In wastewater treatment, lime is not only used as a neutralizer, but also as a phosphorus removal agent. In addition, it also has the effect of coagulation and reducing the moisture content of sludge.
The principle of phosphorus removal by lime method
Quicklime can be used as a neutralizer in water treatment applications. It has high alkalinity, can quickly adjust the PH value of water, and has a low cost of use. When slaked lime is used for sewage treatment, it can effectively adjust the PH value of water quality and has a certain effect on removing total phosphorus in water quality. Because quicklime reacts with water, it will release high heat, so slaked lime is generally used as a phosphorus removal agent for lime phosphorus removal!
The principle of phosphorus removal by the lime method is to add lime to the water, and due to the formation of hydroxide ions, the pH value of the sewage rises. At the same time, the phosphorus in the sewage reacts with the calcium in the lime. Form [Ca5(OH)(PO4)3] (hydroxyapatite), and its reaction formula is as follows:
5Ca2++OH-+3PO43- → Ca5(OH)(PO4)3↓
Since lime enters water, it first reacts with the alkalinity of water to form calcium carbonate precipitation, and then excess calcium ions can react with phosphate to form hydroxyapatite precipitation, so the amount of lime required mainly depends on the alkalinity of the wastewater to be treated. degree, not the phosphate content of the wastewater. Due to the need for a higher pH, it has a certain impact on the normal growth of water organisms. At the same time, calcium salt phosphorus removal agents can also cause scaling on the walls of pools or channels, pipe walls and blockage of aeration pipes. Therefore, calcium salt phosphorus removal agents It is rarely used in river management and urban environmental sewage treatment plants.
Factors affecting lime phosphorus removal
1. pH value
The pH value is a major factor affecting the phosphorus removal effect. The pH value has a great influence on the phosphorus removal of lime. As the pH value increases, the solubility of hydroxyapatite decreases sharply, that is, the phosphorus removal rate increases rapidly, and the pH value is greater than After 9.5, all phosphates in the water are converted to insoluble precipitates. Generally, the pH value is controlled between 9.5 and 10, and the phosphorus removal effect is the best. The dosage of lime for different wastewater should be determined through experiments. If the phosphorus content in the treated water is to be less than 1 mg/L, the pH value of the secondary treated water should be above 9.5, and that of the original wastewater should be above 11.
2. Forms of Phosphorus
The two forms of phosphorus, orthophosphate and polyphosphate, were compared. The removal rate of polyphosphate is lower than that of orthophosphate. Among polyphosphates, the order of ease of removal is: pyrophosphate, tripolyphosphate, metaphosphate, the latter being more difficult. If polyphosphate and orthophosphate coexist, the removal effect of polyphosphate will be the same as that of orthophosphate.
3. The concentration of calcium in the original wastewater
The calcium concentration in the original wastewater has an impact on the removal of phosphorus. When the pH value is 10.5 and the calcium content in the influent water is above 40 mg/L, the phosphorus content in the treated water will be below 0.25 mg/L. In the lime coagulation precipitation dephosphorization treatment process, the effect of using slaked lime (Ca(OH)2) as a coagulant is better than that of quicklime (CaO), and the effect is better if calcium salt is added directly.
4. The influence of magnesium hardness
Magnesium hardness of wastewater is also a factor affecting phosphorus removal by lime, because under high pH conditions, Mg(OH)2 can be precipitated, which is colloidal sediment, which not only consumes lime, but also is not conducive to sludge dewatering.
5. The influence of temperature
As the temperature in the water body increases, the removal rate of phosphorus removal with lime also increases.
Precautions for lime phosphorus removal
Lime as a coagulant cannot be used for simultaneous phosphorus removal, but can only be used for pre or post phosphorus removal.
After lime is used for pre-phosphorus removal, the pH value of sewage is high, and the pH value needs to be adjusted before entering the biological treatment system; when lime is used for post-phosphorus removal, the pH value of the treated effluent must be adjusted to meet the discharge requirements; lime can also be used for Phosphorus removal from sludge anaerobic phosphorus release tank or phosphorus-rich supernatant produced during sludge treatment.
For phosphorus removal with lime, the amount of sludge is much larger than that of aluminum salts or iron salts, so it is rarely used. A small amount of anionic, cationic or anionic-cationic polyelectrolytes, such as polyacrylamide (PAM), is added as a coagulant to facilitate coagulation and precipitation of dispersed free metal phosphate flocs.
Steps of adding lime for phosphorus removal
1. Determination of dosage
2. Direct addition method
That is, the method of adding lime directly into the water body without dilution, but the use of this method has certain limitations. There is a stirring device in the added water body, otherwise the phosphorus removal effect is very limited, because the lime does not fully contact the water body. , the chemical reactions associated with phosphorus will not be sufficient. However, directly adding lime will easily lead to the scattering of powder, which is not friendly to the environment!
3. Addition method after dilution
The lime is directly added to the target water body after being diluted with a certain dilution ratio, and this method is used in a wider range.
Dosing matters needing attention
1. Dosing amount: It needs to be determined by scientific calculation and related experiments.
2. Stirring during dilution: stir thoroughly.
3. Selection of dosing point: It is best to add diluent in places with turbulent water flow and narrow river channels.