7.6 Properties of sludge pumping

Properties of sludge pumping

The properties of sludge pumping is important to understand when selecting pumps for sludge applications. Sludge is the name given to the residue which forms when inorganic and organic particles are separated when a liquid is cleaned. The following mechanical and chemical cleaning methods are used:

  • Sedimentation-settling, the particles fall to the bottom in a separator under their own weight
  • Floatation, the particles are made to float by the injection of small air bubbles
  • Centrifuging, the particles are separated by centrifugal force
  • Filtering, the liquid is passed through a filter which allows the liquid to pass through whilst trapping the particles. The filter may consist of a grill, porous material or one or more beds of filtering media
  • Precipitation flocculation, by the addition of chemical reagents, the particles form insoluble combinations flocs, which can be separated by sedimentation or floatation.

When pumping the properties of sludge like the Dry Substance, DS value or pumpability of the sludge, is of interest as is also the size and hardness of the particles. Since sludge always occurs in the treatment of water and in industrial processes, there can be no general definition.

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The values obtained in the treatment of sludge in municipal sewage works can be used however as a guide.

Dry Substance or DS-content after various treatment stages:

Before thickening

DS content (%)

Chemical sludge (scum) after preciptation, flotation

0,5-1

Mechanical sludge, sedimentation, floatation

2-3

After thickening

Mechanically agitated sludge

6-10

Activated sludge

2-3

Bio-filtration

4-8

Mechanical and activated sludge

5-8

After straining in the centrifuge, filter screen press, vacuum filtering, etc.

>30

In the bottom, or compression zone, of sedimentation basins, sand traps, oil tanks and other containers of stationary liquids, the bottom layer, during sedimentation is subjected to mechanical pressure due to the weight of sludge lying above it, which causes liquid to be compressed out of the bottom layer thus increasing its density. Other names given to sludge are sediment and slurry.

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