What develops as solids settle to the bottom of a sedimentation basin?

As solids settle to the bottom of a sedimentation basin, a sludge layer develops. This process occurs because, during sedimentation, heavier particles in the water column gradually fall under the influence of gravity. The accumulation of these particles at the bottom leads to the formation of the sludge layer, which consists of concentrated settled solids.

This sludge is typically rich in organic material and can contain various contaminants that were present in the water. It is important for water treatment processes to manage this sludge effectively, as it can affect the overall efficiency and safety of the treatment operation.

In contrast, options like a crust layer or a filter layer refer to different phenomena not directly related to the settled solids in this context. While sedimentation might lead to the formation of layers or crusts on the surface, these are not the specific layers formed by the settling of solids at the bottom of the basin. Thus, the correct terminology regarding what accumulates at the base of a sedimentation basin is indeed the sludge layer.