Coagulation and flocculation are two methods used to increase particle size and enhance filtration efficiency. Regardless of the size of the system, coagulation and flocculation are typically the initial steps in water and wastewater treatment.
At their most basic, coagulation and flocculation involve the process of adding positively charged chemicals to the water. These chemicals neutralize negatively charged dirt and other dissolved particles in the water, which causes them to bind with the chemicals to form larger, more easily filtered particles, called floc. Although both processes have the same end goal and are often used together, there are fundamental differences between the two.
Coagulation in water treatment uses specialized chemicals to encourage fine particles to clump together. These chemicals, known as coagulants, create an electronic charge that causes the particles to cluster into larger groupings that make them easier to filter.
Types of Coagulants
Coagulants used in water filtration applications are typically either aluminum or iron-based.
Common aluminum coagulants include:
- Aluminum sulfate
- Aluminum chloride
- Sodium aluminate
Common iron-based coagulants include:
- Ferrous sulfate
- Ferric sulfate
- Ferric chloride
- Ferric chloride sulfate
In addition to the above, water filtration facilities may also use hydrated lime and magnesium carbonate.
The coagulation process uses coagulant chemicals to destabilize negatively charged particles in the water, such as dirt, clay, soil, and other organic particles. Since the negative charge is what keeps these dispersed particles from coalescing, neutralizing that charge allows those solids to stick together, creating submicroscopic clumps of particles known as microflocs.
To facilitate the collision of particles for optimal coagulation, the liquid must be mixed rapidly. This quickly disperses the coagulant into the water, while facilitating the formation of clumps by forcing more of the neutralized particles to collide. To ensure optimal coagulation, it is recommended that the water be agitated at high speed for 1-3 minutes after the coagulant has been added. Once the coagulation process is complete, the water is often treated using flocculation.
While coagulation helps to encourage particles to combine into larger, more easily filtered clumps, the resultant microflocs are still far too small for standard filtration systems to clear. Flocculation takes the coagulation process a step further by gently agitating the microfloc-containing water at varying speeds to encourage more particle adhesion.
The flocculation process takes treated water from the coagulation stage and mixes it slowly to increase the collision rate between suspended microfloc particles. As they collide, the microflocs bond further to create larger flocs, which are visible to the naked eye.
Through additional mixing, these flocs continue to grow and bond with other organic or inorganic polymers which may be produced by the coagulant or coagulant aids added during this step. With these polymers, the flocs form into macroflocs, which become increasingly heavy. The added weight allows the macroflocs to settle at the bottom, where they can be easily removed.
The amount of flocculation induced during the mixing process differs based on the mixing speed and the amount of time for which the material is mixed. There are a variety of mechanisms used to control flocculation speed and particle aggregation to produce the desired particle size and consistency. While rotating blades are the most common flocculation equipment for large-scale water treatment facilities, other methods include granular media beds, diffused air, baffled chambers, and spiral flow chambers.
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