Peak Flows at Sewage Treatment Plants

Dry weather at Treatment Plants

Most municipalities use a series of unit processes to treat wastewater prior to discharge. The most common unit processes at municipal wastewater treatment plants include:

• preliminary treatment or screening to remove large solids,
• primary clarification (or preliminary sedimentation) to remove floating and settleable solids,
• secondary treatment (often using biological processes) to remove biodegradable organic pollutants and suspended solids, and
• disinfection to deactivate pathogens.

Many treatment plants also have advanced treatment to reduce constituents, such as nitrogen and phosphorus, and provide additional polishing.

Wet Weather at Treatment Plants

Some municipalities currently experience high influent flows during periods of wet weather, referred to as peak flows, that exceed the treatment capacity of existing treatment units. This is typically a result of inflow and infiltration (I/I) into the sanitary sewers. The additional flow from I/I can increase the frequency and volume of sewer overflows, including backups of sewage into buildings, as well as the flows conveyed to the treatment plant. 

Peak flows can overwhelm critical biological treatment units, such as those often used for secondary treatment, because they can be sensitive to the large changes in flow rate and wastewater characteristics. This can cause short-term operational challenges and long-term damage to the plant, both of which reduce treatment effectiveness.  

Learn about the many strategies and technologies that support peak flow management.