During which treatment stage are oily materials primarily removed?

The removal of oily materials occurs primarily during the primary treatment stage of wastewater treatment. This initial stage focuses on the physical separation of solids and oils from the wastewater. In primary treatment, processes such as sedimentation and flotation are employed to allow heavier solids to settle at the bottom and lighter materials, including oils and grease, to rise to the surface where they can be skimmed off. This is particularly important because oily substances can interfere with the subsequent biological processes in secondary treatment and can also have negative impacts on the environment if not adequately removed.

Secondary treatment mainly involves biological processes where microorganisms are used to degrade organic matter, but it is not specifically aimed at removing oily substances. Tertiary treatment further polishes the effluent to remove remaining contaminants, but the bulk of oil removal is effectively accomplished in the primary stage. Stabilization, in this context, typically refers to the stabilization of residual sludge or biosolids after digestion, rather than removal of oils from wastewater. Thus, the primary treatment is where oily materials are predominantly removed from the wastewater.