Factors Affecting Total Nitrogen Removal Efficiency in Wastewater Treatment
Denitrification in wastewater treatment is based on the biological nitrification process, with the addition of biological denitrification, which refers to the biochemical process where nitrate in wastewater is reduced to nitrogen gas by microorganisms under anoxic conditions.
The factors influencing the total nitrogen removal efficiency are numerous, and the main ones include:
1. Sludge Load and Sludge Age
Since biological nitrification is the prerequisite for biological denitrification, only good nitrification can achieve efficient and stable denitrification. Therefore, the denitrification system must also adopt low or ultra-low loads and use high sludge age.
2. Internal and External Recycle Ratios
The external recycle ratio for biological denitrification systems is smaller than that for pure biological nitrification systems. This is because the majority of nitrogen in the incoming wastewater has already been removed, and the nitrate (NO3--N) concentration in the secondary clarifier is not high. Consequently, the risk of sludge floating due to denitrification in the secondary clarifier is very low. On the other hand, the sludge settling velocity in the denitrification system is faster. Given the required return sludge concentration, the recycle ratio can be lowered to extend the retention time of wastewater in the aeration tank.
For well-operated wastewater treatment plants, the external recycle ratio can be controlled below 50%. The internal recycle ratio is generally controlled between 300% to 500%.
3. Denitrification Rate
The denitrification rate refers to the amount of nitrate denitrified per unit of active sludge per day. The denitrification rate is related to factors like temperature, with typical values ranging from 0.06 to 0.07gNO3--N/g MLVSS·d.
4. Dissolved Oxygen in the Anoxic Zone
For denitrification, it is ideal to keep the dissolved oxygen (DO) as low as possible, ideally zero, so that the denitrifying bacteria can fully carry out the process, improving nitrogen removal efficiency. However, in practice, maintaining DO in the anoxic zone below 0.5 mg/L is difficult, which affects the denitrification process and thus the total nitrogen in the effluent.
5. BOD5/TKN
Since denitrifying bacteria carry out denitrification while decomposing organic matter, there must be sufficient organic material in the wastewater entering the anoxic zone to ensure smooth denitrification. Due to the lag in the construction of sewage networks in many treatment plants, the incoming BOD5 is often lower than the design value, while nitrogen and phosphorus levels are at or higher than the design value. This results in insufficient carbon sources for denitrification, which may lead to total nitrogen exceeding standards.
6. pH
Denitrifying bacteria are less sensitive to pH changes than nitrifying bacteria. They can carry out normal physiological metabolism within a pH range of 6 to 9, but the optimal pH range for biological denitrification is 6.5 to 8.0.
7. Temperature
Denitrifying bacteria are not as sensitive to temperature changes as nitrifying bacteria, but denitrification efficiency will still vary with temperature. The higher the temperature, the higher the denitrification rate. At temperatures between 30 to 35°C, the denitrification rate peaks. When the temperature is below 15°C, the denitrification rate significantly drops, and at 5°C, denitrification almost stops. Therefore, in winter, to ensure denitrification effectiveness, it is necessary to increase the sludge retention time (SRT), raise sludge concentration, or increase the number of aeration tanks in operation.
