Disposal of human excreta and other waterborne waste products from populated areas to sewage treatment plants. Transport is achieved by use of underground conduits called sewers, sealed to prevent untreated sewage contaminating the environment. Untreated sewage is a major risk to human health, as it is known to carry waterborne pathogens, such as cholera. Pollution associated with untreated sewage is known to cause serious damage to aquatic ecosystems by depleting oxygen levels in the water via eutrophication. Sewage undergoes a series of treatments to remove physical, biological, and chemical contaminants in order to produce an environmentally safe fluid waste stream, called treated effluent, and solid waste stream, called treated sludge. Government agencies impose strict regulations to govern the level of treatment required before these waste streams are considered safe. Treated effluent is discharged into rivers or the sea. Disposal of treated sludge depends on the levels of contaminants remaining; it may be composted to form a soil amendment material, incinerated, or sent to a landfill site.
Contamination of water supplies and the environment from untreated sewage is still a major source of disease in developing countries, especially where open sewers are common and sewage is not treated before being released. Many countries also use untreated human waste products to fertilize agricultural land, increasing the risk of waterborne diseases. A World Health Organization report published in 2008 showed that 200 million farmers in China, India, Vietnam, Latin America, and sub-Saharan Africa used raw sewage to fertilize and irrigate agricultural land.