Introduction: Anaerobic-aerobic biological phosphorus removal process is a high F/M low SRT system. When F/M is higher and SRT is lower, the amount of residual sludge discharge is also higher. Therefore, under the condition of a certain phosphorus content in the sludge, the more phosphorus removal, the better the phosphorus removal effect. For biological systems whose main purpose is phosphorus removal, F/M is usually 0.4～0.7kgBOD/kgMLSS?d, and SRT is 3.5～7d. However, the SRT cannot be too low, and the effective removal of BOD5 must be the premise.

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. Reasons for 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 of the biological nitrification system is generally larger than that of the traditional activated sludge process, mainly because the activated sludge mixture of the biological nitrification system already contains a large amount of nitrate. If the reflux ratio is too small, the activated sludge will stay in the secondary settling tank. The time is longer, and it is easy to produce denitrification and cause the sludge to float up. Usually the reflux ratio is controlled at 50-100%. The hydraulic retention time of the biological nitrification aeration tank is also longer than that of the activated sludge process, and should be at least 8h. This is mainly because the nitrification rate is much lower than the removal rate of organic pollutants, which requires a longer reaction time.

1.3 BOD5/TKN

The larger the BOD5/TKN, the smaller the proportion of nitrifying bacteria in the activated sludge, the smaller the nitrification rate, and the lower the nitrification efficiency under the same operating conditions; conversely, the smaller the BOD5/TKN, the higher the nitrification efficiency. The operation practice of many urban sewage treatment plants has found that the best range of BOD5/TKN value is about 2 to 3.

1.4 Dissolved oxygen

Nitrifying bacteria are obligate aerobic bacteria, which stop their life activities when there is no oxygen, and the oxygen uptake rate of nitrifying bacteria is much lower than that of bacteria that decompose organic matter. If sufficient oxygen is not maintained, nitrifying bacteria will "compete" and lose everything. Need oxygen. Therefore, it is necessary to maintain the dissolved oxygen in the aerobic zone of the biological pond above 2mg/L, and the dissolved oxygen content needs to be increased under special circumstances.

1.5 Temperature and pH

Nitrifying bacteria are also very sensitive to temperature changes. When the sewage temperature is lower than 15°C, the nitrification rate will decrease significantly, and when the sewage temperature is lower than 5°C, its physiological activities will stop completely.

Therefore, in winter, sewage treatment plants, especially sewage treatment plants in northern regions, have more obvious ammonia nitrogen effluents. Nitrifying bacteria are very sensitive to pH response. In the range of pH 8-9, their biological activity is the strongest. When pH<6.0 or >9.6, the biological activity of nitrifying bacteria will be inhibited and tend to stop. Therefore, the pH of the mixed solution of the biological nitrification system should be controlled to be greater than 7.0.

2, the total nitrogen exceeds the standard

2.1 Sludge load and sludge age

Since biological nitrification is the prerequisite of biological denitrification, only good nitrification can obtain efficient and stable denitrification. Therefore, the denitrification system must also use low load or ultra-low load, and use high sludge age.

2.2 Internal and external reflux ratio

The external backflow of the biological denitrification system is smaller than that of the pure biological nitrification system. This is mainly because most of the nitrogen in the influent sewage has been removed, and the NO-N concentration in the secondary sedimentation tank is not high. On the other hand, the sludge sedimentation rate of the denitrification system is relatively fast. Under the premise of ensuring the required return sludge concentration, the return ratio can be reduced to extend the residence time of the sewage in the aeration tank. For a well-functioning sewage treatment plant, the external reflux ratio can be controlled below 50%. The internal reflux ratio is generally controlled between 300 and 500%.

2.3 Dissolved oxygen in hypoxic zone

For denitrification, it is hoped that the DO is as low as possible, preferably zero, so that the denitrifying bacteria can denitrify with “full force” and improve the efficiency of denitrification. However, judging from the actual operation of the sewage treatment plant, it is still difficult to control the DO in the hypoxic zone below 0.5 mg/L, which affects the biological denitrification process and thus the total nitrogen index of the effluent.

2.4 BOD5/TKN

Denitrifying bacteria perform denitrification and denitrification in the process of decomposing organic matter. Therefore, there must be sufficient organic matter in the sewage entering the anoxic zone to ensure the smooth progress of denitrification. Due to the lag in the construction of supporting pipe networks in many sewage treatment plants, the incoming BOD5 is lower than the design value, and indicators such as nitrogen and phosphorus are equal to or higher than the design value, making the influent carbon source unable to meet the carbon source demand for denitrification. It has also led to occasions where the total nitrogen in the effluent exceeds the standard.

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

1. Sludge load and sludge age

Anaerobic-aerobic biological phosphorus removal process is a high F/M low SRT system. When F/M is higher and SRT is lower, the amount of residual sludge discharge is also higher. Therefore, under the condition of a certain phosphorus content in the sludge, the more phosphorus removal, the better the phosphorus removal effect. For biological systems whose main purpose is phosphorus removal, F/M is usually 0.4～0.7kgBOD/kgMLSS•d, and SRT is 3.5～7d. However, the SRT cannot be too low, and the effective removal of BOD5 must be the premise.

2, BOD/TP

To ensure the phosphorus removal effect, the BOD/TP in the sewage entering the anaerobic zone should be controlled to be greater than 20. Because the polyphosphate bacteria belong to the genus Acinetobacter, their physiological activities are weak, and they can only ingest the easily decomposed part of the organic matter. Therefore, the content of BOD5 in the influent should be guaranteed to ensure the normal physiological metabolism of polyphosphoric acid bacteria. However, many urban sewage treatment plants have low carbon sources and high concentrations of nitrogen and phosphorus in the actual influent water. As a result, the BOD5/TP value cannot meet the needs of biological phosphorus removal, which affects the effect of biological phosphorus removal.

3, dissolved oxygen

The anaerobic zone should maintain a strict anaerobic state, that is, the dissolved oxygen is less than 0.2mg/L, then the phosphorus-accumulating bacteria can release the phosphorus effectively to ensure the subsequent treatment effect. The dissolved oxygen in the aerobic zone needs to be kept above 2.0 mg/L for the phosphorus-accumulating bacteria to effectively absorb phosphorus. Therefore, improper control of dissolved oxygen in the anaerobic zone and aerobic zone will greatly affect the effect of biological phosphorus removal.

4, reflux ratio and hydraulic retention time

The reflux ratio of the anaerobic-aerobic phosphorus removal system should not be too low, and a sufficient reflux ratio should be maintained to prevent phosphorus-accumulating bacteria from releasing phosphorus in the secondary sedimentation tank when encountering an anaerobic environment. On the premise of ensuring rapid sludge discharge, the reflux ratio should be reduced as much as possible to avoid shortening the actual residence time of sludge in the anaerobic zone and affecting the release of phosphorus. In the anaerobic-aerobic phosphorus removal system, if the sludge sedimentation performance is good, the reflux ratio is within the range of 50 to 70%, which can ensure rapid sludge discharge. The hydraulic retention time of sewage in the anaerobic zone is generally within the range of 1.5 to 2.0h. The residence time is too short, one is that the effective release of phosphorus cannot be guaranteed, and the other is that the facultative acidifying bacteria in the sludge cannot fully decompose the macromolecular organic matter in the sewage into lower fatty acids for the ingestion of phosphorus-accumulating bacteria, thereby affecting the phosphorus freed. The residence time of sewage in the aerobic zone is generally 4 to 6 hours, so that the full absorption of phosphorus can be ensured.

5, pH

Low pH is good for the release of phosphorus, high pH is good for the absorption of phosphorus, and the effect of phosphorus removal is a combination of phosphorus release and absorption. Therefore, in the biological phosphorus removal system, it is advisable to control the pH of the mixed solution within the range of 6.5 to 8.0.

Article source: Environmental Protection Online