What is the primary purpose of a secondary clarifier?

The primary purpose of a secondary clarifier is to separate treated water from settled biomass, which is often referred to as sludge. In a typical wastewater treatment process, after biological treatment occurs, the mixed liquor (which contains both treated wastewater and microorganisms) flows into the secondary clarifier. Here, the microorganisms, or biomass, settle to the bottom due to gravity, forming a thickened sludge layer.

This separation is crucial because it allows for the efficient removal of excess biomass while enabling the treated water to overflow from the clarifier and proceed to further treatment or discharge. The settled sludge can then be returned to the biological treatment process to help maintain the needed concentration of microorganisms or it can be removed as waste.

While aeration, disinfection, and the removal of inorganic solids are important processes in wastewater treatment, they do not accurately represent the specific role of a secondary clarifier, which is focused on the separation of biomass from the liquid effluent.