What it means
Primary treatment relies mainly on physical separation in screens, grit systems, and primary clarifiers. Secondary treatment uses biological processes such as activated sludge or trickling filters to remove organics that do not settle out in primary treatment.
What operators watch
- Primary performance affects downstream biological loading.
- Secondary treatment depends on living organisms and process control.
- Clarifiers can appear in both primary and secondary treatment but serve different process roles.
Common exam traps
- Calling all clarifiers primary treatment.
- Forgetting that secondary treatment is primarily biological.
- Ignoring how primary solids removal protects downstream processes.
Field example
Poor primary sludge removal can send extra solids and BOD to aeration, increasing oxygen demand and wasting needs.
