Colorado Wastewater (WW) Operator D Practice Exam 2026 - Free Operator D Practice Questions and Study Guide

The slime on trickling filter media is a biofilm. A biofilm is a structured community of microorganisms—bacteria, fungi, and protozoa—attached to a surface and embedded in a sticky extracellular matrix. On trickling filters, wastewater continuously wets the media (rock, plastic, etc.), and these microorganisms attach to the surface, grow, and multiply to form layered, attached colonies. This attached film is what does the biological treatment as water trickles over or through the media, processing organic matter as it passes.

Labeling it a biofilm matters because it emphasizes the organized, surface-attached nature of the microbial community and the protective matrix they produce, rather than just a loose coating. In contrast, a pellicle is a film that forms on the surface of a liquid at the air–water interface and isn’t the same as the attached film on a solid media surface. A generic “slime layer” or “layer” doesn’t convey the same idea of a structured, attached microbial community.