What is the main purpose of secondary treatment in wastewater treatment?

The main purpose of secondary treatment in wastewater treatment is to remove dissolved and suspended organic matter. This stage follows preliminary treatment, which typically focuses on removing large solids and debris, and is designed to significantly reduce the organic load in wastewater.

Secondary treatment employs biological processes, where microorganisms break down organic matter. Techniques such as activated sludge processes, trickling filters, or lagoon systems are commonly used, allowing these microorganisms to metabolize organic pollutants. The outcome is a cleaner effluent with fewer contaminants, which is essential before the water can be considered for discharge or further treatment.

Options that suggest functions such as settling heavy solids, adding nutrients, or disinfecting the effluent do not accurately describe the primary intent of secondary treatment. Instead, those processes occur at different stages or levels within the overall wastewater treatment strategy. For example, settling of heavy solids typically happens during primary treatment, while disinfection is often the final step before effluent release.