A bio-electrochemical reactor and a method for treating low concentration wastewater by using the same

Abstract

The application discloses a kind of bioelectrochemical reactor and the method for processing low concentration wastewater using it, belong to low concentration wastewater treatment technical field.The bioelectrochemical reactor of the application includes reactor top cover and reactor tank body;Gas inlet hole is provided on the reactor top cover, anode, cathode and aeration device are arranged in the reactor tank body, and anode and cathode are connected with external direct-current power supply through titanium wire respectively;Aeration device is connected with gas inlet hole;Anode and cathode are both carbon nanotube modified titanium electrode.When the above-mentioned bioelectrochemical reactor is used to process low concentration wastewater, first, electroactive bacteria are cultivated on the surface of electrode using aerobic sludge, then bioelectrochemical aerobic reaction is carried out, and under aeration condition and low voltage, COD and NH4 + -N removal efficiency can reach more than 95%, electrode corrosion is low, the treatment method is simple and efficient, and no secondary pollution is generated, and has wide application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of low-concentration wastewater treatment, and particularly relates to a bio-electrochemical reactor and a method for treating low-concentration wastewater by using the same. BACKGROUND

[0002] The information disclosed in the background of the present application is only intended to increase the understanding of the overall background of the present application and should not necessarily be regarded as acknowledging or implicitly suggesting that this information constitutes prior art known to those of ordinary skill in the art.

[0003] Typical wastewater discharge indicators include: ammonia nitrogen (NH4 + -N), chemical oxygen demand (COD), heavy metals, etc. Although wastewater is treated by multiple stages in wastewater treatment plants, the wastewater discharged by the wastewater treatment plants is still difficult to meet the required discharge standards. In particular, the advanced treatment of tail water is relatively difficult because the concentration and biodegradability of the organic matter are relatively low. Therefore, some wastewater treatment plants use advanced oxidation methods such as Fenton, ozone oxidation and filtration in the end treatment. However, these methods are costly and there is a risk of secondary pollution. Electrochemical treatment degrades pollutants by the action of free radicals released in the electrochemical cell, for example, electro-oxidation directly relies on the formation of strong oxidants such as hydroxyl radicals (-OH) or hypochlorite ions at the anode, and this treatment technology has low chemical addition and no secondary discharge.

[0004] However, the efficiency of the electrochemical treatment process depends largely on the concentration of organic compounds in the solution and the conditions of electron transfer between the electrode and the solution, and moreover, the anode material is corroded after long-term operation of the electrochemical treatment, resulting in a decrease in the removal efficiency of pollutants. The removal efficiency of pollutants in the electrochemical treatment process is usually improved by increasing the current density during the treatment; however, when the electrochemical system reaches the optimal treatment condition, the high current density used will result in higher power and electrical energy consumption and faster electrode corrosion.

[0005] In order to overcome the shortcomings of electrochemical methods, attempts have been made in recent years to combine electrochemical processes with biological processes as a pretreatment or post-treatment method. Bio-electrochemical system (BES) technology is a new type of biological wastewater treatment technology, which has significant advantages compared to traditional membrane biological systems, including smaller footprint and low-carbon sustainability. Electron exchange within the biofilm or at the biofilm-electrode interface is the key to promoting the degradation of pollutants, and the positive and negative effects of electrical stimulation depend on the direct current or voltage intensity and the physiological characteristics of the microorganisms (aerobic or anaerobic). Therefore, how to provide a bio-electrochemical reactor and method with high efficiency for treating low-concentration wastewater is a problem to be solved. SUMMARY

[0006] Therefore, the present application provides a bio-electrochemical reactor and a method for treating low-concentration wastewater by using the same, which solves the problems of high cost, secondary pollution and serious electrode corrosion in the prior art.

[0007] In a first aspect, the present application provides a bio-electrochemical reactor, comprising a reactor top cover and a reactor tank body, which are detachably connected; the reactor top cover is provided with an air inlet hole, and the reactor tank body is internally provided with an anode, a cathode and an aeration device, the anode and the cathode are respectively connected with an external direct-current power source through titanium wires; the aeration device is connected with the air inlet hole; and the anode and the cathode are both carbon nanotube modified titanium electrodes.

[0008] Preferably, the reactor top cover is provided with a sampling hole, and the titanium wires are connected with the external direct-current power source through the reactor top cover.

[0009] Preferably, the carbon nanotube modified titanium electrode is obtained by adhering carbon nanotube slurry to a titanium mesh through a rolling method, and the loading amount of the carbon nanotubes is controlled to be 10-20 mg / cm 2 ; and then dried.

[0010] Further, the carbon nanotube slurry comprises nitric acid soaked modified carbon nanotubes, a nickel chloride catalyst, a polytetrafluoroethylene emulsion binder and a solvent; and the solvent is a mixed solvent of water and ethanol.

[0011] Further, the amount ratio of the nitric acid soaked modified carbon nanotubes, the nickel chloride catalyst, the polytetrafluoroethylene emulsion binder and the solvent is 1g:(0.005-0.02)g:(0.1-0.2)mL:(30-50)mL.

[0012] Further, the drying step is followed by a step of soaking the carbon nanotube modified titanium electrode in a 0.5-2wt% sodium dodecyl sulfate solution for 20-30h, and then drying.

[0013] In a second aspect, the present application provides a method for treating low-concentration wastewater by using the above bio-electrochemical reactor, which comprises the following steps:

[0014] A mixture of aerobic sludge and water is added to the reactor tank body of the bio-electrochemical reactor, the voltage of the anode and the cathode is controlled to be 0.2-2V, and electrically active bacteria are cultured on the surface of the carbon nanotube modified titanium electrode under aeration conditions.

[0015] The reactor tank body is cleaned, and then low-concentration wastewater is added to the reactor tank body, the voltage of the anode and the cathode is controlled to be 0.2-2V, and bio-electrochemical aerobic reaction is carried out under aeration conditions.

[0016] Preferably, the COD concentration of the low-concentration wastewater is 200-400 mg / L, the NH4 + -N concentration is 10-20 mg / L.

[0017] Preferably, the volume ratio of the aerobic sludge to water is (2-4):(6-8), and the time for culturing the electroactive bacteria on the surface of the carbon nanotube-modified titanium electrode is 12-18 days.

[0018] Preferably, the aeration flow rate is 80-150 mL / min.

[0019] Preferably, the time for performing the bioelectrochemical aerobic reaction under the aeration condition is 8-15 h.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] The bioelectrochemical reactor provided by the present application has simple structure and low cost, and when the reactor is used to treat low-concentration wastewater, the COD and NH4 + -N removal efficiency can reach above 95%, the electrode has low corrosion, the treatment method is simple and efficient, no secondary pollution is generated, and the method has wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0022] The drawings constituting a part of the specification of the present application are used to provide further understanding of the present application, the illustrative embodiments of the present application and the description thereof are used to explain the present application, and do not constitute improper limitation on the present application. Obviously, for those skilled in the art, other drawings can be obtained from these drawings without creative labor.

[0023] Figure 1 is a schematic diagram of the bioelectrochemical reactor of Example 1 of the present application;

[0024] Figure 2 is the COD degradation curve and the NH4 + -N degradation curve of the bioelectrochemical anaerobic reactor of Comparative Example 4 of the present application under different applied voltages;

[0025] Figure 3 is the COD degradation curve and the NH4 + -N degradation curve of the bioelectrochemical aerobic reactor of Examples 2-4 and Comparative Examples 1-3 of the present application;

[0026] Figure 4CV and EIS curves of Examples 2-4 and Comparative Examples 1-4 at the end of the reaction experiment, wherein a is the CV curve of Comparative Example 4, b is the EIS curve of Comparative Example 4, c is the CV curve of Examples 2-4 and Comparative Examples 1-3, and d is the EIS curve of Examples 2-4 and Comparative Examples 1-3;

[0027] Figure 5 SEM images of the electrode surface of Comparative Example 4 of the present application, wherein (a) 0 V, (b) 0.5 V, (c) 1 V, (d) 2 V, (e) 4 V, (f) 8 V;

[0028] Figure 6 SEM images of the electrode surface of Examples 2-4 and Comparative Examples 1-4 of the present application, wherein (a) Comparative Example 1, (b) Example 2, (c) Example 3, (d) Example 4, (e) Comparative Example 2, (f) Comparative Example 3;

[0029] Figure 7 Main phylum level abundance of microbial community in the electrode biofilm of Examples 2-4 and Comparative Examples 1-3 of the present application; (a) abundance of bacterial phylum in the aerobic biofilm of Examples 2-4 and Comparative Examples 1-3 at different voltages, (b) abundance of bacterial genus in the aerobic biofilm of Examples 2-4 and Comparative Examples 1-3 at different voltages;

[0030] Figure 8 Main phylum level abundance of microbial community in the electrode biofilm of Comparative Example 4 of the present application; wherein (a) abundance of bacterial phylum in the anaerobic biofilm of Comparative Example 4 at different voltages, (b) abundance of bacterial genus in the anaerobic biofilm of Comparative Example 4 at different voltages;

[0031] Figure 9 Abundance of microbial metabolic pathways in the anaerobic (a) of Comparative Example 4 and the aerobic (b) of Examples 2-4 and Comparative Examples 1-3 electrochemical biofilm. DETAILED DESCRIPTION

[0032] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0033] The application provides a bio-electrochemical reactor, which comprises a reactor top cover and a reactor tank body, the reactor top cover and the reactor tank body are detachably connected, an air inlet hole is arranged on the reactor top cover, an anode, a cathode and an aeration device are arranged in the reactor tank body, the anode and the cathode are connected with an external direct current power source through titanium wires respectively, the aeration device is connected with the air inlet hole, and the anode and the cathode are both carbon nanotube modified titanium electrodes.

[0034] In the application, the reactor top cover and the reactor tank body are detachably connected, so that the anode, the cathode or the aeration device and other parts can be replaced conveniently. The detachable connection mode is not specially limited in the application, and can be screw connection, buckle connection or other connection modes. The positions and modes of feeding and discharging are not specially limited in the application, and automatic feeding and discharging can be realized by additionally arranging feeding and discharging ports on the top cover, and the positions of the feeding and discharging ports are not specially limited in the application. In a small reactor, the reactor top cover and the reactor tank body can be separated to realize direct injection and pouring of low-concentration wastewater.

[0035] In the application, a sampling hole is arranged on the reactor top cover, so that the wastewater in treatment can be monitored in real time to determine the removal degrees of COD and ammonia nitrogen.

[0036] In the application, the titanium wires are connected with the external direct current power source through the reactor top cover, and further, in order to avoid the adverse effect of the reactor top cover on the conductivity of the titanium wires, insulating adhesive tape or an insulating heat-shrinkable tube is wound on the contact parts of the titanium wires.

[0037] In the application, the carbon nanotube modified titanium electrode is obtained by adhering carbon nanotube slurry to a titanium mesh through a rolling method, and the loading amount of the carbon nanotube is controlled to be 10-20 mg / cm 2 . Then, the carbon nanotube modified titanium electrode is obtained by drying. The rolling method is simple, has low requirements on equipment and low operation cost. The CNT of the carbon nanotube modified titanium electrode can improve the conductivity and electrochemical characteristics of the electrode in the reactor, the use of a high carbon nanotube loading rate reduces the charge transfer resistance, improves the electron transfer rate between the electrode and the biofilm, and further improves the reaction rate of microorganisms on the electrode.

[0038] In the application, the carbon nanotube slurry comprises nitric acid soaked modified carbon nanotubes, a nickel chloride catalyst, a polytetrafluoroethylene emulsion binder and a solvent, and the solvent is a mixed solvent of water and ethanol. The hydrophilicity of the carbon nanotubes soaked in nitric acid is further improved. The nickel chloride catalyst can further improve the reaction rate of the electrode. The polytetrafluoroethylene emulsion binder can firmly adhere the carbon nanotubes to the titanium mesh, and ensure the stability of the carbon nanotube modified titanium electrode.

[0039] In the present application, the use amount ratio of the nitric acid soaking modified carbon nanotube, the nickel chloride catalyst, the polytetrafluoroethylene emulsion binder and the solvent is 1g:(0.005-0.02)g:(0.1-0.2)mL:(30-50)mL.

[0040] In the present application, the drying step is further followed by a step of soaking the carbon nanotube modified titanium electrode in a 0.5-2wt% sodium dodecyl sulfate solution for 20-30h and then drying. This treatment step is to further improve the hydrophilicity of the electrode, which is beneficial to the loading of microorganisms on the electrode.

[0041] The present application also provides a method for treating low-concentration wastewater by using the above-mentioned bio-electrochemical reactor, which comprises the following steps:

[0042] The mixture of aerobic sludge and water is added into the reactor tank of the bio-electrochemical reactor, the voltage of the anode and the cathode is controlled to be 0.2-2V, and the electroactive bacteria are cultivated on the surface of the carbon nanotube modified titanium electrode.

[0043] The reactor tank is cleaned, then the low-concentration wastewater is added into the reactor tank, the voltage of the anode and the cathode is controlled to be 0.2-2V, and the bio-electrochemical aerobic reaction is carried out under the condition of aeration.

[0044] In the present application, the COD concentration of the low-concentration wastewater is 200-400mg / L, the NH4 + -N concentration is 10-20mg / L. The above-mentioned treatment method of the present application is particularly suitable for the treatment of the above-mentioned low-concentration wastewater, and has high treatment efficiency and excellent COD and ammonia nitrogen removal effect.

[0045] In the present application, the volume ratio of the aerobic sludge to water is (2-4):(6-8), and more preferably 3:7. Under the above-mentioned ratio, it is more beneficial to inoculate the electroactive bacteria on the surface of the carbon nanotube modified titanium electrode.

[0046] The present application does not have special limitation on the source of the aerobic sludge, and the sludge from the aerobic treatment stage of a sewage treatment plant is preferably used in the present application, and the technical index thereof preferably meets the following conditions: pH is 7-8; water content is 25-50%; total solid (TS) is 10-25wt%; volatile solid accounts for 8-13wt% of TS; total chemical oxygen demand (TCOD) is 6000-8000mg / L; and soluble chemical oxygen demand (sCOD) is 1000-5000mg / L.

[0047] In the present application, the time for cultivating the electroactive bacteria on the surface of the carbon nanotube modified titanium electrode is 12-18 days, more preferably 14-16 days, and most preferably 15 days.

[0048] The present application needs to clean the reactor tank after culturing the electroactive bacteria, so as to remove the aerobic sludge; or another clean reactor tank can be selected for subsequent bioelectrochemical reaction. However, no matter which way, the electroactive bacteria on the surface of the carbon fiber modified titanium electrode need to be preserved.

[0049] In the present application, the low-concentration wastewater is added into the reactor tank, and the control voltage is 0.3-0.7V.

[0050] In the present application, the aeration flow rate is 80-150mL / min. The aeration of the present application is preferably continuous aeration, so as to continuously provide dissolved oxygen for aerobic microorganisms.

[0051] In the present application, the time for bioelectrochemical aerobic reaction under aeration condition is 8-15h, and is further preferably 12-15h.

[0052] The technical solutions of the present application are further described below in combination with specific examples.

[0053] In the following examples and comparative examples, the aerobic sludge is taken from the aerobic treatment stage of a sewage treatment plant in Jinan, and the anaerobic sludge is taken from the anaerobic digestion stage of a sewage treatment plant in Jinan. The physicochemical properties of the aerobic sludge and the anaerobic sludge are shown in Table 1.

[0054] Table 1 Physicochemical properties of aerobic sludge and anaerobic sludge

[0055] Parameter Anaerobic sludge Aerobic sludge pH 7.8±0.24 7.5±0.45 Moisture content (%) 48±1 30±1.03 Total solids (TS) (wt%) 9.39±0.22 20.79±0.47 Volatile solids (VS) (wt% of TS) 3.44±0.65 11.83±0.72 Total chemical oxygen demand (TCOD) (mg / L) 8537.7±407.55 6438±376.25 Dissolved chemical oxygen demand (sCOD) (mg / L) 679.5±8.51 3941.6±73.37

[0056] The low-concentration wastewater (per liter) used in the following examples and comparative examples is prepared according to the following conditions: 300mg COD (CH3COONa), 45mg NH4 + -N (NH4Cl), 4mg PO4 3- -P (KH2PO4) and 1mL trace element solution. Trace elements (per liter): 2.0mg biotin, 2.0mg folic acid, 10.0mg pyridoxine, 5.0mg thiamine hydrochloride, 5.0mg riboflavin, 5.0mg nicotinic acid, 5.0mg DL-calcium pantothenate, 0.1mg vitamin B12, 5.0mg p-aminobenzoic acid, 5.0mg lipoic acid. The COD value of the artificial low-concentration wastewater prepared is 300mg / L, and the ammonia nitrogen content (NH4 + -N) is 16mg / L.

[0057] Example 1

[0058] This example provides a bioelectrochemical reactor, which comprises a reactor tank, a carbon fiber modified titanium electrode and an anode. Figure 1As shown, including reactor tank 1 and reactor top cover 2, reactor top cover 2 and reactor tank 1 can be detachably connected; reactor top cover 2 is provided with sampling hole 3 and air inlet hole 4, and reactor tank 1 is provided with anode 5, cathode 6 and aeration device 9 inside; anode 5 and cathode 6 are respectively connected with external direct current power supply 8 through titanium wire 7 (φ = 1.8mm) passing through reactor top cover 2, and an insulating heat shrink tube (not shown in the figure) is arranged at the contact position of titanium wire 7 and reactor top cover 2; aeration device 9 is connected with air inlet hole 4; anode 5 and cathode 6 are both carbon nanotube modified titanium electrodes, and the spacing between anode 5 and cathode 6 is 2cm.

[0059] The preparation method of the carbon nanotube modified titanium electrode is as follows:

[0060] The electrode takes a titanium mesh with a size of 6cm x 2cm as a skeleton, and is immersed in anhydrous ethanol for 24 hours in advance to remove surface impurities. Multi-walled carbon nanotubes (CNT) are immersed in a concentrated nitric acid solution for 24 hours to improve their hydrophilicity, and then are continuously rinsed with distilled water to make the pH value greater than 6, and are dried. In 2.5g of CNT modified by nitric acid, 1wt% of nickel chloride is added, and 100mL of 80% ethanol solution is used as a solvent for uniform stirring to make them fully mixed. A polytetrafluoroethylene emulsion (0.375mL, solid content 55%) is added to the CNT as an adhesive, and after ultrasonic treatment for half an hour, the CNT slurry is heated and stirred in a water bath at 80°C until it becomes a paste. After cooling to room temperature, the pretreated carbon nanotube slurry is adhered to the titanium mesh with a roller press, and the carbon nanotube loading amount is controlled to be 15mg / cm 2 ; and then is dried in a 50°C oven. Then it is immersed in a 1wt% sodium dodecyl sulfate solution for 24h to improve the hydrophilicity of the electrode, and is dried to obtain the electrode.

[0061] Example 2

[0062] The present embodiment provides a method for treating low-concentration wastewater by using the bioelectrochemical reactor of Example 1.

[0063] (1) Culturing of electroactive bacteria: inoculate the aerobic sludge and pure water into the bioelectrochemical reactor of Example 1 at a volume ratio of 3:7, control the voltage between anode 5 and cathode 6 to be 0.5V, and culture the electroactive bacteria on the surface of the carbon nanotube modified titanium electrode for 15 days.

[0064] (2) Bioelectrochemical aerobic reaction: clean the reactor tank 1, retain the electroactive bacteria on the surface of the carbon nanotube modified titanium electrode, add 200mL of low-concentration wastewater into the 250mL reactor tank, control the voltage between anode 5 and cathode 6 to be 0.5V, and carry out the bioelectrochemical aerobic reaction under an aeration flow rate of 100mL / min, take samples from sampling hole 3 at different treatment times, and detect the COD concentration and NH4+ -N concentration.

[0065] Example 3

[0066] Compared with Example 2, the difference is that the voltage between the anode 5 and the cathode 6 is controlled to be 1V in the step (1) and the step (2) of the example.

[0067] Example 4

[0068] Compared with Example 2, the difference is that the voltage between the anode 5 and the cathode 6 is controlled to be 2V in the step (1) and the step (2) of the example.

[0069] Comparative Example 1

[0070] Compared with Example 2, the difference is that the voltage between the anode 5 and the cathode 6 is controlled to be 0V in the step (1) and the step (2) of the example.

[0071] Comparative Example 2

[0072] Compared with Example 2, the difference is that the voltage between the anode 5 and the cathode 6 is controlled to be 4V in the step (1) and the step (2) of the example.

[0073] Comparative Example 3

[0074] Compared with Example 2, the difference is that the voltage between the anode 5 and the cathode 6 is controlled to be 8V in the step (1) and the step (2) of the example.

[0075] Comparative Example 4

[0076] Compared with Example 2, the difference is that the bio-electrochemical anaerobic reaction occurs in the example.

[0077] (1) Culturing of electroactive bacteria: anaerobic sludge and pure water are inoculated into the bio-electrochemical reactor of Example 1 at a volume ratio of 3:7, and the voltage is controlled to be 0, 0.5, 1, 2, 4, 8V respectively, and the electroactive bacteria are cultured on the surface of the carbon nanotube modified titanium electrode for 15 days.

[0078] (2) Bio-electrochemical anaerobic reaction: the reactor tank 1 is cleaned, the electroactive bacteria on the surface of the carbon nanotube modified titanium electrode are reserved, low-concentration wastewater is added to the reactor tank 1, and the air inlet hole 4 is sealed; the voltage between the anode 5 and the cathode 6 is controlled to be 0, 0.5, 1, 2, 4, 8V respectively (corresponding to the voltage of step (1)), and the bio-electrochemical reaction is carried out under anaerobic conditions, and the treated wastewater is sampled from the sampling hole 3 at different treatment times, and the COD concentration and NH4 + -N concentration.

[0079] Comparative Example 5

[0080] Compared with Example 2, the difference is that an electrochemical aerobic reaction occurs in this comparative example.

[0081] Low-concentration wastewater was added to the bioelectrochemical reactor of Example 1, and the voltage between the anode 5 and the cathode 6 was controlled to be 0, 0.5, 1, 2, 4, and 8 V, respectively. The electrochemical aerobic reaction was carried out at an aeration flow rate of 100 mL / min. Samples were taken from the sampling hole 3 at different treatment times to detect the COD concentration and NH4 + -N concentration.

[0082] Comparative Example 6

[0083] Compared with Example 2, the difference is that an electrochemical anaerobic reaction occurs in this comparative example.

[0084] Low-concentration wastewater was added to the bioelectrochemical reactor of Example 1, the air inlet 4 was sealed, the voltage between the anode 5 and the cathode 6 was controlled to be 0, 0.5, 1, 2, 4, and 8 V, respectively, and an electrochemical anaerobic reaction was carried out under anaerobic conditions. Samples were taken from the sampling hole 3 at different treatment times to detect the COD concentration and NH4 + -N concentration.

[0085] Test example

[0086] 1. COD, NH4 + -N removal efficiency

[0087] The low-concentration wastewater treatment processes of Examples 2 to 4 and Comparative Examples 1 to 6 were monitored, and the data are summarized in Table 2.

[0088] Table 2 COD and NH4 in low-concentration wastewater treatment process + -N removal efficiency (%)

[0089]

[0090]

[0091] As can be seen from Table 2, the wastewater treatment effect of the bioelectrochemical anaerobic reaction (Comparative Example 4) is better than that of the electrochemical anaerobic reaction (Comparative Example 6), and the wastewater treatment effect of the bioelectrochemical aerobic reaction (Examples 2 to 4, Comparative Examples 2 to 3) is better than that of the electrochemical aerobic reaction (Comparative Example 5).

[0092] COD degradation curves and NH4 degradation curves of the bioelectrochemical anaerobic reactor of Comparative Example 4 under different applied voltages + -N degradation curve is as follows Figure 2The COD removal rate of the bioelectrochemical anaerobic reactor was much higher than that of the electrochemical anaerobic reactor after 30 hours. The COD concentration degradation rate of the electrochemical anaerobic reactor under different applied voltages was quite different. The bioelectrochemical anaerobic reactor had the best degradation effect after 30 hours of wastewater treatment at an applied voltage of 1 V, and the COD concentration decreased from 300 mg / L to 7.04 mg / L. At a lower applied voltage (0-2 V), the applied voltage had a significant stimulating effect on the electroactive biofilm, which could accelerate the metabolism of microorganisms and more effectively degrade organic matter. It is worth noting that when the applied voltage is higher than 2 V, the COD degradation effect in the reactor will decrease. This is because the growth and metabolism of microorganisms are inhibited at high voltage, resulting in a decrease in microbial activity and no longer growth after a long time of power-on. The electron transfer in the biofilm electrode is complex, such as metal conduction and redox conduction. At a higher applied voltage, the mobility of counterions in the conductive biofilm may also be limited, resulting in the biofilm being unable to respond to higher anode potentials. After 30 hours of operation at an applied voltage of 1 V, the NH4 + -N concentration decreased from 16 mg / L to 0.15 mg / L. In the presence of a biofilm, the NH4 + -N in the reactor degraded faster in the first 24 hours of the reaction period. The denitrifying microorganisms in the cathode surface biofilm reduced NO3 - to N2 gas using H2 produced by electrolysis of water. At a low voltage, the removal of NH4 + -N was significantly increased. As the applied voltage increased, the hydrogen produced by electrolysis of water would have a hydrogen inhibition effect, reducing the degradation effect of ammonia nitrogen. And the activity of nitrifying bacteria would also be inhibited at high voltage, slowing down the conversion of NH4 + -N to nitrate in the effluent, resulting in a decrease in denitrification efficiency.

[0093] The COD degradation curves and NH4 + -N degradation curves of the bioelectrochemical aerobic reactors of Examples 2-4 and Comparative Examples 1-3 are shown in Figure 3 Fig. 2. Compared with anaerobic conditions, the degradation effect of the bioelectrochemical reactor under aerobic conditions on the two target pollutants, COD and NH4 + -N, was significantly enhanced. At an applied voltage of 0.5 V, the treatment time for reducing the COD concentration to 3.97 mg / L was shortened from 30 hours to 12 hours. In addition, the minimum concentration of NH4 + -N reached 0.03 mg / L and was achieved and stabilized within 9 hours. Oxygen is the most commonly used electron acceptor in the cathode of a bioelectrochemical system (BES), with strong availability and high redox potential. Under conditions of high dissolved oxygen (DO) concentration, the nitrification reaction can convert NO3- Completely reduced to N2. DO can significantly affect the microbial activity of denitrifying bacteria, which are facultative microorganisms that prefer to use oxygen as an electron acceptor rather than nitrate and may compete with ammonia oxidizing microorganisms (AOB) for oxygen in the reactor. The system voltage of Examples 2 to 4 is lower and has better COD and NH4 + -N removal efficiency. In contrast, under the condition of applying voltage of 0.5V (Example 2), the COD removal rate in 12h reached 98.68±0.38%, and NH4 + -N removal rate reached 99.78 ± 0.10%, showing the best degradation efficiency. This shows that the microbial community structure in the system with an applied voltage of 0.5V is more suitable for denitrification, and the metabolic level is also higher than that of the reactors with different applied voltages. The degradation efficiency of the system with an applied voltage of 8V (Comparative Example 3) is higher than that of the system with a applied voltage of 4V (Comparative Example 2), but due to the high voltage, the corrosion to the electrode is high and it is not suitable for long-term operation.

[0094] 2. Bioelectrochemical characteristics

[0095] The CV and EIS analysis at the end of the reaction experiments of Examples 2 to 4 and Comparative Examples 1 to 4 can show the enrichment of electroactive microorganisms and the electron transfer efficiency of the bioelectrode. The performance under different applied voltages varies greatly, and the biofilm on the bioelectrode also shows different voltammetric characteristics ( Figure 4 From the measured CV curves, the anode does not show obvious oxidation peaks under either aerobic or anaerobic conditions ( Figure 5 a, c). In the experiment under anaerobic conditions, the reduction potential of the reactor under high applied voltage (applied voltage of 2V, 4V, and 8V) was approximately -0.5V (relative to Ag / AgCl). When the applied voltage was 1V, the reduction potential of the reactor was close to 0.2V (relative to Ag / AgCl), and the maximum reduction current was 2.216mA (Table 3). The larger the reduction peak current, the higher the electron transfer efficiency. This shows that under the condition of an applied voltage of 1V, the electroactive microorganisms were well activated in the BES reactor. The reactor with an applied voltage of 0.5V and the control reactor did not show a clear reduction peak. Under aerobic conditions, when the applied voltage was 2V, 4V, and 8V, the reduction potential was -0.07V, -0.2V, and -0.5V, respectively. The control reactor and the reactors with voltages of 1V and 2V did not show a clear reduction peak. The closed area of ​​the CV image shows that the bioelectrochemical aerobic reactor has a higher capacitance, which improves the electron transfer efficiency of the biofilm and further degrades organic matter. Figure 4 The CV image shown in a corresponds to the degradation efficiency of the pollutant.

[0096] The EIS of the BES reactor was analyzed by equivalent circuit, including solution resistance, charge transfer resistance, Warburg impedance and capacitance (Table 3). The use of higher carbon nanotube loading reduced the charge transfer resistance and improved the reaction rate on the electrode. Bacteria formed a biofilm on the electrode surface and metabolized lactic acid, thereby reducing the electrode polarization resistance. In this experiment, the charge transfer resistance of the biofilm electrode under aerobic conditions (83.22-108 Ω) was about half that under anaerobic conditions (145.9-196.6 Ω) Figure 4 The d) shows its excellent electron transfer ability. Low charge transfer resistance means lower electron transfer obstacles inside the electrode biofilm, which is also consistent with the pollutant degradation efficiency under aerobic conditions. This indicates that the smaller the internal resistance and charge transfer resistance of the reactor, the higher the pollutant degradation efficiency. Interestingly, compared with other aerobic BES reactors, the charge transfer resistance of the bioelectrode in the BES reactor under aerobic conditions with an applied voltage of 0.5 V was the largest, at 108 Ω (Table 3). This is mainly due to the higher activity of microorganisms at low voltage, and the overabundance of biofilm may lead to an increase in internal resistance. There is no significant difference between the charge transfer resistance at high voltage and that at low voltage. Studies have shown that the driving force provided by the voltage for BES internal electron transfer is limited, and the applied voltage mainly acts as an activator and enriching agent for microorganisms.

[0097] Table 3 Electrochemical parameters of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in repeated cycles

[0098]

[0099]

[0100] 3. Surface morphology of the bioelectrode

[0101] The changes in the growth of microorganisms on the surface of the bio-cathode in the biodegradation systems of Examples 2-4 and Comparative Examples 1-4 were as follows Figure 5 and Figure 6As shown, the morphology of microorganisms on the bioelectrode surface was observed by SEM. The bioelectrode surface without electric field acclimation displayed a wide variety of microorganisms, exhibiting various characteristics, including varying shapes (spherical and rod-shaped) and size. This indicates the presence of diverse microorganisms on the bioelectrode surface. However, under the influence of applied voltage in the BES system, after electric field acclimation, electroactive microorganisms concentrated relatively uniformly on the carbon-modified bioelectrode at applied voltages of 0.5V, 1V, and 2V, regardless of aerobic or anaerobic conditions. Coccal and rod-shaped microbial cells, as well as other cell types, appeared on the bioelectrode surface under high voltage. The microbial morphology on the bioelectrode surface under aerobic conditions differed significantly from that under anaerobic conditions. The large number of microorganisms adhering to the bioelectrode surface may increase pollutant adsorption capacity. The complex pore structure and large specific surface area of ​​the Ti-CNT electrode surface enhance microbial growth and accumulation, leading to the formation of a stable biofilm. Furthermore, the nitric acid-treated CNT material ensures the availability of more catalytically active sites, thereby improving biofilm formation, electron transfer, and BES performance.

[0102] 4. Microbial community analysis

[0103] As the applied voltage changes and the aerobic and anaerobic conditions change, the composition of the microbial community in the BES reactor also changes during the culture process. The main phylum-level abundance of the microbial community in the electrode biofilm is as follows: Figure 7 and Figure 8 As shown, the main phyla are Actinobacteriota (30.11-71.17%) and Proteobacteria (18.35-71.80%). Actinobacteriota and Proteobacteria play an important role in carbon cycling and nitrogen transformation. These bacterial phyla are the most common bacterial phyla in sewage treatment systems. Different applied voltages have a significant effect on the composition of phylum-level bacterial communities on electroactive biofilm electrodes. Actinobacteriota can promote extracellular electron transfer, such as direct interspecies electron transfer (DIET) between electrodes and microorganisms and between microorganisms and organic matter, and can carry out denitrification and organic matter degradation. It is worth noting that Hydrogenedentes (4.64%) and Hydrogenedensaceae (4.64%) were significantly enriched on the anode biofilm ( Figure 8 ), but no electroactive representatives of this type of microorganism have been reported before. Hydrogenedensaceae and Dethiosulfatibacter are dehalogenating bacteria that can use hydrogen as an electron donor, which is beneficial for biofilm formation and denitrification.

[0104] Under anaerobic conditions, the BES cathode (Figure 8 Bacteroidetes were better enriched at 1 V. Chloroflexi were significantly enriched (13.93%) on the biofilm electrode when a 1 V voltage was applied. Chloroflexi are facultative anaerobes, and their filamentous structure is conducive to the attachment and growth of microorganisms. Alicycliphilus (13.13%) is a denitrifying bacteria that uses NO2 - and NO3 - as electron acceptors to participate in the nitrogen cycle. The specific function of Myxococcota and Deinococcota in the BES system has not been reported, but their abundance was significantly increased in the anaerobic BES system when a low voltage of 0.5 V and 1 V was applied. This means that it may benefit the BES biofilm system through potential microbial mechanisms. The relative abundance of denitrifying bacteria Truepera (2.31%) and nitrifying bacteria Reyranella (1.81%) was higher when a 1 V voltage was applied (0.09%, 0.66%) than in the control group and the reactor with an 8 V voltage (0%, 0.13%), and the activity of these two electroactive microorganisms was greatly inhibited at high voltage. In summary, the above-mentioned dominant strains may be the reason why the reactor with a 1 V voltage showed higher ammonia nitrogen removal efficiency during the degradation process.

[0105] The biological population in the BES under aerobic conditions was significantly different from the biofilm microbial community in the BES under anaerobic conditions. Interestingly, Pseudoxanthomonas (31.51-42.08%, 0.5 V and 1 V, respectively), which is a member of the family of bacteria that can reduce nitrate to nitrogen gas, was better enriched in the BES reactor under aerobic conditions (b) in the above table. Figure 7 Pseudoxanthomonas has denitrification ability and is electroactive. Bosea was also better enriched in the reactor under a voltage of 0.5 V; the abundance of Actinobacteriota and Rhodococcus in the BES reactor under aerobic conditions was 58.34% and 56.90%, respectively, which was higher than in other reactors. Rhodococcus erythropolis is believed to be able to transfer electrons in MFC without the need for exogenous electron shuttling. Bosea, Pseudoxanthomonas, and Rhodococcus are the main denitrifying functional genera in the reactor.

[0106] 5. Bioelectrochemical characteristics and electron transfer pathways

[0107] The degradation and utilization of organic matter in wastewater by electroactive microorganisms requires the synergistic action of multiple microorganisms, which positively influence each other by exchanging compounds. The main pathways for the decomposition and biosynthesis of organic pollutants by electroactive bacteria include the TCA cycle, glycolysis, amino acid metabolism, and menaquinone biosynthesis. The operation of BES depends on the growth and metabolism of microorganisms, which play a crucial role in determining the nitrogen uptake and release capacity of BES. Different applied voltages have a significant impact on the metabolic pathways of microbial communities.

[0108] The denitrification capacity of microorganisms depends on the abundance of functional enzymes in the reactor. Figure 9 The gene abundance distribution of nitrification and denitrification enzymes for Examples 2-4 and Comparative Examples 1-4 is shown. Among them, the NarG, NarI, and NarH genes play a role in the nitrification process, responsible for converting NO2 - -N to NO3 - -N. The relatively important genes in the denitrification process are NarG, NarB, NarI, and NarH. Under anaerobic conditions, the relative abundance of nitrogen metabolism in the reactor is not outstanding, but its role in the degradation of COD and NH4 + -N is confirmed. Under an aerobic voltage of 0.5 V, the abundance of the microbial oxidation / reduction TCA cycle, glycolysis, and manganic acid pathway is high. The increased abundance of the glyoxylate cycle and the TCA cycle produces a large amount of formate and L-glutamate, which are simultaneously supplied to the nitrogen cycle. The products produced by pathway metabolism, such as formate, can be used for nitrogen degradation.

[0109] Electrons can be transported between the inner membrane, periplasm, outer membrane, and electrode through cytochromes and menaquinone (Menaquinone, MQ) chains. The acceptor depends on the electron gradient, and extracellular electron transfer (EET) can be a bidirectional process. As a membrane-integrated electron carrier, MQ accepts electrons through cytochromes or cytoplasm and is reduced to MQH ++ 2. Then, electrons are transferred to the electrode or other cells through cytochrome C. As an important part of the electron transport chain, we found that there is a big difference in the abundance of the MQ biosynthesis pathway under aerobic and anaerobic conditions, and the synthesis pathway of MQ lacks the expression of MqnF under anaerobic conditions. Under aerobic conditions with oxygen and nitrate as electron acceptors, the electron transport process of MQ is significantly higher than that under anaerobic conditions. The synthesis pathway of MQ is also higher than that of other voltage-added reactors under low voltage. The aerobic reactor degrades NH4 + -N through aerobic denitrification, and Nar catalyzes the electron transport pathway in the nitrate reduction process, which is significantly higher than that in other reactors under an external voltage of 0.5 V. The abundant expression of MQ enables the biofilm to form a good electron transport mechanism under aerobic conditions, improving the efficiency of the biofilm in degrading pollutants.

[0110] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. The present application can have various changes and modifications for those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for treating low-concentration wastewater using a bioelectrochemical reactor, characterized in that: The bioelectrochemical reactor includes a reactor top cover and a reactor tank body, which are detachably connected. The reactor top cover is provided with an air inlet, and the reactor tank body is provided with an anode, a cathode, and an aeration device. The anode and cathode are respectively connected to an external DC power supply via titanium wires. The aeration device is connected to the air inlet. The anode and cathode are both carbon nanotube-modified titanium electrodes. A mixture of aerobic sludge and water is added to the reactor tank of the bioelectrochemical reactor, the voltage of the anode and cathode is controlled to be 0.2-2V, and electroactive bacteria are cultured on the surface of the carbon nanotube-modified titanium electrode; The reactor tank is cleaned, and then low-concentration wastewater is added into the reactor tank. The voltage of the anode and cathode is controlled to be 0.2~2V, and the bioelectrochemical aerobic reaction is carried out under aeration conditions.

2. The method according to claim 1, wherein A sampling hole is provided on the top cover of the reactor, and the titanium wire passes through the top cover of the reactor and is connected to an external direct current power supply.

3. The method according to claim 1, wherein The carbon nanotube modified titanium electrode is adhered to the titanium mesh by rolling the carbon nanotube slurry, and the carbon nanotube loading is controlled to be 10-20 mg / cm 2 ; and then dried.

4. The method according to claim 3, wherein The carbon nanotube slurry comprises carbon nanotubes modified by nitric acid soaking, a nickel chloride catalyst, a polytetrafluoroethylene emulsion binder, and a solvent; the solvent is a mixed solvent of water and ethanol; the amount ratio of the carbon nanotubes modified by nitric acid soaking, the nickel chloride catalyst, the polytetrafluoroethylene emulsion binder, and the solvent is 1g: (0.005-0.02)g: (0.1-0.2)mL: (30-50)mL.

5. The method according to claim 3, wherein After the drying step, the method further comprises the steps of soaking the carbon nanotube-modified titanium electrode in a 0.5-2 wt % sodium dodecyl sulfate solution for 20-30 hours and then drying the electrode.

6. The method according to claim 1, wherein The COD concentration of the low-concentration wastewater is 200~400mg / L, NH4 + -N concentration is 10~20mg / L.

7. The method according to claim 1, wherein The volume ratio of the aerobic sludge to water is (2-4):(6-8); and the time for culturing the electroactive bacteria on the surface of the carbon nanotube-modified titanium electrode is 12-18 days.

8. The method according to claim 1, wherein The aeration flow rate is 80~150mL / min.

9. The method according to claim 1, wherein The time for the bioelectrochemical aerobic reaction under aeration conditions is 8 to 15 hours.

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