3.1 The Trickling Filter

A trickling filter operates on a simple, elegant principle: wastewater is sprayed over a bed of media, such as stones or plastic modules, and trickles down through it by gravity.

A slimy layer of microorganisms, called a biofilm, grows on the surface of the media. As wastewater flows over this biofilm, the microorganisms absorb the organic pollutants. Oxygen, which is critical for aerobic treatment, is supplied by air that circulates naturally through the gaps in the media bed.

As the biofilm thickens, the inner layer closest to the media can become deprived of oxygen and turn anaerobic. This anaerobic environment cannot sustain the attached biomass, leading to a natural shedding process, or “sloughing,” where the older biofilm detaches to make way for new growth. These sloughed solids are then carried out with the treated water and removed in a downstream settling tank.

3.2 The Rotating Biological Contactor (RBC)

An RBC consists of large, lightweight plastic disks mounted on a horizontal shaft. These disks are positioned so that they are about half-submerged in a tank of flowing wastewater, and the entire assembly is rotated slowly. The interaction between the biomass, wastewater, and oxygen occurs in a continuous cycle:

1. Food Absorption A layer of biomass grows on the surface of the disks. While the disks are submerged in the wastewater, the microorganisms absorb pollutants (food).
2. Oxygen Absorption As the shaft rotates, the biomass-coated surface is lifted out of the water and exposed directly to the air. During this phase, the microorganisms absorb the oxygen they need to metabolize the pollutants they just consumed.

Now, we will transition from reactors where biomass is stationary to those where it is constantly mixed and in motion.