The treatment of wastewater mainly produces sludge in addition to purified water. These sludges are composed of dead bacteria (those involved in the purifying process) and mineralised organic matters. They may come from urban or private sewage treatment plants.
In the wastewater treatment process, one of the commonly used steps is the aeration phase. This involves bringing oxygen to the water to promote the development of bacteria that digest the organic matter contained in the wastewater. The sludge is mainly produced during this stage.
Depending on their quality, sludge can be spread as a fertiliser directly or digested by biomethanisation before being spread. It is also possible to recover energy by gasification of concentrated sludge. This allows energy to be recovered in addition to the ability to fertilise land, while reducing the costs of water purification.
Biorefining can also be considered to extract platform molecules out of this material. However, this promising technology is still not very widespread.