What is a clarifier in a Wastewater Treatment Plant?

Clarifiers are tanks built with a mechanism for continuous removal of solids deposited by the sedimentation while filtering the water. Its general use is to remove the solid particles from the liquid for clarification and thickening. The concentrated impurities discharged from the bottom of the tank are known as sludge, and the particles that float on the surface of the liquid are known as scum.

Sedimentation tanks play an essential role in treating wastewater, and the primary treatment includes the removal of floating and settleable solids through sedimentation.

Clarification is an essential step in the wastewater treatment process to remove the solids through gravity settling for providing a clarified and clean liquid. These are also used to remove the materials like oil, grease, solids etc. A clarifier is generally divided into two types:

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1. Primary Clarifier: These clarifiers are used to separate the settleable solids from the incoming raw wastewater. They are located the downstream of plant’s screening process. This whole process is known as primary clarification, also known as sedimentation.

2. Secondary Clarifier: These clarifiers are located downstream of biological treatment. Their primary purpose is to separate the treated wastewater from the natural mass. Its main goal is to return the activated sludge. The whole process is known as secondary clarification.

Characteristics of a Clarifier

1. Clarifiers can be installed by modifying the existing facilities, such as a primary sedimentation tank or rainwater tank.

2. The natural flow down process is used in the treatment process, and hence stable operation is possible while depending on the flow rate of fluctuation.

3. Only the maintenance and management cost is needed for electricity due to the availability of the non-flocculation process.

4. It does not require any compulsorily auxiliary tank because washing time for filter media is around 1 minute.

5. The operation management is more accessible. It is because neither a pre-treatment screen nor a bottom screen is needed to prevent the flow out of filter media.


Primary clarifiers are designed to receive raw/waste from the incoming stream, after it has been pre-screened to remove large objects and grit.

The clarifier reduces the velocity of the water, ensuring that the incoming water is distributed uniformly in a radial manner in all the directions. This also provides maximum setting time causing efficient separation of settled particles from water. Collected particles gradually travel down through the slope forming sludge. The sludge is scraped of by a scraper bridge provided. The clear water overflows via a launder provided at the top portion.

Application Areas

1. Municipal Water Treatment

2. Primary Treatment in ETP

3. Lime Soda Softening.


Throughout history, sedimentation has served as a tried and true method for wastewater treatment. Gravity forces contaminants to settle downward in the surrounding liquid whereas clarified water can be removed separately. The same principle is used by wastewater treatment clarifiers: Large tanks provide enough retention time to slow water down and to separate out suspended particles.

Wastewater from aeration basins is usually entering the clarifier from a centrally located pipe. The entering wastewater is also known as the MLSS (mixed liquor suspended solids) which refers to the concentration of suspended solids from the activated sludge process. To slow down the incoming water its directed through a energy dissipating inlet and a feedwell.

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Inside the clarifier solid contaminants will settle down to the bottom of the tank where it is collected by a scraper mechanism and removed by another centrally located pipe. The scraper mechanism arms rotate slowly along the clarifiers perimeter and are attached to a centrally located shaft which is powered by a drive system on the clarifier bridge.

The removed sludge will either by recycled as RAS (return activated sludge) or wasted as WAS (waste activated sludge). The clarified water instead is released over overflow weirs on the top of the clarifier. In properly functioning clarifiers the overflow water looks already very clear. Fats, oil, grease and other floating substances are removed by a rotating skimmer on the water surface.

In some tanks, called tube settler or lamella clarifier tanks, there are additional inclined plates or channels inside the tank to help force solids to settle out of the mixture.

In many cases, flocculants or coagulants are applied to the wastewater before it reaches the clarification tank to make the sedimentation process more effective. The chemicals are usually added in mixing tanks with agitators, which provide a gentle stirring for even distribution of chemicals in the water.