The invention relates to a method for processing sludge, comprising the addition of a lime-based basic agent to a sludge having a first pH value in order to raise the pH to a second value higher than the first one, adding at least one anionic organic flocculating agent that is active at said second pH value, flocculating the sludge, and separating the flocculated sludge between dehydrated sludge and a liquid phase, wherein the sludge to be dehydrated has a first above-mentioned pH value lower than 9, while the lime-based basic agent induces said pH raise to said above-mentioned second value within less than 5 minutes. The invention also relates to a composition for implementing said processing.