Wastewater treatment using biofilms grown on a support media was the first continuous flow bioreactor employed by sanitary engineers nearly a century ago. This wastewater treatment method became known as a trickling filter because wastewater dosed intermittently over support media such as slag or rocks “trickled” down through the media where organics were removed or “filtered” by microorganisms composing the biofilm. Process performance was measured by performing a biochemical oxygen demand (BOD) test on reactor influents and total process effluents. Rock filter heights were typically 1 to 2 m (3 to 6 ft).