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Factors affecting the effect of flocculants

Source:Jiangsu Envset     Time:14 July 2021

In actual operation, the effect of flocculant will vary according to actual conditions, which may be related to the type of flocculant or other factors. Today, let's take a look at the influencing factors of the use of flocculants.

(1) The pH value of water and the effect of flocculant
The pH value of the water has a great influence on the effect of flocculant, and the pH value is related to the type of flocculant selected, the dosage and the coagulation sedimentation effect. The H+ and OH- in the water participate in the hydrolysis reaction of the flocculant. Therefore, the pH value strongly affects the hydrolysis rate of the flocculant, the existence form and performance of the hydrolysate.

Take the aluminum salt that achieves coagulation by generating Al(OH)3 charged colloid as an example. When the pH value is less than 4, Al3+ cannot be hydrolyzed to Al(OH)3 in a large amount, mainly in the form of Al3+ ions, and the coagulation effect is extremely high. difference. When the pH value is between 6.5 and 7.5, Al3+ is hydrolyzed and polymerized into Al(OH)3 neutral colloid with a high degree of polymerization, and the coagulation effect is better. After pH value ﹥8, Al3+ is hydrolyzed into AlO2-, and the coagulation effect becomes very poor.

The alkalinity of water has a buffering effect on the pH value. When the alkalinity is not enough, lime and other agents should be added to supplement it. When the pH of the water is high, you need to add acid to adjust the pH to neutral. In contrast, polymer flocculants are less affected by pH.

(2) Water temperature and the effect of flocculant

Water temperature affects the rate of hydrolysis of the flocculant and the rate and structure of alum formation. The hydrolysis of coagulation is mostly an endothermic reaction. When the water temperature is low, the hydrolysis rate is slow and incomplete.

At low temperatures, the viscosity of water is large, the Brownian motion is weakened, the number of collisions between the flocculant colloidal particles and the impurity particles in the water is reduced, and the shear force of the water increases, which hinders the mutual adhesion of the coagulated flocs; therefore, despite the increase The dosage of flocculant, the formation of flocs is still very slow, and the structure is loose, the particles are small, and it is difficult to remove.

Low temperature has little effect on polymer flocculants. However, it should be noted that when using organic polymer flocculants, the water temperature should not be too high. High temperature will easily cause the organic polymer flocculants to age or even decompose to form insoluble substances, thereby reducing the coagulation effect.

the effect of flocculant
(3) Impurities in water and the effect of flocculant

The uneven particle size of impurities in the water is beneficial to coagulation, and small and uniform particles will lead to poor coagulation effects. Too low concentration of impurity particles is often detrimental to coagulation. At this time, refluxing sediment or adding coagulant can improve 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 an oxidant and other agents that have a coagulation aid. Calcium and magnesium ions, sulfides, and phosphides in water are generally beneficial to coagulation, while certain anions and surface active substances have an adverse effect on coagulation.

(4) Types of flocculants and the effect of flocculants

The choice of flocculant mainly depends on the nature and concentration of colloids and suspended solids in water. If the pollutants in the water are mainly in a colloidal state, inorganic flocculants should be the first choice to make them destabilize and coagulate. If the flocs are small, polymer flocculants should be added or a coagulant aid such as activated silica gel should be added.

In many cases, the combined use of inorganic flocculants and polymer flocculants can significantly improve the coagulation effect and expand the scope of application. For polymers, the greater the amount of charge on the chain molecules, the higher the charge density, the more fully the chain can be stretched, the larger the range of adsorption and bridging, and the better the coagulation effect.

(5) Flocculant dosage and the effect of flocculant

When coagulation is used to treat any wastewater, there is an optimal flocculant and an optimal dosage, which are usually determined through experiments. Excessive dosage may cause the colloid to stabilize again. Generally, the dosage range of ordinary iron salt and aluminum salt is 10-100mg/L, the dosage of polymer salt is 1/2~1/3 of the dosage of ordinary salt, and the dosage range of organic polymer flocculant is 1~5mg/L. 

(6) Flocculant dosing sequence and the effect of flocculant

In order to stabilize the effect of flocculants, when multiple flocculants are used, it is necessary to determine the best dosing sequence through experiments. Generally speaking, when the inorganic flocculant is used together with the organic flocculant, the inorganic flocculant should be added first, and then the organic flocculant should be added.

When the particle size of the treated impurities is above 50μm, the organic flocculant is often added to absorb the bridging, and then the inorganic flocculant is added to compress the electric double layer to destabilize the colloid.

The above are some of the influencing factors of the effect of flocculants. In the use of flocculants, in addition to the types of flocculants, there are many other practical factors.

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