California Wastewater Treatment Operator Practice Exam

What is the purpose of a secondary clarifier?

1. To aerate the wastewater

2. To settle out flocculated particles and remove excess biomass from the effluent

3. To enhance nutrient removal

4. To disinfect wastewater before discharge

The correct answer is: To settle out flocculated particles and remove excess biomass from the effluent

The purpose of a secondary clarifier is to settle out flocculated particles and remove excess biomass from the effluent. During the secondary treatment phase of wastewater treatment, microorganisms break down organic matter, and this process generates a considerable amount of biomass, often referred to as sludge. After the biological treatment is complete, the secondary clarifier functions as a settling tank where the denser flocculated particles, including biomass and other suspended solids, can sediment to the bottom. This allows for the clear liquid, or supernatant, to be captured and directed to further stages in the treatment process, while the settled sludge can be either returned to the biological treatment process to maintain the microbial population or removed for further processing. Aeration is not the primary function of the secondary clarifier; that is typically achieved in the preceding activated sludge process. Although secondary clarifiers can play a role in nutrient removal indirectly by allowing biological floc to settle and thus concentrate nutrients, their main purpose is not to enhance nutrient removal specifically. Disinfection is usually performed in separate stages after clarification and is not a function of the secondary clarifier itself.