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Polluting elements and their processing products separated from the liquid through sedimentation or flotation during any water treatment, irrespective of the nature of this pollution, will finally be collected in more or less concentrated suspensions called "sludge".

The feature common to all this sludge is that it produces a waste that is still very liquid (only 0.5 to 5% suspended solids in most cases) which will contain:

  • suspended solids that will settle naturally;
  • products resulting from the use of chemical treatments (flocculation, neutralisation, precipitation…); e.g. iron or aluminium hydroxides, other metal hydroxides, crystalline products such as carbonates, calcium sulphates, iron phosphates …;
  • excess biological sludge produced by any biological treatment and including, in its floc, excess biomass and solid matter that is not biodegradable or that is too coarse for biodegradation but that has been trapped in or with the bacterial floc.

Some sludge is inert but even sludge that has a low organic matter content and biological sludge will become fermentable and evil smelling after a certain time (the higher the temperature, the shorter this time becomes). The sludge, therefore, needs to be "stabilised".

All sludge calls for specific treatment whether it is recycled, re-used or disposed of in the natural environment.

This treatment usually consists in concentrating/eliminating water (thickening followed by dewatering) and stabilisation.

Sludge processing has become an inevitable corollary that cannot be separated from water treatment. Sometimes, sludge processing may even cost more (investment and/or operating costs) than the water treatment itself.

This chapter addresses two essential aspects required to establish optimum sludge treatment, i.e.:

  • sound knowledge of the sludge to be treated: classification and characterisation of this sludge see sections sludge characterisation/classification and sludge properties;
  • a precise idea of possible conditions governing sludge discharged outside the plant: the final destination of the sludge. Destinations that are backed by a great many regulations that vary from country to country; regulations where the main texts have been reviewed under sludge end uses.

Additionally, chapter liquid sludge treatment, provides a description of the main tools and systems used in liquid sludge treatment; chapter dewatered sludge treatment provides a description of "post-treatment" that has to be carried out on dewatered sludge so that it can be recycled and/or disposed of as ultimate waste.

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