What is the typical role of sand and gravel in wastewater?

In wastewater treatment, sand and gravel primarily serve the role of mechanical filters. They are often used in filtration systems to remove suspended solids and particulates from the wastewater. The structure and size of sand and gravel allow them to trap particles as water flows through them, which helps to improve water quality before further treatment processes.

While it is true that sand and gravel can be found in wastewater, labeling them as unwanted ignores their important function. Instead, they play a crucial role in enhancing the effectiveness of treatment systems, as they help in the physical removal of contaminants. Other options like acting as a nutrient source or facilitating evaporation do not accurately reflect the purpose of sand and gravel in wastewater. Their primary function is tied to filtration, making them an integral part of many sedimentation and filtration processes in sewage treatment.