Alum, lime, or iron salts are added to effluent during which stage of municipal wastewater treatment?

During tertiary treatment, alum, lime, or iron salts are introduced to effluent to aid in the removal of residual solids and nutrients that remain after primary and secondary treatment stages. This phase of treatment is focused on polishing the effluent quality, often addressing specific contaminants, such as phosphates and suspended solids.

The use of chemical coagulants, like alum or iron salts, enhances the process of flocculation, where small particles and impurities agglomerate to form larger particles (flocs) that can subsequently be removed more effectively, resulting in cleaner water. This is crucial for meeting stringent discharge standards and ensuring that the treated effluent is safe for environmental discharge or potential reuse.

In contrast, primary treatment primarily involves physical removal of larger solids via screening and sedimentation, while secondary treatment utilizes biological processes (like activated sludge or trickling filters) to break down organic matter. Disinfection follows tertiary treatment and focuses specifically on reducing pathogenic organisms in the treated effluent. Each stage has distinct functions and methods, but the addition of chemical agents is a characteristic feature of tertiary treatment aimed at enhancing overall water quality.