• Final procedure in preparing clear water for filtering
  • Based on gravity separation
  • Achieved in clarifiers, settling tanks or sedimentation tanks
  • Low velocity (0.1 to 1.0 cm/s; depending on the application)
  • Retention time is in the range of 2 to 4 hours (depending on the application)
  • Residues are collected at the bottom and pumped or scraped out for disposal

Clarification is done using clarifiers that are available in either circular or parallel plate. Clarifiers are often practised in procedures of water and wastewater treatment plants to remove solids, chemicals, microbes and other impurities. Industries that make use of clarifiers are paper industry, the uranium industry, and in alumina, coal, copper, and iron ore production.

Circular clarifiers use slowly rotating rake arms to separate solid particulate. A liquid feed with suspended solids is fed into a tank and left undisturbed to settle by gravity. As the particles settle, angled rake arms move the concentrated slurry toward the centre of the tank, where it is removed. Treated liquid overflows the top of the tank and is collected in a trough surrounding the tank. There are 3 main types of clarifiers: bridge support, column support, and traction.

Rakes and drive mechanism are put off from a bridge that widths the diameter of a tank in a bridge support thickener. These are only economical for tank diameters of < 100 feet and mostly utilized in primary wastewater treatment. After larger contaminants and grit have been filtered from waste water, flow is fed into the primary clarifier. In this stage, floating substances and solids which easily settle out will be separated, resulting in a homogeneous discharge that can be further treated biologically in the secondary clarifier, and a sludge discharge that can be treated or processed.

For a column support clarifier, mostly for secondary wastewater treatment, difference can be seen on the drive and raking mechanisms which are attached to a central column. The feed usually enters through an overhead pipe supported by an access bridge that leads to the centre column. Through this clarifier, presence of bacterial microorganisms consume up to 90% of the organic matter in the wastewater. As the minimum removal standards for biochemical oxygen demand (BOD), total suspended solids (TSS), and pH are met, the treated effluent can be discharged to the environment or sent to further treatment depending on the usage requirements.

Traction units also have a centre column, but the drive power is supplied by a trolley that rides along a track at the tank wall. Unlike centre drives, traction drive units are capable of competently provide a very high amount of torque for high demand applications.

Parallel plate clarifiers aim to create laminar flow, allowing gravity to separate heavier elements from the rest of the fluid. This type of clarifiers are often used to separate solids and low density liquids, such as oils which is suitable for many applications, including chemical processing, coolant systems, food processing, oil refinery, paper making, and industrial waste management. Structure of the parallel plate clarifiers can be horizontal or vertical.

Horizontal parallel plate clarifiers can either have a cone bottom configuration to remove contaminants that cannot be transported with a conveyor or configured with a drag conveyor that removes settled solids nonstop. In horizontal clarifiers, raw material enters from side into a flocculation tank where the fluid is mixed and suspended particles are brought together to create larger and heavier particles that are easier to filter out. In the settling zone, fluid flows though plates that are typically spaced 1 to 4 inches apart. The effluent passes though weirs into collection troughs. Settled sludge is removed though piping by means of pumps or hydraulic head pressure, sometimes mechanical drag conveyors. The configuration used depends on the application, usually used for treating plating wastewater.

Similarly, vertical parallel plate clarifiers operate using the same principle, but the liquid flows vertically. The influent flows to the lower area of the laminar plate sections. Waste fluid flows up the plates under laminar conditions, and then particulates settle onto the plates and slide down to the removal area. Weirs at the top of the plate section maintain a balanced flow though the plates. Effluent flows out the top of the clarifier.