Optimizing Water Clarification: Coagulation, Flocculation, and Sedimentation

Water clarification is very important in water treatment plant dpening the water source you have Clarification is the method used to remove suspended matter from surface water and industrial wastewater. In essence, it makes “turbid” water “clear,” as shown below in Figure

Clarification removes suspended matter from water. Surface waters require clarification because they have moderate to high levels of suspended matter. Well waters do not require clarification because they have low levels of suspended matter.

As shown in above figure, the suspended matter in water includes two kinds of particles:

• Settleable Particles (macroparticles, typically visible to the eye)
• Non-Settleable Particles (microparticles, normally visible through a microscope)

Settleable particles are particles in water that settle out over time. The water itself is clear, indicating an absence of suspended matter (turbidity). If non-settleable particles had been in the water, the water would not be clear. This “turbidity” would have indicated the presence of non-settleable particles. Turbidity is an indirect measurement of the amount of suspended matter (settleable particles and non-settleable particles) in water.

Clarification uses chemicals and sedimentation to remove suspended matter (settleable particles and non-settleable particles). Several steps are involved. First, coagulation destabilizes the particle surface charge that keeps the particles in solution. Once destabilized, the particles no longer repel one another and come together as floc. Second, floc agglomerate into larger particles. Polymers are used to enhance the flocculation process. Third, sedimentation causes agglomerated floc to settle out. The settled floc is collected and concentrated for discharge to waste, called clarifier blow down, or recycled to the coagulation step, called sludge recycle. Clarified water is collected and flows out of the clarifier.

Coagulation

The first step of the clarification process is coagulation. Particles in water have a naturally occurring negative charge. This causes them to repel each other and stay in suspension. When this charge is destabilized, the particles no longer repel one another, and can come together in closer proximity. A chemical salt, called a coagulant, is mixed with the inlet water to destabilize the charge. Common coagulants are aluminum sulfate (alum), ferric sulfate and ferric chloride.

The coagulants provide a positive charge, in the form of metallic cations, that destabilize the natural negative charge of the particles. The metallic cations combine with hydroxide in the water to form a metallic hydroxide that is an insoluble compound. The destabilized particles and metal hydroxide precipitates agglomerate into small, visible particles called floc. Color, organic matter and colloids, including colloidal silica, are removed by becoming bound up in the floc. The precise mechanism for removal- absorption, adsorption, co-precipitation, or a combination not fully understood.

The addition of too much coagulant can cause the suspended matter to be redispersed with the opposite charge. The amount of removal is dependent upon the coagulant dosage and the pH.

Alum (aluminum sulfate), ferric sulfate and ferric chloride coagulants are acidic salts and

decrease the pH of the influent water. Because of this, the pH of the water must be adjusted with caustic (sodium hydroxide) or another alkaline (high pH chemical).

Polymers are added to reduce the amount of coagulant required, broaden the working pH range and create denser, heavier floc that settles out more easily, as shown below in Figure 2.2.-2. Polymers are long-chain organic compounds of high molecular weight that bridge floc particles together or modify their surface charge.

In almost all cases, the water to be treated is disinfected with either gaseous chlorine or sodium hypochlorite. This oxidizes organic matter in the water that has taste and odor and certain metals, such as manganese and iron. When oxidized, these constituents are transformed into a form that can be removed during clarification.

Their removal is important because they can cause fouling of process components.

Flocculation

In the next step of clarification, the small floc (microfloc) is allowed to grow into larger floc, called macrofloc or agglomerated floc. This process, called flocculation, is shown below in Figure 2.2-3. Flocculation is accomplished by gently stirring the coagulated water to assure contact between microfloc particles and polymer. The polymer enhances agglomerated floc formation. As the agglomerated floc continues to grow, it becomes denser and heavier, allowing it to settle.

Mixing too rapidly can create what is called floc shear. Shear is the breaking apart of existing floc particles. The agglomerated floc, or macrofloc, is sheared back into microfloc.

Sedimentation

The final step is Sedimentation in which flocs will settled down as sludge and clear water will move towards the clarifer holding tank for further treatment. It have three zones:

• Mixing Zone
• Reaction Zone
• Settling Zone

Jar Testing

Jar tests are used to establish chemical dosage requirements and predict clarifier effluent turbidity levels. They simulate the chemistry and physical operation of a clarifier. The test equipment is shown below in Figure .

Varying dosages of coagulant, polymer and pH adjustment chemical (if required) are added to the beakers on the gang stirrer. The water in the beaker is stirred for the amount of time equivalent to the retention time of the fast mix chamber and slow mix chamber.

The floc is then allowed to settle for the amount of time equivalent to the retention time of the sedimentation chamber. Then the turbidity of the water is measured and recorded along with the pH, stirring speed and relative volume of floc produced.

Jar tests are repeated with varying dosages of chemicals and alternate chemicals to determine the appropriate combination of chemicals, pH and floc production necessary to obtain optimal performance. Jar tests should be performed prior to the final sizing of the clarifier and the chemical feeders. Once the clarifier is in operation, jar tests should be repeated if the quality of the inlet water changes significantly.