Which treatment stage converts colloidal organics to settleable biological sludge?

The treatment stage that converts colloidal organics into settleable biological sludge is secondary treatment. This stage employs biological processes, primarily through the use of microorganisms, to break down and convert organic matter in wastewater into biomass. During this process, colloidal organics, which are smaller particles that do not settle under gravity, are consumed by bacteria and other microorganisms.

As the organic materials are transformed, these microorganisms grow and multiply, forming a biological mass known as sludge. This biological sludge then becomes settleable, meaning it can be removed from the water through sedimentation processes in secondary clarifiers. Thus, the primary goal of secondary treatment is to enhance the removal of suspended solids and organic matter to improve the effluent quality before it is either discharged or undergoes further treatment in the tertiary stage.

In contrast, primary treatment primarily focuses on the physical removal of larger solids through screening and sedimentation, and preliminary treatment involves the initial physical processes like screening and grit removal that prepare wastewater for further processing. Tertiary treatment is the advanced treatment aimed at further improving water quality, often involving disinfection or nutrient removal.