How is the simultaneous nitrification and denitrification of MBBR realized?
(1) The concept of simultaneous nitrification and denitrification biological nitrogen removal (SND)
Simultaneous nitrification, denitrification and denitrification (SND) is the simultaneous production of nitrification, denitrification and carbon removal in the same reactor. It breaks through the traditional view that nitrification and denitrification cannot occur at the same time, especially under aerobic conditions, denitrification can also occur, making simultaneous nitrification and denitrification possible.
The nitrification process consumes alkalinity, and the denitrification process produces alkalinity, so SND can effectively keep the pH value in the reactor stable, without the need for acid-base neutralization, and without the need for an external carbon source; it saves the volume of the reactor, shortens the reaction time, and reduces the nitrate state. Nitrogen concentration can reduce sludge floating in secondary sedimentation tank, so SND has become a research hotspot of biological denitrification. For the feasibility of SND biological denitrification, there are currently three main views from different perspectives:
Macro-environmental perspective: This point of view believes that a completely uniform mixing state does not exist, and the uneven distribution of DO in the reactor can form aerobic, anoxic, and anaerobic regions, which can occur in the same bioreactor under anoxic/anaerobic environmental conditions Denitrification reaction, combined with the removal of organic matter in the aerobic environment and the nitrification of ammonia nitrogen in the section, SND can be achieved.
From the perspective of microenvironment: This view holds that the anoxic microenvironment in the microbial floc is the main reason for the formation of SND, that is, due to the limitation of oxygen diffusion (transmission), there is a dissolved oxygen gradient in the microbial floc, which is conducive to the realization of simultaneous nitrification and denitrification Microenvironment.
Biological point of view: This view holds that the existence of special microbial populations is considered to be the main reason for the occurrence of SND. Some nitrifying bacteria can perform denitrification in addition to normal nitrification, and some Dutch scholars have isolated aerobic nitrification. , and can carry out aerobic denitrification of Thiococcus pantrophicus; some bacteria cooperate with each other to carry out sequential reactions to convert ammonia into nitrogen, which provides the possibility to complete biological denitrification in the same reactor under the same conditions.
At present, there are many microbiological studies and explanations on biological denitrification, but they are not perfect, and the understanding of SND phenomenon is still under development and exploration. The microenvironment theory is generally accepted. Due to the existence of the dissolved oxygen gradient, the dissolved oxygen concentration on the outer surface of microbial flocs or biofilms is high, mainly aerobic nitrifying bacteria and ammonia bacteria; deep inside, oxygen transfer is blocked and external A large amount of dissolved oxygen is consumed to produce anoxic areas, and denitrifying bacteria are the dominant strains, which can lead to the occurrence of simultaneous nitrification and denitrification. This theory explains the common problem of different bacterial species in the same reactor, but there is also a defect, that is, the problem of organic carbon source. The organic carbon source is not only the electron donor of heterotrophic denitrification, but also the inhibitor of the nitrification process. When the organic carbon source in the sewage passes through the aerobic layer, it is first oxidized by aerobic oxidation. The denitrifying bacteria in the anoxic zone are due to The lack of electron donors reduces the denitrification rate, which may affect the denitrification efficiency of SND. Therefore, the mechanism of simultaneous nitrification and denitrification still needs to be further improved.
(2)The mechanism of simultaneous nitrification, denitrification and denitrification in MBBR biological moving bed
MBBR is a high-efficiency new type of reactor that combines the activated sludge method of suspended growth and the biofilm method of attached growth. The basic design principle is to directly add suspended fillers with a specific gravity close to water and can be suspended in water into the reaction tank as the activity of microorganisms. The carrier, the suspended filler can be in frequent contact with the sewage, and the biofilm (film hanging) gradually grows on the surface of the filler, which strengthens the mass transfer effect of pollutants, dissolved oxygen and biofilm, that is, MBBR is called "mobile biological film". membrane". Based on the research on SND mechanism so far, combined with microenvironment and biological theory, the possible reaction modes of SND in MBBR biofilm are aerobic ammonia oxidizing bacteria, nitrite oxidizing bacteria and aerobic denitrification distributed in the aerobic layer of the biofilm. The bacteria cooperate with anammox bacteria, autotrophic nitrite bacteria and denitrifying bacteria distributed in the biological anoxic layer, and finally achieve the purpose of denitrification.
MBBR relies on the aeration in the aeration tank and the lifting effect of the water flow to make the carrier in a fluidized state, thereby forming suspended activated sludge and attached biofilm, giving full play to the advantages of both attached and suspended organisms. It not only provides macroscopic and microscopic aerobic and anaerobic environments, but also resolves the DO disputes and carbon source disputes between autotrophic nitrifying bacteria, heterotrophic denitrifying bacteria and heterotrophic bacteria. Therefore, MBBR can realize the dynamic balance of the two processes of nitrification and denitrification, and has very good conditions for simultaneous nitrification and denitrification, and can realize MBBR simultaneous nitrification, denitrification and denitrification.
Influencing factors of MBBR simultaneous nitrification and denitrification
The key technology to achieve simultaneous nitrification and denitrification in MBBR is to control the kinetic balance of nitrification and denitrification in MBBR, and to resolve the DO dispute between autotrophic nitrifying bacteria and heterotrophic bacteria, and the carbon source dispute between denitrifying bacteria and heterotrophic bacteria, etc. , so the main control factors are: carbon-nitrogen ratio, dissolved oxygen concentration, temperature and pH.
(1) The influence of fillers on MBBR method
The technical key of the MBBR method lies in the biological fillers whose specific gravity is close to that of water, and which are easy to move freely with water under slight agitation. Usually the filler is made of polyethylene plastic. The shape of each carrier is a small cylinder with a diameter of 10mm and a height of 8mm. There are cross supports in the cylinder and protruding vertical fins on the outer wall. The hollow part of the filler accounts for 0.95% of the entire volume. , that is, in a container full of water and filler, the volume of water in each filler is 95%. Taking into account the rotation of the filler and the total volume of the container, the filling ratio of the filler is defined as the proportion of the space occupied by the carrier. In order to achieve the best mixing effect, the filling ratio of the filler is at most 0.7. Theoretically, the total specific surface area of the filler is defined by the number of specific surface areas per unit volume of biological carrier, which is generally 700m2/m3. When the biofilm grows inside the carrier, the actual effective use of the specific surface area is about 500m2/m3.
This type of biological filler is conducive to the growth of microorganisms on the inside of the filler, forming a relatively stable biofilm, and it is easy to form a fluidized state. When the pretreatment requirements are low or the sewage contains a large amount of fibrous substances, for example, the primary sedimentation tank is not used in the municipal sewage treatment or when the papermaking wastewater containing a large amount of fibers is treated, the biological filler with a small specific surface area and a large size is used. When there is a better pretreatment or for nitrification, the biological filler with a large specific surface area is used.
(2) The effect of dissolved oxygen (DO) on MBBR method
DO concentration is a major limiting factor affecting simultaneous nitrification and denitrification. By controlling DO concentration, different parts of the biofilm can form an aerobic zone or anoxic zone, which has the ability to achieve simultaneous nitrification and denitrification. physical conditions.
Theoretically, when the DO concentration is too high, DO can penetrate into the inside of the biofilm, making it difficult to form an anoxic zone inside, and a large amount of ammonia nitrogen is oxidized to nitrate and nitrite, making the effluent TN still high. On the contrary, if the concentration of DO is very low, a large proportion of the anaerobic zone will be formed inside the biofilm, and the denitrification capacity of the biofilm will be enhanced (the concentrations of nitrate and nitrite in the effluent are very low), but due to insufficient supply of DO, MBBR The nitrification effect of the process decreases, so that the concentration of ammonia nitrogen in the effluent increases, which leads to an increase in the TN of the effluent, which affects the final treatment effect.
Through the research, an optimal value of MBBR method for treating urban domestic sewage DO is finally obtained: when the DO concentration is above 2 mg/L, DO has little effect on the nitrification effect of MBBR, and the removal rate of ammonia nitrogen can reach 97%-99% %, the effluent ammonia nitrogen can be kept below 1.0mg/L; when the DO mass concentration is about 1.0mg/L, the ammonia nitrogen removal rate is about 84%, and the effluent ammonia nitrogen concentration has increased significantly. In addition, the DO in the aeration tank should not be too high. Too high dissolved oxygen can cause the organic pollutants to decompose too quickly, so that the microorganisms lack nutrients, the activated sludge is easy to age, and the structure is loose. In addition, DO is too high and excessive energy consumption is not suitable economically.
Because the MBBR method mainly realizes the final sewage treatment through suspended fillers, the effect of DO on the suspended fillers is also the key to the overall treatment results. Studies have shown that the oxygenation capacity of the reactor increases with the increase of the filling rate of the suspended filler within a certain range. Under the action of aeration, the water is fluidized together with the filler, and the turbulence of the water flow is greater than that without the filler, which accelerates the renewal of the gas-liquid interface and the transfer of oxygen, and increases the rate of oxygen transfer. As the amount of filler increases, the cutting action and turbulent action among the filler, air flow and water flow continue to strengthen. However, when the amount of filler added is 60%, the fluidization effect of the filler in water becomes poor, and the degree of turbulence in the water body also decreases, which reduces the transmission rate of oxygen and the utilization rate of oxygen. Therefore, for different types of water quality, controlling the amount of DO is crucial to the final treatment result of the entire process.
What is MBBR?
The MBBR process is based on the basic principle of the biofilm method. By adding a certain amount of suspended carrier to the reactor, the biomass and biological species in the reactor are increased, thereby improving the processing efficiency of the reactor. Since the density of the filler is close to that of water, it is completely mixed with water during aeration, and the environment for microbial growth is gas, liquid, and solid three-phase.
The collision and shearing of the carrier in the water make the air bubbles smaller and increase the utilization rate of oxygen. In addition, each carrier has different biological species inside and outside, with some anaerobic bacteria or facultative bacteria growing inside, and aerobic bacteria outside, so that each carrier is a microreactor, so that nitrification reaction and denitrification reaction coexist , thereby improving the processing effect.