What occurs during primary wastewater treatment?

During primary wastewater treatment, the focus is on the physical processes involved in the initial stages of treatment rather than biological or chemical interventions. This phase primarily aims to remove large solids and suspended particles from the wastewater.

The physical removal occurs through processes such as screening, where large debris like sticks, leaves, and plastics are filtered out, followed by sedimentation, which allows heavier solids to settle at the bottom of a holding tank. This settled material is often referred to as sludge. By the end of primary treatment, a significant portion of the suspended solids and some organic material has been removed, resulting in a liquid effluent that is less turbid and contains fewer solids.

While biodegradation and disinfection are part of secondary and tertiary treatment phases, respectively, they do not play a role in the primary treatment phase, making physical removal the correct process to identify in this context.