Centrifugation of activated sludge is a frequently used dewatering process but
there is a need for a fitting function for moderate and high sludge concentrations. In
particular, liquid phase accumulation kinetics during centrifugation may be used as a
source of information about sedimentation properties and governing mechanism of
the cake formation. This will allow the obtention of the optimal parameters for the
dewatering control and process optimisation.
For this purpose, activated sludge at different concentrations was investigated on
a laboratory centrifuge with centrifugation factor 2667. The following sludges were
used in the experiments: 1) activated sludge from thickener (with and without
floculant treatment); 2) sludge after industrial centrifuge decanter Flottweg Z62-4
collected from dumping pound; sludge (2) after anaerobic treatment; sludge (2) with a
dispersed solid additive.
Based on the obtained data, the following assumption was made: settling of the
solid phase and cake compaction depends on the hindered settling velocity and
sediment compression. Due to the applied centrifuge force, a primary cake skeleton
from particular aggregates and flocs is formed. During this stage, free water is
displaced from the space between aggregates/flocs with a further transition to the
compaction of the sediment with the water being displaced from the pores of the flocs
by a filtration mechanism until an equilibrium condition is reached.
The liquid volume V vs. time t at hindered settling and sediment dewatering can
be represented in the form Vα tⁿ, where n ≤1.0. Therefore, in log-log coordinates,
the kinetics of liquid accumulation have a linear dependence lg(V ) α n lg(t) in both
settling and compaction stages. This assumption was confirmed for different types of
activated sludge.
Using asymptotical analysis in the function form F(t,V) , a dimensionless fitting
function was obtained that describes the centrifugation of activated sludge. For highly
concentrated sludge, it was found that the dewatering occurs as cake compression.
Analysis of deformation models leads to the conclusion that for the compressible
cake it is necessary to introduce a parameter characterising the cake plasticity
dependence on the centrifugation time.
Developed model was used for fitting numerous experimental data. The main
advantage of proposed model is the possibility to fit the liquid phase accumulation
kinetics during centrifugation in a wide range of the activated sludge concentration,
from suspension up to structured and paste-like cake consistency. The extension of
this model to other slurries requires further investigation