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.