What occurs at the bottom of a sedimentation basin as solids settle?

• A. A sludge layer develops
• B. Water begins to evaporate
• C. Chlorine is added
• D. Flocculation takes place

Answer: (A)

As solids settle at the bottom of a sedimentation basin, a sludge layer develops. This process is a fundamental part of water treatment, where suspended particles, impurities, and sediments gradually drop out of the water column due to gravity. The settled solids accumulate at the bottom, forming thick layers of sludge that can be removed for further treatment or disposal.

The formation of a sludge layer is crucial because it signifies that the sedimentation process is effectively removing contaminants from the water, clarifying it for subsequent treatment steps. This sludge may contain various materials, including silt, clay, organic matter, and other particulates that have been removed from the water, leading to improved water quality.

By contrast, the other options do not accurately describe the processes that occur at the bottom of a sedimentation basin. Water evaporation does not typically occur significantly in this context, nor is chlorine added during the sedimentation phase, which primarily focuses on settling solids. Flocculation, while an important part of the overall treatment process, occurs prior to sedimentation and involves the agglomeration of particles to enhance the settling process, rather than at the bottom of the basin itself.