Trickling Filter & Small Activated Sludge Plants: Alabama Grade II
ADEM classifies Grade II wastewater treatment plants by process type AND size: trickling filter/biological contactor plants of any flow up to 0.999 MGD, or activated sludge plants up to 0.999 MGD. A Grade II operator therefore needs to understand two distinct biological treatment approaches.
Fixed-film treatment: trickling filters and RBCs
A trickling filter passes settled wastewater over a fixed medium (rock or, more commonly today, plastic media) on which a biological slime layer (zoogleal film) grows and consumes organic matter as wastewater trickles past. A rotating biological contactor (RBC) achieves the same fixed-film treatment by rotating discs partially submerged in wastewater, alternately exposing the attached biofilm to wastewater and air. Both are simpler to operate than activated sludge (no return-