Secondary wastewater treatment uses microorganisms to break down organic waste in sewage. There are three main processes: (1) Trickling filters use aerobic bacteria and media to treat sewage as it passes through a bed. (2) Rotating biological contractors have discs coated with microbes that remove organic matter as the discs rotate in wastewater. (3) Activated sludge treatment agitates and aerates a mixture of wastewater and microbes to remove 85% of organic waste over 6-8 hours before solids are removed. Tertiary treatment then uses chlorine or UV light to disinfect the treated water before release.