Sedimentation
The final step of the clarification process is sedimentation. In this step, agglomerated floc settles out to form sludge and the sludge is transported to the sludge concentration chamber by the scraper. The sludge thickening pickets concentrate the sludge. The sludge is discharged to waste in a process called clarifier blow down. Above the sludge, clarified water is collected in the outlet launder and flows forward for use or further treatment.
Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration, or electromagnetism.
Sedimentation is recommended as simple pre-treatment of water prior to application of other purification treatments such as filtration and disinfection methods. It removes undesirable small particulate suspended matters (sand, silt and clay) and some biological contaminants from water under the influence of gravity. The longer the water is sedimented, the more the suspended solids and pathogens will settle to the bottom of the container. Adding special chemicals or some natural coagulants can accelerate sedimentation. Three common chemicals used are aluminium sulphate, polyaluminium chloride (also known as PAC or liquid alum) and ferric sulphate. 'PUR' is a powder product containing both coagulants and disinfectant.