What best describes the operation of trickling filters?

• A. Wastewater trickled over media, bacteria attached removes BOD, creates slime layer; slime layer sloughs off when too heavy.
• B. They are fixed-film units with media that bacteria colonize and a slime layer forms.
• C. They rely on activated sludge in aeration tanks.
• D. They use anaerobic digestion to treat wastewater.

Answer: (A)

Trickling filters operate as fixed-film biological systems: wastewater is distributed over a bed of media, and microorganisms attached to that media form a biofilm. As the water trickles over the biofilm, the attached bacteria oxidize organic matter, removing BOD. A slime layer forms on the biofilm, which helps with attachment and activity. When the biofilm becomes too thick or heavy, portions slough off, releasing biomass and helping regulate the system. This combination of wastewater moving over a biofilm-coated media and the formation and shedding of the slime layer is the hallmark of trickling filter operation.

Other described processes involve suspended growth in aeration tanks (activated sludge) or anaerobic digestion, which operate quite differently from trickling filters.