What factors affect use of flocculants?
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What factors affect use of flocculants?

Views:7     Author:Ryde Environment     Publish Time: 2019-05-25      Origin:Ryde Environment

Flocculants are most commonly used in wastewater treatment and sludge treatment. 

In the process of sludge concentrating and dewatering from secondary clarifier, flocculants should be added to mix with sludge evenly and then enter other equipment such as multi disc screw press or belt filter press. This can improve the sludge dewatering effect and enhance the efficiency of sludge dewatering.

Solid-FlocculantFlocculantsMulti disc screw press

In the process of using flocculants, there are many factors that will affect the effect of using flocculants.

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, dosage and coagulation sedimentation effect of flocculants. H + and OH - in water take part in the hydrolysis of flocculants. Therefore, pH value strongly affects the hydrolysis rate, the morphology and properties of hydrolysates. 

For example, when the pH value is less than 4, Al3+ can not be hydrolyzed into Al (OH)3 in large quantities, mainly in the form of Al3+ ions, and the coagulation effect is very poor. When the pH value is between 6.5 and 7.5, the neutral colloid of Al (OH)3 with high degree of polymerization can be prepared by hydrolysis and polymerization of Al3+ with good coagulation effect. After pH > 8, Al3+ is hydrolyzed to AlO2-, and the coagulation effect becomes very poor. 

The alkalinity of water has a buffer effect on the pH value. When the alkalinity is not enough, lime and other chemicals should be added to supplement it. When the pH value of water is on the high side, it is necessary to add acid to adjust the pH value to neutrality. In contrast, polymer flocculants are less affected by pH value.

2. Water temperature

Water temperature affects the hydrolysis rate of flocculant and the formation rate and structure of alum. Coagulation hydrolysis is mostly endothermic reaction. When the water temperature is low, the hydrolysis speed is slow and incomplete. At low temperature, the viscosity of water is high, Brownian motion is weakened, collision times between colloidal particles of flocculant and impurity particles in water are reduced, and the shear force of water is increased, which hinders the cohesion of flocs. 

Therefore, although the dosage of flocculant is increased, the formation of flocs is very slow, and the structure is loose, the particles are small, and it is difficult to remove them. Low temperature has little effect on polymer flocculant. However, it should be noted that when using organic polymer flocculants, the water temperature should not be too high. High temperature can easily make the organic polymer flocculants aging and even decompose into insoluble substances, thus reducing the coagulation effect.

3. Impurities in water

The heterogeneous size of impurity particles in water is beneficial to coagulation. Fine and uniform particles will lead to poor coagulation effect. When the concentration of impurity particles is too low, the coagulation effect can be improved by refluxing sediment or adding coagulant aids. When the impurity particles in water contain a large amount of organic matter, the coagulation effect will become worse. It is necessary to increase the dosage or add oxidants and other coagulation aids. Calcium and magnesium ions, sulfides and phosphides in water are generally beneficial to coagulation, while some anions and surfactants have adverse effects on coagulation.

4. Types of flocculants

The selection of flocculants depends mainly on the properties and concentration of colloids and suspended solids in water. If the pollutants in water are mainly colloidal, inorganic flocculants should be used to destabilize and flocculate them. If the flocs are small, polymer flocculants or activated silica gels should be added. In many cases, the combination of inorganic flocculants and polymer flocculants can significantly improve the coagulation effect and expand the scope of application. For polymers, the more charged the chain molecule, the higher the charge density, the more fully extended the chain, the wider the range of adsorption bridging, and the better the coagulation effect.

5. Flocculants dosage

There are good flocculants and dosage in coagulation treatment of any wastewater. Usually, it must be determined by experiments. Excessive dosage may lead to the re-stabilization of colloids. The dosage range of common iron salt and aluminium salt is 10-100 mg/L, the dosage of polymer salt is 1/2-1/3 of common salt, and the dosage range of organic polymer flocculant is 1-5 mg/L.

Usually, Flocculants are in a solid state to facilitate transportation and sales. In use, it is necessary to dissolve the solid sludge conditioner into a certain concentration of viscous water solution.

The function of the integrated of dosing device is to make the powder or liquid sludge conditioner dissolve or dilute into sludge conditioner solution.

6. The order of flocculants dosage

When using a variety of flocculants, it is necessary to determine the optimum order of dosage through experiments. Generally speaking, when inorganic flocculants are combined with organic flocculants, inorganic flocculants should be added first and then organic flocculants. When the size of impurity particles is more than 50 micron, organic flocculant is often added to bridge the gap, then inorganic flocculant is added to compress the double layer to destabilize the colloid.

7. Hydraulic Conditions

In the mixing stage, flocculant and water are required to mix rapidly and evenly. In the reaction stage, it is necessary to create sufficient collision opportunities and good adsorption conditions to allow flocs to grow sufficiently, and to prevent small flocs from being broken. Therefore, the stirring intensity should be gradually reduced and the reaction time should be long enough.


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