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Wastewater treatment plants produce a residue that must be usually dewatered to reduce volume before handling and disposal, but operating parameters may impact activated sludge dewatering. On the one hand, the results obtained from tests conducted in the filtration-compression cell (FCC) show an impact of the initial slurry concentration and its amount on filtration kinetic. However, a scaling with respect to the initial amount enables an interesting representation of these data. On the other hand, the working pressure does not modify filtration kinetics, but it has a significant effect on the expression stage. Larger amount of filtrate is extracted when a higher pressure is applied for the expression stage. Moreover, the squeezing ability of the filter cake greatly depends on the time when the pressure step is applied. All these results are not fully modelled by the current theories, and Ruth's equation, which is commonly used, has its limitations in being able to properly describe activated sludge filtration. Because the visco-elastic behaviour of filter cake is generally not considered by classical dewatering models, further work is needed to take into account this phenomenon. 

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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.

In France, more than half of the production of sewage sludge is spread on agricultural fields as a fertilizer. However, these biosolids contain nutrients that can pollute the soil if their distribution on field is heterogeneous: it is then necessary to optimize the spreading in order to limit their environmental impacts.In that purpose, this study focuses on the impact of pasty materials rheological behaviour on spatial distribution parameters obtained with a centrifugal spreader, i.e. a spinning disc system. Kaolin suspensions were employed. To model their rheological properties, a HerscheleBulkley equation was used, which is consistent with the literature. Experiments were performed with a specific spreading device (a vane with no disc) and with the CEMOB, a bench dedicated to the analysis of organic fertilisers spreading. Results showed that radial distribution of the 3D spread patterns is clearly dependent on the size and the density of the particles, both of them being related to the material consistency. Angular distribution also tends to follow similar tendencies. Eventually, advanced imaging shows a relation between fragmentation process and material consistency. This study presents a new approach to analyse organic spreading and opens the way to more developments.