Wastewater treatment
Wastewater treatment uses microbes to decompose organic matter in sewage. If too much untreated sewage or other organic matter is added to a lake or stream, dissolved oxygen levels will drop too low to support sensitive species of fish and other aquatic life. Wastewater treatment systems are designed to digest much of the organic matter before the wastewater is released so that this will not occur. Treatment systems use physical, chemical, and biological processes:

• Primary treatment physically removes large solids using grates, screens, and settling tanks.
• Secondary treatment promotes growth of bacteria and other microbes that break down the organic wastes. These biodegradation processes also take place in streams, lakes, and oceans, but the purification systems in nature can easily be overloaded with input of too much organic waste. Secondary treatment prevents this type of pollution by degrading most of the organic matter before the water is released into the environment.
• Tertiary treatment is used only where it is needed to protect the receiving waters from excess nutrients. In tertiary treatment, the concentrations of phosphorus or nitrogen are reduced through biological or chemical processes.
• Disinfection kills disease-causing organisms, most commonly through chlorination.
  

Sludge, the collection of solids that are removed during wastewater treatment, requires processing to reduce odor and water content. Depending on the disposal method, the sludge also may undergo treatment to decompose organic matter or kill disease-causing organisms.

Wastewater Treatment Links:

• Wastewater Treatment Principles and Regulations
• How Sewer Systems and Septic Systems Work
• Water Purification: An Essential Ecosystem Service (click on "Water Purification")
• Wastewater Treatment Hands-On Activities (Water Environment Federation)
• Cornell University Waste Management Institute: Sewage Sludge
• Water Quality and Treatment (American Society for Microbiology)