Which of the following settles as a solid in the bottom during wastewater treatment? 

Sludge

The correct answer is option 2. Sludge.

Out of the options listed, sludge settles as a solid in the bottom during wastewater treatment.

Here's why:

Sewage refers to the entire wastewater stream, which includes both dissolved and suspended solids. It doesn't specifically refer to the settled material.

Litter is non-biodegradable waste that would generally not be found in raw sewage and wouldn't settle as a solid.

Sludge, on the other hand, is a semi-solid material that separates from the wastewater during the primary treatment stage, which involves sedimentation or settling.

Sludge primarily consists of:

Settled organic matter: This includes organic material like food scraps, human waste, and decaying plant matter that sink to the bottom due to gravity.

Inorganic matter: This includes sand, gravel, and other mineral particles that settle due to their heavier weight.

The settled sludge is then removed from the bottom of the tank and undergoes further treatment and processing, which may involve anaerobic digestion, dewatering, and disposal or reuse.