The secondary treatment stage follows primary settling. During secondary treatment, the wastewater is pumped to one of two treatment units.
Rotary Trickling Filter Unit
One of the units is a large circular distributor that moves and sprays the water over layers of plastic media. This unit is called a "rotary trickling filter".
The wastewater that is sprayed on the plastic media then "trickles down" (hence the name) the plastic to an underdrain system at the bottom where the water is collected, then transferred. While the water is trickling down the plastic media, bacteria that are growing on the media "eat" the leftover dissolved organic material and nutrients that are in the water.