Name the primary treatment process used in most wastewater treatment plants.

Sedimentation is the primary treatment process used in most wastewater treatment plants as it plays a crucial role in removing settleable solids from the wastewater. In this process, wastewater is held in a large tank, allowing heavier solids to sink to the bottom, forming sludge, while lighter materials, such as oils and greases, may float to the surface as scum.

This separation is essential because it significantly reduces the concentration of suspended solids in the water, which is a fundamental goal of primary treatment. The sedimentation process is typically the first step in the treatment train, effectively reducing the organic and inorganic load on subsequent treatment processes, such as biological treatment and secondary clarifications.

While other processes mentioned play critical roles in wastewater treatment, they usually come after sedimentation. For example, aeration is involved in biological treatment and helps to introduce oxygen for microorganisms to break down organic matter, while filtration is more commonly part of tertiary or advanced treatment stages. Activated sludge is a process used in secondary treatment, where microorganisms are used to degrade organic pollutants but relies on the preliminary removal of solids through sedimentation first.