Trickling filters are a form of secondary treatment.

• A. False
• B. True
• C. Not applicable
• D. Both

Answer: (B)

Trickling filters are a form of secondary treatment because they provide biological degradation of organic matter after the primary treatment stage. After solids are settled in a primary clarifier, wastewater is distributed over a bed of media where microorganisms grow as a biofilm and oxidize organic pollutants. This biological polishing reduces the BOD/COD and improves effluent quality, which is the goal of secondary treatment. They’re not primary treatment, which relies mainly on physical settling to remove solids, and not tertiary, which involves polishing steps or advanced nutrient removal and disinfection.