Which stage of wastewater treatment is characterized by the removal of sand and gravel?

The stage characterized by the removal of sand and gravel is known as Primary-Grit Removal. This step is essential in wastewater treatment processes as it effectively eliminates larger inorganic particles, such as sand and gravel, that can cause damage to equipment and interfere with downstream processes.

During the grit removal phase, the wastewater flow is typically slowed down, allowing these heavier particles to settle out of the water due to gravity. This not only protects the mechanical components of treatment facilities but also enhances the performance of subsequent treatment stages by increasing the efficiency of processes like biological treatment or sedimentation.

The other stages mentioned in the options serve different purposes. For instance, Primary-Comminutor focuses on chopping or shredding larger debris into smaller pieces to prevent clogging in the later stages. Primary-Screening involves using screens to remove larger solids and debris but does not specifically target sand and gravel. Tertiary Treatment is a more advanced stage that addresses additional contaminants and is typically employed after primary and secondary treatment processes.