As the global population grows, the volume of human waste inevitably grows as well.
In sewers and drains, this waste is “sewage;” in onsite sanitation systems, where it is primarily solid, it is “septage.” Treated wastewater contains solid residues known as sludge. The solid content of mechanically dewatered sludge typically ranges from 20%-45% solids by weight. Solid residues in onsite systems take the form of septage or fecal sludge, which needs to be emptied either manually or mechanically once the underground facility is filled.
These residues are high in methane content and are therefore a significant passive contributor to greenhouse gas emissions from cities, as methane seeps out from millions of small sources such as septic tanks, waste dumpsites, littered organic wastes, and others.
The massive volume of wastewater generated in cities is disposed of in two primary ways: either treated through public investments in piped sewerage systems and wastewater treatment plants, or through private investments in septic tanks, pit latrines and other infrastructure. In most low-income developing countries, capital investments in wastewater infrastructure is lacking, resulting in a patchwork of private and community solutions through on-site waste disposal.
There is currently no comprehensive data available on this indicator.