Operator terms · Wastewater
What is a trickling filter?
A trickling filter is a secondary treatment process where wastewater is sprayed over a bed of rock or plastic media. A film of microorganisms grows on the media and eats the organic matter as the water trickles down, lowering BOD before the water moves on.
Last reviewed 2026-08-03
How it works
Wastewater is distributed over a deep bed of rock or plastic media. Microorganisms living as a slime layer on the media consume the organic matter in the water as it passes, then treated water collects at the bottom.
How it compares to activated sludge
Both are biological secondary treatment. In a trickling filter the microbes grow attached to fixed media; in activated sludge they float in an aerated tank. A trickling filter uses less energy but can be harder to fine-tune.
Common questions
- Is a trickling filter biological?
- Yes; microbes on the media do the treatment work.
- What is the media?
- A bed of rock or plastic that the microbes grow on.
- What stage is it?
- Secondary treatment, after primary settling.