Which element is NOT typically associated with enhanced primary treatment?

In the context of wastewater treatment, enhanced primary treatment focuses on the removal of solids and some organic material through physical and chemical means, rather than biological processes. This typically includes the use of chemicals to aid solid removal, which helps settle out particles in the wastewater. The goal during this stage is to remove a substantial amount of total suspended solids and gravity-separated oils and greases.

Biological agents are more associated with secondary treatment, where microorganisms break down organic matter. This involves a process that requires specific conditions for bacteria and other biological agents to thrive and is aimed at reducing biochemical oxygen demand (BOD), among other objectives.

Thus, the reference to biological agents for treatment directly relates to a different phase in wastewater processing, making it the correct choice for what is not typically linked with enhanced primary treatment.