A bioelectrochemical denitrification system using a slow-release carbon source as the anode fuel

Abstract

This invention discloses a bioelectrochemical denitrification system using a slow-release carbon source as the anode fuel, belonging to the field of microbial fuel cell technology. It comprises an anode chamber, a cathode chamber, an anode, a cathode, wires, and a separating membrane between the anode and cathode chambers. Electrochemically active microorganisms grow on the anode in the anode chamber, and anaerobic fermentation bacteria are present in the solution. An electrolyte and hydrogel particles with slow-release carbon function are added to the anode chamber. Electrochemically active denitrifying bacteria grow on the cathode in the cathode chamber. The hydrogel-released carbon serves as the solid fuel at the anode, providing a continuous electron donor to the cathode. Simultaneously, the selective permeation membrane controls the transmembrane transport of small organic molecules to the cathode, providing a carbon source for the denitrifying bacteria. The combined supply of electrons and carbon enhances denitrification by the denitrifying microorganisms at the cathode. This system is suitable for enhanced denitrification of polluted water bodies with low C / N ratios.

Description

Technical Field

[0001] This invention belongs to the field of microbial fuel cell technology, and particularly relates to a bioelectrochemical denitrification system using a slow-release carbon source as the anode fuel. Background Technology

[0002] The removal of nitrates from polluted water bodies has received widespread attention. For polluted water bodies with low C / N ratios, the efficiency of microbial denitrification is often limited due to a lack of sufficient organic carbon to provide electrons for heterotrophic denitrifying bacteria. Although denitrification efficiency can be improved by supplementing the polluted water with carbon sources, strict control is required over the amount and frequency of organic carbon addition; otherwise, secondary pollution can easily occur.

[0003] Based on the above limitations, a method has been developed in which electrons generated by the decomposition of organic matter by anode microorganisms in a microbial fuel cell system are transferred to the cathode, and denitrifying bacteria at the cathode can use them for denitrification. This method provides a new approach for the enhanced removal of nitrogen from C / N ratio wastewater.

[0004] Microbial fuel cell (MFC) reactors can generate electricity using various substrates. However, when using various organic wastewaters as substrates, the power density of MFC power generation has remained relatively low, thus limiting its application (Kelly and He, 2014). The maximum power density of MFCs using slow-release carbon sources as substrates is 427.6 ± 15.6 mW / m². Given the low energy recovery and low energy consumption characteristics of MFCs, slow-release carbon source anodes are typically combined with biological cathodes, with the main research focus being on the removal of nitrate pollutants rather than energy recovery.

[0005] Liu et al. (2019) used sediment fuel cells to provide electrons to a biocathode for in-situ nitrate remediation. This study utilized organic matter in the sediment as a carbon source, making it readily available. However, the drawbacks are that the amount of carbon source available to electrogenic bacteria in the sediment is limited, the current density is low, and the improvement on the denitrification efficiency of the cathode is limited.

[0006] In addition, some studies have utilized wastewater containing organic matter as the anode liquid fuel for MFCs. For example, Nguyen et al. (2016) studied the use of denitrifying biocathodes for in-situ microbial electrochemical denitrification in groundwater aquifers, and Zhang and Angelidaki (2013) designed an immersion biocathode denitrification desalination tank for in-situ nitrate reduction. However, the disadvantage is that the organic matter in the liquid fuel degrades rapidly, requiring frequent fuel replacement and making it difficult to use for long-term power generation. Generally, the power generation time of MFCs using organic wastewater as anode fuel in intermittent operation mode is 3-15 days.

[0007] Based on the above problems, there is an urgent need to provide a bioelectrochemical enhanced denitrification system and method that can continuously provide carbon sources to promote the utilization of electrochemically active denitrifying bacteria and continuously supply electrons to the cathode to improve the efficiency of microbial denitrification. Summary of the Invention

[0008] To address the aforementioned problems, this invention provides a bioelectrochemical denitrification system using a slow-release carbon source as the anode fuel, comprising an anode chamber and a cathode chamber separated by a separator membrane; the anode chamber and cathode are respectively housed within the anode chamber and cathode chamber; and a wire connects the anode and cathode without passing through the separator membrane.

[0009] The anode chamber is filled with an electrolyte containing hydrogel particles with slow-release carbon function and anaerobic fermentation bacteria;

[0010] Electrochemically active microorganisms are attached to and grow on the anode of the anode chamber, and the solution contains anaerobic fermentation bacteria;

[0011] The hydrogel particles in the electrolyte in the anode chamber will slowly release carbon source, which will be decomposed into small molecule organic matter by anaerobic fermentation bacteria as anode solid fuel; and ② will be decomposed into electrons by electrochemically active microorganisms.

[0012] The high concentration of small-molecule organic matter generated in the anode chamber will diffuse through the intermediate separator membrane to the cathode chamber or the external water environment to be purified, providing a carbon source for the electrochemically active denitrifying bacteria and the microbial heterotrophic denitrification process in the cathode chamber;

[0013] Electrons generated in the anode chamber are transferred along the wire to the cathode, enabling electrochemically active denitrifying bacteria on the cathode to obtain electrons for the removal of nitrate nitrogen from the water to be purified.

[0014] The hydrogel particles in the electrolyte in the anode chamber provide electrons and small organic molecule carbon sources for the cathode biological denitrification for a long time by utilizing the slow-release carbon function, thereby improving the cathode denitrification rate, sustainability, and nitrogen selectivity.

[0015] The cathode chamber is filled with water to be purified, which contains nitrate nitrogen (NO3-N), that is, nitrogen-containing substances existing in the form of nitrate.

[0016] Electrochemically active denitrifying bacteria attach and grow on the cathode of the cathode chamber.

[0017] The anode chamber and cathode chamber can be either enclosed or open environments.

[0018] The anode chamber is filled with electrolyte, or the electrolyte is drained into the anode chamber, or the anode is exposed to an open environment containing anolyte.

[0019] The cathode chamber is filled with water to be purified, or water containing nitrate nitrogen is introduced into the cathode chamber, or the cathode is exposed to open water containing nitrate nitrogen.

[0020] The wires also include an external resistor, power supply, or voltage.

[0021] In some preferred embodiments, the preparation process of the hydrogel particles includes the following steps:

[0022] 1-1) Dissolve sodium alginate and polyvinyl alcohol in water, heat and stir to prepare a uniform first gel solution;

[0023] The heating temperature is 80-85℃;

[0024] The amount of sodium alginate added to the water is 0.5%-1% w / v (mass of sodium alginate / volume of water);

[0025] The amount of polyvinyl alcohol added to the water is 5%-10% w / v (mass of polyvinyl alcohol / volume of water);

[0026] 1-2) Add starch acetate and / or sucrose to gel solution I and stir until completely dissolved to obtain gel solution II;

[0027] The amount of starch acetate added is 10-25% w / v (mass of starch acetate / volume of the first gel solution);

[0028] The amount of sucrose added is 1-10% w / v (mass of sucrose / volume of the first gel solution);

[0029] 1-3) After cooling the second gel solution, the second gel solution is dropped into a saturated boric acid solution containing 4-6 wt% CaCl2 to carry out a cross-linking reaction, thus obtaining hydrogel particles;

[0030] The crosslinking reaction temperature is 20-35℃, and the crosslinking reaction time is 20-30h;

[0031] The diameter of the hydrogel particles is 3-10 mm; the amount of acetate starch and / or sucrose embedded in the prepared hydrogel particles is controlled at 30-45% (w / w).

[0032] In some embodiments, electrochemically active denitrifying bacteria are attached to the cathode, and the cultivation steps include:

[0033] Anaerobic activated sludge is inoculated into the cathode chamber, and inorganic salt culture medium, easily biodegradable organic matter and nitrate nitrogen are added. The system is run, and the cathode liquid is replaced after the nitrate nitrogen is completely removed. Finally, when there is no organic carbon in the influent, nitrate nitrogen can still be effectively removed, and the electrochemically active denitrifying bacteria on the biological cathode are completed.

[0034] In some preferred embodiments, the amount of biodegradable organic matter and nitrate nitrogen added is controlled to maintain the ratio C / N (CODcr / NO3). - N) is 2-5.

[0035] In some preferred embodiments, the cathode potential during operation is -0.2 to -0.3V vs. SHE.

[0036] In some preferred embodiments, the amount of hydrogel particles added to the electrolyte in the anode chamber is 10g-50g / L.

[0037] In some embodiments, the electrolyte in the anode chamber is a buffer solution; the pH of the electrolyte environment is maintained at 5.5-7.5 during operation.

[0038] In some preferred embodiments, the buffer solution is selected from one of the following: phosphate buffer solution, bicarbonate slow-release solution;

[0039] In some preferred embodiments, when the pH of the anode chamber drops to <5.5 during operation, a buffer solution needs to be added to the anode chamber and the pH adjusted to the neutral range.

[0040] In some preferred embodiments, the dissolved oxygen (DO) in the water to be purified is ≤1.0 mg / L.

[0041] In some preferred embodiments, the separator membrane includes a proton exchange membrane or a cation exchange membrane. The separator membrane allows small molecules to permeate.

[0042] In some preferred embodiments, the anode material includes one or a combination of several of carbon paper, carbon cloth, carbon fiber, carbon felt, and graphite rod.

[0043] In some preferred embodiments, the cathode material includes one or a combination of several of the following: carbon paper, carbon cloth, carbon fiber, carbon felt, granular carbon, and graphite rod.

[0044] In some preferred embodiments, when the nitrate nitrogen removal rate in the water is greater than 95%, the wastewater in the cathode chamber is treated and discharged, and then the wastewater to be treated is added again.

[0045] The beneficial effects of this invention are as follows:

[0046] 1. Using hydrogel-based slow-release carbon as the anode, a solid fuel is continuously supplied to the anode through controlled release. Electrogenous and fermentative microorganisms at the anode work together to produce electrons and small-molecule organic matter. On one hand, electrons are transferred to the cathode via wires, providing electrons for the denitrifying bacteria at the cathode. On the other hand, the small-molecule organic matter produced in the anode chamber diffuses to the cathode through a selective exchange membrane, supplementing the carbon source for the denitrifying bacteria. This combined supply of electrons and carbon enhances the denitrification process by the cathode microorganisms. Compared to a method that solely provides electrons through electrodes, this method can increase the denitrification rate by 6-9 times.

[0047] 2. The bioelectrochemical system uses electrochemically active denitrifying bacteria with denitrification properties as the cathode catalyst. These bacteria can directly accept electrons transferred from the cathode, reducing nitrate nitrogen to nitrogen gas. Compared to cathodes without a catalyst or those using Pt as a catalyst, its advantages are: firstly, it can convert more nitrate nitrogen into nitrogen gas, rather than ammonia nitrogen; secondly, it reduces cathode costs.

[0048] 3. The bioelectrochemical system using hydrogel solid carbon source as slow-release fuel has a long continuous power generation time. A single anode replenishment can sustain power generation for up to 220 days, which is more than 20 times longer than using liquid carbon source as anode fuel.

[0049] 4. Addressing the issue of low microbial denitrification efficiency in wastewater with low C / N ratios, this patent provides a method that utilizes a bioelectrochemical system to couple slow-release carbon diffusion control at the anode with electron flow transfer to a biological cathode, thereby enhancing denitrification under low C / N ratio conditions. The advantages of this technology are: the slow-release carbon at the anode serves as biofuel, providing a continuous electron flow and small-molecule organic matter as a carbon source for the denitrifying bacteria at the cathode; it has a long fuel lifespan and is simple to operate, improving the rate and efficiency of biological denitrification under low C / N water conditions and enhancing nitrate nitrogen removal under conditions of organic carbon scarcity. Attached Figure Description

[0050] Figure 1 The structure and schematic diagram of a bioelectrochemical system using hydrogel slow-release carbon as the anode fuel; wherein, 1-anode, 2-cathode, 3-voltmeter, 4-electrochemically active microorganisms, 5-hydrogel particles, 6-anaerobic fermentation bacteria, 7-electrochemically active denitrifying bacteria, 8-separation membrane.

[0051] Figure 2 This is a graph showing the nitrogen removal rate of the bioelectrochemical system using hydrogel-released carbon as the anode fuel in Example 1.

[0052] Figure 3 Example 3 shows the denitrification effect of a bioelectrochemical system using hydrogel slow-release carbon as the anode fuel immersed in simulated nitrogen-containing water. Detailed Implementation

[0053] The present invention will be further described below with reference to specific embodiments. However, the scope of protection claimed by the present invention is not limited to the scope described in the embodiments. Anyone can derive other various forms of products under the guidance of the present invention. However, regardless of any changes in their shape or composition ratio, any technical solution that is the same as or similar to that of the present application falls within the protection scope of the present invention.

[0054] Example 1

[0055] I) Construction of bioelectrochemical systems:

[0056] like Figure 1 As shown, a two-chamber bioelectrochemical system was constructed using plexiglass as the material, wherein the volume of the anode chamber is 120 mL (4 cm × 5 cm × 6 cm, length × width × height) and the volume of the cathode chamber is 72 mL (4 cm × 3 cm × 6 cm, length × width × height); the separator 8 between the anode and cathode chambers of the reactor is a Nafion 117 proton exchange membrane (DuPont, USA).

[0057] Anode 1 uses a carbon brush (5cm in diameter and 6cm in length) that has been treated at high temperature, and cathode 2 uses a granular carbon biocathode; the cathode and anode are connected in series with an external 20Ω resistor via a wire, and a voltmeter 3 is connected to it.

[0058] II) Preparation of hydrogel particles with sustained-release carbon function:

[0059] 1-1) Dissolve sodium alginate (0.5% w / v) and polyvinyl alcohol (8% w / v) in distilled water, heat and stir at 80°C to prepare a uniform first gel solution.

[0060] 1-2) Add a certain amount of starch acetate to the first gel solution and mix until completely dissolved to obtain the second gel solution; wherein the starch acetate content in the second gel solution is controlled at 20% (w / v).

[0061] 1-3) After cooling the II gel solution to 40°C, it was dripped into a saturated boric acid solution containing 4% CaCl2 using a syringe, and crosslinked at 25±1°C for 24 hours to obtain hydrogel particles 5 with a diameter of 5-7 mm.

[0062] The starch acetate content embedded in the prepared hydrogel particles was controlled at 40% (w / w). The embedding amount is related to the sustained release time and affects the stability of the hydrogel particles; excessively high embedding amounts should be avoided.

[0063] III) Offline pre-culture of anodic and cathodic electrochemically active microorganisms:

[0064] Anaerobic sludge (containing anaerobic fermenting bacteria 6 and electrochemically active microorganisms 4) was inoculated into the anode, and an inorganic salt culture medium (ordinary nitrogen and phosphorus-containing culture medium) was added as the anolyte, with 2000 mg / L glucose as the substrate.

[0065] Add 50 mM PBS buffer to the cathode chamber as the cathodic solution, add 200 mg / L sodium acetate and 50 mg / L nitrate nitrogen; connect a 1000 Ω constant resistor between the anode and cathode to start; when the maximum output voltage remains stable for three consecutive cycles and the cathode nitrate nitrogen removal rate remains >95%, the cathode biofilm is considered to be successfully attached; at this time, electrochemically active microorganisms are enriched and attached to the anode.

[0066] IV) Operation of the slow-release carbon anode bioelectrochemical enhanced denitrification system:

[0067] The anode and cathode of the electrochemically active microorganisms that have completed offline pre-culture in step III) are transferred into the anode chamber and cathode chamber of the bioelectrochemical system in step I, respectively.

[0068] Electrolyte (i.e., anolyte) is added to the anode chamber. The buffer solution consists of 18 g / L Na2HPO4, 9 g / L KH2PO4, 0.1 g / L MgSO4·7H2O, 0.5 g / L NaCl, 0.2 g / L NH4Cl, 0.014 g / L CaCl2, and 12.5 mL / L trace elements.

[0069] Hydrogel particles were added to the anode chamber at a mass ratio of 37.5 g / L to the volume of the solution in the cathode chamber.

[0070] Wastewater containing nitrate nitrogen to be treated is added to the cathode chamber, so that electrochemically active denitrifying bacteria 7 are present on the cathode for the removal of nitrate nitrogen from the water to be purified.

[0071] The reactor was operated at 28±2℃. The removal of nitrate nitrogen in the cathode chamber effluent was monitored. When the removal rate exceeded 95%, the cathode chamber wastewater was discharged, and the reactor was restarted. After a single addition of anode fuel, the power generation time was up to 220 days. During 25 operating cycles, NO3... - -N removal rate see Figure 2 The highest removal rate reached 27 mg / L / d.

[0072] Example 2

[0073] The difference between Example 2 and Example 1 is that in step (1), the anode chamber and cathode chamber of the reactor are separated by a cation exchange membrane; and in step (4), the mass ratio of the slow-release carbon hydrogel added to the anode chamber to the volume of the solution in the cathode chamber is 10 g / L.

[0074] Example 3

[0075] Example 3 differs from Example 1 in that the bioelectrochemical system uses an open cathode, specifically as follows: The bioelectrochemical system comprises an anode chamber, an anode, a diffusion membrane on one side of the anode chamber, a cathode, and wires; the volume of the anode chamber is 120 mL (4 cm × 5 cm × 6 cm, length × width × height), and a proton exchange membrane is installed on one side of the reactor anode chamber; the anode is a high-temperature treated carbon brush anode (5 cm in diameter, 6 cm in length), and the cathode consists of three carbon cloth cathodes (3 cm × 6 cm); the anolyte composition includes 18 g / L Na₂HPO₄, 9 g / L KH₂PO₄, 0.1 g / L MgSO₄·7H₂O, 0.5 g / L NaCl, 0.2 g / L NH₄Cl, and 0.014 g / L... CaCl2, 12.5 mL / L trace elements; hydrogel slow-release carbon particles encapsulated with acetate starch were added to the anode chamber, with a mass-to-volume ratio of 40 g / L to the cathode chamber solution; both the anode and cathode were pre-cultured and enriched with electrochemically active microorganisms; the bioelectrochemical system was immersed in a water tank with a volume of 960 mL (12 cm × 8 cm × 10 cm), and the anode and cathode were connected by wires, and the nitrate-containing nitrogen wastewater to be treated was introduced into the water tank; on the one hand, the cathode exposed in the water tank continuously received electrons from the anode for denitrification, and on the other hand, the exposed exchange membrane of the anode released small molecules from the anode chamber into the water tank solution at a controlled rate, providing a slow-release carbon source for biological denitrification.

[0076] Example 4

[0077] The difference between Example 4 and Example 3 is that a proton exchange membrane is installed on one side of the anode chamber of the reactor; the cathode consists of three sets of carbon brushes (5 cm in diameter and 6 cm in length) that have undergone high-temperature treatment; hydrogel slow-release carbon particles encapsulated with acetate starch and sucrose are added to the anode chamber, and the mass ratio of the particles to the solution volume in the cathode chamber is 10 g / L.

[0078] Comparative Example 1

[0079] Based on Example 1, instead of adding hydrogel particles to the anode chamber, 2000 mg / L glucose liquid fuel was added, while the cathode chamber was retained. The cathode chamber denitrification reaction test was conducted, and the highest nitrate removal rate was 13 mg / L / d, with an anode power generation time of 11 days. The nitrate removal rate and power generation time of the comparative example were both lower than those of the anode chamber in Example 1 where hydrogel particles were added.

[0080] Comparative Example 2

[0081] Based on Example 1, no hydrogel particles were added to the anode chamber, and the cathode chamber was retained. By applying a voltage of 0.8V between the anode and cathode to provide electrons to the cathode, the denitrification reaction in the cathode chamber was tested. The nitrate removal rate in the cathode chamber was 4 mg / L / d, which was lower than the nitrate removal rate in Example 1 when hydrogel particles were added to the anode chamber. This indicates that the diffusion of small molecule organic matter from the anode chamber to the cathode chamber increased the cathode denitrification rate.

[0082] Comparative Example 3

[0083] Based on Example 3, no hydrogel particles were added to the anode chamber, and the denitrification reaction test was conducted in the cathode chamber. Figure 3 As can be seen from the results, after 20 days of reaction, 50 mg / L of NO3-N was completely removed in the experimental group (Example 3), with an average removal rate of 2.5 mg / L / d. In contrast, the control group (Comparative Example 1) only added carbon cloth cathodes and did not have an anode chamber, so almost no denitrification reaction occurred. This proves that adding hydrogel particles to the anode chamber to release carbon source can enhance the denitrification effect of the cathode. The above comparison results prove that the removal of nitrate in Example 3 mainly comes from two aspects: (1) Anode microorganisms generate electrons by oxidizing and releasing carbon source, which reach the cathode through the external circuit and reduce nitrates under the action of autotrophic denitrifying bacteria. (2) Heterotrophic denitrifying bacteria use organic matter synthesized by autotrophic denitrifying bacteria and organic matter permeating from the anode chamber to the cathode chamber as electron donors to reduce nitrates.

Claims

1. A bioelectrochemical denitrification system using a slow-release carbon source as the anode fuel, characterized in that, It includes an anode chamber and a cathode chamber, which are separated by a partition membrane; the anode chamber and the cathode chamber are respectively installed in the anode chamber and the cathode chamber. The wire connects the anode and cathode, but does not pass through the separator membrane; The anode chamber is filled with an electrolyte containing hydrogel particles and anaerobic fermentation bacteria; the hydrogel particles include sodium alginate, polyvinyl alcohol, and starch acetate and / or sucrose. Electrochemically active microorganisms attach and grow on the anode of the anode chamber; Hydrogel particles have the function of slow-release organic carbon; Anaerobic fermentation bacteria break down large organic carbon molecules into smaller organic molecules; Electrochemically active microorganisms decompose organic matter to generate electrons; The cathode chamber is filled with water to be purified, which contains nitrate nitrogen; Electrochemically active denitrifying bacteria attach and grow on the cathode in the cathode chamber. The cultivation steps include: inoculating the cathode chamber with anaerobic activated sludge, adding inorganic salt culture medium, readily biodegradable organic matter, and nitrate nitrogen; operating the system; replacing the cathode solution after nitrate nitrogen removal; and finally, even when there is no organic carbon in the influent, nitrate nitrogen can still be effectively removed, thus completing the biofilm formation of the biological cathode microorganisms; and controlling the ratio of readily biodegradable organic matter to nitrate nitrogen (C / N) (CODcr / NO3). - N) is 2-5; during operation, the cathode potential is -0.2 ~ -0.3 V vs. SHE.

2. The system according to claim 1, characterized in that, The preparation process of the hydrogel particles includes the following steps: 1-1) Dissolve sodium alginate and polyvinyl alcohol in water, heat and stir to prepare a uniform first gel solution; The heating temperature is 80-85 ℃; The amount of sodium alginate added to the water is 0.5%-1% w / v (mass of sodium alginate / volume of water). The amount of polyvinyl alcohol added to the water is 5%-10% w / v (mass of polyvinyl alcohol / volume of water). 1-2) Add starch acetate and / or sucrose to gel solution I and stir until completely dissolved to obtain gel solution II; The amount of starch acetate added is 10-25% w / v (mass of starch acetate / volume of the first gel solution). The amount of sucrose added is 1-10% w / v (mass of sucrose / volume of the first gel solution). 1-3) After cooling the second gel solution, the second gel solution is dropped into a saturated boric acid solution containing 4-6 wt% CaCl2 to carry out a cross-linking reaction, thus obtaining hydrogel particles; The cross-linking reaction temperature is 20-35 ℃, and the cross-linking reaction time is 20-30 h.

3. The system according to claim 1, characterized in that, The amount of hydrogel particles added to the electrolyte in the anode chamber is 10-50 g / L.

4. The system according to claim 1, characterized in that, The electrolyte in the anode chamber is a buffer solution; the pH of the electrolyte environment is maintained at 5.5-7.5 during operation.

5. The system according to claim 1, characterized in that, The dissolved oxygen (DO) in the water to be purified is ≤1.0 mg / L.

6. The system according to claim 1, characterized in that, The separator membrane includes a proton exchange membrane and / or a cation exchange membrane.

7. The system according to claim 1, characterized in that, The anode material includes one or a combination of several of the following: carbon paper, carbon cloth, carbon fiber, carbon felt, and graphite rod.

8. The system according to claim 1, characterized in that, The cathode material includes one or a combination of several of the following: carbon paper, carbon cloth, carbon fiber, carbon felt, granular carbon, and graphite rod.

9. The system according to claim 1, characterized in that, The anode chamber and cathode chamber can be either enclosed or open environments.

Images