What are the factors that affect the nitration reaction?

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1. Sludge load F/M and sludge age SRT: Biological nitrification is a low-load process, with F/M generally being below 0.15 kgBOD/(kgMLVSS·d). The lower the load, the more complete the nitrification occurs, and the higher the efficiency of converting NH3-N to NO3--N. Sometimes, to achieve an extremely low NH3-N in the effluent, ultra-low load rates with an F/M of 0.05 kgBOD/(kgMLVSS·d) are even employed. Corresponding to low load, the sludge age SRT in biological nitrification systems is generally longer. This is mainly because nitrifying bacteria grow slowly and have a long generation time; without a sufficiently long SRT, these bacteria cannot be cultivated, and thus nitrification cannot take place. In actual operation, the level of SRT control depends on factors such as temperature. However, under normal circumstances, to achieve an ideal nitrification effect, the SRT should be at least 15 days. 2. Recirculation ratio R and hydraulic retention time T: The recirculation ratio in biological nitrification systems is generally higher than that in conventional activated sludge processes. This is mainly because the activated sludge mixture in the biological nitrification system already contains large amounts of nitrate; if the recirculation ratio is too low, the residence time of the activated sludge in the secondary sedimentation tank is prolonged, which facilitates denitrification and leads to the sludge floating to the surface. The hydraulic retention time T in the aeration tank of biological nitrification systems is generally longer than that in conventional activated sludge processes, being at least 8 hours. This is mainly because the nitrification rate is much lower than that of organic pollutant removal, thus requiring a longer reaction time. 3. Dissolved oxygen DO: The DO level in the mixture used in the nitrification process should be controlled at 2.0 mg/L, with values generally ranging between 2.0 and 3.0 mg/L. When DO is less than 2.0 mg/L, nitrification will be inhibited ； When DO is less than 1.0 mg/L, nitrification is completely inhibited and tends to stop. Biological nitrification systems require a high concentration of DO for various reasons. Firstly, nitrobacteria are obligate aerobes; they cease their metabolic activities in the absence of oxygen, unlike bacteria that break down organic matter, most of which are facultative organisms. Secondly, nitrifying bacteria have a much lower oxygen uptake rate compared to bacteria that break down organic matter; without an adequate supply of oxygen, nitrifying bacteria will not be able to obtain the oxygen they need. Furthermore, the vast majority of nitrifying bacteria are enclosed within sludge flocs; only by maintaining a high dissolved oxygen concentration in the mixture can this oxygen be \"forced\" into the flocs, thereby facilitating its uptake by the nitrifying bacteria. Under normal conditions, approximately 4.57 g of oxygen are required to convert each gram of NH3-N into NO3--N. For typical urban wastewater, the actual oxygen supply required by biological nitrification systems is generally more than 50% higher than that in conventional activated sludge processes, depending on the TKN concentration in the influent water. 4. Nitrification rate: A specific process parameter in biological nitrification systems is the nitrification rate, which refers to the amount of ammonia nitrogen converted per unit weight of activated sludge per day. It is generally denoted as NR, with the unit typically being gNH3-N/(gMLVSS·d). The value of the NR ratio depends on factors such as the proportion of nitrifying bacteria in the activated sludge and temperature; the typical value is 0.02 gNH3-N/(gMLVSS·d), meaning that each gram of activated sludge can convert approximately 0.02 g of NH3-N into NO3--N per day. 5. The effect of BOD5/TKN on nitrification: TKN refers to the sum of organic nitrogen and ammonia nitrogen in water. The ratio of BOD5 to TKN in the incoming wastewater is an important factor affecting the nitrification effect. The higher the BOD5/TKN ratio, the smaller the proportion of nitrifying bacteria in the activated sludge, and thus the lower the nitrification rate NR; under the same operating conditions, the nitrification efficiency is also lower ； Conversely, the lower the BOD5/TKN ratio, the higher the nitrification efficiency. The BOD5/TKN ratio of urban wastewater is approximately 5–6; at this value, the proportion of nitrifying bacteria in the activated sludge is about 5% ； If the BOD5/TKN ratio of the wastewater increases to 9, the proportion of nitrifying bacteria will drop to 3% ； If BOD5/TKN is reduced to 3, the proportion of nitrifying bacteria can reach up to 9%. Secondly, as the BOD5/TKN ratio decreases, the proportion of nitrifying bacteria increases; some of these bacteria separate from the sludge flocs and become in a free state. They do not settle easily in the secondary sedimentation tank, resulting in turbid effluent. In summary, when BOD5/TKN is too low, although the nitrification efficiency increases, the clarity of the effluent decreases ； When BOD5/TKN is too high, clarity improves, but nitrification efficiency decreases. Therefore, for a particular biological nitrification system, there is an optimal BOD5/TKN value. Operational experience from many treatment plants has shown that the optimal range for the BOD5/TKN ratio is 2 to 3. 6. The effect of pH and alkalinity on nitrification. Nitrifying bacteria are very sensitive to pH; their biological activity is highest within the range of pH 8–9. When pH is less than 6.0 or greater than 9.6, the biological activity of nitrifying bacteria is suppressed and tends to cease. In biological nitrification systems, it is necessary to keep the pH of the mixed solution above 7.0; when the pH is below 7.0, the nitrification rate decreases significantly. When pH < 6.5, alkali must be added to the wastewater. There may be two reasons for the decrease in the pH of the mixture: first, strong acids are discharged into the influent water, causing the pH of the incoming wastewater to drop, and as a result, the pH of the mixture also decreases. If no strong acids are discharged, normal municipal wastewater should be alkaline, meaning its pH is generally above 7.0; in such cases, the pH of the mixture depends mainly on the alkalinity present in the incoming wastewater. As can be seen from the nitration reaction equation, as NH3-N is converted into NO3--N, some mineralized acidity in the form of H+ is generated; this acidity consumes part of the alkalinity. Approximately 7.14 g of alkalinity (expressed in terms of CaCO3) is consumed per gram of NH3-N converted into NO3--N. Therefore, when the alkalinity in the wastewater is insufficient while the TKN load is high, the alkalinity in the wastewater is depleted, causing the pH of the mixture to drop below 7.0 and thereby reducing or inhibiting the nitrification rate. 7. The effect of toxic substances on nitrification: Certain heavy metal ions, complex anions, cyanides, and some organic substances can interfere with or disrupt the normal physiological activities of nitrifying bacteria. When the concentration of these substances in wastewater is high, it inhibits the normal progression of biological nitrification. For example, nitration is inhibited when lead ions exceed 0.5 mg/L, phenol exceeds 5.6 mg/L, and thiourea exceeds 0.076 mg/L. Interestingly, when the NH3-N concentration exceeds 200 mg/L, it can also inhibit the nitrification process; however, urban wastewater generally does not have such high NH3-N concentrations. 8. The effect of temperature on nitrification: Nitrifying bacteria are also very sensitive to changes in temperature. Within the range of 5–35°C, nitrate bacteria can carry out normal physiological metabolic activities, and their biological activity increases as the temperature rises. At around 30°C, its biological activity reaches its maximum, while below 5°C, its physiological activities come to a complete halt. In the operation and management of biological nitrification systems, when the wastewater temperature is above 16°C, an sludge age of 8–10 days is sufficient ； However, when the temperature is below 10°C, the sludge age SRT should be increased to 12–20 days.