sludge centrifugation suitability

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When a sample of sewage fresh mixed sludge is subjected to a 2,000 g centrifugal field for a few minutes in a laboratory centrifuge, we will find the following in the centrifuge pot (figure 29):

  • cloudy supernatant liquid;
  • bottom sediment featuring two main areas:

- one lower and fairly hard zone (DM = 25-35 %);

- one upper and extremely pasty zone (DM = 10-15 %) with a high OM content.

If the same experiment is repeated with sludge that has been flocculated using a polyelectrolyte, we find:

  • a clarified supernatant liquid;.
  • a homogenous sediment having an intermediate dry solids content (DM = 18-22 %) and good cohesion.
centrifugation laboratory urban sludgeSecured image
Figure 29. Example of centrifugation carried out in a laboratory on urban sludge

This capacity of a sludgy suspension for separating into two distinct phases when a centrifugal force is applied (therefore, without the barrier formed by a filtering medium as would be the case with a belt filter or a filter press), expresses sludge «centrifugability».

Polymer conditioning is essential to obtaining a good solid-liquid separation.