urban sewage treatment plant phosphorus removal mainly relies on biological phosphorus removal, that is, add anaerobic segment before the aerobic section, so that polyphosphate bacteria alternately is in an anaerobic and aerobic state, realize the release and absorption of phosphate , and achieve the purpose of phosphorus removal by discharging remaining sludge.
In the condition that it is difficult to meet the standards for biological phosphorus removal, chemical agents can also be considered to assist in phosphorus removal. Chemical phosphorus removal mainly uses coagulation, precipitation and filtration to make phosphorus an insoluble solid precipitate and separate it from sewage. What are the reasons for the total phosphorus exceeding the standard in
? Let's check them one by one.
involves reasons
causes biological phosphorus removal water Total phosphorus exceeds the standard
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 residual sludge emissions are also greater. Therefore, under the condition that the sludge contains a certain amount of phosphorus, the more phosphorus removal, the better the phosphorus removal effect.
For biological systems with phosphorus removal as the main purpose, F/M is usually
0.4～0.7kgBOD5/kgMLSS×d, and SRT is 3.5～7d. However, SRT cannot be too low, and it must be based on ensuring the effective removal of BOD5.
2, BOD5/TP
To ensure the phosphorus removal effect, BOD5/TP should be controlled to be greater than 20 in the sewage entering the anaerobic zone. Because the genus Acinetobacteria genus has weak physiological activities and can only absorb the easily decomposed parts of organic matter.
Therefore, the content of BOD5 should be ensured in the inlet water to ensure the normal physiological metabolism of polyphosphate bacteria. However, the actual water inlet of sewage treatment plants in many urban sewage treatment plants has low carbon source and high concentrations of nitrogen and phosphorus, which leads to the fact that the BOD5/TP value cannot meet the needs of biological phosphorus removal, affecting the effect of biological phosphorus removal.
3, dissolved oxygen
anaerobic zone should remain strictly anaerobic, that is, dissolved oxygen is less than 0.2mg/L, and then the polyphosphine bacteria can effectively release phosphorus to ensure the subsequent treatment effect. The dissolved oxygen in the aerobic zone needs to be maintained above 2.0mg/L so that polyphosphate bacteria can 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. In addition, the water intake of some sewage treatment plants is river water, and the dissolved oxygen content of in the sewage is relatively high. If it enters the anaerobic zone directly, it is not conducive to the control of the anaerobic state and affects the phosphorus release effect of polyphosphate bacteria.
4. The reflux ratio of
anaerobic-aerobic phosphorus removal system should not be too low. A sufficient reflux ratio should be maintained to discharge the sludge in the second sedimentation tank as soon as possible to prevent the release of phosphorus from occurring in the second sedimentation tank. 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 the sludge in the anaerobic zone and affecting the release of phosphorus.
In the anaerobic-aerobic phosphorus removal system, if the sludge settlement performance is good, the reflux ratio is within the range of 50-70%, which ensures rapid sludge discharge.
5, hydraulic residence time
The hydraulic residence time of sewage in the anaerobic zone is generally within the range of 1.5 to 2.0h. The residence time is too short. First, the effective release of phosphorus cannot be guaranteed. Second, the facultative acidified bacteria in the sludge cannot fully decompose the macromolecular organic matter in the sewage into lower fatty acids for the intake of polyphosphorus bacteria, which also affects the release of phosphorus. The residence time of
sewage in the aerobic zone is generally between 4 and 6 hours, so as to ensure the full absorption of phosphorus.
6, pH
Low pH is conducive to the release of phosphorus, and high pH is conducive to the absorption of phosphorus, and the phosphorus removal effect is a combination of phosphorus release and absorption.Therefore, in the biological phosphorus removal system, the pH of the mixture should be controlled within the range of 6.5 to 8.0.
Due to the continuous improvement of the total phosphorus index for effluent water, chemical phosphorus removal has also been increasingly used in addition to biological phosphorus exclusion. However, while chemical phosphorus removal improves the phosphorus removal effect, it will also greatly increase the amount of residual sludge due to the addition of chemical agents, thereby increasing the sludge treatment volume and mud cake disposal.
In practice, the dosage point and dosage of chemical agents should be determined based on experiments, and timely adjustments should be made to ensure that the phosphorus content in the effluent reaches the standard and to reduce drug consumption as much as possible.
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