The Dewatering Properties of Autothermal Thermophilic Aerobic Sludge: A Compressive-Rheology Approach to Characterisation

McCausland,C.1, Bartletta, J.1 and Stickland,A.D.2
1The Centre for Sustainability, Institute of Technology, Ireland, 2The University of Melbourne, Australia


Autothermal Thermophilic Aerobic Digestion (ATAD) is a high-rate aerobic digestion process that is increasingly used as a
treatment method to treat municipal wastewater sludge to Class A standards. However, the resulting sludge is difficult to
dewater, exhibiting a high conditioner demand and impermeable filter cake structure, thus making the ATAD process
undesirable if post-process dewatering is required. To fully understand and optimise the dewatering properties of ATAD
sludge, suitable characterisation techniques are first required. Characterisation techniques such as the capillary suction time
(CST) test and Specific Resistance to Filtration (SRF) are widely applied in the literature and industry, and whilst useful in
predicting dewatering trends, these methods do not readily assist in the design and optimisation of dewatering devices from
first principles. Furthermore, the empirical nature of these methods compromises the meaningful translation and comparison
of results between separate research groups. To address these problems, this work uses recently developed experimental
techniques to measure fundamental material properties of ATAD sludges in an unequivocal manner. The parameters
measured were the compressive yield stress, which characterises the compressibility of the sludge, and the hindered settling
function, which characterises the permeability of the sludge. These parameters when coupled with first-principle models are
useful for the prediction of the performance of solid-liquid separation devices such as filter presses and centrifuges. The
sludges which were characterised in this study included sludge sampled from a specially commissioned 500 litre ATAD pilot
plant, sludges sampled from the full-scale Killarney ATAD plant in County Kerry (the only ATAD plant treating municipal
wastewater sludge in the Republic of Ireland) and sludges from ATAD treatment plants in Germany and Luxembourg. The
characterisation results are presented in conjunction with selected physico-chemical properties allowing conclusions on
factors influencing ATAD dewatering to be drawn. The results show that dewatering properties deteriorated as a function of
increasing retention time for the pilot-plant sludge. The quantity of soluble proteins and polysaccharides in the liquid phase of
the pilot-plant sludge was shown to increase with retention time and these substances may interfere with ATAD dewatering.
In the full-scale plant, ATAD sludge which had been held in post-process holding tanks had significantly better dewatering
characteristics than sludge sampled from the reactors, in terms of both compressibility and permeability, and required 70%
less cationic polymer than fresh ATAD sludge to condition it to an optimal level. This suggests that operators wishing to
optimise ATAD dewatering should store the effluent sludge for as long as possible.

ATAD, Compressibililty, Dewatering, Digestion, Filtration, Permeability, Sludge

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