What happens in a primary clarifier?

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Aerial view of wastewater treatment basins with circular clarifiers; overlaid toilet graphic and arrow with text reading: “Where does it go?” — wastewater treatment All the sewers in your neighborhood end up here.

In a primary clarifier, also known as a sedimentation tank, the primary goal is to remove suspended solids from sewage through the process of gravity settling.

As sewage flows slowly through these tanks—typically providing about two hours of detention time—the suspended solids gradually sink to the bottom. Mechanical scrapers then move this settled material, referred to as raw or primary sludge, into a hopper for collection and removal. Simultaneously, mechanical surface-skimming devices are used to remove grease and other materials that float to the top of the tank.1

This process is a critical component of primary treatment, which focuses on the physical removal of materials that can be separated by gravity before the water undergoes further biological or chemical processing.1