During preliminary treatment, which of the following materials is commonly removed?

During preliminary treatment in wastewater treatment processes, the primary focus is on the removal of larger, non-biodegradable materials that can hinder subsequent treatment processes. This phase typically involves screening and grit removal to eliminate items that could cause mechanical problems or increase operational costs.

Rags, sand, and eggshells are common examples of materials targeted during this phase. Rags can clog equipment and interfere with flow, sand can accumulate and require costly removal, and eggshells, while less common, are part of the larger organic debris that can be found in the influent. By removing these materials early in the treatment process, facilities can ensure that the mechanical components of the plant operate efficiently and reduce wear and tear on equipment.

Other materials, such as microorganisms, heavy metals, and nutrients, are typically addressed in later treatment stages where more advanced methods, such as biological treatment and chemical processes, are employed. Thus, the focus of preliminary treatment is on removing larger physical contaminants to protect the integrity of the treatment plant.