Method for treating nitrate wastewater by using three-dimensional electrode biological membrane

Abstract

The invention provides a method for treating nitrate wastewater through a three-dimensional electrode biological membrane, and belongs to the technical field of nitrate wastewater treatment. The method comprises the following steps: constructing a cylindrical three-dimensional electrode method bio-membrane reactor by taking activated carbon as a substrate, taking a cylindrical shape enclosed by a plurality of graphite rods as a cathode and taking a middle graphite rod as an anode; the reactor cathode generates H2 under the action of impressed current, autotrophic hydrogen type denitrifying bacteria grow and breed depending on H2 and are attached to the surface of a cathode graphite rod to form a biological membrane, nitrate nitrogen in water can be diffused into the biological membrane, and the biological membrane on the surface of the cathode graphite rod utilizes the generated H2 to reduce the nitrate nitrogen; cO2 is generated at the anode of the reactor and is further converted into carbonic acid, bicarbonate radicals and carbonate radicals, and a carbon source required by growth and reproduction is provided for the autotrophic hydrogen type denitrifying bacteria. According to the three-dimensional electrode bio-membrane reactor, a filling material is added in a two-dimensional reactor structure to serve as a supplementary electrode, and stronger wastewater treatment capacity is brought by a larger surface area.

Description

Technical Field

[0001] The invention belongs to the technical field of nitrate wastewater treatment and relates to a method for treating nitrate wastewater with a three-dimensional electrode biofilm. Background Art

[0002] With the growth of population and the intensification of economic activities, the demand for water is increasing, while the supply of water resources is increasingly threatened. In a large number of production and life activities, the discharge of nitrate wastewater has become one of the serious environmental pollution problems. Nitrate wastewater can have a harmful effect on lakes, rivers and groundwater, leading to eutrophication, changes in the water ecological environment, accumulation of toxic substances in water and other problems, affecting human health, ecosystem balance and sustainable economic development. In addition, nitrate wastewater can also have a negative impact on soil health and plant growth. The discharge of wastewater may lead to adverse effects such as soil acidification, increase in microbial biomass and consumption of soil organic matter, ultimately affecting the growth and quality of crops.

[0003] Nitrate wastewater is a serious hazard and is an important link that cannot be ignored in water purification research in recent years. There are many treatment technologies for nitrate wastewater, including photocatalytic oxidation technology, SBR, biological rotary disc, immobilization technology, denitrification technology and adsorption technology. The technologies currently used for nitrate wastewater treatment are mainly physical, chemical and biological methods.

[0004] However, these treatment methods are inefficient, generally between 40% and 70%, and it is difficult to meet the emission standards after one treatment. Secondary treatment is required, which increases the treatment cost and makes the operation more complicated. Some methods have high treatment efficiency, but require complex instrument structure support, and the materials are easily contaminated during the treatment process and need to be replaced regularly, which greatly increases the cost and makes it difficult to be widely used in industrial production. Summary of the invention

[0005] The purpose of the present invention is to address the above-mentioned problems in the prior art and to propose a method for treating nitrate wastewater using a three-dimensional electrode biofilm.

[0006] The purpose of the present invention can be achieved by the following technical solutions:

[0007] A method for treating nitrate wastewater using a three-dimensional electrode biofilm, characterized by comprising:

[0008] Using activated carbon as the substrate, graphite rods as the positive and negative electrodes, and external current, a cylindrical three-dimensional electrode biofilm reactor was built.

[0009] Autotrophic hydrogen-type denitrifying bacteria added to a cylindrical three-dimensional electrode biofilm reactor;

[0010] Water is introduced into the cylindrical three-dimensional electrode biofilm reactor and an external current is applied.

[0011] Preferably, the cylindrical three-dimensional electrode biofilm reactor uses multiple graphite rods to form a cylindrical shape as the cathode. The cathode produces H2 under the action of an external current to provide autotrophic hydrogen-type denitrifying bacteria for growth and reproduction, and a biofilm is formed on the surface of the cathode graphite rod. The biofilm reduces nitrate nitrogen to N2 through the produced H2.

[0012] Preferably, the cylindrical three-dimensional electrode biofilm reactor uses the graphite rod in the middle as the anode, and the anode produces CO2, which is further converted into carbonic acid, bicarbonate and carbonate to provide a carbon source for the growth and reproduction of autotrophic hydrogen denitrifying bacteria. The autotrophic hydrogen denitrifying bacteria use the H2 and CO2 produced at the cathode of the reactor to convert NO3 － -N is converted to N2.

[0013] Preferably, the activated carbon base of the cylindrical three-dimensional electrode biofilm reactor is used as a particle electrode and a carrier for biofilm attachment, thereby increasing the attachment area and diffusion force of autotrophic denitrifying bacteria. Through the application of external current, the H2 and CO2 produced by the electrode can be directly utilized by the denitrifying bacteria.

[0014] Preferably, the nitrate removal rate is measured by UV spectrophotometer based on NO3 － -N, NO2 － -N、NH4 + -N absorbance measurement at different wavelengths.

[0015] Compared with the prior art, the present invention has the following advantages:

[0016] 1. The three-dimensional electrode biofilm reactor adds filling materials as supplementary electrodes to the two-dimensional reactor structure. The filled particle electrodes make up for the shortcoming of the small surface area of ​​the two-dimensional reactor electrodes, and its larger surface area also brings stronger wastewater treatment capabilities.

[0017] 2. The filled particle electrode can also be used as a carrier for biofilm attachment, increasing the attachment area and diffusion capacity of autotrophic denitrifying bacteria. And under the action of external current, the H2 and CO2 produced by the electrode can be directly used by denitrifying bacteria, improving water purification efficiency.

[0018] 3. The previous biological treatment system for nitrate wastewater is prone to excessive growth of autotrophic hydrogen-type denitrifying bacteria due to the addition of carbon sources, which in turn leads to reactor blockage. Therefore, the autotrophic hydrogen-type denitrification system avoids the previous problem of excessive sludge discharge, easy clogging of equipment, and the need for effective methods to deal with discharged sludge. In addition, the addition of external carbon sources is also prone to form by-products, causing secondary pollution.

[0019] 4. The anode of the three-dimensional electrode biofilm reactor will produce CO2, which will be further converted into carbonic acid, bicarbonate and carbonate, which will buffer the pH of the water. CO2 can also create an anaerobic environment, providing a suitable environment for anaerobic hydrogen autotrophic denitrifying bacteria, which is conducive to biological denitrification. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a cathode graphite rod for three-dimensional electrode biofilm reactor.

[0021] Figure 2 This is a comparison chart of the effect of pH value on denitrification in Example 1.

[0022] Figure 3 This is a comparison chart of the effect of current intensity on denitrification in Example 2.

[0023] Figure 4 This is a comparison chart of the C / N effect on denitrification in Example 3.

[0024] Figure 5 It is a comparison chart of the effect of hydraulic retention time on denitrification in Example 4.

[0025] Figure 6 This is a comparison chart of the denitrification effect of the influent concentration in Example 5. DETAILED DESCRIPTION

[0026] The following are specific embodiments of the present invention and the accompanying drawings to further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

[0027] A method for treating nitrate wastewater using a three-dimensional electrode biofilm, comprising:

[0028] Using activated carbon as the substrate and graphite rods as the yin and yang stages, a cylindrical three-dimensional electrode biofilm reactor is built by connecting to external current, and autotrophic hydrogen-type denitrifying bacteria are added to the cylindrical three-dimensional electrode biofilm reactor; water is introduced into the cylindrical three-dimensional electrode biofilm reactor and external current is applied.

[0029] Specific:

[0030] The cylindrical three-dimensional electrode biofilm reactor uses multiple graphite rods to form a cylindrical cathode. The cathode produces H2 under the action of an external current. The principle of H2 production is as follows:

[0031] 2H + +2e － →H2

[0032] 2H2O+2e － →2OH －

[0033] The generated H2 is used for the growth and reproduction of autotrophic hydrogen-type denitrifying bacteria, forming a biofilm on the surface of the cathode graphite rod. The biofilm reduces nitrate nitrogen to N2 through the generated H2. The reaction formula is as follows:

[0034] NO3 － +H2→NO2 － +H2O

[0035] 2NO2 － +2H2+2H + →N2O+3H2O

[0036] N2O+H2→N2+H2O

[0037] 2NO3 － +5H2+2H + →N2+6H2O

[0038] The cylindrical three-dimensional electrode biofilm reactor uses the graphite rod in the middle as the anode. The anode produces CO2, which is further converted into carbonic acid, bicarbonate and carbonate, providing a carbon source for the growth and reproduction of autotrophic hydrogen denitrifying bacteria. The autotrophic hydrogen denitrifying bacteria use the H2 and CO2 produced at the cathode of the reactor to convert NO3 － -N is converted to N2, the reaction is as follows:

[0039] 2.16NO3+7.24H2+0.8CO2→0.16C5H7O2N+N2+5.6H2O+2.16OH － The activated carbon at the base of the cylindrical three-dimensional electrode biofilm reactor serves as a particle electrode and a carrier for biofilm attachment, increasing the attachment area and diffusion force of autotrophic denitrifying bacteria. Through the applied current, the H2 and CO2 produced by the electrode can be directly utilized by the denitrifying bacteria.

[0040] Nitrate removal rate, measured by UV spectrophotometer based on NO3 － -N, NO2 － -N、NH4 + -N absorbance at different wavelengths was measured, and the nitrate removal rate was obtained using the formula:

[0041]

[0043] Embodiment 1

[0044] pH is one of the very important control factors in the denitrification process. Autotrophic hydrogen-type denitrifying bacteria need to absorb and metabolize nutrients from the outside world during their growth and reproduction, and the pH of the water body will directly affect the activity of related enzymes. In addition, the availability of nutrients in the water body to autotrophic hydrogen-type denitrifying bacteria is also affected by pH. In the denitrification process, pH control needs to be adjusted according to specific reaction conditions. Only under appropriate pH conditions can the smooth progress of the reaction be guaranteed. Denitrifying bacteria are sensitive to the influence of pH value. Usually, the optimal pH for hydrogen-autotrophic denitrifying bacteria is around neutral (6.8-7.2). Too high or too low pH will affect the working efficiency of denitrifying bacteria and thus have an adverse effect on the denitrification process. Therefore, in the study of denitrification effects, pH regulation is a very important link.

[0045] The effect of pH on reactor denitrification was investigated under the conditions of influent nitrate nitrogen concentration of 50 mg / L, external current of 35 mA, C / N of 1.5 and hydraulic retention time of 24 h. The pH gradient was selected as 5.68, 6.70, 7.16, 7.74, 8.34 and 9.17.

[0046] like Figure 2 As shown in the results, the optimal pH for denitrification in this study is about 7.5-8.0, and high or low pH will reduce the efficiency of nitrate treatment. The results show that the difference between pH 9.0 and the optimal pH 7.74 in the final nitrate removal rate is no more than 15%, so the higher pH does not significantly reduce the nitrate nitrogen treatment capacity, but the lower pH has a significant attenuation on the nitrate nitrogen treatment effect. In addition, the pH value has a serious effect on the treatment of nitrite nitrogen. When the pH value is greater than 8.5 or less than 7.0, the nitrite accumulation phenomenon is more serious. When the pH increases from the optimal 7.74 to 9.17, the effluent nitrite concentration increases significantly from 0.32 mg N / L to 5.87 mg N / L. Usually, when the pH is greater than 8, the denitrification process will be limited, and this study confirms that the new 3D reactor expands the pH range and obtains better performance. This may be because the generated CO2 plays a good pH buffering role. Without buffering, denitrification consumes hydrogen ions and the pH value increases with denitrification. To explore its mechanism, the experiment was repeated again at pH = 7.68. It was observed that the pH value slightly increased to 7.84 after 2 hours. During the treatment, the pH value decreased and remained at around 7.69. This indicates that carbon dioxide should dissolve and dissociate into bicarbonate and carbonate ions. The pH value should be adjusted as follows:

[0047] CO2+H2O→H2CO3

[0048] H2CO3+OH - →H2O+HCO3-

[0049] HCO3 - +OH - →H2O+CO3 2-

[0050] As shown in the above formula, favorable acid-base conditions are provided for the denitrification process of the entire reactor.

[0051] Embodiment 2

[0052] The three-dimensional electrode biofilm reactor is a technology that uses the microbial film formed by electrodes during the pollutant treatment process to biodegrade or transform pollutants. Electric current can affect the metabolism and community structure of various microorganisms in the three-dimensional electrode biofilm reactor, and plays a key role in the degradation process of pollutants in the water environment. Therefore, in the study of denitrification effect, the regulation of current intensity is a very important link.

[0053] The effect of current intensity on the denitrification of the reactor was investigated under the conditions of influent nitrate nitrogen concentration of 50 mg / L, pH of about 7.5, C / N of 1.5, and hydraulic retention time of 24 h. The current intensity gradient was selected as 10 mA, 20 mA, 40 mA, 60 mA and 80 mA.

[0054] like Figure 3 As shown, the removal rate of nitrate nitrogen first increases and then decreases with the increase of current, reaching a peak value near 40mA. Obviously, in addition to a slight electrochemical effect, the promoting effect of current on autotrophic denitrification is much greater than its promoting effect on heterotrophic denitrification. This result shows that the presence of current is more conducive to the growth of autotrophic denitrifying bacteria. As denitrifying bacteria adapt to the environment, the rate of autotrophic denitrification accelerates. However, SAKAKIBARA found that when the current intensity exceeds 50mA, the excessive current intensity will inhibit the activation of microorganisms, causing part of the biofilm to die and fall off, thereby reducing the removal efficiency of biological action; in addition, studies have shown that if the intensity of the external current is increased, the cathode will produce too much OH - Ions increase the pH in the water, but too high a pH is not conducive to the biological denitrification reaction.

[0055] It should be noted that due to the presence of calcium and magnesium ions in tap water, they will be deposited on the cathode surface during electrolysis. CO2 dissolves in water and generates CO3 according to the following formula - .

[0056] CO2+H2O→HCO3 - +H + (13)

[0058] HCO3- +OH - →CO3 2- +H2O (14)

[0060] CO3 - The way in which calcium ions and magnesium ions produce deposits is as follows:

[0061] Ca 2+ +CO3 2- →CaCO3↓ (15)

[0063] Mg 2+ +CO3 2- →MgCO3↓ (16)

[0065] Ca(Mg) 2+ +CO3 2- →Ca(Mg)CO3↓ (17)

[0067] The precipitate will have an adverse effect on the electrolysis of the cathode. Although the deposited calcium and magnesium can be redissolved by changing the polarity of the electrode, the inevitable precipitation causes the bio-carrier to agglomerate, greatly reducing the activity of the microorganisms and causing the nitrate removal rate to plummet.

[0068] Embodiment 3

[0069] Among the various factors that affect the purification of nitrate wastewater by three-dimensional electrode biofilm reactor, C / N is one of the important indicators. This is because one of the essential substrates for the growth of denitrifying bacteria is the carbon source. When the organic carbon source in the wastewater is insufficient, that is, when the C / N is low, the autotrophic hydrogen-type denitrifying bacteria grow and reproduce slowly, so the treatment efficiency is low.

[0070] Under the conditions of influent nitrate nitrogen concentration of 50 mg / L, pH of about 7.5, current intensity of 35 mA and hydraulic retention time of 24 h, the effect of addition carbon-nitrogen ratio on reactor denitrification was investigated, and the addition carbon-nitrogen ratio gradient was selected as 2, 1.5, 1.25, 1 and 0.75.

[0071] like Figure 4 As shown in the figure, the nitrate removal rate is positively correlated with C / N, and reaches 92.08% when C / N=2:1, which indicates that the microorganisms in the system may be a mixed nutrition system. A high C / N ratio can accelerate the growth of heterotrophic denitrifying bacteria in the biofilm, thereby increasing the total denitrification rate. When C / N is lower than the biometric value, the nitrate nitrogen still has a good treatment effect because the activated carbon matrix provides a larger biofilm area as the third electrode, but the nitrite nitrogen treatment time will be greatly extended.

[0072] The three-dimensional electrode biofilm reactor based on activated carbon does not need to add additional carbon source. This is because activated carbon itself is a highly adsorbable material that can provide a carbon source by adsorbing organic matter in wastewater, and can also provide an attachment surface for the biofilm. In addition, the electrolysis process in the three-dimensional electrode biofilm reactor will also produce a certain amount of H2 and CO2, which can be used by the biofilm as an electron donor and carbon source, thereby achieving wastewater treatment purposes such as denitrification and dephosphorization.

[0073] Embodiment 4

[0074] Hydraulic retention time is one of the most important parameters of 3D-BER, which determines the treatment capacity of the reactor at a certain inflow rate. Increasing the residence time in the reactor allows bacteria to degrade organic matter and consume nitrates to a greater extent.

[0075] The effect of hydraulic retention time on reactor denitrification was investigated under the conditions of influent nitrate nitrogen concentration of 50 mg / L, pH of about 7.5, current intensity of 35 mA, and C / N of 1.5. The hydraulic retention time gradient was selected as 8h, 10h, 12h, 16h and 24h.

[0076] like Figure 5 As shown in the figure, the concentrations of nitrate nitrogen and nitrite nitrogen decrease with the extension of treatment time, and the removal rate of nitrate nitrogen increases with the extension of treatment time. It can be seen that the hydraulic retention time is positively correlated with the nitrate treatment effect. Among them, when the hydraulic retention time is divided into 12h, 16h, and 24h, the removal rates of nitrate nitrogen are 78.93%, 83.10%, and 84.27%, respectively. It can be seen that when the hydraulic retention time exceeds 12h, the nitrate removal efficiency becomes flat. If the hydraulic retention time is only extended, the nitrate treatment effect is not greatly improved. It can be seen that too long hydraulic retention time is not necessary. Therefore, in actual production use, from the perspective of operating costs, it can be considered to shorten the hydraulic retention time of actual treatment to about 12h to reduce costs and achieve better treatment effects.

[0077] Embodiment 5

[0078] The effect of influent concentration on reactor denitrification was investigated under the conditions of pH around 7.5, current intensity of 35 mA, C / N of 1.5 and hydraulic retention time of 24 h. The influent concentration gradient was selected as 50 mg / L, 80 mg / L, 120 mg / L and 160 mg / L.

[0079] like Figure 6As shown in the figure, the nitrate nitrogen removal rate decreases with the increase of influent concentration, but when the influent concentration is less than 80 mg / L, it can be observed that the nitrate nitrogen removal amount is higher than that of 50 mg / L. This is just because the nitrate nitrogen content in the influent increases, so the calculated nitrate nitrogen removal rate is lower due to the larger base number. When the nitrate nitrogen is greater than 80 mg / L, it can be observed that the nitrate nitrogen removal rate drops sharply and decreases with the increase of influent concentration. There are two reasons for this situation:

[0080] (1) As the influent nitrate concentration increases, the mass transfer capacity of nitrate nitrogen in the water also increases, which increases the diffusion capacity of nitrate nitrogen in the wastewater and the possibility of nitrate nitrogen being transferred to the cathode biofilm of the reactor, thereby increasing the amount of nitrate nitrogen removed. However, when the influent nitrate concentration increases to exceed the upper limit of the reactor load, the nitrate nitrogen treatment rate decreases.

[0081] (2) When the nitrate nitrogen concentration in the influent is too high, the hydrogen-type denitrifying bacteria are stressed by the high concentration of nitrate nitrogen, their activity is reduced, and their nitrate nitrogen treatment capacity declines.

[0082] In summary: By changing parameters such as wastewater pH, applied current intensity, carbon-nitrogen ratio, hydraulic retention time and influent nitrate nitrogen concentration, the effects of the above parameters on reactor denitrification were investigated.

[0083] Taking the best parameters, i.e., pH≈7.5, C / N=2, I≈40mA and influent concentration of 50mg / L, the nitrate nitrogen concentration of 24h was measured. The nitrate nitrogen concentration of the effluent was 2.76mg / L, and the nitrate nitrogen removal rate was 94.58%, which was the best treatment effect, confirming the rigor and accuracy of the comparison of each group, and also confirming the high efficiency of the three-dimensional electrode method for denitrification treatment of nitrate wastewater.

[0084] The specific embodiments described herein are merely examples of the spirit of the present invention. Those skilled in the art may make various modifications or additions to the specific embodiments described or replace them in similar ways, but they will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A method for treating nitrate wastewater using a three-dimensional electrode biofilm, characterized in that: include: Using activated carbon as the substrate, graphite rods as the positive and negative electrodes, and external current, a cylindrical three-dimensional electrode biofilm reactor was built. Autotrophic hydrogen-type denitrifying bacteria added to a cylindrical three-dimensional electrode biofilm reactor; Water is introduced into the cylindrical three-dimensional electrode biofilm reactor and an external current is applied.

2. The method for treating nitrate wastewater using a three-dimensional electrode biofilm according to claim 1, characterized in that: The cylindrical three-dimensional electrode biofilm reactor uses multiple graphite rods to form a cylindrical cathode. The cathode produces H2 under the action of an external current to provide autotrophic hydrogen-type denitrifying bacteria for growth and reproduction. A biofilm is formed on the surface of the cathode graphite rod. The biofilm reduces nitrate nitrogen to N2 through the produced H2.

3. The method for treating nitrate wastewater using a three-dimensional electrode biofilm according to claim 1, characterized in that: The cylindrical three-dimensional electrode biofilm reactor uses the graphite rod in the middle as the anode. The anode produces CO2, which is further converted into carbonic acid, bicarbonate and carbonate, providing a carbon source for the growth and reproduction of autotrophic hydrogen denitrifying bacteria. The autotrophic hydrogen denitrifying bacteria use the H2 and CO2 produced by the reactor cathode to convert NO3 － -N is converted to N2.

4. The method for treating nitrate wastewater using a three-dimensional electrode biofilm according to claim 1, characterized in that: The activated carbon base of the cylindrical three-dimensional electrode biofilm reactor is used as a particle electrode and a carrier for biofilm attachment, thereby increasing the attachment area and diffusion force of autotrophic denitrifying bacteria. Through the applied current, the H2 and CO2 produced by the electrode can be directly utilized by the denitrifying bacteria.

5. The method for treating nitrate wastewater using a three-dimensional electrode biofilm according to claim 1, characterized in that: The nitrate removal rate is measured by UV spectrophotometer based on NO3 － -N, NO2 － -N、NH4 + -N absorbance measurement at different wavelengths.

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