The invention concerns a method for dehydrating sludge, comprising adding a basic agent to sludge having a pH not more than 8, adding to the sludge at least one organic flocculating constituent, by adding said basic agent, increasing the sludge pH to a value not more than a pH at which occurs degradation of said at least one organic constituent, flocculating the sludge and separating in the flocculated sludge dehydrated sludge from a liquid phase, said basic agent being a magnesian limestone compound of formula xCaCO.sub.3,(1-x)[yMg(OH).sub.2+(1-Y) MgO], wherein 0,45:=x=0,75, and 0=y=, the treated sludge having, until after said separation, said value less than a pH at which occurs degradation of said at least one organic constituent.