A region at the bottom of a sedimentation tank used to separate solid particles from a supernatant liquid. The particles are allowed to settle in the settling zone allowing them to accumulate as a sludge at the bottom. It therefore provides a storage area for the sludge before its removal for treatment or disposal. The zone is designed to have low velocities such that the sludge accumulates and remains undisturbed, which could otherwise lead to washout. The sludge is removed by scraper or by vacuum devices that move along the bottom.