What is a trickling filter?

A trickling filter is best described as a fixed film process for wastewater treatment. In this system, wastewater is distributed over a bed of media such as stones or plastic, which provides a surface for microorganisms to grow. The biofilm that forms on the media consumes organic matter and nutrients in the wastewater, effectively treating it biologically.

As the wastewater trickles over the media, oxygen from the air is drawn into the biofilm, facilitating aerobic biological processes. This setup allows for effective treatment of the wastewater as the attached microorganisms metabolize the pollutants present in the influent.

Other options do not accurately define a trickling filter. For instance, a mechanical filtration device generally involves physical barriers to remove solids, lacking the biological treatment mechanism of a trickling filter. A biological nutrient removal system is designed specifically to remove nutrients, focusing more on processes like nitrification and denitrification, rather than the simple biological treatment characteristic of trickling filters. An aerobic digestion unit typically refers to a process used to break down organic material in the presence of oxygen, but it is distinct from the trickling filter's operation and structure.

Thus, identifying a trickling filter as a fixed film process highlights its primary function and methodology in wastewater treatment.