Operator terms · Wastewater
What is secondary treatment in wastewater?
Secondary treatment is the biological stage of wastewater treatment, where microorganisms break down the dissolved and fine organic matter that primary treatment leaves behind. Processes like activated sludge or trickling filters do this work, lowering BOD before the water is disinfected.
Last reviewed 2026-08-03
How it works
Microorganisms are given oxygen and time to consume the organic matter still in the water after primary treatment. In the activated sludge process this happens in an aeration tank; in a trickling filter the microbes grow on a media bed the water passes over.
Why it matters
Secondary treatment is what removes most of the remaining BOD and suspended solids, the two common permit limits. Without it, high-BOD water would rob a receiving stream of the oxygen aquatic life needs.
Common questions
- Is secondary treatment biological?
- Yes; it uses microorganisms to break down organic matter.
- What are common secondary processes?
- Activated sludge and trickling filters are the most common.
- What does it lower?
- Mainly BOD and suspended solids before disinfection and discharge.