Which treatment stage typically follows primary treatment in municipal wastewater treatment?

In municipal wastewater treatment, primary treatment is primarily focused on removing large solids and sediments from the wastewater. This stage often involves processes like screening and sedimentation, which help reduce the load of suspended solids and organic matter. The next logical step in the treatment process is secondary treatment, which builds upon the removal achieved during primary treatment.

Secondary treatment aims to further reduce the biochemical oxygen demand (BOD) and suspended solids in the wastewater. This is typically accomplished using biological processes, where microorganisms break down organic matter. Common methods used in secondary treatment include activated sludge systems, trickling filters, and biofilm reactors. These processes are essential for making the wastewater safe enough to either be discharged into the environment or undergo further treatment.

Tertiary treatment, on the other hand, occurs after secondary treatment and focuses on polishing the effluent to remove remaining contaminants, often involving advanced filtration or chemical treatments. Filtration can also be a part of tertiary treatment, but it does not follow primary treatment directly. Sludge disposal pertains to the management of the residual solids that result from primary and secondary treatment but is not a direct stage of treatment.