Compared
Primary vs. secondary treatment
Primary treatment physically settles out solids in a clarifier; secondary treatment uses microorganisms to break down the dissolved and fine organic matter the primary step leaves behind. Primary is mostly physics, secondary is mostly biology, and together they do most of a wastewater plant's cleaning.
Last reviewed 2026-08-04
| Primary | Secondary | |
|---|---|---|
| How it works | Physical settling | Biological breakdown |
| What it removes | Settleable solids | Dissolved and fine organic matter |
| Key equipment | Primary clarifier | Aeration tank and secondary clarifier |
| Order | First, after the headworks | After primary treatment |
| Living microbes involved | No | Yes |
The bottom line
They work in sequence: primary drops out the easy solids so secondary's microbes can handle the rest. A plant needs both to meet its permit.
Common questions
- Which comes first?
- Primary treatment, right after the headworks.
- What is the main difference?
- Primary is physical settling; secondary is biological breakdown.
- Do plants need both?
- Typically yes, to remove enough to meet the discharge permit.