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.
- 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
%.
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
- 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 %.
Final clarifier
Collected By : Khan Rajib Hossain
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