Final answer:
Sludge in the bottom of a reservoir or sedimentation tank is caused by solids settling due to gravity during water treatment. Heavier particles form sludge, while lighter materials like grease and oil form sum that floats on the surface.
Step-by-step explanation:
The sludge at the bottom of a reservoir or in the context of wastewater treatment, a sedimentation tank or primary clarifier, is caused by the settling of solids.
During the water treatment process, heavier particles in the water settle to the bottom over time due to gravity. These solids include organic and inorganic materials, and in a wastewater treatment plant, they form sludge that is eventually pumped out and sent to a solids processing facility. Meanwhile, lighter materials such as grease, oil, soap, and other substances form sum which floats to the top.
To keep these materials separate, slow-moving rakes are used to collect and remove the sum from the surface.
Hence, solids settling in water treatment leads to sludge at the reservoir or sedimentation tank bottom, with heavier particles forming sludge and lighter materials like grease and oil creating sum on the surface.