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Analysis of seven factors affecting the use effect of flocculants

Analysis of seven factors affecting the use effect of flocculants


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1. PH value of water

The pH value of water has a great influence on the use effect of inorganic flocculants. The pH value is related to the type of flocculants, dosage and coagulation sedimentation effect. H + and oh - in water participate in the hydrolysis reaction of flocculant. Therefore, pH value strongly affects the hydrolysis rate of flocculant, the existing form and performance of hydrolysate. For example, when the pH value is less than 4, Al3 + can not be hydrolyzed into Al (OH) 3, mainly in the form of Al3 + ions, and the coagulation effect is very poor. When pH value is between 6.5 and 7.5, Al3 + is hydrolyzed and polymerized to form neutral colloid al Al (OH) 3 with high degree of polymerization, and the coagulation effect is better. When pH value is higher than 8, Al3 + is hydrolyzed to AlO2 -, and the coagulation effect becomes very poor.

The alkalinity of water has buffering effect on pH value. When the alkalinity is not enough, lime and other chemicals should be added to supplement. When the pH value of water is high, ACID should be added to adjust the pH value to neutral. In contrast, the effect of pH value on polymer flocculant is small.

2. Water temperature

Water temperature affects the hydrolysis rate of flocculant and the formation rate and structure of alum. When the water temperature is low, the hydrolysis rate is slow and incomplete. At low temperature, the viscosity of water is high, Brownian motion is weakened, the number of collisions between flocculant colloidal particles and impurity particles in water is reduced, and the shear force of water is increased, which hinders the mutual adhesion of coagulation flocs.

Therefore, although the dosage of flocculant is increased, the formation of flocs is still very slow, and the structure is loose, the particles are small and difficult to remove. The effect of low temperature on polymer flocculant is small. However, it should be noted that when using organic polymer flocculant, the water temperature should not be too high, and the high temperature is easy to cause the organic polymer flocculant to age or even decompose into insoluble substances, thus reducing the coagulation effect.

3. Impurities in water

The uneven size of impurity particles in water is good for coagulation, and the small and uniform size will lead to poor coagulation effect. Too low concentration of impurity particles is often unfavorable to coagulation. At this time, reflux sediment or adding coagulant aid can improve the coagulation effect. When the impurity particles in the water contain a large amount of organic matter, the coagulation effect will become worse, and it is necessary to increase the dosage or add oxidants and other coagulant aids. Calcium and magnesium ions, sulfides and phosphates in water are generally beneficial to coagulation, while some anions and surfactants have adverse effects on coagulation.

4. Flocculant types

The selection of flocculant mainly depends on the properties and concentration of colloid and suspended solids in water. If the pollutants in water are mainly in colloidal state, inorganic flocculant should be preferred to make it destabilize and coagulate. If the floc is small, polymer flocculant or activated silica gel and other coagulant aids should be added. In many cases, the combination of inorganic flocculant and polymer flocculant can significantly improve the coagulation effect and expand the scope of application. For polymer, the larger the amount of charge on the chain molecule, the higher the charge density, the more fully the chain can be extended, the larger the scope of adsorption bridging, and the better the coagulation effect.

5. Dosage of flocculant

When using coagulation to treat any wastewater, there are the best flocculant and the best dosage, which are usually determined by experiments. Excessive dosage may cause colloid re stabilization. Generally, the dosage range of common iron salt and aluminum salt is 10-100mg / L, the dosage of polymerized salt is 1 / 2-1 / 3 of that of common salt, and the dosage range of organic polymer flocculant is 1-5mg / L.

6. Flocculant dosing sequence

When a variety of flocculants are used, it is necessary to determine the optimal dosing sequence through experiments. Generally speaking, when inorganic flocculant and organic flocculant are used together, inorganic flocculant should be added first, and then organic flocculant should be added. When the particle size of the impurity is more than 50 μ m, the organic flocculant is usually added first to adsorb the bridge, and then the inorganic flocculant is added to compress the electric double layer to destabilize the colloid.

7. Hydraulic condition

In the mixing stage, the flocculant and water are required to be mixed rapidly and evenly. In the reaction stage, it is necessary to create enough collision opportunities and good adsorption conditions, so that the flocs have enough growth opportunities, and prevent the generated small flocs from being broken. Therefore, the stirring strength should be gradually reduced, and the reaction time should be long enough.

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