Sludge sorts

Biological wastewater treatment produces different sorts of sludge within the individual process steps. In the wastewater linguistic usage the following terms are used for sludge.

• Raw sludge
  Raw sludge is untreated non-stabilized sludge, which can be taken from wastewater treatment plants. It tends to acidify digestion and produces odour.

• Primary sludge
  Primary sludge is produced through the mechanical wastewater treatment process. It occurs after the screen and the grit chamber and consists of unsolved wastewater contaminations. The sludge amassing at the bottom of the primary sedimentation basin is also called primary sludge. The composition of this sludge depends on the characteristics of the catchment area. Primary sludge consists to a high portion of organic matters, as faeces, vegetables, fruits, textiles, paper ect. The consistence is a thick fluid with a water percentage between 93 % and 97 %.

• Activated Sludge
  The removal of dissolved organic matter and nutrients from the wastewater takes place in the biological treatment step. It is done by the interaction of different types of bacteria and microorganisms, which require oxygen to live, grow and multiply in order to consume the organic matter. The resulting sludge from this process is called activated sludge. The activated sludge exists normally in the form of flakes, which besides living and dead biomass contain adsorbed, stored, as well as organic and mineral parts.
  The sedimentation behaviour of the activated sludge flakes is from great importance for the function of the biological treatment. The flakes must be well removable, so that the biomass can be separated from the cleaned wastewater without problems and a required volume of activated sludge can be pumped back into the aerated part.

Biological aeration basin

Final clarifier

• Return activated sludge
  The activated sludge flows from the biological aeration basin into the final clarifier. The activated sludge flakes settle down to the bottom and can be separated from the cleaned wastewater. The main part of the separated sludge, which is transported back to the aeration basin, is called return activated sludge.

• Excess sludge, secondary sludge
  To reach a constant sludge age the unused biomass has to be removed from the biological treatment system as excess sludge. The excess sludge contains not-hydrolysable particulate materials and biomass due to metabolisms.

• Tertiary sludge
  Tertiary sludge is produced through further wastewater treatment steps e.g. by adding a flocculation agent.

• Bulking sludge, Floating sludge

• Digested sludge
  Digested sludge accrues during the anaerobic digestion process. It has a black colour and smells earthy. As a function of the stabilization degree anaerobic sludge exhibits an organic portion of the solid from 45 to 60 %.

Sludge sorts

More information about sludge treatment and reduction are on the following pages:

Stabilisation
Thickening
Dewatering
Sludge drying
Copious growth of filamentous organisms - problems and solutions

Filter presses for sludge treatment