Method for treating chlorella circulating culture solution through Fenton oxidative coupling activated carbon adsorption and application of chlorella circulating culture solution

Abstract

The invention belongs to the technical field of biological energy and environmental engineering, and particularly discloses a method for treating a chlorella circulating culture solution through Fenton oxidative coupling activated carbon adsorption to solve the growth inhibition phenomenon caused by extracellular polymeric substance accumulation in the microalgae circulating culture process so as to achieve cyclic utilization of the chlorella culture solution and application of the chlorella culture solution. By means of Fenton oxidative coupling activated carbon adsorption, chlorella extracellular polymeric substances (EPS) can be effectively degraded and oxidized and decomposed into small-molecule organic substances, the growth inhibition effect of the small-molecule organic substances on chlorella is relieved, an organic carbon source can be provided for growth and metabolism of chlorella, and growth of chlorella is remarkably promoted. According to the method, the chlorella circulating wastewater is treated and recycled through Fenton oxidative coupling activated carbon adsorption, so that the yield of chlorella biomass can be effectively increased, and the production and culture cost is reduced.

Description

Technical Field

[0001] The invention relates to the technical field of bioenergy and environmental engineering, and in particular to a method for treating a circulating culture solution of Chlorella vulgaris by Fenton oxidation coupling activated carbon adsorption and an application thereof. Background Art

[0002] As an ancient microscopic autotrophic organism, Chlorella has been widely used in food, feed, health products, medical treatment, wastewater and waste gas treatment, biodiesel and other fields due to its fast growth rate, high lipid content, strong environmental adaptability, and various bioactive ingredients. However, the insufficient supply of microalgae biomass raw materials has become one of the main challenges facing the bioenergy field.

[0003] Due to economic and environmental issues, in the actual production process, microalgae farming usually adopts semi-continuous and continuous culture methods to circulate the culture solution, but during the culture process, the microalgae secrete a large amount of extracellular polymers (EPS), which causes the circulating culture solution to age and inhibits the growth of microalgae. Therefore, in order to avoid the negative impact of EPS enriched in the harvested culture solution, it is necessary to pre-treat the culture solution before recycling. However, there is currently no relevant patent reporting an efficient method for treating circulating wastewater to remove or degrade EPS in the circulating wastewater, thereby increasing the number of cycles and reducing production and breeding costs. Summary of the invention

[0004] In view of the above problems, the present invention proposes a method for treating the circulating culture solution of Chlorella by Fenton oxidation coupled activated carbon adsorption, which can oxidatively decompose organic matter into small molecular organic matter or convert it into CO2 and H2O by generating strongly oxidizing hydroxyl radicals (·OH). Coupled activated carbon adsorption can adsorb organic matter that is difficult to oxidize and decompose through intermolecular interactions, enhance the removal efficiency of EPS in aquaculture wastewater, and use the treated aquaculture wastewater to cultivate Chlorella, and the biomass is significantly increased. The present invention has the advantages of simple operation, green and environmental protection, low cost and good treatment effect, and provides a theoretical basis for the large-scale circulating wastewater treatment of Chlorella.

[0005] To achieve the above object, the present invention adopts the following technical solution:

[0006] The pH value of the Chlorella aquaculture wastewater to be treated is adjusted to 3.0-3.5, and then FeSO4·7H2O and 30% H2O2 are added to carry out Fenton oxidation reaction; after the Fenton oxidation is completed, the pH value is adjusted to 7.5-8.0 again to terminate the reaction, and the generated Fe(OH)3 precipitate is coagulated and adsorbed, and then filtered and separated to obtain the aquaculture wastewater treated by Fenton oxidation; finally, the obtained treated wastewater is added to granular activated carbon for adsorption, and filtered and separated to obtain the aquaculture wastewater treated by Fenton oxidation coupling activated carbon adsorption; the aquaculture wastewater obtained by Fenton oxidation coupling activated carbon adsorption treatment is used to cultivate Chlorella, and the biomass of Chlorella is significantly increased compared with the untreated wastewater.

[0007] Compared with single Fenton oxidation and activated carbon adsorption, Fenton oxidation coupled with activated carbon adsorption in treating aquaculture wastewater has a more efficient EPS removal effect, and the recycled cultured Chlorella has a higher biomass.

[0008] The method of the present invention has the following advantages:

[0009] The present invention couples Fenton oxidation and activated carbon adsorption to oxidatively decompose organic matter into small molecular organic matter or convert it into CO2 and H2O. At the same time, organic matter that is difficult to oxidatively decompose can be adsorbed through intermolecular interactions. Compared with single Fenton oxidation and activated carbon adsorption, it has a more efficient EPS removal efficiency.

[0010] The present invention realizes the recycling of Chlorella culture solution through Fenton oxidation coupling activated carbon adsorption, has the advantages of simple operation, green environmental protection, low cost and good treatment effect, reduces the treatment cost of circulating culture solution in the process of microalgae cultivation, and significantly improves the biomass accumulation of microalgae. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 This is the growth curve of Chlorella after 4 cycles of cyclic cultivation.

[0012] Figure 2 The change in the yield of EPS secreted into the culture medium during the cyclic culture of Chlorella

[0013] Figure 3 Effect of different granular activated carbon addition concentrations on the removal of EPS from Chlorella circulating aquaculture wastewater

[0014] Figure 4 Effect of different granular activated carbon treatment time on the removal of EPS from Chlorella recycling aquaculture wastewater

[0015] Figure 5 Effect of different FeSO4·7H2O addition amounts on the removal of EPS from Chlorella circulating culture wastewater in Fenton oxidation system

[0016] Figure 6Effect of different H2O2 addition amounts on the removal of EPS from Chlorella circulating aquaculture wastewater in Fenton oxidation system

[0017] Figure 7 Effects of granular activated carbon adsorption, Fenton oxidation, and Fenton oxidation-coupled activated carbon adsorption on the removal of EPS from Chlorella recirculating aquaculture wastewater

[0018] Figure 8 The growth curve of Chlorella culture after treating Chlorella recycling aquaculture wastewater with different treatment methods DETAILED DESCRIPTION

[0019] The following examples are used to illustrate the present invention but are not intended to limit the scope of the present invention.

[0020] Example

[0021] 1. Construction of Chlorella Circulation Culture System

[0022] A. Selection of Chlorella: The Chlorella species selected was Chlorella vulgaris, which was purchased from the Freshwater Algae Bank of the Chinese Academy of Sciences (FACHB);

[0023] B. Seed culture of Chlorella: Use BG11 liquid medium for semi-continuous seed culture, the culture temperature is 25±1℃, the light intensity is 120μmol / (m 2 ·s), photoperiod of 12h:12h, and manual shaking 3 times a day.

[0024] C. Cyclic culture of Chlorella: Chlorella was cultured in a 20 L column photobioreactor. First, Chlorella was cultured to the logarithmic phase, and then centrifuged at 6000 rpm for 15 min to obtain algal cells. Fresh BG11 was used to adjust the algal cell concentration to OD 680 =0.1. The culture temperature was 25±1°C and the light intensity was 120 μmol / (m 2·s), a photoperiod of 12h:12h, air was introduced at a rate of 4L / min, and 4 cycles of cultivation were carried out, each cycle lasting 7d. In addition, on the 7th day, the algal mud and the supernatant were separated by centrifugation at 6000rpm for 15min. The supernatant of the culture solution was filtered through a 0.45μm filter membrane and then recycled for subsequent cultivation. The reused culture medium was supplemented with distilled water to keep its volume consistent. The concentrations of total nitrogen and total phosphorus were determined using Chinese standard methods HJ 636-2012 and GB 11893-1989, respectively. Subsequently, sodium nitrate (NaNO3) and dipotassium hydrogen phosphate (K2HPO4) were added to ensure that the amounts of these essential nutrients were equal in each cycle. Before each recycling process, the pH value of the culture medium was adjusted to 7.0. The TOC of the recycled wastewater (RW) of the aquaculture wastewater obtained after 4 cycles of Chlorella was 24.84mg / L.

[0025] 2. Fenton oxidation and granular activated carbon adsorption treatment

[0026] A. Fenton oxidation: Add FeSO4·7H2O with a content of 0.033% to 0.061% and 30% H2O2 with a content of 0.004% to 0.024% to a 100mL glass beaker containing 50mL of aquaculture wastewater. The specific operation steps are as follows:

[0027] (1) First, dilute sulfuric acid was added to adjust the pH of the circulating wastewater to 3.0-3.5, and magnetic stirring was performed at 200 rpm for 5 min;

[0028] (2) Add a certain amount of FeSO4·7H2O and stir with a magnetic stirrer at 500 rpm to fully dissolve;

[0029] (3) Add a certain amount of H2O2 and stir with magnetic stirring at 700 rpm for 8 h;

[0030] (4) adding 1 mol / L NaOH to adjust the pH to 7.5-8.0 to terminate the Fenton oxidation reaction;

[0031] (5) Fenton coagulation: Based on the above experiment, the mixture was stirred at 200 rpm for 60 min, then allowed to stand for 30 min, and samples were taken and filtered through a 0.45 μm filter to determine TOC;

[0032] B. Granular activated carbon adsorption: Add 0.1% to 2% granular activated carbon to a 100mL conical flask containing 50mL of aquaculture wastewater, and oscillate and adsorb in an air bath constant temperature shaker at 170rpm at a temperature of 25±1°C for 12 hours. Subsequently, the circulating wastewater after adsorption treatment was filtered through a 0.45μm filter membrane and the residual TOC content was determined. The TOC-L measuring instrument detected that the 1% granular activated carbon addition group basically reached the adsorption saturation limit, and the subsequent 1% addition amount was selected to screen the adsorption time. The optimal addition amount was selected to optimize the oscillation time of 30 to 180 minutes, and other conditions remained the same.

[0033] 3. Cultivation of Chlorella after Fenton oxidation coupled activated carbon adsorption treatment

[0034] After the above experimental optimization, the optimal activated carbon adsorption (GAC), Fenton oxidation (Fenton), and Fenton oxidation coupled activated carbon adsorption (GAC+Fenton) were selected as three experimental groups. Fresh BG11 culture (BG11) and untreated recycled wastewater group (RW) were selected as controls. The initial inoculation concentration was OD 680 =0.5, culture temperature 25±1°C, light intensity 120μmol / (m 2 ·s), photoperiod of 12h:12h, and culture time of 7d.

[0035] 4. Results Analysis

[0036] A. Effects of Circulation Culture on Growth and EPS Accumulation of Chlorella

[0037] The growth of Chlorella was characterized by measuring the absorbance of Chlorella at 680 nm in four culture cycles, and samples were taken every day for testing. After each culture, 50 mL of supernatant was collected, concentrated to about 10 mL using a rotary evaporator, and then concentrated and desalted using a 3000 Da dialysis membrane, and finally vacuum-frozen and freeze-dried to obtain freeze-dried EPS, which was weighed.

[0038] According to the above steps, Figure 1 The results showed that as the number of cycles increased, the growth of Chlorella was gradually inhibited. After 7 days of the fourth culture, the biomass yield of Chlorella was only 81% of that of the initial culture. By measuring the EPS yield in the circulating wastewater in each culture cycle, such as Figure 2 The results show that the EPS production of Chlorella gradually increased with the increase of cycle times, and the EPS content in the Chlorella circulating wastewater reached 26.4 mg / L after the fourth culture.

[0039] B. Effect of optimization of Fenton oxidation conditions on the removal of EPS from Chlorella recycling wastewater

[0040] Figure 3 The results show that as the concentration of FeSO4·7H2O increases, the TOC content gradually decreases, and the TOC removal rate of the 0.055% addition group reaches 40.36%. Excessive ferrous ions easily react with hydroxyl radicals to generate Fe(OH)3, which not only consumes H2O2 but also increases the turbidity of the reaction system, resulting in reduced removal efficiency. Therefore, FeSO4·7H2O was selected as 0.055% to screen the amount of H2O2 added. Figure 4 As shown in the figure, with the increase of H2O2 (30%) concentration, TOC content gradually decreased. The TOC content was the lowest in the 0.016% addition group, and the TOC removal rate reached 43.44%. Excessive H2O2 decomposes to produce more ·OH, which removes Fe 2+ Rapidly oxidized to Fe 3+ , resulting in the reaction actually being at Fe 3+ The catalysis of , while consuming H2O2, inhibits the generation of ·OH, and the excess H2O2 and Fe 2+ There is a competitive relationship.

[0041] C. Effect of Optimization of Granular Activated Carbon Conditions on Removal of EPS from Chlorella Cultivation Wastewater

[0042] Figure 5 The results show that as the amount of GAC added increases, the removal rate of TOC of EPS increases. In the GAC=1% group, the TOC is 18.93 mg / L, and its removal rate is 23.79%. As the amount of addition continues to increase, the removal rate slows down. Therefore, GAC=1% was selected to optimize the adsorption time. Figure 6 As shown, with the increase of adsorption time, the TOC removal rate of EPS increased. At T = 90 min, GAC reached saturation, the residual TOC was 18.88 mg / L, and its removal rate was 23.99%.

[0043] D. Effects of different treatment methods on the re-cultivation of Chlorella after treating the wastewater from circulating aquaculture of Chlorella

[0044] Figure 7 The results show that there are differences in the removal effects of EPS in the circulating wastewater of Chlorella by GAC, Fenton, and GAC+Fenton. GAC adsorption can adsorb organic matter in the water through intermolecular interactions, but there is a phenomenon of adsorption saturation. Fenton oxidation produces highly oxidizing hydroxyl radicals (·OH), which oxidize and decompose organic matter into small molecular organic matter or mineralize it into CO2 and H2O. Therefore, coupling the two has a significant removal effect. The removal rate of EPS and TOC in the circulating wastewater of Chlorella treated with GAC+Fenton reached 73.78%.

[0045] After the above experimental optimization, the optimal activated carbon adsorption (GAC), Fenton oxidation (Fenton), Fenton oxidation coupled activated carbon adsorption (GAC+Fenton), fresh BG11 culture (BG11) and untreated recycled wastewater group (RW) were selected for the cultivation verification experiment of Chlorella. Figure 8 As shown, the growth curve of the GAC group is almost the same as that of BG11, which indicates that GAC can effectively adsorb and remove EPS in the circulating wastewater and restore the growth of Chlorella. Fenton oxidation can produce a large number of free radicals, such as hydroxyl radicals OH, which have strong oxidizing properties. After treatment with Fenton and GAC+Fenton, the growth of Chlorella was significantly higher than that of the control groups BG11 and RW. Compared with RW, the biomass of GAC+Fenton was significantly increased by 50.40%. These results show that advanced oxidation methods can not only eliminate growth inhibitors EPS, but also oxidize these inhibitors into small organic molecules that can be absorbed by algal cells, thereby increasing their biomass.

[0046] V. Experimental Summary

[0047] The present invention utilizes Fenton oxidation coupled activated carbon adsorption to treat chlorella recycling aquaculture wastewater to obtain the best EPS removal efficiency, and the recycling and cultivation of chlorella can significantly promote its growth, and its biomass is significantly increased by 50.40% compared with the untreated group. The optimal treatment conditions are: FeSO4·7H2O=0.055% and H2O2(30%)=0.016% are selected for Fenton oxidation treatment, and then granular activated carbon adsorption is performed, GAC=1% adsorption for 90min. The removal efficiency of EPS by GAC+Fenton treatment is 1.70 times that of Fenton treatment and 3.18 times that of GAC treatment. The present invention has important practical and theoretical significance for the recycling cultivation of microalgae and saving precious water resources.

[0048] Although the present invention has been described in detail above by general description and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements can be made to the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all belong to the scope of protection claimed by the present invention.

Claims

1. A method for treating circulating culture fluid of Chlorella by Fenton oxidation coupling activated carbon adsorption and its application, characterized in that: After the wastewater from the recycling aquaculture of Chlorella is treated by Fenton oxidation coupled activated carbon adsorption, the extracellular polymers (EPS) in the aquaculture wastewater are oxidized and decomposed, and can be recycled to cultivate Chlorella, and its growth is restored and the biomass is significantly increased, including the following steps: (1) Fenton oxidation: The pH of the Chlorella aquaculture wastewater to be treated is adjusted to 3.0-3.5, and then FeSO4·7H2O and 30% H2O2 are added to carry out Fenton oxidation reaction; (2) Fenton coagulation: After the Fenton oxidation in step (1) is completed, the pH is adjusted again to 7.5-8.0 to terminate the reaction, and the generated Fe(OH)3 precipitate is subjected to coagulation and adsorption, and then filtered and separated to obtain the aquaculture wastewater after Fenton oxidation treatment; (3) activated carbon adsorption: adding the treated wastewater obtained in step (2) to granular activated carbon for adsorption, and finally filtering and separating to obtain the aquaculture wastewater treated by Fenton oxidation coupling activated carbon adsorption; (4) Recycling of culture fluid: The aquaculture wastewater obtained by Fenton oxidation coupled activated carbon adsorption treatment in step (3) is used to cultivate Chlorella vulgaris, and the biomass of Chlorella vulgaris is significantly increased compared with the untreated wastewater.

2. The method for recycling the culture solution of Chlorella vulgaris by Fenton oxidation coupling activated carbon adsorption according to claim 1, characterized in that: In the step (1), the content of FeSO4·7H2O added during the Fenton oxidation process is 0.033% to 0.061% (m / v), and the content of 30% H2O2 is 0.004% to 0.024% (v / v).

3. The method for recycling the culture solution of Chlorella vulgaris by Fenton oxidation coupled activated carbon adsorption according to claim 1, characterized in that: In the step (2), the amount of granular activated carbon added for adsorption is 0.1% to 2%.

4. The method for recycling the culture solution of Chlorella vulgaris by Fenton oxidation coupling activated carbon adsorption according to claim 1, characterized in that: In the steps (2) and (3), a 0.45 μm microporous filtration membrane is used for filtration separation.

5. The method for recycling the culture solution of Chlorella vulgaris by Fenton oxidation coupling activated carbon adsorption according to claim 1, characterized in that: In the step (3), the activated carbon adsorption time is 30 to 180 minutes.

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