CN118724248A - A bioelectrochemical reactor and a method for treating low-concentration wastewater using the same - Google Patents

Abstract

The present invention discloses a bioelectrochemical reactor and a method for treating low-concentration wastewater using the same, and belongs to the technical field of low-concentration wastewater treatment. The bioelectrochemical reactor of the present invention comprises a reactor top cover and a reactor tank body; an air inlet is arranged on the reactor top cover, and an anode, a cathode and an aeration device are arranged inside the reactor tank body, and the anode and the cathode are respectively connected to an external DC power supply through a titanium wire; the aeration device is connected to the air inlet; and the anode and the cathode are both carbon nanotube modified titanium electrodes. When the present invention uses the above-mentioned bioelectrochemical reactor to treat low-concentration wastewater, aerobic sludge is first used to cultivate electroactive bacteria on the electrode surface, and then a bioelectrochemical aerobic reaction is carried out, and the COD and _NH4 ⁺ -N removal efficiency of low-concentration wastewater within 12 hours can be achieved under aeration conditions and low voltage, and the excellent effect of up to 95% or more can be achieved, the electrode has low corrosion, the treatment method is simple and efficient, and no secondary pollution is generated, and it has broad application prospects.

Description

A bioelectrochemical reactor and a method for treating low-concentration wastewater using the same

Technical Field

The present invention relates to the technical field of low-concentration wastewater treatment, and in particular to a bioelectrochemical reactor and a method for treating low-concentration wastewater using the same.

Background Art

The information disclosed in the background of the invention is only intended to enhance the understanding of the overall background of the invention and should not be necessarily regarded as an acknowledgment or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

Typical wastewater discharge indicators include: ammonia nitrogen (NH ₄ ⁺ -N), chemical oxygen demand (COD), heavy metals, etc. Although wastewater treatment plants carry out multi-stage treatment of wastewater, it is still difficult for wastewater discharged from wastewater treatment plants to meet the required discharge standards. In particular, the deep treatment of tail water is relatively difficult because its organic matter concentration and biodegradability are relatively low. Therefore, some wastewater treatment plants use advanced oxidation methods such as Fenton, ozone oxidation and filtration in terminal treatment. However, these methods are costly and there is a risk of secondary pollution. Electrochemical treatment degrades pollutants through the action of free radicals released in electrochemical cells. For example, electro-oxidation directly relies on the anode to form strong oxidants such as hydroxyl radicals (-OH) or hypochlorite ions. This treatment technology has low chemical additions and no secondary emissions.

However, the efficiency of the electrochemical treatment process depends largely on the concentration of organic compounds in the solution and the electron transfer conditions between the electrode and the solution. Moreover, after long-term operation of the electrochemical treatment, the anode material will be corroded, resulting in reduced pollutant removal efficiency. The pollutant removal efficiency in the electrochemical treatment process is usually improved by increasing the current density during the treatment process; however, when the electrochemical system reaches the optimal treatment conditions, the high current density used will lead to higher power and energy consumption and faster electrode corrosion.

In order to overcome the shortcomings of electrochemical methods, attempts have been made in recent years to combine electrochemical processes with biological processes as pretreatment or post-treatment methods. Bioelectrochemical system (BES) technology is a new type of biological wastewater treatment technology that has significant advantages over 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 pollutant degradation, 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 bioelectrochemical reactor and method with high efficiency in treating low-concentration sewage is an urgent problem to be solved.

Summary of the invention

In view of this, the present invention provides a bioelectrochemical reactor and a method for treating low-concentration wastewater using the same, which solves the problems of high cost, secondary pollution or severe electrode corrosion in existing low-concentration wastewater treatment processes.

In the first aspect, the present invention provides a bioelectrochemical reactor, comprising a reactor top cover and a reactor tank body, wherein the reactor top cover and the reactor tank body are detachably connected; an air inlet is arranged on the reactor top cover, and an anode, a cathode and an aeration device are arranged inside the reactor tank body, wherein the anode and the cathode are respectively connected to an external DC power supply through titanium wire; the aeration device is connected to the air inlet; and the anode and the cathode are both carbon nanotube-modified titanium electrodes.

Preferably, a sampling hole is provided on the reactor top cover, and the titanium wire passes through the reactor top cover and is connected to an external DC power supply.

Preferably, the carbon nanotube-modified titanium electrode is obtained by adhering carbon nanotube slurry to a titanium mesh by a roller pressing method, controlling the loading amount of carbon nanotubes to be 10-20 mg/cm ² , and then drying.

Furthermore, the carbon nanotube slurry comprises carbon nanotubes modified by immersion in nitric acid, a nickel chloride catalyst, a polytetrafluoroethylene emulsion binder and a solvent; the solvent is a mixed solvent of water and ethanol.

Furthermore, the usage ratio of the carbon nanotubes modified by immersion in nitric acid, the nickel chloride catalyst, the polytetrafluoroethylene emulsion binder and the solvent is 1 g: (0.005-0.02) g: (0.1-0.2) mL: (30-50) mL.

Furthermore, 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.

In a second aspect, the present invention provides a method for treating low-concentration wastewater using the above-mentioned bioelectrochemical reactor, comprising the following steps:

Adding a mixture of aerobic sludge and water into the reactor tank of the bioelectrochemical reactor, controlling the voltage of the anode and cathode to be 0.2-2V, and culturing electroactive bacteria on the surface of the carbon nanotube-modified titanium electrode under aeration conditions;

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 a bioelectrochemical aerobic reaction is carried out under aeration conditions.

Preferably, the COD concentration of the low-concentration wastewater is 200-400 mg/L, and the NH ₄ ⁺ -N concentration is 10-20 mg/L.

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.

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

Preferably, the bioelectrochemical aerobic reaction is carried out under aeration conditions for 8 to 15 hours.

Compared with the prior art, the present invention has achieved the following beneficial effects:

The bioelectrochemical reactor provided by the present invention has a simple structure and low cost. When it is used to treat low-concentration wastewater, the COD and _NH4 ⁺ -N removal efficiency of the low-concentration wastewater can be as high as 95% or more within 12 hours under aeration conditions and low voltage. The electrode has low corrosion, the treatment method is simple and efficient, and no secondary pollution is generated, so it has broad application prospects.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constituting part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their description are used to explain the present invention and do not constitute an improper limitation of the present invention. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative work.

FIG1 is a schematic diagram of a bioelectrochemical reactor according to Example 1 of the present invention;

FIG2 is a COD degradation curve and an NH ₄ ⁺ -N degradation curve of the bioelectrochemical anaerobic reactor of Comparative Example 4 of the present invention under different applied voltages;

FIG3 is a COD degradation curve and NH ₄ ⁺ -N degradation curve of the bioelectrochemical aerobic reactor of Examples 2 to 4 of the present invention and Comparative Examples 1 to 3;

Fig. 4 is the CV and EIS curves of the reaction experiments of Examples 2 to 4 and Comparative Examples 1 to 4 of the present invention at the end of the 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 to 4 and Comparative Examples 1 to 3, and d is the EIS curve of Examples 2 to 4 and Comparative Examples 1 to 3;

5 is a scanning electron microscope (SEM) image of the electrode surface of Comparative Example 4 of the present invention, wherein (a) 0 V, (b) 0.5 V, (c) 1 V, (d) 2 V, (e) 4 V, and (f) 8 V;

FIG6 is a scanning electron microscope (SEM) image of the electrode surface of Examples 2 to 4 and Comparative Examples 1 to 4 of the present invention, wherein (a) Comparative Example 1, (b) Example 2, (c) Example 3, (d) Example 4, (e) Comparative Example 2, and (f) Comparative Example 3;

FIG7 shows the main phylum-level abundance of microbial communities in electrode biofilms of Examples 2 to 4 and Comparative Examples 1 to 3 of the present invention; (a) the abundance of bacterial phyla in aerobic biofilms of Examples 2 to 4 and Comparative Examples 1 to 3 at different voltages, (b) the abundance of bacterial genera in aerobic biofilms of Examples 2 to 4 and Comparative Examples 1 to 3 at different voltages;

FIG8 is a diagram showing the main phylum-level abundance of the microbial community in the electrode biofilm of Comparative Example 4 of the present invention; wherein (a) the abundance of the bacterial phylum of the electrode anaerobic biofilm of Comparative Example 4 at different voltages, and (b) the abundance of the bacterial genus of the electrode anaerobic biofilm of Comparative Example 4 at different voltages;

9 shows the abundance of microbial metabolic pathways in the anaerobic (a) electrochemical biofilms of Comparative Example 4 of the present invention and the aerobic (b) electrochemical biofilms of Examples 2 to 4 and Comparative Examples 1 to 3.

DETAILED DESCRIPTION

It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

The invention provides a bioelectrochemical reactor, comprising a reactor top cover and a reactor tank body, wherein the reactor top cover and the reactor tank body are detachably connected; an air inlet is arranged on the reactor top cover, an anode, a cathode and an aeration device are arranged inside the reactor tank body, the anode and the cathode are respectively connected to an external direct current power supply through a titanium wire; the aeration device is connected to the air inlet; and the anode and the cathode are both carbon nanotube-modified titanium electrodes.

In the present invention, the reactor top cover and the reactor tank body are detachably connected, which is convenient for replacing parts such as the anode, cathode or aeration device. The present invention does not impose any special restrictions on the above-mentioned detachable connection method, which can be a threaded connection or a snap connection or other connection methods. The present invention does not impose any special restrictions on the position and method of feeding and discharging. Automatic feeding and discharging can be achieved by adding a feed port and a discharge port on the top cover. The present invention does not impose any special restrictions on the setting position; in a small reactor, the reactor top cover can also be separated from the reactor tank body to achieve direct injection and pouring of low-concentration wastewater.

In the present invention, a sampling hole is arranged on the top cover of the reactor, and the sampling hole is convenient for real-time monitoring of the wastewater being treated to determine the removal degree of COD and ammonia nitrogen.

In the present invention, the titanium wire passes through the reactor top cover and is connected to an external DC power supply. Furthermore, in order to avoid the adverse effect of the reactor top cover on the conductivity of the titanium wire, an insulating tape is wrapped around the contact portion of the titanium wire or an insulating heat shrink tube is provided.

In the present invention, the carbon nanotube modified titanium electrode is obtained by adhering the carbon nanotube slurry to the titanium mesh by a roller pressing method, controlling the loading amount of the carbon nanotubes to be 10-20 mg/cm ² ; and then drying. The roller pressing method is simple and has low equipment requirements and low operating costs. The CNT of the carbon nanotube modified titanium electrode can improve the conductivity and electrochemical properties of the electrode in the reactor. The use of a higher carbon nanotube loading rate reduces the charge transfer resistance, increases the electron transfer rate between the electrode and the biofilm, and thus increases the reaction rate of the microorganisms on the electrode.

In the present invention, the carbon nanotube slurry comprises carbon nanotubes modified by nitric acid soaking, nickel chloride catalyst, polytetrafluoroethylene emulsion binder and solvent; the solvent is a mixed solvent of water and ethanol. The hydrophilicity of the carbon nanotubes modified by nitric acid soaking is further improved. The nickel chloride catalyst can further increase the reaction rate of the electrode. The polytetrafluoroethylene emulsion binder can make the carbon nanotubes firmly adhere to the titanium mesh, ensuring the stability of the carbon nanotube modified titanium electrode.

In the present invention, the usage 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.

In the present invention, after the drying step, the carbon nanotube modified titanium electrode is further soaked in a 0.5-2wt% sodium dodecyl sulfate solution for 20-30 hours and then dried. This treatment step is to further improve the hydrophilicity of the electrode, which is beneficial to the loading of microorganisms on the electrode.

The present invention also provides a method for treating low-concentration wastewater using the above-mentioned bioelectrochemical reactor, comprising the following steps:

Adding a mixture of aerobic sludge and water into the reactor tank of the bioelectrochemical reactor, controlling the voltage of the anode and cathode to be 0.2-2V, and culturing electroactive bacteria 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 a bioelectrochemical aerobic reaction is carried out under aeration conditions.

In the present invention, the COD concentration of the low-concentration wastewater is 200-400 mg/L, and the NH ₄ ⁺ -N concentration is 10-20 mg/L. The treatment method of the present invention is particularly suitable for treating the low-concentration wastewater, with high treatment efficiency and excellent COD and ammonia nitrogen removal effects.

In the present invention, the volume ratio of aerobic sludge to water is (2-4): (6-8), and more preferably 3: 7. Under the above ratio, it is more conducive to inoculating the surface of the carbon nanotube-modified titanium electrode with electroactive bacteria.

The present invention does not impose any special restrictions on the source of aerobic sludge. The present invention preferably uses sludge from the aerobic treatment stage of a sewage treatment plant, and its technical indicators preferably meet the following conditions: pH is 7-8; moisture content is 25-50%; total solids (TS) is 10-25wt%; volatile solids account for 8-13wt% of TS; total chemical oxygen demand (TCOD) is 6000-8000mg/L; soluble chemical oxygen demand (sCOD) is 1000-5000mg/L.

In the present invention, the time for culturing the electroactive bacteria on the surface of the carbon nanotube-modified titanium electrode is 12 to 18 days, more preferably 14 to 16 days, and most preferably 15 days.

After culturing the electroactive bacteria, the present invention needs to clean the reactor tank to remove the aerobic sludge; another clean reactor tank can also be selected to carry out subsequent bioelectrochemical reactions. However, in either case, the electroactive bacteria on the surface of the carbon fiber modified titanium electrode must be retained.

In the present invention, low-concentration wastewater is added into the reactor tank, and the voltage is controlled to be 0.3-0.7V.

In the present invention, the aeration flow rate is 80-150 mL/min. The aeration in the present invention is preferably continuous aeration to continuously provide dissolved oxygen for aerobic microorganisms.

In the present invention, the time for the bioelectrochemical aerobic reaction under aeration conditions is 8 to 15 hours, more preferably 12 to 15 hours.

The technical solution of the present invention is further described below in conjunction with specific embodiments.

In the following examples and comparative examples, aerobic sludge was taken from the aerobic treatment stage of a sewage treatment plant in Jinan City, and anaerobic sludge was taken from the anaerobic digestion stage of a sewage treatment plant in Jinan City. The physical and chemical properties of aerobic sludge and anaerobic sludge are shown in Table 1.

Table 1 Physical and chemical properties of aerobic sludge and anaerobic sludge

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

The low-concentration wastewater (per liter) used in the following examples and comparative experiments was prepared according to the following conditions: 300 mg COD (CH ₃ COONa), 45 mg NH ₄ ⁺ -N (NH ₄ Cl), 4 mg PO ₄ ^3- -P (KH ₂ PO ₄ ) and 1 mL trace element solution. Trace elements (per liter): 2.0 mg biotin, 2.0 mg folic acid, 10.0 mg pyridoxine, 5.0 mg thiamine hydrochloride, 5.0 mg riboflavin hydrochloride, 5.0 mg nicotinic acid, 5.0 mg DL-calcium pantothenate, 0.1 mg vitamin B12, 5.0 mg p-aminobenzoic acid, 5.0 mg lipoic acid. The COD value of the artificial low-concentration wastewater prepared was measured to be 300 mg/L, and the ammonia nitrogen content (NH ₄ ⁺ -N) was 16 mg/L.

Example 1

This embodiment provides a bioelectrochemical reactor, as shown in FIG1 , comprising a reactor tank body 1 and a reactor top cover 2, wherein the reactor top cover 2 and the reactor tank body 1 are detachably connected; a sampling hole 3 and an air inlet 4 are arranged on the reactor top cover 2, an anode 5, a cathode 6 and an aeration device 9 are arranged inside the reactor tank body 1, the anode 5 and the cathode 6 are respectively connected to an external DC power supply 8 through a titanium wire 7 (φ=1.8 mm) passing through the reactor top cover 2, and an insulating heat shrink tube (not shown in the figure) is arranged where the titanium wire 7 contacts the reactor top cover 2. The aeration device 9 is connected to the air inlet 4; the anode 5 and the cathode 6 are both carbon nanotube modified titanium electrodes, and the distance between the anode 5 and the cathode 6 is 2 cm.

The preparation method of carbon nanotube modified titanium electrode is as follows:

The electrode is based on a titanium mesh of 6 cm × 2 cm and is pre-soaked in anhydrous ethanol for 24 hours to remove surface impurities. Multi-walled carbon nanotubes (CNTs) are soaked in a concentrated nitric acid solution for 24 hours to improve their hydrophilicity, then rinsed continuously with distilled water to make the pH value greater than 6, and dried. Nickel chloride with a concentration of 1wt% is added to 2.5g of nitric acid-modified CNTs, and 100mL of 80% ethanol solution is used as a solvent for uniform stirring to fully mix. Polytetrafluoroethylene emulsion (0.375mL, solid content 55%) is added to CNTs as a binder. 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 pre-treated carbon nanotube slurry is adhered to the titanium mesh with a roller press to control the carbon nanotube loading to 15mg/ ^cm2 ; then dried in an oven at 50°C. Then, the electrode was immersed in a 1 wt % sodium dodecyl sulfate solution for 24 hours to improve its hydrophilicity, and then dried.

Example 2

This embodiment provides a method for treating low-concentration wastewater using the bioelectrochemical reactor of Example 1.

(1) Cultivating electroactive bacteria: aerobic sludge and pure water were inoculated into the bioelectrochemical reactor of Example 1 at a volume ratio of 3:7, the voltage between the anode 5 and the cathode 6 was controlled to be 0.5 V, and the electroactive bacteria were cultured on the surface of the carbon nanotube-modified titanium electrode for 15 days.

(2) Bioelectrochemical aerobic reaction: Clean the reactor tank 1 to retain the electroactive bacteria on the surface of the carbon nanotube-modified titanium electrode. Add 200 mL of low-concentration wastewater into the 250 mL reactor tank. Control the voltage between the anode 5 and the cathode 6 to 0.5 V. Perform the bioelectrochemical aerobic reaction at an aeration flow rate of 100 mL/min. Take samples from the sampling hole 3 at different treatment times to detect the COD concentration and _NH4 ⁺ -N concentration of the treated wastewater.

Example 3

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

Example 4

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

Comparative Example 1

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

Comparative Example 2

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

Comparative Example 3

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

Comparative Example 4

Compared with Example 2, the difference is that a bioelectrochemical anaerobic reaction occurs in this comparative example.

(1) Cultivating electroactive bacteria: Anaerobic sludge and pure water were inoculated into the bioelectrochemical reactor of Example 1 at a volume ratio of 3:7, and the voltage was controlled to be 0, 0.5, 1, 2, 4, and 8 V, respectively. The electroactive bacteria were cultured on the surface of the carbon nanotube-modified titanium electrode for 15 days.

(2) Bioelectrochemical anaerobic reaction: clean the reactor tank 1, retain the electroactive bacteria on the surface of the carbon nanotube-modified titanium electrode, add low-concentration wastewater into the reactor tank 1, and seal the air inlet 4; control the voltage between the anode 5 and the cathode 6 to 0, 0.5, 1, 2, 4, and 8 V (corresponding to the voltage in step (1)), perform bioelectrochemical reaction under anaerobic conditions, take samples from the sampling hole 3 at different treatment times, and detect the COD concentration and _NH4 ⁺ -N concentration of the treated wastewater.

Comparative Example 5

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

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. An 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 NH ₄ ⁺ -N concentration of the treated wastewater.

Comparative Example 6

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

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, an electrochemical anaerobic reaction was carried out under anaerobic conditions, samples were taken from the sampling hole 3 at different treatment times, and the COD concentration and NH ₄ ⁺ -N concentration of the treated wastewater were detected.

Test example

1. COD, NH ₄ ⁺ -N removal efficiency

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.

Table 2 COD and NH ₄ ⁺ -N removal efficiency in low concentration wastewater treatment process (%)

It can be seen from Table 2 that 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).

The COD degradation curve and NH ₄ ⁺ -N degradation curve of the bioelectrochemical anaerobic reactor of comparative example 4 under different applied voltages are shown in Figure 2. After 30 hours, the removal rate of its COD is much higher than that of the electrochemical anaerobic reactor. In the anaerobic degradation process, the degradation rate of the COD concentration of the electrochemical anaerobic reactor under different applied voltages is quite different. The bioelectrochemical anaerobic reactor has the best degradation effect on wastewater treatment after 30 hours when the applied voltage is 1V, and the COD concentration drops from 300mg/L to 7.04mg/L. Under the condition of lower applied voltage (0-2V), the applied voltage has a significant stimulating effect on the electroactive biofilm, which can accelerate the metabolism of microorganisms and degrade organic matter more effectively. It is worth noting that when the applied voltage is higher than 2V, the degradation effect of COD in the reactor will decrease instead. This is because the growth and metabolism of microorganisms are inhibited under high voltage, resulting in a decrease in microbial activity after long-term power-on so that it no longer grows. The electron transfer in the biofilm electrode is complex, such as metal conduction and redox conduction. At higher applied voltages, 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 1V, the _NH4 ⁺ -N concentration dropped from 16mg/L to 0.15mg/L. In the presence of biofilm, the degradation rate of _NH4 ^{+-N in the reactor was faster within the first 24 hours of the reaction cycle. Denitrifying microorganisms in the biofilm on the cathode surface used H2 produced by electrolysis of water to reduce NO3- to N2 gas. At low voltages, the removal effect of NH4+} _{-N increased} ^{significantly} _{. As} ^the applied voltage increased, the hydrogen produced by electrolysis of water produced a hydrogen inhibition effect, reducing the degradation effect of ammonia nitrogen. In addition, the activity of nitrifying bacteria was also inhibited at high voltages, and the conversion rate of _NH4 ⁺ -N to nitrate in the effluent slowed down, resulting in reduced denitrification efficiency.

The COD degradation curves and NH ₄ ⁺ -N degradation curves of the bioelectrochemical aerobic reactors of Examples 2 to 4 and Comparative Examples 1 to 3 are shown in FIG3 . Compared with anaerobic conditions, the bioelectrochemical reactor under aerobic conditions has a significantly enhanced degradation effect on the two target pollutants, COD and NH ₄ ⁺ -N. Under the condition of 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 NH ₄ ⁺ -N reached 0.03 mg/L, and it was reached and stabilized within 9 hours. Oxygen is the most commonly used electron acceptor in the cathode of the bioelectrochemical system (BES), with strong availability and high redox potential. Under high dissolved oxygen (DO) concentrations, the nitrification reaction can completely reduce NO ₃ ^- to N ₂ . 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 nitrates, and may compete with ammonia oxidizing bacteria (AOB) for oxygen in the reactor. The system voltage of Examples 2 to 4 is low, and has better COD and NH ₄ ⁺ -N removal efficiency. In contrast, under the condition of an applied voltage of 0.5V (Example 2), the COD removal rate of 12h reached 98.68 ± 0.38%, and the NH ₄ ⁺ -N removal rate reached 99.78 ± 0.10%, showing the best degradation efficiency. This shows that the microbial community structure in the system under the condition of applying a voltage of 0.5V is more suitable for denitrification, and the metabolic level is also higher than that of the reactors applying different voltages. The degradation efficiency of the system under the condition of applying a voltage of 8V (Comparative Example 3) is higher than that of the system under the condition 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.

2. Bioelectrochemical characteristics

Analysis of CV and EIS at the end of the reaction experiments of Examples 2 to 4 and Comparative Examples 1 to 4 can show the enrichment performance 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 curve, the anode does not show obvious oxidation peaks under either aerobic or anaerobic conditions (a, c in Figure 5). In the experiment under anaerobic conditions, the reduction potential of the reactor under high applied voltage (applied voltage is 2V, 4V, 8V) is about -0.5V (relative to Ag/AgCl). When the applied voltage is 1V, the reduction potential of the reactor is close to 0.2V (relative to Ag/AgCl), and the maximum reduction current is 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 are well activated in the BES reactor. The reactor with an applied voltage of 0.5V and the control reactor did not show obvious reduction peaks. Under aerobic conditions, when the applied voltage was 2V, 4V and 8V, the reduction potential was -0.07V, -0.2V and -0.5V, respectively. No obvious reduction peaks appeared in the control reactor and the reactors with voltages of 1V and 2V. The closed area of the CV image shows that the bioelectrochemical aerobic reactor has a higher capacitance, thereby improving the electron transfer efficiency of the biofilm and further degrading organic matter. The CV image shown in a of Figure 4 corresponds to the degradation efficiency of the pollutants.

Equivalent circuit analysis of the EIS in the BES reactor was performed, including solution internal resistance, charge transfer resistance, Weber impedance and capacitance (Table 3). The use of a higher carbon nanotube loading rate reduced the charge transfer resistance and increased the reaction rate on the electrode. Bacteria form biofilms on the electrode surface and metabolize using lactic acid, thereby reducing the electrode polarization resistance. In this experiment, it was observed that the charge transfer resistance of the biofilm electrode under aerobic conditions (83.22-108Ω) was about half of that under anaerobic conditions (145.9-196.6Ω) (Figure 4d), showing its excellent electron transfer ability. Low charge transfer resistance means that the electron transfer barrier inside the electrode biofilm is low, which is also consistent with the pollutant degradation efficiency under aerobic conditions. This shows 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 exhibited by the bioelectrode in the BES reactor with an applied voltage of 0.5V under aerobic conditions is the largest, at 108Ω (Table 3). This is mainly due to the higher activity of microorganisms at low voltages, and excessive enrichment of biofilms may lead to increased 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 voltage for electron transfer within BES is limited, and the role of applied voltage is mainly as an activator and enricher of microorganisms.

Table 3 Electrochemical parameters of cyclic voltammogram (CV) and electrochemical impedance spectroscopy (EIS) in repeated cycles

3. Bioelectrode Surface Morphology

The changes in the growth of microorganisms on the biocathode surface in the biodegradation systems of Examples 2 to 4 and Comparative Examples 1 to 4 are shown in Figures 5 and 6. The morphology of microorganisms on the surface of the bioelectrode was observed by SEM. There are many types of microorganisms on the surface of the bioelectrode that have not been domesticated by the electric field, and the microorganisms show various characteristics, including different shapes (spherical and rod-shaped) and changes in size. This shows that there are many types of microorganisms on the surface of the bioelectrode. However, under the influence of the applied voltage in the BES system, after domestication by the electric field, the types of electroactive microorganisms in the reactors with applied voltages of 0.5V, 1V, and 2V under aerobic or anaerobic conditions are relatively uniformly gathered on the carbon-modified bioelectrode. Coccal and rod-shaped microbial cells and other types of cells appeared on the surface of the bioelectrode under high voltage. The morphology of microorganisms on the surface of the bioelectrode under aerobic conditions is quite different from that of the bioelectrode under anaerobic conditions. A large number of microorganisms adhere to the surface of the bioelectrode, which may increase the adsorption capacity of pollutants. The surface void structure of the Ti-CNT electrode is complex and the specific surface area is large, which enhances the growth and enrichment of microorganisms to form a stable biofilm. Furthermore, the nitric acid-treated CNT materials ensured the availability of more catalytically active sites, thereby improving biofilm formation, electron transfer, and BES performance.

4. Microbial community analysis

The composition of the microbial community in the BES reactor also changes during the culture process with the change of applied voltage and the change of aerobic and anaerobic conditions. The main phylum-level abundance of the microbial community in the electrode biofilm is shown in Figures 7 and 8, among which the main phyla are Actinobacteriota (30.11-71.17%) and Proteobacteria (18.35-71.80%). Actinobacteriota and Proteobacteria play an important role in carbon cycle 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 perform 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), and the electroactive representatives of this type of microorganisms have not been reported before. Hydrogenedensaceae and Dethiosulfatibacter are dehalogenase bacteria that can use hydrogen as an electron donor, which is beneficial to the formation of biofilm and denitrification.

At the BES cathode under anaerobic conditions (a in Figure 8), Bacteroidetes were better enriched at a voltage of 1V. When a voltage of 1V was applied, the number of Chloroflexi on the biofilm electrode increased significantly (13.93%). Chloroflexi are facultative anaerobes, and their filamentous structure is conducive to the attachment and growth of microorganisms. Alicycliphilus (13.13%) is a denitrifying bacterium that participates in the nitrogen cycle using _NO2- and _NO3- as electron ^acceptors . The specific functions of Myxococcota and Deinococcota in the BES system have not been reported, but their abundance increased significantly in the anaerobic BES system with low voltages of 0.5V and 1V. 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%) when 1V voltage was applied (0.09%, 0.66%) was higher than that of the control group and the reactor with 8V voltage (0%, 0.13%). The activity of these two electroactive microorganisms will be greatly inhibited under high voltage. In summary, the above-mentioned dominant strains may be the reason why the reactor with 1V voltage showed higher ammonia nitrogen removal efficiency during the degradation process.

The biota within the BES under aerobic conditions were significantly different from the bioelectrode microbial community within the BES under anaerobic conditions. Interestingly, Pseudoxanthomonas (31.51-42.08%, b in Figure 7) was better enriched in the BES reactor under aerobic conditions. Pseudoxanthomonas has denitrification ability and is electroactive. Bosea in the reactor was also better enriched under an applied voltage of 0.5V; under aerobic conditions, the abundance of Actinobacteriota and Rhodococcus in the BES reactor was 58.34% and 56.90%, respectively, which was higher than that in other reactors. Rhodococcus erythropolis is believed to be able to transfer electrons in the MFC without the need for exogenous electron shuttling. Bosea, Pseudoxanthomonas and Rhodococcus are the main denitrifying functional bacterial genera in the reactor.

5. Bioelectrochemical characteristics and electron transfer pathways

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 menadione biosynthesis. The operation of BES depends on the growth and metabolism of microorganisms, which play a vital role in determining the nitrogen absorption and release capacity of BES. Different applied voltages have a significant effect on the metabolic pathways of the microbial community.

The denitrification capacity of microorganisms depends on the abundance of functional enzymes in the reactor. Figure 9 shows the gene abundance distribution of nitrification and denitrification enzymes in Examples 2 to 4 and Comparative Examples 1 to 4. Among them, NarG, NarI and NarH genes play a role in the nitrification process and are responsible for converting _NO2 ^-- N into _NO3 ^-- N. 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 prominent, but its degradation effect on COD and _NH4 ⁺ -N has been confirmed. Under the condition of an aerobic voltage of 0.5V, the abundance of microbial oxidation/reduction TCA cycle, glycolysis and manganate pathways is relatively high. The increased abundance of glyoxylate cycle and TCA cycle produces a large amount of formate and L-glutamate, and is simultaneously supplied to the nitrogen cycle. The products produced by pathway metabolism (such as formate) can be used for nitrogen degradation.

Electrons can be transferred between the inner membrane, periplasm, outer membrane and electrode through cytochromes and 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, the electrons are transferred to the electrode or other cells through cytochrome C. As an important component of the electron transport chain, we found that the abundance of the biosynthetic pathway of MQ under aerobic and anaerobic conditions was very different, and the synthesis pathway of MQ under anaerobic conditions lacked the expression of MqnF. Under aerobic conditions with oxygen and nitrate as electron acceptors, the electron transfer process of MQ was significantly higher than that under anaerobic conditions. The synthesis pathway of MQ under low voltage was also higher than that of other reactors with applied voltage. Aerobic reactors degrade NH ₄ ⁺ -N through aerobic denitrification, and the electron transfer pathway in the process of Nar-catalyzed nitrate reduction was relatively clear, which was significantly higher than that of other reactors under an applied voltage of 0.5V reactor. The abundant expression of MQ enables the biofilm to form a good electron transfer mechanism under aerobic conditions, thus improving the efficiency of the biofilm in degrading pollutants.

The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims (10)

1. The bioelectrochemical reactor is characterized by comprising a reactor top cover and a reactor tank body, wherein the reactor top cover is detachably connected with the reactor tank body; an air inlet is formed in the top cover of the reactor, an anode, a cathode and an aeration device are arranged in the reactor tank body, and the anode and the cathode are respectively connected with an external direct current power supply through titanium wires; the aeration device is connected with the air inlet hole; the anode and the cathode are both carbon nanotube modified titanium electrodes.

2. The bioelectrochemical reactor as claimed in claim 1, wherein a sampling hole is provided in the reactor top cover, and the titanium wire is connected with an external direct current power supply through the reactor top cover.

3. The bioelectrochemical reactor according to claim 1, wherein the carbon nanotube modified titanium electrode is adhered to the titanium mesh by a roll method from carbon nanotube slurry, and the loading amount of the carbon nanotubes is controlled to be 10-20 mg/cm ²; and then drying.

4. The bioelectrochemical reactor of claim 3, wherein said carbon nanotube slurry comprises nitric acid-soaked modified carbon nanotubes, a nickel chloride catalyst, a polytetrafluoroethylene emulsion binder, and a solvent; the solvent is a mixed solvent of water and ethanol; the dosage ratio of the nitric acid soaking modified carbon nano tube, 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 bioelectrochemical reactor as claimed in claim 3, wherein said drying step further comprises a step of immersing the carbon nanotube-modified titanium electrode in a 0.5 to 2wt% sodium dodecyl sulfate solution for 20 to 30 hours, followed by drying.

6. A method for treating low concentration wastewater using the bioelectrochemical reactor as claimed in any one of claims 1 to 5, comprising the steps of:

Adding the mixture of aerobic sludge and water into a reactor tank body of the bioelectrochemical reactor, controlling the voltage of an anode and a cathode to be 0.2-2V, and culturing electroactive strains on the surface of the carbon nanotube modified titanium electrode;

Cleaning a reactor tank body, adding low-concentration wastewater into the reactor tank body, controlling the voltage of an anode and a cathode to be 0.2-2V, and performing bioelectrochemical aerobic reaction under the aeration condition.

7. The method according to claim 6, wherein the COD concentration of the low concentration wastewater is 200-400 mg/L and the NH ₄ ⁺ -N concentration is 10-20 mg/L.

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

9. The method according to claim 6, wherein the aeration flow rate is 80 to 150mL/min.

10. The method according to claim 6, wherein the bioelectrochemical aerobic reaction is performed under aeration for a period of 8 to 15 hours.