What is the average detention time for wastewater in a primary clarifier?

The average detention time for wastewater in a primary clarifier is typically around 2 hours. This duration allows for adequate settling of solids present in the influent wastewater, which is critical to effective clarifier operation. During this time, heavier particles settle to the bottom and can be removed for further treatment, while lighter materials and scum rise to the surface for skimming.

A detention time of about 2 hours strikes a balance between the need for effective sedimentation and the potential for providing sufficient flow capacity within the treatment plant. Detention times shorter than this may not allow for sufficient solids removal, while longer times could lead to issues such as flow saturation or the stagnant conditions that might result in the release of odors or other quality concerns. Therefore, a 2-hour detention time is a well-accepted practice that supports optimal performance in a primary clarifier.