Method for treating furfural wastewater through electric fermentation

Abstract

The invention relates to a method for treating furfural-containing wastewater through electric fermentation. By applying 0.4-1.6 V voltage, the removal of furfural in the wastewater and the yield of biological methane are improved. Compared with an open circuit (0.0 V), 1.2 V can significantly enhance furfural degradation and methane yield. And a compact biological membrane is formed on the conductive carbon cloth electrode. In contrast, higher voltage (1.6 V) is found to inhibit microbial activity, resulting in reduced furfural degradation and reduced methane production. The invention provides a new method for efficient removal and resource recovery of furfural and other toxic furan derivatives in electric fermentation.

Description

Technical Field

[0001] The present invention relates to a method for electro-fermentation treatment of furfural wastewater, specifically, by applying an external voltage to regulate the anaerobic fermentation process, effectively removing furfural and generating methane, which can be used for the resource utilization of furan-containing organic wastes in biomass refining wastewater. Background Art

[0002] Furfural is an important organic chemical raw material. Generally, the furfural production process is as follows: The non-mildewed and non-decomposed corncobs are crushed and dried, and then evenly mixed with dilute sulfuric acid with a mass fraction of 5%. Hydrolysis is carried out at 145 - 230 °C and 0.8 MPa. A small amount of acetic acid contained in the furfural vapor after hydrolysis is neutralized with soda solution, and the furfural vapor enters the condenser for condensation. The dilute furfural solution enters from the middle of the rectifying column, and the bottom of the column is heated with indirect steam. The aldehyde-containing vapor concentrated through multiple evaporations is led out from the top of the column, and the residue is discharged from the bottom of the column. The vapor led out from the upper part of the column enters the condenser, and after condensation, it enters the crude furfural collector. The product in the collector is divided into two layers: the lower layer is oily furfural with a concentration of up to 90%, that is, crude furfural; the upper layer is a saturated solution of furfural dissolved in water, containing 7 - 10% of furfural. During the operation, the temperature at the top of the column should be maintained at 94 - 97 °C, the temperature at the bottom of the column at 98 - 102 °C, and the temperature of the distillate below 55 °C. The crude furfural is refined by vacuum distillation or steam distillation to obtain the final product (Xu Yu, Xiao Chuanhao, Yu Yinghui. Furfural production process technology and prospect [J]. Journal of Puyang Vocational and Technical College, 2010, 23(04): 150 - 152.). In the above process, organic wastewater containing furfural and its derivatives will be generated at the bottom of the column (COD is about 11000 - 16500 mg / L, and the furfural content is 0.2% - 0.5%). This wastewater has the characteristics of strong toxicity and difficult biodegradation. If directly discharged without effective treatment, it will cause serious harm to the water environment and ecological system.

[0003] Currently, wastewater treatment methods include: physical adsorption, chemical oxidation, biological fermentation, etc. Among them, biological treatment is widely used because of its environmental friendliness and low operating cost. However, since furfural belongs to furan organic compounds and has a significant toxic inhibitory effect on microorganisms, the efficiency of traditional anaerobic fermentation for treating furfural wastewater is low, the startup time is long, and the reaction system is prone to collapse. Therefore, it is urgent to develop a new type of furfural wastewater resource treatment technology with high treatment efficiency and strong system stability. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for efficiently removing furfural and producing methane from wastewater by stimulating anaerobic fermentation with an external voltage, effectively realizing furfural detoxification and energy recovery, and overcoming the problems of low treatment efficiency, long inhibition period, and low microbial activity in existing anaerobic fermentation.

[0005] Technical Solution of the Present Invention The method includes the following steps: Adding granular sludge into a single-chamber reactor, adding an aqueous furfural solution with a furfural content of 0.1-5 (preferably 0.5-3, more preferably 1.0-2) g / L according to the ratio of granular sludge g to aqueous furfural solution ml of 1 g: 0.5-5 (preferably 0.8-3, more preferably 1.0-2) ml, and performing electro-fermentation in a suitable environment; Electro-fermentation conditions: Performing electro-fermentation under anaerobic conditions at a reaction temperature of 30-40 °C (preferably 35±1 °C) and an initial pH of the aqueous furfural solution of 7.2-8.0 (preferably 7.4-7.6); the electro-fermentation process is to add two electrodes into the aqueous furfural solution and apply a voltage of 0.4-1.6 V (preferably 0.8-1.5 V, more preferably 1.0-1.4 V, optimally 1.1-1.3 V) between the two electrodes. The distance between the two electrodes is 1-10 cm, preferably 2-6 cm, more preferably 2-4 cm. The two electrodes are carbon cloth with an area of 4-25 cm 2 (preferably 12-20 cm 2 ) and the surfaces of the two pieces of carbon cloth are arranged parallel to each other. The granular sludge is the granular sludge at the bottom of the anaerobic fermentation tank for treating wastewater in citric acid production or the granular sludge in the anaerobic fermentation tank of a domestic sewage treatment plant; The granular sludge is pre-enriched with ethanol for the strains before application, and the specific process is as follows: In an anaerobic fermentation bottle, adding sludge and ethanol according to 0.5-5 (preferably 1-4, more preferably 1.5-3) g of granular sludge / L of ethanol, performing strain enrichment for 3-10 days under anaerobic conditions at 30-40 °C (preferably 35±1 °C), and after solid-liquid separation, collecting the solid sludge; Collecting the solid sludge to replace the above granular sludge and performing the strain enrichment process 0-2 times (preferably 1 time) according to the above conditions, and the collected solid sludge is the required granular sludge.

[0006] The specific steps for electro-fermenting the wastewater containing furfural to produce methane are as follows:

[0007] (1) Adding 2 g / L of ethanol to the granular sludge in the anaerobic fermentation bottle for strain enrichment, enriching twice under anaerobic conditions at 35±1 °C, with each enrichment lasting for 7 days.

[0008] (2) Inoculating the enriched granular sludge into the reactor, and then adding wastewater (furfural concentration is 1 g / L). Applying different voltages with a DC power supply. Adjusting the initial pH value of each group, purging with high-purity nitrogen, and placing the reactor in a water bath environment at 35±1 °C for electro-fermentation. Three parallel samples are set for all working conditions.

[0009] (3) Measure the methane content every 24 h and measure the furfural concentration in the liquid phase every other day until no more methane is produced and the electro-fermentation process ends.

[0010] Further, the single-chamber reactor uses hydrophilic carbon cloth (4 cm × 4 cm) as the cathode and anode. The two electrodes are arranged in parallel and are 2.5 cm apart. Before use, ultrasonically clean with absolute ethanol and pure water for 15 min in sequence.

[0011] Further, the external resistor (10 Ω), DC power supply, cathode, and anode are connected by copper wires. During the experiment, a data acquisition system is used to monitor the voltage across the resistor. The current generated in the reaction can be obtained through Ohm's law.

[0012] Further, apply a potential of 0.4 - 1.6 V to the single-chamber reactor with a DC power supply.

[0013] Further, inoculate 100 g of granular sludge into the single-chamber reactor. The granular sludge is taken from the bottom of the anaerobic fermentation tank for wastewater treatment in the citric acid production of Shandong Yingxuan Industry Co., Ltd. The granular sludge is formed by anaerobic microorganisms (including acidogenic bacteria and methanogenic bacteria) in the water body utilizing organic matter and secreting extracellular polymeric substances (EPS) at the same time, which promotes the adhesion of bacteria to each other and forms preliminary flocs. The flocs gradually aggregate into larger and denser particles under continuous hydraulic shear force and are gradually deposited at the bottom of the reactor as granular sludge through sedimentation. The citric acid preparation process is as follows: Disperse the Aspergillus niger spores in the Aspergillus niger bran koji in sterile water to prepare an Aspergillus niger spore suspension with a spore concentration of 5×10 8 spores / mL. Mix 10 mL of the Aspergillus niger spore suspension with 30 g of modified calcium alginate and drop it into 200 mL of a calcium chloride solution with a mass concentration of 2.5%. Stir and react at 35 °C for 4 h, filter, wash, and dry at 30 - 32 °C to obtain solid immobilized bacteria particles. Add the fermentation medium and immobilized bacteria particles to the fermentation tank. The components of the fermentation medium are 110 g / L glucose, 3.5 g / L (NH4)2SO4, 1.5 g / L KH2PO4, 0.5 g / L MgSO4·7H2O, 0.05 g / L FeSO4·7H2O, and the solvent bran hydrolysis solution (obtained by adding 15 g of bran to 800 mL of water, boiling for 1 h, and making the volume of the filtered solution up to 1 L with water). Ferment at 30 - 32 °C, and citric acid is obtained after 38 - 52 h. The modified calcium alginate is prepared as follows: 10 g of chitosan and 20 g of glycidyltrimethylammonium chloride are reacted at pH = 8 - 10 and 50 °C for 20 h to obtain quaternized chitosan; the quaternized chitosan is dissolved in a 2% acetic acid aqueous solution to obtain a chitosan solution; the calcium alginate is dissolved in deionized water to obtain a calcium alginate solution; 10 mL of a 2.0% chitosan solution by mass is slowly added dropwise to 10 mL of a 3.0% calcium alginate solution by mass, stirred for 1.5 h, and the pH is adjusted to 4.5 - 5.0 to obtain the modified calcium alginate (solid). The granular sludge was taken from the bottom of the wastewater treatment tower for molasses in the citric acid factory. The organic matter concentration in the molasses wastewater is relatively high, with a COD of about 20000 - 25000 mg / L, and it is treated using an anaerobic circulation reaction tower. The wastewater enters the bottom of the tower and undergoes an anaerobic reaction after being mixed with the anaerobic granular sludge in the reactor.

[0014] Further, 100 of wastewater is added, where the furfural concentration is 1 g / L, that is, 100 mL of water contains 172 μL of furfural.

[0015] Further, the initial pH of the reaction is adjusted to 7.4 - 7.6 using a 3 mol / L hydrochloric acid solution and a NaOH solution.

[0016] Advantages of the present invention: By introducing voltage assistance on the basis of the anaerobic fermentation system, the degradation efficiency of microorganisms for furfural and the production of biogas are significantly improved, a large number of microorganisms are enriched, and the start-up time of the system is shortened. It is applicable to the treatment of furfural-containing wastewater and has good prospects for engineering application. The present invention improves the removal of furfural and the production of biogas in wastewater by applying a voltage of 0.4 - 1.6 V. Compared with open circuit (0.0 V), 1.2 V can significantly enhance furfural degradation and methane production. A dense biofilm is formed on the conductive carbon cloth electrode. In contrast, it is found that a higher voltage (1.6 V) will inhibit microbial activity, resulting in reduced furfural degradation and methane production. The present invention provides a new method for the efficient removal and resource recovery of furfural and other toxic furan derivatives in electro-fermentation. Description of the Drawings

[0017] Figure 1 Schematic structural diagram of the electro-fermentation reaction device used in the examples and comparative examples of the present invention; wherein: 1 is a heparin cap, 2 is a DC power supply, 3 is a stopcock, 4 is sampling port a, 5 is sampling port b, 6 is a fermentation tank, 7 is a cathode, 8 is an anode, 9 is a silica gel hose, 10 is a 100 mL air bag;

[0018] Figure 2 Variation of the cumulative methane production of Examples 1 - 4 and Comparative Example 1 with time;

[0019] Figure 3 Variation of the furfural concentration in Examples 1 - 4 and Comparative Example 1 with time;

[0020] Figure 4 SEM images of the electrodes and the biofilms thereon before and after the reaction in Examples 1-4 and Comparative Example 1: (a) carbon cloth before the reaction; (b) cathode after the reaction at 0.0 V; (c) cathode after the reaction at 1.2 V; (d) anode after the reaction at 1.2 V; (e) cathode after the reaction at 1.6 V; (f) anode after the reaction at 1.6 V; Detailed implementation manners

[0021] The present invention will be further described in detail below through specific examples. The granular sludge taken from the bottom of the anaerobic fermentation tank for treating citric acid wastewater produced by Shandong Yingxuan Industry Co., Ltd. was added into an anaerobic fermentation bottle. Ethanol was added to the anaerobic fermentation bottle at 200 g of granular sludge / L of ethanol to enrich the strains. It was enriched under anaerobic conditions at 35 ± 1 °C for 7 days, and then solid-liquid separation was carried out to collect the granular sludge; the collected granular sludge was mixed with ethanol at 2 g of granular sludge / L of ethanol in the anaerobic fermentation bottle again, and it was enriched under anaerobic conditions at 35 ± 1 °C for 7 days, and then solid-liquid separation was carried out to collect the granular sludge, which was used as granular sludge A; it was used in the following Comparative Example 1 and Examples 1-4.

[0022] Example 1:

[0023] 1) A sealed Shuniou 250 mL high borosilicate reagent bottle produced by Sichuan Shubo (Group) Co., Ltd. was used as the fermentation reactor (i.e.). Two stainless steel wires with a diameter of 1 mm passed through the silicone stopper at the top of the reactor, and clean hydrophilic carbon cloths with the same area size (length × width = 4 cm × 4 cm) were fixed at the lower ends of the two stainless steel wires in the fermentation reactor as the cathode and the anode respectively. The surfaces of the two carbon cloths were arranged parallel to each other with a spacing of 2.5 cm, and it was ensured that the two carbon cloths were completely immersed under the liquid level when adding furfural solution or furfural-containing wastewater. The silicone hose was used as the sampling tube. The sampling port a at the top was sealed with a heparin cap for extracting the gas generated during fermentation for detection, and the sampling port b was sealed with a stopcock for taking liquid samples. The fermentation reactor was connected to a gas bag (a 100 mL double-valve aluminum foil gas sampling bag from Ningbo Hongpu Experimental Technology Co., Ltd.) through a conduit;

[0024] 2) 100 g of granular sludge A after enrichment and 100 mL of aqueous solution containing 1 g / L of furfural were added to the single-chamber reactor. The pH was adjusted to 7.4 - 7.6 (here it was 7.5) with 3 mol / L hydrochloric acid solution and 3 mol / L NaOH solution, and it was purged with high-purity nitrogen for 5 min to replace the atmosphere in the reactor with an anaerobic atmosphere of nitrogen. The cathode and the anode were respectively connected to the negative and positive poles of a DC power supply through stainless steel wires, and the applied voltage was set to 0.4 V, and an electro-fermentation experiment was carried out in a constant temperature water bath at 35 ± 1 °C.

[0025] 3) The methane content was measured every 24 h for a total of 12 days, and the cumulative methane production was detected to be 4.18 mL. Every 24 h, 3 mL of liquid was taken, filtered through a 0.45-μm pore size filter membrane, and the furfural concentration was determined by high-performance liquid chromatography. At the end of the reaction, there was still 0.1 g / L of furfural residue. This indicates that the applied voltage can alleviate the inhibition of furfural on microorganisms, promote the degradation of furfural, and increase methane production. The current value during the reaction was monitored with a data collector. The average current at 0.4 V was only 0.02 mA, and the maximum value was 0.04 mA.

[0026] 4) After no more methane was produced in the reactor, the fermentation stopped, lasting for a total of 12 days.

[0027] Example 2:

[0028] The process and conditions were the same as those in Example 1, except that an external voltage of 0.8 V was applied between the two electrodes of the reactor for the reaction of furfural degradation to produce methane, and the other conditions were the same as those in Example 1. The detected cumulative methane production was 11.79 mL. On the 9th day, the furfural had been completely degraded and the fermentation stopped. The average current was 1.09 mA, and the maximum current during the reaction was 2.21 mA. After applying the external voltage, the methane production and the furfural degradation rate were improved compared with 0.4 V, indicating that 0.8 V is a favorable condition.

[0029] Example 3:

[0030] The process and conditions were the same as those in Example 1, except that an external voltage of 1.2 V was applied between the two electrodes of the reactor for the reaction of furfural degradation to produce methane, and the other conditions were the same as those in Example 1. At this voltage, there was the shortest inhibition period, which was two days shorter than that in the open circuit (0.0 V); the largest cumulative methane production, which was 37.50 mL; the fastest furfural degradation rate. On the 7th day, the furfural concentration was 0 and the fermentation stopped; the average current was 2.50 mA, and the highest current value was 5.23 mA. It was shown that 1.2 V is the optimal voltage for electro-fermentation to degrade furfural. Then, the microbial morphology and community on its cathode and anode were tested and analyzed. Through Figure 4 The SEM image of the biofilm on the 1.2-V electrode showed the densest biofilm with a thickness of 5 - 6 μm.

[0031] Example 4:

[0032] The process and conditions were the same as those in Example 1, except that an external voltage of 1.6 V was applied between the two electrodes of the reactor to carry out the reaction of furfural degradation to produce methane, and the other conditions were the same as those in Example 1. The average current generated by this voltage was 1.86 mA, but the cumulative methane production was lower than that at 1.2 V, being 23.23 mL, and the furfural degradation rate was also lower than that at 1.2 V. The maximum current value was 4.49 mA. In addition, it was seen from the SEM images that the biofilm on the 1.6-V electrode was sparser than that on the 1.2-V electrode, indicating that too high a voltage would reduce the methane production and furfural degradation rate and was not conducive to the enrichment of microorganisms on the electrode. Therefore, during implementation, an appropriate voltage range (1.0 - 1.4 V) can be selected to ensure the rapid degradation of furfural while producing a large amount of methane.

[0033] Comparative Example 1:

[0034] The process and conditions were the same as those in Example 1, except that the process of furfural degradation to produce methane was carried out in a reactor without applying an external voltage (0.0 V) between the two electrodes. Except for not applying the voltage, the other conditions were the same as those in Example 1. The cumulative methane production under this condition was the lowest, only 0.51 mL, and the most furfural remained at the end of the reaction, being 0.21 g / L. The sparest biofilm was observed in the SEM images, indicating that furfural could not be completely degraded and the fermentation performance and the vital activities of microorganisms were severely inhibited when no voltage was applied.

[0038] Example 5:

[0039] The process of Comparative Example 1 and Examples 1 - 4 was respectively repeated by using granular sludge with the same mass taken from the anaerobic fermentation tank of Yinhe Domestic Sewage Treatment Plant in Zhucheng, Shandong instead of granular sludge A (the process and conditions were the same as the corresponding Comparative Example 1 and Examples 1 - 4 respectively), and similar methane production was obtained. The cumulative methane production at 0.0 - 1.6 V in the processes corresponding to Comparative Example 1 and Examples 1 - 4 was respectively: 0.49, 5.23, 13.84, 40.21, 28.99 mL. This shows that applying an external voltage is beneficial to increasing methane production.

[0039] Example 6:

[0040] The process for preparing furfural from corncobs was as follows: Corncobs with a particle size of 0.2 - 0.8 cm after crushing and 5% dilute sulfuric acid were added into a hydrolysis tank (mass ratio, dilute sulfuric acid: corncobs = 0.35). The temperature in the hydrolysis tank was 250 °C, the steam pressure was 0.8 Mp, and the reaction lasted for 5 h. The steam containing a small amount of acetic acid was neutralized to neutrality with soda solution (3 mol / L NaOH solution), and then entered a condenser for condensation to form a dilute furfural solution. Then distillation was carried out. The bottom temperature of the tower was 105 °C, the top temperature of the tower was 100 °C, the residual liquid was discharged from the bottom of the tower, and the distillate entered the condenser from the top of the tower. After condensation, crude furfural was obtained, and after further rectification, furfural products were obtained.

[0041] In the above process, organic wastewater containing furfural and its derivatives is generated (COD is about 11,000 - 16,500 mg / L, and the furfural content is 0.2% - 0.5%). After dilution with water, wastewater containing 1 g / L of furfural is obtained (i.e., the mass content of furfural is 0.1%). Using this as a raw material to replace the 100 mL aqueous solution containing 1 g / L of furfural in Example 3 for the experiment, the process and conditions are the same as those in Example 3. At the 7th day, the furfural concentration is 0 and the fermentation stops; and the highest current value is 5.31 mA.

Claims

1. A method for electro-fermentation treatment of furfural-containing wastewater, characterized in that, The method comprises the following steps: Adding granular sludge into a single-chamber reactor, adding an aqueous furfural solution with a furfural content of 0.1-5 (preferably 0.5-3, more preferably 1.0-2) g / L according to the ratio of granular sludge g to aqueous furfural solution ml of 1 g: 0.5-5 (preferably 0.8-3, more preferably 1.0-2) ml, and performing electro-fermentation in a suitable environment; Electro-fermentation conditions: Performing electro-fermentation under anaerobic conditions at a reaction temperature of 30-40 °C (preferably 35 ± 1 °C) and an initial pH of the aqueous furfural solution of 7.2-8.0 (preferably 7.4-7.6); the electro-fermentation process is to add two electrodes into the aqueous furfural solution and apply a voltage of 0.4-1.6 V (preferably 0.8-1.5 V, more preferably 1.0-1.4 V, optimally 1.1-1.3 V) between the two electrodes.

2. The method according to claim 1, wherein: The distance between the two electrodes is 1-10 cm, preferably 2-6 cm, more preferably 2-4 cm.

3. The method according to claim 1 or 2, wherein: The two electrodes are carbon cloth with an area of 4 - 25 cm 2 (preferably 12 - 20 cm 2 ), and the surfaces of the two pieces of carbon cloth are arranged parallel to each other.

4. The method according to claim 1, wherein: The granular sludge is the granular sludge at the bottom of an anaerobic fermentation tank for treating wastewater in citric acid production or the granular sludge in an anaerobic fermentation tank of a domestic sewage treatment plant; The granular sludge is pre-enriched with ethanol before being applied, and the specific process is as follows: In an anaerobic fermentation bottle, adding sludge and ethanol according to 0.5-5 (preferably 1-4, more preferably 1.5-3) g of granular sludge / L of ethanol, performing strain enrichment for 3-10 days under anaerobic conditions at 30-40 °C (preferably 35 ± 1 °C), and after solid-liquid separation, collecting the solid sludge; Collecting the solid sludge to replace the above granular sludge and performing the strain enrichment process 0-2 times (preferably 1 time) under the above conditions, and the collected solid sludge is the required granular sludge.

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