All the treatment steps so far are referred to as primary treatment. The sewage now enters an aeration tank and held for six to eight hours, while compressed air is pumped through the sewage. This provides oxygen for aerobic bacteria which decompose suspended and dissolved organic wastes. The wastewater and bacteria flow into a final clarifying tank where bacteria form clumps. These steps are referred to as secondary treatment.
Some treatment plants add additional tertiary steps which remove phosphorus and nitrogen.
The final step is to add chlorine which kills any pathogens that may have survived the other treatment steps. When a treatment plant malfunctions, or when water supplies are contaminated by floodwaters or when raw sewage is diverted around a treatment plant, waterborne epidemics can result. Water should be boiled before used for drinking, or bottled water should be purchased. Some major types of water pollutants are pathogens, oxygen-demanding wastes, aquatic plant nutrients, sediments, toxic chemicals, oil spills, and heat. To humans, the pathogens would be the greatest threat to their health.