteria solution was inoculated into acclimatized sludge (anaerobic granular sludge) for anaerobic fermentation, which was consistent with Example 3.

[0086] The hot-alkali treatment method combines heat treatment and alkali treatment. The hot-alkali method increased production by approximately 200% compared to the heat treatment method alone (without alkali), indicating that the addition of alkali played a significant role. After hot-alkali treatment, the yield of organic acids increased across the board, with the highest yield at pH 10, nearly doubling that of the control group. At pH 12, acid production decreased slightly compared to the control group, possibly because the overly alkaline environment caused excessive denaturation of the cyanobacteria, reducing the concentration of available organic matter in the substrate for microbial use.

[0087] Comparative Example 3: Effect of Acid Pretreatment on Anaerobic Fermentation

[0088] (1) Acid pretreatment: Dissolve 5g of cyanobacteria powder in 50g of water, then add HCl to make the pH values ​​2, 3, 4 and 5 respectively, and stir gently to mix evenly; after reacting at room temperature for 24h, adjust the pH with NaOH to make it between 4.9-5.1 to obtain the acid-treated cyanobacteria solution.

[0089] (2) Anaerobic fermentation: The acid-treated cyanobacteria solution is inoculated into acclimatized sludge (anaerobic granular sludge) for anaerobic fermentation, which is consistent with the specific embodiment 3.

[0090] Adjusting the pH value with acid can accelerate the dissolution of cell walls. In addition, soaking cyanobacteria with a certain amount of acid before anaerobic fermentation helps dissolve the organic matter in the cyanobacteria, turning it into smaller molecules that are more readily utilized by microorganisms, thus improving the efficiency of anaerobic digestion of cyanobacteria.

[0091] Appropriate acid pretreatment significantly increased the yield of organic acids. The increase was particularly pronounced at pretreatment pH 4 and 5, indicating that milder acid pretreatment effectively promoted the anaerobic fermentation of cyanobacteria. However, at pH 2 and 3, pretreatment not only failed to increase the yield of organic acids but also resulted in lower yields than the control group. This may be because the extremely acidic conditions severely damaged or decomposed the organic matter in the cyanobacteria, greatly reducing the readily available components and ultimately leading to low yields.

[0092] Comparative Example 4: Effect of Microwave Pretreatment on Anaerobic Fermentation for Biogas Production

[0093] (1) Microwave pretreatment: Dissolve 5g of blue-green algae powder in 50g of water, then put it into a microwave oven and treat it for 3min, 4min and 5min respectively under medium heat. Stir well and set aside.

[0094] (2) Anaerobic fermentation: Microwave-treated cyanobacteria are inoculated into domesticated sludge (anaerobic granular sludge) for anaerobic fermentation, consistent with the specific embodiment 3.

[0095] Microwave pretreatment for 3 min, 4 min, and 5 min all increased the yield of organic acids from anaerobic fermentation of cyanobacteria, but the increase was not significant, and the effects were not very different.

[0096] The test data for Example 3 and Comparative Examples 1 (blank group), 2 (pH=10), 3 (pH=4), and 4 (4 min) are shown in Table 5:

[0097] Table 5. Experimental results of Example 3 and Comparative Examples 1-4

[0098] Organic acid yield from anaerobic fermentation (mg / L) Percentage increase compared to blank (%) Example 3 4664 85 Comparative Example 1 (Blank Group) 2521 / Comparative Example 2 4084 62 Comparative Example 3 3630 44 Comparative Example 4 3983 58

[0099] Table 5 shows that, comparing the effects of the four pretreatment methods on the organic acid yield after anaerobic fermentation of cyanobacteria, each pretreatment method can increase the organic acid yield to varying degrees, indicating that pretreatment can maximize the dissolution of organic matter in cyanobacterial cells, thereby improving fermentation efficiency and acid production potential. The organic acid yields after electrochemical treatment, hot alkaline treatment, acid treatment, microwave treatment, and untreated anaerobic fermentation of cyanobacteria were increased by 85%, 62%, 44%, and 58%, respectively, compared to the control group. Among the four pretreatment methods, electrolytic pretreatment resulted in the highest organic acid yield.

[0100] Comparative Example 5

[0101] The anode plate in step (2) of Example 1 was adjusted to be ruthenium-iridium-titanium, while the rest remained the same as in Example 1, to obtain a pretreated cyanobacteria solution; then, anaerobic fermentation to produce acid was carried out according to Example 3.

[0102] Test data is shown in Table 6:

[0103] As can be seen from Table 6, under the same conditions, the effect of using ruthenium-iridium titanium plates is not much different from the results obtained in Experiment 1. However, in terms of economic efficiency, 316 stainless steel plates (2mm, 428 yuan / m) are more cost-effective. 2 The price is much lower than that of ruthenium-iridium titanium plate (2mm, 24,000 yuan / m). 2 Therefore, this invention ultimately selects stainless steel plates to electrolyze cyanobacteria, and then inoculates the pretreated cyanobacteria solution into acclimatized sludge (anaerobic granular sludge) for fermentation.

[0104] Table 6 Test data for Example 3 and Comparative Example 5

[0105] Electrode material SCOD (mg / L) Acid production from anaerobic fermentation (mg / L) <![CDATA[Unit price of electrode plate (yuan / m 2 )]]> Stainless steel sheet (Example 1) 2108 4664 428 Ruthenium-iridium titanium plate (Comparative Example 5) 2311 4982 24000

[0106] Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Anyone skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention should be determined by the claims.

Claims

1. A method for electrolytic pretreatment of cyanobacteria, characterized in that, Includes the following steps: (1) Electrolyte preparation: Dissolve cyanobacteria powder in water, shake well, centrifuge, remove the supernatant, dilute with water to obtain cyanobacteria solution; then add Na2SO4 electrolyte to the cyanobacteria solution, stir well to obtain electrolyte; (2) Electrolytic pretreatment: Stainless steel plates were used as the cathode and anode, and a DC power supply was used. The electrolyte was continuously stirred, and electrolytic pretreatment was carried out to obtain a pretreated cyanobacteria solution. In step (1), the concentration of Na2SO4 electrolyte is 5-15 mM / mL; In step (2), the current density during the electrolytic pretreatment is 10 - 100 mA / cm². 2 The electrolysis time is 10-60 min.

2. The method according to claim 1, characterized in that, Step (1) The concentration of cyanobacterial cells in the cyanobacterial solution is (4.5-5.5) × 10⁻⁶. 6 cells / mL.

3. The method according to claim 1, characterized in that, The stainless steel plate mentioned in step (2) includes 316 stainless steel and has a size of 50 mm × 80 mm.

4. The pretreated cyanobacteria solution prepared by the method according to any one of claims 1-3.

5. A method for anaerobic fermentation to produce acid, characterized in that, It uses the pretreated cyanobacteria solution as described in claim 4.

6. The method according to claim 5, characterized in that, The method for producing acid through anaerobic fermentation involves inoculating a pretreated cyanobacteria solution into domesticated sludge for anaerobic fermentation.

7. The method according to claim 6, characterized in that, The volume ratio of the pretreated cyanobacteria solution to the acclimated sludge in the anaerobic fermentation acid production process is 4-6:

1.

8. The method according to claim 6, characterized in that, The conditions for anaerobic fermentation to produce acid are as follows: after mixing the pretreated cyanobacteria solution and the acclimated sludge, the mixture is placed in an opaque container and anaerobic fermentation is carried out at 35°C and 300 rpm.

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