Denitrification of wastewater from microbial fuel cell coupling system

The rapid development of industrialization and urbanization has brought extensive attention to wastewater treatment in the microbial fuel cell coupling system. Especially in some industrial production areas, excessive nitrogen will increase the pressure of self-cleaning in the environment, leading to serious eutrophication of rivers, lakes and other water bodies, degrading ecosystem services, or even completely losing them. Therefore, controlling the discharge of nitrogen-containing sewage is an urgent environmental pollution control task.

1. Common wastewater denitrification technology methods

(1) Physical Chemistry Law

The chemical precipitation method uses metal ions and sulfate precipitation to remove pollutant elements in wastewater. Metal ions often used in this process technology include calcium ions, barium ions, and so on. Using calcium ion chemical precipitation method can remove sulfate in printing and dyeing wastewater with a removal efficiency of 30%, and the removal rate of barium ions can reach 100%. The chemical precipitation method has a simple process principle and is relatively convenient to operate, with a high removal rate. However, in practical applications, heavy metal ions need to be introduced, so the initial investment is relatively high. The adsorption method is to use natural barium ore to adsorb harmful pollutants in the wastewater, and stay in wastewater with a pH of 9 for about 80 minutes, and the adsorption efficiency can reach 50%.

The electrodialysis method utilizes the characteristic of selective permeability of ion exchange membranes and the action of an external electric field to separate pollutants from wastewater. However, this method requires ion-exchange membranes and consumes high electrical energy, so the economic cost is high and the use is less.

(2) Biological Law

The most representative biological method is the single-phase anaerobic process technology, which mainly uses the sulfate anaerobic reaction to produce primary and secondary inhibition of anaerobic nitrification. The single-phase stripping process is developed on the basis of traditional single-phase anaerobic. The advantage is that it can use gas inertness to carry out the reduction reaction, reduce the toxic effect of pollutants in the wastewater, and weaken the inhibitory effect on other anaerobic bacteria, so that the system Can effectively remove more than 30% of pollutants.

2. Research on the Denitrification Test of Wastewater from Microbial Fuel Cell Coupling System

(1) Test materials and methods

1). Material

A dual-chamber microbial fuel cell was selected as the main material for this study. The two-electrode chambers were inoculated with anammox activated sludge, and the cathode was potassium ferricyanide electrolyte to receive electrons. The effective volume of the anode and cathode chambers are both 400 ml. The two electrode chambers are separated by a proton exchange membrane. The electrodes are made of carbon felt. Four plates are placed in each electrode chamber and fixed by titanium wires. The external circuit uses a 1000 ohm fixed-value resistor in series, and the data acquisition card is connected in parallel, and then the data acquisition card is connected to the computer to ensure that the collected data can be directly saved to the computer.

2). Method

The inoculated sludge is a culture of anammox bacteria. A UASB reactor that has been operating stably and has been running for more than one year is taken from the laboratory. The sludge is brick-red granular and has high anammox activity. The reaction is obtained by measurement. The dominant bacteria in the vessel, the inoculation amount is 5% of the effective gas in the anode chamber.

The study uses simulated wastewater to ensure that the ratio of nitrogen oxide ions and nitrogen hydrogen ions is 1:1.32 to adapt to the anaerobic ammonia oxidation reaction. The volume of trace element concentrate added per liter of solution is 1 ml, and nitrogen is used for 10 minutes aeration. Make sure that the dissolution rate is within 0.2 mg/L, and then adjust the pH to about 7.5.

The anode chamber undergoes a substrate replacement once a day, and each replacement is defined as a cycle. The cathode chamber adopts a continuous flow water inlet mode, adjust the speed of the water pump so that the daily flow of electrolyte is 400 ml, and a magnetic rotor is placed in the cathode and anode chambers to ensure the consistency of the solution concentration. Use the water bath heating mode to control the operating temperature of the microbial fuel cell to 30 degrees Celsius, and use the data acquisition card to pressurize at a frequency of once per minute, and the data is directly saved to the computer system connected to it.

(2) Analysis of test results

1). Test results

The anode microbial fuel cell coupling system locks the ammonia nitrogen concentration and nitros nitrogen concentration of the influent water under stable operation, and can ensure that the ammonia nitrogen, nitrate nitrogen, and nitrous nitrogen concentration of the effluent water are below 5.28, 4.78, and 0.54 mg/L, respectively. The total removal rate is 0.11 kg/standard cubic meter per day, the total nitrogen in the effluent is less than 5.63 mg/L, and the total nitrogen removal efficiency is above 95%.

2). Result analysis

In the study, it was found that the concentration of nitrous nitrogen in the effluent was low, mainly because a small amount of organic carbon source was added to the influent part to promote the denitrification reaction and slightly increase the ratio of ammonia to nitrogen in the water. When nitrogen oxide ions react with nitrogen hydrogen ions, the stoichiometric number is about 1.54, and the reaction stoichiometric number is 0.07. The results show that the anode has a certain denitrification ability, and the advantage is that it can promote the reduction of the ratio of nitrous nitrogen to nitrate nitrogen in the water. However, if it is operated for a long time, it may cause excessive denitrification, leading to the occurrence of anaerobic ammonia oxidation reaction ion electron acceptor. Make the anammox reaction end early. The increase in the concentration of ammonia nitrogen in wastewater indicates that the balance regulation of anaerobic ammonia oxidation and denitrification is the key point to ensure the stable denitrification of wastewater in the coupled system of microbial fuel cells.

When the anode anammox microbial battery is in a continuous and stable state, it can lock the Anyang concentration and the nitrosamine concentration of the water inlet part. When the maximum output voltage and power density of the system in a single day are relatively stable, the maximum output voltage is 62 mV, and the power is 0.74 MW/m3. Through the observation of the system for two weeks, the highest output voltage in a single day showed a fluctuating state. At the same time, the lowest output voltage was only 21 millivolts. After statistical analysis, it is found that there is no obvious regular change in electric energy output. Therefore, it is speculated that only anaerobic ammonia oxidation is the main body of the anode, and it is difficult for the reaction to generate a high output voltage. The electricity generation mechanism of anammox reaction has not been thoroughly explored, so the reason for electricity generation is still unknown. This research result is consistent with the results obtained by other researches on the wastewater denitrification of microbial fuel cell coupling system in the same field. Some scholars use anaerobic ammonia oxidation biological cathode to remove ammonia and generate electricity, and the maximum output power is 92 MW/m3. Meter. In addition, some anaerobic ammonium bacteria are inoculated into the microbial fuel cell coupling system, and the landfill leachate with high ammonia nitrogen content is processed in the anode of the battery, and the output power is about 0.22 watts/m3. The high power density is due to the high concentration of influent chemical oxygen demand, but denitrification will completely inhibit anaerobic ammonia oxidation, so that the device cannot always maintain a stable operating state.

When anammox and denitrification are in equilibrium, the microbial fuel cell can achieve better denitrification effects, but the electricity generated at this time is not strong, and the system productivity effect is not ideal. By controlling the influent chemical oxygen demand and optimizing the denitrification effect of the device, it is difficult to maintain its long-term stable operation, and periodic fluctuations in water and nitrogen concentration will occur. In order to avoid similar problems, increase the anode influent chemical oxygen demand concentration method and conduct a special test for this. Studies have proved that keeping the influent ammonia nitrogen and nitros nitrogen concentrations unchanged, only adjust the influent chemical oxygen demand, when its concentration reaches 125-150 mg/L, the single-cycle maximum voltage reaches 65-59 millivolts, the result is related to the production capacity Compared with the previous, it has remained almost unchanged. It is proved that when the single index of influent chemical oxygen demand is adjusted, the effluent ammonia nitrogen content will increase, which will not increase the power generation capacity of the device. By increasing the nitrous nitrogen content of the influent water, the effluent ammonia nitrogen content will be slightly reduced.

Summary

In summary, as the research on the coupled treatment method of microbial fuel cell continues to deepen, the related biological denitrification technology has also been developed correspondingly. The research of this technology is moving in the direction of more practical and modern development, and is continuously optimized and upgraded. It will eventually become a new technology that can provide high efficiency and high productivity for wastewater treatment.