Introduction: The anaerobic–aerobic biological phosphorus removal process is a high F/M, low SRT system. When the F/M ratio is higher and the SRT is lower, the amount of excess sludge discharged also increases. Therefore, under conditions where the phosphorus content in the sludge remains constant, the greater the extent of phosphorus removal, the better the overall phosphorus removal performance. For biological systems whose primary objective is phosphorus removal, the F/M ratio is typically 0.4–0.7 kg BOD/kg MLSS·d, and the SRT is 3.5–7 days. However, the SRT must not be set too low; effective BOD5 removal must be ensured as a prerequisite.

Nitrogen and phosphorus removal processes are increasingly applied to sewage treatment, but in actual operation, the excessive nitrogen and phosphorus content of the effluent often troubles the staff of the water plant. Therefore, clarifying and controlling the important parameters of the nitrogen and phosphorus removal process can ensure the normal operation of the system and the nitrogen and phosphorus content of the effluent reaches the standard.

1. Causes of Excessive Nitrogen Content in Sewage and Control Methods

1. Excessive ammonia nitrogen

1.1 Sludge Load and Sludge Age

Biological nitrification is a low-load process, and the F/M is generally 0.05～0.15kgBOD/kgMLVSS•d. The lower the load, the more fully the nitrification is carried out, and the higher the conversion efficiency of NH-N to NO-N. Corresponding to the low load, the SRT of the biological nitrification system is generally longer, because the generation cycle of nitrifying bacteria is longer. No nitrification effect can be obtained. How much SRT is controlled depends on factors such as temperature. For biological systems whose main purpose is denitrification, SRT usually takes 11 to 23 days.

1.2 Reflux Ratio and Hydraulic Retention Time

The reflux ratio in biological nitrification systems is generally higher than that in the conventional activated sludge process, primarily because the mixed liquor in biological nitrification systems already contains a substantial amount of nitrate. If the reflux ratio is too low, the activated sludge will remain in the secondary clarifier for an extended period, creating conditions conducive to denitrification and resulting in sludge bulking and floating. Typically, the reflux ratio is maintained within the range of 50% to 100%. Additionally, the hydraulic retention time in the biological nitrification aeration tank is longer than that in the activated sludge process, usually not less than 8 hours. This is mainly due to the fact that the nitrification rate is significantly lower than the rate of organic pollutant removal, necessitating a longer reaction time.

1.3 BOD5/TKN

The higher the BOD5/TKN ratio, the lower the proportion of nitrifying bacteria in the activated sludge, the slower the nitrification rate, and the lower the nitrification efficiency under the same operating conditions; conversely, the lower the BOD5/TKN ratio, the higher the nitrification efficiency. Operational experience from numerous municipal wastewater treatment plants has shown that the optimal BOD5/TKN range is approximately 2 to 3.

1.4 Dissolved oxygen

Nitrifying bacteria are obligate aerobes; their metabolic activity ceases in the absence of oxygen, and their oxygen uptake rate is significantly lower than that of heterotrophic bacteria that degrade organic matter. Without adequate oxygen supply, nitrifying bacteria will be outcompeted and ultimately fail. Therefore, it is essential to maintain dissolved oxygen levels in the aerobic zone of the biological pond above 2 mg/L, with the dissolved oxygen concentration further increased under specific conditions.

1.5 Temperature and pH

Nitrifying bacteria are also highly sensitive to temperature fluctuations. When the wastewater temperature falls below 15°C, the nitrification rate declines markedly; and when the temperature drops below 5°C, their physiological activities cease entirely.

Therefore, in winter, sewage treatment plants, especially those in northern regions, tend to exhibit more pronounced ammonia-nitrogen effluent concentrations. Nitrifying bacteria are highly sensitive to pH; their biological activity is maximized within the pH range of 8–9. When the pH falls below 6.0 or exceeds 9.6, nitrifying bacterial activity is inhibited and may even cease. Consequently, the pH of the mixed liquor in biological nitrification systems should be maintained above 7.0.

2. Total nitrogen exceeds the standard

2.1 Sludge Load and Sludge Age

Since biological nitrification is the prerequisite for biological denitrification, only effective nitrification can achieve efficient and stable denitrification. Therefore, the denitrification system must operate at low or ultra-low organic loading and maintain a high sludge retention time.

2.2 Internal and External Reflux Ratio

The external return flow in a biological denitrification system is lower than that in a pure biological nitrification system. This is primarily due to the fact that most of the nitrogen in the influent wastewater has already been removed, resulting in a relatively low NO₃⁻-N concentration in the secondary clarifier. Additionally, the sludge settling rate in denitrification systems is comparatively high; therefore, provided that the required return sludge concentration is maintained, the external return ratio can be reduced to prolong the hydraulic retention time of the wastewater in the aeration tank. For a well-operating wastewater treatment plant, the external return ratio can typically be kept below 50%, while the internal return ratio is generally maintained within the range of 300% to 500%.

2.3 Dissolved oxygen in hypoxic zone

For denitrification, it is desirable to maintain dissolved oxygen (DO) at the lowest possible level, ideally close to zero, so that denitrifying bacteria can operate at full capacity and enhance denitrification efficiency. However, based on the actual operation of the wastewater treatment plant, it remains challenging to keep DO in the anoxic zone below 0.5 mg/L, which adversely affects the biological denitrification process and, consequently, the total nitrogen concentration in the effluent.

2.4 BOD5/TKN

Denitrifying bacteria carry out denitrification during the decomposition of organic matter. Therefore, sufficient organic matter must be present in the wastewater entering the anoxic zone to ensure the smooth progression of denitrification. However, due to delays in the construction of supporting sewer networks at many wastewater treatment plants, the influent BOD5 is often lower than the design value, while nitrogen and phosphorus concentrations are equal to or exceed the design levels. This results in an insufficient carbon source in the influent to meet the carbon demand for denitrification, leading to instances where the total nitrogen in the effluent exceeds regulatory limits.

2.5 Temperature and pH

Although denitrifying bacteria are not as sensitive to temperature changes as nitrifying bacteria, the denitrification effect will also change with temperature changes. The higher the temperature, the higher the denitrification rate, and the denitrification rate increases to the maximum at 30-35°C. When it is lower than 15°C, the denitrification rate will obviously decrease, and when it reaches 5°C, the denitrification will tend to stop. Denitrifying bacteria are not as sensitive to pH changes as nitrifying bacteria. They can perform normal physiological metabolism within the range of pH 6-9, but the optimal pH range for biological denitrification is 6.5-8.0.

2. Causes and countermeasures for total phosphorus in biological removal of sewage from sewage

1. Sludge load and sludge age

The anaerobic–aerobic biological phosphorus removal process is a high F/M, low SRT system. As the F/M ratio increases and the SRT decreases, the volume of excess sludge discharged also rises. Consequently, for a given phosphorus concentration in the sludge, greater phosphorus removal leads to better overall phosphorus removal performance. In biological systems primarily designed for phosphorus removal, the F/M ratio is typically 0.4–0.7 kg BOD/kg MLSS·d, and the SRT is 3.5–7 days. However, the SRT must not be set too low; effective BOD5 removal must remain the fundamental prerequisite.

2. BOD/TP

To ensure effective phosphorus removal, the BOD/TP ratio in the wastewater entering the anaerobic zone should be maintained above 20. Since polyphosphate-accumulating organisms belong to the genus Acinetobacter and exhibit relatively weak physiological activity, they are capable of only assimilating the readily biodegradable fraction of organic matter. Therefore, it is essential to ensure an adequate BOD5 concentration in the influent to support the normal physiological metabolism of these polyphosphate-accumulating bacteria. However, in many urban wastewater treatment plants, the actual influent typically has low carbon availability and high concentrations of nitrogen and phosphorus. Consequently, the BOD5/TP ratio often fails to meet the requirements for biological phosphorus removal, thereby compromising its effectiveness.

3. Dissolved oxygen

The anaerobic zone must be maintained under strictly anaerobic conditions, with dissolved oxygen levels below 0.2 mg/L, so that polyphosphate-accumulating organisms can efficiently release phosphorus and thereby ensure the effectiveness of subsequent treatment. In the aerobic zone, dissolved oxygen must be kept above 2.0 mg/L to enable these organisms to effectively take up phosphorus. Consequently, improper control of dissolved oxygen in both the anaerobic and aerobic zones can significantly impair the efficiency of biological phosphorus removal.

4. Reflux ratio and hydraulic retention time

The reflux ratio in an anaerobic–aerobic phosphorus removal system should not be too low; a sufficient reflux ratio must be maintained to prevent polyphosphate-accumulating organisms from releasing phosphorus in the secondary clarifier when exposed to an anaerobic environment. While ensuring rapid sludge discharge, the reflux ratio should be reduced as much as possible to avoid shortening the actual hydraulic retention time of sludge in the anaerobic zone, which could impair phosphorus release. In such systems, if sludge settling performance is good, a reflux ratio within the range of 50% to 70% can facilitate rapid sludge discharge. The hydraulic retention time of wastewater in the anaerobic zone is generally between 1.5 and 2.0 hours; if this retention time is too short, phosphorus release will be inadequate, and facultative acidogenic bacteria in the sludge will be unable to fully degrade the macromolecular organic matter in the influent into lower fatty acids that can be utilized by polyphosphate-accumulating organisms, thereby reducing phosphorus release. In the aerobic zone, the hydraulic retention time is typically 4 to 6 hours, which ensures complete phosphorus uptake.

5, pH

Low pH promotes phosphorus release, while high pH favors phosphorus uptake; the overall phosphorus removal efficiency results from the combined effects of phosphorus release and uptake. Therefore, in biological phosphorus removal systems, it is advisable to maintain the pH of the mixed liquor within the range of 6.5 to 8.0.

Article source: Environmental Protection Online