Reasons Affecting Ammonia Nitrogen Removal Efficiency in Wastewater Treatment
The removal of ammonia nitrogen in wastewater is mainly achieved by applying a nitrification process based on the conventional activated sludge process, typically by extended aeration and reducing the system loading.
The factors affecting ammonia nitrogen removal efficiency involve many aspects, mainly including the following:
1. Sludge Loading and Sludge Retention Time (SRT)
Biological nitrification is a low-loading process. The F/M ratio is generally 0.05–0.15 kg BOD/kg MLVSS·d. The lower the loading, the more complete the nitrification process, and the higher the conversion efficiency of NH3-N to NO3--N.
Corresponding to low loading, the SRT of a biological nitrification system is usually long, because nitrifying bacteria have a long generation cycle. If the sludge retention time is too short (i.e., SRT is insufficient) and the sludge concentration is low, nitrifying bacteria cannot be sufficiently cultivated, resulting in poor nitrification performance. The appropriate SRT depends on factors such as temperature. For biological systems mainly designed for nitrogen removal, the SRT is typically 11–23 days.
2. Return Sludge Ratio
The return sludge ratio of biological nitrification systems is generally higher than that of conventional activated sludge processes. This is mainly because the mixed liquor in nitrification systems contains a large amount of nitrate. If the return ratio is too low, the sludge residence time in the secondary clarifier becomes too long, which can easily lead to denitrification and cause sludge flotation. The return sludge ratio is usually controlled at 50–100%.
3. Hydraulic Retention Time (HRT)
The HRT of nitrification aeration tanks is longer than that of conventional activated sludge processes and should generally be no less than 8 hours. This is because the nitrification rate is much lower than the removal rate of organic pollutants, thus requiring a longer reaction time.
4. BOD5/TKN Ratio
TKN refers to the sum of organic nitrogen and ammonia nitrogen in water. The BOD5/TKN ratio in influent wastewater is an important factor affecting nitrification efficiency. A higher BOD5/TKN ratio means a lower proportion of nitrifying bacteria in the activated sludge, resulting in a lower nitrification rate and efficiency under the same operating conditions. Conversely, a lower BOD5/TKN ratio leads to higher nitrification efficiency. Operational experience from many wastewater treatment plants shows that the optimal BOD5/TKN range is approximately 2–3.
5. Nitrification Rate
A specific process parameter of biological nitrification systems is the nitrification rate, which refers to the amount of ammonia nitrogen converted per unit mass of activated sludge per day. The nitrification rate depends on factors such as the proportion of nitrifying bacteria in the activated sludge and temperature. A typical value is 0.02 g NH3-N/(g MLVSS·d).
6. Dissolved Oxygen (DO)
Nitrifying bacteria are obligate aerobic microorganisms and cease metabolic activity in the absence of oxygen. Moreover, their oxygen uptake rate is much lower than that of heterotrophic bacteria that degrade organic matter. If sufficient oxygen is not maintained, nitrifying bacteria will be unable to compete for the required oxygen. Therefore, the dissolved oxygen concentration in the aerobic zone of the bioreactor should be maintained above 2 mg/L, and in special cases, even higher DO levels may be required.
7. Temperature
Nitrifying bacteria are highly sensitive to temperature changes. When the wastewater temperature drops below 15 °C, the nitrification rate decreases significantly, and when it falls below 5 °C, nitrifying bacteria completely cease physiological activity. As a result, ammonia nitrogen exceedances in effluent are particularly common in winter, especially in wastewater treatment plants located in northern regions.
8. pH
Nitrifying bacteria are also very sensitive to pH. Their biological activity is strongest within a pH range of 8–9. When the pH is below 6.0 or above 9.6, nitrification activity is inhibited and tends to stop. Therefore, the mixed liquor pH in biological nitrification systems should be controlled to be above 7.0 whenever possible.
