Sewage sludge is the residue of wastewater treatment, which must be stored before reused in agricultural or incineration. But storage areas appear often under-dimensioned due to creep flow of sludge. In this paper, we intend to demonstrate that a sludge can't be stored in heap because its flowing structure. Using rheometric experiments with pasty sludge and the pressure filtration with non-limed solid sludge, we demonstrate that a sludge is deformed as long as an external stress is applied, whichever its structure. Moreover, we show that fermentation affects the heap behavior : the sludge is more fluid during aging, but paradoxically, solid-liquid interactions are stronger. Finally, we highlight that leaching is a consequence of the creep flow of sludge in storage areas.