What distinguishes "primary" effluent from "secondary" effluent?

Primary effluent and secondary effluent are key terms used to describe the stages of wastewater treatment. Primary effluent is the result of the initial phase of treatment, where physical separation processes such as screening and sedimentation remove a significant portion of the suspended solids and some organic material. However, at this stage, the effluent has still not undergone comprehensive treatment to further reduce organic matter and nutrients.

In contrast, secondary effluent has undergone biological treatment processes, which typically involve the action of microorganisms breaking down organic matter that remains after primary treatment. This biological treatment phase aims to significantly reduce the concentration of organic pollutants and nutrients in the effluent.

The distinction between these two types of effluent is critical because it indicates the different levels of treatment and purification the wastewater has undergone. Understanding this distinction helps in the evaluation of effluent quality and the effectiveness of the wastewater treatment process.