Experimental characterisation of activated sludge behaviour during mechanical expression

Abstract

Even after mechanical dewatering, activated sludge contains a large amount of water. Due to its biological nature, composition and also the type of treatment it comes from, this material is usually highly compressible and known to be difficult to dewater. In the present work various tests are proposed to try to highlight some aspects which could explain the poor dewaterability of activated sludge. Experiments of expression were carried out in a Filtration-Compression Cell for semi-solid samples of sludge. Operating conditions (intensity of the applied pressure, initial thickness of sludge sample) and the composition of the sludge (ionic strength by adding monovalent salt) were modified for the different series of experiments. It was observed that the removal of water from the sludge depended significantly on the pressure gradient throughout the sludge cake. However, the development of a dense layer of cake at the interface cake/filter medium seems to generate a pressure gradient non-uniformly distributed throughout the thickness of the cake. This phenomenon should control in part the efficiency of dewatering. This study also discusses the link between sludge dewaterability and cake relaxation on the basic of an osmotic effect within the flocculated matrix, which tends to resist against compression or deformation. This effect was reduced when both the applied pressure and the time increased.Finally, the activated sludge dewatering during expression stage depends on both the formation Author-produced version of the paper published in Process Safety and Environmental Protection, vol. 88, 200-206. May 2010 http://www.sciencedirect.com ( Elsevier http://dx.doi.org/10.1016/j.psep.2010 2 of a dense layer of cake at the interface cake/filter medium and an osmotic effect.

Optimization Study of Municipal Sludge Conditioning, Filtering, and Expressing Dewatering by Partial Least Squares Regression

“…Although several researchers have studied the expression characteristics of sludge, the above questions have not been explained in detail. [16][17][18] Novak et al [14] reported the effect of filtration pressure on expression performance when the expression pressure was limited to below 300 kPa, which is much less than that in practice. On the other hand, the filtration and expression apparatus they employed was batch feed and the expressed water may be as little as 2-4 mL, neither of which matches with plant-scale operation.…”