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In the simultaneous nitrification-denitrification biological denitrification process in MBBR, DO concentration is a major limiting factor affecting simultaneous nitrification-denitrification. By controlling the DO concentration, different parts of the biofilm can form aerobic zones or anoxic zones, thus providing the physical conditions to achieve simultaneous nitrification and denitrification. Theoretically, when the DO mass concentration is too high, DO can penetrate into the biofilm, making it difficult to form an anoxic zone inside. A large amount of ammonia nitrogen is oxidized into nitrate and nitrite, so that the effluent TN remains high. ; On the contrary, if the DO concentration 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 effluent nitrate nitrogen and nitrite nitrogen concentrations are both very low). However, due to insufficient DO supply, MBBR The nitrification effect of the process decreases, causing the ammonia nitrogen concentration of the effluent to increase, which causes the TN of the effluent to increase, affecting the final treatment effect.
An optimal value for the MBBR method to treat urban domestic sewage DO: when the DO mass concentration is above 2 mg/L, DO has little impact on the nitrification effect of MBBR. The ammonia nitrogen removal rate can reach 97%-99%, and the effluent ammonia nitrogen can be removed. Keep it below 1.0mg/L; when the DO mass concentration is around 1.0mg/L, the ammonia nitrogen removal rate is around 84%, and the effluent ammonia nitrogen concentration increases significantly. In addition, the DO in the aeration tank should not be too high. Excessive dissolved oxygen can cause organic pollutants to decompose too quickly, resulting in a lack of nutrients for microorganisms, and the activated sludge is prone to aging and has a loose structure. In addition, if the DO is too high, it consumes excessive energy, which is also economically unsuitable.
Because the MBBR method mainly uses suspended fillers to achieve final sewage treatment, the impact of DO on suspended fillers is also the key to the entire treatment results. Under the action of aeration, water fluidizes together with the filler, and the degree of turbulence of the water flow is greater than that without filler, which accelerates the renewal of the gas-liquid interface and the transfer of oxygen, increasing the oxygen transfer rate. As the number of fillers increases, the cutting and turbulence effects between the fillers, air flow and water flow continue to strengthen. When the filling rate of the filler reaches 60%, the fluidization effect of the filler in the water becomes worse, and the degree of turbulence of the water body also decreases, causing the oxygen transfer rate to decrease and the oxygen utilization rate to decrease. Therefore, for different types of water quality, controlling the amount of DO is crucial to the final treatment result of the entire process.
Appropriate hydraulic retention time (HRT) is an important control factor to ensure purification effect and economical project investment. The length of hydraulic retention time will directly affect the contact time between organic matter in the water and biofilm, which will in turn affect the adsorption and degradation efficiency of organic matter by microorganisms. Therefore, finding an economical and reasonable HRT for different sewage types is one of the key issues. Research on HRT at home and abroad is not limited to studying the impact of HRT itself, but to grasp the macroscopic effects through experiments.
Under normal circumstances, with the gradual extension of HRT, the effluent COD concentration will gradually decrease. Most domestic experiments believe that the average COD concentration of effluent decreases with the extension of hydraulic retention time. To shorten the hydraulic retention time, it can be achieved by increasing the proportion of filler (up to 70%). When the requirements for effluent water quality are not high, The proportion of filler can be reduced. In addition, test results show that: under medium and low ammonia nitrogen load conditions, as HRT decreases, the surface load of ammonia nitrogen gradually increases, while the removal rate maintains the original level or increases to a certain extent; when the ammonia nitrogen load rises to a high level, as the As HRT decreases, the ammonia nitrogen removal rate gradually decreases.
Among the various factors that affect the physiological activities of microorganisms, the role of temperature is very important. A suitable temperature can promote and strengthen the physiological activities of microorganisms; an inappropriate temperature can weaken or even destroy the physiological activities of microorganisms. Inappropriate temperature can also lead to changes in the morphology and physiological characteristics of microorganisms, and may even cause the death of microorganisms. The optimal temperature of microorganisms means that under this temperature condition, the physiological activities of microorganisms are strong and vigorous, which is manifested in fast fission speed and short generation time in terms of proliferation. The MBBR method mainly degrades organic pollutants in wastewater through the metabolism of various types of microorganisms in biofilms. Therefore, the quality of biofilm growth will be directly related to the final result of wastewater treatment, especially for nitrifying bacteria and denitrifying bacteria. Generally speaking, they have a long growth cycle and are very sensitive to environmental changes. The suitable temperature for nitrifying bacteria is 20℃-30℃, and the suitable temperature for denitrifying bacteria is 20℃-40℃. When the temperature is lower than 15℃, this The activity of both types of bacteria decreases and stops completely at 5~C, so changes in temperature will directly affect the growth of this type of bacteria.
The change of surface load of ammonia nitrogen filler is basically consistent with the change trend of water temperature. When the water temperature is low, the surface load of the filler is low. When the water temperature is high, the surface load of the filler is about 15 times that of when the water temperature is low. It can be seen that nitrifying bacteria are greatly affected by temperature, and their activity is weak under low temperature conditions.
The physiological activities of microorganisms are closely related to the pH of the environment. Only under suitable pH conditions can microorganisms carry out normal physiological activities. If the pH value deviates too much from the appropriate value, the catalytic function of the microbial enzyme system will weaken or even disappear. The pH values to which the physiological activities of different species of microorganisms are adapted have a certain range. Within this range, they can also be divided into the lowest pH value, the optimal pH value and the highest pH value. In the lowest or highest pH environment, although microorganisms can survive, their physiological activities are weak, they are prone to death, and their proliferation rate is greatly reduced. The optimal pH range for microorganisms involved in biological treatment of sewage is generally between 6.5-8.5. As a process that combines biofilm method and activated sludge method, MBBR method also relies on the growth of microorganisms to achieve the purpose of organic matter degradation. Therefore, maintaining the optimal pH range of microorganisms is a necessary condition for achieving good sewage treatment results. When the pH value of sewage (especially industrial wastewater) changes greatly, it is necessary to consider setting up a regulating tank to adjust the pH value of the sewage to a suitable range. Perform aeration.
Depending on each specific test condition, there are many different influencing factors. For example, the size of the aeration volume. If the aeration volume is too small, it will be difficult for the filler to roll and fluidize. If the aeration volume is too large, it will be difficult for the biofilm to form in the early stage. For example, the air-water ratio is generally controlled at (3~4). Such air volume It can make the filler in the reactor circulate and rotate evenly; the turbidity also needs to be controlled within a certain range. Relevant research results show that high turbidity makes certain suspended solids easily cover the surface of the biofilm, hindering the progress of biological oxidation. , leading to a significant decrease in treatment efficiency, and at the same time, it is easy to cause packing clogging. The COD volumetric load also has a great impact on the removal rate. Research shows that the COD removal rate is within the range of COD volumetric load of 0.48-2.93kg/(m3·d). Basically stable at 60%-80%. Under the same hydraulic retention time, the COD removal rate increases proportionally with the load. This is because when the inlet water COD concentration is low, the rate of microbial degradation of organic matter is also small, and its degradation ability cannot be fully exerted. When the inlet water COD concentration is When it increases, it promotes the growth of biofilm microorganisms and increases the degradation rate, so the COD removal rate is improved. Each of the above factors will have varying degrees of impact on sewage treatment. In addition, there are nutrients, toxic substances, etc. If these substances deviate too much from the growth needs of microorganisms, they will have an impact on the final results of sewage treatment. We must determine which factor mainly affects the final result of the MBBR method based on specific conditions and requirements.
