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Total phosphorus, in a broad sense, refers to the general term for various forms of phosphorus compounds such as orthophosphate, polyphosphate, and organic phosphoric acid. In water quality indicators, total phosphorus refers to the result of the determination of the conversion of various forms of phosphorus into orthophosphate after the water sample is digested. It is measured in milligrams of phosphorus per liter of water sample and is a pollutant index often measured in sewage. . In order to avoid water eutrophication caused by phosphorus enrichment, many industries have relevant requirements for total phosphorus in water pollutants. Among them, the pulp and paper industry requires total phosphorus in water pollutants to be controlled below 1.0mg/L, the sugar industry requires it to be controlled below 0.5mg/L, and the saponin industry requires total phosphorus to not exceed 0.5mg/L.
Urban sewage plants have different limits on total phosphorus control according to the construction time and the water area where the wastewater is discharged.
In addition, surface water is divided into 5 types of water bodies according to the different functions of the water body, and the requirements for total phosphorus in each type of water body are also different.
The phosphorus removal of urban sewage treatment plants mainly relies on biological phosphorus removal. Today, we will analyze several reasons that cause the total phosphorus in the biological phosphorus removal effluent to exceed the standard:
1. Sludge load and sludge age
The anaerobic-aerobic biological phosphorus removal process is a high F/M low SRT system. When the F/M is high and the SRT is low, the excess sludge discharge is also more. Therefore, under the condition of certain phosphorus content in the sludge, the more phosphorus removal, the better the phosphorus removal effect.
To ensure the phosphorus removal effect, the BOD5/TP in the sewage entering the anaerobic zone should be controlled to be greater than 20. Because polyphosphoric acid bacteria belong to the Acinetobacter genus, their physiological activity is weak, and it can only ingest the easily decomposed part of organic matter. Therefore, the content of BOD5 should be guaranteed in the influent water to ensure the normal physiological metabolism of polyphosphate bacteria. However, the actual influent of many urban sewage treatment plants has low carbon sources and high concentrations of nitrogen and phosphorus, resulting in the BOD5/TP value being unable to meet the needs of biological phosphorus removal and affecting the effect of biological phosphorus removal.
3. Dissolved oxygen
Improper control of dissolved oxygen in the anaerobic zone and the aerobic zone will greatly affect the effect of biological phosphorus removal. In addition, the influent of some sewage treatment plants is river water, and the dissolved oxygen content in the sewage is high. If it directly enters the anaerobic zone, it is not conducive to the control of the anaerobic state and affects the phosphorus release effect of phosphorus accumulating bacteria.
4. Reflux ratio
The reflux ratio of the anaerobic-aerobic phosphorus removal system should not be too low, and a sufficient reflux ratio should be maintained to discharge the sludge in the secondary sedimentation tank as soon as possible to prevent phosphorus accumulation bacteria from encountering an anaerobic environment in the secondary 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 sludge in the anaerobic zone and affecting the release of phosphorus. In the anaerobic-aerobic phosphorus removal system, if the sludge settling performance is good, the reflux ratio is in the range of 50-70%, which can ensure rapid sludge discharge.
5. Hydraulic retention time
The hydraulic retention time of sewage in the anaerobic zone is generally in the range of 1.5-2.0h. 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 intake of phosphorus-accumulating bacteria, which also affects the release of phosphorus.
Low pH is favorable for phosphorus release, high pH is favorable for phosphorus absorption, and the phosphorus removal effect is a combination of phosphorus release and absorption. Therefore, in the biological phosphorus removal system, the pH of the mixed solution should be controlled within the range of 6.5 to 8.0.
Due to the continuous improvement of the requirements for the total phosphorus index of effluent, in addition to biological phosphorus removal, chemical phosphorus removal has also been used more and more. However, while chemical phosphorus removal improves the phosphorus removal effect, the amount of excess sludge will be greatly increased due to the addition of chemicals, thereby increasing the amount of sludge treatment and mud cake treatment. In practice, the dosing point and dosage of chemical agents should be determined according to the experiment and adjusted in time to ensure that the total phosphorus in the effluent reaches the standard stably and the consumption of the chemical agent is reduced as much as possible.
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