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During the treatment of wastewater, sewage sludge is removed at an early stage. This then goes through a process to separate out the useful parts from the pathogens and other harmful things before safe disposal.

Sludge Thickening

Some of the specialized tools used to treat and dispose of sewage from wastewater include sludge dewatering equipment and gravity thickeners. Reducing sludge volume with gravity thickeners or dissolved air flotation solidifies it for easier handling.

Sludge Digestion

A series of tanks then hold the sludge while specific bacteria digest the organic solids into more stable substances, this reduces the total mass and destroys pathogens for easier dewatering. This is a two-part process where the dry sludge is heated and mixed in a sealed tank to aid the digestion process of acid-forming bacteria and then moved to a second tank where more bacteria produce carbon dioxide and methane. The methane can be collected and sometimes used to fuel the heating of the first tank.


There is still quite a bit of water present in the sludge at this point and drying it out as much as possible helps disposal methods. This can be done on drying racks, which can take weeks, or through mechanical processes such as a centrifuge, rotary drum vacuum filter or a belt filter press for a quicker result.


The dried sludge can then be buried in a sanitary landfill, used as fertilizer or incinerated depending on the chemical composition. The more toxic the composition, the more likely it is to be turned to ash instead of buried or used as fertilizer.

Sewage sludge is often the first byproduct of wastewater reclamation and can pose the most health risks if left untreated. This mass includes organic and inorganic compounds and is reduced in volume and mass by thickening and then by bacterial digestion which destroys any pathogens present. It can then be dried out and disposed of.