What area of a basin receives the settled solids and keeps them separated from other particles?

The sludge zone is the correct answer because it specifically refers to the area within a basin or sedimentation tank where settled solids, also known as sludge, accumulate. This zone is crucial for separating these heavier particles from the water and any remaining lighter suspended solids.

In a typical sedimentation process, water flows through the basin and allows particles to settle out under the influence of gravity. The sludge zone collects these settled solids at the bottom, preventing them from mixing back with the clearer water above.

In contrast, the settling zone is where the settling of particles begins, but it does not specifically denote the area where solids accumulate. The influent zone is the area where untreated water enters the system, and the effluent zone is where the treated water exits, both of which are not involved in the direct accumulation of settled solids. Understanding these distinctions is important for proper management and operation of treatment facilities.