Secondary treatment of wastewater is primarily a __________ process.

The secondary treatment of wastewater is primarily a biological process. This stage focuses on the removal of dissolved and suspended organic matter primarily through the action of microorganisms. During this treatment, bacteria and other microorganisms metabolize the organic pollutants present in the wastewater, converting them into biomass and more stable forms of matter that can be further processed and removed.

Biological treatment methods, such as activated sludge systems and trickling filters, are commonly employed in secondary treatment facilities because they are effective in breaking down organic materials and significantly reducing the biochemical oxygen demand (BOD) of the wastewater. This crucial step is essential for ensuring that the water is suitable for discharge into the environment or for further treatment before reuse.

The other processes mentioned—physical, chemical, and thermal—are typically more relevant in different contexts or stages of wastewater treatment, such as during preliminary treatment or in specific disinfection or sludge processing stages, but they do not define the nature of secondary treatment as effectively as biological processes do.