What factors contribute to excess total nitrogen?
1) Sludge load and sludge age
As biological nitrification is the premise of biological denitrification, only good nitrification can obtain stable denitrification. Therefore, the denitrification system must also adopt low load or ultra-low load, and adopt high sludge age.
(2) Internal and external reflux ratio
The external reflux of the biological denitrification system is smaller than that of the simple biological nitrification system, mainly because most of the nitrogen in the effluent has been removed, and the concentration of NO3--N in the secondary sedimentation tank is not high. Relatively speaking, the risk of sludge floatation due to denitrification in secondary sedimentation tank is very small. On the other hand, the sludge settling rate of denitrification system is faster, and the reflux ratio can be reduced on the premise of ensuring the required reflux sludge concentration, so as to prolong the residence time of sewage in the aeration tank.
A well-run sewage treatment plant can control the reflux ratio below 50%. The internal reflux ratio is generally controlled between 300 ~ 500%.
(3) Denitrification rate
Denitrification rate refers to the amount of nitrate denitrification per unit of activated sludge per day. The denitrification rate was related to temperature and other factors, and the typical value was 0.06 ~ 0.07gNO3--N/gMLVSS×d.
(4) Dissolved oxygen in anoxic zone
For denitrification, DO is expected to be as low as possible, preferably zero, so that denitrifying bacteria can "go all out" for denitrification and improve denitrification efficiency. However, from the actual operation of the sewage treatment plant, it is still difficult to control the DO in the anoxic zone below 0.5mg/L, so it affects the process of biological denitrification, and then affects the total nitrogen index of effluent.
(5) the BOD5 / TKN
Because denitrifying bacteria carry out denitrification and denitrification in the process of decomposition of organic matter, there must be enough organic matter in the wastewater entering the anoxic zone to ensure the smooth denitrification. BOD5 is lower than the design value, while nitrogen, phosphorus and other indicators are equivalent to or higher than the design value, which makes the influent carbon source cannot meet the demand for carbon source for denitrification, and also leads to the situation that the total nitrogen in the effluent exceeds the standard.
(6) pH
Denitrifying bacteria are not as sensitive to pH changes as nitrifying bacteria. They can carry out normal physiological metabolism in the range of pH 6 ~ 9, but the optimal pH range of biological denitrification is 6.5 ~ 8.0.
(7) Temperature
Denitrifying bacteria are not as sensitive to temperature change as nitrifying bacteria, but the denitrifying effect will also change with temperature change. The higher the temperature is, the higher the denitrification rate is, and the denitrification rate increases to the maximum at 30-35 ℃. When the temperature is below 15℃, the denitrification rate decreases obviously, and when it reaches 5℃, the denitrification tends to stop. Therefore, to ensure the effect of nitrogen removal in winter, it is necessary to increase SRT, increase sludge concentration or increase the number of operating ponds.
