Operator terms · Wastewater
What is primary treatment in wastewater?
Primary treatment is the first main stage of wastewater treatment, where solids are removed physically. Wastewater sits in a settling tank so heavy solids sink and grease floats, and both are skimmed or scraped off before the water moves on to biological treatment.
Last reviewed 2026-08-03
What it removes
After screening and grit removal, wastewater enters a primary clarifier where it slows down. Heavy solids settle to the bottom as sludge, and lighter grease and scum float to the top. Both are removed, taking out a large share of the solids.
What comes next
Primary treatment removes settleable solids but leaves dissolved and fine organic matter. Secondary treatment, usually a biological process, then removes that remaining organic load before the water is disinfected and discharged.
Common questions
- Is primary treatment physical or biological?
- Physical; it relies on settling and skimming, not microbes.
- What settles out?
- Heavy solids sink as primary sludge; grease and scum float and are skimmed.
- What follows primary treatment?
- Secondary, usually biological, treatment of the remaining organic matter.