the dry solids contend approach of an industrial centrifuge

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This laboratory method is used to forecast the dry solids content that can be achieved with an industrial clarifier and applies to all sludge.

test principle

Sludge is first flocculated using an appropriate polyelectrolyte, i.e. a polymer that produces a very resistant large floc, releasing a maximum of interstitial liquid and for minimum amounts of said polymer: selection made according to the method detailed in the drainability tests taking care to select the floc that is best at withstanding vigorous agitation.

Flocculated sludge is then drained through the sample screens.

The sludge drained and, therefore, thickened in this way is placed in the filter scoop of a laboratory centrifuge. The sludge is centrifuged at 2000 g for 5-10-15 and 20 minutes.

apparatus

  • GDD screen, GDE6 screen or GDE8 screen (for drainage);
  • laboratory centrifuge:
    • 2 000 g centrifugal field;
    • fitted with a spindle (four-point star) to hold the centrate collection scoops and the metal filter scoops.
  • solid scoops: optimum capacity = 200 to 250 mL.
  • filter scoops: optimum capacity = 100 mL (photo 8 and figure 23):
    • four 100 µm mesh scoops: for «fine» sludge, without support, with a high proportion of biological sludge and hydroxides;
    • four 200 µm mesh scoops: for more heterogeneous dense or fibrous sludge such as sludge containing a high proportion of primary sludge.
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Photo 8. Solid scoops – filter scoops
Apparatus capillary suction timeSecured image
Figure 23. Apparatus for measuring the capillary suction time

operating method

sample to be processed

  • minimum of 10 litres of sludge;
  • establish sludge properties:
    • suspended solids (g· L-1)
    • Dry solids content (%)
    • pH
    • Loss to heat (105-550 °C) = DS-VM;
  • choice of polymer (on samples of at least 500 mL): refer to Büch­ner drainability tests (see drainability tests);
  • test carried out with a classic solid scoop and sludge that has been flocculated at 2 000 g for 5 minutes.

filter scoop test (100 or 200 mm)

Having selected the mesh:

  • drain the 500 mL of flocculated sludge as specified for the drainability tests using screens (see drainability tests);
  • fill each of the four filter scoops up to the flange.

N.B: the amounts of polymer used for test purposes produce a 20% excess amount compared with the normal beaker amount.

Centrifuge for 5-10-15 and 20 minutes.

Comment: interrupt centrifugation every 5 minutes to mash up the cakes in the scoops using a metal spatula in order to break down any water pockets that may have formed.

interpretation of results

Our experience has shown that an industrial centrifuge used at its nominal operating rate and with an optimum amount of reagent, will produce a cake having a dry solids content close to that obtained with measurements carried out on filter scoop pellets (depending on sludge type, we will use the 10 minute or 20 minute test to forecast industrial dry solids content).