Trickling filters are an example of which treatment type?

• A. Primary
• B. Secondary
• C. Tertiary
• D. Quaternary

Answer: (B)

The concept being tested is how wastewater treatment stages are classified by the level of treatment, with trickling filters falling into the biological, secondary stage. Trickling filters use a fixed bed of media (like rocks or plastic) covered with a biofilm of microorganisms. When wastewater is distributed over the bed, the biofilm oxidizes organic matter as air fills the voids, reducing pollutants such as BOD and suspended solids. This biological oxidation happens after the primary stage, which mainly removes settleable solids by gravity, and before any polishing steps that come with tertiary treatment. Because trickling filters rely on microorganisms to break down organic material rather than just physically separating solids, they’re considered secondary treatment. They aren’t primary, since they don’t focus on solids removal alone; they aren’t tertiary, since tertiary would involve further polishing like filtration or disinfection; and they aren’t quaternary, which denotes advanced, beyond-tertiary processes.